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Appears in Collections:Biological and Environmental Sciences Journal Articles
Peer Review Status: Refereed
Title: Citrate influences microbial Fe hydroxide reduction via a dissolution-disaggregation mechanism
Author(s): Braunschweig, Juliane
Klier, Christine
Schröder, Christian
Handel, Matthias
Bosch, Julian
Totsche, Kai U
Meckenstock, Rainer U
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Keywords: Geobacter
electrostatic stabilization
mathematical modelling
Issue Date: 15-Aug-2014
Date Deposited: 26-May-2014
Citation: Braunschweig J, Klier C, Schröder C, Handel M, Bosch J, Totsche KU & Meckenstock RU (2014) Citrate influences microbial Fe hydroxide reduction via a dissolution-disaggregation mechanism. Geochimica et Cosmochimica Acta, 139, pp. 434-446.
Abstract: Microbial reduction of ferric iron is partly dependent on Fe hydroxide particle size. Nanosized Fe hydroxides greatly exceed the bioavailability of their counterparts larger than 1 μm. Citrate as a low molecular weight organic acid can likewise stabilize colloidal suspensions against aggregation by electrostatic repulsion but also increase Fe bioavailability by enhancing Fe hydroxide solubility. The aim of this study was to see whether adsorption of citrate onto surfaces of large ferrihydrite aggregates results in the formation of a stable colloidal suspension by electrostatic repulsion and how this effect influences microbial Fe reduction. Furthermore, we wanted to discriminate between citrate-mediated colloid stabilization out of larger aggregates and ferrihydrite dissolution and their influence on microbial Fe hydroxide reduction. Dissolution kinetics of ferrihydrite aggregates induced by different concentrations of citrate and humic acids were compared to microbial reduction kinetics with Geobacter sulfurreducens. Dynamic light scattering results showed the formation of a stable colloidal suspension and colloids with hydrodynamic diameters of 69 (± 37) to 165 (± 65) nm for molar citrate:Fe ratios of 0.1 to 0.5 and partial dissolution of ferrihydrite at citrate:Fe ratios ≥ 0.1. No dissolution or colloid stabilization was detected in the presence of humic acids. Adsorption of citrate, necessary for dissolution, reversed the surface charge and led to electrostatic repulsion between sub-aggregates of ferrihydrite and colloid stabilization when the citrate:Fe ratio was above a critical value (≤ 0.1). Lower ratios resulted in stronger ferrihydrite aggregation instead of formation of a stable colloidal suspension, owing to neutralization of the positive surface charge. At the same time, microbial ferrihydrite reduction increased from 0.029 to 0.184 mM h-1 indicating that colloids stabilized by citrate addition enhanced microbial Fe reduction. Modelling of abiotic dissolution kinetics revealed that colloid stabilization was most pronounced at citrate:Fe ratios of 0.1 – 0.5, whereas higher ratios led to enhanced dissolution of both colloidal and larger aggregated fractions. Mathematical simulation of the microbial reduction kinetics under consideration of partial dissolution and colloid stabilization showed that the bioaccessibility increases in the order large aggregates < stable colloids < Fe-citrate. These findings indicate that much of the organic acid driven mobilization of Fe oxy(hydr)oxides is most likely due to colloid formation and stabilization rather than solubilisation.
DOI Link: 10.1016/j.gca.2014.05.006
Rights: Published in Geochimica et Cosmochimica Acta by Elsevier; The Elsevier Policy is as follows: Authors retain the right to use the accepted author manuscript for personal use, internal institutional use and for permitted scholarly posting provided that these are not for purposes of commercial use or systematic distribution. An "accepted author manuscript" is the author’s version of the manuscript of an article that has been accepted for publication and which may include any author-incorporated changes suggested through the processes of submission processing, peer review, and editor-author communications. The research that generated this article was funded by the research group FOR 580 of the German Research Foundation (DFG) “Electron Transfer Processes in Anoxic Aquifers”, the Nanosan project of the German Federal Ministry of Education (BMBF, Grant ID 03X0085A), and the EU-project NANOREM (FP7-Grant Agreement #309517). However the funding by the EU predates the 2014 mandatory Open Access policy.

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