Effects of Sulfide and pH on Microbial Sulfate Reducing Efficiency
AdvisorMiller, Glenn C
Natural Resources and Environmental Science
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Semi&minuspassive bioreactors using sulfate reducing bacteria (SRB) have beenincreasingly utilized in acid mine drainage (AMD) affected sites. When designedappropriately, these systems are sustainable, cost effective, and have relatively low laborrequirements. This study examined the effects of sulfide toxicity and pH on the activityof SRB, in order to improve the efficiency of sulfate reducing bioreactors. It was foundthat compared to sulfide, pH had a bigger influence on the sulfate reducing efficiency.The results indicated a high sensitivity of the SRB to pH, and a relatively lowersensitivity to sulfide. Two types of studies were undertaken. First, long-term columnstudies were utilized that employed five rock-filled columns. Second, 21-day static bottlestudies were utilized to examine the effect of removal of sulfide by purging with nitrogengas on the ability of SRB to reduce sulfate. For the column studies, the columns were feda glycerol and methanol mixture that could theoretically remove 50 % or 200 % of the1000 mg/L sulfate concentration. The influent pH was in the range of 8.0 to 9.3 for thecolumn SRB consortium which ran for 2 years. The 200 % carbon columns were able toreduce more sulfate than the 50 % columns, although the carbon utilization efficiency ofthe 50 % columns was greater. For the 21-day static bottle SRB experiments, the best pHrange was 7 to 8. The difference between the columns and the static bottles is probablyrelated to the stronger buffering capacity of the columns, compared to the static bottles.Even when the column influent pH was 9, the effluent pH was one or two pH units lower.These pH results point to the microorganism consortium being dominated by incompleteSRB oxidizers initially while later being outcompeted by complete oxidizers, whichprefer a higher pH range. The highest sulfate reduction reached in the 21-day staticbottles was 50 % using a 200 % carbon source supply. The low sulfate reduction in the200 % carbon fed bottles may be due to the short acclimation period, the species of SRBconsortium, and the inhibition of a series factors such as sulfide, and pH.The toxic effect of sulfide is difficult to separate from pH effects, but there are trendsthat suggest decreases in SRB activity when hydrogen sulfide is higher than 60-70 mg/L.Purging nitrogen into static bottles to remove sulfide significantly improved the SRBreducing efficiency.