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The investigations described show that the formation of elemental sulfur from the biological oxidation of sulfide can be optimized by controling the redox state of the solution. The nonsoluble sulfur can be removed by gravity sedimentation and re-used as a raw material, i.e., in bioleaching processes. It was shown that, by supplying an almost stoichiometrical amount of oxygen to the recirculated gas phase, the formation of sulfate is minimized. The redox potential is mainly determined by the sulfide concentration because this compound has a high standard exchange current density with the platinum electrode surface. By maintaining a particular redox setpoint value, in fact, the reactor becomes a "sulfide-stat." It was shown that in a sulfide-oxidizing bioreactor the measured redox potential, using a polished redox electrode, is kinetically determined rather than thermodynamically. The optimal redox value for sulfur formation is between -147 and -137 mV (H2 reference electrode, 30 degrees C, pH 8). The presented results are currently used for controling several full-scale installations, which desulfurize biogas and high-pressure natural gas. Copyright 1998 John Wiley & Sons, Inc.

This study shows that, in a sulphide-oxidizing bioreactor with a mixed culture of Thiobacilli, the formation of sulphur and sulphate as end-products from the oxidation of sulphide can be controiledinstantaneously and reversibiy by the amount of oxygen supplied. It was found that at sulphide loading rates of up to 2.33 mmol7/L . h, both products can be formed already at oxygen concentrations below 0.1 mg/L. Because the microorganisms tend to form sulphate rather than forming sulphur, the oxygen concentration is not appropriate to optimize the sulphur production. Within less than 2 h, the system can be switched reversibly from sulphur to sulphate formation by adjusting the oxygen flow. This is below the minimum doubling time (2.85 h) of, e.g., Thiobacillus neapolitanus and Thiobacillus 0,(18) which indicates that one metabolic type of organism can probably perform both reactions. Under highly oxygen-limited circumstances, that is, at an oxygen/sulphide consumption ratio below 0.7 mol . h(-1) mol . h(-1) thiosulphate is abundantly formed. Because the chemical sulphide oxidation results mainly in the formation of thiosulphate, it is concluded that, under these circumstances, the biological oxidation capacity of the system is lower than the chemical oxidation capacity. The oxidation rate of the chemical sulphide oxidation can be described by a first-order process (k =-0.87 h(-1)).(c) 1995 John Wiley & Sons, Inc.