Solution pH and Jarosite Management during Ferrous Iron Biooxidation in a Novel Packed-Column Bioreactor

Abstract

Jarosite formation is undesirable in bioleaching processes as it depletes the needed ferric reagent for the oxidation of most sulfide minerals. Although it creates kinetic barriers thereby retarding the leach rates of most minerals, jarosite serves as support for the attachment of bioleaching microbes, facilitating biooxidation rate. Microbial ferrous-oxidation by mesophilic microbe was studied in a recently reported novel packed-column bioreactor with a view to investigate the potential of using solution pH to manage jarosite accumulation in the bioreactor in addition to establishing a base case data for the bioreactor. Experiments were conducted in the bioreactor packed with glass balls (15mm diameter) at constant temperature of 38.6 °C, residence time of 18 hours, airflow rate of 20 mLs-1and pH values of 1.3, 1.5 and 1.7. The results showed that the amount of jarosite accumulation is proportional to the solution pH, and to the duration of operation of the bioreactor. Jarosite precipitation concentrations of 4.95, 5.89 and 7.08 gL-1were obtained after 10 days of continuous operation at solution pH values of 1.3, 1.5 and 1.7 respectively, while after 15 days the precipitations concentrations increased to 5.50, 7.90, 9.98 gL-1respectively. The results also showed that 33% and 52% precipitate reduction could be achieved by gradual decrease in the bioreactor solution pH to 1.5 and 1.3 after being continuously operated for 10 days at pH 1.7 respectively after an addition of 5 days. A maximum ferrous oxidation rate (), 6.85 mmol.L-1.h-1and the affinity kinetic constants (,), of 0.001 and 0.006 for Hansford and Monod models respectively. Although a directly relationship exist between jarosite formation and pH, the results of this study may be relevant in bioleach heaps or at least in column bioreactors to manage/control jarosite accumulation thereby improving the leach kinetics of mineral sulfides.