Effects of Sulfide and pH on Microbial Sulfate Reducing Efficiency

Abstract

Semi and minuspassive bioreactors using sulfate reducing bacteria (SRB) have been increasingly utilized in acid mine drainage (AMD) affected sites. When designed appropriately, these systems are sustainable, cost effective, and have relatively low labor requirements. This study examined the effects of sulfide toxicity and pH on the activity of SRB, in order to improve the efficiency of sulfate reducing bioreactors. It was found that 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 lower sensitivity to sulfide. Two types of studies were undertaken. First, long-term column studies were utilized that employed five rock-filled columns. Second, 21-day static bottle studies were utilized to examine the effect of removal of sulfide by purging with nitrogen gas 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 the 1000 mg/L sulfate concentration. The influent pH was in the range of 8.0 to 9.3 for the column SRB consortium which ran for 2 years. The 200 % carbon columns were able to reduce more sulfate than the 50 % columns, although the carbon utilization efficiency of the 50 % columns was greater. For the 21-day static bottle SRB experiments, the best pH range was 7 to 8. The difference between the columns and the static bottles is probably related 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 incomplete SRB oxidizers initially while later being out competed by complete oxidizers, which prefer a higher pH range. The highest sulfate reduction reached in the 21-day static bottles was 50% using a 200% carbon source supply. The low sulfate reduction in the 200% carbon fed bottles may be due to the short acclimation period, the species of SRB consortium, 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 trends that 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 SRB reducing efficiency.