Investigation of Bioremediation of Arsenic-Contaminated Mine Waste Rock Water in a Challenging Low Temperature and Ph Anaerobic Environment

Arsenic (As) released from uranium waste rock stored in pit lakes can be challenging to treat given their low pH, low temperature (~8 °C), and anaerobic environment. Bioremediation via sulfate reducing bacteria (SRB) can immobilize As under anaerobic conditions and has been shown to be cost-effective and environmentally friendly. However, the potential of stimulating SRBs for As bioremediation of pit lake water has not been studied. Thus, we investigated the As bioremediation of waste rock stored in water at in situ temperatures and under anaerobic conditions. Treatments included molasses-only, molasses with sulfate, lactate-only, and lactate with sulfate. Negative controls had autoclaved waste rock with sodium azide, while positive controls included waste rock with no carbon addition. Treatment reactors had the highest bacterial activities with all treatments subjected to bacterial community analysis after 130 days. There was wide variation in community compositions at both the class and genus levels having 15 and 38 groups, respectively, including both SRBs and iron-reducing bacteria (FeRBs). Overall, all treatments resulted in As being released into the overlying waters with concentrations ranging from 1.4 to 2.7 mg/L. In contrast, control treatments had concentrations of 0.1 mg/L. Clearly, the experimental duration was not long enough to allow for As removal from water via mechanisms such as biosorption and precipitation, attributed to the low temperature environment limiting bacterial activity. Molasses was deemed as the optimum carbon source overall and was considered for use in longer term (~1.5 year) experiments using collected mine pit lake water