Review of recent findings on occurrence and fates of siloxanes in environmental compartments

Advances in the Separation and Detection of Secondary Organic Aerosol Produced by Decamethylcyclopentasiloxane (D5) in Laboratory-Generated and Ambient Aerosol

Decamethylcyclopentasiloxane (D5), a common ingredient in many personal care products (PCPs), undergoes oxidation in the atmosphere, leading to the formation of secondary organic aerosol (SOA). Yet, the specific contributions of D5-derived SOA on ambient fine particulate matter (PM2.5) have not been characterized. This study addresses this knowledge gap by introducing a new analytical method to advance the molecular characterization of oxidized D5 and its detection in ambient aerosol. The newly developed reversed phase liquid chromatography method, in conjunction with high-resolution mass spectrometry, separates and detects D5 oxidation products, enabling new insights into their molecular and isomeric composition. Application of this method to laboratory-generated SOA and urban PM2.5 in New York City expands the number of D5 oxidation products observed in ambient aerosol and informs a list of molecular candidates to track D5-derived SOA in the atmosphere. An oxidation series was observed in which one or more methyl groups in D5 (C10H30O5Si5) is replaced by a hydroxyl group, which indicates the presence of multistep oxidation products in ambient PM2.5. Because of their specificity to PCPs and demonstrated detectability in ambient PM2.5, several oxidation products are proposed as molecular tracers for D5-derived SOA and may prove useful in assessing the impact of PCPs-derived SOA in the atmosphere.

Emerging contaminants and their potential impacts on estuarine ecosystems: Are we aware of it?

Emerging contaminants (ECs) are becoming more prevalent in estuaries and constitute a danger to both human health and ecosystems. These pollutants can infiltrate the ecosystem and spread throughout the food chain. Because of the diversified sources and extensive human activities, estuaries are particularly susceptible to increased pollution levels. A thorough review on recent ECs (platinum group elements, pharmaceuticals and personal care products, pesticides, siloxanes, liquid crystal monomers, cationic surfactant, antibiotic resistance genes, and microplastics) in estuaries, including their incidence, detection levels, and toxic effects, was performed. The inclusion of studies from different regions highlights the global nature of this issue, with each location having its unique set of contaminants. The diverse range of contaminants detected in estuary samples worldwide underscores the intricacy of ECs. A significant drawback is the scarcity of research on the toxic mechanisms of ECs on estuarine organisms, the prospect of unidentified ECs, warrant research scopes.

Cyclic and linear siloxane contamination in sediment and invertebrates around a thermal power plant in Korea: Source impact, distribution, seasonal variation, and potential for bioaccumulation

Siloxanes have been commonly used as additives in a variety of industrial and consumer products. Media and government investigations have revealed that defoamers containing siloxanes are used in the effluent of thermal power plants in Korea. However, investigations of the source impact of siloxane contamination from the discharge of thermal power plants into coastal environments are scarce. In this study, sediment and invertebrates were collected around a thermal power plant to assess source impact, seasonal variation, and a potential for bioaccumulation. Although siloxanes were detectable in sediment and invertebrates, the spatial distribution and composition (which differed between the siloxanes found in sediment and invertebrates and those in defoamer used in the plant) suggest they were likely transported by long-distance migration as well as the discharge of thermal power plant. Seasonal differences might affect sedimentary contamination and the bioaccumulation potential of siloxanes. Specifically, octamethylcyclotetrasiloxane (D4) may have limited adsorption capacity and potential for long-distance migration, as its contribution in sediment far from the coastline was greater than that of decamethylcyclopentasiloxane (D5) and dodecamethylcyclohexasiloxane (D6). However, higher D5 accumulation in invertebrates, and D5 has a potential bioaccumulation. A molecular docking analysis showed that the binding affinity between D5 and the cytochrome enzyme in invertebrates was weaker than that with other siloxanes, which could lead to higher D5 accumulation in invertebrates.

Are volatile methylsiloxanes in downcycled tire microplastics? Levels and human exposure estimation in synthetic turf football fields

Downcycled rubber, derived from end-of-life tires (ELTs), is frequently applied as crumb rubber (CR) as infill of synthetic turf in sports facilities. This practice has been questioned in recent years as numerous studies have reported the presence of potentially hazardous chemicals in this material. CR particles fall into the category of microplastics (MPs), making them possible vectors for emerging micropollutants. A preliminary study where volatile methylsiloxanes (VMSs) were found in CR originated the hypothesis that VMSs are present in this material worldwide. Consequently, the present work evaluates for the first time the levels and trends of seven VMSs in CR from synthetic turf football fields, while attempting to identify the main sources and impacts of these chemicals. A total of 135 CR samples and 12 other of alternative materials were analyzed, employing an ultrasound-assisted dispersive solid-phase extraction followed by gas chromatography-mass spectrometry (GC-MS), and the presence of VMSs was confirmed in all samples, in total concentrations ranging from 1.60 to 5089 ng.g-1. The levels were higher in commercial CR (before field application), a reflection of the use of VMS-containing additives in tire production and/or the degradation of silicone polymers employed in vehicles. The VMSs generally decreased over time on the turf, as expected given their volatile nature and the wearing of the material. Finally, the human exposure doses to VMSs in CR (by dermal absorption and ingestion) for people in contact with synthetic turf in football fields were negligible (maximum total exposure of 20.5 ng.kgBW-1.year-1) in comparison with the European Chemicals Agency (ECHA) reference doses: 1.35 × 109 ng.kgBW-1.year-1 for D4 and 1.83 × 109 ng.kgBW-1.year-1 for D5. Nevertheless, more knowledge on exposure through inhalation and the combined effects of all substances is necessary to provide further corroboration. This work proved the presence of VMSs in CR from ELTs, another family of chemical of concern to take into account when studying MPs as vectors of other contaminants.

Directed evolution of enzymatic silicon-carbon bond cleavage in siloxanes

Volatile methylsiloxanes (VMS) are man-made, nonbiodegradable chemicals produced at a megaton-per-year scale, which leads to concern over their potential for environmental persistence, long-range transport, and bioaccumulation. We used directed evolution to engineer a variant of bacterial cytochrome P450BM3 to break silicon-carbon bonds in linear and cyclic VMS. To accomplish silicon-carbon bond cleavage, the enzyme catalyzes two tandem oxidations of a siloxane methyl group, which is followed by putative [1,2]-Brook rearrangement and hydrolysis. Discovery of this so-called siloxane oxidase opens possibilities for the eventual biodegradation of VMS.

Factors determining contamination and time trends in cyclic and linear siloxanes in sediments from an industrialized lake in Korea

Siloxanes, widely used in various consumer and industrial products, are emerging concerns of contaminants. Despite this, limited studies have been conducted on contamination and time trends on siloxanes in coastal environments. In the present study, four cyclic and 15 linear siloxanes were measured in sediments collected from an artificial saltwater lake in Korea during 2001-2016 to investigate contamination, time trends, and ecotoxicological concerns. Cyclic siloxanes were detected in all sediment samples, whereas linear siloxanes were not frequently detected. The highest siloxane concentrations were observed in creeks passing through various industrial complexes, indicating that industrial activities predominantly contributed to siloxane contamination in coastal environments. Decamethylcyclopentasiloxane (D5) and dodecylcyclohexasiloxane (D6) were predominant siloxanes in sediments over the last two decades. Siloxane concentrations significantly increased in creek sediments from 2008 to 2016, whereas those in inshore and offshore regions significantly decreased due to a strong dilution effect by the operation of tidal power plant. This suggests that consumption patterns and coastal development activities are crucial factors determining the contamination and time trends in the sedimentary siloxanes. The sedimentary concentrations of octamethylcyclotetrasiloxane (D4) and D5 exceeded several thresholds, raising the potentials for ecological risks to aquatic organisms.

Physiological impact of personal care product constituents on non-target aquatic organisms

Personal care products (PCPs) are products used in cleaning, beautification, grooming, and personal hygiene. The rise in diversity, usage, and availability of PCPs has resulted in their higher accumulation in the environment. Thus, these constitute an emerging category of environmental contaminants due to the potential of its constituents (chemical and non-chemical) to induce various physiological effects even at lower concentrations (ng/L). For analyzing the impact of the PCPs constituents on the non-target organism about 300 article including research articles, review articles and guidelines were studied from 2000 to 2023. This review aims to firstly discuss the fate and accumulation of PCPs in the aquatic environment and organisms; secondly provides overview of environmental risks that are linked to PCPs; thirdly review the trends, current status of regulations and risks associated with PCPs and finally discuss the knowledge gaps and future perspectives for future research. The article discusses important constituents of PCPs such as antimicrobials, cleansing agents and disinfectants, fragrances, insect repellent, moisturizers, plasticizers, preservatives, surfactants, UV filters, and UV stabilizers. Each of them has been found to display certain toxic impact on the aquatic organisms especially the plasticizers and UV filters. These continuously and persistently release biologically active and inactive components which interferes with the physiological system of the non-target organism such as fish, corals, shrimps, bivalves, algae, etc. With a rise in the number of toxicity reports, concerns are being raised over the potential impacts of these contaminant on aquatic organism and humans. The rate of adoption of nanotechnology in PCPs is greater than the evaluation of the safety risk associated with the nano-additives. Hence, this review article presents the current state of knowledge on PCPs in aquatic ecosystems.

Occurrence, time trends, and human exposure of siloxanes and synthetic musk compounds in indoor dust from Korean homes

Siloxanes and synthetic musk compounds (SMCs) have been widely used as additives in household and personal care products. Humans are easily exposed to siloxanes and SMCs originating from these products through ingestion and dermal absorption of indoor dust. In the present study, indoor dust samples were analyzed for 19 siloxanes (cyclic and linear) and 12 SMCs (polycyclic, macrocyclic, and nitro musks) to assess their occurrence, time trends over time, source, and health risks. A total of 18 siloxanes and 10 SMCs were detected in all indoor dust samples obtained from 2011⎯2021, indicating widespread and long-term contamination. Higher detection frequencies and concentrations were associated with siloxanes and SMCs with higher use and strong resistance against degradation processes. Indoor dust samples were dominated by linear siloxanes (L11-L13), 1,3,4,6,7,8-hexahydro-4,6,6,7,8,8-hexamethylcyclopenta-(g)-2-benzopyran (HHCB), musk ketone (MK), and 7-acetyl-1,1,3,4,4,6-hexamethyl-1,2,3,4-tetrahydronaphthalene (AHTN). The frequent use of household and personal care products is likely an important source of siloxane and SMC contamination in indoor environments. The concentrations of siloxanes and SMCs in indoor dust increased from 2011 to 2021, particularly, those of linear siloxanes, reflecting the impact of regulatory actions addressing cyclic siloxanes. The profiles of siloxanes remained stable throughout the study period, whereas those of SMCs shifted from nitro to polycyclic musks. The estimated daily intakes (EDIs) of siloxanes and SMCs arising from ingestion were greater than from dermal absorption of indoor dust. The EDIs of siloxanes and SMCs associated with indoor dust indicated that children are exposed to these pollutants.

Directed evolution for Si-C bond cleavage of volatile siloxanes in glass bioreactors

Volatile methylsiloxanes (VMS) are a class of non-biodegradable anthropogenic compounds with propensity for long-range transport and potential for bioaccumulation in the environment. As a proof-of-principle for biological degradation of these compounds, we engineered P450 enzymes to oxidatively cleave Si-C bonds in linear and cyclic VMS. Enzymatic reactions with VMS are challenging to screen with conventional tools, however, due to their volatility, poor aqueous solubility, and tendency to extract polypropylene from standard 96-well deep-well plates. To address these challenges, we developed a new biocatalytic reactor consisting of individual 2-mL glass shells assembled in conventional 96-well plate format. In this chapter, we provide a detailed account of the assembly and use of the 96-well glass shell reactors for screening biocatalytic reactions. Additionally, we discuss the application of GC/MS analysis techniques for VMS oxidase reactions and modified procedures for validating improved variants. This protocol can be adopted broadly for biocatalytic reactions with substrates that are volatile or not suitable for polypropylene plates.

Are Si-C bonds formed in the environment and/or in technical microbiological systems?

Organosiloxanes are industrially produced worldwide in millions of tons per annum and are widely used by industry, professionals, and consumers. Some of these compounds are PBT (persistent, biaccumulative and toxic) or vPvB (very persistent and very bioaccumulative). If organosiloxanes react at all in the environment, Si-O bonds are hydrolyzed or Si-C bonds are oxidatively cleaved, to result finally in silica and carbon dioxide. In strong contrast and very unexpectedly, recently formation of new Si-CH3 bonds from siloxanes and methane by the action of microorganisms under mild ambient conditions was proposed (in landfills or digesters) and even reported (in a biotrickling filter, 30 °C). This is very surprising in view of the harsh conditions required in industrial Si-CH3 synthesis. Here, we scrutinized the pertinent papers, with the result that evidence put forward for Si-C bond formation from siloxanes and methane in technical microbiological systems is invalid, suggesting such reactions will not occur in the environment where they are even less favored by conditions. The claim of such reactions followed from erroneous calculations and misinterpretation of experimental results. We propose an alternative explanation of the experimental observations, i.e., the putative observation of such reactions was presumably due to confusion of two compounds, hexamethyldisiloxane and dimethylsilanediol, that elute at similar retention times from standard GC columns.

A review on the pollution assessment of hazardous materials and the resultant biorefinery products in Palm oil mill effluent

The voluminous nature of palm oil mill effluent (POME) is directly associated with environmental hazards and could be turned into biorefinery products. The POME, rich in BOD, COD, and oil and grease, with few hazardous materials such as siloxanes, fatty acid methyl ester, and phenolic compounds that may significantly increase the risk of violating the effluent quality standards. Recently, the application of chemical and biological risk assessment that can use electrochemical sensors and microalgae-like species has gained paramount attention towards its remediation. This review describes the existing risk assessment for POME and recommends a novel assessment approach using fish species including invasive ones as suitable for identifying the toxicants. Various physico-chemical and biological treatments such as adsorption, coagulation-flocculation, photo-oxidation, solar-assisted extraction, anaerobic digestion, integrated anaerobic-aerobic, and microalgae cultivation has been investigated. This paper offers an overview of anaerobic technologies, with particular emphasis on advanced bioreactors and their prospects for industrial-level applications. To illustrate, palmitic acid and oleic acid, the precursors of fatty acid methyl ester found in POME pave the way to produce biodiesel with 91.45%. Although there are some challenges in attaining production at an economic scale, this review offers some opportunities that could help in overcoming these challenges.

Occurrence and fate of chlorinated methylsiloxanes in surrounding aqueous systems of Shengli oilfield, China

Mono-chlorinated products of cyclic volatile methylsiloxanes (cVMS), i.e., Monochlormet-hylheptamethylcyclotetrasiloxane [D3D(CH₂Cl)], monochlormethylnonamethylcyclopenta-siloxane [D4D(CH₂Cl)], and monochlormethylundemethylcyclohexasiloxane [D5D(CH₂Cl)], were detected in water [<LOQ (Limit of quantitation) -86.3 ng/L, df (detection frequency) = 23%-38%, n=112] and sediment samples [<LOQ-504 ng/g dw (dry weight), df = 33%-38%, n=112] from 16 lakes located in Shengli oilfield of China, and had apparent increasing trends (31%-34% per annum) in sediments during Year 2014-2020. Simulated experiments showed that chloro-cVMS in sediment-water system had approximately 1.7-2.0 times slower elimination rates than parent cVMS. More specifically, compared with those of parent cVMS, volatilization (86-2558 days) and hydrolysis (135-549 days) half-lives of chloro-cVMS were respectively 1.3-2.0 and 1.8-2.1 times longer. In two species of freshwater mussels (n=1050) collected from six lakes, concentrations of chloro-cVMS ranged from 9.8-998 ng/g dw in Anodonta woodiana and 8.4-970 ng/g dw in Corbicula fluminea. Compared with parent cVMS, chloro-cVMS had 1.1-1.5 times larger biota-sediment accumulation factors (2.1-3.0) and 1.1-1.7 times longer half-lives (13-42 days). Their stronger persistence in sediment and bioaccumulation in freshwater mussels suggested that environmental emission, distribution and risks of chloro-cVMS deserve further attention.

Developmental effects of siloxane exposure in zebrafish: a comparison study using laboratory-mixed and environmental water samples

Siloxanes are used in personal care, biomedical, and industrial products. Their worldwide use and persistence in the environment cause consistent exposure for both humans and aquatic animals. Two siloxane congeners, decamethylcyclopentasiloxane (D5; CAS 541-02-6) and octamethylcyclotetrasiloxane (D4; CAS 556-67-2), are among the most prevalent, with measurable levels in air, sediment, water, and biological samples. However, few studies have examined the impact of developmental (embryo/larva) exposure. To address this gap, we performed parallel experiments using wildtype zebrafish (Danio rerio). One set of experiments used laboratory-mixed individual solutions containing either D4, D5, or 2,4,6,8-tetramethylcyclotetrasiloxane (D₄ ^H ; CAS 2370-88-9); the other used environmental water samples containing a mixture of siloxanes, including D4 and D5. These samples were collected from Bladensburg Waterfront Park (BWP) a site along the Anacostia River, Washington, DC. In both experiments, zebrafish (24-48 hours postfertilization, hpf) were exposed until 7 or 14 days (d)pf. Chronic exposure to D4, D5, or BWP water until 7 dpf caused stress-like behaviors and reduced swim velocities; anatomical differences were noted only in BWP-exposed larvae. At 14 dpf, BWP-treated larvae still showed slower swimming velocities and increased immobility; anatomical differences were no longer evident and thigmotactic behavior was reduced. D4 and D5-exposed larvae did not survive after 10 dpf. Larvae exposed to D₄ ^H showed no decreases in behavior or growth at either age. These results suggest early developmental sensitivity to siloxane exposure and point to the need to consider embryonic/larval endpoints when assessing aquatic contaminants.

Reply to Thomas, K. Comment on “Hossain et al. Exposure to Dodecamethylcyclohexasiloxane (D6) Affects the Antioxidant Response and Gene Expression of Procambarus clarkii. Sustainability 2021, 13, 3495”

We have reviewed and studied Karluss T.’s comment [...]

Considerations on the Analysis of E-900 Food Additive: An NMR Perspective

Food additives are in widespread use in the food industry to extend the shelf life of food, improve its organoleptic characteristics or facilitate industrial processing. Their use is not without controversy, which makes regulation and control crucial for food safety and public health. Among food additives, silicone-based antifoaming agents (polysiloxanes or E900) are difficult to analyze and quantify due to their polymeric nature. Currently, there is no official method of quantifying this additive in foods. In this context, nuclear magnetic resonance (NMR) is a quantitative method for speciation analysis of silicon compounds almost without known interferents. In this work, we describe the evolution of the regulation of the E900 additive, discuss different analytic methods quantifying polydimethylsiloxanes (PDMS), and propose a new method based on NMR suitable for analyzing the content of E900 in the form of PDMS in various types of food from dietary oils to marmalades and jellies, among others. The proposed method consists of a previous quantitative concentration of PDMS by liquid–liquid extraction and the monitoring of the quantification using a bis(trimethylsilyl)benzene (BTMSB) standard to control the variability, ranging within 2–7%, depending on the food. This simple, direct, and reproducible procedure for aqueous and lipidic foods may help to monitor and fill a gap in regulatory legislation regarding the E900 additive.

A Mini-Review on Recent Developments in Anti-Icing Methods

An aggressive impact of the formed ice on the surface of man-made objects can ultimately lead to serious consequences in their work. When icing occurs, the quality and characteristics of equipment, instruments, and building structures deteriorate, which affects the durability of their use. Delays in the adoption of measures against icing endanger the safety of air travel and road traffic. Various methods have been developed to combat de-icing, such as mechanical de-icing, the use of salts, the application of a hydrophobic coating to the surfaces, ultrasonic treatment and electric heating. In this review, we summarize the recent advances in the field of anti-icing and analyze the role of various additives and their operating mechanisms.