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Letter pubs.acs.org/journal/estlcu
Temperature Dependence of the Organic Carbon/Water Partition Ratios (KOC) of Volatile Methylsiloxanes Dimitri Panagopoulos,†,‡ Annika Jahnke,§ Amelie Kierkegaard,† and Matthew MacLeod*,† †
Department of Environmental Science and Analytical Chemistry (ACES), Stockholm University, Svante Arrhenius väg 8, SE-11418 Stockholm, Sweden ‡ Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, United States § Department of Cell Toxicology, Helmholtz Centre for Environmental Research (UFZ), Permoserstrasse 15, DE-04318 Leipzig, Germany S Supporting Information *
ABSTRACT: Knowing the temperature dependence of the organic carbon/water partition ratios (KOC) of volatile methylsiloxanes (VMS) is required to understand their environmental fate. We measured the KOC of two linear VMS (lVMS), three cyclic VMS (cVMS), and six polychlorinated biphenyls (PCBs) at 25, 15, 10, and 5 °C and calculated their enthalpies and entropies of sorption to organic carbon (ΔHOC and ΔSOC, respectively). The ΔHOC of VMS ranged from −79.2 to −45.8 kJ mol−1, while the ΔHOC of the PCBs ranged from −68.7 to −29.3 kJ mol−1. Previously reported measurements of the enthalpy of phase change between octanol and water (ΔHOW) for cVMS (11.3−68.8 kJ mol−1) differed substantially from our ΔHOC measurements, even showing different signs (negative versus positive). Literature data of ΔHOC and ΔHOW for PCBs (−61 to −17 kJ mol−1) are closer to our measured values of ΔHOC for the PCBs showing the same sign (negative) with differences within a factor of 2 in the majority of the cases. Comparison of all available data for PCBs and VMS indicated that there may be important differences between ΔHOC and ΔHOW, especially for the VMS. Therefore, assuming ΔHOC equals ΔHOW in environmental fate models may be a source of substantial error.
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INTRODUCTION Volatile methylsiloxanes (VMS) are organosilicon polymers consisting of Si(CH3)2O- units that are found in both cyclic (cVMS) and linear (lVMS) form.1−8 cVMS are used as carriers in personal care products such as deodorants, hand creams, and lotions because of their high volatility and the smooth texture they impart to the products.1−8 cVMS and lVMS are also present in silicone polymers used as coatings and sealants.4−7 VMS are attracting an increasing amount of interest from environmental chemists and regulatory agencies because of their large production volumes,1−8 their presence in the aquatic environment,9 and their bioaccumulation potential.10 VMS are highly hydrophobic and have partition ratios between organic carbon and water (KOC)11−15 that are comparable to those of high-molecular weight PCBs.13,14 A typical problem when attempting to measure a very high KOC with conventional equilibrium approaches is the accurate determination of the freely dissolved concentration of the chemicals in the water.16,17 In previous studies,13,14 we measured the KOC of VMS in fresh and salt water using an indirect method that circumvents the problem of measuring low concentrations in the dissolved phase by instead measuring © 2017 American Chemical Society
the rate of volatilization of the chemicals from a mixture of organic carbon and water. Using a fugacity model, we calculated the KOC of the chemicals from their volatilization rate, assuming equilibrium partitioning between water and organic carbon in the experimental system. The effect of temperature on KOC is important in determining the fate of chemicals released to aquatic systems where water temperatures deviate substantially from the range of 20−25 °C in which most laboratory measurements are conducted. Colder water and sediment temperatures typically favor partitioning out of the dissolved phase to organic carbon18 and could thus prolong the residence times of VMS in aquatic environments by decreasing their availability for loss processes, such as hydrolysis and volatilization. The effect of temperature on KOC can be described by the van’t Hoff equation:18 Received: Revised: Accepted: Published: 240
April 17, 2017 May 12, 2017 May 12, 2017 May 12, 2017 DOI: 10.1021/acs.estlett.7b00138 Environ. Sci. Technol. Lett. 2017, 4, 240−245
Letter
Environmental Science & Technology Letters
Figure 1. van’t Hoff plots of ln KOC as a function of inverse temperature (1/T) for three cVMS and two lVMS. The data points represent the ln KOC measurements (n = 3 at each of the different temperatures). The black lines are the regression lines, and the blue lines are the 95% confidence intervals of the regression lines. Measured KOC values for all chemicals at each temperature are listed in Table S11.
ln K OC =
−ΔHOC ΔSOC + RT R
(1,4-DCB) and α-hexachlorocyclohexane (α-HCH), in 10 μL of hexane in a closed vial and left to equilibrate while rotating overnight. For more details about the spiking procedure and analysis of the spiked sediment, see refs 13 and 14. The spiked amounts and the spiking efficiencies are listed in Table S1. The spiked sediment was submerged in 300 mL of Milli-Q water in a purge-and-trap system that was placed in a temperaturecontrolled bath (±1 °C). The water inside the purge-and-trap system was continuously stirred, and the headspace was purged with a stream of nitrogen [50 ± 0.5 mL/min, filtered through a solid-phase extraction (SPE) column packed with 25 mg of ENV+ sorbent]. The chemicals from the purged headspace were collected on a second SPE column containing 25 mg of ENV+. At eight time points over the course of 4 days, the SPE columns were exchanged, eluted with 1 mL of dichloromethane, and analyzed. Extraction blanks were processed together with the samples. At the end of the experiment, the bulk water/sediment was extracted and analyzed to assess the mass balance of the system according to ref 13. Organic carbon/water partition ratios were calculated by adjusting the KOC parameter in a fugacity model of the purge-and-trap system calibrated using the benchmarking chemicals, so that the volatilization rate in the model fitted the observed volatilization rates of the VMS and PCBs. The benchmarking chemicals were used to calibrate the mass transfer coefficients on the air side (MTCa) and the water side (MTCw) of the air−water interface, and these MTCs were assumed to be the same for all chemicals. Our method does not account for the possibility of facilitated transport of chemicals bound to dissolved organic carbon (DOC) on the water side of the air−water interface because of solvation of a small fraction of organic carbon in the sediment as the sediment particles come in contact with the water. The process of facilitated mass transfer has been discussed by ter Laak et al.21 and Mayer et al.22 Because our KOC and ΔHOC measurements are derived from model fits with epistemic uncertainties, when we refer to our KOC and ΔHOC values, we refer to the apparent values of KOC and ΔHOC. Our model is presented in detail in ref 13. A brief description of the model and the parametrization details are included in Text S2 of the Supporting Information.
(1)
where T (kelvin) is the temperature, ΔHOC (joules per mole) and ΔSOC (joules per mole per kelvin) are the changes in enthalpy and entropy, respectively, of phase change for sorption of the chemical to organic carbon from water, and R is the ideal gas constant (joules per mole per kelvin). Because of the scarcity of experimental data on ΔHOC in the literature, it is common practice in multimedia chemical fate modeling to assume that ΔHOC is equal to the enthalpy of phase change between octanol and water (ΔHOW) when correcting KOC for temperature.19,20 The aim of this study was to measure ΔHOC for VMS alongside polychlorinated biphenyls (PCBs) that were used as reference chemicals because of their well-characterized properties. Specifically, we report new measurements of the KOC (sediment from lake Ången, Sweden) of three cVMS [octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5), and dodecamethylcyclohexasiloxane (D6)], two lVMS [decamethyltetrasiloxane (L4) and dodecamethylpentasiloxane (L5)], and PCBs 28, 52, 101, 118, 138, and 153 at a range of temperatures, and their ΔHOC and ΔSOC. Our measurements provide property data for VMS to support fate modeling and, by comparison with data and polyparameter linear free energy relationships (PP-LFERs) from the literature, provide a basis for critically evaluating the validity of the assumption that ΔHOW can be used as a surrogate for ΔHOC of these compounds for modeling their environmental fate.
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MATERIALS AND METHODS Materials. The materials used in this study are presented in Text S1 in the Supporting Information. Methods. Our method for measuring KOC is described in detail in our previous study,13 in which experiments were conducted at room temperature (21 °C). Here, we determined ΔHOC for the VMS and PCBs from new measurements of KOC of these chemicals at four different temperatures (5, 10, 15, and 25 °C). The experiments were conducted in triplicate at each temperature. Briefly, dry sediment (6 mg) was spiked with cVMS, lVMS, PCBs, and two benchmarking chemicals, 1,4-dichlorobenzene 241
DOI: 10.1021/acs.estlett.7b00138 Environ. Sci. Technol. Lett. 2017, 4, 240−245
Letter
Environmental Science & Technology Letters Literature Data and PP-LFERs. For comparison to our measured values, we collected measured ΔHOC values for PCBs,23 previously published ΔHOW measurements for PCBs and VMS,24−26 and PP-LFER predictions for ΔHOW of PCBs and VMS.26
and consequently the smaller amount of organic carbon used in ref 14. Previous studies have presented evidence that KOC may depend on the amount of organic carbon in the system.27,28 In small amounts of sediment, the available sites in the organic carbon to sorb chemicals may become saturated. This situation would result in higher partitioning of chemicals to water and lower measured KOC values in the experiments in ref 14 compared to those reported here and in ref 13, which is observed for PCBs, but not siloxanes (Table 1). Another factor that could be driving the variability between the measurements is the possibility of facilitated transport of DOC-bound chemicals on the water side of the air−water interface, as discussed by ter Laak et al.20 and Mayer et al.21 Different amounts of OC would result in different amounts of DOC and consequently in differences in the facilitated transport of DOCbound chemicals. However, if this mechanism was important, we would most likely observe higher KOC values in smaller amounts of sediment and lower KOC values in larger amounts of sediment. We consider each of the KOC values reported in Table 1 to be an independent and valid measurement. For VMS, the maximal variability between the measured KOC values in different experiments was 0.35 log units for D5, and this value could therefore be selected as a conservative estimate of uncertainty in measured KOC values of the siloxanes. For PCBs, the variability between different experiments was larger, and the maximal value of 0.78 log units for PCB 52 could be selected as a conservative estimate of uncertainty in the KOC values of the PCBs. Enthalpy and Entropy Measurements. The ΔHOC values were calculated from the slopes of the regression lines (slope) in the van’t Hoff plots shown in Figure 1 and Figure S3 according to eq 2:
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RESULTS AND DISCUSSION Mass Balance Control. The average total recoveries ranged from 76.2 to 122% for cVMS and lVMS and from 65.9 to 103% for the PCBs and the benchmarking chemicals (Table S2). The loss of chemicals due to degradation or due to formation of a non-extractable residue is accounted for in our modeling calculations and described in Text S2 of the Supporting Information. In all cases, the concentrations of the chemicals in the blanks were lower than 15% of the concentration of the chemicals in the samples. No blank correction was made. Volatilization Curves and Calculations of KOC. Of 156 volatilization curves, R2 was greater than 0.9 in 151 cases (Text S3 and Tables S6−S9). In four cases, R2 was between 0.9 and 0.8, and in only one case was it lower than 0.8 [one of the three replicates for D4 at 5 °C; R2 = 0.78 (Table S9)]. The measured KOC was higher at lower temperatures for all VMS and PCBs (Figure 1 for VMS and Figure S3 for PCBs). The VMS that showed the strongest temperature dependency was D4, where the KOC increased by 1.18 log units over the temperature range from 25 to 5 °C, whereas the PCB that showed the strongest temperature dependency was PCB 28, where the KOC increased by 0.97 log units. Our newly measured KOC values for PCBs and siloxanes at 25 °C are in all cases
