FACTORS AFFECTING METABOLISM AND FERROUS IRON OXIDATION IN SUSPENSIONS AND BATCH CULTURES OF THIOBACILLUS FERROOXIDANS: RELEVANCE TO FERRIC IRON LEACH SOLUTION REGENERATION

Abstract

Ferric iron in acid solution is the essential reagent for the bacterially–assisted solubilization of uranium from ores such as uraninite, and is also significant in the dissolution of metal sulphides. The regeneration of ferric iron by bacterial reoxidation of ferrous sulphate is an essential step in the recycling of leach liquors in heap or other percolation leach systems. Consequently it is of use to know the factors likely to affect bacterial development and activity. The principal factor limiting growth in batch culture of Thiobacillus ferrooxidans on ferrous iron is ferric iron product inhibition of the rate of ferrous iron oxidation. Ferric iron inhibits oxidation competitively, lowering substrate saturation coefficient (K s ) without affecting maximum attainable specific growth rate (μ max ). In normal batch culture, μ max was 0.143h −1 , minimum observed K S was 36mM ferrous sulphate, μ max and growth yield coefficient was 0.35g dry wt/g atom iron oxidized. Growth rates were exponential, measured as iron oxidation and fixation of labelled carbon dioxide. Growth rate was not limited by carbon dioxide concentration, but exhaustion of carbon dioxide in sealed systems caused a switch from exponential to linear iron oxidation, indicating growth–uncoupled oxidation of the residual iron by the static population. High potassium concentrations partially relieved ferric iron inhibition. With non–growing cell suspensions, oxygen electrode methods indicated multiple K m values for ferrous iron oxidation: 0.7mM FeSO 4 between 0.4-43mM; 20-123mM between 100-400mM; and unmeasurably high between 400-700mM. Above 0.7M ferrous sulphate became an inhibitory substrate. Increased H + or uranyl sulphate inhibited non-competitively. Efficiency of coupling of carbon dioxide fixation to Fe oxidation was decreased at low (25mM) and high (500mM) ferrous sulphate concentrations and was maximal in the 100-300mM range. Ferric sulphate did not inhibit carbon dioxide fixation and so affected metabolism and growth only by its effect on ferrous iron oxidation.