Fate of Cyclic Methylsiloxanes inSoils. 2. Rates of Degradation andVolatilizationS H I H E X U * A N D G R I S H C H A N D R A

Health and Environmental Sciences, Dow CorningCorporation, Midland, Michigan 48686-0994

Cyclic volatile methylsiloxane (cVMS) compounds arevolatile, low-viscosity silicone fluids used as precursors inthe synthesis of high molecular weight PDMS and asingredients in certain personal care products. This studyinvestigates cVMS degradation and evaporation rates in soilsas influenced by molecular size, soil type, and moisturelevel. A temperate Michigan soil and a highly weatheredHawaiian soil were incubated with ∼40 µg of 14C-labeledcVMS/g of soil at ∼22 °C; samples were kept at 32%,50%, 92%, and 100% relative humidity (RH) both in openand closed tubes. At each designated incubation time (from0 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 chro-matography (RP-HPLC). The results showed that cVMSdegradation was more significant than loss by volatilizationin soil with low moisture levels. Degradation reactionsfollowed pseudo-first-order kinetics. The half-life of cVMSfluids in air-dried soils ranged from 50 min to 5 days,depending on soil type and cVMS molecular sizes. At highhumidity (particularly at 100% RH), the degradationslowed, while volatilization was accelerated and becamea predominant process in regulating the cVMS removal fromsoil. At any given moisture level, the degradation ratesof cVMS were much greater in highly weathered soils (e.g.,Oxisols) than in temperate soils, and the differenceswere more profound for small cVMS (e.g., D4). These findingsdemonstrate that cVMS fluids are unlikely to persist inany soils within the wide range of moisture conditions tested.

IntroductionThis study is part of a long-term project investigating theenvironmental 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 octa-methylcyclotetrasiloxane {[(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 theeffects of soil type, moisture level, and molecular weight oncVMS degradation rates. Second, the degradation andevaporation rates of cVMS were determined in open tubesto evaluate the importance of volatilization as a competingprocess in cVMS removal from soil.

Cyclic volatile methylsiloxane compounds are low-viscosity silicone fluids used as site-limited precursors in theproduction of high molecular weight PDMS (poly(dimeth-

ylsiloxane)) and as ingredients in some personal care products(1, 2). They can enter terrestrial environments through theuse of treated sludge as fertilizer (3), by accidental spill, orvia landfill disposal of cVMS-containing product residues.They may also be generated in soil as transient intermediatesin the natural process of PDMS degradation (4, 5). The fateof cVMS in soil is determined by several environmentalprocesses, including sorption, degradation, and volatilization.

Cyclic volatile methylsiloxane compounds have very lowwater solubility (<60 ppb) (6), and partitioning coefficients(Kow) of 104.45-105.86 have been estimated for D4, D5, and D6

in octanol-water systems (7). These Kow values correspondto estimated partitioning coefficients in soil, with (Koc) around104.17-105.08 (7). Although there has been no direct study onsorption of these compounds in soil, they are expected tomainly partition into the solid phase based on their high Koc

values.The degradation of cVMS in soil is presumably abiotic,

based on several lines of evidence. First, significant degrada-tion of cVMS was observed in air-dried soil (e.g., at 32% RH,equivalent to a water potential of -1500 bar) (8). The drynessof soil under this condition severely limits the biologicalactivity but promotes abiotic reactions such as surface-acid-catalyzed hydrolysis of poly(dimethylsiloxane) (PDMS), apolymer whose skeletal structure is based on siloxane bondssuch as those present in cVMS (4, 9, 10).

In addition, no significant amount of 14CO2 is evolvedfrom 14C-labeled D4 in a water/sediment system with activemicroflora (11), implying that cVMS cannot be mineralizeddirectly by microorganisms. Finally, the predominant deg-radation intermediates for cVMS in air-dried soil are initiallyoligomeric siloxane diols with the same number of dimeth-ylsiloxane units as the parent cVMS (8), implying surface-catalyzed ring-opening hydrolysis as the initial degradationstep (8). These linear oligomers are subsequently hydrolyzedto smaller siloxane diols and ultimately to a monomer:dimethylsilanediol or DMSD (8). So far, no demethylationhas been observed in soil for any cVMS compound (8).

Several important factors have been found to determinethe degradation rates of PDMS in soil. They include soilmoisture (10), clay type, and clay content (4, 9). Since cVMScompounds have the same Si-O-Si bonds as linear PDMS,moisture level and soil mineralogy should have the sameeffects on cVMS degradation rates. In addition, the size ofthe specific cVMS molecule should determine the rate ofdiffusion to the surface catalytic sites and thus may alsoinfluence degradation rate.

The volatilization of cVMS has been studied in aqueoussystems. For example, the Henry’s law constant for D4 hasbeen determined to be ∼3.4 at 20 °C in freshwater, and it canbe increased by inorganic and organic cosolutes (12).Although the volatilization of cVMS from soil has not beenreported, it is expected that sorption of cVMS by soil particlesplays a significant role in retarding the loss of cVMS by thismechanism, based on knowledge of other volatile organiccompounds (13). Finally, degradation and volatilization aretwo competing processes in the removal of cVMS from soil.Their relative importance in determining the environmentalfate of cVMS will be shown in this study to depend on thesoil conditions that influence those processes.

Materials and MethodsSoils and Chemicals. Two soils were used in this study:Londo soil from Bay County, MI, and Wahiawa soil fromKunia area, Oahu, HI. The choices were based on the degreeof soil weathering. Londo is a coarse-textured Alfisol with an

* Corresponding author telephone: (517)496-5961; fax: (517)496-5956; e-mail: shihe.xu@dowcorning.com.

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intermediate organic matter and clay content (Table 1). Thepredominant clay minerals in this soil are illite and chlorite,which is typical of the temperate soils. In contrast, Wahiawais a clay Oxisol (Table 1). The predominant clay minerals arekaolinite, gibbsite, and goethite (14), which is representativeof highly weathered soils.

14C-labeled D4, D5, and D6 were obtained from WizardLaboratories (Davis, CA). The chemical and radiochemicalpurities of these compounds were >99%. Additional infor-mation about these compounds can be found elsewhere (8).Solvents such as tetrahydrofuran (THF), pentane, acetonitrile(ACN), and hexane (all HPLC grade) were obtained fromFisher Scientific (Pittsburgh, PA) and used without furtherpurification.

Soil Spiking and Incubation. Closed Tubes. To determinethe degradation kinetics of cVMS as influenced by soil type,moisture level, and the compounds’ molecular weights, theair-dried Londo and Wahiawa soils were weighed into 30-mL Teflon tubes. Each tube contained 5 g of air-dried soil.The tubes were left open and placed into three desiccatorsthat 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. Afterpreequilibration for 7 days, each tube was spiked with 0.25mL of 14C-labeled cVMS (D4, D5, and D6) pentane solutionand closed with a Teflon cap. After 2 min, each tube washooked to a moisture control apparatus (9), and the cVMS-spiked soil was flushed with humidity-adjusted air toevaporate the pentane. After 90-120 s of flushing, each tubewas 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 volatilizationas a competing process in removal of cVMS from soil, threesets of soil samples were preconditioned at 32% and 100%RH in desiccators. They were spiked with D4 (at 32% and100% RH) and D5 (at 100% RH) in the same way as the closedtubes but left open in the desiccators (container i.d. of 250mm) during incubation. A tube with 5 g of activated carbonwas also placed in each desiccator to adsorb cVMS evaporatedto the headspace. (These carbon tubes were discarded afterthe sample incubation was completed.) In addition, soilsamples were also preconditioned, spiked, and incubated inthe open laboratory air (∼50% RH). Finally, air-dried soilsamples that had been spiked with D4 were incubated inopen tubes in the laboratory for 1 day and then rewettedwith 3 mL of 0.01 M CaCl2 solution. After the rewetted soiltubes were left in a hood for specific times (ranging from 1.5h to 14 days), the soil samples were extracted and analyzedas described.

Soil Extraction. At the end of each incubation time, twotubes of each soil were extracted sequentially as follows: threetimes 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 ofhexane was added to each tube. The tubes were shaken for1 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 wasrepeated twice more for each sample. The organosilicon inall hexane extracts combined is referred to as the hexanes-extractable organosilicon fraction.

CaCl2 Extraction. A total of 20 mL of 0.01 M CaCl2 solutionwas added to each tube after the third hexane extraction.The tubes were shaken for about 3 h and then centrifugedat 3000 rpm for 25 min. The supernatant from each tube wasseparated into two phases. The upper hexane residue fromeach tube was transferred into a glass vial and analyzed aspart of the hexane extract. The lower aqueous phase fromeach sample was transferred into five glass vials for deter-mination of 14C radioactivity. The CaCl2 extraction wasrepeated once more with 20 mL of 0.01 M CaCl2 solution.After the second CaCl2 extraction, the tubes with soil residuewere weighed to determine the CaCl2 extract residueremaining, which was used to correct the 14C radioactivitycarried into the next extraction step. The organosilicon in allCaCl2 extracts is hereafter referred to as the water-extractableorganosilicon fraction.

HCl Extraction. Approximately 20 mL of 0.1 M HCl wasadded to each tube. The tubes were shaken overnight beforethey were centrifuged at 3000 rpm for 25 min. The supernatantfrom each tube was transferred to five glass vials fordetermination of 14C radioactivity, and each tube was weighedto determine the amount of HCl extract remaining, whichwas used to correct the remaining aqueous 14C radioactivityin the soil residue. The organosilicon in this fraction ishereafter referred to as the HCl-extractable organosiliconfraction.

Soil Combustion. Each tube containing the soil residueafter HCl extraction was combusted to determine thenonextractable organosilicon using a biological oxidizer(OX500, Harvey Instrument Corporation). Combustion timewas 4 min each. About 14 mL of alkaline cocktail for eachsample (14C-Cocktail, R. J. Harvey Instrument Corporation)was used to absorb the 14CO2 converted from any residual14C-labeled organosilicon species by combustion. The or-ganosilicon recovered by combustion is hereafter referredto as the nonextractable organosilicon fraction after cor-rection of the 14C carried over by the residual HCl extract.

Reverse-Phase HPLC Analysis. To determine the spe-ciation of organosilicon obtained in each step of the abovesequential extraction procedure, 1 g of Wahiawa soil (air-dried) was spiked with 0.25 mL of 14C-labeled D4 solution,incubated for 45 min at 32% RH, and then extractedsequentially as before. Each fraction of the sequentialprocedure was analyzed by RP-HPLC equipped with a flowscintillation analyzer, following the technique outlined byXu (8). In addition, a replicate sample was amended with 10mL of saturated MgSO4 solution and then extracted by 15mL of THF. (This procedure is hereafter referred to as THF/H2O extraction.) The THF extract so obtained should containmost organosilicon species, including residual parent cVMS,degradation intermediates, and the final product formed (8).Therefore, the speciation of the THF extract can be used asa basis to examine the extraction selectivity of various solventsin the preceding sequential extraction procedure.

Results and DiscussionOrganosilicon Species Recovered by the Sequential Extrac-tion Procedure. As reported previously, cVMS hydrolyzed toform oligomeric siloxane diols {HO[Si(CH3)2O]xH, x ) 2-6},i.e., hexamer, pentamer, tetramer, trimer, and dimer siloxanediols and DMSD (8). Obviously, it is necessary to know whichspecies was extracted in each step of the sequential procedurebefore the results can be understood.

Ideally, the sequential extraction procedure should havequantitative recovery and sufficient selectivity to separatethe degradation products from the parent cVMS to simplify

TABLE 1. Characterization of Soils

soils Wahiawa Londo (16)origin Ohua, HI Bay County, MIclassification/great

groupeutrustoxes glossaqualfs

pH 4.9 7.6organic matter (%) 2.2 2.4sand (%) 21.2 50silt (%) 24.0 28clay (%) 54.8 22predominant clay

mineralskaolinite, gibbsite,

goethiteillite, chlorite

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analysis. This selectivity requirement has been met asdemonstrated by HPLC data (Figure 1). Although varioussiloxane diols and silane diols coexisted with D4 in soil asindicated by the HPLC chromatogram of the THF/H2O extract(Figure 1a), only D4 was found in the hexane extract of soilincubated at 32% RH (Figure 1b). Small amounts of D3, D5,and D6 were also found in the hexane extract from soilincubated at a higher humidity (e.g., 100% RH). Even in sucha situation, no polar degradation product was found in thehexane extract (data not shown).

This extraction selectivity was partially due to the hugedifference in hydrophobicity between D4 and its degradationproducts. In a separate experiment, 14C-labeled DMSD (thefinal degradation product of cVMS in soil (8)) samples in 5mL of water were shaken with 5 mL of hexane for 30 min andfor 12 h. The ratio of DMSD in hexane to that in water was∼2-3 × 10-4, suggesting the highly preferential partitioningof hydrophobic cVMS into hexane and the hydrophilicdegradation product (DMSD) into water.

When similar HPLC analysis was applied to the diluteCaCl2 extract of the soil (the second solvent in the sequentialextraction scheme), two peaks were identified in the chro-matogram (Figure 1c). They corresponded to DMSD anddimer diol. The DMSD peak accounted for 90% or more oftotal 14C in this extract (Figure 1). The complete absence ofother diols (e.g., trimer and tetramer diols in Figure 1a)suggested that the dilute CaCl2 solution can extract mainlyDMSD and some small amount of dimer diol.

For the HCl-extractable fraction, only DMSD can be foundin the HPLC profile (Figure 1d), suggesting that the orga-

nosilicon species in this fraction can be readily converted toa monomer diol in dilute acid. The nature of the organosiliconspecies in this fraction cannot be directly determined. Theycould be any diols with low water solubility or diols boundonto clay surfaces. According to Figure 1, siloxane diols largerthan dimer diol were the only organosilicon species that werenot extractable by hexane or dilute CaCl2 solution but wereextractable by THF in the presence of H2O. Obviously, theHCl-extractable fraction at the early stage must include thoselarge diols.

We also noticed that as degradation proceeded, the HCl-extractable fraction increased, even when no larger diol wasfound in the THF/H2O extract. This implied that largeoligomeric diols cannot account for all of the organosiliconspecies in the HCl-extractable fraction. The soil-bound DMSDshould account for most of the HCl-extractable organosiliconat the late stage however.

In addition, the sequential extraction procedure resultedin high recovery of the organosilicon compounds. In closedtubes, the total recovery of 14C originally added as D4 rangedfrom 94.5 to 101.2%, with an average of 98.7% after D4 wasincubated for 0.25-7 days in soil at 32-100% RH. Thequantitative recovery and good separation of parentalcompounds from the degradation products demonstratedthat the sequential extraction method is suitable for cVMSin soil.

D4 Degradation Kinetics As Influenced by Soil Type andMoisture. With the knowledge that hexane-extractable 14Crepresents the amount of D4 remaining, we can examine thedegradation kinetics of cVMS in soil. It is evident that theamount of D4 remaining at any given time is a function ofhumidity and soil type (Figure 2). In Londo soil, most of theD4 was intact within 21 days at 100% RH (Figure 2). At <100%RH, the amount of D4 remaining decreased significantly withincubation time (Figure 2a). In Wahiawa soil, the exponentialdecrease of D4 relative to incubation time was more rapidand significant even at 100% RH (Figure 2b). In both cases,the degradation rates increased with a decrease in humidity.

Similar moisture effects have been well demonstrated onthe hydrolysis of linear PDMS on clay minerals (4) and insoils (10). Under water-saturated conditions, PDMS degra-dation is generally very slow (10). As moisture decreases, thePDMS degradation rates increase (4, 10). This moisture effecthas been explained in terms of PDMS degradation mech-anisms (4). Briefly, hydrolysis of the Si-O-Si linkage inpolyorganosiloxanes is believed to be catalyzed by surfaceacidity of soil clays (4, 9). The increase in soil moisturedecreases the surface acidity (15) and, thus, the hydrolysisrates of polysiloxanes such as cVMS.

In addition to the influence of surface acidity on degra-dation rates, physical separation between the substrate (i.e.,D4) and the catalysts (i.e., soil clays) may also contribute tolower degradation rates at high humidity, possibly becausea significant portion of the D4 was actually vaporized to theheadspace at high moisture levels, which will be discussedlater (e.g., Figure 5).

Even though D4 tends to evaporate to the headspace athigh moisture levels, D4 is different from high molecularweight PDMS in its order of degradation kinetics. As shownin Figure 2 and Table 3, the logarithm of the D4 fractionremaining was linearly related to incubation time (t) untilthe fraction of D4 remaining was less than 5% in Londo and0.1% in Wahiawa soil. These linear relations implied that D4

degradation was a pseudo-first-order reaction with respectto D4 concentration in soil. The pseudo-first-order kineticswere entirely different from those of PDMS, which typicallydisplays pseudo-zero-order kinetics with respect to PDMSconcentration in both clays (4) and soils (10).

The effect of soil type on the degradation rate of D4 asdemonstrated in Figure 2 was due to a change in soil

FIGURE 1. Comparison of organosilicon species in various extractsof the air-dried Wahiawa soil samples after they were spiked with14C-labeled D4 and incubated for 45 min at 22 °C and 32% RH inclosed tubes. The THF extract (a) is a nonselective extract. Hexane(b), 0.01 M CaCl2 (c), and 0.1 M HCl (d) solutions were selectiveextracts from the sequential extraction procedure.

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mineralogy (4, 9). Wahiawa is a highly weathered soil with∼55% clay content (Table 1). Half of that clay is kaolinite,and ∼10% is gibbsite (14). Kaolinite and gibbsite are bothhighly effective catalysts for the hydrolysis of Si-O-Silinkages in poly(dimethylsiloxane) (4). On the other hand,Londo soil from Michigan is an Alfisol with less clay content(∼20%) (16). The dominant clay minerals in this soil are illiteand chlorite (Table 1), the former being one of the leasteffective catalysts known for hydrolysis of Si-O-Si linkages(4). The low clay content and less effective catalysts both

contribute to the much lower degradation rates of D4 in Londosoil than in Wahiawa samples.

Degradation Kinetics of Other cVMS. Two importantpoints were demonstrated by comparing the degradationrates of the cVMS (Figure 3) samples. First, as with D4, thedegradation reactions of D5 and D6 also followed pseudo-first-order kinetics (Figure 3b). Second, the degradation ratesamong the cVMS increased as the molecular weight de-creased: D6 , D5 < D4.

According to the composition of the intermediatesextracted at different incubation times, cVMS degradation isdescribed as a multistep hydrolysis process, initiated withthe ring-opening hydrolysis (8). The first-order kinetics forall these compounds are probably due to the relatively lowconcentration of these ring-opening intermediates as com-pared to that of the parental cVMS. The slower degradationrates for larger cVMS may be due to the lower surface mobilityof larger cVMS and its degradation products.

Distribution of Degradation Products. As mentioned,the HCl-extractable and nonextractable degradation productsrepresented the fraction of degradation products not ex-tractable by infiltrating water in natural soil. Figure 4 showedthat the ratio of this fraction to the water-extractable fraction(nonextractable/extractable ratio) depended on incubationtime, moisture level, and the parent compounds. Generallyspeaking, the initial nonextractable/extractable ratios (e.g.,at incubation time <1 day) varied substantially. For a givencVMS (e.g., D4), a high humidity corresponded to a greaterfraction of nonwater-extractable degradation products. At agiven humidity, the higher the molecular weight of cVMS,the greater the nonwater-extractable fraction. As incubationtime increased, the variation decreased, and the ratio passeda minimum to converge in an upward trend with incubationtime.

The variation of the nonextractable/extractable ratiounder different conditions was consistent with the multistep

FIGURE 2. D4 remaining in Londo and Wahiawa soils in closed tubes as function of soil moisture and incubation time (a) and the first-orderkinetics plots (b).

TABLE 2. Selected Properties of cVMS at 25 °C (Ref 7)

cVMS D4 D5 D6water solubility (mg L-1) 0.056 0.017 0.005vapor pressure (mmHg) 0.99 0.17 0.03log Kow 4.45 5.2 5.86log Koc (estimated) 4.17 4.6 5.08evaporate mass transfer

coefficient0.12-0.55 1.64

Henry’s law constant 3.4 5.5

TABLE 3. Pseudo-First-Order Degradation Rate Constants (kapp)and Half-Life (τ) for cVMS in Soils in Closed Tubes

soil cVMS RH (%) kappa r 2 (df)b τ (day)

Wahiawa D4 32 19.54 0.988 (5) 0.0492 8.69 0.965 (4) 0.08

100 0.77 0.998 (3) 0.89D5 32 8.23 0.966 (5) 0.08D6 32 0.51 0.974 (6) 1.38

Londo D4 32 0.20 0.999 (3) 3.5492 0.13 0.999 (3) 5.25

100 ∼0.001 0.165 (3)a The regression equation: log[Dn]/[Dn]o ) - kappt, where [Dn]o and

[Dn] were initial and final concentration, respectively, of cVMS (withnSi(CH3)2O units) at any given time t. b df, degree of freedom.

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degradation process. As discussed previously, the speciationand bonding of degradation products in soil are twounderlying factors determining the fraction of the degradationproducts extractable by water. The large variation in non-extractable/extractable ratio at short incubation time wascaused by the temporary accumulation of various largeintermediates (8) that are nonextractable by 0.01 M CaCl2

solution (Figure 1). The upward trend of the ratio in Figure4 at long incubation times (e.g., <3 days) may be due to theincreased fraction of DMSD bonding to soil clay particles (4)or diffusion of DMSD into the less accessible interior of thesoil aggregates.

It should be noted that the boundary between the water-and HCl-extractable fractions is rather arbitrary, since theHCl-extractable fraction can be slowly converted to the water-extractable fraction (8). However, such a division is mean-ingful in an environmental context. For example, the HCl-extractable fraction can be regarded as sorbed and non-leachable by infiltrating water in soil during a short rain event.

Volatilization as a Competing Process in cVMS Removalfrom Soil. The preceding discussion focused on cVMSdegradation in a closed system. In an open system, like naturalsoil, volatilization and degradation are both involved in thedissipation of cVMS in soil (Figure 5). The relative importanceof each process in cVMS loss from soil depends on soilmoisture level and the volatility of the cVMS compoundsinvolved (Figure 5).

For a soil at 50% RH, the degradation products couldaccount for up to 60% of 14C originally added as D4.Volatilization accounted for up to 40% of D4 loss based ontotal recovery of 14C (Figure 5a), suggesting that bothdegradation and volatilization of D4 were significant indissipation of D4 at this moisture level. When soil was evendrier, e.g., at ∼32% RH, volatilization was negligible (Figure5b). At this dryness, the rapid degradation was the predomi-nant process in the dissipation of D4 (Figure 2).

For soil in equilibrium with moisture-saturated air (i.e.,100% RH), degradation products accounted for less than 5%of the total 14C added over the entire incubation time (datanot shown), while more than 80% of the applied D4 wasevaporated from soil in the same period (Figure 5b). Clearly,volatilization of D4 was the predominant process at highmoisture levels.

The effect of moisture on the relative importance ofdegradation vs volatilization in dissipating D4 in soil can beunderstood in terms of moisture’s influence on degradationand adsorption of D4 by soil. As discussed previously,degradation rates of D4 decreased with raised soil moisture(Figure 2). Typically, the sorption of hydrophobic compoundsby soil is decreased with increases in soil moisture due to thehydration of soil minerals (17-20). Therefore, the negligiblevolatilization of D4 at low moisture levels was a result of highsorption and fast degradation of D4 in dry soil. Likewise, theincreased volatilization at high humidity was due to the slowdegradation and low sorption of D4 in moist soil.

It should be noted that the volatilization of cVMS at higherhumidity also depended on volatility (e.g., the vapor pressurein Table 2) of the specific cVMS compound. This point waswell supported by the lower volatilization rate for D5 relativeto that for D4 (Figure 5) under 100% RH.

Although volatilization of D4 from moist soil at 100% RHwas rapid, it was much slower from water-saturated soil(Figure 6). The decrease in volatilization rates due to watersaturation is directly related to the low water solubility of D4

and the high proportion expected to partition into soil organicmatter in wet soil. The low volatilization rate may be due tothe slow mass transfer of D4 from soil organic matter to theair/water interface where D4 evaporation took place.

Obviously, any process that increases the mass transferof cVMS to the interface, such as soil mixing or reduced watercontent, should increase the volatilization rates. This wasexactly what happened when soil samples were subjected toair-drying after day 6 of the study (point A in Figure 6). Assoil water content gradually dropped, the volatilizationbecame more significant (point B vs point A in Figure 6).

In summary, cVMS such as D4, D5, and D6 were readilydegradable in soils. In closed systems, the degradation

FIGURE 3. Comparison of the D4, D5, and D6 remaining in Wahiawasoil in closed tubes at 32% RH (a) and the first-order plot (b).

FIGURE 4. Extractability of degradation products of cVMS in soilsas a function of humidity and incubation time.

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followed first-order kinetics with D4 half-life varying from afew minutes to 4.5 days depending on soil type and moisturelevel. As with high molecular weight PDMS, when soilmoisture increased, degradation rates decreased. In highlyweathered soil, the half-life of D4 was less than 1 day evenat 100% RH, whereas a very small fraction of D4 degradedwhen incubated for 21 days in a less weathered, temperatesoil.

Volatilization is a complementary process to degradationfor loss of cVMS from soil in open systems. In air-dried soilsat low humidity where degradation was rapid, volatilizationwas insignificant. However, when degradation rates werelow at raised moisture levels, volatilization was the pre-dominant process responsible for cVMS removal from soil.The simultaneous action of both processes is the bestassurance that cVMS, regardless of its origin, will not persistin soil environments.

AcknowledgmentsWe are grateful to Drs. Robert G. Lehmann and Cecil Fryefor their comments on the draft of this manuscript.

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Received for review January 29, 1999. Revised manuscriptreceived August 26, 1999. Accepted September 1, 1999.

ES990099D

FIGURE 5. Percent of cVMS and their degradation products in Londo soil as a function of incubation time in open tubes in laboratoryhood (∼50% RH) (a) and the loss of cVMS due to volatilization as influenced by moisture (b).

FIGURE 6. Influence of water saturation and subsequent drying onvolatilization of D4 from soil. The dash line represents the evaporationof D4 from the same soil at 100% RH. The soil was maintained toa water saturation state by daily addition of water until day 6 (pointA). No water was added after day 6, and the soil became air-driedat day 13 (point B).

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