Microbial Fermentation of Organic Carbon Substrates Drives Rapid pH Neutralization and Element Removal in Bauxite Residue Leachate

Globally, mineral processing activities produce an estimated 680 GL/yr of alkaline wastewater. Neutralizing pH and removing dissolved elements are the main goals of wastewater treatment prior to discharge. Here, we present the first study to explicitly evaluate the role of microbial communities in driving pH neutralization and element removal in alkaline wastewaters by fermentation of organic carbon, using bauxite residue leachate as a model system, and evaluate the effects of organic carbon complexity and microbial inoculum addition rates on the performance of these treatment systems at laboratory scale. Rates and extents of pH neutralization were higher in bioreactors fed with simpler organic carbon substrates (glucose and banana: 6 days to reach pH ≤ 8) than those fed with more complex organic carbon substrates (eucalyptus mulch: 15 days to reach pH ≤ 8; woodchips: equilibrium pH around 9). Concentrations of dissolved Al, As, B, Mo, Na, S, and V all significantly decreased after bioremediation. Increasing soil inoculant addition rate accelerated rates and extent of pH neutralization and element removal up to 0.1 wt %; further increases had little effect. Overall, glucose added at 1.8 wt % and soil inoculum added at 0.1 wt % provided the most effective minimal combination of carbon substrate and inoculum to drive pH neutralization and element removal.