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Ferrihydrite is a widespread poorly crystalline Fe oxide which becomes easily coated by natural organic mat- ter in the environment. This mineral-bound organic matter entirely changes the mineral surface properties and therefore the reactivity of the original mineral. Here, we investigated 2-line ferrihydrite, ferrihydrite with adsorbed organic matter, and ferrihydrite coprecipitated with organic matter for mi- crobial and abiotic reduction of Fe(III). Ferrihydrite-organic matter associations with different organic matter loadings were reduced either by Geobacter bremensis or abiotically by Na-dithionite. Both types of experiments showed decreas- ing initial Fe-reduction rates and decreasing degrees of re- duction with increasing amounts of mineral-bound organic matter. At similar organic matter loadings, coprecipitated ferrihydrites were more reactive than ferrihydrites with ad- sorbed organic matter. The difference can be explained by the smaller crystal size and poor crystallinity of such coprecipi- tates. At small organic matter loadings the poor crystallinity of coprecipitates led to even faster Fe-reduction rates than found for pure ferrihydrite. The amount of mineral-bound organic matter also affected the formation of secondary min- erals: goethite was only found after reduction of organic matter-free ferrihydrite and siderite was only detected when ferrihydrites with relatively low amounts of mineral-bound organic matter were reduced. We conclude that direct contact of G. bremensisto the Fe oxide mineral surface was inhibited by attached organic matter. Consequently, mineral-bound or- ganic matter shall be taken into account as a factor in slowing down reductive dissolution.

biogeosciences

Reduction of ferrihydrite with adsorbed and coprecipitated organic matter: microbial reduction by &amp;lt;i&amp;gt;Geobacter bremensis&amp;lt;/i&amp;gt; vs. abiotic reduction by Na-dithionite