Electron transfer reactions at iron mineral surfaces in the presence of organic sorbates
FOR 580 HAD
From 02/2006Principal Investigator: Stefan Haderlein
Staff: Christine Laskov, Anke Buchholz, Silvia Orsetti
Grant: FOR 580 Electron Transfer Processes in Anoxic Aquifers
Redox reactions at iron mineral surfaces play an important role in determining the overall biogeochemical milieu in anoxic groundwater systems. Previous studies have shown that oxidation of sorbed ferrous iron at mineral phases may cause remodelling of the mineralwater interphase and thus may affect electron transfer processes in anoxic aquifers. In the first funding period, we studied in detail how and at which conditions oxidation of ferrous iron at mineral surfaces affects electron transfer processes. Using carbon tetrachloride (CCl4) as model oxidant, we could further demonstrate, that the proposed reactive tracer approach, which is based on changes of the stable isotopic composition of model oxidants, could be successfully applied to characterize the surface reactivity and dynamics of surface bound Fe(II) species at iron(III)hydroxides. Up to date, process based studies on surface mediated transformation of redox active solutes in iron mineral systems have been conducted primarily in model systems devoid of natural organic matter. In natural systems, however, mineral surfaces are inevitably in contact with OM. Sorbed DOM is likely to affect heterogeneous electron transfer processes due to its interactions with iron both in aqueous solution and at the mineral surface. On one hand, DOM sorption at iron hydroxides may interfere with the formation of reactive Fe(II) surface sites. On the other hand, DOM contain redox active quinone moieties and may act as a mediator enhancing the electron-transfer across the mineral surface. In this follow-up project we propose to investigate the effects of various organic sorbates such as redox-inert organic acids as well as redox-active quinones, humic substances and DOM on electron transfer reactions at iron mineral surfaces. Furthermore, we will investigate the effects of sulfide as additional redox active natural component on DOM-iron interfacial redox processes.
Here, we focus on an improved understanding of heterogeneous redox reactions and subsequent phase transformations taking place at iron minerals. In situ characterization of the type and the dynamics of active redox species at mineral surfaces with existing spectroscopic methods, however, is very difficult if feasible at all. We therefore propose here to develop, validate and apply an indirect method (reactive tracer approach) based on changes of the stable isotopic composition of model oxidants reacting specifically with surface bound Fe(II) species. Using this novel approach we intend to characterize the properties, the dynamics of formation and eventually the significance of reactive iron coatings at geochemically relevant settings with experiments conducted under well controlled conditions in batch and column systems.