Ferrous sulphate oxidation using Thiobacillus ferrooxidans cells immobilised on sand for the purpose of treating acid mine-drainage

Thiobacillus ferrooxidans was immobilised on sand (size 0.85 mm to 1.18 mm) for use in a repeated batch and continuously operated packed-bed bioreactor which has not been previously reported in the literature. Repeated batch operation resulted in the complete oxidation of ferrous to ferric iron. The bacteria were active immediately after 3–4 weeks in a non-aqueous medium; i.e. the sand was allowed to dry out, demonstrating the stability of the system. A lag phase of 28 days was recorded when the sand was stored dried in a sealed container for 16 weeks compared with a lag phase of 13 days for a sample frozen for 18 weeks. After a period of 10 days, continuous operation of the reactor at a dilution rate of 0.64 h–1 resulted in 95–99% oxidation of ferrous iron or 0.31–0.33 kg m–3 h–1. With the use of a scanning electron microscope, images were recorded of Thiobacillus ferrooxidans on sand.