Kinetics of chemical and biological sulphide oxidation in aqueous solutions

Abstract

A new equation for the non-catalysed sulphide chemical oxidation rate in a phosphate buffered system at pH 8.0 and 25°C is found. Our experiments show that the reaction order with respect to the oxygen concentration, n, depends on the sulphide concentration. The following equation is proposed: i = k[S] m[O] n log[S] (mg/1 h). The values for the constants m, n, k were found to be 0.41, 0.39 and 0.57 respectively. The biological oxidation rate in cell suspensions from a bioreactor, also measured in a phosphate buffered system at pH 8.0 and 25°C, was found to be a factor 75 faster than the chemical non-catalysed oxidation rate at sulphide concentrations around 10 mg/l. At higher sulphide concentrations this difference becomes less, e.g. at 100 mg/l the biological oxidation rate is only 7 times faster than the chemical oxidation rate. The two cell suspensions used in the experiments behave quite differently towards the sulphide concentration. Cell suspension 1 (taken from a reactor operated at a sulphide concentration of 7 mg/1) exerts its maximal oxidation rate (230 mg/l h) at a sulphide concentration of 10 mg/l. Cell suspension 2 (taken from a reactor operated at a sulphide concentration of 95 mg/1) exerts its maximal biological oxidation capacity (120 mg/1 h) at a sulphide concentration of 150 mg/1. The total oxidation rate (chemical and biological) of cell suspension 2 at 150 mg/1 is 210 mg/l h (of which only 5% is chemical). Cell suspension 1 shows severe substrate inhibition at sulphide concentrations exceeding 10 mg/l, while cell suspension 2 shows no sulphide inhibition up to a sulphide concentration of 600 mg/1.