Biogeochemical reactivity of Fe-organic matter coprecipitates
Zusammenfassung der Projektergebnisse
Ferric oxyhydroxides play an important role in controlling the bioavailability of oxyanions in soil and participate in the stabilization of OM. This project aimed at the determination of structural properties of Fe-organic coprecipitates and the linkage of physicochemical properties to their environmental reactivity. The reactivity of these phases has been tested by adsorption and (abiotic and biotic) dissolution experiments, and was also compared to adsorption complexes where NOM has not been precipitated but adsorbed to an already existing mineral phase (ferrihydrite). Coprecipitates and adsorption complexes were synthesized at pH 4 using two natural organic matter (NOM) types extracted from forest floor layers (Oi and Oa horizon) of a Haplic Podzol. Iron-organic coprecipitates were formed at initial molar metal-to-carbon (M/C) ratios of 1.0 and 0.1 and an Al-to-Fe(III) ratio of 0.01 to 0.2. Sample properties were studied by multiple techniques including XRD, XPS, XANES and EXAFS as well as Mössbauer spectroscopy, TEM, N 2 gas adsorption, dynamic light scattering, and electrophoretic mobility measurements. The most relevant property that overwhelmingly governed both the structure and reactivity of Fe-organic coprecupitates was the initial molar M/C ratio. The OC contents of the 'M/C 0.1' coprecipitates were higher than those of 'M/C 1.0' coprecipitates and adsorption complexes, leading to pronounced variations in specific surface area, average pore radii, and total pore volumes but being independent of the NOM type or the presence of aluminum. The occlusion of Fe solids by OM caused comparable pHPZC (1.5-2) of adsorption complexes and coprecipitates. Iron oxide particles in 'M/C 0.1' coprecipitates covered to a larger extent the outermost aggregate surfaces, for some 'M/C 1.0' coprecipitates OM effectively enveloped the Fe oxides, while OM in the adsorption complexes primarily covered the outer aggregate surfaces. Coprecipitates formed at high NOM availability contained largely X-ray amorphous Fe oxides whereas in 'M/C 1.0' coprecipitates, ferrihydrite was the main mineral component. The presence of NOM caused the formation of more perturbed ferrihydrite structures and of smaller primary particles with less scattering domains. The same held true for the presence of Al in case of Oa-derived NOM ('M/C 1.0' coprecipitates only). Coprecipitation also caused fractionation of NOM with strong retention of acidic and phenolic components by both innersphere as well as outersphere complexes. The latter were more prominent for coprecipitates formed at an M/C 1.0 ratio. Despite of their much larger OC contents, both the adsorption of oxyanions (As(V)) as well as the ligandinduced Fe dissolution by low-molecular weight organic acids (oxalate, ascorbate) were fastest to and from coprecipitates formed at low Fe availability (M/C 0.1) and facilitated by desorption of weakly bonded OC and disaggregation. These findings strongly contrast the common view that Fe oxide-bound OM acts as a "barrier" for negatively charged solution compounds. The structural properties of 'M/C 0.1' coprecipitates were the main reason for their pronounced reactivity. Particularly their larger volumes contained in mesopores (2-10 nm) and their wider average pore radii allowed for a faster approach of soil solution components towards the Fe-oxide/water interface. Coprecipitates containing more OM are thus likely more vulnerable to exchange reactions with the soil solution (e.g., anion sorption, OM desorption) and also for being colloidally transported within soil. In contrast to the abiotic reactivity tests (oxyanion adsorption, ligand-mediated Fe dissolution), the biologically driven Fe dissolution by Shewanella putrefaciens revealed a reverse pattern: Coprecipitates with less OM and more Fe (M/C 1.0) had a higher electron acceptor capacity and thus were also dissolved faster by the facultative Fe reducer than those coprecipitates holding more OM (M/C 0.1). We assume that the "biologically active" surface area was larger for the 'M/C 1.0' coprecipitates. Likewise in these experiments, the M/C ratio was more crucial in determining the reactivity than the type of NOM or the presence of Al. In summary, the project has shown that both the structure and reactivity of poorly crystalline Fe(III) oxides in terrestrial and aquatic systems can largely vary depending on the soil solution conditions and on the environmental processes considered. Carbon-rich Fe phases precipitated at low molar M/C ratios may play a more important role in oxyanion immobilization and abiotic Fe cycling than phases formed at higher M/C ratios or respective adsorption complexes. Contrary, coprecipitates with high M/C ratios and adsorption complexes were more reactive towards bacteria-mediated dissimilatory Fe reduction due to their smaller OM contents and thus less decisive surface passivation by the entrained OM. Further studies dealing with biological processes should therefore focus on defining the biologically "active or accessible" surface area of such mixed Fe-organic phases.
Projektbezogene Publikationen (Auswahl)
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2011. Strukturelle Variabilität pedogener Fe(III) Hydr(oxid)-Mischphasen. Biannual Meeting of the German Soil Science Society. Berlin-Potsdam
Freund A., Jung S., Warr, L., Guggenberger G., Mikutta R.
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2012. Soil mineral-organic associations - How structure rules reactivity. 5th International workshop on soil and sedimentary organic matter stabilization and destabilization. Ascona, Switzerland
Mikutta R.
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2013. Fe (hydr)oxides: From Structure to Reactivity. Monte Verità Conference on Iron Biogeochemistry. March 3. - 8., Ascona, Switzerland
Freund A., Mikutta R., Guggenberger G., Mikutta C., Homenya P.
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2013. Impact of formation conditions on the reactivity of ferrihydrite. Joint Annual Meeting Deutsche Mineralogische Gesellschaft und Geologische Vereinigung e.V., September 16. - 19., Tübingen
Mikutta R., Freund A., Lorenz D., Guggenberger G.
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2013. Role of iron in redox processes in soil. Summer School "Ferrum" of the Centre of Solid State Chemistry and New Materials (ZFM), July 22, Goslar
Thompson A., Mikutta R.
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2014. Properties and Reactivity of Fe- Organic Matter Associations formed by Coprecipitation versus Adsorption: Clues from Arsenate Batch Adsorption. Geochmica et Cosmochimica Acta 144, 258 - 276
Mikutta R., Lorenz D., Guggenberger G., Haumeier L., Freund A.
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2015. Mineral–Organic Associations: Formation, Properties, and Relevance in Soil Environments. Advances in Agronomy, Volume 130, 2015, Pages 1-140
Kleber M., Eusterhues K., Keiluweit M., Mikutta C., Mikutta R., Nico P.S.
