Electron Transfer Processes between Hydrogen Sulfide and Humic Substances - Implications for Anaerobic Sulfur Cycling in Freshwater Ecosystems

The relevance of biogeochemical gradients for turnover of organic matter is yet poorly understood. This study aims at the identification and quantification of the interaction of different redox processes along gradients with particular emphasis on the impact of redox active humic substances. The interactions between sulfide, sulfate (SO42-) reduction, methanogenesis, and organic matter (OM) redox processes were investigated in controlled abiotic and biotic incubation experiments. In the first study, we investigated the abiotic transformation of sulfide upon reaction with reduced and non-reduced Sigma Aldrich humic acid (HA), under anoxic conditions. Sulfide reacted with non-reduced HA at rates comparable to sulfide oxidation by iron oxides or molecular oxygen. The main transformation products were elemental S (S0), and thiosulfate (S2O32-), yielding electron accepting capacities (EACs) of 2.82~1.75 µmol e- (mg C) -1. Native iron contents in the HA explained only 6~9% of these EACs. Another important fraction of the reaction of sulfide with HA was organic S (Sorg). For HA reduced by hydrogen (H2) on a Palladium (Pd) catalyst, even only a formation of Sorg was observed and no inorganic transformation products occurred. X-ray absorption near edge structure (XANES) spectroscopy supported Sorg to be mainly about zerovalent, such as thiols, organic di- and polysulfides, or heterocycles. In a second study, we addressed the impact of electrochemical and wet chemical (hydrogen (H2)/Pd-catalyst) reduction of Sigma Aldrich humic acid (HA) on its reactivity towards sulfide. Moreover, we tested the impact of HA reaction with sulfide on electron transfer capacities (ETC) as detected by mediated electrochemical reduction and oxidation. The reactivity of HA towards sulfide was clearly related to the initial redox state of HA, as measured initial values of EAC of HA had a strong and positive correlation with the amount of transformed sulfide. H2/Pd treatment of HA obviously changed HA structures and lead to a different reactivity towards sulfide, limiting a direct comparison to electrochemically reduced organic matter. In a third study, we incubated peat samples virtually devoid of inorganic electron acceptors under anoxic conditions and monitored CO2 and CH4 production to estimate EAC from organic matter. From excess CO2 production, i.e. from CO2:CH4 ratios of 3.2:1, we calculated an EAC of OM of 2.36 µmol e- cm-3 d-1. Addition of sulfate (SO42-) increased CO2 production and suppressed CH4 production as expected. However, after subtracting the EAC provided though SO42- (0.97~2.81 µmol e- cm-3 d-1), OM provided even higher EAC of 3.88 to 4.85 µmol e- cm-3 d-1.The contribution of organic sulfur was again evaluated by sulfur K-edge XANES and using δ34S natural abundance as a tracer. Bacterial sulfate reduction (BSR) presumably involved a re-oxidation of sulfide by organic matter as proposed earlier, but also a sulfurization of OM yielding reduced organic sulfur, and changes in oxidized organic sulfur species. In conclusion, our results quantitatively demonstrated both HA and reduced HA can abiotically re-oxidize sulfide in anoxic environments at rates competitive to sulfide oxidation by molecular oxygen or iron oxides. H2/Pd pre-treatment of HA alters redox properties and reactivity of organic matter and may therefore lead to biased results when being employed in experimental approaches. Peat incubation experiment confirmed that organic matter contributes to anaerobic respiration i) directly by EAC of redox active functional groups ii) directly by provision of EAC from oxidized organic sulfur and iii) indirectly by re-oxidation of sulfide to maintain BSR. Overall, our results indicated the importance of anaerobic sulfur cycling through organic matter and identified limitations of common approaches addressing redox properties of organic matter solely by H2/Pd reduction or electrochemical approaches.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Universität Bayreuth

Year

2015

Author dc:creator

• Zhiguo, Yu

Contributors dc:contributor

• Knorr, Klaus-Holger

Repository record source_url

https://epub.uni-bayreuth.de/id/eprint/2132/

OAI identifier oai:identifier

oai:epub.uni-bayreuth.de:2132