The effect of dissolved cations on microbial ferrous-iron oxidation by Leptospirillum ferriphilum in continuous culture

In heap bioleaching the dissolution of gangue minerals from igneous ore materials can lead to the build-up of considerable concentrations of Mg and Al sulphates in the recycled leach solution. This may interfere with microbial ferrous iron oxidation, which drives the oxidation of the target minerals.In the present study the effect of solution concentrations of Mg and Al as sulphate at individual concentrations of 0 to 10 g·dm− 3 and combined concentrations 0 to 16 g·dm− 3 each (or total ionic strength from 0.2 to 1.3 M) has been investigated in continuous culture using Leptospirillum ferriphilum. Increasing the concentrations of the salts increasingly depresses the specific rate of ferrous iron oxidation and also shifts the viable range more and more into the low potential region. Aluminium significantly reduces the amount of carbon biomass maintained in the reactor, whereas magnesium actually enhances it at low concentrations. The experimental data was correlated using the Pirt equation and a simplified substrate utilisation model. The results suggest that the maximum microbial growth rate and growth yield decline significantly only at total ionic strengths above about 1 mol·dm−3.The implications of this study are that heap cultures are likely to perform sub-optimally in those operations where build-up of dissolved gangue minerals is not controlled.