Effective bioreduction of hexavalent chromium–contaminated water in fixed-film bioreactors

Mf Deflaun,R Borch,J Howell,Mm Maleke,E Botes,Pj Williams,R Jordan,A Ojo,E Van Heerden

doi:10.4314/wsa.v40i3.19

Abstract

Hexavalent chromium (Cr6+) contamination from a dolomite stone mine in Limpopo Province, South Africa, has resulted in extensive groundwater contamination. In order to circumvent any further negative environmental impact at this site, an effective and sustainable treatment strategy for the removal of up to 6.49 mg/l Cr 6+ from the groundwater was developed. Laboratory-scale, continuous up-flow bioreactors were constructed to evaluate reduction of Cr 6+ , with a residence time of 24 h, an efficiency porosity of 44% and a flow rate of 1.5 ml/min. Stoichiometrically balancing terminal electron acceptors in the feed water with a selected electron donor, directed reactor balance for complete Cr6+ reduction. The microbial community shifted in relative dominance during operation to establish an optimal metal-reducing community, including Enterobacter cloacae, Flavobacterium sp. and Ralstonia sp., which achieved 100% reduction. Evaluation after reactor termination with SEM-EDX and XRD confirmed the establishment of biofilm on the reactor matrix, as well as trivalent chromium (Cr 3+ ) precipitation within the reactor. Due to gravitational force, high concentrations of Cr 3+ were found in the bottom third of the reactor. Based on the results from the laboratory investigation, a 24 000 l fixed-film pilot bioreactor was designed and constructed at this site. Influent flow rates, electron donor injection and automated sampling were remotely controlled by a programmable logic controller (PLC). Similar to the laboratory column study, steady state conditions could be achieved and successful Cr 6+

Highlights

The study source site is located in the Limpopo Province of South Africa at a facility that actively mines and processes marble and dolomite stone as a graded aggregate for landscaping and construction purposes
The design of the up-flow bioreactor can be applied to a myriad of metal potential contaminants commonly found in aqueous environments by effective management of the redox potential and electron donor/acceptor balancing to direct bacterial biofilm development and steady-state activity
The presence of a suitably adapted microbial community effective for chromium reduction is essential for this system

Summary

Introduction

The study source site is located in the Limpopo Province of South Africa at a facility that actively mines and processes marble and dolomite stone as a graded aggregate for landscaping and construction purposes. Despite efforts to prevent migration of site wastes to the surrounding environment, leaching of Cr6+ into the groundwater has occurred over the 80-year history of the site. Hexavalent chromium is classified as a SABS 0228 danger group 8(II) compound leading to birth defects and reproductive impairment (Arlauskas et al 1985; DWAF, 1998; Kanojia et al, 1998). It can cross the cellular membrane via sulphate transporters and, once inside the cell, can generate reactive oxygen species (ROS), implicated as mutagens and carcinogens, from the cyclic single electron transfer between Cr6+, molecular oxygen and Cr5+ (Ackerley et al, 2004). On the other hand, is considered to be relatively innocuous and usually occurs as insoluble organic and inorganic complexes due to its strong Lewis acid nature (Barceloux, 1999)

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