Humic acid-enhanced illite and talc formation associated with microbial reduction of Fe(III) in nontronite

Humic substances are a chemically heterogeneous class of organic macromolecules ubiquitous in the environment and have been reported to function as an electron shuttle for microbial iron reduction. In most experimental research, synthetic quinone compounds (e.g., anthraquinone-2,6-disulfonate, AQDS) were used as the surrogate for humic substances. However, comparative studies of the differences between humic substances and AQDS in microbial reduction of iron-containing clay minerals and subsequent mineral transformations are rare. To achieve this goal, anaerobic batch experiments were performed in non-growth medium (bicarbonate buffered, pH = 7.0), with lactate as the electron donor, iron-rich smectite (nontronite NAu-2) as the electron acceptor, and a typical dissimilatory iron reducing bacterium Shewanella oneidensis MR-1 as the reaction mediator in the absence or presence of electron shuttles (Pahokee peat humic acid and AQDS). Pahokee peat humic acid (PPHA) and AQDS were amended in the media to achieve similar electron-accepting capacity. Our results indicated that both PPHA and AQDS greatly enhanced the final extent of smectite reduction by S. oneidensis MR-1 but to a different extent. The reduction extent at the end of experiments was 27.5% with PPHA, lower than that with AQDS (34.1%). Combined mineralogical analyses showed the conversion of smectite to illite upon bioreduction of nontronite with PPHA or AQDS, but this reaction was facilitated to a higher degree by PPHA than by AQDS. Biogenic talc was also observed when NAu-2 was bio-reduced with PPHA.