Partitioning and persistence of volatile methylsiloxanes in aquatic environments

Abstract: The presence of volatile methylsiloxanes (VMS) in the environment has raised concerns among environmental chemists and regulators about their persistence and the risks they may pose to the environment. This thesis explores the partitioning and persistence of VMS in aquatic environments. In Paper I, we reported new measurements of the organic carbon/water (KOC) and dissolved organic carbon/water (KDOC) partition ratios of three cyclic volatile methylsiloxanes (cVMS) and of three polychlorinated biphenyls (PCBs), which were used as reference chemicals. We combined new measurements with existing data to construct polyparameter linear free energy relationships (PP-LFER) that describe the KOC and KDOC of diverse sets of chemicals. The findings suggest that cVMS do not conform to single-parameter regressions that relate the chemicals’ KOC to their octanol/water partition ratio (KOW). PP-LFERs can accurately describe the KOC and KDOC of cVMS but only if cVMS are included in their training sets. In Paper II, we studied the effect of salinity on the KOC and KDOC of three cVMS, two linear volatile methylsiloxanes (lVMS) and three PCBs. We also evaluated the predictive power of the PP-LFERs constructed in Paper I by testing them on the newly measured KOC values of lVMS. The KOC and KDOC increased with increasing salinities similarly to those of the PCBs. PP-LFERs that were trained with datasets that included siloxanes could predict the KOC and KDOC of other siloxanes more accurately than PP-LFERs without siloxanes in the training set. In Paper III, we evaluated the effect of temperature on the KOC of VMS and we compared our measurements of the enthalpy of sorption to organic carbon (ΔHOC) to existing measurements of the enthalpy of phase change between octanol and water (ΔHOW). Due to the scarcity of ΔHOC data in the literature it is common practice in modeling calculations to use ΔHOW instead when correcting for temperature changes. The KOC of cVMS increased with decreasing temperatures. Moreover, our results indicate that ΔHOC and ΔHOW may be intrinsically different and hence replacing ΔHOC with ΔHOW in modeling calculations could lead to substantial errors, especially for VMS. In Paper IV, we explored the environmental fate of VMS in aquatic environments using multimedia models. In particular, we assessed the differences that may occur in calculations of persistence due to (i) the reported KOC measurements of VMS differing by one log unit (ii) the influence of salinity on KOC, and (iii) the differences in the reported ΔHOC and ΔHOW measurements of VMS. The calculated residence times for decamethylcyclopentasiloxane (D5) in a site-specific scenario for a Norwegian fjord receiving siloxanes in wastewater ranged from 200 to 1000 days, and demonstrated that the selection of KOC values can result in substantially different calculated persistence. Future partitioning measurements of VMS in the real environment and mass-balance modeling studies in aquatic environments combined with field measurements could help us to deepen our understanding about their persistence and to assess the risks VMS may pose to the environment.