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Layer silicate clays modified with cationic surfactants (organoclays) such as hexadecyltrimethylammonium (HDTMA) are effective sorbents for neutral organic contaminants (NOCs) and may be useful for in-situ and ex-situ remediation of contaminated soils and waters. In this study, the sorptive characteristics of organoclays were compared in terms of magnitude and mechanism to those of natural soil organic matter. The data presented here support multiple mechanisms controlling the sorption of aqueous-phase NOCs on organoclays including solvation of the cationic ammonium centers, the alkyl chains of HDTMA, and the mineral surfaces and solute partitioning. In contrast, solute partitioning appears to be the singular mechanism for the sorption of aqueous-phase NOCs by natural soil organic matter. Organic contaminant sorption by the surfactant-derived organic matter of organoclays is substantially higher than by natural soil organic matter due to the existence of multiple sorptive mechanisms for organoclays and the greater solvency of their organic phases.

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Cyclic volatile methyl siloxanes (cVMS) such as octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5), and dodecamethylcyclohexasiloxane (D6) may enter the environment through industrial activities and the use of various consumer products. Reliable air/water (K(AW)), 1-octanol/water (K(OW)), and octanol/air partition coefficients (K(OA)) for those compounds and their common degradation product, dimethylsilanediol, are critical for accurate prediction of the environmental fate, distribution, and transport of these materials. Challenges have been encountered in determining these properties for cVMS and their degradation products mainly due to the extremely low water solubility of the organosiloxanes, low volatility of their degradation products, and reactivity of those compounds in the water/1-octanol system that can lead to inconsistent and inaccurate partition coefficients. A novel direct method is presented for the simultaneous determination of K(AW), K(OW), and K(OA) of organic compounds and was applied to these organosilicon compounds. It was tested in a range of log K(AW) values from -6.8 to 3.1, log K(OW) values from -0.4 to 8.9, and log K(OA) values up to 7. The advantages of the new direct method include the improved accuracy, a shortened measurement time, simultaneous measurement of three partition coefficients of multiple compounds, self-consistency among resultant partition coefficients, and a wide range of applicability including materials that may be slowly reactive in the water/1-octanol system.

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The environmental sources, fate, transport, and routes of exposure of decamethylcyclopentasiloxane (D5; CAS no. 541-02-6) are reviewed in the present study, with the objective of contributing to effective risk evaluation and assessment of this and related substances. The present review, which is part of a series of studies discussing aspects of an effective risk evaluation and assessment, was prompted in part by the findings of a Board of Review undertaken to comment on a decision by Environment Canada made in 2008 to subject D5 to regulation as a toxic substance. The present review focuses on the early stages of the assessment process and how information on D5's physical-chemical properties, uses, and fate in the environment can be integrated to give a quantitative description of fate and exposure that is consistent with available monitoring data. Emphasis is placed on long-range atmospheric transport and fate in water bodies receiving effluents from wastewater treatment plants (along with associated sediments) and soils receiving biosolids. The resulting exposure estimates form the basis for assessments of the resulting risk presented in other studies in this series. Recommendations are made for developing an improved process by which D5 and related substances can be evaluated effectively for risk to humans and the environment.

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