a critical analysis on the efﬁciency of activated carbons from … · · 2015-05-10treatment...

Critical Reviews in Environmental Science and Technology, 45:613–668, 2015Copyright © Taylor & Francis Group, LLCISSN: 1064-3389 print / 1547-6537 onlineDOI: 10.1080/10643389.2013.876526

A Critical Analysis on the Efficiencyof Activated Carbons from Low-Cost

Precursors for Heavy Metals Remediation

VINOD KUMAR GUPTA,1 ARUNIMA NAYAK,1 BRIJ BHUSHAN,2

and SHILPI AGARWAL1

1Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee, India2Department of Chemistry, Graphic Era Deemed University, Dehradun, India

A vast array of industrial waste products, agricultural by-products,and biological materials, such as bacteria, algae, yeasts, and fungihave received increased attention for heavy metal removal and re-covery due to their good performance, low cost, and availabilityin large quantities. Such materials are cheaper, more effective al-ternatives to commercial activated carbon (AC) for the removal ofheavy metals from aqueous solution. The authors review in greaterdetail the various factors affecting the adsorption capacity and costefficiency based on the literatures and present research results. Spe-cial emphasis has been made on nature of the precursors, theirpretreatment and modification, the activation conditions of timeand temperature, pH of aqueous media, and regeneration/reusepotential. The authors present some of the latest important resultsand give a source of up-to-date literature on the ACs prepared fromvarious low-cost precursors from agro-industrial wastes as well asfrom biological materials. The evolution of adsorption has turnedfrom an interesting alternative approach into a powerful standardtechnique by offering a numbers of advantages: better performancein terms of ulterior adsorption capacity, rate of adsorption, solvingwastewaters pollution in a cost-effective way, and overcoming partof the solid wastes problem around the world. Various advantagesand challenges have been identified and a widespread and greatprogress in this area can be expected in the future.

Address correspondence to Vinod Kumar Gupta, Department of Chemistry, Indian Insti-tute of Technology Roorkee, Roorkee 247667, India. E-mail: [email protected]

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614 V. K. Gupta et al.

KEY WORDS: toxic metal ions, low-cost activated carbon, ad-sorption capacity, chemical modification

1. INTRODUCTION

It is well documented that the quality of water bodies is deteriorating dueto the presence of agricultural, urban, and industrial wastes containing con-taminants such as sewage, pesticides, fertilizer, dyes, and heavy metals. Thishas proven to be very damaging to human health, aquatic habitats, andspecies. Heavy metals have been classified as major pollutants as they aretoxic, are not metabolized by the body and get easily accumulated in thesoft tissues.1,2 As a result of their increased usage, they are present in vir-tually every aspect of modern consumerism such as construction materials,cosmetics, medicines, processed foods, fuel sources, agents of destruction,appliances, and various personal care products.3–5 It is extremely difficultfor anyone to avoid exposure to any of the many harmful heavy metals thatare so prevalent in our environment. The heavy metals hazardous to hu-mans and aquatic life include lead, mercury, cadmium, arsenic, copper, zinc,and chromium. At high exposure levels, they result in damaged or reducedmental and central nervous function, lower energy levels, and damage toblood compositions, lungs, kidneys, livers, and other vital organs.6 Long-termexposure may result in slowly progressing physical, muscular, and neuro-logical degenerative processes that mimic Alzheimer’s disease, Parkinson’sdisease, muscular dystrophy, and multiple sclerosis. All these severe injurieshave made the public opinion ecologically sensitive and extremely aware ofthe environmental problems. Some information regarding the principal toxicmetal ions are reported in Table 1.

Different treatment techniques for wastewater have been developedin recent years both to decrease the amount of wastewater produced andto improve the quality of the treated effluent. The waste waters emanatingfrom electroplating, metal finishing operations, electronic–circuit production,steel and nonferrous processes, chemical, pharmaceutical production indus-tries consist of different metal pollutants and varied types of other pollutantshaving different physical and chemical characteristics. So a series of primary,secondary, and tertiary treatment processes are used for their treatment al-though depending on the wastewater characteristics, such processes are alsoused as standalone processes.19 The primary treatment includes preliminarypurification processes of physical and chemical nature while the secondarytreatment deals with the biological treatment of wastewater. In tertiary treat-ment process, wastewater treated by primary and secondary processes isconverted into good quality water that can be used for different types ofpurposes (i.e., drinking, industrial, and medicinal supplies). In the tertiaryprocess, the pollutants are removed up to 99% and water is converted into

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Efficiency of Activated Carbons for Heavy Metals Remediation 615

TABLE 1. Sources, health effects, and permissible limits of some toxic metal species

Toxicmetal Main anthropogenic source Health hazards

Limits indrinking

water (ppm)7

ArsenicAs(V)

Smelting process of Cu, Zn, Pb;Manufacturing of chemicals,glasses, pesticides, paints

Carcinogenicproducing tumor inliver, gastrointestinaleffects

0.01

LeadPb(II)

Burning fossil fuels, mining andmanufacturing, production ofbatteries, cable coverings,plumbing, ammunitions, paintpigments, PVC plastics, crystalglass, fuel additives, devices toshield X-ray.8,9

Suspected carcinogen,loss of appetite,anemia, muscle andjoint pain, causesterility, kidneyproblem, high BP

0.003

MercuryHg(II)

Mining operations, chloralkaliplants, paper industry; burningcoal and wastes, production ofchlorine gas, caustic soda10;use in thermometers, dentalfillings and batteries; use inantiseptic cremes, skinlightening cremes, ointments

Ataxia, numbness,muscle weakness,damage to hearing,speech and vision,insanity, paralysis,coma, death

0.006

CadmiumCd(II)

By product of mining andsmelting of lead, zinc andcopper; natural occurrence inrocks, soils and mineralfertilizers, Used innickel-cadmium batteries, PVCplastics, and paint pigments. incigarettes, dental alloys,electro-plating, motor oil andexhaust.11

Carcinogenic, bonesoftening, kidneyfailure, Itai-itaidisease13

0.003

Chromium(VI)

Release from manufacture, useand disposal of chromiumbased products and during themanufacturing process, Usedfor chrome plating, dyes andpigments, leather tanning,wood preserving.12

Suspected humancarcinogenproducing lungtumors, allergicdermatitis

0.05

NickelNi(II)

Alloy, batteries, electrolytic andwelding procedures, glass,Ceramic industries, catalysts.14

Allergenic,carcinogenic15

0.07

CopperCu(II)

Mining, farming andmanufacturing operations,Used to make wire, plumbingpipes; combined with metals toform brass and bronze pipes,faucets used in agriculture totreat plant diseases.16,17

Lung cancer18 2.00

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TABLE 2. Treatment technologies for the removal of heavy metals from wastewaters withassociated advantages and disadvantages

Technology Advantages Disadvantages Reference

Chemicalprecipitation

Simplicity of process, notmetal selective, inexpensivecapital cost

Huge sludge generation;Sludge disposal cost; Highmaintenance cost.

[20–23]

Ion exchange Metal selective, limited pHtolerance, high regeneration

High initial capital cost; Highmaintenance cost

[24–26]

Coagulation/flocculation

Bacterial inactivationcapability, good sludgesettling and dewateringcharacteristics

Chemical consumption;increased sludge volumegeneration.

[27,28]

Flotation Metal selective, low retentiontimes, removal of smallparticles

High initial capital cost; Highmaintenance cost.

[29–31]

Membranefiltration

Low solid waste generation,low chemical consumption,small space requirement,possible to be metalselective

High initial capital cost, highmaintenance and operationcost, membrane fouling,limited flow rates.

[32]

Electrochemicaltreatment

No chemicals required; can beengineered to toleratesuspended solids, metalselective

High capital cost, Highrunning cost.

[33]

Adsorptionusingactivatedcarbon

Wide variety of targetpollutants, high capacity,fast kinetics

Performance depends on typeof adsorbent; high cost; noregeneration.

[34]

the safe quality for a specific use. The advantages and disadvantages associ-ated with each method are listed in Table 2.

2. ADSORPTION TECHNIQUES

The adsorption technology, which is widely used in the tertiary wastewatertreatment step, is essentially an accumulation of adsorbate molecules on thesurface of a porous adsorbent. It has been widely documented as superiorto other treatment processes in terms of its high efficiency for the removalof pollutants present at very low concentrations, has wider applicability inthe removal of diverse types of pollutants, environmental friendliness andexhibiting resistance to fouling from toxic pollutants.34,35 It has been reportedto be used successfully as a single treatment process mainly because of itsefficiency in removing diverse types of pollutants, greater versatility and lowcost of installation and operation.

Several materials such as activated carbon (AC), zeolite, alumina, andsilica, have shown good capacity for the adsorption of heavy metals ions.36–46

The structural features of well-developed porosity and high interparticulate

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TABLE 3. Adsorption capacities of various inorganic and bio-based precursors as low-costadsorbents for toxic metal ions removal from waste water

Waste ads capacity OptimumAs(V) source Adsorbent (mg/g) conditions Ref

Inorganic Leatherindustry

Leather 26.00 pH 1,temp25◦C

[115]

Inorganic Thermalpower

Fly ash (ultrasonictreatment)

13.04 pH 2.5,time 7hr

[80]

Inorganic Aluminumindustry

Red Mud (Acid treated) 0.82 pH 3.5 [98]

As(III)Inorganic Steel

industryBF (Slag) 1.40 temp 25◦C [99]


Red Mud (Acid treated) 0.67 pH 7.2 [98]

Cd(II)Bio-based agricultural Rice husk (phosphate

treated)2000.00 pH 12,

time60 min,temp40◦C

[130]

Bio-based biomass Spirulina 357.00 na [205]Bio-based agricultural Puresorbe 285.70 pH 7 [158]Bio-based agricultural Orange peel (chem mod) 136.05 pH 7, time

120 min[194]

Bio-based agricultural Rice husk (alkali treated) 125.94 pH 6.5 [134]Bio-based agricultural Rice husk (H3PO4

treated)102.00 pH 6 [133]

Bio-based agricultural coir pith 93.40 pH 5;temp30◦C

[162]

Bio-based biomass Fucusceranoides 90.00 pH 5, time25 min

[211]

Bio-based biomass Oedogoniumh. 88.90 pH 5, time55 min,temp25◦C

[203]

Bio-based agricultural Natural Rice husk 73.96 pH 6.5 [134]Bio-based agricultural Mango peel 68.92 pH 5, time

60 min,temp25◦C

[191]


Red Mud 68.00 pH 6, time8–10 hr,temp30◦C

[97]

Bio-based biomass Pseudomonasf . 66.25 pH 5 [213]Bio-based agricultural Wheat bran(ultrasonic

treatment)51.58 pH 5, time

60 min[154]

(Continued on next page)

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TABLE 3. Adsorption capacities of various inorganic and bio-based precursors as low-costadsorbents for toxic metal ions removal from waste water (Continued)

Waste ads capacity Optimumsource Adsorbent (mg/g) conditions Ref

Bio-based agricultural Orange peel 47.60 pH 7, time120 min

[194]

Bio-based agricultural Rice husk(sulfuric acidtreatment)

40.92 na [136]

Bio-based agricultural Rice husk ash 39.87 pH 6 [135]Bio-based agricultural Wheat straw 39.22 pH 6, time

10 min[138]

Bio-based agricultural raw date pit 35.90 na [199]Bio-based agricultural Banana peel 35.52 pH 8, time

30 min[188]

Bio-based agricultural Wheat bran(ultrasonictreatment)

22.78 pH 5, time60 min

[154]

Bio-based agricultural Pomelo peel 21.83 pH 5, time20 min

[196]

Bio-based agricultural Wheat straw 21.00 pH 5, time15 min,temp36◦C

[139]

Bio-based agricultural Wheat bran 21.00 pH 5, time20 min

[155]

Bio-based agricultural NaOH treated rice husk(NRH)

20.24 time 4 hr [132]

Inorganic Steelindustry

BF (Slag) 18.72 pH 5,temp40◦C

[102]

Bio-based agricultural Cicerarientinum 18.00 pH 8, time60 min

[198]

Bio-based agricultural sodium bicarbonatetreated rice husk(NCRH)

16.18 time 1 hr [132]

Inorganic Municipality sewage sludge 16.00 pH 5.8,temp25◦C

[126]

Bio-based agricultural Wheat bran 15.82 pH 5, time25 min,temp20◦C

[155]

Bio-based agricultural raw coffee powder 15.65 pH 7, time120 min,temp20◦C

[172]

Inorganic Municipality iron oxide coated sludge 14.70 pH 7 [127]Bio-based agricultural Wheat straw 14.56 pH 6, time

210 min[141]


Red Mud+H2O2 13.00 pH 4, time8–10 hr,temp30◦C

[96]


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Bio-based agricultural Wheat straw 11.60 pH 5, time60 min

[140]

Bio-based agricultural tea waste 11.29 na [170]Bio-based agricultural epichlorohydrin treated

rice husk ERH)11.12 time 2 hr [132]


Red Mud 10.57 na [93]


BF (Sludge) 10.15 temp 25◦C [106]

Bio-based agricultural Banana peel 5.71 pH 3, time20 min,temp25◦C

[187]


Coal fly ash 5.00 pH 8, time2–3 hr

[84]

Bio-based agricultural copra meal 4.99 pH 6;temp26◦C

[164]

Bio-based agricultural Wheat straw (ureatreated)

4.25 pH 6, time20 min

[138]

Bio-based agricultural Pinus roxburghii bark 3.01 pH 6.5,time 1hr, temp30◦C

[123]


Red Mud (Acid treated) 2.47 pH 5.5–5.9 [95]

Inorganic Sugarindustry

Bagasse Fly Ash 2.00 pH 6, time60 min,temp50◦C

[89]


Roll Mill Scale 1.20 temp 80◦C [106]


lignite-based fly ash 0.83 pH 7–7.5,time 0.5hr, temp20◦C

[85]

Cr(III)Bio-based agricultural Wheat straw(chem mod) 322.58 temp 55◦C [142]Bio-based agricultural Wheat bran(chem mod) 93.00 pH 5, time

20 min[145]

Inorganic Paperindustry

Black Liquor 17.97 pH 5 [118]

Inorganic Municipality chemically activatedsewage sludge

15.40 pH 3 [128]

Inorganic Fertilizerindustry

Carbon slurry 1213.80 pH 2, time24 hr,temp30◦C

[112]


Carbon slurry 728.00 na [111]


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[148]


Leather 133.00 pH 1,temp25◦C

[115]

Bio-based agricultural Banana peel 131.56 pH 2,time30 min,temp25◦C

[189]

Bio-based biomass U. lactuca (activated) 112.36 pH 1 [212]Bio-based agricultural Sapotaceae seed

(chitosan +acid)84.31 pH 3, time

90 min,temp30◦C

[200]

Bio-based agricultural Sapotaceae seed(chitosan coated)

76.23 pH 3, time90 min,temp30◦C

[200]

Bio-based agricultural Sapotaceae seed (acidcoated)


[200]


Iron complexed proteinwaste

51.00 pH 4, time3 hr,

[116]

Bio-based agricultural Wheat bran 40.80 pH 1, time12 hr

[153]



[94]

Bio-based biomass Oedogonium h. (acidtreated)


[202]

Bio-based biomass Oedogonium h. (raw) 31.00 pH 2, time110 min,temp45◦C

[202]

Inorganic Automobile Scrap tyre 29.93 pH 2 [114]Inorganic Thermal

powerFly ash 23.86 pH 2, time

3 hr[82]

Bio-based biomass Nostoc 22.92 pH 3, time120 min,temp25◦C

[209]

Bio-based agricultural Wheat straw 21.34 na [143]Bio-based biomass Chrococcus 21.36 pH 2, time

30 min[217]

Bio-based agricultural Bael fruit 17.27 pH 2, time240 min

[175]


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Carbon slurry 15.24 pH 2, time70 min,temp30◦C

[110]

Bio-based biomass Spirogyra 14.70 pH 2, time120 min

[207]

Bio-based biomass N. calicola 12.23 pH 3, time30 min

[217]

Bio-based agricultural coir pith 11.56 pH 3.3 [161]Bio-based agricultural Groundnut husk (Ag

coated)11.40 pH 3, time

5 hr[185]

Bio-based agricultural CSC(chitosan+HNO3) 10.88 na [165]Bio-based biomass U. lactuca(dry) 10.61 pH 1 [212]Bio-based agricultural Hazelnut shell 8.28 pH 3.4,


[179]

Bio-based agricultural Walnut shell 8.01 pH 3.2,time100 min,temp25◦C

[179]


BF (Slag) 7.50 na [100]

Bio-based agricultural Groundnut husk 7.00 pH 3, time5 hr

[185]

Inorganic Municipalilty Distillery Sludge 5.70 pH 3, time105 min

[124]



[90]


[123]

Bio-based agricultural CSC(chitosan+H2SO4) 4.05 na [165]Bio-based agricultural CSCCC(chitosan) 3.65 na [165]Bio-based agricultural Almond shell 3.40 pH 3.5,


[179]

Bio-based agricultural Peanut husk 3.34 na [184]Cu(II)Bio-based biomass Spirogyra 133.30 pH 5, time

120 min[208]

Bio-based biomass Pseudomonas p. 89.60 na [216]Bio-based agricultural Potato peel (ZnCl2

treatment)74.00 na [190]


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Bio-based agricultural Orange peel (chem mod) 70.67 pH 7, time120 min

[194]


Modified fly ash (NaOH) 59.19 pH 6.2,time 100hr, temp30◦C

[83]

Bio-based agricultural Wheat bran(dehydrated) 51.50 pH 5, time30 min,temp60◦C

[152]

Bio-based agricultural Orange peel 50.94 pH 7, time120 min

[194]

Bio-based biomass Sphigomonas p. 50.10 pH 5,temp20◦C

[214]

Bio-based agricultural tea waste 48.00 pH 5, time15 min,temp22◦C

[169]

Bio-based agricultural Mango peel 46.09 pH 5, time60 min,temp25◦C

[192]

Bio-based agricultural Rice husk ash 40.82 pH 6 [135]Inorganic Municipality chemically activated

sewage sludge30.70 pH 4 [128]


Activated Slag 30.00 pH 5, time6–8 hr,temp40◦C

[103]

Bio-based agricultural Rice husk (acid treated) 29.00 pH 5.2,time 4hr, temp27◦C

[129]

Bio-based agricultural Peanut hull 21.25 pH 5.5,time 2hr, temp30◦C

[182]


Red Mud 19.72 na [93]


[198]

Bio-based agricultural Wheat bran 17.42 pH 6, time3 hr,temp60◦C

[149]


Slag 17.40 na [104]

Inorganic Municipality iron oxide coated sludge 17.30 pH 6 [127](Continued on next page)

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Bio-based agricultural Casuarina Equisetifoliabark

16.58 pH 5 [122]


[146]

Bio-based agricultural Wheat bran 12.70 pH 4.5,time 24hr

[151]

Bio-based agricultural Chestnut shell 12.56 pH 5,temp20◦C

[177]

Bio-based agricultural Peanut hull pellet 12.00 pH 6.5,time120 min

[183]

Bio-based agricultural Wheat straw 11.43 pH 6, time210 min

[141]

Bio-based agricultural Peanut hull 9.00 pH 7.5,time120 min

[183]

Bio-based agricultural Rhizophoraapiculatatannin

8.78 na [121]

Bio-based agricultural tea waste 8.64 na [170]Bio-based agricultural Wheat bran 8.34 pH 5, time

120 min[150]


[153]

Bio-based agricultural Chestnut shell(acidtreated)

5.48 temp 25◦C [176]


Red Mud 5.35 pH 5.5,time60 min,temp30◦C

[91]


Coal fly ash 4.71 pH 4.5,time 24hr

[81]

Inorganic Municipality Sludge sewage ash 4.10 pH 6.2 [125]Bio-based agricultural Pinus roxburghii bark 3.81 pH 6.5,

time 1hr, temp30◦C

[123]

Bio-based agricultural Peanut husk 3.34 na [184]Inorganic Thermal

powerCoal fly ash 2.80 pH 8, time

2–3 hr[84]



[87]


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Red Mud 2.28 pH 5.5,time60 min,temp30◦C

[92]

Bio-based agricultural blended coffee 2.00 na [173]Co(II)Bio-based agricultural coir pith 12.82 pH 4.3 [161]Bio-based agricultural Lemon peel 22.00 na [195]Hg(II)Bio-based agricultural Rice husk(sulfuric acid

treatment)384.62 na [137]

Inorganic Automobileindustry

Scrap tyre 211.00 na [113]


175.40 pH 5 [128]

Bio-based agricultural Walnut shell (ZnCl2 mod) 151.50 na [180]Bio-based agricultural Wheat bran(chem mod) 70.00 pH 5, time

20 min[145]


Fe(III)/Cr(III) hydroxide 37.30 pH 5.6 [108]

Bio-based agricultural Chem mod coir pith(PGCP-COOH)

13.73 pH 5.5,time 3hr, temp60◦C

[163]

Pb(II)Inorganic Paper

industryBlack Liquor 1865.00 temp 40◦C [117]


Carbon slurry 1618.00 pH 4,temp30◦C

[112]


Carbon slurry 1110.60 na [111]

Bio-based biomass Thiobacillus ferrodoxins 443.00 pH 5, time30 min,temp25◦C

[218]

Bio-based agricultural Coir pith waste 263.00 pH 4 [160]Inorganic Steel

industryBF (Sludge) 227.00 temp 80◦C [107]

Bio-based biomass Oedogonium h. 145.00 pH 5, time90 min,temp45◦C

[204]


BF(Dust) 142.00 temp 25◦C [107]

Bio-based biomass Spirogyra 140.00 pH 5, time100 min,temp40◦C

[206]


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Bio-based agricultural spent black tea 129.90 pH 5.5,temp25◦C

[167]


Fe(III)/Cr(III) hydroxide 126.55 pH 3.5,temp30◦C

[109]


BF (Slag) 125.00 temp 25◦C [101]

Bio-based agricultural Rice husk (acid treated) 108.00 pH 5.2,time 4hr, temp27◦C

[129]

Bio-based agricultural Mango peel 99.05 pH 5, time60 min,temp25◦C

[191]

Bio-based biomass Nostoc 93.50 pH 5, time70 min,temp45◦C

[204]


BOF Sludge 92.50 pH 5, time1 hr,temp30◦C

[105]

Bio-based agricultural Rice husk ash 91.74 pH 5, time1 hr

[131]

Bio-based agricultural spent green tea 90.10 pH 5.5,temp25◦C

[167]

Bio-based agricultural Wheat bran 87.00 pH 4–7,time60 min,temp60◦C

[144]


BF (Sludge) 79.87 temp 80◦C [106]

Bio-based agricultural tea waste 65.00 pH 6, time20 min,temp22◦C

[169]



[94]


64.10 pH 4 [128]

Bio-based agricultural coffee(ZnCl2 mod) 63.00 pH 5.8,time 2hr, temp25◦C

[174]

Bio-based agricultural Wheat bran(chem mod) 62.00 pH 5, time20 min

[145]


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Inorganic Municipality iron oxide coated sludge 42.40 pH 5 [127]Inorganic Steel

industryBF (Slag) 40.00 na [100]

Bio-based agricultural Rice husk ash 39.74 pH 6 [135]Bio-based agricultural Moringa oleifera bark 34.60 pH 5 [119]Bio-based agricultural Rhizophoraapiculata

tannin31.32 na [121]

Bio-based agricultural shell carbon 30.00 na [156]Bio-based agricultural Hazelnut shell 28.18 time 2 hr [178]Bio-based agricultural Cicerarientinum 20.00 pH 6, time

80 min[198]


8.50 temp 25◦C [176]

Bio-based agricultural Almond shell 8.08 time 2 hr [178]Bio-based agricultural Peanut husk 4.59 na [184]Inorganic Steel

industryRoll Mill Scale 2.74 temp 60◦C [106]


Bagasse Fly Ash 2.73 pH 3, time6–8 hr,temp30◦C

[86]



[90]

Bio-based agricultural Banana peel 2.18 pH 5, time20 min,temp25◦C

[187]

InorganicInorganic Aluminum

industryRed Mud (Acid treated) 1.80 pH 5.5–5.9 [95]


Red Mud (untreated) 0.77 pH 5.5–5.9 [95]

Ni(II)Bio-based Agricultural Acacia leucocephala bark 294.10 pH 5,

temp30◦C

[120]

Bio-based biomass Sargassum (acid treated) 250.00 pH 5, time120 min,temp30◦C

[210]

Bio-based biomass Sargassum (raw) 181.00 pH 5, time120 min,temp30◦C

[210]

Bio-based Agricultural Orange peel 158.00 pH 6, time2 hr,temp50◦C

[193]


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Bio-based Agricultural tea waste 73.00 na [168]Inorganic Thermal

powerModified fly ash (NaOH) 61.20 pH 6.2,


[83]

Bio-based Agricultural Pomegranate peel 52.00 pH 5.5,time 2hr, temp25◦C

[186]

Bio-based biomass Oedogonium h. (acidtreated)


[201]

Bio-based biomass Oedogonium h. (raw) 40.90 pH 5, time80 min,temp25◦C

[201]

Bio-based Agricultural Mango peel 39.75 pH 5, time60 min,temp25◦C

[192]

Bio-based Agricultural Guava seed (chem mod) 32.05 pH 6 [197]Inorganic Steel

industryActivated Slag 29.35 pH 4, time

6–8 hr,temp40◦C

[103]

Bio-based Agricultural Guava seed 18.05 pH 6 [197]Bio-based Agricultural coir pith 15.95 pH 5.3 [161]Inorganic Aluminum

industryRed Mud 10.95 na [93]

Inorganic Municipality sewage sludge 9.00 pH 5.8,temp25◦C

[126]

Inorganic Municipality iron oxide coated sludge 7.80 pH 7 [127]Bio-based Agricultural Pinus roxburghii bark 3.53 pH 6.5,


[123]


Bagasse Fly Ash 1.70 pH 6.5,time80 min,temp50◦C

[89]

Zn(II)Inorganic Aluminum

industryRed Mud 133.00 pH, time

8–10 hr,temp30◦C

[97]

Inorganic Paperindustry

Black Liquor 95.00 temp 40◦C [117]


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Bio-based biomass Cyanobacterium 71.42 pH 8, time90 min

[215]

Bio-based agricultural shell carbon(H3PO4

+chitosan)60.41 pH 6, time

3 hr,temp25◦C

[157]

Bio-based agricultural shell carbon(chitosanmod)

50.93 pH 6, time3 hr,temp25◦C

[157]

Bio-based agricultural raw date pit 39.50 na [199]Bio-based agricultural Rice husk ash 39.17 pH 6 [135]Bio-based agricultural Mango peel 28.21 pH 5, time

60 min,temp25◦C

[192]


[198]

Bio-based agricultural Rice husk(sulfuric acidtreatment)

19.38 na [137]


BF (Slag) 17.66 pH 6,temp40◦C

[102]

Bio-based agricultural Wheat bran 16.40 pH 6.5,time 24hr

[151]


Red Mud 14.50 pH 5, time8–10 hr,temp30◦C

[96]


Bagasse Fly Ash 13.21 pH 4, time6–8 hr,temp30◦C

[88]


Red Mud 12.59 na [93]



[84]


BF (Sludge) 9.65 temp 80◦C [106]

Bio-based agricultural tea waste 8.90 pH 4.2,time30 min,temp60◦C

[171]


Coal fly ash 5.75 pH 4.5,time 24hr

[81]


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[84]


lignite-based fly ash 2.78 pH 7–7.5,time 0.5hr, temp20◦C

[85]



[87]


Red Mud (Acid treated) 2.47 pH 5.5–5.9 [95]


2.41 temp 25◦C [176]


Roll Mill Scale 2.17 temp 80◦C [106]

Se(IV)Bio-based agricultural Rice husk(sulfuric acid

treatment)41.15 Na [136]

Ga(III)Bio-based agricultural oxidized coir 13.75 pH 3 [166]Bio-based agricultural raw coir 19.42 pH 3 [166]Fe(III)Bio-based biomass Polyporous squamosus 31.20 pH 2,

temp28◦C

[219]

surface area are responsible for their good adsorption capacity.40 While acti-vated alumina and silica are reported to have a mean surface area of 250 and750 m2/g, the AC has a high surface area ranging from 800 to 1500 m2/g.35,47

Zeolites that are an important class of hydrated aluminosilicates possesscage-like structures and have internal and external surface areas of up toseveral hundred square meters per gram. In comparison to alumina, zeo-lites, and silica, the AC is not only the oldest adsorbent but also the mostwidely used standard adsorbent in the purification of industrial effluents andgroundwater treatment. It is well known that ACs have a high degree ofporosity and an extensive internal surface area ranging from approximately800 to 1000 m2/g. They can be produced from a variety of carbonaceousmaterials including wood, coal, and coconut shells and can be fabricatedin the form of fine powders, large sized granules, pellets, and fibers. Thewidespread usage of ACs as versatile adsorbents in comparison to zeolites,

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silica, and alumina is due to their relatively high resistance to heat and radia-tion, stability in acidic and basic solutions, good mechanical strength, and arecost effective from the viewpoint of regeneration. Silica on the other handis known to degrade in solution of high pH and zeolites usually are low inefficiency and high in cost with respect to regeneration. Another advantageenjoyed by ACs is that they can be tailored based on the requirements of itsdifferent applications. This is evident from the vast literature survey whichalso reveals that ACs have been used successfully for the removal of metalions, besides other pollutants such as phenols, detergents, pesticides, chlo-rinated hydrocarbons.47–53 The high adsorption capacity and faster kineticsas demonstrated by ACs for heavy metal ion removal from liquid phases isprimarily due to their high micropore volume, large specific surface area,and favorable pore size distribution.8 Despite the major success of AC ascommercial adsorbent in waste water treatment, its use is restricted due tothe high cost of its precursors such as coal, coconut shells, wood, peat, andpetroleum-based residues.54,55

3. AC FROM LOW-COST PRECURSORS

Using inexpensive raw materials with high amount of carbon and lowerlevels of inorganic compounds to produce low-cost AC has been the fo-cus of research efforts in the past years. Waste products from industrialand agricultural operations and biosorbents are among the widely used pre-cursors for the production of low-cost AC. The potential and feasibility ofsuch low-cost ACs for the removal of metal pollutants from waste water hasbeen documented in various review articles.56–74 Various precursors havebeen converted to ACs and assessed for their metal ion adsorption capac-ity by various researchers and documented by well known reviewers. Forexample, inexpensive and efficient biosorbents have been documented asreplacement strategy for existing conventional systems by Shukla et al.60

Crini61 demonstrated chitosan, sawdust, and polysaccharide-based materi-als as economic alternatives. Wan-Ngah et al.69 reviewed an extensive listof studies on chemically treated plant wastes as adsorbents including ricehusks, spent grain, sawdust, sugarcane bagasse, fruit wastes, and weeds andfound that the treated adsorbents showed good adsorption capacities for Cd,Cu, Pb, Zn, and Ni. A similar review was made on rice husk ash by Fooand Hameed.75 Farooq76 reviewed scattered information on the utilizationof wheat straw and bran for the removal/minimization of metal ions fromwaters. A vast array of agro waste biomass for the removal of heavy metalswas reviewed by Garcia-Reyes et al.77

Such reviews have well documented the replacement of waste by-products as inexpensive and effective metal ion adsorbents for exist-ing commercial AC materials. High adsorption capacity, faster kinetics,

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cost-effectiveness, and renewability are the important parameters makingthese materials as economical alternatives for metal removal and waste re-mediation.78,79

But, on closer examination, it can be seen that such review studieshave been restricted to a particular type of waste by-product for metal ionremediation from aqueous phase. Keeping in focus the huge success ofthe waste by-products from various precursors for recycling and reuse asadsorbent, the present work is firstly, an attempt to compile an up-to dateliterature and related results on their adsorption properties followed by acomparative study on their efficiency for heavy metal ion remediation aspresented in Table 3. Second, and more important, a critical in-depth studyis presented and discussed in detail in the forthcoming sections as to thevarious factors affecting the efficiency of such adsorbents; this being themajor objective of the present review.

4. FACTORS AFFECTING THE METAL ION ADSORPTIONCAPACITY OF AC

The huge success of the use of AC from various low-cost precursors forthe removal of heavy metal ions warrants a critical analysis on the variousfactors affecting its adsorption capacity as well as its cost effectiveness forlarge-scale use. Adsorption is basically controlled by the precursors fromwhich the adsorbent is developed, inherent characteristics of the adsorbents,the nature of adsorbates, and the background solution chemistry. A point-to-point analysis of such factors is discussed in the forthcoming sections, andthis is the objective of the present review. The criteria of various adsorbentsapplicable for waste water treatment are generally based on the adsorptioncapacity and kinetics.

4.1 Nature of the Precursor and Its Inherent Characteristics

Various sources ranging from industrial wastes, agricultural by-products, andbiomass have been successfully used to prepare AC for heavy metal ions.The efficiency of the low-cost ACs as adsorbents is revealed to some ex-tent by the maximum adsorption capacity of the adsorbent-adsorbate systemas derived from the various adsorption models and the data collected areshown in Table 3. Although it is difficult to compare the adsorption capacityof ACs produced from various sources due to the differences in the exper-imental conditions, still an increasing trend of high adsorption capacity isseen for agricultural and biomass generated ACs followed by their coun-terparts from industrial and municipal sources as seen from Figures 1–4.The low-cost ACs that stand out in terms of maximum adsorption capacity(in the range > 100 mg/g; in descending order) are rice husk for Cd(II)

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0

200

400

600

800

1000

1200

1400

1600

1800

2000

Fly

ash

-Cr(V

I)

M/f

ly a

sh-C

u(I

I)

Co

al

fly

ash

-Ni(

II)

BF

A-Z

n(I

I)

Red

Mu

d-P

b(I

I)

BF

/Sla

g-P

b(I

I)

A/S

lag-C

u(I

I)

A/S

lag-N

i(II

)

Sla

g-C

u(I

I)

BO

F S

lud

ge-P

b(I

I)

R/M

ill

Scale

-Pb

(II)

BF

/Slu

dg

e-P

b(I

I)

BF

/Slu

dg

e-P

b(I

I)

BF

/Du

st-P

b(I

I)

Fe/C

r h

yd

ro

xid

e-H

g

Fe/C

r h

yd

ro

xid

e-P

b(I

I)

Ca

rb

on

slu

rry

-Pb

(II)

Ca

rb

on

slu

rry

-Cr(V

I)

Ca

rb

on

slu

rry

-Pb

(II)

Ca

rb

on

slu

rry

-Cr(V

I)

Lea

ther

wa

ste-C

r(V

I)

iro

n/p

ro

tein

wast

e-C

r(V

I)

Bla

ck

liq

uor-C

r(I

II)

Bla

ck

liq

uor-P

b(I

I)

Bla

ck

liq

uo

r-Z

n(I

I)

ma

x a

ds.

Cap

acit

y (

mg/g

)

adsorbent-adsorbate system

inorganic based industrial wastes derived adsorbent-metal ion system

FIGURE 1. Comparative study of adsorption capacity of the various inorganic based pre-cursors derived from industrial wastes selected as low-cost adsorbents for toxic metal ionremoval from waste water.

020406080

100120140160180200

Dis

till

ery

Slu

dge-C

r(V

I)

Win

e s

lud

ge-C

r(I

II)

sew

ag

e s

lud

ge-C

d(I

I)

sew

ag

e s

lud

ge-N

i(II

)

iro

n o

xid

e/s

lud

ge-C

u(I

I)

iro

n o

xid

e/s

lud

ge-C

d(I

I)

iro

n o

xid

e/s

lud

ge-N

i(II

)

iro

n o

xid

e/s

lud

ge-P

b(I

I)

ch

em

ica

lly

acti

va

ted

ss-

Hg(I

I)

ch

em

ica

lly

acti

vate

d s

s-P

b(I

I)

ch

em

ica

lly

acti

vate

d s

s-C

u(I

I)

ch

em

icall

y a

cti

vate

d s

s-C

r(I

II)

ma

x a

ds.

Cap

acit

y (

mg/g

)


inorganic based municipal waste adsorbent-metal ion system

FIGURE 2. Comparative study of adsorption capacity of various the various inorganic basedprecursors derived from the municipal wastes selected as low-cost adsorbents for toxic metalion removal from waste water.

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50

150

250

350R

ice

hus

k (

acid

)-P

b

Ric

e h

usk

(ac

id)-

Cd

Ric

e h

usk

ash

-Pb

Ric

e h

usk

(ac

id)-

Cd

Wh

eat

stra

w(c

hem

)-C

r

Wh

eat

bra

n-P

b

Wh

eat

bra

n-P

b

Wh

eat

bran

-C

r

Wh

eat

bra

n-C

r(II

I)

Wh

eat

bran

-Cu

Wh

eat

bran

-Hg

Wh

eat

bran

-Cd

Pur

esor

be-

Cr(

VI)

coco

nut

she

ll (

chem

)-Z

n

coco

nut

she

ll(c

hem

)-Z

n

coir

pit

h-P

b

coir

pit

h-C

d

spen

t gr

een

tea

-Pb

spen

t bl

ack

tea-

Pb

tea

was

te-P

b

coff

ee(Z

nC

l2 m

od)-

Pb

Wal

nut

she

ll (

chem

)-H

g

Pom

egra

nat

e pe

el-N

i

Ban

ana

peel

-Cr

Man

go p

eel-

Cd

Man

go p

eel-

Pb

Ora

nge

pee

l-N

i

Ora

nge

pee

l (ch

em)-

Cu

Ora

nge

pee

l -C

u

Ora

nge

pee

l (ch

em)-

Cd

Sap

otac

eae

seed

(ch

em)-

Cr(

VI)

max

ads

. Cap

acit

y (m

g/g)


bio-based agricultural waste adsorbent-metal ion system

FIGURE 3. Comparative study of adsorption capacity of the various bio-based precursorsderived from agricultural wastes selected as low-cost adsorbents for toxic metal ion removalfrom waste water.

(2000 mg/g),130 black liquor for Pb(II) (1865 mg/g),117 carbon slurry forPb(II) (1618 mg/g),112 carbon slurry for Cr(VI) (1213.80 mg/g),112 carbonslurry for Pb(II) (1110.60 mg/g),111 carbon slurry for Cr(VI) (728.00 mg/g),111

Thiobacillus ferrodoxins for Pb(II) (443.00 mg/g),218 rice husk for Hg(II)(384.62 mg/g),137 Spirulina sp. (357.00 mg/g),205 wheat straw for Cr(III)(322.00 mg/g),142 wheat bran for Cr(VI) (310.00 mg/g),148 coconut coir pith(298 mg/g),174 Acacia bark for Ni(II) (294 mg/g),120 puresorb for Cd(II)(285.70 mg/g),158 Coir pith waste for Pb(II) (263 mg/g),160 Sargassum sp. forNi(II) (250 mg/g),210 blast furnace sludge (227 mg/g),107 scrap tyre for Hg (II)(211 mg/g),113 Sargassum for Ni(II) (181 mg/g),210 sewage sludge for Hg(II)(175.40 mg/g),128 orange peel for Ni(II) (158 mg/g),193 walnut shell for Hg(II)(151.50 mg/g),180 Oedogonium h. for Pb(II) (145.00 mg/g),204 blast furnacedust for Pb(II) (142 mg/g),107 Spirogyra for Pb(II) (140 mg/g),206 orange peelfor Cd(II) (136.05 mg/g),194 Spirogyra for Cu(II) (133.30 mg/g),208 leather forCr(VI) (133 mg/g),115 red mud for Zn(II) (133 mg/g),97 banana peel for Cr(VI)(131.56 mg/g),189 spent black tea for Pb(II) (129.9 mg/g),167 Fe(III)/Cr(III)hydroxide for Pb(II) (126.5 mg/g),109 rice husk for Cd(II) (125.94 mg/g),34

blast furnace slag for Pb(II) (125 mg/g),101 U. lactuca for Cr(VI)(112.36 mg/g),212 rice husk for Pb(II) (108 mg/g),129 and rice husk for Cd(II)(102 mg/g).133

On the basis of this observation, it is worthwhile considering the divisionof precursors as bio based precursors and inorganic based precursors. The

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0

100

200

300

400

500

Oed

og

on

ium

-Ni

Oed

ogon

ium

(c)-

Ni

Oed

ogon

ium

-C

r

Oed

ogon

ium

(c)-

Cr

Oed

og

on

ium

-Cd

Oed

og

on

ium

-Pb

Nost

oc-P

b

Sp

iru

lin

a-C

d

Sp

iro

gy

ra

-Pb

Sp

iro

gy

ra

-Cr

Sp

iro

gy

ra

-Cu

Nost

oc-C

r

Sargass

um

-Ni

Sargass

um

-Ni

Pse

ud

om

on

as-

Cd

Sp

hig

om

on

as-

Cu

Cy

an

ob

acte

riu

m-Z

n

Pse

ud

om

on

as-

Cu

Ch

rococcu

s-C

r

Th

iob

acil

lus-

Pbmax

. ads

orp�

on ca

paci

ty (m

g/g)


bio-based biomass adsorbent-metal ion system

FIGURE 4. Comparative study of adsorption capacity of the various bio-based precursorsderived from the biomasses selected as low-cost adsorbents for toxic metal ion removal fromwaste water.

bio based precursors for AC preparation include agricultural by-productsand biomass such as the bark, tannin rich materials, saw dust, rice hulls,coconut shell, fruit stones, peat moss, algae, chitosan, sea weeds, and resins.The inorganic precursors include the waste produced from industrial andmunicipal activities such as hydroxides, metal oxides, red mud, clays, blastfurnace slag, zeolites, soil, sediment, ore minerals, and sludge.

A critical analysis of such ACs vis a vis their role as metal ion adsorbentreveals that their chemical and physical characteristics have a contributoryrole.

4.1.1 INORGANIC PRECURSORS

Statistical analysis of data from Table 3reveals that among the industrial andmunicipal wastes, irrespective of metal ions, it was found that the fly ashhas demonstrated an average metal ion adsorption capacity of 17.41 mg/gin the range from 0.83 to 61.20 mg/g. Bagasse fly ash—a residue from sugarindustry—has demonstrated an average adsorption capacity of 3.92 mg/gfor metal ions such as lead, chromium, copper, zinc, and cadmium fromwastewater ranging from 1.7 to 13.21 mg/g. Red mud—an aluminum indus-try waste—exhibited promising adsorption capacity of 14.85 mg/g rangingfrom 0.67 to 133 mg/g for various metal ions from waste water and acid minedrainage. An average metal ion adsorption of 47.46 mg/g in the range from1.2 to 227.0 mg/g was demonstrated by ACs prepared from steel industry

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wastes. Fertilizer industry wastes such as the carbon slurry revealed high av-erage metal ion adsorption capacity of 692.0 mg/g with a range from 15.24 to1618.0 mg/g. Similarly, black liquor—a paper industry waste—showed highaverage adsorption capacity of 659.0 mg/g ranging from 17.97 to 1865.0 mg/gfor metal ion adsorption. Also, as revealed from Table 3, incidences of ACsprepared from inorganic based precursors showing adsorption capacities inthe range greater than 100 mg/g are 14 (17%) and those in the range from50 to 100 mg/g are 9 (11%). Out of these, seven of such incidences havetheir source from fertilizer and paper industry waste and five have theirsource from steel industry wastes. Thereby, ample conclusion can be de-rived that wastes from fertilizer and paper industry have great potential asmetal ion adsorbents from wastewaters followed by their counterpart fromsteel industry.

On closer examination of their inherent characteristics, it is revealed thatthere is a great dependence of the physical and chemical characteristics ofthe ACs on the adsorption capacity of the adsorbent from which they arederived. Study revealed that the carbon slurry is carbonaceous having highcarbon content.111,112 Appropriate heat treatment conditions during activationresulted in the development of a porous morphology and high surface areato the tune of 630 m2/g. Such characteristics favored its adsorptive removalof Cr(VI), Hg(II), Pb(II), Cu(II), and Mo(II) metal ions from metallurgicaland electroplating wastewaters. Black liquor, which reported an apprecia-ble adsorption capacity of 1865.0 mg/g for Pb(II), 95 mg/g for As(V) and17.97 mg/g for Cr(VI),117,118 was found to have a high carbon content of60.8% and the presence of phenolic, hydroxyl, carboxyl, benzyl alcohol,methoxyl, and aldehyde groups on lignin extracted from black liquor re-vealed its usefulness as an adsorbent material for removal of heavy metalsfrom wastewater.

Dimitrova227 reported that the highly alkaline nature of various steelindustry wastes such as blast furnace slag, sludge, and flue dust that is basi-cally due to the presence of calcium and silica created conditions for effectivemetal ion adsorption in a wide pH range. It was also reported that metal ionssorption took place mainly in the form of hydrooxocomplexes. Studies byMartın et al.106 revealed that blast furnace sludge reported an adsorptioncapacity of 79.87 mg/g for Pb(II) ions. They reported that the sludge was acomplex heterogeneous crystalline material composed mainly of hematite (α-Fe2O3) and coke with minor quantities of wustite (FeO), magnetite (Fe3O4),maghaemite (γ -Fe2O3), calcium ferrite (CaO.Fe2O3), quartz (SiO2), and cal-cium and aluminum silicates. The high carbon content of 34.05%, besidesthe presence of other elements such as Fe (33.00%), Si (3.65%), Ca (2.30%),Al (1.70%), Zn (1.20%) and Pb (0.75%), and S (1.15%) was responsible forthe high physical adsorption of Pb(II), Zn(II), and Cd(II) on the surface ofthe AC.227

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Closer looks at the internal characteristics of the other adsorbents vis–a vis their metal ion adsorption capacities has further demonstrated thisconclusion.

Literature reveals that although the primary components of fly ash (a coalcombustion residue) are silica (60–65%), alumina (25–30%), and magnetiteand Fe2O3 (6–15%), but it was the unburnt carbon residues (ranging from1% to 10%) that had an important role in the adsorption capacity.220,221

Other important physical characteristics such as bulk density, particle size,water holding capacity, and surface area have made fly ash suitable for useas adsorbent. In general, fly ash has a hydrophilic surface and a porousstructure. Studies revealed that virgin fly ash reported a lower adsorptioncapacity and its adsorption performance strongly depends on its origin andchemical treatment.83 The high aluminum and silica content of fly ash hasmade it a potential source for zeolite synthesis which in turn has tremendousapplication as adsorbent and ion exchanger for heavy metal remediation.This is evident from few researches in this field.220 It is also seen that theunburnt carbon when converted to AC has been found to enhance theadsorption capacity of fly ash when separated from its inorganic counterpart.

Studies have shown that the chemical composition of bagasse fly ashcomprised of 60.5% silica, 15.4% alumina 15.4%, 2.9% calcium oxide, 4.9%iron oxide, and 0.81% magnesium oxide; thereby confirming their inorganicnature. The density, porosity and surface area of this material were 1.01gcm−3, 0.36 fractions, and 450 m2/g respectively. The X-ray spectra of bagassefly-ash showed the presence of different minerals such as kaolinite, mullite,goethite, α-quartz, γ -alumina, and hematite and the scanning electron mi-croscope (SEM) studies indicated flocs and porous nature of the material.224

Red mud which is an abundantly available bauxite industry waste ishighly alkaline in nature and is principally composed of fine particles ofSiO2, Al2O3, Fe2O3, and TiO2 and these have been found to be responsiblefor its high surface reactivity.225 The surface electrical properties have beenfound to be dependent on these metal oxides.226 The surface charge of redmud as determined from the zero point of charge (pHpzc) was found to be8.3, which is close to the pHpzc of Fe2O3, Al2O3, and TiO2; this consequentlyshowed a strong uptake of protons.226 Wang et al.225 in their review showedthat it has a specific surface area (BET) of around 10–25 m2/g. Variousmineral phases are known to exist based on XRD studies ranging fromhematite (Fe2O3), goethite (a-FeOOH), boehmite (c-AlOOH), titania (TiO2),quartz (SiO2), sodalite (Na4Al3Si3O12Cl), or cancrinite-type sodium aluminumsilicate (CAN), and gypsum (Ca-SO4.2H2O), with a minor presence of calcite(CaCO3), whewellite (CaC2O4.H2O), and gibbsite (Al(OH)3). This reveals ahighly crystalline nature of red mud particles but besides this, it basicallyhas a porous surface morphology as concluded by Wang et al.225 In termsof particle size distribution, it is seen that red mud particles are very fine,with an average particle size < 10 μm. Such characteristics have been found

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to be favorable for red mud as a low-cost adsorbent which has exhibitedpromising adsorption capacity for various metal ions from waste water andacid mine drainage as revealed from Table 3.91–93

It can be inferred that the relatively poor performance of the bagasse flyash and red mud as heavy metal ion adsorbent may be due to their inorganicnature.

Scrap rubber tires are composed of a mixture of polymers of styrenebutadiene rubber, natural rubber and butadiene rubber plus other additivessuch as carbon black, sulfur and zinc oxide.229 They have been reportedto represent an enormous environmental disposal problem since the sameproperties that make them desirable as tires, most notably durability, alsomake their disposal and reprocessing difficult, they are almost immune tobiological degradation. Approximately 32% by weight of the waste tire230 ismainly constituted of carbon black in which the carbon content is as highas 70–75 wt%.231 This carbonaceous adsorbent is rather similar to AC andthe only apparent physical difference is that carbon black has a much lessinternal surface area.232 Appropriate treatment procedures such as physicalactivation of carbon black by heating in air, carbon dioxide, or steam atmo-sphere helped in developing its surface area and porosity.233–246 and henceimproved its adsorption behavior as is evident from high adsorption capac-ity of 211 mg/g for Hg(II) and 29.93 mg/g for Cr(VI) ions.113,114 Althoughless work has been undertaken to tap its potential, scrap tyres has positiveinherent characteristics as adsorbent for heavy metal ions.

4.1.2 BIO-BASED PRECURSORS

A statistical analysis revealed that among the bio-based precursors, thebiomass derived from algae, fungi, bacteria etc showed an average metalion adsorption capacity of 110.86 mg/g ranging from 14.7 to 443.0 mg/gwhereas those from agricultural by-products showed 83.34 mg/g averagemetal ion adsorption capacity with a range from 0.94 to 2000 mg/g. Irre-spective of the metal ions, the incidences of ACs reporting an adsorptioncapacity in the range greater than 100 mg/g are 21 (14%) and those in therange from 50 to 100 mg/g are 31 (21%). Of these, 15 are from the ACs frombiomass-based precursors.

The high efficiency for metal ion adsorption is mainly because agricul-tural by-products such as rice bran and husk, wheat bran and husk, saw dust,bark, groundnut shells, coconut shells, hazelnut shells, walnut shells, cottonseed hulls, waste tea leaves, maize corn cob, apple, banana, orange peels,soybean hulls, grapes stalks, water hyacinth, sugar beet pulp, sunflowerstalks, coffee beans, and cotton stalks have showed high carbon and lowash content and reasonable hardness. These have been identified to possesssome of the unique properties that have made them suitable for heavy metalion adsorption.247–249 Their high relative abundance and easy processing to

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develop AC is another criterion for their demand as low-cost precursor forsolving environmental problems. The main components of such low-cost ACprecursors such as rice husk and rice bran was found to be 32.24% cellulose,21.34% hemicellulose, 21.44% lignin, and 15.05% mineral ash.250 Rahmanet al.250 further demonstrated in their studies that the insoluble nature, goodchemical stability, high mechanical strength, and a granular structure weresome of the attributes that resulted in rice based ACs to be good metal ionadsorbent. The main components found in wheat based precursors were cel-lulose (37–39%), hemicellulose (30–35%), lignin (14%), and sugars as well asother compounds carrying different functional groups such as carboxyl, hy-droxyl, sulfhydryl, amide, and amine.251 Similar agro based precursors suchas tea, coffee, and shells and nuts of various fruits and seeds were found tohave cellulose, hemicellulose, and lignin252,253 and various adsorption chro-matographic studies have proved that cellulose is a proven adsorbent.254–257

The wheat based ACs showed good metal ion adsorption capacity and wasreviewed by Farooq et al.76 to be due to the presence of different functionalgroups, large amounts of cellulose and the porosity of their surface. The ex-istence of a variety of surface functional groups such as acetamido groups,carbonyl, phenolic, structural polysaccharides, amido, amino, sulfhydryl car-boxyl groups, alcohols and esters was proved by FTIR studies as collectedfrom various reference works on agro-based precursors such as moringaoleifera bark,119 rhizophoraapiculata tannin,121 rice husk ash,135 raw cof-fee powder,172 hazelnut shell,178 peanut hull pellet,183 and mango peel.191

These groups have been able to bind heavy metals through replacement ofhydrogen ions with metal ions in solution or by donation of an electronpair from these groups to form complexes with metal ions in solution. Themetal binding processes with such low-cost ACs involving chemisorption,complexation, ion exchange, chelation, and physical adsorption have beenreported.252,253

Metal biosorption by dead microbial biomass such as algae, bacteria,fungi, and yeast depends mainly on their cell wall components. Variousworkers have reported that the cell walls of algae irrespective of their typegenerally contain three components: cellulose, the structural support; alginicacid, a polymer of mannuronic and guluronic acids and the correspondingsalts of sodium, potassium, magnesium, and calcium; and sulfated polysac-charides. These compounds contain several functional groups (e.g., amino,carboxyl, sulfate, hydroxyl) that play an important role in the biosorptionprocess.258–267 The involvement of various surface functional groups such asthe carboxyl, carbonyl, sulfonated, thioester, amine, amide, and phospho-nate in metal binding have been established by various evidences in theform of FTIR, EDS, XPS, XAFS, and Raman spectroscopy.258–267 Importantchemical constituents of the bacterial surface are identified as peptidoglycan,teichoic acids and lipoteichoic acids. Various polysaccharides such as cellu-lose, chitin, alginate, and glycan have been found to be present on fungal

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and algal cell wall and these have been directly involved in metal binding.74

For certain biomasses, proteins have been involved in metal binding. Variousmetal binding mechanisms such as ion exchange, complexation, extracellularprecipitation reactions, intracellular accumulation, oxidation and reductionreactions, methylation and demethylation, and extracellular binding havebeen reported268–270 and these processes take advantage of the negativelycharged functional groups on cell walls of microorganisms to bind to thecationic metals. Hence biosorbent behavior for metallic ions is a function ofthe chemical make-up of the microbial cells of which it consists as verifiedby the works of Volesky and Holan.257

In addition it has been found that the biosorption technique involvingthe use of dead microbial cells is more advantageous and favored for watertreatment since the biosorbents are not affected by toxic waste, and alsodo not require a continuous supply of nutrients. Moreover they can beregenerated and reused for a number of cycles with no significant loss inefficiency.

4.2 Method of Preparation of Low Cost AC and ActivationConditions

The effect of surface porosity, pore size distribution, and surface chemistry ofAC has greatly affected its metal ion adsorption capacity and it is widely doc-umented. Such features depend on the method of preparation viz. physicalor chemical activation and the activation conditions of temperature, time etc.It is well known that the carbonization step eliminates the volatile compo-nents of hydrogen and oxygen to produce chars of desired porosity. It is theactivation of such chars at elevated temperatures in the presence of steam,carbon dioxide, air that refines the pore structure and produces the greatestpossible number of randomly distributed pores of various sizes and shapes,thereby giving rise to an extended and extremely high surface area of theAC.271 Mesopores and micropores are formed yielding surface areas up to2000 m2/g.272 Various research works have proved that low-cost precursors ifactivated under appropriate optimum conditions of activating agent, temper-ature and retention time can develop ACs of high surface area. Olive stonepowders after carbonization at 850◦C and steam activation at the same tem-perature resulted in AC having a surface area in the range of 600–680 m2/g.273

Carbonization of almond tree prunings at 800◦C for 1 h followed by CO2 ac-tivation resulted in ACs with a surface area up to 840 m2/g.274 Again, whenalmond tree prunings were exposed to a two-stage physical activation withsteam in a range of temperatures from 650–800◦C, the surface areas of theACs produced increased from 193 m2/g to 840 m2/g.274 These results showedthat increased activation temperature and prolonged retention times had abeneficial effect on the development of surface area. In order to determinethe effect of activating agent,275 walnut shell chars were activated at 850◦C

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in a mixture of CO2 and N2 for 5, 10, and 15 h. Pecan shell chars weresubjected to the same treatment at 800◦C. Nut shell chars were subjectedto steam activation at 800◦C for 12 h. Steam-produced ACs resulted in sig-nificantly higher surface areas than those activated with N2/CO2 mixture.In order to determine the effect of activation temperatures on the physico-chemical characteristics of the AC, Lua and Guo276 prepared AC from oilpalm stones by carbonization at 600◦C for 2 h followed by an activation stepwith CO2 in a range of temperatures between 500 and 900◦C for retentiontimes 10–60 min. An activation condition of 900◦C and 30 min retention timeresulted in an AC with highest surface area of 1366 m2/g, whereas an AC withlowest surface area of 356 m2/g was observed at an activation temperature of600◦C. Thus increasing the temperature increased the surface area. Retentiontime also played an important role. Steam activation of bagasse in the tem-perature range 750–840◦C and 2 h retention time produced ACs with surfaceareas ranging from 446 to 607 m2/g, respectively.277 But high surface area isnot always related to the high adsorption capacity of AC. Other parametersthat have been found to affect the adsorption capacity are the mesopore andmicropore volume. This is explained by citing the following example278: arubber tire was found to be a purely nonporous material. Its physical acti-vation by heating at 900◦C for 2 hr resulted in a mesoporous AC but havinga surface area comparatively lower than a purely microporous commercialAC. The adsorption studies revealed that it was the higher mesoporositybut lower surface area of the rubber tire AC and not the higher surfacearea of the commercial AC that resulted in higher Pb(II) and Ni(II) uptake.Hence surface area may not be a primary factor for metal ion adsorption onAC.278

In many instances, it has been seen that irrespective of the precur-sor, the chemically modified AC, has shown enhanced metal ion adsorp-tion as compared to its virgin or the original form where physical acti-vation is employed (Figure 5). An average metal ion adsorption capac-ity of 230.71, 148.09, 109.8, and 45.98 mg/g was reported by chemicallytreated ACs from industrial, agricultural, biomass, and municipal-based pre-cursors, respectively, as compared to their same counterparts in their vir-gin form and which have undergone a physical activation viz. 76.6, 46.39,107.02, and 14.36 mg/g. Such conclusion can be amply seen from Table3.83,85,94,95,127–129,132–134,136,138,142,157,162,165,174,176,180,181,185,200,201 The chemicalmodification of the surface can greatly alter the adsorption capacity as theadsorption process is a surface phenomenon where metal ions are adsorbedonto the surface of the adsorbent. Chemical activation involves a pretreat-ment scheme where the charred material obtained from carbonization ismixed with a chemical before the activation process. The chemicals or mod-ifying agents used for surface modification by various workers have beenidentified as basic solutions (sodium hydroxide, calcium hydroxide, sodiumcarbonate) mineral and organic acid solutions (hydrochloric acid, nitric acid,

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76.66

46.39

14.36

107.02

230.71

148.09

45.98

109.8

0

50

100

150

200

250

300

Inorganic Industrialwaste

Bio-based Agriculturalwaste

Inorganic Municipalwaste

Bio-based Biomass

ad

sorp

tion

cap

acit

y (

mg/g

) virgin undergoing physical activation

virgin chemically modified and activated

FIGURE 5. Comparative study of activation conditions on the adsorption capacity of variouswaste categories selected as low-cost adsorbents for toxic metal ion removal from wastewater.

sulfuric acid, tartaric acid, citric acid, thioglycolic acid), organic compounds(ethylenediamine, formaldehyde, epichlorohydrin, methanol), and oxidizingagent (hydrogen peroxide). The functional surface groups are found to becovalently and chemically bonded onto the surface of AC.

In a more typical example, Wong et al.129 synthesized various modifiedrice husks by various kinds of carboxylic acids (citric acid, salicylic acid,tartaric acid, oxalic acid, mandelic acid, maleic acid, and nitrilotriacetic acid)and carried out an adsorption study of copper and lead on such modifiedrice husk. They reported that the highest adsorption capacity was achievedby tartaric acid modified rice husk. Esterified tartaric acid modified rice huskhowever significantly reduced the uptake of Cu and Pb. The maximum ad-sorption capacities for Pb and Cu were reported as 108.0 and 29.0 mg/g,respectively. Ye et al.134 similarly tested natural and alkali modified rice huskto remove Cd(II) ions from water and the results showed the Cd(II) adsorp-tion capacity was 73.96 and 125.94 mg/g, respectively, for the natural andmodified rice husk. The modified rice husk had faster kinetics and higheradsorption capacities than the natural rice husk, which was attributed tothe surface structural changes of the material. In yet another example, theapplication of a strong dehydrating agent such as sulfuric acid was found tohave a significant effect on the surface area of the AC prepared from wheatbran, and resulted in better adsorption capacity for copper ions (51.5 mg/gat pH 5 and contact time 30 min).152 The authors explained that there was asignificant conversion of macropores to micropores resulting in higher sur-face area as a result of acid treatment. The extent of the effect of chemicalmodification on the adsorption capacity is seen irrespective of the nature of

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metal ions in the studies conducted by Sha et al.194 who proved that un-der identical conditions of pH and contact time, chemically modified orangepeel showed a higher adsorption capacity of 70.67 mg/g and 136.05 mg/gfor Cu(II) and Cd(II) removals than its unmodified counterpart. Such obser-vations of improved adsorption capacity are again seen in the case of rawand acid treated algal species of Oedogonium h. and Sargassum by studiesconducted by Gupta and Rastogi201–203 and Kalyani et al.210 Acid treated Oe-dogonium h. showed 44.20 mg/g Ni(II) removal under pH 5, temperature25◦C, and contact time of 80 min. Among the industrial wastes, acid treatedred mud showed a higher Pb(II) removal under same conditions of pH thanthe untreated red mud.95 In all such cases, it has been seen that the surfacegroups such as the carboxyl, lactones and phenols render the AC surfacepolar and hydrophilic thereby facilitating the adsorption of cationic speciesof metal ions. Although many of the modifiers have been known to enhanceadsorption capacity of ACs, but there have been exceptions too. Kumar andBandyopadhyay132 showed that hydrochloric acid treated rice husk showedlower adsorption capacity of cadmium than the untreated rice husk. Whenrice husk was treated with hydrochloric acid, surface functional groups onthe surface of rice husk were protonated, thereby repelling the heavy metalions in the aqueous phase rather than being adsorbed on the adsorbentsurface.

Yet again literature study reveals that chemical modification of ACcan occur by using macromolecules such as polymer chains. Suchmacromolecules are bound by physical methods (through impregnation)or chemical methods (via diverse grafting methods) onto the surfaceof AC.

The physical method of activation of AC from its precursors employingconventional heating methods has no doubt improved its porous propertiesand hence metal ion adsorption as discussed earlier, but it suffers from thedisadvantage of adding to the cost and time of its preparation. Researchershave used a microwave radiation method of activation of AC279 and theprocess has not only resulted in development of refined porous propertiesin AC but have shown other properties such as higher sintering temper-atures, selective heating, higher efficiency, shorter processing times and,therefore, higher energy savings.280 Maldhure and Ekhe281 made a compara-tive study of the microwave and conventional heating methods using usedblack liquor obtained from the Kraft pulping process as a precursor to pre-pare ACs for Cu(II) adsorption. The SBET and total pore volume of the ACprepared by microwave heating increased to 1164 m2/g and 0.6 cc/g, re-spectively, whereas its counterpart prepared by conventional heating had asurface area and pore volume of 921 m2/g and 0.5 cc/g, respectively. Be-sides this, AC prepared by microwave heating has clearly shown formationof more number of active surface functional groups than those obtainedby conventional thermal process. As a result, the ACs preparedly microwave

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treatment showed higher capacity for Cu(II) adsorption compared to ACs pre-pared by simple conventional heating method. Many more promising resultshave been thus obtained by for the preparation of relatively homogeneousinexpensive AC particles with high surface area and significant adsorptioncapacity.282–284

4.3 pH of Aqueous Media

In the adsorption of metal ions from aqueous solutions, it has been seen thatthe speciation of metal ions and hence the pH of the aqueous medium playa dominant role. When designing an adsorption process for optimum metalremoval from wastewater, the pH of the medium must be taken into consid-eration. This is because the pH of the aqueous medium affects the behaviorand chemical speciation of metal adsorbates. Similarly, Ni(II), Cd(II), Pb(II)and Zn(II) are known to exist as Ni2+, Cd2+, Pb2+, and Zn2+ at pH values of≤ 6.5, 8, 7, and 8, respectively. But above these pH values, such metal ionsexist as their hydroxides. A perusal of literature reveals that irrespective ofthe physical characteristics of the AC or its precursor source, the optimumpH at which maximum adsorption capacity is achieved depends ultimatelyon the nature of metal ion. For example, the optimum pH as determinedby various researchers for maximum Cd(II) uptake ranges from 3–7 for ACfrom industrial precursors such as coal fly ash,84 bagasse fly ash,89 red mud,96

acid treated red mud,97 blast furnace slag,102 and acid treated slag.103 The pHrange for Cd(II) uptake was observed to be from 6 to 7 for agricultural andbiomass precursors.123,130,132,135,136,138,162,170,172,187,202,211

While Cr(III) has shown an optimum uptake at a pH of 5,118,128,139 Cr(VII)was best adsorbed at acidic pH values of 1–5. Among inorganic based pre-cursors, maximum uptake for Cr(VI) was seen at pH 2 for fly ash,82 at pH 5for bagasse fly ash,90 at pH 2 for scrap tire,114 at pH 1 for leather industrywaste,115 and at pH 3 for distillery sludge.124 Similar acidic pH range wasobserved to be suitable for maximum Cr(VI) uptake in the case of bio-basedprecursors. pH 2 was observed as optimum for Cr(VI) uptake for wheatbran,147,148 at pH 2 for bael fruit shell,175 at pH 3 for groundnut husk,185

at pH 2 for Oedogonium h.,202 and at pH 2 for Chrococcus sp. bacteria.217

Cr(VI) is known to exist as H2CrO4 at a pH of 1, as HCrO4− as the pH

increases to 4 and almost exclusively as CrO42− at pH > 7. Thus, within the

pH range of 1–5, HCrO4− is the major species existing in solution and its sta-

bility depends on the pH and Cr(VI) concentration.71 Such negative speciesget easily adsorbed via electrostatic attraction onto the highly protonatedpositively charged AC surface.

The optimum pH values for Cu(II) uptake for various ACs irrespective oftheir precursor was observed to range from 4–8 with both Cu2+ and Cu(OH)2

being dominant species. As can be observed from Table 3, the optimum pHwas 4.5 and 8 for coal fly ash,81,84 6.2 for modified fly ash,83 4 for bagasse

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fly ash,87 5.5 for red mud,91,92 5 for activated slag,103 5.2 for acid treatedrice husk129, 4.5–6 for wheat waste,141,146,149,150,151–153 5 for tea waste,169 5 forchestnut shell,177 5.5 for peanut hull,182,183 5 for mango peel,192 7 for orangepeel,194 7 for Cicer seed,198 and 5 for Sphigomonassp bacteria.214

Similarly the optimum pH values for Ni(II) and Zn(II) uptake rangedfrom 5 to 7 and 4 to 8, respectively (Table 3). The reason behind thisphenomenon is that within the specified optimum pH ranges, the respectivemetal ions are in their divalent cationic form and their adsorption mainlyoccurs by electrostatic attraction to the negative active sites on the AC surfaceproduced by the ionization of the acidic groups.

The AC surface charge arising mainly from the carbon-oxygen func-tional groups thus can change with the pH of the aqueous medium asrevealed by electrokinetic studies thus interpreted from various research pa-pers.85,98,121,124,125,127,163 The surface of AC derived from fly ash had a positivecharge below pHzpc of 7 (zero point charge) and a negative charge abovepHzpc as reported by Bayat.85 The origin of the positive charge on the acti-vated carbon surface is attributed to the presence of basic surface groups,the excessive protonation of the surface at low pH values and to graphenelayers that act as Lewis bases resulting in the formation of acceptor-donorcomplexes important for the adsorption of many organic compounds fromaqueous solutions. At higher pH values, the carbon surface has a nega-tive charge, due to the ionization of acidic carbon-oxygen surface groups.Thus, at the optimum pH of 7.5, the adsorption of metal ions such as Zn(II) and Cd(II) mainly involved electrostatic attractive interactions betweenmetal ionic species (Zn2+, Zn(OH)+, Cd2+, or Cd(OH)+) in the solution andthe negative sites on the carbon surface.85 Selvaraj et al.124 found that theadsorption potential of distillery sludge for Cr(VI) removal exhibited its max-imum at pH 3. The observations were explained by the strong electrostaticattraction between the anionic chromium species (Cr2O7

2−, HCrO4−, andCrO4

2−) and a protonated adsorbent surface (pHpzc = 5.9). Similarly, in astudy by Genc-Fuhrman et al.285 while assessing the adsorption capacity ofAC derived from seawater neutralized red mud, it was observed that whenthe pH was below the pHpzc the solid AC surface was positively charged andfavored the adsorption of As(V) anions of H2AsO4

− and HAsO42− due to

the Columbic attraction, but when the pH was above the pHpzc the surfaceof the solids was negatively charged and anion adsorption competed withColumbic repulsion. The pHpzc of AC estimated was 6.9, which helped to ex-plain the observed decrease in As(V) adsorption above pH 7.0. In yet anotherexample of a study conducted by Oo et al.121on the adsorption of Cu(II) andPb(II) on mangrove tannins (MT), the adsorption of Cu (II) and Pb (II)on ACbased on MT were greatly influenced by the solution pH which determinedthe surface property of the AC and the speciation of metal ions and whichin turn affected the interaction between the adsorbate and the adsorbent. AtpH 5.0 and above, the removal of Cu (II) and Pb (II) reached 100%. More

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metalions were adsorbed when the pH of the solution was greater than thepHpzc of AC based on MT (4.09) as its negative charge was able to attractmore cations through the electrostatic interaction. The electrostatic attrac-tion mechanism of metal ion-AC adsorption system has been reported byvarious other researchers.98,124,125,127,163 The dispersive interactions betweenthe ionic species in the solution and the graphene layers of the carbon sur-face play a smaller role in the adsorption of inorganics. Some other workershave reported the adsorption of metal ions may occur by an ion exchangemechanism between the protons, alkaline or alkaline earth metals presenton the AC surface and the metal ion adsorbates. ACs from agricultural pre-cursors acts as normal ion exchangers as they contain weak acidic and basicfunctional groups in the form of alcohols and carboxylic acids. In the pHrange of 3–8 as observed from literature studies of agricultural precursors aslow-cost adsorbents, the weak acidic groups are dissociated thereby facili-tating in the binding of metal cations by ion exchange. Reddy et al in theirFTIR spectral analysis during their study on the adsorption of Pb(II) ontoAC based on Moringa oleifera bark (MOB), reported that lead ions werechelated to hydroxyl and carboxyl functional groups present on the surfaceof AC.119 The ion exchange mechanism of metal ion adsorption has beencorroborated by studies on wheat straw, Pinus radiata bark, and coirpith,which revealed the release of K+, Mg2+, and Ca2+.142,162,286 Batch adsorptionstudies showed that AC based on wheat straw modified with epichlorohy-drin and triethylamine had great Cr(VI) anion-adsorbing capacity, due to theexistence of a large number of introduced amino groups, and the value ofpHPZC was around 5.0. The mechanism was determined as ion exchange.142

In their study of assessing the adsorption capacity of coconut shell carbonfor removal of Cd(II), at the optimum pH value of 7, maximum adsorptioncapacity of 285.7 mg/g was attributed to ion exchange mechanism.158 Someresearchers have pointed to a complex formation of metal ions with the ACsurface functional groups.

4.4 Regeneration Capacity

As already stated and as established by various publications, AC from thelow-cost precursors is one of the best adsorbent for heavy metal ion re-mediation due to its various factors such as an extended surface area, highpore volume, well developed porous structure and modifiable surface func-tionality.287 However, its use is limited in that the metal ion pollutants aremerely transferred to the solid phase onto the AC after adsorption; thus, thespent AC is another potential hazardous waste. Hence, in order to make theprocess more economical and in order to comply with the environmentalregulations, the spent AC can be reused after an appropriate regeneration

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step. Literature survey reveals that currently, the most widely used regener-ation methods include thermal regeneration,288 solvent regeneration,289 andbiological regeneration.290

In thermal process, regeneration is conventionally accomplished viaelectrically heated or gas-fired rotary kilns or vertical tube furnaces, wherein,the spent AC is gasified at temperatures of up to 900◦C and steam or inertgas serve as transport medium for the desorbed components and reduce theconcentration of adsorbed pollutant.291 Thermal regeneration is expensive interms of energy consumption, is time consuming, and suffers from carbonloss due to the oxidation and attrition.292 Also, and after several successiveheating and cooling cycles, the thermal treatment will cause significant de-terioration of the carbon pore structure reducing the specific surface areaavailable for the adsorption process. In many cases, conventional thermalprocess is replaced by heating the spent AC electrically, with graphite ormetallic fabric inserted into the fixed bed, but this approach has limitationsin the sense that it causes hot spots in the fixed column.

Chemical regeneration involves desorption of adsorbed compounds us-ing specific solvents or by decomposition of adsorbed species using oxi-dizing chemical agents under either subcritical or supercritical conditions.293

The major drawback of this process is the regeneration efficiency dependsprimarily on the solubility of the adsorbed substances and moreover, an extrastep for recovery of extraction agent is usually involved after the desorptionprocess, which results in higher investment cost.

The biological regeneration is economical process, yet it usually re-quires long reaction time and can only be applied to the biodegradablesubstances.294 A number of new alternative methods, such as microwave re-generation.295–297 electrochemical regeneration,298,299 supercritical water oxi-dation regeneration,300 ultrasonic regeneration,301 and dielectric barrier dis-charge (DBD) plasma regeneration,302–305 have recently been developed forthe reutilization of exhausted AC.

Although various methods have been adopted for regeneration of thespent AC, yet it is worthwhile observing that the adsorbates desorbed areorganic compounds. A study of the literature data from Table 3reveals thatonly chemical regeneration method via the use of chemicals has been usedfor desorption of metal ions and simultaneous regeneration of the spentAC.90,105,110,134,148,163,198,201–204,209 This is true because it has been observed andproved beyond doubt that the adsorption of metal ions is highly dependenton the pH of the medium. Hence, desorption of metal ions will be facilitatedby controlling the pH. With the exception of Cr(VI), which gets adsorbed atan acidic medium, irrespective of the adsorbent source, a basic medium isrequired for the adsorption of the other metal ions. Thus an acidic desorbingmedium would facilitate desorption of metal ions such as Cd(II), Hg(II),Zn(II), Pb(II), Cu(II) etc, where the H+ ions would replace the metal ionson the metal loaded adsorbent. In the case of Cr(VI) desorption, a basic

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medium would maximize the removal from the spent AC. Based on thishypothesis, an ion exchange mechanism would be operative during themetal-adsorbent binding. This hypothesis is clearly proved by observingTable 4, which shows a summary of work carried out by various workers onthe desorption potential of metal loaded ACs from low-cost precursors. As isclearly evident from the table,

1. Desorption of metal ions is a pH dependent process.2. HCl and HNO3 are the most used desorbing medium for heavy metal ions

recovery, thereby highlighting with the exception of a few stray cases, thesuperiority of acids over bases as desorbing medium.

3. Metal loaded adsorbent act as cation exchanger and proves that ion ex-change mechanism is operative during the metal binding onto the adsor-bents.

4. The AC from the low-cost precursors has adequate stability for their reuseon a continuous scale for industrial waste water treatment.

5. The adsorption/desorption process not only has made the adsorptiontechnology more economical, but also have helped shed insight onto theunderlying mechanism.

5. FUTURE PROSPECTS

The evolution of adsorption has turned from an interesting alternative ap-proach into a powerful standard technique by offering a numbers of ad-vantages: better performance in terms of ulterior adsorption capacity, rateof adsorption, solving wastewaters pollution in a cost-effective way andovercoming part of the solid wastes problem around the world. Various ad-vantages and challenges have been identified and a widespread and greatprogress in this area can be expected in the future.

The pros and cons on the effect of various such factors on the adsorptioncapacity and cost efficiency of ACs converge into one conclusion: moreeffort is required to modify cellulose based precursors as efficient metal ionadsorbents. This is based on their high adsorption capacity as reflected fromthis study as compared to other precursors, their high relative abundance,low cost, and relative ease of chemical modification. Literature reveals thatvarious methods have been adopted for their modification such as directchemical modification or by grafting of suitable polymer chains followedby functionalization.307–310 Photochemical initiation, high energy initiation,and chemical initiation are some of the grafting techniques adopted whereasesterification, etherification, halogenation, and oxidation are the main routesof direct chemical modification. Various studies have already been carriedout in this direction.157,163,165 Details of the modification of cellulose based

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precursors for effective metal ion adsorbent preparation and their effects onthe adsorption capacity were outlined by O’Connell et al.306

The optimum pH for maximum metal ion adsorption irrespective of theprecursor is studied as 4–6, with the exception of Cr(VI). With respect tolarge-scale waste water treatment, this pH range is very narrow whereasmost of the commercial ion exchange resins are operative at a wider pHrange. Focus should be made in broadening the pH range for metal ionadsorption and also, enhancing the stability of ACs prepared from suchlow-cost precursors via crosslinking, introduction of various ligands, andstrengthening the metal-ligand interaction. Cost effectiveness of the modifiedcellulose adsorbents can be improved by enhancing their regeneration.

6. CONCLUSION

Thus, it is apparent that adsorption is an effective, efficient and univer-sal method of water treatment providing risk-free treated water as per theguidelines of the World Health Organization and the U.S. EnvironmentalProtection Agency. The ACs thus developed from low-cost waste/by prod-ucts are unique adsorbents that have great potential for the removal of metalions from waste water and from potable drinking waters. This is due to thefact that the activated carbons have a high surface area, a highly developedporous character, and a high degree of chemical reactivity of their surface.Furthermore, the surface of an activated carbon can be modified by theformation of different types of carbon-oxygen, carbon-sulfur, and carbon-nitrogen surface groups, by impregnation or by degassing to make themsuitable for the removal of inorganic metal pollutants. By comparison andstatistical analysis of the adsorbents used for metal remediation, based onthe performance, adsorption capabilities and cost, it was found that the mostimportant and feasible low-cost adsorbents are bio-based agricultural wastessuch as rice husk, wheat bran, coconut coir pith, barks, and algal and bac-terial biomass. The industrial wastes that stand out in terms of low cost andadsorption efficacy are the carbon slurry of fertilizer industry, black liquorof paper industry followed by steel industry wastes. Such wastes are freelyavailable and have no economic value but have served a dual role to oursociety as a whole. Due to their good adsorption capacities and requiring lit-tle pretreatment, they have been successfully used in waste water treatmentand have also solved the problem of solid waste management. Moreover,the management of the exhausted adsorbents is not a problem as they arebeing used in construction purposes.

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FUNDING

The authors are thankful to the Department of Science and Technology, NewDelhi, India, for financial support under Water Technology Initiative (ProjectNo. DST/TM/WTI/2K11/352).

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