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(44) (45) (46) (47)
Berner, R. A. J. Sediment. Petrol. 1981, 51, 359. Cogger, C. G.; Carlile, B. L. J. Environ. Qual. 1984,13,137. Capone, D. G.; Bautista, M. F. Nature 1985, 313, 214. Hem, J. D. US.Geol. Surv. Water-Supply Pap. 1985, No.
2254. (48) Goldberg, S.; Sposito, G. Soil Sei. Am. J . 1984, 48, 772. (49) Wield, A. C.; Ketchum, B. H.; Richards, F. A. In The Sea; Hill, M. N., Ed.; Interscience: New York, 1963; Vol. 2, pp 26-77. (50) Fetter, C. W. Ground Water 1977, 15, 365. (51) Reneau, R. B.; Pettry, D. E. J. Environ. Qual. 1976,5,34.
(52) Johannes, R. E. Mar. Ecol. Prog. Ser. 1980, 3, 365. (53) Valiela, I.; Costa, J.; Foreman, K.; Teal, J.; Howes, B.; Aubrey, D. Biogeochemistry 1990, 10, 177.
Received for review November 5, 1990. Revised manuscript received July 15, 1991. Accepted August 15,1991. This work was supported by U S . EPA Grant CX812886-01-3, Sea Grant N A 86-AA-D-SG090, and the Jessie B. Cox Charitable Trust. Contribution No. 7050 of the Woods Hole Oceanographic Institution.
Cotransport of Cadmium and Hexachlorobiphenyl by Dissolved Organic Carbon through Columns Containing Aquifer Material Frank M. Dunnivant, Philip M. Jardine,” Davld L. Taylor, and John F. McCarthy
Environmental Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831-6038 The cotransport of an ionic contaminant (cadmium) and two nonionic contaminants (2,2’,4,4’,5,5’-hexachlorobiphenyl and 2,2’,4,4’,6,6’-hexachlorobiphenyl) by naturally occurring dissolved organic carbon (DOC) was evaluated using columns containing aquifer material. Contaminant mobility was found to increase as solution DOC concentrations were incrementally changed from 0 to 20.4 mg of DOC/L for polychlorinated biphenyl (PCB) and from 0 to 58.1 mg of DOC/L for cadmium. Desorption processes were similarly affected by the presence of mobile DOC. Experimental adsorption breakthrough curves (BTCs) were predicted independently of column experiments using measured contaminant distribution coefficients (batch technique) and the convection-dispersion (CD) transport equation. The increased contaminant mobility, observed in the presence of mobile DOC, was explained by incorporating a three-phase system (two mobile, one stationary) into the CD transport equation. Results support the hypothesis that contaminants can be cotransported by mobile DOC in groundwater systems.
Introduction A current topic of intense research interest is the quantification and characterization of inorganic and organic colloids in groundwater systems (1). The recent interest in colloid research results from field investigations documenting the presence of contaminant migration plumes at distances greater than those predicted by traditional groundwater transport models. This anomaly can result from inaccurate field characterization (high dispersion and preferential flow paths through aquifer macropores) or from the presence of contaminant-enriched mobile colloids leading to the inappropriate use of twophase distribution transport models (1). Physical transport processes, such as preferential flow, can be quantified by conservative tracer studies, but quantification of a contaminant-enriched mobile colloid presents considerable analytical challenges. Documentation and quantification of colloids and associated contaminants are the first steps in understanding the migration of contaminants in groundwater systems. The exact physical state of the “colloid”is sometimes uncertain. In this article, discussions are limited to dissolved organic carbon (DOC), where the exact nature of naturally occurring DOC, as a truly dissolved species, an organic colloid, or a separate mobile
* Corresponding author. 360
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“phasen, is a subject of debate. For the purposes of this study, no distinction between these terms is made and they are used interchangeably, as is commonly portrayed in the relevant literature (1). Our ability to predict contaminant mobility is also limited by current groundwater models, which do not adequately account for immobilization-mobilization interactions between the colloid and the immobile solid phase. Until recently, classical convective-dispersive (CD) transport models were based on equilibrium or kinetic reactions of a contaminant between an immobile (solid) phase and a single mobile (solution) phase and did not consider the possible influence of a second mobile colloidal phase on the transport process. One approach used to account for multiple mobile phases has been to reparameterize the CD equation to include size-exclusionprocesses. This reparameterized CD equation has been used to describe the transport of macromolecule-associated contaminants in soil columns (2, 3 ) ; however, the reported exclusion of the macromolecules from portions of the porous media may have been the result of the experimental conditions imparted (3). The use of commercial humic material, large column dispersivities, and low solution pH values may have minimized chemical adsorption processes (3). The phenomenon of size exclusion in soils and aquifer material has been observed for larger particles that are 0.1-10 pm in cross-sectional diameter (4). Generally, unaggregated macromolecules of truly dissolved organic carbon have cross-sectional diameters smaller than 0.01 pm and appear not to be affected by size-exclusion processes. This has been shown by Abdul et al. (5), who evaluated the ability of commercial humic materials to remove organic contaminants from soils. Although DOC transport processes were not specifically addressed, the published data can be replotted to clearly show the breakthrough of DOC after the breakthrough of conservative tracers; thus size-exclusion processes were not present in the soil system. Dunnivant et al. (6) and Jardine et al. (7) also observed the breakthrough of naturally occurring DOC from soil columns concomitant with or significantly after the elution of conservative tracers, and the movement of the DOC through the soil columns was effectively described by chemical adsorption processes. Numerous studies have as well shown that DOC may be immobilized through complex interactions with mineral surfaces (8-11). Additional exDeriments bv Dunnivant et al. (6) evaluated the transpori of DOC in“the absence of chemical adsorption processes and conclusively showed
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0 1992 American Chemical Society
that size-exclusion processes were not significant. The common discrepancies between transport models and field results emphasize the potential need for incorporating three-phase distribution relationships into groundwater models (1). Similar three-phase distribution concepts are widely accepted for the partitioning of pollutants in surface waters (12-15). The dissolved phase has been operationally defined as the constituents in solution that pass through a 0.45-pm filter, and filtrates containing DOC have been shown to increase the solution-phase affinity for nonionic compounds and significantly complex ionic compounds under a variety of experimental conditions (16-19). Similarly, organic carbon levels (OC >0.1% by weight) in soils and sediments control the sorption processes of nonionic compounds by increasing contaminant sorption (or partitioning) (19-23). Similar effects are expected for ionic compounds. It follows that dissolved and solid-phase organic carbon can be a dominant factor controlling the fate and transport of contaminants in groundwaters, thus providing a basis for developing three-phase transport models. Recently, Kan and Tomson (24) considered the influence of a second mobile phase on the transport process by redefining the distribution coefficient of the classical CD equation. This approach allows a contaminant species to be distributed between an immobile solid phase, a mobile solution phase, and a mobile colloidal phase (Le., DOC). Such a description of three-phase cotransport of contaminants is an essential step toward our understanding and predicting the fate of colloid-mediated contaminant transport through the subsurface environment. Unfortunately, the approach described by Kan and Tomson (24) did not use naturally occurring DOC or consider the interaction of the “mobile” colloids with the porous media, which can result in large errors in predicting the mobility of colloid-associated contaminants. The purpose of this investigation was to evaluate the cotransport of two distinctly different contaminants by naturally occurring DOC in laboratory columns and batch experiments. The contaminants included cadmium and polychlorinated biphenyls, thus allowing both ionic metal complexation and hydrophobic partitioning mechanisms to be evaluated. Other publications (6, 7) have documented the transport of naturally occurring DOC through aquifer materials that were identical to those used here; this investigation focuses on the cotransport of contaminants by DOC. Results from this investigation can be used to evaluate the cotransport hypothesis and, if appropriate, modify current transport codes to describe contaminant mobility in porous media. Theoretical Background The one-dimensional transport of a single solute in porous media, assuming constant fluid flow in a homogeneous matrix, can be described by the convection-dispersion equation
where p is the porous medium bulk density, 8 is the volumetric water content, S is the total adsorbed solute per unit mass of solid, t is time, C is the resident concentration of solute in the mobile phase, D is a dispersion coefficient reflecting the combined effects of diffusion and hydrodynamic dispersion on transport, X is distance, and V is the mean pore water velocity. Contaminant distributions in field samples are usually characterized by total solution and solid-phase measurements; thus, C (in eq 1)is defined
as the solute concentration in the mobile phase and can contain solutes in the truly dissolved form and/or solutes in a colloidal or colloid-associated phase. It has been shown for solutions containing DOC that the three-phase distribution relationships, represented by eqs 2-4, can be incorporated into eq 1 using eq 6 to account for the presence of two mobile solute phases in porous media (24). Kd KApp
KApp
= s/cAQ
= s/ (CAQ + CDOC [DOC] Kd/(l
+ KDOC[DOC])
R = 1 i- ( P K A ~ ~ / @
(2)
(3) (4) (5)
R = 1 (P/B)[Kd/(l + K~oc[DocI)1 (6) In these equations, Kd represents the equilibrium distribution of the contaminant in the absence of mobile DOC, KAp (apparent Kd)represents the equilibrium distribution of tke contaminant in the presence of mobile DOC, S is the solid-phase contaminant concentration (mg of pollutant/g of solid), CAQ is the aqueous-phase contaminant concentration (truly dissolved phase given in mg of contaminant/mL of solution), CDoc is the colloidal-associated contaminant in the mobile phase (given in mg of contaminant/g of C), R is the net retardation factor, [DOC] is the concentration of dissolved organic carbon in the mobile is the phase (given in g of C/mL), and KDOC CDOC/CAQ distribution coefficient of the contaminant between DOC and water. Although “K””(true and apparent) have traditionally been used to describe the “distribution” of a contaminant between two phases, we will use this term to represent both the partitioning of PCBs and the complexation of Cd in our systems. We fully appreciate the mechanistic differences between the processes (16,17,20, 23, 25); however, the convention of K as a distribution coefficient will be used here in an effort to simplify discussions and model comparisons between each contaminant. Combining eqs 1 and 5 yields the general form of the transport equation R -ac =D--V a2c ac (7) at ax2 Selection of eq 2 or 4 (for use with eq 7) will depend on the presence or absence of mobile DOC. Since the assumption of local equilibrium may not be valid for contaminant interactions with the solid phase during transport, eq 7 may be modified to consider an additional region or site which exhibits time-dependent reactions with the contaminant (26,27). These transport models are known as two-region or two-site models and are mathematically identical (28). For convenience, we utilized the two-site model, which assumes instantaneous adsorption on type-1 sites and first-order kinetic controlled adsorption on type-2 sites. The adsorption rate for type-2 sites is described by
where S2is the concentration of adsorbed solute on type-2 sites, F is the fraction of type-1 sites, and a is the firstorder rate coefficient. For a two-site adsorption process, eq 7 now takes the form of
where @ = (8 + F P K A ~ ~+) /pKApp) ( ~ is a dimensionless Envlron. Sci. Technol., Vol. 26, No. 2, 1992 381
variable related to the fraction of type-1 sites. A special case of this model develops when F is assumed to be negligible. Under this condition, a one-site, nonequilibrium model develops where CY defines a first-order rate coefficient in a system having only one type of adsorption site. Equations 7-9 are solved subject to a uniform equilibrium initial condition and first-type inlet boundary condition for a semiinfinite medium appropriate for continuous injection and for flux-averaged solute detection in the effluent (27). In our studies, the column physical parameters and V, t , and R were measured directly, while D was estimated from the observed transport of a conservative tracer through the system. Previous investigations (6, 7) found that the transport of DOC through laboratory columns containing aquifer material was influenced by chemical adsorption processes between the solution and solid phases and dispersion coefficient values for the conservative tracer and DOC were similar. Since most of the aquifer material used here was saturated with respect to DOC prior to initiation of cotransport experiments, the overall dispersion of DOC was assumed to be identical to that of the conservative tracer (6).
Materials and Methods Aqueous Carrier Solutions. Water containing DOC was obtained from a stream channel adjacent a peak deposit located in Hyde County, NC. The water was collected in aged, high-density polyethylene carboys, immediately transported to the Oak Ridge National Laboratory (Oak Ridge, TN), and stored at 277 K. Prior to use, water was filtered through 0.8-pm polycarbonate filters (Nuclepore, Inc.) and diluted with deionized distilled water (Millipore Milli-Q System, Bedford, MA) to yield predetermined organic carbon levels of 5.2,10.2,20.4, 29.5,40.1, and 58.1 mg of DOC/L. The original stock solution, pH = 5.6, had a total organic carbon concentration of approximately 62 mg of DOC/L with 69% of the total carbon as hydrophobic solutes (55% hydrophobic acid and 14% hydrophilic neutral) and 31% as hydrophilic solutes as determined by the DOC fractionation method described by Leenheer and Huffman (29). Total organic carbon (TOC) analysis was performed on an organic carbon analyzer (IO, Inc., College Station, TX). Although TOC concentrations were measured, the term “dissolved organic carbon” and “DOC”will be used in the following discussion because the carbon quantification technique only measures organic carbon in the solution phase (i.e., the organic carbon is operationally defined as dissolved). Background concentrations of polychlorinated biphenyls (PCBs) and cadmium were insignificant in the DOC water (98% recovery of PCBs at all DOC concentrations. Batch Experiments. Batch experiments were conducted to determine the apparent distribution coefficients for Cd and PCB 153 between (1)DOC and water (KDOC) and (2)solid and total solution phases (Kd). Distribution coefficients for each contaminant were also determined for a variety of DOC concentrations, thus allowing the characterization of each three-phase system under conditions identical to those used in column experiments (e.g., KC1, constant ionic strength, DOC concentration, and pH). Batch techniques for the Cd KDOc determinations were similar to the dialysis method described by Carter and Suffet (17) except that DOC solutions were predialyzed to remove low molecular weight molecules that could pass through the dialysis membrane and bias KDOC results. Aquifer material used in the Cd solid/water experiments was prewashed three times with equilibration water (DOC ranging from 0 to 60 mg/L but containing no Cd), centrifuged, and fmally washed in 0.025 M KC1 to remove any residual DOC. Suspensions, 1000 mg of solid/L, were prepared with the OC-saturated aquifer material and different solution-phase DOC concentrations at varying concentrations of Cd (0, 0.25, 0.5, and 1.0 mg of Cd/L). Suspensions were equilibrated for 3 days and centrifuged to remove solid-phase Cd; the supernatant was analyzed using AA. The PCB 153 water/soil experiments were conducted using lo00 mg/L aquifer material suspensions, which were spiked as described earlier and equilibrated for 30 days at 295 K. Aqueous-phase concentrations of DOC ranged
from 0 to 60 mg/L. Samples were centrifuged and the aqueous phase analyzed for PCBs using the isooctane extraction technique described earlier. Attempts to independently measure the contaminant DOC/water distribution coefficients (KDOC) using the method of Chiou et al. (16) resulted in highly inconsistent observations; thus an estimate of KDOc was obtained by a nonlinear least-squares fit of eq 4 to the KAppdata set (KAp measured at five different DOC concentrations). It shoujd be noted that Kd in eq 4 was determined in the absence of DOC while a KAppwas determined at each DOC concentration; thus the only unknown variable in eq 4 is KDW. The validity of this approach is supported by results given elsewhere (12, 14, 15). Data Comparisons. The techniques described above allowed the cotransport of contaminants by DOC to be evaluated using two independent techniques. First, results from the column experiments were obtained and transformed into reduced contaminant BTCs. These data were modeled using the equations described earlier and the method of Parker and van Genuchten (27), which produced estimates of R (and subsequently estimates of KApp, Kd, and KDOC). Alternatively, KApp,Kd, and KD0cdata, independently obtained from the batch experiments, were used with the transport equations to simulate BTCs under the physical and experimental conditions used in the column experiments. BTC simulations using data from the batch technique were plotted along with the observed data collected in the column experiments, thus allowing comparison of each technique and evaluation of the cotransport hypothesis.
Results and Discussion DOC Transport in Soil Columns. Earlier investigations, utilizing the same experimental conditions described here, evaluated the mobility of naturally occurring DOC through laboratory columns (6, 7). A brief summary of these results will be given here because of the strong dependence of contaminant sorption to soils in the presence of dissolved and solid bound OC. Breakthrough curves for DOC showed a rapid, but partial, breakthrough (C/C,= 0.1) of DOC followed by a slow approach to saturation of the aquifer material with OC. Extensive tailing of BTCs to long times was not due to dispersions or preferential flow paths but resulted from slow adsorption kinetics of the DOC as well as the nonlinear nature of the DOC adsorption isotherm (6, 7). It was also noted that the desorption of DOC in the presence of KC1 was negligible. It was suspected that the slow breakthrough of DOC could significantly complicate the concomitant breakthrough of contaminants due to varying solid-phase OC gradients along the length of the aquifer column. In an effort to simplify the experimental design with respect to the solution and solid OC interactions and provide a consistent basis of comparison among the column experiments using different concentrations of DOC, all aquifer materials (except where noted) were equilibrated with DOC prior to initiating cotransport experiments (see Materials and Methods). This treatment increased the solid-phase OC of the aquifer material from its original level of 0.039% to 0.058% OC. Cotransport of PCBs by DOC. As noted in the previous section, solid-phase OC levels of
