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Environmental fate & pathways

Phototransformation in air

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Phototransformation in air: Rate constant of 1.55 E-12 cm3 molecule-1 second-1 at 24°C (half-life 10.4 days, (calculated using tropospheric concentration of OH radicals 5E+05 molecule/cm3 over 24 -hour period (ECHA, 2016))) for reaction with OH radicals.

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An experimental relative rates study (Atkinson, 1991) found that the NO3 radical and O3 reactions are of no importance as tropospheric removal processes for this compound. The dominant gas-phase chemical loss process is by reaction with the OH radical, with measured rate constant of 1.55 E-12 cm3 molecule-1 second-1 at 24°C (calculated half-life 10.4 days, using tropospheric concentration of OH radicals 5 E+05 molecule/cm3 over 24 -hour period (ECHA, 2016)).

A reaction with the OH radical rate constant of 1.50 E-12 cm3 molecule-1 second-1 (calculated half-life 10.7 days, using tropospheric concentration of OH radicals 5 E+05 molecule/cm3 over 24 -hour period (ECHA, 2016)) was obtained using an accepted calculation method ( AOPWIN ver. 1.92). The result is considered to be reliable.

Sommerlade et al. (1993) identified the major products from the reaction of the related substance octamethylcyclotetrasiloxane (D4) with hydroxyl radicals as heptamethylhydroxycyclotetrasiloxane, along with smaller amounts of heptamethyl(hydroperoxymethyl)cyclotetrasiloxane and 1,2- bis(heptamethylcyclotetrasiloxanyl)ethane, and trace amounts of heptamethyl(hydroxymethyl)cyclotetrasiloxane and bis(heptamethylcyclotetrasiloxanyl)ether.

Two recent publications which provide the reaction rate constant with the hydroxyl radical studies agree closely with the key study, but with somewhat faster reaction rates.

These degradation products are expected to be more soluble in water than D4 and D5, and to have a lower vapour pressure, and so are likely to be removed from the atmosphere by wet and dry deposition (Chandra, 1997).  Chandramouli and Kamens (2001) confirmed this deposition process for D5. In this study nonamethylhydroxycyclopentasiloxane was identified as the main degradation product from D5 using an outdoor smog chamber that contained fine road dust. More than 99% of the hydroxy derivative formed partitioned onto the dust particles.

Whelan et al. (2004) assessed the atmospheric fate of volatile methyl siloxanes (VMS) and their degradation products. The assessment used a simple equilibrium-partitioning model to investigate the relative rates of removal of two representative VMSs (the linear siloxane decamethyltetrasiloxane, L4, and the cyclic siloxane, D4) and their hydroxyl-substituted degradation products by reaction and atmospheric deposition. The modelling is based on the work of Atkinson (1991) and Sommerlade et al. (1993), which demonstrates that siloxanes break down in the atmosphere to form hydroxyl-substituted degradation products by reaction with OH radicals. As substitution proceeds the silanols become increasingly water-soluble and less volatile, and so tend to be washed out of the atmosphere by wet deposition. The silanols are also assumed to undergo hydrolysis reactions when dissolved in liquid water droplets. Removal of the silanols from the atmosphere by dry deposition is also accounted for. The model indicated that L4 and D4 and the monohydroxy degradation products occur mainly in the vapour phase, whereas the further degradation products occur mainly in the dissolved and particulate phases. Overall, it is concluded that >99% of L4 and D4 are removed from the atmosphere as silanols in wet deposition and <1% are removed in dry deposition.

All the aforementioned studies are focused on homogenous reactions. Although they definitely represent the major characteristics of D5 degradation in the atmospheric environment, the real atmosphere is much more complex. For example, the atmosphere contains a combination of multiple oxidants such as O3, OH and other free radicals, as well as UV radiation and aerosols. In addition, cVMS release also follows distinguishable spatial and temporary patterns: they are released mostly to the urban and suburban atmosphere where the O3, OH radical and aerosol concentrations may be much higher than the rural or remote regions. When cVMS are transported from the source region through the air, they move along with those oxidants, which may increase their exposure to the intensified atmospheric degradation processes. The overall half-lives of cVMS therefore may largely depend on the resident time of cVMS in urban and suburban atmosphere.


In order to better understand the environmental fate of cVMS under the more realistic atmospheric conditions, several projects were initiated both at the University Iowa and Dow Corning Corporation. The specific objectives of those studies are twofold: To determine the removal of gas-phase cVMS by combination of multiple oxidants in the presence of UV radiation and aerosols; to determine the effects of the unique release pattern of cVMS and any additional removal mechanism on the overall half-lives of cVMS in the atmosphere.


In University Iowa studies, the uptake of D4 and D5 vapours (up to saturated vapour pressure) by carbon black and several types of reactive mineral dust aerosols in the absence and presence of O3 and hydroxyl radicals was simulated in an atmospheric chamber at room temperature and monitored by FT-IR spectroscopy (Navea et al., 2009a,b,c). It was found that the heterogeneous uptake (removal from gas phase) of D4 and D5 by mineral aerosols such as kaolinite, hematite and quartz was rapid and significant (Navea et al., 2009a). Under dry conditions (< 1% RH) in the absence of O3 and hydroxyl radicals, removal of both D4 and D5 by hematite and kaolinite were characterised by two processes. The initial fast removal process completed within one-minute accounts for 30 to 50% total removal for D4, but 50~70% for D5. The subsequent slow removal process had a pseudo-first order kinetics with rate constant k at room temperature varying in the range of 0.5~0.9´10-3 s-1 (corresponding to half-lives from 24~13 minutes) for D4, and in a range of 0.2~4´10-3 s-1 (corresponding to half-lives from 53 minutes to 12 minutes) for D5 for hematite and kaolinite, respectively. The reactivity of various aerosols was in the order, kaolinite > hematite > quartz, for both D4 and D5 after the reaction rates were normalised to surface area of the aerosols.


In the presence of O3 but no solar radiation, no detectable change was observed in the concentration of the gas phase D4 or D5 after 50 minutes exposure (Navea et al., 2009b), consistent with the low rate constant obtained for O3/D4 reactions in the previous study (Atkinson, 1991). Under dry (< 1% RH) conditions, the introduction of aerosols such as hematite and kaolinite triggered immediate removal of both cVMS and O3 (Navea et al., 2009b). The kinetics of the gas phase removal apparently implied multiple heterogeneous processes, significantly different than that obtained with aerosols in the absence of O3. The major differences were in two aspects: First, addition of O3 slowed the uptake of the both D4 and D5 by the aerosols, while existence of D4 and D5 in the gas phase also slowed the decomposition of gas phase O3 relative to the aerosol-containing control with no cVMS, suggesting the competition of cVMS with O3 for surface sites of the aerosols (Navea et al., 2009b). In addition, the constant concentration profile observed at reaction times greater than 200 minutes in the cVMS/aerosol systems without O3 was replaced with a linear decline in D4 and D5 concentrations with increase of the reaction time when O3 was present. The disappearance of the surface saturation characteristics (constant concentration profile as time increases) increased the total removal over a longer time period (Navea et al., 2009b).


Under simulated solar radiation and in the presence of O3, hematite and kaolinite aerosols remove up to 50 to 70 % gas phase D4 and 60 to 90% gas phase D5 within 400 minutes under dry condition (< 1% RH) (Navea et al., 2009c). An increase in humidity under those conditions actually accelerates the removal presumably due to formation of hydroxyl radicals through photolysis of O3 in the presence of water.


The University of Iowa’s studies were conducted under relatively high concentrations of cVMS (> 10 mg L-1) due to limitation of the non-destructive analytical technique employed for cVMS analysis. In follow-up studies conducted at Dow Corning Corporation (Kim et al., 2009; Kim and Xu, 2009 a,b), the mechanism for D4 sorption in the high concentration range is verified as polymerisation of the sorbed D4 catalysed by clay surface. Modelling assessment results suggested that aerosol effect on the overall D4 degradation in natural environment should be relatively small by this polymerization mechanism (Navea et al., 2010). However, D4 concentration in atmosphere is in nanogram to micrograms per cubic meter, or 3 to 6 orders of magnitude lower than those tested in Navea’s studies.


At low D4 concentration range, reactive adsorption via depolymerization was observed on aerosol surface (Kim and Xu, 2009b). Under those conditions (initialCD4< 0.3 mg/L, RH 10~80%), 60% to 97% of sorbed D4 was not desorbable (Kim and Xu, 2009a and b). Surface speciation analysis of the sorbed D4 revealed that almost all sorbed D4 in this low concentration range was transformed to silanols within 2 hrs at 28% RH, due to surface-facilitated hydrolysis (or depolymerisation) (Kim and Xu, 2009b), similar to that found in dry soil (Xu 1999; Xu and Chandra, 1999).

Similar studies have been carried out with D5 (Kim and Xu, 2010 and 2011), and showed similar results, i.e. the mechanism for adsorption at high concentration range is polymerisation of the sorbed D5 by the clay surface, whereas at the low D5 concentration range, reactive adsorption via depolymerisation was observed on the aerosol surface (sorbed D5 being transformed to silanols and smaller cyclics (D3 and D4) as transient intermediates).

A recent study (Kim, J. and Xu, S., 2016) investigated the sorption and desorption behaviours of airborne VMSs (including D4) on nine major primary and secondary atmospheric aerosols (RH 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 favoured especially at low concentrations. Some aerosols such as carbon black and sea salts interacted reversibly with D4 whereas other aerosols such as kaolinite and sulfates showed highly irreversible sorption for the VMS, especially at low concentrations. Values of apparent aerosol-air partition coefficients ranged 0.09-50.4 L/m(2) for D4, with carbon black having the largest values.

These results suggest that the heterogeneous interaction of D5 with mineral aerosols, therefore, can be an important mechanism in reducing the concentrations and transport of this volatile siloxane compound in the environment. The exact effects from this depolymerisation by aerosol on the half-life of airborne D5 could not be estimated at this juncture. Nevertheless, the actual half-life of D5 in air should be shorter than that calculated based solely on the homogenous reaction rate. 

More recent work using actual field monitoring data has tested this hypothesis (Xu et al., 2016).  The objective of this work was to explore how the air monitoring data may be used to extract P and LRTP of cyclic volatile methylsiloxanes (cVMS). Data (~ 700 individual measurements) on atmospheric concentrations of cVMS, i.e., D4, D5 and D6, included all published air concentrations of the three at various places including urban, suburban/background, rural and remote Arctic locations. For all data sets, no screening was performed except that the correlation between concentrations of different cVMS compounds measured at the same location were used to check if any given set of data fall in the 99% confidence intervals of the entire data set. Data falling outside the 99% intervals were considered as outliers and were excluded from spatial pattern analysis. All measured values that passed the above screening were incorporated into the spatial analysis to minimize data bias. However, measurements below method detection limits (MDL) were given a value of half of the corresponding MDL values from the same studies. Published values between MDL and limit of quantitation (LOQ) were used without change. If no specific value was accessible, the average values of LOQ and MDL were assumed for measurements recorded as “< LOQ.”

On an assumption that airborne cVMS removal roughly follows the first order kinetic the same as that shown for other environmental contaminants by Shen et al. (2005), the concentration of any given cVMS ([cVMS], in ng m-3) may be related to the latitude (LA in °N) in a south-to-north transect in the northern hemisphere:

[cVMS] = [cVMS]°exp(-kappt) = [cVMS]°exp(-kapp(111.5/v)LA)              (1)

where [cVMS]°is the hypothetical concentration at zero °N; kappis the pseudo-first-order rate constant; t the time travel from the source to the given latitude; 111.5 (km/°N) is the average displacement on earth surface per °N of latitude; v is the average wind velocity in a south-to-north direction. Stated simply, the log [cVMS] should show a negative linear correlation with latitude (“SL1”) that can be used to calculate the empirical characteristic travel distance (eCTD, in km) (see SI for details):

eCTD = 111.5´log (1/e)/SL1 = - 48.4/SL1                (2)

where e is the Euler’s number (~ 2.718). The globally averaged half-life (t, in day) of the cVMS compound (see SI for details) should be:

t= -0.099/SL1              (3)

based on an assumption of 4.5 m s-1for v. 

Composition change of airborne cVMS at different latitude

On assumption that [cVMS] during transport is linearly related to the travel displacement projection at a south-to-north direction, the ratio of cVMS compounds with different average half-lives, (tAvstB, log ([cVMS A]/[cVMS B])) may be related to the latitude (LA) where they are measured by the following equation:

log ([cVMS A]/[cVMS B]) = constant + ((1/11.6)(1/tB– 1/tA))×LA       (4)

 where A and B referred to any two cVMS Compound A and B, respectively. Therefore, a plot of log ([cVMS A]/[cVMS B]) against LA should be a straight line with a slope (SL2) related to the corresponding averaged half-lives:

(1/tB– 1/tA) = 11.6´SL2        (5)

Based on the above relationships, the composition (D4:D5:D6) changes measured at non-source regions may also be compared with those at the source regions to extract average air half-lives.

D4 and D6 were found to be correlated with the D5 concentrations in a majority of the data sets measured at the same times and same locations. Average D4, D5 and D6 concentrations in outdoor air, excluding point sources, decreased exponentially by a factor of 100 in a south-to north transect from the source (urban) to the high latitude remote regions (the Arctic). In addition, concentration ratio of D5-to-D6 was found to decrease exponentially along this spatial gradient. However, the same correlation was not observed for D4-to-D6 ratio due to poor data quality surrounding D4 measurements. The yearly-averaged empirical characteristic travel distances (eCTD) of cVMS extracted from these spatial patterns were much less than the model estimations (mCTD) for all three compounds. Similarly, the empirical air half-lives of cVMS were substantially less than those derived from laboratory studies (see attached table " New Characteristic Travel Distance and Air Half-lives based on existing monitoring data"). These findings support that additional removal processes not accounted for in modelling and laboratory assessments are acting upon cVMS and require further elucidation.



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