Bio-reduction of soluble chromate using a hydrogen-based membrane biofilm reactor

Abstract

Hexavalent chromium (Cr(VI)) is a mutagen and carcinogen that is a significant concern in water and wastewater. A simple and non-hazardous means to remove Cr(VI) is bioreduction to Cr(III), which should precipitate as Cr(OH)_3(s). Since Cr(VI)-reducing bacteria can use hydrogen (H₂) as an electron donor, we tested the potential of the H₂-based membrane biofilm reactor (MBfR) for chromate reduction and removal from water and wastewater. When Cr(VI) was added to a denitrifying MBfR, Cr(VI) reduction was immediate and increased over 11 days. Short-term experiments investigated the effects of Cr(VI) loading, H₂ pressure, and nitrate loading on Cr(VI) reduction. Increasing the H₂ pressure improved Cr(VI) reduction. Cr(VI) reduction also was sensitive to pH, with an optimum near 7.0, a sharp drop off below 7.0, and a gradual decline to 8.2. Cr(III) precipitated after a small upward adjustment of the pH. These experiments confirm that a denitrifying, H₂-based MBfR can be used to reduce Cr(VI) to Cr(III) and remove Cr from water. The research shows that critical operational parameters include the H₂ concentration, nitrate concentration, and pH.

Introduction

The widespread use of chromate in industries such as leather tanning, metallurgy, electroplating, petroleum refining, textile manufacturing, and pulp production has resulted in large quantities of chromium being discharged into the environment (Barnhart, 1997). In the natural environment, chromium is found in trivalent (Cr(III)) and hexavalent (Cr(VI)) forms. Cr(III) has relatively low toxicity and tends to form insoluble complexes with hydroxides at neutral pH (Anderson and Kozlovsky, 1985; Palmer and Wittbrodt, 1991). On the other hand, Cr(VI) is highly soluble and, therefore, mobile and bio-available in aquatic systems (Dragun, 1988). At relatively high concentrations, Cr(VI) compounds are potent irritants whose acute effects include ulceration of skin, eyes, mucous membranes, and the gastrointestinal tract. At low concentrations, typical of those found in the environment, Cr(VI) has mutagenic and carcinogenic effects (DeLeo and Ehrlich, 1994; National Toxicology Program, 1991; US EPA, 1992). The maximum contaminant level (MCL) for drinking water is 100 μg/l total chromium in the United States (US EPA, 2003).

Conventional drinking water treatment is not effective for removing chromate. Advanced treatment techniques, such as reverse osmosis, ion exchange, membrane filtration, and electrodialysis, are more effective for removing Cr(VI), but are expensive and generate concentrated wastes that require subsequent treatment and disposal (Komori et al., 1990; Srivastava et al., 1986).

Biological reduction may provide a suitable means for Cr(VI) removal from water and wastewater (Lovley and Coates, 1997; Rittmann et al., 2004). Once biologically reduced, Cr(III) can precipitate as Cr(OH)_3(s), which can be removed by filtration.

Cr(VI) is bio-reduced to Cr(III) under aerobic (Bopp and Ehrlich, 1988; Gopalan and Veeramani, 1994) and anaerobic (Lovley, 1993; Wang et al., 1989) conditions. Direct reduction of Cr(VI) in the absence of sulfide positively correlated with nitrate reduction by a nitrate-reducing consortium when straw of cattail was used as the organic carbon source (Vainshtein et al., 2003), and the addition of molasses and nitrate to a microcosm stimulated chromate reduction (Oliver et al., 2003). Chen and Hao (1996) and Wang (2000) reported that environmental factors, including pH, temperature, and other electron acceptors, affected Cr(VI) reduction in an anaerobically enriched mixed culture. Cervantes (1991) indicated that chromate reduction did not stimulate growth. If growth with chromate were possible, the yields would be low since chromate reduction provides less free energy per electron than sulfate (Marsh and McInerney, 2001).

Some Cr(VI)-reducing bacteria can use hydrogen (H₂) as an electron donor (Marsh and McInerney, 2001), and the H₂-based membrane biofilm reactor (MBfR) (Lee and Rittmann, 2000, Lee and Rittmann, 2002, Lee and Rittmann, 2003; Nerenberg and Rittmann, 2002; 2004; Nerenberg et al., 2002; Rittmann et al., 2004; Ergas and Reuss, 2001) is an ideal biological reactor configuration for autotrophic bioreduction of chromate. H₂ is non-toxic to humans, is inexpensive compared to organic donors, leaves no residuals that could cause bacterial re-growth or add oxygen demand, and is used with nearly 100% efficiency in the MBfR setting (Lee and Rittmann, 2002; Nerenberg et al., 2002; Rittmann et al., 2004). H₂-based bio-reduction also supports autotrophic microorganisms that have inherently low biomass yields (Rittmann and McCarty, 2001). In addition, screening studies (Nerenberg and Rittmann, 2004) demonstrated that Cr(VI) was reduced without lag in MBfRs in which oxygen or nitrate was the primary electron acceptor.

In this study, we examine chromate reduction in a denitrifying, H₂-based MBfR. We use a denitrifying reactor because nitrate is a common co-contaminant in surface and ground waters and because nitrate can serve as a primary electron acceptor for chromate-reducing bacteria. In particular, we investigate factors that ought to affect the kinetics of Cr(VI) reduction or the removal of the resulting Cr(III): H₂ pressure, chromate surface loading, the addition of other electron acceptors, and pH. The results provide information important for optimizing chromate reduction and removal in a H₂-based MBfR.