Determination of Henry's Law Constants for Organosilicones in Acutal and Simulated Wastewater

Abstract

The organosilicone compounds hexamethyldisiloxane (HMDS) and decamethylcyclopentasiloxane (D5) are used in consumer products that are disposed of down the drain and may be released to municipal wastewater treatment facilities. In such systems, their fate is determined largely by partitioning between air and water, which is affected by components of the aqueous phase. The Henry's law constant (H) is a commonly used expression for a compound's air−water partitioning behavior. The Equilibrium Partitioning in Closed Systems (EPICS) method of determination of H (dimensionless) was used to assess the fate of HMDS and D5 in wastewater systems. Three chlorinated hydrocarbons and toluene were used to validate the method. The pure water H is measured and reported for D5 and estimated for HMDS, which proved too volatile to be accurately determined using the EPICS technique. The apparent Henry's law constants (Ha) for all compounds were measured in raw wastewater and simulated wastewater solutions of KCl, humic acid, and resuspended secondary treatment sludge solids. A significant difference between pure water H and Ha in actual and simulated wastewater was observed for all compounds tested. In the simulated wastewater, relationships were observed for organosilicone air−water partitioning over ranges of suspended solids and humic acid and were best correlated to liquid-phase organic carbon content. An equilibrium partitioning model of the system is presented and used to estimate an organic carbon partitioning coefficient (Koc) and H for the organosilicones from the relationship between Ha and liquid-phase organic carbon content. Values of Koc were estimated at 24 000 for D5 and 29 000 for HMDS, with corresponding H values of 3.11 and 32.2 for D5 and HMDS, respectively. Observations from these experiments indicated that the components of natural wastewater can affect the volatility of the test chemicals by a significant amount.