American Journal of Agricultural and Biological Sciences 3 (1): 299-314, 2008 ISSN 1557-4989 © 2008 Science Publications Polymer in Agriculture: a Review Francesco Puoci, Francesca Iemma, Umile Gianfranco Spizzirri, Giuseppe Cirillo, Manuela Curcio and Nevio Picci Dipartimento di Scienze Farmaceutiche, Università della Calabria Arcavacata di Rende (CS) 87036, Italy Abstract: In agricultural field, polymers are widely used for many applications. Although they were used, in the first time, just as structural materials (inhert polymers), in the last decades functionalized polymers revolutionized the agricultural and food industry with new tools for several applications. Smart polymeric materials and smart delivery systems helped the agricultural industry to combat viruses and other crop pathogens, functionalized polymers were used to increase the efficiency of pesticides and herbicides, allowing lower doses to be used and to indirectly protect the environment through filters or catalysts to reduce pollution and clean-up existing pollutants. This report will review the key aspects of used polymers in agricultural area, highlighting current research in this field and the future impacts they may have. Keywords: Fuctional polymers, smart materials. INTRODUCTION Smart polymeric materials and smart delivery systems helped the agricultural industry combat viruses and Polymers are an important part in everyday life; other crop pathogens, functionalized polymers were products made from polymers range from sophisticated used to increase the efficiency of pesticides and articles, such as biomaterials (prosthetic hip and knee herbicides, allowing lower doses to be used and to joints), to aerospace materials. One of the reasons for protect the environment indirectly through filters or the great popularity exhibited by polymers is their ease catalysts to reduce pollution and clean-up existing of processing. Polymer properties can be tailored to pollutants[2-4]. This report will review the key aspects of meet specific needs by varying the “atomic used polymers in agricultural area, highlighting current composition” of the repeat structure and by varying research in this field and the future impacts they may molecular weight. The flexibility can also be varied have. through the presence of side chain branching and according to the lengths and the polarities on the side BIODEGRADABLE POLYMERS IN THE chains. The degree of crystallinity can be controlled AGRICULTURAL FIELD through the amount of orientation imparted to the polymers during processing , through copolymerization, In the last decade, one of the problems affecting by blending with other polymers, and via the the environment has been the increased use of plastic incorporation (via covalent and non-covalent materials and their subsequent disposal. Plastics have interactions) of an enormous range of compounds. been used in innumerable applications with little In agricultural field, polymers are also widely used for consideration for their ultimate disposability. many applications[1]. Although they were used, in the Conventional polymers, such as polyethylene and first time, just as structural materials for creating a polypropylene persist for many years after disposal. climate beneficial to plant growth (inhert polymers), in Built for the long haul, these polymers seem the last decades functionalized polymers revolutionized inappropriate for applications in which plastics are used the agricultural and food industry with new tools for the for short time periods and then disposed. Furthermore, molecular treatment of diseases, rapid disease detection, plastics are often soiled by food and other biological enhancing the ability of plants to absorb nutrients etc.. substances. In other words, the resistance of synthetic Corresponding Author: Francesco Puoci ,Dipartimento di Scienze Farmaceutiche, Università della Calabria - Edificio Polifunzionale Arcavacata di Rende (CS) 87036, Italy, Tel. +390984493151 Fax. +390984493298 299 Am. J. Agri. & Biol. Sci., 3 (1): 299-314, 2008 polymers to the action of living systems is becoming By varying the synthetic blend component and its more and more problematic in several domains where miscibility with starch, the morphology and hence the are used for a limited period of time before becoming properties can be regulated easily and efficiently.[14] waste. Among the various possible routes to eliminate Blends containing thermoplastic starch (non crystalline polymeric wastes, biodegradation and biorecycling via starch) may be blended or grafted with biodegradable bioassimilation are regarded as attractive solutions for polyesters, such as polycaprolactone, to increase environmental protection, when incineration is not flexibility and resistance to moisture. Blends, mainly feasible because it is a source of unacceptable pollution. formed into films and sheets, with more than 85% Biodegradable polymers (BPs) have increasingly starch are used for foaming and injection molding[15]. been used such as plastics substitutes for several By mixing thermoplastic starch with cellulose applications in the agriculture field[5-7]. BPs, disposed in derivatives, rigid and dimensionally stable injection- bioactive environments, degrade by the enzymatic molded articles result. Chemically modified plant action of microorganisms such as bacteria, fungi, and cellulose is used in a remarkably diverse set of algae and their polymer chains may also be broken applications. For example, cellulose acetate is used in down by non enzymatic processes such as chemical many common applications, including toothbrush hydrolysis. Unfortunately, in the majority of cases, the handles and adhesive tape backing. Studies in simulated properties of natural polymers do not fit the needs of compost environments revealed that cellulose acetates specific applications, and blending with synthetic with degrees of substitution of up to 2.5 are polymers is a route largely used to gain the desired biodegradable. A decrease in the degree of substitution properties[8,9] Convenient candidates for those of cellulose acetate from 2.5 to 1.7 results in a large . applications are natural polymers such as agar, starches, increase in the rate of their biodegradation[16]. An alginates, pectins and cellulose derivatives, along with important application of these biodegradable polymers synthetic biodegradable polymers, such as is their employment in solarization process. Indeed, one polycaprolactone, polylactide and poly (vinyl of the problems afflicting agricultural production is the alcohol).[10-13] (Fig. 1). presence of parasites in the soil that, along with spontaneous weeds, take away nourishment from the soil. In the past, the elimination of parasites and seeds CH2OH CH2OH CH2OH CH2OH of undesirable plants, before a new sowing, was O O O O O n * performed through fumigation with methyl bromide, * OH O OH O n * * OH O OH which has been indefinitely banned for its toxicity. In OH OH OH OH the 1970s, a new approach, solarization,[17,18] which involves covering the soil to be reconditioned with Amylose Cellulose polymeric films, was introduced. The polymeric films COOH OH COOH for this application have to be mechanically resistant, O O O O OHOH OHOH OH OH transparent to visible light, and opaque to infrared * O O n * * O O n * radiation. The optical properties are important because, COOH COOH OH during the day, visible radiation, which passes through Alginate Pectin the film, warms the soil. During the night, when the soil cools by emitting infrared radiation, the film, which is O O * O * n * O n * impermeable to infrared radiation, traps it and thus O n* prevents heat loss. Actually, a film with these optical H OH Polylactide Poly(vinylalchol) Polycaprolactone properties has a micro greenhouse effect on the soil. This technique is largely used today, particularly at Fig. 1: Natural and synthetic biodegradable polymers those latitudes with temperate climate. It makes use of low-density polyethylene with fillers, such as Starch is an inexpensive, annually renewable material phosphates, that increase the opacity to infrared derived from corn and other crops. All starches contain radiation. Solarization guarantees the decontamination amylose and amylopectin, at ratios that vary with the of soils assigned to insemination within 4–6 weeks. At starch source. This variation provides a natural the end of the treatment, the problem of the removal mechanism for regulating starch material properties. and disposal of films has to be resolved. Films based on Starch-based biodegradable polymers can be produced synthetic polymers have to be treated as special waste by blending or mixing them with synthetic polymers. with additional costs. Moreover, there are several 300 Am. J. Agri. & Biol. Sci., 3 (1): 299-314, 2008 problems related to environmental pollution for all acrylates monomers such as diethylene glycol films that, in violation of the law, are burned after their methacrylate (DEGMA), octaethylene glycol use. A biodegradable films, made of natural polymers, methacrylate (OEGMA), acrylamide (AA), for solarization offers the advantage that it does not tetraethylene glycol methacrylate (TEGMA) and N,N’- have to be removed from the soil after they are used. In methylenebisacrylamide (MNBA) as cross-linking literature film for solarization containing alginates, agent in 2,4-D and CMPA release[23]. The same authors poly(vinyl alcohol) and glycerol are reported.[19,20] proposed a device characterized by Poly(methacrylic Alginates are water-soluble linear copolymer, acid) cross-linked with divinylbenzene (DVB), containing (cid:1)-gluronic acid and (cid:2)-mannuronic acid units, derivatized with hydrazine, ethylene diamine and present in seaweed.[21,22] The proposed materials were examethylendiamine followed by modification with the characterized in terms of the mechanical parameters acid chlorides of 2,4-D and CMPA. Polymeric and optical properties. The films were transparent in the materials, which have properties of fertilizers and visible region and opaque in the infrared region to herbicides at the same time, were also obtained by ensure a micro greenhouse effect on the soil. binding 2,4-D derivatives (tartrate and glutarate) to hydrophilic polymers such as polyamides, polyureas, POLYMERS FOR AGROCHEMICALS poly(Schiff base)s and polyesters and their release was DELIVERY investigated. Akelah et al. proposed one easy synthesis of polyamides containing pendant hydroxyl groups The use of conventional agrochemicals produce from diethyil-L-tartrate and various diamines[2]. The undesirable side-effects such as their incorporation into polymers obtained in this way represent a potentially the food chain and the contamination of the surrounding bioactive degradable supports able to release an environment. Persistent and less persistent herbicides herbicid, 2,4-D, in the soil (Figure 2). are either inadequate because the first may be hazardous for humans care while the seconds necessitate of multiple applications with high exposure CHNH COCH CH CONHCH to operators and high cost. Polymer supported 2 x OCOCHO 2 y herbicides overcome the serious environmental OH 2 Cl n problems of the conventional herbicides because they release the herbicide to the plant at a controlled rate in the quantities required over a specified period of time. Cl Fig. 2: 2,4-D derivatized polyamides. There are two different approaches in combining the agrochemicals agents with polymeric materials: either by physical combination (encapsulation, or The rate of release of 2,4-D from copolymers is heterogeneous dispersion) to act as a rate controlling dependent on the polymeric hydrophilicity, the pH and device, or by chemical combination to act as carrier for the temperature of medium. When the hydrophilicity increase the increased diffusion of medium in the the agent. The choice of the best system to release the polymer allow the attack upon the bonding group. The active agent is highly dependent on the biological and chemical properties of the agent and on its change in the pH and temperatures of the medium physicochemical interactions in the system. A wide and surrounding the polymers, showed an increase in the diverse range of polymer compositions have been used rate of the hydrolysis reaction. Cross-linked gel based to fabricate agrochemicals delivery devices. Particular on polyacrylamide derivatized with various diammine were also tested by Kenawy et al. in order to realize a interest was focussed on polymers containing herbicides as pendant groups (Pentachlorophenol slow release of chemically bounded 2,4-D[24](Figure 3). (PCP), 2,4-dichlorophenoxyacetic acid (2,4-D) and 4- chloro-2-methylphenoxyacetic acid (CMPA). Materials CP CONHRNHCOCH2O CP = crosslinked able to release an herbicide in the soil, and at the same Cl polyacrylamide time, to increase the water retention by sandy soil, were R = -(CH)-;-(CH)O(CH)O(CH)-; 22 22 22 22 realized linking covalently the herbicides to hydrophilic -(CH)O[(CH)O]-(CH)-; polymers. Hydrogels prepared from oligooxyethylene Cl -(CH22)33O[(CH22)44O]49-(CH22)33-; methacrylates were used to link CMPA via ionic and Fig. 3: 2,4-D chemically bounded to crosslinked covalent interactions. So1aro and coworkers polyacrylamide via various diamines. synthesized hydrophilic polymers based on different 301 Am. J. Agri. & Biol. Sci., 3 (1): 299-314, 2008 In vitro release experiments of 2,4-D herbicide external phase and internal phase solvent etc.. The drug from acrylamide-based gels suggested that the release entrapment can be improved especially by increasing rate is dependent on the pH of the medium; it was polymer-solvent ratio or/and decreasing pH of slower in acidic medium than in neutral or alkaline continuous phase. The drug loading can increase from medium, and on the length of the side-chain spacer 7.7 to 27% by modifying the process parameters. group (R). Instead 2,4,5-trichlorophenoxyacetic acid Particle size can be controlled absolutely by stirring (2,4,5-T) was encapsulated in a matrices based on speed and polymer-solvent ratio. starch-g-poly(vinyl alcohol) (starch-g-PVA). These ones were prepared by saponification of the corresponding poly(vinyl acetate) (starch-g-PVAc) with NaOH[25]. Starch-g-poly(butyl acrylate) was employed by Zhu et al. to encapsulate alkali-sensitive chemicals, such as carboxylic-containing herbicides. 2,4-D and 2,4,5-T were indeed efficiently encapsulated in the modified starch matrix and slow release behaviour was observed[26]. Fig. 4: SEM micrographs of ethylcellulose The Variation of grafting ratio, molar ratio of microspheres loaded by 2,4-D NaOH to acetate groups, herbicide content and particle size all influence the swellability and release rate. In The release rate depends strongly on the particle particular increasing the grafting ratio, herbicide size because that drug liberation is governed by content, particle size or decreasing the molar ratio of diffusion process throughout ethylcellulose NaOH to acetate, a slow release of herbicide was microspheres. Microcapsules of natural rubber, PE, observed. Moreover the interaction between the copolymers of VC-acrylic acid esters and copolymers hydroxyl group present on the matrices and the of cyclopentadiene with a glyceryl ester of an unsatured carboxylic-containing herbicides is favourable to slow fatty acid were also prepared by film-forming on the release. Taki Et al.reported on a controlled delivery fertilizer granules in order to obtain a water insoluble systems for the herbicide N-(3,4-dichlorophenyl)-N,N- coating around the fertilizer which should be modulate dimethyl urea (Diuron) obtained by coprecipitation, in the delivery of the drug[1]. Coating fertilizer granules methylene chloride, using a supercritical antisolvent based on phenol or urea-formaldehyde, urea or furfuril technique (SAS) which allows to load the herbicide in alcohol, urethane, epoxyresins have been employed[1]. amorphous microparticles of a biodegradable polymer Controlled-release urea fertilizers based on acrylamide (L-polylactic acid, L-PLA)[27]. The SAS process copolymers was evaluated by Abraham et al.[30]. involves the spraying of the solution composed of the Acrylamide was crosslinked with comonomers with solute and of the organic solvent into a continuous different chemical properties, such as divynilbenzene supercritical phase flowing cocurrently. The paper (DVB), N,N’-methylenbisacrylamide (NNMBA), reports the first results of this work and concerns the tetraethyleneglycol diacrylate (TEGDA), and influence of various process parameters upon the size pentaerythrytol triacrylate (PETA) as crosslinking and morphology of the particles formed, i.e. pressure, agents, in order to provide properties like rigidity and temperature, composition and flow rates. Successively proper hydrophobic-hydrophilic balance. The release other coating materials such as the poly(methyl experiments showed that the slow-release property is methacrylate) (PMMA) were also tested but did not maximum for the TEGDA containing system. give convincing results for the researchers[28]. A Rudzinsky et al. instead proposed pH-responsive biodegradable polymeric microspheres (Figure 4) based hydrogels for the encapsulation of a water insoluble on ethyl cellulose loaded with 2,4-D were prepared by pesticide and a water soluble micronutrient, such as the emulsion solvent-evaporation technique to develop cypermethrin and cupric sulphate respectively, based on controlled release formulations that protect the acrylic monomers[31]. herbicide from photodegradation and evaporation and Copolymers, with different hydrophilic mechanical to reduce the environment pollution[29]. In this work and thermodynamic properties, based on methyl Elbahri et al. studied the process control parameters of methacrylate (MMA), methacrylic acid (MAA) and 2- microencapsulation and factors controlling the drug hydroxyethyl methacrylate (HEMA), using ethylene release like stirring speed, stabiliser concentration, glycol dimethacrylate as crosslinker, were prepared by polymer concentration, drug-polymer ratio, pH of either a solution or bulk polymerization. 302 Am. J. Agri. & Biol. Sci., 3 (1): 299-314, 2008 Dynamic swelling and equilibrium studies, varying pH the active agents in order to improve the life span and conditions, indicated a pH dependency of water uptake quality of plants in dry arid areas. properties. Swelling was higher in alkaline medium than in water or acidic media (Figure 5). H H H H C C C C C CH C C C C * HC2HOCH n * + H3PO4 D HHO2 OP OH2 OHHHO2 OP OH2OHHHO2 OP O PVA-DP OH O OH H H H C C C C C CH C C C C H2 O H2 H H2 OH H2O H2OH HO P O HO P O PVA-P OH OH H H H H C C C C C CH C C C C Na2CO3 H2 O H2 OHH2 O H2OH H2 O Fig. 5: Photographs showing different hydrophilicities NaO P O HO P O HO P O PVA-PNa at 30°C in 0.1N NaOH: (A) polymer samples ONa O ONa H H H iInII ,t haenidr (dDry) Psotalytem, e(rB I)IPI.o lymer I, (C) Polymer HC2 OC HC2 HC HC2 OCHH HC2OC HC2OCH HO P O NaO P O Encapsulation of cypermethrin and cupric sulfate ONa ONa PVA-PNa2 was possible in all the hydrogels synthesized. The in vitro release experiments showed that of Cu2+ release Fig. 6: Preparation of PVA-P and PVA-DP depends on the hydrophilicity of polymer sample and nature of incapsulation solvent (Acetone, DMSO, Guo et al. described another superabsorbent and aqueous NH OH), while it is not affected by the release devices on the preparation for the slow-release 4 chemical interaction between the cupric ions and the membrane-encapsulated urea fertilizer with polymeric structure, and the release of cypermethrin superabsorbent and moisture preservation depends upon the nature of the solute as well as the (SMUSMP)[33]. SMUSMP granules were formed by a rigidity of the polymeric matrix. An important class of core of urea, coated by a crosslinked starch (first layer) agrochemicals delivery devices are the superabsorbent and a copolymer of acrylic acid (AAc) and acrylamide polymers, able to absorb large amounts of waters. A (AA) (second layer) (Figure 7). superabsorbent polymer with slow-release phosphate fertilizer was prepared by esterification of poly-vinyl alcohol (PVA) and phosphoric acid (SAPSRPF) by Zhan et. al. PVA and phosphoric acid react in water at 80°C and carried out to a mixture of polyvinyl alcohol mono- and diphosphate, (PVA-P and PVA-DP respectively) under granular shape (Figure 6)[32]. The authors reported on the effects of the reaction time, the amount of Na CO employed in the 2 3 precipitation of polymer, the reaction temperature and the concentration of phosphoric acid on water affinity (WA) of SAPSRPF. The release profile depicted Fig. 7: Swelling Profile for SMUSMP in Soil showed that the total amount of phosphate was released out in 28 day. The phosphate amount (47%) released On the basis of the same mechanism, a coated out in the previous 72 hours is due to fast dissolution of nitrogen fertilizer with slow release and water retention the fertilizer on the SAPSRPF surface layer The authors (CNSW), was prepared by Liang et al[34]. The authors demonstrated that the PVA hydrogel decrease the water described the preparation of urea granules (core) coated evaporation in the soil, and at the same time release the by an inner layer based on urea-formaldehyde (UF) and phosphate content gradually. The authors suggested this by a cross-linked poly(acrylic acid)/ organ-attapulgite water-absorbing hydrogels suitable to reduce the loss of (O-APT) as outer layer (Figure 8). water by evaporation, and at the same time to release 303 Am. J. Agri. & Biol. Sci., 3 (1): 299-314, 2008 the polymers make their incorporation in the food chain very difficult. Biocides polymers could be incorporated into textile fibers, and used for contact disinfectant in many agri-food applications such as clothing but also as cartridge filter of potable or irrigation water. One of most important class of biocide polymers is the quaternary ammonium salt (QAS) one. Polymers containing QAS with at least a long alkyl chain (number of carbon atom (cid:3)8) are usually very effective against a large spectrum of microorganism such as Fig. 8: Cross-sectional schematic view of a CNSW bacteria, algae, etc. The polymers are believed to be fertilizer granule adsorbed onto the negatively charged cell surfaces by electrostatic interaction, followed by the diffusion of Urea-formaldehyde granules coated by a cross- the long alkyl chain through the cell wall. This results linked calcium alginate (inner layer) and by a in a weakening of the cytoplasmic membrane, leading to a leakage of cytoplasmic contents and eventual death superabsorbent polymer based on ammonium acrylate of the cell[40]. Chen et al. reported the synthesis of (outsider layer) were developed as fertilizer with slow functionalized poly(propyleneimine) dendrimers release, superabsorbent and moisture preservation properties (USFSMP)[35]. Du et al. studied the evaluated their antibacterial properties (Figure 9) [41]. differential release rate and patterns of nitrate, ammonium, potassium and phosphate from two polyurethane-coated controlled release fertilizer (CRFs) and the effect of the temperature, the water content and type of release medium on it [36]. A superabsorbent with slow release nitrogen fertilizer (SSRNF) was prepared by inverse suspension polymerization of partially neutralized acrylic acid using N,N’-methylene bisacrylamide as a crosslinker and ammonium persulfate as an initiator in the presence of urea [37]. All these superabsorbent polymers showed excellent water absorbency, water retention, moisture Fig. 9: Scheme of generation 2 poly(propyleneimine) preservation besides its slow-release properties and dendrimer quaternary ammonium biocides generally the water absorbency is strongly influenced with 8 QAC groups on the surface. These by the amount of initiator, crosslinker, and the degree quaternary ammonium dendrimers are very of neutralization of acrylic acid. Moreover the release powerful biocides. The antibacterial properties ratio of the effective nutrient in it was not above 75% depend on the size of the dendrimer, the length on the 30th day. of hydrophobic chains in the quaternary ammonium groups, and the counteranion. Biocide Polymers : As described before, one of the Since these dendrimers are well characterized main problem in the use of conventional biocides is the and monodisperse, they also serve as an extra amount of these compounds respect to the effective system to study the structure-activity required one. This is due to the need to compensate the relationship. The antimicrobial properties of amount waste by environmental agents (rain, sun, wind these dendrimer biocides have a parabolic etc.)[38,39]. dependence on molecular weight, which is In order to solve this undesirable side effects either different from the bell-shaped molecular to the plant or to public health, polymeric biocides were weight dependence of conventional polymer synthesized. Indeed, functionalized polymers biocides. Lee et al. prepared a new polymeric containing biocidal moieties offer several advantages quaternary ammonium salts via coupling respect to traditional herbicides, bactericides etc.. reaction between the dimethylamino group of The polymeric biocides are more persistent and do not N,N-dimethyl-3-aminophenol (DMAP) and leach out any toxic chemicals. Furthermore the size of alkyl halide group of the polymer (Figure 10)[42]. 304 Am. J. Agri. & Biol. Sci., 3 (1): 299-314, 2008 This polymer seems to be of interest because of polymers have attracted considerable interest in the antibacterial activity due to its QAS in addition to the polymeric biocides area[46]. phenol group. It exhibits a stronger antibacterial activity An N-halamine may be defined as a compound than free DMAP. Microbeads based on quaternization containing one or more nitrogen-halogen covalent of poly (4-vinyl pyridine) were reported by Hu et al.. bonds (Figure 12). * OH n O - O Cl ClO + O N NH N H R R Fig. 10: Ammonium salt polymeric biocide O + OH - They showed good antibacterial and antifungal NCl properties and could be successfully utilized for R application in the areas related to biomedical, environmental protection, and water treatment[43]. Fig. 12: synthesis of N-halamine polymeric biocide Instead Kanazawa et al. reported a new classes of polymeric biocides characterized by Phosphonium and The biocidal action of N-halamines was believed to Sulfonium salts (Figure 11). be a manifestation of a chemical reaction involving the direct transfer of positive halogens from the N- halamines to appropriate receptors and/or following oxidative reactions in the microorganism cells. This n* n* transfer reaction could effectively destroy or inhibit enzymatic or metabolic cell processes, consequently resulting in the expiration of the organisms[47]. Moreover, after the antimicrobial activity is lost because of extensive usage, the antimicrobial functions P+ Cl S+ BF - could be regenerated by simply treating the 4 R corresponding materials with halogen-releasing agents R' R'' again. Therefore, N-halamines are powerful and refreshable biocides against a wide spectrum of Fig. 11: Phosphonium and Sulfonium salts polymeric microorganisms[48]. Although several stable water- biocides soluble monomers were developed that possessed excellent biocidal properties, the commercialization of The antibacterial activities of these polymers were these compounds has been impossible because of the examined against E.coli and S.aureus (Gram-positive expensive, exhaustive toxicity testing that must be car- and Gram negative respectively) by the viable cell ried out to gain regulatory approval for new counting method in sterilized distilled water[44,45]. These compounds. However an insoluble polymeric biocide studies also regarding various polymeric Phosphonium might obtain approval for a biocidal water-filter and Sulfonium Salts containing long alkyl chains and application with less expensive testing if it could be their model compounds which possess the same shown that not significant amounts of chemicals are hydrophobic structure as that of the common leached into water flowing through it. Generally in disinfectants (QAS). The polymer with the decyl group order to prepare them, two approaches have been exhibited a higher activity than that of the employed - polymerization of unsaturated side chains corresponding model compound, particularly against on biocidal monomers and modification of commercial S.aurus. Although polymeric phosphonium salts exhibit polymers by introduction of biocidal moieties. Sun et a bactericide properties both Gram-positive and Grma al. proposed modifications of commercial polyamides negative, polymers with sulfonium salts exhibited only (Kevlar, Nomex, etc.) via direct N-chlorination[49]. a high antibacterial activity against Gram-positive These N-halamine provided powerful, durable and bacteria and it was found that their activity increased regenerable antibacterial against both Gram-negative with an increase in MW. Recently, novel N-halamines and Gram-positive bacteria. 305 Am. J. Agri. & Biol. Sci., 3 (1): 299-314, 2008 The same authors studied a synthesis of polymers practices, transport (automotive, aerospace), industrial based on imidazolidin-4-one and hydantoin derivatives activities (miming, metallurgy, chemical), and water monomers. After chlorine bleach treatment, hydantoin and wastewater streams. As a consequence of these and Imidazolidin-4-one units in the grafted copolymers activities, a concentration of these toxic metals in the were transformed into N-halamine structures. The biosphere is created; thus there is ample opportunity for antimicrobial efficacies and stabilities of the polymeric exposure to toxic metals[54]. Considering the harmful N-halamines could be rationalized by the effects of heavy metals, it is necessary to almost totally hydrophilic/hydrophobic property of the substrates and remove them from waste effluents[55], for this reason all mobility/swellability of the polymeric molecules, as over the world industry is forced to diminish down the well as the surface area of the materials and migration acceptable level contents of heavy metal in water and of potentially active chlorine. The hydrophobic industrial wastewaters[56]. When soils are contaminated characteristic of synthetic fabrics had a great influence with heavy metals, the clean-up is one of the most on their antibacterial properties[50]. Because the aqueous difficult tasks for environmental engineering. For bacterial suspension could not make sufficient surface remediating sites contaminated with inorganic contact with the fabrics, a longer contact time was pollutants, several techniques have been developed. All necessary for satisfactory antibacterial results. Other these technique are based on two principles referred to types of polymenc biocides that have recently been as immobilization or mobilization principle[57]. In the prepared and studied; Oh et al. and coworkers have first case, the retention of heavy metals on soil is prepared several series of copolymers and blends, increased, but in this way a not permanent solution is which could be classified as benzimidazoles, polyethers raised, because metals still remain in the soil with and polyesters[51]. The materials appear to have possible long-term side effect on bio-systems. The potential as fungicidal and bactericidal films, but the mobilization methods, which is recently become the monomers from which the polymers are produced seem most common technique for soil decontamination, to have greater activity than the polymers. other recent involve the removal of metals from the soil matrix biocidal polymers that have been prepared and using various washing solution agents. Generally, the evaluated for antimicrobial efficacy include conventional processes to treat contaminated water are poly(hexamethylene guanidine) reacted with chemical precipitation, electrodeposition, poly(acrylic acid) and grafted onto cellulose, crystallization, evaporation, liquid-liquid extraction and polyacrylate with biguanides pendant groups[52]. Rojo membrane processes such as electrodialysis (ED), and coworkers studied modifications of the chemical nanofiltration (NF) and reverse osmosis (RO). structure of eugenol by the incorporation of a However, when contaminants are in low concentrations, polymerizable group, e.g., the methacrylic group[53]. these methods can become inefficient[58]. Furthermore, This approach allows the eugenol derivatives to waste solutions generally contain large quantities of participate in polymerization reactions rather than to various ions which are hazardous for the environment, inhibit them. The new derivatives could be incorporated and selective removal of only one kind of ions is to permanent restorative materials, giving to the needed. One way to obtain the selective macromolecular chains the bactericide effects of decontamination of effluent streams is to employ of eugenol. polymeric ion exchange resins, with the replacement of the undesirable ion by another one which is neutral within environment[56]. Generally, the synthetic ion- POLYMERS FOR HEAVY METALS REMOVAL exchange resins are very versatile materials based on Soil pollution has become an important crossilnked polymers with various geometric pore environmental issue due to changes in the land use structures (microporous, mesoporous, and pattern (urban-industrial expansion, infrastructure macroporous)[59]. The most common crosslinking development, and tourism increase). The soil is a long- agents are divinylbenzene[60] and ethylene glycol term sink for the group of potentially toxic elements dimethacrylate (EGDMA)[61], while many different often referred to as heavy metals, including zinc (Zn), surface functionalities are raised by using comonomers copper (Cu), nickel (Ni), lead (Pb), chromium (Cr) with the desired functional groups at the stage of cadmium (Cd), mercury (Hg) and arsenic (As). The copolymerization or by chemical transformation of the concentrations of these metals in soils are associated formed polymer using appropriate modifying agents. with biological and geochemical cycles and are For the preparation of this kind of materials, the most influenced by human activities, such as agricultural common functional groups responsible of the 306 Am. J. Agri. & Biol. Sci., 3 (1): 299-314, 2008 interaction with the metal ions are styrene[62] and carbonaceous materials, agricultural by-products have weakly acidic or basic functionalities: in particular a been widely studied for metal removal from water. resin based on acrylic acid was employed by Kocaoba et al.[63] for Cr(III) removal, acrylamide (AAm)-based hydrogel with different water absorption properties (Figure 13) are very useful in Pb (II), Hg (II), Cd (II) and Cr (VI) removal[64,65] and amine resins for Cr (VI)[66], Ni (II), Zn (II), Cu (II), Cd (II) and Pb (II)[67]. Fig. 14: Adsorption-desorption mechanism of Pb (II) imprinted polymers. These include peat, wood, pine bark, banana pith, rice bran, soybean and cottonseed hulls, etc..[71]. Most of these works have shown that plant material could replace water insoluble support rather than the typical water insoluble, synthetic polymers made from polystyrene and divinylbenzene polymer resins, activated carbon, activated alumina, which are often Fig. 13: Optical micrograph of poly(EGDMA-co- expensive and non-regenerable. Unfortunately, AAm) beads: a) dry, and b) wet state. lignocellulosic material in the form of various agricultural by-products possesses little ability to bind Several articles showed that others versatile cations and anions found in solution. In order to functionalities are sulfonate groups and phosphoric enhance this ability, cationic or anionic groups must be groups.[68,69]. They were able to create thermosensitive added to the particular by-product, e.g. cellulose- resins with LCST able to bind Cu (II) ions; finally containing agricultural by-products modified with the thiacrown polymers immobilized on polystyrene- cross-linking reagent dimethyloldihydroxyethylene urea divinylbenzene were employed for Hg (II) removal[70]. and the quaternary amine,[72]. In this way, resins with a The remotion of metals pollulant was also performed by dual functionality for both cations and anions were solid phase extraction (SPE) technique. SPE technique prepared by two separate reaction of by-products. In the is based on the distribution of analyte between aqueous first one, the cross-linking reagent solution and sorbent by mechanisms, such as dimethyloldihydroxyethylene urea and a quaternary adsorption, co-precipitation, complex formation and amine (choline chloride) were employed to add positive other chemical reactions on or in the sorbents. A new charge to the lignocellulosic material. In the second approach to this methodology has been developed using two-step process, the order of reaction was reversed, Ion Imprinted Polymers (IIPs) as sorbents to join high with positive charge added first, followed by the molecular recognition properties to the separation addition of negative charge. These combined reactions technique. Like Molecularly Imprinted Polymers added both cationic and anionic character to the by- (MIPs), IIPs are materials with high recognition products as evidenced by the increased removal properties toward the template molecules around which capacity towards copper cations and chromate anions. they have been synthesized. In IIPs case, the template is One of the most abundant and interesting biopolymers a metal ion, and with this technique polymers for the which can be applied for the recovery of heavy metal selective removal heavy metals ions have been ions from industrial effluents is chitin, and in particular successfully synthesized. Important results were chitosan, the polymer obtained by its alkaline reported for Pb (II) (Figure 14), (Cd (II) and Co (II) deacylation. The main commercial sources of chitin are removal. crab and shrimp shells, though other sources such as In recent years, attention has focused on the fungal biomass, insect cuticle or squid pen may be application of cost effective alternative technologies or used[73]. In chitosan structure, many amine groups can sorbents for treatment of metals contaminated waste be observed, due to the presence of acetylglucosamine streams. Natural materials that are available in large and glucosamine units. This functionalities are quantities, or certain waste products from industrial or responsible for the uptake of metal cations by a agricultural operations, may have potential as chelation mechanism. Indeed, nitrogen atoms hold free inexpensive sorbents, thus several product including electron doublets that can react with metal cations. 307 Am. J. Agri. & Biol. Sci., 3 (1): 299-314, 2008 Furthermore, the amine groups are easily employed in this technique is polyethylenimine, a protonated in acidic solutions, carrying out the highly branched aliphatic polyamine. This formation of electrostatic attraction of anionic macromolecules is useful in mercury removal: Sauer et compounds, including metal anions or anionic dyes[74]. al.[80] reported that when it is functionalized with With physical modification, it is possible to prepare bromo- or chloroacetic acid to give an differently conditioned polymer forms such as powder, aminocarboxylate chelating group, it is able to nano-particles, gel beads, membranes, sponge, effectively binds lead with binding constants similar to honeycomb, fibers or hollow fibers for varied fields of the molecular chelator ethylenediaminetetraacetic acid. application (waste water treatment, biomedical, textiles, By functionalization with sulphur donors, Gohdes et etc.). Chemical modifications are represented by the al.[81] were able to prepare a polymers with loading crossinking of chitosan with opportune agents[75] such capacities substantially higher than those for the base as glutaraldehyde, 1,1,3,3-tetramethoxypropane, one. oxidized-cyclodextrin, ethyleneglycol diglycidyl ether, or hexamethylenediisocyanate. A particular chemical SUPERABSORBENT POLYMERS modification of chitosan involves the imprinting method, which allows to increased sorption capacity for The water lack and the desertification are very metals such as rhodium, copper and gallium[74]. The serious problems for many regions of the world selected metal ions is adsorbed prior to the chemical because, first of all, they compromise agriculture modification, which may consist of a chemical grafting development. or cross-linking step. After this step, a cavity tailored to Desertification is the degradation of land in arid, semi the volumetric space of the target metal for further arid and dry areas resulting from various factors sorption operation. Finally, derivatized chitosan is a including climatic variations, but primarly human base material to prepare Ion Imprinted Polymers, as activities. A valid aim to these problems could come demonstred in Sun et al. work[76]. Starch-based from the use of synthetic materials with good water polymers are another group of useful low-cost sorbent absorption and retention capacities even under high for heavy metals, in particular in Delval et al. work[77] a pressure or temperature. Systems of this type are the very effective starch-based polymer was prepared with Superabsorbent polymers (SAPs). Because of their a crosslinking reaction between starch-enriched flour excellent properties, these SAPs were already well and epichlorohydrin as crosslinking agent in the established in various applications such as disposable presence of NH OH. Cork is also effective in heavy diapers, hygienic napkins, cement, drug delivery 4 metals removal by virtue of its interaction with Ni(II) systems, sensors, and agriculture. In such applications, and Cu(II) ions, as reported by Villaescusa et al.[78]. water absorbency and water retention are essentials. Another technique for removal of metal ions from Their use for agricultural applications has shown wastewaters is the use of functionalized water-soluble encouraging results; they have been observed to help polymers combined with the membrane-based reduce irrigation water consumption and the death rate separation method of ultrafiltration[79]. It consists in of plants, improve fertilizer retention in the soil, and making heavy metals react with a water-soluble increase plant growth rate. Recent articles reported the macromolecular ligand to form a macromolecular modification of these superabsorbent copolymers with a complex. Solution containing macromolecular complex view to enhance their absorbency, gel strength, and is pumped through an ultrafiltration membrane. absorption rate. Raju et al. [82] prepared a series of Unbound chelates pass through the membrane, while superabsorbent copolymers by using acrylamide (Am), metal-loaded polymers are of sufficient molecular size potassium methacrylate (KMA) and 2- to be retained. The advantages of Polymer Filtration hydroxyethilmethacrylate (HEMA) as monomers, and over others metal ion separation techniques are the N,N-methylenebisacrylamide (MNBA) as a cross- following: 1) high selectivity for the metal ion to be linking agent. The influence of various synthetic separated, 2) high reaction rate for the sorption of the parameters such as the monomer concentration, metal ions on the bonding agents, 3) the possibility to crosslinker concentration, and initiator concentration employ commercially available polymers modified to were studied. The experimental results showed that selectively bind the target metal ions 4) The large these superabsorbents polymers have a good quantities of wastewater can be treated in a short period absorbency both in water and NaCl solutions and a fast of time, with a comparatively low investment in swelling capacity. Furthermore, it was observed that equipment. The most common water soluble polymer SAPs determine a considerable increase of the water 308