Volatile Methylsiloxanes in the Environment

The main applications of siloxanes, sources of their release, their production volume, and the reasons for the growing interest in them are presented. The premises for which they are currently considered as potential environmental pollutants are explained and commented on. The physical and chemical properties of selected volatile methylsiloxanes are characterized with regard to both the increasing number of their applications and the impact on ecosystems and humans. On the basis of the available scientific literature and official risk assessments, their toxic potential, durability, circulations and transformations in the environment, long-range transport potential, degradation mechanisms, and fate in some environmental compartments are discussed. It is argued that they can interfere with the female reproductive system, the liver, and the lungs and irritate the skin, the eyes, and the respiratory system, considering that their bioaccumulation potential is relatively high. Due to their relative durability, they can be accumulated in various environmental matrices and transported to regions distant from the release sites. It is shown that the main mechanisms of their degradation are hydrolysis and demethylation with OH radicals in, respectively, the water and soil environment and in the air (referred to as the final sink in which the fundamental process of their degradation and decay take place). In conclusion, the need for further research on their impact on human health, their bioaccumulation and biodegradation, their life time and long-term impact on ecosystems, and the development of methods of removing VMSs from wastewater, sludge, and biogas is highlighted.

The main uses and environmental emissions of cyclic methylsiloxanes (CMSs) and linear methylsiloxanes (LMSs), especially the three volatile CMSs (D4, D5, and D6), were reviewed. This chapter provides information on production, use, concentrations in various products, as well as emission of volatile methylsiloxanes (VMSs) into the environment. Many silicone-based products contain residues of CMSs as impurities, and hence the occurrence of VMSs in silicone-based materials (such as rubber products) has been described. CMSs are mainly used as intermediates in the production of silicone polymers, silicone fluids, elastomers, and resins, all of which have diverse industrial and consumer applications. CMSs are also used directly in personal care products (PCPs), as carriers. The concentrations and profiles of CMSs and LMSs in PCPs and household products from North America, Europe, and Asia varied widely across and within the product categories. The measured concentrations ranged from 0.01% in body wash to 70% (by weight) in deodorants. D5 was the predominant CMS found with high detection frequency in most PCPs. The correlations among VMSs in consumer products suggested incorporation of different blends of silicones to the raw material or as additives in those products. The industrial production of VMSs and direct use of PCPs result in significant emissions of VMSs into the environment. High production volume, high mobility, and environmental persistence of VMSs are causes of concern. VMSs, especially CMSs, were found globally in various environmental matrices including air, water, and sludge. This chapter also provides information on the concentrations and patterns of VMSs in the environment surrounding silicone factories, paper production facilities, and oil fields.

Silicon materials are widespread in our daily life and in numerous industrial applications, and this started raising concerns in the scientific community a couple of decades ago regarding the potential negative effects these chemicals could have in the environment and human health. Naturally, analytical methodologies were required to assess their presence around us. In particular, volatile methylsiloxanes (VMSs) have been the focus of research in this field, and their presence has been determined in many environmental matrices. However, this extended presence tends to provoke problems of external contamination during sampling and analysis, as, for instance, personal care products or chromatograph parts have VMSs in their formulations. Also, the volatility of these compounds advises against a large number of sample handling steps. This chapter reviews the analytical choices for the analysis of VMSs in water, air, sediments, soil and sewage sludge reported so far in literature, giving an overview of the sampling and sample processing precautions and the strategies employed for the extraction/clean-up (or lack thereof) before the typical analysis by gas chromatography coupled with mass spectrometry detection (GC-MS), which in some cases presented different injection options.

Concentration, distribution, fate, removal efficiencies, daily and seasonal variations of volatile methylsiloxanes (VMS) in wastewater treatment plants (WWTPs) were reviewed in this chapter. Purge-and-trap, headspace, liquid-liquid extraction, liquid-solid extraction, membrane-assisted solvent extraction, and modified QuEChERS methods have been developed to analyze of VMS in samples from WWTPs. The different consumption quantities of commercial products containing siloxanes result in the difference of concentrations and proportion of VMS in the world. Daily fluctuations of VMS concentrations in water usage induce in flow variation to WWTP and VMS show seasonal variation in the WWTPs with different types of processes. In cold seasons, VMS prefer to stay in water phase rather than air or sludge because the air/water and organic carbon/water partition coefficients decrease with temperature. Although most WWTP remove siloxanes efficiently, long-term environmental monitoring of VMS is necessary in certain environments, considering the potential of VMS to bioaccumulate in biota and its toxicity to sensitive aquatic organisms.

Biogas produced in wastewater treatment plants (WWTPs) by microorganisms during the anaerobic degradation process of organic compounds is commonly used in energy production. Due to the increasing interest in renewable fuels, biogas has become a notable alternative to conventional fuels in the production of electricity and heat. Biomethane, upgraded from biogas, has also become an interesting alternative for vehicle fuel. Biogas contains mainly methane (from 40 to 60%) and carbon dioxide (40 to 55%), but it also contains trace compounds, such as hydrogen sulphide, halogenated compounds and volatile methyl siloxanes (VMS), which pose a risk on its energetic valorization.

It is reported that the concentrations of siloxanes in biogas are increasing in the recent years due to an increase in the use of silicon-containing compounds in personal care products, silicone oils and production of food, among others. This chapter reviews the presence of VMS in sewage biogas, depicting their concentrations and their speciation between linear and cyclic compounds depending on the wastewater treatment processes and operating conditions.

WWTP operators face therefore a choice between installing a gas purification equipment and controlling the problem with more frequent maintenance. Available technologies for siloxane removal are studied, and their impact on the performance of Energy Conversion Systems (ECS) is reported. The performance of adsorption systems using activated carbon, silica gel and zeolites is reviewed as it is a well-known and widespread used technology for siloxane abatement both at the scientific and industrial studies.

Volatile methylsiloxanes are high-volume synthetic chemicals that are included in a plethora of domestic and industrial formulations. Because of their widespread use, these organosilicon molecules are emitted to the environment and reach the aquatic systems, where they may cause potential adverse effects to some aquatic organisms. The study of the occurrence and fate of volatile methylsiloxanes in the aquatic media has progressed considerably during the last years thanks to the development of new analytical methods, which decrease the limits of detection substantially while minimising and stabilising the contamination levels of the blank assays. The present chapter briefly reviews the most relevant analytical strategies that have been developed for the analysis of volatile methylsiloxanes in the aquatic environment, with a focus in water matrices, sediments and biota. The behaviour and fate of cyclic and linear methylsiloxanes in the aquatic environment are summarised, as well as the levels at which these compounds have been reported.

Methylsiloxanes (MSs) are an important class of additive chemicals that due to their physicochemical properties have been broadly used in several industrial applications and consumer products. The purpose of this chapter is to provide a literature review on the current state of knowledge on the occurrence and distribution of MSs in air samples from different indoor environments, including, for example, residential houses, offices, public buildings, cars, industries or hair salons. Literature studies on the levels of cyclic and linear siloxanes in indoor dust, which is a major source of MS due their particle-binding affinity, are discussed. A wide range of MS concentrations in air and dust samples has been reported together with an evident different level in indoor air samples from building of different classification. Among cyclic methylsiloxanes, D5 was usually the dominant congener in the investigated samples. In general, the levels from industrial facilities were one or more orders of magnitude higher than those in residential buildings. The mean inhalation exposure doses to total siloxanes for infants, toddlers, children, teenagers and adults are also presented. Recent investigations on human exposure to MSs through dust ingestion were also included.

Field data showing volatile methyl siloxane (VMS) concentrations in the atmosphere is still limited. Outdoor air concentrations are highly conditioned by population, being VMS values much higher in urban locations than in remote regions, generally in the range of ng m⁻³, one to three orders of magnitude lower than other volatile organic compounds commonly found in the atmosphere. Cyclic VMS (cVMS) are the most abundant compounds, with concentrations up to 2–3 orders of magnitude higher than those of the linear VMS (lVMS). This abundance is related to the large production and use of cVMS globally. In urban areas, lVMS are generally in the range of 1–20 ng m⁻³. On the other hand, cVMS present much higher concentrations, ranging from a few hundred to several thousand ng m⁻³. A limited number of studies evaluating VMS in outdoor air include background and rural locations. Background regions generally present VMS levels one order of magnitude lower than those usually found in urban areas, with lVMS concentrations between 0.01 and 1 ng m⁻³. In contrast, cVMS concentrations range from 1 to 100 of ng m⁻³. In the Arctic, lVMS are seldom observed, but cVMS are usually found in the range of 0.1 and 4 ng m⁻³. The regulation and establishment of air quality criterions for VMS are still very limited worldwide. Evaluations regarding human and environmental exposure to these compounds would be mandatory in the future, as well as the establishment of air quality standards.

Volatile methyl siloxanes (VMS) are emitted primarily to air, and the bulk of VMS present in the environment resides in the atmosphere. Therefore, the atmospheric fate of VMS is a core component of the environmental chemistry of these chemicals. In this chapter the phase partitioning of VMS in the atmosphere is first examined, and then the different mechanisms by which they can be removed from the atmosphere are evaluated, both physical removal via deposition and chemical removal via reactions. We find that VMS are almost entirely present in gaseous form and that reaction with OH radicals is the dominant process for their removal. Consequently, for most purposes, the atmospheric fate of VMS can be simplified to three processes: the emission function, advection, and removal via reaction with OH radicals. However, each of these processes is complex, so we explore how mathematical models have been used to capture this complexity, quantify the expected atmospheric fate, and describe the variability of VMS concentrations in time and space.

Volatile methylsiloxanes (VMS) are synthetic chemicals that have been extensively used in the manufacture of many industrial and consumer products and in the formulation of personal and health-care products. Due to their extensive use, VMS have been found in a diversity of abiotic media (air, soil, water, sediments) and in a wide range of aquatic and terrestrial organisms. The ubiquitous presence of VMS has raised concerns regarding whether these chemicals are prone to accumulate in aquatic and terrestrial life to levels higher than those found in the environment and ultimately to affect human and ecosystem health. The purpose of this chapter is to provide an overview of the studies that have been developed to understand if VMS have the potential to bioconcentrate, bioaccumulate, and biomagnify. Key factors affecting bioaccumulation of VMS by different organisms will be described, including physicochemical properties, environmental conditions, characteristics of the exposed organism, and the respective food chains. A review of the studies reporting VMS in different biota samples will be provided.

This chapter reviews volatile methyl siloxanes (VMS) in polar regions (i.e., at latitudes above the polar circles), including their sources, measured concentrations, and the effect of polar environmental conditions on behavior of VMS. Knowledge about VMS in polar regions has been centered on cyclic VMS (cVMS) due to their widespread use and presence in the environment. Due to their high volatility, cVMS are mainly emitted to and remain in the atmosphere, where they eventually degrade. cVMS are present in Arctic air due to both long-range atmospheric transport and local sources within the Arctic. There is no evidence that cVMS deposit to surface media to a significant extent, not even under polar environmental conditions. However, cVMS are emitted via wastewater, and many Arctic communities have limited wastewater treatment where low removal efficiency of cVMS from wastewater can result in high emissions. cVMS concentrations in sediments and aquatic biota close to wastewater outlets in the Norwegian Arctic are comparable to those at temperate latitudes. Sporadic detections of cVMS in biota and surface media in remote Arctic and Antarctic regions need further investigation to be confirmed. Very few measurements are reported for linear VMS (lVMS) in polar regions, with a majority of studies reporting findings below detection limits. The understanding of how Arctic conditions, including low temperatures and strong seasonality, affect the environmental behavior and bioaccumulation of VMS has been expanded through modelling studies. However, important knowledge gaps remain regarding temperature dependence of partitioning behavior in aquatic environments, biotransformation rates in polar biota, and the influence of physiological and behavioral adaptations of polar biota on bioaccumulation of VMS.

This final chapter intends to convey to the readers the main lessons learned so far, the most important challenges found and an overview of the possible research trends yet to explore in the study of volatile methylsiloxanes (VMSs) in the environment. From all the excellent contributions to this book, besides a comprehensive and thorough display of properties, levels, trends and advances of the current state of the art in several environmental matrices, a number of gaps were detected and discussed and new solutions suggested. Therefore, it is expected that in the near future, the development of new management strategies; new findings regarding the occurrence, behaviour and toxicity of volatile methylsiloxanes; the improvement of risk assessment studies; and the inclusion of chronic exposure assays of mixtures of these substances are the pathways to enhance the expertise of scientists and stakeholders on the science of siloxanes.