Adsorbents and Their Applications in Water and Wastewater Treatment

Carbon Adsorbents

There are several types of carbon adsorbents used in the adsorption process, for example, activated carbon, activated carbon fibers, fullerene, etc. Activated carbon is the most widely used carbon adsorbent. It is a crude form of graphite with a random or amorphous highly porous structure with a broad range of pore sizes, from visible cracks and crevices to crevices of molecular dimensions.145! Generally, it is the carbon material derived mostly from charcoal. Besides coal, agricultural by-products are conventional sources of commercial activated carbon. Many types of agricultural wastes were proposed as raw materials for producing carbon adsorbents, including cork;146471 sucrose chars;1481 corncob;149-501 jackfruit peels;!51-521 wood;153-541 oil palm;1181 stones; fruit shells, coats, and husks;148,55-641 and wastes from cherries,165-661 plums,1671 coconut, apricot, almond, and nuts.166-681 Conditions in synthesizing activated carbon from these materials are provided in Table 2. The comparison of adsorption capacity of each type of activated carbon is shown in Table 3. Sources of the raw materials used and the preparation and treatment conditions such as pyrolysis temperature and activation time are major factors that have an effect on the adsorption capacity of activated carbon. Many other factors can also affect the adsorption capacity in the same sorption conditions such as surface chemistry (heteroatom content), surface charge, and pore structure.

The adsorbent properties of activated carbon depend on their composition, physicochemical properties, and mechanical strength. Activation by physical means, by chemical means, or by a combination of both has been employed to control the pore size and distribution of activated carbon and/or to increase porosity, surface modification, and improvement of carbonization.192-981 Normally, activated carbons are made up of small hydrophobic graphite layers with disordered, irregular, and heterogeneous surfaces bearing hydrophilic functional groups. The surface chemistry of activated carbon depends mainly upon the activation conditions and temperatures employed. The activated carbon has strong heterogeneous surfaces. Its geometrical heterogeneity is the result of differences in size and shape of pores, and cracks, pits, and steps. The chemical heterogeneity is involved with different functional groups, mainly oxygen groups that are located most frequently at the edges of the crystallites among with various surface impurities. These heterogeneity surfaces contribute to the unique sorption properties of activated carbon.112,99-1001 The presence of oxygen and hydrogen in surface groups affects strongly the adsorptive properties of the activated carbon. The apparent chemical character of an activated carbon surface is determined by functional groups and delocalized electrons of the graphitic structure.11011 Oxygen on an activated carbon surface may be present in various forms, such as carboxyls, aldehydes, ketones, phenols, lactones, quinines, hydroquinones, anhydrides, or ethereal structures. Some of the groups, e.g., carbonyl, carboxyl, phenolic hydroxyl, and lactonic ones, are acidic. Consequently, the pH value of the liquid bulk phase can have an effect on the acidic and/or basic functional groups of the carbon

TABLE 2 Conditions in Synthesizing Activated Carbon from Natural Materials

Natural Material

Conditions

Refs,

Algerian coal

930°C with KOH/NaOH

Alvim Ferraz1691

Almond and pecan shells

Chemical activation with H3P04/physical C02

Tancredi et al.1701

Almond shell, olive stones, and peach stones

Heating in C02 at 606°C

Ferro-Garcia et al.1711

Bituminous coal

N2/400-700°C with ZnCl,

Hall and Holmes1721

Coal or coconut shell

Phosgene or chlorine gas at 180°C

Otowa et al.1731

Coconut shell

Parts by weight H2S04 for 24 h at 150°C

Manju et al.1741

Coconut shell

450°C with H3P04

Laine et al.1751

Coconut shells and coconut shell fibers

Carbonized with H2S04 and activated at 600°C for 1 hr

Mohan et al.,1761 Mohan et al.,1771 Mohan et al.,1781 and Mohan et al.1791

Eucalyptus wood chars

C02 activation, 400-800°C

Kumar and Sivanesan1801

Fertilizer slurry

450°C, 1 hr with H20,/H,0, N,

Marungrueng and Pavasant|S11

Fly ash

Froth flotation, hydrophobic char was separated from hydrophilic ash with the help of methyl isobutyl ketone

Basava Rao and Mohan Rao1821

Lignite

Inert atmosphere/600-800°C with Na2Mo04/ NaW04/ NH4VOj/(NH4),Mo04/FeCV Fe(N03)3

HI Qada et al.1831

Oat hulls

Fast pyrolysis at 500°C with inert nitrogen

Tamai et al.1841

Palm tree cobs

730°C, 6 hr with H,P04/H2S04

Banat et al.1851

Petroleum coke

700-850°C, 4 hr with КОН/НЧ)

McKay et al.1861

KOH dehydration at 400°C followed by activation at 500-900°C

McKay et al.1871

Pine sawdust

850°C, 1 hr; 825°C, 6 hr with Fe(N03),/C0,

Kannan and Sundaramiss|

Raffination earth

10% (v/v), 350°C with H,S04

Bestani et al.1891

Solvent-extracted olive pulp and olive stones

Under vacuum and atmospheric pressure; 60°C/min; 800°C; activation under N2 at 10°C/min with K2C03

Stavropoulos and Zabaniotou'9"1

TABLE 3 Adsorption Capacity of Each Type of Activated Carbon

Adsorbent

Adsorption Capacity (mg/g)

Refs.

Activated carbon

  • 400
  • 238
  • 9.81

Kumar and Sivanesan1801 Marungrueng and Pavasant1811 Basava Rao et al.1821

Activated carbon produced from New Zealand coal

588

El Qada et al.|8J|

Activated carbon produced from Venezuelan bituminous coal

380

El Qada et al.|8J|

Bituminous coal

176

Tamai et al.1841

Charcoal

62.7

Banat et al.1851

Coal

  • 323.68
  • 230

McKay et al.1861 McKay et al.1871

Commercial activated carbon

  • 980.3
  • 200

Kannan and Sundaram1881 Bestani et al.1891

Peat

324

Fernandes et al.1911

surface. Thus, the surface charge of carbon is a function of pH of the solution. Considering the point of zero charge (PZC) and the isoelectric point (IEP), the surface is positively charged at pH < pH,,zc and negatively charged at pH > pHPZC. In practice, pH№P is usually close to pHPZC, but it is lower than pHPZC for activated carbon.11021 For pH < pKa adsorption of non-ionized organics does not depend on the surface charge of activated carbon. However, for pH > pKa, the adsorption of its ionic form depends on the surface charge. As a result, the activated carbon possesses perfect adsorption ability for relatively low-molecular-weight organic compounds from drinking water and wastewater streams.

Several methods have been used to removal organic pollutants from water. However, the use of activated carbons is perhaps the best broad-spectrum technology available at present.111 Accordingly, the use of activated carbons in water treatment has increased tremendously. Generally, the three main physical carbon types are granular, powder, and extruded (pellet). The granular activated carbon (GAC) adsorption, the most widely used type, is an effective treatment technology for organic contaminant removal from drinking water to improve taste and odor. The use of GAC for treatment of municipal and industrial wastewaters has developed rapidly in the last three decades from small size for household units to large scale for industrial wastewater application. Moving beds, downflow fixed beds, and upflow expanded beds have been widely used for water purification for industry.

It is well known that activated carbon can remove several types of pollutants including metal ions,1102'1041 phenols,146,1051 pesticides,11061 chlorinated hydrocarbons11071 detergents,11081 and many other chemicals and organisms. Application of activated carbon in removal of various heavy metals and organic contaminants with the Langmuir and Freundlich capacities is shown in Table 4.

Clay Minerals

Clay minerals are hydrous aluminosilicates composed of minerals that make up the colloid fraction (<2 pm) of soils, sediments, rocks, and water11171 and may be composed of mixtures of fine-grained clay minerals and clay-sized crystals of other minerals such as quartz, carbonate, and metal oxides. Their structures are similar to micas with the formation of flat hexagonal sheets. Clay minerals and oxides are widespread and abundant in aquatic and terrestrial environments.

Clay contains various types of exchangeable ions on its surface. The prominent ions found on the clay surface are СаЧ, Mg2-!-, H+, K+, NH4+, Na+, S042', CL, P045“, and N03“. These ions can be exchanged

TABLE 4 Adsorption Capacities of Activated Carbon for Heavy Metal and Organic Contaminant Removal from Water and Wastewater

Pollutant

Activated Carbon

Adsorption Capacity (mg/g)

Isotherm

Refs.

Cr(VI)

Commercial activated carbon

4.7

Langmuir

Babel and Kurniawan11091

Commercial activated carbon oxidized with H2S04

8.9

Langmuir

Babel and Kurniawan11091

Commercial activated carbon oxidized with hnoj

10.4

Langmuir

Babel and Kurniawan11091

Fe(III)

Granular activated carbon

0.1

Freundlich

Kim11101

Ni(II)

Granular activated carbon

6.5

Langmuir

Satapathy and Natarajan11111

Modified activated carbon

7.0

Langmuir

Satapathy and Natarajan11111

Catechol

Activated charcoal

320

Langmuir

Richard et al.|m|

Gallic

acid Activated charcoal

408-488

Langmuir

Figaro et al.|m|

Tannin

Activated charcoal

0.39

Langmuir

Mohan and Karthikeyan11141

Vanillin

Activated charcoal

93.18-121.72

Langmuir

Michailof et al.1631

Phenol

Rice husk activated carbon

27.58

Langmuir

Kalderis et al.11151

Nonylphenol

Activated charcoal

83.1

Langmuir

Lang et al.11161

with other ions easily without affecting the structure of the clay mineral.11181 Clay can adsorb the cationic, anionic, and neutral metal species. They act as a natural scavenger of pollutants by taking up cations and/or anions through either ion exchange or adsorption, or both.

Currently, several types of clay minerals such as mont-morillonite and kaolinite are widely used in the water purification process. Because of their low cost, abundance in most continents of the world, high sorption properties, and potential for ion exchange, clay materials are strong adsorbents. Montmorillonite is a clay mineral with substantial isomorphic substitution. It is composed of units made up of two silica tetrahedral sheets with a central alumina octahedral sheet. The theoretical composition without the interlayer material is Si02,66.7%; A1,03,28.3%; and H,0,5%. There is substitution of Si4 + by Al3+ in the tetrahedral layer and of Al3+ by Mg2+ in the octahedral layer. Exchangeable cations in the 2:1 layers balance the negative charges generated by isomorphic substitution. The uptake kinetics of cation exchange is fast, and the cations such as Na+ and Ca2+ form outer-sphere surface complexes, which are easily exchanged with solute ions by varying the cationic composition of the solution.

Kaolinite is the least reactive clay. It has the theoretical composition of Si02, 46.54%; A1203, 39.50%; and H20,13.96%, expressed in terms of the oxides. It has a small net negative charge, which is responsible for the surface not being completely inert. Its high pH dependency enhances or inhibits the adsorption of metals according to the pH of the environment.11191 The metal adsorption is usually accompanied by the release of hydrogen (H‘) ions from the edge sites of the mineral. The substitution of H* ions for metal ions could influence the van der Waals force within the kaolinite structure.

Their applications are mainly found in dye and heavy metal removal. From previous research, it was reported that the sorption capacity of clay minerals can vary strongly with pH. Gupta and Bhattacharyya1120-1211 used kaolinite and montmorillonite along with their poly(oxo zirconium) and tet- rabutylammonium derivatives for Cd(II) removal from water. The adsorption of Cd(II) was influenced by pH of the aqueous medium, and the amount adsorbed increased with gradually decreasing acidity. By increasing the solution pH from 1.0 to 10.0, the extent of adsorption increased from 4.3% to 29.5% for kaolinite and 74.7% to 94.5% for montmorillonite. In dye removal, Bagane and Guiza11221 reported an adsorption capacity of 300 mg/g and suggested that clay is a good adsorbent for methylene blue removal due to its high surface area. Almeida et al.11231 studied the removal of methylene blue from synthetic wastewater by using montmorillonite and described it as an efficient adsorbent where the equilibrium was attained in less than 30 min. The adsorption of dyes on kaolinite was also studied by Ghosh and Bhattacharyya,11241 who reported that its adsorption capacity can be improved by purification and by treatment with NaOH solution.

The adsorption capacities vary from metal to metal and also depend on the type of clay used.11181 When a comparison is made with other low-cost adsorbents, the clays have been found to be either better or equivalent in adsorption capacity. Type of pollutant and adsorption capacity of each clay mineral are summarized in Table 5.

Natural Zeolites

Zeolites are highly porous aluminosilicates with different cavity structures. They consist of a three- dimensional framework, having a negatively charged lattice. A well-defined pore structure in the microporous range of zeolite can accommodate a wide variety of cations such as Na+, K+, Ca2+, Mg2+, and others. These charge-compensating cations are free to migrate in and out of zeolite structures, and they are rather loosely held so that they can readily be exchanged for others in a contact solution. Accordingly, zeolites are not only good adsorbates but also good ion exchangers. This property can be used to introduce different cations into the structure, creating selective sites for adsorption purposes or catalysis. Their narrow pore size and tuneable affinity for certain molecules make them ideal adsorbents for selective purification to encapsulate hazardous compounds. Zeolites are characterized not only by a high selectivity separation mechanism but also by the ability to separate substances based on differences in sizes and shapes of molecules’ steric separation mechanism.

TABLE 5 Adsorption Capacities of Clay Minerals for Heavy Metal Removal from Water and Wastewater

Pollutant

Clay Mineral

Langmuir

Capacity

Freundlich

Capacity

Refs.

Cd(II)

Kaolinite

9.9

0.5

Gupta and Bhattacharyya[125]

Montmorillonite

32.7

8.6

Gupta and Bhattacharyya[125]

Ni(II)

Acid-activated

montmorillonite

29.5

6.0

Bhattacharyya and Gupta[126]

Kaolinite

10.4

1.1

Gupta and Bhattacharyya[127]

Montmorillonite

28.4

4.5

Gupta and Bhattacharyya[127]

Cr(Vl)

Kaolinite

11.6

-

Bhattacharyya and Gupta[128]

Acid-activated kaolinite

13.9

-

Bhattacharyya and Gupta[128]

Co(II)

Raw kaolinite

11.5

-

Yavuz et al.[129]

Kaolinite

11.2

1.1

Bhattacharyya and Gupta[130]

Acid-activated kaolinite

12.1

1.5

Bhattacharyya and Gupta[130]

Montmorillonite

28.6

4.6

Bhattacharyya and Gupta[130]

Acid-activated

montmorillonite

29.7

6.0

Bhattacharyya and Gupta[130]

Pb(II)

Kaolinite

11.2

0.7

Gupta and Bhattacharyya[131] and Bhattacharyya and Gupta[132]

Acid-activated kaolinite

12.1

1.0

Gupta and Bhattacharyya[131] and Bhattacharyya and Gupta[132]

Montmorillonite

33.0

8.9

Gupta and Bhattacharyya[131] and Bhattacharyya and Gupta[132]

Acid-activated

montmorillonite

34.0

11.3

Gupta and Bhattacharyya[131] and Bhattacharyya and Gupta[132]

Fe(III)

Kaolinite

11.2

1.3

Bhattacharyya and Gupta[133]

Acid-activated kaolinite

12.1

1.7

Bhattacharyya and Gupta[133]

Montmorillonite

28.9

5.2

Bhattacharyya and Gupta[133]

Acid-activated

montmorillonite

30.0

6.4

Bhattacharyya and Gupta[133]

Cu(II)

Kaolinite

4.4

1.1

Bhattacharyya and Gupta[134]

Acid-activated kaolinite

5.6

1.3

Bhattacharyya and Gupta[126]

Montmorillonite

25.5

9.2

Bhattacharyya and Gupta[134]

Acid-activated

montmorillonite

28.0

12.4

Bhattacharyya and Gupta[126]

Note: Units of Langmuir capacity and Freundlich capacity are mg/g and mg'lto Ll,n/g, respectively.

Zeolites have been widely used for pollution control due to their ion exchange and adsorption properties. They have been used for the selective separation of cations from aqueous solution. The diffusion, adsorption, and ion exchange in zeolites have been extensively reviewed in many previous works.1135-1371 Kesraoui-Ouki, Cheeseman, and Perry11381 reviewed natural zeolite utilization in metal effluent treatment applications. Dewatered zeolites produce channels that can adsorb molecules small enough to access the internal cavities while excluding larger species. Zeolites, modified by ion exchange, can be used for adsorption of different metal ions according to requirements and costs. The characteristics and applications of zeolites have been extensively reviewed by Ghobarkar, Schaf, and Guth.11391 High ion- exchange capacity and relatively high specific surface areas, and more importantly, their relatively cheap prices, make zeolites more attractive adsorbents.

Besides zeolite, other siliceous materials such as perlite and glass have been proposed for contaminant removal. The use of natural siliceous adsorbents such as silica, glass fibers, and perlite for wastewater is increasing because of their high abundance, easy availability, and low cost. The other commonly applied inorganic sorbents are silica gels, activated alumina, and oxide and hydroxide metals. Perlite is another siliceous material that exhibits a good adsorbent for decontamination purposes. It has been used as a low-cost adsorbent for the removal of methylene blue.11401411 Methylene blue is physically adsorbed onto the perlite. However, perlites of different types (expanded and unexpanded) and of different origins have different properties because of the differences in composition. Chakrabarti and Dutta11421 also investigated glass fiber for the adsorption of methylene blue. They stated that a considerable amount of the dye is adsorbed on soft glass even at ambient temperature. Accordingly, several siliceous materials become widely used as adsorbate materials in the adsorption process.

Currently, a new family of mesopore materials, so-called MCM materials or Mobil Composition of Matter (MCM), was developed by Mobil Oil Corporation, which proposed a revolutionary synthesis method to obtain such materials that comprise strictly uniform pores. An organic surfactant like an alkyltrimethylammonium bromide in an aqueous medium forms rod-like micelles, which are used as templates to form two or three monolayers of silica or alumina particles encapsulating the micelles’ external surface. By removing the organic species from a well-ordered organic-inorganic condensed phase, a porous silicate or alumina material with uniformly porous structure remains. The mesopore size can be controlled by the molecular size template of the surfactant. Nowadays, MCM materials have been widely used in heavy metal removal, and they are currently the adsorption material that plays an important role in water and wastewater treatment.

Chitin and Chitosan

Chitin is a nontoxic, biodegradable polymer of high molecular weight. It contains 2-acetamido-2-deoxy- P-D-glucose through a p (1^4) linkage. Chitin is the most abundant natural fiber next to the cellulose and is similar to cellulose in many respects. The most abundant source of chitin is the shell of crab and shrimp. Chitin has presented exceptional chemical and biological qualities that can be used in water and wastewater purification through the adsorption process.

Chitin and chitosan have their chemical structures in common. Chitin is made up of a linear chain of acetyl-glucosamine groups. Chitosan is obtained by removing enough acetyl groups (CH3-CO) for the molecule to be soluble in most diluted acids. This process, called deacetylation, releases amine groups (NH) and gives the chitosan a cationic characteristic. Chitosan contains 2-acetamido-2-deoxy-P-D- glucopyranose and 2-amino-2-deoxy-P-D-glucopyranose residues. Chitosan is known as an ideal natural support for enzyme immobilization because of its special characteristics such as hydrophilicity, biocompatibility, biodegradability, non-toxicity, adsorption properties, etc.11431

Chitosan has drawn particular attention as an effective biosorbent due to its high content of amino and hydroxyl functional groups, giving it high adsorption potential for various aquatic pollutants.1143-1471 This biopolymer represents an attractive alternative to other biomaterials because of its physicochemical characteristics, chemical stability, high reactivity, excellent chelation behavior, and high selectivity toward pollutants. Chitin and chitosan derivatives have been extensively investigated as adsorbents for the removal of organic molecules and metal ions from water and wastewater. The high adsorption potential of chitosan can be attributed to the following: 1) high hydrophilicity due to a large number of hydroxyl groups of glucose units; 2) presence of a large number of functional groups; 3) high chemical reactivity of these groups; and 4) flexible structure of the polymer chain.1148,1491

To enhance the adsorption capacity for pollutant removal, chitosan has been modified by several methods, either physical or chemical processes. Different shapes of chitosan, e.g., membranes, microspheres, gel beads, and films, have been synthesized and tested for their performance in pollutant removal from water and wastewater.1143-1471 A cross-linked chitosan bead is one type of chemical modification for chitosan to increase the uptake capacity in the adsorption process.11501 This method using the chemical reaction of ethylenediamine and carbodiimide in modifying chitosan provided a high uptake capacity for mercury (Hg2t) ions, which is considered to be one of the highest uptake capacities among various biosorbents.

Beads of 1 and 3 mm diameter were prepared as one type of modified chitosan.11511 The gelled chitosan beads were cross-linked with glutaraldehyde and then freeze-dried. Beads of 1mm diameter possessed surface areas exceeding 150m2/g and mean pore sizes of 560 A and were insoluble in acid media at pH 2. A new composite chitosan biosorbent was also prepared by coating chitosan onto perlite ore. It was used in the removal of Cu(II) and Ni(II) from aqueous solution.11521 The magnetic chitosan nanocomposites were synthesized on the basis of amine-functionalized magnetite nanoparticles.11531 These nanocomposites provide a very efficient, fast, and convenient tool for removing Pb2*, Cu2+, and Cd2+ from water. It was suggested that synthesized magnetic chitosan nanocomposites can be used as a recyclable tool for heavy metal ion removal. Several types of heavy metals and organic contaminants removed by chitosan are shown in Table 6.

Agricultural-Based Waste Materials

Agricultural by products usually are composed of lignin and cellulose as major constituents that have the ability to some extent to bind some type of pollutants, for example, heavy metals, by donation of an electron pair from these groups to from complexes with the metal ions.|16S| Currently, many types of agricultural-based waste materials play a significant role in the adsorption process. They are normally organic materials from plants, trees, crops, and algae. Two larger carbohydrate that play a significant role in the adsorption process are cellulose and hemicelluloses (holocellulose.) Cellulose is a remarkable pure organic polymer, consisting solely of units of anhydroglucose held together in a giant straight- chain molecule.11681 These anhydroglucose units are bound together byp-(l,4)-glycosidic linkages. Hemicelluloses consist of different monosaccharide units. The polymer chains of hemicelluloses have

TABLE 6 Adsorption Capacities of Chitosan and Its Composite for Removal of Heavy Metals and Some Organic Contaminants from Water and Wastewater

Pollutant

Chitosan

Adsorption Capacity (mg/g)

Isotherm

Refs.

Hg(II)

Chitosan/cotton fibers

104.31

Langmuir

Qu et al.[154]

Cd(II)

Chitosan/cotton fibers

15.74

Langmuir

Zhang et al. [155]

Cr(VI)

Magnetic chitosan

69.40

Langmuir

Huang et al.[156]

Chitosan/cellulose

13.05

Langmuir

Sun et al. [157]

Chitosan/perlite

153.8

Langmuir

Shameem et al.[158]

Chitosan/ceramic alumina

153.8

Freundlich

Veera et al. [159]

Pb(ll)

Chitosan/cotton fibers

101.53

Freundlich

Zhang et al. [155]

Chitosan/magnetite

63.33

Langmuir

Tran et al.[160]

Chitosan/cellulose

26.31

Langmuir

Sun et al. [157]

Chitosan/sand

12.32

Langmuir

Rorrer et al.[151]

Cu(III)

Chitosan/cellulose

26.50

Langmuir

Sun et al. [157]

Chitosan/perlite

196.07

Langmuir

Kalyani et al. [152]

Chitosan/polyvinylchloride

87.9

Langmuir

Srinivasa et al.[162]

Ni(ll)

Chitosan/magnetite

52.55

Langmuir

Tran et al.[160]

Chitosan/cellulose

13.21

Langmuir

Sun et al.[157]

Chitosan/perlite

114.94

Langmuir

Kalyani et al. [152]

Chitosan/silica

254.3

Langmuir

Vijaya et al.[163]

Phenol

Chemically modified chitosan

2.22-151.50

Langmuir

Li et al.[164]

Chitosan/calcium alginate beads

108.69

Langmuir

Nadavala et al. [165]

4-Chlorophenol

Chemically modified chitosan

2.58-179.73

Langmuir

Li et al.[164]

Nonylphenol

Chitosan

56.3

Langmuir

Langet al.[116]

short branches and are amorphous. Hemicelluloses are derived mainly from chains of pentose sugars and act as the cement material holding together the cellulose micelles and fiber.I1681 Hemicelluloses are partially soluble in water. Currently, chemical modification is widely used to alter the biochemical component of the biomaterials to obtain higher efficiency in pollutant removal by biosorption pro- cess.I1691 Biomass chemical modifications include delignification, esterification of carboxyl and phosphate groups, methylation of amino groups, and hydrolysis of carboxylate groups. Sawamiappan and Krishnamoorthy1170! replaced phenol-formaldehyde cationic matrices with sulfonated bagasse. Odozi et al.l1711 polymerized corncob, sawdust, and onion. However, the disadvantages of chemical modification are a high expense to pay and unwanted problems, such as bleeding of excessive quantities of colored organic compounds, odor, and further pollution through the use of toxic chemicals. Several types of agricultural wastes have been used in the adsorption process, with the differences in adsorption capacity as shown in Table 7.

Mechanisms involved in the biosorption process include chemical adsorption, complexation, adsorption-complexation on surfaces and in pores, ion exchange, microprecipitation, heavy metal hydroxide condensation onto the biosurface, and surface adsorption.1172-1741 In the adsorption process, functional groups are responsible for pollutant binding on the surface of biomaterial. Most of the functional groups involved in the binding process are found in cell walls. Plant cell walls are generally considered as structures built by cellulose molecules, organized in microfibrils and surrounded by hemicellulosic materials (xylans, mannans, glucomannans, galactans, arabogalactans), lignin, and

TABLE 7 Adsorption Capacities of Agricultural Waste for Heavy Metal And Organic Contaminant Removal from Water and Wastewater

Pollutant

Agriculture Waste

Adsorption Capacity (mg/g)

Isotherm

Refs.

Cd(II)

Juniper fiber

9,2

Langmuir

Min et al.[181]

Base-treated juniper fiber

29,5

Langmuir

Min et al.[181]

Cr(Vl)

Cactus

7,1

Langmuir

Dakikyet al. [182]

Coconut shell carbon

2,2

Langmuir

Babel and Kurniawan[109]

Coconut shell carbon oxidized with H2S04

4.1

Langmuir

Babel and Kurniawan[109]

Coconut shell carbon oxidized with HN03

10.9

Langmuir

Babel and Kurniawan[109]

Sawdust

15.8

Langmuir

Dakikyet al. [182]

Pb(II)

Carbonaceous adsorbent

25.0

Langmuir

Bhatnagar et al.[183]

Sawdust

22.2

Langmuir,

Freundlich

Taty-Costodes et al.[184]

Fe(III)

Maize cobs

2.5

Langmuir

Nassar et al.[185]

Cu(II)

Tree fern

7.6

Langmuir

Ho et al.[186]

Ni(II)

Peat

28.3

Langmuir,

Freundlich

Chen et al.[187]

Phenol

Banana pith

49.9-129.4

Langmuir

Sathishkumar et al.[188]

Banana peel

688.9

Langmuir

Achak et al. [189]

Corn grain

256

Langmuir

Park et al. [190]

2-Nitrophenol

Lessonia itigrescens

71.28

Langmuir

Navarro et al.[191]

Macrocystis integrifolia

97.37

Langmuir

Navarro et al.[191]

2,4-

Pomegranate peel

65.7

Langmuir

Bhatnagar and Minocha[192]

Dicholorophenol

Nonylphenol

Rhizopus arrhizus

4.5-43.7

Langmuir

Lang et al.[116]

pectin along with small amounts of protein.11751 During biosorption, water is able to permeate the noncrystalline portion of cellulose and all of the hemicellulose and lignin. The aqueous solution comes into contact with a very large surface area of different cell wall components. The disordered structure of amorphous cellulose allows easier access to reagents than highly structured crystalline cellulose. While water penetrates through the cell wall components, water adsorption of fibers causes swelling. The bigger the amount of water adsorption, the bigger the swelling. Swelling also depends on the fiber’s structure, on the degree of crystallinity, and on the amorphous and void regions.11761 Swelling occurs when polar solvents such as water and alcohols come into contact with wood.11771 These polar solvent molecules are attracted to the dry solid matrix and held by hydrogen bonding forces between the -OH or -COOH groups in the wood structure and cause the biosorption of pollutants in aqueous solution. Many research works1178-1801 have reported the wide use of biosorption process in heavy metal removal. Thus, the agricultural-based waste materials become the adsorption material that plays an important role in water and wastewater treatment nowadays (Table 7).

 
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