Bioreduction of Cr(VI) from acidic wastewaters in a sulfidogenic ABR

In the present study, the biotreatment of synthetic acidic wastewater containing Cr(VI) (35–200 mg L−1), Fe(III) (100 mg L−1) and sulfate (3500 mg L−1), was studied in an ethanol supplemented (2730 mg L−1 COD) anaerobic baffled reactor (ABR) at constant hydraulic retention time (HRT) of 1.0 day. Reactor performance was very high even at low pH (2.5–3.5) and high Cr(VI) concentration (200 mg L−1). Sulfate reduction and COD oxidation degrees exceeded 80% and 90%, respectively, in all cases. The alkalinity generated due to the sulfidogenic oxidation of ethanol increased wastewater pH to neutral values (7.3–8.0) and facilitated precipitation of Cr(III) compounds. Electron flow from ethanol oxidation to sulfate reduction averaged 85.4 ± 13%.The produced precipitates were retained in the ABR as a result of its compartmentalized structure and the presence of a sludge blanket acting as filter. XRD, XRF and SEM studies were carried out to identify the type and morphology of the precipitates formed and further elucidate Cr(VI) removal mechanisms. Cr(VI) precipitates as CrO(OH) as well as chromium sulfide. Part of chromium may be also adsorbed in amorphous precipitated phases.The experimental results prove the high efficiency of the process and its potential application to tackle environmental issues related with acidic wastewaters/leachates generated in the mining and metallurgical industry as well as in chromium containing waste disposal sites.

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► Sulfate reducing bioreactors are viable alternative for the removal of metals.
► Sulfide reduces Cr(VI) to the much less toxic and immobile form of Cr(III).
► Reduced form of chromium precipitates in the bioreactor.
► ABR is efficient for Cr(VI) containing acid mine drainage treatment.
► Cr(VI) precipitates as CrO(OH) as well as chromium sulfide.