Elsevier

Chemosphere

Volume 93, Issue 5, October 2013, Pages 711-725
Chemosphere

Review of recent advances in research on the toxicity, detection, occurrence and fate of cyclic volatile methyl siloxanes in the environment

https://doi.org/10.1016/j.chemosphere.2012.10.041Get rights and content

Abstract

The fate and behavior of cyclic volatile methylsiloxanes (cVMS) octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5), and dodecamethylcyclohexasiloxane (D6) in the environment were reviewed. We evaluated their usage data and patterns, physico-chemical properties, toxicology, partitioning and degradation, methods of detection, and concentrations. The use of cVMS as an intermediate in the formation of silicone polymers, personal care and household products has resulted in their widespread environmental exposure; they have been detected in biogas, air, water, soil, biosolid, sediment, and biota samples. Modeled and experimental results suggest that cVMS may be subject to long-range atmospheric transport, but have low potential to contaminate the Arctic. For D4 and D5, there was no evidence of trophic biomagnification in aquatic food webs, while some aquatic organisms demonstrated a high degree of bioconcentration and bioaccumulation. High concentrations of cVMS observed in indoor air and biosolids resulted from point sources. Concentrations of cVMS in water, sediment, and soil were all below their no-observed-effect-concentrations.

Highlights

► A critical assessment of the current state of knowledge of toxicity, fate, and environmental levels of cVMS is presented. ► An overview of the use, physico-chemical properties, methods of detection, of the three most widely used cVMS is reported. ► Knowledge gaps and recommendations for future research on cVMS have been identified.

Introduction

Organosilicon compounds have a backbone of alternating silicon (Si) and oxygen (O) atoms, with each silicon atom bearing one or several hydrocarbon groups, such as methyl, ethyl or phenyl. Three organosilicon classes, volatile methylsiloxane, polydimethylsiloxane, and polyethermethylsiloxane, have noteworthy environmental loadings. Cyclic volatile methylsiloxanes (cVMS) are a subgroup of volatile methylsiloxanes that favor partitioning into the atmosphere due to their high vapor pressures, low water solubilities, and high Henry’s Law constants. cVMS consist of repeating units of [Me2SiO]n, and the Si–O atoms are singly bonded forming a ring. Three widely used cVMS are octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5), and dodecamethylcyclohexasiloxane (D6) (Fig. 1).

Starting in the 1940s, cVMS were produced commercially for use as specialty materials for consumer and industrial products (Hunter et al., 1946, Patnode and Wilcock, 1946, Chandra, 1997). The main uses of cVMS are as an intermediate for the production of other chemicals (silicone polymers), in personal care products (e.g. cosmetic, skin- and hair-care products), in household products, in industrial/institutional cleaning, and construction.

As a result of their widespread use, dispersion and accumulation in aquatic sediments, the US. EPA signed a testing consent order with six industrial manufacturers of D4 in the 1980s (Kent et al., 1996). A comprehensive aquatic toxicity and environmental fate testing program of D4 was sponsored by the Silicones Environmental Health and Safety Council of North America (SEHSC) supported by the silicone industry in the 1990s (Kent et al., 1994, Fackler et al., 1995, Hobson, 1995, Hobson and Silberhorn, 1995, Mueller et al., 1995, Potts and Guy, 1995, Sousa et al., 1995, Hamelink et al., 1996, Varaprath et al., 1996). Based on these results, in 1994 the US EPA concluded that D4 represents a low risk to aquatic species (Walker and Smock, 1995).

Recently, several regulatory jurisdictions have prioritized a number of cVMS, including D4, D5, and D6 based on concerns regarding their persistence and bioaccumulation potential in the environment. Consequently, there have been extensive reviews assessing the environmental risks of cVMS as potential priority pollutants in Europe (Brooke et al., 2009a, Brooke et al., 2009b, Brooke et al., 2009c) and Canada (Environment Canada and Health Canada, 2008a, Environment Canada and Health Canada, 2008b, Environment Canada and Health Canada, 2008c).

Several countries that have been systematically screening the environment for potentially hazardous substances have identified cVMS (Lassen et al., 2005, Kaj et al., 2005a, Kaj et al., 2005b). The Nordic screening project has collected samples of biota, sediment, biosolids, soil, and water in Denmark, the Faroe Islands, Finland, Iceland, Norway, and Sweden since 2004 (Kaj et al., 2005a, Kaj et al., 2005b). This project reported the widespread distribution of cVMS in the Nordic environment, except for in soils, although, the observed concentrations were not of immediate concern.

This review summarized the use and consumption, physico-chemical properties, toxicity, fate, methods of detection, environmental concentrations, and global distributions of the three most widely used cVMS, D4, D5, and D6. The goal of this review was to provide a comprehensive assessment of the current state of knowledge concerning the environmental risk of cVMS. Knowledge gaps and recommendations for future research were also identified.

Section snippets

Use and consumption

cVMS belong to a group of substances used in a number of industrial applications, cosmetics and personal care products (Graiver et al., 2003, Dewil et al., 2006). D4 has been used primarily as an off-site intermediate for the production of silicone polymers. However, the majority of cVMS used currently in personal care products have been identified as D5 and D6, including fragrances, hair care products, deodorants, antiperspirants, nail polishes, lotions, and skin cleansers. (Horii and Kannan,

Water solubility and vapor pressure

cVMS possess a rather unusual combination of physico-chemical properties, including both hydrophobicity and volatility. The water solubilities of cVMS in distilled water are very low (56, 17, and 5 μg L−1 for D4, D5, and D6) (Varaprath et al., 1996), and the vapor pressures of cVMS are relatively high (122, 25, and 2.2 Pa for D4, D5, and D6) (Flaningam, 1986, Lei et al., 2010).

Octanol/water partitioning coefficient

The octanol/water partition coefficient (KOW) is an important physico-chemical property, used for predicting the

Aquatic toxicity

Experimental data indicate that exposure to D4 can cause harm to some aquatic organisms at very low concentrations, yet other species were not affected. Kent et al. (1994) studied the toxicity of D4 to midge using 14-d aqueous exposures at five concentrations ranging from 0.49 to 15 μg L−1. No adverse effects were observed among exposed midges at any concentration, and the 14-d no-observed-effect-concentration (NOEC) was >15 μg L−1. Sousa et al. (1995) investigated the acute and chronic toxicity of

Atmospheric degradation

cVMS compounds are sparingly water soluble and have high vapor pressures (see Table 1). Therefore, they readily partition to the atmosphere in a multi-media environment (Powell and Kozerski, 2007). In the gas-phase, cVMS compounds reacted with OH and NO3 radicals and O3, but reaction with OH radicals was by far the most dominant tropospheric removal process, with NO3 radical and O3 reactions being negligible. The calculated atmospheric half-lives of these cVMS were ∼10 d for D4 and ∼20 d for D5,

Air

Biogas such as landfill gas and sewage gas has been demonstrated to be an important source of cVMS in the atmosphere. Schweigkofler and Niessner (1999) determined that D4 and D5 concentrations ranged from 4240–8840 μg m−3 to 400–1090 μg m−3 in landfill gas, and 2870–6980 μg m−3 to 2750–9650 μg m−3 in sewage gas (Schweigkofler and Niessner, 1999). However, D6 was not detected in either type of gases. Similar concentration ranges for D4 and D5, 4800–5100 μg m−3 and 600–650 μg m−3 in landfill gas, and

Background contamination

Avoiding background contamination is one of the greatest challenges of cVMS analysis at trace levels in the environment (Chainet et al., 2011). During method development, particular attention must been given to avoid or minimize sources of cVMS. It is critical to avoid using products containing cVMS during sample preparation and analysis (Carpenter and Gerhards, 1997). Solvent, silicone-based GC injection septa, the o-ring, glass-liner containing silanized glass–wool in the injection port, and

Conclusions and future recommendations

cVMS have been detected in air, biogas, biosolids, and wastewater influent and effluent from most WWTPs in many countries, although, concentrations varied greatly. The highest concentrations of cVMS were measured in biogas (100–10 000 μg m−3), followed by point source air (0.1–30 μg m−3), and outdoor air (<1 μg m−3). cVMS concentrations in effluent from WWTPs and receiving waters were <2 μg L−1. There was a large concentration range in biosolids (0.1–100 μg g−1 dw) compared to sediment and soil, which were

Acknowledgements

This study was funded by the Chemicals Management Plan (CMP); managed by Environment Canada and Health Canada. Dr. De-Gao Wang acknowledges support from the Natural Sciences and Engineering Research Council of Canada, Visiting Fellowship Program and the National Natural Science Foundation Program of China (Grants 21077015 and 20807008).

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