PRODUCTION OF H_2S BY SULFATE-REDUCING BACTERIA IN A TWO-COLUMN GAS/LIQUID REACTOR FOR THE PURIFICATION OF METAL-CONTAINING EFFLUENTS

摘要

Effluents containing high levels of sulfate and metals constitute a threat to the environment. Among the biotechnological processes developed over the past few years to deal with such effluents, the one described here Consists of two distinct steps: (1) sulfate is fed to sulfate-reducing bacteria (SRB) to produce H_2S, which is then (2) put into contact with the effluent to precipitate the metals in a separate reactor. A 21-liter reactor composed of two linked columns, namely a fixed-film biological reactor and a N_2 stripping system, was designed to test the sulfate-reducing and sulfide-recovery process under continuous feed operating conditions. The H_2S-producing laboratory-scale plant was inoculated with SRB using hydrogen as electron source. Stripping was optimized in order to recover more than 95 percent of the produced H_2S in the outlet gas. The CO_2 concentration in the bin-column was adjusted in the range 5÷30 percent, in order to maintain pH between 7.5 and 8.5; stripping with pure CO_2 inhibited the SRB. The increase in H_2 flowrate had a positive effect on the SO_4 - reduction rate, which reached a maximum value of 0.2 g/l centre dot h. The H_2S production was then used to precipitate the metals in a real effluent from a Spanish mine (Tharsis). A 10-liter agitated reactor was first used in batch experiments to determine the conditions for a selective recovery of metals. Precipitation of the metal sulfides was performed both with and without a preliminary treatment of bacterial iron oxidation and neutralization. Copper and zinc were selectively produced whereas all other metals, except manganese, were precipitated at pH = 6. The resulting solution, with a high concentration of sulfate, was then used to feed the bioreactor.