Ethane-driven chromate and nitrate bioreductions in a membrane biofilm reactor

Abstract

Chromium (Cr(VI)) and co-contaminating nitrate (NO3–) pose major threats to human health. Hydrogen- and methane-driven Cr(VI) and NO3– bioreductions have been documented, but unknown is whether or not non-methane short-chain gaseous alkanes (SCGAs), such as ethane (C2H6), also can drive Cr(VI) and NO3– reductions. This study demonstrated simultaneous Cr(VI) and NO3– bioreductions driven by C2H6 oxidation in a membrane biofilm reactor (MBfR) that delivered C2H6 directly to a biofilm by its diffusion through non-porous membranes. Scanning electron microscopy and X-ray photoelectron spectroscopy showed that the biofilm was mainly composed of cocci and bacillus and that a Cr(III) precipitate was the major product of microbial Cr(VI) reduction. Microbial community analysis showed that the C2H6-oxidizing bacterium Mycobacterium oxidized C2H6 and delivered organic intermediates to Cr(VI)-reducing bacteria TM7a, Sediminibacterium, and Deinococcus. Because the first step of ethane catabolism is a monooxygenation, the influent had to have dissolved oxygen, and the dominant respiratory process was O2 reduction. By providing insights into how Cr(VI) and NO3– bioreductions can be driven by C2H6 oxidation, these finding support the application of SCGAs for ex-situ bioremediation of waters contaminated with Cr(VI) and NO3–.