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.

Low Dietary Uptake Efficiencies and Biotransformation Prevent Biomagnification of Octamethylcyclotetrasiloxane (D4) and Decamethylcyclopentasiloxane (D5) in Rainbow Trout.

With octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5) being considered for evaluation under the UN Stockholm Convention on Persistent Organic Pollutants, which specifically acknowledges risks of biomagnification of persistent organic pollutants in traditional foods, a study into the mechanism of the biomagnification process of D4 and D5 in Rainbow trout was conducted by combining the absorption-distribution-metabolism-excretion for bioaccumulation (ADME-B) approach to determine intestinal and somatic biotransformation rates and radiochemical analyses to identify metabolite formation. High rates of intestinal biotransformation of D4 and D5 (i.e., 2.1 (0.70 SE) and 0.88 (0.67 SE) day-1, respectively) and metabolite formation [i.e., 52.0 (17 SD)% of D4 and 56.5% (8.2 SD)% of D5 were metabolized] were observed that caused low dietary uptake efficiencies of D4 and D5 in fish of 15.5 (2.9 SE)% and 21.0 (6.5 SE)% and biomagnification factors of 0.44 (0.08 SE) for D4 and 0.78 (0.24 SE) kg-lipid·kg-lipid-1 for D5. Bioaccumulation profiles indicated little effect of growth dilution on the bioaccumulation of D4 and D5 in fish and were substantially different from those of PCB153. The study highlights the importance of intestinal biotransformation in negating biomagnification of substances in organisms and explains differences between laboratory tests and field observations of bioaccumulation of D4 and D5.

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.

Development of collection, storage and analysis procedures for the quantification of cyclic volatile methylsiloxanes in wastewater treatment plant effluent and influent.

A reliable and accurate method for collection and analysis of octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5), and dodecamethylcyclohexasiloxane (D6) in wastewater treatment plant influent, effluent and surface waters was developed. Due to the use of cyclic volatile methylsiloxanes (cVMS) in industrial and consumer products including some personal care products, the wastewater stream represents a potential post-use disposal route and cVMS may subsequently enter the environment through wastewater treatment plant effluents. cVMS in the environment has come under increased regulatory scrutiny with regard to their potential for persistence, bioaccumulation and toxicity indicating a need for monitoring programs with reliable analytical methods. The developed method is unique in that it utilizes low density polyethylene (LDPE) to inhibit loss of cVMS during sampling and transport to the laboratory. The samples are then processed with a simple solvent extraction and analyzed by gas chromatography mass spectrometry with stable isotope internal standard calibration. This method utilizes readily available laboratory supplies and requires minimal field processing, reducing contamination potential. Method detection limits of 17 ng/L, 57 ng/L, and 20 ng/L were obtained for D4, D5, and D6, respectively. Additionally a robust quality control program was employed to ensure sample integrity. The method described herein can readily be adopted for use in monitoring studies where the amount of cVMS in water samples will be quantified.

Metabolism of 14C-octamethylcyclotetrasiloxane ([14C]D4) or 14C-decamethylcyclopentasiloxane ([14C]D5) orally gavaged in rainbow trout (Oncorhynchus mykiss).

Critical factors (uptake, distribution, metabolism and elimination) for understanding the bioaccumulation/biomagnification potential of Octamethylcyclotetrasiloxane (D4) and Decamethylcyclopentasiloxane (D5) siloxanes in fish were investigated to address whether these chemicals meet the "B" criteria of the Persistent, Bioaccumulative, and Toxic (PBT) classification. A metabolism study was conducted in rainbow trout whereby a 15mg [14C]D4/kg bw or [14C]D5/kg bw as a single bolus oral dose was administered via gavage. Of the administered dose, 79% (D4) and 78% (D5) was recovered by the end of the study (96-h). Eighty-two percent and 25% of the recovered dose was absorbed based on the percentage of recovered dose in carcass (69% and 17%), tissues, bile and blood (12% and 8%) and urine (1%) for D4 and D5, respectively. A significant portion of the recovered dose (i.e. 18% for D4 and 75% for D5) was eliminated in feces. Maximum blood concentrations were 1.6 and 1.4µg D4 or D5/g blood at 24h post-dosing, with elimination half-lives of 39h (D4) and 70h (D5). Modeling of parent and metabolite blood concentrations resulted in estimated metabolism rate constants (km(blood)) of 0.15 (D4) and 0.17day-1(D5). Metabolites in tissues, bile, blood, and urine totaled a minimum of 2% (D4) and 14% (D5) of the absorbed dose. The highest concentration of 14C-activity in the fish following D4 administration was in mesenteric fat followed by bile, but the opposite was true for D5. Metabolites were not detected in fat, only parent chemical. In bile, 94% (D4) and 99% (D5) of the 14C-activity was due to metabolites. Metabolites were also detected in the digestive tract, liver and gonads. Approximately 40% of the 14C-activity detected in the liver was due to the presence of metabolites. Urinary elimination represented a minor pathway, but all the 14C-activity in the urine was associated with metabolites. Clearance may occur via enterohepatic circulation of metabolic products in bile with excretion via the digestive tract and urinary clearance of polar metabolites.

Disposition of decamethylcyclopentasiloxane in Fischer 344 rats following single or repeated inhalation exposure to 14C-decamethylcyclopentasiloxane (14C-D5).

The disposition of decamethylcyclopentasiloxane (D5) in male and female Fischer 344 rats following single or repeated inhalation exposures was evaluated. Animals were administered a single 6-h nose-only exposure to 7 or 160 ppm 14C-D5 or fourteen 6-h nose-only exposures to unlabeled D5 followed on day 15 by a 6-h exposure to 14C-D5. Subgroups of exposed animals were used to evaluate body burden, distribution, elimination, and deposition on the fur. Retention of radioactivity following single and repeated exposures was relatively low (approximately 1-2% of inhaled D5). Radioactivity and parent D5 were widely distributed to tissues of both male and female rats, with the maximum concentration of radioactivity observed in most tissues by 3 h postexposure. Fat was a depot for D5, with elimination occurring much slower than observed for plasma and other tissues. In all groups, the primary route for elimination of radioactivity was through expired air. Analyses for parent D5 indicated that essentially all the radioactivity in the expired volatiles was unchanged D5. Repeated exposure gave rise to higher levels of parent D5 in the lung and fat of both sexes and in female liver relative to the single exposure. In fat, immediately after sacrifice approximately 50% of the radioactivity was attributed to parent. Five polar metabolites of D5 were identified in urine, with no parent D5 detected. Radiochromatograms demonstrated two peaks in feces. One corresponded to the retention time for D5. The second has been putatively identified as hydroxylated D5.