Biodegradation of thiocyanate in mining-contaminated groundwater

Abstract:

In-situ SCN^- biodegradation as a strategy for remediating contaminated groundwater remains largely unproven. This study aimed to culture and characterise a community of SCN^--degrading microbes from mining-contaminated groundwater, and to optimize the efficiency of this process under varied geochemical conditions.

A gold ore processing plant in Victoria, Australia, has generated high amounts of thiocyanate (SCN^-)-contaminated waste effluent. This effluent collects in a tailings storage facility (TSF) on site and seepage has contaminated local groundwater. This SCN^- plume recently escaped the mine lease in a plume flowing partly through a confined aquifer and partly along buried paleochannel gravels.

Groundwater samples were collected using a low-flow pump from two bores near the TSF. The pH of the SCN^- contaminated groundwater typically varies between 4 and 6, and dissolved O₂ varies between 1 and 40 ppm. SCN^- concentrations in off-lease groundwater have increased from 10 ppm in 2010 to over 150 ppm in 2015.

Cultures were inoculated directly from the groundwater, and filtered groundwater was used with amendments as the basal growth medium Cultures were subjected to geochemical amendments including changes in dissolved O₂, pH, SCN^- concentration and additions of organic carbon, phosphate or both. The enriched microbial consortia could not degrade thiocyanate under anoxic conditions, but some could completely degrade high concentrations of SCN^- (>800mg L^-1) under oxic conditions. Biodegradation accelerated with the addition of phosphate, while the addition of organic carbon actually limited the rate. SCN^- degrading cultures are undergoing DNA sequencing for species identification and comparison to SCN^--degrading cultures inoculated from surface waters in the TSF.