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 (Fh) is a widespread poorly crystalline Fe oxide which becomes easily coated by natural organic matter (OM) in the environment. This mineral-bound OM entirely changes the mineral surface properties and therefore the reactivity of the original mineral. Here, we investigated the reactivity of 2-line Fh, Fh with adsorbed OM and Fh coprecipitated with OM towards microbial and abiotic reduction of Fe(III). As a surrogate for dissolved soil OM we used a water extract of a Podzol forest floor. Fh-OM associations with different OM-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 OM. At similar OM-loadings, coprecipitated Fhs were more reactive than Fhs with adsorbed OM. The difference can be explained by the smaller crystal size and poor crystallinity of such coprecipitates. At small OM loadings this led to even faster Fe reduction rates than found for pure Fh. The amount of mineral-bound OM also affected the formation of secondary minerals: goethite was only found after reduction of OM-free Fh and siderite was only detected when Fhs with relatively low amounts of mineral-bound OM were reduced. We conclude that direct contact of G. bremensis to the Fe oxide mineral surface was inhibited when blocked by OM. Consequently, mineral-bound OM shall be taken into account besides Fe(II) accumulation as a further widespread mechanism to slow down reductive dissolution.

Millennial scale persistence of organic carbon bound to iron in Arctic marine sediments

“…For example, experimental laboratory studies showed that the organic matter composition can influence the OC:Fe ratio regardless of the adsorption and coprecipitation bonding mechanism 15 , 58 , 59 . Moreover, as our sediment cores represent a time span of several thousands of years, microbial degradation might have, over time, selectively modified the adsorbed or coprecipitated OC-Fe R content 60 , 61 . Additionally, the determination of Fe R via chemical extraction yields an operationally defined reactive iron pool, including Fe R that is not associated with OC.…”

“…Moreover, the association between OC and Fe R via coprecipitation does not necessarily result in higher fOC-Fe R and is, therefore, not the dominant factor controlling fOC-Fe R in the Barents Sea sediments. Other factors such as organic matter composition 15 , 58 , 59 , inconsistent effects of different binding mechanisms on organic matter loadings 60 , 61 , and changes in Fe(III) phase reactivity due to incorporation and adsorption of other elements with a high affinity to Fe R 16 , likely all play a (combined) role in natural environments.…”

Asynchronous reductive release of iron and organic carbon from hematite–humic acid complexes

“…The difference in the half-life for Fe reduction of the hematite-HA complexes compared to pure hematite was below 20% for most of the samples, except hematite-HA3 synthesized at pH 5 and 7, indicating that the reduction of Fe(III) was affected by the sorbed HA to a minor degree. Previous studies showed that humic substances can increase the microbial reduction rate of Fe (Lovley et al, 1996;Roden et al, 2010;Scott et al, 1998), although Eusterhues et al (2014) found that the sorption of humic substances onto ferrihydrite decreased the abiotic and biotic reduction rate constant of Fe(III) by a factor of up to ten times. The relatively minor effect of HA on Fe reduction rates in our study was due to the strong reduction induced by dithionite and the small amount of organic matter sorbed by hematite.…”

“…Dithionite is one of the most commonly used Fe(III)-reducing agents for abiotic Fe reduction experiments (Eusterhues et al, 2014;Poulton and Canfield, 2005). We used a relatively low dithionite concentration to slow down the reduction kinetics and simulate natural environmental conditions.…”

Role of root exudates on the sorption of arsenate by ferrihydrite

“…The surface area of ferrihydrite was determined by H 2 O sorption at 20% relative humidity (Quirk, ) and by N 2 gas adsorption (BET method) as reported by Eusterhues et al . (). The point of zero charge (PZC) was determined by laser Doppler velocimetry‐photon correlation spectroscopy (Beckham Coulter Electronics, Hialeah, Fl, USA).…”