Microbially Mediated Thallium Immobilization in Bench Scale Systems

Results from bench-scale tests for thallium remediation in mining-impacted water are presented and removal mechanisms are discussed. The source water consisted of surface runoff mixed with groundwater from an inactive gold mine in central Montana. Bench scale columns were operated under continuous flow for 225 days to test for microbially-mediated thallium immobilization. Various compositions of straw and steer manure in a gravel matrix provided a source of organic nutrients and sulfate-reducing bacteria sufficient to initiate and maintain microbial sulfate reduction up to 270 mmol/m3d. Hydraulic residence times of 2.7 days produced an aqueous thallium effluent concentration below the analytical detection limit of 2.5 μg/L at 20°C in all the tested columns. These effluent levels were achieved for influent dissolved thallium concentrations varying from 450 to 790 μg/L. An increase in pH between influent (pH 6.9) and effluents (pH 7.5) was observed. Hydraulic conductivity remained relatively constant during the course of the experiments and varied for the different test columns between 0.2 and 10 cm/s. The highest k-values were observed in the horizontal flow column. In addition to the column tests, sterile serum-vial experiments were performed to confirm that thallium sulfide (Tl2S) formation and precipitation was the most likely mechanism for thallium removal. Data from the bench-scale experiments were utilized for the design of an on-site pilot-scale passive treatment system.