Potentiometric titration method for determining rates of sulfate reduction in a constructed wetland

The Big Five wetland in Idaho Springs, Colorado, was constructed as a pilot‐scale system to treat metal‐laden acidic mine drainage and to determine optimal design criteria for full‐scale treatment systems. Removal of dissolved metals from the mine drainage occurred primarily through bacterial sulfate reduction and metal‐sulfide precipitation in the substrate of the wetland treatment system. A potentiometric titration method was performed to measure the rate of sulfate reduction in the substrate of the Big Five wetland under optimal conditions for design criteria purposes. Rates of sulfate reduction of 1.6 and 0.94 μmol cnr^‐3 day^‐1 over incubation time periods of 35 and 17 days, respectively, were determined by a titrimetric method from samples of treatment‐system substrate. The titrimetric method was compared with the more commonly employed ³⁵S radiotracer method. Both the titrimetric and radiotracer methods gave similar rates of sulfate reduction based on results of analyses of split samples of treatment‐system substrate. Advantages of the titrimetric method included avoidance of the necessary certification requirements, safety precautions, and expensive equipment and waste disposal associated with the use of ³⁵S‐labeled sulfate in the most commonly applied radiotracer methods. These advantages allowed easier measurement and correlation of additional system parameters including metals and sulfate. For these reasons, the titrimetric method may be useful for inexpensive, initial treatability studies of acidic mine drainages to provide estimates of sulfate reduction and metal loading rates for various mine drainage and wetland substrate compositions.

Keywords:

• acid mine drainage
• constructed wetland
• potentiometric titration
• ³⁵S radiotracer
• sulfate reduction rate