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Investigation of bacterial ferrous iron oxidation kinetics in a novel packed-column reactor: pH and jarosite management
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Jarosite formation is regarded as undesirable in the bioleaching processes as it depletes ferric reagent; a critical reagent for the oxidation of most sulphide minerals, from bioleach solution. It creates kinetic barriers and clogs on mineral surfaces, thereby retarding leach rates of most minerals. However, jarosite has also been shown to serve as support for the attachment of bioleaching microbes, facilitating a high ferric-iron generation rate. In this study, a series of experiments on microbial ferrous-iron oxidation by a mesophilic microbe were carried out in a novel packed-column bioreactor with a view to investigating the potential use of solution pH to manage jarosite accumulation in the bioreactor. The kinetics of the oxidation was also investigated to establish base case data for the novel bioreactor. The bioreactor was packed with glass balls 15 mm in diameter. The experiments were conducted at a constant temperature of 38.6 °C, residence time of 18 hrs, airflow rate of 20 mL.s-1 and at desired solution pHs (1.3, 1.5 and 1.7). The results showed that the amount of jarosite accumulation is proportional to the operating solution pH and also to the duration of operation of the bioreactor. Jarosite precipitate of 4.95, 5.89 and 7.08 g.L-1 were obtained after 10 days of continuous operation at solution pH of 1.3, 1.5 and 1.7 respectively, while after 15 days the precipitate concentration increased to 5.50, 7.90 and 9.98 g.L-1respectively. The results also showed that a 33% and 52% reduction in jarosite accumulation could be achieved by a gradual decrease of the bioreactor solution pH after being continuously operated for 10 days from pH 1.7 to 1.5 and pH 1.7 to 1.3, respectively, for an additional five days of continuous operation. The results of the ferrous-iron biooxidation kinetics investigated at pH 1.3 show a maximum ferrous oxidation rate ( max 2 Fe r ) of 6.85 mmol.L-1.h-1 and apparent affinity kinetics constants ( 2 Fe K , 2 Fe K ) of 0.001 mmol Fe2+.L-1 and 0.006 (dimensionless) using Hansford and Monod equations, respectively. Although a direct relationship exists between jarosite formation and solution pH, the results of this study may be relevant in bioleach heaps, or at least in column bioreactors, to manage and control jarosite accumulation, thereby improving leach kinetics of sulphide minerals.