Environ. Sci. Technol. 1996, 30, 1220-1226
Soil Component Interactions
with 2,4-Dichlorophenoxyacetic
Acid under Supercritical Fluid
Conditions
E . A . R O C H E T T E , * ,† J . B . H A R S H , † A N D
H. H. HILL, JR.‡
Department of Crop and Soil Sciences and Department of
Chemistry, Washington State University,
Pullman, Washington 99164
Interest in using supercritical CO2 as an alternative
to nonpolar liquid solvents to extract toxic organics
from soils is growing. Unfortunately, supercritical
CO2 alone is a poor solvent for many polar compounds,
including acid herbicides, in soils. In this study,
supercritical CO2 was modified with benzoic acid/
methanol to extract 2,4-dichlorophenoxyacetic acid (2,4D) from selected model soil components, analogs
of soil components that potentially limit its extraction
from soils. The components included four minerals,
silica gel, sodium humate, and humic acid. These model
materials were chosen to test three potential factors
inhibiting 2,4-D extraction: (1) adsorption to mineral
surfaces, (2) diffusion-limited release from porous
materials, and (3) pH-dependent partitioning between
the solid and supercritical fluid phases. High
recoveries were obtained from gibbsite (100 ( 3%),
goethite (91 ( 3%), and illite (88 ( 6%). Porous
materials such as the silica gels and humic acid yielded
lower recoveries, 70 ( 4% to 87 ( 7% and 80 (
3%, respectively. We extracted only 11 ( 2% of the
spiked 2,4-D from sodium humate. An inverse
relationship existed between the pH of the solidbenzoic acid/methanol suspension and 2,4-D recovery.
Overall, soil pH was the main chemical factor affecting
2,4-D recovery. Due to its porosity, pH buffering
capacity, and ubiquitous occurrence, we contend
organic matter will generally be the main component
limiting extraction of 2,4-D from soils. Furthermore,
it appears methanol enhances recovery, in part,
because the protonated form of 2,4-D is favored due
to the higher pKa of 2,4-D in this solvent compared
to water, since the ionized form will not dissolve in a
nonpolar fluid unless an ion pair is formed.
Introduction
Supercritical fluid extraction (SFE) with CO2 above critical
temperatures and pressures has provided a clean alternative
to liquid/liquid extraction techniques, especially for nonpolar pesticides in soils (1, 2). It has both industrial and
analytical applications, the former including the regeneration of adsorbents used in wastewater treatment (3, 4), and,
potentially, treatment of diesel oil- and PCB-contaminated
soil (5). Discussions of the theory, advantages, and some
of the applications of modified and unmodified supercritical
fluids for environmental analysis have been described by
others (6-10). Extraction of polar compounds has proved
difficult and will require a better understanding of both the
chemical and physical processes influencing SFE from soils.
The acid herbicide 2,4-D (2,4-dichlorophenoxyacetic
acid) was selected for this investigation due to its heavy
agronomic, industrial, and private usage and because it
represents a large class of herbicides, phenoxy acids.
Unfortunately, the recommended EPA techniques (Methods
8150 or 8151) for extracting chlorinated herbicides from
soil are time-consuming and require large volumes of the
toxic solvents diethyl ether or methylene chloride (11, 12).
A need to eliminate laboratory use of these solvents provides
incentives to understand the capabilities of modified
supercritical carbon dioxide as an extractant.
Supercritical fluid extraction of polar compounds, such
as 2,4-D, from environmental samples is enhanced by polar
modifiers added to the matrix or the supercritical fluid. For
instance, extractions using supercritical CO2 alone recovered less than 5% of the applied 2,4-D from soil samples
(13, 14), whereas the addition of derivatizing reagents and
ion-pairing reagents (14-16) and/or inorganic salts in
methanol (14) as modifiers improved the SFE of 2,4-D from
soils. Co-extraction of native soil organic matter compounds by ion pairing reagents may interfere with 2,4-D
extraction or analysis and may warrant the investigation of
alternative modifiers.
Testing of various extraction schemes has been largely
by trial and error, because too little is known about the
mechanisms governing polar solute sorption and extractability in supercritical fluids. The objective of this research
was to determine which of several common soil constituents
are likely to limit 2,4-D recovery by SFE. Model soil
components included representatives of common soil
minerals, porous materials, and organic matter to test
surface reactions, diffusion in porous materials, and pH
buffering as potentially limiting factors in 2,4-D extraction.
Materials and Methods
We considered and tested several extraction systems for
this study. For a spiked Palouse silt loam, soil recoveries
of 2,4-D decreased in the following order: BF3/methanol
methylation during SFE (66 ( 6%) ) a modified EPA Method
8150 (64 ( 5%) > 0.3 M CaCl2‚2H2O/methanol-modified
* Present address: Department of Plant, Soil, and Entomological
Sciences, University of Idaho, Moscow, ID 83844-2339; telephone
208-885-2947; fax: 208-885-7760; e-mail addrss: brochette@
marvin.csrv.uidaho.edu.
† Department of Crop and Soil Sciences.
‡ Department of Chemistry.
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 30, NO. 4, 1996
0013-936X/96/0930-1220$12.00/0
 1996 American Chemical Society
SFE (55 ( 2%) > 0.6 M benzoic acid/methanol-modified
SFE (51 ( 1%) g 0.6 M acetic acid/methanol/acetonitrilemodified SFE (42 ( 3%) > methanol-modified SFE
(26 ( 3) ) acetone-modified SFE (25 ( 5%) > hexamethyldisilazane/trimethylchlorosilane silylation during SFE (12
( 3%) (17). Because BF3/methanol and CaCl2‚2H2O/
methanol were harmful to PEEK (polyether ether ketone)
and/or stainless steel (14, 17), we selected 0.6 M benzoic
acid/methanol as a modifier for this study. Chromatographic interferences were not a problem. For this study,
it was necessary to use spiked samples since the materials
of interest were not available from the field in a 2,4-Dcontaminated form; spiked soils do not necessarily provide
an accurate representation of yields obtained from soils
that have been contaminated in the field by environmental
processes (18).
Minerals. Gibbsite [Al(OH)3], a single-layer type clay
common in highly weathered soils, was obtained from Alcoa
Chemicals and used without modification. It easily passed
through a 250-µm sieve. The supernatant from the deionized water/gibbsite slurry had a pH of 9.4 after 4 h of
equilibration. Goethite (FeOOH), an abundant soil iron
oxide mineral, and calcite (CaCO3), a common carbonate
mineral, were both obtained from Minerals Unlimited
(Ridgecrest, CA). The goethite was from the Cary Mine of
Gogebic County, MI, and was ground to approximately 50
µm in a stainless steel shatterbox. The calcite was a very
fine chalk from Bedfordshire, England, and was ground in
a porcelain mortar and pestle to pass through a 250-µm
sieve. Illite (K,Al,Si)4(Mg3Fe2Al2)O10(OH)2, a 2:1 clay abundant in soils of eastern Washington, was from a mine in
Pennsylvanian shale from the Goose Lake Area of Illinois
(19) and easily passed through a 250-µm sieve. Illite was
washed with organic solvents to remove impurities that
interfered with GC/ECD analysis of the supercritical fluid
extracts of the unwashed illite. The supernatant from a
deionized water slurry of cleaned illite had a pH of 3.9 after
4 h of equilibration.
Silica Gels. Three silica gels donated by Waters Chromatography Division of Millipore (Milford, MA) had particle
diameters of 15-20, 37-55, and 55-105 µm. All were
porous normal-phase packing materials. Waters provided
the following data with the silicas: surface areas (base silica)
of 320 m2/g, total carbon less than 0.1%, pore volume of
1.0 mL g-1, and median pore diameter of 125 Å for all three
silica size ranges. The silica gels served as model porous,
amorphous soil inorganic components.
Humic Materials. Sodium humate obtained from
Aldrich Chemical Company (Milwaukee, WI) and characterized by Malcom and McCarthy (20) was used without
modification. The pH of the sodium humate solution in
deionized water (0.29 g of sodium humate/mL of deionized
water) was 8.9. Humic acid was prepared by titrating 60
g of sodium humate dissolved in 1 L of deionized water
with concentrated HCl to a pH of 0.9. The humic acid
slurry was transferred to Spectrapor standard cellulose
dialysis tubing (Fisher Scientific, Pittsburgh, PA), with a
molecular mass cutoff of 12 000-14 000 Da and dialyzed
against deionized water in a 6-L glass vessel until the
electrolytic conductivity of the dialyzate dropped to 0.0003
S/m. After being dried, the humic acid was ground to pass
through a 1-mm sieve.
Chemicals. All organic solvents and NaCl used in this
study were Baker analyzed reagent grade and were used as
received. Absolute methanol was used as an SFE collection
solvent and modifier. For pH measurements in methanol,
HPLC-grade methanol was used. Boron trifluoride/
methanol (14%) was obtained from Alltech Associates
(Deerfield, IL). Benzoic acid (99% purity) was from Aldrich
Chemical Company (Milwaukee, WI). The 2,4-D acid (2,4dichlorophenoxyacetic acid; 98.6% purity), 2,4-D methyl
ester (97% purity), and lindane (1,2,3,4,5,6-hexachlorocyclohexane; 99% purity) were obtained from Sigma Chemical
Company (St. Louis, MO). The 2,4-D acid and 2,4-D methyl
ester were stored in a refrigerated desiccator with anhydrous
CaSO4 prior to use. Instrument-grade carbon dioxide was
obtained from Alphagaz (San Francisco, CA).
Spiking Procedure. Minerals, silica gels, and humic
materials were spiked to 10.8 ppm (and 1.08 ppm for
goethite) by preparing saturation pastes with deionized
water solutions as described by Rhoades (21) and adding
2,4-D in acetone to the paste. The ratio of acetone to
solution was 0.03 by volume. The materials were allowed
to dry, with the drying time controlled to 24 h either by
periodic addition of deionized water to fast-drying minerals,
or by mixing the pastes (illite and humic materials) with a
spatula to facilitate drying. Because the humic materials
and the 9:1 mixture of gibbsite plus humic acid formed
aggregates during the drying process, they were gently
crushed to pass through a 1-mm sieve. The material was
then homogenized, split, and placed in a freezer until use.
The high concentration of 2,4-D applied to the model soil
components was to account for the typically low mass
fraction of a given component in soil (generally less than
0.1).
In addition to spiking the silica gels with aqueous
solutions of 2,4-D, subsamples of all the silica gels were
analogously spiked with acetone solutions of 2,4-D. The
treatment was identical except that water was excluded by
drying the silicas at 105-107 °C overnight in an oven and
cooling in a desiccator prior to spiking; also, the silicas
were maintained in a desiccator during the spiking, drying,
and splitting processes. Gravimetric analysis after spiking
indicated that less than 3% acetone (by mass) remained on
the silica after spiking.
Supercritical Fluid Extractions. Extractions of the
minerals, silica gels, and humic materials were performed
with an Isco SFX-210 supercritical fluid extractor with an
Isco 260D syringe pump. Lindane, used as our internal
standard, was injected on the exiting filter frit of the SFE
cell, and 5.0 ( 0.02 g (goethite and gibbsite), 2.50 g (illite,
calcite and silica gels), or 1.00 g (humic acid and sodium
humate) was placed in the 10-mL stainless steel extraction
cell. The reduced mass of material for illite, calcite, and
humic material prevented problems of reduced flow rate
and restrictor clogging encountered when larger masses of
these materials were used. For silica gels, it was not possible
to fit 5 g into the cell due to its low bulk density. In all
cases, the ratio of modifier to solid was 0.3 mL:1 g per
modifier addition. This volume of modifier was selected
since many soils are saturated with solution at 30% moisture
content. After the modifier was injected onto the sample,
the extraction chamber was brought to 80 °C and pressurized to 350 atm. Replicate samples were extracted in
series. Static extractions were performed for 15 min;
dynamic extractions followed with CO2 exiting through 30
cm of 50 µm i.d. fused silica tubing that emptied into vented,
capped test tubes. One milliliter of methanol was placed
in the collection tubes. Dynamic extractions lasted approximately 15 min each and were performed by passing
VOL. 30, NO. 4, 1996 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
1221
20 mL of pressurized CO2 through the sample at a flow rate
of 1-2 mL of pressurized CO2/min. The flow restrictor
was inserted through a section of aluminum pipe, which
was heated to approximately 100 °C. The extractor was
allowed to vent (the remaining CO2 depressurized) for 20
min into the collection tubes after each extraction and then
was completely vented through the instrument vent valves.
Additional modifier (0.3 mL/g solid) was added to the
cell, the restrictor was placed into a new tube containing
1 mL of methanol, and the extraction continued. Final
extract volumes were adjusted to 2 mL with methanol.
Esterification. All extracts were esterified with BF3/
methanol, based on the protocols of Pierce (Rockford, IL),
Alltech Associates (Deerfield, IL), and Erner and Coggins
(22): (a) 1-mL aliquots of the extracts were placed in 5-mL
derivatization vials; (b) 1 mL of BF3/methanol was added
to the extracts; the vials were capped, heated on a hot plate
to 60 °C for 15 min, cooled for at least 15 min, and 1 mL
of aqueous 1 M NaCl was added to each vial and shaken;
(c) the samples were partitioned into a total of 1 mL of
benzene using three portions of benzene; (d) the benzene
layers were removed, combined, and centrifuged for
approximately 5 min; and (e) the final benzene solutions
were placed in vials and sealed. Standards containing
lindane were prepared from 2,4-D methyl ester stocks and
were treated in the same way as extracts except that 1 mL
of methanol (instead of BF3/methanol) was added in step
b and the standards were not heated. Also, standards were
prepared from 2,4-D stocks and went through all steps used
for the extracts. These standards always agreed with each
other within 10%, and the latter were used for recovery
calculations.
Gas Chromatography. Analyses were performed with
a Tracor 560 gas chromatograph, which was equipped with
an 18-m DB-5 capillary column, a split-mode injection
system, and an electron capture detector (ECD). Analyses
of methyl esters were performed at a column temperature
of 180 °C (isothermal), injector at 250 °C, and ECD at 325
°C.
Surface Area Measurements. Surface areas of all of the
model components were measured using the ethylene glycol
monoethyl ether (EGME) method of Carter et al. (23).
Samples were dried for 24 ( 2 h in an oven at 110 °C instead
of P2O5 drying prior to analysis. Sample mass measurements, followed by desiccator evacuations (45 min each),
were made until weight losses of EGME did not exceed
0.0002 g. A standard of freeze-dried Ca2(-SWy-1 (Wyoming
bentonite; Clay Minerals Society) was analyzed with each
group of four samples. The surface areas of all three silica
gels, goethite, and humic acid were also determined by
BET-N2 on an Autosorb-6 instrument (Quantachrome
Corporation, Syosset, NY). The BET-N2 surface area values
were from multipoint BET measurements.
pH Measurements in Methanol. The pH of slurries of
minerals, silica gels, and humic materials with methanol
and benzoic acid/methanol were measured with a Radiometer GK 2321 C combination glass electrode attached to
a Beckman φ45 pH meter. Probe filling solutions were
aqueous saturated KCl as recommended by Radiometer
for aqueous solutions and 0.08 M LiCl in methanol. Lithium
chloride in methanol was recommended for titrations in
methanol (24). The two filling solutions gave essentially
the same pH results.
Modifiers of 0.6 M benzoic acid/methanol and HPLCgrade methanol were applied to the samples in a ratio of
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 30, NO. 4, 1996
FIGURE 1. SFE recoveries of 2,4-D from materials spiked with aqueous
solutions of 2,4-D. The modifier was 0.6 M benzoic acid/methanol.
Error bars represent absolute standard deviations for triplicate
extractions. The concentration of 2,4-D (µg g-1) is indicated for
goethite and was 10 µg g-1 for all other materials. The silica data
are from the 37-55-µm particle diameter silica treated with 2,4-D
in acetone.
0.3 mL/g of sample as was done for SFE modification. The
modifiers were left in contact with the sample for 15 min
and diluted with 2 mL of methanol/g of sample (2:1
dilutions). The slurries were thoroughly mixed and centrifuged, and the supernatant solution was decanted for
measurement. For silica gels, it was necessary to make 3:1
dilutions due to their large surface areas; however, it was
possible to obtain sufficient sample from a 2:1 dilution of
the 55-105-µm silica, which allowed a comparison with
the 3:1 dilution. There was no significant difference at the
0.05 significance level between these dilutions. The pH
was calculated by using a sodium salicylate (0.01 M)/salicylic
acid (0.01 M)/methanol buffer and the following equation:
pH sample )
Ebuffer (mV) - Etest solution (mV)
(0.198) (temperature (K))
+
buffer pH
where 0.198 is a constant in units of mV K-1 (25) and was
obtained by combining constants from the Bates-Guggenheim convention (26). The buffer pH was assumed equal
to 7.53 as given by Dean (26). A second buffer of ammonium
hydrogen oxalate (0.01 M)/oxalic acid (0.01 M)/methanol
having a theoretical pH value of 5.79 (26) and a 0.0008 M
HCl/methanol solution were utilized to verify the linearity
of the probe response.
Results
Our experiments were conducted to determine the influence
individual soil components may have on 2,4-D partitioning
into a supercritical fluid. Soil component effects may result
from interactions of 2,4-D with specific surface functional
groups or from the effects of pH, surface area, and porosity,
which vary from one soil component to another.
Minerals. Total recoveries from experiments with 0.6
M benzoic acid/methanol-modified SFE of 2,4-D from the
minerals are given in Figure 1 and Table 1. The recovery
from gibbsite (100 ( 3%) is consistent with the finding that
2,4-D is weakly adsorbed by alumina (27). Recovery of
2,4-D from goethite was similar whether spiked at 10 µg g-1
(91 ( 3%) or at 1 µg g-1 (83 ( 3%). Further additions of
TABLE 1
Physical Characteristics and Total 2,4-D Recoveries from Model Soil Components
component
goethite
gibbsite
calcite
humic acid
sodium humate
illite
15-20 µm silica
water
acetone
37-55 µm silica
water
acetone
55-105 µm silica
water
acetone
EGME surface
(m2 g-1)
total surface in
extraction cella
(m2)
N2-BET surface
(m2 g-1)
total surface in
extraction cella
(m2)
5(1
5(2
23 ( 1
51 ( 1
54 ( 6
136 ( 7
241 ( 9
25 ( 5
25 ( 10
57.5 ( 2.5
51 ( 1
54 ( 6
340 ( 18
603 ( 23
0.88
4.4
0.034
0.034
271
677
376 ( 17
940 ( 43
253
633
363 ( 9
907 ( 23
305
763
total recovery (
absolute SD (%)
91 ( 3
100 ( 3
69 ( 4
80 ( 3
11 ( 2
88 ( 6
87 ( 7
70 ( 4
83 ( 6
87 ( 6
72 ( 3
81 ( 1
a Calculated using the mass of the component in the extraction cell (g) × the respective surface area (m2 g-1). The modifier volume was always
0.3 mL/g of component in the extraction cell.
TABLE 2
Silica Recovery Ratios, Micropore Characteristics,
and Water Contents after Spiking
silica
37-55 µm
water
acetone
15-20 µm
water
acetone
55-105 µm
water
acetone
microporeb
recovery surface area micropore vol
water
ratioa
(mL g-1)
content (%)
(m2 g-1)
8.5
0.0035
0.68
0.62
14.0
14.4
0.0065
0.88
0.70
31.5
19.5
0.96
0.80
0.0091
41.0
a Recovery ratio ) (recovery from 1st modifier addition/total recovery). b Combined surface area of pores less than 20 Å wide.
modifier may have increased 2,4-D recovery from illite and
calcite given the relatively large amounts of 2,4-D in the
second extractions of these minerals. In general, higher
recoveries of 2,4-D were obtained from gibbsite, goethite,
illite, and calcite than from soils extracted in previous
studies.
Silica Gels. Total recoveries of 0.6 M benzoic acid/
methanol-modified SFE of 2,4-D from spiked silica gel are
given in Table 1. Prior to spiking, the silicas all contained
between 2.5 and 2.9% moisture; however, after treating the
samples with aqueous spike solutions and air drying, all
silicas had moisture contents greater than 10% (Table 2).
The recovery ratio (the 2,4-D recovery from the first addition
of modifier divided by the total 2,4-D recovery from the
first and second additions of modifier) correlated with the
silica moisture contents for the aqueous 2,4-D-spiked silicas
[recovery ratio ) 0.011 (moisture %) + 0.537, r2 ) 0.995].
The recovery ratio is assumed to be positively related to the
2,4-D extraction rate, though each 30-min static/dynamic
extraction coincides with the addition of more modifier.
Since essentially no 2,4-D could be recovered from soils
without modifier over equivalent periods of time in a
previous study (14), the extraction rate is not strictly timedependent in these experiments and is probably more
dependent on solid/fluid (especially modifier) partition
coefficients.
Additionally, a strong correlation between micropore
surface area and volume and the moisture content of watertreated silicas was seen. Because water content introduced
a complicating chemical factor, it was not possible to
determine if microporosity was directly or indirectly related
to the extraction rate. Extractions of 2,4-D from silicas
spiked with acetone solutions of the herbicide were made
to avoid the possible complications that may arise from
the variable amounts of adsorbed water present in the
water-treated silicas after air drying. Overall, the mean
total 2,4-D recoveries from acetone-treated silicas (79%) is
essentially the same as that from the water-treated silicas
(81%) after two additions of modifier; however, the rate of
2,4-D recovery from the water-treated silicas was greater
than from the acetone-treated silicas.
Humic Materials. The results of 0.6 M benzoic acid/
methanol modified SFE of 2,4-D from humic materials are
given in Figure 1 and Table 1. The quantity of 2,4-D
extracted from sodium humate (11 ( 2%) was much lower
than that from humic acid (80 ( 3%). Mixing humic acid
with gibbsite (prior to spiking) reduced the amount of 2,4-D
recovered in the first modifier addition compared to pure
gibbsite. A third modifier addition to the mixture brought
the total 2,4-D recovery to 97 ( 5%.
Surface Areas. Results from both EGME and N2-BET
surface areas are presented in Table 1 for all of the
components measured. The surface area of the Ca2+saturated SWy-1 EGME standard, 691 ( 39 m2 g-1,was
similar to the value reported by Madsen (28). In general,
2,4-D recoveries were poorly correlated (r2 ) 0.007) with
EGME surface areas. Correlations improved to r2 ) 0.724
when calcite, sodium humate, and humic acid were
removed from the data set. Poor correlations (r2 ) 0.192)
between surface area and recovery were also observed with
N2-BET data, which were only available for five samples.
The EGME surface areas for humic acid were higher than
N2-BET surface areas. Other differences between N2 and
EGME surface areas were not large enough to affect overall
relationships between recovery and surface areas.
The surface areas of humic acid and sodium humate
were low compared to literature values for soil organic
matter surface areas measured with polar liquids. Bower
and Gschwend (29) reported ethylene glycol surface areas
VOL. 30, NO. 4, 1996 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
1223
TABLE 3
pH Values of 2:1 Mineral, Silica, and Humic
Material Slurries in Modifiers, with Aqueous KCl
Probe Filling Solutiona
sample
sodium humate
silica (55-105 µm)
acetone-2,4-D spike solutions
aqueous-2,4-D spike solutions
calcite
gibbsite
illite
goethite
humic acid
benzoic acid/
methanol
pH ( SD
methanol
pH ( SD
9.37 ( 0.01
12.01 ( 0.10
7.89
7.53
7.42 ( 0.01
(7.23 ( 0.01)
6.99 ( 0.02
6.59 ( 0.08
6.22 ( 0.11
4.74 ( 0.05
10.93 ( 0.08b
10.81 ( 0.01b
11.03 ( 0.04
11.34 ( 0.00
10.93 ( 0.08
9.62 ( 0.23
4.77 ( 0.01
a Values measured with LiCl/methanol probe filling solution are given
in parentheses when they differed statistically. b Prepared as 3:1
methanol slurries.
for soil organic matter of 558-803 m2 g-1, based on soil
samples before and after treatment with H2O2. The humic
acid and sodium humate in this study, like the other
materials analyzed by the EGME technique, were ovendried at 110 °C for 24 h to remove water before the EGME
treatment. This may have resulted in some collapse of the
humic materials, as has been observed for air-dried humic
and fulvic acids (30), and in the resulting loss of specific
surface area (29).
pH Experiments. The effects of SFE modifiers on pH
were examined by measuring the pH of the model soil
components in the presence of methanol and 0.6 M benzoic
acid/methanol (Table 3). No significant differences existed
between H+ activities of the three particle sizes of silica
(0.05 significance level). On the other hand, the slurry of
moist 55-105-µm silica in benzoic acid/methanol was 0.4
pH unit lower than that for the dry silica, so the presence
of water in the silica appears to slightly lower the pH in
silica/benzoic acid/methanol slurries.
Discussion
Physical Factors Affecting Extraction. Discerning chemically controlled processes from physically controlled processes was done in previous studies (14, 17) by varying
chemical modifiers while the soil type was kept constant.
Additional chemical information was obtained in this study
by changing the solid phase from which 2,4-D was extracted
while maintaining a constant chemical modifier to the SFE
system. The latter type of experiment, involving surfaces
representative of soils, inherently also changed the physical
features of the system. The surface area data (Table 1)
were collected to help distinguish this potential source of
variability in extraction results from purely chemical effects.
Recoveries of 2,4-D cannot be explained by surface area
variations. The poor correlation coefficient (0.007) was due
mainly to humic acid, sodium humate, and calcite, implying
that chemical factors are important determinants of
extractability. General relationships aside, the relatively
lower recovery of 2,4-D from illite compared to gibbsite
may be due to the higher surface area of illite.
When silica gels were considered exclusively, relationships between 2,4-D recovery and surface area were
expected to be free of chemical variability, whereas physical
characteristics such as surface area, particle size, porosity,
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 30, NO. 4, 1996
and probably pore tortuosity varied. Several important
physical factors relate to diffusion path geometries: tortuosity, the presence of “dead-end” pores, and variable
pore diameters (31). Janda et al. (9) point out that the rate
of supercritical fluid extraction depends on the radius of
the matrix particles. In this study, few if any relationships
between 2,4-D recovery and the measured physical characteristics of the silicas could be found. For the most part,
the differences between the physical characteristics of the
silicas and the differences in 2,4-D recoveries were relatively
small. The silicas may actually be very similar in their
micropore geometries and, if so, 2,4-D extractability may
be independent of the variable macropore characteristics.
Nonetheless, there are some differences between the
recoveries from the individual silicas, suggesting that there
may be physical effects on 2,4-D extraction that are not yet
understood. Indirectly, physical factors may have affected
the chemistry of the extractions of the silicas spiked with
aqueous solutions of 2,4-D. These silicas contained variable
amounts of water, which increased as the micropore surface
areas and volumes increased (Table 2). The retained water
may have acted as a supercritical fluid modifier (to be
discussed later).
Chemical Factors Affecting Extraction. Previous studies with soils in which the 2,4-D recoveries changed with
modifier chemistry (14, 17) provide evidence that chemical
interactions among 2,4-D, the supercritical solvent, and
soil are important in controlling extractability by SFE. For
instance, boron trifluoride/methanol enhanced 2,4-D extraction by derivatizing the compound to an ester, which
is a less polar compound than the acid form of 2,4-D and
is expected to have a higher affinity for nonpolar solvents.
The role of supercritical fluid modifiers (aside from derivatizing and ion pairing agents) in the extraction of polar
and nonpolar compounds from solids has been attributed
to modification of the polarity of the supercritical fluid (32),
solubility enhancement (of dioxin) by methanol-modified
supercritical CO2 (33), H-bonding-enhanced solubility (34),
effects on pore diffusional resistances and desorption from
soils (35), and improved wetting of soil by polar methanol
and dimethyl sulfoxide (36). There have been, however,
no suggestions that methanol enhances extraction efficiencies by affecting the pKa values of ionizable solutes. It has
been shown that the pKa of 2,4-D is roughly 5 pH units
higher in methanol than in water (pKa values of 7.6 and
2.64, respectively), favoring the protonated form in methanol solvent (37). The methanol modifier should, therefore,
enhance extraction by increasing the abundance of the acid
form of 2,4-D in the soil. The nonionic acid form of 2,4-D
is likely to be more extractable in solvents of low polarity,
such as supercritical CO2, than the ionic form. Thus,
CaCl2‚2H2O/methanol modifier used in previous studies
(14, 17) and benzoic acid/methanol modifier may have
improved 2,4-D recovery compared to methanol alone by
decreasing the pH of the soil solution prior to and during
SFE.
The chemical nature of the soil components may also
influence 2,4-D extractability. Solution pH appears to be
an important factor in the case of minerals as well as soils.
Mineral surfaces, with the exception of calcite, do not appear
to limit 2,4-D extraction, as the highest 2,4-D recoveries
from the entire study were obtained from gibbsite, goethite,
and illite. Slurries of these minerals also had relatively low
benzoic acid/methanol pH values; only humic acid had a
lower value than illite. Slurries of calcite in benzoic acid/
methanol maintained slightly higher pH values than the
other minerals, and 2,4-D yields from calcite were the lowest.
On the other hand, a small effect of 2,4-D adsorption to
oxide surfaces is possible. Recovery from gibbsite is rapid
and complete, whereas goethite yielded less 2,4-D and
required two extractions to reach 90% recovery. Consistent
with this, 2,4-D is reversibly adsorbed to goethite (38) but
negatively adsorbed to aluminum oxide (27). It is tempting
to conclude that the 10% of 2,4-D not recovered from
goethite was adsorbed by specific surface sites; however,
if sites capable of irreversibly sorbing 2,4-D existed, the 1
µg g-1 spike should have been unrecoverable. It is neither
possible to invoke nor rule out surface effects on 2,4-D
recovery with these data.
Moisture also affects 2,4-D extractability. The recovery
ratio (Table 2) correlated strongly with the moisture retained
on the air-dried silicas. This suggests that water acted as
an additional modifier, increasing 2,4-D affinity for the
methanol/CO2, particularly in the first extraction where
the water content was higher. This is supported by
extractions from silicas spiked from acetone solutions,
where the recovery ratios were comparable to that of the
low-moisture, aqueous 2,4-D-treated silica (37-55 µm).
Water also appears to slightly lower the pH of the silica/
benzoic acid/methanol slurries, as described in the Results
section for the moist 55-105-µm silica. Snyder et al. (39)
found that when water was added to soil before SFE, the
extraction of polar pesticides by supercritical CO2 improved.
The tendency for water to enhance the recovery of polar
compounds although limit the extraction of nonpolar
compounds was discussed by Camel et al. (8). After two
modifier additions, however, the mean 2,4-D recovery from
the silicas treated with aqueous and acetone spiking
solutions were equivalent (81% and 79%, respectively).
Organic Matter. Humic materials in soils are among
the most important buffers of soil acidity and are highly
porous. Therefore, organic matter likely affects 2,4-D
extractability both via physical and chemical processes.
Though the Aldrich humic acid used in this study is not an
exact analog of soil organic matter (20), it does have many
of the above features in common with soil organic matter
and allows comparison of 2,4-D extraction from a wellcharacterized organic material with that of several minerals
and silica.
The highest and lowest pH values in benzoic acid/
methanol were those of sodium humate and humic acid,
respectively, and 2,4-D extractability differed dramatically
for these two components (Tables 1 and 3). The distribution
of 2,4-D between acid and base forms is difficult to quantify
because the pKa’s of benzoic acid and 2,4-D and the solution
pH depend on the soil moisture, methanol concentration,
and CO2 pressure. Any attempt to quantitatively model
this complex system without knowledge of the appropriate
pKa values would be fraught with uncertainty. Nonetheless,
the pHs of humic acid (4.74 ( 0.05) and sodium humate
(9.37 ( 0.01) in benzoic acid/methanol slurries provide
relative measures of the pH of these materials before
supercritical CO2 was added to them.
The pKa of 2,4-D was shown to be 7.6 in methanol (37),
which is roughly 1.8 units below the pH of the sodium
humate/benzoic acid/methanol slurries and almost 3 units
above the pH of humic acid/benzoic acid/methanol slurries.
The presence of mineral or organic surfaces may alter the
reactivity of H+ or 2,4-D such that protonation exceeds
that predicted from the pH of the bulk solution and the
FIGURE 2. Recoveries of 2,4-D from benzoic acid/methanol-modified
SFE (first modifier addition) plotted against the pH of 2:1 slurries of
minerals, silica gel, and humic materials in benzoic acid/methanol.
Error bars represent absolute standard deviations for triplicate
extractions. The curve represents the “expected recovery” of 2,4-D
based on the acid/conjugate base ratios at various pHs. The pKa of
2,4-D in methanol used for this calculation was 7.6 (37).
pKa. Harter and Ahlrichs (40) showed that significant
protonation and sorption of 2,4-D on Na+- and H+-saturated
montmorillonite occurred at pHs 2 or more units above its
pKa. This may explain recovery of as much as 11% of the
2,4-D even when the suspension pH exceeded the pKa by
almost 1.8 units in the case of sodium humate.
Intraorganic matter diffusion, known to be the dominant
process limiting the sorption of hydrophobic organic
compounds onto soils (41), could also limit the extraction
of 2,4-D from humic materials. Khan (42) found rapid
adsorption of 2,4-D by humic acid (in water) for the first
1-1.5 h of treatment. Continued adsorption thereafter was
linearly related to the square root of treatment time and
was attributed to intraparticle transport. Molecular diffusion slows sorption of organic compounds in soils, with
sorption to pore walls of small pores and organic gels
responsible for retarding diffusion (42). Slow diffusion
through the humic matrix may explain the reduced rate of
extraction from gibbsite with humic acid relative to gibbsite
alone (Figure 1).
If the solution pH of the soil components in the presence
of the modifier is the only factor controlling the recovery
of 2,4-D, then an expected 2,4-D recovery can be calculated
using the pH in the modifier and the pKa of 2,4-D in
methanol. An “expected recovery” curve has been plotted
on Figure 2. While many components plot near the line,
humic acid yielded a relatively low recovery given its pH.
Slower diffusion through the 3-D organic matrix than
through inorganic components, and/or a relatively lower
partition coefficient for 2,4-D between humic acid and the
fluid, compared to that for inorganic components, may
cause this discrepancy. Almost 30% more 2,4-D was
obtained when a second extraction of the humic acid was
made (Figure 1). Sodium humate has such a high pH and
such a low 2,4-D recovery compared to inorganic materials
that diffusional limitations are difficult to discern from the
effects of pH. The effects of soil organic matter both on the
proportion of 2,4-D in the extractable form (protonated in
this case), on 2,4-D diffusion rates, and on solid-fluid
partitioning are likely to be the most important factors
limiting the rate and extent of extraction of 2,4-D from soils
by modified SFE.
VOL. 30, NO. 4, 1996 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
1225
Acknowledgments
The activities on which this paper was based were financed
in part by the Department of the Interior, U.S. Geological
Survey, through the State of Washington Water Research
Center. The contents of this paper do not necessarily reflect
the views and policies of the Department of the Interior,
nor does mention of trade names or commercial products
constitute their endorsement by the United States Government. The authors thank Doug Wittmer of Waters Chromatography Division of Millipore for donating the silica
gels used in this study. The assistance of Keith D. Miller
and Gerard P. Irzyk with the lyophilizer is gratefully
acknowledged. We thank William C. Koskinen for helpful
suggestions and Clarke Maxwell for performing N2-BET
surface area measurements.
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Received for review June 20, 1995. Revised manuscript received November 9, 1995. Accepted November 14, 1995.X
ES950432N
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Abstract published in Advance ACS Abstracts, February 1, 1996.