FOR 580: Electron Transfer Processes in Anoxic Aquifers
Final Report Abstract
Research Unit 580 has focused on the mechanisms and kinetics underlying electron transfer processes related to the biotic and abiotic electron flow from organic carbon to ferric iron minerals and to identify and characterize important pathways of electron flow linked to the redox properties of iron mineral surfaces, humic substances, sulfur compounds and iron colloids. To these ends we have performed laboratory and field experiments using state of the art methodology and kinetic modeling approaches. We were able to demonstrate that many environments have the potential for reversible electron transfer between microorganisms and solid-phase electron acceptors (e.g. Fe(III) minerals) via humic substances as electron shuttles since i) a large variety of microorganisms and diverse microbial population can use dissolved and solid-phase humic substances as electron acceptors and ii) the reduced humic substances react rapidly with different abiogenic and biogenically produced Fe(III) minerals. Electron transfer is associated with significant alteration of surface properties of the ferric mineral, leading to highly reactive Fe(III)/Fe(II) surface sites with a significant redoy buffering capacity. We could constrain the redox potential of such Fe(II)/Fe(III) surface sites at Goethite to ≈ -170 mV at pH7 using a quinone redox probe. In addition to electron shuttling humic substances function also as a significant redox buffer in environments that undergo dynamic redox cycling. Sulfidation of ferric hydroxides lead to rapid formation of hitherto overlooked surface bound polysulphides and subsequently a novel pathway for pyrite formation was discovered which we assume to be prominent under oscillating redox conditions, i. e. under conditions of rapid cycling of electrons. The role of surface bound polysulphide for the sulphur cycle and as electron shuttle remains to be clarified. Disproportionation of elemental sulfur to polysulfide and sulfate involved incorporation of oxygen atoms from FeOOH, indicating that the oxygen isotope composition of sulfate in the environment can be affected by the presence of oxygen-containing minerals. Our research revealed a rapid (< 24 h) reactions between dissolved sulphide and organic matter, clearly affecting its redox properties. Electron accepting functional groups in organic matter contribute to the recycling of inorganic sulphur. Moreover, organic matter also allows for storage of sulphur making it a highly relevant reactant also in dynamic systems. Specific degradation products of organic matter revealed that electron transfer processes are linked in anoxic aquifers via fluxes of methane across large scales. Progressive degradation of source substances (organic contaminants but also natural organic material) leads to increasing amounts of acidic metabolites and sulphurisation and reflects the extent of the metabolic network. Sulphate reduction, sulphide re-oxidation and thus sulphur cycling in general turned out to determine organic matter redox properties and highlight the need to consider these interactions when evaluating electron transfer in anaerobic systems. Overall, the results obtained in Research Unit 580 clearly demonstrate that electron transfer processes are tightly linked to the generation of surface precipitates or solid phases that are of poor order but high redox reactivity. Such species do rapidly form but due to their high reactivity are of metastable nature. Our research provides clear evidence that occurrence of redoxactive metastable phases is rather the rule than the exception under transient or oscillating redox conditions, implying a completely new perspective on the mechanistic controls of redox processes occurring at dynamic hydrological interfaces.
