Applications of polymeric materials as adsorbents for dyes removal from aqueous medium

Applications of polymeric materials as adsorbents for dyes removal from aqueous medium D. Suteu* 1 and S. Coseri 2 1 Faculty of Chemical Engineering and Environment Protection, Gheorghe Asachi Technical University of Iasi, 73 D. Mangeron Blv., Iasi, 700050, Romania 2 Petru Poni Institute of Macromolecular Chemistry, 41A, Gr. Ghica Voda Alley, 700487 Iasi, Romania The scientific literature contains some valuable information about the applicability of polymeric materials, such as ion exchange resins or cellulosic materials in removal of harmful, pollutants organic compounds (i.e. dyes) from aqueous medium, probably due to their advantages related to ease of use in dynamic systems with increased adsorption efficiency, the possibility of being employed in several consecutive cycles of sorption - desorption and high performance in terms of degree of discoloration and treatment of aqueous effluents. The aim of this chapter is to review the latest developments of the authors in the field of using polymeric materials (ion-exchange resins, cellulose ion exchange, cellulose functionalized or modified physical / chemical) for retaining dyes textile from diverse aqueous media. Keywords: adsorption, aqueous medium, cellulose, dyes, ion exchange resin 1. Organic dyes in aqueous media Dyes represent a class of organic compounds, which could have a pollutant action of aquatic ecosystems. These compounds originating from effluents of different industries, such as textile and leather, production of pharmaceuticals and cosmetics, printing, food processing, paper and pulp. Besides from the aesthetic problems related to colored effluents, the presence of dyes in aquatic environment can reduce light penetration affecting photosynthetic activity and oxygen transfer in the water. Generally, dyes are synthetic chemical compounds with complex aromatic structures, which determine their low degree of biodegradability or even non-biodegradability, the long time presence in environment and accumulation in sediments but especially in fishes or other aquatic organisms. Most of dyes are risky substances to living organisms causing allergies, dermatitis, skin irritation or different tissular changes. Moreover, aromatic amines, benzidine, naphthalene and other aromatic compounds resulted as by-products of azo dyes degradation by microorganisms have been reported to be toxic, carcinogenic and mutagenic to humans [1,2]. In this context, the removal of dyes from industrial effluents before their discharge into aquatic ecosystems is of considerable concern. Therefore, in many countries strict laws and regulations have been introduced and more stringent standard about discharge limits of colored industrial effluents, what makes necessary the introduction a stage for discoloring the industrial effluent before their discharge. There is a diversity of methods and technologies for treatment of colored wastewater such as chemical oxidation, coagulation/flocculation, ozonation, ion exchange, adsorption, electrochemical reduction, membrane processes, biological degradation; the advantages and the limitations of each method were highlighted by different authors. Adsorption (process that consists in the transfer of soluble organic dyes (solutes) from wastewater to the surface of solid, highly porous, particles (the adsorbent), represents even now a viable alternative due to their major advantages appreciated in terms of efficiency and cost. Also, adsorption is one of the methods very useful in the treatment of coloured wastewaters because of their design simplicity, inexpensiveness, specific physic-chemical interactions between dyes and the solid adsorbent, and low matrix effects [3-6]. Dye removal by adsorption in batch or dynamic conditions is a relatively simple method which can be carried out without sophisticated equipments. Among other advantages of this method we can mention: increase of process sensibility; increase of selectivity; reduction of matrix effects; possibility of simultaneous achievement of pre-concentration and the proper estimation. But the main advantage of adsorption is the possibility to use to use as adsorbents many types of materials: synthetic to natural low-cost materials (natural as well as wasted materials from different industries and agriculture) as suitable adsorbents for decolourization of industrial effluents. The intention of this chapter is to make a review of our works developed over time about the use of polymeric materials (ion-exchange resins, cellulose ion exchange cellulose functionalized or physical /chemical modified) for retaining dyes textile diverse aqueous media. The results are presented in the context of development in this area and presented in the literature. 62

2. Adsorbents based on polymeric materials The choice of an adsorbent was based on requirements concerning high selectivity, large capacity of adsorption, favorable kinetic features, physical-chemical stability, mechanical strength, easy regeneration and availability at low cost [6]. Because of compatibleness between these criteria and their adsorptive features, the synthetic materials, such as ion exchange resins (Amberlite IRA-401S, Vionit AT-1, Purolite, A-400, Purolite, A-500, Purolite C-100), polyamide, ion exchange celluloses, chitosan, functionalized polymers (with chelating group), and activated charcoal constitute, in many instances, a solution of election [1,6-9]. In order to avoid some disadvantages of conventional adsorbents based on synthetic polymers (high prices, difficulties in obtaining, pollution produced during their synthesis), and in accordance with the tendency of replacing chemically synthesized compounds, unconventional materials are increasingly used for wastewaters analyzes and treatment [6]. These materials can be: synthetic (e.g., ashes, different charcoals in the presence or not of biodegradable polymers such as polyelectrolytes); natural, such as: peat, algae, seashell; different types of wastes: industrial waste [10,11]: coal ash from thermo-electric power plant, for retaining metal ions and textile dyes; cellolignin - residual product obtained after treating the wood with dilute mineral acid; sawdust, from wood industries; textile fibers (hemp, polyacryl-amidoxime chelating fiber, PAN, etc.); microbial biomass resulted from industrial fermentative technology (food and pharmaceutical industry) [10,11]. agro-industry waste - cellulosic and/or lignocellulosic materials which present different chelatic groups in their structure, (e.g., pumpkin core peels, seed pods, straws, jute fibers, hemp, cotton [10-12]. 3. Ion exchange resins 3.1 Structure Synthetic resins are largely used as adsorptive supports in various fields, including ecology and biotechnology. They are used in their active form, or can be physical or chemical modified, therefore the adsorptive properties are dictate by the macromolecular structure or by the adopted physical/chemical routes to obtain the resin. The most used ones contains cross-linked divinylbenzene (for instance with styrene or metacrylic esters, see Tables 1, 2.). These resins are characterized by high stability over the whole range of ph, thus being possible to use eluents which have extreme ph-s. Another advantage of these resins is their physical-chemical stability, which allows several utilizations (large number of cycles). Table 1 Properties of Purolite anionic resins [13] Type of Purolite resins Parameter A 400 A 500 Matrix polystyrene-divinylbenbeze polystyrene-divinylbenzene Structure gel macroporous Functional group R (CH 3 ) 3 N + R (CH 3 ) 3 N + Ionic form Cl - Cl - Mean particle diameter/ mm 0.64 ( 0.03) 0.64 ( 0.03) Capacity * / meqg -1 3.72 3.93 * - determined by ph-metric titration of resins dried at room temperature for 72 hours 63

Table 2 Physico-chemical characteristics of used cationic Purolite resins [1] Characteristics Type of Purolite resins C 107E C 145 specific gravity g/ml 1.15 1.22 bead size range/ µm 630-1300 300-1200 physical form transparent white beads operating temperature 100 o C (max.) 120 0 C (max.) polymer matrix structure polymethacrylic crosslinked with divinylbenzene polystyrene crosslinked with divinylbenzene structure macroporous macroporous functional group R COO - - R SO 3 ionic form as shipped H + Na + type of resins weak acid strong acid ph range, operating ph limits (stability) 5-14 0-14 total exchange capacity * eq/l 3.7 1.5 * - determined by ph-metric titration of resins dried at room temperature for 72 hours Unlike rigid structures, as for example silica gel, whose properties depends by the porosity and pore size distribution, the physical characteristic of the styrene-divinylbenzene resins are related by the polymerization conditions (initiator type, temperature, etc) and the cross-linked degree. These parameters are manipulated during fabrication, and strongly influenced the resins performances: specific surface, size and shape pores, and also pore size distribution [14, 15]. As lower as the cross-linking, the higher the adsorption capacity is. For example, in the case of microporous divinylbenzene-styrene resins, or those gel type, the optimum cross-linking degree is between 2 and 8%, which ensures both good mechanical properties and adsorption efficiency [14, 15]. Macroporous resins could be obtained in polar solvents by suspension polymerization. To achieve the macroporous structure, often are introduced in the polymerization system an inert solvent or inorganic fine salts, for example CaCO 3. In this way, spherical particles are obtained, having a high specific surface (25-800 m 2 /g) and an average pore size between 20 500 Å. The high crosslinking degree of these resins gives them a good mechanical strength. [14, 16]. Internal structure of the resins depends on the presence of micro/macro pores. Macropores facilitates the migration of the solute through adsorbant during adsorption/desorption processes. Micropores role is very similar with capillars, providing small centers on the surface for adsorption process. [14, 17]. A wide variety of conditioning resins exist: from rigid or soft granules, nonporous or porous materials, with pore sizes of 600 nm, with specific surfaces ranging from 2 to 1000 m 2 /g, anionic or cationic charged 14, 18. The adsorption selectivity of the macro cross-linked resins is influenced by the pores shape, size and distribution, but also by the polarity of the surface, being affected by polymerization process: monomer type, cross-linking agent, initiator, filler material, etc. The three-dimensional structure ensures a good stability in harmful media (acid, alkaline, organic solvents), so these adsorbents are easily regenerated, being used in dynamic systems. Adsorptive properties of the macro cross-linked porous polymers are influenced by formation of the cavities in solvated adsorbent and also by the hydrogen bond, which can modify the selectivity [16]. In order to be used in chromatographic columns for separation in dynamic conditions, the degree of impregnating resin in solvent is an important factor in their design because swelling can block resin beads or glass columns break due to pressure. Also, solvent impregnation modifies pore size by more than 20-30% as compared to a hydrated form. 3.2 Applications of ion exchange resins The resins, styrene-divinylbenzene type copolymers may be adsorbents for various organic compounds. These resins are effective extraction systems from different fermentation in lightening and removing organic compounds from waste streams and recovery, separation and purification of reaction mixtures produced [19]. This class of resin includes polymers derivatives of styrene crosslinked with divinylbenzene (XAD-2, XAD-4, XAD-1180 XAD-16, HP-20, HP-21) and polymers based on acrylic esters type: XAD-7, XAD-8. The scientific literature contains some valuable information about the applicability of ion exchange resins in removal of dyes from aqueous solutions, probably due to their advantages related to ease of use in dynamic systems with increased adsorption efficiency, the possibility of employ in several consecutive cycles of adsorption - desorption and high performance in terms of degree of discoloration and treatment of aqueous effluents [1, 20-25]. The major advantages outweighed high cost of the synthetic resins involved. Various ion exchange resins investigated recently for the removal of dyes from aqueous solutions are summarized in Table 3. 64

Table 3 Application of ion exchange resins as adsorbents for removal of different dyes Ion exchange resin used/ main characteristics Removed dye Ref. Gel anion exchangers (weakly basic Amberlite IRA-67 and strongly basic Amberlite Acid Orange 7 [23] IRA-458) Strongly basic polystyrene anion exchangers: Amberlite IRA-900 and Amberlite Tartrazine [26] IRA-910 Acrylic resins: Amberlite IRA-458 with gel structure and Amberlite IRA-958 with Reactive Black 5 [27] macroporous structure Gel anion exchanger Purolite A-850 of N+(CH 3 ) 3 functional groups Acid Blue 29 [28] Strongly basic anion exchanger of macroporous structure Purolite A-520E Acid Blue 29 [29] Purolite ion exchange resins of macroporous (A 500) and gel (A 400) structure Blue M-EB [13] Purolite ion exchange resins C-100 Methyl Violet [30] Purolite macroporous ion exchange C-107E (polymethacrylic crosslinked with Basic Blue 9 [1] divinylbenzene) and C-145 (polystyrene crosslinked with divinylbenzene) Strong cation - exchange resin obtained by poly(glycidylmethacrylate) grafted via Crystal Violet [31] surface-initiated-atom transfer radical polymerization on a cross-linked acrylate based resin and Basic Fuchsine Acrylic weak base anion exchange resin with ethylenediamine-functional groups Acid Green 9 [32] Polystyrene anion exchangers Amberlite IRA-900 and Amberlite IRA-910 with macroporous structure Allura Red and Sunset Yellow [33] 4. Cellulosic materials 4.1 Structure Cellulose [(C 6 H 10 O 5 ) n ], represent the most abundant regenerable polymer on Earth, with applications almost on every field of the human needs. Cellulose possesses a fibrous structure, high internal surface (caused by the size and distribution of pores), higher higroscopicity (correlated with porous structure and the presence of hydroxyl groups) and offers the possibility of functionalization with organic reagents, under certain conditions. The most important functionalization processes on cellulose are the esterification and etherification reactions which were firstly developed since the nineteen century, with a great impact on the humanity. Adsorptive properties of cellulose can be improved by changing the physical and / or chemical functional groups of constituents. For the synthesis of ion exchange pulps often are used the cellulose from cotton or wood. The main cellulose cation exchange (Table 4) are those cellulose sorts containing carboxyl groups, introduced by selective oxidation using appropriate oxidizing agent (nitrogen dioxide, perchloric acid, periodic acid, nitroxyl radical, i.e. TEMPO, N-hydroxyphthalimide). These selective reagents are able to oxidize only the primary hydroxyl groups on the cellulose unit, converting them to carboxylic acids, via an aldehyde structure intermediate, Figure 1[34-39]. Fig. 1 Possible oxidation routes for the cellulose selective oxidation, in presence of nitroxyl radicals or periodates. Cation exchange cellulosic materials with radicals of citric acid, phthalic acid, maleic acid, succinic acid, glutaric or salicylic acid are of great interest. 65

Celluloses anion exchange (Table 5) containing anionic groups, which gives them basic character. They are more hydrophilic materials than synthetic resins anionites, but shows smaller exchange capacity. Instead there is the possibility of increasing the adsorption capacity, maintaining fibrous structure by chemical treatments based on bifunctional crosslinking reagents and introducing ionogene groups, operations that can be performed sequentially or simultaneously. Some crosslinking agents are: formaldehyde, epichlorohydrin, divinyl sulfone, 1,3-dichloro-2-propanol, diepoxide derivatives [14]. Table 4. Characterization of some cation exchange celluloses Cation exchange cellulose / Ionogen Acid strength Functionalization method CM a -cellulose / - weak Alkali cellulose treated with OCH 2 COOH monochloroacetic acid medium CE b -cellulose / -OC 2 H 4 COOH P c -cellulose / -O-PO 3 H 2 medium Nominal exchange capacity, meq/g 0 7 0,1 0,7 Alkali cellulose treated with POCl 3 /NH 3 or esters-phosporic acid 0,85 0,1 SE d -cellulose / -O-C 2 H 4 SO 3 H strong Alkali cellulose treated with 2- sodium chloroethyl sulfate 0,2 0,06 SM e -cellulose / -O-CH 2 -SO 3 H strong Alkali cellulose treated with 2- sodium chloromethyl sulfate - Notes: a-carboxymethyl cellulose; b- carboxyethyl cellulose; c-cellulose phosphate; d-sulfoethyl cellulose; e-sulfomethyl celuloza Table 5. Characterization of some anion exchange celluloses Anion exchange cellulose / Ionogen Base strength Functionalization method Nominal exchange capacity, meq/g AE a /-OCH 2 CH 2 NH 2 medium Cellulose reaction with 2-aminoethyl sulfonic acid, in the presence of NaOH, or ethyl amine in the presence of acids catalysts. 0,8 0,1 DEAE b /-O(CH 2 ) 2 N(C 2 H 5 ) 2 medium Cellulose treated with 2-chlorotriethyl amine, in the presence of NaOH. 0,7 0,1 TEAE c /-O(CH 2 ) 2 N + (C 2 H 5 ) 3 ]X - strong Diethylaminoethyl cellulose reaction with alkil halides 0,5 0,08 medium Alkali cellulose treated with epichlorohydrin and triethanol amine. 0,3 0,05 ECTEOLA/-OCH 2 CHOHCH 2 O- CH 2 CH 2 N((CH 2 ) 2 OH) 2 GE d / -O(CH 2 ) 2 NHC(NH)NH 2 strong Aminoethyl cellulose treated with thiourea in tampon solution carbonatebicarbonate. PAB e /-O-CH 2 -C 6 H 4 -NH 2 weak Reaction of cellulose with para nitrobenzil, in the presence of NaOH, followed by reduction with sodium hydrosulphite. 0,4 0,1 0,2 0,1 Notes: a-aminoethyl cellulose; b-diethylaminoethyl cellulose; c-triethylaminoethyl cellulose; d-guanidoethyl cellulose; e-p-amino benzyl cellulose. Special ion exchange cellulosic materials (DEAE Sephacel) based on microcrystalline cellulose were prepared, from cotton linters by partial hydrolysis, yielding products with low molecular weight and satisfactory mechanical strength. A new type of cellulose materials type is Cellets 200 and 350, a versatile product which combines different properties such as perfect sphericity, narrow particle size distribution, low friability, low solubility and inertness [40]. Cellets are microcrystalline cellulose beads, produced exclusively by microcrystalline cellulose and purified water, without any additive. They possess high spherical starter cores with extreme stability and low friability (Table 6) [40]. Recently, an extensive work has been dedicated to produce pellets of Cellets 200 by a continuous mode of operation, using a Wurster fluidized bed, in contrast with batch-wise technique, to reduce the production costs and improve the process control [41]. The authors found that the particle size distribution (PSD) of the obtained Cellets 200 pellets is narrow, ranging from 200 to 400 m [41]. The coordination number for Cellets 200, which is a basic attribute that influences many properties of products made of particulate materials, has been determined as 8.1±1.5 by using 66

DBSCAN (Density-Based Spatial Clustering of Applications with Noise) for segmentation of X-ray micro-tomographic images [42]. Table 6 Physical and chemical characteristics of microcrystalline cellulose Cellets Cellets 200 Cellets 350 Particle size distribution 200 355 m ( 85 %) 350 500 m ( 85 %) Loss on drying 7.0 % Bulk density (g/cm) 0.80 ± 5 % Sphericity degree (average) 0.90 ± 0.05 Degree of polymerization 350 ph value 5.0 7.0 Conductivity/(µS/cm) 75 Made for biomedical applications, this material has been studied also as a potential adsorbent for retaining dyes from aqueous medium due to outstanding mechanical properties that confer them resistance to repeated manipulations (such as repeated cycles of adsorption - desorption) [40]. Granular cellulose and its derivatives are characterized by availability at a low price, spherical particle, availability with selectable particle size, strength, having already applications as industrial filters, high porosity, hydrophilicity, compatibility with biological structures, and are readily for various functionalization reactions. These properties are considered performance factors as compared with other materials such as agarose (Sepharose) and cross-linked dextran (Sephadex) [14]. Depending on the technological process used to manufacture them, celluloses may be create in many forms and types ranging from fibers, linters, microcrystalline powders, softwood pulp, bacterial cellulose and many others [40, 43-53]. 4.2 Applications of cellulosic materials Cellulose-based materials have successfully been used as adsorbents for retaining various types of dyes from aqueous media (Table 7). Their adsorption capacity depends primarily on the adopted synthesis protocol, existing functional groups and the presentation form, and secondly by the structure and properties of the studied dyes (molecular weight, functional groups). Table 7 Application of cellulosic materials as adsorbents for removal of different dyes Adsorbents based on cellulose Dye Adsorption capacity, q Ref. (mg/g) Acrylic acid grafted cellulosic Luffa Methylen Blue 65.15 [54] cylindrical fiber Bead cellulose Methylene Blue / 0.6 10 5 /2.3 10 5 /4.4 10 5 [55] Alizarin Red / Congo Red Carboxymethyl cellulose g- Methyl Orange 1825 [56] poly(2-(dimethylamino) ethyl methacrylate) hidrogel B Carboxymethyl cellulose Methylene Blue 369-652 [57] Ion exchanger cellulosic materials Acid Clue 25 / Acid Yellow 9 / 294/333/175/127 [58] Reactive Yellow 23/Acid Blue 79 Cellulose multicarboxyl Malachite Green / Basic 458.72/1155.76 [47] fuchsine Carboxylate functionalized Methylene Blue / Rhodamine 185.63 / 118.21 [44] cellulose from waste cotton fabrics 6G Cellulose nanocrystal alginate Methylene Blue 256.41 [43] hydrogel beads Cellulose nanocrystal Methylene Blue 118 [59] Microcrystalline cellulose Cellets Methylene Blue Brilliant Red HE-3B 4.45 [40] 5. Conclusions Synthesizing information from literature allows the assertion that polymer remain a valuable category of adsorbents for retaining the dyes, in dynamic or static, continuous or discontinuous systems, present in aqueous media due to more 67

than a few advantages such as: (1) use easily in dynamic systems, (2) Their excellent mechanical properties facilitates the use in consecutive repeated cycles of adsorption desorption, (3) a good report between efficiency and price and (4) high performance in terms of degree of discoloration and treatment of aqueous effluents [1, 23, 25, 40]. Also, the morphological properties and the physicochemical characteristics of these classes of adsorbents allow their use to treat large volumes of wastewater which is an advantage for situations where the pollutant is found dispersed in large volumes of influent or in the case of the effluent of sewage treatment plants, that it allows both removal and concentrating the pollutant. 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