Application of multimedia models for understanding the environmental behavior of volatile methylsiloxanes: Fate, transport, and bioaccumulation

Abstract

Multimedia fate and transport models (MFTMs) describe how chemicals behave in the environment based on their inherent properties and the characteristics of receiving systems. We critically review the use of MFTMs for understanding the behavior of volatile methylsiloxanes (VMS). MFTMs have been used to predict the fate of VMS in wastewater treatment, rivers, lakes, marine systems, and the atmosphere, and to assess bioaccumulation and trophic transfers. More widely, they have been used to assess the overall persistence, long-range transport potential (LRTP), and the propensity for atmospheresurface exchange. The application of MFTMs for VMS requires particularly careful selection of model inputs because the properties of VMS differ from those of most organic compounds. For example, although n-octanol/water partition coefficient (K OW ) values are high, air:water partition coefficient (K AW ) values are also high and n-octanol/air partition coefficient (K OA ) values are relatively low. In addition, organic carbon/water partition coefficient (K OC ) values are substantially lower than expectations based on K OW . This means that most empirical relationships between K OC and K OW are not appropriate. Good agreement between modeled and measured concentrations in air, sediment, and biota indicates that our understanding of environmental fate is reasonable. VMS compounds are "fliers" that principally partition to the atmosphere, implying high LRTP, although they have low redeposition potential. They are degraded in air (half-lives 3-10 days) and, thus, have low overall persistence. In water, exposure can be limited by hydrolysis, volatilization, and partitioning to sediments (where degradation half-lives are likely to be high). In food webs, they are influenced by metabolism in biota, which tends to drive trophic dilution (i.e., trophic magnification factors are often but not always <1). Key remaining uncertainties include the following: (i) the strength and direction of the temperature dependence for K OC ; (ii) the fate of atmospheric reaction products; and (iii) the magnitude of emissions to wastewater.

Chemical characterization and formation of secondary organosiloxane aerosol (SOSiA) from OH oxidation of decamethylcyclopentasiloxane

“…Due to the properties of persistent, bioaccumulative, toxic and long-range atmospheric transport, cVMS are currently being considered as emerging persistent organic pollutants (POPs) for regulation. 14 Recent results have also detected low-volatility siloxanes in the particle phase from indoor cooking events 15 or ship emissions. 16 Evidence from laboratory studies indicated that oxidized cVMS products could also contribute to the formation of secondary organosiloxane aerosol (SOSiA), 17,18 especially decamethylcyclopentasiloxane (D 5 -siloxane, C 10 H 30 O 5 Si 5 ), the most abundant siloxane in urban areas.…”

“…Due to the properties of persistent, bioaccumulative, toxic and long-range atmospheric transport, cVMS are currently being considered as emerging persistent organic pollutants (POPs) for regulation. 14…”

Comparison of the Yield and Chemical Composition of Secondary Organic Aerosol Generated from the OH and Cl Oxidation of Decamethylcyclopentasiloxane

“…As such, recent studies have focused on the atmospheric chemistry of cVMS. D 5 is usually emitted in higher concentrations than other cVMS, with typical indoor and outdoor D 5 mixing ratios ranging from approximately 4 to 20 ppbv and from 0.4 to 20 pptv, respectively. ,− …”

“…14,16,22 OFRs use residence times that are on the order of minutes and oxidant concentrations that are typically 100−1000 times higher than ambient levels. Their ability to access photochemical aging time scales of up to several days was critical in previous studies examining D 5 + OH SOA formation as a result of the relatively long OH lifetime of D 5 . While these factors may make the chemistry and microphysics in the OFR somewhat different from the chemistry and microphysics in the atmosphere, 23,24 here, investigating D 5 Cl-SOA formation in an OFR enables a more direct comparison of the relative SOA yields obtained via OH and Cl oxidation (hereafter referred to as "OH-SOA" and "Cl-SOA", respectively).…”