Sources and Fate of Cyclic Phenylmethylsiloxanes in One Municipal Wastewater Treatment Plant and Biosolids-Amended Soil

Assessing the impact of mining on cyclic and linear methylsiloxane distribution in Tibetan soils: Source contribution and transport pattern

The pervasive presence of methylsiloxanes (MSs), comprising linear and cyclic congeners, in the environment poses significant ecological risks, yet the understanding of their transport mechanisms and deposition patterns remains limited. This study analyzed the concentrations of 12 linear-MSs (L3-L14) and 7 cyclic-MSs (D3-D9) in 29 surface soil samples collected across varying altitudes (3726 to 4863 m) near the Jiama mining sector in Tibet, aiming to investigate the distribution and transport dynamics of MSs from the emission source. The distribution of total MS concentration (ranging from 50.1 to 593 ng/g) showed a remarkable correlation with proximity to the mining site, suggesting the emergent source of mining activities for the MSs in the remote environment of the Tibetan Plateau. Employing the innovative model of robust absolute principal component scores-robust geographically weighted regression (RAPCS-RGWR), the analysis predicted that the mining operations contributing 57.1 % of the total soil MSs, would significantly surpass contributions from traffic emissions (14.7 %), residential activities (13.2 %), and the environmental factor of total organic matter content (14.9 %). The Boltzmann equation effectively modeled the distribution pattern of soil MSs, highlighting atmospheric transport and gravitational settling as key distribution mechanisms. However, linear-MSs exhibited longer transport distances than cyclic-MSs and were more profoundly affected by prevailing wind directions, suggesting their differential environmental behaviors and risks. Our study underscored that the mining sector possibly emerged as a significant source of Tibetan MSs, and provided insights into the transport and fate of MSs in remote, high-altitude environments. The findings emphasize the need for targeted pollution control strategies to mitigate the environmental footprint of mining activities in Tibet and similar regions.

Phenylmethylsiloxanes in indoor dust from residential area of China: Source, occurrence, bioavailability and exposure assessment

Phenylmethylsiloxanes, as modified products of dimethylsiloxanes, have been used in personal care products (PCPs) and household appliances, with indoor dust serving as one potential reservoir due to their particle-binding properties. This study measured six isomers of two phenylmethylsiloxanes (P3 and P4) in PCPs (99 %) intakes of phenylmethylsiloxanes for adults, while dust ingestion/adsorption (0.19 ng/d) may play important roles for toddlers/infants with little usage of phenylmethylsiloxanes-containing PCPs. Additionally, total daily intakes of PhMeSi(OH)2 (0.30-0.84 ng/d) via ingestion and dermal absorption of dust were higher than P3 (0.06-0.31 ng/d) and P4 (0.02-0.09 ng/d), suggesting exposure risk of degradation product of phenylmethylsiloxanes deserving attention.

Vinylmethylsiloxanes in Municipal Wastewater Treatment Plant and Biosolid-Amended Soils: Their Distribution and Backbone/Vinyl Branch Degradation

This study is the first to investigate the emission and environmental fate of one type of modified methylsiloxane with double-bond (vinyl) groups. During 2018-2020, 2,4,6-trimethyl-2,4,6-trivinylcyclotrisiloxane (V3), 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane (V4), and 2,4,6,8,10-pentavinyl-2,4,6,8,10-pentamethylcyclopentasiloxane (V5) were found in aqueous (<LOD-72.9 ng/L) and solid [13.0-371 ng/g dw (dry weight)] phases of wastewater samples from one Chinese municipal wastewater treatment plant (WWTP) as well as the corresponding biosolid-amended soils [<LOD-36.9 ng/g dw, df (detection frequency) = 37.5-41.7%, n = 48]. Based on the measure of environmental samples, simulated experiment, and product analysis by ESI-FT-ICR-MS, it was found that (1) in addition to sorption to sludge, abiotic degradation of vinylmethylsiloxanes (especially V3, t_1/2 = 0.5-1.9 h at pH = 5.2-9.2) should have an important contribution to their sufficient removal in WWTP; (2) different from siloxane analogues with saturated branches and aromatic branches, abiotic degradation pathways of vinylmethylsiloxane might include both the hydrolysis of Si-O backbone and the oxidation/addition reactions of vinyl branches; (3) although vinylmethylsiloxanes in wastewater could be transferred to soil by biosolids application, these compounds had no accumulation in soil, which should arise from their fast elimination, such as volatilization (t_1/2 = 3.2 h-20.9 days) and degradation (t_1/2 =9.1 h-96.3 days); and (4) degradation of the Si-O backbone and vinyl branches had slowing trends with the increase in the soil organic matter.

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

In view of their vast global usage in both consumer products and industrial processes, environmental emission and fates of siloxanes have become concerned issue. This review summarized the research progress, especially in the last decade, on production/consumption data, toxicities, analysis methods, environmental distribution, migration and degradation/transformation of both dimethylsiloxanes and modified siloxanes in atmospheric, aquatic and terrestrial compartments from various areas (especially in China). In spite of their fast degradation (hydrolysis and hydroxylation, etc) in various matrices (except sediment), dimethylsiloxane oligomers have been found in various environmental matrices from many countries due to their constant usage and emission. Moreover, recent literatures have paid attention to behaviors of dimethylsiloxanes in industrial areas, e.g., their higher residual levels compared with residential areas and unique transformed products (such as halogenated products) arose from special industrial production scenarios. Meanwhile, although most prior studies focused on dimethylsiloxanes, identification of modified-siloxanes with other functional groups in environment have been beginning to attract the attention of scientists. Furthermore, related literatures indicated that compared with dimethylsiloxanes, both halogenated-dimethylsiloxanes and modified methylsiloxanes (phenylsiloxanes and trifluoropropylsiloxanes) could have stronger persistence due to their weaker volatilization and degradation, especially in terrestrial matrices.

Cyclic Phenylmethylsiloxane Oligomers in Municipal Landfills and Their Elimination Mechanisms in Leachate Treatment Processes

This study investigated some sources and elimination mechanisms of phenylmethylsiloxanes in landfill leachates. During a 20-day leaching experiment for electronic wastes collected from one Chinese landfill, significant release (4.9 ng/L to 1.3 μg/L) of cis-/trans-2,4,6-triphenyl-2,4,6-trimethylcyclotrisiloxanes (cis-P3 and trans-P3) and cis-/trans-2,4,6,8-tetraphenyl-2,4,6,8-tetramethylcyclotetrasiloxanes (cis-P4 and trans-P4a,b,c) in simulated leachates was found. From January 2017 to December 2018, P3 and P4 isomers were detected in raw leachates from active cells [<LOQ-990 ng/L, detection frequency (df) = 67-100%] and closed cells (<LOQ-282 ng/L, df = 2.1-98%) of this landfill. Generally, mean mass loads of total phenylmethylsiloxanes in raw leachates were larger in the summer (380 mg/d) and winter (295 mg/d) for active cells, while they decreased from 36.1 mg/d to <LOQ for closed cells during the entire period. During leachate treatment processes, sorption to sludge was responsible for major removal (64-84%) of phenylmethylsiloxanes, while Fenton treatment accounted for 8.5-25% removal. Simulated Fenton experiments indicated that hydroxylation half-lives of P3 (1.3-1.5 h) and P4 (0.65-0.86 h) were 15-91 times faster than their hydrolysis half-lives (22-59 h, pH 3.5). Furthermore, monohydroxylated P4 isomers with a hydroxyphenyl group had larger (3.2-3.9 times) concentrations than those with a hydroxymethyl group, meaning that a phenyl group may be more likely to be hydroxylated than a methyl group.

Identification of organosiloxanes in ambient fine particulate matters using an untargeted strategy via gas chromatography and time-of-flight mass spectrometry

Organosilicons are widely used in consumer products and are ubiquitous in living environments. However, there is little systemic information on this group of pollutants in ambient particles. This study proposes a novel untargeted strategy based mainly on the mass difference of three silicon isotopes to screen organosilicon compounds from 2-year PM_2.5 samples of Beijing using gas chromatography and high-resolution time-of-flight mass spectrometry. 61 organosilicons were filtered from 1019 peaks, and 35 ones were identified as organosiloxanes including 17 methylsiloxanes and 18 phenylmethylsiloxanes, of which 6 and 3 species were confirmed using reference standards, respectively. These organosiloxanes could be clustered into three groups: low-silicon-number methylsiloxanes, high-silicon-number methylsiloxanes, and phenylmethylsiloxanes. Low-silicon-number methylsiloxanes showed high abundance in the heating season but low abundance in the non-heating season, whereas high-silicon-number methylsiloxanes showed the opposite seasonal variation. This study provides a promising strategy for screening organosilicon compounds through an untargeted approach and gives insights for further investigation of the sources and health risks of organosiloxanes.

Distribution and Elimination of Trifluoropropylmethylsiloxane Oligomers in Both Biosolid-Amended Soils and Earthworms

During a primary screening in 2015 and 2016, tris(trifluoropropyl)trimethylcyclotrisiloxane (D3F) and cis-/trans-tetrakis(trifluoropropyl)tetramethylcyclotetrasiloxane isomers (cis-D4F, trans-D4Fa,b,c) were detected in 12 biosolid-amended soils from Laixi and Shijiazhuang Cities of China, with mean concentrations being 10.3 ng/g dry weight (dw) and 2.7 ng/g dw for D3F and D4F, respectively. Subsequently, one further systematical survey found that although repeatedly amended by biosolids containing trifluoropropylmethylsiloxanes (4.2-724 ng/g dw), these compounds had no increasing trend in biosolid-amended soils (n = 100) collected from Laixi City at five sampling events from February 2017 to June 2019. Simulated experiments indicated that hydrolysis half-lives (1.8-28.0 days) of trifluoropropylmethylsiloxanes in soils were 3.0-18.3 times shorter than volatilization half-lives (7.4-362 days). Compared with those of octamethylcyclotetrasiloxane (D4), the hydrolysis rates of D4F isomers were faster in soils with total organic carbon (TOC) ≤80 mg/g but lower in soils with TOC ≥ 150 mg/g. In earthworm bodies, trifluoropropylmethylsiloxanes had 1.03-1.5 times lower biota-soil accumulation factors (1.3-3.2) but 1.4-3.0 times longer half-lives (2.6-5.7 days) than D4. The stronger persistence of fluorinated-siloxane than the corresponding dimethylsiloxane in both soils (at high TOC levels) and earthworms indicated that environmental risks of these compounds deserve further investigation.

Phenylmethylsiloxanes and trifluoropropylmethylsiloxanes in municipal sludges from wastewater treatment plants in China: Their distribution, degradation and risk assessment

Based on wastewater and raw/digested sludge samples from 29 wastewater treatment plants in 25 Chinese cities, the nationwide profiles of cis- and trans- isomers of phenylmethylsiloxanes (P3 and P4) and trifluoropropylmethylsiloxanes (D3F and D4F) were investigated. Calculated with paired influents/sludges positive for these compounds, majority (93% at mean) of them were found accumulating in raw sludges [<LOQ-188 ng/g dw (dry weight), df (detection frequency) = 0-96.6%, n = 58] during wastewater treatment. Overall, trifluoropropylmethylsiloxanes were merely found in 6 cities, while phenylmethylsiloxanes were found in all cities distributed over seven geographic regions of China. Sludge emissions of phenylmethylsiloxanes in each region correlated with their reported consumption volume (R² = 0.81, except for the Southwest China), per capita gross domestic products (R² = 0.20), and annual average temperature (R² = 0.63). Although not found in wastewater biological treatment processes, degradation of phenylmethylsiloxanes and trifluoropropylmethylsiloxanes was apparent (3.0-25.9%) during sludge-digestion processes. In digestion experiments, detection of silanediols and silanetriols indicated possible biodegradation pathway, i.e., hydrolysis of Si-O and Si-C bones, with half-lives ranging from 33.7-57.7 d The calculated hazard quotients of these compounds in soils undergoing one year sludge-fertilization were less than 0.01, but their ecological risks should be further studied in view of their potential accumulation in soils.

Can low-temperature point discharge Be used as atomic emission source for sensitive determination of cyclic volatile methylsiloxanes?

Despite of increased interest in the application of miniature microplasma atomic spectrometry for environmental analytical chemistry, the amenable element detection range is limited to some metal elements and carbon due to it low power consumption. In this work, the generation of silicon atomic emission (251.6 nm and 288.2 nm) from the organosiloxanes was found possible in a low-temperature, low-power, and compact point discharge. Consequence, a tiny point discharge silicon optical emission spectrometer (μPD-OES) was exploited, and used as a novel GC detector for the determination of various cyclic volatile methyl siloxanes (cVMSs). Under the optimized conditions, the developed system provided limits of detection (LODs) of 0.2 mg L^-1, 0.04 mg L^-1, 0.03 mg L^-1 and 0.02 mg L^-1 of Si for hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane, decamethylcyclopentasiloxane and dodecamethylcyclohexasiloxane, respectively. Meanwhile, relative standard deviations (RSDs) of better than 2.3% were obtained. In contrast to gas chromatography mass spectrometer, GC-μPD-OES significantly simplifies the experimental setup with low power consumption and a miniature configuration. As far as we know, this work reports for the first time that silicon atomic emission can be generated in such low temperature microplasma. The accuracy of this system was validated by determining cVMSs in five daily-used shampoo samples collected from retail store, providing satisfactory recoveries (84%-114%) and excellent agreement with values determined by GC-MS at the 95% confidence level.

Development of a solid-phase microextraction method for fast analysis of cyclic volatile methylsiloxanes in water

Cyclic volatile methylsiloxanes (cVMS) are widely used in consumer products and commonly detected in the environment. There are challenges in the analysis of cVMS because of their ubiquitous use which can introduce high background contamination. The current study introduces a sample preparation method based on headspace of solid-phase microextraction (SPME) for monitoring the cVMS in waters. Efforts were made to reduce the background contamination during sample preparation and instrument analysis. A laboratory prepared MIL-101 coating was prepared using polysulfone instead of polydimethylsiloxane as adhesive to avoid the contamination. The extraction performance of the MIL-101 fiber was optimized and evaluated. The optimized extraction time and temperature were 60 min and 40 °C, respectively. The method quantification limits of the MIL-101 fiber for octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5) and dodecylcyclohexasiloxane (D6) in water were 0.15 ng mL^-1, 0.14 ng mL^-1, and 0.27 ng mL^-1, respectively. The extraction efficiency of the proposed MIL-101 fiber was comparable to the commercial polydimethylsiloxane/divinylbenzene fiber. The developed method was applied to analyze the cVMS in wastewater treatment plant and the concentrations in the barscreen and in the aeration tank ranged from 0.73 to 3.3 ng mL^-1 and 7.74-85.1 ng mL^-1, respectively. The MIL-101 fiber was also applied to study the photodegradation of the cVMS in water under simulated sunlight. Approximately 25%, 20%, and 45% of D4, D5, and D6, respectively, were degraded after 10 h exposure.

Investigation of volatile methyl siloxanes in biogas and the ambient environment in a landfill

Landfill biogas is a potential alternative for fossil fuel, but the containing impurities, volatile methyl siloxanes (simplified as siloxanes), often cause serious problems in gas turbines when applied to generate electricity. In this research, a collecting and analyzing method based on solvent adsorption and purge and trap-gas chromatography-mass spectrometry was established to determine the siloxanes in biogas from a landfill in Jinan, China, and adjacent ambient samples, such as soil, air, and leachate of the landfill. The results showed that, octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5) accounted for 63% of total siloxanes; and without considering D4 and D5, the order of detected siloxanes in concentration was found relating to Gibbs free energies of molecules, namely that higher abundant siloxane (except for D4 and D5) usually had lower Gibbs free energy. Additionally, the mass ratio between D4 and octamethyltrisiloxane (L3) in the biogas varied with different garbage age in landfills, possibly revealing the breaking-down of larger siloxane molecules with time. The samples, which were collected from environmental samples adjacent to the landfill, such as soil, water, and air, presented much higher siloxane level than urban or rural area away from landfills. The current H₂S scrubber of the landfill biogas could decrease the total siloxanes from 10.7 to 5.75 mg/m³ due to Fe₂O₃ and a refrigerant drier in a purification system and cyclic siloxanes were more easily removed than linear ones.