Environ. Sci. Technol. 1996, 30, 1946-1952
Henry’s Law Constant, Volatilization Rate, and Aquatic Half-Life of Octamethylcyclotetrasiloxane J E R R Y L . H A M E L I N K * ,† Health & Environmental Sciences, Dow Corning Corporation, Midland, Michigan 48686-0994
PHILIP B. SIMON Ann Arbor Technical Services, Ann Arbor, Michigan 48103
ERIC M. SILBERHORN‡ Technology Sciences Group Inc., Washington, D.C. 20036
The undimensioned Henry’s law constant (Hc) and volatilization rate constant (kvc) of the silicone fluid octamethylcyclotetrasiloxane (OMCTS) were determined experimentally. The grand mean Hc for five experiments conducted under dynamic conditions with initial concentrations ranging from 4 to 50 µg/L (20 °C; 48-h equilibration period) was 3.4 ( 1.37. An Hc of >17 was observed under kinetically limited, static conditions where equilibrium was not reached in 96 h. Hc increased about 10-fold when humic acids were added to OMCTS test solutions, suggesting typical environmental cosolutes will increase the Henry’s law constant for OMCTS. The measured volatilization/ re-aeration ratio (kvc/kvo) for OMCTS was 0.57, a value similar to that measured for trichloroethylene (0.57) and benzene (0.56), indicating that OMCTS will readily volatilize from water.
Introduction Substantial amounts of octamethylcyclotetrasiloxane (OMCTS; [-(CH3)2SiO-]4 are formed during the manufacture of polydimethylsiloxane (PDMS) by the hydrolysis of dimethyldichlorosilane (1). Most of the OMCTS is used to make various siloxane polymers, especially the gums and rubbers (2). Substantial amounts are also used in a variety of personal care products. For example, cyclomethicone, a mixture of cyclic siloxanes with a large proportion of OMCTS, is typically used at levels of 5% in stick formulation antiperspirants (3). Unlike the more common, high molecular weight PDMS silicones, OMCTS has a molecular mass of only 296 Da and a molecular cross-sectional size of 1.08 × 1.03 nm (4). OMCTS is also volatile and sparingly soluble in water. Using the equation and constants given by Flaningam (5), the * Address correspondence to this author. † Present address 4209 Blair, Hudsonville, MI 49426-9343. ‡ Present address Environ Corporation, Arlington, VA 22203.
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 30, NO. 6, 1996
vapor pressure for OMCTS was estimated to be 0.681 mmHg at 20 °C. The measured aqueous solubility of OMCTS is approximately 0.074 mg/L (0.00025 g mol m-3) in freshwater (6). Like most PDMS fluids, OMCTS is lighter than water, with a density of 0.9558 g/cm3 at 20 °C. Hence, its molar volume (296/0.9558 ) 309 cm3/mol) is surprisingly close to the 300 cm3/mol acute toxicity threshold recognized for nonpolar organic chemicals (7). Consistent with its octanol-water partition coefficient (log Kow) of 4.45-5.1 (8, 9), a bioconcentration factor (BCF) of up to 12 400 has been measured for OMCTS using fathead minnows (10, 11). Because of the relative magnitude of the Kow and BCF, the presence of OMCTS in water bodies may represent a potential hazard to aquatic organisms unless it readily degrades or is removed from aqueous environments by some other mechanism (e.g., volatilization). The Henry’s law constant (H) and volatilization/reaeration rate ratio (kvc/kvo) are useful parameters for determining the distribution and fate of a compound in the environment. Chemical substances with a small H and little volatility are likely to remain in aquatic environments, while those with a large H and high volatility are likely to partition to the atmosphere. The “dimensioned” Henry’s law constant (H′) can be estimated with the solute vapor pressure (Pv) and aqueous solubility (S) using the following equation:
H′ ) Pv/S where Pv is given in atmospheres and S is given in g mol m-3. At 20 °C, the undimensioned Henry’s law constant (Hc) ) H′ (atm m3 mol-1) × 41.5. For OMCTS, Hc was estimated to be 148 using the data for Pv and S given above. However, this value must be considered as only a rough estimate of Hc since Pv was not actually measured and it is difficult to determine S in the parts per billion range, accurately. Furthermore, use of this equation assumes ideal behavior of the solute. This is a particular weakness for OMCTS, which has a relatively high vapor pressure and a very low aqueous solubility, such that the method falsely assumes that the solubility limit will not be exceeded at the vapor pressure of the compound. In order to more accurately predict the fate of OMCTS in the aquatic environment, several studies were undertaken to determine the actual value for Hc and the influence of several environmental variables (equilibration time, agitation, temperature, cosolutes) on it. Experiments were carried out using a combination of the methods of Munz and Roberts (12) and Gossett (13) where both gas- and liquid-phase concentrations of OMCTS were measured directly. In order to confirm the high volatility for OMCTS, the volatilization of OMCTS from deoxygenated water was studied directly using the basic procedures of Smith et al. (14). The volatilization rate constant (kvc) and volatilization/re-aeration ratio (kvc/kvo) for OMCTS were determined, and these data were then used to estimate the aquatic halflife for OMCTS based on the type of water body (i.e., lake, river) and oxygen re-aeration rate.
Experimental Section Experiments to determine the Hc for OMCTS were conducted under both static (preliminary study and study I)
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1996 American Chemical Society
and dynamic conditions (study II). Hc measurements should be reproducible if the measurements are taken under equilibrium conditions. Equilibrium had previously been attained within 48 h under static conditions when this apparatus was used for a large variety of more water-soluble, volatile halocarbons. Therefore, measurements in a preliminary study (n ) 9) were made after 48 h of equilibration using static conditions. The Hc obtained from the preliminary study was substantially less than that estimated from the aqueous solubility and vapor pressure data. Therefore, the kinetics of equilibration in the quiescent test system were evaluated by recording the gas and liquid concentrations after contact periods of 24, 48, 72, and 96 h. The Hc values obtained in the static study increased with time, suggesting that the quiescent systems did not attain equilibrium even after 96 h. Consequently, a second study, incorporating a modest amount of agitation to help eliminate the diffusion-limited conditions encountered with quiescent storage, was undertaken to increase the likelihood of achieving equilibrium during the contact period. Equilibrium was obtained in about 48 h when test chambers were agitated (rotated). Therefore, all subsequent experiments used “dynamic” conditions and an equilibration period of 48 h. These experiments assessed the effects of temperature, cosolutes, and the initial OMCTS concentration on Hc. Apparatus. Hc was determined using 100-mL gas-tight borosilicate glass syringes with Teflon-faced plungers and Kel-F rotary valves as test vessels; graduations were readable to 1 mL. The syringe barrels and plungers were triple rinsed with methanol to remove residues, acid-washed with 6 N HCl, distilled water rinsed, “conditioned” with a dilute (5 ppb) aqueous solution of OMCTS to minimize surface activity toward the analyte, and finally rerinsed with methanol to remove any residual OMCTS. This pretreatment was found to increase the recovery of OMCTS and to improve the precision of the experiments. Two sets of experiments were conducted to measure Hc. Initially, the syringes were held in a water bath at a constant temperature under static conditions. The second set of experiments was conducted with the syringes attached to the 60 cm (24 in.) diameter circular rack of a Glas-Col Model RD350 variable speed laboratory rotator with the plane of rotation set ∼10° from vertical. The rotators were housed in light-free, temperature-controlled environmental chambers and continuously rotated at 10 rpm in order to create dynamic conditions to overcome the diffusionlimited conditions encountered initially. Volatilization of OMCTS from water was determined using a modified version of the apparatus described by Smith et al. (14). Initial studies using the apparatus of Smith et al. indicated leaks at the joint of the dissolved oxygen (DO) probe and its holder in the system and apparent sorptive losses of OMCTS. Consequently, the circulating loop and pump were eliminated, and a YSI Model 5739 polarographic DO probe was immersed directly in the test solution. A ring stand and clamp were used to mount the probe against the wall of a 2-L borosilicate glass beaker so that it was out of the vortex created by the stirrer blade at high stirring rates. The YSI Model 5739 DO probe was selected because it automatically compensates for changes in temperature and pressure allowing accurate measurement of DO over a range of stirring speeds. Subsequent studies using this simplified apparatus indicated satisfactory results, even at low re-aeration rates.
Before conducting all experiments, the 2-L beaker was acid-washed with 6 N HCl, distilled water rinsed, conditioned with a dilute (5 ppb) aqueous solution of OMCTS to minimize surface activity toward the analyte, and finally rinsed with methanol to remove any residual OMCTS. Stock Solutions. A 100 ppm stock solution of OMCTS was prepared by diluting a precisely weighed amount of the neat material into chromatographic grade methanol. This stock solution was used throughout the first study to spike the test solutions. A similar 10 000 ppm stock solution was prepared for the second study. Intermediate stock solutions of 5, 25, 125, and 500 ppm were prepared by diluting the 10 000 ppm stock solution with the chromatographic grade methanol. ASTM Type IV reagent-grade water (
