The kinetics of ferrous-iron oxidation by Leptospirillum ferriphilum in continuous culture: the effect of temperature

Abstract

A typical bioleach heap is characterized by wide variation of temperature across the heap bed, leading to oxidation of target minerals occurring at different rates. Previous studies on the effect of temperature on the microbial oxidation of ferrous-iron were limited to a narrow range of temperatures (30–40 °C) near optimum conditions and mostly toAcidithiobacillus ferrooxidans. The kinetics of ferrous-iron oxidation by Leptospirillum ferriphilum were studied in continuous culture. In this paper we focus on the effect of temperature (18–45 °C) on these kinetics. The study was based on the assumption that the effect of temperature can be studied independently of other, equally important factors such as pH, dissolved salts, etc. and independent of the reactor context. The experimental data were correlated using both, a simplified ferric-iron inhibitory model and the Pirt Equation. The results showed that the maximum specific ferrous-iron oxidation rate, increased with increasing temperature to a maximum at 42 °C. This trend can be described adequately by the Arrhenius Equation with an activation energy, Ea of 34.46 kJ mol−1 and frequency factor,K0 of 1.05 × 107 mmol Fe2+(mmolC)−1 h−1. An increase in temperature slightly reduces the steady state carbon biomass in the reactor, while the apparent affinity constant, K′Fe2+ increases. The investigation further suggests that at low temperature (18 °C) and beyond the maximum temperature (42 °C), the culture cannot be sustained in a continuous mode. The maximum biomass yield followed a linear decline with increasing temperature, while cell maintenance on ferrous-iron followed a quadratic trend, although the small values indicates that it is not significant, as would be expected in continuous culture. The results indicate that L. ferriphilum is likely to perform optimally, at warm temperatures (25–42 °C) in heap bioleach operations before being taken over by thermophiles at higher temperatures.