Environ. Sci. Technol. 1991,25,323-331
Transport of Dissolved Organic Macromolecules and Their Effect on the Transport of Phenanthrene in Porous Media Brian R. Magee,+ Leonard W. Lion,*!*and Ann T. Lemley5 Cornell University, Ithaca, New York 14853
The retardation factor ( R ) of phenanthrene in a sand column was reduced by an average factor of 1.8 in the presence of dissolved organic matter (DOM) derived from soil, suggesting that a phenanthrene-DOM “complex” enhanced the transport of phenanthrene. Distribution coefficients (Kd’s)were determined in batch and column studies for combinations of phenanthrene and DOM with sand. The retardation factor in the advective-dispersive transport equation was modified to reflect the presence of a carrier by incorporating both the retardation and pore exclusion of the carrier itself. The best prediction of phenanthrene transport in the presence of DOM was provided by modeling the retardation by using two Kd’s derived from column experiments of DOM alone and phenanthrene alone, along with the Kd for phenanthrene binding to DOM. Sensitivity analyses indicated that the critical model parameters are the distribution coefficients for the hydrophobic pollutant binding to the stationary phase and binding to the carrier, as well as the carrier concentration.
Introduction Many environmental contaminants, such as polynuclear aromatic hydrocarbons (PAHs), polychlorinated biphenyls, and dioxins, are hydrophobic and have low aqueous solubilities. Consequently, these compounds typically sorb strongly to the porous matrix, and the risk to water users distant from the contaminant source is assumed to be low. However, if dissolved macromolecules or colloidal particles are dispersed in the groundwater, they may sorb the pollutants, allowing them to be transported with these mobile carriers. The processes resulting in such “facilitated transport” are generally not incorporated into environmental fate models. Some investigators have found poor or no correlation between sorptive partition coefficients or calculated retardation values and observed transport of trace hydrophobic organics (1-4). Hydrophobic chemicals have also been observed to migrate farther and to achieve greater concentrations in the environment than predicted based upon their sorptive partitioning (5-8). Enhanced transport may result from physical or hydrodynamic processes such as fingering or the existence of macropores. Binding of hydrophobic pollutants to “carriers” such as colloidal particles or dissolved macromolecules may also serve as a mechanism for enhanced mobility, even in homogenous isotropic porous media. McCarthy and Zachara (9) provided an excellent review of this topic. Binding to carriers has additional implications, including effects on biodegradation, volatilization, hydrolysis, photolysis, and bioaccumulation of pollutants (10-12). For carriers to increase PAH mobility, two conditions must be met: (1)the carrier must be able to bind these pollutants to an appreciable extent, and (2) the carrier ‘Present affiliation: Roy F. Weston, Inc.; West Chester, PA 19380.
#Addresscorrespondence to this author at the School of Civil and Environmental Engineering. College of Human Ecology. 0013-936X/91/0925-0323$02.50/0
must have a greater mobility than the pollutant that it binds. Given these constraints, it is interesting that most studies of potential carriers have focused exclusively on their binding properties (7, 10, 13-18). However, a few recent investigations have helped to develop a qualitative understanding of carrier effects on mobility. Enhanced transport of hydrophobic pollutants has been observed in laboratory columns in the presence of suspended solids from sewage effluent (19), humic material (20), and well-defined surrogates for dissolved and colloidal organics (21) . Enfield et al. (22) recently demonstrated the enhanced transport of several pollutants, including PAHs, in laboratory soil columns using dextran as a macromolecular carrier. Both dextran and groundwater dissolved organic carbon (DOC) were demonstrated to undergo apparent size exclusion from some pore space in the column, while a third potential carrier (humic acid) was retarded. These investigators developed a modified form of the advective-dispersion equation in which pollutant binding to the carrier and its pore exclusion were incorporated. Retardation of the carrier was not included in the model. The objectives of this research were (1)to determine the extent to which transport may be enhanced by soil-derived organic matter, (2) to revise existing miscible transport models to permit evaluation of the enhanced transport by carriers subject to retardation, (3) to test the performance of the model with the results of column and batch isotherm experiments, and (4)to conduct a sensitivity analysis on the parameters of the model.
Pollutant Retardation Factors The average transport velocity of a solute (u,) relative to that of water (u,) is defined as the retardation factor, R , such that R = u,/u,. (Note, all notation is summarized in the Glossary at the end of this article.) The retardation factor is also defined in the advective-dispersion transport equation (23, 24) as (1) R = 1 + p&d/n where Pb is the bulk density of the porous medium, Kd is the equilibrium distribution coefficient of the pollutant, and n is the effective porosity (water content). In the presence of potential carriers such as dissolved organic matter, West (20) and Hutchins et al. (2) have shown that R can be redefined to account for additional pollutant sorbed to the carrier:
R‘= 1 +
K8Pb/n
1+ KgDOM
where K i and KZm are the distribution coefficients for the pollutant with the stationary phase (soil or aquifer material) and mobile (dissolved or colloidal) organic matter, respectively, and DOM is the aqueous concentration of mobile organic matter. This relationship describes transport of a compound in the presence of a carrier that migrates with a velocity equal to that of water. A carrier that sorbs to the porous matrix will not enhance transport to the extent represented by eq 2, since the average velocity of the carrier would not
0 1991 American Chemical Society
Environ. Sci. Technol., Vol. 25, No. 2, 1991
323
Table I. Soil and Sand Characteristics Rhinebeck soil'.*
Flgure 1.
Partitioning of phenanthrene and
DOM with soil.
be the same as the water velocity. Therefore, the expression for R'must be modified to account for the partitioning of the carrier. The interactions between the carrier, pollutant, and the stationary phase are illustrated in Figure 1. A model that describes the retardation of a pollutant in the presence of a carrier that partitions onto the stationary phase is developed in the Appendix. The resulting retardation factor, R*, is shown to be a function of Pd and Kim as well as the partition coefficient of the carrier with the stationary phase, K&, and is given by R*
(1 + K3"DOM
I+
Environ. Sci. Technol., VoI.
5.9 3.13 35.5 52.3
ND 12.2 ND ND ND ND 6.7
0.4 0.11 ND ND 1.2 98.7 3.7 47.2 47.6 0.2 8.6
'Used as the source of DOM. for the porous medium in column studies, and as the sorbent in batch studies. 'ND, not determined.
exclusion, then eq 3 may be readily modified to include this effect (see Appendix) to give a retardation coefficient, R**, for a pollutant in the presence of a carrier that is both sorbed by the stationary phase and excluded from some pores:
+ Kipb/n)
KyDOM
(3)
1 -k (Ki,,Pb/n) The difference between the equations for R'and R* is the term 1 + Pdompb/n.When the carrier does not partition into the stationary phase (Ka, = O), this term equals 1 and eq 3 becomes equivalent to eq 2. If the interactions illustrated in Figure 1are sufficient to describe behavior of the hydrophobic pollutant in the presence of a carrier, then the individual distribution coefficients Kim,KBd, and Pdom determined in independent experiments should predict the behavior of pollutantcarrier mixtures. This hypothesis was tested in the present study with data obtained from batch and column experiments. Examination of eqs 2 and 3 indicates that carrier effects increase at high levels of DOM and with increasing pollutant Kim. The experimental data that are provided here are, therefore, used to test the plausibility of the generic pollutant and carrier interactions that are indicated in Figure 1. Laboratory experiments, by nature, restrict our ability to observe some phenomena. For example, column experiments are typically carried out over short distances with test solutes that are sufficiently mobile to permit experiments of a reasonable duration. Phenanthrene was employed as the test solute in this investigation and is less hydrophobic than many other contaminants for which it is a surrogate (PCBs and PAHs of four or more rings). Therefore, high levels of DOM were employed, relative to those that occur naturally, to enhance the likelihood of observable differences. Anthropogenic sources such as landfill leachate and land application of wastewater can, however, result in DOM concentrations that exceed the levels used in this study (25, 26). The relevance of the observed effects at lower levels of DOM or for different pollutants can, at present, be most readily assessed through use of the model calculations and awaits experimental confirmation. In the development of eq 3, it is assumed that free DOM in the aqueous phase moves with a velocity equal to that of the water, uv However, pore exclusion of large macromolecules has been demonstrated to occur in column studies with soil (8,22). Pore exclusion of large molecules gives them an average velocity greater than that of water molecules in the mobile phase. If DOM undergoes pore 324
oreanie matter. % " organic carbon, % clay, % silt, 70 silt and clay, 70 sand, % very fine (0.1-0.05 mm) fine (0.25-0.1 mm) medium (0.5-0.25 mm) coarse P0.5 mm) pH, H,O extract
dark
25. No. 2. 1991
Kiompb/n
where u;, is the velocity of the carrier in the aqueous phase. Equation 4 is equal to eq 3 if u:, = u,. The term u:, cannot be determined from batch studies and can be determined from column experiments only if the carrier is known not to sorh to the porous medium, or if Pdom is independently determined in a batch experiment. If carrier sorption does occur, exclusion of the carrier from pores will reduce the apparent retardation of the carrier and of the pollutant-carrier complex compared to that estimated on the basis of partitioning in batch experiments. Pore exclusion of DOM derived from groundwater has been demonstrated by Enfield et al. (22). Equation 4 is included to provide a comprehensive discussion of the possible influences of DOM as a pollutant carrier. The effect of variations in u;, is addressed through sensitivity analysis using eqs 3 and 4. Materials and Methods A dark sand obtained from a quarry in Newfield, NY, was used in batch and column experiments. The characteristics of the sand are provided in Table I. Radiolabeled phenanthrene-9J4C with a specific activity of 10.9 mCi/mmol was obtained from Sigma Chemical Co., St. Louis, MO. Phenanthrene stock solutions were prepared by dissolving the ["Clphenanthrene in a 5050 mixture of HPLC-grade methanol and distilled-deionized water (Corning Mega-Pure system). Methanol fractions in all experiments ranged from 0.0010 to 0.0015 (by volume) and were deemed small enough to avoid significant solvophobic effects (27). Nonradiolabeled zone-refined phenanthrene (Aldrich Chemical Co.) was used to measure phenanthrene binding to DOM. Solutions of DOM were prepared by extraction from soil (see details below). One soil (Rhinebeck) was a clayey silty loam (A horizon) from east-central New York State (see Table I for characteristics). DOM from a second soil, an organic muck from Sapsucker Woods in Ithaca, NY, was also prepared. While DOM from both sources had a high DOC concentration, DOM from Sapsucker Woods did not pass through the sand column as readily; therefore, DOM from the Rhinebeck soil was selected for further study. It
Table 11. Initial Parameters of Soil Column Experimentsu
DOM phenanthrene phenanthrene alone + DOM alone DOM concn, mg OC/L column length, cm column diam, mm bulk density, g/cm3 wetted pore vol, mL volumetric flow, mL/h phenanthrene, rg/L
59.3 21.4 25 1.94 29.1 13.6 0
0 5.5 25 1.65, 1.65 8.54, 9.26 14.7, 16.8 47.6, 52.9
99.0, 77.5 5.5 25 1.61, 1.60 10.2, 9.34 18.6, 16.8 39.2, 99.1
a All experiments were conducted at a pore water velocity of 10 cm/h.
should be noted that, while there may be many sources of DOM in groundwater or surface water, not all types are mobile, and only the mobile types are likely to serve as carriers. To produce DOM, a sample of soil (50 g) and a solution (250 mL) of 0.005 M calcium sulfate and 1.0 g/L sodium azide were combined in 250-mL Nalgene bottles. Calcium sulfate provided a background electrolyte and sodium azide inhibited microbial degradation. After approximately 18 h on a wrist-action shaker, the bottles were centrifuged for 30 min at 5400g. The supernatant was filtered through a Whatman 2V filter and refrigerated until use. A typical DOM sample contained approximately 80 ppm total organic carbon and had a pH in the range of 6.4-7.5. Although this concentration of DOM is higher than would be encountered in pristine groundwater, it was used in experiments to enhance the likelihood of obtaining an observable effect. An infrared (IR) spectrum of the DOM indicated the presence of hydroxyl (OH), aromatic (C=C), and carboxyl (COOH) groups; the spectrum was not significantly altered by DOM transport through the sand (28). Acidification of the DOM did not produce a visible precipitate, a significant drop in DOC, or significant change in the IR spectrum. Therefore, the DOM appears to have insignificant humic acid content. Fifty-nine percent of the DOM freely diffused through dialysis tubing with a molecular weight cutoff of 1000. The DOM produced three distinct peaks on passage through an HPLC size exclusion column (Supelco LC-diol packing, 100-A pore size), providing qualitative indication of a nonuniform size distribution. Madhun et al. (29) also found three molecular size fractions in soil-derived DOM: 47% below 1000 daltons (Da), 36% between 1000 and 5000 Da, and 17% greater than 5000 Da. Column Experiments. The transport of the carrier or phenanthrene was measured by continuous delivery at a constant pore water velocity of a solution containing the compound of interest through a glass preparative HPLC column (Beckman Instruments) packed with the dark sand. The solution was delivered by a continuous syringe pump (Sage Model 220) through Teflon tubing. Experiments were run in the dark to inhibit photolysis of phenanthrene and in a temperature-controlled room maintained at 15 “C. Parameters for the column experiments are given in Table 11. Prior to the start of each column experiment, approximately 20 pore volumes of the calcium sulfate-sodium azide solution was applied in an upflow mode to equilibrate the column packing. If DOM was to be used as a carrier, it was also included in the equilibration solution so that DOM concentrations would not vary during the experiment. In order to minimize adsorption of hydrophobic phenanthrene to the experimental apparatus, phenanthrene was
pumped through a second pump until the concentration in the effluent from the pump was stable. Once equilibration of the column was complete, delivery was switched to the second pump containing a solution of the substance of interest (phenanthrene, phenanthrene plus DOM, or DOM). The input also contained a nonsorbing tracer, 3H20,with which the phenanthrene or DOM breakthrough was compared. The column was inverted throughout the experiment so that the effluent would drip from the bottom of the column directly to test tubes in a fraction collector. Tubes in the collector were subsequently covered with Parafilm to minimize volatilization. By weighing the column at the beginning and at the end of the experiment, it was determined that the column remained saturated in the downflow mode. Three types of column experiments were conducted: breakthrough of DOM alone, breakthrough of phenanthrene alone, and breakthrough of phenanthrene with DOM. Each phenanthrene experiment was replicated, and initial parameters for each experiment are presented in Table 11. Retardation factors were determined by
R =
sow(l 0
- C / C o ) d8
(5)
where C is the column effluent concentration, Co is the column influent concentration, 8 are the pore volumes, and d, are the total pore volumes displaced when C = Co. The area above the breakthrough curve up to C/Co = 1.0 thus provided a measure of retardation (30). Isotherm Experiments. Distribution coefficients for DOM and phenanthrene were determined by batch isotherm experiments for various combinations of DOM, phenanthrene, and dark sand. Like the column experiments, these were conducted in the dark a t 15 “C. (a) Phenanthrene on Sand. The binding of phenanthrene onto sand was determined by combining 10 mL of phenanthrene solution and 1.00 g of dark sand in a 50-mL screw-top glass centrifuge tube. Aluminum foil was placed inside the screw top so that the solution would not contact the hydrophobic septum. Six samples with varying phenanthrene concentrations (10-250 pg/L) were mixed on a wrist-action shaker for 48 h. It was determined by preliminary experiments that equilibrium was attained in 24 h. The samples were then centrifuged for 30 min at lOOOg, and the supernatant was analyzed for phenanthrene by liquid scintillation counting. To account for phenanthrene losses via sorption to container surfaces, controls (without sorbent) were run at similar phenanthrene concentrations to produce a “blank sorption isotherm”. The concentration of phenanthrene sorbed to sand corrected for the blank isotherm was determined by difference. An amount of phenanthrene equal to that initially added to each container was added directly to scintillation cocktail to determine the amount of phenanthrene in the initial solution. The concentration of phenanthrene sorbed versus that in solution at equilibrium was plotted, and the distribution coefficient was calculated as the slope of the isotherm. (b) Phenanthrene on Sand in the Presence of DOM. The partitioning of phenanthrene into sand in the presence of DOM (77.5 ppm DOC) was determined by combining 1.00 g of dark sand and 10 mL of DOM-phenanthrene solution at varying phenanthrene concentrations in centrifuge tubes and following the procedures described above. Controls were also run at varying phenanthrene concentrations to determine the extent of phenanthrene sorption to the container surfaces. The amount of sorbed phenanthrene was determined as described above. The disEnviron. Sci. Technol., Vol. 25, No. 2, 1991 325
tribution coefficient for phenanthrene in the presence of DOM, K J , could presumably be used in eq 1 to model phenanthrene retardation in the presence of the carrier. (c) DOM on Sand. The partitioning of DOM was determined by combining 20 mL of DOM solution (103.1 ppm) and dark sand at 5,10,15,20,25, and 30 g in 50-mL centrifuge tubes and following the procedure outlined above. DOC was measured by heated persulfate digestion and IR analysis of the C 0 2 evolved using an 01 International Model 700 Carbon analyzer. The DOC released by the dark sand during the experiment was accounted for by analysis of blanks. No decrease in DOC due to sorption to the containers was observed in control samples (without sand), so no correction for sorption by container walls was needed. Assuming DOM obeys a linear isotherm within the concentration range investigated, then its distribution coefficient, Ki,,, is given by PdOm = DOM,/DOM,. Therefore, from mass balance DOMo/DOM, = Kiom(MJ/(Vw) + 1 (6) where DOM, is the initial DOM concentration, DOM, is the equilibrium aqueous DOM concentration in the tube containing sand, DOM, is the amount of DOM sorbed per mass of sorbent, V, is the volume of solution, and M , is the mass of sorbent. The ratio of the initial to final DOM concentrations versus the mass of sand per volume of solution was plotted, and Pdom was determined from the slope. (a) Phenanthrene Binding to DOM. The fluorescence of phenanthrene has been shown to be quenched when bound to dissolved or colloidal material and can be used to measure the extent of binding (18, 31). Fluorescence was measured on a Perkin-Elmer MPF-44B fluorescence spectrophotometer with band widths of 2 nm on the excitation monochromator and 4 nm on the emission monochromator. Fluorescence was measured as a function of added DOM at 288-nm excitation wavelength and 364-nm emission wavelength. A procedure similar to that described by Gauthier et al. (31) was employed. The fluorescence of 2 mL of a phenanthrene solution in a quartz fluorescence cuvette was measured after equilibration with the cuvette walls. Aliquots (0.050 mL) of DOM (99.0 ppm) were added to the cuvette. After each addition, the solution was shaken for 30 s and allowed to stand quiescent for -4 min before the fluorescence intensity was recorded. The fluorescence intensity was stable after this period of equilibration, indicating equilibrium had been attained. While DOM was being added and during equilibration, the shutter of the spectrofluorometer was closed to protect the phenanthrene from photodegradation. In a trial run in which a phenanthrene solution sample was continually exposed to the UV excitation source, no decrease in intensity was observed for the time period required for the addition of 10 aliquots (-1 h). To correct for possible fluorescence by added DOM, the fluorescence of a solution of DOM alone was measured with the same concentrations and instrumental conditions as for the measurements with DOM and phenanthrene. The fluorescence intensity measured for DOM was subtracted from the total fluorescence intensity measured for phenanthrene in the presence of DOM. The maximum fluorescence intensity of DOM was 2.6% of the total. The binding of phenanthrene with DOM can be represented by K2m = [pheneDOM]/ [phen][DOM] (7) where [phenVDOM] is the concentration of phenanthrene 326
Environ. Sci. Technol., Vol. 25, No. 2, 1991
-
1 .o
0
Y 2.
0.8
C
.-
CI
0.6
R
CI
= 0.99
0
8
0.4
al
.-
-
I
0.2
2
0.04' 0.0
.
i
1 0
30
20
Pore Volumes
Figure 2. Breakthrough of 3H,0 from dark sand. 1
0.40
0.20
0
10
5
15
Pore Volumes Figure 3. Breakthrough of DOM and 3H,0 from dark sand.
bound to DOM, and [phen] and [DOM] are the concentrations of free phenanthrene and free DOM. By performing a mass balance on phenanthrene and assuming that the fluorescence intensity is proportional to the free phenanthrene concentration, Gauthier et al. (31) have shown
Fo/F = 1 + K8"[DOM]
(8)
where Fo and F are the fluorescence intensities in the absence and presence of DOM, respectively. Since a significant excess of DOM over phenanthrene was present, [DOM] was taken as the amount added without correction for the fraction of DOM that was associated with phenanthrene. K,: was then obtained from a plot of F o / F versus DOM concentration.
Results and Discussion Column Transport. The breakthrough curves (BTC) of 3H20and DOM from a dark sand column are shown in Figures 2 and 3. By measuring the area above the 3H20 BTC up to C/Co = 1 (Figure 2), R was estimated at 0.99, indicating that 3H20behaved as a nonsorbing tracer. The BTC of DOM (measured as DOC), indicated that the carrier was retarded compared to the tritiated water, with a measured R of 3.2. The curve was very asymmetric, with rapid breakthrough of approximately 75% of the DOM, almost matching the breakthrough of 3H20. HOWever, there appeared to be a DOM fraction that eluted much more slowly. West (20) found a similarly shaped BTC for a humic material. Since DOM is a heterogeneous mixture of molecular sizes and functional groups, it is possible that a fraction was more hydrophobic or smaller (i.e., less subject to pore exclusion) and took longer to elute.
1
0.8
CIC,
Replicate Experiments
0.4-
Without DOM
0.2-
00 0
*P
'
50
100
150
Pore Volumes Flgure 4. Phenanthrene breakthrough from dark sand, with and without DOM.
{
1.60
1.40-
-
1.20
1.00-
0.60-
-
0.60
0.40-
0.04
0.08
0.12
0.16
0.20
0.24
pg PhenanthrenelmL soln Figure 5. Isotherms for phenanthrene binding to dark sand, with and without DOM.
The measured R can be considered an average for all DOM fractions and takes into account the asymmetry of the BTC. Skewed breakthrough curves of this type may also be produced in cases where local equilibrium assumptions break down and kinetic or mass-transfer limitations occur (32). Phenanthrene eluted more quickly in the presence of DOM than in its absence (Figure 4). The average R (from duplicate experiments, extrapolating the BTC to C/Co = 1.0) was reduced from 79 without the carrier to 44 with it; i.e., transport was, on average, 1.8 times faster. The BTCs exhibited a long, slow increase in eluent phenanthrene concentration, indicating nonideal sorption, i.e., failure to conform to the assumption of local equilibrium (32). It is likely that the sorption of phenanthrene involves a fast and a slow kinetic component. The slow component may involve phenanthrene diffusion to sorptive sites within sand particles or within sand organic matter (32). Furthermore, diffusion of phenanthrene into the polymeric matrix of the Teflon retainers on either end of the column may have contributed to the slow approach to C/Co = 1 (33). Isotherms. (a) Phenanthrene on Sand. The isotherm for sorption of phenanthrene to the dark sand is shown in Figure 5. A linear regression of the data (correlation coefficient r2 = 0.98) yields p d = 12.9 mL/g. On an organic carbon basis, Po,= 11700 mL/g of sand OC. Other estimates of K$ for phenanthrene include values of 5200 mL/g (34)and 14 000 mL/g (35). Karickhoff et al. (36)found K!, = 22 900 mL/g for phenanthrene sorption to a sediment. These investigators found the sand fraction to be a less effective sorbent than the fines fraction on an organic carbon basis, with a 50-90% reduction in KB,, for sand. This is borne out by the above data, in which the dark sand K:, is approximately 51% of the value obtained
by Karickhoff et al. (36) for sediment. By use of the value of K i = 12.9 mL/g obtained from the batch isotherm and the column parameters in Table 11, an R value of 64 is calculated from eq 1. The value of R was measured to be 79 in the column experiment, so the batch-predicted R underestimated the column-observed R by 19%. Differences between batch and column measurements of R have been previously reported and discussed by several investigators (30, 37, 38). Given the difficulty in reconciling such values, Jury has argued that it is essential to perform adsorption measurements in the field (38). Diffusion of phenanthrene into Teflon has recently been described by Reynolds et al. (33)and is one process that may be responsible for the greater column R. Incomplete separation of the liquid and solid phases in the batch equilibrium study would have the same effect (39). Phenanthrene exhibited less affinity for the dark sand in the presence of DOM (77.5 ppm DOC). Linear regression of the batch data (r2 = 0.983) gave Kd' = 6.61 mL/g (Figure 5 ) . With this value of Kd', an average R of 30 is predicted for the phenanthrene breakthrough experiments with DOM, again underestimating the average column-observed R of 44. The average DOM DOC was slightly greater in the column experiments (85.8 ppm) than in the batch (77.5 ppm), so a somewhat lower average R in the column experiments would have been expected. It is possible that a DOM fraction, subject to removal by filtration in the column experiments, sorbed phenanthrene but remained suspended in the batch experiments. Phenanthrene sorption by such a DOM fraction would cause the batch Kd'to underestimate column retardation, as occurred here. (b) DOM on Sand. The sorption of DOM by the dark sand appeared to be quite weak. A linear regression of the batch data (r2= 0.95) produced a KiOmof 0.132 mL/g. The linearity of the data supports the assumption of a linear isotherm in eq 8. On an organic carbon basis, KB,,,, = 120 mL/g. With a P d o m of 0.132 mL/g, the predicted R,, is 1.9, compared to the column-measured value of 3.2. The occurrence of significant pore exclusion of DOM in the sand column is not indicated, since the column R value exceeds that estimated from the batch study. The difference between the predicted and observed R values may be due, in part, to the low DOM affinity for sand, making an accurate measurement of Kiomdifficult. In addition, although the DOC produced by shaking the dark sand (2.3-3.0 ppm) in the absence of DOM was measured and accounted for, it was an additional source of uncertainty concerning DOC. The amount of sand DOC released may have changed in the presence of DOM, but this could not be determined. If high levels of DOM reduced the release of sand DOC, K;,, would be underestimated. ( c ) Phenanthrene on DOM. Measurements of fluorescence quenching of phenanthrene by DOM were used to determine Kim. The resulting isotherm has a slope (Kim)of 43 800 mL/g (r2= 0.989). Since DOM was measured as DOC, the distribution coefficient is on an organic carbon basis (Kim= K",). For comparison, Gauthier et al. (31) measured KEF = 50000 mL/g for phenanthrene with a humic acid by the fluorescence quenching technique. Based on the observed data, phenanthrene has a greater affinity for the organic carbon in DOM than that in the dark sand (KEF = 3.7 K:,). (d) Modeling Phenanthrene Transport in the Presence of DOM. With the three distribution coefficients obtained from the batch experiments, eq 3 can be used to predict R* for phenanthrene in the presence of DOM. By use of the average DOM carbon concentration Environ. Sci. Technol., Vol. 25, No. 2, 1991
327
Table 111. Effect of Model Parameters on Retardation
Kz?,
DOC,
Pboclr2,
mL/g
mg/L
g/mL
0 1 10 100
0.005 0.005 0.005 0.005
1 1 1 1
1 1
106 106
106 106 106 106
1
5001 2501 456 50.5
104
104
0
51
1
0.005 0.005
1 1 1
50.5 46.5
0.05
1
1
50001
1 1
25001
106
104 104 104 106 106 106
1 1 1 1
1
104 104
0.005 0.005
106
106
100
106 106 104 104
106
10
106
K",c,
mL/g 106 106
104
108
106
10
100 0 1
R,,
u~,Iu, 1
0.05
1
0.05 0.05
1 1
0.005 0.005
104 104
10
0.005
2 10 2
1
10
10
0.005
10
1
106 106 104
106
10
2
1.2
106 104
10
0.005 0.005
2 2
104
104
10
2 1.2 2 1 1 1 1
10
10 lo6
106
107
10 10
104
2 x 104 105
io
2
lo6 104
l l l l l l l l l _ l l _ l
X
10
0.005 0.005 0.005 0.005
0.005 0.005 -
2 2
2 2 2
retardn factor
26
1 1
--t
DOC
:1
, Koc
z
_,,'
1 O6
,*e'
,/'
100
46.5 34.7
of 85.8 ppm (8.58 X 10 g/mL) and the average pb/n of 4.38 g/mL, the estimated R* is 18. This value underestimates the observed column R* by approximately a factor of 2. Siiice the values of batch K:,, and K i underestimated both DOM and phenanthrene retardation in the column experiments, R* may also be predicted with &, and K; obtained from eq 1 by using results from the separate phenanthrene and DOM column experiments. By use of the column Miorn= 0.601 and the average column K i = 16.0, the calculated R* = 37, a reasonable estimate of the observed R* of 44. This computation, along with the computation using the batch Kd' obtained in experiments with Phenanthrene in the presence of DOM, avoids the difficulty in determining Kiomin a batch isotherm experiment, which i s complicated by the release of DOC from the sand (although correction for this effect was made) and may present the greatest source of experimental error. ( e ) Model Sensitivity Analysis. It appears that enhanced PAH transport by DOM (and possibly other carriers) can occur in some porous media. Even though an aquifer material (sand, for example) does not produce much organic carbon, the aquifer recharge water originating from a soil type containing significant organic matter (e.g., most surface soils) may control the concentration of carriers (25). Therefore, significant levels of DOC can be present in aquifers of low organic carbon content. DOC levels in aquifers are typically 1ppm, but higher concentrations can occur from both natural and anthropogenic sources (25, 26). It is informative to predict pollutant mobility for those stationary phases that may allow facilitated transport. West (20) tabulated the extent of predicted facilitated transport at various values of DOC and Kd, but used eq 2 and assumed that Kzm = K i . This analysis showed that DOC can dramatically reduce the retardation factor at high values of KO,and DOC. With eq 3 or 4, carrier sorption characteristics may be incorporated into a sensitivity analysis. Table 111sumniarizes the calculated retardation factors that result under various benchmark conditions. The calculations assume pboc/n= 0.005 g of OC/mL of
-
328 Environ. Sci. Technol., Vel. 25, No. 2 , 1991
'.'.
4546 496 835 2505 48.7 50.6 716 456 48.2 46.5 456
1
2
0
4
6
8
10
12
16
14
Rorn
Figure 7. Effect of carrier retardation on pollutant retardation factor. DOC in milligrams per liter.
0.21
17 Ioj, 1 --t-
DOC Koc 10' DOC z 1 . KOC z l o e DOC = 100, K o c = l o 6
1
,
0.0
,
1
2
3
,
4
Vom'IVw
Figure 8. Effect of increased carrier velocity on pollutant retardation factor. DOC in milligrams per liter. (Ram = 2).
water. This value would describe, for example, an aquifer material with a bulk density of 1.62 g/cm3, an organic carbon content (foe) of 0.0011, and an effective porosity of 0.36, similar to the values measured for the sand columns. It is emphasized that such calculations are speculative and have not been confirmed by experiment. The calculations assume the interactions between the stationary phase, carrier, and pollutant depicted in Figure 1 are valid. The plausibility of this assumption is supported by the ex-
i l1
should be noted that many of the model parameter? are not independent. For example, a more hydrophobic carrier (i.e., higher KZf) may bring more of a pollutant into the mobile phase, but the carrier itself will tend to be more retarded (greater R,,), counteracting this effect. Consequently, the modified retardation equations presented here are better viewed as a whole without isolating various parameters.
.01
K,W S ,
,
l,o,4, , , ,
,
K;;,O.
,
- Kooom-Koa Kocom Kocom KOCS ..- - -.-. Kacom Kocr --.----Kocom 10
I
1
'.Iv
J
Kocom IKocs I2 KOCS
-2
001
,
1
I
10 Kws
10
100
DOC (mgiL)
Figure 9. Effect of
> Gc(Ram = 2).
perimental data presented above. The importance of several model parameters is illustrated in Figures 6-9 (pboc/n= 0.005 g/mL for all calculations). In Figure 6, the effect of a retarded carrier on pollutant transport is illustrated as a function of DOC. The ratio of R* (eq 3) to R (eq 1) is plotted for the case where R,, = 2.0 (Kt,,, = 200 mL/g) and for an unretarded carrier (R,, = 1.0). (The relationship between R,, and K;,, is provided in the Appendix.) In the latter case, R* = R' (eq 2). It is assumed for purposes of illustration that KZr = K$. Under these conditions, at a DOC concentration of 10 mg/L, the retardation factor for a compound with K, = lo6 mL/g would be 835, as opposed to 455 with an unretarded carrier. In other words, the average velocity of transport would be about half as fast when the carrier retardation is taken into account. A similar change in R,, for a less hydrophobic compound with K , = lo4 increases R from 46.4 to 48.7, or only -5%. Figure 7 shows the effect that a range of R,, values would have on pollutant retardation (R*)relative to pollutant retardation without taking carrier retardation into account (R?. Carrier retardation appears to have an order of magnitude effect only when both DOC and K , are high. (The curve for 100 mg/L DOC and KO,= lo4 is indistinguishable from the curve for 1mg/L DOC and KO,= lo6.) Nevertheless, pollutant retardation could be underestimated by 50-100% a t low KO,values or low DOC concentrations (where R*/R' is between 1.5 and 2.0). Based on eq 4, an increase in aqueous carrier velocity (uz,) relative to that of water (u,) due to pore exclusion can also impact pollutant mobility. The curves in Figure 8 reflect varying u:,/uw, K,, and DOC concentration while assuming R,, = 2, and KZF = Kt,. (The curve for DOC = 100 ppm and KO,= lo4 mL/g coincides with the curve for DOC = 1 ppm and KO,= lo6 mL/g.) At large values of DOC (e.g., 100 ppm) and KO,(e.g., lo6 mL/g), the relationship between u:,,,/v, and R**lR* becomes inversely proportional; Le., a 2-fold increase in u:,/uw reduces R**/R* by half. The sorptive capability of soil organic carbon relative to dissolved organic carbon is still being debated. KZf values as high as 35 KS,, have been reported (40). If DOC possesses a higher sorptive capability than carbon on the stationary phase, facilitated transport might occur to a greater degree. In this study, KZT was measured to be approximately 3.7 K:,. Figure 9 illustrates the effect on pollutant mobility a t various relationships between KZY and Po, holding R,, = 2. The more KEF exceeds KtC,the greater R is overestimated. In summary, any of the parameters in eq 3 or 4 can affect the extent of facilitated transport. However, it
Conclusions Water-soluble soil organic matter (DOM) can be an effective carrier, enhancing the transport of a hydrophobic compound through a sand. DOM derived from a clayey silty loam increased the transport of phenanthrene through sand by an average factor of 1.8, even though DOM itself was retarded ( R = 3.2). The generality of this phenomenon for other potential carriers, pollutants, and soils is suspected but is not established. Dissolved or colloidal material has also been observed to have no effect or even to inhibit the transport of hydrophobic pollutants in other studies (41,42). Further study is needed to determine the specific characteristics of the carrier or stationary phase that permit enhanced transport (9). With a model incorporating three distribution coefficients, distribution coefficients obtained in batch and column studies provided reasonable estimates of pollutant retardation in the presence of a carrier that is also retarded. Better estimates of column R* were obtained by using column instead of batch values for Pdom and K; (within 16%),or by using the overall batch distribution coefficient for phenanthrene in the presence of DOM, Kd' (within 32%). The effects of carrier-enhanced transport are calculated to be greatest on the most hydrophobic compounds and at high levels of DOC. Transport is also sensitive to the velocity of the carrier itself and the relative sorptive partition coefficients of the soil organic carbon (Po,) and the dissolved organic carbon (KZY). Enhanced transport appears to be relatively insensitive to changes in the weight fraction of soil organic carbon. While a more hydrophobic carrier may yield a greater KEF, this increase may be counteracted by a concomitant increase in carrier retardation (greater KZ,, and Ram). Appendix Under circumstances in which the carrier as well as the pollutant can sorb to the porous medium, the expression for the pollutant's average velocity becomes (see Glossary for definition of terms) (AI) ~ ' a v= fwuw + fornuom + f s ~ = s fwuw + fornuom The velocity of the stationary phase, us is typically assumed equal to zero. This conceptualization, and the derivation that appears below, is comparable to that employed by West (20). However, an additional relationship is required if the velocity of the carrier has a dissolved and a sorbed component. Assuming the dissolved carrier component has a velocity equal to that of water gives u,, = fow"uw + p u s = p u , (A2) The term f"," is defined as = M;!/(M;,
em
+ M;")
(A3) The partition coefficient of the DOM with the stationary phase can be defined as
Environ. Sci. Technol., Vol. 25, No. 2, 1991
329
Solving for Mi" and substituting into eq A3 yields l/&, = 1 + K%,,MS/Vw= Ro,
(A5)
With this expression, the average velocity of the organic carrier becomes
and so fwuw
1
-
uw
R* =
647)
fw + [fo,Vw/(l + K~omMs)l
+ fornuom
The fraction of pollutant that is truly dissolved, f,, may be defined as fw = M&/(M&+ MZm + Mi)
(A81
Substitution of the distribution coefficients for the binding of pollutant to the DOM (KZm),and to stationary phase, (K:), yields 1 (A9) fw = 1 + (K2"Mom/Vw) + (K$Ms/Vw) Similarly, the fraction of pollutant bound to the DOM is
where u t is the velocity of the aqueous organic matter. Therefore UW
R** = fwuw
=
fnm
1 + (K8"Mom/ Vw)
+ (KIM,/ Vw)
(A101
The expression for the average velocity of the pollutant becomes uav
=
fwuw
As the velocity of the carrier (ug,) increases, the retardation of the pollutant R* decreases. If pore exclusion occurs, but is not taken into account, use of the PdOm from a batch experiment will cause overestimation of carrier retardation, R,,, and pollutant retardation, R*.
Glossa,Y
c
DOC DOM
F Fo foc
fs
or
e" f"," fw Kd Kd'
Therefore
R * = -uw = uav
1
1+
KYMom / Vw (1 + KiomMs/ Vw) 6412)
or (1
R* =
+ K g D O M + Kipb/n) 1+
K8,DOM
(A13)
1 + (K%nmPb/n)
A further complication in carrier-mediated transport occurs if pore exclusion from small dead-end pores causes the velocity of the carrier ( u f ) to exceed that of the water (uw). Pore exclusion has been demonstrated for groundwater DOM and a macromolecule (dextran) in soil columns (8, 22). The present model can readily be expanded to incorporate the effect of pore exclusion:
330
Environ. Sci. Technol., Vol. 25, No. 2, 1991
(A151
(ucL/uw)fom
+ K80mpb/n
f om
+ fornuom
1
The term uzm/ uw is the extent of enhanced transport of carrier due to pore exclusion; the rest of the terms are as defined in the above derivation of R*. After substituting for fw, f o m , and Ms/Vw 1 + K8"DOM + Kipb/n R** = (A161 (vim/ U,)K;~DOM 1+
CO
KYMom/ Vw
+ fornuom
-
M:" Mi! n [phenl [phen. DOM] R
concentration in column effluent concentration in column influent concentration of dissolved organic carbon concentration of dissolved organic matter fluorescence intensity in the presence of DOM fluorescence intensity in the absence of DOM weight fraction of organic carbon in the stationary phase fraction of solute bound to DOM fraction of solute that is sorbed to the stationary phase fraction of DOM that is sorbed to the stationary phase fraction of DOM in the aqueous phase fraction of the solute dissolved in water distribution coefficient distribution coefficient for the pollutant with the stationary phase in the presence of DOM distribution coefficient for binding of the pollutant to DOM, (M:,/Mo,J/(Mk/ V,) distribution coefficient for the pollutant with the stationary phase, (M:/Ms)/(M&/V,) distribution coefficient for the binding of DOM to the stationary phase distribution coefficient on an organic carbon basis, &Ifoc Kim on an organic carbon basis K: on an organic carbon basis Kio, on an organic carbon basis mass of DOM mass of pollutant associated with DOM mass of pollutant associated with the stationary phase mass of the solid phase mass of pollutant associated with the aqueous phase mass of DOM associated with the stationary phase mass of DOM associated with the aqueous phase effective porosity of the stationary phase aqueous concentration of free phenanthrene aqueous concentration of phenanthrene bound to DOM retardation factor
R' R* R** Rom VW
retardation factor for the pollutant in the presence of an unretarded carrier retardation factor for the pollutant in the presence of a retarded carrier retardation factor for the pollutant in the presence of a retarded carrier that undergoes pore exclusion retardation factor for DOM volume of the aqueous phase average velocity of the pollutant average velocity of DOM aqueous phase velocity of DOM experiencing pore exclusion velocity of the stationary phase pore water velocity bulk density of the stationary phase bulk density of organic carbon in the stationary phase, P h f o c pore volume pore volumes displaced when C = Co
Registry No. Phenanthrene, 85-01-8.
Literature Cited (1) Tomson, M. B.; Dauchy, J.; Hutchins, S. R.; Curran, C.; Cook, C. J.; Ward, C. H. Water Res. 1981,15, 1109-1116. (2) Hutchins, S. R.; Tomson, M. B.; Bedient, P. B.; Ward, C. H. CRC Crit. Rev. Environ. Control 1985, 15, 355-416. (3) Hamaker, J. W.; Thompson, J. M. In Organic Chemicals in the Soil Environment; Goring, C. A. I., Hamaker, J. W., Eds.; Marcel-Dekker: New York, 1972; Vol. 1, pp 49-143. (4) Rao, P. S.; Jessup, R. E. In Chemical Mobility and Reactivity in Soil System; Nelson, D. W., Elrick, D. G., Tanji, K. K., Eds.; American Society of Agronomy: Madison, WI, 1983; pp 183-201. (5) Enfield, C. G.; Walters, D. M.; Carsell, R. F.; Cohen, S. Z. Ground Water 1982, 20, 711-722. (6) Baker, J. E.; Capel, P. D.; Eisenreich, S. J. Enuiron. Sci. Technol. 1986,20, 1136-1143. (7) Pierce, R. H., Jr.; Olney, C. E.; Felbeck, G. T., Jr. Geochim. Cosmochim. Acta 1974, 30, 1061-1073. (8) Enfield, C. G.; Bengtsson, G. Ground Water 1988,26,64-70. (9) McCarthy, J. F.; Zachara, J. M. Environ. Sci. Technol. 1989, 23, 496-502. (10) Carter, C. W.; Suffet, I. H. Enuiron. Sci. Technol. 1982,16, 735-740. (11) McCarthy, J. F.; Jimenez, B. K. Environ. Tonicol. Chem. 1985, 4, 511-521. (12) Guy, R. D.; Narine, D. R.; DeSilva, S. Can. J. Chem. 1980, 58, 547-554. (13) Caron, G.; Suffet, I. H.; Belton, T. Chemosphere 1985,14, 993-1000. (14) Chiou, C. T.; Kile, D. E.; Brinton, T. I.; Malcolm, R. L.; Leenheer, J. A.; MacCarthy, P. Enuiron. Sci. Technol. 1987, 21, 1231-1234. (15) Chiou, C. T.; Malcolm, R. L.; Brinton, T. I.; Kile, D. E. Enuiron. Sci. Technol. 1986, 20, 502-508. (16) Gamble, D. S.; Haniff, M. I.; Zienius, R. H. Anal. Chem. 1986,58, 727-731. (17) Gauthier, T. D.; Seitz, W. R.; Grant, C. L. Enuiron. Sci. Technol. 1987, 21, 243-252.
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Received f o r review September 25, 1989. Revised manuscript received March 19, 1990. Accepted August 23, 1990.
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