Inhibition of Microbial Sulfate Reduction by Molybdate

Hydrogen sulfide generation in oil production is a constant cause of concern. In several cases this phenomenon occurs due to the action of sulfate-reducing bacteria. Various methods, including biocides and nitrate addition, have been used to control the activity of these microorganisms, but they are expensive and have side effects. Other inhibition methods, such as the use of molybdate, have received much less attention. This study performed a kinetic test for the inhibition of sulfate reduction, and short-term tests to determine the minimum inhibitory concentration of molybdate in liquid medium containing 10.4 and 20.8 mM of sulfate (1000 mg L-1 and 2000 mg L-1), respectively. An inoculum was prepared from a microbial consortium of sulfate-reducing bacteria dominated by Desulfovibrio vulgaris enriched from produced water from oil wells of the Reconcavo Basin, Brazil. The indicators of bacterial activity used were: sulfate reduction, sulfide production, culture medium color, pH, and redox potential. The tests show that, regardless of the concentration of molybdate, sulfate reduction is inhibited after the first hours of the start of the test. The conversion of sulfate in the kinetic experiment without molybdate is 78%, whereas the average conversion in the experiment with molybdate is approximately 30% in 216 h. The short-term test results show that 0.08 mM (12.8 mg L-1) molybdate, a molar ratio molybdate/sulfate of 0.004, is sufficient to inhibit the activity of sulfate-reducing bacteria for 168 h.