Chapter 1: Preparation and Modification of New Functional Materials for Organic Pollutant Elimination

Water is one of the most important resources in the world to ensure a safe life for billions of people. Considering the increasing population and the rapid industrialization and urbanization, the demand for water is constantly growing, and the need for drinking water, as well as fresh water for industrial facilities, is becoming a major issue.

According to the Food and Agriculture Organization (FAO), due to water pollution and misuse, billions of people will still lack access to basic water services in 2030 unless progress quadruples in speed.¹ In order to resolve this problem, novel and effective methods for removing pollutants from water are needed, in particular for emerging pollutants such as pharmaceuticals, perfluorinated compounds (PFC), solvents and nano-plastics.² 

Processes for treatment of contaminated ground water include many techniques such as micro/ultra/nano filtration, centrifugation, (co)precipitation, (photo)oxidation, reverse osmosis, etc. which have been thoroughly discussed in the literature.^3–5 

Adsorption can efficiently be applied for the removal of different pollutants (organic, inorganic, and biological) and can be considered the cheaper way to produce purified water. In general, the adsorbate (pollutant) containing solution is kept in contact for a certain time, passed through, shaken, or stirred with the adsorbent, and the former adheres onto the surface of the adsorbent, via chemical or physicochemical interactions, under specific conditions, until equilibrium is established. Each type of adsorption depends on the way the adsorbent and adsorbate interact. Chemisorption is irreversible, because of the strong chemical bonds established between the adsorbate and the adsorbent surface. In contrast, physisorption is reversible, due to weak intermolecular physical forces involved (π–π and dipole–dipole interactions, Van der Waals forces, hydrogen bonds), between the adsorbate and adsorbent.⁶ In developing a water remediation strategy, tests are usually performed at a lab-scale with single or multipollutant systems to simulate wastewater model systems. In this context, unselective adsorbents based on cheap or readily available materials can be useful to remove most pollutants reaching removal efficiencies close to complete removal. In some cases, the water resulting from the first round of remediation could be subjected to further cycles reaching the desired level of removal and water purity. However, when the residual pollutants are of a different chemical family with respect to the main ones, using the same type of adsorbent for subsequent cycles may not be a suitable solution for the complete removal of impurities. In these cases, the degradation of residual pollutant could be performed by several approaches that might include photocatalysis or other advanced oxidation processes (AOPs). Nevertheless, in case of persistent pollutants that are recalcitrant to most degradation processes or in the case of degradation that does not mineralize the pollutant but produces smaller/different by-products, the removal of such substances should be approached by specific adsorbing systems. This last step of water purification, that could be defined as fine water remediation (FWR), requires methods that are economically and energy-wise competitive with “simple” distillation and that are based on the availability of adsorbents for specific classes of residual persistent pollutants.

For this reason, many different approaches have been attempted, and different materials were synthesized depending on the nature of the pollutants to be removed. Organic materials such as biomass derived polymers, metal and covalent organic frameworks, carbon nanomaterials (graphene oxide, carbon nanotubes), and hybrid materials have been recently synthesized or functionalized for pollutant removal. In this chapter selected recent examples will be presented to give a general overview of this interesting and wide research field.

The scientific community has shown increasing interest in biopolymers and their derivatives because of their unique characteristics, such as biodegradability and biocompatibility. Most biopolymer’s monomer units are made up of repeating molecules of nucleic acid, nucleotides, amino acids, proteins, or saccharides.^7,8

They can be classified into three categories: natural (polysaccharides like chitosan, alginate, cellulose, and proteins like gelatine, collagen), synthetic (made by chemical or biological modification of natural ones) and microbial biopolymers (produced through a fermentation process of monomers in bacteria and fungi like xanthan, cellulose).^9–11 

In terms of functional properties and manufacturing costs, biopolymers have created an ideal competition with their fuel fossil-based equivalents. In fact, research in this area is still ongoing with the goal of developing new biopolymers with ever-improving functional properties and ever-expanding uses. Biopolymers have recently found to have a variety of applications, from additives and bioplastics to personal hygiene, and medical products, whilst also offering the advantage of ecologic degradation.¹² Biopolymer applications for wastewater treatment are receiving more attention thanks to their properties and those of their composites. The expense of the clean-up procedure is decreased by using economical polymers as adsorbents. Because of their superior durability, high functionality, processing capabilities, and huge surface area, biopolymers are more effective at removing pollutants from the environment through adsorption.^13,14

In this chapter, some biomaterials were selected for each class. The number of applications for cellulose, chitosan, alginate, and lignin has expanded in recent years, indeed, they are frequently found as by-products or waste and are becoming basic and freely accessible resources according to the principles of the circular economy.¹⁵ 

The distinctive macroscopic fibrous structure of cellulose is made up of a linear polymer chain formed by multiple d-glucose units connected by β-1,4-glycosidic linkages, and alternating sections of amorphous and crystalline zones. Several natural sources, including plants, seeds, cane, sand, and certain microbes, can yield highly pure cellulose.¹⁶ 

The raw materials balsa wood, loofah and pomelo peel have been combined with polyethyleneimine (PEI) to produce high-efficiency adsorbents for the removal of perfluorooctanoic acid (PFOA) from water. The three different starting materials were chosen for their easy modification and abundant presence as by-products, PEI was chosen because of the presence of a repeating amino group capable of binding the negatively charged residues of PFOA.¹⁷ Essentially, hydroxyl groups work as nucleophiles in almost all cellulose reactions, and this type of nucleophilic substitution reaction provides a greater range of functionalization possibilities. In this case epichlorohydrin, under alkaline conditions, produces an epoxy cellulose ether structure. On this intermediate, a further substitution occurs with the PEI (grafting approach), Figure 1.1.¹⁸ 

The maximum adsorption of PFOA is reached at pH 3 with a capacity of 279.3 mg g⁻¹ for the material obtained from balsa wood. The systems were saturated for 4 h and the presence of salts promoted the adsorption through salting-out phenomena in addition to the electrostatic and hydrophobic interactions.

To remove polystyrene nanoparticles, cellulose’s porous qualities and layered double hydroxides (LDHs) distinct positive charge were combined to create cellulose/MgAl LDHs composite beads. Van der Waals and electrostatic attractions dominated the interaction between nano-plastics and Mg/Al LDHs.¹⁹ In this case the cellulose source comes from the Pennisetum sinese Roxb, an affordable and sustainable supply for polysaccharide. Using the entrapment approach, composite beads made of cellulose and LDHs have been produced.²⁰ The removal of nano-plastics using cellulose and hydrotalcites had a synergistic effect, with the maximum removal rate observed at a ratio of 1 : 1. The maximum activity is reached under acidic conditions, achieving 90% of efficiency. The efficient removal of nano-plastics is facilitated by the hydrotalcite’s electrostatic attraction to the nano-plastics. Cellulose/LDHs composite beads were more successful in removing pollutants and they can be simply recovered and separated and are not toxic in the environment.²⁰ 

The combination of polyaniline (PANI) and polypyrrole (PPy) with hydroxyehtylcellulose (HEC) is investigated for the removal of dyes (Methyl Orange and Rhodamine B) from aqueous media. PANI and PPy were chosen for the affordable cost, biocompatibility, non-toxicity, easy synthetic process (Figure 1.2), and significant surface area.²¹ 

Alginate is a water-soluble linear polysaccharide derived from brown seaweed. It can collect metallic ions due to its surface which is rich in carboxylic and hydroxyl functional groups. Nevertheless, it is characterized by excessive rigidity and fragility, as well as weak elasticity and mechanical characteristics.

Chitosan combined with alginate solution forms an interpenetrated network, which, with the aid of a metal precursor (Fe³⁺, Zr⁴⁺, Al³⁺), was concurrently transformed into an interconnected polymer chain mesh²² by using two hybrid polysaccharides in a complex matrix distribution with Zr–Al sites forming a strong adsorbent for Cr⁶⁺ and F⁻ ions. The system works at pH = 4 and has a capacity of 315.86 mg g⁻¹ for Cr^6+, and 52.75 mg g⁻¹ for F⁻. The mechanism of action is attributed to the ion exchange capacity and electrostatic interactions.

In recent years, novel materials have been developed by combining alginate with other synthetic or natural polymers, such as polyacrylic acid and chitosan, to improve mechanical and thermal stability and adsorption capabilities. Furthermore, numerous organic and inorganic–alginate materials have been produced and tested for the removal of antibiotics from wastewater.

Encapsulating or incorporating graphene oxide (GO) into an alginate-based hybrid material not only improves the mechanical and physical properties of the alginate, but also reduces GO nano-toxicity. Because hydrogels may be easily removed from water, they are ideal for the treatment of aqueous matrices.

Alginate–graphene hydrogels can be made by dissolving sodium alginate and graphene oxide in water or by dropping the combined dispersion into a CaCl₂ solution.^23,24 In acidic conditions, these materials demonstrate the highest adsorption capacity for ciprofloxacin and tetracycline, whereas in alkaline conditions, the gel structure is disrupted, and the adsorbed chemicals are liberated.

Ciprofloxacin and norfloxacin antibiotics were removed using alginate-Fe₃O₄ fibres. Adsorption capacity was higher under acidic conditions since charge repulsion between ionized carboxylic groups is reduced, and intermolecular hydrogen bonding contributes to the stability of the hydrogel structure to a larger extent.

Lignin is a heterogeneous aromatic polymer found in nature, made up of cross-linked phenylpropanoid units (e.g. p-hydroxyphenyl, guaiacyl and syringyl units).²⁵ 

For the removal of phosphate from water the application of sawdust functionalized with iron-binding ligands is interesting. The amount of phosphate that sawdust may bind is insufficient (0.3 g kg⁻¹). But because iron and phosphate have a great affinity for each other, the formation of iron–phosphate linkages presents a possible path for the creation of recyclable resins. The three-step, fully aqueous synthesis of ethylene diamine/Fe(iii)-functionalized lignin produces a material that has shown remarkable efficacy (40 g kg⁻¹) and a recyclable nature as a phosphate-binding ligand.²⁶ Previously synthetized iron–chitosan composite has a capacity of 8.2 g kg⁻¹, but recently the price of this biopolymer has increased; therefore, lignin could be a cheaper option demonstrating better efficiency as well.

When most pollutants have been removed from wastewater, the porosity of the adsorbent is one of the main properties that needs to be considered for residual pollutant removal. In this framework, metal–organic frameworks (MOFs),^27–29 a type of crystalline organic–inorganic hybrid compound and the corresponding covalent organic frameworks (COFs),³⁰ thanks to their high porosity, have strongly been applied for environmental remediation.^31,32 MOFs and COFs have demonstrated great efficiency in the removal of both inorganic and organic pollutants.^33,34 The reason behind the recent large use of these materials for environmental remediation lies in the easy fine tuning of their channels through pre- or post-synthetic approaches.^35,36 The channels can be varied in terms of size, shape, and functionality giving rise to a controlled host–guest chemistry. For all these reasons they have also recently been applied for the removal of some emerging pollutants, however, the impact of the synthetic approach of these materials is also important considering the environmental application.³⁷ 

A lanthanum/palladium-metal organic framework (La/Pd-MOF) has been used to remove triclosan, an antimicrobial agent often used in detergents, soaps, toothpaste and cosmetics, becoming an increasingly dangerous presence for water purity.³⁸ La/Pd-MOF was easily synthesized mixing solutions of palladium and lanthanum nitrate to a solution of 2-methylimidazole.³⁹ The resulting MOF was characterized by an amorphous nature, that supported a high level of antimicrobial elimination effectiveness. After adsorption, the surface area and the pore volume of La/Pd-MOF decreased dramatically indicating the effective adsorption. The adsorption process was influenced by pH and a chemisorption process was identified which was involved in the overall process. In particular, the mechanism of interaction between the La/Pd-MOF and pollutant could involve π–π interaction, pore filling, H-bonding, or electrostatic interaction. In addition, the synthesized MOF adsorbent can be recyclable up to five cycles of adsorption–desorption.⁴⁰ 

On the other hand, a Zn-MOF synthesized from an amide containing carboxylic acid N,N″-(3,5-dicarboxylphenyl)benzene-1,4-dicarbox-amide and 4,4′-bipyridine (Figure 1.3) has been recently used for sensing activities toward biomarker (2-methoxyethoxy)acetic acid and antibiotic tetracycline hydrochloride.⁴¹ A MOF structure is formed by two-dimensional layers stacked to form a stable supramolecular structure with one-dimensional channels, the single unit is held together by the bond between carboxylate ligand and 4,4′-bipyridine with Zn(ii) centers. Luminescence measurements indicated that the complex exhibited excellent multiple sensing activities.⁴¹ 

Another emerging pollutant is the endotoxin secreted from bacteria; cyanobacteria can cause severe health problems such as impairment of the immune system and toxic shock. To avoid its presence in water, a novel modified amino Cu-MOF was synthesized.⁴² Firstly, the Cu-MOF was synthesized reacting Cu(NO₃)₂ and benzenetricarboxylic acid, to this preformed crystalline MOF a post-functionalization reaction involving lysine to obtain an ammine tail was carried out (Figure 1.4).

X-ray and SEM analysis showed the cubic crystalline nature of modified MOF and this was used for the adsorption of endotoxin. Adsorption experiments performed in several conditions demonstrated great efficiency of MOF with over 90% of endotoxin degradation and indicated that the adsorption was driven by electrostatic, hydrophobic bonding and hydrophobic interactions held together by endotoxin and NH₂-Cu(MOF). Recycling tests also showed good reusability and stability of NH₂-Cu(MOF), with a decrease in removal efficiency of only 93.4–89.1% after five subsequent recycles.

To test the adsorption ability of materials against organic contaminants dyes are often used, for example an anionic sulfonate-functionalized covalent organic framework (COF) was applied. The selected COF was yielded by the Schiff-base condensation between 2,4,6-triformylphloroglucinol and 4,4′-diaminostilbene-2,2′-disulfonic acid under solvothermal conditions in the presence of sodium acetate (Figure 1.5). Thanks to the dense sulfonate groups in the periodic one-dimensional channels, in only 10 min the negatively charged COF showed a 99% uptake of multiple cationic dyes such as methylene blue, crystal violet, malachite green and janus green.⁴³ The efficiency was higher with respect to those of MOFs, also demonstrating good recyclability. In addition, COF presented low adsorption capacity for anionic dyes rendering it selective for the separation of cationic and anionic dyes.

Recently, a copper–gallic acid metal–organic framework (CuGA MOF) has been synthesized with an eco-friendly approach in order to use the material for basic dye removal.⁴⁴ The molar ratio of NaOH and GA, temperature, and reaction time, were tuned to obtain the optimal yield and crystallinity percentage with the aid of computational investigations. The adsorption capacity of this MOF was higher than 115.08 mg g⁻¹ and this efficiency remained even after 3 adsorption–desorption cycles.

Furthermore, a series of Bi₂O₃-MOF functionalized with iron succinic acid was prepared varying the Fe : Bi molar ratios using a hydrothermal method at low temperature. The modified MOF showed enhanced properties than the pristine MOF overall for the photocatalytic degradation of the cationic organic dye, Rhodamine B. Modified MOF with molar ratios of Fe : Bi (1 : 0.1) exhibited the highest degradation efficiency of 80% in 120 min under visible irradiation.⁴⁵ The degradation mechanism proposed that the introduction of Bi₂O₃ into the Fe-succinic acid framework, facilitated the charge carrier’s separation at the interface, responsible for the enhanced photodegradation efficiency of the nanocomposite photocatalysts. In addition, trapping experiments of the reactive species revealed the order hydroxyl radicals > superoxide anion radicals > holes for their role in the degradation process. Indeed, a new eco-friendly synthesis of the composite iron–succinic frameworks was designed without the use of any toxic organic solvents, taking care of the overall processes from the synthesis of the material to the application for the cost effective and potential technology for purifying contaminated water.

Other emerging pollutants in addition to the common heavy metals are rare earth elements that have started to be extensively used in renewable energy technologies and electronic devices. In addition, the depletion of natural rare earth elements from mineral deposits has made selective recovery of rare earth elements from alternative sources essential. In this context, a chromium-based MOF was first synthesized and then modified with N-(phosphonomethyl)iminodiacetic acid for the selective recovery of Eu. The modified MOF was achieved following a hydrothermal process, where the pristine Cr-MOF bearing an ammino free functionality was modified with N-(phosphonomethyl)-iminodiacetic acid in the presence of N,N-dicyclohexylcarbodiimide under reflux.⁴⁶ The adsorbent showed a good adsorption capacity and enhanced selectivity for Eu uptake from acidic solution over competing transitional metal ions (Na, Mg, Al, Ca, Mn, Fe, Ni, Cu, Co, and Zn). The high selectivity was ascribed to the formation of coordinative complexes with grafted carboxylate, phosphonic, and residual amine functional groups of MOF (Figure 1.6). Once again, the material was recyclable without any loss in efficiency for several adsorption cycles.

For the selective separation of thorium from rare earth elements and uranium, useful for the application of thorium in nuclear energy, new ionic and neutral imidazole-based COFs have been designed (Figure 1.7).⁴⁷ Both COFs were highly crystalline and predominantly mesoporous. The pore sizes were not significantly affected by the presence of the anion. Nevertheless, the neutral based COF exhibits a significantly higher Th(iv) uptake capacity and adsorption rate than the ionic one, indicating that the adsorption uptake of Th is governed by the Th–N coordination interaction as Th(iv) preferentially binds at different N sites with a mechanism of selective coordination.

Several MOFs and COFs have been efficiently tested for the uptake of heavy metals from wastewater,^48,49 but recently a new water-stable MOF prepared from the combination of two different oxamide-based metallo-ligands derived from l-serine and l-methionine has been synthesized and applied for the efficient removal of both inorganic and organic pollutants.⁵⁰ 

MOF is characterized by hexagonal channels bearing two types of flexible and functional tails (–CH₂OH and –CH₂CH₂SCH₃) allowing the simultaneous and efficient removal of both kinds of pollutant: heavy metals, such as Hg²⁺, Pb²⁺, and Tl⁺ and organic dyes such as Pyronin Y, Auramine O, Brilliant green, and Methylene blue. This MOF can be used for the removal of many kinds of pollutants from water resources and can be recyclable, thus opening new perspectives for this emerging type of multivariant MOF.

Carbon-based nanomaterials (CMs) such as graphene-based materials, carbon nanotubes, nanofibers, and fullerene, are compounds whose structure, mainly represented by carbon atoms, allows them to self-assemble into many different two- and three-dimensional structures, each with its own properties. CMs have recently attracted attention as pollutant-removing agents thanks to their excellent physicochemical and mechanical stability, the presence of multiple active surface sites, the high biocompatibility and large possibility of functionalization,^6,51 and lastly, whether the pollutant in charged, ion-exchange can be performed by giving the CM the opposite charge.⁵² 

In batch processes CMs have shown high capability in binding small pharmaceutical molecules, allowing them to be used in the purification of wastewater. Granularly activated carbon F-400, carbon nanotubes, and nanofibers have shown high adsorption rates of 242 mg g⁻¹ for carbamazepine and 264 mg g⁻¹ for ciprofloxacin.⁵³ 

Graphene oxide (GO) has been able to remove 500 mg g⁻¹ of diclofenac and 3709 mg g⁻¹ of sulfamethoxazole. The mechanism for the adsorption was due to hydrogen bonds, hydrophobic and π–π interactions. Furthermore, an adsorption value of 149.4 mg g⁻¹ of 17β-estradiol has been calculated for GO. This happens due to the hydrogen bonds established between hydroxyl groups and π–π interactions because of the presence of the aromatic ring in these hormone (Figure 1.8).⁵⁴ 

In addition, biochar, a porous carbonaceous material produced by pyrolysis of biomass, has demonstrated an adsorption efficiency for ibuprofen of 167 mg g⁻¹ at pH = 2. However, if the biochar is functionalized with iron chloride, becoming magnetic, it can be easily recovered after absorption but its efficiency decreases to 140 mg g⁻¹ at pH = 4.⁵⁵ 

Another example occurs in polymeric magnetic functionalized GO composites, which has been used for the removal of chlorinated pesticide such as chlorpyrifos and, moreover, exceptional adsorption capacities for bisphenol-A, with a value of 182 mg g⁻¹ at pH range of 2–4.⁵⁶ This can be due to the formation of H-bonds between –OH/–COOH groups (Figure 1.9).

Verma et al. performed an ammonia functionalization of the GO sheets, by redispersing 100 mg of GO in 40 mL of ethylene glycol, adding 0.1 µL mg⁻¹ of liquid ammonia until the dispersion color turned black. The resulting NH₃-GO (Figure 1.10) was able to remove dyes from an aqueous solution in relevant percentages: 56.2% of Basic Blue 41, 52.3% of Methyl Orange and 28.3% and of 4-nitrophenol.^57,58

Sajid et al. investigated the possibility of using CMs to remove microplastics. Because they are less than 3 µm microplastics are difficult to remove from aqueous media, it is therefore crucial to design non-toxic and biodegradable adsorbents with the ability to remove small size MPs, to avoid the phenomenon of secondary pollution.⁵⁹ Graphene oxide (GO) sponges have been developed as materials for the efficient removal of microplastics from aqueous solutions. These sponges are made by incorporating GO powder into a chitin hydrogel (a non-toxic and biodegradable polysaccharide), which increases their strength and enhances their pollutant removal performance. The procedure involves crafting a chitin hydrogel through a specific blend, establishing a dual-crosslink structure (Figure 1.11)⁶⁰ with solid GO homogeneously distributed, eliminating residues, and transforming the hydrogel into sponges via cryogenic treatment and vacuum freeze-drying. The overall method demonstrates a methodical and controlled approach to produce sponges with the desired characteristics from hydrogels. The presence of functional groups on GO contributes to the removal of positively, negatively, and neutrally charged pollutants (such as polystyrene (PS), PS-COOH, PS-NH₂). The chitin-based sponges created with GO have shown a removal efficiency of 71.6–92.1% for different types of MPs. The adsorption mechanism involves hydrogen bonding, π–π interactions, and electrostatic interactions.^54,59

With the aim of increasing the sp² clusters in GO to enhance π–π or other hydrophobic interactions for increasing adsorption of organic compounds, another material has been synthetized by the introduction of a lipophilic functional group on GO. In detail, GO was functionalized by the formation of the covalent bonds between carboxyl groups of GO and amine groups of 9-aminoanthracene to obtain GO-9-AA (Figure 1.12). This material was utilized for the removal of naphthalene, acenaphthylene, and phenanthrene.⁶¹ 

Hybrid materials are commonly employed due to their ability to combine the advantages of organic frameworks (easy functionalization and modification, bio-compatibility, and renewability) and inorganic scaffolds (easy synthesis, stability, and durability).^23,62 The hybrid materials can be classified depending on their obtainment method as: (i) Structurally Hybridized Materials (SHM) if the organic–inorganic parts are composites that interact through non-covalent bonds; (ii) Materials Hybridized in Chemical Bonds (MHCB) if the presence of covalent bonds between the organic–inorganic parts can provide new properties to the material. In recent years many different materials have been synthesized and currently the most common building blocks are bio-compatible organic polymers such as chitosan, gelatine, alginate, cellulose, carboxymethylcellulose (CMC), etc.

Concerning the inorganic portions, minerals, clay, and oxides, such as silica, have been preferred for their low cost and toxicity. The use of metal organic frameworks (MOF) or carbon-based materials such as graphene oxide (GO) is readily expanding in recent years. Below we will present some recent examples focusing on the employed inorganic building blocks and the different synthetic strategies.

Silica and/or metal oxides are widely used as pollutant adsorbents due to their high surface area, defined and tuneable pore size, thermal stability, and availability.^63,64 Streide Machado and co-worker⁶⁵ designed a silica–chitosan composite crosslinked with glutaraldehyde with promising properties for the removal of diclofenac from water. The synthetic process starts with the hydrolysis of tetraethoxysilane (TEOS) in the presence of 1% chitosan. The obtained silica particles, coated with the biopolymer, were crosslinked with glutaraldehyde, with different ratios, thus forming xerogels named XE presenting covalent imino-bond (Figure 1.13).

The hybrid was characterized by means of FT-IR, BET, SEM, and elemental analysis. The adsorption isotherms revealed a high adsorption capacity of 237.8 mg g⁻¹ for diclofenac. Notably, due the presence of free carbonyl groups material XE can also covalently link the amino moiety of the adsorbed drug. This feature was not previously observed with other similar composites and it is quite interesting for the prospective removal of other emerging contaminants.

Bionanocomposite-based aerogel (BNC-AG) is an α-FeOOH and γ-AlOOH-based nanocomposite formed within a chitosan and agarose aerogel reinforced with ethylenediaminetetraacetate dianhydride (EDTAD) and can be used for membrane filtration of multi-polluted water.⁶⁶ Fe–Al oxyhydroxide nanocomposite (NC) was formed in situ on preformed chitosan/agarose aerogel, obtained by means of the freeze-drying method, by dipping in the presence of EDTAD, which is hydrolysed and acts as a reinforcement of the membrane through covalent bonding with the hydroxyl groups of the polymers (Figure 1.14).

The microporous structure of the membranes was evidenced with FE-SEM and the material was investigated with multiple techniques: PXRD, FT-IR, BET, and XPS. The membrane can act as a filter for the removal of dyes, emerging pollutants, arsenate, and fluoride. In particular, the removal of ciprofloxacin, peptide hormone oxytocin, and non-ionic surfactant Triton X 100 from water was 91%, 89%, and 96%, respectively. Also, inorganic pollutants can be effectively removed, in fact, 93% As(v) and 99% F can be retained. The membrane can work under continuous flow and can be recycled up to 50 times with low efficiency loss after acidic washing, therefore it can be considered to be of practical application for wastewater purification.

MOF are MHCB materials with the great advantage of a regular and highly porous structure that ensures good adsorption properties.^67,68

The advantage of using MOF is clearly shown in a recent example, surface functionalized metal–organic framework and biological macromolecule (SFMOF/BM) bio-composite was synthesized starting from MIL-53(Fe), a common MOF synthesized using the hydrothermal method. Surface modification with aminopropyl trimethoxysilane (APTMS) of the formed MOF was followed by mixing with chitosan (Figure 1.15).⁶⁹ 

The obtained SFMOF/BM composite was studied for adsorption capacities toward dyes such as Direct Red 23 (DR23) and emerging pollutants such as tetracycline and doxycycline. The adsorption capacity of SFMOF/BM was 388 and 264 mg g⁻¹, for tetracycline and doxycycline respectively. The saturated adsorbent can be recycled five times and can be considered a promising adsorbent for water treatment applications to remove emerging pollutants.

Other significant recent examples are sodium alginate (SA), MXene, MIL-101(Fe), composite hydrogel beads,⁷⁰ efficiently employed for the removal of naproxene in fixed-bed adsorption columns. Analysis of relevant parameters predict the potential design of an industrial-scale column. The regeneration of composites involved the indirect electrooxidation of the materials in the presence of OH radicals or indirect oxidation in the presence of active chlorine species.

A novel adsorbent formed combining poly(acrylamide-co-acrylic acid)/chitosan (P(AM-co-AA)/CS) composite with Zr-based MOF-808, was developed for the adsorption of U(vi) from seawater, through a rational design of the hybrids aiming to obtain a low swelling ratio in water reducing the solvent penetration filling the copolymer with CS.⁷¹ 

In fact, the adsorbent exhibited good mechanical properties because of the hydrogen bonding and covalent cross-linking between CS, AM, and AA and the coordinate bond between MOF-808 and P(AM-co-AA). The adsorption capacity of the adsorbent is 159.56 mg g⁻¹ at pH = 8.0 also showing a U(vi) removal up to 95% in the presence of other ions.

Graphene, GO and other carbon-based compounds are effective nanomaterials for the removal of organic pollutants, heavy metals, anions, and bacteria, due to their high surface area and adsorption capacity.^58,72–74

GO can adsorb organic compounds by means of π–π interactions and/or hydrogen bonding. GO, synthesized via Hummers’ method, composites with commercially available cellulose with the aim to remove Flupentixol (FPLL), an antidepressant and emerging micropollutant.⁷⁵ XRD, Raman spectra, SEM and TEM micrographs were employed to confirm the presence of GO and its nanosheet structure, while an artificial neural network (ANN) model was developed to optimize the adsorption process parameters with a maximum of (99%) FPL adsorption. Docking studies also confirmed the absorption of FPL@GO by means of π–π interactions, hydrogen bonding, hydrophobic interactions, sulphur interaction and lone pair interactions (Figure 1.16).

Another multifunctional material containing carboxymethyl cellulose/graphene oxide/polyaniline (CMC/GO/PANI) was synthesized as a thin film by mixing and evaporation and applied for the adsorptive removal of Cu(ii), and oxytetracycline (OTC) from wastewater.⁷⁶ The results showed that a hybrid CMC/GO/PANI thin film had a higher adsorption efficiency than pure materials due to the multifunctional synergetic effect. Hybrid CMC/GO/PANI thin film is a reusable adsorbent for eliminating pharmaceuticals and metal ions from wastewater, with a performance comparable to other previously reported systems, but unfortunately the cost of the GO and PANI is quite high.

Other recent examples consist of a novel eco-sustainable chitosan/gelatine/GO hybrid as an adsorbent for water purification from fluoroquinolonic antibiotics ofloxacin and ciprofloxacin as well as Pb.⁷⁷ The aerogel presents an antimicrobial effects after 1 h of incubation when employed as a filter for drinking water. Finally, efficient beads adsorbers for diclofenac (DIF) were obtained combining activated carbon (CAC) with chitosan in a 1 : 1 ratio. Notably, the active beads were obtained by switching the pH to basic conditions and the maximum adsorption capacity was 99.29 mg g⁻¹. Moreover, the hybrid materials were able to reduce DIF toxicity toward Vigna mungo seeds during germination. In fact, after filtration and DIF absorption the seed’s growth was considerably improved.

The great variety of synthetic tools available to organic chemists to produce new functional materials or to add new functions to existing materials makes the possibility to design advanced materials for pollutant removal practically unlimited. However, the more complex is the need then the more expensive is the solution in terms of production costs of the advanced material, especially if it is needed in large quantities. In this context, the feasibility of a sustainable approach to water remediation by using advanced materials for pollutant removal is based upon a careful choice of the most appropriate technology/material as a function of the qualitative/quantitative composition of the water that needs to be treated. The suitability of a material for a given purpose, especially when removal is mainly based on adsorption processes, depends on the specificity of the interactions which, in turn, depend on the chemical moieties of the adsorbent framework and of the pollutant. Given the fact that the chemical features of the pollutant cannot be “designed”, but rather their classification depends on the pollution source, it is important to be able to design the features of the adsorbent to optimize the response to the removal need. The choice of the best technology to implement as pollutant removal, will then rely on an accurate costs-versus-benefits analysis, including not only the efficiency of the removal, but also the costs of advanced material production and the availability of the source of the starting raw material. Once the efficacy of a given material is established, further developments should be focused on the possibility to re-use, regenerate, or recycle the used adsorbent, also through immobilization strategies for batch reactors or innovative separation approaches such as magnetic separation of heterogeneous components. In this framework, without the intention of providing exhaustive examples, this chapter intends to offer to the reader an overview of strategies available to obtain advanced materials, including hybrid composite materials, with features that can be combined as a function of the available primary source of the raw material (secondary waste product, biomasses, etc.), of the functionalization method, and of the type of target class of pollutants.

Carla Rizzo thanks PNR next generation EU-DM737/2021-CUP B79J21038330001 and FFR2022 for funding.