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 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 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. Integr Environ Assess Manag 2022;18:599–621. © 2021 The Authors. Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC).

Key Points

• We critically review the use of MFTMs as a framework for understanding the environmental behavior of volatile methylsiloxanes (VMS).

• Careful selection of model inputs is needed because the properties of VMS differ from those of most organic compounds which means that some empirical estimation methods are not appropriate.

• Key remaining uncertainties include: (i) the strength and direction of the temperature‐dependence of partition coefficients; (ii) the fate of atmospheric reaction products; (iii) the magnitude of emissions to wastewater and (iv) the extent to which trophic transfer is influenced by metabolism.