Reduction of toxic hexavalent chromium by Ochrobactrum intermedium strain SDCr-5 stimulated by heavy metals

Abstract

A Cr(VI) resistant bacterial strain SDCr-5, identified as Ochrobactrum intermedium on the basis of 16S rRNA gene sequencing, was tolerant to high concentrations of Cr(VI) up to 15 mg ml⁻¹ in acetate minimal medium. O. intermedium SDCr-5 reduced Cr(VI) under a wide range of concentrations from 100 to 1500 μg ml⁻¹ and reduction was optimum at 37 °C and pH 7. It reduced 200 and 721 μg ml⁻¹ Cr(VI) within 72 and 96 h, respectively. The rate of Cr(VI) reduction increased with concentration from 100 to 1500 μg ml⁻¹. The presence of heavy metal cations such as Cu²⁺, Co²⁺, Mn²⁺ and Ni²⁺ stimulated Cr(VI) reduction. Strain SDCr-5 might be useful for Cr(VI) detoxification under a wide range of environmental conditions.

Introduction

Chromium is widely used in industrial operations such as leather tanning, electroplating, paints, pigment production, steel manufacture and others (Wang and Xiao, 1995, Pattanapipitpaisal et al., 2001). Cr(VI) containing wastewaters from these industries have become a well recognized bio-hazard as Cr(VI) is a known carcinogen and mutagen (Suzuki et al., 1992, Wang and Shen, 1995). In aquatic environments, Cr(VI) usually exists as ${\mathrm{CrO}}_4^{2-}$ that can enter cells via sulphate transport system. Inside the cell, ${\mathrm{CrO}}_4^{2-}$ can oxidatively damage DNA and other cell components via the production of more reactive intermediate species Cr(V) and Cr(IV) to produce its toxic, mutagenic and carcinogenic effects on biological systems (Kaim and Schwederski, 1994, McLean and Beveridge, 2001).

Cr(III) is not carcinogenic or mutagenic. Reduction of Cr(VI) to Cr(III) is a potentially effective method of detoxification of Cr(VI) polluted wastewaters (Mabbett and Macaskie, 2001, Michel et al., 2001). Conventional methods for treatment of toxic Cr(VI) include chemical reduction followed by precipitation, ion exchange and adsorption on alum or kaolinite. Most of these methods, however, need either high energy or large inputs of chemicals (Ohtake and Silver, 1994, Fujie et al., 1996). Microbial reduction of toxic Cr(VI) to less toxic Cr(III) has emerged as an useful bioremedial process and an alternative to the expensive conventional methods (Badar et al., 2000, Mabbett and Macaskie, 2001, Camargo et al., 2003a, Megharaj et al., 2003, Faisal and Hasnain, 2004). Many bacteria including Bacillus (Campos et al., 1995, Camargo et al., 2003a), Enterobacter (Wang et al., 1989), Microbacterium (Pattanapipitpaisal et al., 2001), Pseudomonas (DeLeo and Ehrlich, 1994, Badar et al., 2000, McLean et al., 2000), etc., can reduce Cr(VI) to the less toxic Cr(III). Biological reduction of toxic Cr(VI) offers potential cost effective bioremediation approach. The objective of the present study was to characterize the Cr(VI) resistance and reduction ability of an indigenous bacterial strain SDCr-5 previously isolated from the effluent of dying industry (Sultan and Hasnain, 2003).