Total dissolved solids and their effects on iron oxidation by chemolithotrophic cells

Continuous growth of an acidophilic, chemolithotrophic bacterial culture in minimal iron media, was investigated over a range of TDS values. The specific cell parameters, iron oxidation rates, growth rates and observed yields at fixed solution potentials were compared over a range of TDS values but with the same total iron concentrations. By perturbing the steady state at any set point it was possible to estimate the population of sessile cells and calculate values for the specific cell parameters. The TDS was increased by addition of Na2SO4 which produced no toxic effects and allowed a flourishing culture. There was however a significant inhibition of the specific iron oxidation rates which were reduced by more than 75% by the increase in TDS from 0.05 to 0.4 M. A framework for understanding the observed result, based on the ionic strength (I) rather than TDS, is suggested. The oxidation of iron is an important sub-process in hydrometallurgy and TDS values of 0.4 M are modest from an operational perspective so these results may point to potential problems during long term operation where TDS can accumulate without otherwise interfering.

► Planktonic and sessile cell numbers in a continuous reactor have been estimated.
► Specific iron oxidation rates under various conditions have been determined.
► The effect of electrolyte ions is to slow the cell's iron oxidation rate.
► The extent of this inhibition can be predicted using the ‘primary salt effect’.