Mechanisms of Fe~Ⅱ sequestration following dissimilatory iron reduction of structurally diverse Fe~Ⅲ (hydr)oxides

摘要

The structural order of ferric (hydr)oxides may have an appreciable impact on the extent and rate of dissimilatory Fe~Ⅲ reduction. Here we compare the reductive biomineralization and Fe~Ⅱ sequestration mechanisms of 2-line ferrihydrite, goethite, and hematite. While 83% of Fe~Ⅱ produced following ferrihydrite reduction is retained within the solid-phase, merely 17% is retained by goethite and hematite. Iron(Ⅱ) sequestration ranges from magnetite precipitation and crystal growth on ferrihydrite to localized spinel-like domains within surface hydrated layers on goethite and hematite. The lower solubility of goethite and hematite relative to ferrihydrite, resulting in lower Fe_(aq)~Ⅲ activity and bacterially-generated Fe_(aq)~Ⅱ concentrations, may hinder magnetite precipitation beyond mere surface reorganization into nm-sized spinel-like domains. A decline in microbial reduction rates correlates with the initial phase disorder and termination of Fe~Ⅱ sequestration. As such, sustained microbial reduction of 2-line ferrihydrite, goethite, and hematite appears to be controlled, in large part, by changes in available reactive (higher energy) surface sites.