Removal of hexavalent chromium by new quaternized crosslinked poly(4-vinylpyridines)

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Abstract

New quaternized crosslinked poly(4-vinylpyridines) prepared by nucleophilic substitution reactions of 4-vinylpyridine: divinylbenzene copolymers of gel and porous structure with halogenated compounds such as benzyl chloride and 2-chloracetone, were used to remove Cr(VI) from the aqueous solution. Batch adsorption studies were carried out to determine the effect of the initial concentration of Cr(VI), pH, temperature and the presence of sulfate anions. The process was found to be pH and concentration dependent. The adsorption capacities increase with the increase of the initial concentration of Cr(VI) and both resins exhibited the degrees of usage of the exchange capacities higher than 90% and good efficiency in the chromium removal. Equilibrium modeling of the process of Cr(VI) removal was carried out by using the Langmuir and Freundlich isotherms. The experimental data obeyed these isotherm models. The thermodynamic parameters (free energy change ΔG, enthalpy change ΔS and entropy change ΔH) for the adsorption have been evaluated and therefore, it was showed the spontaneous and endothermic process of the adsorption of Cr(VI) on the pyridine resins. In the competitive adsorption studies, chromate/sulfate revealed the selectivity of the pyridine adsorbents towards chromium ions. At acidic pH the synthesized pyridine resins offer much greater chromate removal capacities compared to alkaline pH. In the competitive adsorption studies, chromate/sulfate revealed the selectivity of the pyridine adsorbents towards chromium ions due to the formation a sandwich arrangement with the chromium anion and functional groups attached to the quaternary nitrogen atom.

Introduction

Metals and metallic compounds are essential for economic growth of any nation. Like many other metallic species chromium also has variety of applications. Several industries such as paint and pigment manufacturing, stainless steel production, corrosion control, leather tanning, chrome plating, wood preservation, fertilizers, textile, photography, etc., discharge effluents containing Cr(VI) [1]. Disposal of untreated industrial effluents is a source of pollution of aquatic systems. Despite a number of alternative technologies suggested to replace chromium, it is particularly difficult to substitute metal finishing industries because of its hardness, bright appearance, corrosion resistance ease of application, and low cost.

Spent chrome liquors and waste from tanneries contain 2900–4500 and 10–50 mg L−1 of chromium, respectively [2]. As redox active metal chromium usually exists as Cr(III) and Cr(VI) species in the environment. The hexavalent species may be in the form of dichromate (Cr2O72−), hydrochromate (HCrO4) or chromate (CrO42−) in a solution depending on concentration and pH. Due to the repulsive electrostatic interactions these anion species are generally poorly adsorbed by the soil particles in the environment and transfer freely in the aqueous environment. Cr(VI) is reported to be 100 times more toxic than Cr(III) which makes the former a major environmental hazard [3]. Exposure to Cr(VI) causes dermatitis, allergic skin reactions and ulceration of intestine. It is a known carcinogenic agent. The maximum permissible levels of Cr(VI) in potable and industrial wastewater are 0.05 and 0.25 mg L−1, respectively [4]. Installation of a physico-chemical treatment plant to destroy or remove toxic materials is the main method used to treat the wastes generated from electroplating operations. Kinetics of chromium transformations under typical environmental conditions were investigated using batch and column experiments. Some of the investigated reactions such as the reduction of Cr(VI) by Fe(II) under anaerobic conditions were instantaneous and the results followed the stoichiometry of the red-ox equation [5]. The goal of all industry should be the recovery and reuse of the valuable material contained in the generated wastes, and especially in the case of hazardous wastes.

Alternative approaches such as adsorption, reverse osmosis, electrolytic recovery techniques, precipitation, ion exchange and liquid–liquid extraction have also been shown to work for treatment of chromium containing effluents and waters. A number of different types of adsorbents such as, including activated carbon [6], [7], [8], [9], [10], [11], bioadsorbents [12], [13], [14], [15], [16], [17], wine processing [18] and distillery [19] sludges, residual lignin [20], amine modified coconut [21], and synthetic polymers [22], [23], [24] were shown to remove chromium from waste water. Solvent extraction is a convenient but expensive technique for the removal of chromate ions from aqueous solutions. Literature data report that the extractants containing tertiary amine group Alamine 336 and Aliquat 336, are effective reagents for the removal of Cr(VI) [25], [26], [27]. Among the various treatment techniques available, the most commonly used ones are ion exchange and adsorption.

The main advantages of ion exchange are high degree of metal recovery, selectivity, lesser sludge volume produced and the ability to meet strict discharge specification and reduce dissolved chromate concentration below detectable levels in the water treatment plants.

Ion exchange using synthetic resins is the method of choice for removing toxic contaminants from water or waste water. The anion exchange processes can be used for the removal of chromium from wastewater at alkaline and acidic pH in the presence of a high concentration of chloride, sulfate, bicarbonate and nitrate ions [28], [29].

The selectivity of the commercial polystyrene or polyacrylic based strong or weak base anion exchangers for chromate anions was studied in the pH range of 3–8 [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41]. The synthesis of the anion exchangers with higher affinity with chromate than the commercially available anion exchangers at neutral to alkaline pH in the presence of other competing anions, namely, sulfate, chloride, bicarbonate, and nitrate was reported [42].

Strong base anion exchangers containing pyridine moieties have shown high selectivity for Cr(VI) and could represent a viable alternative for the selective retention of chromate ions. The previous work focused on the thermodynamic and kinetic studies of Cr(VI) removal from aqueous solutions by the pyridine resins with different lengths of alkyl substituent namely, methyl, ethyl, butyl or amide functional groups at the quaternary nitrogen atom [43], [44], [45], [46]. Chromium retention capacity depends on the ionic form of the strong base anion exchangers and the substituent type at the quaternary ammonium atoms. Selectivity of these strong base anion exchangers was influenced by the concentration of chromium, the ionic form of the resin and the substituent type at the quaternary ammonium atoms.

In this paper the primary objective is to synthesize pyridine resins with novel functional groups for selective retention of Cr(VI). The functional groups of the new anion exchange resins namely, benzyl and ketone groups are attached to the quaternary nitrogen atoms by the nucleophilic substitution reactions of the 4-vinylpyridine (4-VP): divinylbenzene (DVB) copolymer of gel and porous structure with halogenated compounds such as, benzyl chloride (BCl) and 2-chloroacetone (ClA). The retention studies of the synthesized pyridine resins towards Cr(VI) were performed for the different conditions, the parameters that influence adsorption are initial chromium concentration, contact time, pH, resin amount, temperature and the presence of sulfate anions as competitive anions.

Section snippets

Materials

4-Vinylpyridine supplied from Fluka was purified by vacuum distillation immediately prior to use. Divinylbenzene supplied from Fluka, of technically purity (80%) contains residual mainly 3- and 4-ethyl vinylbenzene.

The starting 4-VP:8%DVB copolymers of gel morphological type was synthesized by aqueous suspension copolymerization of 4-VP with DVB using 1.5 wt. % of benzoyl peroxide as initiator according to [35], [36]. To obtain a porous copolymer the copolymerization reaction was carried out in

Synthesis of the pyridine resins

Physicochemical characteristics of yielded these resins are presented in Table 1.

From this table it can be seen that both the synthesized strong base anion exchangers have high values of the strong base exchange capacities in comparison with those of the commercial strong base anion exchanger, Amberlite IRA 400, i.e., 4.57 meq g−1 and 1.25 meq mL−1. The gel and porous structures of the pyridine resins were evidenced by scanning electron microscopy of the beads cross-section, SEM images clearly show

Conclusions

Nucleophilic substitution reaction of 4-VP:DVB copolymers of gel and porous type with benzyl chloride and 2-chloracetone as halogenated compounds led to the novel pyridine strong base anion exchange resin with benzyl and ketone functional groups attached to the quaternary nitrogen atoms. These resins exhibited good values of the strong base exchange capacity in comparison with the commercial strong base anion exchange resin, Amberlite IRA 400.

Adsorption study of hexavalent chromium on new

Acknowledgement

Dr. Violeta Neagu expresses her thanks to the Royal Society, UK which has financed the project entitled “Removal of heavy elements from water by novel strong base anion exchangers” in 2008, a part of which is presented in this study.

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