Removal of Heavy Metals from Wastewater by Adsorption

Adsorption processes are extensively used in wastewater treatment for heavy metal removal. The most widely used adsorbent is activated carbon giving the best of results but it’s high cost limits its use. It has a high cost of production and regeneration. As the world today faces a shortage of freshwater resources, it is inevitable to look for alternatives that lessen the burden on existing resources. Also, heavy metals are toxic even in trace concentrations, so an environmentally safe method of their removal necessitated the requirement of low cost adsorbents. Adsorption is a cost-effective technique and gained recognition due to its minimum waste disposal advantage. This chapter focuses on the process of adsorption and the types of adsorbent available today. It also encompasses the low-cost adsorbents ranging from agricultural waste to industrial waste explaining the adsorption reaction condition. The cost-effectiveness, technical applicability and easy availability of raw material with low negative impact on the system are the precursors in selecting the adsorbents. The novelty of the chapter lies in covering a wide range of adsorbents with their efficiency in removal of heavy metals from wastewater.

[1]. The main natural sources of heavy metals include volcanic processes, weathering of rocks and soil erosion. While the anthropogenic sources include mineral processing, fuel combustion and industrial activities like mining, metal processing, chemical fertilizers and dye manufacturing etc. Heavy metals are non-biodegradable, recalcitrant and have high mobility in aqueous media, so they tend to accumulate in soils and living organisms leading to environmental repercussions. Heavy metals are taken up by plants which biomagnify through food chains in animals and humans causing serious negative health effects due to their carcinogenicity [2][3][4][5]. Table 1 gives the maximum contaminant level (MCL) in drinking water given by USEPA [7] along with their harmful effects.
Heavy metals have a high tendency to form complexes, are highly reactive and have increased biochemical activity which makes them very persistent in the environment. They are transported through aqueous medium and can concentrate in soil and water resources. This makes them extremely dangerous to all kinds of life forms and the environment. Hence, it is necessary to remove these toxic metals

Activated carbon adsorbents
Activated carbon (AC) is one of the most widely used adsorbents due to its high efficiency, porosity and high surface area. It is commercially manufactured from the carbonization of like coal and wood, so it is expensive and its use is limited [24,[60][61][62]. They are mainly produced by pyrolysis of carbonaceous material at temperatures lower than 1000°C. The preparation of activated carbon involves two steps, one is the carbonization of raw material at temperature less than 800°C in inert atmosphere, second is activation of the produced product at temperature between 950°C and 1000°C [63]. Hence, most of the carbonaceous material can be used as raw material for activated carbon production, though the characteristics of the final product will rely on the raw material used and activated conditions [63]. Carbon is the main component of activated carbon adsorbent, other elements such as hydrogen, oxygen sulfur and nitrogen are also present. They are produced in both powdered and granular forms. The powdered one has large pores and smaller internal surface area; while the granular one has large internal area and small pores. The adsorptive capacity of an activated carbon is determined by its high porosity and surface are along with its chemical structure. Hence, other low cost raw materials such as agricultural wastes are looked upon for increasing the cost effectiveness of activated carbon.
Kobya studied adsorptive removal of Cr 4+ from aqueous solutions by AC prepared from hazelnut shell and reached a maximum removal of 170mg/g at pH 1.0 [64]. This was found to be higher than other adsorbents like coconut shell and wood AC [65] which had a removal of 58.5 and 87.6mg/g respectively. Karthikeyan et al. studied removal of Cr 6+ from wastewater using activated carbon prepared from wood saw dust. The adsorption capacity of Cr 6+

Zeolites
They are alumino silicates with a crystalline structure that occur naturally or are manufactured industrially. They are one of the best adsorbents for heavy metal removal as they consist of hydrated aluminosilicate minerals comprising of interlinked alumina and silica. Zeolites possess appreciable ion exchange capacities, hydrophilic properties and high specific surface area which makes then exceedingly good adsorbents for heavy metal remediation [75]. Zeolites can also be modified which attain a better adsorption capacity as compared to unmodified ones. NaX zeolite is one of the most widely used nanosized zeolite for removal of heavy metals from wastewater [76][77][78][79]. Rad et al. prepared NaX nanozeolite followed by polyvinylacetate polymer/NaX nanocomposite nanofibers to study removal of Cd 2+ . The maximum adsorption capacity was reported to 838.7mg/g at pH 5.0 [79]. Javadian et al. used fly ash for preparation of amorphous zeolite and obtained a maximum adsorption capacity of 26.246mg/g for Cd 2+ at 5 optimum pH [80]. Similar studies were conducted by Visa who reported that zeolites have a high surface area and porosity which aid in adsorption of heavy metals [81]. Kobayashi et al. studied removal of Hg 2+ and Pb 2+ from aqueous solutions using zeolites prepared from fly ash. The maximum amount of Hg 2+ and Pb 2+ adsorbed were 22.4 mg/g and 30.7mg/g respectively at optimum pH of 5 [82].

Clay minerals
Bentonite, a clay mineral holds the highest cation exchange capacity, is regenerable and around 20 times cheaper than activated carbon [83,84]. Clay minerals have less removal capacity of heavy metals when compared to zeolites. But they are still used owing to their advantages such as brilliant physical, chemical and have occupied an important place in the area of adsorption of heavy metals from effluents. They possess exceptional mechanical and chemical properties, strength, exchange capacity, electrical conductivity and thermal stability. A high surface area along with numerous intermolecular interactions gives them an edge over other adsorbents in remediation of heavy metals.

Carbon nanotubes, fullerenes and graphene
Iijima discovered carbon nanotubes (CNTs) in 1991 [94]. They exist as long carbon cylindrical in shape with a continuous hexagonal graphite sheets. They are of two types: single walled CNT, which have a single graphite sheet and multi walled CNTs which have multiple sheets. They have portrayed excellent potential for heavy metal from wastewater for copper [95,96] lead, [97,98], chromium [99,100], nickel [100,101] and cadmium [100,102]. CNTs prove to be excellent adsorbents owing to the advantages such as mechanical and surface properties electrical and semiconductor properties [102,103]. They also provide a high specific surface area (150-1500m 2 /g) and the presence of mesopores increases their adsorption efficiency [104][105][106][107]. The presence of different functional groups containing elements such as oxygen, nitrogen and sulfur directly and indirectly affect the adsorption mechanisms that enhance the adsorption of heavy metals [108][109][110][111].
Oxidized CNTs also portray exceedingly high adsorption capacity for the removal of Cr 6+ , Pb 2+ and Cd 2+ from wastewater [112][113][114][115]. Wang et al. (2007a) carried out a study using MWCNTs activated with conc. HNO 3 which escalated the adsorption capacity due to creation of more oxygen functional groups. The equilibrium time for Pb 2+ adsorption was found to be 120 min at an optimum pH 2.0 [116]. Nanocomposites are also prepared using CNTs with ferrous, zirconium, aluminium oxides by coprecipitation method for removal of Pb, As, Cu, Ni and Cr ions [117][118][119][120][121][122]. Luo et al. synthesized Fe 2 O 3 /MnO 2 /acid oxidized MWCNT nanocomposites for removal of Cr 6+ . At an optimum pH of 2.0 a maximum removal capacity of 85% was achieved by the nanocomposite [123]. Ge et al. prepared magnetic Fe@MgO nanocomposites for the removal of Pb 2+ from water. A maximum adsorption efficiency of 14746.4 mg/g was achieved for Pb 2+ at 120 min contact time [124]. Stafiej and Pyrzynska stated some facts related to adsorption capacity of CNTs and reached a conclusion that pH and concentration of heavy metals significantly affect the CNTs efficiency [125]. CNTs portray excellent adsorption efficiency due to their surface morphology, electrochemical potential and ion exchange capacity [126,127]. The ability of CNTs to be easily modified makes them selective adsorbents with the merit of enhanced adsorption efficiency [113,[127][128][129][130]. They are instituted as great adsorbents in the field of wastewater treatment due to their appreciable mechanical and surface characteristics, mechanical and magnetic properties and high stability [131]. But the use is restricted due to the accumulation of the active sites by the adsorbate. Hence, activation of CNTs offers the advantage of increasing the sites with functional groups which in turn increases their adsorption efficiency for heavy metal removal from water and wastewater [132][133][134][135][136][137][138].
The discovery of fullerenes in 1985 led to another breakthrough in adsorption science [139,140]. They have a closed-cage structure containing pentagonal and hexagonal carbon rings with the formula C 20+m , m being an integer. Their adsorption efficiency can also be attributed to their surface morphology and presence of mesopores which gives then higher ion affinity and higher specific surface area for remediation of heavy metal ions from water and wastewater [141,142]. Alekseeva et al. conducted a study using fullerenes for the removal of Cu 2+ and explained the mechanism through Langmuir model [143]. The maximum adsorption efficiency was found to be 14.6 mmol/g. Spherical fullerene containing 60 carbon atoms is the most explored one. Its striking features comprise of hydroxyl and epoxy functional groups on surface, large surface to volume ratio, hydrophobicity, high electron affinity and low aggregation capacity which make it beneficial for heavy metal removal [144][145][146]. But their use is often restricted due to their high price. So, research on incorporation of other conventional adsorbents with fullerenes has come up. It was revealed that fullerenes enhance the porous structure of adsorbent leading to increase in the removal efficiency of heavy metals. It was found that adsorption capacity of ACs escalated by 1.5-2.5 times after introduction of fullerenes into their structure [147,148].
Graphene came into the scene in 2004 and is a 2-D hexagonal lattice of carbon atoms. It also possesses structural, chemical and mechanical properties which aid its use in wastewater treatment. It has a high surface area, active functional groups and sites on its surface which enhance its adsorption capacity [149][150][151]. Graphene can also be activated by oxidation to increase functional groups which surge the adsorption capacity for removal of heavy metals [114,[152][153][154][155]. Deng et al. 2010 conducted a study using functionalized graphene for removal of Pb 2+ from aqueous solution. At an optimum pH of 5.0 the maximum adsorption capacity reached was 406.6mn/g within 40 min [156]. Several studies were conducted to study the properties of graphene oxides for adsorption [157][158][159][160][161]. It was revealed that graphene oxides can also be magnetically modified which increases their adsorption capacity [162,163]. A study by Zhao et al. used layered graphene oxide for removal of Pb 2+ from aqueous solution. The adsorbent layers had oxygen functional groups which greatly enhanced the adsorption capacity reaching a maximum of 1850 mg/g [164]. Jian et al. synthesized a bio-adsorbent polyacrylamide/graphene oxide hydrogel grafted with sodium alginate and studied the removal of Cu 2+ and Pb 2+ from aqueous solution. The maximum adsorption capacity of Cu 2+ was 68.76 mg/g at pH 5 and 240.69 mg/g for Pb 2+ at 5.5 pH [165].

Low cost adsorbents
Although, ACs are the most widely used adsorbents, their use is limited due to their high cost and low regeneration. Same is with other developed adsorbents such as carbon nanotubes, fullerenes and nanocomposites. To make the process of wastewater treatment speed up and effective, it is vital to look for adsorbents that are cost effective as well administer a high adsorption efficiency. Thus, the need for low cost adsorbents came to be realized. Low cost adsorbents comprise of those non-conventional materials that are easily available and cost effective mainly agricultural and industrial waste.   Agricultural wastes have the constitution of lignin, cellulose, hydrocarbons, sugars, water and starch along with other functional groups which enhances the adsorption capacity of these agricultural wastes. These wastes can range from rice husk to wheat shells, egg shells, coconut husk, palm fruit, bagasse, groundnut shell, fruit peels, biochar etc. These wastes can be used directly in which they are washed and grounded first. Then they are sieved to get the desirable particle size which are used for adsorption tests. They can also be modified into chars and further activated to increase the adsorption sites [166,167]. Table 2 shows the different agricultural wastes used for heavy metal ions removal.

Biochar
Biochar is the charred solid material obtained from the carbonization of biomass. The most common method of production of biomass is by pyrolysis which is the thermal decomposition of biomass in absence or limited oxygen. Biochars are less carbonized than AC so more carbon, hydrogen and oxygen remain in their structure. Biochar has shown remarkable potential for remediation of heavy metals from wastewater than other conventional and low-cost adsorbents. They have a mesoporous structure leading to a high surface area and presence of different functional groups and a low ash content makes them excellent and effective adsorbents. Feedstock such as rice husk [180][181][182][183][184][185], corn husk [180], tea waste [181,184,186,187] and digested sludge have [185,188] been employed for removal of heavy metals from aqueous solutions as well as wastewater.

Industrial waste
Industrial activities generate huge quantities of waste that are usually sent to landfill sites for disposal. These wastes possess a good adsorption capacity and are solve the problem of waste treatment. Waste products like fly ash [189][190][191][192][193][194][195][196], red mud [197][198][199][200][201][202][203] and slag [204,205] have been effectively used owing to their appreciable capacity for removal of heavy metals from wastewater. Many industrial waste adsorbents have been employed for the remediation of Zn 2+ from effluents. The maximum adsorption capacity for lignin was 73.2mg/g, 168mg/g for waste sludge and 55.82mg/g for cassava waste [206,207]. Table 3 gives various industrial wastes used for removal of heavy metals from wastewater and aqueous solutions.

Comparison of conventional and non-conventional adsorbents
For the adsorption process to be efficient, selection of the most appropriate adsorbent is a crucial step. The main basis of selection of an adsorbent include low cost, high adsorption capacity, effective for a broad spectrum of pollutants and having a low footprint [208,209]. There has been extensive research in the field of conventional and non-conventional adsorption performances and mechanisms. Different adsorbents follow varied mechanisms because of difference in raw material and adsorbent production conditions. Mainly four mechanisms have been identified for efficient adsorption of pollutants; chemisorption, physisorption, ion-exchange and precipitation [210,211]. Davis et al. stated that ion-exchange does not necessarily describe the mechanism of adsorption but a lot of other factors and mechanisms co-aid to make the process successful [210]. Some other researchers also explained the adsorption mechanisms [212][213][214][215]. Literature evidently points

Author details
Athar Hussain 1 , Sangeeta Madan 2 and Richa Madan 2 * 1 Department of Civil Engineering, Netaji Subhas Institute of Technology, West Campus, Jaffarpur, Delhi, India 2 Department of Environment Science, Gurukula Kangri (Deemed to be University), Haridwar, Uttarakhand, India *Address all correspondence to: richa.madan.92@gmail.com out that activated carbons have proven themselves as brilliant adsorbents due to their high specific surface area, mechanical and structural surface morphology and presence of functional groups which can also be modified. However, non-conventional adsorbents are increasingly employed as low-cost and effectual adsorbents. Their commercialization remains a task but there as upcoming as their available in abundant. More focused research into their engineering and modification can bring them at par some commercial solid adsorbents.

Conclusion
Heavy metal pollution is one the most dangerous situations being faced today. They harmful even in trace concentrations. Many of them are carcinogenic, cause birth defects and are extremely fatal. Hence, it is necessary to remove these toxic metals from wastewater before it is discharged into open waters. Adsorption is one such technique that caters not only to the remediation of heavy metal from wastewater, it is also eco-friendly with a low footprint. Adsorbents like activated are widely used, but it is restricted due to its high cost. So, it is necessary to look for options that are sustainable and aim at remediating the larger prospect of the problem. Low-cost adsorbents like agricultural wastes, industrial wastes and biochar aid not only in removal of heavy metals but are also cheap methods. Their raw material is easily available and these adsorbents can be easily manufactured. So, it is a green technology that greatly enhances the process of wastewater treatment. Further research into developing more low-cost adsorbents can help in further remedial of heavy metals.
© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. [17] Abo-Farha, SA, Abdel-Aal, AY, Ashour, IA, Garamon, SE. Removal of some heavy metal cations by synthetic resin purolite C100, J. Hazard. Mater. 169 (2009)190-194. [18] Alyuz, B, Veli, S. Kinetics and equilibrium studies for the removal of nickel and zinc from aqueous solutions by ion exchange resins, J. Hazard. Mater. 167 (2009)    [82] Kobayashi Y, Ogata F, Nakamura T, Kawasaki N. Synthesis of novel zeolites produced from fly ash by hydrothermal treatment in alkaline solution and its evaluation as an adsorbent for heavy metal removal. Journal of Environmental Chemical Engineering. 2020 Apr 1;8(2):103687.