Effects of soil organic fractions on iron oxide biodissolution under anaerobic conditions

Soil iron oxides are poorly soluble; however, their dissolution may favor not only an increase in the amount of available iron but also the release of anions and cations, adsorbed or occluded on Fe-III oxides, which may have a great impact on environmental quality. In this study, the reductive biodissolution of iron oxides by microorganisms was evaluated using different soil humic fractions as electron donors. A mixed population of microorganisms extracted from an anaerobic mud was incubated for 15 days under anaerobic conditions in the presence of ferrihydrite or goethite, after adding, as electron donors, dissolved organic matter (DOM), fulvic acids (FA), humic acids (HA) or, for comparison, acetate. At regular time intervals Eh, pH, dissolved Fe-II and the concentration of dissolved organic compounds that remained in solution were measured. The redox potential (Eh) decreased rapidly in the first days of incubation, more with ferrihydrite than with goethite, and then returned to the initial values with some differences in specific values, depending on the organic fraction. The amounts of Fe-II released from ferrihydrite reached a maximum of 11.0 mug Fe mL(-1) after 15 days of waterlogging compared with 7.5 mug Fe mL(-1) released from goethite. Ferrihydrite released more Fe-II because of its higher specific surface area and its low crystallinity. DOM and FA fractions were the most effective in Fe reduction and as a C source for bacteria, suggesting that microorganisms can also use DOM for N supply as well as for C, selecting fractions rich in monosaccharides, amino acids, and carboxyl acids.