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Page 1: Heavy Metals In Water || CHAPTER 1. Contamination of Heavy Metals in Aquatic Media: Transport, Toxicity and Technologies for Remediation

CHAPTER 1

Contamination of HeavyMetals in Aquatic Media:Transport, Toxicity andTechnologies for Remediation

RAVINDRA K. GAUTAM,a SANJAY K. SHARMA,*b

SURESH MAHIYAb AND MAHESH C. CHATTOPADHYAYA*a

a Environmental Chemistry Research Laboratory, Department ofChemistry, University of Allahabad, Allahabad, 211 002, India; b GreenChemistry & Sustainability Research Group, Department of Chemistry,JECRC University, Jaipur, 303905, India*Email: [email protected]; [email protected]

1.1 IntroductionThe term ‘‘heavy metal’’ refers to any metal and metalloid element that has arelatively high density ranging from 3.5 to 7 g cm�3 and is toxic or poisonousat low concentrations, and includes mercury (Hg), cadmium (Cd), arsenic(As), chromium (Cr), thallium (Tl), zinc (Zn), nickel (Ni), copper (Cu) andlead (Pb). Although ‘‘heavy metals’’ is a general term defined in the litera-ture, it is widely documented and frequently applied to the widespreadpollutants of soils and water bodies.1 These metals are found widely in theearth’s crust and are non-biodegradable in nature. They enter into thehuman body via air, water and food. A small number have an essential rolein the metabolism of humans and animals in very trace amounts but theirhigher concentration may cause toxicity and health hazards. The hazardous

Heavy Metals in Water: Presence, Removal and SafetyEdited by Sanjay K. Sharmar The Royal Society of Chemistry 2015Published by the Royal Society of Chemistry, www.rsc.org

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nature of heavy metals has been recognized because of their bioaccumula-tive nature in biotic systems. They can enter into the environment throughmining activities, industrial discharge and from household applications,into nearby bodies of water.

1.2 Sources of Heavy MetalsHeavy metals differ widely in their chemical properties, and are used ex-tensively in electronics, machines and the artifacts of everyday life, as well asin high-tech applications. As a result they are able to enter into the aquaticand food chains of humans and animals from a variety of anthropogenicsources as well as from the natural geochemical weathering of soil androcks. The main sources of contamination include mining wastes, landfillleaches, municipal wastewater, urban runoff and industrial wastewaters,particularly from the electroplating, electronic and metal-finishing indus-tries. With increasing generation of metals from technologies activities, theproblem of waste disposal has become one of paramount importance. Manyaquatic environments face metal concentrations that exceed water qualitycriteria designed to protect the environment, animals and humans. Theproblems are exacerbated because metals have a tendency to be transportedwith sediments, are persistent in the environment and can bioaccumulate inthe food chain. Some of the oldest cases of environmental pollution in theworld are due to heavy metal use, for example, Cu, Hg and Pb mining,smelting and utilization by ancient civilizations, such as the Romans and thePhoenicians.

The heavy metals are among the most common pollutants found in was-tewater. These metals pose a toxicity threat to human beings and animalseven at low concentration. Lead is extremely toxic and shows toxicity to thenervous system, kidneys and reproductive system. Exposure to lead causesirreversible brain damage and encephalopathic symptoms.2 Cadmium isused widely in electroplating industries, solders, batteries, television sets,ceramics, photography, insecticides, electronics, metal-finishing industriesand metallurgical activities. It can be introduced into the environment bymetal-ore refining, cadmium containing pigments, alloys and electroniccompounds, cadmium containing phosphate fertilizers, detergents andrefined petroleum products. Rechargeable batteries with nickel–cadmiumcompounds are also sources of cadmium.3–5 Cadmium exposure causesrenal dysfunction, bone degeneration, liver and blood damage. It hasbeen reported that there is sufficient evidence for the carcinogenicity ofcadmium.3

Copper, as an essential trace element, is required by biological systems forthe activation of some enzymes during photosynthesis but at higher con-centrations it shows harmful effects on the human body. High-level exposureof copper dust causes nose, eyes and mouth irritation and may cause nauseaand diarrhea. Continuous exposure may lead to kidney damage and evendeath. Copper is also toxic to a variety of aquatic organisms even at very low

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concentrations. Mining, metallurgy and industrial applications are themajor sources of copper exposure in the environment.

Zinc is also an essential element in our diet. Too much zinc, however, canalso be damaging to health. Zinc toxicity in large amounts causes nauseaand vomiting in children. A higher concentration of zinc may cause anemiaand cholesterol problems in human beings. Mining and metallurgical pro-cessing of zinc ores and its industrial application are the major sources ofzinc 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 areused in several industrial applications such as in electroplating, automobileand aircraft parts, batteries, coins, spark plugs, cosmetics and stainlesssteel, and is used extensively in the production of nickel–cadmium batterieson an industrial scale. It enters into the water bodies naturally by weatheringof rocks and soils and through the leaching of the minerals.4 The watersoluble salts of nickel are the major problems of contamination in aquaticsystems.5 Paint formulation and enameling industries discharges nickelcontaining effluents to the nearby bodies of water.6 Nickel is also found incigarettes, as a volatile compound commonly known as nickel carbonyl.7

Arsenic is found naturally in the deposits of earth’s crust worldwide. Theword arsenic is taken from Zarnikh in Persian literature, which means yelloworpiment.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. Incertain areas the concentration of arsenic may be higher than its normaldose and creates severe health hazards to human beings and animals. Itenters the environment through the natural weathering of rocks and an-thropogenic activities, mining and smelting processes, pesticide use andcoal combustion. The toxicity of arsenic as a result of the contamination ofgroundwater bodies and surface waters is of great concern. Arsenic exists asarsenate, As(V), and arsenite, As(III), in most of the groundwater.9–12 Ad-sorption and solution pH commonly controls the mobility of arsenic in theaqueous environment.13–17 Metal oxides of Fe, Al and Mn play a role in theadsorption of arsenic in aquatic bodies.18–20 Arsenic has been found nat-urally at high concentration in groundwater in countries such as India,Bangladesh, Taiwan, Brazil and Chile. Its high concentration in drinkingwater causes toxic effects on humans and animals.

The toxicity of mercury has been recognized worldwide, such as inMinamata Bay of Japan. Mentally disturbed and physically deformed babieswere born to mothers who were exposed to toxic mercury due to con-sumption of contaminated fish. The natural sources of mercury are volcaniceruption, weathering of rocks and soils, whereas anthropogenic mercurycomes from the extensive use of the metal in industrial applications, itsmining 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 hasdischarged huge amounts of wastewater containing toxic chromium speciesinto water bodies. Chromium enters into the environment by natural inputs

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and anthropogenic sources. Volcanic eruptions, geological weathering ofrocks, soils and sediments are the natural sources of chromium, whereasanthropogenic contributions of chromium come from the burning of fossilfuels, production of chromates, plastic manufacturing, electroplating ofmetals and extensive use in the leather and tannery industries.21 Hexavalentchromium is more toxic than trivalent chromium.

1.3 Environmental and Health RisksCadmium is the most toxic element, even at its low concentration in the foodchain and has been found to cause of itai-itai disease in Japan. Unlike otherheavy metals, cadmium is not essential for biological systems. Hence it hasno benefit to the ecosystem and only harmful effects have been reported. It isused in the manufacturing of nickel–cadmium batteries, plastics and pig-ments. Phosphate fertilizers and waste dumping are both routes for cad-mium transference into the environment. Concern regarding the role andtoxicity of cadmium in the environment is on the increase, because it can behighly toxic to human beings and animals at very low concentrations. Cad-mium toxicity causes renal dysfunction and lung cancer, and also osteo-malacia in the human population and animals, in addition to increasingblood pressure. Smoking of cigarettes is one of the sources of cadmiumpoisoning in humans.

Chromium is commonly used in the leather and tanning industries, paperand pulp and rubber manufacturing applications. High levels of exposurecause liver and kidney damage, skin ulceration and also affects the centralnervous system. With plant species it reduces the rate of photosynthesis. It isalso associated with the toxic effects on hematological problems and im-mune response in freshwater fish. Chromium(VI) causes greater toxicity thanchromium(III) in animal and human health.

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

Human exposure to lead causes severe toxicity. Higher doses may damagethe fetus and be toxic to the central nervous system. Newborn babies aremore sensitive than the adults. Lead toxicity may harm hemoglobin syn-thesis, the kidneys and reproductive systems. Exposure to higher doses oflead may disrupt the function of the central nervous system and gastro-intestinal tract. Airborne lead may cause the poisoning of agricultural foodby the deposition on fruits, soils and water.7

Mercury is a very toxic element in its organic form and has been the causeof Minamata disease in Japan. It shows toxicity to the physiology of animalsand human beings. Mercury toxicity has been found to be associated withphysiological stress, abortion and tremors. Methyl mercury is highly toxicand causes toxic effects on the central nervous system in the human

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population. Mercury can result from volcanic eruptions and degassing. Theexposure to mercury causes toxicity to the brain, blindness, mental retar-dation 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 notdamage biological cells, but exposure to a high dose for a longer time maydamage cells, decrease body weight and damage the liver and heart. Nickelpoisoning may cause reduction in cell growth, cancer and nervous systemdamage.5–7

The undesirable presence of iron and manganese in drinking water maypose a toxicity threat to health. However, iron and manganese are requiredby the biological system as they play major roles in the hemoglobin synthesisand functioning of cells. The presence of these metals in water may causestaining of cotton clothes and give a rusty taste to drinking water. The majorconcerns focus on the dietary intake of iron because a higher dose may poseacute toxicity to newborn babies and young children. The gastrointestinaltract rapidly absorbs iron that may pose a toxicity risk to the cells andcytoplasm. The liver, kidneys and cardiovascular systems are the majortoxicity targets of iron. Neurological disturbances and muscle functiondamage are the result of toxic effects of manganese in human bodies.

Heavy metals are highly toxic to the fetus and newborn babies, wherehigher levels of exposure exist for human beings, mainly to industrialworkers. Metal ions exposure to newborn babies may damage brain memory,disrupt the function of red blood cells, the central nervous system, physio-logical and behavioral problems. Severe toxicity from these metals may causecancers. Exposure of plants to heavy metals may lead to physiological andmorphological changes and damage to cell function and reduce photo-synthesis rates. Mutagenic changes have also been observed in several plantspecies. Metal ion toxicities may lead to chlorosis, bleaching, nutrient de-ficiencies and increased oxidation stress in plants. Heavy metals obstruct thegrowth of microbes.22 Table 1.1 shows the standards for metal concentrationin drinking water and the health effects.

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

Severe toxic effects and poisoning by heavy metal ions worldwide andstrict discharge regulations for wastewater effluents to aquatic bodiesrequires better treatment techniques. Environmental scientists have de-veloped several procedures such as coprecipitation, membrane filtration,

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Table 1.1 The standard metal concentration in drinking water and the healtheffects.

Metal Effects Drinking water standards

Lead � Toxic to humans, aquaticfauna and livestock

� High doses causemetabolic poison

� Tiredness, irritability anemiaand behavioral changes ofchildren

� Hypertension andbrain damage

� Phytotoxic

� By the EnvironmentalProtection Agencymaximum concentration:0.1 mg L�1

� By European Community:0.5 mg L�1

� Regulation of water quality(India) 0.1 mg L�1

Nickel � High conc. can causeDNA damage

� Eczema of hands� High phytotoxicity� Damaging fauna

� By the EnvironmentalProtection Agencymaximum concentration:0.1 mg L�1

� By European Community:0.1 mg L�1

� Regulation of water quality(India) 0.1 mg L�1

Chromium � Necrosis nephritis and deathin man (10 mg kg�1 of bodyweight as hexavalentchromium)

� Irritation of gastrointestinalmucosa

� By the EnvironmentalProtection Agencymaximum concentration:(hexavalent and trivalent)total 0.1 mg L�1

� By European Community:0.5 mg L�1

� Regulation of water quality(India) 0.1 mg L�1

Copper � Causes damage in a varietyof aquatic fauna

� Phytotoxic� Mucosal irritation and

corrosion� Central nervous system

irritation followed bydepression

� By the EnvironmentalProtection Agencymaximum concentration:1.0 mg L�1

� By European Community:3 mg L�1

� Regulation of water quality(India) 0.01 mg L�1

Zinc � Phytotoxic� Anemia� Lack of muscular

coordination� Abdominal pain etc.

� By the EnvironmentalProtection Agencymaximum concentration:5 mg L�1

� By European Community:5 mg L�1

� Regulation of water quality(India) 0.1 mg L�1

Cadmium � Cause serious damage tokidneys and bones in humans

� Bronchitis, emphysema,anemia

� Acute effects in children

� By the EnvironmentalProtection Agencymaximum concentration:0.005 mg L�1

� By European Community:0.2 mg L�1

� Regulation of water quality(India) 0.001 mg L�1

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ion-exchange resins, photocatalytic reduction and adsorption for treatmentof wastewater effluents containing heavy metals.

1.4 Remediation TechnologiesBioaccumulation of heavy metals in food chains and their toxicity to bio-logical systems due to increased concentration over time have led to tre-mendous pressure for their separation and purification. Heavy metals canenter into water bodies through agricultural runoff, industrial effluents,household uses and from commercial applications. We can remove heavymetals from drinking water very easily with reliable technology. Severaltechnologies available in the market remove a huge range of metals com-monly found in drinking water and wastewater effluents. There are variousremediation technologies that have been used for the removal of heavy metalsfrom water/wastewater. These remediation technologies are summarized as:

� Precipitation and coagulation� Ion exchange� Membrane filtration� Bioremediation� Heterogeneous photocatalysts� Adsorption

1.4.1 Membrane Filtration

Membranes are complex structures that contain active elements on thenanometer scale. Modern day reverse osmosis membranes are typically

Table 1.1 (Continued)

Metal Effects Drinking water standards

Mercury � Poisonous� Causes mutagenic effects� Disturbs the cholesterol

� By the EnvironmentalProtection Agencymaximum concentration:0.002 mg L�1

� By European Community:0.001 mg L�1

� Regulation of water quality(India) 0.004 mg L�1

Arsenic � Causes toxicological andcarcinogenic effects

� Causes melanosis, keratosisand hyperpigmentationin humans

� Genotoxicity throughgeneration of reactive oxygenspecies and lipid peroxidation

� Immunotoxic� Modulation of co-receptor

expression

� World Health Organizationguideline of 10 mg L�1

� By European Community:0.01 mg L�1

� Regulation of water quality(India): 0.05 mg L�1

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homogeneous polymer thin films supported by a porous support structure.Partitioning water and dissolved salts between the membrane and the bulksolution, and transport of water and salts across the membrane, depend onthe chemical properties of the membrane as well as the physical structureson nano- to microscales. The nanometer length scale is defined as betweenthe scale of macroscopic particles suspended in water and dissolved atomicand molecular species. From a filtration perspective, this intermediate rangecontains, for example, colloidal solids, large organic and biological mol-ecules, polymers and viruses. It also corresponds to the dimensions at whichthat we recognize distinct modes of material transport across a membrane.For a larger dimension of porous membranes, transport is described interms of convective flow through pores. On the other hand, transport in adense reverse osmosis membrane is typically described in terms of diffusiveflow through a homogeneous material.

1.4.2 Phytoremediation

Bioremediation is the technological process whereby biological systems,plants and animals, including microorganisms, are harnessed to effect thecleanup of pollutants from environmental matrices.23 During the past fewyears, microbe-assisted bioremediations have been widely applied for thetreatment of wastewater contaminated with heavy metals and metalloids.Here we will address the global problem of heavy metal pollution originatingfrom increased industrialization and urbanization and its amelioration byusing plants from various environmental conditions. Conventional tech-nologies are not cost effective and may produce adverse impacts on aquaticecosystems. Microbe-assisted bioremediation and phytoremediation ofheavy metals are cost-effective technologies and metal ion accumulatingplants have been successfully used for the treatment of wastewater.24

Aquatic plants, especially ‘‘wetland ecosystems’’, have unique properties tosequester heavy metals and metalloids.

Wetland ecosystems are much superior in comparison with other con-ventional methods, for example because of the low cost, frequent growth ofmicroorganisms, easy handling and low maintenance cost. The rhizospheresin wetlands provide an enhanced nutrients supply to the microbial eco-systems of plants, which actively transform and sequester heavy metalsin their biological functions. Constructed wetlands have been actively usedfor the treatment of heavy metals from agricultural runoff, mine drainageand municipal wastes. Many aquatic plants such as Phragmites, Lemna,Eichchornia, Azolla and Typha have been used for the treatment of wastewatercontaining heavy metals.

Phytoremediation is a low-cost, low-tech and emerging cleanup technol-ogy for contaminated soils, groundwater and wastewater.25 Plants are verysensitive to metals but in phytoremediation wild and genetically modifiedplants, including grasses, herbs, forbs and woody species, are mainly used.The plants take up heavy metals and metalloids through the process of

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phytostabilization, phytoextraction, phytofiltration or rhizoremediation.However, in contrast to organic compounds the heavy metals and metalloidscannot be metabolized but accumulate in the plant biomass.26 The biomassgenerated 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 thatmetals are toxic to many plants, they have developed some internal mech-anisms that allow the uptake, tolerance and accumulation of high concen-trations of metals that would be toxic to other organisms. Many researchershave reported that aquatic macrophytes viz. Typha, Phragmites, Eichhornia,Azolla and Lemna are potential wetland plants for removal of heavy metaland metalloids due to their morphological change.24,28 Being a cost-effectiveand easily applicable technique, phytoremediation can be implemented fortheir enhancement to metal accumulations and translocations. In general,two strategies of phytoextraction have been developed, which are: (1) normalphytoremediation of heavy metals from aquatic bodies through the plants intheir entire growth cycle29–31 and (2) chemically induced phytoextractiontechniques to cleanup contaminated water by using metal-tolerant plants toremove heavy metals and metalloids.32 The efficiency of phytoextraction canbe increased by using more biomass producing plant species and with theapplication of suitable chelates. Hyperaccumulators or hyperaccumulatingplants are capable of accumulating large amounts of heavy metals andmetalloids, including Ni, As, Zn, Cd and Pb, in their aboveground tissueswithout any toxic symptoms.33

Metals uptake in relation to the external concentration of the toxic heavymetals may differ due to the different genotypes of plants. Those plants thathave low uptake of metals at quite high metal concentrations are calledexcluders. These plants have some kind of barrier to avoid uptake of heavymetals, however, when metal concentrations are at a high level this barrierlosses its function, probably due to the toxic action of the metals. Someplants have certain detoxification mechanism within their tissue, whichallow the plant to accumulate high amounts of metals.34 Several reports areavailable in the literature on the hyperaccumulator plants: Pteris vittata L.and Thlaspi caerulescens were found to hyperaccumulate As, Minuartia vernafor 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 bio-mass production, limited growth rate and time consumption.35 In order tocompensate for the low metal accumulation, much research has been con-ducted using synthetic chelators or ligands such as ethylenediaminetet-raacetic acid (EDTA); S,S-ethylenediaminedisuccinic acid (S,S-EDDS);nitrilotriacetate (NTA) and naturally occurring low molecular weight organicacids to enhance the availability of heavy metals and increase phytoextr-action efficiency.36,37

Phytoextraction is a publically appealing ‘‘green’’ remediation technique.However, phytoextraction can be effectively applied only for soils and

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wetlands contaminated with specific potentially toxic metals and metalloids.Many researchers have reported that common crop plants with a high bio-mass can be triggered to accumulate large amounts of low bioavailabilitymetals when applied the phytochelates.38,39 In such chemically enhancedphytoextractions, chelating agents are used almost exclusively as the mo-bilizing agents.40 However, EDTA was the most efficient chelate to increasemetal uptake by plants of Pb, but the slow degradation of chelating com-pounds in the root zone limits its application on an industrial scale.41

Nevertheless, more biodegradable chelates, such as NTA, (S,S-EDDS) andother chelates are also recognized for metals removal. Application of thesechelating agents with plants for the uptake of metal ions is gaining morepopularity and has become an interesting field of research. Several studieshave been carried out using EDTA as a metal chelator for sequestration ofmetals.42 The full-scale application for treating wastewater on an industrialscale should be based on optimization of several parameters such as solu-bilization of metals, chelates stability, plant roots and the capacity of metaltransport through the shoots of plants.43

1.4.3 Heterogeneous Catalysts and Catalysis

In 1972 Fujishima and Honda discovered the photocatalytic splitting ofwater on titanium dioxide (TiO2) electrodes.44,45 Their discovery provided thefoundation stone for photocatalysis. Since this remarkable discovery muchresearch has been carried out on the efficiency of TiO2 as a photo-catalyst.46–48 During the past few years, the applications of TiO2 for en-vironmental cleanups have been performed by several laboratories for thetreatment of industrial effluents.49,50

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

1.4.4 Photocatalysts

Reduction of Cr(VI) using semiconductor heterogeneous photocatalystshas been carried out as an economical and simple method of wastewatertreatment.51,52 Surface-catalyzed Cr(VI) reduction is a very slow reaction andhas been described as a feasible process in the presence of oxide surfaces

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such as TiO2.53 Furthermore, organic donors have a chelation capacity forthe 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-UVradiation. At pH 2, the addition of oxalate facilitated Cr(VI) reduction. It hasbeen 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 photo-catalyst exhibited efficient photocatalytic activity for the photoreduction ofCr(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 beenperformed by He et al.60 The surface fluorination facilitated the adsorptionprocess by increasing the number of surface OH groups generated. The re-duction of Cr(VI) occurred because of the oxidative dissolution of H2O on{001} facets and the Cr(VI) reductions that occurred on {101} facets weresimultaneous reactions.

1.4.5 Electrocoagulation

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

Removal of Cr31 from aqueous solution by electrocoagulation using ironelectrodes is a feasible process. Golder et al.78 investigated the removal ofCr31 from water by electrocoagulation methods. It was found that the co-agulation and adsorption play very important roles in the removal of Cr31

during electrocoagulation. The removal of Cr31 from aqueous solution washighest at a higher current density. A multiple electrode was used in theelectrocoagulation system for the removal of Cr31 from aqueous solutionwith both bipolar and monopolar configurations.79 This technique can beused for the treatment of pollutants down to the ppb level, but the high costof resin makes the process costly for industrial scale applications.80,81 Gaoet al.82 used a combined electrocoagulation and electroflotation system forthe removal of Cr61 from aqueous solutions. The performance of an elec-trocoagulation system with aluminium electrodes for removing heavy metalions on a laboratory scale was studied systematically by Heidmann andCalmano.83

Removal of heavy metal ions from wastewater by electrocoagulation withiron and aluminium electrodes with monopolar configurations was in-vestigated by Akbal and Camcı.84 They explored the influence of electrode

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material, current density, wastewater pH and conductivity on removal per-formance. The results indicated that an electrocoagulation system with anFe–Al electrode was useful and 100% of the Cu, Cr and Ni were observedwithin 20 min with a current density of 10 mA cm�2 and a pH of 3.0. Theperformance of electrocoagulation, with an aluminium sacrificial anode, inthe treatment of wastewater containing metal ions has been investigated byAdhoum et al.85 Cu, Zn and Cr were removed successfully by using thistechnique. The method was found to be highly efficient and relatively fastcompared with conventional existing techniques. Direct electrochemicalreduction of Cr61 can be carried out at the cathode.86 The hydroxyl ionsproduced at the cathode induce the coprecipitation of Cu, Zn and Cr.87–89

1.4.6 Clays/Layered Double Hydroxides (LDHs)

Clays have been widely used for the removal of heavy metals from aqueoussolutions due to their outstanding properties.90,91 Heavy metals can be re-moved by ion exchange or a complexation reaction at the surface of clays.During the past few years, surface modifications of natural clays with re-agents containing metal binding groups have been explored.91–93 Severalmodification techniques such as intercalation of organic molecules into theinterlayer space and grafting of organic moieties have been applied.94,95

Organic-modified clays based on montmorillonite were prepared by em-bedding ammonium organic derivatives with different chelating function-alities for heavy metal removal.96 Montmorillonite intercalated with poly-hydroxyl Fe(III) complexes was used for the sorption of Cd(II).97 Sodiumdodecyl sulfate modified iron pillared montmorillonite has been success-fully applied for the removal of aqueous Cu(II) and Co(II).98 Smectite inter-calated with a non-ionic surfactant shows a good performance for theremoval of heavy metals.99 Through the grafting of inorganic and organiccomponents, natural clay can be functionalized to obtain a better sorptioncapacity.100,101 Heavy metals have been removed through the grafting ofamino or mercapto by reaction with the silanol groups onto the surface ofclays.102,103 Synthesis of layered magnesium organosilicates for the removalof heavy metals has been carried out with different organosiloxanes.104

Sepiolite can be grafted with organic moieties due to its high content ofsilanol groups. Liang et al.90 have functionalized the sepiolite by nano-texturization in aqueous sepiolite gel and surface grafting in toluene withmercaptopropyltrimethoxysilane. The sorption of Pb(II) and Cd(II) werestudied and it was found that the surface modification can obviously in-crease the sorption capacities for Pb(II) and Cd(II).

LDH materials appear in nature and can be easily synthesized in the la-boratory. In nature they are formed from the weathering of basalts or pre-cipitation in saline solution. All natural LDH minerals have a structuresimilar to hydrotalcite, which has the formula [Mg6Al2(OH)16]CO3 � 4H2O.LDHs have been prepared using many combinations of divalent to trivalentcations including Mg, Al, Zn, Ni, Cr, Fe, Cu, Ga and Ca.105–118 A number of

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synthetic techniques has been successfully employed in the preparation ofLDHs. There are a number of methods used to synthesize LDHs includingcoprecipitation methods, hydrothermal synthesis, urea hydrolysis methods,sol–gel methods, ion-exchange methods and rehydration methods.

LDHs have been investigated intensively for anion-exchange propertiesdue to recent interest in developing the use of anionic clays for environ-mental remediation. The main characteristic that has been studied is toclearly characterize the adsorption properties of the materials under vigor-ous solid–liquid interface conditions. The effect of sorbent composition,surface and bulk adsorption and concentration of adsorption site have beenassessed. The adsorption capacity is significantly affected by the nature ofthe counter anion of the LDHs layer. LDHs can be used as precipitatingagents of heavy metal cations for the decontamination of wastewater. Mn21,Fe21 and Cu21 cations have been removed by synthetic hydrotalcite-likecompounds, with zaccagnaite and hydrotalcite thin films being used for theremediation of aqueous wastes containing hazardous metal ions.119

1.4.7 Biomass and Biosorption of Metal Ions

During the last few years numerous new processes have been tested suc-cessfully, many of which have gone into operation and a great number ofpapers have been published on biosorption. In this section we will discuss‘‘Biomass based biosorbents and biosorption of heavy metals’’. Biosorptionhas been defined as the ‘‘property of certain bio-molecules to sequestermetal ions or other molecules from aqueous solutions’’.120,121 It differs frombioaccumulation, where active metabolic transport takes place, as biosorp-tion involves a passive process in which interaction between sorbent andsorbate occurs. Biosorption of heavy metals has become a popular and activefield of research in environmental science.122–126

Rao et al.127 have studied the removal of Cr(VI) and Ni(II) from aqueoussolution using bagasse based biosorbents. The bagasse was chemicallytreated with 0.1 N NaOH followed by 0.1 N CH3COOH. The materials ad-sorption capacity in order of selectivity for Cr(VI) and Ni(II) was powderedactivated carbon 4bagasse 4 fly ash and powdered activated carbon 4 flyash 4bagasse, respectively. Values for Langmuir and Freundlich isothermconstants 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 wassuitable for Ni(II) ions removal. However, an increase in pH values of thesolution reduces the Cr(VI) adsorption because of the abundance of OH�

ions, causing hindrance to the diffusion of dichromate.128,129 However, theadsorption capacity was very low and their application for industrial effluenttreatment cannot be justified.

Recently, pectin-rich fruit wastes have been investigated as biosorbentsfor heavy metal ion removal.130 It has been observed that biosorption ofcadmium by pectin-rich fruit materials and citrus peels were found to be

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most suitable. Equilibrium kinetics were achieved within 30–90 min, de-pending upon particle size. A pseudo-second order model was found to bemore suitable than a first-order model to describe the kinetics. Isothermstudies show that the data were well fitted to a Langmuir model. It has alsobeen observed that the metal uptake decreased with decreasing pH, indi-cating competition of protons for binding to acidic sites. Gurgel and Gil131

have described the preparation of two new chelating materials, MMSCB 3and 5, derived from succinylated twice-mercerized sugarcane bagasse(MMSCB 1). MMSCB 3 and 5 were synthesized from MMSCB 1 using twodifferent methods. In the first method, MMSCB 1 was activated with 1,3-diisopropylcarbodiimide and in the second with acetic anhydride, and laterboth were reacted with triethylenetetramine in order to obtain MMSCB 3 and5. The capacity of MMSCB 3 and 5 to adsorb Cu21, Cd21 and Pb21 fromaqueous single metal ion solutions was evaluated at different contact times,pH and initial metal ion concentrations. Adsorption isotherms were wellfitted by a Langmuir model. Maximum adsorption capacities of MMSCB 3and 5 for Cu21, Cd21 and Pb21 were found to be 59.5 and 69.4, 86.2 and106.4, 158.7 and 222.2 mg g�1, respectively.

A few biosorbents have been reported for the adsorption of heavy metalsnot only in the form of metallic ions but also organometallic compounds.Saglam et al.132 have prepared the biosorbents from the biomass ofPhanerochaete chrysosporium, which adsorbed inorganic mercury and alkyl-mercury species with an affinity of CH3HgCl 4 C2H5HgCl 4 Hg21, withmaximum sorption capacities of 79, 67 and 61 mg g�1, respectively.

The efficiency of Parthenium hysterophorous weed for the removal and re-covery of Cd(II) ions from wastewater has been studied by Ajmal et al.133

These workers reported that the kinetics data for the adsorption processobeyed the second-order rate equation. The adsorption process was foundto be endothermic and spontaneous in nature. The maximum adsorptioncapacity of Cd(II) ions was 99.7% in the pH range 3–4. The desorption studiesconfirm 82% recovery of Cd(II) when 0.1 M HCl solution was used as theeffluent. Coconut copra meal, a waste product of the coconut industry, wasused for the removal of cadmium from water.134 The biosorption processwas a spontaneous and exothermic process in nature.

Rao et al.135 tested the biosorption potential of fennel biomass (Foenicu-lum vulgari) for the removal of Cd(II) from water. It was found that the bio-sorption of Cd(II) was a chemically controlled process. Removal of Cd(II) wasconcentration dependent and increased with an increase in metal ion con-centration, which showed that the multilayer adsorption takes place at thesurface of the biosorbent and it was best described by a Freundlich isothermmodel and pseudo-second order rate kinetics. El-Said et al.136 utilized ricehusk ash for the removal of Zn(II) and Se(IV) from water. A higher removalcapacity of Zn(II) was found than for Se(IV). The removal capacity increaseswith an increase in biosorbent dose from 1 to 10 g L�1.

Recently, Schiewer and Iqbal137 investigated the role of pectin for theremoval of cadmium from water. The carboxyl group plays an important role

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in the surface charge and was responsible for the binding of cadmium ontothe biosorbent surface. Typically, metal binding experiments were carriedout at an optimized pH of 5. A Langmuir isotherm model provided the bestfit. Metal binding kinetics were better described by the first-order modelthan by the second-order model.

Removal of mercury from water was carried out using Carica papaya as abiosorbent.138 Sulfuric acid treated almond husk based activated carbon wasprepared and used for the sorption of Ni(II) ions from water.139 The ad-sorption capacity was very high and 97.8% Ni(II) ions were removed by anadsorbent dose of 5 g L�1.

1.4.8 Magnetic Nanoparticles as Nanosorbents

Magnetic nanomaterials are one of the recently highlighted branches ofmaterials science and technology that have been utilized in the removal ofpollutants from aqueous solutions. Owing to their magnetic properties, highchemical stability, low toxicity, ease of synthesis and excellent recyclingcapability, magnetic nanoparticles have been studied to remove toxic metalions from water.

Magnetic nanoparticles are of great interest for researchers from a widerange of disciplines, including magnetic fluids, catalysis, biomedicine,drug delivery, magnetic resonance imaging, data storage and environ-mental remediation.140,141 Although several suitable methods have beendeveloped for the synthesis of magnetic nanoparticles for a variety of dif-ferent compositions, successful application of such magnetic nano-particles in the areas listed here is particularly dependent on the stability ofthe particles under a range of different conditions. In the majority of theenvisaged applications, the particles perform best when the size of thenanoparticles is below a critical value, which is dependent on the sourcematerial but is typically around 10–20 nm.142 The design and fabrication ofnanoparticle-based adsorbents has generated great interest in a variety ofscientific communities ranging from chemical, biological and environ-mental science to engineering. Magnetic nanoparticle-based adsorbentscan be used in the separation and purification of biologically as wellas environmentally relevant target species with high precision andaccuracy.143,144

1.4.9 Removal of Iron and Manganese from Water

The presence of iron and manganese gives an astringent and metallic tasteto drinking water, which causes problems in cooking and in the productionof beverages.145 A simple method of iron and manganese removal consists ofoxidation and ion-exchange resins. The oxidation of iron is dependent onthe solution’s pH, and organic matter and carbonate concentration. Oxi-dation of iron and manganese can be achieved by introducing an oxidizingagent and it may be done through the application of methods that include

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the addition of oxidants such as chlorine and potassium permanganate.Activated carbons have also been applied for the removal of iron and man-ganese from aqueous solutions.146 Klueh and Robinson147 investigatedthe sequestration of iron by polyphosphate addition while providing thenecessary disinfection through chlorine addition. They observed that thepresence of calcium in the groundwater inhibited the removal of iron.The addition of polyphosphate to the groundwater first and the simul-taneous addition of polyphosphate and chlorine were both fairly successfulat removing the iron.

1.4.9.1 Ion Exchange

Ion-exchange resins provide many advantages and are one of the mostwidely techniques used for treatment of wastewater effluents.148 Lee andNicol149 have used the Diphonix resin to remove ferric iron from a cobaltsulfate solution with various pH ranges. A lower pH and higher dose of resingives a higher removal of iron from solution. Elution of iron was observedwith an increase of Ti(III) in the sulfuric acid eluent. These workers foundthat the iron elution enhancement with Ti(III) was due to the combined ef-fects 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 Fe31 in different solutions by a chelating ion ex-change resin. A mathematical model was used to predict the equilibrium,which gave a good fit for the experimental data in various solutions. It hadbeen observed that solvent type influences the adsorption capacity. Khalilet al.151 studied the removal of ferric ions by using crosslinked chitosanresins immobilized with diethylenetriamine and tetraethylenepentamine. Ithad been found that the tetraethylenepentamine containing chitosan resinshowed a higher uptake capacity towards Fe(III) compared with diethylene-triamine containing chitosan resin. Kinetic data showed that the adsorptionprocess followed the pseudo-second order kinetics. Thermodynamic studiesindicated that the adsorption process was exothermic and spontaneousin nature.

1.4.9.2 Activated Carbons

Omri and Benzina152 achieved the removal of Mn(II) ions from aqueous so-lutions by adsorption on activated carbons derived from Ziziphus spina-christi seeds. The effects of process parameters such as solution pH, initialmetal ion concentration and temperature on the adsorption performance ofactivated carbons for Mn(II) ions removal were tested to optimize the system.Maximum adsorption was obtained at pH 4. Freundlich isotherms followedthe adsorption system and the higher adsorption capacity for a Langmuirisotherm was 172 mg g�1. Adsorption of iron and manganese ions fromaqueous solution by low-cost adsorbents of palm fruit bunch and maize cobswas carried out.153 Adsorption of iron ions on palm fruit bunch and maize

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cobs was in the range of 80–57%, for initial concentrations ranging between1 and 10 ppm.

Recently, Mengistie et al.154 performed the adsorption of Mn(II) byusing activated carbons of Militia ferruginea leaves from aqueous solutionsin the batch mode. Adsorption equilibrium was achieved within 2 h. It hadbeen found that pH 4 was appropriate for Mn(II) removal and 95.8% metalions were removed. The adsorption isotherms were best fitted to a Freun-dlich model, which showed multilayer adsorption at the surface of theactivated carbons. The adsorption kinetics were best fitted to a first-orderkinetic model. Thermodynamic analysis showed that the adsorption pro-cess was endothermic and spontaneous in nature. Emmanuel and Rao155

studied the adsorption of Mn(II) by activated carbons of Pithacelobiumdulce from aqueous solutions and found a good sorption capacity for metalions. The sorption equilibrium was achieved within 50 min. The equi-librium isotherm was best fitted to a Langmuir isotherm model, whichindicates the adsorption of Mn(II) onto activated carbons was as amonolayer.

1.4.9.3 Other Treatment Methods

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 byAmbikadevi and Lalithambika.156 It was found that the oxalic acid gives thebest results, both at room temperature as well as at high temperatures, be-cause of its high acid strength, good complexing capacity and reducingpower. The effects of several parameters such as time, temperature and re-agent concentration were studied for the optimization process. The removalof iron was found to be B80% 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 ironas the cathode. Several removal parameters such as solution pH, currentdensity, electrode configuration, inter-electrode distance, effects of coexist-ing ions and temperature were studied. The results obtained suggested thatthe highest removal of 97.2% at a pH of 7.0 was for a current density0.05 A dm�2 with an energy consumption of 1.151 kWh m�3. Thermodynamicparameters indicated that the Mn(II) removal was feasible, spontaneous andendothermic in nature. A Langmuir adsorption isotherm well fitted to theadsorption system. The kinetic model was best described by a pseudo-secondorder rate at the various current densities. Taffarel and Rubio158 appliedChilean zeolite as an adsorbent for removal of Mn(II) ions from aqueoussolutions. The solution pH significantly influenced the adsorption of Mn(II)removal and the best results were been found at pH 6–6.8. The removalkinetics was best fitted with a pseudo-second order model. The equilibriumisotherm data were best fitted to a Langmuir isotherm model. It was foundthat the Chilean zeolite treated with NaCl, NaOH, Na2CO3 and NH4Cl in-creased its uptake ability in comparison with natural Chilean zeolite.

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1.5 Concluding NotesThe presence of heavy metals and their toxicity to the environment and tohuman beings is posing a serious challenge to environmental engineers withrespect to the treatment of wastewater effluents prior to discharge into thenearby water bodies. Several removal techniques have been developed andapplied 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 adsorptionhave their own advantages and disadvantages over metal ion sequestrationsfrom environmental matrices. During recent years the developments in ad-sorption of heavy metals from aqueous solutions have gained tremendouspopularity among the scientific community as methods to treat industrialwastewater. Several adsorbents such as clays, LDHs, zeolites, carbon nano-tubes and their composites, activated carbons, biomass derived biosorbents,inorganic nanomaterials, inorganic organic hybrid nanocomposites andmagnetic nanomaterials have been synthesized and investigated for theirability to sequester metal ions from water.

Functionalized magnetic nanoparticles are very promising for appli-cations in catalysis, biolabelling and bioseparation. In liquid-phase ex-traction of heavy metals and dyes in particular, such small and magneticallyseparable particles may be useful as they combine the advantages of highdispersion, high reactivity, high stability under acidic conditions and easyseparation. In this chapter we focused mainly on recent developments in thesynthesis of active adsorbents and nanoparticles. Further, functionalizationand application of magnetic nanoparticles and their nanosorbents for theseparation and purification of hazardous metal ions from the environmentare discussed in detail in a separate chapter in this book.

AcknowledgementsR.K. Gautam thanks the University Grants Commission for the award of aJunior Research Fellowship (JRF). Suresh Mahiya is grateful to the President,JECRC University, for the award of Scholarship for his PhD. The authorsequally acknowledge the support and provision of the necessary facilities bythe University of Allahabad, Allahabad, India and JECRC University, Jaipur,India. The support and encouragement of Prof. V.S. Tripathi from the De-partment of Chemistry, University of Allahabad, is also appreciated. We alsothank the anonymous editors and reviewers for giving their kind criticismsand comments, which fuelled the zeal for the manuscript.

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