Reduction-Driven Mobilization of Structural Fe in Clay Minerals with High Fe Content

Clay minerals contain significant amounts of Fe in their alumosilicate framework, and this structural Fe can be reduced and re-oxidized, constituting a potentially renewable source of reduction equivalents in sedimentary environments. However, dissolution and/or clay mineral transformations during microbial Fe reduction contradict this concept. Here, we investigate how Fe reduction and re-oxidation affect the propensity of Fe to be released from the clay mineral structure and use selective sequential extractions in combination with Mössbauer spectroscopy. Negligible amounts of Fe were released in the sequential extraction of high Fe content clay minerals NAu-1 and NAu-2. Once aqueous Fe(II) was added as a reductant, the extraction procedure recovered the initially added Fe amount and up to 30% of the Fe from the clay mineral structure as both Fe(II) and Fe(III). Similar extents of Fe mobilization were found for clay minerals partly reduced (7%–20%) with dithionite, suggesting that mobilization was reduction-induced and independent of the source of reduction equivalents (Fe(II), dithionite). Although higher Fe reduction extents mobilized more structural Fe, i.e., >90% in fully reduced clay minerals, re-oxidation largely reverted the reduction-induced Fe mobilization in clay minerals. Our finding of reduction-driven Fe mobilization provides a plausible explanation for conflicting reports on Fe release from clay minerals and how extensive Fe atom exchange between aqueous and clay mineral Fe occurs.