Influence of organic acids, complexing agents and heavy metals on the bioleaching of iron from kaolin using Fe(III)-reducing bacteria

Nutrients in leaching solution play an important role in removal of iron impurities from kaolin when using dissimilatory Fe(III)-reducing bacteria (DIRB). However, the effect of organic acids, complexing agents and heavy metals on Fe leaching from kaolin when added to the leaching solution was not well understood and was investigated in this study. The results showed that the removal of iron impurities increased in the presence of oxalic acid in the leaching solution where the removal of iron reached 3.76 mg Fe g clay−1. In contrast, acetic and salicylic acids inhibited or ceased microbial growth. Furthermore, iron dissolution was enhanced by nitrilotriacetic acid (NTA), while the addition of EDTA to the solution medium had little impact on iron reduction. The reduction of Fe(III) was completely inhibited by the addition of 0.08 g L−1 Cu(II) to the solution, while dissolved iron was enhanced by the addition of Mn(II) and Fe(II). Low concentrations of Fe(III) caused a slight decline in iron reduction, but reduction of Fe(III) was significantly inhibited at an initial Fe(III) concentration of 0.08 g L−1. Fourier transform infrared analysis (FTIR) and differential thermal analysis–thermogravimetry (DTA–TG) techniques were used to characterize kaolin before and after bioleaching demonstrating that there was no major change in structural composition or physicochemical properties.