CHAPTER 1: Contamination of Heavy Metals in Aquatic Media: Transport, Toxicity and Technologies for Remediation Free

The term “heavy metal” refers to any metal and metalloid element that has a relatively high density ranging from 3.5 to 7 g cm⁻³ and is toxic or poisonous at low concentrations, and includes mercury (Hg), cadmium (Cd), arsenic (As), chromium (Cr), thallium (Tl), zinc (Zn), nickel (Ni), copper (Cu) and lead (Pb). Although “heavy metals” is a general term defined in the literature, it is widely documented and frequently applied to the widespread pollutants of soils and water bodies.¹  These metals are found widely in the earth's crust and are non-biodegradable in nature. They enter into the human body via air, water and food. A small number have an essential role in the metabolism of humans and animals in very trace amounts but their higher concentration may cause toxicity and health hazards. The hazardous nature of heavy metals has been recognized because of their bioaccumulative nature in biotic systems. They can enter into the environment through mining activities, industrial discharge and from household applications, into nearby bodies of water.

Heavy metals differ widely in their chemical properties, and are used extensively in electronics, machines and the artifacts of everyday life, as well as in high-tech applications. As a result they are able to enter into the aquatic and food chains of humans and animals from a variety of anthropogenic sources as well as from the natural geochemical weathering of soil and rocks. The main sources of contamination include mining wastes, landfill leaches, municipal wastewater, urban runoff and industrial wastewaters, particularly from the electroplating, electronic and metal-finishing industries. With increasing generation of metals from technologies activities, the problem of waste disposal has become one of paramount importance. Many aquatic environments face metal concentrations that exceed water quality criteria designed to protect the environment, animals and humans. The problems are exacerbated because metals have a tendency to be transported with sediments, are persistent in the environment and can bioaccumulate in the food chain. Some of the oldest cases of environmental pollution in the world are due to heavy metal use, for example, Cu, Hg and Pb mining, smelting and utilization by ancient civilizations, such as the Romans and the Phoenicians.

The heavy metals are among the most common pollutants found in wastewater. These metals pose a toxicity threat to human beings and animals even at low concentration. Lead is extremely toxic and shows toxicity to the nervous system, kidneys and reproductive system. Exposure to lead causes irreversible brain damage and encephalopathic symptoms.²  Cadmium is used widely in electroplating industries, solders, batteries, television sets, ceramics, photography, insecticides, electronics, metal-finishing industries and metallurgical activities. It can be introduced into the environment by metal-ore refining, cadmium containing pigments, alloys and electronic compounds, cadmium containing phosphate fertilizers, detergents and refined petroleum products. Rechargeable batteries with nickel–cadmium compounds are also sources of cadmium.^3–5  Cadmium exposure causes renal dysfunction, bone degeneration, liver and blood damage. It has been reported that there is sufficient evidence for the carcinogenicity of cadmium.³ 

Copper, as an essential trace element, is required by biological systems for the activation of some enzymes during photosynthesis but at higher concentrations it shows harmful effects on the human body. High-level exposure of copper dust causes nose, eyes and mouth irritation and may cause nausea and diarrhea. Continuous exposure may lead to kidney damage and even death. Copper is also toxic to a variety of aquatic organisms even at very low concentrations. Mining, metallurgy and industrial applications are the major sources of copper exposure in the environment.

Zinc is also an essential element in our diet. Too much zinc, however, can also be damaging to health. Zinc toxicity in large amounts causes nausea and vomiting in children. A higher concentration of zinc may cause anemia and cholesterol problems in human beings. Mining and metallurgical processing of zinc ores and its industrial application are the major sources of zinc in the air, soil and water. It also comes from the burning of coal.

Nickel occurs naturally in soils and volcanic rocks. Nickel and its salts are used in several industrial applications such as in electroplating, automobile and aircraft parts, batteries, coins, spark plugs, cosmetics and stainless steel, and is used extensively in the production of nickel–cadmium batteries on an industrial scale. It enters into the water bodies naturally by weathering of rocks and soils and through the leaching of the minerals.⁴  The water soluble salts of nickel are the major problems of contamination in aquatic systems.⁵  Paint formulation and enameling industries discharges nickel containing effluents to the nearby bodies of water.⁶  Nickel is also found in cigarettes, as a volatile compound commonly known as nickel carbonyl.⁷ 

Arsenic is found naturally in the deposits of earth's crust worldwide. The word arsenic is taken from Zarnikh in Persian literature, which means yellow orpiment.⁸  It was first isolated as an element by Albert Magnus in 1250 AD. Arsenic exists in powdery amorphous and crystalline forms in the ores. In certain areas the concentration of arsenic may be higher than its normal dose and creates severe health hazards to human beings and animals. It enters the environment through the natural weathering of rocks and anthropogenic activities, mining and smelting processes, pesticide use and coal combustion. The toxicity of arsenic as a result of the contamination of groundwater bodies and surface waters is of great concern. Arsenic exists as arsenate, As(v), and arsenite, As(iii), in most of the groundwater.^9–12  Adsorption and solution pH commonly controls the mobility of arsenic in the aqueous environment.^13–17  Metal oxides of Fe, Al and Mn play a role in the adsorption of arsenic in aquatic bodies.^18–20  Arsenic has been found naturally at high concentration in groundwater in countries such as India, Bangladesh, Taiwan, Brazil and Chile. Its high concentration in drinking water causes toxic effects on humans and animals.

The toxicity of mercury has been recognized worldwide, such as in Minamata Bay of Japan. Mentally disturbed and physically deformed babies were born to mothers who were exposed to toxic mercury due to consumption of contaminated fish. The natural sources of mercury are volcanic eruption, weathering of rocks and soils, whereas anthropogenic mercury comes from the extensive use of the metal in industrial applications, its mining and processing, applications in batteries and mercury vapor lamps. Methyl mercury is more toxic than any other species of mercury.

Extensive use of chromium compounds in industrial applications has discharged huge amounts of wastewater containing toxic chromium species into water bodies. Chromium enters into the environment by natural inputs and anthropogenic sources. Volcanic eruptions, geological weathering of rocks, soils and sediments are the natural sources of chromium, whereas anthropogenic contributions of chromium come from the burning of fossil fuels, production of chromates, plastic manufacturing, electroplating of metals and extensive use in the leather and tannery industries.²¹  Hexavalent chromium is more toxic than trivalent chromium.

Cadmium is the most toxic element, even at its low concentration in the food chain and has been found to cause of itai-itai disease in Japan. Unlike other heavy metals, cadmium is not essential for biological systems. Hence it has no benefit to the ecosystem and only harmful effects have been reported. It is used in the manufacturing of nickel–cadmium batteries, plastics and pigments. Phosphate fertilizers and waste dumping are both routes for cadmium transference into the environment. Concern regarding the role and toxicity of cadmium in the environment is on the increase, because it can be highly toxic to human beings and animals at very low concentrations. Cadmium toxicity causes renal dysfunction and lung cancer, and also osteomalacia in the human population and animals, in addition to increasing blood pressure. Smoking of cigarettes is one of the sources of cadmium poisoning in humans.

Chromium is commonly used in the leather and tanning industries, paper and pulp and rubber manufacturing applications. High levels of exposure cause liver and kidney damage, skin ulceration and also affects the central nervous system. With plant species it reduces the rate of photosynthesis. It is also associated with the toxic effects on hematological problems and immune response in freshwater fish. Chromium(vi) causes greater toxicity than chromium(iii) in animal and human health.

Copper has been used by man since prehistoric times. It is used in the production of utensils, electrical wires, pipes and in the manufacture of brass and bronze. It has a role as an essential element in human and animal bodies. However, at a higher dose it shows toxic effects, such as kidney and stomach damage, vomiting, diarrhea and loss of strength.

Human exposure to lead causes severe toxicity. Higher doses may damage the fetus and be toxic to the central nervous system. Newborn babies are more sensitive than the adults. Lead toxicity may harm hemoglobin synthesis, the kidneys and reproductive systems. Exposure to higher doses of lead may disrupt the function of the central nervous system and gastrointestinal tract. Airborne lead may cause the poisoning of agricultural food by the deposition on fruits, soils and water.⁷ 

Mercury is a very toxic element in its organic form and has been the cause of Minamata disease in Japan. It shows toxicity to the physiology of animals and human beings. Mercury toxicity has been found to be associated with physiological stress, abortion and tremors. Methyl mercury is highly toxic and causes toxic effects on the central nervous system in the human population. Mercury can result from volcanic eruptions and degassing. The exposure to mercury causes toxicity to the brain, blindness, mental retardation and kidney damage.

Nickel plays an essential role in the synthesis of red blood cells; however, it becomes toxic when taken in higher doses. Trace amounts of nickel do not damage biological cells, but exposure to a high dose for a longer time may damage cells, decrease body weight and damage the liver and heart. Nickel poisoning may cause reduction in cell growth, cancer and nervous system damage.^5–7 

The undesirable presence of iron and manganese in drinking water may pose a toxicity threat to health. However, iron and manganese are required by the biological system as they play major roles in the hemoglobin synthesis and functioning of cells. The presence of these metals in water may cause staining of cotton clothes and give a rusty taste to drinking water. The major concerns focus on the dietary intake of iron because a higher dose may pose acute toxicity to newborn babies and young children. The gastrointestinal tract rapidly absorbs iron that may pose a toxicity risk to the cells and cytoplasm. The liver, kidneys and cardiovascular systems are the major toxicity targets of iron. Neurological disturbances and muscle function damage are the result of toxic effects of manganese in human bodies.

Heavy metals are highly toxic to the fetus and newborn babies, where higher levels of exposure exist for human beings, mainly to industrial workers. Metal ions exposure to newborn babies may damage brain memory, disrupt the function of red blood cells, the central nervous system, physiological and behavioral problems. Severe toxicity from these metals may cause cancers. Exposure of plants to heavy metals may lead to physiological and morphological changes and damage to cell function and reduce photosynthesis rates. Mutagenic changes have also been observed in several plant species. Metal ion toxicities may lead to chlorosis, bleaching, nutrient deficiencies and increased oxidation stress in plants. Heavy metals obstruct the growth of microbes.²²  Table 1.1 shows the standards for metal concentration in drinking water and the health effects.

An arsenic presence in groundwater through the weathering of rocks and sediments and drinking of arsenic contaminated water causes poisoning to the blood, central nervous system, lung and skin cancer, breathing problems, vomiting and nausea. Its presence in Third World countries is becoming hazardous. The countries that are suffering with the problems of arsenic are India, Bangladesh, Taiwan, China, Brazil, Chile, South Korea, Thailand and Indonesia. Arsenic is a geogenic problem worldwide but anthropogenic sources, such as the processing of metals and manufacture of pesticides and their byproducts, are contributing equally to the levels of arsenic in the environment.

Severe toxic effects and poisoning by heavy metal ions worldwide and strict discharge regulations for wastewater effluents to aquatic bodies requires better treatment techniques. Environmental scientists have developed several procedures such as coprecipitation, membrane filtration, ion-exchange resins, photocatalytic reduction and adsorption for treatment of wastewater effluents containing heavy metals.

Bioaccumulation of heavy metals in food chains and their toxicity to biological systems due to increased concentration over time have led to tremendous pressure for their separation and purification. Heavy metals can enter into water bodies through agricultural runoff, industrial effluents, household uses and from commercial applications. We can remove heavy metals from drinking water very easily with reliable technology. Several technologies available in the market remove a huge range of metals commonly found in drinking water and wastewater effluents. There are various remediation technologies that have been used for the removal of heavy metals from water/wastewater. These remediation technologies are summarized as:

• Precipitation and coagulation

• Ion exchange

• Membrane filtration

• Bioremediation

• Heterogeneous photocatalysts

• Adsorption

Membranes are complex structures that contain active elements on the nanometer scale. Modern day reverse osmosis membranes are typically homogeneous polymer thin films supported by a porous support structure. Partitioning water and dissolved salts between the membrane and the bulk solution, and transport of water and salts across the membrane, depend on the chemical properties of the membrane as well as the physical structures on nano- to microscales. The nanometer length scale is defined as between the scale of macroscopic particles suspended in water and dissolved atomic and molecular species. From a filtration perspective, this intermediate range contains, for example, colloidal solids, large organic and biological molecules, polymers and viruses. It also corresponds to the dimensions at which that we recognize distinct modes of material transport across a membrane. For a larger dimension of porous membranes, transport is described in terms of convective flow through pores. On the other hand, transport in a dense reverse osmosis membrane is typically described in terms of diffusive flow through a homogeneous material.

Bioremediation is the technological process whereby biological systems, plants and animals, including microorganisms, are harnessed to effect the cleanup of pollutants from environmental matrices.²³  During the past few years, microbe-assisted bioremediations have been widely applied for the treatment of wastewater contaminated with heavy metals and metalloids. Here we will address the global problem of heavy metal pollution originating from increased industrialization and urbanization and its amelioration by using plants from various environmental conditions. Conventional technologies are not cost effective and may produce adverse impacts on aquatic ecosystems. Microbe-assisted bioremediation and phytoremediation of heavy metals are cost-effective technologies and metal ion accumulating plants have been successfully used for the treatment of wastewater.²⁴  Aquatic plants, especially “wetland ecosystems”, have unique properties to sequester heavy metals and metalloids.

Wetland ecosystems are much superior in comparison with other conventional methods, for example because of the low cost, frequent growth of microorganisms, easy handling and low maintenance cost. The rhizospheres in wetlands provide an enhanced nutrients supply to the microbial ecosystems of plants, which actively transform and sequester heavy metals in their biological functions. Constructed wetlands have been actively used for the treatment of heavy metals from agricultural runoff, mine drainage and municipal wastes. Many aquatic plants such as Phragmites, Lemna, Eichchornia, Azolla and Typha have been used for the treatment of wastewater containing heavy metals.

Phytoremediation is a low-cost, low-tech and emerging cleanup technology for contaminated soils, groundwater and wastewater.²⁵  Plants are very sensitive to metals but in phytoremediation wild and genetically modified plants, including grasses, herbs, forbs and woody species, are mainly used. The plants take up heavy metals and metalloids through the process of phytostabilization, phytoextraction, phytofiltration or rhizoremediation. However, in contrast to organic compounds the heavy metals and metalloids cannot be metabolized but accumulate in the plant biomass.²⁶  The biomass generated by phytoremediation remains very limited in amount and persists, whereas all the biomass can be utilized in the form of fertilizer, forage, mulch or for the production of bio-gas.²⁷  Even though it is well known that metals are toxic to many plants, they have developed some internal mechanisms that allow the uptake, tolerance and accumulation of high concentrations of metals that would be toxic to other organisms. Many researchers have reported that aquatic macrophytes viz. Typha, Phragmites, Eichhornia, Azolla and Lemna are potential wetland plants for removal of heavy metal and metalloids due to their morphological change.^24,28  Being a cost-effective and easily applicable technique, phytoremediation can be implemented for their enhancement to metal accumulations and translocations. In general, two strategies of phytoextraction have been developed, which are: (1) normal phytoremediation of heavy metals from aquatic bodies through the plants in their entire growth cycle^29–31  and (2) chemically induced phytoextraction techniques to cleanup contaminated water by using metal-tolerant plants to remove heavy metals and metalloids.³²  The efficiency of phytoextraction can be increased by using more biomass producing plant species and with the application of suitable chelates. Hyperaccumulators or hyperaccumulating plants are capable of accumulating large amounts of heavy metals and metalloids, including Ni, As, Zn, Cd and Pb, in their aboveground tissues without any toxic symptoms.³³ 

Metals uptake in relation to the external concentration of the toxic heavy metals may differ due to the different genotypes of plants. Those plants that have low uptake of metals at quite high metal concentrations are called excluders. These plants have some kind of barrier to avoid uptake of heavy metals, however, when metal concentrations are at a high level this barrier losses its function, probably due to the toxic action of the metals. Some plants have certain detoxification mechanism within their tissue, which allow the plant to accumulate high amounts of metals.³⁴  Several reports are available in the literature on the hyperaccumulator plants: Pteris vittata L. and Thlaspi caerulescens were found to hyperaccumulate As, Minuartia verna for Pb, Aellanthus biformifolius for Co and Cu, Berkheya coddi for Ni, Macadamia neurophylla for Mn and Thlaspi caerulescens for Zn.^34,35  However, phytoremediation on a commercial scale is limited because of its low biomass production, limited growth rate and time consumption.³⁵  In order to compensate for the low metal accumulation, much research has been conducted using synthetic chelators or ligands such as ethylenediaminetetraacetic acid (EDTA); S,S-ethylenediaminedisuccinic acid (S,S-EDDS); nitrilotriacetate (NTA) and naturally occurring low molecular weight organic acids to enhance the availability of heavy metals and increase phytoextraction efficiency.^36,37 

Phytoextraction is a publically appealing “green” remediation technique. However, phytoextraction can be effectively applied only for soils and wetlands contaminated with specific potentially toxic metals and metalloids. Many researchers have reported that common crop plants with a high biomass can be triggered to accumulate large amounts of low bioavailability metals when applied the phytochelates.^38,39  In such chemically enhanced phytoextractions, chelating agents are used almost exclusively as the mobilizing agents.⁴⁰  However, EDTA was the most efficient chelate to increase metal uptake by plants of Pb, but the slow degradation of chelating compounds in the root zone limits its application on an industrial scale.⁴¹  Nevertheless, more biodegradable chelates, such as NTA, (S,S-EDDS) and other chelates are also recognized for metals removal. Application of these chelating agents with plants for the uptake of metal ions is gaining more popularity and has become an interesting field of research. Several studies have been carried out using EDTA as a metal chelator for sequestration of metals.⁴²  The full-scale application for treating wastewater on an industrial scale should be based on optimization of several parameters such as solubilization of metals, chelates stability, plant roots and the capacity of metal transport through the shoots of plants.⁴³ 

In 1972 Fujishima and Honda discovered the photocatalytic splitting of water on titanium dioxide (TiO₂) electrodes.^44,45  Their discovery provided the foundation stone for photocatalysis. Since this remarkable discovery much research has been carried out on the efficiency of TiO₂ as a photocatalyst.^46–48  During the past few years, the applications of TiO₂ for environmental cleanups have been performed by several laboratories for the treatment of industrial effluents.^49,50 

During the photocatalysis system, photo-induced reactions take place at the surface of a catalyst. Depending on where the initial excitation occurs, photocatalysis can be generally divided into two classes of processes. When the initial photo-excitation occurs in an adsorbate molecule, which then interacts with the ground state catalyst substrate, the process is referred to as a catalyzed photoreaction. When the initial photo-excitation takes place in the catalyst substrate and the photo-excited catalyst then transfers an electron or energy into a ground state molecule, the process is referred to as a sensitized photoreaction. The initial excitation of the system is followed by subsequent electron transfer and/or energy transfer. It is the subsequent de-excitation process that leads to chemical reactions in the heterogeneous photocatalysis process.

Reduction of Cr(vi) using semiconductor heterogeneous photocatalysts has been carried out as an economical and simple method of wastewater treatment.^51,52  Surface-catalyzed Cr(vi) reduction is a very slow reaction and has been described as a feasible process in the presence of oxide surfaces such as TiO₂.⁵³  Furthermore, organic donors have a chelation capacity for the TiO₂ surface, which accelerates the reduction of Cr(vi).^54–57 

Testa et al.⁵⁸  carried out the reduction of Cr(vi) over TiO₂ under near-UV radiation. At pH 2, the addition of oxalate facilitated Cr(vi) reduction. It has been found that the oxalic acid accelerates the reduction of Cr(vi) over TiO₂ particles. Guo et al.⁵⁹  have synthesized a plasmonic photocatalyst of Ag–AgCl@TiO₂ by deposition–precipitation and photoreduction. This photocatalyst exhibited efficient photocatalytic activity for the photoreduction of Cr(vi) ion under irradiation with visible light.

Photocatalytic reduction of Cr(vi) in an aqueous suspension of surface-fluorinated anatase TiO₂ nanosheets with exposed {001} facets has been performed by He et al.⁶⁰  The surface fluorination facilitated the adsorption process by increasing the number of surface OH groups generated. The reduction of Cr(vi) occurred because of the oxidative dissolution of H₂O on {001} facets and the Cr(vi) reductions that occurred on {101} facets were simultaneous reactions.

Electrocoagulation consists of electrodes that act as the anode and cathode, where oxidation and reduction takes place. Many physicochemical processes such as oxidation, reduction, coagulation and adsorption govern the electrocoagulation.^61,62  Similarly to other treatment techniques, the electrocoagulation of heavy metals offers a cost-effective and easy-handling technique on an industrial scale.⁶³  This technique has been used for the treatment of dyes, heavy metals, nitrates, fluorides and phenolic compounds from wastewater.^64–74  Recently, various workers have investigated electrocoagulation for the removal of heavy metals from wastewater.^75–77 

Removal of Cr³⁺ from aqueous solution by electrocoagulation using iron electrodes is a feasible process. Golder et al.⁷⁸  investigated the removal of Cr³⁺ from water by electrocoagulation methods. It was found that the coagulation and adsorption play very important roles in the removal of Cr³⁺ during electrocoagulation. The removal of Cr³⁺ from aqueous solution was highest at a higher current density. A multiple electrode was used in the electrocoagulation system for the removal of Cr³⁺ from aqueous solution with both bipolar and monopolar configurations.⁷⁹  This technique can be used for the treatment of pollutants down to the ppb level, but the high cost of resin makes the process costly for industrial scale applications.^80,81  Gao et al.⁸²  used a combined electrocoagulation and electroflotation system for the removal of Cr⁶⁺ from aqueous solutions. The performance of an electrocoagulation system with aluminium electrodes for removing heavy metal ions on a laboratory scale was studied systematically by Heidmann and Calmano.⁸³ 

Removal of heavy metal ions from wastewater by electrocoagulation with iron and aluminium electrodes with monopolar configurations was investigated by Akbal and Camcı.⁸⁴  They explored the influence of electrode material, current density, wastewater pH and conductivity on removal performance. The results indicated that an electrocoagulation system with an Fe–Al electrode was useful and 100% of the Cu, Cr and Ni were observed within 20 min with a current density of 10 mA cm⁻² and a pH of 3.0. The performance of electrocoagulation, with an aluminium sacrificial anode, in the treatment of wastewater containing metal ions has been investigated by Adhoum et al.⁸⁵  Cu, Zn and Cr were removed successfully by using this technique. The method was found to be highly efficient and relatively fast compared with conventional existing techniques. Direct electrochemical reduction of Cr⁶⁺ can be carried out at the cathode.⁸⁶  The hydroxyl ions produced at the cathode induce the coprecipitation of Cu, Zn and Cr.^87–89 

Clays have been widely used for the removal of heavy metals from aqueous solutions due to their outstanding properties.^90,91  Heavy metals can be removed by ion exchange or a complexation reaction at the surface of clays. During the past few years, surface modifications of natural clays with reagents containing metal binding groups have been explored.^91–93  Several modification techniques such as intercalation of organic molecules into the interlayer space and grafting of organic moieties have been applied.^94,95  Organic-modified clays based on montmorillonite were prepared by embedding ammonium organic derivatives with different chelating functionalities for heavy metal removal.⁹⁶  Montmorillonite intercalated with poly-hydroxyl Fe(iii) complexes was used for the sorption of Cd(ii).⁹⁷  Sodium dodecyl sulfate modified iron pillared montmorillonite has been successfully applied for the removal of aqueous Cu(ii) and Co(ii).⁹⁸  Smectite intercalated with a non-ionic surfactant shows a good performance for the removal of heavy metals.⁹⁹  Through the grafting of inorganic and organic components, natural clay can be functionalized to obtain a better sorption capacity.^100,101  Heavy metals have been removed through the grafting of amino or mercapto by reaction with the silanol groups onto the surface of clays.^102,103  Synthesis of layered magnesium organosilicates for the removal of heavy metals has been carried out with different organosiloxanes.¹⁰⁴  Sepiolite can be grafted with organic moieties due to its high content of silanol groups. Liang et al.⁹⁰  have functionalized the sepiolite by nanotexturization in aqueous sepiolite gel and surface grafting in toluene with mercaptopropyltrimethoxysilane. The sorption of Pb(ii) and Cd(ii) were studied and it was found that the surface modification can obviously increase the sorption capacities for Pb(ii) and Cd(ii).

LDH materials appear in nature and can be easily synthesized in the laboratory. In nature they are formed from the weathering of basalts or precipitation in saline solution. All natural LDH minerals have a structure similar to hydrotalcite, which has the formula [Mg₆Al₂(OH)₁₆]CO₃·4H₂O. LDHs have been prepared using many combinations of divalent to trivalent cations including Mg, Al, Zn, Ni, Cr, Fe, Cu, Ga and Ca.^105–118  A number of synthetic techniques has been successfully employed in the preparation of LDHs. There are a number of methods used to synthesize LDHs including coprecipitation methods, hydrothermal synthesis, urea hydrolysis methods, sol–gel methods, ion-exchange methods and rehydration methods.

LDHs have been investigated intensively for anion-exchange properties due to recent interest in developing the use of anionic clays for environmental remediation. The main characteristic that has been studied is to clearly characterize the adsorption properties of the materials under vigorous solid–liquid interface conditions. The effect of sorbent composition, surface and bulk adsorption and concentration of adsorption site have been assessed. The adsorption capacity is significantly affected by the nature of the counter anion of the LDHs layer. LDHs can be used as precipitating agents of heavy metal cations for the decontamination of wastewater. Mn²⁺, Fe²⁺ and Cu²⁺ cations have been removed by synthetic hydrotalcite-like compounds, with zaccagnaite and hydrotalcite thin films being used for the remediation of aqueous wastes containing hazardous metal ions.¹¹⁹ 

During the last few years numerous new processes have been tested successfully, many of which have gone into operation and a great number of papers have been published on biosorption. In this section we will discuss “Biomass based biosorbents and biosorption of heavy metals”. Biosorption has been defined as the “property of certain bio-molecules to sequester metal ions or other molecules from aqueous solutions”.^120,121  It differs from bioaccumulation, where active metabolic transport takes place, as biosorption involves a passive process in which interaction between sorbent and sorbate occurs. Biosorption of heavy metals has become a popular and active field of research in environmental science.^122–126 

Rao et al.¹²⁷  have studied the removal of Cr(vi) and Ni(ii) from aqueous solution using bagasse based biosorbents. The bagasse was chemically treated with 0.1 N NaOH followed by 0.1 N CH₃COOH. The materials adsorption capacity in order of selectivity for Cr(vi) and Ni(ii) was powdered activated carbon>bagasse>fly ash and powdered activated carbon>fly ash>bagasse, respectively. Values for Langmuir and Freundlich isotherm constants for sorption of Cr(vi) ions onto powdered activated carbon, bagasse and fly ash were 0.03, 0.0005 and 0.001, and 0.12, 0.03 and 0.01, respectively. A lower pH of 6.0 favors the uptake of Cr(vi) and pH 8.0 was suitable for Ni(ii) ions removal. However, an increase in pH values of the solution reduces the Cr(vi) adsorption because of the abundance of OH⁻ ions, causing hindrance to the diffusion of dichromate.^128,129  However, the adsorption capacity was very low and their application for industrial effluent treatment cannot be justified.

Recently, pectin-rich fruit wastes have been investigated as biosorbents for heavy metal ion removal.¹³⁰  It has been observed that biosorption of cadmium by pectin-rich fruit materials and citrus peels were found to be most suitable. Equilibrium kinetics were achieved within 30–90 min, depending upon particle size. A pseudo-second order model was found to be more suitable than a first-order model to describe the kinetics. Isotherm studies show that the data were well fitted to a Langmuir model. It has also been observed that the metal uptake decreased with decreasing pH, indicating competition of protons for binding to acidic sites. Gurgel and Gil¹³¹  have described the preparation of two new chelating materials, MMSCB 3 and 5, derived from succinylated twice-mercerized sugarcane bagasse (MMSCB 1). MMSCB 3 and 5 were synthesized from MMSCB 1 using two different methods. In the first method, MMSCB 1 was activated with 1,3-diisopropylcarbodiimide and in the second with acetic anhydride, and later both were reacted with triethylenetetramine in order to obtain MMSCB 3 and 5. The capacity of MMSCB 3 and 5 to adsorb Cu²⁺, Cd²⁺ and Pb²⁺ from aqueous single metal ion solutions was evaluated at different contact times, pH and initial metal ion concentrations. Adsorption isotherms were well fitted by a Langmuir model. Maximum adsorption capacities of MMSCB 3 and 5 for Cu²⁺, Cd²⁺ and Pb²⁺ were found to be 59.5 and 69.4, 86.2 and 106.4, 158.7 and 222.2 mg g⁻¹, respectively.

A few biosorbents have been reported for the adsorption of heavy metals not only in the form of metallic ions but also organometallic compounds. Saglam et al.¹³²  have prepared the biosorbents from the biomass of Phanerochaete chrysosporium, which adsorbed inorganic mercury and alkylmercury species with an affinity of CH₃HgCl > C₂H₅HgCl > Hg²⁺, with maximum sorption capacities of 79, 67 and 61 mg g⁻¹, respectively.

The efficiency of Parthenium hysterophorous weed for the removal and recovery of Cd(ii) ions from wastewater has been studied by Ajmal et al.¹³³  These workers reported that the kinetics data for the adsorption process obeyed the second-order rate equation. The adsorption process was found to be endothermic and spontaneous in nature. The maximum adsorption capacity of Cd(ii) ions was 99.7% in the pH range 3–4. The desorption studies confirm 82% recovery of Cd(ii) when 0.1 M HCl solution was used as the effluent. Coconut copra meal, a waste product of the coconut industry, was used for the removal of cadmium from water.¹³⁴  The biosorption process was a spontaneous and exothermic process in nature.

Rao et al.¹³⁵  tested the biosorption potential of fennel biomass (Foeniculum vulgari) for the removal of Cd(ii) from water. It was found that the biosorption of Cd(ii) was a chemically controlled process. Removal of Cd(ii) was concentration dependent and increased with an increase in metal ion concentration, which showed that the multilayer adsorption takes place at the surface of the biosorbent and it was best described by a Freundlich isotherm model and pseudo-second order rate kinetics. El-Said et al.¹³⁶  utilized rice husk ash for the removal of Zn(ii) and Se(iv) from water. A higher removal capacity of Zn(ii) was found than for Se(iv). The removal capacity increases with an increase in biosorbent dose from 1 to 10 g L⁻¹.

Recently, Schiewer and Iqbal¹³⁷  investigated the role of pectin for the removal of cadmium from water. The carboxyl group plays an important role in the surface charge and was responsible for the binding of cadmium onto the biosorbent surface. Typically, metal binding experiments were carried out at an optimized pH of 5. A Langmuir isotherm model provided the best fit. Metal binding kinetics were better described by the first-order model than by the second-order model.

Removal of mercury from water was carried out using Carica papaya as a biosorbent.¹³⁸  Sulfuric acid treated almond husk based activated carbon was prepared and used for the sorption of Ni(ii) ions from water.¹³⁹  The adsorption capacity was very high and 97.8% Ni(ii) ions were removed by an adsorbent dose of 5 g L⁻¹.

Magnetic nanomaterials are one of the recently highlighted branches of materials science and technology that have been utilized in the removal of pollutants from aqueous solutions. Owing to their magnetic properties, high chemical stability, low toxicity, ease of synthesis and excellent recycling capability, magnetic nanoparticles have been studied to remove toxic metal ions from water.

Magnetic nanoparticles are of great interest for researchers from a wide range of disciplines, including magnetic fluids, catalysis, biomedicine, drug delivery, magnetic resonance imaging, data storage and environmental remediation.^140,141  Although several suitable methods have been developed for the synthesis of magnetic nanoparticles for a variety of different compositions, successful application of such magnetic nanoparticles in the areas listed here is particularly dependent on the stability of the particles under a range of different conditions. In the majority of the envisaged applications, the particles perform best when the size of the nanoparticles is below a critical value, which is dependent on the source material but is typically around 10–20 nm.¹⁴²  The design and fabrication of nanoparticle-based adsorbents has generated great interest in a variety of scientific communities ranging from chemical, biological and environmental science to engineering. Magnetic nanoparticle-based adsorbents can be used in the separation and purification of biologically as well as environmentally relevant target species with high precision and accuracy.^143,144 

The presence of iron and manganese gives an astringent and metallic taste to drinking water, which causes problems in cooking and in the production of beverages.¹⁴⁵  A simple method of iron and manganese removal consists of oxidation and ion-exchange resins. The oxidation of iron is dependent on the solution's pH, and organic matter and carbonate concentration. Oxidation of iron and manganese can be achieved by introducing an oxidizing agent and it may be done through the application of methods that include the addition of oxidants such as chlorine and potassium permanganate. Activated carbons have also been applied for the removal of iron and manganese from aqueous solutions.¹⁴⁶  Klueh and Robinson¹⁴⁷  investigated the sequestration of iron by polyphosphate addition while providing the necessary disinfection through chlorine addition. They observed that the presence of calcium in the groundwater inhibited the removal of iron. The addition of polyphosphate to the groundwater first and the simultaneous addition of polyphosphate and chlorine were both fairly successful at removing the iron.

Ion-exchange resins provide many advantages and are one of the most widely techniques used for treatment of wastewater effluents.¹⁴⁸  Lee and Nicol¹⁴⁹  have used the Diphonix resin to remove ferric iron from a cobalt sulfate solution with various pH ranges. A lower pH and higher dose of resin gives a higher removal of iron from solution. Elution of iron was observed with an increase of Ti(iii) in the sulfuric acid eluent. These workers found that the iron elution enhancement with Ti(iii) was due to the combined effects of a reduction of Fe(iii) and competitive adsorption of Ti(iii) and Ti(iv) ions. Lasanta et al.¹⁵⁰  studied the equilibrium diagrams for ionic exchange, which occurs between Fe³⁺ in different solutions by a chelating ion exchange resin. A mathematical model was used to predict the equilibrium, which gave a good fit for the experimental data in various solutions. It had been observed that solvent type influences the adsorption capacity. Khalil et al.¹⁵¹  studied the removal of ferric ions by using crosslinked chitosan resins immobilized with diethylenetriamine and tetraethylenepentamine. It had been found that the tetraethylenepentamine containing chitosan resin showed a higher uptake capacity towards Fe(iii) compared with diethylenetriamine containing chitosan resin. Kinetic data showed that the adsorption process followed the pseudo-second order kinetics. Thermodynamic studies indicated that the adsorption process was exothermic and spontaneous in nature.

Omri and Benzina¹⁵²  achieved the removal of Mn(ii) ions from aqueous solutions by adsorption on activated carbons derived from Ziziphus spina-christi seeds. The effects of process parameters such as solution pH, initial metal ion concentration and temperature on the adsorption performance of activated carbons for Mn(ii) ions removal were tested to optimize the system. Maximum adsorption was obtained at pH 4. Freundlich isotherms followed the adsorption system and the higher adsorption capacity for a Langmuir isotherm was 172 mg g⁻¹. Adsorption of iron and manganese ions from aqueous solution by low-cost adsorbents of palm fruit bunch and maize cobs was carried out.¹⁵³  Adsorption of iron ions on palm fruit bunch and maize cobs was in the range of 80–57%, for initial concentrations ranging between 1 and 10 ppm.

Recently, Mengistie et al.¹⁵⁴  performed the adsorption of Mn(ii) by using activated carbons of Militia ferruginea leaves from aqueous solutions in the batch mode. Adsorption equilibrium was achieved within 2 h. It had been found that pH 4 was appropriate for Mn(ii) removal and 95.8% metal ions were removed. The adsorption isotherms were best fitted to a Freundlich model, which showed multilayer adsorption at the surface of the activated carbons. The adsorption kinetics were best fitted to a first-order kinetic model. Thermodynamic analysis showed that the adsorption process was endothermic and spontaneous in nature. Emmanuel and Rao¹⁵⁵  studied the adsorption of Mn(ii) by activated carbons of Pithacelobium dulce from aqueous solutions and found a good sorption capacity for metal ions. The sorption equilibrium was achieved within 50 min. The equilibrium isotherm was best fitted to a Langmuir isotherm model, which indicates the adsorption of Mn(ii) onto activated carbons was as a monolayer.

The effect of various organic acids, such as acetic, formic, citric, ascorbic, succinic, tartaric and oxalic acids, on the removal of iron has been studied by Ambikadevi and Lalithambika.¹⁵⁶  It was found that the oxalic acid gives the best results, both at room temperature as well as at high temperatures, because of its high acid strength, good complexing capacity and reducing power. The effects of several parameters such as time, temperature and reagent concentration were studied for the optimization process. The removal of iron was found to be ∼80% by the authors.

Ganesan et al.¹⁵⁷  used an electrocoagulation process for removal of Mn(ii) from aqueous solutions using magnesium as the anode and galvanized iron as the cathode. Several removal parameters such as solution pH, current density, electrode configuration, inter-electrode distance, effects of coexisting ions and temperature were studied. The results obtained suggested that the highest removal of 97.2% at a pH of 7.0 was for a current density 0.05 A dm⁻² with an energy consumption of 1.151 kWh m⁻³. Thermodynamic parameters indicated that the Mn(ii) removal was feasible, spontaneous and endothermic in nature. A Langmuir adsorption isotherm well fitted to the adsorption system. The kinetic model was best described by a pseudo-second order rate at the various current densities. Taffarel and Rubio¹⁵⁸  applied Chilean zeolite as an adsorbent for removal of Mn(ii) ions from aqueous solutions. The solution pH significantly influenced the adsorption of Mn(ii) removal and the best results were been found at pH 6–6.8. The removal kinetics was best fitted with a pseudo-second order model. The equilibrium isotherm data were best fitted to a Langmuir isotherm model. It was found that the Chilean zeolite treated with NaCl, NaOH, Na₂CO₃ and NH₄Cl increased its uptake ability in comparison with natural Chilean zeolite.

The presence of heavy metals and their toxicity to the environment and to human beings is posing a serious challenge to environmental engineers with respect to the treatment of wastewater effluents prior to discharge into the nearby water bodies. Several removal techniques have been developed and applied for the treatment of these wastes to remove the toxic metal ions. Technologies such as microbe-assisted phytoremediation, ion exchange, membrane filtration, photocatalytic oxidation and reduction and adsorption have their own advantages and disadvantages over metal ion sequestrations from environmental matrices. During recent years the developments in adsorption of heavy metals from aqueous solutions have gained tremendous popularity among the scientific community as methods to treat industrial wastewater. Several adsorbents such as clays, LDHs, zeolites, carbon nanotubes and their composites, activated carbons, biomass derived biosorbents, inorganic nanomaterials, inorganic organic hybrid nanocomposites and magnetic nanomaterials have been synthesized and investigated for their ability to sequester metal ions from water.

Functionalized magnetic nanoparticles are very promising for applications in catalysis, biolabelling and bioseparation. In liquid-phase extraction of heavy metals and dyes in particular, such small and magnetically separable particles may be useful as they combine the advantages of high dispersion, high reactivity, high stability under acidic conditions and easy separation. In this chapter we focused mainly on recent developments in the synthesis of active adsorbents and nanoparticles. Further, functionalization and application of magnetic nanoparticles and their nanosorbents for the separation and purification of hazardous metal ions from the environment are discussed in detail in a separate chapter in this book.

R.K. Gautam thanks the University Grants Commission for the award of a Junior Research Fellowship (JRF). Suresh Mahiya is grateful to the President, JECRC University, for the award of Scholarship for his PhD. The authors equally acknowledge the support and provision of the necessary facilities by the University of Allahabad, Allahabad, India and JECRC University, Jaipur, India. The support and encouragement of Prof. V.S. Tripathi from the Department of Chemistry, University of Allahabad, is also appreciated. We also thank the anonymous editors and reviewers for giving their kind criticisms and comments, which fuelled the zeal for the manuscript.