High-rate iron oxidation at below pH 1 and at elevated iron and copper concentrations by a Leptospirillum ferriphilum dominated biofilm

The influences of H⁺, Fe²⁺, Fe³⁺ and Cu²⁺ ion concentrations and their different combinations on the iron oxidation by an enrichment culture dominated by Leptospirillum ferriphilum were studied in batch experiments and continuous-flow fluidized-bed reactors. The iron oxidation rate did not significantly vary in the pH range of 0.9-1.5 and was only partially inhibited at pH 0.7. Ferric and ferrous iron at 5 and 24 g L^-1, respectively, had little or no effect on iron oxidation by the enrichment culture and the iron oxidation rate slightly decreased at higher concentrations. At a loading rate of 3 g Fe²⁺ L^-1 h ^-1, 2 g L^-1 Cu²⁺ did not affect the iron oxidation rate, whereas at a loading rate of 10 g Fe²⁺ L^-1 h^-1 it reduced the rate by 69%. The maximum iron oxidation rate was 10 g Fe²⁺ L^-1 h^-1 at pH 0.9 and in the presence of 21 g L^-1 Fe²⁺ and 2 g L^-1 Cu²⁺.