Organic Carbon/Water and Dissolved Organic Carbon/Water Partitioning of Cyclic Volatile Methylsiloxanes: Measurements and Polyparameter Linear Free Energy Relationships

Abstract

The sorption of cyclic volatile methyl siloxanes (cVMS) to organic matter has a strong influence on their fate in the aquatic environment. We report new measurements of the partition ratios between freshwater sediment organic carbon and water (KOC) and between Aldrich humic acid dissolved organic carbon and water (KDOC) for three cVMS, and for three polychlorinated biphenyls (PCBs) that were used as reference chemicals. Our measurements were made using a purge-and-trap method that employs benchmark chemicals to calibrate mass transfer at the air/water interface in a fugacity-based multimedia model. The measured log KOC of octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5), and dodecamethylcyclohexasiloxane (D6) were 5.06, 6.12, and 7.07, and log KDOC were 5.05, 6.13, and 6.79. To our knowledge, our measurements for KOC of D6 and KDOC of D4 and D6 are the first reported. Polyparameter linear free energy relationships (PP-LFERs) derived from training sets of empirical data that did not include cVMS generally did not predict our measured partition ratios of cVMS accurately (root-mean-squared-error (RMSE) for logKOC 0.76 and for logKDOC 0.73). We constructed new PP-LFERs that accurately describe partition ratios for the cVMS as well as for other chemicals by including our new measurements in the existing training sets (logKOC RMSEcVMS: 0.09, logKDOC RMSEcVMS: 0.12). The PP-LFERs we have developed here should be further evaluated and perhaps recalibrated when experimental data for other siloxanes become available.

A critical assessment of the environmental fate of linear and cyclic volatile methylsiloxanes using multimedia fugacity models

“…Mackay et al 19 in their modeling calculations used a log K OC value of 5.17, which was measured by Kozerski et al 26 In a recent study, we measured the log K OC for D 5 to be 6.12. 16 This K OC value is of one order of magnitude higher than the measurements of Kozerski et al, 26 which would substantially increase the modeled residence time of D 5 in aquatic environments, and potentially indicate a more marked exceedance of the P criterion for sediment set by REACH. 24 A parameter that could have great inuence on K OC and thus on the residence times of VMS in aquatic environments is the enthalpy of sorption to OC from water (DH OC ).…”

“…Advection and reaction in the air are the main removal mechanisms (Table 4). Again, there are no notable differences between calculations using the K OC measurements from Panagopoulos et al 16,17 and those of Kozerski et al 26 The Level III EQC modeling assessment shows that for all VMS the medium of release strongly affects the distribution of the chemicals between air, water, soil and sediment. When released in the air all VMS tend to remain in air and they are removed from the environment through advection and reaction.…”

“…The values for K OA were calculated by subtracting the values for log K AW from those for log K OW . c The values for the salting-out constants (K s ) of VMS were measured by Panagopoulos et al 16,17 d The values for DH OW , DH AW and DH OA of D 4 , D 5 and L 4 were measured in the study of Xu and Kropscott. 31 The values for DH OW , DH AW and DH OA of D 6 and L 5 were calculated by linearly extrapolating from the measured values based on the chemicals' molecular weight.…”

“…12 In the air, VMS degrade within days because of their reaction with hydroxyl radicals, 13 but estimated lifetimes in sediment are substantially longer. [14][15][16][17] Whelan 18 explored the fate of cVMS in two contrasting North American lakes using multimedia models and underlined the importance of obtaining accurate measurements of K OC , as this was the parameter that was shown to be the most sensitive in the model calculations.…”

Screening-level models to estimate partition ratios of organic chemicals between polymeric materials, air and water

“…However, even the ppLFERs should be applied with caution to substances that are not represented in the training set, such as siloxanes, which are not well described by ppLFER models that do not include siloxanes in their training set. 55,56 Similarly, compounds with molecular weight greater than 500 g mol À1 and those that combine many different functional groups are not represented in our database and are likely outside the range of applicability of our models.…”