Irradiation of Oil / Water Biphasic Systems: the Importance of Interfacial Surface Area on the Production of Hydrogen and Other Deleterious Products

Occasionally, organic materials, such as lubricating oils, can enter irradiated aqueous reactor systems. This can upset the chemistry control of the reactor, resulting in elevated hydrogen gas concentrations, changes in system pH and the formation of unwanted degradation products. Most available information on the radiation chemistry of oil is extrapolated from irradiations of neat simple hydrocarbons like hexane; there is little information available as to the radiolytic breakdown of larger hydrocarbons in the presence of water. In the absence of water, the general radiation effects on hydrocarbons can be divided into fragmentation and polymerizations reactions. Some factors that can influence the degradation of hydrocarbons include the extent of hydrocarbon branching, the degree of bond-saturation, and the presence of scavenging molecules and dissolved gases. The mechanism of water radiolysis is well understood and tools are available to simulate such radiation chemistry. Additionally, irradiations of aqueous systems containing trace quantities of soluble organic species and ion exchange resins have also been studied. However, at least initially, oils that enter irradiated aqueous systems are essentially insoluble in water. This leads to a non-homogeneous system where radiation energy is deposited in both water and organic phases, each of which will have distinct irradiation behaviours. In addition to the irradiation effects in the aqueous and organic phases, the effects of irradiation on the chemistry at the interface between the phases and the rate of production of soluble hydrocarbon fragments from the degradation of the oil are unknown. A program was initiated to examine the radiation chemistry effects on aqueous systems contaminated with insoluble hydrocarbon-based oils. A unique vessel has been designed and fabricated for the irradiation of hydrocarbon-water mixtures in a Gammacell ⁶⁰Co γ-irradiator. The design allows for variation of the hydrocarbon-water interfacial surface area, control of headspace gas composition, and removal of sample aliquots. Results highlight the importance of interfacial surface area in affecting the radiolytic degradation of the studied hydrocarbons. In particular, experiments having higher oil-water interfacial surface areas generate greater quantities of oil degradation products as compared with lower surface area samples. As expected, one notable result from these irradiations was the formation of significant quantities of hydrogen, which was found to be dependent on the interfacial surface area. Presented here is a review of the radiolytic degradation of insoluble organic material in aqueous systems, a summary of experimental results focusing on biphasic systems and a description of a strategy to mitigate the effects of insoluble organic material ingress and to aid in developing station-appropriate responses. (authors)