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

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^–.