Immobilization of Cr(VI) and its reduction to Cr(III) phosphate by granular biofilms comprising a mixture of microbes

Abstract

We assessed the potential of mixed microbial consortia, in the form of granular biofilms, to reduce chromate and remove it from synthetic minimal medium. In batch experiments, acetate-fed granular biofilms incubated aerobically reduced 0.2 mM Cr(VI) from a minimal medium at 0.15 mM day(-1) g(-1), with reduction of 0.17 mM day(-1) g(-1) under anaerobic conditions. There was negligible removal of Cr(VI) (i) without granular biofilms, (ii) with lyophilized granular biofilms, and (iii) with granules in the absence of an electron donor. Analyses by X-ray absorption near edge spectroscopy (XANES) of the granular biofilms revealed the conversion of soluble Cr(VI) to Cr(III). Extended X-ray absorption fine-structure (EXAFS) analysis of the Cr-laden granular biofilms demonstrated similarity to Cr(III) phosphate, indicating that Cr(III) was immobilized with phosphate on the biomass subsequent to microbial reduction. The sustained reduction of Cr(VI) by granular biofilms was confirmed in fed-batch experiments. Our study demonstrates the promise of granular-biofilm-based systems in treating Cr(VI)-containing effluents and wastewater.

FIG. 1.

Stereo zoom microscopic (A) and scanning electron microscopic (B) images of aerobic granular biofilms. Scale bar in panel A, 1 mm.

FIG. 2.

Effect of granular biomass on Cr(VI) removal under aerobic conditions. The blank lacked granular biofilm. MM (minimal medium) received 5 g granular biomass and lacked carbon source. Each data point is a mean of three independent experiments. Error bars represent ± 1 standard deviation.

FIG. 3.

Cr(VI) removal by granular biofilm incubated anaerobically. Each data point is a mean of duplicate experiments. Error bars represent ± standard deviation.

FIG. 4.

Cr(VI) removal by granular biofilms under anaerobic conditions. The initial granule content was constant (10 g wet weight). Each data point is mean of three independent experiments. Error bars represent ± 1 standard deviation.

FIG. 5.

Chromium immobilization by aerobic granular sludge in anaerobic fed-batch conditions. The arrows indicate replacement of spent medium with fresh AMM-Cr(VI) medium. The initial concentration of Cr(VI) was 0.2 mM.

FIG. 6.

XANES spectra of Cr speciation in granule samples incubated aerobically and anaerobically.

FIG. 7.

The fitted k²-weighted (2.5 to 12.5 Å) (A) and Fourier-transformed (B) spectra showing association of 1 mM Cr(III) with bacterial cells. Solid lines represent experimental data, and dashed lines represent fitted data.