Redox Properties of Sorbed Natural Organic Matter

Abstract

Natural organic matter (NOM) is a complex mixture that originates from the decay of organic residues from plants and microbial sources. NOM, typically present in sorbed state as mineral coating or in particulate form rather than in solution, plays a key role in biogeochemical processes, i.e., electron shuttling between redox active minerals and bacteria. The main goal of this work was to investigate the influence of sorption of NOM on its redox properties (namely, electron exchange capacities (EEC) and redox state). Given the inherent heterogeneity of NOM, sorption of natural organic matter to mineral surfaces might induce changes in the redox properties of NOM fractions (sorbed vs. aqueous). Moreover, the electron transfer between redox active moieties in NOM and mineral surfaces might also alter its redox properties. Batch sorption experiments with standard humic acids (HA) isolates at defined redox states and redox-inert sorbents (Al2O3 and DAX-8 resin) were conducted. Mediated electrochemical approaches were applied to determine the Electron -Accepting (EAC) and -Donating (EDC) Capacities of HA fractions (dissolved, aqueous and in suspension). Adsorption of native HA onto Al2O3 resulted in large decreases in EEC of HA filtrates, while the EDC of sorbed HA increased significantly. These findings are consistent with preferential sorption of redox active components (e.g., quinone/hydroquinone, phenols) in HA. The increase in EDC of sorbed HA might be linked to: i) conformational changes of HA moieties on the Al2O3 surface or/and ii) surface catalyzed polymerization of polyphenol compounds under oxidizing conditions. The latter hypothesis was confirmed by up to 50% increase in EDC of a polyphenolic compound sorbed to Al2O3. Sorption of HA to DAX-8 had little effects on the EEC of HA fractions. Furthermore, sorption of electrochemically reduced HA to Al2O3 led to slightly lower EEC of HA in suspensions. Strong depletion of aromatic components in aqueous fractions of reduced HA caused large decreases in EEC. In addition to mediated electrochemical approaches, molecular scale (high-field Fourier Transform Ion Cyclotron Resonance Mass Spectrometry; FT-ICRMS) methods were applied to investigate the changes in electrochemical properties and the concomitant changes in chemical composition of HA fractions upon sorption. FT-ICRMS analysis provided clear evidence of preferential sorption of tannin-like compounds (rich in polyphenolic groups) onto Al2O3. Moreover, sorption of Pahokee Peat HA standard isolate (PPHA) on Na-saturated montmorillonite (Na-SWy-2) was studied at pH 7. While the EEC of PPHA in aqueous phase after sorption decreased up to 60 % compared to PPHA stock solution, the adsorbed fraction became more oxidized (up to 70 % lower EDC). This was associated to preferential sorption of redox active functional groups and/or electron transfer between Fe(III) in Na-SWy-2 and electron donor groups in PPHA. This study demonstrates that sorption of NOM considerably influences its redox properties to various degrees depending on the characteristics of the sorbent and the occurrence of electron transfer reactions. These effects can be related to molecular scale processes at the NOM-mineral interface. Considering that NOM in soil and groundwater is usually present in sorbed state, these findings are fundamental for an improved understanding of biogeochemical processes involving NOM. Additional work is needed to confirm the effects of sorption of NOM on its redox properties as well as the additivity of changes in EEC of NOM upon sorption to Fe-bearing minerals.