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Advancement of Chitosan-Based Adsorbents for Enhanced and Selective Adsorption Performance in Water/Wastewater

Treatment-Review

ABSTRACT This paper gives an overview of the results obtained by various researchers in the treatment of various suspensions and solutions by using chitosan. Adsorption techniques are widely used to remove certain classes of pollutants from water and wastewater. Besides conventional adsorbents there are so many alternative non-conventional sorbents have been investigated. It is well-known that natural materials, waste materials from industry and agriculture and biosorbents can be obtained and employed as inexpensive sorbents. In this review, Chitosan, a partially deacetylated polymer obtained from the alkaline deacetylation of chitin, a biopolymer extracted from shellfish sources has been reviewed for its application in water and wastewater pollution as an adsorbent. Chitosan exhibits a variety of physicochemical and biological properties resulting in numerous applications in fields such as cosmetics, biomedical engineering, pharmaceuticals, ophthalmology, biotechnology, agriculture, textiles, oenology, food processing and nutrition. Application of chitinous products in wastewater treatment has received considerable attention in recent years in the literature. This review highlights some of the notable examples in the use of chitosan and its grafted and cross linked derivatives for removal of metal ions, dyes from aqueous solutions. The review provides a summary of recent information obtained using batch studies and deals with the various adsorption mechanisms involved. The effects of parameters such as the chitosan characteristics, the process variables, the chemistry and the solution conditions used in batch studies on the biosorption capacity are presented and discussed. The review also summarizes and attempts to compare the equilibrium and kinetic modeling. It is attempted to identify the gaps in the use of Chitosan as an adsorbent and to indicate future directions useful for research. Key Words: Chitosan, Chitin, Adsorption, Sorbents, Batch Process, Modeling, 1.0 INTRODUCTION: Pollution is the most serious ecological crisis to which we are subjected today. The main causes include the rapid urban-industrial technology revolution, hurriedly exploitation of natural resources by man, population explosion etc. Today the environment has become foul, contaminated, undesirable, and therefore harmful for the health of living organisms, including man. The fabulous plentifulness of nature is a heritage that should never be spoiled. But the unlimited voracious exploitation of nature by man has disturbed the delicate ecological balance existing between living and non-living components on the

planet earth. The root cause of environmental pollution has been the man’s misbehavior with the nature under the false ego of being the master of nature. This undesirable situation created by man has threatened the survival of man himself and other living biota on the earth. 1.1 Water pollution: The earth is the blue planet. Water is a vital natural resource, which is essential for variety of purposes. It is an essential constituent of all animal and vegetable matters. It is also an essential ingredient of animal and plant life. Its

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uses include drinking and other domestic uses, industrial cooling, power generation, agriculture, transportation and also waste disposal. At the present state of national development, agricultural productivity in India is profoundly dependent on rainfall. Droughts due to less rain in various parts of our country during the last decade, have given a series of jolts to the growth of our economy. Growing population, accelerating pace of industrialization and intensification of agriculture and urbanization exert heavy pressure on our water resources. With the increase in the age of the earth, clean water is becoming more precious because water is being polluted by several manmade activities, e.g. rapid population growth, alarming speed of industrialization and deforestation, urbanizations, increasing living standards and wide spheres of other human activities. Ground water, surface water and water from river, sea, lakes, ponds etc one getting polluted very rapidly. The term water pollution refers to anything causing change in the diversity of aquatic life. The presence of too much of undesirable foreign substance in water is responsible for water pollution. Water pollution is one of the most serious problems faced by man today. Since water is the vital concern for mankind and essential for erudition, animal and aquatic life. It is the universal enabling chemical which is capable of dissolving or carrying in suspension of a variety of a toxic materials from mainly heavy flux of sewage, industrial effluents, domestic and agricultural waste. Plant nutrients, pesticides, insecticides, herbicides and fertilizers plants and animal debris are reported to cause heavy pollution to water sources. Nowadays fertilizers containing phosphates and nitrates are added to soil, some of these are washed off through rainfall, irrigation and drainage into water bodies’ thereby severely disturbing aquatic system. Organic wastes increase the BOD of the receiving water body. Some pesticides, which are non-biodegradable, when sprayed, remain in the soil for longer time and then are carried in water bodies during rainfall. That is why it is of special interest to study the water pollution. 1.2 Sources of Water Pollution: Sources of contamination of water can be classified as follows: i) Sewage and Domestic Wastes. ii) Industrial Effluents iii) Agricultural Discharges

iv) Pesticides and Fertilizers v) Soap and Detergents vi) Thermal Pollution etc. Heavy metals concentration in above sources found to be more; consequently it is essential to study in lengths. 1.3 Pollution by Heavy Metals: Pollution by heavy metals is a serious threat to aquatic ecosystems because some of these metals are potentially toxic, even at very low concentrations. Additionally, heavy metals are not biodegradable and tend to accumulate in living organisms, and they can cause severe problems to both human health and wildlife (Crini, 2005). Natural processes and human activities have polluted and reduced the quality of water resources all over the world. Groundwater and superficial water have been contaminated in various ways; e.g. by mining wastes including cyanide and toxic heavy metals, by agricultural chemicals, by industrial and domestic sewage that is sometimes discharged without treatment into waterways and by natural trace elements (e.g. arsenic). Since it is important to eliminate or reduce the concentration of heavy metals in the aquatic ecosystems, various methods and technologies are commonly applied in the treatment of mining and refining industry effluents before they are discharged to receiving water. These methods include precipitation, ion exchange, and membrane processes. However, the application of some of these methods may be impractical due to economic constraints or may be insufficient to meet strict regulatory requirements. Furthermore, they may generate hazardous products or products which are difficult to treat (Gavrilescu, 2004; Reddad et al., 2002). The traditional coagulation and flocculation processes use inorganic coagulants such as aluminium hydroxides in drinking water treatment. Metal coagulants can be used to partially remove heavy metal from wastewater ,but the use of metal coagulants is not fully effective for removing metal cations from water at pH 7 (Bell and Saunders, 2005). The high cost of adsorbents such as activated carbon and some ion-exchange resins used for the treatment of water and wastewater have found to be more effective and cheaper adsorbents. Bailey et al., (1999) mentioned that natural materials that are available in large quantities or industrial waste products can also be used as adsorbents. Chitin and its deacetylated form, chitosan, are two biopolymers that come from crustacean

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shells and have the ability to fix a great variety of heavy metals (Muzzarelli, 1977). The relatively high proportion of nitrogen sites explains the strong affinity of metal ions for these sorbents. The use of these biopolymers can be a low cost alternative for the removal of contaminants from industrial effluents and from the natural water supply and is hence chosen for detailed study in this paper. 2.0 CHITOSAN AS AN ADSORBENT: The metal sorption capacity of chitosan varies with crystallinity, affinity for water, deacetylation degree and amino group content. Kinetic studies have demonstrated that the rate of metallic ion sorption onto chitosan differs depending on the raw material (shrimp, crab or lobster shells), preparation method, chemical modification, and chitosan particle shape. Wu (2000) evaluated the sorption capacities and rates for Cu (II) onto flakes and bead forms of chitosan prepared from fishery wastes. They found that the bead type of chitosan exhibited a greater sorption rate than the flake type. On the other hand, Ngah Wan et al., (2004) performed kinetic studies of Cu (II) sorption on chitosan beads and chitosan/PVA beads, and reported that the pseudo-second-order rate constant of chitosan beads was higher than the rate constant of chitosan/PVA beads. The sorption process also depends on the physicochemical characteristics of the aqueous solutions, such as pH, temperature, metallic ion concentration and the form of the main species in the solution (Guibal, 2004). For example, it is observed that the uptake of As (V) is greater than As (III) onto chitosan (Boddu et al., 2007), and that it depends greatly on the pH and redox conditions and on the temperature (Gerente et al., 2005) .The design of a chitosan filter for the removal of metallic ions from contaminated effluents requires equilibrium and kinetic data for the system. Numerous studies have demonstrated that chitosan possesses a great sorption capacity and favorable kinetics for most metals. Simplified kinetic models have been used to determine the sorption mechanisms and potential rate-controlling steps, such as external and intra-particle mass transfer as well as adsorption of copper and zinc onto chitosan. These models have incorporated the pseudo first-order, pseudo-second-order, and intra-particle diffusion equations.

2.1 Chitin and Chitin: Chitin and chitosan are nitrogenous polysaccharides that are made up of acetylglucosamine and glucosamine units. In fact, these two polymers have exactly the same basic chemical structure: (1→4)-2-acetamido-2-deoxy-β-D-glucan and(1→4)-2-amino-2-deoxy-β-D-glucan , respectively. The difference between them is the deacetylation degree (DD) and their respective solubility in dilute acidic media. Sorlier et al., (2001) showed that chitosan is the only derivative to be soluble at a DD above 40%. Chitosan is a biodegradable, nontoxic extract from shellfish shells used in a variety of water purification applications. Chitosan is derived from chitin (pronounced ky-tin), nature’s second most abundant biopolymer and primary constituent of shellfish shells, insect exoskeletons, and fungi cell walls. Chitosan is so safe that it is used in commercial aquariums to clarify the water in the aquarium exhibits and it is also used to clarify public pools and spas. In addition to its safety record, the secret of its success is the way in which it interacts with sediment particles. Chitosan creates a fibrous web linking the sediment particles together in a three-dimensional matrix. When this matrix enters a sand filter it is caught in the sand but allows the water through, but other polymers and coagulants form a gelatinous floc that rapidly clogs filters. Chitosan allows extremely long filtration cycles at sediment loading rates that are well above industry standards. Chitin occurs in nature as ordered crystalline micro fibrils forming structural components in the exoskeleton of arthropods (Rinaudo, 2006), its major source being the seafood crustacean (crab, shrimp, prawn, and lobster shells) that is usually disposed as waste material. Depending on its source, three different crystalline polymorphic forms of chitin have been identified: α-chitin (shrimp and crab shells), β-chitin (squid pen), and γ-chitin (stomach cuticles of cephalopoda) (Jang et al., 2004) Chitin is a white, hard, inelastic, and inert solid. It is highly hydrophobic and is insoluble in water and most other organic solvents. It is soluble in hexafluoroisopropanol, hexafluoroacetone, and chloroalcohols in conjugation with aqueous solutions of mineral acids (Ravi Kumar, 2000). When the deacetylation degree of chitin drops to about 50%, it becomes soluble in aqueous acidic media and is called

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chitosan (Rinaudo, 2006). The production of chitin involves chemical or enzymatic deproteinisztion and acid treatment to dissolve calcium carbonate. In addition, discoloration is

often to remove residual pigments. Chitosan is produced at an industrial level by chemical deacetylation of chitin using sodium hydroxide (see Figure ‘1’)

Figure (1): Scheme of chemical deacetylation of chitin to produce chitosan but chitosan can also be produced by enzymatic deacetylation of chitin using lysozyme, snailase, neutral protease, and chitin deacetylase (Cai et al., 2006). Due to the free amino groups in chitosan, this polymer chelates five to six times’ greatest amounts of metals than chitin (Bailey et al., 1999). Muzzarrelli (1977) found that chitosan is selective with regard to the sorption of metal ions. It does not take up alkali and alkali earth metal ions but it collects transition and post-transition metal ions from the aqueous solution. These sorption properties have been used for environmental purposes (uptake of heavy metals), separation processes (recovery

of valuable metals), and analytical purposes. However, chitosan has not found any practical applications on an industrial scale because of the cost of material, the variability in the characteristics of the material, and the availability of the resource (Guibal, 2004). 2.2 Characteristics of Chitosan: Chitosan is a semi crystalline polymer in the solid state (Rinaudo, 2006). Chitosan has been shown to be biologically renewable, biodegradable, biocompatible, non-antigenic, non-toxic and biofunctional (Malafaya et al., 2007). The characteristics of seafresh chitosan are mentioned in Table -1.

Table-1: Characteristics of Chitosan

Item Characteristic

1.Appearance 2.particle size 3.ash content 4.moisture content 5.deacetylation degree(%DAC) 6.Solution(1% in 1% acetic acid) Insoluble Viscosity 7.Heavy metals Total Plate count Yeast and Mould E.coli Salmonelia

Yellowish Mesh no.60 0.83% 8.5% 86% 0.59% 152cps 0 ppm 50 20 Nil Nil

Source: Seafresh Chitosan (Lab), (2000). The main parameters for its characterization are the deacetylation degree (DD), the crystallinity and the polymer molecular weight (Guibal, 2004). These parameters may affect its conformation in solution, and its physico-

chemical and biological properties (Sorlier et al., 2001). The deacetylation degree controls the fraction of free amino groups that will be available for interactions with metals ions. Infrared spectroscopy and NMR analysis are

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the most common methods to evaluate the deacetylation degree. Infrared spectra of chitosan are usually obtained with a frequency range of 4000–400 per cm and the degree of deacetylation (DD) is given by equation (Domszy and Roberts, 1985), In contrast, Guibal (2004) asserted that rather than the deacelylation degree it is better to consider the total number of free amino groups accessible to metal uptake, since some amino groups can be involved in hydrogen bonds. This can be controlled by the residual crystallinity, which can be influenced by the experimental preparation procedure and the origin of the raw material. Methods used to decrease the crystallinity involve the dissolution of chitosan (in acid solution) followed by a coagulation process and direct freeze-drying of the polymer solution. The crystallinity of the polymer can be measured by X-ray diffraction. A simple method to determine the chitosan molecular weigh is viscometry (Ravi Kumar, 2000). The polymer molecular weigh may be found by applying the Mark-Houwink equation: [η] = KMα, where [η] is the intrinsic viscosity and M is the molecular weight. α and K are experimental values which can be determined in several solvents. 2.3 Modified Chitosan Chitosan can be modified by chemical or physical processes in order to control the reactivity of the polymer or enhance the sorption kinetics depending on the field of application (Guibal, 2004). Two basic types, namely, physical and chemical are discussed below: 2.3.1 Physical Modification Various techniques have been used to physically modify chitosan, obtaining conditioned polymer forms such as powders, nano particles, and gels (beads, membranes, sponge, honeycomb, fibers or hollow fibers). Several studies have demonstrated that the particle size plays an important role in the uptake of metallic ions. Ng et al. (2003) found that the uptake of Pb ions depends on the inverse of the particle sizes. Chitosan nano particles (between 40 and 110 nm) produced by ionic chelation of sorbent were used to remove lead from aqueous solution. A decrease in the particle size of the chitosan improved the adsorption capacity for lead ions. In addition, a decrease in the crystallinity of sorbent was observed (Qi and Xu, 2004).

Guibal (2004) mentioned that, due to the resistance of intra-particle mass transfer, the use of small particles is necessary. However, these can be inappropriate for a column system because they can lead to hydrodynamic limitations. To improve diffusion properties and hydrodynamic behaviour, chitosan gel beads can be used. However, the adsorption capacity for metallic ions usually decreases. Chen and Chung (2006) reported a low uptake for As(III) and As(V) using gel beads compared with other studies using chitosan. Spherical gel beads of different sizes and porosities, membranes, and fibers can be prepared using neutralization methods. Gel beads are obtained by adding an acetic acid chitosan solution drop wise to a 1M NaOH solution with a micro syringe (Krajewska, 2005). Chitosan membranes can also be prepared from chitosan solutions in acetic acid. The solution is poured into a Petri dish and, after the solvent has evaporated, the membrane is neutralized with sodium hydroxide (Guibal, 2004). Furthermore, porous three-dimensional sponges can be prepared by freeze-drying, where chitosan solutions or gels are frozen followed by lyophilisation. The porosity and morphology of the material produced depends on the chitosan molecular weight and on the composition and concentration of the starting solution, and most importantly on the freezing temperature and freezing rate (Krajewska, 2005). 2.3.2 Chemical Modification: Chemical modification of chitosan has two main aims: (i) to improve the metal adsorption properties, and (ii) to change the solubility properties of chitosan in water or acidic medium. These include substitution reactions, chain elongation (cross-linking, graft copolymerization, and polymer networks), and depolymerisation (chemical, physical, and enzymatic) (Harish Prashanth and Tharanathan, 2007).The insertion of functional groups in the chitosan may involve the –NH2 group at the C-2 position (specific reactions) or –OH groups at the C-3 and C-6 positions (non specific reactions) (Rinaudo, 2006). In fact, the preparation of water-soluble chitosan derivatives was carried out by simple N, O–acetylation using AcCl and Ac2O in MeSO3H, where the degree of substitution of the NHAc group was 0.15–0.29 and that of the OAc group was around 1.0 (Hitoshi et al.,

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2002).The modification of chitosan to produce Schiff bases may improve its capacity to interact with metallic ions. These Schiff bases can be obtained via a reaction with aldehydes and ketones (Muzzarelli et al., 1985), with aromatic aldehydes in acetic acid (Tirkistani, 1998), and with salicylaldehyde and five derivatives: 5–brome, 5–chlorine, 5–nitro, 5–methyl, and 5–methoxy (Dos Santos et al., 2005). Other chitosan derivatives produced by substitution reactions are O– and N– carboximethyl chitosan, an amphoteric polymer; chitosan 6–O–sulphate, an anticoagulant; N–methylene phosphonic chitosan, an anionic derivative; and trimethyl chitosan ammonium, a cationic derivative (Rinaudo, 2006). An important chitosan derivative for the effective uptake of As (III) and As (V) from aqueous solution is molybdate-impregnated chitosan beads (Dambies et al., 2002). This derivative can be prepared by the molybdate adsorption and coagulation methods. The possibility of extending the uptake of metallic ions from acidic medium has motivated the production of cross linked chitosan. The cross-linking method improves the acidic stability of chitosan. However, this process may cause a decrease in the adsorption capacity of the sorbent, especially in the case of chemical reactions involving amino groups (Guibal, 2004). These derivatives can be obtained by reaction of chitosan with different di/polyfunctional reagents such as tripolyphosphate (Lee et al., 2001), formaldehyde (Desai, 2005), gluteraldehyde (Jeon and Höll, 2003), ethylene glycol diglycidyl ether (Li and Bai, 2006), hexamethylene diisocyanate (Arrascue et al., 2003) or cross-linked N-carboximethyl chitosan (Muzzarelli et al., 1989).Chitosan

derivates have also been obtained by grafting new functional groups: (a) to increase the density of sorption sites, (b) to change the pH range for metal sorption, (c) to change the sorption sites with the purpose of increasing sorption selectivity for the target metal (Guibal, 2004). The feasibility of grafting poly (methyl acrylate) and poly [1-(methoxycarbonyl) ethylene] onto chitosan, poly-β(1← 4)-2-amino-2-deoxy-d-glucose, has also been investigated. The grafting reaction was carried out in aqueous solution using ferrous ammonium sulphate (FAS) in combination with H2O2 as redox initiator. The grafted chitosan was found to be insoluble in solvents, which normally dissolve chitosan. The results showed that the graft copolymer was thermally more stable than pure chitosan (Yazdani-Pedram et al., 1995). Additionally, the grafting of phosphate or phosphonic groups onto chitosan promoted the uptake of some alkaline and alkaline-earth metals (e.g. calcium and sodium) (Guibal, 2004). The degradation products of chitosan, such as low molecular weight chitosan (LMWC), chitooligosaccharides (COS), and monomers, were found useful for biomedical applications. Chemical, physical or enzymatic depolymerisation has been used to produce these products (Harish Prashanth and Tharanathan, 2007). 2.4 Applications of Chitosan: Chitosan has a wide range of applications which depend on its physical, chemical, and biological properties. The principal areas are agriculture, drinking water and wastewater treatment, food and beverages, cosmetics and toiletries, biomedics and pharmaceutics, fibers and textiles, and paper technology (Rinaudo, 2006; Ravi Kumar, 2000). A summary of the main applications are presented in Table-2.

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Table- 2: Applications of Chitosan

Area Application Reference Agricultural Coating of fertilizers, pesticides, herbicides,

nematocides and insecticides for their controlled release to soil. Coating of seeds and leaves to prevent microbial infections

Krajewska (2005)

Biomedical and Pharmaceutical

The tissue engineering and drug delivery fields, ranging from skin, bone, cartilage, and vascular graft. For the lowering of serum cholesterol, its application in enzyme and cell immobilizations, as material for the production of contact lenses or eye bandages.

Bodnar et al. (2007); Malafaya et al.(2007);

Synowiecki (1986); Sandford (1989).

Cosmetic Component of toothpaste, hand and body creams, shampoos, cosmetics and toiletries.

Synowiecki (1986)

Environmental In drinking water and wastewater treatment. Chitosan is used as flocculating and chelating agents, for the removal of heavy metals and dyes, as an ecological polymer (eliminates synthetic polymers and reduces odours).

Gerente et al. (2007); Guibal (2004);

Rinaudo (2006); Wan et al. (2004); Wu et al. (2000).

Food Industry Clarification of juices, production of biodegradable packaging films, antimicrobial agents, beverage clarification additives, flavour extenders and colour and texture stabilizers.

Krajewska (2005); Mayer et al. (1989).

Pulp and Paper Industry

As a carbonless copy paper, as a processing additive for surface treatment applications, and for incorporation into photographic papers.

Gerente et al. (2007)

Textile Industry Inclusion of chitosan into mixtures, blends, and coatings, of other textiles such as silk, wool, viscose, cotton, and others are based on chitosan’s properties to repel water.

Gerente et al. (2007)

3.0 ADSORPTION: Adsorption techniques are widely used to remove certain classes of pollutants from waters, especially those that are not easily biodegradable. Adsorption is commonly defined as the concentration of a substance at an interface or surface. In other words Adsorption is a process where one or more components (adsorbates) are attracted and bonded to the surface of a solid (adsorbent) with which they are in contact. The process can occur at an interface between any two phases, such as, liquid-liquid, gas-liquid, gas-solid, or liquid-solid interfaces. The interface of interest in water and wastewater treatment is the liquid-solid interface. The exact nature of the bonding (ionic, covalent, or metallic) depends on the properties of the species involved, but the adsorbed material is generally classified as exhibiting

physisorption, chemisorption or electrostatic sorption. 3.1 Adsorption Equilibrium: Adsorption is usually described by isotherms which show how much solute can be adsorbed by the adsorbent at a given temperature. An adsorption isotherm relates the concentration of solute on the surface of the adsorbent to the concentration of the solute in the fluid with which the adsorbent is in contact. These values are usually determined experimentally, but there are also models to predict them, both for single metal adsorption and multi component adsorption. Although the Langmuir and the Freundlich adsorption isotherms are the two well established types of adsorption isotherms for single metal adsorption, there are other equations which model adsorption equilibrium.

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Table -3: The four main classes of chitosan derivatives A) Modified polymers: 1) Carboxymethyl chitosans 2) Alkylated chitosans 3) Chitosan sulfate derivatives 4) Carbohydrate-branched chitosans 5) Grafted chitosans 6)Ligand-bound chitosan B) Cross linked Chitosan: 1)Covalently cross linked particles 2)Ionically cross linked particles 3)Nano particles 4)Physical gels C) Chitosan-based composites: 1) Chitosan-dendrimer hybrids 2) Chitosan-supported on inert materials (silica gel, glass beads, alumina, etc.) D) Membranes: Chitosan Membranes from gels as a filter

Source: Harish Prashanth, K. V. and R. N. Tharanathan (2007).

Table- 4: Some methods for preparation of chitosan particles A) Cross linking with chemicals: 1) (Single) emulsion cross linking 2) multiple emulsion 3) Precipitation/cross linking

B) Cross linking and interactions with charged ions, molecules and Polymers: 1) Ionotropic gelation 2) Wet-phase inversion 3) Emulsification and ionotropic gelation 4) Emulsification and solvent evaporation 5) Simple or complex coacervation (precipitation, complexation C) Miscellaneous methods: 1)Thermal cross linking 2) Solvent evaporation method 3) Neutralization method 4) Spray drying 5) Freeze drying 6) Reverse micellar 7) Coating 8) Interfacial acylation Source: Harish Prashanth, K. V. and R. N. Tharanathan (2007). Table -3 gives four main classes of chitosan derivatives and Table-4 outlines various methods and approaches which have been proposed for the preparation of chitosan particles including microspheres/micro particles, and nano particles. Selection of any of the methods depends upon factors such as particle size requirement, thermal and chemical stability. In practice, the methods are often combined and different follow-up treatments are carried out [Krajewska B, 2005]. The emulsion cross linking method is widely used for the synthesis of micro spheres. With this method, the size of the particles can be controlled by modifying the size of the aqueous droplets. Another interesting method is spray drying. This is a complex operation

with the movement of countless droplets/particles in turbulent drying medium flows under changing temperature and humidity. 3.2 Adsorption Mechanisms: The first major challenge for the adsorption field is to select the most promising types of adsorbent, mainly in terms of efficiency and low cost. The next real challenge is to clearly identify the adsorption mechanism(s), in particular the interactions occurring at the adsorbent/adsorbate interface. Two mechanisms are clearly established for the interpretation of metal adsorption on chitosan materials, i.e. electrostatic interactions in acid media (ion exchange) and metal chelation

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(coordination), although the formation of ion pairs has also been reported [ Guibal E., 2004, Varma & Deshpande, 2002, Ravi Kumar MNV.,2000, No & Meyers, 2000]. Metal ion adsorption is assumed to occur through single or mixed mechanisms including coordination on amino groups in a pendant fashion or in combination with vicinal hydroxyl groups, and ion-exchange with protonated amino groups through proton exchange or anion exchange, the counter ion being exchanged with the metal anion. The nature of the reaction depends upon several parameters related to the adsorbent (ionic charge), to the solution (pH) and the chemistry of the metal ion (ionic charge, ability to be hydrolyzed and to form polynuclear species). For more details on these mechanisms, two recent reviews can be consulted [Guibal E., 2004, Varma & Deshpande, 2002]. 4.0. M ODELING: Equilibrium isotherm models, adsorption properties and equilibrium data, commonly known as adsorption isotherms, illustrate how pollutants interact with adsorbent materials and so, are important in optimizing the use of adsorbents. In order to optimize the design of an adsorption system to remove any impurity from solutions, it is important to establish the most appropriate correlation for the equilibrium curve. An accurate mathematical description of equilibrium adsorption capacity is obligatory for reliable prediction of adsorption parameters and quantitative comparison of adsorption behavior for different adsorbent systems within any given system. The distribution of impurity molecule between the liquid phase and the biosorbent is a measure of the position of equilibrium in the adsorption process and can generally be expressed by one or more of a series of isotherm models [Tien C., 1994, Langmuir I., 1906, Freundlich HMF., 1906, Giles CH, Smith D, Huitson A, 1974, Giles CH, D’Silva AP, Easton IA.,1974]. The shape of an isotherm may be considered with a view to predicting if a sorption system is ‘‘favorable’’ or ‘‘unfavorable’’. The isotherm shape can also provide qualitative information on the nature of the solute–surface interaction. In addition, adsorption isotherms have been developed to evaluate the capacity of chitosan

materials for the adsorption of a particular impurity molecule. The most popular classification of adsorption isotherms of solutes from aqueous solutions has been proposed by Giles et al. [Giles CH, Smith D, Huitson A,1974, Giles CH, D’Silva AP, Easton IA.,1974]. Four characteristic classes are identified, based on the configuration of the initial part of the isotherm (i.e., class S, L, H, C). The subgroups relate to the behavior at higher concentrations. The Langmuir class (L) is the most widespread in the case of adsorption of dye compounds from water, and it is characterized by an initial region, which is concave to the concentration axis. Type L also suggests that no strong competition exists between the adsorbate and the solvent to occupy the adsorption sites. However, the H class (high affinity) results from extremely strong adsorption at very low concentrations giving rise to an apparent intercept on the ordinate. The H-type isotherms suggest the uptake of pollutants by materials through chemical forces rather than physical attraction. There are several isotherm models available for analyzing experimental data and for describing the equilibrium of adsorption, including Langmuir, Freundlich, BET, Toth, Temkin, Redlich-Peterson, Sips, Frumkin, Harkins-Jura, Halsey, Henderson and Dubinin-Radushkevich isotherms. These equilibrium isotherm equations are used to describe experimental adsorption data. The different equation parameters and the underlying thermodynamic assumptions of these models often provide insight into both the adsorption mechanism, and the surface properties and affinity of the adsorbent. Therefore, it is important to establish the most appropriate correlation of equilibrium curves to optimize the condition for designing adsorption systems. Various researchers have used these isotherms to examine the importance of different factors on dye molecule sorption by chitosan. However, the two most frequently used equations applied in solid/ liquid systems for describing sorption isotherms are the Langmuir and the Freundlich models and the most popular isotherm theory is the Langmuir one which is commonly used. Table- 5 reports the corresponding equations that can be used for fitting experimental data.

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Table- 5: The two most popular equilibrium isotherm equations and their linear forms and equation parameters

Isotherm Equation Assumptions Linear form

Langmuir x KL Ce

qe = = m 1+aLCe

1) Monolayer adsorption 2) The sorption takes place at specific sites within the adsorbent 3) Once a dye molecule occupies a site 4) The adsorbent has a finite capacity for the adsorbate (at equilibrium, a saturation point is reached where No further adsorption can occur) 5) All sites are identical and energetically equivalent 6)The adsorbent is structurally homogeneous

Ce 1 aL --- = --- + --- Ce Qe KL KL

Freundlich qe =KFCe1/nF 1) Multilayer adsorption

2)The model applies to adsorption on heterogeneous surfaces with interaction between adsorbed molecules 3) The adsorption energy exponentially decreases on completion of the sorptional centers of an adsorbent 4) This is an empirical equation employed to describe heterogeneous systems.

1 ln qe= ln KF + ---- ln Ce nF

Source: Metcalf and Eddy 3rd edition, (1995). 4.1 Kinetic Modeling: An ideal adsorbent for wastewater pollution control must not only have a large adsorbate capacity but also a fast adsorption rate. Therefore, the adsorption rate is another important factor for the selection of the material and adsorption kinetics must be taken into account since these explain how fast the chemical reaction occurs and also provides information on the factors affecting the reaction rate. The kinetics of adsorption is also another area of debate, and once again, a difference in chitosan type, preparation and methodology examined makes any comparison of results difficult.

Three kinetic models (Table 6) have been widely used in the literature for adsorption processes: (i) pseudo-first-order kinetic model (Lagergren model) ; (ii) pseudo-second-order kinetic model (Ho and McKay model) ; (iii) and intraparticle diffusion model (Webber and Morris model) .These kinetic models are used to examine the controlling mechanism of adsorption process such significance of chemisorption mechanism during the processes. They supposed that the coordination and reaction between the dyes and the amino and hydroxy groups on chitosan chains would be significant and chemisorption controlled the process.

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Table -6: The three most popular kinetic models and their linear forms

Model Equation Linear form Lagergren

q k1 Log = t qe-qt 2.303

Log(qe _ qt) = logqe = k1 t 2.303

Ho and McKay

1/(qe-qt) = 1/qe = k2 t t/qt = 1/qc2 + 1/qe t

Webber and Morris

------ Qt = ki t1/2 +c

Source: Metcalf and Eddy 3rd edition, (1995). Cestari et al. [Cestari et al.,2004,Dos Anjos et al.,2002] indicated that the pseudo models did not take into account the influence of parameters such as temperature and kind of dye. So, they suggested using the Avrami model, which is the best kinetic model to evaluate multi step adsorption phenomena at the solid/solution interface. However, this model cannot give interaction mechanisms. Mass transfer involves several steps including (i) bulk diffusion, (ii) film diffusion, (iii) intraparticle diffusion and (iv) (physical and/or chemical) adsorption reactions. Numerous authors consider that bulk and film diffusion can be ignored if a sufficient stirring speed is used. This is correct for bulk diffusion but is more controversial regarding film diffusion. Moreover, it is usually accepted that, in the case of physical adsorption, the adsorption itself can be considered as an instantaneous process, and the adsorption kinetics is controlled either by intraparticle diffusion or by both diffusion mechanisms at the same time . [Tien C.,1994] .In the case of chemical reactions, their own kinetic rates may interfere in the control of the adsorption rate. For complete modeling of adsorption kinetics it would be necessary to take into accounts not only the diffusion equations but also boundary conditions including the adsorption isotherm equation [Guibal E., 2004, Tien C, 1994]. This means that the system of equations is very complex but, generally, it is possible to simplify the system by separating diffusion steps or taking into account only diffusion steps in the control of kinetic rates. In different adsorption studies, the diffusion mechanisms were considered independently in accordance with the assumptions that the kinetics was controlled by external diffusion at the

beginning of the experiment and then controlled by intraparticle diffusion. The adsorption system obeys the pseudo-second-order kinetic model for the entire adsorption period and thus supports the assumption behind the model that the adsorption is due to chemisorption. They also showed that the kinetic parameters decreased markedly with increasing initial dye adsorption. The adsorption of dye probably takes place via surface exchange reactions until the surface functional sites are fully occupied; thereafter dye molecules diffuse into the polymer network for further interactions and/or reactions. Both the Lagergren, and Ho and McKay models basically include all steps of adsorption (i.e. external film diffusion, adsorption and intraparticle diffusion), they are thus pseudo-models. However, using the so-called pseudo-first and pseudo second- order equations for data interpretation is questionable since the equations have no physical significance. It is more reasonable to interpret the kinetic data in terms of mass transfer [Crank J., 1975, Findon A. et al., 1993, McKay G. et al., 1996]. During the past several decades, a large number of studies of batch adsorption have been reported in the literature and a summary of these studies can be found in the excellent compilation reported by Tien. Because the above two-lumped kinetic pseudo- models cannot identify adsorption mechanisms, several investigators proposed to use the diffusion mechanisms such as intraparticle diffusion using the Weber and Morris equation, the Avrami model and the Elovich equation [ Chang MY& Juang RS.,2005] .The former model originates from Fick’s second law. The validity of the Elovich equation suggests that the chemisorption (chemical

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reaction) mechanism is probably rate controlling in the adsorption mechanism. 5.0 LITERATURE REVIEW: Water resources, in many countries are heavily polluted by nitrate (Laigla et al., 1990). The major concern due to nitrate is the blue-baby syndrome resulting from the conversion of hemoglobin into methaemoglobin, which cannot carry oxygen (Golden and Weinstein, 1998). Nitrate, contained in surface or groundwater, can be removed by sorption on protonated cross-linked chitosan gel beads was studied by K. Jaafari. et al., (2001).The sorption capacity is pH-dependent and large enough to meet the standard of drinkable water. The isothermal equilibrium curves are straight lines, which imply that the removal is independent of the initial concentration. If required, the sorption capacity is easily recovered by increasing the pH to 12. The main competitor is fluoride but, even in its presence, the sorption capacity of nitrate remains significant. In conventional water treatment systems, alum is the most widely used coagulant McLachlan (1995) discovered that intake of large quantity of alum salt may cause Alzheimer disease. To minimize the detrimental effect accompanied with the use of alum, polymers are added either with alum or alone and have gradually gained popularity in water treatment process. Chitosan can be a promising substitute for alum in the coagulation process, because of its potential feasibility in coagulation without posing any health threat as the residual aluminium and other synthetic polymers do. In this study, Chihpin Huang et al. (1999) studied various pretreatment conditions for the optimum chitosan modification. Batch tests with synthetic source water suggest that the optimal pretreatment condition to prepare modified chitosan coagulant is deacetylation by 45% alkali solution for 60 min, followed by dissolution in 0.1% hydrochloric acid. Elvan Yilmaz and Nesrin Hasirci in (2000) have studied two properties of chitosan and investigated its potential as a new iron (III) ion (ferric ion) adsorbing agent. In this study, the physicochemical parameters affecting the ability of chitosan flakes to adsorb iron (III) ions were studied by complexmetric titration. The results showed that the iron (III) adsorption capacity of chitosan increases with the amount of chitosan, degree of deacetylation of chitosan, concentration of

ferric ions in solution and with the pH of the medium. The amount of ferric ions that adsorb on the polymer increases with time until an equilibrium is reached between adsorbed iron (III) ions and those in solution. Divakaran and Pillai (2001), have studied flocculation of three freshwater algae, Spirulina, Oscillatoria and Chlorella, and one brackish alga, Synechocystis, using chitosan in the pH range 4 to 9, and chlorophyll-a concentrations in the range 80 to 800 mg/m3, which produces a turbidity of 10 to 100 nephelometric turbidity units (NTU) in water. Chitosan reduced the algal content effectively by flocculation and settling. The flocculation efficiency is very sensitive to pH, and reached a maximum at pH 7.0 for the freshwater species, but lower for the marine species. Flocculation and settling were faster when concentrations of chitosan higher than optimal were used. The rates of adsorption of three commercial reactive dyes and Cu (II) from water in the absence and presence of complexing agents using chitosan were measured by Feng-Chin Wul et al., (2001). They have tested three simplified kinetic models, i.e., pseudo-first-order, pseudo-second-order, and intraparticle diffusion, to investigate the adsorption mechanisms. It was shown that the adsorption of reactive dyes and Cu(II) in the absence of complexing agents could be best described by the intraparticle diffusion model, whereas that of Cu(II) in the absence of complexing agents such as EDTA, citric acid, and tartaric acid by the pseudo-second-order equation. The design procedure for batch study has been investigated by Gurusamy Annadurai (2002) for Dye stuffs. The experimental results show that chitosan is a suitable adsorbent for adsorption of basic methylene blue. Adsorption of 100 percentages is possible in the higher temperature, increasing adsorbent dosages and decreasing particle size ranges. They found maximum amount of dye adsorption occurred at temperature level 60°C (29 .9 mg/g); at pH level 9 .5 (29 .8 mg/g) and particle size 0 .177 mm (30 .0 mg/g) S. Annouar and Mountadar et al., (2004) studied the elimination of fluorides from underground water by adsorption on chitosan followed by electro dialysis with the help of the CMX-ACS membranes. After determining the optimal conditions of the Defluoridation of a synthetic water by the chitosan extract from

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the chitin of the pink shrimps, they applied it to underground water of the city of Youssoufia. Another application has been made by electro dialysis of which the optimal conditions of treatment have been pre-established. A comparison of the two processes was also given. Torres et el., (2004), prepared chitosan micro spheres with a mean size of 140 ± 119 µm by the spray and coagulation methods. The micro spheres were chemically modified using a) cross linking with glutaraldehyde b) cross linking with epychlorohydrin and c) acetylation. They performed adsorption experiments using a static method. The adsorption media and equilibrium concentration of adsorbates were varied in the ranges of pH 4-11 and 0.07-0.70 mg.ml-1, respectively. The maximum adsorption capacities (qm) and the constant of the Langmuir model (Ks) were shown to be dependent on charge interactions and on the kind of treatment performed on chitosan micro spheres. Moattar and Hayeripour (2004) have reported two types of Shrimp Chitin derivatives and two types of Iranian natural Zeolite derivatives (Firuzkooh linoptiliolite) for adsorption and treatment of low-level radioactive liquid waste (LLW). Chitin with lowers than 10% and Chitosan with higher than 90% deacetylation factor were selected as natural organic adsorbents. They found that the adsorption efficiency depend on particle size, PH, adsorbent type, deacetylation factor and cation type. The best Cs (Caesum) adsorption occurred in Na form Clinoptilolite. Chitin derivatives, particularly Chitosan, are more efficient than Zeolite adsorbents for removing of radio nuclides. Nomanbhay (2005) prepared a new composite biosorbent by coating chitosan onto acid treated oil palm shell charcoal (AOPSC). The adsorption capacity of the composite biosorbent was evaluated by measuring the extent of adsorption of chromium metal ions from water under equilibrium conditions at 25ºC. Using Langmuir isotherm model, the equilibrium data yielded the following ultimate capacity values for the coated biosorbent on a per gram basis of chitosan: 154 mg Cr/g. After the biosorbent was saturated with the metal ions, the adsorbent was regenerated with 0.1 M sodium hydroxide. Maximum desorption of the metal

takes place within 5 bed volumes while complete desorption occurs within 10 bed volumes. Rashmi Sanghi and Bani Bhattacharya (2005), were studied decolourization by coagulation. For optimum results the variables studied with polyaluminium chloride (PAC) were pH, dosage and temperature. Coagulation efficiencies of natural polyelectrolyte such as psyllium and chitosan alone or as coagulant aids found to be very effective for the decolourization of acidic and direct dyes. Chitosan and other similar coagulants were used in the treatment of an olive oil water suspension as a model for the processing wastewater. The effect of chitosan, starch, alum and ferric chloride on the coagulation of oil droplets were determined by the jar test apparatus and turbidometric measurements. At optimum conditions of coagulation and flotation stages, the COD of the olive oil emulsion could be reduced by 95%. (B. Meyssami et al., 2005). Liu and Bai (2006) have reported their study on the effect of fabrication factors influencing the structures and morphologies of the chitosan and cellulose acetate blend hollow fibers as adsorptive membranes to achieve highly porous and macro voids-free structures with different pore sizes. They found that by increasing the alkalinity of the coagulants, the coagulation rate of the blend hollow fibers was increased. Chunxiu and Renbi(2006) in their study prepared highly porous adsorptive hollow fiber membranes from chitosan (CS) and cellulose acetate (CA) blend solutions. Adsorption experiments showed that the CS/CA blend hollow fiber membranes had good adsorption capacity (up to 35.3–48.2 mg/g), fast adsorption rates and short adsorption equilibrium times (less than 20–70 min) for copper ions, and can work effectively at low copper ion concentrations (<6.5 mg/l) to reduce the residual level to as low as 0.1–0.6 mg/l in the solution. They found that the copper ions adsorbed on the hollow fiber membranes can be effectively desorbed in an EDTA solution (up to 99% desorption efficiency) and the hollow fiber membranes can be reused almost without loss of the adsorption capacity for copper ions Ahmed et al. (2006) have used Chitosan as a coagulant for oily wastewater. They compared performance of chitosan with alum and PAC.

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The results obtained proved that chitosan was comparatively more efficient and economical to alum and PAC. In the study, shingling et. al.,(2006), attempted to find a new adsorbent material based on chitosan to improve adsorption selectivity for heavy metals, so that the cross linked N,O-car boxy methyl-chitosan resin with Pb(II) as template ions (cross linked CMC template) were synthesized by using CMC adsorbed Pb(II) ions cross linked with glutaraldehyde. The adsorption experiments demonstrated the cross linked CMC template has high adsorption selectivity for Pb (II) ions in solution containing single metal ions or coexistence of three metals ions of Cu(II), Zn(II)and Pb(II). They also investigated that the cross linked CMC template has a good reusability and stability as compared to CMC. Fung Hwa et al., (2006) studied the suitability of chitosan to treat the wastewater from milk processing plant. Chitosan is a natural material, the sludge cake from the coagulation after dehydrated could be used directly as feed supplement, therefore not only saving the spent on waste disposal but also recycling useful material. The result shows that the optimal result was reached under the condition of pH 7 with the coagulant dosage of 25 mg/l. The analysis of cost-effective shows that no extra cost to use chitosan as coagulant in the wastewater treatment, and it is an expanded application for chitosan. The arsenic speciation and arsenic removal in chitosan packed column were studied by Martha Benavente et al., (2006). They found that arsenic adsorption depends mainly on the pH as well as the activity of functional groups that compose the chitosan structure. At pH 3 and volume of adsorbent material of 337.8 cm3 an adsorption of 94% was obtained from arsenic standard solution. Jian Ping Wang et al., (2007), used chitosan-g-PDMC (poly (2-methacryloyloxyethyl) trimethyl ammonium chloride), as coagulant to treat a paper-recycling wastewater with aluminum chloride. The optimal conditions obtained from the compromise of the two desirable responses, turbidity and SVI, were coagulant dosage of 759 mg/l, flocculent dosage of 22.3 mg/l and pH 5.4, respectively. The RSM was demonstrated as an appropriate approach for the optimization of the coagulation–flocculation process by confirmation experiments.

Luigi Rizzo et.al., (2007) were compared chitosan and conventional coagulants (aluminum sulphate and ferric chloride) in terms of turbidity and natural organic matter (NOM) removal, as well as acute toxicity on Daphnia magna of coagulated and coagulated/chlorinated surface water. All coagulants decreased toxicity on D. magna from 100% to 0% immobilization. However, the addition of humic acids affected final toxicity in different ways according to coagulant type and dose. Yimin Qin et.al, (2007), examined the combined use of chitosan and sodium alginate in the treatment of wastewater containing heavy-metal ions and dye. They found that when used in combination, sodium alginate and chitosan were effective in removing copper, cadmium, lead, and silver ions as well as acid dye molecules. Antonio R. Cestari et al., (2008) used a factorial design to evaluate the quantitative removal of an anionic red dye from aqueous solutions on epichlorohydrin-cross-linked chitosan. The results indicated that increasing the dye concentration from25 to 600mg/L increases the dye adsorption. The factorial results also demonstrate the existence of statistically significant binary interactions of the experimental factors. The cross linked chitosan synthesized by the homogeneous reaction of chitosan in aqueous acetic acid solution with epichlorohydrin were used to investigate the adsorptions of three metals of Cu(II), Zn(II), and Pb(II) ions in an aqueous solution. Arh-Hwang Chen et al., (2008) in their study found that the adsorption process was followed the second-order kinetic equation. The order of adsorption capacity for three metal ions follows: Cu2+ >Pb2+ >Zn2+. The results obtained from the equilibrium isotherms adsorption studies well fitted to the Langmuir isotherm equation under the concentration range studied. Baohong Guan et al., (2008) have studied the removal of Heavy metals Mn(II) and Zn(II) from wastewater with water-soluble chitosan. The results showed that compared with the settling of metal hydroxide, the chitosan–metal complex had better separating performance. Application of chitosan solution for chelation could remove Mn(II) and Zn(II) efficiently and make it easy to separate sediment from dual-alkali FGD wastewater.

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Decontamination of lead ions from aqueous media has been investigated using cross linked xanthated chitosan (CMC) as an adsorbent by Divya Chauhan and Nalini Sankararamakrishnan (2008). Maximum adsorption was observed at both pH 4 and 5. The adsorption data followed both Freundlich and Langmuir isotherms. Droppo et al., (2008) studied behavior of Hamilton Barbour sediments mixed with three different amendments: alum, chitosan (both coagulants) and a polyacrylamide (a flocculant). Results show that amendment addition is an effective method for the management of high turbidity. Floc formed with chitosan has high settling value. The removal of arsenic (As) species, such as As(III) and As(V), from water by molybdate-impregnated chitosan beads (M ICB) in both batch and continuous operations was studied by Chih-Yu Chen et al.,(2008). The results indicated that MICB favor the adsorption of both As(V) and As(III). The optimal pH value for As(III) and As(V) removal was 5. The adsorption of arsenic on the MICB is most likely an exothermic reaction. The optimal desorption solution for arsenic recovery was 1 M H2SO4, which resulted in a 95% efficiency for As(III) and 99% for As(V). Batch adsorption experiments were carried out for the removal of methylene blue (MB) cationic dye from its aqueous solution using chitosan-g-poly (acrylic acid)/montmorillonite nano composites as adsorbent. In this study Li Wang et al., (2008) showed that the nano composites have great influence on adsorption capacities and introducing a small amount of MMT could improve adsorption ability. The adsorption behaviors of the nano composite showed that the adsorption kinetics and isotherms were in good agreement with pseudo-second-order equation and the Langmuir equation, respectively, and the maximum adsorption capacity is 1859 mg/g. Ona Gyliene et al.,(2008) studied the use of biosorbent chitosan to remove both heavy metals and organic compounds from fresh water. They found, the essential microelements calcium and magnesium are not removed from water after the treatment with chitosan In work the combined pre-treatment of actual olive mill wastewaters by coagulation with natural organic coagulant, such as chitosan, and the advanced oxidation processes,

specifically photo catalysis (PC), Fenton (F) and photo-Fenton (PF), was investigated by Luigi Rizzo et al.,(2008). The optimum removal of total suspended solids (TSS, 81%) by chitosan coagulation was achieved at actual pH (4.3) for 400mg/l coagulant dose. The maximum organic matter removals by F (85%) and PF (95%) processes were achieved after 2.0 and 1.0 h, respectively by using 15,000/1852 (w/w) ratio of H2O2/FeSO4. The applicability of neodymium-modified chitosan as adsorbents for the removal of excess fluoride ions from water was studied by Ruihua Yao et al.,(2008). The equilibrium sorption data were fitted reasonably well for Langmuir isotherm model, the maximum equilibrium sorption had found to be11.411–22.380mg/g. Vandana Singh et al.,(2008) in their study reported the effect of optimization of persulfate/ascorbic acid initiated synthesis of chitosan-graft-poly (acrylamide) (Ch-g-PAM) and its application in the removal of azo dyes. The Ch-g-PAM was found to be very efficient in removing color from real industrial wastewater. The adsorption data of the Ch-g-PAM and Ch for both the dyes were modeled by Langmuir and Freundlich isotherms where the data fitted better to Langmuir isotherms. Both Ch-g-PAM and Ch followed pseudo-second-order adsorption kinetics. The thermodynamic study revealed a positive heat of adsorption indicating spontaneous and endothermic nature of the adsorption. The effect of chitosan immobilization of Scenedesmus spp. cells on its viability, growth and nitrate and phosphate uptake was investigated by Sashenka Fierro et al.,(2008). Scenedesmus sp. (strains 1 and 2) and Scenedesmus obliquus immobilized in chitosan beads showed high viability after the immobilization process. Immobilized cells accomplished a 70% nitrate and 94% phosphate removal within 12 h of incubation Sirlei Rosa et al.,(2008) studied the use of adsorption of reactive orange 16 by quaternary chitosan salt (QCS) to remove reactive dyes from textile effluents. The results obtained demonstrate that the adsorbent material could be utilized to remove dyes from textile effluents independent of the pH of the aqueous medium. Suparna Dutta et al.,(2008) have done a comparative study between the two radiation grafted chitosan derivatives viz. cross-linked

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chitosan (CRC) and cross-linked chitosan after hydrolysis (CRCH). It was seen that CRCH had a greater uptake of metal ions compared to CRC but CRC was more selective of the two. Sudipta Chatterjee & Seung Han Woo (2008) has done a physico-chemical investigation of the adsorption of nitrate by chitosan hydro beads. The maximum adsorption capacity was found to be 92.1 mg/g at 30oC. The kinetic result corresponds with the pseudo-second-order rate equation. They found adsorption process is a spontaneous, exothermic process and has positive entropy. Desorption of nitrate from the loaded beads was accomplished by increasing the pH of the solution to the alkaline range, and desorption ratio of 87% was achieved at pH 12.0. Tania Chatterjee et al., (2008) showed the optimum concentration of chitosan required for maximum coagulation of bentonite suspension was 5 mg/l. They used five additive for conditioning, NaHSO4, at all concentrations, was found to be the most effective for enhancing the coagulating efficiency of chitosan and it was at its maximum when conditioned by 0.05M NaHSO4. NaHSO4 conditioned chitosan showed better coagulating efficiency than unconditioned chitosan at all pH. Using 5 mg/l of NaHSO4 conditioned chitosan at pH 6, the removal percentage of bentonite was increased from 76% to 88 % compared to unconditioned chitosan. Wan Ngah W.S. & Fatinathan (2008) found use of chitosan beads, chitosan–GLA beads, and chitosan–alginate beads were very effective for the removal of Cu(II) ions from aqueous solution at different initial pH, agitation periods, adsorbent dosage, and initial concentrations. Chitosan–alginate beads showed a better fit to the non-linear Langmuir isotherm giving an adsorption capacity of 67.66 mg/g. Wen F. Sye et al.,(2008) described a new method for the removal of colour from textile wastewater. They used porous crab shell and chitosan beads as adsorbents for removal of water-soluble dyes present in textile dyeing wastewater. In the work, W. A. Morais et al.,(2008) analyzed the sorption of a model dye, methyl orange, on chitosan hydro beads is in terms of equilibrium and kinetic approaches. Equilibrium studies showed that dye adsorption had a mixed Freundlich–Langmuir

behavior that had its Langmuir character increased as the pH was increased. Investigation was carried out by E. Biro et al., (2009) to elucidate the influence of process parameters on the mean particle size of chitosan micro spheres produced by water-in-oil (w/o) emulsion cross linking method. They found that model equation proved to be suitable for the prediction of the mean particle size as a function of the studied process parameters. The removal of a reactive (Remazol Yellow Gelb 3RS) and a basic (Basic Yellow 37) dye from aqueous solutions was investigated by George Z. Kyzas et al., (2009) using cross-linked chitosan derivatives as sorbents (either powder or beads), which have been grafted with carboxyl and amide groups. Chitosan powder presented higher sorption capacity than beads. Chitosan grafted with amide groups was found superior sorbent for reactive dye at pH 2 (Qmax = 1211 mg/g), while chitosan grafted with carboxyl groups for basic dye at pH 10 (Qmax = 595 mg/g). A new biosorbent was developed (Srinivasa R. Popuri et al., 2009) by coating chitosan on to polyvinyl chloride (PVC) beads. The biosorbent was characterized by FTIR spectra, porosity and surface area analyses. The experimental data were fitted to Langmuir and Freundlich adsorption isotherms. The maximum monolayer adsorption capacity of chitosan coated PVC sorbent as obtained from Langmuir adsorption isotherm was found to be 87.9 mg/ g for Cu(II) and 120.5 mg/ g for Ni(II) ions, respectively. 6.0 ECONOMIC ASPECTS: ● Many Researchers have mainly focused on the technical performances of chitin, chitosan and its derivatives, while their economic aspect is usually neglected. Cost is actually an important parameter for comparing adsorbent materials. According to Bailey et al. [Bailey SE, Olin TJ, Bricka M, Adrian DD.,1999] a sorbent can be considered low cost if it requires little processing, is abundant in nature, or is a by-product or waste material from another industry. ● Chitosan-based materials exhibit economic advantages: Chitin is a material obtained from natural raw resources. It is only commercially extracted from crustaceans which are conveniently available as waste from processing shellfish.

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The wastes consists of chitin (20–30%), proteins (20–40%), salts (mainly carbonate and phosphate, 30–60%) and lipids (0–14%) [Agullo E, Rodriguez M S, Ramos V, Albertengo L., 2003]. These proportions vary with species and season. Several countries possess large unexploited crustacean resources, especially in Asia. ● Chitin and chitosan are now produced commercially at low cost and their production is also economically interesting, especially if it includes the recovery of carotenoids. A prerequisite for the greater use of chitin in industry is cheap manufacturing processes and/or the development of profitable processes to recover chitin and byproducts such as proteins and pigments. It is important to note that the recovery of these products from waste is an additional source of revenue Crustacean shells contains considerable quantities of carotenoids which so far have not been synthesized, and which are marketed as a fish food additive in aquaculture, mainly for salmon.[ Ravi Kumar MNV.,2004] In addition, calcium carbonate which is another major component of crab shells is converted to calcium oxide and sodium carbonate .[ Peter MG.,1995] Pigments may be also recovered as high value side products. ● The production of the chitosan-based materials is economically possible because they are easy to prepare with relatively inexpensive chemical reagents under mild conditions. The conventional method of extraction creates its own environmental problems as it generates large quantities of concentrated effluent containing polluting bases and degradation products and presenting inconsistent physicochemical properties. ● Chitosan possesses several intrinsic properties such as its non-toxicity, its biodegradability and its outstanding chelation behaviour that make it an effective coagulant and/or flocculant for the removal of contaminants in the dissolved state. 7.0 CONCLUDING REMARKS: The state-of-the-art advancement in the field of biosorption by Chitosan using batch systems is reviewed in this paper, based on a significant number of references published in recent times. Evidently, it is very difficult task to make a comparison of data obtained using different materials. The experimental conditions used by various researchers also are

not the same, logically. The following conclusions may be made: ●The works reviewed above indicate that bio-adsorption onto chitosan is becoming a promising alternative to replace conventional adsorbents used in water and wastewater Engineering for adsorption of various impurities. ●Excellent progress has been made, representing the application of chitosan and cross linked chitosan in bio-adsorption of fluoride, arsenic and many of the heavy metals. These materials are efficient in removal of above mentioned impurities with the additional advantage of being cheap, non-toxic and biocompatible. ●Chitosan is characterized for its easy dissolution in many dilute mineral acids, with the remarkable exception of sulfuric acid. Several methods have been developed to reinforce chitosan stability. The advantage of chitosan over other polysaccharides is that its polymeric structure allows specific modifications without too many difficulties. ● It has the physico-chemical characteristics of both coagulants and flocculants, i.e. high cationic charge density and long polymer chains, leading to bridging of aggregates and precipitation. Numerous works have demonstrated that chitosan and its derivatives (in particular grafted biopolymers) can be a potential substitute for metallic salts and synthetic polyelectrolytes in the treatment of water and wastewater for the removal of both particulate and dissolved substances. ● Several authors concluded that the binding was a chemisorption reaction and the adsorption phenomenon mainly depended on the interactions between the surface of the adsorbent and the adsorbed species. However, all the studies arrive at contrasting conclusions showing the difficulty of using simple models for the interpretation of the interactions of these polymeric materials with metal ions. Much work is necessary to clearly demonstrate the adsorption mechanism for the different types of chitosan-based materials. ● Researchers have demonstrated that chitosan and its derivatives (in particular grafted biopolymers) can be a potential substitute for metallic salts and synthetic polyelectrolytes in the treatment of wastewater for the removal of both particulate and dissolved substances. However, more studies are required to refine the optimization of the properties of chitosan

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