Chapter 1: Methods of Persulfate Activation for the Degradation of Pollutants: Fundamentals and Influencing Parameters

Rapid economic growth and industrialization continue to result in ubiquitous contamination of the environment and serious problems all over the world. A number of technologies have been demonstrated to protect human health, maintain ecological balance, and improve sustainability. Advanced oxidation processes (AOPs), based on radical reactions, have been the most important development methods in recent decades and involve the degradation of contaminants with reactive radical species. Sulfate radical-based AOPs have gained much success in water and wastewater treatment and are usually preferred over the other AOPs. This is because the sulfate radical anion has a higher redox potential (E⁰ = 2.5–3.1 V_NHE compared to 1.9–2.7 V_NHE for HO˙), a longer half-life (30–40 μs versus 20 ns for HO˙) and high selectivity to electron-rich compounds. Furthermore, sulfate radicals are more stable and diffuse further to contact pollutants and readily react with organic compounds resulting in complete or partial degradation of pollutants.

The conventional sources for generating sulfate radicals are persulfate (S₂O₈²⁻) and peroxymonosulfate (HSO₅⁻) species with symmetrical and asymmetrical structures, respectively. Among three persulfate salts of sodium, potassium and ammonia, sodium persulfate salt (Na₂S₂O₈) with a high-water solubility is the most conventional salt for in situ degradation of pollutants. Meanwhile, K₂S₂O₈ is also used with a lower price despite its much lower water solubility but is adequate for the desired concentrations. The salt form of peroxymonosulfate as potassium peroxymonosulfate, which is also called Oxone (2KHSO₅, KHSO₄, K₂SO₄) has good potential applicability due to its high solubility in water and is safe to handle. It is noteworthy that peroxymonosulfate requires more energy to produce sulfate radicals compared to persulfate because of the shorter length of the peroxide bond and higher dissociation energy.

Considering the difficult reaction of persulfate or peroxymonosulfate with organic pollutants, utilizing an activation method is essential to generate sulfate radicals by (1) imparting energy to the persulfate anion resulting in cleavage of a peroxide bond and forming two sulfate radicals, and (2) accepting one electron from an electron donor generating a single sulfate radical. Generally, activation methods based on utilizing synergy between multifunctional phenomena provide improved degradation.

The persulfate (or peroxymonosulfate) can be activated with no external energy through alkaline, organic substrate, or catalytic activation or with external energy such as heat, microwave, or ultrasound, as well as with photo- and electrochemical techniques. The present chapter aims to offer an extensive survey of the main theories and applications relevant to persulfate activation. The basic concepts, dominant mechanisms, as well as influencing parameters, particularly persulfate concentration, pH, temperature, involved materials and reactor configuration are discussed for each technique. Comprehensive comparison of the activation methods provides essential knowledge to select a reaction and design a suitable reactor with the aim of boosting degradation efficiency.

Based on the outline above, the content is organized into different sections. The first section focuses on persulfate activation methods with no external energy. The subsequent sections concern utilizing external energy as well as combination with a catalyst if relevant. Each process in this classification can be considered as a homogeneous or heterogeneous operation depending on the presence of solid-state reagents, catalysts, and/or electrodes. Figure 1.1 illustrates a schematic of this chapter content, organized in the order of this classification. The influence of relevant operating parameters and employed reactors are also discussed in each section.

Alkaline activation of persulfate, as a low cost and high-efficiency method, has been widely used for degradation of organic pollutants. The mechanism of sulfate radical generation from alkaline activation is based on the catalyzed hydrolysis of persulfate to hydroperoxide anion and sulfate ion (eqn (1.1)). Subsequently, another persulfate molecule is reduced with a hydroperoxide ion and sulfate radical and superoxide radical (O₂˙⁻) are generated:¹ 

Meanwhile, effective hydroxyl radicals are rapidly produced from a sulfate radical and hydroxide ion as

Activation of peroxymonosulfate under alkaline conditions has also been introduced.²  Base-catalyzed hydrolysis of peroxymonosulfate to a sulfate anion and hydrogen peroxide is the initial step of the proposed mechanism (eqn (1.4)). Then, the generated hydrogen peroxide reacts with excessive hydrogen peroxide to produce a superoxide anion radical according to eqn (1.5)–(1.7)). Highly reactive singlet oxygen is then generated through two possible pathways: (1) reaction of a superoxide anion with a hydroxyl radical (eqn (1.8)), and (2) reaction of the superoxide with itself (eqn (1.9)):

It is worth mentioning that although alkaline activation of persulfate has extensive applications in pollutant degradation, the main drawback of this technique is the consumption of large amounts of an alkaline which may potentially cause undesirable effects. Indeed, considering the generation of H⁺ during persulfate activation (eqn (1.1)–(1.2)), there is a significant demand for OH⁻ to maintain the alkaline conditions.³ 

The use of a catalyst under alkaline conditions has been proven to be beneficial for both alkaline and catalytic activations. In this regard, the efficiency of supported copper oxidate (CuO) for activation of persulfate under alkaline conditions has been confirmed. It has to be mentioned that under strong alkaline conditions, the promoted degradation is attributed to the reaction of sulfate radicals with hydroxide ions leading to the generation of hydroxyl radicals with a higher redox potential compared to sulfate radicals.⁴  The proposed mechanism for alkaline activation of persulfate in the presence of a copper oxide catalyst is illustrated in Figure 1.2.

Persulfate concentration has a significant influence on the performance of the alkaline activation process. By increasing the persulfate concentration, degradation efficiency is initially improved but further increases after the optimum concentration leading to a decrease in degradation efficiency because of the competition of excess persulfate with pollutant in reacting with the sulfate and hydroxyl radical anion:⁵ 

Different bases can be utilized in persulfate activation with different dissociation abilities. In this regard, the capability of NaOH, KOH and Ca(OH)₂ in peroxymonosulfate activation has been examined. The effectiveness of bases for alkaline activation was in the order of NaOH > KOH > Ca(OH)₂, i.e. NaOH facilitates a stronger effect due to the highest dissociation ability.² 

Considering the nature of pollutants, different degradation mechanisms are dominant in the alkaline activation of persulfate. For instance, some pollutants with low carbon oxidation states are prone to degradation through oxidation via SO₄˙⁻ and HO˙; whereas, in the degradation of chlorinated pollutants with high carbon oxidation states (such as carbon tetrachloride), a O₂˙⁻ superoxide radical is more beneficial;⁶  however, the efficiency of superoxide radicals is limited due to a very short lifetime in aqueous solution.⁷  Accordingly, the accelerating effect of methanol co-solvent in the alkaline activation of persulfate for carbon tetrachloride treatment has been confirmed.⁸ 

Introducing other less polar co-solvents can significantly promote the stability and reactivity of a superoxide radical in aqueous media. Notably, in the presence of co-solvents with a hydroxyl radical-scavenging nature, the consumption of persulfate and producing superoxide radicals is considerably promoted.

Persulfate activation with organic compounds particularly non-toxic and abundant materials, as a highly efficient technique, has attracted great interest. In this regard, the capability of various organic substrates for persulfate or peroxymonosulfate activation is evaluated here.

The anionic form of phenol, phenoxide (PHO⁻), which is produced under alkaline conditions, has the ability to activate persulfate. Indeed, applying strong alkaline conditions is effective not only in direct activation of persulfate but also in generating the active anionic species from phenol. Two main mechanisms have been proposed for phenoxide activation of persulfate:⁹  (1) reduction of persulfate by phenoxide according to eqn (1.12) and (2) nucleophilic attack on persulfate by the phenoxide in which hydroperoxide (HO₂⁻) is firstly produced, then reduces another persulfate molecule, generating a sulfate radical and superoxide radical anions (eqn (1.13)–(1.14)). Sulfate radicals react with water forming hydroxyl radicals (eqn (1.15)). In both the proposed mechanisms, a sulfate radical may react with a hydroxide ion to produce a hydroxyl radical according to eqn (1.3).

Aiming to develop the performance of phenoxide activation of persulfate, in addition to phenol and catechol, the effect of phenol ring substitution with chlorine has been examined. In this regard, organic compounds with no chlorine substitution exhibit the highest ability for persulfate activation since chlorine substitution results in both increasing acidity and deactivation of the ring with less reducing potential.¹⁰ 

Considering the toxicity of phenol, the effectiveness of some organic functional groups such as ketones, primary alcohols, and low carbon chain aldehydes for persulfate activation under alkaline conditions has been confirmed.¹¹ 

Quinone (Q), as a reactive organic compound, with the ability of reversible reduction to a semiquinone anion radical (SQ˙⁻) or hydroquinone (H₂Q), is another organic substrate, which has been utilized for persulfate activation. According to the proposed mechanism, illustrated in Figure 1.3, the comproportionation reaction between quinone and hydroquinone generates a semiquinone radical (eqn (1.16)), the electron transfer agent, which activates persulfate:¹² 

The ability of quinone to activate peroxymonosulfate has also been confirmed. In contrast to persulfate activation through a radical mechanism, a non-radical process is correspondingly through reactive singlet oxygen, ¹O₂. Nucleophilic addition of peroxymonosulfate to the quinone is the initial step of the proposed mechanism which leads to the generation of intermediates in the subsequent steps and conversion to ¹O₂ as a selective oxidizing species with high reactivity in pollutant degradation.

The persulfate concentration is important in the organic substrate activation. Degradation efficiency is initially improved with the persulfate concentration, but high concentrations result in the production of less reactive species due to the reaction between sulfate and hydroxyl radicals with excess persulfate and a decrease in the activation as presented in eqn (1.10)–(1.11).

The pH of solutions has substantial effects on the persulfate activation utilizing organic substrates through decomposition of persulfate and the nature of an organic substrate. Under alkaline conditions, the decomposition of persulfate and peroxymonosulfate increases. When utilizing phenoxide for persulfate activation, pH will have an important role in both persulfate activation and in converting phenol to its active form, phenoxide.

Increasing the concentration of quinone for persulfate activation promotes the degradation efficiency through producing sulfate radicals. However, after an optimum point more semiquinone radical and hydroquinone are produced since excess hydroquinone quenches the sulfate radical. By using phenoxide, a slight decline in degradation efficiency by the excess phenoxide after an optimum point is due to the scavenging of hydroxyl radicals, since phenoxide is slightly more reactive in the presence of hydroxyl radicals.

Similarly, in the activation of peroxymonosulfate with quinone, the degradation efficiency is initially increased with the concentration of quinone; however, it is slightly decreased with the progress in pollutant degradation which is due to undesired side reactions consuming the quinone as well as destroying quinone with ¹O₂.¹³ 

Activation of persulfate with the aim of generating sulfate radicals has attracted increasing attention. The catalytic activation of persulfate with the advantages of adaptability, capability, and ease of operations, provides a sustainable and economic approach without the aid of external energy. Accordingly, the main kinds of utilized catalysts are metal-based and carbon-based materials.

Metal-based catalysts in homogeneous or heterogeneous systems can activate persulfate through a one-electron transfer process. Iron is the most widely used catalyst since it is relatively inexpensive and nontoxic; while the catalytic ability of cobalt, manganese, and copper has also been confirmed.¹⁴ 

Carbon-based catalysts with unique features of a metal-free nature, high availability, superior resistance, porous structure, and large specific surface area are other promising heterogeneous persulfate activators through both radical and non-radical mechanisms. In this regard, the catalytic performance of various carbon materials including biochar, activated carbon, carbon nanotubes, and graphene and its derivatives (graphene oxide and reduced graphene oxide) has been assessed.¹⁵ 

As has been partially mentioned, the superiority of iron over other metal-based catalysts is due to the low price of iron salts, its easy availability, and high catalytic efficiency. In the homogeneous activation of persulfate, with the benefits of lower mass-transfer resistance and higher reaction rates, iron ions contribute in generating sulfate radicals.

In the activation of persulfate with Fe²⁺, the active species are generated through breakage of the O–O bond and transferring an electron to the oxidant (eqn (1.18)). Similarly, peroxymonosulfate could be activated with a ferrous ion to generate an active SO₄˙⁻ radical anion and HO˙ radical in different pathways, as depicted in eqn (1.19)–(1.20).

The chelate modified homogeneous catalyst contributes greatly to the development of Fe-based catalysts. Chelation of metal ions considerably regulates and maintains the concentration of Fe²⁺ aiming to minimize the losses and allow the long-term formation of radicals. Controlling the faster oxidation of Fe²⁺ to Fe³⁺ and stabilization of the dissolved Fe²⁺ are the other uses of these complexing agents.¹⁶  In this regard, various chelating agents are used to form complexes. Although ethylene diamine tetra acetic acid (EDTA) is prevalently utilized to chelate iron, it could act as a contaminant. Thus, ethylene diamine-N,N′-disuccinic acid (EDDS), a structural isomer of EDTA, has been introduced as an environmentally-friendly alternative. Besides, the ability of oxalic acid, citrate, and pyrophosphate along with an inorganic chelating agent such as sodium thiosulfate for enhancing persulfate activation has been evaluated.¹⁷ 

The addition of reducing agents can assist the process of persulfate activation through the enhanced regeneration of Fe²⁺ ions. For instance, utilizing an α-amino acid cysteine as a reducing agent not only accelerates the redox cycle of Fe³⁺ to Fe²⁺ but also causes faster oxidation of Fe²⁺, which in turn leads to promoting the persulfate activation.¹⁸ 

Due to the drawbacks of persulfate activation via a homogeneous system including its high pH dependence, secondary pollution, difficulties in catalyst recovery, and requirement of further disposal of iron sludge, heterogeneous activation as an appropriate alternate has been developed. Accordingly, the performance of heterogeneous iron catalysts including zero-valent iron, iron oxides and iron sulfides, as well as immobilized iron catalysts on various supports have been confirmed for persulfate activation.

Use of zero-valent iron avoids the accumulation of excess iron and reduces the production of iron sludge through recycling Fe³⁺ to Fe²⁺ ion. Besides, it can be easily removed by filtration. Persulfate activation with zero-valent iron follows two mechanisms: (1) direct electron transfer from the surface of zero-valent iron to reduce persulfate and generating sulfate radicals (eqn (1.21)), and (2) corrosion of zero-valent iron and producing dissolved Fe²⁺ under aerobic or anaerobic and acidic conditions (eqn (1.22)–(1.25)). The generated Fe³⁺ could be regenerated on the surface of zero-valent iron (eqn (1.26)).¹⁹ 

The nano-sized zero-valent iron has a large specific surface area and requires complete utilization of the catalyst; however, the easy agglomeration of iron particles has to be considered and the fact that the catalytic activity of the nanoparticle is reduced due to the surface oxidation. For the latter case, supporting a nano zero-valent iron on appropriate materials boosts the efficiency in persulfate activation by improving dispersion and avoiding accumulation.

Another intensification method is the use of iron sulfide, which has a higher activity due to the active sites, leading to significant promotion in persulfate activation over a wide range of pH.²⁰ 

The magnetic properties and the stability are unique features of the iron oxide particles of Fe₃O₄, Fe₂O₃, FeOOH, and hematite that are beneficial for their potential use in persulfate activation. Fe₃O₄, for instance, has an inverse spinel crystal with octahedral sites that firmly accommodates Fe²⁺ and Fe³⁺. Likewise, as a semiconductor with a narrow band gap of 0.1 eV, it easily provides electron transfer.

Obviously, Fe²⁺ ions play the main role in the activation of persulfate (or peroxymonosulfate); but in contrast to homogeneous Fe²⁺, by utilizing Fe₃O₄, the activation process occurs on the surface of solids and avoids full contact and fast consumption of Fe²⁺. Moreover, recycling the catalyst at the end of the reaction through virtue of magnetic properties is possible.

The mechanism of persulfate activation utilizing Fe₂O₃ and FeOOH with only Fe³⁺ species is based on the reduction of Fe³⁺ to Fe²⁺. Indeed, ferric iron surface sites (Fe³⁺) on the surface of iron oxide are reduced to surface bound divalent iron (Fe²⁺) along with the generation of the persulfate radical (S₂O₈²⁻) according to eqn (1.27). Then, the provided Fe²⁺ sites activate the persulfate and produce sulfate radicals as:²¹ 

Considering the superior oxygen mobility and relatively high activity of the manganese oxides, the magnetic nanocomposite of γ-Fe₂O₃/α-MnO₂ with an improved catalytic efficiency has been introduced. In this composite, along with iron-based activation, continuous conversion of Mn⁴⁺ and Mn³⁺ surface species in a redox cycle to one another contribute to the persulfate activation as:²¹ 

Besides, reaction of a sulfate radical with water generates a hydroxyl radical as follows:

Degradation of pollutants utilizing a γ-Fe₂O₃/α-MnO₂ composite follows two main pathways of pollutant adsorption on the catalyst surface and heterogeneous degradation with active species as well as homogeneous reaction with sulfate radicals at the interfacial zone.²¹  The proposed activation mechanism of the γ-Fe₂O₃/α-MnO₂ composite is illustrated in Figure 1.4.

The possibility of aggregation due to high surface energy and magnetic forces, easy oxidation, and leaching are problems relevant to heterogeneous iron-based catalysts. Thus, supported iron catalysts have been introduced in which the appropriate interaction between iron species and supports provides novel physicochemical properties and improves the stability and reactivity.

Metal oxides not only contribute to the catalytic activation of persulfate but also act as a support for iron species, providing a larger contact surface area and serving as a charged particle carrier. The simultaneous contributions of various metal oxides including MnO₂, Al₂O₃, TiO₂, CuO, CeO and ZnO, as catalysts by themselves as well as support for iron species, have been confirmed.²² 

Another emerging alternative catalyst support for iron species is metal organic frameworks (MOFs). In the inorganic–organic porous structure of MOFs, metal ions are linked by organic bridging ligands. Among various MOFs, Fe-based Materials of Institute Lavoisier (MILs) series have been used in persulfate activation.²³  The superior activation capacity is attributed to coordinatively unsaturated Fe(ii)/Fe(iii) sites existing in the framework of MILs, which are converted to each other continuously, accompanied by the persulfate activation. In this regard, the ability of the heterogeneous catalyst metal–organic framework MIL-53(Fe) has been confirmed²⁴  and the proposed mechanism is depicted in Figure 1.5.

The catalytic ability of other transition metals such as Ag, Mn, Cu, and Zn for the activation of persulfate has also been evaluated. Furthermore, Co has been recognized as a suitable homogeneous catalyst for the activation of peroxymonosulfate.²⁵  Regarding Co-based catalysts, the toxicity of excessive cobalt ions has been determined. Accordingly, various novel supported cobalt catalysts have been introduced and their capability in persulfate activation has been evaluated. For a detailed description, the review of cobalt-catalyzed persulfate activation by Hu et al. is suggested.²⁶ 

The application of copper in persulfate activation has also been investigated. The interaction of Cu⁺ and Cu²⁺ with persulfate leads to the generation of sulfate radicals as follows:²⁷ 

Mn-based catalysts, particularly manganese oxides, are another category of transition metals with the advantages of low toxicity, high natural abundance, and environmental friendliness. MnO₂, and other MnO_x species have also been introduced to effectively activate persulfate, including MnO, Mn(iii)(oxyhydr) oxides, and nano zero-valent manganese. With respect to the development of Mn-based catalysts, introducing single ions such as Pt⁺, Ce³⁺, Co²⁺, Fe³⁺, and Cu²⁺ into the structures of MnO_x improves their properties for enhancing the catalytic reactivity.²⁸  For instance, utilizing cerium as a dopant into Mn₂O₃ boosts the charge transfer ability as well as additional active sites, which in turn increases the generation of active species according to eqn (1.34)–(1.35).²⁹ 

Carbon-based catalysts (as metal-free natural materials) with unique physiochemical properties, low cost, chemical and thermal stabilities, as well as an appropriate porous structure and surface chemistry overcoming the metal leaching problem, exhibit improved catalytic performance and good environmental benefits. Various carbon-based catalysts including pristine carbon, graphene and its derivatives, carbon nanotubes, activated carbon, heteroatom-doped carbon materials, and carbon materials with metal particles have been confirmed to be effective in the activation of persulfate.

The mechanism of persulfate activation by utilizing carbon-based catalysts is relatively complicated since both radical and non-radical pathways contribute to the activation process. Generally, oxygen-functionalized carbon, structure defects (including vacancies and zigzag/armchair defective edges, sp²-hybridized carbon), and doped heteroatoms are introduced to affect the catalytic activity.

In a radical pathway, the oxygen-containing functional groups and defects are responsible for persulfate activation. In carbon materials, oxygen-containing functional groups including ketone, carboxyl and hydroxyl could exist. Among them, the electron-rich ketone group provides critical reactive sites and exhibits the best catalytic activity to convert persulfate to active radicals as follows:³⁰ 

As for the defects, it has been found that delocalized electrons from the defects can be transferred to activate persulfate, leading to peroxy bond breaking and production of active radicals. For instance, the versatile defects and the proportion of a graphitic carbon layer (outer sphere of the annealed nanodiamonds) in the sp²/sp³ configurations promote the catalytic capability for persulfate activation by serving as a tunnel for electron transfer.³¹ 

Another kind of structural defect i.e. zigzag/armchair edges facilitates active sites with the aim of improving the persulfate activation process. Though edges sites are favorable for electron transfer, the vacancy sites, similarly, enhance the adsorption of persulfate molecules on the carbon-based catalyst. The adsorption of persulfate on the edge and vacancy sites of carbon nanofiber catalysts in both the zigzag and armchair structures are presented in Figure 1.6.³² 

For improving the performance of carbon-based catalysts, a chemical composition modification with the aim of generating active sites for adsorption and persulfate activation has been attempted. In this regard, nitrogen has been the most prevailing heteroatom as the dopant in carbon materials. Considering nitrogen atoms with higher electronegativity, they can change the electronic structure of nearby carbon atoms and accelerate the electron transfer, thus enabling sites for the surface reactions.³³ 

Utilizing metal nanoparticles on a carbonaceous material support is another important strategy to facilitate electron transfer and to generate more free radicals. The reason for this has been attributed to the fact that new atomic configurations prevent local functionalization and formation of negatively-charged domains at the surface of carbonaceous materials; therefore, persulfate activation will be assisted through reducing electrostatic repulsion between the carbon surface and the persulfate ions. In addition, transition metals, themselves, rapidly activate persulfates to generate sulfate radicals.³⁴ 

It has been revealed that both radical and non-radical pathways participate in the persulfate activation process. Non-radical persulfate activation takes place on the surface of catalysts, whereas radical activation occurs in the bulk solution.

Generally, the three mechanisms of surface electron transfer are as follows:

• (1) In the surface electron-transfer mechanism, carbon-based catalysts act as an electron bridge and accelerate the electron transfer from the organic pollutant (electron donor) to persulfate molecules (electron acceptor) adsorbed on the surface of the catalysts.³⁵ 

• (2) Another pathway is through the surface-bound sulfate radical or the surface-activated complex. For instance, in non-radical activation of persulfate utilizing carbon nanotubes, transferring an electron leads to producing “CNT-activated persulfate” complexes which improve the oxidative potential of the carbon nanotubes as a catalyst.³⁶ 

• (3) In the non-radical pathway, singlet oxygen (¹O₂) is possibly produced, which is a highly reactive, electrophilic, nonradical, and selective reactive oxygen species which degrades electron-rich pollutants. By utilizing benzoquinone as a chemical additive, persulfate has been efficiently activated to produce singlet oxygen.³⁷  Likewise, the presence of carbonyl groups in N-doped graphene provides active sites to generate ¹O₂ and the corresponding electrophilic reaction of the carbonyl group with persulfate. Furthermore, the self-decomposition of peroxymonosulfate could generate single oxygen as

In addition to non-radical pathways, generation of ¹O₂ through the radical mechanism is conceivable. For instance, singlet oxygen in the catalytic activation of persulfate with a carbon nanotube is formed through the radical pathway.³⁸  In this regard, the hydrolysis of persulfate catalyzed by a carbon nanotube generates superoxide radicals (O₂˙⁻) and then singlet oxygen is produced as

The proposed mechanism of persulfate and peroxymonosulfate activation on a carbon-based catalyst containing radical and non-radical pathways is illustrated in Figure 1.7.³⁹ 

Graphene is a single 2D planar sheet, exfoliated from graphite, with carbon atoms packed in a hexagonal lattice. The many fascinating properties of graphene and its derivatives have led to their promising applications as catalyst, dopant, and support.

Although the relatively lower oxygen content of graphene oxide compared to graphene causes its higher catalytic activity, heteroatom doping and chemical/thermal reduction of graphene oxide to reduced graphene oxide have been employed to boost its catalytic activity. Considering improvement in the catalytic performance, reduced graphene oxide with the merits associated with its intrinsic carbon hybridization, defective sites such as vacancies and zigzag/armchair edges, and with suitable functional groups has been introduced as an efficient catalyst for persulfate activation.⁴⁰ 

Carbon nanotubes (CNTs) as a novel carbon-based tubular structure, are composed of single or multi-layer graphite sheets and dominated by sp² hybridization. The superiority of CNT catalysts is attributed to the cavity structure, low mass transfer limitation, and high stability.

The catalytic ability of both single-walled CNTs (SWCNTs) and multi-walled CNTs (MWCNTs) for persulfate activation has been confirmed. However, the degradation performance is strongly dependent on the type of CNTs. In this regard, the higher reactivity of SWCNTs is due to their different specific surface areas. Furthermore, the source of sulfate radicals influences the activation process. This is attributed to the different molecular structures of peroxymonosulfate and persulfate and the relevant activation mechanism. Indeed, the active species generated in the peroxymonosulfate/CNTs system are radicals and non-radicals; whereas, non-radical reactive complexes are dominant in the persulfate/CNTs system.⁴¹ 

High porosity, high specific surface area, and abundant oxygen-containing functional groups in addition to its relatively low cost and the availability of activated carbon make it a suitable potential catalyst for utilization in persulfate activation. Assessing the performance of powder and granular activated carbon has revealed that powder activated carbon has a better catalytic ability due to a greater surface area; whereas, granular activated carbon provides easy separation and reusability.

Activated carbon fiber with strong adsorption ability as well as easy handling has also been investigated and its persulfate activation ability for degrading azo dyes has been assessed. Incorporation of cobalt ions in activated carbon fiber supports has been beneficial for the degradation of various dyes such as Reactive Brilliant Red, Acid Orange 7, Acid Red, Methylene Blue, and Rhodamine B through peroxymonosulfate activation. The enhanced catalytic ability of the composite is due to the high adsorption and the specific role of activated carbon fiber as an electron donor to accelerate the Co²⁺/Co³⁺ cycle.⁴² 

Evidently, the concentration of persulfate plays an important role in the catalytic activation process for the aim of pollutant treatment. Generally, increased degradation efficiency is initially obtained with increased persulfate concentration. However, higher concentrations after a maximum, have negative effects on the efficiency of persulfate activation. The scavenging effect of oxidants toward radicals and the production of less reactive S₂O₈²⁻ species are the main reasons for the diminishing degradation level as presented in eqn (1.10) and (1.11).

The pH of a solution has a substantial effect on persulfate activation utilizing metal-based and carbon-based catalysts in the decomposition of persulfate, speciation of radicals and structural form of homogeneous catalyst species, as well as surface charges of the heterogeneous catalysts. Regardless of the utilized catalysts, the pH range can determine the active radicals in the solutions. Accordingly, the prevailing reactive species under acidic conditions is the sulfate radical; however, with increasing pH and under alkaline conditions, the dominant reactive species is the hydroxyl radical. Meanwhile, both the hydroxyl and sulfate radicals participate in the degradation of pollutants around neutral pH.

In metal-based catalytic activation, particularly homogeneous iron-based processes, a low pH is desired since within the pH range of 2–9, Fe²⁺ is readily soluble; whereas, at pHs higher than 3, the precipitation of Fe³⁺ into ferric hydroxides occurs. The generated ferrous and ferric complexes, including FeOH⁺, Fe(OH)₂, FeOH²⁺, Fe₂(OH)₂⁴⁺, Fe(OH)₂⁺, Fe(OH)₃ and Fe(OH)₄⁻ is not as effective as free Fe²⁺ ion for persulfate (or peroxymonosulfate) catalytic activation. Additionally, the availability of Fe²⁺ and the subsequent activation performance is significantly reduced under acidic conditions due to the formation of Fe(H₂O)²⁺ (pH < 2.5) and Fe(OH)²⁺ (pH < 3).⁴³ 

Conversely, the limited effect of solution pH on heterogeneous iron-based catalysts makes their application over a wide pH range feasible. Considering the effect of pH, persulfate activation by using zero-valent iron is remarkably enhanced under acidic conditions through surface corrosion and the production of more soluble Fe²⁺ ions. However, low pH has an influence on catalyst stability and iron leaching, which hinders the reusability of heterogeneous catalysts. On the other hand, when the pH is high, the formation of a layer of iron oxides or oxyhydroxide complexes on the surface of catalysts, due to hydrolysis, prevents further corrosion and subsequent persulfate activation.⁴⁴  Generally, the surface charge of catalysts is determined based on a pH of zero-point charge, pH_pzc. The surface of a catalyst has a positive charge if pHs < pH_pzc and a negative charge if pHs > pH_pzc. The electrostatic repulsion between catalysts, pollutants, and persulfate (or peroxymonosulfate) is strongly influenced by the pH of the solution, which in turn, restrains their contact and leads to the significant decrease of persulfate activation and degradation performance. For instance, Rhodamine B is completely degraded at pH values below 11 in carbon nanotube persulfate activation, but at a pH around 11, more negative charge sites on the surface of the CNTs reduce the adsorption of persulfate and pollutant onto the CNT's surface and subsequent activation.⁴⁵ 

Heat activation of persulfate for the degradation of various pollutants, due to its unique advantages, has been utilized as a conventional method. In the heat activation process no additional chemical is needed. Thus the persulfate consumption is minimized; whereas, side and undesired reactions in other activation strategies cause more persulfate consumption. Moreover, by raising the temperature during the process, the solubility of the pollutants into the aqueous phase, as well as the mass transfer and reaction rates are enhanced.

Energy supply in the heat activation of persulfate results in homolysis of the peroxide bond and allows the formation of two sulfate radicals according to eqn (1.45). Consequently, a chain of reactions can generate hydroxyl radicals according to eqn (1.3)–(1.15):

A heat activation process has been successfully combined with various catalysts to improve the performance. Here temperature assists the diffusion of reactants toward the catalyst and increases the movement and solubility of the oxidants, pollutants, and intermediates.

In this regard, the enhanced efficiency due to the simultaneous effects of transition metal catalysts and heat-activated persulfate has been confirmed. Indeed, at lower temperatures (25–50 °C) due to the insufficient thermal energy, metal ions (Fe²⁺ and Cu²⁺) are responsible for the generation of sulfate radicals from persulfate, while at high temperatures, close to 90 °C, the system obtains sufficient energy for the activation of persulfate and dramatic enhancement in degradation has been attained for the pollutant p-nitrophenol.⁴⁶  A combination of heat-activated persulfate with a reduced graphene oxide-supported nano zero-valent iron catalyst for oil refinery wastewater treatment has also been reported.⁴⁷ 

A similar synergetic effect of carbocatalysis and heat activation of persulfate has been found. A faster electron transfer would be facilitated via the carbon bridge which leads to generating more free radicals for pollutant degradation. Furthermore, the increased efficiency of the simultaneous utilization of a nitrogen-doped single-walled carbon nanotube along with the heat activation for treatment of various pollutants such as nitrobenzene, phenol, benzoquinone, and sulfachlorpyridazine has been confirmed.⁴⁸ 

The performance of the heat activation process is strongly dependent on the persulfate concentration. Degradation efficiency is initially promoted with increasing persulfate concentration but it decreases above a certain concentration. The competition between excess persulfate ions and pollutants in reacting with sulfate and hydroxyl radicals leads to significant reduction in degradation efficiency.

The influence of pH on persulfate activation and in determining the dominant radical species, as well as the nature of catalysts is important. Under acidic conditions, sulfate radicals (with a redox potential of 2.6 V) are the prevailing reactive species. Under neutral pH, both the hydroxyl and sulfate radicals contribute to the degradation of pollutants. However, at elevated pH levels hydroxyl radicals are dominant. Thus, the reactive species can be dominate via adjusting the pH of solutions.

Temperature is a definite influencing parameter for the heat activation of persulfate. As aforementioned, an increasing trend of degradation efficiency has been observed with temperature, attributed to the increased solubility and movement of reactants and pollutants. However, there is usually an optimum temperature with respect to cost-effectiveness of input energy versus degradation outcomes. In heat-assisted catalytic activation of persulfate, the necessity of applying the optimum temperature is due to the retarding effect of high temperatures. For instance, in homogeneous iron-based catalytic activation, elevated temperatures increase the formation rate of Fe³⁺, resulting in ferric hydroxide precipitation.

The design of reactors for persulfate heat activation should be determined by the type of heat source, fluid dynamics, and temperature control. In this regard, the reactors utilized for heat activation can be categorized into batch and continuous operations. Numerous persulfate heat activation processes are conducted in batch systems which usually consist of an automatic heater, a reflux condenser along with cold water flow for reducing the evaporation of the solutions, a mechanical stirrer for mixing, and a thermostat for adjusting the temperature. A schematic of a typical batch reactor for persulfate heat activation is presented in Figure 1.8.⁴⁷ 

In another reactor configuration, continuous operation has been employed. This was performed in a soft coil copper tube in which the thermal energy was supplied by halogen light bulbs. The feed aqueous solution containing the metronidazole pollutant and persulfate, in stoichiometric amounts, was fed to the reactor using a peristaltic pump. Interestingly, the continuous operation in a soft coil copper tube supplies the zero-valent copper and leads to simultaneous heat and catalytic activation of persulfate.⁴⁹ 

A novel solar reactor with considerable economic and environmental benefits has been introduced for heat activation of persulfate with the advantage of storing the heat energy collected from sunlight. The set-up is mainly composed of two tanks, three flat solar panels with a light harnessing surface installed in series, and a heat exchanger. Figure 1.9 shows a picture of the solar pilot reactor and its flow-diagram scheme. Degradation of pollutants based on persulfate heat activation in this set-up comprises of two steps: (1) heating the wastewater effluent to the desired temperature, and (2) oxidation of pollutants with persulfate, introduced from a reagent vessel. The closed fluid loop of wastewater connected to the upper tank with the thermal receiver (solar heating panels) is facilitated by utilizing a volumetric pump. After complete degradation and with the aim of heat energy management, an integrated heat exchange system is utilized for heat transfer from the outflowing heat treated wastewater to a new polluted wastewater flow entering the system.⁵⁰ 

Microwave activation of persulfate, as an economic and highly efficient method with the outstanding advantage of rapid heating without a temperature gradient, prominently boosts the pollutant degradation efficiency by reducing the energy consumption and reaction time. Meanwhile, this method facilitates easy control and decreases the equipment size.

Microwave irradiation on the electromagnetic spectrum has a frequency between 300 MHz and 300 GHz and a wavelength of 1–0.001 m. The interaction between a microwave and a material consists of penetration, reflection, and adsorption, depending on their properties. In this regard, insulators with a large penetration depth and a small dielectric constant are transparent, while conductors reflect the microwave irradiation with the generation of electromagnetic shielding. Absorbing microwaves is a unique feature of dielectric materials with a large dielectric constant, which leads to immediate conversion of the absorbed microwave energy into thermal energy. The principles of converting microwave energy to thermal energy are (1) dipolar polarization, (2) a conduction mechanism, and (3) interfacial polarization.

The dominant mechanism for microwave persulfate activation is dipolar polarization which is responsible for the majority of microwave heating in the solvent. When the high-frequency alternative electric field of the microwave is applied, the rotation of the dipole cannot adequately follow the rate of change in the direction of the electric field. This causes a time delay, leading to abundant energy consumption and conversion into thermal energy. Based on this mechanism, high temperatures in short times are achieved, compared to conventional heating methods. Microwave irradiation with a power range of 300–900 W increases the reaction temperature to approximately 100 °C by instantaneously initiating the self-rotation of water molecules. Under these conditions, persulfate could be activated rapidly to generate sulfate radicals. Persulfate (or peroxymonosulfate) is also a good dielectric that can be activated to generate free active radicals under microwave irradiation. In this way, oxidants might also be activated at high temperatures, but activation could also be realized by reducing the reaction barrier due to rapid self-rotation.⁵¹ 

With the development of microwave activation of persulfate, the combination of microwave irradiation and catalysts has been introduced. The used catalysts include transition metals and carbon-based materials. Considering the relatively low efficiency of persulfate activation with microwave irradiation for the degradation of complex pollutants, different catalysts for utilization have been sought.

Among transition metal cations, the potential of Fe²⁺ cations with distinctive features of low cost and low toxicity has been assessed under microwave irradiation. Simultaneous persulfate activation with transition metals (eqn (1.18)–(1.20)) and microwave irradiation could provide a remarkable synergetic effect in the microwave/Fe²⁺/persulfate system.⁵¹ 

A major problem in the microwave activation of persulfate in the presence of homogeneous catalysts is producing a large number of secondary pollutants, particularly iron sludge. However, this drawback can be overcome by introducing zero-valent iron as a heterogeneous metallic catalyst which not only provides Fe²⁺ with the aim of persulfate activation (eqn (1.21)–(1.26)) but can also easily be separated by means of a simple magnet, avoiding undesired environmental effects.

In the microwave/Fe⁰/persulfate system, microwave irradiation provides a high temperature in a short time for generating sulfate radicals. Besides, molecules spin at high speed which enhances the collision probability between pollutants and active species. The iron particles act as hotspots under microwave irradiation which create many reaction sites for pollutant degradation.⁵²  The proposed mechanism for the persulfate/microwave irradiation/zero-valent iron system is represented in Figure 1.10.

A considerable drawback of utilizing transition metal catalysts in microwave activation of persulfate is metal leaching; hence, novel non-metals particularly carbon-based catalysts have been introduced.

The porous surface of activated carbon, as a promising carbon-based catalyst, usually has functional groups which act as an electron-transfer mediator in activating persulfate and in the generation of sulfate radicals.⁵³  However, the activation of persulfate without microwave irradiation is also appropriate (eqn (1.37)–(1.38)). When a microwave irradiates a carbonaceous material like activated carbon, hot spots with a high temperature of 1200 °C or more are generated on its surface through permanent dipole rotation and thus, persulfate is effectively activated.⁵⁴  Besides, in aqueous solutions, water molecules are thermally decomposed around the hot spots leading to the generation of a series of active species according to eqn (1.46)–(1.51). Moreover, direct degradation of pollutants can occur due to the effect of the high temperature of the hot spots.⁵⁵ 

The persulfate concentration has a significant influence on the ultrasonic activation process for the degradation of pollutants. Although an increase in persulfate concentration causes a corresponding increase in the production of sulfate radicals, due to the unfavorable reaction between sulfate and hydroxyl radicals with excess persulfate and the generation of less reactive species, the degradation efficiency is considerably decreased.

pH has a great impact on the microwave activation of persulfate and has an influence on both the oxidant and catalyst. Regarding the dependency of active species, the degradation efficiency is enhanced with increasing solution acidity. This is because sulfate radicals with a longer lifetime have an advantage under acidic conditions. Meanwhile, the effect of pH is different and strongly depends on the features of the catalyst. For instance, in a microwave/Fe⁰/persulfate system and under acidic conditions, hydrogen adsorption corrosion of Fe⁰ is increased under acidic conditions and more Fe²⁺ is generated. Additionally, Fe(OH)₃ precipitates on the surface of corroded Fe⁰ under alkaline conditions which reduces the effective reaction area and obstructs the transfer of electrons.⁵² 

The performance of carbon-based catalysts in the microwave activation of persulfate is strongly dependent on their adsorption capability. A variation in pH can influence the surface properties of carbon-based catalysts. Specifically, the dependency on pH is related to the catalyst zero-point charge. The catalyst surface charge is negative or positive if the pH of the solution is lower or greater than the catalyst pH_pzc, respectively.⁵⁶ 

The microwave activation of persulfate is considerably dependent on the microwave output power which directly influences the activation temperature. Initially, with increasing the microwave irradiation power, the degradation efficiency is promoted; however, further power leads to extra-high temperatures in which the sulfate radical anion can act as a self-scavenger.⁵⁴  Besides, catalyst stability can be influenced at high temperatures. Hence, regarding the energy saving and catalytic efficiency, it seems important to find the optimum microwave power.

In the design of reactors for the microwave activation of persulfate, batch or continuous operations, distribution of the catalyst (fixed or suspended), temperature control, and energy consumption are crucial. Generally, a modified domestic microwave oven is utilized as a source of microwave irradiation for the aim of persulfate activation.

The used microwave reactors can be categorized into major operating modes of batch and continuous which are shown in Figure 1.11.⁵⁴  Batch reactors are usually made with a glass/Pyrex vessel (Figure 1.11a) and a glass column (Figure 1.11b). A microwave generator, infrared pyrometer for measuring temperature, and heat controllers are the main components of batch reactors. Besides, mixing the reactor content with a magnetic stirrer and with a perforated quartz plate for the glass column has been employed. In addition, in the batch reactors, part of the oven bottom can be replaced with a round plate for stirring and a water-cooled condenser. In the event of microwave leakage, a solid absorber is utilized. In a typical continuous flow operation, the glass column reactor is connected to a metering pump and a pressure gauge at the inlet and outlet points. The reactor is also connected to a heat exchanger and a pressure regulating valve (Figure 1.11c).

Microwave reactors can be further classified into two main groups: atmospheric pressure ones with the requirement for a cooling water system to avoid evaporation of the solution (Figure 1.12a); and high pressure ones, which are also called hydrothermal microwave reactors, equipped with pressure and temperature sensors (Figure 1.12b).⁵⁷  Both types have been widely utilized for activation of persulfate with the aim of degradation of various pollutants such as Methylene Blue,⁵⁸  pentachlorophenol,⁵⁹  and sulfamethoxazole.⁶⁰ 

Finally, the cost-effectiveness of a pollutant degradation process is an important issue in the design of microwave reactors, which can be evaluated through analysis of energy consumption. In this regard, the total power consumed in each process and the total power consumed per unit mass of degraded pollutant are calculated as:

where C₀ and C_t are the initial and final concentrations of a pollutant at a certain time, t, respectively, and V is the working volume of the microwave reactor.

Ultrasonic activation of persulfate, as an emerging method, has gained increasing attention due to its unique characteristics of cavitation and providing extremely high temperatures and pressures, under which persulfate could be activated. Upon ultrasonic irradiation, the cavitation phenomenon will occur which consists of formation, growth and violent collapse steps. Ultrasound irradiation includes a series of compression and rarefaction waves, acting on the molecules of the liquid media. The rarefaction cycle, under sufficient power, first increases the attractive forces of the liquid molecules and cavitation bubbles are formed. By growing bubbles through a rectified diffusion process, small amounts of vapor transfer from liquid to bubbles during the expansion. Further growth of bubbles leads to their collapse as they reach a resonant size and results in the generation of a local hotspot with a high temperature and pressure.

In the ultrasonic activation of persulfate, localized high temperatures and pressures from ultrasonically produced cavitation give enough energy for the homolysis of the O–O bond, producing two sulfate radicals (eqn (1.54)). Peroxymonosulfate can also be decomposed to generate active sulfate and hydroxyl radicals according to eqn (1.55), and a chain of reactions occurs (eqn (1.3)–(1.15)).

where the symbol “)))” stands for ultrasonic irradiation. Besides, in aqueous solutions, the thermal dissociation of water due to collapsing cavitation bubbles causes the generation of various reactive radicals (eqn (1.56)–(1.61)). In fact a kind of pyrolysis is appropriate under high temperatures inside the bubbles or near the interphase, resulting in effective degradation of organic pollutants.⁶¹ 

In addition to persulfate activation, utilizing ultrasound waves results in increased mass transfer of pollutants in the solutions, due to to the violent turbulence.

To facilitate the potential of ultrasonic irradiation, hybrid systems of homogeneous and heterogeneous catalysts with ultrasound waves have been introduced as they are beneficial in promoting persulfate activation.

Nano zero-valent iron, as a promising alternative to iron salts, with unique features of large specific surface area, high surface reactivity, and gradual release of Fe²⁺ has been widely used in persulfate activation.⁶²  The corrosion of nano zero-valent iron results in gradual release of Fe²⁺ as an activator for persulfate to generate sulfate radicals (eqn (1.21)–(1.22)). Besides, the regeneration of Fe³⁺ would be possible on the surface of nano-sized zero-valent iron (eqn (1.26)).

Notably, the solid–liquid interface in heterogeneous catalysts limits the efficiency because of the mass transfer limitation in heterogeneous catalysis; however, this problem could be reduced by using ultrasonic irradiation as it provides more reactive surfaces.

The main contribution of ultrasound in the hybrid systems is the continuous cleaning of the catalyst surface from catalytic residues leading to the regeneration of a more reactive surface as well as increasing the corrosion of heterogeneous catalysts, which in turn enhances the efficiency. Indeed, utilizing nano zero-valent iron particles under ultrasonic irradiation promotes the degradation of pollutants, which can be attributed to the action of ultrasound in removing the oxide layers and impurities from the catalyst surface. Besides, the mass transfer in solid–liquid interfaces during the heterogeneous reaction is remarkably enhanced via the strong effect of ultrasound agitation.

With respect to developing a nano zero-valent iron catalyst, supports like reduced graphene oxide (rGO) not only preserve the surface area, reactivity, and the mobility of the catalyst and inhibit agglomeration but also improve persulfate activation due to their carbocatalytic capabilities. In this regard, the higher efficiency of peroxydisulfate/ultrasound/nano zero-valent iron catalyst/rGO has been confirmed in the degradation of pollutants.⁶³ 

The enhancing influence of ultrasonic irradiation on persulfate activation has also been confirmed using heterogeneous zero-valent copper as the catalyst. In this regard, copper ions could be generated simultaneously from chemical corrosion under acidic conditions (eqn (1.62)) with the effect of ultrasonic irradiation (eqn (1.63)). Subsequently, persulfate is activated with Cu⁺ through the homogeneous reaction in the solution and the heterogeneous reaction on the catalyst surface (eqn (1.64)). Considering the fact that Cu²⁺ is rather stable, it will gradually diffuse into the solution and provide homogeneous persulfate activation (eqn (1.65)). The proposed mechanism of persulfate activation with zero-valent copper under ultrasound irradiation is depicted in Figure 1.13. Indeed, the effect of ultrasound irradiation is not limited to generating active species through a cavitation phenomenon, it also provides continuous catalyst surface cleaning and boosts the corrosion of metal catalysts.⁶⁴ 

The persulfate concentration is an important parameter in ultrasonic activation. Degradation efficiency is usually promoted with the persulfate concentration, but high values cause the degradation to decline due to the reaction between sulfate and hydroxyl radicals with excess persulfate, leading to the production of less reactive species.

Another reason for the retarding effect of persulfate concentration after an optimum value is related to the increased solution viscosity leading to difficulties in inducing cavitation and a lower number of cavities per unit volume.⁶⁵ 

pH is of great significance for the ultrasonic activation of persulfate. Generally, the increasing trend of degradation efficiency with solution acidity is attributed to the fact that sulfate radicals with longer lifetime (compared to hydroxyl radicals) are dominant under acidic conditions. Besides, solution pH can influence pollutant hydrophobicity and its tendency to accumulate near cavitation bubbles. For instance, diclofenac is more hydrophobic under acidic pH, whereas it is rather hydrophilic under alkaline pH. Therefore, the tendency of diclofenac to accumulate near cavitation bubbles results in an improvement in the pollutant degradation.⁶⁶ 

On the contrary, the degradation of a number of pollutants is significantly accelerated with increasing pH to the alkaline conditions. Tetracycline, with amphoteric characteristics, has been a pollutant, which is more degraded at high pH levels. This is because under high pH, the negatively charged tetracycline molecules tend to attract reactive species because of the high electrical density on the ring system.⁶⁷  Certainly, to determine the optimum pH for the ultrasonic activation of persulfate, considering the prevailing active species as well as chemistry of the pollutant is essential.

Temperature, as an important aspect in chemical processes, has to be adjusted in the ultrasonic activation of persulfate. Generally, the observed ascending trend of degradation efficiency with temperature has been attributed to the intensified movement of oxidants, pollutants, intermediates, and direct sonolysis of pollutants. Another reason is the direct sonolysis of the pollutants which is increased with temperature. Nevertheless, since the solutions have a lower vapor pressure and higher viscidity at lower temperatures, the cavitation is restricted at high temperatures.⁶¹  Besides, increasing the temperature results in the reduction of dissolved oxygen and the formation of hydroxyl radicals which in turn decreases the formation of active species (see eqn (1.57)–(1.61)). Thus, the appropriate temperature for the ultrasonic activation of persulfate should be determined with regard to different conditions.

Ultrasonic power intensity (power per unit surface area) has a significant influence on performance. High ultrasonic power intensity considerably enhances pollutant degradation and assists in mass transfer. This can be attributed to the more violent generated turbulences at high intensities. Thus, there is an optimum power intensity for ultrasound for each system and higher values would give lower degradation efficiency. The reason for this has been attributed to the production of additional gas bubbles via cavitation which could scatter the acoustic waves from the solution to the walls of the container or back to the transducer.⁶⁸  Moreover, increasing the power intensity leads to more generation of extensive amounts of tiny bubbles which in turn would coalesce to form larger bubbles and reduce cavitation.⁶⁹ 

The design of a sonoreactor is based on maximizing the cavitation yield and energy efficiencies. The cavitation yield for acoustic equipment determines the ability to produce the desired change per the total supplied electrical energy. Therefore, manipulation of the operating conditions and the geometry should be considered in the design of sonoreactors. It is worth mentioning that in the ultrasound-assisted hybrid process i.e. the combination of photo- and sono-persulfate activation, a variety of operating parameters have to be considered for better configuration.⁷⁰ 

Ultrasonic baths are the most common sonoreactors in the laboratory and pilot scales and an schematic is presented in Figure 1.14a. However, the constant working frequency and power, and low power transmission are the main drawbacks of ultrasonic baths. With respect to enhancing the performance of sonoreactors, utilizing directly dipped ultrasonic transducers in solutions has been introduced. The outstanding advantage of this configuration is the direct contact of ultrasonic probes with solutions providing more efficient power control. The benefits of this configuration on a laboratory scale have promoted its application on an industrial scale.⁷¹  Two approaches of utilizing ultrasonic probes consisting of batch and continuous systems are illustrated in Figures 1.14b and c.

A simple ultrasonic bath reactor, equipped with cooling water circulation, has been utilized for the ultrasonic activation of persulfate in the degradation of carbamazepine as a pharmaceutical pollutant.⁷²  The continuous water circulation adjusts the temperature of the reaction to the intended value.

The combination of an ultrasonic probe and mechanical stirrer in a sonoreactor has been utilized for the degradation of propranolol through persulfate activation.⁷³  In this regard and considering some dead zones in the reactor chamber, the mechanical stirrer increases the mass transfer. A schematic of a typical sonoreactor equipped with a mechanical stirrer and cooling water circulation is depicted in Figure 1.15.

The photo activation of persulfate has attracted much attention due to its many sustainable and environmentally friendly benefits. This method provides a high rate of degradation and mineralization of organic pollutants under mild conditions. It is a cost-effective method in terms of energy consumption compared to other activation methods. In this regard, investigations have proven the possibility of using solar energy as a free, clean, and not just a renewable but an inexhaustible energy source for this method.

It has to be mentioned that peroxymonosulfate (HSO₅⁻), can be utilized as an oxidant for the generation of sulfate radicals. Considering the symmetrical persulfate (S₂O₈²⁻) structure, from which two sulfate radical anions are produced upon activation; the unsymmetrical peroxymonosulfate generates a hydroxyl and a sulfate radical species, both very active in the degradation of pollutants.

There has been an escalating growth in the photo-activation of persulfate via dye sensitizing and catalyzing under photon irradiation with ultraviolet (UV) light, visible light, and solar light sources. The crucial role of light irradiation can be categorized in two distinct pathways:

• (1) direct activation;

• (2) assisting with homogeneous or heterogeneous catalysts.

The various strategies that have been employed so far in persulfate photo-activation are schematically presented in Figure 1.16. Different parts are discussed in detail in the following sections.

In direct photo activation, UV irradiation is often utilized as a cost-effective way to activate persulfate or peroxymonosulfate aiming to generate sulfate radicals for pollutant degradation. However, a rather slow reaction rate is found for this method.

Two simultaneous pathways are attributed to the pollutant degradation in this method. One corresponds to the direct effect of UV energy on the molecules of organic pollutants (eqn (1.66)) and another is due to the action of active radicals, which are produced from either persulfate or peroxymonosulfate upon breaking the peroxide bond in their chemical structure (eqn (1.67)–(1.68)) as well as the generation of hydroxyl radicals according to eqn (1.3)–(1.15).

Degradation efficiency is not usually significant in direct photolysis of pollutants with only UV irradiation. The relevant wavelengths in the UV region are within 100–400 nm including UV-A: 315–400, UV-B: 280–315, and UV-C: 100–280 nm and frequently give a high quantum efficiency (the ratio of extracted photons over the number of injected electrons).

Although UV light provides a higher photon energy, it accounts for only 3% of the sunlight spectrum, while about 44% of this falls in the visible region.⁷⁴  Utilizing solar energy, as an available, sustainable, and economic source, has encouraged investigators in the development of visible light-activation of persulfate. In this regard, special organic dyes including Rhodamine B, Eosin Y, Methylene Blue, and Methyl Orange can be visible-light-excited from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), and the excited electrons are accepted by persulfate to generate active radicals (photosensitization phenomenon).

In the presence of metal ions, dye sensitization can be improved by attaching transition metal ions to photosensitive dye molecules and subsequently forming metal-containing complexes which in turn facilitate the redox cycling of metal ions in persulfate activation systems.

Concerning the slow rate and low efficiency of direct photo activation, a great number of emerging catalysts including metal-based, metal free, and a series of novel catalysts have been utilized. Catalyst design with the aim of expanding visible-light absorption, utilizing solar light to a greater extent and promoting efficiency, has been persued. Stability, environmental friendliness, economic viability, and reusability are the factors that have been considered in this regard.

In addition to the wide applications of semiconductors, in the photocatalytic degradation of pollutants, their potential for persulfate photo activation is considerable. As presented in Figure 1.17a, UV light irradiation on TiO₂, as the most conventional semiconductor, results in an electron being excited from the valence band (VB) to the conduction band (CB), leaving behind an electron (e_cb⁻) and a hole (h_vb⁺) (eqn (1.69)). Consequently, the powerful oxidizing agents of SO₄˙⁻ and HO˙ are produced due to breaking of the peroxide bond of persulfate (eqn (1.70)–(1.71)). At the same time, the reaction of photo-excited holes with water molecules generates hydroxyl radicals (eqn (1.72)) and prevents unfavorable electron–hole recombination. Superoxide radicals (O₂˙⁻) are also produced (eqn (1.73)) because of the reduction of oxygen molecules via the conduction band electrons (e_cb⁻). This active oxidant provides a significant contribution to the degradation of organic hazardous materials in solutions.

Despite many attempts utilizing semiconductor catalysts in persulfate activation, the short lifetime of photogenerated electron–hole pairs is still a relevant problem. This drawback could be overcome by using heterojunction photocatalysts. In the photocatalytic activation of persulfate via heterojunction photocatalysts (Figure 1.17b), electron–hole pairs are initially generated due to photo excitation of both semiconductors (eqn (1.69)). Transferring electrons from the CB of semiconductor B to semiconductor A occurs when the energy of the CB in semiconductor A is lower than that in semiconductor B. When the VB of semiconductor A has a lower energy than that in semiconductor B, the holes in the VB of semiconductor A can be transferred to semiconductor B. Therefore, the recombination of electron–hole pairs is significantly suppressed using the heterojunction photocatalysts which in turn leads to promoting photo redox reactions and improving the degradation of pollutants.

Broad prospects of combining low valance state transition metals with light irradiation (UV or visible light) have been demonstrated to be useful for persulfate activation. Among the common transition metals, unique features of Fe²⁺ cations (usually supplied in the form of FeSO₄ salt) such as low cost, low toxicity, and high abundance in the natural environment, have expanded their application in homogeneous catalysis in aqueous solutions. However, activation of persulfate with Fe²⁺ in the absence of light, i.e. under dark conditions, is also achieved with low performance (eqn (1.18)–(1.20)). Generating Fe³⁺ and regeneration of Fe²⁺ in the presence of light leads to continuous activation of persulfate. Indeed, the Fe³⁺ occurs as a photosensitive ferric hydroxide ion, Fe(OH)²⁺, which strongly absorbs UV light and causes rapid regeneration of Fe²⁺ (eqn (1.74)).

On the other hand, Fe-based heterogeneous catalysts are more prominent due to the smooth release of Fe²⁺, avoiding excess amounts, and ease of separation. In this regard, utilizing Fe-containing materials i.e. zero-valent iron, hematite, magnetite, and zinc ferrite as useful catalysts for the photo activation of persulfate has been the subject of various investigations. With respect to developing Fe-based catalysts, the regeneration process could be promoted using electron donors, such as persistent free radicals. In this regard, the positive role of free radicals in activated carbon fibers (ACFs) by donating the electrons to ferric citrate has been confirmed. This accelerates the transformation of Fe³⁺ to Fe²⁺ and enhances the visible-light photosensitivity of the FeCit@ACFs composite for degradation of organic contaminants via persulfate activation.⁷⁵ 

Cobalt-based materials are another kind of metal catalyst with high capability in the degradation and mineralization of pollutants. The mechanism for the photo activation of persulfate using Co-based homogeneous catalysts has been proposed to be carried out in two ways: (1) activation of persulfate under UV light, where both the UV light and the Co²⁺ cation play as activators, and (2) activation under visible light where transferring electrons to oxidants and Co³⁺ happens as a result of dye sensitization with a dye, like Acid Orange 7.⁷⁶  Indeed, some dye molecules act both as a photosensitizer and as a substrate to be degraded.

To assess the performance of heterogeneous catalysts, novel bifunctional catalysts, formed by cobalt ions and TiO₂, have also been introduced.⁷⁷  As depicted in Figure 1.18,⁷⁸  the superiority of Co-based catalysts has been attributed to the synergetic influence of UV light, the activation of oxidants, and the formation of electron–hole pairs to complete the redox cycle.

Along with Fe-based and Co-based catalysts, other transition metals such as Ag, Mn, Cu, and Zn have also been utilized in some catalysts with the aim of persulfate activation. In this regard, the capability of Ag-based catalysts for enhancing heterogeneous catalysis efficiency has been assessed via heterojunction formation. Utilization of Ag doped on the Co₃O₄ surface evidently promoted photo degradation of pollutants, which was attributed to the decreased band gap and suppressed recombination of electron–hole pairs.⁷⁹  Mn-based catalysts are another example that have the same proposed mechanism for persulfate activation similar to Co-based catalysts. In a typical study, catalytic degradation of pollutants with peroxymonosulfate via cryptomelane-type Mn oxide octahedral molecular sieves (OMS-2) has led to only a redox reaction between Mn⁴⁺/Mn³⁺ pairs under dark conditions; while, under visible-light irradiation, two redox pairs of Mn⁴⁺/Mn³⁺ and Mn³⁺/Mn²⁺ are involved and high efficiency has been reported for the OMS-2/peroxymonosulfate/visible light process.⁸⁰ 

A major problem in utilizing metal-based catalysts in the photo activation of persulfate is metal leaching, which causes catalyst deactivation and secondary pollution induced by the metal ions. In this regard, the next generation of catalysts, composed of non-metal elements (essentially P, S, N, and C), has attracted attention. The application of these novel catalysts in the photo activation of persulfate have been expanded due to their low cost and superior physiochemical properties, such as chemical and thermal stability, as well as the desired porous structure and surface chemistry. Graphitic carbon nitride, orthorhombic α-sulfur (α-S), and polyimide are the most used metal-free catalysts. Similar to semiconductors, photo-induced electron–hole pairs are formed in persulfate photo activation by using metal-free catalysts. Meanwhile, the activation of persulfate molecules by accepting electrons leads to the generation of SO₄˙⁻ and HO˙ which are powerful oxidizing agents (eqn (1.70)–(1.72)).

The unique features of graphitic carbon nitride (g-C₃N₄) such as eco-friendliness, desirable chemical and thermal stabilities, and ease of large-scale preparation are amazing. These and favorable optoelectronic characteristics, as well as its mdium band gap and p-type semiconductivity, have persuaded researchers to assess its application in photocatalytic energy conversion and environmental remediation. The capabilities of the g-C₃N₄/persulfate/visible light process in the degradation of different dyes such as the azo dye Reactive Brilliant Red, the xanthene dye Rhodamine B, the quinone imine dye Methylene Blue, the anthraquinone dye Methyl Orange, the reactive dye Orange 5, and the reactive Brilliant Blue have been confirmed.⁸¹  The performance, of course, has limitations in some cases with respect to low specific surface area and the fast recombination of photo generated electron–hole pairs. In this regard and with respect to developing metal-free catalysts, the novel fabricated sulfur-doped carbon nitride can establish complete degradation of pollutants through activation of persulfate under visible-light irradiation. Since sulfur atoms are located on the zigzag edges of graphene-analog materials in the used composite structure and induce a high spin and charge density to the neighboring carbon atoms, more active sites are available for the persulfate activation.⁸² 

Orthorhombic α-sulfur (α-S)and polyimides are two other metal-free catalysts with a huge specific surface area, and they are inexpensive and have remarkable stability, which means they have potential for the photo activation of persulfate. The removal of contaminants in the α-S/persulfate/visible light process with high efficiency as well as excellent recyclability and stability has been reported.⁸³ 

Meanwhile, polyimides with desirable visible light-response activity and reusability have received much attention. They have stable structures after several cycles and are considered to be environmentally friendly persulfate activators with no heavy metal in the media.⁸⁴ 

Carbon quantum dots (CQDs), a novel class of carbon materials, consist of numerous carbon atoms assembled in amorphous or crystalline quasi-spherical nanostructure assemblies. They can be introduced as a visible-light-responsive photocatalyst for the activation of persulfate.⁸⁵ 

CQDs can be used under light emitting diode (LED) sources. Here, electrons are initially excited from the top of the valence band and injected into the bottom of the conduction band due to the irradiation of photons with equal or greater energy than the band gap of CQDs and simultaneously produce holes and electrons. Then, hydrogen peroxide and sulfate ion radicals are generated since persulfate molecules accept the electrons. Afterwards, photocatalytic generation of highly reactive superoxide ions, generated from the decomposition of hydrogen peroxide, along with the oxidative holes on the valence band of CQDs contribute to the degradation of pollutants as follows:

Recently, the use of CQDs in nanocomposites, with outstanding photon-conversion and excellent electron transfer properties, has been attempted. For instance, oxygen-deficient titanium dioxide/reduced graphene oxide (TiO_2−x/rGO) nanocomposites have been evaluated as a promising catalyst, in a novel structure Ti³⁺-self-doped TiO_2−x/rGO nanocomposite. Better light absorption and charge separation occurs due to the narrower band gap and formation of a Ti³⁺ induced mid-gap (the mid-gap as a deep-level state can extend the light absorption spectral range of wide band gap semiconductors from the UV light to the visible-light range). Coupling CQDs with this nanocomposite gives a narrow band gap and facilitates visible-light harvesting and benefits the synergistic effect of CQDs, Ti³⁺, and rGO for photo activation of persulfate in wastewater treatment.⁸⁶ 

Metal organic framework (MOF) materials are synthesized by the combination of transition metal-containing units of Mn, Fe, Co, Ni, and Cu, and organic ligands (containing C, H, O, N, and S) in a framework. They have remarkable features of high porosity, stable network, and enormous surface area and have been widely used for different purposes.

The photocatalytic ability of MOFs as inorganic–organic porous materials, is somewhat related to their HOMO–LUMO gap and the energy transfer that can take place from an organic linker to the metal–oxo cluster within some MOF catalysts under light irradiation. For instance, porphyrin derivatives have been employed as a photosensitizer that can promote the separation of photo excited electron–hole pairs.⁸⁷  It has been demonstrated that the combination of hemin, a natural porphyrin compound, with non-metal photocatalysts significantly enhances the degradation performance. In this regard, the benefits of this photosensitizer have also been assessed in the field of photo activation of persulfate with the design of a kind of MOF consisting of hemin as the organic linkage and copper ions as the metal containing unit. This MOF has been combined with boron nitride (BN), which has a two-dimensional (2D) graphite-like structure and some unique physical and chemical properties such as high specific surface area, numerous structural defects, high thermal conductivity, high chemical stability, and high oxidation resistance.⁸⁸  In a Cu–hemin MOFs/BN/persulfate/visible light process, BN acts not only as a stabilizer to support the hemin-MOFs but also adsorbs pollutants itself. Apart from the main role of copper ions and coordinated Fe²⁺ ions in the hemin which provide sites for the activation of persulfate (even in the dark); the Cu–hemin MOFs/BN composites are also able to generate electrons under visible light irradiation (eqn (1.79)). The photo-induced electrons then act as initiators for generating sulfate radicals from persulfate (eqn (1.80)). Moreover, direct transformation of the free hydroxide ions into active hydroxyl radicals can occur via an excited Cu–hemin/MOFs/BN composite (eqn (1.81)).

A synthetic layered double hydroxide (LDH), is a type of two-dimensional anionic material with a layered structure containing divalent (usually Mg²⁺, Zn²⁺, and Cu²⁺) and trivalent (usually Fe³⁺, Al³⁺, and Ce³⁺) metal cations. Application of LDHs is gaining great attention as they are novel ecosystem friendly semiconductors with distinctive properties such as large specific surface area and special structure, and they have a relatively simple synthesis process compared to conventional metal oxide semiconductors. Among different LDH materials, the photocatalytic performance of Fe-based, Zn-based, and Co-based LDHs has gained more attention due to their chemical stability, high catalytic performance, and low toxicity.⁸⁹ 

Transition metals in metal-based LDHs play an important role in photocatalytic activation. Initially, metal-based-LDH composites are excited by visible-light irradiation and then photo-induced electrons and holes are generated. Immediately after, the electrons are captured by persulfate to produce sulfate radicals and hinder the recombination of electrons and holes. The transition metal conversion is also responsible for improving the oxidation activity of the whole system. Indeed, the synergetic effects of the conduction electron capture (excitation of CuFe-LDH with visible-light photons and capturing electrons by persulfate) and the transition metal conversion could boost the photo activation of persulfate.

In a case study, a Cu–Fe layered doubled hydroxide (CuFe-LDH) has been introduced as an efficient heterogeneous photocatalyst for persulfate activation under visible light irradiation.⁹⁰  The presented mechanism in Figure 1.19 shows that CuFe-LDH first absorbs visible light and generate selectron–hole pairs. Then, capturing electrons by persulfate causes the generation of sulfate radicals and assists the separation of electrons and holes. Meanwhile, Cu²⁺ and Fe²⁺ transform into Cu³⁺ and Fe³⁺ ions upon reacting with persulfate. On the other hand, the trivalent metals, by capturing photo-induced electrons, accelerate the regeneration of Cu²⁺ and Fe²⁺ in a redox cycle while sulfate and hydroxyl radicals are also generated. Ultimately, the recalcitrant pollutants are removed with the generated radicals.

Obviously, the concentration of persulfate plays an important role in the photo activation process with the aim of pollutant treatment. Generally, an ascending trend of degradation efficiency with the persulfate concentration is observed. However, higher concentrations than a maximum value exhibit negative effects on the efficiency of persulfate photo activation. The main reason for this can be attributed to the scavenging effect of oxidants toward radicals and the production of less reactive S₂O₈^•− species.

pH is another influencing parameter in persulfate photo activation processes as it affects the catalysts, radicals and the degradation of pollutants. Active radicals are strongly dependent on the pH range. Under acidic conditions sulfate radicals are the prevailing reactive species while both the hydroxyl and sulfate radicals contribute to the degradation of pollutants around neutral pH. However, by increasing pH and under alkaline conditions, hydroxyl radicals are the dominant reactive species. The effect of pH on the performance and stability of catalysts through the surface charge or the ions released from solid catalysts in catalyst-involved processes is also noticeable. The surface charge of catalysts is assessed with the criterion of pH_pzc. Indeed, at pH values < pH_pzc, and at pHs > pH_pzc positive and negative surface charges of a catalyst would be appropriate, respectively. In these cases, the formation of active radicals is strongly dependent on the electrostatic forces between the catalyst and the present organic materials.

Similar to oxidants, increasing catalyst concentrations first lead to an increase in persulfate activation efficiency, which then decreases at higher catalyst concentrations. For common heterogeneous catalysis, the increasing photocatalyst dosage leads to more availability of active sites. Consequently, the number of electrons and holes generated is increased and enhanced degradation would be achieved. Nevertheless, in the case of homogeneous Fe-catalysis, for instance, side reactions of the ferrous ion (Fe²⁺) result in generating inactive or less active species such as the ferryl ion (FeO²⁺), which not only decrease the amount of catalyst for persulfate activation but also inhibit the generation of radicals.⁹¹  Moreover, excess amounts of metals could scavenge active radicals as for instance:

Furthermore, by raising the turbidity of the suspensions, a fraction of the incident irradiation is lost via scattering and is no longer available for the photo activation of persulfate.

Investigations show that a light source with 260 nm wavelength is the most effective in the photo activation of persulfate compared to higher wavelengths. This is because the excitation energy at this wavelength (461.5 kJ per mole of persulfate) is high enough to provide vibration in the peroxide bond of the persulfate structure and to generate sulfate and hydroxyl radicals.⁹²  Nevertheless, in determining the optimum wavelength, the energy consumption and cost considerations should be accounted for.

Progress in the design and synthesis of novel photocatalysts has persuaded investigators to utilize a new generation of light sources with low cost, and environmentally friendly LEDs. Emitting a relatively narrow spectrum of light, the long life span, high spectral purity, uniform illumination, energy efficiency and flexible configuration are interesting features of LEDs. They are ideal candidates for photocatalytic applications in environmental remediation procedures considering their low power consumption and high potential in the design of different photo reactors.⁹³ 

In the design of photo reactors for persulfate activation, it is essential to consider major factors including the type and particle size of the photocatalyst, the distribution of the particles (fixed or suspended), the content and distribution of pollutants, the type and position of the light source, mass transfer, fluid dynamics, and temperature control. Photo reactor classification into different groups, based on the main criteria, is listed in Table 1.1.

Considering the mass transfer limitation, the performance of homogeneous processes in the photo activation of persulfate has to be improved with efficient mixing i.e. using a micro-air compressor to bubble air through a distributor from the reactor bottom (Figure 1.20a).⁹⁴  In another configuration (Figure 1.20b), employing a circulating pump provides a thin film on discharge around a quartz tube which leads to better utilizing light irradiation and minimizing the mass transfer resistance.⁹⁵ 

In heterogeneous photo activation of persulfate, slurry reactors in which catalyst particles are suspended in the liquid phase with the help of mechanical agitation are conventional. The high total available surface area of catalysts per unit volume, simple construction, well-mixed catalyst suspension, feasibility for large capacity and low pressure drop are advantages of the slurry photo reactors. Notably, difficult separation and recovery of photocatalysts as well as turbidity and light scattering in these reactors have restricted their applications on a large scale.

Photocatalytic reactors with immobilized photocatalysts are those in which the photocatalyst is fixed on supports by physical surface forces or chemical bonds. Typical supports are polymer films, glass beads, stainless steel and carbon fibers. The main advantages of these reactors are easy catalyst recovery and continuous operations. Among photocatalyst supports, activation of persulfate with clay-based catalysts in the presence of an LED has also been investigated with prominent results. Natural clays are abundant and are widely used as catalyst supports.⁹⁶  However, low surface-area to reactor-volume ratio, significant pressure drop and catalyst fouling are some problems for the immobilized photo reactors.

The comparative performance of slurry and immobilized photo reactors on wire gauze structured packing and on random quartz Raschig rings in a helical flow configuration and under visible light irradiation has demonstrated that random packing results in the highest efficiency and the lowest energy consumption. The different mentioned configurations in a helical flow photo reactor are illustrated in Figure 1.21.⁹⁷ 

In another configuration solar photo reactors have been employed for persulfate activation. Solar photocatalytic reactors can be categorized into major groups of concentrating and non-concentrating systems. Indeed, collecting the solar irradiation with a reflecting surface in the concentrating reactors results in decreasing the reactor volume with a higher efficiency than in the case of non-concentrating systems. The most common solar reactors include parabolic trough reactors (PTRs), compound parabolic collectors (CPCs) and double-skin sheet reactors (DSSRs).⁹⁸  A parabolic trough collector (PTR) is a kind of concentrating solar reactor which consists of a platform with one or two motors for tracking solar irradiation, as shown in Figure 1.22a.⁹⁹  Non-concentrating photo reactors particularly double-skin sheet reactors (DSSRs) have no moving parts or solar tracking devices (Figure 1.22b). Hence, their performance does not depend on reflection and solar tracking issues. The benefits of a parabolic trough concentrator and non-concentrating systems are combined in compound parabolic collectors (CPCs) as the most promising photocatalytic solar reactors. CPCs are low-concentration static collectors via a reflective surface with the advantage of simplicity and low capital investment (Figure 1.22c).¹⁰⁰ 

In this regard, a specific solar reactor with automatic rotation against the sun has been designed for persulfate photo activation for the treatment of petroleum refinery wastewater. Sunlight-sensitive sensors are installed to provide automatic rotation and utilize both direct and single reflective sunlight beams.¹⁰¹  A schematic of the photo reactor and the reflector surface feature is presented in Figure 1.23.

Electro activation of persulfate, as a powerful and controllable process for the degradation of organic pollutants, has drawn considerable attention. The advantages of low sludge production, in-situ generation of coactivators, small reactor volume, relatively low investment costs and improved utilization efficiency have persuaded researchers to work on different concepts of this method. The degradation of organic pollutants is significantly promoted by utilizing electro activation of persulfate because of in-situ and fast activation. Meanwhile, investigations have proven the possibility of implementing this process on a large scale.

Electro activation of persulfate (S₂O₈²⁻) and peroxymonosulfate (HSO₅⁻) with peroxy bonds generates sulfate radical anions with high oxidation potential. The mechanism of sulfate radical generation from persulfate through the desired redox reactions is based on accepting an electron from an electron donor (substance or electrode). The dominant mechanism, with respect to the material of electrodes can be classified into the following major approaches:

• (1) electro activation with a sacrificial iron electrode,

• (2) electro activation with no iron being used.

The use of conventional iron electrodes provides an anodic reaction in the electrochemical activation of persulfate leading to in-situ and smooth formation of useful Fe²⁺ ions (eqn (1.83)). As a consequence, persulfate and peroxymonosulfate are converted to active SO₄˙⁻ radical anions and HO˙ radicals by accepting electrons from Fe²⁺ ions (eqn (1.84)–(1.87)). Moreover, regeneration of Fe²⁺ from Fe³⁺ through cathodic reduction (eqn (1.88)) strongly enhances the activation efficiency of persulfate and reduces the required amounts of iron. Therefore, smaller amounts of iron sludge would be produced (Figure 1.24a).

In the absence of iron, the proposed mechanism for electrochemical activation of persulfate with platinum electrodes (cathode and anode) is based on cathodic activation (Figure 1.24b). Indeed, through accepting an electron on a platinum cathode, persulfate and peroxymonosulfate will be activated to SO₄˙⁻ radical anions and HO˙ radicals (eqn (1.89)–(1.91)). On the anode, direct electro-oxidation as well as generation of hydroxyl radicals from water discharge are responsible for the degradation of pollutants (eqn (1.92)).

The mechanism of anodic activation of persulfate utilizing the carbon-based anode, as an emerging electrode material (Figure 1.24c), is rather complicated and consists of adsorption, direct oxidation, and radical and non-radical oxidation.¹⁰²  Considering the fact that the electrochemical reaction happens at the interfacial zone between the electrode surface and the bulk electrolyte solution, adsorption of pollutants into this zone is an essential step. Both the organic pollutant degradation and persulfate activation occur at the electrode surface.

Direct oxidation on the anode surface is also relevant due to the media electrolysis, contributing to the degradation of adsorbed pollutants. Investigations on the potential of carbon materials for activation of persulfate show that the sp²  hybridized carbon (mainly CO group) is the main active site, and sp³ hybridized carbons (C–H, CH₂, and C–OH) are unable to activate persulfate. The freshly prepared carbon-based electrode terminated with hydrogen as C–H and CH₂ could be electrochemically oxidized to oxygen-containing functional groups such as a carbonyl group and provide the active sites, which promotes the activation of persulfate.¹⁰³  The mechanism of this nonradical oxidation in the electrochemical activation is based on the structure of the persulfate activated state.¹⁰⁴  Persulfate is initially adsorbed on the electrode surface, then by applying current, the adsorbed persulfate is converted from its stable into its active state (S₂O₈²⁻*). Consequently, the activated persulfate degrades the pollutants.

Radical oxidation has been accounted for as another step in which the powerful radical species HO˙ and SO₄˙⁻ are electrochemically generated. This is so that decomposition of activated persulfate, adsorbed on the carbon-based anode i.e. “Carbon(S₂O₈²⁻)*” generates SO₄˙⁻.

Higher concentrations of hydroxyl radicals are produced in the electro activation of persulfates process compared to those generated with electrolysis alone. This confirms that the electro activation of persulfate can enhance the production of hydroxyl radicals via water dissociation at the surface of the anode. The adsorbed water molecules on the electrode surface are also oxidized to HO˙ via persulfate activation.¹⁰⁵ 

The electro-generation of HO˙ via water electrolysis is another proposed mechanism for producing hydroxyl radicals. In this mechanism, the activated persulfate molecule increases the overpotential of water oxidation and enhances the hydroxyl radical generation via water electrolysis. It is worth mentioning that the cyclic voltammetry analysis verifies the positive effect of persulfate addition on water dissociation.¹⁰³ 

Optimization of persulfate concentration, due to the generation of sulfate radicals, results in high degradation efficiency for various pollutants. Related investigations indicate that despite an initial increase in the degradation efficiency with persulfate concentration, inhibition of sulfate radical generation is appropriate under high persulfate doses. This phenomenon is attributed to the self-quenching of sulfate radicals or an undesired reaction with persulfate resulting in the scavenging of sulfate radicals and generation of less reactive S₂O₈^2– radical anions.

The multiple effects of pH depend on the electrolyte, electrochemical reactor configuration, and operational conditions and are important in the electro activation of persulfate. Concerning the hydrogen ion generation through the reaction of sulfate radicals with water (eqn (1.15)) the sulfate radical anion (redox potential of 2.6 V) is the prevailing reactive species under acidic conditions. Acid-catalyzation plays an important role in generating more sulfate radicals and subsequent degradation efficiency.¹⁰⁶  Conversely, degradation efficiency is remarkably decreased with a large decrease in pH, attributed to the competitive side reaction of the cathodic hydrogen evolution as

Under neutral pH, both the hydroxyl and sulfate radicals contribute to the degradation of pollutants. However, with increasing pH and under alkaline conditions, the hydroxyl radical (eqn (1.3)) plays the key role in forming more reactive intermediates and degradation of pollutants. Thus, the reactive species can be determined via adjusting the pH of solutions.

Considering the above-mentioned advantages of electro activation of persulfate, selecting a low-cost electrode material with high current efficiency seems essential. The dominant mechanism of the electro activation process is also significantly influenced by the electrode material. Various materials like Fe, Pt, boron-doped diamond (BDD), Ti/RuO₂–IrO₂ (known as dimensionally stable anodes, DSAs, as mixed metal oxides with high conductivity and corrosion resistance), and carbon-based electrodes like carbon nanotubes, graphite and black carbon have been used as electrodes for the degradation of organic pollutants via persulfate electro activation.¹⁰⁷ 

Compared to other transition metals, iron is the most widely applied material for the anode electrode due to its unique features including non-toxicity, abundance, and low price. The activation of persulfate for the degradation of 2,4-dinitrophenol in aqueous solutions, by using an annular iron sheet has been recently investigated in which the dual role of the iron anode as the electrode and the source of the activator has remarkably promoted the degradation efficiency with persulfate.¹⁰⁸  However, iron rust and the slow regeneration rate are the main problems when utilizing an iron anode in persulfate activation.

Considering the inferiorities of iron electrodes, carbon-based electrodes have been introduced as efficient electrodes for the activation of persulfate. Obviously, the utilization of carbon-based electrodes has been expanded due to fixing the carbon material into the electrode frame and inhibiting carbon leaching. To assess the performance of carbon electrodes, the potential of BDD and DSAs for the degradation of bisphenol A has been investigated.¹⁰⁹  High efficiency has been revealed and attributed to the contribution of direct electrolysis of pollutants and electro activation of persulfate with both nonradical and radical oxidation mechanisms. However, rather low current efficiency is the main limitation of the BDD electrode due to mass transfer restrictions, particularly for low soluble pollutants. To deal with this problem, electro generation of stable oxidants with high diffusivity and reactivity including peroxo and oxo-compounds on the BDD surface has been reported.¹¹⁰ 

Carbon-based materials including multi-walled carbon nanotubes, graphite, black carbon and granular activated carbon are other efficient materials for electrode construction. Simultaneous persulfate electro activation and adsorption of pollutants on the surface of porous carbon electrodes can provide the electron transfer between the generated reactive species and the target pollutants, useful in the degradation of pollutants. Accordingly, the capability of multi-walled carbon nanotubes in the degradation of aniline via persulfate electro activation has been confirmed.¹¹¹ 

It is noteworthy that carbon electrodes are usually more efficient and economical compared to others. Additionally, modification of the carbon nanotubes provides an enhanced catalyzing ability while using them as the electrode. For instance, doping carbon nanotubes with metal oxides like SnO₂, CeO₂, and MoO₃ has been attempted with the aim of obtaining a higher oxygen evolution potential and lower charge transfer resistance.¹¹² 

The use of nitrogen-doped carbon material electrodes is another technique for enhancing persulfate activation taking into account the electron-withdrawing nature of pollutants like pyridinic and pyrrolic nitrogen atoms and subsequently positively charged carbon.¹¹³  Moreover, cathodic reduction and the production of reactive species from oxygen decomposition are significantly improved in Ni encapsulated carbonaceous materials. In this regard, an electrochemical system composed of an SnO₂/Ni@NCNT composite as the anode and Ni@NCNT as the cathode has been used to promote the electrochemical activation of persulfate, and the complete degradation of the drug cephalexin (antibiotic) has been achieved.¹¹⁴ 

The persulfate electro activation process is easily controlled with optimization of current density i.e. the amount of current flowing per unit area of the electrode surface as the influencing parameter. Generally, the degradation efficiency increases with current density; however, this trend may be kinetically limited due to the mass transfer constraint on the anode surface. Moreover, a current density higher than an optimum value accelerates the undesirable and energy-consuming side reactions.¹¹⁵  Therefore, taking degradation efficiency into consideration, current density should be optimized aiming to achieve higher efficiency and lower costs.

The reactor configuration is important in the persulfate electro activation process. The design of an electrochemical reactor should be such that greater and faster pollutant degradation and improved current efficiency as well as lower energy input should be attainable. Hence, evaluating the trade-off between energy cost and degradation efficiency, the electro activation of persulfate could be introduced as a promising approach for water treatment in large-scale applications.

By utilizing a sacrificial iron anode in single-cells, gradual and continuous generation of Fe²⁺ ions as well as degradation of various kinds of pollutants are achieved but electrode corrosion due to direct contact of persulfate also occurs. Furthermore, in a typical cell configuration, thte pH increases during electrochemical reactions, attributed to the production of OH⁻ at the cathode surface, and in turn leads to precipitation and deactivation of Fe²⁺.

Concerning the mentioned problems, a novel configuration has been introduced for an electrochemical reactor which is depicted in Figure 1.25.¹¹⁶  In this design, three chambers (anode, reaction, and cathode) are separated with two cation exchange membranes. In the anodic chamber chemical corrosion of an iron electrode is prevented because there is no direct contact between the wastewater and oxidizing agent. In this chamber, applying the electrical current not only contributes to anodic electrochemical corrosion as a unique approach to precisely control the Fe²⁺ generation but also drives the ions to move toward the reaction chamber. The migration of Fe²⁺ ions to the reaction chamber is the main cause of electro activation of persulfate. Consequently, the generated active species of the SO₄˙⁻ radical anion and hydroxyl radical could degrade the pollutants. In this device, the cathodic chamber has been separated from the reaction chamber with the aim of reducing the undesired effects of generated OH⁻ on the iron catalysts.

In a unique configuration, a flow-through cathode with well-ordered micro-channels has been utilized aiming to boost the electro activation via confining persulfate anions.¹¹⁷  As previously described, electro activation of persulfate can occur at the cathode surface. The repulsive electrostatic interaction between persulfate anions and a negatively-charged cathode was originally recognized as the main reason for the low performance compared to anodic electro activation. Accordingly, the design of electrochemical reactors with a shorter diffusion distance and enhanced contact between the persulfate anion and cathode has been facilitated. A carbon cathode derived from natural wood with well-ordered microchannels is a novel electrode material.

The flow mode is another influencing parameter in cathodic microchannels. The cathode is contrived along the direction of the microchannels. Hence, the persulfate-containing solution flows through the microchannels. There are two kinds of corresponding flow modes: the flow through cathode (FTC) which allows the solution to flow through the microchannels, and the flow by cathode (FBC) in which the cathode is directly sealed and the solution is prohibited from flowing through the microchannels. Comparing flow modes, the flow through cathode is superior in terms of pollutant degradation efficiency. The performance of these flow modes is presented in Figure 1.26.

The authors wish to thank the authorities of Bu-Ali Sina University for the financial support of this work.