The Mechanism of Microbial Refinement of Kaolin by Fe(Ⅲ)-reducing Bacteria with Oxalic Acid

The influence of oxalic acid concentration in Fe(Ⅲ) reduction by microbes on Fe leaching from kaolin was examined through microbial test. Environmental scanning electron microscopy (ESEM) and energy-dispersion microanalysis (EDS) techniques were used to characterize kaolin before and after bioleaching and chemical leaching. The results showed that the addition of oxalic acid (0.2 g/L) to the leaching solution best enhanced the amount of iron leached from the kaolin with dissimilatory iron-reducing bacteria (DIRB). Too low or too high concentrations of oxalic acid would decrease iron reduction, while reduction of Fe(Ⅲ) was completely inhibited by the addition of 1.5 g/L. The chemical comparative test indicated that the reduction of Fe(Ⅲ) was enhanced with increasing concentration of oxalic acid. When the concentration of oxalic acid increased to 13.0 g/L, the reduction of Fe(Ⅲ) after 1day without DIRB was near to the reduction of Fe(Ⅲ) after 4 days using iron reduction bacteria. The results of ESEM and EDS demonstrated that there was no major change in structural composition or physicochemical properties, but the iron content was reduced selectively.