EVOLUTION OF δ13C VALUES DURING ABIOTIC OXIDATION OF ORGANIC COMPOUNDS IN CIRCUM-NEUTRAL ANAEROBIC CONDITIONS UNDER BICARBONATE AND PHOSPHATE BUFFERED SYSTEMS

We conducted experiments to investigate abiotic oxidation of organic compounds as a function of electron acceptors, media composition, and pH in the presence and absence of fluorescent light. Methodologies involved were time series determinations of aqueous iron (II)/(III), dissolved inorganic carbon (DIC) and its carbon isotope ratio (d¹³C). The evolution of DIC and stable isotope of carbon were used as proxies for the oxidation of citrate and lactate under strict anaerobic conditions, variable pH (5-9) and 30^°C, dark and light (fluorescent) conditions. Ferric citrate (δ¹³C, -25‰), citric acid (δ¹³C, -32‰) and sodium lactate (δ¹³C, -25‰) were used as organic compounds, while HFO and Ferric citrate were used as electron acceptors. Sodium bicarbonate (δ¹³C, +10‰) or potassium phosphate was used as buffering agent. Mixing between carbon isotopes from the added bicarbonate and carbon dioxide produced from the oxidation of organic compounds were observed. Type of buffer showed little effect on the redox process. HFO media generated more DIC compared to ferric citrate containing media. Light and pH played major roles in the oxidation of citrate and lactate in the presence of ferric iron. Under dark conditions in the presence or absence of Fe (III) the DIC produced was very low in all pH conditions. The slower redox processes observed in our study, compared to previous studies, could be due to the inhibiting effect of chloride ions on photochemical reactions due to the scavenging effect of chloride ion on hydroxyl radicals. d¹³C values of DIC in both bicarbonate and phosphate buffered systems, at neutral pH condition, approached the ¹³C values of citrate/lactate used. Inhibition of DIC production was also observed upon photo exposure when Fe (III) was absent. Isotopically, the bicarbonate system showed mixing between the bicarbonate and the carbon dioxide produced from the oxidation. Such studies have implications in understanding geochemical processes and their reaction mechanisms in waste water treatment, enhancing in situ degradation of organic and inorganic compounds, cycling of nutrients in wetland ecosystems, sources and sinks of chemicals in the atmosphere, and are also important in extreme environments and even on other planetary atmospheres.