Environ. Sci. Technol. 1998, 32, 3162-3168
Hydrolysis of
Poly(dimethylsiloxanes) on Clay
Minerals As Influenced by
Exchangeable Cations and Moisture
SHIHE XU*
Health and Environmental Sciences, Dow Corning
Corporation, Midland, Michigan 48686-0994
Silicone polymers undergo clay-catalyzed hydrolytic
degradation in soil. This study investigated the effects of
moisture levels and exchangeable cations on degradation
of a poly(dimethylsiloxane) (PDMS) fluid on clay minerals.
Kaolinite, talc, and Arizona montmorillonite saturated
with Na+, Ca2+, or Al3+ were incubated with 14C-labeled
PDMS (∼2000 µg g-1) at 32% and 100% RH. The hydrolytic
products were extracted sequentially and analyzed with
HPLC/GPC, GC/MS, and LSC. It was found that PDMS
hydrolyzed predominantly through random scission of its
Si-O-Si backbone, regardless of clay type, exchangeable
cation, or humidity. The hydrolytic degradation had two
stages; both were zero-order reactions. The degradation
rates in the initial stage rose with an increase in the
polarizing power of the exchangeable cations (i.e., Al3+
. Ca2+ > Na+) and decreased humidity. Although high
humidity may result in the formation of some volatile cyclic
methylsiloxanes on an artificial catalyst, Al-saturated
montmorillonite, the ultimate degradation product was
otherwise water-soluble dimethylsilanediol. The conclusion
was that exchangeable cation type, moisture level, and
clay type can all influence the degradation rates and products
of silicone polymers.
Introduction
Poly(dimethylsiloxane) (PDMS) fluids are widely used in both
industrial and consumer products. They typically enter the
soil environment through land application of sewage sludge
containing PDMS. Although PDMS polymers are well-known
for stability under a wide range of environmental conditions,
they undergo hydrolytic degradation in soil to form siloxanols
and ultimately dimethylsilanediol or DMSD (1-4).
Soil is a complex system, with substantial variation in
both mineralogy and chemistry. Although the catalysis of
PDMS degradation by soil is widely accepted, the process is
not completely understood. In a previous study, we observed
that 12 different clay minerals commonly found in soils (such
as montmorillonite, nontronite, beidellite, illite, chlorite,
allophane, gibbsite, and goethite) all catalyze PDMS degradation at 32% relative humidity (RH) (5). This suggested
that soil catalysis of PDMS degradation is a general phenomenon.
To better understand and predict catalytic activity of
various soils, however, we need to know not only what soil
* To whom all correspondence should be addressed; telephone: (517) 496-5961; fax: (517) 496-6609; e-mail: USDCCMG5@
IBMMAIL.COM.
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 20, 1998
minerals are responsible but also how various factors may
influence PDMS degradation in soil. Xu et al. (5) found that
the effectiveness of Ca-saturated clay minerals in catalyzing
PDMS degradation varied substantially under the same water
activity (e.g., 32% RH). For example, Ca-saturated kaolinite,
the most effective catalyst, was ∼26 times as fast as
Ca-saturated Wyoming montmorillonite, one of the less
effective catalysts. This variation arises from a difference in
type of surface functional groups and specific surface area.
For Ca-saturated clay minerals, the more effective clay
minerals were crystalline, with large amounts of Al-OH
functional groups on their surfaces (5). As would be expected
for a heterogeneous reaction, an increase in the specific
surface area of minerals also increased the PDMS degradation
rates (5).
All of the aforementioned data were obtained with a single
type of exchangeable cation (Ca2+) at one moisture level:
32% RH (5). However, in any given soil, many types of
exchangeable cations coexist on clay surfaces (6). The
composition of these cations on clay influences hydrolysis
for many organic compounds in soil (7-10) and may also
affect PDMS hydrolysis. In addition, moisture has been
identified as the most important factor, one that dramatically
influences the PDMS degradation rates in soils (1, 2, 11).
Obviously, it is difficult to predict PDMS degradation in soil
without knowing exactly how exchangeable cations and
moisture levels influence the silicone hydrolysis process on
soil clay surfaces.
This study investigated the effects of exchangeable cations
and moisture levels on PDMS depolymerization mechanisms,
degradation rates, and hydrolysis products on clay minerals.
The results enable us to better understand the catalytic
activities of soil clays and provide a basis for predicting PDMS
degradation in various soils.
Materials and Methods
Freeze-dried Ca-saturated talc and kaolinite as well as Na-,
Ca- and Al-saturated Arizona montmorillonite (SAz-1) were
prepared according to the procedure described by Xu et al.
(5). Freeze-dried clay was weighed into 35-mL glass tubes
(0.1 g per tube) with Teflon-lined caps. The open tubes with
clay minerals were placed in the upper compartments of
two desiccators, one with saturated CaCl2 solution (32% RH)
in its lower compartment and the other with Milli-Q water
(100% RH).
After 5 days of equilibrium at 22 ( 2 °C, ∼100 µL of 14Clabeled PDMS spiking solution (∼2 mg g-1) was added to
each tube using a 250-µL syringe. The tubes were immediately closed with Teflon-lined caps. After 2 min, the
tubes were hooked to a moisture-controlled air flushing
apparatus (5) with four gas washing bottles connected in
series. The tubes were immediately flushed for 15 min, either
with air passed through the saturated CaCl2 solution (32%
RH) or Milli-Q water (100%RH) in the gas washing bottles,
to evaporate tetrahydrofuran (THF) introduced via the spiking
solution. The tubes were then closed with Teflon-lined caps
and incubated at 22 ( 2 °C for different time periods.
At the end of each incubation time, two tubes from each
mineral group were sacrificed for sequential extraction and
organosilicon analysis. Details of these procedures have been
described elsewhere (5). Briefly, the clay minerals with PDMS
and its degradation products were twice extracted with 0.01
M CaCl2 solution to remove any water-extractable organosilicon, mostly free silanols (5). In the first extraction, a 6-mL
CaCl2 solution was used for each tube, and all the extract was
S0013-936X(98)00338-1 CCC: $15.00
 1998 American Chemical Society
Published on Web 09/03/1998
FIGURE 1. Fraction of the THF-extractable polymeric residue
remaining as a function of time and exchangeable cations for Arizona
montmorillonite (SAz-1).
analyzed by liquid scintillation counting or LSC (Tri-Carb
2500 TR, Packard Instrument Co., Meriden, CT). In the
second extraction, a 20-mL CaCl2 solution was used for each
tube, and only three 2-mL fractions were analyzed by LSC.
The contact time for each extraction was 3 h.
After the second CaCl2 extraction, the clay minerals in
each tube were extracted 3 times using 20 mL of THF each
time. The contact time for each was 1 h. All of the THF
extracts were analyzed by LSC, except that a 0.9-mL aliquot
of the THF extract was taken from the first THF extraction
for each sample and analyzed by high performance size
exclusion chromatography or GPC (5).
The clay residue from each tube was extracted once with
20 mL of 0.1 M HCl to remove the bound silanols (5). For
each sample, three 2-mL aliquots of 0.1 M HCl extract were
analyzed by LSC. The residue after the extraction was
weighed and air-dried in a hood. All of the air-dried clay
from each sample was transferred into a quartz boat and
combusted in a biological oxidizer (R. J. Harvey Instrument
Corp., Hillsdale, NJ). The [14C]CO2 was trapped in an alkaline
scintillation cocktail (R. J. Harvey Instrument Corp.) and
analyzed by LSC.
Results
PDMS Degradation Rates. For SAz-1 saturated with three
different cations, the THF-extracted polymeric residue
decreased in two stages in a linear fashion over time (Figure
1), suggesting apparent zero-order degradation kinetics with
respect to the polymer concentration on clays. As indicated
by the slopes of the straight lines in the initial stages of Figure
1, exchangeable cations had a significant influence on PDMS
degradation. Thus, on Na+-saturated SAz-1 (Na-SAz-1), 0.62
mg (g of PDMS)-1 was converted to nonpolymeric silanols
over the first 10 days, corresponding to a very small rate
constant (0.74 µg m-2 day-1). On Ca2+-saturated SAz-1 (CaSAz-1), 0.68 mg (g of PDMS)-1 was converted to nonpolymeric
silanols over 7 days, corresponding to a greater rate constant
(1.1 µg m-2 day-1). The degradation of PDMS on Al3+saturated SAz-1 (Al-SAz-1) was much faster, requiring only
0.145 days to convert 1.65 mg (g of PDMS)-1 to nonpolymeric
silanols, corresponding to a rate constant of 91 µg m-2 day-1.
Similarly, humidity had substantial effects on the catalytic
activity of the clay minerals. For example, initial rates of
PDMS degradation in the presence of Ca-kaolinite and AlSAz-1 were between 3 and 11 mg g-1 day-1 at 32% RH (Figure
FIGURE 2. Fraction of the THF-extractable polymeric residue
remaining as a function of time and humidity for Al-Arizona
montmorillonite, Ca-kaolinite, and Ca-talc.
TABLE 1. Rate Constants of the Initial Phase (IP) of PDMS
Degradation in the Presence of Three Clay Minerals
at 100% RH
clay
minerals
length
of IP
(days)
PDMS
degraded
in IP
(mg g-1)
rate constant
(mg g-1 day-1 )
% compared
with rates
at 32% RHb
Al-SAz-1
Ca-kaolinite
talc
8.3
30.0
3.0
1.66
0.275
0.21
0.200
0.009
0.070
1.76
0.35
50.00
a % compared with rate at 32% RH ) rate constant of IP at 100% RH
× 100/rate constant of IP at 32% RH.
2). Increasing humidity to 100% RH dramatically reduced
the initial degradation rates, regardless of clay type (Table
1). The decrease was especially significant for kaolinite and
Al-SAz-1; their rate constants at 100% RH were only 0.35%
and 1.76% of the values at 32% RH (Table 1).
GPC Chromatograms of the THF-Extractable Polymeric
Residue. Exchangeable cations and moistures influence not
only the quantity of THF-extractable PDMS residue as
discussed but also the molecular weight distribution of the
residue at any given incubation time (Figure 3). As the figure
shows, regardless of exchangeable cations and moisture
levels, GPC peaks of the polymeric residue shifted with
incubation time toward lower molecular weights (i.e., longer
retention time). The common influence of the exchangeable
cations and moistures was reflected on the rates of GPC peak
shifting. For example, the peak molecular size was about
5-10 Si-O [i.e., -Si(Me2)O-, where Me ) methyl group]
units after PDMS was incubated with Al-SAz-1 for just 0.5 h
at 32% RH. However, it took 3-7 days for Ca-SAz-1 to reduce
the PDMS residue to the same size range at the same humidity
(Figure 3a), and Na-SAz-1 required 21-30 days.
Increased moisture had similar effects on GPC profiles
(Figure 3b). For Al-SAz-1, it took 3 days to reduce the
molecular size of polymeric residue to about 7 Si-O units
at 100% RH (Figure 3b), compared to 0.5 h at 32% RH (Figure
3a). A similar reduction in GPC peak shifting rates was
evident for kaolinite and talc when the data for 100% RH
(Figure 3b) were compared with those for 32% RH (5).
Extractable and Nonextractable Degradation Products
and Overall 14C Mass Balance. Nonextractable organosilicon
accounted for a very small fraction (<5%) of the degradation
products, regardless of humidity (Table 2) and exchangeable
VOL. 32, NO. 20, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. GPC profiles of the THF-extractable polymeric residue after PDMS was incubated for different times with Arizona montmorillonite
saturated with three different cations at 32% RH (a) and with Al-saturated Arizona montmorillonite (Al-SAz-1) and Ca-saturated kaolinite
and talc at 100% RH (b).
cations (data not shown). However, an increase in humidity
reduced the HCl-extractable fraction (i.e., the bound silanols)
for talc and Al-SAz-1 but increased the HCl fraction for
kaolinite.
In addition, humidity had effects on total recovery of 14C
added as 14C-labeled PDMS. The recovery of the added 14C
by sequential extraction and combustion was close to 100%
for kaolinite and talc at both 100% and 32% RH (Table 2). For
Al-SAz-1 at 32% RH, a slight under-recovery was evident at
short incubation times, such as 0.063 day (Table 2). In
contrast to this trend, the recovery of 14C for Al-SAz-1 at
100% RH decreased substantially with incubation time after
3 days (Table 2). Since the clay residue after all extractions
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was combusted, the 14C that was not recovered from AlSAz-1 must have been lost as volatile organosilicon compounds during the incubation and/or extraction processes.
Formation of Volatile Methylsiloxane (VMS). To test
the hypothesis that volatile organosilicon compounds may
be responsible for low recoveries of 14C for Al-SAz-1 at 100%
RH, eight separate Al-SAz-1 samples were weighed into 35mL glass centrifuge tubes. Four were predried at 32% RH,
and four were equilibrated with air at 100% RH in a desiccator.
These clay samples were then spiked with PDMS at two
loading levels (Table 3) and incubated at corresponding
humidities in the same way as previous samples, except that
a charcoal tube (SKC Inc., Eighty Four, PA) was inserted into
TABLE 2. Distribution of Degradation Products As Influenced
by Humidity
fraction of degradation products
recovered by
incub time
(days)
Al-SAz-1, 32%RH
0.021
0.063
1
3
10
21
Al-SAz-1, 100% RH
1
3
10
14
17.7
30
kaolinite, 32% RH
0.021
0.042
0.479
1
3
10
14
21
kaolinite, 100% RH
1
3
7
14
17.7
30
talc, 32% RH
1
3
10
21
30
talc, 100% RH
1
3
10
17.7
30
CaCl2
extr
0.154
0.367
0.455
0.424
0.332
0.420
total
HCl
sub- % HCl recovery
(%)
extr nonextr total extra
0.064
0.050
0.242
0.334
0.442
0.542
0.002
0.002
0.004
0.005
0.009
0.011
0.220
0.419
0.701
0.763
0.783
0.973
29.1
11.9
34.5
43.8
56.4
55.7
101.1
90.2
95.1
97.5
93.9
106.4
TABLE 3. Comparison of the Volatile Methylsiloxanes Formed
on Al-SAz-1 at Two Different Humidities and Initial PDMS
Concentrations
relative
humidity
32%
100%
32%
0.092
0.145
0.292
0.313
0.253
0.226
0.009
0.022
0.048
0.058
0.069
0.069
0.000
0.002
0.004
0.000
0.005
0.000
0.101
0.169
0.344
0.371
0.327
0.295
8.9
13.0
14.0
15.6
21.1
23.4
102.5
98.4
73.3
57.3
53.6
36.6
0.079
0.186
0.593
0.698
0.696
0.657
0.588
0.634
0.010
0.016
0.067
0.080
0.146
0.203
0.245
0.270
0.000
0.001
0.000
0.002
0.005
0.007
0.007
0.011
0.089
0.203
0.660
0.780
0.847
0.867
0.840
0.915
11.2
7.9
10.2
10.3
17.2
23.4
29.2
29.5
103.5
104.4
95.9
98.0
99.5
100.7
98.4
100.6
0.001
0.002
0.006
0.012
0.028
0.077
0.000
0.002
0.014
0.013
0.033
0.063
0.000
0.000
0.002
0.001
0.002
0.002
0.001
0.004
0.022
0.026
0.063
0.142
0.0
50.0
63.6
50.0
52.4
44.4
101.6
100.9
101.9
103.3
100.1
102.5
0.019
0.052
0.148
0.250
0.210
0.079
0.143
0.192
0.232
0.213
0.001
0.010
0.016
0.037
0.044
0.099
0.205
0.356
0.519
0.467
79.8
69.8
53.9
44.7
45.6
99.5
102.6
104.4
104.2
97.9
0.010
0.039
0.024
0.032
0.060
0.017
0.031
0.053
0.057
0.086
0.003
0.005
0.005
0.022
0.007
0.030
0.075
0.082
0.111
0.153
56.7
41.3
64.6
51.4
56.2
99.1
98.5
101.0
100.2
101.6
a Percent of degradation products recovered as the HCl-extractable
fraction (to subtotal).
100%
a
incubation
time (d)
% PDMS
degraded to
monomer diol
% PDMS
recovered as VMS
on
C-trap
in
clay
Initial PDMS Concn ) 2350 mg kg-1
0.23
20.4
1.7
7.0
0.4
7.0
68.0
16.2
14.0
82.9
5.9
11.8
nda
3.1
1.8
Initial PDMS Concn ) 500 mg kg-1
0.23
79.2
0.4
7.0
85.9
0.6
7.0
66.1
18.4
14.0
71.9
16.3
3.7
nda
0.7
1.8
nd, not detectable.
FIGURE 4. GC/MS profiles of the THF extract from Al-saturated
Arizona montmorillonite (Al-SAz-1) (b) and the charcoal trap (a)
after the 14C-labeled PDMS spiked Al-SAz-1 was incubated at 100%
RH for 7 days with a charcoal trap suspended over the clay in a
closed glass tube.
each centrifuge tube before incubation. At the end of the
designated incubation time, the clay and charcoal were both
extracted with THF.
found in the charcoal than on the clay, whereas at 32% RH
more cyclic methylsiloxanes were found on clay than in the
charcoal.
The GC/MS chromatograms of these THF extracts after
incubation for 7 and 14 days at 100% RH were characterized
by three peaks (Figure 4) regardless of the initial PDMS
loadings. The major peak had a retention time of 7.7-7.8
min, and two minor peaks had retention times of 5.5 and 9.5
min, respectively (Figure 4). Mass spectra revealed that these
peaks arose from volatile cyclic methylsiloxanes. The peak
at about 7.7 min was octamethylcyclotetrasiloxane (D4), the
one at 5.5 min was hexamethylcyclotrisiloxane (D3), and the
one at 9.5 min was decamethylcyclopentasiloxane (D5). For
clay at 32% RH, the peaks of the cyclic methylsiloxanes were
found only for high PDMS loading at short incubation times,
such as 5.5 h (Table 3). In addition to the differences in the
time of cyclic formation, humidity influenced the distribution
of the cyclic methylsiloxanes between the clay and charcoal
(Table 3). At 100% RH, more cyclic methylsiloxanes were
Discussion
PDMS Depolymerization Mechanisms. The GPC traces for
the THF extracts after PDMS was incubated with Al-SAz-1,
kaolinite, and talc at 100% RH (Figure 3b) or SAz-1 saturated
with different cations at 32% RH (Figure 3a) were similar to
those GPC traces obtained for other minerals at 32% RH (5).
The similarity was indicated by continuous shifting of the
GPC peaks with incubation time. However, the rate of the
peak shifting was much slower at high humidity (i.e., 100%
RH) or when clay minerals were saturated with cations of
lower valency (e.g., Al3+ vs Ca2+ or Na+), reflecting the slower
PDMS degradation rates under those conditions. These GPC
traces can be compared with the fraction of PDMS converted
to nonpolymeric materials (i.e., the summation of waterand HCl-extractable and nonextractable degradation prodVOL. 32, NO. 20, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 5. Comparison of the percentage of degradation products
with the reduction in the average molecular size of polymeric
residue.
ucts as shown in Figure 5) to elucidate the PDMS depolymerization mechanism.
Theoretically, linear PDMS polymers can be potentially
degraded by either an end-cutting or random scission
mechanism. By the end-cutting mechanism, -Si(Me2)Ounits are cut off one by one from the end of the polymer
chain. By the random scission mechanism, the cleavage of
Si-O-Si bonds occurs at random places along the backbone,
producing segments with average size decreasing with
incubation time. If end-cutting is the only operative mechanism, the percentage of 14C recovered as degradation
products should correspond to the equal percentage of
reduction in the average molecular size of the polymeric
residue. In other words, a negatively linear relationship
should be observed between the amount of degradation
products and the average molecular size of the polymeric
residue (Figure 5).
However, if random scission is the only operative mechanism, no significant amount of DMSD should be observed
until the average molecular size is reduced to a very small
number, resulting in an obviously concave line in Figure 5.
The data for all three minerals at both 32% and 100% RH fall
between the two lines representing the mechanisms but very
close to the concave line. This suggests that both end-cutting
and random scission were operative, but the latter is the
predominant mechanism regardless of clay type, exchangeable cations, or humidity.
Effects on Degradation Rates. Both exchangeable cations
and moisture levels had substantial effects on PDMS
degradation rates (Figures 1 and 2). The variation of the rate
constants with exchangeable cations (Figure 1) is related to
the catalysis of PDMS degradation by surface acidity. In the
absence of catalysts, PDMS does not undergo hydrolysis at
a significant rate at an ambient temperature (12). However,
the hydrolysis can be catalyzed by both Brønsted acidity and
Lewis acidity (5). The Brønsted acidity on soil clay minerals
is generated by the polarizing power of exchangeable cations
and structural metal cations (6). For example,
Al(H2O)3+ T Al(OH)2+ + H+
(1)
The acidity strongly depends on type of exchangeable cations
and the moisture content (13). The ranking of potential
Brønsted acid strength for common exchangeable cations
follows the order of polarizing power (6):
Al3+ > Mg2+ > Ca2+ > Na+ > K+
(2)
This order is consistent with the PDMS degradation rates on
SAz-1 as shown in Figure 1, which strongly supports the
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involvement of surface Brønsted acidity in catalyzing PDMS
hydrolytic degradation.
It should be pointed out that the presence of exchangeable
cations may also have a negative influence on the catalytic
activity of clay minerals. For example, the initial rate
constants for Na+- and Ca2+-saturated smectites [e.g., SAz-1,
nontronite, Wyoming montmorillonite (5)] and illite (5) are
much smaller than those of their uncharged structural
analogues talc and pyrophyllite (5) after normalizing to their
surface areas. This may be attributed to the strong hydration
of the exchangeable cations on negatively charged clay
minerals, possibly obscuring the catalytic sites on those
minerals and precluding the approach of the hydrophobic
PDMS molecules to these sites. This negative effect seems
most profound in illite and SAz-1 (5), presumably due to the
high charge density on the surfaces of these clays.
Additional evidence for the involvement of surface Brønsted acidity in catalyzing PDMS degradation is the observation
that a high water content slows down PDMS degradation on
clay minerals (Table 1) or in soils (2). Two mechanisms may
account for this moisture effect, both based on the effect of
clay hydration on catalytic activity of a clay surface. The
first mechanism emphasizes the physical contact of PDMS
with clay surfaces (e.g., coordination of Si-O to Lewis acid
sites). At low humidity, surfaces may not be fully hydrated,
and part or all of the PDMS segments may be adsorbed onto
the surface. This close proximity of PDMS to the surface
facilitates the catalysis. As clay surfaces become more fully
hydrated at high humidity, part or all of the PDMS molecule
segments will be repelled from surface sites due to hydrophobicity, resulting in a dramatic reduction of the catalytic
activity of clay minerals.
The above mechanism involves the direct contact of PDMS
with specific sites on the clay surface, which may be the case
if Lewis acidity is the only catalytic mechanism operative. In
contrast, no direct contact with surface sites is implied if
surface Brønsted acidity is involved in PDMS degradation.
As discussed previously, Surface Brønsted acidity generated
by exchangeable cations depends not only on exchangeable
cations but also on humidity. When a great deal of water is
present (e.g., at 100% RH), polarization forces of the
exchangeable cations are distributed among a large number
of water molecules; therefore, no significant effects on
Brønsted acidity from exchangeable cations can be observed
(13). As humidity decreases, the polarization forces of
exchangeable cations become concentrated on the smaller
number of remaining water molecules, dramatically increasing the ability of H2O to donate H+ and thus increasing the
surface Brønsted acidity (13).
Using IR techniques, Mortland and Raman (13) measured
the conversion of NH3 to NH4+ on smectites under different
humidity. They reported that, as the relative humidity
decreased from 98% to 20%, formation of NH4+ increased
300-400%. The researchers attributed the increased NH4+
to increased surface Brønsted acidity as the moisture
decreased (13). Russell et al. (14) found that 3-aminotriazole
is converted to its imino form on dry Mg2+-saturated
montmorillonite via protonation. The same degree of
protonation can be achieved in solution without the clay at
an acidity of 6 M HCl (14). Obviously, such high acidity on
dry clay surfaces will facilitate PDMS hydrolysis if PDMS is
in contact with the surfaces.
Although an increase in humidity generally slowed PDMS
degradation, different minerals had very different sensitivities
to humidity change (Table 1):
kaolinite > Al-SAz-1 . talc
The variation in moisture sensitivities for different minerals
was due to the different surface types for each mineral.
Kaolinite at near-neutral pH has a very low cation exchange
capacity (CEC). Therefore, surface Lewis acidity should be
the predominant contributor to the catalytic activity for this
mineral. The high moisture sensitivity of kaolinite may be
due to its highly hydrophilic surfaces. That is, the adsorbed
water at high humidity on kaolinite surfaces may prevent
the coordination of Si-O-Si of the hydrophobic PDMS to
the Lewis acid sites.
The surfaces of Al-SAz-1 were as hydrophilic as those of
kaolinite. The lower sensitivity of the PDMS degradation
rate on Al-SAz-1 could be due to the fact that surface Brønsted
acidity played a major role in catalyzing PDMS degradation
on this mineral, mainly a result of the large CEC of the mineral
and the high polarizing power of exchangeable Al3+.
Talc is an ideal layer silicate [Si4Mg3O10(OH)2] with siloxane
groups on its platelet surfaces and Si-OH and Mg-OH on
the edge sites. The catalytic activity of this mineral should
be mainly due to Lewis acid sites. The lower sensitivity of
the PDMS degradation rate to moisture can be attributed to
the less polar surface of talc than either kaolinite and AlSAz-1. Oxygen atoms on the talc surface cannot interact
strongly with water (i.e., cannot form hydrogen bonds with
water); therefore, hydrophobic molecules such as PDMS can
be adsorbed to the clay surface, even at high humidity.
Effects on Type and Distribution of Degradation Products. Moisture levels had a significant influence on the type
and distribution of degradation products in two respects.
First, higher humidity decreased the percentage of the HClextractable degradation products for both talc and Al-SAz-1
(Table 2), suggesting a reduction of DMSD bonding to clay
at high water content. This is consistent with results of a
previous study (15), which demonstrated that DMSD does
not bind to soil at high moisture content but binds
substantially when soil becomes air-dry.
It should be noted that the bonding of DMSD was a slow
process. For Ca-kaolinite at 32% RH, the process was slower
than the production of DMSD during PDMS degradation,
resulting in a low percentage of the HCl-extractable fraction
relative to the water-extractable DMSD concentration (5).
As the degradation rate of PDMS on kaolinite decreased at
100% RH, the percentage of the HCl-extractable fraction at
a given water-extractable DMSD concentration increased by
this mechanism. This kinetic effect may obscure the
depression of DMSD bonding by humidity and thus result
in an actual increased percentage of the HCl fraction for
kaolinite in Table 2.
The most important humidity effect on degradation
products was the formation of VMS compounds on Al-SAz-1
at 100% RH (Figures 4 and 5, Table 3). The formation of VMS
in the presence of clay and soil has been poorly understood.
When PDMS is degraded via random scission as previously
discussed, siloxane diols with 3-5 -Si(Me2)O- units will
eventually be formed. The intramolecular condensation of
these diols should lead to formation of VMS such as D3, D4,
and D5. In nonsoil environments, including high temperature
with no catalyst (16) or with an acid catalyst at an ambient
temperature (17), VMS are often found as products of PDMS
degradation. The formation of VMS in soil samples is still
a subject of debate.
Buch and Ingebrigtson (1) reported the formation of D4
in the presence of kaolinite but not with montmorillonite.
Griessbach (personal communication) did not determine
whether VMS had been formed but observed a low recovery
of 14C with clay minerals spiked with 14C-labeled PDMS,
especially after 4 weeks of incubation, suggesting a loss of
degradation products as volatile compounds. In contrast,
Lehmann et al. (18) and Carpenter et al. (19) did not detect
any significant amount of VMS in soil spiked with PDMS.
The lack of consensus in the formation of VMS in
environmental samples is partially due to the complexity of
FIGURE 6. Processes contributing to the formation of VMS during
PDMS degradation on clays.
the PDMS degradation processes (Figure 6); many factors
can affect the outcome. As a competing transformation
mechanism of oligomeric diols such as decamethylpentasilanediol (L5), octamethyltetrasilanediol (L4), and hexamethyltrisilanediol (L3), the formation of VMS depends on
the concentration of these oligomers and the rate of oligomer
transformation to DMSD (i.e., k2) relative to the rate of
intramolecular condensation (k3). In turn, the accumulation
of the oligomeric diols depends on the rates of their formation
from PDMS degradation (k1) relative to those of their
dissipation (k2 and k3).
Obviously, only when k1 > k2 is there any possibility of
VMS formation. Otherwise, these linear oligomers will be
further degraded to DMSD once they form. Even when k1
> k2, there will be no significant VMS accumulation if k3 ,
k2. Furthermore, the VMS formed may not be subject to
volatilization because they can be adsorbed onto clay
particles. In the early stage of PDMS degradation on AlSAz-1 at 32% RH (e.g., 5.5 h in Table 3), most of the D4 formed
was adsorbed on clay surfaces, which is why more VMS were
found on the clay mineral than on the charcoal. The VMS
adsorbed on clay surfaces are unstable and undergo surfacecatalyzed degradation to ultimately form DMSD (20). The
continuous shifting of the reaction toward the ultimate
degradation product (DMSD) is the reason VMS extracted
from clay decreased with incubation time (Table 3).
Volatilization of VMS from Al-SAz-1 at 100% RH is related
to the low affinity of VMS for a wet clay surface. In a parallel
study with pure D4, most of the D4 was adsorbed to air-dry
soil (e.g., at 32% RH) and further degraded to DMSD (Xu,
unpublished data). However, D4 evaporated to the air at
100% RH. The tendency of VMS to be adsorbed on air-dry
soil is consistent with the fact that more VMS were found in
clay than in charcoal at 32% RH (Table 3). In contrast, the
tendency of D4 to be desorbed from clay surfaces and
evaporated to the air at high humidity explains why more
VMS have been found in charcoals than in clay at 100% RH
(Table 3).
It should be pointed out that the L3, L4, and L5 silanols are
necessary precursors to forming D4 from PDMS degradation.
Little volatile loss for Al-SAz-1 was observed during the first
3 days of incubation at 100% RH (Table 2), because the
polymeric residue was >7 Si-O units (Figure 3b). Similarly,
there was no volatile loss during the entire incubation for
kaolinite and talc at 100% RH (Table 2), because oligomers
in these clay systems were still too big (15-35 Si-O units)
even at the last sampling time (Figure 3b). In these cases,
the lack of L3, L4, and L5 siloxanols was probably responsible
for the absence of VMS formation.
Two specific conditions for VMS formation (i.e., rapid
PDMS degradation to produce the precursors and high
humidity to drive off VMS) are important in understanding
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9
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the PDMS degradation in soils. Unlike the situation for pure
Al-SAz-1, VMS were not found during PDMS degradation in
soil (11) because natural soil conditions rarely satisfy the
two requirements simultaneously. At high water content,
PDMS degradation rates (i.e, k1) in soils are often low (11),
resulting in a lack of the precursors. This is because in soil,
the catalytic constituent (soil clays), are diluted by sand, silt,
and soil organic matter, which are not effective in catalyzing
PDMS degradation. In addition, clays in a given soil are
normally a mixture of various minerals including those not
effective in catalyzing PDMS degradation at high humidity.
For example, in acid soil where Al-montmorillonite occurs,
kaolinite, which can be deactivated dramatically by moisture
(Table 1), often coexists and may be the dominate clay (6).
Therefore, at high humidity, natural soil can never be as
effective as pure Al-SAz-1 clay in catalyzing PDMS.
Although PDMS degradation rate increases as soil becomes dry, the VMS release may still be negligible. This is
because under dry conditions, VMS, once formed, will be
adsorbed onto clay surfaces and then undergoes rapid
surface-catalyzed degradation to form DMSD as discussed
previously. Besides, PDMS concentration in field soils is
often much lower (4) than the lowest PDMS loading in Table
3. Such a low PDMS loading should reduce the formation
of VMS to an insignificant level (Table 3).
In conclusion, PDMS on clay minerals can be degraded
via both random scission and end-cutting mechanisms, with
the former as predominant regardless of mineral type,
exchangeable cations, or humidity. However, both exchangeable cations and humidity have tremendous influence
on PDMS degradation rates on clay minerals. Degradation
of PDMS on Na-saturated SAz-1 is very slow (0.71 µg m-2
day-1), but a change in the exchangeable cation from Na+
to Al3+ increased the rate by 120-fold. An increase in humidity
did not change the PDMS depolymerization mechanism but
generally decreased the reaction rate.
This moisture effect was more profound in highly hydrated
clays such as kaolinite and Al-SAz-1 and not as pronounced
in less hydrophilic clay like talc. For clay minerals with
significant PDMS degradation rates at 100% RH (such as
Al-SAz-1), an increase in humidity also changed the composition of the degradation products. For example, at 100%
RH, the VMS such as D3, D4, and D5 formed as intermediates
of PDMS degradation. They may be desorbed from clay
surfaces and vaporized to the air. At 32% RH, most VMS
were adsorbed on clay surfaces and converted to DMSD as
degradation progressed. Further, higher humidity decreased
the bonding of DMSD to clay. The specific combination of
conditions for VMS formation, high moisture, and rapid
PDMS degradation rate seldom exists in natural soils,
explaining why VMS were not found in soil samples. These
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observations indicate that clay type, exchangeable cations
type, and moisture level are all involved in determining the
rates and degradation products of PDMS hydrolysis on clay
minerals.
Acknowledgments
I am grateful to Drs. Robert G. Lehmann and Grish Chandra
for many constructive comments in reviewing the draft of
this paper. I also want to extend my thanks to Dr. Don Kleyer,
Dr. Sudarsanan Varaprath, Mrs. Vicki Sible, and Mr. Jack R.
Miller for assistance in analysis and to Mr. Maris Ziemelis for
helpful discussion on some of the data.
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Received for review April 3, 1998. Revised manuscript received July 27, 1998. Accepted July 27, 1998.
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