J. Agric. Food Chem. 2001, 49, 2899−2907
2899
Potential Contributions of Smectite Clays and Organic Matter to
Pesticide Retention in Soils
Guangyao Sheng,† Cliff T. Johnston,‡ Brian J. Teppen,§ and Stephen A. Boyd*,§
Department of Crop, Soil, and Environmental Sciences, University of Arkansas,
Fayetteville, Arkansas 72701, Department of Agronomy, Purdue University,
West Lafayette, Indiana 47907, and Department of Crop and Soil Sciences, Michigan State University,
East Lansing, Michigan 48824
Soil organic matter (SOM) is often considered the dominant sorptive phase for organic contaminants
and pesticides in soil-water systems. This is evidenced by the widespread use of organic-matternormalized sorption coefficients (KOM) to predict soil-water distribution of pesticides, an approach
that ignores the potential contribution of soil minerals to sorption. To gain additional perspective
on the potential contributions of clays and SOM to pesticide retention in soils, we measured sorption
of seven pesticides by a K-saturated reference smectite clay (SWy-2) and SOM (represented by a
muck soil). In addition, we measured the adsorption of atrazine by five different K-saturated
smectites and Ca-saturated SWy-2. On a unit mass basis, the K-SWy-2 clay was a more effective
sorbent than SOM for 4,6-dinitro-o-cresol (DNOC), dichlobenil, and carbaryl of the seven pesticides
evaluated, of which, DNOC was sorbed to the greatest extent. Atrazine was sorbed to a similar
extent by K-SWy-2 and SOM. Parathion, diuron, and biphenyl were sorbed to a greater extent by
SOM than by K-SWy-2. Atrazine was adsorbed by Ca-SWy-2 to a much lesser extent than by
K-SWy-2. This appears to be related to the larger hydration sphere of Ca2+ (compared to that of
K+) which shrinks the effective size of the adsorption domains between exchangeable cations, and
which expands the clay layers beyond the apparently optimal spacing of ∼12.2 Å for sorption of
aromatic pesticide structures. Although a simple relation between atrazine adsorption by different
K-smectites and charge properties of clay was not observed, the highest charge clay was the least
effective sorbent; a higher charge density would result in a loss of adsorption domains. These results
indicate that for certain pesticides, expandable soil clays have the potential to be an equal or
dominant sorptive phase when compared to SOM for pesticide retention in soil.
Keywords: Sorption; pesticide; clay; organic matter; smectite
INTRODUCTION
Clay minerals and soil organic matter (SOM) are
considered the two most chemically active components
of soils. Among clay minerals commonly found in soil
environments, expandable 2:1 layer silicate clays are
especially important because of their high surface areas
and cation exchange capacities (CECs), as well as their
surface reactivities (1). Although often present in relatively low amounts in soils, SOM disproportionately
influences many important soil processes, including
sorption of aqueous-phase organic contaminants and
pesticides. Currently, about 4.5 billion pounds of chemicals are used as pesticides each year in the U.S., and
agricultural usage accounts for ∼77% of the total (2).
Sorption of soil-applied pesticides is an important
determinant of their environmental fate and behavior,
including bioavailability, persistence, and potential for
leaching. Over the past two decades, research on
pesticide sorption by soil has focused mainly on the
singular role of SOM as the dominant sorptive phase
(e.g., 3-6). Reliance on organic-matter-normalized sorp* Corresponding author. Tel: 517-353-3993. Fax: 517-3550270. E-mail: boyds@msu.edu.
† University of Arkansas.
‡
Purdue University.
§
Michigan State University.
tion coefficients (KOM) to predict soil-water distribution
of pesticides implicitly assumes that SOM is the dominant sorptive phase (4, 7, 8) and ignores the contribution
of clays (hereafter used to indicate clay minerals rather
than clay-sized particles) to pesticide sorption. The
surfaces of clays have often been viewed as being polar
in nature (9-12). It has been suggested that the
preferential adsorption of water by these purportedly
polar surfaces rendered clays ineffective as sorbents for
neutral organic compounds (13-15).
The prevailing view of organic solute sorption by SOM
generally ignores the role of soil minerals (including
clays) in the sorption of organic contaminants and
pesticides. Although this may be valid for relatively
nonpolar molecules (e.g., benzene and trichloroethylene), it is possible that other neutral organic molecules,
including important categories of pesticides, are effectively adsorbed by clays even in the presence of bulk
water. In fact, a few recent studies support this contention. Atrazine adsorption by 13 Ca-saturated smectites
ranged from very low to nearly complete removal from
water, and was inversely dependent on clay properties
such as surface charge density and cation-exchange
capacity (16). With the measured pH of clay-water
suspensions ranging between 4.75 and 6.45, and a pKa
of 1.7 for atrazine, it seemed apparent that atrazine was
adsorbed as the neutral species. Atrazine adsorption by
10.1021/jf001485d CCC: $20.00 © 2001 American Chemical Society
Published on Web 05/23/2001
2900 J. Agric. Food Chem., Vol. 49, No. 6, 2001
a montmorillonite (from Clay Spur, Wyoming) saturated
with Ca2+ or Al3+ was attributed to the formation of
H-bonds between atrazine and polarized waters of
hydration (17). Nitro-substituted aromatics, including
explosives and some dinitrophenol pesticides, are also
significantly adsorbed by clays (18-20). Adsorption of
these compounds was shown to be affected by the type
of clay and its charge, the type of exchangeable cation,
and the type and position of substituents on the
aromatic ring. High adsorption of nitroaromatics by
clays was attributed to the formation of an electron
donor-acceptor complex, in which polarized aromatic
rings parallel to the basal surfaces accept electrons from
the siloxane oxygens to form the pesticide-clay complex.
In addition to the proposed electron donor-acceptor
mechanism, smectite clays may also adsorb neutral
organic molecules by hydrophobic interactions. Recently,
Laird and Fleming (21) presented data on the sorption
of butylpyridine from water by Ca-smectite. As much
as 95% of added butylpyridine was adsorbed, and Ca2+
release measurements indicated that ion exchange
accounted for <20% of the amount sorbed. These results
suggested a hydrophobic interaction between the butyl
group of butylpyridine and the siloxane surface of
smectite. Earlier work by Jaynes and Boyd (22) indicated that the siloxane surfaces of smectites possessed
a hydrophobic character consistent with the results of
Laird and Fleming (21).
The studies cited above, along with substantial experimental evidence from earlier studies (e.g., 23, 24),
clearly documents the ability of pure clays, particularly
expandable 2:1 clays, to effectively bind organic molecules, including pesticides. These studies suggest the
potential importance of clays in organic contaminant
and pesticide retention by soils. Using sorption data
from soils of variable clay and SOM contents, Karickhoff
(25) attempted to evaluate conditions under which
mineral-phase sorption of organic contaminants in
whole soils was important. For the N-heterocyclic simazine and biquinoline, mineral contribution to overall
sorption became apparent (i.e., greater than that predicted from Kom values) at clay/organic matter ratios of
>30. Sorption of pyrene, however, was not affected by
clay content. It was concluded that the contribution of
mineral-phase sorption was a direct function of the
polarity of the compound. Grundl and Small (26) evaluated the role of mineral-phase sorption of atrazine and
alachlor by a suite of sediments, and concluded that the
pesticides were sorbed by both natural organic carbon
(OC) and clays. The critical clay/OC ratios at which
mineral-phase sorption accounted for 50% of the overall
sorption were ca. 62 for atrazine and 84 for alachlor.
Hassett et al. (27) evaluated the effect of clay content
on sorption of R-naphthol by soils and sediments with
different OC and montmorillonite contents. They concluded that the contribution of mineral-phase sorption
to overall uptake was apparent at clay/OC ratios of >10.
Although these studies established the importance of
clays as sorbents for organic contaminants in soils, they
did not isolate or quantify the individual sorptive
contributions of clays and SOM.
The adsorption of aqueous phase pesticides by clays
differs significantly among the various combinations of
pesticides and clays. It is often unclear whether the
sorption of pesticides by clays is comparable to that by
SOM. In this study, representative pesticides from
several major classes were used to compare sorption by
Sheng et al.
a reference smectite clay to that of a muck soil representing SOM. These data were used to provide some
additional perspective on the potential importance of
smectite clays and SOM as sorbents for aqueous-phase
pesticides.
EXPERIMENTAL PROCEDURES
Pesticides. Seven pesticides were used in the adsorption
experiments (Table 1). The pesticides were purchased from
ChemService, Inc., West Chestnut, PA, with a purity of >99%,
and used as received. Six of them were chosen to represent
major classes of pesticides. The seventh, biphenyl, was included as a representative nonpolar unsubstituted pesticide.
The pesticides were also selected to encompass a variety of
different structural and physicochemical properties. They
contained various substituents, including strongly electronwithdrawing groups such as -NO2 and -CN, and also have a
wide range of water solubilities, dipole moments, and octanol/
water partition coefficients (KOW). The gas-phase dipole moments were computed using B3LYP density functional calculations (28-30). These are quantum mechanical calculations
of reasonably good quality using the Becke three-parameter
hybrid method (28) for the exchange functional along with the
Lee, Yang, and Parr correlation functional (30). These electronic structure calculations show that molecular thicknesses,
as estimated by the 0.02 electron/Å3 isodensity surfaces, were
3.5 (0.2 Å for 4,6-dinitro-o-cresol (DNOC) and a wide variety
of other planar nitroaromatic molecules. To estimate the
effective surface area occupied by DNOC on clay surfaces, we
used our force field (31) for organic-clay interactions to perform
molecular mechanics calculations. We used Grand Canonical
Monte Carlo calculations (32) to model adsorption of DNOC
into an uncharged clay with just enough interlayer space (12.8
Å d spacing) for the DNOC. For three simulations at two
different unit cell sizes (in-layer unit cell vector products ab
were 4534 and 5922 Å2), we found similar values of 51.9, 54.3,
and 52.1 Å2 per DNOC. These values should be the upper limit,
as the DNOC packing was not perfect. In summary, the surface
area of DNOC should be e 52 Å2, and its thickness should be
about 3.5 Å. The interlayer space available in a 12.5-Å
monolayer hydrate clay mineral can be estimated by comparing the 9.19-Å d spacing of pyrophyllite (nothing in the
interlayer), which yields about 3.3 Å for the available interlayer space, agreeing moderately well with the thickness of
the organic molecule.
Sorbents. A reference smectite, SWy-2, was used in most
of the measurements of pesticide adsorption by clays. Five
other clays were also studied for atrazine adsorption measurements. The clays were chosen to provide a range of surface
charge densities, and differences in the location of charge
deficit and the structure of octahedral sheet (dioctahedral vs
trioctahedral) (Table 2). All clays were obtained from the Clay
Minerals Society Source Clay Repository (Columbia, MO)
except beidellite which was from Ward’s (Rochester, NY). The
<2 µm clay particles were separated by wet sedimentation.
The clays were subject to K+ saturation, and also Ca2+
saturation for SWy-2, by dispersing 10-g clay samples in 1 L
of KCl (0.1 M) or CaCl2 (0.1 M) solution. The clay suspensions
were shaken for 24 h, and then fresh chloride solutions were
used to displace the original solutions after centrifugation. This
process was repeated four times to ensure complete K- or Casaturation. Distilled water (1 L) was used to wash the clays
to remove excess K+ or Ca2+. The clays were freeze-dried and
stored for later use. The Houghton muck soil was collected
from the Michigan State University Muck Farm and air-dried.
Its OC content (49.5%) was measured by a carbon analyzer
(Rosemount Analytical, Inc., Santa Clara, CA).
Sorption Isotherms. Pesticides were dissolved in 0.1 M
KCl solution with concentrations up to 50% of their water
solubilities. A pH 3 KCl solution was used for DNOC. In the
case of Ca-SWy-2 and the muck soil, 0.1 M CaCl2 was used.
Up to 4.4 mL of KCl (or CaCl2) solution was pipetted into the
7-mL borosilicate glass vials containing clay or muck soil (0.05
Pesticides in Smectite Clays and Organic Matter
J. Agric. Food Chem., Vol. 49, No. 6, 2001 2901
Table 1. Selected Physicochemical Properties of Pesticides Used for Sorption Study
Table 2. Properties of Clays and the Freundlich Coefficients for Atrazine Adsorption by K-clays
sorbent
mineralogy
cation-exchange
capacity
(cmolc/kg)
K-SHCa-1
K-SWy-2
K-BPC
K-SapCa-2
K-SWa-1
K-SAz-1
hectorite
montmorillonite
beidellite
saponite
nontronite
montmorillonite
43.9
81.6
84.2
94.9
107
130
to 1 g). Pesticide solution was then added into each vial to
make up a total volume of 5 mL. For DNOC (pKa 4.35 to 4.46),
sorption was measured at pH 3 to ensure the molecular form
of the pesticide. The pH of clay suspension before DNOC
sorption was adjusted until it remained stable at ∼3, and was
also measured after sorption to further ensure the stable pH.
Clay dissolution under such a pH was not noted. Consistent
with previous studies (18, 19) we found that DNOC sorption
by clays reached equilibrium within 10 min. In the case of the
muck soil the pH of the aqueous CaCl2 suspension was
adjusted to 3.0 ( 0.1 before DNOC solution was introduced.
The pH adjustment was repeated as needed until it remained
stable after 3 days. The vials were continuously rotated
overnight at room temperature and then centrifuged at 1667g
for 20 min to separate the liquid and solid phases. The
concentrations of pesticides in supernatants were analyzed,
by direct injection of supernatants (between 10 and 190 µL),
using a Perkin-Elmer reversed-phase HPLC (Perkin-Elmer,
Norwalk, CT) fitted with an UV-visible detector set at the
maximum absorption wavelength for each pesticide (Table 1).
A platinum extended polar selectivity (EPS) C18 column was
used. The mobile phase was a mixture of methanol and water
ranging from 55% to 75% methanol with a flow rate of 1.0 mL/
min. The amount of pesticide sorption was calculated from the
difference between the amount added and that remaining in
the final solution.
%
tetrahedral
charge
tri-/dioctahedral
Kf
(mg/kg)/
(mg/L)n
n
0
0
52
>50
73
0
trididitrididi-
120
74.6
31.4
135
95.3
8.47
0.821
0.784
0.925
0.841
0.899
0.864
After sampling for HPLC analysis, the remaining DNOCK-SWy-2 suspensions were used for X-ray diffraction analysis.
The supernatants of 1-2 mL were retained in the vials to
resuspend the clay by hand-shaking, and then dropped on
glass and air-dried overnight to obtain the oriented films. X-ray
diffraction patterns were recorded using Cu-KR radiation and
a Philips APD 3720 automated X-ray diffractometer using an
APD 3521 goniometer fit with a θ-compensating slit, a 0.2mm receiving slit, and a diffracted-beam graphite monochromator, from 3 to 14 °2θ, in steps of 0.02 °θ, at 1 s/step.
RESULTS AND DISCUSSION
Pesticide sorption isotherms based on per unit mass
of clay or muck (SOM) versus the equilibrium aqueous
concentration of each pesticide are shown in Figure 1(a-g). The Houghton muck soil was used to evaluate
the potential contribution of soil organic matter (SOM)
to pesticide sorption. The representation of SOM by the
muck soil is supported by the similarities of organiccarbon-normalized sorption coefficients (Koc) between a
peat soil (49.3% OC) and a mineral soil (1.26% OC) for
both nonpolar (ethylene dibromide, EDB) and polar
(dichlorophenol, DCP) compounds (33). The reported log
Koc values for the peat and mineral soils were 1.28 and
2902 J. Agric. Food Chem., Vol. 49, No. 6, 2001
Sheng et al.
Figure 1. Sorption isotherms representing pesticide uptake from water by a reference homoionic K-smectite (SWy-2) and muck
soil representing soil organic matter (a-g), and atrazine uptake by several different K-saturated smectites and Ca-SWy-2 (h).
1.23 for EDB and 2.03 and 1.87 for DCP, respectively.
The isotherms representing pesticide sorption on the
muck soil display some nonlinearity. Isotherm nonlinearity has been observed previously for polar and
nonpolar organic solutes and attributed to a glassy
phase in SOM (5, 6, 34, 35), to the presence of a small
quantity of high-surface-area carbonaceous material (33,
36), and/or to solute-SOM specific interactions (36, 37).
The isotherms for pesticide sorption by K-SWy-2 are
either linear, type I, or type III (38), depending on the
specific compound. This reflects different types and/or
strengths of pesticide-clay interactions. The calculated
distribution coefficients (expressed as the ratio of the
concentration of sorbed pesticide to the aqueous-phase
pesticide concentration) for both K-SWy-2 and muck
at a relative concentration (i.e., the ratio of the aqueousphase concentration to water solubility) of 0.1 are given
in Table 3 for each pesticide. The term KK-SWy-2/Kmuck
Table 3. Distribution Coefficients (L/kg) of Pesticides
Sorbed by K-SWy-2, Muck, or Ca-SWy-2 at the Relative
Concentration of 0.1a
pesticide
KK-SWy-2
Kmuck
KK-SWy-2/Kmuck
4,6-dinitro-o-cresol
carbaryl
diuron
atrazine
dichlobenil
parathion
biphenyl
2.49 × 103
235
103
54.2 (7.68)b
275
125
6.40
184
54.2
173
47.1
179
1.08 × 103
791
13.6
4.34
0.593
1.15
1.54
0.116
8.09 × 10-3
a Relative concentration equals equilibrium aqueous-phase
concentration/water solubility. b Distribution coefficient for sorption by Ca-SWy-2.
is the ratio of the two distribution coefficients for each
pesticide and allows comparison of the relative effectiveness of K-SWy-2 clay and SOM as sorbents for pesticides. It shows that some pesticides, i.e., DNOC, car-
Pesticides in Smectite Clays and Organic Matter
J. Agric. Food Chem., Vol. 49, No. 6, 2001 2903
baryl, and dichlobenil, were more effectively sorbed by
K-SWy-2 clay than by SOM. Others, i.e., diuron,
parathion, and biphenyl, were sorbed more effectively
by SOM than by clay. Atrazine is sorbed by K-SWy-2
clay and SOM to a similar extent. Direct correlations
between pesticide adsorption (Kf values) by K-SWy-2
and water solubility, KOW, or dipole moment were not
found, reflecting the complexity of pesticide-clay interactions.
Biphenyl was highly sorbed by the muck soil. In
contrast, K-SWy-2 clay was a relatively ineffective
sorbent for biphenyl (Figure 1g). The smectite was
approximately 120 times less effective than SOM for
biphenyl sorption (Table 3). Biphenyl is a nonpolar,
poorly water-soluble aromatic molecule. These characteristics manifest its high uptake by SOM, consistent
with previous findings (39). These molecular properties,
however, were apparently not sufficient for significant
sorption by K-SWy-2, despite indications that the
siloxane surfaces of smectites are hydrophobic in nature
(21, 22). This suggests that, in addition to hydrophobic
interactions, other mechanisms may be necessary for
substantial retention by smectites.
Sorption of DNOC by K-SWy-2 was highest among
all pesticides studied (Figure 1a), although it is pHdependent due to ionization of the phenolic hydroxyl.
For example, raising the pH from 3 to 5 caused ∼50%
reduction in the sorption of DNOC by K-SWy-2 (data
not shown), but the sorption was still substantially
higher than that by SOM. We have observed similar or
slightly higher sorption of the nonionizable compounds
1,3- and 1,4-dinitrobenzene by K-SWy-2 (40). Based on
distribution coefficients at a relative aqueous concentration of 0.1 (Table 3), sorption of DNOC was ∼9 times
greater than the next most highly sorbed pesticide
(dichlobenil); the difference is even greater at lower
concentrations. Also, K-SWy-2 was ∼14 times more
effective than SOM as a sorbent for DNOC (Table 3).
The basal spacing of K-SWy-2 increased gradually
from ∼11.1 to ∼12.2 Å with increasing DNOC adsorption (Figure 2a). Selected X-ray diffraction (XRD) patterns (Figure 2b) show that the diffraction peaks were
broad and generally symmetrical, indicating the random
interstratification of DNOC molecules in the interlayers
of K-SWy-2. The X-ray diffraction peaks corresponded
to distributions of clay mineral d spacings from roughly
10 to 13 Å. Thus, the clay films typically contained a
continuum of domains ranging from dehydrated Ksmectite (10 Å) to an interlayer containing a monolayer
of K+, water, and/or DNOC (12 to 13 Å). As the DNOC
loading increased, the d spacing corresponding to the
centroid of this diffraction peak increased to 12.2 Å for
the DNOC loading corresponding to a solution-phase
DNOC concentration of 20 mg/L. The increase in basal
spacing suggests that DNOC is intercalated in the
interlamellar region of the clay, with an increasing
fraction of the clay domains at larger d spacings (12 to
13 Å). The surface area of K-SWy-2 is 750 m2/g and
the estimated cross-sectional area of the DNOC molecule is 52 Å2. The surface area occupied by DNOC
molecules (f) was ca. 10%. This was calculated as
follows:
Figure 2. (a) 4,6-dinitro-o-cresol adsorption-dependent variation in the basal spacing of K-SWy-2, and (b) selected X-ray
diffraction patterns.
f)
49 mg/g
× 10-3 mol/mmol × 6.023 × 1023/mol × 52 Å2 × 10-20 m2/Å2
198 g/mol
750 m2
× 100% ) 10.3%
If the aromatic ring of DNOC is oriented parallel to the
siloxane surface, and it simultaneously interacts with
the opposing clay layers, as much as 21% of the smectite
clay surface may be occupied at this loading.
The reasons that K-SWy-2 is a highly effective
adsorbent for DNOC are not fully understood. The
molecule is planar and aromatic, and it has two nitro
groups that are strongly electron-withdrawing. According to Haderlein and Schwarzenbach (18), these molecular characteristics favor the formation of an electron
donor-acceptor complex, in which the aromatic molecule acts as an acceptor of electrons donated from sites
of negative charge in the clay. FTIR studies showed that
the aromatic ring of adsorbed DNOC molecule is nearly
parallel with respect to the siloxane surfaces of K-SWy-2
(41). The partial negative charge associated with the
nitro groups may result in electrostatic interactions
between the nitro groups and interlayer K+. This is
indicated by the fact that the FTIR stretching vibration
bands of nitro groups shift depending on the interlayer
cation (42). Thus, the high effectiveness of K-SWy-2
for DNOC appears to result from a combination of more
than one mechanism.
Two other pesticides, carbaryl and dichlobenil, are
also more favorably sorbed by K-SWy-2 than by SOM
2904 J. Agric. Food Chem., Vol. 49, No. 6, 2001
(Figure 1b and e). The dichlobenil molecule may form
an electron donor-acceptor complex with the siloxane
surfaces because of the strong electron-withdrawing
character of the cyano group on the aromatic ring, in
analogy with nitroaromatics. The two-ring π-electron
system of carbaryl may participate in the formation of
an electron donor-acceptor complex depending on the
inductive and resonance properties of the N-methylcarbamate (-OCONHCH3) moiety. The functional groups
of carbaryl and dichlobenil may also interact with the
exchangeable cations in analogy with DNOC, but we
have no direct evidence for this currently.
Atrazine adsorption by K-SWy-2 was comparable to
that by SOM (Figure 1d) but considerably less than that
for DNOC. In comparison to DNOC, atrazine is larger
and more structurally complex. In addition to the
presence of -Cl, atrazine possesses two large substituted amino groups. These three substituents are
positioned meta to each other on the triazine ring and
may manifest a degree of steric hindrance in the
adsorption of atrazine by K-SWy-2. Steric effects were
also found in the adsorption of nitroaromatics with ortho
substitution or large alkyl substituents (18, 19). Laird
et al. (16) reported that smectite clays with lower charge
densities and CECs were more effective adsorbents of
atrazine; these characteristics may increase the size of
the adsorptive domains between exchangeable cations.
The adsorption of diuron and parathion is comparably
low and less extensive by K-SWy-2 than by SOM
(Figure 1c and f). These two pesticides contain large
substituents on the aromatic ring (two -Cls and N,Ndimethylurea for diuron, nitro and diethylphosphorothionate for parathion) (Table 1) which may sterically
hinder adsorption. The presence of bulky alkyl substituents has been observed to substantially diminish the
adsorption of nitroaromatics by clay minerals (19). The
type III isotherm for parathion adsorption by K-SWy-2
indicates weak parathion-clay interactions. However,
both of these pesticides are sorbed by K-SWy-2 to a
greater extent than is biphenyl.
Adsorption of atrazine was affected by the type of
exchangeable cation on the smectite clay. Homoionic
Ca-SWy-2 was a much less effective sorbent for atrazine than homoionic K-SWy-2 (Figure 1h). Atrazine
adsorption indicates that K-SWy-2 (Kf ) 54.2) is seven
times more effective than Ca-SWy-2 (Kf ) 7.68) (Table
3). Similar effects were observed by Haderlein and
Schwarzenbach (18) and Haderlein et al. (19) for the
adsorption of nitroaromatics by clays saturated with
different cations. The lower effectiveness of Ca-SWy-2
may be due in part to the higher hydration energy and
hence larger hydrated radius of Ca2+ than that of K+.
Presumably, the waters of hydration associated with
Ca2+ obscure a greater portion of the clay surface than
those associated with K+, and this may effectively
shrink the size of the adsorptive domains between
exchangeable cations. Similar effects were observed by
Kukkadapu and Boyd (43) for the adsorption of aromatic
and chlorinated hydrocarbons by tetramethylphosphonium- and tetramethylammonium-smectites.
Differences in swelling behavior between K- and CaSWy-2 may also contribute to their differential pesticide
adsorption. Many K-smectites equilibrate with ∼12.5-Å
layer spacings (monolayer structures) at 100% humidity
and even in aqueous suspension (44-47), although some
low-charge K-smectites can swell more. For example,
K-hectorite (48) and K-SWy-2 (47) are likely to swell
Sheng et al.
to 15 Å and beyond at 100% humidity in the absence of
organic sorbates. All Ca-smectites, on the other hand,
swell to more than 15 Å. Because there is apparently a
fine line between ∼12.5-Å or larger swelling for Ksmectites, it is quite possible that the presence of
pesticides could cause the monolayer structure to be
more favored, even in cases where the smectite might
swell further in the absence of pesticide. In this scenario,
each pesticide molecule contacts both clay surfaces
simultaneously and thereby avoids contact with most
water molecules. As the free energy of hydration for
many small organic solutes is in the range of +10 to
+30 kJ/mol (45, i.e., n-hexane, +27.5; cyclohexane,
+23.7; toluene, +18.4 kJ/mol), removal of these solutes
from aqueous solution may well provide enough energy
to prevent K-smectite from swelling beyond 12.5 Å. For
a Ca-smectite, in contrast, the energy penalty for
compressing the interlayer and thereby dehydrating the
pesticide (and the cation) is presumably too large.
The specific type of smectite clay also affects pesticide
adsorption. This is illustrated by atrazine adsorption
by six different species of K-smectites (Figure 1h). In a
study of atrazine adsorption by 13 Ca-saturated smectites, Laird et al. (16) found that the logarithm of the
Freundlich adsorption constant (Kf) was inversely correlated to clay CEC. We obtained Kf values (ranging
from 8.5 to 135 L/kg, Table 2) for atrazine adsorption
to six K-smectites, but we did not observe an obvious
relationship between Kf and CEC. However, it was
apparent that the clay with the highest CEC (and
charge density), i.e., K-SAz-1, was the least effective
adsorbent for atrazine. For this clay, it seems likely that
atrazine adsorption decreased because of loss of adsorption domains of sufficient size to accommodate atrazine.
Decreasing adsorption with increasing clay layer charge
has been observed previously for the adsorption of
nonpolar aromatics from water by tetramethylammonium- and trimethylphenylammonium-smectites (22,
49).
The potential contributions of smectite clay (K-SWy2) and SOM to pesticide retention in soils (at relative
pesticide concentration of 0.1, Table 3) can be calculated
by the following equations, assuming these sorbent
phases function independently:
Cclay ) KK-SWy-2 × Ce × clay %
CSOM ) Kmuck × Ce × SOM %
where Cclay and CSOM are the respective clay-sorbed and
SOM-sorbed pesticide concentrations normalized to the
whole soil at the aqueous phase concentration of Ce.
Calculations using the distribution coefficients listed in
Table 3 for model (hypothetical) soils over a range of
SOM (0-10%) and smectite clay (0-40%) contents that
might reasonably be found in mineral soils are graphically shown in Figure 3. These plots can be used to
compare the potential contributions of smectite clays
and SOM to the pesticide retention in a soil with known
smectite clay and SOM contents. For example, from
Figure 3, we estimate that clay (K-SWy-2) contributes
(∼390 mg/kg) ∼35 times more to carbaryl retention than
SOM (∼11 mg/kg) in a soil with 16% clay and 2% SOM.
For parathion in the same soil, contributions by smectite
clay and SOM are about equivalent. For some pesticides,
even low amounts of smectite clay could potentially
exert a strong influence on sorption. For example,
assuming a soil containing 2% K-SWy-2 and 2% SOM,
Pesticides in Smectite Clays and Organic Matter
J. Agric. Food Chem., Vol. 49, No. 6, 2001 2905
Figure 3. Potential contributions of smectite clay (represented by K-smectite, SWy-2) and soil organic matter (SOM, represented
by a muck soil) to pesticide immobilization in model (hypothetical) soils.
the amounts of DNOC sorbed at the relative concentration of 0.1 are ca. ∼980 mg/kg by clay and ∼70 mg/kg
by SOM. Although this illustrates the potential dominant role of certain clay minerals in the retention of
specific pesticides in soils, caution must be taken in
extrapolating these results. Humic substances in soils
may form coatings on clay particles thereby rendering
these surfaces unavailable for pesticide adsorption.
However, this effect is probably limited to the external
surfaces of expandable clays, as there is no evidence for
the intercalation of humics by naturally occurring clays.
This effect is further minimized in subsoils and aquifer
materials with inherently low organic matter contents.
In considering the applicability of these results to real
soil clays, K+ is one of the most abundant exchangeable
cations in soils and subsoils, but it is not normally the
dominant exchangeable cation. The phenomenon of
cation demixing in clays (50-52) results in entire
interlayer regions being occupied by a single exchangeable cation (e.g., K+) even in the presence of other
cations (e.g., Ca2+) because of differences in cation
hydration. Thus, K-rich domains with high affinity for
pesticides such as nitroaromatics are likely to exist in
many soils. Indeed, the widely occurring phenomenon
of K-fixation by soil clays (53-55) proves that K-rich
domains are common. Even for a variety of Ca-saturated
smectites, Laird et al. (16) reported very high removal
of atrazine from water, so the efficacy of smectites as
adsorbents of pesticides is not limited to K-saturated
clays. Improved understanding of pesticide adsorption
by clays makes it possible to enhance the retentive
properties of subsoils and aquifer materials for certain
pesticides by controlling the base saturation of the
matrix via in-situ electrolyte injection (e.g., KCl) as
suggested by the recent findings of Weissmahr et al.
(56).
In summary, pesticides can be sorbed effectively by
either clays or SOM. Sorption depends on the specific
type of clay, saturating cation, and pesticide structure.
Whereas pesticide sorption by SOM is generally deter-
2906 J. Agric. Food Chem., Vol. 49, No. 6, 2001
mined by pesticide water solubility, adsorption by clays
is dependent on both pesticide and clay properties. Our
study builds upon previous work to probe the dependence of pesticide sorption on the structure of the
pesticide, on the exchangeable cation saturating the
smectite clay, and on the smectite clay layer charge.
Understanding these dependencies will help constrain
the interpretation of more mechanistic spectroscopic
studies underway in our laboratories. The existence of
multiple sorption mechanisms appears to favor pesticide
sorption by smectite clays. Planar aromatic structures
and electron withdrawing substituents (e.g. -NO2,
-CN) seem to favor pesticide adsorption, possibly via
the formation of an electron donor-acceptor complex
between pesticide molecules and siloxane surfaces. Polar
substituents may interact via a H2O-bridge with hydrated exchangeable cations; substituents with negative
charge character (e.g. -NO2) may interact directly with
exchangeable cations. Hydrophobic interactions between
the pesticide and the siloxane surfaces may also contribute to adsorption. Large substituents associated
with pesticide structure may cause steric constraints
that diminish adsorption. A steric effect may also arise
from the hydration of cations saturating clays, i.e., a
large hydration sphere may diminish the size of the
adsorption domains between exchangeable cations. Exchangeable cations may also influence sorption due to
effects on basal spacing. A spacing of ∼12.2 Å, such as
that associated with K-smectites, appears optimal for
adsorption of nitro-aromatic pesticides. Finally, low
surface charge density may increase the size of the
adsorptive domain, hence increasing adsorption. In this
limited study of seven pesticides and reference smectite
clays, the K-SWy-2 clay was shown to be a more
dominant sorptive phase than SOM in over half the
cases.
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Received for review December 13, 2000. Revised manuscript
received April 12, 2001. Accepted April 16, 2001. This research
was supported by USDA-NRICGP Grants No. 98-35107-6348
and 99-35107-7782, and by the Michigan Agricultural Experiment Station.
JF001485D