Chlorine isotope fractionation during catalytic reductive dechlorination of Trichloromethane (CHCl₃) over palladium-on-alumina in hydrogen-saturated water: implication to managed aquifer recharge as sustainable storage solution for desalinated water (MAR-DSW) in Menashe recharge basin, Israel

During the storage of excess desalinated water in a managed coastal aquifer in Israel, chlorine in desalinated water may react with natural organic matter, forming toxic disinfection by-products (DBPs), like trihalomethanes (THMs). In order to facilitate managed aquifer recharge as storage solution for desalinated water (MAR-DSW), it is important to understand the mechanisms of the formation and degradation of such DBPs.

In this work, degradation of chloroform (CHCl₃), a main pollutant of interest at the Menashe recharge basin in Israel, was investigated to derive chlorine isotope enrichment factors for the educt and degradation products as a part of the Israel-German joint research “aquifer recharge as sustainable storage solution for desalinated water (MAR-DSW)”.

100ml of deionized water spiked with 30mg/l chloroform was saturated with hydrogen, which serves as an electron donor. Then, 0.25g/l palladium was added as palladium-on-alumina (10% wt.) to catalyze the reaction. Samples were taken at specified intervals from a sampling port designed at the bottom of the reactor, which is closed with a plunger from the top so that no headspace was created as samples were taken out or during the entire experimental period.

During degradation of chloroform, the change in chlorine isotope ratios of CHCl₃ as well as its chlorinated degradation products was determined using a gas-chromatograph-mass spectrometer online connected to a purge and trap system (P&T-GC/MS). This simple online method was developed and optimized at TU Darmstadt. In addition to the reaction kinetics, isotope fractionation of CHCl₃ and its degradation products dichloromethane (CH₂Cl₂) as well as methyl chloride (CH₃Cl) were investigated. A chlorine isotope enrichment factor (ε) of -2.7‰ was derived for chloroform using the Rayleigh equation.

The investigation of isotope fractionation during the reductive dechlorination of chloroform i.e the Cl-CSIA can potentially be used as a tool to discriminate the source, pathway and fate of these compounds so as to foresee the feasibility of medium and long term use of managed aquifer recharge as a storage solution.