Sorption- and diffusion-associated isotope effects for chlorinated and non chlorinated aromatic hydrocarbons in a sediment pore water diffusion sampler

Compound Specific Isotope Analysis (CSIA) has gained prominence for evaluation of microbial and abiotic degradation processes governing the fate of organic contaminants in groundwater. At the sediment pore water interface, in wetland or river bottom sediments, variations in oxidation-reduction conditions can affect reaction mechanisms and hence the contaminant mass flux discharged to surface waters. Carbon isotope fractionation has been shown to be an important tool in identifying the effects of degradation and differentiating between different degradation pathways. To date, while passive diffusion samplers (commonly called 'peepers') have provided a powerful tool for high spatial resolution sampling for dissolved VOC across the sediment water interface, peepers' compatibility with CSIA has never been evaluated. The operating principle of peepers involves compound diffusion from the sediment pore water to the peeper chambers via a membrane. In this study, we evaluated the isotope effects of diffusion through, and possible adsorption to a polysulfone membrane for priority groundwater contaminants including chlorinated and non-chlorinated aromatic hydrocarbons. Chlorinated benzenes tend to accumulate in the food web and therefore represent a significant threat to water resources. This is due to their larger sorption coefficients (Koc) and higher hydrophobicity properties (logKow) compared to other commonly-studied compounds (e.g., chlorinated ethenes). Application of CSIA to BTEX and chlorinated ethenes has demonstrated that non-degradative processes (e.g., sorption, volatilization, diffusion) typically result in smaller carbon isotope fractionation compared to degradative processes that involve breaking bonds. The large sorption properties of chlorinated benzenes preclude a direct extrapolation to these compounds of existing data on sorption-associated isotope effects obtained on other compounds. To date, similar studies have not been done for chlorinated aromatics. Isotopic signatures of benzene, toluene, monochlorobenzene, and 1,2-dichlorobenzene was measured in laboratory peeper experiments over a two-month period. Isotope values remained constant (within × 0.5‰ error) during diffusion through the peepers' polysulfone membrane, and were similar to molecules' characterized values (× 0.5‰). The results showed a consistent increase in compound adsorption on the peeper material with increasing chlorine substituent number and Koc values, without significantly affecting isotope values. This study demonstrated that even for strongly sorbing molecules such as chlorinated benzenes, adsorption and diffusion did not significantly affect contaminant isotope signatures. As a consequence, peeper sediment pore water samplers made of polysulfone membrane are compatible for CSIA application for both chlorinated and non-chlorinated aromatic hydrocarbons.