Detoxification of Acid Drainage Using Inorganic pH Buffers in a Constant Flow Bioreactor

Abstract

To substantially reduce the acclimatization period for SRB in a highly acidic medium, we studied some effective pH buffers for flowing AMD. Silicate buffers increased pH before SRB was introduced. A mixture of AMD and sewage-sludge-impregnated ryegrass from a previous incubation contained an enriched strain of Desulfovibrio vulgaris, one of two major SRBs. They were active at a pH above 5 at 30 °C for six days, producing a black coloration probably because of the precipitation of sulfide, thereby initially suggesting the concentrated presence of viable SRB. The in situ pH was 6.3. The odor of H₂S was detected from the inoculants. An up-flow bioreactor made from rigid PVC tubing with sampling ports placed 5 cm apart in the column was plugged with rubber seals. The treatment consisted of ground ryegrass mixed thoroughly with a mollisol soil. Freshly cut ryegrass ground to <1 mm diameter was hand-mixed with the mollisol in 1:5 ratio (w/w) and the mixture placed on the sand in the column to occupy the column volume, except for a 10 cm wide space at the top of the column. An electrically powered peristaltic pump slowly pumped the AMD into the column from the bottom, until the top of the treatment material in the column was just covered with the influent. The pH was measured after 1 day. At the end of 20 days without any added lime, the oxide achieved the highest pH increase of all the buffers. The bioreactor supported sulphate reduction at continuous flow rates of 30, 60, 90, and 180 ml day⁻¹. The results of this study show that it is possible to establish an SRB population in a constant flow reactor, using readily decomposable material and a fast-acting soil-based buffer. It was concluded that cation exchange capacity (CEC), and not merely free lime, was the crucial buffer characteristic at the low pH level of <3.0 units. The CEC of the substrate used in this study was three times that of a sludge buffer used by Harris and Ragusa (2000). The only effective substrate difference (excluding the influent flow rate) between the present study and that of Harris and Ragusa (2000) is that of the buffers used. Hence, it was concluded that for pH increases, and decreases of soluble cations in AMD, CEC is a major determining factor among the buffering materials. Where sources of lime are insufficient, the use of medium to high organic C soil-based buffers with small quantities of lime can be an effective alternative for activating SRB in a reactor.

Original article: Bioremediation of acid mine drainage using decomposable plant material in a constant flow bioreactor. Environmental Geology Volume 40, Issue 10, pp. 1192–1204.