Reduction of ferrihydrite with adsorbed and coprecipitated organic matter: microbial reduction by <i>Geobacter bremensis</i> vs. abiotic reduction by Na-dithionite

Abstract

Abstract. Ferrihydrite is a widespread poorly crystalline Fe oxide which becomes easily coated by natural organic matter 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 microbial 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 decreasing initial Fe-reduction rates and decreasing degrees of reduction with increasing amounts of mineral-bound organic matter. At similar organic matter loadings, coprecipitated ferrihydrites were more reactive than ferrihydrites with adsorbed organic matter. The difference can be explained by the smaller crystal size and poor crystallinity of such coprecipitates. 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 minerals: 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. bremensis to the Fe oxide mineral surface was inhibited by attached organic matter. Consequently, mineral-bound organic matter shall be taken into account as a factor in slowing down reductive dissolution.

Ferrihydrite-associated organic matter (OM) stimulates reduction by <i>Shewanella oneidensis</i> MR-1 and a complex microbial consortia

“…This minerotrophic, slightly acidic (pH ∼ 5) fen has been previously described in detail (Blodau et al, 2004;Eusterhues et al, 2014;Hausmann et al, 2016;Küsel et al, 2008;Loy et al, 2004;Pester et al, 2012). Briefly, granite bedrock is covered by a Fibric Histosol, often referred to as peat, with a thickness between 40 and 70 cm.…”

“…Not surprisingly, microbial reduction of Fe(III) oxides like nanometer-sized lepidocrocite and ferrihydrite by Shewanella species can be boosted by the presence of humic acids (Adhikari et al, 2017;Amstaetter et al, 2012;Pé-drot et al, 2011;Shimizu et al, 2013) and quinone moieties (Newman and Kolter, 2000) when the amount of dissolved organic carbon is greater than a threshold concentration of 5-10 mg C L −1 (Jiang and Kappler, 2008). Conversely, a study by Eusterhues et al (2014) reported that high C / Fe ratios in coprecipitated and adsorbed ferrihydrite complexes inhibited the reduction of ferrihydrite by G. bremensis. Poggenburg et al (2016) produced ferrihydrite OM complexes with different organic materials (extracted extracellular polymeric substances, water extracts of soil litter) and used S. putrefaciens and G. metallireducens to reduce them.…”

“…Iron minerals such as Fe(III) oxides exhibit a high reactivity towards dissolved organic matter (OM); thus Fe(III) oxides are either partially or completely covered by OM in natural environments (Eusterhues et al, 2005;Kaiser and Zech, 2000;Lalonde et al, 2012;Torn et al, 1997). Fe(III) oxides enveloped by OM potentially lead to changes in the surface properties in comparison to non-modified Fe(III) oxides, which ultimately may influence mobility, solubility, and aggregation (Eusterhues et al, 2014;Narvekar et al, 2017;Pédrot et al, 2011;Vindedahl et al, 2016). The poorly crystalline ferrihydrite is one of the most common Fe(III) oxides and typically forms aggregates of nanometer-sized individual crystals (Bigham et al, 2002;Cornell and Schwertmann, 2003;Jambor and Dutrizac, 1998).…”

“…The poorly crystalline ferrihydrite is one of the most common Fe(III) oxides and typically forms aggregates of nanometer-sized individual crystals (Bigham et al, 2002;Cornell and Schwertmann, 2003;Jambor and Dutrizac, 1998). The coprecipitation of OM with Fe results in the adsorption and occlusion of organic molecules within the interstices between individual ferrihydrite crystals, while adsorption of OM occurs on pre-existing ferrihydrite surfaces (Eusterhues et al, 2014). Moreover, the presence of dissolved OM impedes ferrihydrite growth (Cismasu et al, 2011;Eusterhues et al, 2008;Mikutta et al, 2008;Schwertmann et al, 2005); therefore, ferrihydrite with coprecipitated OM have smaller crystal sizes and more crystallographic defects.…”

“…In this study, we used the same experimental design described in the earlier study of Eusterhues et al (2014) but replaced G. bremensis with S. oneidensis MR-1 to test the effect of increasing amounts of adsorbed and coprecipitated OM on microbial Fe(III) reduction. In a parallel set of incubation studies we used a microbial consortia as inoculum derived from an iron-rich peatland where the microbial reduction of Fe(III) is the dominant electron-accepting processes for the degradation of OM under anoxic conditions.…”