Volatilization of dimethylsilanediol (DMSD) under environmentally relevant conditions: Sampling method and impact of water and soil materials.

Dimethylsilanediol (DMSD) is the common breakdown product of methylsiloxanes such as polydimethylsiloxane (PDMS) and volatile methylsiloxanes (VMS) in soil. In this work, we first present a sorbent selection experiment aiming to identify a sorbent that can trap gas-phase DMSD without causing DMSD condensation and VMS hydrolysis at environmentally relevant humidities. With a proper sorbent (Tenax) identified, the volatilization of DMSD from water and various wet soil and soil materials were measured in a controlled environment. It was demonstrated that DMSD underwent volatilization after soil water was completely evaporated. Various types of soil constituents show drastic differences in preventing DMSD from volatilization. Analysis of the sorbent-captured products provides further insight, most notably that virtually no cyclic methylsiloxanes are formed during the volatilization of DMSD from water or soil materials, except in one extreme case where only traces are detected.

Fate of dimethylsilanediol (DMSD) in soil-plant systems.

Dimethylsilanediol (DMSD) is the degradation product of methylsiloxane polymers and oligomers such as volatile cyclic methylsiloxanes (cVMS). To better understand the environmental fate of this key degradation product, we conducted a three-part study on the movement of DMSD in soil. The objective of this third and final study was to determine the fate of DMSD in soil-plant systems under constant irrigation. Soil columns were constructed using two soils with the upper 20 cm layers spiked with 14C-labeled DMSD. Corn seedlings were transplanted into the soil columns and placed in a field plot underneath a transparent cover that prevented rainwater from reaching the soil columns while allowing soil water to be volatilized freely. The soil-plant columns were regularly irrigated with known amounts of DMSD-free plant growth solution to sustain the plant growth. At pre-determined time intervals (15-67 days), the plant and soil columns were sectioned and the distribution of 14Corganosilicon species in the soil profile and plant parts was determined using a combination of Liquid Scintillation Counting and High-Performance Liquid Chromatography-Flow Scintillation Analysis, while soil water loss was determined gravimetrically. It was found that the majority (>92 %) of DMSD initially spiked into the soil was removed from the soil-plant systems. Although DMSD was transported from the soil to the plant, it was subsequently volatilized from the plant via transpiration, with only a small fraction (∼5%) remaining at the conclusion of the experiments. In addition, little non-extractable DMSD was found in the top layer of soil in the soil-plant systems, suggesting that the air-drying of soil is a necessary pre-condition for the formation of such non-extractable silanol residue on topsoil.

Fate of dimethylsilanediol (DMSD) in outdoor bare surface soil and its relation to soil water loss.

Dimethylsilanediol (DMSD) is a primary degradation product of silicone materials in the environment. Due to its low air/water partition coefficient and low soil/water distribution coefficient, this compound is not expected to undergo sorption and volatilization in wet soil. In an accompanying paper, we confirm that under controlled indoor conditions in test tubes, there is little to no volatilization of DMSD from soil and soil constituents if soil is wet. However, a significant amount of DMSD was volatilized when the soil substrates became air dried. Given the importance of water on the partition and fate of DMSD, we now report a continuation of this study focusing on the relation between DMSD removal and water loss in re-constituted soil columns under outdoor conditions. Consistent with predictions based on its partition properties and reconciling this evidence with previously reported field and laboratory studies, DMSD distribution was found to be largely dependent on water partitioning. The results suggested that DMSD moved upward in soil profile as soil water was evaporated from topmost layers with little DMSD retention by the soil matrix. As soil dried, a fraction of DMSD was sorbed by the soil matrix in the topmost layer, while most of the spiked radio-labeled DMSD was removed from soil through volatilization.

Seasonal and latitudinal variability in the atmospheric concentrations of cyclic volatile methyl siloxanes in the Northern Hemisphere.

Field data from two latitudinal transects in Europe and Canada were gathered to better characterize the atmospheric fate of three cyclic methylsiloxanes (cVMSs), i.e., octamethyl-cyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5) and dodecamethylcyclohexasiloxane (D6). During a year-long, seasonally resolved outdoor air sampling campaign, passive samplers with an ultra-clean sorbent were deployed at 15 sampling sites covering latitudes ranging from the source regions (43.7-50.7 °N) to the Arctic (79-82.5 °N). For each site, one of two passive samplers and one of two field blanks were separately extracted and analyzed for the cVMSs at two different laboratories using gas-chromatography-mass spectrometry. Whereas the use of a particular batch of sorbent and the applied cleaning procedure to a large extent controlled the levels of cVMS in field blanks, and therefore also the method detection and quantification limits, minor site-specific differences in field blank contamination were apparent. Excellent agreement between duplicates was obtained, with 95% of the concentrations reported by the two laboratories falling within a factor of 1.6 of each other. Nearly all data show a monotonic relationship between the concentration and distance from the major source regions. Concentrations in source regions were comparatively constant throughout the year, while the concentration gradient towards remote regions became steeper during summer when removal via OH radicals is at its maximum. Concentrations of the different cVMS oligomers were highly correlated within a given transect. Changes in relative abundance of cVMS oligomers along the transect were in agreement with relative atmospheric degradation rates via OH radicals.

Basic considerations to minimize bias in collection and analysis of volatile methyl siloxanes in environmental samples.

Monitoring studies that aim to quantify volatile methyl siloxanes (VMS) in environmental matrices may encounter a multitude of issues, most of which relate to the unique combination of physical-chemical characteristics of VMS that distinguish them from other classes of organic compounds. These properties, which are critical to their function in various applications, also control their fate and distribution in the environment, as well as the analytical chemistry of their measurement. Polycondensation and rearrangement reactions of VMS oligomers are possible during sample storage and analysis. Thus, care should be exercised to suppress these types of reactions by avoiding any catalytic substances or surfaces in sample collection and analysis equipment. Another factor complicating sample integrity in the analysis of trace levels of VMS, is their ubiquitous presence in many common products and components of instrumentation in the laboratory. For example, some gas chromatography columns and inlet septa have been identified as sources of VMS due to surface-catalyzed transformation of silicones to VMS promoted by moisture under high temperature in some silicone-based GC columns. Possible chemical transformation of the analytes, contamination from other sources, and potential loss of analytes need to be assessed throughout all aspects of the study, from sample collection through analysis, by establishing a rigorous quality assurance and quality control program. The implementation of such a robust QA/QC program facilitates the identification and minimization of potential analytical biases and ensures the validity and usability of data generated from environmental monitoring campaigns for VMS. The objective of this paper is to focus on aspects of collection, processing, and analysis of environmental samples that may influence the quality of the VMS analytical results. This information should then be employed in the design and implementation of future monitoring studies and can used to assess the validity of analytical results from VMS monitoring studies.

Measuring snow scavenging of two airborne cyclic volatile methylsiloxanes under controlled conditions.

The objective of this study was to determine snow scavenging of cVMS and its potential effect on the cVMS concentrations in snowmelt water and surrounding soil. Snow scavenging of two cVMS, octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5), was examined in two steps. First, sorption and desorption of D4 and D5 on snowflakes, including snow sorption coefficients (KiA), were measured against a benchmark compound, cyclopentanone, at different temperatures from 0 to -20 °C. Measurements were made using a custom-made snow chamber and 14C-labeled D4 and D5. In addition, the transfer of snow-bound cVMS to snowmelt water and surrounding soil was studied with 14C-D4 and 14C-D5-spiked snowpack placed both in a closed snow chamber and on top of a layer of frozen soil in an open chemical hood. KiA values measured in both sorption and desorption processes were very small (<10-2 m). They increased with decreasing temperature and were higher for the D5 compared to D4. The calculated gas scavenging of D4 and D5 was small because of the small KiA values, while particle scavenging of cVMS is predicted to be negligible due to their low octanol/air partition coefficients (KOA). Most importantly, almost all 14C-D4 and 14C-D5 sorbed by a snowpack was lost during the snow melting process through re-volatilization and hydrolysis and became non-detectable in snowmelt water. In short, the experimental measurements demonstrated that snow scavenging could not be a valid deposition mechanism for these volatile hydrophobic compounds.

Comment on "Optimization of suspect and non-target analytical methods using GC/TOF for prioritization of emerging contaminants in the Arctic environment".

Lee et al. (2019) recently proposed that volatile methylsiloxanes (VMS) be considered as emerging contaminants in the Arctic environment based on the results of suspect and non-target screening of environmental samples collected from Ny-Ålesund, Svalbard. In any analytical program, it is of critical importance to be able to discern if the identification of analytes is due to true presence in the sampled environmental media or if contamination occurred during sample handling and analysis, leading to false positive detection. This is particularly important for VMS due to their ubiquity in consumer products, sample containers, and analytical instrumentation, thus requiring robust quality control (QC) procedures to support the validity of results. Although Lee et al. (2019) concluded that VMS in the environmental samples originated from potential long-range transport and deposition, it is most likely that local point sources account for their presence. Additionally, there is low confidence in the validity of the reported detection of VMS in the sampled environmental media as this study does not include any of the necessary QC to determine whether the VMS detected would be due to contamination or indicative of presence in the environment.

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 [

Nickel-Catalyzed Cross-Coupling of Aryl 2-Pyridyl Ethers with Organozinc Reagents: Removal of the Directing Group via Cleavage of the Carbon-Oxygen Bonds.

Reaction of aryl 2-pyridyl ethers with arylzinc reagents under catalysis of NiCl2(PCy3)2 affords aryl-aryl cross-coupling products via selective cleavage of CAr-OPy bonds. The reaction features a wide substrate range and good compatibility of functional groups. ß-H-free alkylzinc reagents are also applicable as the nucleophiles in the transformation, whereas ß-H-containing alkylzinc reagents lead to a mixture of cross-coupling and hydrogenation products.

Comment on "Bioaccumulation of Methyl Siloxanes in Common Carp (Cyprinus carpio) and in an Estuarine Food Web in Northeastern China".

We have reviewed a paper titled "Bioaccumulation of Methyl Siloxanes in Common Carp (Cyprinus carpio) and in an Estuarine Food Web in Northeastern China" by Xue et al., which was published in the Archives of Environmental Contamination and Toxicology in 2019. In the paper, the authors presented and discussed the measured bioconcentration factors (BCFs), biomagnification factors (BMFs), and trophic magnification factors (TMFs) of selected volatile methylsiloxanes in Shuangtaizi estuary in China. Although we appreciate their efforts for sample collection and data analysis, we have identified significant errors in calculations of BCFs, TMFs, and BMFs, as well as animal welfare issues and food web trophic level assumptions. Based on the data, we have attempted to correct some of the analysis and offered a more complete and robust interpretation of the related data, when possible. Collectively, these errors would likely lead to very different conclusions than yours in the paper.

Extraction and quantitative analysis of water by GC/MS for trace-level dimethylsilanediol (DMSD).

Dimethylsilanediol (DMSD) is related to the most important bifunctional building block for silicone oligomers and polymers, although DMSD itself is not used in any commercial applications. The environmental release of DMSD is linked to the hydrolytic degradation of other silicone materials in soil and water as DMSD is usually one of the major products. Most common extraction and quantification methods are not suitable for the analysis of trace- and ultratrace-levels of DMSD in water. This is because DMSD is highly water soluble and can readily undergo self-condensation when concentrated. In addition, DMSD may also coexist with DMSD precusors such methylsiloxanes in water. In the present study, solid-phase extraction (SPE) in combination with gas chromatography-mass spectrometry (GC/MS) without pre-column derivatization was tested for analyzing water samples for DMSD. It was found that direct analysis by GC/MS can be used for a wide range of concentrations if DMSD was extracted into a dry organic solvent. Isolute® ENV + may be used for such extraction at higher DMSD concentrations, while Supelclean™ ENVI-Carb™ Plus was found to be better for trace and ultratrace analysis. Increased salt content in water can increase its DMSD extraction efficiency, while polarity of the eluting solvents is a determining factor for eluting efficiency. Moisture in the final extract is a detrimental factor for direct GC/MS analysis. With a proper moisture removal procedure and a suitable internal standard, coupling of SPE and direct GC/MS analysis reduces the method detection limits for DMSD to lower ppb levels. Based on field sample analysis, solvent and instrumental background, not instrumental sensitivity, was found to be the limiting factor in lowering the detection limits for this analysis.

Long-range transport potential and atmospheric persistence of cyclic volatile methylsiloxanes based on global measurements.

This study investigates persistence (P) and long-range transport potential (LRTP) of cyclic volatile methylsiloxanes (cVMS) based on the field measurements in the Northern Hemisphere. The field data consisted of published outdoor air concentrations of octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5) and dodecamethylcyclohexasiloxane (D6) at urban, suburban, rural and remote locations excluding the point sources. Three major trends were observed. First, D4 and D6 concentrations were correlated with measured concentrations for D5 at the same times and locations in the majority of the datasets, reflecting the common sources and similar removal mechanism(s) for these compounds. Second, as the sampling sites changed from the source to remote locations along a south-to-north transect, average cVMS concentrations in air decreased in an exponential manner. The empirical characteristic travel distances (eCTD) extracted from these spatial patterns were smaller than model estimated values and differed in order among individual compounds (D4 ∼ D5 < D6). Finally, D5/D6 concentration ratios were also found to decrease exponentially along the same spatial gradient, contrary to model predictions of an increase based on current knowledge of mechanisms controlling atmospheric cVMS degradation. These findings suggest that there may be additional removal process(es) for airborne cVMS, currently not accounted for, that requires further elucidation.

Chlorinated-Methylsiloxanes in Shengli Oilfield: Their Generation in Oil-Production Wastewater Treatment Plant and Presence in the Surrounding Soils.

In two oil-wastewater treatment stations of Shengli Oilfield, cyclic volatile methylsiloxanes (cVMS, D4-D6) in the wastewater stream were found to undergo chlorination during electro-oxidation process for wastewater containing chlorine ions (16.1-42.0 g/L). Their converted fractions were 4.71-28.0% for monochlorinated D4-D6 and 0.22-7.96% for dichlorinated D4, which were ∼2 orders of magnitude higher than those for hydroxylated products. Furthermore, portions of chlorinated methylsiloxanes retained in excess sludge were released to the surrounding soils. In soil samples ( n = 500), chlorinated methylsiloxanes concentrations (

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

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) were detected in personal care products [

Asynchronous exponential growth for a two-phase size-structured population model and comparison with the corresponding one-phase model.

In this paper we study a two-phase size-structured population model with distributed delay in the birth process. This model distinguishes individuals by 'active' or 'resting' status. The individuals in the two life-stages have different growth rates. Only individuals in the 'active' stage give birth to the individuals in the 'active' stage or the 'resting' stage. The size of all the newborns is 0. By using the method of semigroups, we obtain that the model is globally well-posed and its solution possesses the property of asynchronous exponential growth. Moreover, we give a comparison between this two-phase model with the corresponding one-phase model and show that the asymptotic behaviours of the sum of the densities of individuals in the 'active' stage and the 'resting' stage and the solution of the corresponding one-phase model are different.

Analysis of a time-delayed mathematical model for tumour growth with an almost periodic supply of external nutrients.

In this paper, the existence, uniqueness and exponential stability of almost periodic solutions for a mathematical model of tumour growth are studied. The establishment of the model is based on the reaction-diffusion dynamics and mass conservation law and is considered with a delay in the cell proliferation process. Using a fixed-point theorem in cones, the existence and uniqueness of almost periodic solutions for different parameter values of the model is proved. Moreover, by the Gronwall inequality, sufficient conditions are established for the exponential stability of the unique almost periodic solution. Results are illustrated by computer simulations.

Methylsiloxanes Release from One Landfill through Yearly Cycle and Their Removal Mechanisms (Especially Hydroxylation) In Leachates.

In one yearly cycle (2016), D4 and D5 were detected in biogas samples (n = 36, 0.105-2.33 mg/m3) from a Chinese municipal landfill, while D4-D6 were detected in influents/effluents of leachate storage pond (n = 72, < LOQ-30.5 µg/L). Mass loads of cVMS in both biogas (591-6575 mg/d) and leachate influents (659-5760 mg/d) increased from January to July (summer), and then decreased from July to December (winter). Removal experiments indicated that 1) hydrolysis and volatilization were predominant removal mechanism for D4 and D5, respectively, in leachate storage pond, responsible for their more significant removal (94.5-100%) in August; 2) indirect phototransformation (t1/2 = 25.5-87.0 days), such as hydroxylation by OH radical generated in leachates, was the predominant (50.0-75.5%) removal pathway for D6, which led to the largest removal efficiencies (65.2-73.7%) in June, the month with the largest sun light intensity and highest photosensitizer (e.g., Fe2+ and NO3-) concentrations. Monohydroxylated products of D5 and D6, D4TOH and D5TOH, were detected in leachate effluents (39.6-187 ng/L) during May-July. Compared to D5 and D6, volatilization half-lives of D4TOH (86.3 days) and D5TOH (177 days) in leachates were 2.9 and 1.4 times longer, while their hydrolysis half-lives (7.50 days for D4TOH and 21.5 days for D5TOH) were 7.1 and 10 times shorter, respectively.

Quantitative structure-reactivity relationships of hydroxyl radical rate constants for linear and cyclic volatile methylsiloxanes.

An accurate understanding of the fate of volatile methylsiloxanes (VMS) in air is crucial for determining their persistence and concentrations in the environment. Although oxidation by atmospheric hydroxyl radicals (•OH) is considered as a major degradation mechanism for airborne VMS, the existing bimolecular rate constants with •OH measured and modeled for any given VMS compound varied greatly, depending on the approaches used to generate the data. The objectives of the present study were to measure •OH reaction rate constants for 4 cyclic and 4 linear VMS based on a relative rate method using a newly designed atmospheric chamber and to establish structure-reactivity relationships for the kinetics. In the past, the reaction rate constants for VMS were generally recognized to increase with the number of the methyl groups per molecule, the only differential factor in the existing models. However, the new measurements indicated that molecular structure should also be considered in the prediction of the reaction rates. Better empirical models were developed by simple and multiple linear regressions of the measured values from the present study and the literature. A high correlation existed for the reaction rates with the number of the methyl group attached at 2 distinct siloxane structures (i.e., linear and cyclic VMS). Even better correlations were obtained with one or 2 molecular descriptors that are directly related to the size of VMS, which, in turn, not only depend on the number of methyl groups, but the linear/cyclic structures as well for permethylsiloxanes. Environ Toxicol Chem 2017;36:3240-3245. © 2017 SETAC.

A Time-Delayed Mathematical Model for Tumor Growth with the Effect of a Periodic Therapy.

A time-delayed mathematical model for tumor growth with the effect of periodic therapy is studied. The establishment of the model is based on the reaction-diffusion dynamics and mass conservation law and is considered with a time delay in cell proliferation process. Sufficient conditions for the global stability of tumor free equilibrium are given. We also prove that if external concentration of nutrients is large the tumor will not disappear and the conditions under which there exist periodic solutions to the model are also determined. Results are illustrated by computer simulations.

Sorption and desorption kinetics and isotherms of volatile methylsiloxanes with atmospheric aerosols.

This study investigated sorption and desorption behaviors of airborne volatile methylsiloxanes (VMS) such as octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5) on nine major primary and secondary atmospheric aerosols at a relative humidity (RH) of 30%. It was found that sorption and desorption of VMS took place via a two-phase process, which included an initial rapid step, followed by slower subsequent step. The initial rapid step was favored especially at low concentrations. Equilibrium sorption isotherms were slightly better fitted to Polanyi-Manes sorption model than Langmuir model except D4 on black carbon and D5 on sea salt. Values of apparent aerosol-air partition coefficients ranged 0.09-50.4 L/m(2) for D4 and 2.1-284 L/m(2) for D5 with carbon black having the largest values. Some of aerosols such as carbon black and sea salts reversibly interacted with D4 and D5 whereas other aerosols such as kaolinite and sulfates showed highly irreversible sorption for the VMS, especially at low concentrations. As sorption density of D4 and D5 on kaolinite was decreased from 1100 to 250 µg/m(2), the irreversible fraction was increased from 27% to 80%.The mechanism responsible for these differences is essential for a better understanding and prediction of atmospheric chemistry of VMS as affected by the presence of atmospheric aerosols.