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

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 overall persistence, long-range transport potential (LRTP), and the propensity for atmosphere-surface 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 modelled and measured concentrations in air, sediment and biota indicate that our understanding of environmental fate is reasonable. VMS compounds are “fliers” which principally partition to the atmosphere, implying high LRTP, although they have low re-deposition 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: (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.