Thiocyanate (SCN-) is a relatively stable contaminant in aquatic systems with both natural and anthropogenic origins. In gold mining, this compound is generated by a reaction between cyanide, which is commonly used for gold extraction, and sulphur compounds present in waste streams. Some microorganisms are capable of thiocyanate degradation as energy and/or carbon source and also to obtain nitrogen and/or sulphur. The inherent advantages of microbial consortia over single strains or co-cultures, especially when performed in large-scale bioremediation systems, necessitate detailed study of the effects of operational parameters on the efficiency of thiocyanate biodegradation.
Two field-scale treatment systems were implemented at a Victorian gold mine to remediate thiocyanate-contaminated tailings pondwater and groundwater. Each system was comprised of separate containment tanks dedicated to SCN- degradation, nitrification and denitrification. Samples were collected at regular intervals from each tank for metagenomic analysis using the Illumina Novaseq platform. Concentrations of SCN-, ammonium, nitrate, sulphate, dissolved oxygen, pH, and temperature were tracked over time.
Results showed that simple amendments of phosphate solutions could trigger the onset of microbial thiocyanate biodegradation. However, tailings pondwater tanks showed considerable algal growth, as they were open to sunlight, while closed bioreactor tanks for groundwater treatment were dominated by prokaryotes (autotrophic bacteria). Variation in the type and relative contribution of thiocyanate degrading pathway (algal vs bacteria vs photochemical) in each treatment system are discussed in the context of changing environmental conditions and water quality.