Selective removal of chromium (VI) from sulphates and other metal anions using an ion-imprinted polymer

L Chimuka,E Cukrowska,V Pakade,N Torto,J Darkwa

doi:10.4314/wsa.v37i4.11

Abstract

A linear copolymer was prepared from 4-vinylpyridine and styrene. An ion-imprinted polymer (IIP) specific for Cr (VI) adsorption was prepared by copolymerisation of the quaternised linear copolymer (quaternised with 1,4-chlorobutane), 2-vinylpyridine functional monomer and ethylene glycol dimethacrylate (EGDMA), as the cross-linking monomer, in the presence of 1,1’-azobis(cyclohexanecarbonitrile) as initiator. Ammonium dichromate and aqueous methanol were used as a template and porogenic solvent, respectively. Leaching of the chromate template from the polymer particles was achieved with successive stirring of the ion-imprinted polymer (IIP) particles in 4 M HNO3 solutions to obtain leached materials, which were then used for selective rebinding of Cr (VI) ions from aqueous solutions. Similarly, the non-imprinted polymer/control polymer (NIP/CP) material was also prepared under exactly the same conditions as the IIP but without the chromate anion template. Various parameters, such as solution pH, initial concentration, aqueous phase volume, sorbent dosage, contact time and leaching solution volumes, were investigated. Scanning electron microscopy (SEM), Fourier Transform Infrared (FTIR) spectroscopy, BET surface area and pore size analysis were used for the characterisation of IIP (both unleached and leached) and CP materials. Optimal parameters were as follows: solution pH, 3; contact time, 120 min; eluent, 20 mℓ of 0.1 M NaOH; and sorbent amount, 125 mg. Maximum retention capacity of IIP and CP was 37.58 and 25.44 mg∙g-1, respectively. The extraction efficiencies of the IIP and CP were compared using a batch and SPE mode of extraction. In the absence of high concentrations of ions, especially sulphate ions, both CP and IIP demonstrated no differences in binding of Cr (VI), which was almost 100%. However, in the presence of high concentrations of sulphate ions, the selectivity on the CP completely collapsed. The study clearly demonstrates the suitably of the developed IIP for selective extraction of Cr (VI) in complex samples such as those from acid mine drainage. The selectivity was also compared by direct injection of the real-world sample, both spiked and non-spiked, into that obtained after IIP selective extraction. Despite the method’s very low detection limits for direct injection (below 1 μg∙ℓ-1), no Cr (VI) was obtained. However, after IIP selective extraction, spiked Cr (VI) was detected in the spiked sample.Keywords: Ion-imprinted polymer, chromium (VI), acid mine drainage, selectivity

Highlights

Hexavalent chromium Cr (VI)), the most toxic and carcinogenic form of chromium, exists in most aquatic environments as water soluble oxyanions, HCrO41- or CrO42
The pore size distribution for specific surface areas of the sample was determined via N2 adsorption/desorption according to the BET method, using a Micromeritics Tristar surface area and porosity analyser
In order to confirm the accuracy of the results, the analysis was repeated at least twice for all samples and the measurements were in good agreement

Summary

Introduction

Hexavalent chromium Cr (VI)), the most toxic and carcinogenic form of chromium, exists in most aquatic environments as water soluble oxyanions, HCrO41- or CrO42-. Chromium is mostly present in the +3 oxidation state in chromite, but can exist in other oxidation states, ranging from -2 to +6 in different samples Of these various oxidation states that chromium can exhibit, only the hexavalent and trivalent forms are known to have environmental importance (Goyal et al, 2003). Hexavalent chromium has to be strictly regulated and as such the maximum permissible levels set out by the US EPA for both drinking water and wastewater are 0.05 mg∙l-1 for Cr (VI) and 5 mg∙l-1 for Cr (III), respectively (Acar and Malkoc, 2004). Anthropogenic sources, such as mining operations, leather tanning, metal plating, water cooling and pigment manufacturing, are responsible for water and soil contamination by chromium (Katz and Salem, 1994)

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Conclusion