Batch equilibrium experiments and modeling reveal weak temperature dependence of cyclic volatile methylsiloxane sorption in soil/sediment organic carbon-water systems.

The organic carbon normalized partition coefficient, KOC, describes the equilibrium distribution of a chemical between water and organic carbon in soil or sediment. It is a key parameter in evaluating chemical persistence, mass distribution, and transport using multimedia fate and transport models. Considerable uncertainty remains about the KOC values of cyclic volatile methylsiloxane (cVMS) compounds, and in particular the dependence of KOC on temperature. In this study, we used a batch equilibrium (BE) method to measure KOC values and their temperature dependence between ∼5 and 25 °C for octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5) with soil and sediments. Approximate log KOC values at 25 °C were 4.5-5.0 for D4 and 5.5-6.1 for D5 with different sorbents, and decreased by 0.3 log units or less at 4-5 °C. Enthalpies of sorption, ΔHOC, obtained for the different sorbents ranged from +7.2 to +16 kJ mol-1, with average values of +7.9 and +13 kJ mol-1 for D4 and D5, respectively. These values differ in magnitude and direction from those reported elsewhere based on KOC values determined by a novel dynamic purge-and-trap (PnT) method, but are consistent with predictions based on their solvation properties. A new fugacity-based multimedia model incorporating sorption/desorption kinetics was developed and used to predict concentrations in the phases of BE and PnT systems during desorption of cVMS under different experimental and ideal conditions. Model simulations suggested that KOC values for cVMS compounds derived from the PnT systems could be influenced by sorption disequilibrium between water and solids controlled by desorption rates from the particle phase to water, and subsequent losses due to volatilization and degradation. This has the potential to result in overestimation of KOC values when fitting the experimental data of cVMS mass remaining in a PnT system over time, which could explain the observed differences between the methods.

Basic considerations to minimize bias in collection and analysis of volatile methyl siloxanes in environmental samples.

Monitoring studies that aim to quantify volatile methyl siloxanes (VMS) in environmental matrices may encounter a multitude of issues, most of which relate to the unique combination of physical-chemical characteristics of VMS that distinguish them from other classes of organic compounds. These properties, which are critical to their function in various applications, also control their fate and distribution in the environment, as well as the analytical chemistry of their measurement. Polycondensation and rearrangement reactions of VMS oligomers are possible during sample storage and analysis. Thus, care should be exercised to suppress these types of reactions by avoiding any catalytic substances or surfaces in sample collection and analysis equipment. Another factor complicating sample integrity in the analysis of trace levels of VMS, is their ubiquitous presence in many common products and components of instrumentation in the laboratory. For example, some gas chromatography columns and inlet septa have been identified as sources of VMS due to surface-catalyzed transformation of silicones to VMS promoted by moisture under high temperature in some silicone-based GC columns. Possible chemical transformation of the analytes, contamination from other sources, and potential loss of analytes need to be assessed throughout all aspects of the study, from sample collection through analysis, by establishing a rigorous quality assurance and quality control program. The implementation of such a robust QA/QC program facilitates the identification and minimization of potential analytical biases and ensures the validity and usability of data generated from environmental monitoring campaigns for VMS. The objective of this paper is to focus on aspects of collection, processing, and analysis of environmental samples that may influence the quality of the VMS analytical results. This information should then be employed in the design and implementation of future monitoring studies and can used to assess the validity of analytical results from VMS monitoring studies.

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

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.

Fugacity and activity analysis of the bioaccumulation and environmental risks of decamethylcyclopentasiloxane (D5).

As part of an initiative to evaluate commercial chemicals for their effects on human and environmental health, Canada recently evaluated decamethylcyclopentasiloxane (D5; CAS no. 541-02-06), a high-volume production chemical used in many personal care products. The evaluation illustrated the challenges encountered in environmental risk assessments and the need for the development of better tools to increase the weight of evidence in environmental risk assessments. The present study presents a new risk analysis method that applies thermodynamic principles of fugacity and activity to express the results of field monitoring and laboratory bioaccumulation and toxicity studies in a comprehensive risk analysis that can support risk assessments. Fugacity and activity ratios of D5 derived from bioaccumulation measures indicate that D5 does not biomagnify in food webs, likely because of biotransformation. The fugacity and activity analysis further demonstrates that reported no-observed-effect concentrations of D5 normally cannot occur in the environment. Observed fugacities and activities in the environment are, without exception, far below those corresponding with no observed effects, in many cases by several orders of magnitude. This analysis supports the conclusion of the Canadian Board of Review and the Minister of the Environment that D5 does not pose a danger to the environment. The present study further illustrates some of the limitations of a persistence-bioaccumulation-toxicity-type criteria-based risk assessment approach and discusses the merits of the fugacity and activity approach to increase the weight of evidence and consistency in environmental risk assessments of commercial chemicals.

Critical review and interpretation of environmental data for volatile methylsiloxanes: partition properties.

Volatile methylsiloxanes (VMS) enter the environment through industrial activities and the use of various consumer products. Reliable measurements of environmental partition properties for these compounds are critical for accurate prediction of their environmental fate, distribution, transport, exposure and potential effects. In this study, the measured partition properties including air/water (K(AW)), octanol/water (K(OW)), and octanol/air partitioning coefficients (K(OA)), soil organic carbon/water distribution coefficient (K(OC)), and biological medium/fluid partition coefficients, and their temperature dependence were critically reviewed. Based on these results, organosilicon compounds such as methylsiloxanes are expected to behave differently in the environment compared to conventional hydrophobic environmental contaminants, as a result of their inherent characteristics related to molecular size and capacity for different types of molecular interactions that control partitioning. The differences are critical and need to be taken into consideration in environmental exposure and risk analyses of these compounds.

Determination of soil-water sorption coefficients of volatile methylsiloxanes.

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 using(13)C-enriched sorbates. Sorption and desorption kinetics and isotherms were determined for octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5), octamethyltrisiloxane (L3), and decamethyltetrasiloxane (L4). Concentrations of [(13)C]-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 (logK(OC)) 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 K(OC) values were systematically greater by 0.1 log units to 0.3 log units. The linear isotherms and low variation in K(OC) values across soils suggested partitioning-dominated sorption of the VMS. Compared with traditional hydrophobic organic compounds, K(OC) 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.

Positive vs. false detection: a comparison of analytical methods and performance for analysis of cyclic volatile methylsiloxanes (cVMS) in environmental samples from remote regions.

Contamination and analytical variation can significantly hinder trace analysis of cyclic methyl volatile siloxanes (cVMS); potentially resulting in the report of false positives at concentrations approaching detection limits. To assess detection and variation associated with trace cVMS analysis in environmental matrices, a co-operative laboratory comparison for the analysis of octametylcyclotetrasiloxane (D4), decamethylcylcopentasiloxane (D5), and dodecametylcyclohexasiloxane (D6) in sediment and biota from the Svalbard Archipelago was conducted. Two definitions of detection limits were evaluated in this study; method detection limits (MDL, matrix defined) and limits of detection (LOD, solvent defined). D5 was the only cVMS detected above both LOD (0.08-0.81ngg(-1)ww) and MDL (0.47-2.36ngg(-1)ww) within sediment by all laboratories where concentrations ranged from 0.55 to 3.91ngg(-1)ww. The percentage of positive detects for D5 decreased by 80% when MDL was defined as the detection limit. D5 was also detected at the highest frequency among all laboratories in fish liver with concentrations ranging from 0.72 to 345ngg(-1)ww. Similar to sediment, percentage of positive detects for D5 decreased by 60% across all laboratories for fish livers when using MDL (0.68-3.49ngg(-1)ww). Similar observations were seen with both D4 and D6, indicating that sample matrix significantly contributes to analytical response variation. Despite differences in analytical methods used between laboratories, good agreement was obtained when using MDL to define detection limits. This study shows the importance of incorporating variation introduced by sample matrices into detection limit calculations to insure data accuracy of cVMS at low concentrations.

Determination of descriptors for organosilicon compounds by gas chromatography and non-aqueous liquid-liquid partitioning.

Measurements of retention factors by gas chromatography on up to 10 complementary stationary phases at up to 5 temperatures for each stationary phase and liquid-liquid partition coefficients in three biphasic organic solvent systems (n-hexane-acetonitrile, n-heptane-N,N-dimethylformamide and n-heptane-2,2,2-trifluoroethanol) were used to estimate solute descriptors for 54 organosilicon compounds for use in the solvation parameter model. Many of the E descriptor values (electron lone pair interactions) are negative for simple siloxanes and silanes indicating that these compound bind electron lone pairs more tightly than n-alkanes. Silanes and siloxanes with alkyl groups have near zero dipolarity/polarizability (S descriptor). The S descriptor is only modest for simple phenylsilanes, silazanes, silanols, orthosilicates, and alkoxides. All organosilicon compounds with silicon-oxygen bonds are reasonably strong hydrogen-bond bases (B descriptor) but only the silanol group is a reasonably strong hydrogen-bond acid (A descriptor). Silanes (SiH) and silazanes (SiNHSi) are weak hydrogen-bond acids. Cavity formation and dispersion interactions (V or L descriptor) are often the main component of solvation models for siloxanes and silanes that have simple alkyl and aromatic substituents. A number of physicochemical properties (vapor pressure, aqueous solubility, biphasic partition coefficients, sorption coefficients, etc.) for linear and cyclic dimethylsiloxanes can be reliably predicted from their descriptors in established models for organic compounds.