nai

Project Progress

Radiolytic dissociation of water produces a highly reactive combination of oxidizing (e.g., H₂O₂, OH radicals, and O₂) and reducing (e.g., H atoms and H₂) species [1]. In subsurface environments, radiolysis can produce gradients of both electron acceptors and electron donors that are possible sources of metabolic energy [2]. Radiation-induced chemical reactions have particular significance in geologic environments where molecular oxygen derived from the atmosphere is a negligible input. Results from geochemical studies of unconformity-related uranium deposits indicate that radiolysis is considerably under recognized as a naturally occurring source of chemical energy for biotic and abiotic reactions. In particular, radiolysis of water coupled to oxidation of sulfide minerals [3] or elemental sulfur [4] can produce gradients of partially to fully oxidized sulfur species that might be suitable for microbial metabolism.

In order to evaluate the efficiency of radiolytic sulfide oxidation in the production of sulfate gradients and the stable isotope signatures of sulfur products, L. Lefticariu performed a series of radiation experiments using Park City pyrite and DI water. Radiation experiments were carried out in collaboration with J. Laverne at Notre Dame using a ⁶⁰Co gamma source at the Radiation Laboratory. The dose rate was of 11.3 krad/min (113 Gy/min), as determined by the Fricke dosimeter. Water used in these experiments was de-oxygenated to minimize competing reactions with O₂. These water/pyrite mixtures were degassed and flame sealed in quartz tubes of 2 cm diameter and 10 cm length. The tubes were irradiated from 1 to 14 hours. After irradiation, samples were analyzed for gaseous, aqueous, and solid species produced during radiolysis.

{{ 1 }}

Molecular hydrogen was the dominant gas collected at the end of pyrite-water irradiation experiment. Molecular oxygen was more than an order of magnitude lower than the H₂. The γ radiolysis of liquid water in closed systems leads to a low, constant H₂ concentration due to back reactions [8]. The initial experiments showed that the radiation chemical yield of molecular hydrogen increases with increase in the total irradiation dose. For the same total irradiation dose, the yield of H₂ increases with decrease in the initial amount of water. The dominant sulfur products collected during the initial experiments were aqueous sulfate and trace amounts of gaseous sulfur dioxide. Sulfate was the only aqueous sulfur species detected by ion chromatography methods in the final solution.

{{ 2 }}

In addition to studies of pyrite, we investigated whether an oxidation pathway exists with converts NH₃ to NO₃. An undergraduate at Princeton, R. Raymond, carried out these experiments in collaboration with D. Sigman at Princeton University and L. Lefticariu at Indiana University. Initial results verify oxidation of NH₃ to NO₃ by γ irradiation. The isotopic composition of the NO₃ produced by irradiation is distinct from that of contaminating NO₃.

{{ 3 }}