Inhibition of Microbial Sulfate Reduction by Molybdate

Abstract

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 shortterm 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.

Simulation of the inhibition of microbial sulfate reduction in a two-compartment upflow bioreactor subjected to molybdate injection

“…The starting point for the development of inhibition model associated with the results of the tests of inhibition of microbial sulfate reduction using molybdate previously presented [18] and showed that the sulfate reduction activity and generating sulfide by sulfate-reducing bacteria (SRB) are inhibited when the medium has a molar ratio molybdate/sulfate equal to 0.004. Given this, molybdate transport simulations in the bioreactor were performed for different scenarios.…”

“…Consider first the case in which the reactor is continuously fed with a synthetic saline modified Postgate C with 960 mg/L (10 mM) of sulfate ions and all other nutrients necessary for complete reduction of the sulfate present in the medium are available [18]. According to the results obtained by de Jesus et al (2015) in this situation the inhibition of bacterial activity is achieved when throughout the bioreactor a molybdate concentration profile of 6.4 mg/ L is reached.…”

“…In the inhibition assays the activity of SRB (previously presented [18]), molybdate was dosed in salt form dihydrate sodium molybdate (Na 2 MoO 4 Á2H 2 O). Figure 9a shows the mass of salt used according to the different durations of the experiments.…”

“…The control of SRB using alternative methods based on the application of inhibitors of inorganic origin such as the molybdate has received considerable attention [13][14][15][16][17][18]. Newport and Nedwell (1998) found that 6.4-32.0 mg/L molybdate was sufficient to stop the growth of pure cultures of SRB in a culture medium low in sulfate 480 mg/L.…”

“…It has been found that the inhibition of bacterial activity occurs when a molar ratio of molybdate/sulfate is greater than or equal to 0.004. The variability of results suggests that each system must be investigated individually [18].…”