Effects of dissolved low molecular weight organic acids on oxidation of ferrous iron by Acidithiobacillus ferrooxidans

A few researchers have reported on work concerning bioleaching of heavy-metal-contaminated soil using Acidithiobacillus ferrooxidans, since this acidophile is sensitive to dissolved low molecular weight (LMW) organic acids. Iron oxidation by A. ferrooxidans R2 as well as growth on ferrous iron was inhibited by a variety of dissolved LMW organic acids. Growth experiments with ferrous iron as an oxidant showed that the inhibition capability sequence was formic acid > acetic acid > propionic acid > oxalic acid > malic acid > citric acid. The concentrations that R2 might tolerate were formic acid 0.1 mmol L−1 (2 mmol kg−1 soil), acetic and propionic acids 0.4 mmol L−1 (8 mmol kg−1 soil), oxalic acid 2.0 mmol L−1 (40 mmol kg−1 soil), malic acid 20 mmol L−1 (400 mmol kg−1 soil), citric acid 40 mmol L−1 (800 mmol kg−1 soil), respectively. Although R2 was sensitive to organic acids, the concentrations of LMW organic acids in the contaminated soils were rather lower than the tolerable levels. Hence, it is feasible that R2 might be used for bioleaching of soils contaminated with metals or metals coupled with organic compounds because of the higher concentrations of LMW organic acids to which R2 is tolerant.