Acute toxicity of heavy metals to acetate-utilizing mixed cultures of sulfate-reducing bacteria: EC100 and EC50

Acid mine drainage from abandoned mines and acid mine pit lakes is an impor tant environmental concern and usually contains appreciable concentrations of heavy metals. Because sulfate-reducing bacteria (SRB) are involved in th e treatment of acid mine drainage, knowledge of acute metal toxicity levels for SRB is essential for the proper functioning of the treatment system fo r acid mine drainage. Quantification of heavy metal toxicity to mixed cultu res of SRB is complicated by the confounding effects of metal hydroxide and sulfide precipitation, biosorption, and complexation with the constituents of the reaction matrix. The objective of this paper was to demonstrate tha t measurements of dissolved metal concentrations could be used to determine the toxicity parameters for mixed cultures of sulfate-reducing bacteria. T he effective concentration. 100% (EC 100), the lowest initial dissolved met al concentrations at which no sulfate reduction is observed, and the effect ive concentration, 50% (EC50), the initial dissolved metal concentrations r esulting in a 50% decrease in sulfate reduction, for copper and zinc were d etermined in the present study by means of nondestructive, rapid physical a nd chemical analytical techniques. The reaction medium used in the experime nts was designed specifically (in terms of pH and chemical composition) to provide the nutrients necessary for the sulfidogenic activity of the SRB an d to preclude chemical precipitation of the metals under investigation. The toxicity-mitigating effects of biosorption of dissolved metals were also q uantified. Anaerobic Hungate tubes were set up (at least in triplicate) and monitored for sulfate-reduction activity. The onset of SRB activity was de tected by the blackening of the reaction mixture because of formation of in soluble ferrous sulfide. The EC 100 values were found to be 12 mg/L for cop per and 20 mg/L for zinc. The dissolved metal concentration measurements we re effective as the indicators of the effect of the heavy metals at concent rations below EC100. The 7-d EC50 values obtained from the difference betwe en the dissolved metal concentrations for the control tubes (tubes not cont aining copper or zinc) and tubes containing metals were found to be 10.5 mg /L for copper and 16.5 mg/L for zinc. Measurements of the turbidity and pH, bacterial population estimations by means of a most-probable number techni que, and metal recovery in the sulfide precipitate were found to have only a limited applicability in these determinations.