Detoxification of mercury by immobilized mercuric reductase

Abstract

Mercuric reductase which originated from a recombinant Escherichia coli PWS1 was purified and immobilized on a chemically modified diatomaceous earth support. The mercury reduction kinetics, pH dependence, storage stability, and reusability of the immobilized enzyme were investigated. Four dyes were examined for their electron transfer efficiency with the soluble and bound mercuric reductase. Continuous mercury detoxification by the immobilized mercuric reductase was also performed in fixed-bed processes. The effects of bed-length, mercury loading rate, and electron donor on the performance of the fixed beds were assessed. Immobilized mercuric reductase exhibited substrate-inhibition-type kinetics with a maximal activity (1.2 nmol Hg mg-1 protein s-1) occurring at an initial Hg2+ concentration of 50 μmoldm-3. The optimal pH was 7.0 for the soluble and immobilized mercuric reductase, but the immobilized enzyme maintained higher relative activity for less favorable pH values. Immobilazation of the enzyme appeared to significantly enhance its storage stability and reusability. Of four artificial electron donors tested, azure A (5 mmol dm-3) demonstrated the highest relative activity (78%) for soluble mercuric reductase. For the immobilized enzyme, neutral red (5 mmol dm-3) gave a relative activity of nearly 82%. With a fixed-bed, the mercury-reducing efficiency of using neutral red was only 30-40% of that obtained using NADPH. Fixed-bed operations also showed that increased bed length facilitated mercury reduction rate, and the optimal performance of the beds was achieved at a flow rate of approximately 100-200 cm3 h-1.