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Sorption of volatile organic solvents from aqueous solution onto subsurface solids
Journal of Contaminant Hydrology, 4 (1989) 163-179
163
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
SORPTION OF VOLATILE ORGANIC SOLVENTS FROM AQUEOUS SOLUTION ONTO SUBSURFACE SOLIDS
MARVIN D. PIWONI1 and PINAKI BANERJEE2 i Hazardous Waste Research and Information Center, 1808 Woodfield Drive, Savoy, IL 61874, U.S.A. 2PRC Environmental Management Inc., 303 E Wacker Drive, Chicago, IL 60601, U.S.A.
(Received May 6, 1988; revised and accepted July 15, 1988
ABSTRACT Piwoni, M.D. and Banerjee, P., 1989. Sorption of volatile organic solvents from aqueous solution onto subsurface solids. J. Contam. Hydrol., 4: 162-179. Sorption isotherms for tetrachloroethene on low-carbon subsurface core samples were linear to equilibrium solution concentrations of 2mgL -1. Concentrations above this value produced pronounced curvature in the sorption isotherms. Sorption of tetrachloroethene, benzene, trichloroethene, and 1,2-dichlorobenzene on low-organic-carbon aquifer materials (foe < 0.001) was 2~4 times that predicted based solely on sorbent organic carbon content. Estimates of the mineral surface contribution to sorption and the relationship of this mineral component to sorbate Koware presented. An approach to predicting organic solvent sorption in low-carbon natural environments is proposed.
INTRODUCTION
O r g a n i c c a r b o n c o n t e n t of the s o r b e n t a n d o c t a n o l - w a t e r p a r t i t i o n coefficient (Kow) of the s o r b a t e h a v e evolved as the two p a r a m e t e r s most c o m m o n l y used for e s t i m a t i n g s o r p t i o n of n o n p o l a r o r g a n i c c o m p o u n d s to sediments and soils. The t h e o r y t h a t forms the basis for these estimates is called the hydrophobic b o n d i n g t h e o r y , a n d is p r e d i c a t e d on the fact t h a t the process of part i t i o n i n g of a n o n p o l a r o r g a n i c m o l e c u l e b e t w e e n the a q u e o u s phase and the s o r b e n t o r g a n i c c a r b o n is d r i v e n p r i m a r i l y by the large excess of free e n e r g y of the solute in a q u e o u s solution. The l i t e r a t u r e c o n t a i n s a n u m b e r of linear r e g r e s s i o n models d e s c r i b i n g the r e l a t i o n s h i p b e t w e e n s o r p t i o n coefficients and Kow v a l u e s ( K a r i c k h o f f et al., 1979; S c h w a r z e n b a c h and Westall, 1981; C h i o u et al., 1983; a m o n g others). A p p l i c a t i o n of these models to estimate s o r p t i o n based on k n o w n s o r b e n t o r g a n i c c a r b o n c o n t e n t are g e n e r a l l y a c c u r a t e to w i t h i n a f a c t o r of two to five, d e p e n d e n t on the similarity of the o r g a n i c c o m p o u n d s of i n t e r e s t with t h o s e on w h i c h the r e g r e s s i o n models were developed. The capabilities of these r e l a t i o n s h i p s are best w h e n p r e d i c t i n g s o r p t i o n coefficients u n d e r e x p e r i m e n t a l c o n d i t i o n s and with o r g a n i c c o m p o u n d s similar to t h o s e used in model development. The s o r p t i o n limits over w h i c h these c a r b o n - b a s e d
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164
models are applicable are defined at the low end by some minimum sorbent fraction organic carbon, foe, and at the high end by some fraction of sorbate solubility. These limits are likely variable, dependent on sorbate Kow and sorbent foe (Curtis et al., 1986a). Unfortunately, many investigations of organic contaminant transport through the subsurface involve predictions on systems where these regression models do not apply. Nonpolar, water-immiscible, organic solvents have gained notoriety as common groundwater pollutants. Most such solvents have relatively high water solubilities and low Kowvalues, so consequently interact only to a limited degree with natural sorbents. These properties, the naturally low foe of the subsurface, and the extensive use of solvents in society combine to contribute to the widespread contamination of groundwaters by these compounds (Pye and Patrick, 1983; Westrick et al., 1984), many of which are also known or suspected human carcinogens. Although it should be obvious that the degree of sorption of organic solvents in aquifer systems will be relatively less than that in high foc sediments and soils, interactions between these molecules and mineral surfaces are likely to be important. Over long times and travel distances, even a small retardation factor will contribute to the size of a pollutant plume (Roberts et al., 1986) and result in a substantial increase in the volume of water that must be pumped and treated to realize aquifer remediation. Workers addressing subsurface contamination by organic solvents need improved tools for predicting the sorptive behavior of these contaminants. For very low foc sorbents, mineral surfaces can provide an alternate site for sorption of nonpolar organic molecules. The subsurface environment often provides an abundance of such surfaces with little organic carbon. The foc of subsurface materials tends to decrease with depth (Banerjee et al., 1985) and is typically less than 0.001 in unconsolidated aquifers. Although the clay content, particularly in the saturated zone, is often low, the mineral surfaces could play a significant role in the sorption of all types of organic molecules. For sorption of organic molecules on mineral surfaces to become important, one of both of two conditions must be satisfied. Either the molecules must exhibit some polar functionality that can result in site-specific interactions with the mineral surface (Curtis et al., 1986a), or mineral surfaces must be present in such relative abundance that sorbate interactions with these surfaces become of the same magnitude as sorbate interactions with the organic matter on the sorbent (Karickhoff, 1984). For nonpolar organics, the second condition would be of most significance. Hassett et al. (1980) presented data for aromatics, including polar compounds, that showed sorption to soils well above that predicted based on the foc of the sorbents. Using these same data, Karickhoff (1984) demonstrated that '~mineral phase" sorption was most pronounced in those sorbents having a substantial clay mineral/organic carbon ratio. Little attention has been paid to sorption of organic solvent molecules to aquifer material. Schwarzenbach and Westall (1981) found more sorption of four halogenated and aromatic organics to an aquifer material than could be
165 attributed to the sorbent foc. Curtis et al. (1986b) measured sorption of tetrachloroethene (PCE), 1,2-dichlorobenzene (DCB) and other solvents on Borden aquifer material and found sorption in excess of what would be expected based on foc. Curtis and Roberts (1985) suggested that mineral surfaces play an important role in the sorption process in Borden aquifer material. A need exists to better understand the characteristics of the types of surfaces available for sorption in the subsurface and to determine to what degree these various surfaces affect the sorption of organic contaminants (Curtis et al., 1986b). The existing database does not permit this type of interpretation. In this paper, we present sorption data for selected organic solvents on several subsurface materials. The goals of the study were to advance the qualitative understanding of sorption of four widely-used organic solvents from aqueous solution onto natural sorbents with low foe and to attempt to more quantitatively define a mineral component to that sorption. The ultimate objective was to facilitate a sound basis for predicting transport behavior of dissolved solvents in aquifers. The work first focused on characterizing PCE sorption on samples obtained primarily from two semi-continuous subsurface cores, and then on sorption of four nonpolar organic solvents on three saturated zone sorbents with low organic carbon content. MATERIALSAND METHODS Materials Benzene (BZ), trichloroethene (TCE), PCE and DCB were obtained as ~4Clabeled material from Pathfinder Laboratories (St. Louis, MO). GC and GC/MS analysis showed the isotopes to be 95% pure or better, so they were used as received. The isotopes were diluted with unlabeled compound from Chem Service (West Chester, PA) or Aldrich Chemical (Milwaukee, WI) with purities of 98% or better. The stock solutions were prepared in reagent grade methanol, stored in Teflon and glass vials at - 20°C, and checked occasionally by GC/MS to assure purity. The sorbents used were, with one exception, collected by Robert S. Kerr Environmental Research Laboratory (RSKERL) personnel using a rotary drilling rig fitted with 10-cm diameter custom-made brass core barrels. Cores were collected by alternately pressing the core barrel (50cm) into the sediments, retracting the core, and redrilling to the top of the next sampling interval. Cores were hydraulically extruded on site into polyethylene bags and transported to RSKERL for further processing. Most of the sorbents were collected in two cores, designated as the J and N cores, near Lula, Pontotoc County, OK. The J core consisted of 20 contiguous sections of the unsaturated zone down to a depth of 675 cm below the soil surface. The N core samples were taken at approximately 90-cm intervals down to a depth of 600 cm and included two samples of the saturated zone material. Two additional aquifer material samples were used in this study. The C-1 sample was composited from a series of saturated zone samples collected from
166 b o r e h o l e s d r i l l e d n e a r L u l a , O K . T h e c o l l e c t i o n a n d p r o c e s s i n g o f t h e B-1 s a m p l e , f r o m t h e B o r d e n A i r F o r c e B a s e s i t e i n O n t a r i o , C a n a d a , is d i s c u s s e d by C u r t i s e t al. (1986b).
Methods A l l c o r e s a m p l e s c o l l e c t e d by R S K E R L w e r e a i r - d r i e d , g r o u n d w i t h m o r t a r a n d p e s t l e a n d s i e v e d t h r o u g h a 1-mm s c r e e n . S e l e c t e d s a m p l e s w e r e s u b s e q u e n t l y s i e v e d t o p a s s e i t h e r a 500 o r 250-pm s c r e e n as n o t e d i n T a b l e 1. T h e b u l k - s i e v e d s a m p l e s w e r e s p l i t i n t o a p p r o x i m a t e l y 1-kg s u b s a m p l e s u s i n g a S o i l t e s t splitter. T h e s o r b e n t s used to m e a s u r e s o r p t i o n of the four o r g a n i c s o l v e n t s w e r e s e q u e n t i a l l y s p l i t , f i r s t t o a p p r o x i m a t e l y 100-g s u b s a m p l e s a n d t h e n t o 2 -4 -g a l i q u o t s , o n a Q u a n t a c h r o m e riffle s p l i t t e r . T h e s e a l i q u o t s w e r e t r a n s f e r r e d d i r e c t l y to the i s o t h e r m r e a c t i o n tubes. The m e a n mass of s o r b e n t t r a n s f e r r e d t o t h e 24 t u b e s o f a n i s o t h e r m s e r i e s r o u t i n e l y s h o w e d a r e l a t i v e s t a n d a r d d e v i a t i o n o f 1.5% o r less. T h i s s p l i t t i n g p r o c e d u r e i n s u r e d t h a t e a c h tube contained a reproducible sorbent sample representative of the bulk material. A combustion method using a Leco Model WR12 Carbon Analyzer was used t o m e a s u r e t h e o r g a n i c c a r b o n c o n t e n t o f al l s o r b e n t s . T h i s p r o c e d u r e , TABLE 1 Characteristics of subsurface samples Sample
Size fraction a (mm)
Organic carbon b (%)
Clay-sized fraction (%)
Saturated zone sample
N-1 N-2 N-3 N-5 N-6 N-6 N-7 J-6 J-7 J-9 J-10 J-10 J-11 J-12 J-14 J-19 B-I C-1
<1 <1 <1 <1 <1 < 0.25 <1 <1 <1 <1 <1 < 0.25 <1 <1 <1 <1 < 0.5 <0.5
1.33 (0.11) 0.054 (0.002) 0.039 (0.003) 0.034 (0.002) 0.028 (0.007) 0.028 (0.002) 0.042 (0.004) 0.39 0.89 0.39 0.19 0.26 (0.01) 0.15 0.18 0.08 0.23 0.031 (0.001) 0.021 (0.004)
13 24 21 34 14 23 12 13 17 13 13 14 9 6 9 12 11 14
No No No No Yes Yes Yes No No No No No No No No No Yes Yes
TSA (m2g - 1)
CEC (meq NH 4/g)
30 +_ 2.0
4.5 + 0.4
46 + 3.2
7.2 _+ 0.4
0.8° 11 + 1.2
1.5 + 0.3 2.5 _+ 0.2
a Size fraction of sorbent used for isotherm measurements. bValues in parentheses indicate + 1 standard deviation. Values without standard deviations from Banerjee et al. (1985). c Our measured value was < 10 m2g-1. The value presented is from Curtis et al. (1986b).
167 performed on 1-g samples preleached with 5% HC1 to remove inorganic carbon, generally yielded + 10% precision. Attempts to use a Dohrmann analyzer for low foc sorbents proved unsuccessful because of the precision problems inherent in subsampling such sorbents in 50-100mg quantities (see Powell et al., 1989). Cation exchange capacity (CEC) was estimated by an ammonia electrode procedure described by Miller et al. (1975). Particle size distribution was determined using the hydrometer method of Black (1965). Total surface area (TSA) was estimated using an ethylene glycol monoethyl ether (EGME) technique (Heilman et al., 1965, Chihacek and Bremer, 1979) applicable down to about 10m2g 1. The limited clay mineral data were generated by X-ray diffraction techniques and were graciously provided by M. Rockley, Oklahoma State University. All PCE/subsurface core experiments were performed in either 25- or 10-mL glass tubes with Teflon-lined screw caps. The sorbent was accurately weighed into 24 tubes, then wet with 15-20mL (alternatively 5-7mL in the smaller tubes) of 0.01 M CaCl~. Serial dilutions of a stock ,4 C-labeled PCE solution were prepared in CaC12 solution such that the initial concentration ratios in the tubes would be approximately 1:3:10:30. Measured volumes of these solutions were added directly to tubes containing the sorbent and to a parallel set of 24 tubes without sorbent (control tubes). The tubes were then filled to just overflowing with additional CaC12 solution and carefully capped to exclude air. Sorbent to water ratios (gg 1) were maximized for each sorbent within the operational constraints of the procedure and were generally in the range of 1:2 to 1:5. Five replicates at each of four PCE concentrations (alternatively four replicates at five concentrations) and four blank replicates comprised each set of tubes. The control tubes were not prepared in all runs and served only to provide a check on tube-to-tube variability in concentration and on mass balances. Concerns for increased loss with more volatile sorbates prompted some minor modifications in the sorption experiments on the aquifer materials. Two working stock solutions of each of the sorbates were prepared in methanol. Concentrations of these stock solutions were selected to yield, after addition to the 10-mL glass reaction tubes, initial sorbate concentrations of roughly 10, 30, 100, and 300 pg L 1. Headspace air was virtually eliminated by first prewetting the sorbent with 0.01 M CaC12 solution, allowing the tubes to stand overnight and then filling to just overflowing with the CaCl~ solution. Two to 10#L of methanol stock solution were injected well below the liquid meniscus into each tube, and the tube was immediately capped tightly. Methanol concentrations in the tubes never exceeded 0.1% by volume, so were assumed to cause no discernible effect on sorbate aqueous phase activity and sorption (Nkedi-Kizza et al., 1985). All tubes were placed into a rotating box mixer producing 360 inversions/hr and were mixed for 21-24 hr. After the contact period, tubes containing sorbent were centrifuged for 1 hour at 900 × g in a Sorvall RC-2 centrifuge. Approximately 1-mL aliquots were withdrawn from the supernatant of each tube and
168 transferred to scintillation vials for counting. The remaining supernatant was carefully decanted from the tubes containing sorbent. The sorbent was then extracted twice in succession with scintillation cocktail (Note: By appropriately adjusting the initial sorbent mass in the tubes, the water retention volume of the sorbent in the tubes was balanced against the water handling capacity of the cocktail such that clear extracts were obtained in over 95% of the extractions. Cloudy extracts were rejected.) Earlier work (Banerjee et al., 1985) had shown that sorption coefficient values calculated from the equilibrium solution concentration in each tube and the mass of sorbate extracted from the sorbent were more precise than those calculated from the more traditional determination of differences in initial versus equilibrium solution concentrations. Problems inherent in handling aqueous solutions of volatile organic compounds and in subtracting two relatively large numbers to attain the small mass sorbed on these low foc sorbents produce inherently imprecise results. Two extracts of the sorbent were shown to account for 98% of the total extractable sorbate. Mass balances routinely accounted for 9 5 + % of the total sorbate added to the tubes, indicating that losses of sorbate during sample handling were small. Extractions were performed by first loosening the sorbent plug by vigorous shaking, then mixing the tube contents for two minutes on a vortex mixer. In the determination of sorption coefficients for the four solvents, the second extraction was performed by mixing overnight in the rotating box mixer after tests revealed a slightly improved (~ 5%) recovery of sorbate by this procedure. The extracts were counted separately with the counts combined after correction for blanks, counting efficiency and residual liquid volumes. Weights of tubes and scintillation vials were monitored at each stage of handling so counted sample volumes and volumes of residual water (typically 1 mL/4g sorbent) or cocktail in the sorbent were accurately known. All sorption data presented here are calculated from the amount of chemical sorbed, as determined by direct extraction of the sorbent, compared to sorbate concentration of the supernatant in each tube after centrifugation. Coefficients were obtained for linear and Freundlich interpretations, in the manner of Curtis et al. (1986b), using eqns. (1) and (2): S = Kp C
(1)
where S is the mass of chemical sorbed per mass sorbent (mg kg-1); Kp is the partition coefficient (L kg-1); and C is the equilibrium solution concentration (mgL 1). S
=
K f C 1In
(2)
where Ks is the Freundlich coefficient (in mg ~ ~/nkg ~ L TM) and 1/n is an empirical fitting parameter indicative of the deviation of the data from linearity. To assist in data interpretation, a third model was used in the evaluation of the data: S = K' C + B
(3)
169
K' in this case represents the slope of the sorption data regression line (in L k g 1), and B, the intercept (inmgkg-1). This model is used in discussing the nonlinearity observed in several of the PCE isotherms at higher sorbate concentrations. RESULTS AND DISCUSSION
Sorbent characteristics Characteristics of the subsurface samples are given in Table 1. Several of the sorbents appear twice in the table because they were used as the < l mm fraction in the earlier PCE studies, but were subsequently sieved through a finer mesh for use in the sorption coefficient determinations for the four solvents. The aquifer samples and several of the unsaturated zone sorbents from the N core had foc values well below the 0.001 value suggested by Schwarzenbach and Westall (1981) as a lower limit for sorption dominated by organic carbon. The unsaturated zone sample, J-10, also used in the sorption coefficient determinations for the four solvents, appeared to have sufficient organic carbon to control sorption of the sorbates. TSA and CEC were quite low for the aquifer sorbents, with N-6 yielding the highest CEC and TSA values. Limited x-ray diffraction data for N-6 and C-1 revealed these sorbents to be predominantly quartz with only 1-2% of kaolinite and illite. No measurable quantities of smectite clays were found in these sorbents. The B-1 sample was reported to contain about 2% chlorite (Curtis et al., 1986a). Equilibration time Curtis and Roberts (1985) reported that, on B-1 aquifer material, PCE and DCB approached equilibrium over about three days. Their data show that roughly 90% of the ultimate sorption was reached in 24 hr and that losses from the controls made data interpretation more difficult as contact time increased. We found that prolonging mixing time resulted in increasing loss of sorbate from the test vessels through volatilization or other processes. Preliminary tests revealed that sorption equilibrium was reached in about 24 hr with data at longer contact intervals statistically indistinguishable from the 24 hr data. A 21-24 hr mixing time was used for all batch tests, yielding data of reasonable precision ( + 20% or better) t h a t facilitated our comparisons of sorption coefficients between different sorbates and sorbents. Isotherm linearity PCE isotherms were run on J and N core samples and on the 0.5-mm fraction of aquifer materials B-1 and C-1. The solution contentration range selected for many of the PCE isotherms included concentrations (12 15 mg L 1) well above the 10 5 M upper limit for isotherm linearity suggested by Karickhoff (1980). The data show definite trends to nonlinearity at higher equilibrium concentrations, typified by PCE sorption on sorbent J-11 depicted in Figure 1. Table 2 contains isotherm data interpretations for each of the regression models, eqns.
170
4"
Sorption
3-
(rag kg-1) 2-
1-
0 0
I
l
I
I
I
!
2
4
6
8
10
12
14
Concentration
(rng L"1) Fig. 1. Sorption isotherm of tetrachloroethene on sorbent J-11. (1), (2), a n d (3). T h e F r e u n d l i c h (2) a n d K ' (3) i n t e r p r e t a t i o n s h a v e b e e n a p p l i e d to the entire data sets. The regressions from which K, values were determined incorporate only those data for which equilibrium solution concentrations were at or below 2 mg L-1. Examination of these data reveals several interesting trends. First, the K' v a l u e s a r e a l l l e s s t h a n t h e c o r r e s p o n d i n g Kp v a l u e s . T h e i n t e r c e p t s , B, w i t h t w o e x c e p t i o n s , a r e s i g n i f i c a n t l y g r e a t e r t h a n 0 (~ = 0.05), a n d a r e i n v e r s e l y c o r r e l a t e d , i n a q u a l i t a t i v e f a s h i o n , w i t h t h e F r e u n d l i c h 1/n v a l u e s . O n e i n t e r p r e t a t i o n o f t h e s e o b s e r v a t i o n s is t h a t t h e h i g h e r s o r b a t e c o n c e n t r a t i o n s o l u t i o n s h a v e n o t r e a c h e d e q u i l i b r i u m i n t h e r e a c t i o n p e r i o d p r o v i d e d , ie., that at higher solution concentrations, longer times to sorption equilibrium a r e r e q u i r e d . T h e e x c e p t i o n s t o t h i s o b s e r v a t i o n a r e f o r t h e N-5 a n d N-7 TABLE 2 Partition coefficient values as indicators of nonlinearity of PCE sorption from high concentration solutions Sorbent
K~
1/n
~
K"
N-5 N-7 J-6 J-7 J-9 J-11 J-12 J-14
0.26 0.46 0.58 1.60 0.98 0.60 0.50 1.74
0.95 0.92 0.88 0.89 0.89 0.81 0.81 0.93
0.94 1.00 1.00 0.99 1.00 0.98 0.98 1.00
0.25 0.37 0.41 1.14 0.69 0.35 0.29 1.48
B 0.03 0.07 0.20 0.63 0.41 0.26 0.24 0.19
~
K,
0.89 0.99 1.00 0.99 0.99 0.97 0.94 1.00
0.28 0.44 0.59 1.53 0.99 0.61 0.53 1.68
Kp, K~ and K" are calculated from equations (1), (2), and (3), respectively. B represents the regression line intercept. For Kp, intercepts were zero and r 2 values exceeded 0.97. Kp and K' are expressed in L/kg. K~ is in mg ~ 1/nkg ~LTM
171 TABLE 3 Subsurface sediment sorption of PCE from low concentration aqueous solutions Sorbent
Kf
1/n
r2
Kp
r2
N-1 N-2 N-3 N-6a J-10a J-19 B-1b C-1b
2.93 0.32 0.31 0.19 0.48 0.37 0.35 0.20
0.96 0.93 0.94 0.99 0.94 0.97 0.93 0.95
1.00 0.98 0.99 0.99 0.99 0.99 0.98 0.98
2.98 0.34 0.31 0.18 0.48 0.37 0.35 0.19
1.00 0.97 0.98 0.98 0.99 0.99 0.99 0.97
aSamples were passed through a 0.25mm sieve. hSamples were passed through a 0.5 mm sieve. Kp is in L kg- and Kf is in mg1-1/.kg- 1Ll/n sorbents, b o t h with very low foc values. These exceptions suggest t h a t the n o n l i n e a r i t y observed here for h i g h e r s o r b a t e s o l u t i o n c o n c e n t r a t i o n s has a k i n e t i c basis and t h a t the k i n e t i c p h e n o m e n a are of more i m p o r t a n c e at h i g h e r s o r b e n t o r g a n i c c a r b o n c o n c e n t r a t i o n s . Finally, the K, values are of similar m a g n i t u d e , in e v e r y case, to the Kf values. This o b s e r v a t i o n confirms the a p p l i c a b i l i t y and usefulness of the F r e u n d l i c h model (2) to this type of s o r p t i o n d a t a over the entire c o n c e n t r a t i o n range. The d o w n w a r d c u r v a t u r e of these i s o t h e r m s (exemplified in Fig. 1) is reflected in the 1/n values, all of w h i c h are less t h a n one. E a c h of these i s o t h e r m s e n c o m p a s s e s a set of 20 individual d a t a points at e i t h e r four or five c o n c e n t r a t i o n levels. The 95% confidence i n t e r v a l s a r o u n d the p a r t i t i o n coefficient values are on the o r d e r of + 10% for most of the isotherms, i n c r e a s i n g to + 20% w h e n r 2 v a l u e s drop below 0.90. The d a t a for t h o s e s o r p t i o n i s o t h e r m runs in w h i c h the h i g h e s t equilibrium P C E s o l u t i o n c o n c e n t r a t i o n s did n o t exceed 2 mg/L are presented in Table 3. Coefficients for the l i n e a r a n d F r e u n d l i c h i n t e r p r e t a t i o n s of the d a t a are of n e a r l y equal m a g n i t u d e t h r o u g h o u t . The 1/n values are 0.93 or g r e a t e r t h r o u g h o u t the d a t a set and r 2 values reflect 95% confidence i n t e r v a l s a r o u n d the p a r t i t i o n coefficient v a l u e s on the o r d e r of + 10%. These results confirm the applicability of (1) to P C E s o r p t i o n d a t a w h e r e C < 2 m g L -~. This conc e n t r a t i o n limit for i s o t h e r m l i n e a r i t y m a y be d e p e n d e n t on e x p e r i m e n t a l ~time to e q u i l i b r i u m " and on w h e t h e r s o r b e n t o r g a n i c c a r b o n c o n t r o l s s o r p t i o n of PCE. A m o r e definitive r e s o l u t i o n of these p h e n o m e n a awaits f u r t h e r research.
S o r p t i o n on m i n e r a l c o m p o n e n t s
The r e l a t i o n s h i p b e t w e e n s o r p t i o n of P C E and s o r b e n t foc was also invest i g a t e d u s i n g the P C E i s o t h e r m data. The p a r t i t i o n coefficients for the l i n e a r p o r t i o n of the i s o t h e r m are plotted in F i g u r e 2 a g a i n s t foc for t h o s e sorbents in Tables 2 a n d 3 w i t h foe g r e a t e r t h a n 0.001. T h e r e is a s t r o n g c o r r e l a t i o n b e t w e e n
172 3"
Kp (L
k g -1 )
3/."
o0.
J 0
-0
I 2
I 4
I 6
I 8
I 10
I 12
14
- 3
foc x 10
Fig. 2. Tetrachloroethene sorption dependencyon sorbent organic carbon content. PCE sorption and organic carbon content for these subsurface samples. Linear regression of the data yields the following empirical relationship: Kp = 202 (foe) + 0.06
(4)
(The coefficient of determination, r 2, for the regression is 0.93; the standard errors of the estimate for the slope and intercept are 24 and 0.16, respectively.) This relationship expresses the magnitude of partitioning of PCE into the organic matter of these natural sorbents as measured in our experimental system. Using this relationship, we felt we could predict the partition coefficients for PCE on other subsurface samples to within a factor of two or better. We tested the hypothesis that sorption of PCE and other nonpolar organic sorbates occurs on low-foe aquifer materials in excess of what might be anticipated based on the organic carbon content of the sorbents. Four subsurface samples were selected as sorbents, and four organic solvents were chosen as sorbates. Three of the sorbents were low foe aquifer materials while the fourth was from the unsaturated zone J core. All were sieved to yield size fractions of < 0.5 or < 0.25mm (see Table 1). Our intent was to determine the magnitude of the carbon-based sorption of each of the sorbates on the higher foc J core sample (under the assumption t h a t this sorbent's organic carbon dominates sorption), and to use these data to predict the magnitude of the carbon-based sorption on the low fo¢ aquifer material. Any sorption in excess of that predicted might then be attributable to mineral surface interactions in accordance with the model proposed by McCarty et al. (1981): gp = focgoc + fiogio
(5)
where rio is the mineral equivalent to foc, and Koc and Kio are the partition coefficients for the organic carbon and inorganic fractions, respectively. Measured Kp values for these sorption runs are presented in Table 4. The
173 TABLE 4 M e a s u r e d partition coefficients for selected o r g a n i c s o l v e n t s on aquifer solids Sorbent/Sorbate B-1 BZ (2.1) a T C E (2.3) b P C E (2.9) b D C B (3.4) ~ C-1 BZ TCE PCE DCB N-6 BZ TCE PCE DCB J-10 BZ TCE PCE DCB(1) DCB(2) Kp values are in L k g ~. CI is duplicate isotherm run w a s a Log Kow from Miller et al. h L o g Kowfrom Callahan et
Kp
CI
r~
0.038 0.088 0.35 0.34
0.0304).045 0.0760.098 0.33-0.37 0.31-0.37
0.86 0.94 0.99 0.97
0.026 0.032 0.19 0.082
0.021-0.031 0.029-0.036 0.17-0.21 0.072-0.091
0.89 0.95 0.98 0.94
0.035 0.076 0.18 0.23
0.031-0.039 0.0694).083 0.17-0.19 0.22-0.24
0.95 0.96 0.98 1.00
0.12 0.16 0.48 1.04 0.85
0.11-0.13 0.1542.18 0.454).50 1.01 1.08 0.82-0.89
0.97 0.97 0.99 1.00 0.99
the 95% confidence interval around the m e a n partition coefficient. A made for D C B on sorbent J-10. (1985). al. (1979).
95% confidence intervals for the Kp estimates are narrow, and generally improve as the sorbate volatility decreases or the sorbent foc increases. Replicate isotherms for DCB on J-10 show reproducibility of the Kp estimate to within 80%. The Kp value observed for PCE sorption on J-10 compares favorably (within 20%) with the value of 0.58 predicted from eqn. (4) based on the measured foc of the sorbent, supporting our assumption that this sorbent has sufficient foc to control the sorption of these sorbates. Further evaluation of this point can be made by examining the relationship between the sorption coefficients and the octanol-water partition coefficients of the sorbates. This is commonly done by normalizing the Kp values to the sorbent foe, as in eqn. (6), then plotting the log of the normalized value, designated Koc, against log Kow:
Koc =
Kp/fo¢
(6)
Such a plot of the J-10 data ~for the four sorbates is presented in Figure 3. Schwarzenbach and Westall (1981), as well as numerous other researchers, have presented similar data for sorption of a variety of nonpolar organic
174
3.0.
[]
2.8. 2.6
~'~
2.4
J
~>
oc,
)
/ 2.0. 1.8.
e
~
~(TCE)
o J-lO ~
B-1
e
C-1
1.6 (BZ) 2.1
2.3
2.5
2.7 2.9 Log Kow
i
i
3.1
3.3
Fig. 3. C a r b o n - n o r m a l i z e d s o r p t i o n o n s u b s u r f a c e s e d i m e n t s as a f u n c t i o n of t h e o c t a n o l - w a t e r p a r t i t i o n coefficients of t h e sorbates.
compounds on numerous soils, sediments and sludges. We chose to compare our data with those of Schwarzenbach and Westall (SW) because they worked with similar sorbates and sorbents under similar testing conditions. Our previous work (Banerjee et al., 1985) on subsurface materials has shown our sorption data to be consistently within a factor of two of that of SW. This margin of difference is to be expected based on differences in experimental conditions and is representative of similar data in the literature (see, for example, figure 2 in Curtis et al., 1986 a). The SW regression line is also plotted in Figure 3. Our data for J-10 fall parallel to and about a factor of 2 below the SW line. The assumption that organic carbon is controlling the sorption of these compounds on J-10 seems justified. The regression line (7) generated from the J-10 data can be used to predict the extent of carbon-based sorption on the other sorbents: log Koc = 0.69 log Kow + 0.22
(7)
The r 2 value for this regression is 0.99 with the standard errors of the estimate for slope and intercept equal to 0.04 and 0.12, respectively. Use of Koc generally implies that the sorbent foe adequately accounts for the observed sorption. We make no such assumption when we plot in Figure 3 the sorption data for the four sorbates on the aquifer sorbents. Koc does, however, serve as a useful concept in demonstrating the differences in sorption between the aquifer materials and the J-10 sorbent. If foe were dominating sorption on these aquifer samples, the data points should be co-linear with the J-10
175 regression line. Instead, data for each of the sorbates except DCB are above the SW regression line. The implication is that mechanisms other than partitioning into sorbent foc are important in sorption of these nonpolar sorbates in many saturated subsurface systems. The Kp values for DCB are consistently lower than anticipated for all three aquifer sorbents. The B-1 value for DCB is about 40% of the partition coefficient of 0.8 L kg 1 on Borden aquifer material presented by Curtis et al. (1986b). In contrast, our value for PCE on the same sorbent is about 75% of the Curtis value. This discrepancy can not be explained with the existing data although nonequilibrium conditions for sorption of this sorbate in our experimental procedure is certainly a possibility. We have attempted to estimate the relative importance of the mineral contribution to sorption on the aquifer sorbents. Using eqn. (7), we calculated the amount of sorption expected for these sorbents based on foo and the Kow of the sorbates. Then, assuming the applicability of eqn. (5) to these systems, we calculated values for the mineral phase contribution floKio, to Kp. These calculations are summarized in Table 5. The data provide evidence that hydrophobic partitioning of these sorbates to organic carbon does not explain a substantial portion of the sorption observed on these very low fo~ sorbents. Values of fioKio a r e greater than associated focKoc values for all sorbate/sorbent combinations except one, consistent with the findings of Schwarzenbach and Westall (1981) and Curtis et al. (1986b). For PCE, the calculated mineral phase contribution is 3-5 times the sorption attributable to the sorbent organic carbon. The existing database for sorption of nonpolar organic solvents to low organic carbon subsurface sorbents remains extremely limited, preventing any substantive correlations explaining the nature and magnitude of rio and Kio. We assumed that f~ofor a given sorbent is a constant (an assumption that presumes that any differences in behaviour between the sorbates is accounted for by Kow), and calculated the Kio relative to BZ for the other three sorbates. These values, expressed as Kio(x)/Kio(BZ) and presented in Table 5, provide some indication of the relative magnitude of these partition coefficients. It is obvious that considerable scatter exists in these ratios between sorbents, but, excepting the DCB data, the ratios are within a factor of two to three of and reasonably correlated with comparable ratios of Kow values for these sorbates, also given in Table 5. This is consistent with expectations that solution free energy considerations drive sorption of nonpolar organic molecules to both the organic or inorganic components of the sorbent. Further refinement of the rio concept must await better data on the mineralogy of the low carbon natural sorbents and the hydrophilic nature of these surfaces (Curtis et al., 1986a). Our data shows no correlation between the sorption observed on the various sorbents and such parameters as clay-sized fraction, clay minerals, or TSA, representative of the mineral components. The limited number of sorbents available and the lack of more quantitative mineralogical data hindered this evaluation.
176 TABLE 5 E s t i m a t i o n of role of m i n e r a l s u r f a c e s in s o r p t i o n of n o n p o l a r o r g a n i c s on low-organic c a r b o n aquifer sediments Measured
fio Kiob
Kio (x)/Kio(BZ) c
0.015 0.020 0.052 0.111
0.038 0.088 0.350 0.340
0.023 0.068 0.298 0.229
1.00 2.98 13.1 10.0
0.010 0.013 0.035 0.075
0.026 0.032 0.190 0.082
0.016 0.019 0.155 0.007
1.00 1.18 9.87 0.45
0.014 0.018 0.047 0.100
0.035 0.076 0.180 0.230
0.021 0.058 0.133 0.130
1.00 2.73 6.27 6.11
foc Koca
gp
B-l: (fo~ = 0.O0031) BZ TCE PCE DCB 0.00021) C-l:(foc BZ TCE PCE DCB N-6: (fo~ = 0.00028) BZ TCE PCE DCB Ko~ r a t i o s d : BZ TCE PCE DCB
1.00 1.48 5.88 17.8
a V a l u e s c a l c u l a t e d from (7) a n d Kow of t h e s o r b a t e s . b T h e difference b e t w e e n m e a s u r e d Kp a n d c a l c u l a t e d o r g a n i c s o r b e n t c o n t r i b u t i o n c o n s i s t e n t w i t h eqn. (5). c R a t i o s of t h e c a l c u l a t e d i n o r g a n i c p a r t i t i o n coefficients for e a c h s o l v e n t to t h a t for BZ a s s u m i n g t h a t rio is c o n s t a n t for e a c h sorbent. d T h e r a t i o s for Kow a r e a g a i n for e a c h s o r b a t e r e l a t i v e to BZ.
Estimating sorption in low-carbon environments Workers in groundwater pollution are faced with the task of making defensible estimates of pollutant retardation in aquifer environments where existing carbon-based sorption models probably are not appropriate. While the type of semi-quantitative estimates that apply to sorption on higher carbon sediments are probably still some years away for low carbon systems, it is useful to look for trends in existing low carbon sorption data. Figure 4 is a plot of log Kp on log Kow for low carbon sorbents (foc < 0.001) and sorbates with log Kow < 3.7. The data for sorption of similar compounds to a Swiss aquifer material (Schwarzenbach and Westall, 1981) are generally consistent with that in Figure 4, but the sorbent used in their studies had foc = 0.0015. While the data do not support any firm conclusions as to the relationship between solvent sorption on low carbon sediments and octanol-water partition coefficients, there are some qualitative aspects to Figure 4 which might be of
177
•
From
Curtis,
et
al.
(1986b).
•
• oo-o.,
Kp
'
~
J
foc foc
J
= 0.0002 = 0.0002
-1 . 2 '
-1.
* Log y = 0.96x r2
I ( K p as L kg -1)
2.2
I
I
I
2.6
I 3
I
- 3.40
= 0.85
I
I
3.4
Log Kow Fig. 4. Estimating from existing data the sorption of relatively low Kow sorbates onto low carbon sorbents. The lines designated by foc values represent the substitution of these values into the Schwarzenbach and Westall (1981) regression model. some use. T h e r e g r e s s i o n line eqn. (8) does p r o v i d e a r e a s o n a b l e r e p r e s e n t a t i o n of the t e n d e n c y of t h e d a t a (r 2 = 0.85; s t a n d a r d e r r o r s of the e s t i m a t e = 0.12 a n d 0.31 for slope a n d i n t e r c e p t , respectively): log Kp =
1.01 log Kow - 3.46
(8)
A Kp v a l u e e s t i m a t e for s o r b e n t s w i t h foc < 0.001 t h a t is w i t h i n a f a c t o r of 2 or 3 of the a c t u a l v a l u e s h o u l d r e s u l t from the a p p l i c a t i o n of this r e g r e s s i o n line if applied to n o n p o l a r s o r b a t e s o f l o g K o ~ < 3.7. I f for exceeds 0.001, an o r g a n i c c a r b o n - b a s e d r e g r e s s i o n s u c h as t h a t of S c h w a r z e n b a c h a n d W e s t a l l (1981) would g e n e r a l l y be m o r e a p p r o p r i a t e . N e i t h e r t y p e of r e g r e s s i o n - b a s e d e s t i m a t e should be e x t e n d e d to s o r b a t e s of p o l a r or ionic c h a r a c t e r or to s o r b a t e s o u t of the Kow r a n g e e x p l o r e d in the g e n e r a t i o n of the r e g r e s s i o n e q u a t i o n of interest. In F i g u r e 4 we h a v e also used S c h w a r z e n b a c h and W e s t a l l ' s (1981) r e g r e s s i o n line, s u b s t i t u t i n g foc v a l u e s equal to 0.0015 and 0.0002 to g e n e r a t e lines t h a t b o u n d the e n t i r e d a t a set. If, for t h o s e s o r b e n t s w h e r e foe < 0.001, one chooses to a s s u m e foc = 0.001 a n d s u b s t i t u t e s this v a l u e into the Schwarzenb a c h a n d W e s t a l l e q u a t i o n , the d a t a in F i g u r e 4 s u g g e s t t h a t the r e s u l t i n g e s t i m a t e of Kp w o u l d be of s i m i l a r u n c e r t a i n t y to t h a t o b t a i n e d from eqn. (8). B o t h a p p r o a c h e s will yield m o r e r e a l i s t i c v a l u e s t h a n o b t a i n e d from the r i g o r o u s a p p l i c a t i o n of a n o r g a n i c c a r b o n - b a s e d model to geologic m a t e r i a l s c o n t a i n i n g v e r y low o r g a n i c c a r b o n levels. CONCLUSIONS AND RECOMMENDATIONS O u r r e s u l t s w i t h P C E s o r p t i o n on s u b s u r f a c e s a m p l e s i n d i c a t e t h a t the i s o t h e r m s c a n be l i n e a r l y i n t e r p r e t e d up to a n e q u i l i b r i u m s o l u t i o n c o n c e n t r a -
178 tion of about 2 mg 1 1. Above this value, we observe considerable curvature in the isotherms and recommend the application of the Freundlich equation in interpretation of this data. Limited data suggest that this threshold concentration may not apply rigorously to very low foe sorbents and that kinetics associated with sorption to organic carbon may be an influencing factor. Research is needed to more adequately define high concentration limitations of linear sorption models. We caution that applying transport models with linear sorption components in subsurface environments where contaminant concentrations exceed 10 g M may significantly overestimate sorption. Sorption of organic solvent molecules onto aquifer material from aqueous solution is several times greater than would be predicted from a carboncontrolled model of the system. This sorption appears to be attributable to the mineral surfaces of the sorbent. The mineral sorption component, which appears to be roughly correlatable to sorbate Kow, can dominate in low foc systems. Site specific sorption experiments remain the method of choice for gauging aquifer transport of nonpolar organic pollutants. Alternatively, a Kp prediction based on assuming foo = 0.001 in existing Koc/Kowsorption models should be within a factor of three of the environmental value for sorbents with foc < 0.001. The question of sorption kinetics, in low carbon aquifer systems, as well as in higher carbon environments, looms as an unresolved research issue that could significantly affect the movement of pollutants in the subsurface. The gradual approach to sorption equilibrium discussed by Curtis et al. (1986b), and observed in our studies, as well as the column data of Schwarzenbach and Westall (1981), Bouchard et al. (1988) and Lee et al. (1988) suggest time-dependent sorption and desorption that has important implications to modeling pollutant transport and to aquifer remediation. DISCLAIMER Although the research described in this pape'r was partially funded by the US Environmental Protection Agency through an in-house project at the Robert S. Kerr Environmental Research Laboratory, it has not been subjected to the Agency's peer review system. The views expressed herein are not necessarily the views of the Agency, and no official endorsement should be inferred. ACKNOWLEDGEMENTS Partial support for this work was provided by the US Air Force through the Engineering and Services Center, Tyndall AFB, FL. We wish to thank Khawla Ebeid for experimental assistance, Greg Belcher for his help with the sample characterization, Dave Walters for computer services, and Doug MacKay for providing the B-1 aquifer material. We extend special thanks to Mark Rockley for the mineral identification. We also t h a n k Gary Curtis, Sam Karickhoff, Bill Mabey, Ted Mill, and Peter Nkedi-Kizza for their helpful comments on the manuscript.
179 REFERENCES Banerjee, P., Piwoni, M.D. and Ebeid, K., 1985. Sorption of organic contaminants to a low carbon subsurface core. Chemosphere, 14:1057 1067. Black, C.A., (Editor-in-chief), 1965. Methods of Soil Analyses, Part 1. ASA, Inc., Madison, Wisconsin. Bouchard, D.C., Wood, A.L., Campbell, M.L., Nkedi-Kizza, P. and Rao, P.S.C., 1988. Sorption nonequilibrium during solute transport. J. Contam. Hydrol., 2: 20~223. Callahan, M.A., Slimak, M., Gabel, N., May, I., Fowler, F., Freed, R., Jennings, P., Durfee, R.. Whitmore, F., Maestri, B. and Mabey, W.R., 1979. Water-related environmental fate of 129 priority pollutants. EPA-440/4-79-029b. Chihacek, L.J. and Bremner, J.M., 1979. A simplified ethylene glycol monoethyl ether procedure for assessment of soil surface area. Soil Sci. Soc. Am. J., 43:821 822. Chiou, C.T., Porter, P.E. and Schmedding, D.W., 1983. Partition equilibria of nonionic organic compounds between soil organic matter and water. Environ. Sci. Technol., 17:227 231. Curtis, G.P. and Roberts, P.W., 1985. Sorption ofhalogenated organic solutes by aquifer materials: Comparison between batch equilibrium measurements and field observations. Tech. Rep. 285, Dept. of Civil Eng., Stanford University, Stanford, C.A. Curtis, G.P., Reinhard, M. and Roberts. P.V., 1986a. Sorption of hydrophobic organic compounds by sediments. In: J.A. Davis and K.F. Hayes (Editors), Geochemical Processes at Mineral Surfaces, ACS Symp. Ser., 323:191 216. Curtis, G., Roberts, P. and Reinhard, M., 1986b. A natural gradient experiment on solute transport in a sand aquifer. 4. Sorption of organic solutes and its influence on mobility. Water Resour. Res., 22 (13): 2059 2067. Hassett, J.J., Means, J.C., Banwart, W.L. and Wood, S.G., 1980. Sorption properties of sediments and energy related pollutants. EPA-600/3-80-041. Heilman, M.D., Carter, D.L. and Gonzalez, C.L., 1965. The ethylene glycol monoethyl ether (EGME) technique for determining soil-surface area. Soil Sci., 100:409 413. Karickhoff, S.W., 1980. Sorption kinetics of hydrophobic pollutants in natural sediments. In: R.A. Baker (Editor), Contaminants and Sediments, Vol. 2. Ann Arbor Science, Ann Arbor. MI, pp. 193 205. Karickhoff, S.W., 1984. Organic pollutant sorption in aquatic systems. J. Hydraulic Engrg. ll0: 707 735. Karickhoff, S.W., Brown, D.S. and Scott, T.A., 1979. Sorption of hydrophobic pollutants on natural sediments. Water Res., 13:241 248. Lee, L.S., Rao, P.S.C., Brusseau, M.L. and Ogwada, R.A., 1988. Nonequilibrium sorption of organic contaminants during flow through columns of aquifer materials. Environ. Toxicol. Chem., 7 (10): 779 793. McCarty, P.L., Reinhard, M. and Rittman, B.L., 1981. Trace Organics in Groundwater. Environ. Sci. Technol., 15:40 51. Miller, G.A., Riecken, F.F. and Walter, N.F., 1975. Use of an ammonia electrode for determination of cation exchange capacity in soil studies. Soil Sci. Soc. Am. Proc., 39:372 372. Miller, M.M., Wasik, S.P., Huang, G., Shiu, W. and Mackay, D., 1985. Relationships between octanol-water partition coefficients and aqueous solubility. Environ. Sci. Technol., 19:522 529. Nkedi-Kizza, P., Rao, P.S.C. and Hornsby, A.G., 1985. Influence of organic cosolvents on sorption of hydrophobic organic chemicals by soils. Environ. Sci. Technol., 19:975 979. Powell, R.M., Bledsoe, B.E., Curtis, G.E., and Johnson, R.L., 1989. Interlaboratory methods comparison for the total organic carbon analysis of soils. Environ. Sci. Technol., in press. Pye, V.I. and Patrick, R., 1983. Groundwater contamination in the United States. Science, 221: 713 721. Roberts, P.V., Goltz, M.N. and Mackay, D.N., 1986. A natural gradient experiment on solute transport in a sand aquifer. 3. Retardation estimates and mass balances for organic solutes. Water Resour. Res., 22 (13): 2047 2058. Schwarzenbach, R.P. and Westall, J., 1981. Transport of nonpolar organic compounds from surface water to groundwater. Laboratory sorption studies. Environ. Sci. Technol., 15:1360 1367. Westrick, J.J., Mello, J.W. and Thomas, R.F., 1984. The groundwater supply survey. J. AWWA, May: 52 59.
163
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
SORPTION OF VOLATILE ORGANIC SOLVENTS FROM AQUEOUS SOLUTION ONTO SUBSURFACE SOLIDS
MARVIN D. PIWONI1 and PINAKI BANERJEE2 i Hazardous Waste Research and Information Center, 1808 Woodfield Drive, Savoy, IL 61874, U.S.A. 2PRC Environmental Management Inc., 303 E Wacker Drive, Chicago, IL 60601, U.S.A.
(Received May 6, 1988; revised and accepted July 15, 1988
ABSTRACT Piwoni, M.D. and Banerjee, P., 1989. Sorption of volatile organic solvents from aqueous solution onto subsurface solids. J. Contam. Hydrol., 4: 162-179. Sorption isotherms for tetrachloroethene on low-carbon subsurface core samples were linear to equilibrium solution concentrations of 2mgL -1. Concentrations above this value produced pronounced curvature in the sorption isotherms. Sorption of tetrachloroethene, benzene, trichloroethene, and 1,2-dichlorobenzene on low-organic-carbon aquifer materials (foe < 0.001) was 2~4 times that predicted based solely on sorbent organic carbon content. Estimates of the mineral surface contribution to sorption and the relationship of this mineral component to sorbate Koware presented. An approach to predicting organic solvent sorption in low-carbon natural environments is proposed.
INTRODUCTION
O r g a n i c c a r b o n c o n t e n t of the s o r b e n t a n d o c t a n o l - w a t e r p a r t i t i o n coefficient (Kow) of the s o r b a t e h a v e evolved as the two p a r a m e t e r s most c o m m o n l y used for e s t i m a t i n g s o r p t i o n of n o n p o l a r o r g a n i c c o m p o u n d s to sediments and soils. The t h e o r y t h a t forms the basis for these estimates is called the hydrophobic b o n d i n g t h e o r y , a n d is p r e d i c a t e d on the fact t h a t the process of part i t i o n i n g of a n o n p o l a r o r g a n i c m o l e c u l e b e t w e e n the a q u e o u s phase and the s o r b e n t o r g a n i c c a r b o n is d r i v e n p r i m a r i l y by the large excess of free e n e r g y of the solute in a q u e o u s solution. The l i t e r a t u r e c o n t a i n s a n u m b e r of linear r e g r e s s i o n models d e s c r i b i n g the r e l a t i o n s h i p b e t w e e n s o r p t i o n coefficients and Kow v a l u e s ( K a r i c k h o f f et al., 1979; S c h w a r z e n b a c h and Westall, 1981; C h i o u et al., 1983; a m o n g others). A p p l i c a t i o n of these models to estimate s o r p t i o n based on k n o w n s o r b e n t o r g a n i c c a r b o n c o n t e n t are g e n e r a l l y a c c u r a t e to w i t h i n a f a c t o r of two to five, d e p e n d e n t on the similarity of the o r g a n i c c o m p o u n d s of i n t e r e s t with t h o s e on w h i c h the r e g r e s s i o n models were developed. The capabilities of these r e l a t i o n s h i p s are best w h e n p r e d i c t i n g s o r p t i o n coefficients u n d e r e x p e r i m e n t a l c o n d i t i o n s and with o r g a n i c c o m p o u n d s similar to t h o s e used in model development. The s o r p t i o n limits over w h i c h these c a r b o n - b a s e d
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© 1989 Elsevier Science Publishers B.V.
164
models are applicable are defined at the low end by some minimum sorbent fraction organic carbon, foe, and at the high end by some fraction of sorbate solubility. These limits are likely variable, dependent on sorbate Kow and sorbent foe (Curtis et al., 1986a). Unfortunately, many investigations of organic contaminant transport through the subsurface involve predictions on systems where these regression models do not apply. Nonpolar, water-immiscible, organic solvents have gained notoriety as common groundwater pollutants. Most such solvents have relatively high water solubilities and low Kowvalues, so consequently interact only to a limited degree with natural sorbents. These properties, the naturally low foe of the subsurface, and the extensive use of solvents in society combine to contribute to the widespread contamination of groundwaters by these compounds (Pye and Patrick, 1983; Westrick et al., 1984), many of which are also known or suspected human carcinogens. Although it should be obvious that the degree of sorption of organic solvents in aquifer systems will be relatively less than that in high foc sediments and soils, interactions between these molecules and mineral surfaces are likely to be important. Over long times and travel distances, even a small retardation factor will contribute to the size of a pollutant plume (Roberts et al., 1986) and result in a substantial increase in the volume of water that must be pumped and treated to realize aquifer remediation. Workers addressing subsurface contamination by organic solvents need improved tools for predicting the sorptive behavior of these contaminants. For very low foc sorbents, mineral surfaces can provide an alternate site for sorption of nonpolar organic molecules. The subsurface environment often provides an abundance of such surfaces with little organic carbon. The foc of subsurface materials tends to decrease with depth (Banerjee et al., 1985) and is typically less than 0.001 in unconsolidated aquifers. Although the clay content, particularly in the saturated zone, is often low, the mineral surfaces could play a significant role in the sorption of all types of organic molecules. For sorption of organic molecules on mineral surfaces to become important, one of both of two conditions must be satisfied. Either the molecules must exhibit some polar functionality that can result in site-specific interactions with the mineral surface (Curtis et al., 1986a), or mineral surfaces must be present in such relative abundance that sorbate interactions with these surfaces become of the same magnitude as sorbate interactions with the organic matter on the sorbent (Karickhoff, 1984). For nonpolar organics, the second condition would be of most significance. Hassett et al. (1980) presented data for aromatics, including polar compounds, that showed sorption to soils well above that predicted based on the foc of the sorbents. Using these same data, Karickhoff (1984) demonstrated that '~mineral phase" sorption was most pronounced in those sorbents having a substantial clay mineral/organic carbon ratio. Little attention has been paid to sorption of organic solvent molecules to aquifer material. Schwarzenbach and Westall (1981) found more sorption of four halogenated and aromatic organics to an aquifer material than could be
165 attributed to the sorbent foc. Curtis et al. (1986b) measured sorption of tetrachloroethene (PCE), 1,2-dichlorobenzene (DCB) and other solvents on Borden aquifer material and found sorption in excess of what would be expected based on foc. Curtis and Roberts (1985) suggested that mineral surfaces play an important role in the sorption process in Borden aquifer material. A need exists to better understand the characteristics of the types of surfaces available for sorption in the subsurface and to determine to what degree these various surfaces affect the sorption of organic contaminants (Curtis et al., 1986b). The existing database does not permit this type of interpretation. In this paper, we present sorption data for selected organic solvents on several subsurface materials. The goals of the study were to advance the qualitative understanding of sorption of four widely-used organic solvents from aqueous solution onto natural sorbents with low foe and to attempt to more quantitatively define a mineral component to that sorption. The ultimate objective was to facilitate a sound basis for predicting transport behavior of dissolved solvents in aquifers. The work first focused on characterizing PCE sorption on samples obtained primarily from two semi-continuous subsurface cores, and then on sorption of four nonpolar organic solvents on three saturated zone sorbents with low organic carbon content. MATERIALSAND METHODS Materials Benzene (BZ), trichloroethene (TCE), PCE and DCB were obtained as ~4Clabeled material from Pathfinder Laboratories (St. Louis, MO). GC and GC/MS analysis showed the isotopes to be 95% pure or better, so they were used as received. The isotopes were diluted with unlabeled compound from Chem Service (West Chester, PA) or Aldrich Chemical (Milwaukee, WI) with purities of 98% or better. The stock solutions were prepared in reagent grade methanol, stored in Teflon and glass vials at - 20°C, and checked occasionally by GC/MS to assure purity. The sorbents used were, with one exception, collected by Robert S. Kerr Environmental Research Laboratory (RSKERL) personnel using a rotary drilling rig fitted with 10-cm diameter custom-made brass core barrels. Cores were collected by alternately pressing the core barrel (50cm) into the sediments, retracting the core, and redrilling to the top of the next sampling interval. Cores were hydraulically extruded on site into polyethylene bags and transported to RSKERL for further processing. Most of the sorbents were collected in two cores, designated as the J and N cores, near Lula, Pontotoc County, OK. The J core consisted of 20 contiguous sections of the unsaturated zone down to a depth of 675 cm below the soil surface. The N core samples were taken at approximately 90-cm intervals down to a depth of 600 cm and included two samples of the saturated zone material. Two additional aquifer material samples were used in this study. The C-1 sample was composited from a series of saturated zone samples collected from
166 b o r e h o l e s d r i l l e d n e a r L u l a , O K . T h e c o l l e c t i o n a n d p r o c e s s i n g o f t h e B-1 s a m p l e , f r o m t h e B o r d e n A i r F o r c e B a s e s i t e i n O n t a r i o , C a n a d a , is d i s c u s s e d by C u r t i s e t al. (1986b).
Methods A l l c o r e s a m p l e s c o l l e c t e d by R S K E R L w e r e a i r - d r i e d , g r o u n d w i t h m o r t a r a n d p e s t l e a n d s i e v e d t h r o u g h a 1-mm s c r e e n . S e l e c t e d s a m p l e s w e r e s u b s e q u e n t l y s i e v e d t o p a s s e i t h e r a 500 o r 250-pm s c r e e n as n o t e d i n T a b l e 1. T h e b u l k - s i e v e d s a m p l e s w e r e s p l i t i n t o a p p r o x i m a t e l y 1-kg s u b s a m p l e s u s i n g a S o i l t e s t splitter. T h e s o r b e n t s used to m e a s u r e s o r p t i o n of the four o r g a n i c s o l v e n t s w e r e s e q u e n t i a l l y s p l i t , f i r s t t o a p p r o x i m a t e l y 100-g s u b s a m p l e s a n d t h e n t o 2 -4 -g a l i q u o t s , o n a Q u a n t a c h r o m e riffle s p l i t t e r . T h e s e a l i q u o t s w e r e t r a n s f e r r e d d i r e c t l y to the i s o t h e r m r e a c t i o n tubes. The m e a n mass of s o r b e n t t r a n s f e r r e d t o t h e 24 t u b e s o f a n i s o t h e r m s e r i e s r o u t i n e l y s h o w e d a r e l a t i v e s t a n d a r d d e v i a t i o n o f 1.5% o r less. T h i s s p l i t t i n g p r o c e d u r e i n s u r e d t h a t e a c h tube contained a reproducible sorbent sample representative of the bulk material. A combustion method using a Leco Model WR12 Carbon Analyzer was used t o m e a s u r e t h e o r g a n i c c a r b o n c o n t e n t o f al l s o r b e n t s . T h i s p r o c e d u r e , TABLE 1 Characteristics of subsurface samples Sample
Size fraction a (mm)
Organic carbon b (%)
Clay-sized fraction (%)
Saturated zone sample
N-1 N-2 N-3 N-5 N-6 N-6 N-7 J-6 J-7 J-9 J-10 J-10 J-11 J-12 J-14 J-19 B-I C-1
<1 <1 <1 <1 <1 < 0.25 <1 <1 <1 <1 <1 < 0.25 <1 <1 <1 <1 < 0.5 <0.5
1.33 (0.11) 0.054 (0.002) 0.039 (0.003) 0.034 (0.002) 0.028 (0.007) 0.028 (0.002) 0.042 (0.004) 0.39 0.89 0.39 0.19 0.26 (0.01) 0.15 0.18 0.08 0.23 0.031 (0.001) 0.021 (0.004)
13 24 21 34 14 23 12 13 17 13 13 14 9 6 9 12 11 14
No No No No Yes Yes Yes No No No No No No No No No Yes Yes
TSA (m2g - 1)
CEC (meq NH 4/g)
30 +_ 2.0
4.5 + 0.4
46 + 3.2
7.2 _+ 0.4
0.8° 11 + 1.2
1.5 + 0.3 2.5 _+ 0.2
a Size fraction of sorbent used for isotherm measurements. bValues in parentheses indicate + 1 standard deviation. Values without standard deviations from Banerjee et al. (1985). c Our measured value was < 10 m2g-1. The value presented is from Curtis et al. (1986b).
167 performed on 1-g samples preleached with 5% HC1 to remove inorganic carbon, generally yielded + 10% precision. Attempts to use a Dohrmann analyzer for low foc sorbents proved unsuccessful because of the precision problems inherent in subsampling such sorbents in 50-100mg quantities (see Powell et al., 1989). Cation exchange capacity (CEC) was estimated by an ammonia electrode procedure described by Miller et al. (1975). Particle size distribution was determined using the hydrometer method of Black (1965). Total surface area (TSA) was estimated using an ethylene glycol monoethyl ether (EGME) technique (Heilman et al., 1965, Chihacek and Bremer, 1979) applicable down to about 10m2g 1. The limited clay mineral data were generated by X-ray diffraction techniques and were graciously provided by M. Rockley, Oklahoma State University. All PCE/subsurface core experiments were performed in either 25- or 10-mL glass tubes with Teflon-lined screw caps. The sorbent was accurately weighed into 24 tubes, then wet with 15-20mL (alternatively 5-7mL in the smaller tubes) of 0.01 M CaCl~. Serial dilutions of a stock ,4 C-labeled PCE solution were prepared in CaC12 solution such that the initial concentration ratios in the tubes would be approximately 1:3:10:30. Measured volumes of these solutions were added directly to tubes containing the sorbent and to a parallel set of 24 tubes without sorbent (control tubes). The tubes were then filled to just overflowing with additional CaC12 solution and carefully capped to exclude air. Sorbent to water ratios (gg 1) were maximized for each sorbent within the operational constraints of the procedure and were generally in the range of 1:2 to 1:5. Five replicates at each of four PCE concentrations (alternatively four replicates at five concentrations) and four blank replicates comprised each set of tubes. The control tubes were not prepared in all runs and served only to provide a check on tube-to-tube variability in concentration and on mass balances. Concerns for increased loss with more volatile sorbates prompted some minor modifications in the sorption experiments on the aquifer materials. Two working stock solutions of each of the sorbates were prepared in methanol. Concentrations of these stock solutions were selected to yield, after addition to the 10-mL glass reaction tubes, initial sorbate concentrations of roughly 10, 30, 100, and 300 pg L 1. Headspace air was virtually eliminated by first prewetting the sorbent with 0.01 M CaC12 solution, allowing the tubes to stand overnight and then filling to just overflowing with the CaCl~ solution. Two to 10#L of methanol stock solution were injected well below the liquid meniscus into each tube, and the tube was immediately capped tightly. Methanol concentrations in the tubes never exceeded 0.1% by volume, so were assumed to cause no discernible effect on sorbate aqueous phase activity and sorption (Nkedi-Kizza et al., 1985). All tubes were placed into a rotating box mixer producing 360 inversions/hr and were mixed for 21-24 hr. After the contact period, tubes containing sorbent were centrifuged for 1 hour at 900 × g in a Sorvall RC-2 centrifuge. Approximately 1-mL aliquots were withdrawn from the supernatant of each tube and
168 transferred to scintillation vials for counting. The remaining supernatant was carefully decanted from the tubes containing sorbent. The sorbent was then extracted twice in succession with scintillation cocktail (Note: By appropriately adjusting the initial sorbent mass in the tubes, the water retention volume of the sorbent in the tubes was balanced against the water handling capacity of the cocktail such that clear extracts were obtained in over 95% of the extractions. Cloudy extracts were rejected.) Earlier work (Banerjee et al., 1985) had shown that sorption coefficient values calculated from the equilibrium solution concentration in each tube and the mass of sorbate extracted from the sorbent were more precise than those calculated from the more traditional determination of differences in initial versus equilibrium solution concentrations. Problems inherent in handling aqueous solutions of volatile organic compounds and in subtracting two relatively large numbers to attain the small mass sorbed on these low foc sorbents produce inherently imprecise results. Two extracts of the sorbent were shown to account for 98% of the total extractable sorbate. Mass balances routinely accounted for 9 5 + % of the total sorbate added to the tubes, indicating that losses of sorbate during sample handling were small. Extractions were performed by first loosening the sorbent plug by vigorous shaking, then mixing the tube contents for two minutes on a vortex mixer. In the determination of sorption coefficients for the four solvents, the second extraction was performed by mixing overnight in the rotating box mixer after tests revealed a slightly improved (~ 5%) recovery of sorbate by this procedure. The extracts were counted separately with the counts combined after correction for blanks, counting efficiency and residual liquid volumes. Weights of tubes and scintillation vials were monitored at each stage of handling so counted sample volumes and volumes of residual water (typically 1 mL/4g sorbent) or cocktail in the sorbent were accurately known. All sorption data presented here are calculated from the amount of chemical sorbed, as determined by direct extraction of the sorbent, compared to sorbate concentration of the supernatant in each tube after centrifugation. Coefficients were obtained for linear and Freundlich interpretations, in the manner of Curtis et al. (1986b), using eqns. (1) and (2): S = Kp C
(1)
where S is the mass of chemical sorbed per mass sorbent (mg kg-1); Kp is the partition coefficient (L kg-1); and C is the equilibrium solution concentration (mgL 1). S
=
K f C 1In
(2)
where Ks is the Freundlich coefficient (in mg ~ ~/nkg ~ L TM) and 1/n is an empirical fitting parameter indicative of the deviation of the data from linearity. To assist in data interpretation, a third model was used in the evaluation of the data: S = K' C + B
(3)
169
K' in this case represents the slope of the sorption data regression line (in L k g 1), and B, the intercept (inmgkg-1). This model is used in discussing the nonlinearity observed in several of the PCE isotherms at higher sorbate concentrations. RESULTS AND DISCUSSION
Sorbent characteristics Characteristics of the subsurface samples are given in Table 1. Several of the sorbents appear twice in the table because they were used as the < l mm fraction in the earlier PCE studies, but were subsequently sieved through a finer mesh for use in the sorption coefficient determinations for the four solvents. The aquifer samples and several of the unsaturated zone sorbents from the N core had foc values well below the 0.001 value suggested by Schwarzenbach and Westall (1981) as a lower limit for sorption dominated by organic carbon. The unsaturated zone sample, J-10, also used in the sorption coefficient determinations for the four solvents, appeared to have sufficient organic carbon to control sorption of the sorbates. TSA and CEC were quite low for the aquifer sorbents, with N-6 yielding the highest CEC and TSA values. Limited x-ray diffraction data for N-6 and C-1 revealed these sorbents to be predominantly quartz with only 1-2% of kaolinite and illite. No measurable quantities of smectite clays were found in these sorbents. The B-1 sample was reported to contain about 2% chlorite (Curtis et al., 1986a). Equilibration time Curtis and Roberts (1985) reported that, on B-1 aquifer material, PCE and DCB approached equilibrium over about three days. Their data show that roughly 90% of the ultimate sorption was reached in 24 hr and that losses from the controls made data interpretation more difficult as contact time increased. We found that prolonging mixing time resulted in increasing loss of sorbate from the test vessels through volatilization or other processes. Preliminary tests revealed that sorption equilibrium was reached in about 24 hr with data at longer contact intervals statistically indistinguishable from the 24 hr data. A 21-24 hr mixing time was used for all batch tests, yielding data of reasonable precision ( + 20% or better) t h a t facilitated our comparisons of sorption coefficients between different sorbates and sorbents. Isotherm linearity PCE isotherms were run on J and N core samples and on the 0.5-mm fraction of aquifer materials B-1 and C-1. The solution contentration range selected for many of the PCE isotherms included concentrations (12 15 mg L 1) well above the 10 5 M upper limit for isotherm linearity suggested by Karickhoff (1980). The data show definite trends to nonlinearity at higher equilibrium concentrations, typified by PCE sorption on sorbent J-11 depicted in Figure 1. Table 2 contains isotherm data interpretations for each of the regression models, eqns.
170
4"
Sorption
3-
(rag kg-1) 2-
1-
0 0
I
l
I
I
I
!
2
4
6
8
10
12
14
Concentration
(rng L"1) Fig. 1. Sorption isotherm of tetrachloroethene on sorbent J-11. (1), (2), a n d (3). T h e F r e u n d l i c h (2) a n d K ' (3) i n t e r p r e t a t i o n s h a v e b e e n a p p l i e d to the entire data sets. The regressions from which K, values were determined incorporate only those data for which equilibrium solution concentrations were at or below 2 mg L-1. Examination of these data reveals several interesting trends. First, the K' v a l u e s a r e a l l l e s s t h a n t h e c o r r e s p o n d i n g Kp v a l u e s . T h e i n t e r c e p t s , B, w i t h t w o e x c e p t i o n s , a r e s i g n i f i c a n t l y g r e a t e r t h a n 0 (~ = 0.05), a n d a r e i n v e r s e l y c o r r e l a t e d , i n a q u a l i t a t i v e f a s h i o n , w i t h t h e F r e u n d l i c h 1/n v a l u e s . O n e i n t e r p r e t a t i o n o f t h e s e o b s e r v a t i o n s is t h a t t h e h i g h e r s o r b a t e c o n c e n t r a t i o n s o l u t i o n s h a v e n o t r e a c h e d e q u i l i b r i u m i n t h e r e a c t i o n p e r i o d p r o v i d e d , ie., that at higher solution concentrations, longer times to sorption equilibrium a r e r e q u i r e d . T h e e x c e p t i o n s t o t h i s o b s e r v a t i o n a r e f o r t h e N-5 a n d N-7 TABLE 2 Partition coefficient values as indicators of nonlinearity of PCE sorption from high concentration solutions Sorbent
K~
1/n
~
K"
N-5 N-7 J-6 J-7 J-9 J-11 J-12 J-14
0.26 0.46 0.58 1.60 0.98 0.60 0.50 1.74
0.95 0.92 0.88 0.89 0.89 0.81 0.81 0.93
0.94 1.00 1.00 0.99 1.00 0.98 0.98 1.00
0.25 0.37 0.41 1.14 0.69 0.35 0.29 1.48
B 0.03 0.07 0.20 0.63 0.41 0.26 0.24 0.19
~
K,
0.89 0.99 1.00 0.99 0.99 0.97 0.94 1.00
0.28 0.44 0.59 1.53 0.99 0.61 0.53 1.68
Kp, K~ and K" are calculated from equations (1), (2), and (3), respectively. B represents the regression line intercept. For Kp, intercepts were zero and r 2 values exceeded 0.97. Kp and K' are expressed in L/kg. K~ is in mg ~ 1/nkg ~LTM
171 TABLE 3 Subsurface sediment sorption of PCE from low concentration aqueous solutions Sorbent
Kf
1/n
r2
Kp
r2
N-1 N-2 N-3 N-6a J-10a J-19 B-1b C-1b
2.93 0.32 0.31 0.19 0.48 0.37 0.35 0.20
0.96 0.93 0.94 0.99 0.94 0.97 0.93 0.95
1.00 0.98 0.99 0.99 0.99 0.99 0.98 0.98
2.98 0.34 0.31 0.18 0.48 0.37 0.35 0.19
1.00 0.97 0.98 0.98 0.99 0.99 0.99 0.97
aSamples were passed through a 0.25mm sieve. hSamples were passed through a 0.5 mm sieve. Kp is in L kg- and Kf is in mg1-1/.kg- 1Ll/n sorbents, b o t h with very low foc values. These exceptions suggest t h a t the n o n l i n e a r i t y observed here for h i g h e r s o r b a t e s o l u t i o n c o n c e n t r a t i o n s has a k i n e t i c basis and t h a t the k i n e t i c p h e n o m e n a are of more i m p o r t a n c e at h i g h e r s o r b e n t o r g a n i c c a r b o n c o n c e n t r a t i o n s . Finally, the K, values are of similar m a g n i t u d e , in e v e r y case, to the Kf values. This o b s e r v a t i o n confirms the a p p l i c a b i l i t y and usefulness of the F r e u n d l i c h model (2) to this type of s o r p t i o n d a t a over the entire c o n c e n t r a t i o n range. The d o w n w a r d c u r v a t u r e of these i s o t h e r m s (exemplified in Fig. 1) is reflected in the 1/n values, all of w h i c h are less t h a n one. E a c h of these i s o t h e r m s e n c o m p a s s e s a set of 20 individual d a t a points at e i t h e r four or five c o n c e n t r a t i o n levels. The 95% confidence i n t e r v a l s a r o u n d the p a r t i t i o n coefficient values are on the o r d e r of + 10% for most of the isotherms, i n c r e a s i n g to + 20% w h e n r 2 v a l u e s drop below 0.90. The d a t a for t h o s e s o r p t i o n i s o t h e r m runs in w h i c h the h i g h e s t equilibrium P C E s o l u t i o n c o n c e n t r a t i o n s did n o t exceed 2 mg/L are presented in Table 3. Coefficients for the l i n e a r a n d F r e u n d l i c h i n t e r p r e t a t i o n s of the d a t a are of n e a r l y equal m a g n i t u d e t h r o u g h o u t . The 1/n values are 0.93 or g r e a t e r t h r o u g h o u t the d a t a set and r 2 values reflect 95% confidence i n t e r v a l s a r o u n d the p a r t i t i o n coefficient v a l u e s on the o r d e r of + 10%. These results confirm the applicability of (1) to P C E s o r p t i o n d a t a w h e r e C < 2 m g L -~. This conc e n t r a t i o n limit for i s o t h e r m l i n e a r i t y m a y be d e p e n d e n t on e x p e r i m e n t a l ~time to e q u i l i b r i u m " and on w h e t h e r s o r b e n t o r g a n i c c a r b o n c o n t r o l s s o r p t i o n of PCE. A m o r e definitive r e s o l u t i o n of these p h e n o m e n a awaits f u r t h e r research.
S o r p t i o n on m i n e r a l c o m p o n e n t s
The r e l a t i o n s h i p b e t w e e n s o r p t i o n of P C E and s o r b e n t foc was also invest i g a t e d u s i n g the P C E i s o t h e r m data. The p a r t i t i o n coefficients for the l i n e a r p o r t i o n of the i s o t h e r m are plotted in F i g u r e 2 a g a i n s t foc for t h o s e sorbents in Tables 2 a n d 3 w i t h foe g r e a t e r t h a n 0.001. T h e r e is a s t r o n g c o r r e l a t i o n b e t w e e n
172 3"
Kp (L
k g -1 )
3/."
o0.
J 0
-0
I 2
I 4
I 6
I 8
I 10
I 12
14
- 3
foc x 10
Fig. 2. Tetrachloroethene sorption dependencyon sorbent organic carbon content. PCE sorption and organic carbon content for these subsurface samples. Linear regression of the data yields the following empirical relationship: Kp = 202 (foe) + 0.06
(4)
(The coefficient of determination, r 2, for the regression is 0.93; the standard errors of the estimate for the slope and intercept are 24 and 0.16, respectively.) This relationship expresses the magnitude of partitioning of PCE into the organic matter of these natural sorbents as measured in our experimental system. Using this relationship, we felt we could predict the partition coefficients for PCE on other subsurface samples to within a factor of two or better. We tested the hypothesis that sorption of PCE and other nonpolar organic sorbates occurs on low-foe aquifer materials in excess of what might be anticipated based on the organic carbon content of the sorbents. Four subsurface samples were selected as sorbents, and four organic solvents were chosen as sorbates. Three of the sorbents were low foe aquifer materials while the fourth was from the unsaturated zone J core. All were sieved to yield size fractions of < 0.5 or < 0.25mm (see Table 1). Our intent was to determine the magnitude of the carbon-based sorption of each of the sorbates on the higher foc J core sample (under the assumption t h a t this sorbent's organic carbon dominates sorption), and to use these data to predict the magnitude of the carbon-based sorption on the low fo¢ aquifer material. Any sorption in excess of that predicted might then be attributable to mineral surface interactions in accordance with the model proposed by McCarty et al. (1981): gp = focgoc + fiogio
(5)
where rio is the mineral equivalent to foc, and Koc and Kio are the partition coefficients for the organic carbon and inorganic fractions, respectively. Measured Kp values for these sorption runs are presented in Table 4. The
173 TABLE 4 M e a s u r e d partition coefficients for selected o r g a n i c s o l v e n t s on aquifer solids Sorbent/Sorbate B-1 BZ (2.1) a T C E (2.3) b P C E (2.9) b D C B (3.4) ~ C-1 BZ TCE PCE DCB N-6 BZ TCE PCE DCB J-10 BZ TCE PCE DCB(1) DCB(2) Kp values are in L k g ~. CI is duplicate isotherm run w a s a Log Kow from Miller et al. h L o g Kowfrom Callahan et
Kp
CI
r~
0.038 0.088 0.35 0.34
0.0304).045 0.0760.098 0.33-0.37 0.31-0.37
0.86 0.94 0.99 0.97
0.026 0.032 0.19 0.082
0.021-0.031 0.029-0.036 0.17-0.21 0.072-0.091
0.89 0.95 0.98 0.94
0.035 0.076 0.18 0.23
0.031-0.039 0.0694).083 0.17-0.19 0.22-0.24
0.95 0.96 0.98 1.00
0.12 0.16 0.48 1.04 0.85
0.11-0.13 0.1542.18 0.454).50 1.01 1.08 0.82-0.89
0.97 0.97 0.99 1.00 0.99
the 95% confidence interval around the m e a n partition coefficient. A made for D C B on sorbent J-10. (1985). al. (1979).
95% confidence intervals for the Kp estimates are narrow, and generally improve as the sorbate volatility decreases or the sorbent foc increases. Replicate isotherms for DCB on J-10 show reproducibility of the Kp estimate to within 80%. The Kp value observed for PCE sorption on J-10 compares favorably (within 20%) with the value of 0.58 predicted from eqn. (4) based on the measured foc of the sorbent, supporting our assumption that this sorbent has sufficient foc to control the sorption of these sorbates. Further evaluation of this point can be made by examining the relationship between the sorption coefficients and the octanol-water partition coefficients of the sorbates. This is commonly done by normalizing the Kp values to the sorbent foe, as in eqn. (6), then plotting the log of the normalized value, designated Koc, against log Kow:
Koc =
Kp/fo¢
(6)
Such a plot of the J-10 data ~for the four sorbates is presented in Figure 3. Schwarzenbach and Westall (1981), as well as numerous other researchers, have presented similar data for sorption of a variety of nonpolar organic
174
3.0.
[]
2.8. 2.6
~'~
2.4
J
~>
oc,
)
/ 2.0. 1.8.
e
~
~(TCE)
o J-lO ~
B-1
e
C-1
1.6 (BZ) 2.1
2.3
2.5
2.7 2.9 Log Kow
i
i
3.1
3.3
Fig. 3. C a r b o n - n o r m a l i z e d s o r p t i o n o n s u b s u r f a c e s e d i m e n t s as a f u n c t i o n of t h e o c t a n o l - w a t e r p a r t i t i o n coefficients of t h e sorbates.
compounds on numerous soils, sediments and sludges. We chose to compare our data with those of Schwarzenbach and Westall (SW) because they worked with similar sorbates and sorbents under similar testing conditions. Our previous work (Banerjee et al., 1985) on subsurface materials has shown our sorption data to be consistently within a factor of two of that of SW. This margin of difference is to be expected based on differences in experimental conditions and is representative of similar data in the literature (see, for example, figure 2 in Curtis et al., 1986 a). The SW regression line is also plotted in Figure 3. Our data for J-10 fall parallel to and about a factor of 2 below the SW line. The assumption that organic carbon is controlling the sorption of these compounds on J-10 seems justified. The regression line (7) generated from the J-10 data can be used to predict the extent of carbon-based sorption on the other sorbents: log Koc = 0.69 log Kow + 0.22
(7)
The r 2 value for this regression is 0.99 with the standard errors of the estimate for slope and intercept equal to 0.04 and 0.12, respectively. Use of Koc generally implies that the sorbent foe adequately accounts for the observed sorption. We make no such assumption when we plot in Figure 3 the sorption data for the four sorbates on the aquifer sorbents. Koc does, however, serve as a useful concept in demonstrating the differences in sorption between the aquifer materials and the J-10 sorbent. If foe were dominating sorption on these aquifer samples, the data points should be co-linear with the J-10
175 regression line. Instead, data for each of the sorbates except DCB are above the SW regression line. The implication is that mechanisms other than partitioning into sorbent foc are important in sorption of these nonpolar sorbates in many saturated subsurface systems. The Kp values for DCB are consistently lower than anticipated for all three aquifer sorbents. The B-1 value for DCB is about 40% of the partition coefficient of 0.8 L kg 1 on Borden aquifer material presented by Curtis et al. (1986b). In contrast, our value for PCE on the same sorbent is about 75% of the Curtis value. This discrepancy can not be explained with the existing data although nonequilibrium conditions for sorption of this sorbate in our experimental procedure is certainly a possibility. We have attempted to estimate the relative importance of the mineral contribution to sorption on the aquifer sorbents. Using eqn. (7), we calculated the amount of sorption expected for these sorbents based on foo and the Kow of the sorbates. Then, assuming the applicability of eqn. (5) to these systems, we calculated values for the mineral phase contribution floKio, to Kp. These calculations are summarized in Table 5. The data provide evidence that hydrophobic partitioning of these sorbates to organic carbon does not explain a substantial portion of the sorption observed on these very low fo~ sorbents. Values of fioKio a r e greater than associated focKoc values for all sorbate/sorbent combinations except one, consistent with the findings of Schwarzenbach and Westall (1981) and Curtis et al. (1986b). For PCE, the calculated mineral phase contribution is 3-5 times the sorption attributable to the sorbent organic carbon. The existing database for sorption of nonpolar organic solvents to low organic carbon subsurface sorbents remains extremely limited, preventing any substantive correlations explaining the nature and magnitude of rio and Kio. We assumed that f~ofor a given sorbent is a constant (an assumption that presumes that any differences in behaviour between the sorbates is accounted for by Kow), and calculated the Kio relative to BZ for the other three sorbates. These values, expressed as Kio(x)/Kio(BZ) and presented in Table 5, provide some indication of the relative magnitude of these partition coefficients. It is obvious that considerable scatter exists in these ratios between sorbents, but, excepting the DCB data, the ratios are within a factor of two to three of and reasonably correlated with comparable ratios of Kow values for these sorbates, also given in Table 5. This is consistent with expectations that solution free energy considerations drive sorption of nonpolar organic molecules to both the organic or inorganic components of the sorbent. Further refinement of the rio concept must await better data on the mineralogy of the low carbon natural sorbents and the hydrophilic nature of these surfaces (Curtis et al., 1986a). Our data shows no correlation between the sorption observed on the various sorbents and such parameters as clay-sized fraction, clay minerals, or TSA, representative of the mineral components. The limited number of sorbents available and the lack of more quantitative mineralogical data hindered this evaluation.
176 TABLE 5 E s t i m a t i o n of role of m i n e r a l s u r f a c e s in s o r p t i o n of n o n p o l a r o r g a n i c s on low-organic c a r b o n aquifer sediments Measured
fio Kiob
Kio (x)/Kio(BZ) c
0.015 0.020 0.052 0.111
0.038 0.088 0.350 0.340
0.023 0.068 0.298 0.229
1.00 2.98 13.1 10.0
0.010 0.013 0.035 0.075
0.026 0.032 0.190 0.082
0.016 0.019 0.155 0.007
1.00 1.18 9.87 0.45
0.014 0.018 0.047 0.100
0.035 0.076 0.180 0.230
0.021 0.058 0.133 0.130
1.00 2.73 6.27 6.11
foc Koca
gp
B-l: (fo~ = 0.O0031) BZ TCE PCE DCB 0.00021) C-l:(foc BZ TCE PCE DCB N-6: (fo~ = 0.00028) BZ TCE PCE DCB Ko~ r a t i o s d : BZ TCE PCE DCB
1.00 1.48 5.88 17.8
a V a l u e s c a l c u l a t e d from (7) a n d Kow of t h e s o r b a t e s . b T h e difference b e t w e e n m e a s u r e d Kp a n d c a l c u l a t e d o r g a n i c s o r b e n t c o n t r i b u t i o n c o n s i s t e n t w i t h eqn. (5). c R a t i o s of t h e c a l c u l a t e d i n o r g a n i c p a r t i t i o n coefficients for e a c h s o l v e n t to t h a t for BZ a s s u m i n g t h a t rio is c o n s t a n t for e a c h sorbent. d T h e r a t i o s for Kow a r e a g a i n for e a c h s o r b a t e r e l a t i v e to BZ.
Estimating sorption in low-carbon environments Workers in groundwater pollution are faced with the task of making defensible estimates of pollutant retardation in aquifer environments where existing carbon-based sorption models probably are not appropriate. While the type of semi-quantitative estimates that apply to sorption on higher carbon sediments are probably still some years away for low carbon systems, it is useful to look for trends in existing low carbon sorption data. Figure 4 is a plot of log Kp on log Kow for low carbon sorbents (foc < 0.001) and sorbates with log Kow < 3.7. The data for sorption of similar compounds to a Swiss aquifer material (Schwarzenbach and Westall, 1981) are generally consistent with that in Figure 4, but the sorbent used in their studies had foc = 0.0015. While the data do not support any firm conclusions as to the relationship between solvent sorption on low carbon sediments and octanol-water partition coefficients, there are some qualitative aspects to Figure 4 which might be of
177
•
From
Curtis,
et
al.
(1986b).
•
• oo-o.,
Kp
'
~
J
foc foc
J
= 0.0002 = 0.0002
-1 . 2 '
-1.
* Log y = 0.96x r2
I ( K p as L kg -1)
2.2
I
I
I
2.6
I 3
I
- 3.40
= 0.85
I
I
3.4
Log Kow Fig. 4. Estimating from existing data the sorption of relatively low Kow sorbates onto low carbon sorbents. The lines designated by foc values represent the substitution of these values into the Schwarzenbach and Westall (1981) regression model. some use. T h e r e g r e s s i o n line eqn. (8) does p r o v i d e a r e a s o n a b l e r e p r e s e n t a t i o n of the t e n d e n c y of t h e d a t a (r 2 = 0.85; s t a n d a r d e r r o r s of the e s t i m a t e = 0.12 a n d 0.31 for slope a n d i n t e r c e p t , respectively): log Kp =
1.01 log Kow - 3.46
(8)
A Kp v a l u e e s t i m a t e for s o r b e n t s w i t h foc < 0.001 t h a t is w i t h i n a f a c t o r of 2 or 3 of the a c t u a l v a l u e s h o u l d r e s u l t from the a p p l i c a t i o n of this r e g r e s s i o n line if applied to n o n p o l a r s o r b a t e s o f l o g K o ~ < 3.7. I f for exceeds 0.001, an o r g a n i c c a r b o n - b a s e d r e g r e s s i o n s u c h as t h a t of S c h w a r z e n b a c h a n d W e s t a l l (1981) would g e n e r a l l y be m o r e a p p r o p r i a t e . N e i t h e r t y p e of r e g r e s s i o n - b a s e d e s t i m a t e should be e x t e n d e d to s o r b a t e s of p o l a r or ionic c h a r a c t e r or to s o r b a t e s o u t of the Kow r a n g e e x p l o r e d in the g e n e r a t i o n of the r e g r e s s i o n e q u a t i o n of interest. In F i g u r e 4 we h a v e also used S c h w a r z e n b a c h and W e s t a l l ' s (1981) r e g r e s s i o n line, s u b s t i t u t i n g foc v a l u e s equal to 0.0015 and 0.0002 to g e n e r a t e lines t h a t b o u n d the e n t i r e d a t a set. If, for t h o s e s o r b e n t s w h e r e foe < 0.001, one chooses to a s s u m e foc = 0.001 a n d s u b s t i t u t e s this v a l u e into the Schwarzenb a c h a n d W e s t a l l e q u a t i o n , the d a t a in F i g u r e 4 s u g g e s t t h a t the r e s u l t i n g e s t i m a t e of Kp w o u l d be of s i m i l a r u n c e r t a i n t y to t h a t o b t a i n e d from eqn. (8). B o t h a p p r o a c h e s will yield m o r e r e a l i s t i c v a l u e s t h a n o b t a i n e d from the r i g o r o u s a p p l i c a t i o n of a n o r g a n i c c a r b o n - b a s e d model to geologic m a t e r i a l s c o n t a i n i n g v e r y low o r g a n i c c a r b o n levels. CONCLUSIONS AND RECOMMENDATIONS O u r r e s u l t s w i t h P C E s o r p t i o n on s u b s u r f a c e s a m p l e s i n d i c a t e t h a t the i s o t h e r m s c a n be l i n e a r l y i n t e r p r e t e d up to a n e q u i l i b r i u m s o l u t i o n c o n c e n t r a -
178 tion of about 2 mg 1 1. Above this value, we observe considerable curvature in the isotherms and recommend the application of the Freundlich equation in interpretation of this data. Limited data suggest that this threshold concentration may not apply rigorously to very low foe sorbents and that kinetics associated with sorption to organic carbon may be an influencing factor. Research is needed to more adequately define high concentration limitations of linear sorption models. We caution that applying transport models with linear sorption components in subsurface environments where contaminant concentrations exceed 10 g M may significantly overestimate sorption. Sorption of organic solvent molecules onto aquifer material from aqueous solution is several times greater than would be predicted from a carboncontrolled model of the system. This sorption appears to be attributable to the mineral surfaces of the sorbent. The mineral sorption component, which appears to be roughly correlatable to sorbate Kow, can dominate in low foc systems. Site specific sorption experiments remain the method of choice for gauging aquifer transport of nonpolar organic pollutants. Alternatively, a Kp prediction based on assuming foo = 0.001 in existing Koc/Kowsorption models should be within a factor of three of the environmental value for sorbents with foc < 0.001. The question of sorption kinetics, in low carbon aquifer systems, as well as in higher carbon environments, looms as an unresolved research issue that could significantly affect the movement of pollutants in the subsurface. The gradual approach to sorption equilibrium discussed by Curtis et al. (1986b), and observed in our studies, as well as the column data of Schwarzenbach and Westall (1981), Bouchard et al. (1988) and Lee et al. (1988) suggest time-dependent sorption and desorption that has important implications to modeling pollutant transport and to aquifer remediation. DISCLAIMER Although the research described in this pape'r was partially funded by the US Environmental Protection Agency through an in-house project at the Robert S. Kerr Environmental Research Laboratory, it has not been subjected to the Agency's peer review system. The views expressed herein are not necessarily the views of the Agency, and no official endorsement should be inferred. ACKNOWLEDGEMENTS Partial support for this work was provided by the US Air Force through the Engineering and Services Center, Tyndall AFB, FL. We wish to thank Khawla Ebeid for experimental assistance, Greg Belcher for his help with the sample characterization, Dave Walters for computer services, and Doug MacKay for providing the B-1 aquifer material. We extend special thanks to Mark Rockley for the mineral identification. We also t h a n k Gary Curtis, Sam Karickhoff, Bill Mabey, Ted Mill, and Peter Nkedi-Kizza for their helpful comments on the manuscript.
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