Rehabilitating mine tailings water using constructed wetlands
Outukumpu Zinc-Tara Mines Ireland, located approximately 50 km north of Dublin, is Europe's largest producer of lead and zinc ores. Water used during extraction and purification of these ores is enriched with sulfate and heavy metals. Presently, this water together with suspended tailings, is pumped from the mine and stored in a large tailings settling pond. Two experimental artificial wetlands were built beside this settling pond to treat seepage. The primary strategy employed for treatment of the water is the reduction of sulfate to sulfide. The wetlands are maintained anaerobic to provide optimum conditions conducive to reduction reactions. This process is mediated by sulfate reducing bacteria, which are indigenous in the bedding substrate (spent mushroom compost) used. The alkaline nature of the mine tailings water (due to the strong buffering capacity of the prevailing limestone geology) facilitates the subsequent precipitation of metal sulfide complexes which form following reduction of sulfate. The constructed wetlands have been in operation for two years but have exhibited consistent removal of sulfate from water. Recent results suggest that algae, having spontaneously invaded the wetland ecosystems, also play a substantial role in the removal of heavy metals from the tailings water. To-date, this decontamination approach is showing to be efficacious and cost-effective.