Environ. Sci. Technol. 1993, 27, 1553-1562
Modeling Partitioning and Transport Interactions between Natural Organic Matter and Polynuclear Aromatic Hydrocarbons in Groundwater Houmao Llu and Gary Amy'
Department of Civil, Environmental, and Architectural Engineering, University of Colorado, Boulder, Colorado 80309-0428 The influences of aqueous-phase natural organic matter (NOM) and solid-phase (mineral-phase) NOM on the partitioning and transport of polynuclear aromatic hydrocarbons (PAH) were studied in both batch and column experiments. The column experiments were conducted in two stages. In the first stage, the soil matrix was simulated with quartz, with which NOM does not interact; the effect of aqueous-phase NOM on PAH transport was studied in such a soil matrix. In the second stage, the soil matrix was simulated with quartz coated with aluminum oxide (mixed bed), with which NOM can interact (sorb); the effect of soil-phaseNOM on PAH transport was studied in this stage. The effects of PAH water solubility, NOM source, and Darcy velocity on PAH transport were also addressed. Data from the column experiments were modeled with a one-dimensional advection-dispersion transport model (CFITIM) to independently estimate partition coefficients as well as retardation factors and kinetic parameters. Observations based on the results of experiments include the following: (1) the interaction between PAH and NOM is NOM source dependent, and the interaction between PAH and minerals is mineralogy dependent, (2) aqueous-phase NOM facilitates PAH transport, and (3) solid-phase NOM retards PAH transport.
Introduction Polynuclear aromatic hydrocarbons (PAH) comprise a group of hydrophobic neutral organic compounds. Their presence in groundwater impairs beneficial uses of groundwater resources, necessitating cleanup of PAH-contaminated aquifer systems. However, the efficiency of remediation efforts mainly depends on further developing an understanding of the geochemical behavior of PAH during their subsurface transport. The influence of chemical interactions of PAH on their transport in aquifers is as important as those of hydrodynamic properties (i.e., groundwater flow rate, hydraulic conductivity, dispersion coefficient, etc.). In particular, sorption reactions between PAH and aquifer materials are of importance. The affinity of natural organic matter (NOM) for neutral organic pollutants (e.g., PAH) as well as to mineral surfaces adds complexity to transport phenomena and may play an important role in the geochemical behavior of PAH, particularly their transport. Natural organic matter (NOM) occurs ubiquitously in aquatic systems, measured as dissolved organic carbon (DOC) or as UV absorbance. It consists of humic substances such as humic and fulvic acids, and nonhumic Substances. Humic and fulvic acids are macromolecules with a complex three-dimensional molecular structure comprised of hydrophobic and hydrophilic sites (1);soil humic substances contain more aromatic constituents and
* Corresponding author. 0013-936X/93/0927-1553$04.00/0
0 1993 American Chemical Society
less aliphatic constituents than aquatic humic substances (2, 3). The hydrophobic sites can form a complex with neutral organic pollutants such as PAH, while the hydrophilic sites can form a complex with positive-charged mineral surfaces. PAH are classified as hydrophobic organic compounds which are relatively insoluble in water and tend to absorb onto other nonaqueous phases, either through hydrophobic interaction when the nonaqueous phase is a nonpolar compound (4) or through conjugate ?r bonding when the nonaqueous phase is a polar compound (e.g., silica, A1203) (5). Humic materials represent a separate nonaqueous phase when they are present in water; the hydrophobic sites of humic substances are an important phase for PAH compounds. The interactions between PAH compounds and dissolved humic substances may not only enhance the concentration of hydrophobic organic compounds in the aqueous phase (6-14)but may also effectively desorb pollutants in aquifer media (15). The major interactions that occur during PAH transport in groundwater are the partitioning of PAH compounds between the aqueous (mobile)and solid (immobile)phases, represented by pure water and mineral soil, respectively. However, if there is a dissolved NOM phase within the aqueous phase, hydrophobic compounds such as PAH will have a tendency to bind to the dissolved NOM and stay in the solution, potentially facilitating their subsurface transport downgradient. In contrast, where an organic matter phase is associated with a soil matrix (mineralbound NOM), PAH may sorb onto the soil phase, and its transport will be retarded. Furthermore, if both dissolved NOM and mineral-bound NOM are present in the aquifer, the movement of PAH will depend on the partitioning competition of PAH between the aqueous-phase NOM and the mineral-phase NOM. The aqueous-phase concentration enhancement of moderately hydrophobic organic compounds (e.g., DDT) in the presence of dissolved organic matter has been observed by many researchers (6-14)and has been attributed to the interaction of organic compounds with dissolved NOM. Such interaction follows a partitioning process (IO),with the partitioning of PAH into the hydrophobic interior of NOM. Hydrophobic compounds may also undergo sorption with soil materials (10, 13). In the presence of mineralbound NOM, the sorption of organic compounds onto the soil matrix is simply a partitioning process from the aqueous phase to the organic phase associated with the soil (mineral-bound NOM) (16). In this case, the partitioning coefficientof PAH and the soil is highly correlated with the organic content of the soil (fraction organic carbon, f0d (17,18), with the contribution of the mineral surface to sorption considered negligible (19). Means et al. (18) found that suspended colloidal material of an organic character was able to bind atrazine more effectively than sediment or soil organic matter, on an organic carbon basis. Murphy et al. (20)observed that NOM witha high aromatic content is the strongest sorbent for hydrophobic organic Environ. Scl. Technoi., Vol. 27, NO. 8, 1993 1553
compounds. Amy et al. (7) observed a decrease in the binding capacity of aqueous-phase NOM for PAH upon its association with a mineral surface. The extent of concentration enhancement of PAH in aqueous phase by NOM may depend on NOM characteristics. NOM is a heterogeneous mixture with individual sources having different molecular weight distributions and humic substance contents. Davis et al. (21) showed that NOM with higher molecular weight exhibits stronger sorption to Murphy et al. (20) concluded that the source, molecular weight, and polarity of NOM affects both NOM binding to a mineral surface and NOM binding to organic compounds. Amy et al. (7) observed a higher binding capacity of both high molecular weight NOM fraction and NOM humic fraction for PAH. Though much research has been conducted on PAHNOM interactions, a systematic understanding of the effect of NOM on PAH transport is still necessary. Issues that relate to the study of NOM effects on PAH transport mechanisms in groundwater include (1)the degree to which NOM may facilitate the transport of PAH, (2) the possibility of using NOM to facilitate the cleanup of a PAH-contaminated aquifer, (3) the comparisons of results between equilibrium methods (batch experiments) and kinetic methods (column experiments), and (4) the transport behavior of NOM in aquifer media and its concomitant effects on PAH transport. In order to provide an improved understanding of the mechanisms of NOM effects on PAH transport in groundwater, this research had the following specific objectives: (1)to study the transport behavior of NOM, (2) to study the facilitated transport of PAH by aqueous-phase NOM, (3) to study the retardation effects of solid-phase NOM on PAH transport, (4) to study the possibility of enhanced desorption of a PAH-contaminated aquifer by using NOM as a flushing solution, and (5) to further develop the mathematical description of PAH partitioning and transport in heterogeneous systems.
Materials and Methods Materials. The three main groups of materials used in this research were polynuclear aromatic hydrocarbons (PAH), minerals, and natural organic matter (NOM). PAH. The specific polynuclear aromatic hydrocarbons (PAH) used in this study were phenanthrene and anthracene (Aldrich Chemical Co.). The solubility of phenanthrene in water (S) and its octanol-water partition coefficients [log(K,,)] are 960 pg/L and 4.4, respectively. For anthracene, these two values are 40 pg/L and 4.5. The quantification of PAH here was based on fluorescence intensity with a spectrofluorometer (Hitachi 3100),according to Gauthier’s method (22). Standard curves of PAH in water were made by (1) dissolving PAH in methanol, (2) transferring a small volume of PAH-methanol solution into water, (3) conducting a serial dilution of the solution made in step 2 with water, and (4) obtaining the relation between PAH concentration and fluoroescence intensity. All of the samples used for the standard curve contained less than 0.1 % of methanol in volume, under which no interference of methanol on PAH fluorescence intensity was observed. Likewise, standard curves of PAH in hexane were made by dissolving PAH in hexane following a serial dilution in hexane. 1554
Envlron. Scl. Technol., Vol. 27, No. 5. 1993
To avoid the cosolvent effect of organic solvent on PAHNOM or PAH-mineral interaction, the stock PAH solutions used in experiments were generated with the followingprocedure: (1)dissolve a certain amount of PAH in a certain volume of acetone (HPLC grade, Aldrich), (2) transfer a certain volume of PAH-acetone solution into a beaker, (3) completely evaporate acetone under dark condition, (4) introduce a certain volume of Milli-Q water into the beaker and stir the water with a magnetic stir bar for 24 h, and (5) check the concentration of the stock solution with PAH standard curves to ascertain that the solution is not oversaturated. Minerals. Both crystalline quartz (Feldspar Industry) and Linde B aluminum oxide (Union Carbide), employed as the solid phase in experiments, were treated for the removal of impurities prior to their use in experiments. For the silica quartz (SiOz), its pH,,, is about 2.0; its size (in diameter) ranges from 75 to 300 pm, with an average diameter of about 200 pm and an average surface area of about 0.007 m2/g. For the aluminum oxide (Al2O3), it is composed with 85 % of y-component and 15 % of cy-component; its pH,,, is about 8.0; its size is about 0.05 pm (in diameter); its surface area is about 70-95 m2/g. NOM. Both bulk natural organic matter (NOM) and commercially available organic matter isolates were used in this study. The bulk NOM sources included filtered water from a DOE field site in Georgetown, SC, NOM(1) and NOM(B). NOM(1) was collected from a local surface water source at this DOE field site and was used for ”injection”into a shallow aquifer. NOM(B) was collected (recovered)as ”breakthrough”water withdrawn by a pump downgradient in the aquifer within which NOM(1) was injected. Both NOM(1) and NOM(B) were characterized according to dissolved organic carbon (DOC), UV absorbance at 254 nm, molecular weight (MW) distribution, and humic content, following the protocols described in the paper of Amy et al. (7). The commercially available organic matter source was a soil humic acid (SHA) from the International Humic Substances Society (IHSS). The soil humic acid was prepared by dissolution into Milli-Q at a pH of 10, adjustment to the pH to 7.0, and filtration through a 0.45 filter. Batch-Mode Experiments. Batch-mode experiments were designed to evaluate the partitioning of PAH in the binary systems of interest: PAH/NOM, NOM/mineral, and PAH/mineral surface. All experiments in the batch experiments were conducted in glass bottles with glass stoppers following the protocol described in a previous paper (7). Data derived from batch binary systems were modeled as linear equilibrium isotherms; alinear partition coefficient (K) was determined from the slope of the respective isotherm. In all cases, a preliminary kinetic experiment was performed to select an appropriate equilibrium time. Continuous-Flow Column Experiments. Two stages of column experiments were designed. The experiments in the first stage were conducted in the column packed with silica quartz (quartz system) to address the effect of aqueous NOM on PAH transport. Those in the second stage were conducted in the column packed with aluminacoated quartz (mixed bed system) to study NOM transport and the effect of solid-phase NOM on PAH transport. All experiments were conducted using stainless steel minicolumns [Supelco, 15 cm (length) X 0.425 cm (diameter)] filled with a given porous media &e., silica quartz
or quartz coated with aluminum oxide). For mixed-bed systems, 0.5% by weight of aluminum oxide was coated onto quartz according to the method described by Odem (23). Operating conditions (e.g., flow rate) were chosen to achieve laminar flow in order to keep hydrodynamic dispersion to a minimum. A Darcy velocity (u)of 3.1 cm/ min or 9.4 cm/min was used in the experiments. The porosity of quartz (e) is 0.4. In the setup of column experiments, two reservoirs were used: one containing the solution with solutes of interest (adsorption mode) and the other containing a flushing solution (desorption mode). A HPLC pump (Eldex) was used to control the flow rate. A three-way valve was located before the pump for selecting the desired feed solution. The effluent from the column was connected to either a flow-through cell in a given instrument (fluorometer, UV spectrophotometer) for continuous direct measurement or to a fraction collector for discrete sample collection and subsequent measurement. In a quartz system, repeated experiments were conducted using the same bed of media. After each PAH adsorption and desorption experiment, the media was flushed first with methanol until residual PAH were completely removed and then with Milli-Qwater to remove all methanol residuals from the column. Specific experiments in this system included (1) tracer adsorption (NaNO3) onto quartz by feeding NaN03 solution and its subsequent desorption by feeding Milli-Q water, to study the hydraulic characteristics (i.e., dispersion coefficient) of the system; (2) NOM adsorption onto quartz by feeding NOM solution and its subsequent desorption by feeding Milli-Q water, to study NOM transport behavior in the system; (3) PAH adsorption onto quartz by feeding PAH solution (made with Milli-Q water) and its subsequent desorption by feeding Milli-Q water, to study PAH transport behavior through the system; (4) PAH adsorption onto quartz by feeding PAH solution and its subsequent desorption by feeding NOM solution, to study the effect of aqueous-phase NOM on PAH desorption; and (5) PAH adsorption onto quartz by feeding PAH-NOM preequilibriated solution, to study the effect of aqueous-phase NOM on PAH adsorption. In the mixed-bed system, the bed was discarded after each experiment due to the irreversible adsorption of NOM. Thus, each experiment was run with anewly packed column. To avoid the desorption of alumina oxide from the quartz, all experiments were performed at a pH of 6 (a pH between the pHzpc's of the two minerals). The experiments regarding the transport behaviors of tracer (NaN03), NOM, and PAH through the bare mixed-bed quartz system (Le., without the presence of NOM in the mixed bed) were performed, following the same protocol of the respective first three experiments conducted in the quartz system. In addition to these experiments, the effect of soil-phase NOM on PAH transport in the mixed bed was studied by (1)generating a soil-phase NOM through NOM adsorption onto the mixed bed by feeding NOM solution followed with its desorption by feeding Milli-Q water, (2) calculating the ratio of NOM mass retained in the column (Le., the difference between NOM mass adsorbed onto the column in the adsorption limb and NOM mass desorbed from the column in the desorption limb) over the mass of the solid media, a ratio reflecting the fw content (fraction organic carbon) of the mixed bed, and (3) conducting PAH adsorption onto soil NOM in the
mixed bed by feeding PAH solution and its desorption by feeding Milli-Q water. During experiments, UV absorbances of the breakthrough samples were also measured to detect the potential desorption of soil NOM from the mixed bed. The introduction of feed solution to the column corresponded to a continuous input during the adsorption mode, with the feed solution not containing the sorbate of interest introduced during the desorption mode. The breakthrough samples of NOM were continuously measured with a UV spectrophotometer at 254 nm and those of PAH were measured with a fluorescence spectrophotometer. For the PAH from PAH-NOM mixture samples, a hexane extraction method was used to measure total PAH (24). Breakthrough curves from the column experiments were constructed based on normalized effluent concentrations (CIC,).One pore volume was defined by the time required for a conservative tracer to reach C/C,= 0.5, where C is the observed concentration and C, is the influent concentration. The breakthrough results from the column experiments were analyzed with a one-dimensional advective-dispersive transport model (CFITIM) developed by Van Genuchten (25). A least-squares method is used in the model to predict the parameters for a solute transported through a column under equilibrium or various kinetic conditions. The kinetic conditions evaluated here were physical nonequilibium, two-site kinetic, and onesite kinetic models. The equilibrium model was used for estimating the transport parameters of the tracer. For PAH and NOM transport, kinetic models were employed. The parameters of interest in this research were retardation factor, hydraulic dispersion coefficient, /3 (dimensionless parameter, representing the fraction of instantaneous retardation in the two-site model), and w (the ratio of hydrodynamic residence time to the characteristic time of sorption) (25,26). The retardation factors for the solute of interest were obtained with the followingmethods and compared among each other: Modeled Retardation Factor (Rm). The retardation factor obtained from the CFITIM model. Observed Retardation Factor (Ro). The number of pore volumes at which C/Cois 0.5. Integrated Retardation Factor (Ri). The integration above (or below) the breakthrough curve for adsorption (or for desorption), as presented in eqs 1 and 2
Ri =
soTm- ") (1
CO T C dT Ri = T-
c
dT
for adsorption
for desorption
(1) (2)
0
where Tm, is the maximum pore volume (i.e., pulse) in the adsorption limb, and T, is the maximum pore volume at the end of desorption limb. Specific Retardation Factor (R,). The ratio of integrated retardation factor over observed retardation factor. This parameter was used to represent the sorption kinetics during solute transport and the degree of asymmetry of the breakthrough curve. Only near equilibrium conditions will R, be equal to 1(e.g., tracer, and NOM). Otherwise, R, is always larger than 1. The higher the R, is, the more kinetically controlled the transport is. Environ. Sci. Technol., Vol. 27. No. 8, 1993 1555
Table I. Summary of Properties of Bulk NOM Sources and PAH-NOM Binding Constants
NOM source
DOC: (mg/L)
UVabs (cm-9
PH
cond. (S/cm)
spec abs (UV/DOC)
NOM(1) NOM(B) SHA
60.7 18.4 6.3
2.83 0.76 0.36
7.8 8.2 8.0
120 137 na
0.041
( % DOC)
>10K (% DOC)
humic content (% DOC)
89.3 80.7 na
58.6 45.8 na
87.4 83.0 na
>1K
0.046 0.058
Kdw ( ~ 1 0(mL/g) ~) phenanthrene anthracene
11.9 8.20 16.0
na 8.5 17.6
Results IJ
NOM Characteristics. The important characteristics of NOM such as DOC, humic fraction, and molecular weight (MW) fraction were summarized in Table I. In general, NOMU) has a higher DOC value, a higher humic content, and a higher MW fraction than NOM(B). The difference between NOM(1) and NOM(B) is presumably the amount of NOM retained in the aquifer media. Batch Experiments. Linear isotherms were observed for the interactions between PAH and NOM, PAH and minerals, and NOM and minerals (24). The binding constants summarized in Table I show that (1) soil humic acid (SHA) exhibited a stronger affinity for PAH than aquatic NOM [NOM(I) and NOM(B)I, (2) NOM(1) showed a higher binding capacity for PAH than NOM(B), and (3) no discernible differences were observed between the binding constants for phenanthrene-NOM interaction versus anthracene-NOM interaction. The higher binding capacity of soil humic acid for PAH is possibly due to its higher aromatic content and lower polarity than those of aquatic NOM. The higher affinity of NOM(1) for PAH than NOM(B) is likely due to its higher humic content and its higher molecular weight. For the interaction between phenanthrene and minerals, aluminum oxide exhibited a higher binding constant for phenanthrene (53 mL/g) than quartz (5 mL/g), a result mainly attributable to the greater surface area of aluminum oxide. The interactions between either NOM(1) or NOM(B) and aluminum oxide were quantified by UV and DOC measurements. Binding constants of NOM(1) and NOM(B) obtained from UV measurements were 900 and 1430 mL/g, respectively, and those obtained from DOC measurement were 640 and 870 mL/g, respectively. The discrepancy between the binding constants obtained from UV measurements and those obtained from DOC measurements is possibly due to the heterogeneity of NOM and due to the fact that not all DOC absorbs UV light. Column Experiments with Quartz System. The conservative transport behaviors of both tracer and NOM in the quartz system were observed, with their retardation factors of approximately 1.0 and their mass recovery of loo%,in both adsorption and desorption limbs, as shown in Figure 1. The observation that NOM from a NOMalone solution behaves like a tracer is mainly due to the electrical repulsion between NOM and the acidic quartz surface. The presence of PAH had an insignificant effect on NOM transport. This was possibly attributed to the fact that (1) the concentration of PAH is too low (pg/L) compared to the concentration of NOM (mg/L) and (2) the PAH associated with the PAH-NOM complex is located within the interior of the NOM macromolecule, thus it does not have an opportunity to interact with the soil matrix and play a "bridging role" for NOM adsorption. The results of phenanthrene and anthracene transport through a quartz-packed column are presented in Table 11. The table describes the values of the retardation factor 1556
Envlron. Scl. Technol., Vol. 27, No. 8, 1993
'-1
0.8
4 l
0
-
PORE VOLUME NOM(1) Alone -a- NOM(B) Alone -Tracer -!a.NOM(B)tPAH
-*-NOM(1)tPAH
Flgure 1. Transport of NOM(1)andNOM(B)through the quartz system, adsorption with NOM-alone, and desorption wlth MIiII-Q water; PAH effects on NOM transport, adsorption with preequliibrated NOM-PAH solutions; conservative tracer (NaN03) transport.
obtained from model (R,), observation (&), and integration (Ri)approaches; the values of specific retardation factor (R,) for both adsorption and desorption limbs; and mass recoveries. Also presented in the table are both modeled and observed results for PAH compounds transported through the system with different flow velocities. The analysis of results leads to the following observations: (1) In general, the breakthrough curve of the tracer (NaN03) is symmetrical, and the retardation factors from the different approaches are close to 1. However,for PAH transport, the breakthrough curves are not symmetrical, and differences exist among the retardation factors obtained from model, observation, and integration approaches, with R, and Ri agreeing most closely. (2) PAH transport through the column is kinetically controlled. This kinetic phenomena is reflected by the following characteristics: (a) the breakthrough curves were asymmetrical, with a long tailing, (b) the values of R, for all cases were larger than 1, and (c) only the two-site kinetic model simulated the breakthrough data closely. (3) Aqueous-phase NOM facilitates the transport of PAH in the adsorption mode, and the retardation factor is reduced to about one-half compared to the retardation factor in the absence of NOM, as shown in Figure 2 and Table 11. The presence of aqueous NOM also affects the transport kinetics of PAH, with a shorter adsorption time and a transport behavior closer to equilibrium transport. (4) Aqueous-phase NOM does not facilitate (enhance) the desorption of PAH, as shown in Figure 3 and Table 11. To demonstrate the potential facilitated desorption behavior of PAH, methanol was used as a flushing solution. The comparison of PAH desorption behavior by NOM with that by methanol shows that the former does not exhibit any of the following characteristics of the latter: (1) a shorter desorption time is required; (2) the effluent concentration of the solute is higher than that of the feed solution during some initial time period of desorption, reflecting solubility enhancement, and after this period,
Table 11. Summary of P A H Breakthrough Curves in Quartz-Packed Columna adsorption limb
desorption limb
retardation factor (R) feed solution
Rm
retardation factor (R)
Ri
R O
Rs
Milli-Q + PHE water + PHE
2.7 f 0.2 2.5 f 0.3
1.9 f 0.3 1.9 f 0.3
3.0 f 0.2 3.1 f 0.2
1.57 1.63
NOM(1) + PHE NOM(B) + PHE SHA + PHE water + ANT water + ANT
1.3 1.4 1.6 2.7 f 0.2 2.6 f 0.3
1.18 1.13 1.48 2.0 f 0.3 2.0 f 0.5
1.34 1.41 1.70 3.8k 0.4 2.7 f 0.2
1.13 1.25 1.15 1.9 1.35
+ NOM(1)
1.7 f 0.3
1.6 f 0.2
1.9 f 0.2
1.19
ANT
Rm
feedsolution
RO
Ri
Rs
water water NOM(1) NOM(B)
3.0f 0.1 2.8f0.3 2.9f 0.1 2.8f 0.1
2.0f 0.2 2.1 f 0 , 3 2.4f0.3 2.3 f 0.2
3.8 f 0.5 3.3i0.1 3.1 f 0 . 2 3.1 f 0.1
1.9 1.57 1.30 1.35
water water NOM(1)
4.0 f 0.3 3 . 3 f 0.3 2.9 f 0.4
1.9 f 0.3 2.7 f 0 . 4 1.6 f 0.3
4.7 3.9f0.3 3.8f 0.7
2.5 1.4 2.4
mass recovery (%)
darcy velocity (cm/min)
-100
3.1 9.4 9.4 9.4 9.4 9.4 9.4 9.4 3.1 3.1 3.1
-100 -100
-100
-100 -100 -100
a PHE represents phenanthrene: ANT represents anthracene: all experiments were done at pH 6.0; Rgis the specific retardation factor: relative errors in the table were baaed on standard deviation.
I.
I
I
15 pore volume
+ wRulilli-Q +wMOM(I) -6- w/SHA t Tracer
-*-
Pore Volume
wMOM(B)
1
Flgure 2, Effects of aqueous-phase NOM on phenanthrene adsorption in the quartz system; comparison of the PAH adsorption from preequlllbrated PAH-NOM solutions wRh those from PAH-Milli-Q solutions.
+
wiMEOH
W/NOM(I)
I
Flgure 4. Comparison of anthracene desorption with NOM(1) versus that with methanol: preadsorptlon with PAH alone and desorption with NOM(1) or methanol.
"-1
1
0
1 +
WMOM(I) O b . ....... W/NOM(I) Mod. W/NOM(B) Mod. A W/Milli-Q O h .
W/NOM(B) Ob.
-w/Milli-Q Mod.
Flgure3. Effects of aqueous-phase NOM on phenanthrene desorption in the quartz system, preadsorptlon with phenanthrene alone and desorption with NOM solutions (Mod. and Obs. in the figure represent the two-site modeled results and observed data, respectively).
the concentration decreases rapidly; (3) the breakthrough curve is of a bell shape rather than a S shape (Figure 4). (5) A reversible interaction occurs between PAH and quartz; no discernible difference was observed between anthracene and phenanthrene transport, as shown in Figure 5. (6) Flow rate affects the transport kinetics of PAH; the lower the flow rate is, the closer the PAH breakthrough behavior is to equilibrium transport. Column Experiments with Mixed-Bed System. Table 111 summarizes the results of NOM transport
+
20
40
60
80
Pore Volume Phe. Observed ...-... Phe. Modeled A Ant. Observed
100
0
- Ant. Modeled
1
Flgure 5. PAH transport through the quartz system: study of hydrophobicity effects embodied by phenanthrene versus anthracene: adsorptlon with PAH solutions and desorptlon with Miili-Q water (the model used here is the two-site model).
through the mixed bed. The breakthrough curves of NOM and mass of NOM retained in the column are shown in Figures 6 and 7, respectively. A close examination of these results reveals the following observations: (1)NOM transport is retarded by the mineral phase to some degree, depending on the source of NOM, soil humic acid exhibited the strongest affinity for the mineral surfaces. (2) NOM interaction with the mixed bed is partially irreversible, with a high percentage of NOM being retained by the basic alumina surface in the mixed bed, as shown in Table I11 and Figure 7. (3) The humic fraction and >1K MW fraction contribute the most to NOM adsorption (Figure 7). Envlron. Scl. Technol., Vol. 27, No. 8, 1993
1557
Table 111. Summary of NOM Breakthrough Curves in Mixed-Bed System. adsorption limb
desorption limb
retardation factor (R)
R,
feed solution
R.
mass recovery
retardation factor (R)
R.
Ri
.R
R.
Ri
R.
(%I 25.0 15.9 3.3
3.72f1.2 2.01 3.16k0.4 1.85k0.4 1.13 1.36 2.12f0.01 1.50f0.01 2.54f0.03 1.70 32.4 1.03 1.94 1.63 2.08 1.28 NOM adsorption at pH 6.0 in the mixed-bedsystem with 0.5% by weight of Y-AI~OS coating; flow velocity of 3 cmimin; in desorption limb,
NOMU) NOM(B) SHA
11.6f1.8 9.95k1.5 31.9
10.4k1.6 7.8k0.9
11.9k1.7 10.7k1.4 33.4
the feed solution was Milli-Q water at pH 6.0.
of retardation depends on NOM sources; soil humic acid exhibited the highest retardation capacity. (3)The interaction betweenPAH and soil organicmatter is reversible (Figure 8).
3 $0 $01 3
."0.
20
0
SO
40
1W
120
Pore Volume
Figure 8. NOM transport through the mixed-bed system, adsorption with NOM and SHA soiutions. and desorption with Milli-Q water (Mod. and Obs. represent two-sne modeled resuns and observed data,
respectively).
I
z l K MW
Humic
Mass Retained
I
Flgure'l. Comparison of percentageof NOM mass retained(heversibiy adsorbed) In the mlxed-bed system with the characteristics of the corresponding bulk NOM (fraction shown as % of bulk NOM), based on DOC.
Discussion PAH-Mineral Interactions. The adsorption of hydrophobiccompounds, such as PAH, ontominerals is more energetically favorable than their dissolution in water. The mechanism of PAH adsorption onto a mineral phase is a conjugate *-bonding interaction (27). Results from both batch and column experiments confirm the occurrence of PAH-mineral interactions. The results also show that the binding constants between PAH and A1203are larger than those between PAH and quartz, a result attributable to the greater specific surface area of A1203than that of quartz. The 100% mass recovery of PAH during their transport through a quartz system and a mixed-bed system, as shown in Tables I1 and IV, shows that the reaction between PAH and the minerials is reversible, suggesting that adsorption of PAH onto mineral surfaces is through a weak physical bonding. A comparison of binding constants between phenanthrene and quartz obtained from batch and column experimentashowedtbat thebindingconstantsfrom batch experiments (i.e., 5 mL/g with quartz) were significantly larger than the corresponding values (Le., 0.75 mL/g with quartz) from column experiments (24). Our explanation for this discrepancy is that the binding constants from batch experiments were obtained under equilibrium conditions, while those from column experiments were under kinetically controlled nonequilibrium conditions. A lower apparent bulk density and higher apparent porosity in the batch experiment may also contribute to this difference (28). The breakthroueb results of PAH throueh the mineral systems were modeled with different kinetic models. The results from the two-site kinetic model most closely simulate observed results. Since the columns were packed with nonporous solid particles, it isunlikely that the kinetic transport of PAH is due to transport-related (physical) nonequilibrium. The different reaction rates in the interactions between PAH and different chemical groups from the mineral surfaces are a more reasonable explanation. Interaction a n d Transport of NOM through Mineral Phases. The mechanism of NOM adsorption on A1203surfaces is still largely unknown. Jardine et al. (29) proposed that NOM adsorption onto A1203is mostly due to hydrophobic adsorption. Parfitt et al. (30)and Tipping (31) suggested that the adsorption is due to a ligand exchange between humic materials and surface OH-. Davis (21)indicated that adsorption is due to complex formation between mineral-surface metal sites and acidic functional Y
Table IV summarizes the results of PAH transport through a mixed bed in the presence and absence of soilphase NOM. Soil-phase NOM in the mixed bed was simulated with the mass of NOM retained (as DOC) on the solid phase and characterized by the fraction organic carbon, jm, which is defined as the ratio of NOM mass (as C) over the mass of solid matrix. Though the soil organic content in the soil phase is very low (f,is about 0.00007), PAH transport was retarded by soil-phase NOM, as shown in Figure 8. Specific observations are summarized as follow: (1)The retardation factor from the transport of PAH through the mixed bed in the absence of soil organic matter is slightly higher than that through the quartz system, which agrees with the batch experiment results. (2) The transport of PAH through the mixed bed in the presence of soil organic matter is retarded, and the degree 1558 Envlron. Scl. Technd.. VoI. 27. No. 8. i993
I
Table IV. Transport of PAH through Mixed-Bed System with NOM Coatings
adsorption limb retardation factor ( R ) NOM residual (foe) without NOM NOM(1) (0.00009) NOM(1) (0.00007) NOM(B) (0.00004) NOM(B) (0.00006) SHA (0.00005) NOM(1) (0.00007)
feed solution
R,
R O
desorption limb retardation factor (R) Rm R O Ri
Ra
Ri
anthracene + Milli-Q 3.57 f 0.5 2.7 f 0.7 3.65 f 0.4 anthracene + Milli-Q 7.1 4.0 7.0 anthracene + Milli-Q 4.63 3.0 5.71 anthracene + Milli-Q 3.99 2.46 5.49 3.27 3.06 3.77 anthracene + Milli-Q anthracene + Milli-Q 10.4 10.1 11.77 4.90 7.13 phenanthrene + Milli-Q 5.56
1.35 4.5 f 0.8 1.75 7.5 1.9 6.77 2.23 2.88 1.23 4.76 1.16 10.4 1.45 6.69
3.0 f 0.8 5.0 6.2 2.46 4.42 8.73 5.98
mass recovery
4.4 k 0.7 7.2 8.0 3.78 6.50 11.1 7.23
Rn
(7%)
1.47 1.44 1.29 1.54 1.39 1.27 1.20
-100 -100 -100 -100 -100 -100 -100
a Experiments were conducted at pH 6.0 under flow velocity = 3 cm/min; in the desorption limb, Milli-Q water at pH = 6.0 was used as feed solution.
1 0.9-
0.80.70.6-
I
0.5-
0.4-
0.3-
0
Pore Volume wiwater
- w/NOM(I) --.-... w/NOM(B)
20
10
30
50
40
60
Pore Volume
-wiSHA
Flgure 8. Anthracene transport through the mixed-bed system precoatedwith NOM;adsorptionwithanthracenesolution and desorption with Milii-Q water.
groups of the organic matter. However, all the abovementioned statements were based on the results of batch experiments. The purpose here is to try and obtain some insight into NOM adsorption by examining data from column experiments. The breakthrough results of NOM through a mixed bed show that a significant amount of NOM was retained. A comparison of the percentage of NOM mass retained with the percentage of humic fraction content and >1K MW fraction content of the corresponding NOM in Figure 7 suggests that it is the humic fraction and the >1K MW fraction of NOM that contribute most to NOM adsorption. Examination of the retardation factors (Table 111) and the mass retained (Figure 7) also show that soil humic acid (SHA) exhibits a much stronger binding with A1203 than that of the aquatic NOM sources [NOM(I) and NOM-
031. Although the results suggest that NOM humic materials and higher MW materials contribute most to NOM-Al203 interaction, it is still difficult to clearly elucidate the interaction mechanism. Even though they possess some nonpolar characteristics, humic (and fulvic) adsorption onto A1203 surfaces may not solely be hydrophobic adsorption, as proposed by Jardine et al. (29). The reasons for making this statement are as follows: (1)Hydrophobic adsorption is a weak physical interaction, and it is usually a reversible process (such as the adsorption of PAH onto minerals); however,NOM adsorption is largely irreversible. (2) If NOM adsorption is a hydrophobic interaction, some degree of interaction should be observed between NOM and quartz (such as the interaction that has been observed between PAH and quartz). Humic (and fulvic) acid consists of various acidic functional groups that enable interaction with the A1203 surface through surface com-
I
Obs.
-Two-site
One-site
I
Flgure 9. NOM(1)transport throughthe mixed-bed system, adsorption with NOM(1) solution and desorption wlth MIiIi-Q water: model comparison.
plexation and ligand exchange. Therefore, perhaps a more reasonable interpretation of NOM adsorption onto A1203 surface involves some combination of hydrophobic effects and surface complexation. NOM transport through the mixed-bed system was modeled with different kinetic models. Results in Figure 9 show that although both one-site and two-site kinetic models agree closely with the observed data in the adsorption limb, the two-site model is more accurate in describing the desorption behavior of NOM. This observation suggests that NOM transport through the mixed bed is best simulated by two-site kinetically controlled transport. Role of Aqueous NOM in PAH Partitioning and Transport. The one-dimensional PAH transport through a column can be expressed as follows:
as + 8-ac = 0D-a2c at at ax2
p-
8V-ac
ax
(3)
where S is the concentration of PAH adsorbed on the solid phase, C is the PAH concentration remaining in the aqueous phase, 0 is the porosity of solid phase, D is the hydraulic dispersion coefficient, (0v) is Darcy velocity, p is the bulk density of solid phase, t is the time, and X is the distance. By assuming that the partition coefficient of PAH between the aqueous phase and the solid phase is K,, the retardation factor, R,, without the presence of aqueous-phase NOM can be calculated as follow (32): (4)
If there is aqueous-phase NOM in solution, some of the PAH will partition with NOM and stay within the NOM Envlron. Scl. Technol.. Vol. 27, No. 8, 1993 1659
Table V. Effect of NOM on PAH Transport: Comparison of Retardation Factor from Experiments vs Those Calculated from Eq 8
feed solution PHE + NOM(1) PHE + NOM(B) PHE + SHA ANT + NOM(1)
DOC
Kdm
(mg/L)
(L/mg)
retardation factor from from expt eq 8
Phenanthrene Feed Solution" 30 9.0 6.3
0.119 0.082 0.16
1.37 1.98 1.85
1.3 1.4 1.7
Anthracene Feed Solutionb 30
0.119
1.37
1.7
Ro = 2.7 without the presence of aqueous-phase NOM. Ro = 2.7 without the presence of aqueous-phase NOM. a
phase. Upon equilibrium conditions, the PAH (C) in the overall aqueous phase actually consists of free PAH (Cfre,) and bound PAH (Cbound). The free PAH is the PAH that stays in solution, and the bound PAH is the PAH that binds with NOM. Only the free PAH contributes to PAH adsorption onto quartz. By assuming that the partition coefficient between NOM and PAH iSKdocand that NOM does not interact with the solid phase (which is true in quartz system), the following relationships are obtained: C = Cfree + Cto,
(5)
n
bound
Kdoc=
Cfr,,DOC
From eqs 5 and 6, the relation between S and Kdoccan be derived as follows:
(7) By substituting eq 7 into eq 3 and reorganizing the equation, the retardation factor of PAH in the presence of aqueous-phase NOM ( R ) can be expressed as follows:
Comparing the retardation factor in eq 4 with that in eq 8 shows that the presence of aqueous-phase NOM can reduce the magnitude of the retardation factor for PAH transport to some degree, depending on NOM concentration and the binding constant between PAH and NOM. For a given Kdoc, R is closer to R, at lower DOC concentration and is closer to 1 at higher DOC concentration. The facilitated transport of PAH in the presence of NOM is mainly due to the formation of a NOM-PAH complex; the interaction between PAH and NOM leads to some of the PAH binding into the hydrophobic interior of the NOM. This part of PAH does not interact with the solid phase, and thus, may be transported at the same rate as NOM. In the quartz system, NOM transport through the column has been found to behave like a conservative tracer. Thus, the PAH that is bound with NOM may migrate through the column conservatively and contribute to the faster movement of total PAH. We found close agreement between the retardation factors obtained from the experimental data and those calculated from eq 8, further supporting this interpretation of the role of aqueous-phase NOM in PAH transport (Table V). The presence of aqueous NOM also affects the transport kinetics of PAH. The results of model comparisons for 1580 Environ. Scl. Technol., VoI. 27, No. 8 , 1993
Table VI. Model Comparison for PAH Breakthrough from PAH-NOM Prequilibrated Solution through the Quartz System
Adsorption Limb parameters feed solution phenanthrene NOM(1)
+
phenanthrene NOM(B)
+
phenanthrene SHA
+
anthracene + NOM(1)
model equilibrium two-site kinetic one-site kinetic equilibrium two-site kinetic one-site kinetic equilibrium two-site kinetic one-site kinetic equilibrium two-site kinetic one-site kinetic
Rm
P
1.12 1.07 0.0 1.27 1.22 1.20 0.0 1.39 1.56 1.50 0.00 1.58 1.61
w
Ri(R0)' 1.34 (1.18)
2.85 0.34 1.41 (1.13) 4.83 0.34 1.70 (1.48) 6.72 1.14 1.9 (1.6)
1.56 0.00 4.30 1.70
0.85
Retardation factor from integration method (Ri)and observation method (Ito), respectively.
the breakthrough results of PAH compounds in the presence of NOM (preequilibrated) are summarized in Table VI. The P values from the two-site kinetic model for all cases are 0, which means that there is no instantaneous reaction sites during PAH transport and that all reactions are kinetically controlled. This can be attributed to the presence of aqueous-phase NOM. During its transport, free PAH may largely interact with aqueous NOM in the column instead of with the solid surface itself. NOM has been shown to behave like a tracer in the quartz system, and PAH-NOM interaction can quickly reach equilibrium conditions (< 3 min); thus, the free PAH that interacts with NOM will migrate through the column in a shorter time. This also explains the lesser mass of PAH adsorbed onto the solid phase in the presence of aqueousphase NOM. Therefore, PAH transport from a PAHNOM preequilibrated solution in the quartz system ismore likely one-site kinetically controlled transport with the dominant interaction between the free PAH and aqueous NOM in the column. Furthermore, since the interaction between PAH and NOM is so rapid, the transport is very close to equilibrium transport. Role of Aqueous NOM in the Enhanced Desorption of PAH-Contaminated Solid Phase The observation that NOM did not enhance the desorption of PAH is presumably due to the weak-binding between PAH and NOM. Since both PAH-NOM interactions and PAHquartz interactions are the results of hydrophobic interaction, it is difficult to judge which interaction is stronger. On the basis of binding constants measured by weight, NOM exhibits a much higher binding capacity (lo4mL/g) to PAH than quartz (5 mL/g). However, since quartz has amuchsmaller surface area (0.007m2/g)than NOM (30000 m2/g DOC) (7),its binding capacity per unit surface area to PAH is much greater than NOM. Therefore, the observation that NOM has no effect on PAH desorption may be attributed to NOM-PAH binding not being strong enough to break the existing PAH-quartz binding and form a new NOM-PAH complex. A comparison between the desorption behavior of PAH by methanol and that by NOM further supports the conclusion that NOM does not enhance PAH desorption under the conditions of our study. Role of Soil N O M on PAH Transport. The breakthrough results of PAH in the presence of soil NOM show the effects of different NOM sources. Among the NOM
.
Table VII. Model Comparison for Anthracene Breakthrough in Mixed-Bed System desorption limb parameters
adsomtion limb Darameters
mixed-bed system
model
P
w
0.64 f 0.02
0.50 f 0.10 5.92 f 1.6
0.45 f 0.1
0.58 f 0.1 7.00 f 4.0
0.61 f 0.12
0.79 f 0.4 5.41 f 1.7
Rm
2.80 f 0.5 without NOM equilibrium coating two-site kinetic 3.57 f 0.6 one-site kinetic 3.00 f 0.6 3.74 f 0.3 with NOM(1) equilibrium two-site kinetic 6.35 f 0.6 coating one-site kinetic 4.32 f 0.2 3.61 f 0.4 with NOM(B) equilibrium coating two-site kinetic 4.05 f 0.5 one-site kinetic 3.64 f 0.6 10.52 with SHA equilibrium coating two-site kinetic 12.0 one-site kinetic 10.4
0.82
0.17 71.52
R i W
B
Rm
2.89 f 0.4 4.53 f 0.8 3.20 f 0.7 5.99 f 0.6 6.35 f 0.6 (3.50 f 0.5) 9.54 f 1.5 6.10 f 0.6 4.63 f 0.8 3.82 f 0.9 (2.76 f 0.7) 6.69 f 2.2 3.81 f 0.9 9.19 11.77 10.18 (10.10) 9.78 3.65 f 0.4 (2.70 f 0.7)
RdRJ
w
4.4 f 0.7 0.32 f 0.05 (3.0 f 0.8) 8.60 f 4.0 7.6 f 0.6 0.30 f 0.01 (5.6 f 0.6) 0.54 f 0.6 14.70 f 4.5 5.14 f 1.2 0.49 f 0.05 0.26 f 0.07 (3.44 f 1.0) 3.61
0.58 f 0.01
11.1
0.58
(8.73)
1.63 8.77
Table VIII. Comparison of Binding Constants between PAH and Soil-Phase NOM (&) Calculated from Column Experiments (Ii&) with Those between PAH and Corresponding Aqueous NOM (&m) from Batch Experiments
K , obtained from column expt
R
solid-phase NOM
ads
foc
K (mL/g)" ads des
des
K , (mLMb ads
des
Kdoc (mL/g) from batch expts
Anthracene 3.03 X 104 11.5 X 104 2.22 2.73 2.45 x 104 11.5 x 104 1.88 x 104 3.00 x 104 1.32 2.10 2.70 x 104 1.70 x 104 8.5 x 104 1.09 0.68 0.83 1.36 1.38 x 104 2.28 x 104 8.5 x 104 17.6 x 104 3.42 3.42 6.83 x 104 6.84 x 104 Phenanthrene 11.9 x 104 NOM(1) 0.00007 5.5 1.63 2.34 x 104 K is defined as the binding constants between PAH and solid phase, it is calculated from R, where R = 1 + (Kp/O).b K , = K/foc. csd Replication of PAH transport through the mixed bed coated with NOM(1) and NOM(B), respectively. NOM(1)C NOM(I)c NOM(B)d NOM(B)d SHA
0
i0
0.00009 0.00007 0.00005 0.00006 0.00005
40
60
7.11 4.63 3.99 3.27 10.4
100
EO
7.5 6.77 2.88 4.76 10.4
120
140
160
Pore Volume
1
O b . Values
- Equil.
One-site
- Two-site
I
Flgure 10. Transport of anthracene alone through the mixed bed precoated with NOM(I), adsorption with anthracene solution, and desorption with Miili-Q water: model comparison.
sources studied, soil humic acid showed the strongest retardation effect, NOM(1) showed a moderate effect, and NOM(B) showed the weakest effect. This observation is in agreement with the results of batch experiments regarding the effect of NOM source on NOM-PAH partitioning. Again, the higher retardation effect of soil humic acid than aquatic NOM is probably due to its high aromatic content; the higher retardation effect of NOM(I) than that of NOM(B) is probably due to its higher molecular weight and its higher humic content. The results of model comparisons for anthracene transport through the mixed-bed system are summarized in Table VI1 and shown in Figure 10. All the results indicate that results from the two-site kinetic model agree more with the observed data than that of the one-site kinetic model, a result attributable to the heterogeneous
character of NOM on mineral surfaces. The reversibility of anthracene adsorption onto the soil-phase NOM was observed in our study, an observation agreeing with the results of other researchers (33). Again, such reversibility of PAH adsorption onto soil-phase NOM indicates a weak physical interaction. The binding constants between PAH and soil-phase NOM can be calculated from the retardation factors of PAH transport in the mixed-bed system. Table VI11 summarizes the binding constants between PAH and soilphase NOM and those between PAH and the corresponding bulk aqueous-phase NOM. The comparison shows that the binding constants of aqueous-phase NOM toPAH are higher than those of soil-phaseNOM. This observation is consistent with the results from the batch experiments of Amy et al. (7). Effects of Other Experimental Variables on the Transport of PAH. The experimental variables discussed here are flow velocity and PAH hydrophobicity. Most of the relevant studies were conducted in the quartz system. The effects of flow velocity on PAH transport were evaluated with two parameters from the two-site kinetic model; P and w. The degree of nonequilibrium decreases with an increase in either of these two parameters (33). The modeled results for anthracene transport at different flow velocities in the quartz showed the increase of both P and w with the decrease in flow velocity, suggesting that the degree of nonequilibrium decreases with the decrease of flow velocity. This is consistent with observed breakthrough curves, with breakthrough curves from the lower flow veloxity exhibiting a shorter tailing period than those from the higher flow velocity (24,33). Envlron. Sci. Technoi.. Voi. 27,
No. 8, 1993 I561
Besides these modeled parameters, the symmetries of the breakthrough curves also reflect the degree of nonequilibrium. Generally,the closer the breakthrough curves are to symmetry, the lower the degree of nonequilibrium. The symmetry of curves can be characterized by the specific retardation factor. The degree of symmetry increases with a decrease in the magnitude of the specific retardation factor; it was observed that the magnitude of specific retardation factor decreases with a decrease in flow rate. Regarding the critical flow velocity under which PAH transport is in an equilibrium mode, there is still no agreement on the exact value. Schwarzenbach et al. (33) observed that for velocities 1K MW fraction contributing most of the NOM adsorption, (3) the presence of aqueous-phase NOM facilitates the transport of PAH, (4)aqueous-phase NOM does not enhance the desorption of moderate hydrophobic PAH, ( 5 ) the presence of soil NOM retards PAH transport to some degree, depending on the sources of NOM, and (6) PAH-NOM interactions are NOM source dependent. Acknowledgments
This research has been supported by the Subsurface Science Program of the Ecological Research Division, Office of Health and Environmental Research, US. Department of Energy, Washington, DC (Program Director, Dr. F. J. Wobber). 1562 Envlron. Scl. Technol., Vol. 27,
No. 8, 1993
L i t e r a t u r e Cited
(1) Wersahw, R.J. Contam. Hydrol. 1986,1, 29-45. (2) Stuermer, D. H.; Peter, K. E.; Kaplan, I. R. Geochim. Cosmochim. Acta 1978,42, 989-997. (3) Malcolm L. R. In Humic Substances in Soil, Sediment, and Water;Aiken, et al., Eds.; Wiley-Interscience: John Wiley & Sons, 1985;Chapter 7. (4) Tinoco, I.; Sauer, K.; Wang, J. Physical Chemistry, 2nd ed.; Prentice-Hall: Old Tappan, NJ, 1978. (5) Lee, L. M.; Novotny, V. M.; Bartle, K. D. Analytical Chemistryof PolynuclearAromatic Compounds;Academic Press: New York, 1981. (6) Amy, G.; Liu, H. Preprints; 199thACS Conference, Division of Environmental Chemistry; American Chemical Society: Washington, DC, 1990;pp 243-246. ( 7 ) Amy, G.; Conklin, M.; Liu, H.; Cawein, C. In Organic Substances in Sediments and Water;Baker, R., Ed.; Lewis Publishers: Chelsea, MI, 1991;Chapter 7. (8) Carter, C.; Suffet, I. Environ. Sci. Technol. 1982,16,735740. (9) Carter, C.; Suffet, I. In Fate of Chemicals in the Environment; ACS Symposium Series 225;American Chemical Society: Washington, DC, 1983;pp 191-215. (10) Chiou, C.; Porter, P.;Schmedding, D. Enuiron. Sci. Technol. 1983,17, 227-231. (11) Chiou, C.; Schmedding, D.; Manes,M. Enuiron.Sci. Technol. 1982,16,4-10. (12) Chiou, C.; Kile, D.; Brinton, T.; Malcolm, R.; Leenheer, L.; MacCarthy, P. Enuiron. Sci. Technol. 1987,21,1231-1234. (13) Chiou, C.; Kile, D.; Brinton, T.; Malcolm, R. Environ. Sci. Technol. 1988,22,298-303. (14) Kan, T.;Tomson, M. Environ. Chem. 1990,253-263. (15) Abdul, S.A.;Gibson,L. T.; Rai,D. N. Environ. Sci. Technol. 1990,24, 328-333. (16) Gschwend, P.; Wu, S. Environ. Sci. Technol. 1985,19,9096. (17) Means, J.;Wood, S.; Hassett, J.; Banwart, W. Enuiron. Sci. Technol. 1980,14,1524-1528. (18) Means, J.; Wood, S.; Hassett, J.; Banwart, W. Environ. Sci. Technol. 1982,16,93-98. (19) Lion, W.; Stauffer, B.; MacIntyre, W. J. Contam. Hydrol. 1990,5, 215-234. (20) Murphy, E.;Zachara, J.; Smith, S. Environ. Sci. Technol. 1990,24,1507. (21) Davis, J. Geochim. Cosmochim. Acta 1982,46,2381. (22) Gauthier, T.; Shane, E.; Guerin, W.; Seltz, W.; Grant, C. Enuiron. Sci. Technol. 1986,20,1162-1166. (23) Odem, W. Ph.D. Dissertation, University of Arizona a t Tucson, 1991. (24) Liu, H. Ph.D. Dissertation, University of Colorado a t Boulder, 1992. (25) Van Genuchten, M. Th. Non-equilibrium transport parameters from miscible displacement experiments; Research Report 119;U.s.Salinity Lab: Riverside, CA, 1981. (26) Brusseau, M.; Larsen, T.; Christensen, H. Water Resour. Res. 1991,27, 1137-1142. (27) Giles, H.; Easton, I. Adu. Chromatogr. (N.Y.) 1966,3,6783. (28) Palvik, H.L.; Baker, F. G.; Runnells, D. D. Proceedings of the 7th International Symposium on Water-Rock Interactions: Salt Lake City, UT, 1992,pp 59-62. (29) Jardine, P.; Weber, N.; McCarthy, J. Soil Sci. SOC.Am. J. 1989,53,1378-1385. (30) Parfitt, L.; Fraser, A,; Farmer, V. J.Soil Sci. 1977,28,289292. (31) Tipping, E. Geochim. Cosmochim.Acta 1981,45,191-199. (32) Magee, R. B.; Lion, W. L.; Lemley, A. T. Enuiron. Sci. Technol. 1991,25,323-331. (33) Schwarzenbach,R.;Westall, J. Enuiron.Sci. Technol. 1981, 11,1360-1367.
Received for review August 10, 1992.Revised manuscript received March 19,1993.Accepted March 24, 1993.
