Determination of soil–water sorption coefficients of volatile methylsiloxanes

Abstract

The sorption behaviors of 4 cyclic and linear volatile methyl siloxane (VMS) compounds between water and organic matter in 3 United Kingdom soils were studied by a batch equilibrium method using13C-enriched sorbates. Sorption and desorption kinetics and isotherms were determined for octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5), octamethyltrisiloxane (L3), and decamethyltetrasiloxane (L4). Concentrations of [13C]-VMS in the soil and aqueous phases were measured directly by extraction and gas chromatography–mass spectrometry techniques. All VMS compounds were sorbed rapidly, reaching constant distributions in all soils by 24 h. Desorption kinetics were very rapid, with reattainment of equilibrium within 1 h. In the main, linear isotherms were observed for aqueous concentrations at or below 4% of the solubility limits. The average sorption organic carbon partition coefficient (log KOC) values across soils were 4.23 for D4, 5.17 for D5, 4.32 for L3, and 5.13 for L4, with standard deviations of 0.09 to 0.34. Desorption KOC values were systematically greater by 0.1 log units to 0.3 log units. The linear isotherms and low variation in KOC values across soils suggested partitioning-dominated sorption of the VMS. Compared with traditional hydrophobic organic compounds, KOC values for the VMS compounds were significantly lower than expected on the basis of their octanol–water partition coefficients. A linear free energy relationship analysis showed that these differences could be rationalized quantitatively in terms of the inherent characteristics of the VMS compounds, combined with the differences in solvation properties of organic matter and octanol. Environ Toxicol Chem 2014; 33:1937–1945.

Critical Review and Interpretation of Environmental Data for Volatile Methylsiloxanes: Partition Properties

“…Thus, when adequately trained, LFERs have wider domains of applicability, which can include VMS. 42 The same conclusion has been reached by other researchers for different organic compounds. 64,67 VIII.2.…”

“…In a laboratory study, a batch equilibrium method based on OECD test guideline 106 41 was employed. 42 The measured log K OC values as listed in Table 8 demonstrated a general trend of increasing log K OC as the molar mass of VMS increases, similar to that of log K OW . In addition to the wellcontrolled laboratory study cited above, K OC values were also measured in isolated soil organic matter/water systems, 43 and river water/sediment systems.…”

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

“…Previous experimental studies have reported considerably lower log K OC values that range from 4.98 to 6.33 for D 5 . 10,11,16 These values for D 5 are all more than 1 order of magnitude lower than the estimate from the Seth regression. Existing SP-LFERs fail to provide good estimates for cVMS because SP-LFERs implicitly assume that controlling interactions are identical for all chemicals.…”

“…Evidence of the steric hindrance of the ring structure of cVMS reducing the potential for London forces and dipole−induced dipole interactions can be found in comparing the log K OC of the cyclic D 4 to linear decamethyltetrasiloxane (L 4 ). Kozerski 16 Differences in Sorption between OC and DOC. As discussed above, cVMS have a lower L compared to other chemicals with similar V. For that reason, we focused the comparison of OC and DOC on their differences in v and l. In the PP-LFER equations that include cVMS in the training set v DOC is slightly larger than v OC (1.84 and 1.76) (Figure 3) implying that the energy cost for cavity formation on the water Environmental Science & Technology Article side will be slightly higher in a K DOC system than in a K OC system.…”

Decamethylcyclopentasiloxane (D5) environmental sources, fate, transport, and routes of exposure

“…Table provides a summary of the recommended physical and chemical properties of D5, including key intermedia partitioning and degradation properties (i.e., half‐lives). A more detailed discussion on methods for determining these properties and justification of these recommended properties and their temperature dependence is given in Kozerski et al , Xu and Kropscott , and Xu et al . In the present study, we focus on aspects of these properties that are of particular importance for evaluating fate and transport.…”