Environ. Sci. Technol. 1999, 33, 4034-4039
Fate of Cyclic Methylsiloxanes in Soils. 2. Rates of Degradation and Volatilization SHIHE XU* AND GRISH CHANDRA Health and Environmental Sciences, Dow Corning Corporation, Midland, Michigan 48686-0994
Cyclic volatile methylsiloxane (cVMS) compounds are volatile, low-viscosity silicone fluids used as precursors in the synthesis of high molecular weight PDMS and as ingredients in certain personal care products. This study investigates cVMS degradation and evaporation rates in soils as influenced by molecular size, soil type, and moisture level. A temperate Michigan soil and a highly weathered Hawaiian soil were incubated with ∼40 µg of 14C-labeled cVMS/g of soil at ∼22 °C; samples were kept at 32%, 50%, 92%, and 100% relative humidity (RH) both in open and closed tubes. At each designated incubation time (from 0 to 21 days), the cVMS-containing soils were extracted, and the extracts were analyzed by liquid scintillation counting (LSC) and reverse-phase high-performance liquid chromatography (RP-HPLC). The results showed that cVMS degradation was more significant than loss by volatilization in soil with low moisture levels. Degradation reactions followed pseudo-first-order kinetics. The half-life of cVMS fluids in air-dried soils ranged from 50 min to 5 days, depending on soil type and cVMS molecular sizes. At high humidity (particularly at 100% RH), the degradation slowed, while volatilization was accelerated and became a predominant process in regulating the cVMS removal from soil. At any given moisture level, the degradation rates of cVMS were much greater in highly weathered soils (e.g., Oxisols) than in temperate soils, and the differences were more profound for small cVMS (e.g., D4). These findings demonstrate that cVMS fluids are unlikely to persist in any soils within the wide range of moisture conditions tested.
Introduction This study is part of a long-term project investigating the environmental fate of cyclic volatile methylsiloxane (cVMS) fluids in soils. There were two objectives for this work: first, the degradation kinetics were determined in soil for octamethylcyclotetrasiloxane {[(CH3)2SiO]4, referred to as D4}, decamethylcyclopentasiloxane {[(CH3)2SiO]5, known as D5}, and dodecamethylcyclohexasiloxane {[(CH3)2SiO]6 or D6}. Testing was then conducted in closed tubes to estimate the effects of soil type, moisture level, and molecular weight on cVMS degradation rates. Second, the degradation and evaporation rates of cVMS were determined in open tubes to evaluate the importance of volatilization as a competing process in cVMS removal from soil. Cyclic volatile methylsiloxane compounds are lowviscosity silicone fluids used as site-limited precursors in the production of high molecular weight PDMS (poly(dimeth* Corresponding author telephone: (517)496-5961; fax: (517)4965956; e-mail:
[email protected]. 4034
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ylsiloxane)) and as ingredients in some personal care products (1, 2). They can enter terrestrial environments through the use of treated sludge as fertilizer (3), by accidental spill, or via landfill disposal of cVMS-containing product residues. They may also be generated in soil as transient intermediates in the natural process of PDMS degradation (4, 5). The fate of cVMS in soil is determined by several environmental processes, including sorption, degradation, and volatilization. Cyclic volatile methylsiloxane compounds have very low water solubility (99%. Additional information about these compounds can be found elsewhere (8). Solvents such as tetrahydrofuran (THF), pentane, acetonitrile (ACN), and hexane (all HPLC grade) were obtained from Fisher Scientific (Pittsburgh, PA) and used without further purification. Soil Spiking and Incubation. Closed Tubes. To determine the degradation kinetics of cVMS as influenced by soil type, moisture level, and the compounds’ molecular weights, the air-dried Londo and Wahiawa soils were weighed into 30mL Teflon tubes. Each tube contained 5 g of air-dried soil. The tubes were left open and placed into three desiccators that had humidity controlled by a saturated CaCl2 solution (32% RH), K2HPO4 solution (92%RH), or Milli-Q water (100% RH) in the lower compartments of the desiccators. After preequilibration for 7 days, each tube was spiked with 0.25 mL of 14C-labeled cVMS (D4, D5, and D6) pentane solution and closed with a Teflon cap. After 2 min, each tube was hooked to a moisture control apparatus (9), and the cVMSspiked soil was flushed with humidity-adjusted air to evaporate the pentane. After 90-120 s of flushing, each tube was recapped and incubated at room temperature (22 ( 2 °C) for various times, ranging from 0 to 21 days. Open Tubes. To determine the importance of volatilization as a competing process in removal of cVMS from soil, three sets of soil samples were preconditioned at 32% and 100% RH in desiccators. They were spiked with D4 (at 32% and 100% RH) and D5 (at 100% RH) in the same way as the closed tubes but left open in the desiccators (container i.d. of 250 mm) during incubation. A tube with 5 g of activated carbon was also placed in each desiccator to adsorb cVMS evaporated to the headspace. (These carbon tubes were discarded after the sample incubation was completed.) In addition, soil samples were also preconditioned, spiked, and incubated in the open laboratory air (∼50% RH). Finally, air-dried soil samples that had been spiked with D4 were incubated in open tubes in the laboratory for 1 day and then rewetted with 3 mL of 0.01 M CaCl2 solution. After the rewetted soil tubes were left in a hood for specific times (ranging from 1.5 h to 14 days), the soil samples were extracted and analyzed as described. Soil Extraction. At the end of each incubation time, two tubes of each soil were extracted sequentially as follows: three times with hexane, then twice with 0.01 M CaCl2 solution, and finally once with 0.1 M HCl solution. Hexane Extraction. For each hexane extraction, 20 mL of hexane was added to each tube. The tubes were shaken for 1 h and then centrifuged (Beckman, GS-6) at 3000 rpm (RCF ) 2960g) for at least 5 min. The hexane extract for each sample
was analyzed by LSC. This hexane extraction procedure was repeated twice more for each sample. The organosilicon in all hexane extracts combined is referred to as the hexanesextractable organosilicon fraction. CaCl2 Extraction. A total of 20 mL of 0.01 M CaCl2 solution was added to each tube after the third hexane extraction. The tubes were shaken for about 3 h and then centrifuged at 3000 rpm for 25 min. The supernatant from each tube was separated into two phases. The upper hexane residue from each tube was transferred into a glass vial and analyzed as part of the hexane extract. The lower aqueous phase from each sample was transferred into five glass vials for determination of 14C radioactivity. The CaCl2 extraction was repeated once more with 20 mL of 0.01 M CaCl2 solution. After the second CaCl2 extraction, the tubes with soil residue were weighed to determine the CaCl2 extract residue remaining, which was used to correct the 14C radioactivity carried into the next extraction step. The organosilicon in all CaCl2 extracts is hereafter referred to as the water-extractable organosilicon fraction. HCl Extraction. Approximately 20 mL of 0.1 M HCl was added to each tube. The tubes were shaken overnight before they were centrifuged at 3000 rpm for 25 min. The supernatant from each tube was transferred to five glass vials for determination of 14C radioactivity, and each tube was weighed to determine the amount of HCl extract remaining, which was used to correct the remaining aqueous 14C radioactivity in the soil residue. The organosilicon in this fraction is hereafter referred to as the HCl-extractable organosilicon fraction. Soil Combustion. Each tube containing the soil residue after HCl extraction was combusted to determine the nonextractable organosilicon using a biological oxidizer (OX500, Harvey Instrument Corporation). Combustion time was 4 min each. About 14 mL of alkaline cocktail for each sample (14C-Cocktail, R. J. Harvey Instrument Corporation) was used to absorb the 14CO2 converted from any residual 14C-labeled organosilicon species by combustion. The organosilicon recovered by combustion is hereafter referred to as the nonextractable organosilicon fraction after correction of the 14C carried over by the residual HCl extract. Reverse-Phase HPLC Analysis. To determine the speciation of organosilicon obtained in each step of the above sequential extraction procedure, 1 g of Wahiawa soil (airdried) was spiked with 0.25 mL of 14C-labeled D4 solution, incubated for 45 min at 32% RH, and then extracted sequentially as before. Each fraction of the sequential procedure was analyzed by RP-HPLC equipped with a flow scintillation analyzer, following the technique outlined by Xu (8). In addition, a replicate sample was amended with 10 mL of saturated MgSO4 solution and then extracted by 15 mL of THF. (This procedure is hereafter referred to as THF/ H2O extraction.) The THF extract so obtained should contain most organosilicon species, including residual parent cVMS, degradation intermediates, and the final product formed (8). Therefore, the speciation of the THF extract can be used as a basis to examine the extraction selectivity of various solvents in the preceding sequential extraction procedure.
Results and Discussion Organosilicon Species Recovered by the Sequential Extraction Procedure. As reported previously, cVMS hydrolyzed to form oligomeric siloxane diols {HO[Si(CH3)2O]xH, x ) 2-6}, i.e., hexamer, pentamer, tetramer, trimer, and dimer siloxane diols and DMSD (8). Obviously, it is necessary to know which species was extracted in each step of the sequential procedure before the results can be understood. Ideally, the sequential extraction procedure should have quantitative recovery and sufficient selectivity to separate the degradation products from the parent cVMS to simplify VOL. 33, NO. 22, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Comparison of organosilicon species in various extracts of the air-dried Wahiawa soil samples after they were spiked with 14 C-labeled D4 and incubated for 45 min at 22 °C and 32% RH in closed tubes. The THF extract (a) is a nonselective extract. Hexane (b), 0.01 M CaCl2 (c), and 0.1 M HCl (d) solutions were selective extracts from the sequential extraction procedure. analysis. This selectivity requirement has been met as demonstrated by HPLC data (Figure 1). Although various siloxane diols and silane diols coexisted with D4 in soil as indicated by the HPLC chromatogram of the THF/H2O extract (Figure 1a), only D4 was found in the hexane extract of soil incubated at 32% RH (Figure 1b). Small amounts of D3, D5, and D6 were also found in the hexane extract from soil incubated at a higher humidity (e.g., 100% RH). Even in such a situation, no polar degradation product was found in the hexane extract (data not shown). This extraction selectivity was partially due to the huge difference in hydrophobicity between D4 and its degradation products. In a separate experiment, 14C-labeled DMSD (the final degradation product of cVMS in soil (8)) samples in 5 mL of water were shaken with 5 mL of hexane for 30 min and for 12 h. The ratio of DMSD in hexane to that in water was ∼2-3 × 10-4, suggesting the highly preferential partitioning of hydrophobic cVMS into hexane and the hydrophilic degradation product (DMSD) into water. When similar HPLC analysis was applied to the dilute CaCl2 extract of the soil (the second solvent in the sequential extraction scheme), two peaks were identified in the chromatogram (Figure 1c). They corresponded to DMSD and dimer diol. The DMSD peak accounted for 90% or more of total 14C in this extract (Figure 1). The complete absence of other diols (e.g., trimer and tetramer diols in Figure 1a) suggested that the dilute CaCl2 solution can extract mainly DMSD and some small amount of dimer diol. For the HCl-extractable fraction, only DMSD can be found in the HPLC profile (Figure 1d), suggesting that the orga4036
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nosilicon species in this fraction can be readily converted to a monomer diol in dilute acid. The nature of the organosilicon species in this fraction cannot be directly determined. They could be any diols with low water solubility or diols bound onto clay surfaces. According to Figure 1, siloxane diols larger than dimer diol were the only organosilicon species that were not extractable by hexane or dilute CaCl2 solution but were extractable by THF in the presence of H2O. Obviously, the HCl-extractable fraction at the early stage must include those large diols. We also noticed that as degradation proceeded, the HClextractable fraction increased, even when no larger diol was found in the THF/H2O extract. This implied that large oligomeric diols cannot account for all of the organosilicon species in the HCl-extractable fraction. The soil-bound DMSD should account for most of the HCl-extractable organosilicon at the late stage however. In addition, the sequential extraction procedure resulted in high recovery of the organosilicon compounds. In closed tubes, the total recovery of 14C originally added as D4 ranged from 94.5 to 101.2%, with an average of 98.7% after D4 was incubated for 0.25-7 days in soil at 32-100% RH. The quantitative recovery and good separation of parental compounds from the degradation products demonstrated that the sequential extraction method is suitable for cVMS in soil. D4 Degradation Kinetics As Influenced by Soil Type and Moisture. With the knowledge that hexane-extractable 14C represents the amount of D4 remaining, we can examine the degradation kinetics of cVMS in soil. It is evident that the amount of D4 remaining at any given time is a function of humidity and soil type (Figure 2). In Londo soil, most of the D4 was intact within 21 days at 100% RH (Figure 2). At
