Dynamic multi-phase partitioning of decamethylcyclopentasiloxane (D5) in river water

Abstract

The behaviour of decamethylcyclopentasiloxane (D5) in river water was evaluated by measuring concentration changes in open beakers. Effective values for the partition coefficient between organic carbon and water (K(OC)) were derived by least-squares optimisation of a dynamic model which accounted for partitioning between the sorbed and dissolved phases of D5, and for losses via volatilisation and hydrolysis. Partial mass transfer coefficients for volatilisation were derived from model fits to controls containing deionised water. Effective values of log (K(OC)) were between 5.8 and 6.33 (mean 6.16). These figures are higher than some other experimentally-derived values but much lower than those estimated from the octanol: water partition coefficient using single-parameter linear free energy relationships (LFERs). A poly-parameter LFER gave a predicted log (K(OC)) of 5.5. Differences in partitioning are believed to be due to the nature of the organic matter present. The new value for effective K(OC) was employed in a simple model of D5 behaviour in rivers to ascertain the extent to which a higher affinity for organic carbon would depress volatility. The results suggest that despite the revised K(OC) value, volatilisation of D5 remains a significant removal mechanism in surface waters.

Determination of soil–water sorption coefficients of volatile methylsiloxanes

“…The D5 sorption K OC value reported in the present study is approximately 1 log unit lower than values reported by Whelan et al for volatilization experiments with filtered river water . However, direct comparison with the present study is complicated by methodological differences, including the assumption of equilibrium partitioning in the dynamic system.…”

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

“…Our measured values of log K OC and log K DOC of D 5 are in good agreement with the measurements of Whelan et al 10,11 and 0.93 log units higher than the measurements of Kozerski et al 16 Our measured log K OC of D 4 is similarly 0.83 log units higher than the measurements of Kozerski et al 16 The differences observed between our measurements and those of Whelan et al 10,11 and Kozerski et al 16 (Table S8) could be attributed to a myriad of factors, including differences in the source of organic carbon, differences in dosing the chemicals on the organic carbon and differences in the experimental setup. Whelan et al 11 used suspended particles in river water as a source of organic carbon, Kozerski et al 16 used soil and in our study we used sediment. Since organic carbon is not a well-defined standard material such as for example octanol, differences in the molecular composition of the source of organic carbon may result in differences in the partitioning of a chemical between organic carbon and water.…”

“…The volatilization losses of the cVMS over time are shown in Figure 2 and those of the PCBs in Figure S1. In the similar studies conducted by Whelan et al 10,11 for D 5 , the fraction that was not available for volatilization was between 5% and 25%. When we treated the maximum amount of the nonavailable fraction, B, as an unconstrained fitting parameter, its value was unrealistically high in experiments where less than 60% of the spiked chemical was volatilized.…”

“…10,11 The training data set from Neale et al 25 for K DOC is from studies that used Aldrich humic acid sodium salt as a source of DOC. This difference could have resulted in underestimation or overestimation of the K DOC measurements because of potential differences in the size of the dissolved matter or because of a salting-out effect from dissolved salts.…”

Predicting rapid herbicide leaching to surface waters from an artificially drained headwater catchment using a one dimensional two-domain model coupled with a simple groundwater model

“…Uncertainty in K OC is well recognised as it is known to vary significantly with the type of organic carbon (e.g. Niederer et al, 2007;Whelan et al, 2010). An initial value was set at 840 L kg −1 but this resulted in an underestimation of measured losses.…”