N-139. Developing a Kinetic Model for Bacterial Oxidation of Iron in the Presence of Ferric Iron, Zinc and Nickel

P. Nurmi1, B. Özkaya1, A. H. Kaksonen1, J. A. Puhakka1, O. H. Tuovinen1,2;
1Tampere Univ. of Technology, Tampere, FINLAND, 2Ohio State Univ., Columbus, OH.

During the bioleaching of metals from sulfide ores, acidophilic iron-oxidizers are exposed to high concentrations of Fe3+ and other dissolved metals. A number of kinetic models for bacterial Fe2+ oxidation have been previously proposed and they can be broadly classified as either empirical or those based on the Michaelis-Menten/Monod rate expressions. The previous kinetic models of Fe2+ oxidation and inhibition are applicable only to single toxicants such as a metal ion. The purpose of this study was to develop a kinetic model to predict the Fe2+ oxidation rate under conditions of high concentrations of Fe3+ and multiple other, potentially toxic metals. Iron oxidation was studied in a Leptospirillum ferriphilum-dominated fluidized bed bioreactor culture, which was challenged with various concentrations of Fe3+, Zn2+, and Ni2+. A combined method for determining the type and coefficients of inhibition was developed and validated with data from Fe2+ oxidation experiments at pH 1. Fe3+ competitively inhibited Fe2+ oxidation, whereas Zn2+ and Ni2+ conformed to non-competitive inhibition model with inhibition coefficients (Kii) of 49.1 and 62.7 g/L, respectively. Mg2+ and Na+ had a compounding effect on Zn2+ toxicity. A combined competitive and non-competitive inhibition model gave a good fit for data on iron oxidation in the presence of Fe3+ - Zn2+ and Fe3+ - Ni2+. Fe2+ was oxidized by the culture at high concentrations of 30 g Fe3+/L - 10 g Zn2+/L and 30 g Fe3+/L - 10 g Ni2+/L. The model was further developed to take into account the simultaneous effects of Ni2+ and Zn2+, and good fits with the experimental data were obtained. The data indicated that L. ferriphilum was relatively insensitive to the test metals, suggesting that the bacterium has application in ferric iron regeneration processes in circuits involving tank bioleaching of Zn- and Ni-sulfide concentrates. This model is a useful tool for predicting Fe2+ oxidation rates in the presence of Fe3+ and other metals. Such predictions will help to determine the solids retention time and the volume of bioreactors for Fe2+ oxidation in bioleaching applications, in which acidophilic iron oxidizers are in contact with high Fe3+ and other metal concentrations.