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

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.