Essay: Polysaccharides

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  • Polysaccharides
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Introduction and Literature Review
1- Introduction
In each step of our life, we came across things were fruits of polymers researches. So, the middle of the last century, the working on these fields have been so rapidly and its applications are very useful.
1.1- Polysaccharides
Polysaccharides are natural based polymers (carboxymethyl cellulose, starch, cellulose, chitin, alginate, hyaluronate, etc.). currently, applications of natural based polymers are promising for different fields such as wastewater treatment.
Any of a class of carbohydrates, such as carboxymethyl cellulose, starch, chitin and cellulose, consisting of a number of monosaccharaides joined by glycosidic bonds. The large class of long-chain sugars composed of monosaccharaides. Because the chains may be unbranched or branched and the monosaccharaides may be of one, two, or occasionally more kinds, polysaccharides can be categorized in various ways. Cellulose, starch, glycogen, and dextran are all polysaccharides of glucose, with different configurations. Pectins are composed of a galactose derivative, chitin of a glucose derivative. Connective tissues, joint fluid, and cartilage contain two-component polysaccharides, including heparin cellulose, the most abundant renewable resource on earth, will become the main chemical resource in the future [1]. There are a new functional cellulosic substances are being shared over a wide range of applications, because of the raising demand for environmental safety and compatibility[2]. Cellulose has hydroxyl groups used for preparing hydrogels simply with chemical and physical properties [3]. There is a need to study cellulose-based hydrogels in both fundamental research and industrial application.

Figure (1) show the structure of cellulose.

1.2- Radiation Synthesis of natural based hydrogels
Radiation processing is based on the use of high energy ionizing radiation to induce chemical and biological changes in irradiated systems.
The use of similar sources like glow discharge, ultraviolet, laser and visible light, and the recently developed incoherent dielectric exciter lamps are closely related. Since high energy electromagnetic and particle radiation exhibit properties of controlled penetration and intensities, which are especially suitable for synthesis and modification of polymeric materials without the need of usually toxic additives, the interest to use radiation techniques in industrial, agricultural, wastewater and biotechnology is growing rapidly. These methods are being used for synthesis of functional polymers in forms of macro and microgels, micro- and nanospheres, functionalization of surfaces and radiation processing of naturally derived biomaterials among others. Potential biomedical and biotechnological applications of these new materials include implants, topical dressings, injectable formulations, drug delivery devices, diagnostic assays, separation and purification processes, immobilized enzyme and cell bioprocesses and cell culture surfaces.
A natural based polymer is defined as any substance or combination of substances of synthetic or natural origin that can be used for any period of time, as a whole or as a part of a system which treats, augments, or replaces any tissue, organ or function of the body without causing local adverse reaction or systemic toxicity.
There are a number of ways to synthesize new functional natural based polymers or to modify surface properties of existing materials, but radiation initiation of the suitable chemical reactions has a number of advantages over classical initiation methods:
‘ the absence of additives (initiators, catalysts’), usually toxic materials that could contaminate the product, thus a higher possibility to produce clean materials
‘ the possibility of initiating the reaction at any temperature a wide variety of monomers and polymers to choose from including those that cannot be polymerized by classical chemical initiation
‘ the possibility to control the degree of crosslinking and grafting
‘ the possibility to control the depth of surface modification, thus surfaces of finished products can also be modified without affecting the bulk properties
‘ the possibility of simultaneous synthesis and sterilization
‘ the possibility of simultaneous immobilization of bioactive materials without any loss in their activity.

1.2.1 – Effect of radiation on the polymer chains.
Radiation based processes have many advantages over other conventional methods. For initiation processes, radiation differs from chemical initiation where no catalyst or additives are required to initiate the reaction. Generally, with the radiation technique, absorption of energy by the backbone polymer initiates a free radical process. With chemical initiation, free radicals are brought fourth by the decomposition of the initiator into fragments which then attack the base polymer leading to free radicals [4].
Two reaction processes occur when electromagnetic radiation passes through matter, it may interact either with the atomic nucleus or with orbital electrons. In the case of polymeric materials reactions with orbital electrons are more frequent [5]. The removal of the orbital electrons disrupt the stability of the macromolecules, giving rise to a positive ion as follows:-

When the thermal electrons discharge the ionized molecules, highly excited molecules are formed.

These excited molecules are decomposed into free radicals.

Thus, the primary effect of radiation on the polymer molecules is the formation of free radicals and the loss of atomic hydrogen. The free radicals can undergo a variety of reactions as indicated below [6].

1.2.2 – Abstraction: –
The hydrogen atom formed in reaction (3) may abstract a hydrogen atom from another polymer molecule, forming molecular hydrogen and a new radical.

1.2.3- Recombination: –
The recombination of free radicals is a process converse to the decomposition of excited molecules and leads to crosslinking of free radicals.

1.2.4- Disproportionation:-
In this reaction, two radicals form a molecule by transfer of an atom or group of atoms.

1.2.5- Polymerization
A polymerization reaction occurs if a free radical can combine with another molecule without losing its characteristics. For example, a methyl radical can combine with an ethylene molecule to form a propyl radical.

This radical can combine further with other ethylene molecules to form larger molecules. This process can continue until the radical reactivity is lost by recombination or disproportionation reactions with another radical. In general, the effect of radiation will depends mainly on the total quantity of energy deposited in the material regardless of the nature of the incident photons or particles. New chemical bonds are formed and thus alter the structure of the polymeric material and result in changing the chemical and physical properties of the irradiated substances.

1.3 – Chemical effects of radiation
Chemical changes are center to an understanding of the effect of radiation on polymer. Chemical changes are the basis for the explanation of physical changes, and the establishment of the exact nature of a chemical change is a prerequisite to an understanding of the full reaction mechanism. The changes are generally analogous to those found with the more tractable low molecular weight materials of similar composition. These changes can be summarized as follows: –

1.3.1- Crosslinking and degradation
The most important effects are cross-linking, which are analogous to dimerization and degradation, which is analogous to main-chain scission processes. In most polymers, one of these processes predominates. If cross-linking predominates, the ultimate effect of irradiation will be to produce a network polymer, but if degradation predominates molecules become smaller and smaller as the irradiation proceeds and the material loses its polymeric properties. There are no absolute rules for determining whether or not any given polymer will cross-link or degrade on irradiation [7].

1.3.2- Gas evolution.
Gas evolution has been observed in the radiolysis of all polymers investigated so far as a result of the rupture of small molecules from the polymer. The nature of the polymer, the total absorbed dose and the temperatures of irradiation considerably affect evolution of gaseous products. It has been found that the yield of gas is much higher if the irradiation is carried out at high temperature.

1.3.3- Double bond formation.
It was found that, the extent of unsaturation increases with irradiation. The irradiation of polyvinyl chloride (PVC) leads to a dehydrochlorination reaction and as a result, conjugate structure appears in the polymer. The evolution of HCl from PVC and corresponding production of conjugated double bonds in this polymer presumably proceeds through basically the same free radical mechanism [8].

This decomposition is responsible for the pink or yellow color of degraded polymer and a proof of the formation of double bond.

1.4 – Hydrogels prepared by ”- radiation
Hydrogels can be obtained by radiation technique. More frequently the method of irradiation of monomer is applied. In this technique, polymerization takes place by crosslinking of the formed chains. This way is possibly when the chosen monomer is easily available but its polymer is not. Since many of the monomers used are harmful or even toxic, particular care has to be taken when using this method for the formation of hydrogels for biomedical use to ensure that either all the monomer has reacted or its un reacted residues have been fully extracted in a separate operation [9,10]. When a mixture of monomers in aqueous solution is subjected to ionizing radiation, reactive intermediates are formed in the macromolecules. Among of them, the three main reactive species formed upon water irradiation as well as, hydrated electrons, hydroxyl radicals and hydrogen radicals.

Hydrogels have the ability to absorb water, and water-soluble molecules and ions without loss of shape and mechanical strength. Natural hydrogels are present in living matter under the form of cartilage, muscles and tendons. There is great research interest in hydrogels because they find already industrial application and are promising materials for instance in applications in prosthetic materials, soft lenses, biomedical materials for controlled drug release [11], food processing and heavy ions binding. Natural hydrogels have the unique advantage of being compatible with living tissues. The insolubility and stability of the hydrogel shape are due to the presence of a three-dimensional network which acts as a cage for the water molecules and other dissolved molecules and ions. The swollen state results from a balance between dispersing forces acting on hydrated chains and cohesive forces that do not hinder the penetration of the water inside the network. The circle represented in the Figure (2) is accessible for diffusion of solutes. In swollen state, this space contains water or the solvent molecules. The dimension of this region and the mesh size depend on the degree of crosslinking of the polymer chains or the density of junction points. The amount of water absorbed also depends on the degree of crosslinking of polymer chain.

Figure (2) Hydrogel network

The higher the degree of crosslinking is the smaller the amount of water/solvent absorbed in this space. Such a schematic representation of a hydrogel network is treated as a network of ‘phantom type’. Hydrogels are polymeric materials that do not dissolve in water at physiological temperature and pH. They swell considerably in an aqueous medium and demonstrate extraordinary capacity (‘20%) for imbibing water into the network structure. Hydrogels exhibiting a phase transition in response to change in external condition such as pH, ionic strength, temperature and electric currents are known as “stimuli-responsive” or “smart” hydrogel [12]. Being insoluble, these three-dimensional hydrophilic networks can retain a large amount of water that maintains a certain degree of structural integrity and elasticity [13]. Hydrophilic functional groups such as ‘OH, ‘COOH, ‘CONH2, and ‘SO3H present in the hydrogel are capable of absorbing water without undergoing dissolution.
Hydrogels can be prepared from natural or synthetic polymers [14]. The hydrogels are polymeric materials, which exhibit the ability to swell in water; i.e. they retain a significant fraction of water within themselves but do not dissolve in water. The soft non-abrasive quality, their wettability and permeability to biologically active substances of low molecular weight make them possible to prepare materials which can serve the bio-medical purposes excellently [11]. The hydrogels can be prepared by ionizing radiation. On ionizing radiation of macromolecules, at first macro radicals are formed, which when recombined form hydrogels. If the macro radicals are favorably positioned, new covalent bonds between the polymer chains are formed and thus the hydrogel turns into an insoluble fraction depending upon the amount of these new bonds. On further irradiation, the amount of gel increases, although a part of macromolecules or their fragments) may still be unbound. However, the mechanism is rather complicated in presence of water as a solvent and it has been deeply studied [15].
They are gaining tremendous importance in a wide variety of applications in medical, pharmaceutical and related fields, e.g. wound dressings, treatment of wastewater, [16], contact lenses [17], artificial organs and drug delivery systems [18].
Since in radiation polymerization no catalyst or heat is required, there is no possibility to bring toxicity in the system and decomposition of the drugs. Thus, it is a very convenient method to prepare drug delivery system. Moreover, by using radiation technique, the hydrophilicity as well as porosity can be controlled at a low temperature based on its properties of hydrogel, it can be used as carriers for the immobilization of bioactive compounds and can be used in the controlled release of drug [19].
Immobilization is the technique by which mobility of the biological species has been lowered. It is the technique to fix the biological component in the surface of the polymer or to entrap it into polymer matrix. A small molecule like a drug is leakage by different rates according to the porosity of the polymer and the nature of the drug [20]. To control the rates of leakage, drug delivery system is needed. Slow controlled release of drug, low or no secondary reaction and better targeting the affected cells, are the bases of the drug delivery system. The polymer-based drug delivery devices have the ability to control the drug release function at desired rate and for the necessary period.
1.4.1- Advantages of Radiation Processing for Hydrogel Production.
Regarding hydrogels and its industrial production as market product, which are the advantages of radiation processed hydrogels in respect to its characteristics and industrial production.
1. Simultaneous sterilization and crosslinking; one should understand that this feature is really important as the combination of sterilization and polymer modification in just one industrial procedure simplifies the process and reduces costs.
2. Take advantage of the electron penetration profile to produce hydrogels with a sandwich structure, i.e. the crosslinking density depends on the depth of electron penetration. This characteristic can be very important for wound care management, as for instance, the main advantage of hydrogels dressings are its tender contact with nerves points, reducing the pain, so softer surfaces can achieve this effect much better.
3. For matrices designed to slow release of drugs sometimes is important to compound monomers with polymers, in that case, radiation can show all its usefulness, as is the technique that reduces at minimum level the amount of toxic residual monomers.
4. Easy control of physical properties by combining dose with polymer composition; for instance, the same basic formula of dressings can produce solid, elastic ones and fluid gels.
1.4.2- Classification of Hydrogels.
The classification of the hydrogels was taken into the consideration from the view of water absorption capacity. Hydrophilic polymer network structure is that may be absorbed water in the amount from 10% up to thousands of times their dry weight. Also Hydrogels can be classified into two categories, conventional hydrogels and stimuli responsive hydrogels [21].
The first is light crosslinked hydrophilic polymers which swell significantly in water but do not dissolve. They are usually uncharged and exhibit no significant change in swelling with change in pH, temperature, electric field, light or other stimuli. The second is the stimuli responsive hydrogels which may exhibit significant volume changes in response to small changes in pH, temperature, electric field and light. They usually contain an important hydrophobic component and may or may not be charged; if so, they contain pH-sensitive ionic groups.
When a dry hydrogel begins to absorb water, the water molecules first hydrate the most polar, hydrophilic groups, which are the ionic (if present) and H-bonding groups. This kind of water is sometimes called primary bound water. After these groups are hydrated, the chains begin to expand, and as the hydrophobic groups are exposed to water molecules, they interact via hydrophobic interactions, leading to a kind of bound water coating the surroundings of those groups which is often called secondary bound water. These two types of water are often combined and simply called bound water. When all of these short-range interactions of water with polymer backbone groups are satisfied, the network may imbibe additional water, causing it to expand to an equilibrium swelling level [22].
Polymeric hydrogels are classified in accordance to many categories as shown in Table 1.
Table 1: various criteria for the classification of hydrogels.
Origin Natural
Synthetic
Water content or degree of Swelling Low swelling
Medium swelling
High swelling
Superabsorbent
Porosity Nonporous
Micro-porous
Macro-porous
Super- porous
Cross-linking Chemically
Physically
Biodegradability Biodegradable
Non-Biodegradable

Hydrogels can be classified in accordance to their monomeric composition based on the method of preparation giving some important classes of hydrogels namely:
1) Homopolymeric hydrogels: They are cross-linked networks of one type of hydrophilic monomer unit.
2) Copolymeric hydrogels. They are cross-linked networks of two or more different monomers species with at least one hydrophilic component along the chain of the polymer network. The co-polymeric hydrogel networks are generally covalently or ionically cross-linked structures, which are not water soluble.
3) Interpenetrating polymeric hydrogels: They are two independent crosslinked synthetic and/or natural polymers components contained in a network form.
Furthermore, the chemical constituent of monomers used in the preparation of hydrogels plays an important role in classifying the hydrogels. The hydrogels are classed as either:
1) Neutral hydrogels (non-ionic hydrogels). They are homopolymeric or copolymeric networks, which do not bear any charged groups in their structure.
2) Ionic hydrogels: They are prepared from monomers accompanying ionic charges. The charges could be positive or negative thus classing the hydrogels as anionic, cationic or ampholytic based on the presence of ionic charges on the monomer.
Hydrogels are also classed as amorphous or semi-crystalline materials based on their physical nature.
1.5- Application of the hydrogels in wastewater treatment.
Water is the most essential commodity for our civilization to flourish. Availability of safe drinking water is the most important prerequisite for a sound public health system. Most of the dreaded epidemics of the past were water related. Water borne diseases still continue to be the major contributor of illness in developing and underdeveloped countries [23,24]. In the recent past, most of the water borne diseases were effectively controlled by the establishment of municipal water supply systems guaranteeing the supply of safe drinking water. However, as the population of cities are increasing and the known sources of water are depleting (e.g. due to global warming the glaciers are melting, leading to the threat of decrease in water flow in all season glacier fed rivers like Ganga), it is becoming more and more challenging to increase our safe drinking/potable water needs. The situation is further complicated in the face of increasing water pollution and more stringent standards for potable water.

1.5.1- Pollution by Heavy Metals.
Recently, much attention focused on the waste treatment of heavy and toxic metals from water because of its the severe problems of environmental pollution. There are many chemical compounds whose presence in water could be harmful or fatal to human life and it is necessary to consider two aspect of the problem is assessing potential hazard. One of the most important sources of pollution of the natural environment is industrial waste, because of their composition and large content of non-degradable or toxic substances, physicochemical methods are required for their treatment. Conventional mechanico-biological methods are frequently ineffective. Many industries such as chemical, metal, textile or tanning carry away in their effluents large amounts of anionic and nonionic surface agents. The presence of these substances in wastes impedes or, at sufficiently high concentrations, inhibits the processes of their biological treatment and exerts toxic effect on the biosensors of receiving waters.
A heavy metal is a member of a loosely defined subset of elements that exhibit metallic properties. It mainly includes the transition metals, some metalloids, lanthanides, and actinides. Many different definitions have been proposed’some based on density, some on or atomic weight and some on chemical properties, or toxicity [25]. The term heavy metal has been called a considered to be as toxic metal. Heavy metals are generally considered to be those whose density exceeds 5 g per cubic centimeter. Most commonly releasing heavy metals are arsenic, chromium, copper, mercury, cadmium, lead. These are causing hazardous effect on humans and other living organisms.

Heavy metals have high solubility, and are having acidic or neutral pH. These heavy metals are generally present in very low concentration. Because of high solubility these are easily taken by living organisms and get accumulated in the body. On increasing pH to basic and by changing concentration of metal to more amount metal gets precipitated as metal hydroxide and can be easily removed from water.

Industrial wastewater streams containing heavy metals are produced from different industries. Electroplating and metal surface treatment processes generate significant quantities of wastewaters containing heavy metals (such as cadmium, zinc, lead, chromium, nickel, copper, vanadium, platinum, silver, and titanium) from a variety of applications. These include electroplating, electrolysis depositions, conversion-coating, anodizing-cleaning, milling, and etching. Another significant source of heavy metals wastes result from printed circuit board (PCB) manufacturing. Tin, lead, and nickel solder plates are the most widely used resistant over plates. Other sources for the metal wastes include; the wood processing industry where a chromated copper-arsenate wood treatment produces arsenic-containing wastes; inorganic pigment manufacturing producing pigments that contain chromium compounds and cadmium sulfide; petroleum refining which generates conversion catalysts contaminated with nickel, vanadium, and chromium; and photographic operations producing film with high concentrations of silver and ferrocyanide. All of these generators produce a large quantity of wastewaters, residues, and sludges that can be categorized as hazardous wastes requiring extensive waste treatment [26].
Some studies will be focused on investigation the applicability of the complexation’ultrafiltration process for removal of toxic heavy metals, in particular; Cu(II), Ni(II), and Cr (III), from synthetic wastewater solutions. To highlight the metals’ removal performance, the main operating conditions such as pH, pollutants and ligand concentrations, and permeate flux, have been investigated. The permeate and retentate parameters have also been determined. Pollution of aquatic bodies because of indiscriminate disposal of heavy metals has been causing worldwide attention in recent years. Heavy metals are susceptible to accumulate in organisms which are significantly toxic to human beings and ecological environments, some even in relatively low concentrations [27].

1.5.2- Removal of Dyes.
Removal of cationic dyes from industrial effluents is still an important but challenging subject in the field of environmental remediation. Annually, millions of tons of cationic dyes are consumed by the textile, rubber, paper, and plastic industries, in which about 10-20% is discharged in wastewater effluents [28]. These dyes cover thousands of different chemical structures. Most of them exhibit specific properties, such as high hydrophilicity, stability to light or heat. From the view of industrial application, removal of cationic dyes by adsorption is a promising approach because of its low performance cost and easy technical access [29]. Recently, particular attention has been paid to producing dye adsorbents from cellulosic polymers due to their advantages of abundant, rapidly renewable, and biodegradable in nature [30]. The adsorbents products are usually prepared in the form of hydrogels owing to their three-dimensional porous inner structure, readily swelling behavior, and strong adsorption capacity of dyes [31]. Many cellulose derivatives, such as carboxymethyl cellulose and hydroxymethyl cellulose have been used for synthesizing hydrogels adsorbents with acrylic acid. Some efforts have also been made to synthesize hydrogels from bacterial cellulose. However, few reports focus on the swelling behavior and dye adsorption performance of hydrogels made from unmodified cotton cellulose. The most probable reason is that the poor-solubility of cellulose in most common solvents hinders the synthesis of hydrogels and finally limits the adsorption capacity of dye. Solvents may play an important role in synthesis of hydrogels and may affect the properties of the final products, such as swelling-shrinking kinetics [32],
Polyvinyl alcohol (PVA) is an efficient, less costly and non-toxic polymer. Physical modifications such as boiling, heating, autoclaving and chemical treatments with acids, alkali, salts, surfactants and some organic compounds enhanced or decreased the biosorption capacities of pollutants such as metals and dyes [33]. The pretreatments or modifications of the biomass may increase the surface area of the biomass and enhanced the amount of pollutant sorbed from aqueous solution [34,35]. Dyes are a relatively large group of organic chemicals classified based on their molecular structure as azobenzene, anthraquinone, or triphenylmethane dyes. Methyl Orange (MO; C.I. 35780), Disperse Blue 2BLN (DB; C.I.63285), and malachite green chloride (MG; C.I. 42000) are frequently used dyes representing the above classes. These dyes are difficult to degrade because of their complex aromatic structures and can cause allergy, dermatitis, irritation and even cancer to humans [36,37]. Residual and unspent dyes are usually discharged into the environment, thereby causing pollution problems [38]. With the increasing concern on environment protection, removal of such dyes is gaining public and technological attention [39]. For dye removal from wastewater, methods such as flocculation, oxidation, and electrolysis are common methods [40-43].

Sorption can transform dyes from the effluent to a solid phase, this method is generally regarded as an effective technique for dealing with wastewater dye. Among conventional adsorbents, activated carbon has been widely investigated and used for dye adsorption from various effluents; its application is limited because of its high cost [44].

In recent years, there is growing interest in finding an effective alternative to activated carbon such as starch, cellulose, chitosan and lignin [45-48]. Cellulose is one of the most abundant natural substances in nature. However, the compact and inactive molecular structure of cellulose, the modification is required to improve its hydrophilicity as an adsorbent for dye removal. Cellulose-based materials are prepared by carboxymethylization, grafting and cross-linking [39,49,50]. Among the cellulose-based materials, carboxymethyl cellulose (CMC) is a representative water-soluble cellulose derivative. CMC itself or grafted with other water-soluble compounds is used as an adsorbent for dye sorption from various wastewaters systems [39,51].

Several reports have also indicated that a particular kind of cellulose-based material can remove dyes with different molecular structures. In this study, CMC prepared by our previous research [52] was used as the basic adsorption material to prepare a novel absorbent. This absorbent was produced by grafting acrylic acid (AAc) along the chains of CMC to improve the mechanical properties, swelling capacity and dyes adsorption capacity of CMC. Methyl Orange, Disperse Blue and Malachite Green were chosen to investigate the adsorption behavior and universality of CMC-AAc to the dyes. The preparation method of CMC-AAc adsorbent has many advantages as lower temperature, short time and cheap raw material. The CMC-AAc adsorbent has high adsorption to multiple dyes, therefore, it can effectively reduce the production and application cost. The present research will provide the technical basis for preparation and application of CMC based adsorbent. So far many adsorbents have been evaluated as candidates for the removal of Methyl Orange from water and their adsorption capacities have varied widely depending on the adsorbent. Even though the adsorption capacity of CMC-AAc is less than that of iron terephthalate and activated carbon [53,50].

1.5.3- Removal of organic pollutants.

Chlorophenols with at least two chlorines either have been used directly as pesticides or converted into pesticides. Also, chlorophenols, especially 4”chlorophenol, have been used as antiseptics. In addition to being produced commercially, small amounts of some chlorophenols, especially the mono- and dichlorophenols, may be produced when waste water or drinking water is disinfected with chlorine, if certain contaminants are present in the raw water. They are also produced during the bleaching of wood pulp with chlorine when paper is being produced. Chlorophenols can enter the environment while they are being made or used as pesticides. Most of the chlorophenols released into the environment go into water, with very little entering the air. The compounds that are most likely to go into the air are the mono- and dichlorophenols because they are the most volatile (that is, has the greatest tendency to form vapors or gases). Once in the air, sunlight helps destroy these compounds and rain washes them out of the air. Chlorophenols stick to soil and to sediments at the bottom of lakes, rivers, or streams. However, low levels of chlorophenols in water, soil, or sediment are broken down by microorganisms and are removed from the environment within a few days or weeks. The spatial configuration and resonance effect of 2-chlorophenol may suppress the activity of the halogen atom by hydrogen bonding, which partly accounts for the lower toxicity than the 3- and 4-chlorophenol isomers [54].

2,4-Dichlorophenoxyacetic acid (2,4-D) is the third most widely used herbicide in North America and the most widely used herbicide in the world [55]. However, its extensive use in agriculture has become a serious environmental concern. Due to its high solubility, 2,4-D can be easily transferred into water and cause consequential harm to public health and the environment [56].

1.6- Literature Review
Cellulose is the most abundant, renewable biopolymer and a very promising raw material available at low cost for the preparation of various functional polymers. Carboxymethylcellulose sodium salt (CMC) is the first water soluble ionic derivative [57]. Hydrogels are one of the promising and versatile materials with enormous possibilities and potential. In particular, hydrogels are cross-linked polymeric networks that can imbibe large quantities of fluid. In recent years considerable attention has been drawn to hydrogels.
1.6.1- Preparation of hydrogels based on carboxymethyl cellulose by gamma- radiation.

1.6.1.1- CMC hydrogel.
The synthesis of carboxymethyl cellulose (CMC) hydrogels by gamma-ray with an absorbed dose of 50 kGy from a 60Co source has been studied [58]. The CMC hydrogels were absorbed and swelled in silver nitrate aqueous solution (0.01 M) by dipping for 1 hour, and then irradiated by gamma ray at various doses to form silver nano-particles (Ag NPs). The UV-Vis analysis indicated that the concentration of Ag NPs was enhanced by increasing of absorbed dose from 1 to 5 kGy in this situ reducing system. The FE-SEM and XPS measurements provided further evidence for the successful formation of Ag NPs. These CMC hydrogels stabilized Ag NPs also have been investigated for inhibiting the growth of Staphylococcus aureus and Escherichia coli strains in liquid as well as on solid growth media. The antibacterial tests indicated that the hydrogels containing Ag NPs have antibacterial activity.
The sodium carboxymethyl cellulose (CMC) is a kind of degraded polymer by gamma irradiation. However, in this work, it has been found that CMC crosslinks partially to form hydrogel by radiation technique at more than 20% CMC aqueous solution [59]. The gel fraction increases with the dose. The crosslinking reaction of CMC is promoted in the presence of N2 or N2O due to the increase of free radicals on CMC backbone, but gel fraction of CMC hydrogel is not high (<40%). Some important values related to this kind of new CMC hydrogel synthesized under different conditions, such as radiation yield of crosslinking G(x), gelation dose Rg, number average molecular weight of network Mc were calculated according to the Charlesby’Pinner equation. The results indicated that although crosslinked CMC hydrogel could be prepared by radiation method, the rate of radiation degradation of CMC was faster than that of radiation crosslinking due to the character of CMC itself. Swelling dynamics of CMC hydrogel and its swelling behavior at different conditions, such as acidic, basic, inorganic salt as well as temperature were also investigated. Strong acidity, strong basicity, small amount of inorganic salts and lower temperature can reduce swelling ratio.
The slight radiation-crosslinked Na-CMC as a substitute for hydrogel was prepared by gamma irradiation below gelation dose [60]. The effects of various parameters such as absorbed dose, concentration of inorganic salts, pH, swelling temperature and swelling time on the swelling ratio in water were investigated in detail. This kind of slight crosslinked Na-CMC showed good water absorption below 60 oC, whereas, it became solution when heated up to 70 oC. Na-CMC gel is different from the true gel that is insoluble in boiled water; nevertheless, it can be used as hydrogel at room temperature and produced at low dose. Due to its low cost, it might be useful for its application in agriculture or other applications.
Study the mechanism of gelation of aqueous solutions of sodium carboxymethylcellulose (Na-CMC) irradiated with gamma ray, we have measured viscoelasticity, dynamic light scattering (DLS) and microwave dielectric spectrum of the solutions with varying dose of gamma ray and Na-CMC concentration [61]. Na-CMC was a commercial product obtained from Daicel Company. Its degree of carboxymethyl substitution and the weight-average molecular weight were 2.2 and 5.2 x 105, respectively. Elastic modulus and viscosity were measured by a coaxial cylinder-type torsional viscoelastometer. The elastic properties could be determined by a competitive effect of degradation and crosslinking. This effect was also recognized in behaviors of the relaxation time obtained from dielectric measurements and the diffusion coefficient determined by DLS.
Studied the preparation of hydrogels from carboxymethyl cellulose (CMC) and chitosan (CHI) with different ratios by gamma irradiation different dose was carried out in presence of different methylene bisacrylamide (MBA) concentrations as a crosslinking agent [62]. The hydrogels were characterized by FT-IR spectroscopy which confirmed complexation between carboxylic group in CMC and amino group in CHI. The swelling behavior in different buffers of different pH values was also studied. The results indicated the formation of network structure of pH-sensitive hydrogels. The CMC/CHI hydrogels were evaluated for the possible use in drug delivery field, in which the release profile of aspirin, as a drug model, was investigated. Scanning electron microscopy was carried out before and after aspirin release proving the drug release.
Investigation of swelling properties and gel fraction of CMC hydrogel prepared by gamma radiation in presence of mono-and divalent salts (NaCl and CaCl2) were studied [63]. Two parameters such as effect of radiation dose and ionic strength of salts on the properties of hydrogel were observed. Swelling ratio and gel fraction of hydrogels depend on radiation dose and ionic strength of salts. Results also indicate that CMC with divalent salt, the gelation capacity is higher than that of mono-valent salt and swelling capacity of CMC hydrogel with mono-valent salt is higher than that of CMC hydrogel with di-valent salt. In addition, gel fraction increases and swelling ratio decreases with increased ionic strength of salt, but at higher ionic strength > 0.41 M, an unexpected inversion of the trend is observed and gives low cross-linking value.
1.6.1.2- Graft copolymerization of different monomers onto Carboxymethyl cellulose by gamma radiation.
Noted that the carboxymethyl cellulose (CMC) is a cellulose derivative with a wide range of applications and markets [64]. In order to tailor the molecular weight and the rheological profile of CMC, hydrogels based on acrylamide monomer (AM) and different ratios (5’20 wt%) of carboxymethyl cellulose CMC) were synthesized by gamma irradiation. The hydrogels were characterized in terms of gel content, swelling and drug release characters.
Studied that the CMC/AAm hydrogels were prepared by radiation crosslinking with an electron-beam accelerator. The gelation dose was identified and equal to 30 kGy [65]. The optimum concentration ratio of CMC/AAm was 90/10. The swelling properties of the hydrogels were higher in distilled water than in inorganic salts. The swelling increased with increasing temperature. The results of this work indicate that these hydrogels possess good water-retention capacity.
Investigation of the preparation of carboxymethyl cellulose/acrylamide (CMC/AAm) hydrogels have been prepared by gamma radiation at room temperature (27 ”C) were carried out [66]. The preparation conditions such as effect of CMC concentration and radiation dose on gel fraction and swelling behaviors were investigated. The maximum value of gel fraction is obtained at 25 kGy radiation dose. The swelling properties were investigated in distilled water, saline solution (NaCl) and buffer medium. The maximum swelling value of hydrogel was obtained at 48 hours in water. The swelling ratio of hydrogel decreased with increase in concentration of NaCl in swelling medium. In buffer, it was found that swelling ratio increased with increasing pH of medium.
1.6.1.3- Carboxymethyl cellulose (CMC) blend hydrogels.

Investigate the copolymer hydrogel Poly (vinyl alcohol) (PVA) and carboxymethyl cellulose (CMC) have received increasing attention in biomedical and biochemical applications because of their properties such as being water-soluble and biocompatible [67]. In this study, a PVA/CMC hydrogel applicable to artificial cartilage was prepared by a freezing-thawing technique and a gamma-ray irradiation. The concentration of PVA was 7 wt% and the concentration of CMC was 4 wt%. The freezing/thawing process was repeated twice and the dose of gamma-ray irradiated was 30 kGy. Results of gelation before and after gamma-ray irradiation were similar, but the swelling degree decreased and compressive strength increased. The cytotoxicity was investigated with CCK-8 assay.

Investigate copolymer hydrogels composed of poly(vinyl alcohol) (PVA) and carboxymethyl cellulose (CMC) was prepared by using electron beam irradiation as crosslinking agent [68]. The copolymers were characterized by FTIR and the physical properties such as gelation. The thermal behavior and swelling properties of the prepared hydrogels were investigated as a function of PVA/CMC composition. The factors affecting adsorption capacity of acid, reactive and direct dyes onto PVA/CMC hydrogel, such as CMC content, pH value of the dye solution, initial concentration and adsorption temperature for dyes were investigated. Thermodynamic study indicated that the negative values of ”H suggested that the adsorption process is exothermic. The value of ”H (38.81 kJ/mol) suggested that the electrostatic interaction is the dominant mechanism for the adsorption of dyes by hydrogel.
The preparation of carboxymethyl cellulose (CMC) with poly (N-vinyl pyrrolidone) (PVP) blend by 60Co gamma ray irradiation has been studied [69]. A series of hydrogels were prepared with different compositions (PVP:CMC). The properties of the hydrogels such as gel fraction, gel strength, swelling behavior and moisture retention were investigated.
Synthesis series superabsorbent hydrogel based on carboxymethyl cellulose (CMC) and polyvinyl pyrrolidone (PVP) crosslinked by gamma irradiation which has been proposed for agriculture application [70]. The effect of preparation conditions such as feed solution composition and irradiation dose on the gelation (%) and swelling degree was evaluated. The structure and the morphology of the superabsorbent CMC/PVP hydrogel were characterized using Fourier transform infrared technique (FTIR), and scanning electron microscope (SEM). Effect of ionic strength and cationic and anionic kinds on the swelling behavior of the obtained hydrogel was investigated. Urea as an agrochemical model was loaded onto the obtained hydrogel to provide nitrogen (N) nutrients. The water retention capability and the urea release behavior of the CMC/PVP hydrogels were investigated. It was found that, the obtained CMC/ PVP hydrogels have good swelling degree that greatly affected by its composition and dose. The swelling was also extremely sensitive to the ionic strength and cationic kind. Owing to its considerable slow urea release, good water retention capacity, being economical, and environment-friendly, it might be useful for its application in agriculture field.

Blends of styrene butadiene rubber (SBR) with varying loading degree from 60 wt% to 100 wt% of carboxymethylcellulose (CMC) have been prepared by Gamma radiation. Vulcanization of prepared blends was carried out with doses varying between 50 kGy and 250 kGy. Mechanical properties, namely, tensile strength (Ts), elongation at break (Eb) and hardness were followed up as a function of loading degree of CMC and gamma irradiation dose. Moreover, physical properties, specifically swelling number (SN) and gel % (G%) were undertaken. Results obtained showed an improvement in mechanical as well as in physical properties with increasing either CMC content or irradiation dose. Thermal properties namely thermo gravimetric analysis (TGA) was carried out [71].
Studied the Blend hydrogels based on the carboxymethyl cellulose (CMC) and carboxymethyl chitosan (CMCts) were prepared by gamma irradiation of a high concentrated CMC/CMCts aqueous solution [72]. Properties of the hydrogels, such as gel fraction, swelling ratio, gel strength, and metal adsorption for Pb and Au were investigated. The gel fraction increased with increasing irradiation dose, while the swelling ratio decreased with increasing with irradiation dose. The obtained blend hydrogels had high adsorption performance which was controlled by adjusting the composition of CMC/CMCts.
1.6.2- Environmental applications using Carboxymethyl cellulose based hydrogels.
1.6.2.1- Removal of heavy metals.
Suggested that, the ion exchange hydrogels are a class of functional materials that display ion exchange properties owing to fixed ionic sites attached to their framework [73], which may involve, e.g., chemical bonds or lattice energy and may be called poly ions. Oppositely charged ions move throughout the framework and may be replaced by ions of similar charge. The majority of commercial ion exchange polymers are in the form of beads (particles) and membranes. Resins in other physical forms, such as gels, fibers and fabrics, are often prepared for special separation purposes.
The carboxymethyl cellulose (CMC) is well known as a safe and biodegradable material which is widely used as food additives, wash paste, etc.in our daily life [74]. Aiming at the environmental purification Cu+2 absorption property of CMC gel that was crosslinked by gamma irradiation without toxic crosslinker has been investigated. The CMC gel has revealed to capture Cu+2 of which number depends on the gamma ray dose as well as the CMC concentration, indicating the chelation by carboxyl group at the end of the side chain of CMC. The adsorbed Cu+2 ion amount increases with the CMC-Na concentration and with the gamma irradiation dose. Since the polymerization rate is superior to that of decomposition in the gel forming concentration range as mentioned above, the gamma irradiation with higher dosage can cause higher degree of the crosslinking even after the gelation, with this maturing reaction, more rigid configurations can be established between the CMC-Na network polymers to stabilize their dense relative arrangement. As a result, the chelation of Cu+2 ion with the carboxyl groups will take place more effectively compared with the unstable fluctuating configuration in the lower polymerization degree.
A series of functional copolymer hydrogels composed of carboxymethyl cellulose (CMC) and 2-acryla-mido-2-methyl propane sulfonic acid (AMPS) were synthesized by using gamma radiations-induced copolymerization and crosslinking [75]. Preparation conditions were optimized, and the swelling characteristics were investigated. The ability of the prepared hydrogels to recover some toxic metal ions from their aqueous solutions was studied. The prepared hydrogel showed a great capability to recover metal ions such as: Mn+’2, Co+’2, Cu+’2, and Fe+’3 from their solutions. The data revealed that the chelating ability of the prepared hydrogels is mainly dependent on their internal composition, in addition to the physical properties of the metal ion solution such as pH and metal ion concentration. The data show that the chelating ability of the prepared hydrogels increases by increasing the AMPS content in the hydrogel as well as the increment in the pH of the solution and the metal ion concentration. The prepared CMC/AMPS copolymer hydrogels are chemically stable enough to be regenerated and reused for at least five times with good efficiency.

Environment-friendly Carboxymethyl cellulose (CMC) hydrogel beads were successfully prepared using epichlorohydrin (ECH) as a crosslinking agent in the suspension of fluid wax [76]. There was an ether linkage formed between ECH and CMC, which was identified from bands in FTIR spectra of the prepared hydrogel. The prepared hydrogel beads with diameters about 4 mm were apparently spherical and fully transparent. The X-ray diffraction (XRD) spectra indicated that the adsorption of metal ion onto the oxygen atom of carboxyl group led to change in crystallinity patterns of hydrogels. The scanning electron microscope (SEM) images clearly showed that the hydrogels had an internal porous structure. The adsorption capacity increased as initial concentrations of metal ions and the pH value of metal ion solution increased. Freundlich and Langmuir isotherm models were employed to analyze the data from batch adsorption experiments. There are very good correlation coefficients of linearized equations for Langmuir model, which indicated that the sorption isotherm of the hydrogel beads for metal ions can be fitted to the Langmuir model. The maximum adsorption amount of hydrogel beads for metal ions is 6.49, 4.06, and 5.15 mmol/g for Cu(II), Ni(II), and Pb(II), respectively.

1.6.2.2- Removal of Dyes.
Water pollution by toxic contaminants is a serious environmental problem caused by rapid industrialization and growth of population. Among all the contaminants, colored species appear to have a wide impact on various segments of the environment. The partially treated/untreated effluents generated from dyestuffs manufacturing and consuming industries including textile, pulp and paper production, tanneries, chemical production, paints and varnishes cause the presence of such harmful color species. Development of effective treatment technology for the color removal from dyes wastes has been rather baffling. This is primarily due to diversity of dyes and their continuously changing character, complex chemical nature, persistent color, inhibitory and non-biodegradable nature and toxicity [77].

The removal of noxious dyes is gaining public and technological attention. Herein grafting polymerization was employed to produce a novel adsorbent using acrylic acid and Carboxymethyl cellulose for dye removal [78]. Scanning electron microscopy and Fourier-transform infrared spectroscopy verified the adsorbent formed under optimized reaction conditions. The removal ratio of adsorbent to methyl mrange, disperse blue 2BLN and malachite green chloride reached to 84.2%, 79.6% and 99.9%, respectively. The results implied that this new cellulose-based absorbent had the universality for removal of dyes through the chemical adsorption mechanism. A new cellulose-based absorbent CMC-AA was prepared by polymerization of AA onto CMC. The highest gelation value of 85.6% is obtained under the optimum synthesis conditions. The prepared CMC-AA absorbent is proved to be a potential adsorbent for removal of methyl orange (MO), disperse blue 2BLN (DB) and malachite green chloride (MG) from aqueous solutions, although the adsorption depends on different temperatures and pH values. Pseudo second order kinetic model better fits the kinetics of current adsorption, showing the chemical adsorption of CMC-AA adsorbent to MO, DB and MG. Temkin isotherm better fits the experimental equilibrium data of dye adsorption on the prepared CMC-AA adsorbent.

The adsorptive properties of carboxymethyl cellulose (CMC) prepared from sugarcane bagasse (SB) for the removal of methylene blue (MB) from aqueous solution was investigated by two batches of CMC, CTSB-CMC-B1 and CTSB-CMC-B2, were prepared from chlorite treated sugarcane bagasse (CTSB) [79]. The prepared CMCs were characterized by FT-IR spectral analysis. Degree of substitution (DS) value of prepared CMCs was estimated. Batch adsorption experiments show that the adsorption of MB on CMCs reaches equilibrium within 30 minutes. The MB adsorption capacity of CTSB-CMC-B1 and CTSB-CMC-B2 were found to be 652.0 and 369.0 mg/g, respectively. CMC with the higher DS value (CTSB-CMC-B1) shows higher adsorption capacity than the CMC having lower DS value (CTSB-CMC-B2). The uptake of MB was minimum at pH 2 and gradually increases with the increase of pH. From the desorption studies, it was found that large amount of MB was released in strong acidic (pH 3.0) conditions.

The sorption behaviors of carboxymethyl cellulose (CMC) for methylene blue (MB) has been studied [80]. The experimental results indicated that the sorption capacity increased from 50 mgg’1 for unmodified cellulose (UmC) to more than 300 mgg’1 for CMC. The most favorable sorption of MB was observed at an alkaline condition. The sorption isotherms closely followed the Langmuir mode, and the sorption kinetics was in agreement with the pseudo-second order equation. The results from the batch experiments illuminated that the sorption mechanism was ion-exchange controlled process. In fixed-bed tests, CMC also exhibited high efficiency for removal for MB, in which sorption behaviors followed Thomas model. Desorption of the dye from the MB – sorbed CMC (MBsCMC) indicated that MBsCMC was stable, and MB was seldom released at neutral and alkaline conditions. Furthermore, a more efficient method for reuse of the disused sorbents was tried. MBsCMC was employed for removal of methyl orange (MO) in a secondary sorption at neutral or alkaline conditions. The maximal MO uptake of MBsCMC was over 100 mg/g, which was much higher than those of CMC and UmC. It was indicated that MBsCMC was efficient in sorption of MO for the electrostatic interaction between MO and MBsCMC, and secondary sorption was an appropriate way for reuse of this kind of disused sorbents.
In the present study, free carboxymethyl cellulose (CMC)-immobilized, polyvinyl alcohol (PVA)-alginate immobilized and chemically treated rice husk biomass was used for the biosorption of ever direct orange-3GL and direct blue-67 dyes [81]. Maximum biosorption capacity of free, immobilized and hydrochloric acid treated biomass was observed for both dyes at low pH. Comparative study of free, immobilized and HCl treated biomass showed that, the HCl treated biomass exhibited more biosorption capacity (29.98 and 37.92 mg/g) for Ever direct Orange-3GL and Direct Blue-67, respectively. Equilibrium time was less for HCl treated biomass when compared with immobilized biomass. The Langmuir type 1 and 2 models were best fitted to experimental data for free CMC, polyvinyl alcohol-alginate immobilized and HCl treated biomass in case of ever direct Orange-3GL, while the equilibrium data of Direct Blue-67 followed the Langmuir type 2 isotherm. Pseudo-second-order and Elovich kinetic models illustrated good fitness to all types of biomasses showing chemisorption nature of biosorption. The amount of dyes sorbed (mg/g) increased with increase in temperature. The values of Gibbs free energy (”G”) showed that reaction was spontaneous at high temperature.
Prepared two samples of partially carboxymethyl cellulose derivatives of different degree of substitution (D.S) values were prepared from Egyptian rice straw via pulping followed etherification using different concentrations of monochloroacetic acid under the catalytic action of sodium hydroxide [82]. The prepared derivatives were assessed for D.S. and evaluated as dye adsorbent for different classes of dyestuff. The results obtained indicate that, the D.S. increases from 0.09 to 0.14 by increasing monochloroacetic acid from 5 to 10g/100g cellulose pulp. The rate of dye absorbance increases by increasing the amount of adsorbent as well as the time of adsorption. While as the dye concentration increases from 0.01 to 0.5 the percent dye absorption decrease regularly. However, the magnitude of the percent decrease in the colour depends on:
(a) the nature of the dyestuff used, (b) the D.S. of the adsorbent and (c) on the technique applied . The magnitude of colour removal in case of using ultrasonic technique is relatively higher than the mechanical shaking irrespective of the nature of the dye used and/or the conditions of adsorbance. The percent colour removal follows the order Basic green> Basic yellow> Acid green> Acid blue respectively.
1.6.2.3- Removal of organic pollutants.
Increasing in using of pesticides in agriculture and domestic activities for controlling pests cause water pollution of our water resources a day. The pesticides structures a strong class of water pollution as they are sometimes non-biodegradable and carcinogenic in nature. Therefore, toxicity of pesticides and their degraded compounds lead to formation of dangerous chemical substances which help in environmental contamination. Despite the considerable advances in the field of water treatment, the elimination of certain types of pollutants to fulfill the standards in force still raises some problems and remains a topic of concern.
The organic compounds of polyphenols type, organochlorinated or aromatic, considered as being the most toxic for living species. The origin of these pollutants are different, the chemical industrial, leaching and run-off from agricultural and forest land (through the intensive use of pesticides and weed-killers) deposition from aerial applications, discharge from industrial wastewater [83,84]. In the absence of any treatment, the organic compounds and the persistent organic pollutants (POP), like as pesticides, accumulate in water and favor the risk of contamination of the underground sources in an irreversible way [85]. This effect is worsened by the fact that the chemical stability of these pollutants is high and even their chemical decomposition can generate very stable products, such as polychlorinated biphenyl (PCB) and dioxins, which are extremely toxic at low concentration.

The synthesis of sodium carboxymethyl cellulose (CMC)-stabilized Pd/Fe nano-particles and its applications to remove chloride of 2,4-dichlorophenoxy acetic acid (2,4-D) under controlled conditions [86]. For this purpose batch mode experiments were conducted to understand the effects of CMC on the surface characteristics of Pd/Fe nano-particles, suitable condition for removal of 2,4-D and other surface interactions mechanism. Our experimental results demonstrated considerable enhancements in particle stability and chemical reactivity with the addition of CMC to Pd/Fe nano-particles. (TEM) analysis indicated that CMC-stabilized Pd/Fe nano-particles were well dispersed, and nano-particles remained in suspension for days compared to non-stabilized Pd/Fe nano-particles precipitated within minutes. The isoelectric point (IEP) of the nano-particles shifted from pH 6.5 to 2.5, suggesting that CMC-stabilized Pd/Fe nano-particles were negatively charged over a wider pH range. Our batch experiments demonstrated that CMC-stabilized Pd/Fe nano-particles (0.6 g Fe/L) were able to remove much higher levels of 2,4-D with only one intermediate 2-chlorophenoxyacetic acid (2-CPA) and the final organic product phenoxyacetic acid (PA), than non-stabilized Pd/Fe nano-particles or microsized Pd/Fe particles. The removal percentage of 2,4-D increased from 10% to nearly 100% as the reaction pH decreased from 11.5 to 2.5. The optimal CMC/Fe mass ratio for the dechlorination of 2,4-D was determined to be 5/1, and the removal of 2,4-D was evidently hindered by an overdose of CMC.
Noveel polymer beads have molecular adsorption abilities which prepared from carboxymethyl cellulose sodium salt (CMC-Na) and ” -cyclodextrin (”-CD) by suspended crosslinker, utilize ethylene glycol diglycidyl ether (EGDE) in Alkaline media as a crosslinking agent [87]. FTIR and solid NMR spectroscopy analysis showed that the incorporated ”-CD and crosslinking densities within the hydrogels bead structures are strongly depending on the molar feed ratio of ”-CD to CMC during sysnthesis. The beads showed water-swelling capacities of 70’200 mL/g-polymer, with reducing in capacities combined with increasing amounts of ” -CD incorporated in the gel structure. The hydrogel beads also have highly adsorption capacity for biphenol A (BPA).

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