Influence of Coprecipitated Organic Matter on Fe²⁺_(aq)-Catalyzed Transformation of Ferrihydrite: Implications for Carbon Dynamics

Abstract

Aqueous Fe(II) is known to catalyze the abiotic transformation of ferrihydrite to more stable Fe minerals. However, little is known about the impacts of coprecipitated OM on Fe(II)-catalyzed ferrihydrite transformation and its consequences for C dynamics. Accordingly, we investigated the extent and pathway of Fe(II)-induced transformation of OM-ferrihydrite coprecipitates as a function of C/Fe ratios and aqueous Fe(II) concentrations, and its implications for subsequent C dynamics. The coprecipitated OM resulted in a linear decrease in ferrihydrite transformation with increasing C/Fe ratios. The secondary mineral profiles upon Fe(II) reaction with OM-ferrihydrite coprecipitates depend on Fe(II) concentrations At 0.2 mM Fe(II), OM completely inhibited goethite formation and stimulated lepidocrocite formation. At 2 mM Fe(II), whereas goethite was formed in the presence of OM, OM reduced the amount of goethite and magnetite formation and increased the formation of lepidocrocite. The solid-phase C content remained unchanged after reaction, suggesting that OM remains associated with Fe minerals following ferrihydrite transformation to more stable Fe minerals. However, C desorbability by H2PO4(-) from the resulting Fe minerals following reaction was enhanced. The study indicates a "lepidocrocite favoring effect" by OM and suggests that Fe(II)-catalyzed transformation of ferrihydrite may decrease OM stability in natural environments under moderately reducing conditions.

Geochemical controls on the distribution and composition of biogenic and sedimentary carbon

“…Similarly, organic carbon was retained during transformation of ferrihydrite to more stable Fe oxides under surface conditions (Chen et al, 2015) and simulated diagenesis ; however, in the surface-condition experiments, at the final experiment time point (90 days), a greater proportion of OC was desorbable with H 2 PO 4 - (Chen et al, 2015), implying decreased preservation potential. Organic carbon associated with Fe oxides formed by the Fe(II)-oxidizing M. ferroxydans became more enriched in aromatic-and unsaturated-C throughout the growth cycle and as the Fe oxide ripened .…”

Fe(II)-catalyzed transformation of ferrihydrite associated with natural organic matter

“…The effect of NOM on Fe(II)-catalyzed ferrihydrite transformation is of growing interest because it not only impacts the stability of associated NOM but also affects the fate of trace metals and nutrients that associate with ferrihydrite. From previous studies, we know NOM can inhibit Fe(II)-catalyzed ferrihydrite transformation (42,85,86), however, the mechanism behind this inhibition is unclear. In addition, little is known about the effect of NOM species on this Fe(II)catalyzed ferrihydrite transformation, which is important because there exists diverse NOM compositions in different soils.…”

“…Despite the smaller crystal sizes, however, ferrihydrite in the presence of OM tends to have smaller surface areas compared to pure ferrihydrite, which has been attributed to OM blocking surface sites and facilitating aggregation (75,96). In addition to the physical changes in both crystal size and surface area, OM is has also been shown to inhibit the crystallization of ferrihydrite and transformation to secondary minerals (42,86,98).…”

“…Since both Fe(II), which accelerates ferrihydrite transformation, and OM, which inhibits ferrihydrite transformation, are often present together in reducing environments, recent studies have explored how OM influences both DIRB and Fe(II)-catalyzed ferrihydrite mineral transformation (42,86,(101)(102)(103). Organic matter that was coprecipitated or adsorbed with ferrihydrite has been found to inhibit Fe(II)-catalyzed ferrihydrite mineral transformation (42,86,103). For example, ferrihydrite coprecipitated with OM extracted from the O horizon of an Ultisol resulted in less transformation with higher C/Fe ratio.…”

“…For example, ferrihydrite coprecipitated with OM extracted from the O horizon of an Ultisol resulted in less transformation with higher C/Fe ratio. At C/Fe ratios equal to or greater than 1.6, no ferrihydrite transformation was observed (86). Adsorbed fulvic acid also resulted in significant inhibition of ferrihydrite transformation (C/Fe ratio 4.18) (42).…”