Environ. Sci. Technol. 1989, 23, 1278-1286
Handa, T.; Yamauchi, T.; Sawai, K.; Yarnamura, T.; Koseki, Y.; Ishii, T. Environ. Sci. Technol. 1984, 18, 895-902. Andrews, G. E.; Iheozor-Ejiofor, I. E.; Pang, S. W.; Oeapiptanukul, S. Znt. Conf. Combust. Eng., Inst. Mech. Eng. 1983, C73/83, 63-77. Henry, R., The Application of Factor Analysis to Urban Aerosol Source Identification. Preprint volume from The
Fifth Conference on Probability and Statistics, Las Vegas, NV, 1977. (36) Pierson, W. R.; Brachaczek, W. P. Aerosol Sci. Technol. 1983, 2, 1-40. Received for review December 29,1988.Accepted June 21,1989.
Influence of Macromolecules on Chemical Transport Carl 0. Enfleld,*
st
Goran Bengtsson,$and Roland LIndqvlst§
R. S.Kerr Environmental Research Laboratory, U.S. Environmental Protection Agency, Ada, Oklahoma, and Department of Ecological Chemistry, University of Lund, Lund, Sweden Macromolecules in the pore fluid influence the mobility of hydrophobic compounds through soils. This study evaluated the significance of macromolecules in facilitating chemical transport under laboratory conditions. Partition coefficients between 14C-labeledhexachlorobenzene and three macromolecules [dextran, humic acid, and groundwater dissolved organic carbon (DOC)] were determined in a three-phase (water-macromolecule-soil) system. There were significant differences between the macromolecu1e:water partition coefficients, which ranged from 1 X lo3to 1 X 106. Soikwater partitioning for humic acid was demonstrated by using column breakthrough curves where the breakthrough curve for humic acid was retarded behind 3H20. Breakthrough curves for dextran and groundwater DOC demonstrated apparent size exclusion, as these compounds eluted from the soil column before the 3H20. The impact of the dextran was demonstrated under dynamic conditions by use of hexachlorobenzene, anthracene, and pyrene with and without macromolecules in replicated, biologically inhibited (sodium azide), saturated soil columns. The results may help explain the mechanism by which hydrophobic pollutants appear in deep groundwater aquifers.
Introduction The transport of organic pollutants in a saturated porous medium is determined by many physical, chemical, and biological processes. Mathematical models recently reviewed by Rao and Jessup ( I ) , Boeaten and Leistra (2))and Addiscott and Wagenet (3) describe the impact of dispersion, advection, sorption, and transformation on the movement of chemicals in soils. A fairly extensive literature (e.g., ref 4-11) has emphasized the linear relationship between sorption of nonpolar hydrophobic compounds, the organic carbon content of the soil, and the octano1:water partition coefficient of the sorbate. Roy and Griffin (12), in a review of methods of predicting soibwater partition coefficients from octanokwater and solubility relationships, suggested that soil organic carbon:water partition coefficients could be estimated within an order of magnitude if the octano1:water partition coefficient were known. This model of hydrophobic sorption as a linear phase equilibrium relationship has been challenged by laboratory measurements showing solid concentration induced nonlinearities (13). This observation has been attributed to slow desorption rates (14) and incomplete phase separaSoil Scientist, U.S. EPA.
tions (15, 16) due to nonsettling organic matter that remains in the aqueous phase, so that solute molecules can reside in three phases rather than two. The concept of two aqueous subphases is in agreement with earlier observations on the binding of hydrophobic compounds to humic macromolecules or colloids (17-21). In recent years, the partitioning between a solute and macromolecules in the aqueous phase has been addressed in a number of papers (22-29). Both concentration and chemical composition of dissolved organic material (DOM) may vary considerably in natural waters on a geographical basis, and structural differences between similar macromolecules may modify partition coefficients by a factor of 10 or more (25, 29-31). If hazardous chemicals partition to mobile macromolecules in subsurface environments, the current approaches of making environmental exposure and risk analysis, assuming partitioning between soil organic carbon and dilute aqueous solutions of the solute, may significantly underestimate the mobility of relatively immobile solutes. Enfield (32) suggested that macromolecules or immiscible substances moving with water may, under certain circumstances, enhance the movement of hydrophobic chemicals through soils. Enfield and Bengtsson (33) demonstrated that under some conditions macromolecules, on the average, will actually move faster through soils than water due to exclusion of the macromolecules from the smaller pores. Bengtsson et al. (34) showed that 500 mgL-' of a polysaccharide enhanced mobility of hexachlorobenzene by about 25% in a saturated sandy soil. The present study was conducted to further explore the theory for facilitated transport with solutes of different hydrophobicity and more heterogeneous macromolecules. The objective of this study was to propose a mathematical model that offers some insight into the mechanisms of facilitated transport and to test the predictions of the model by independently measuring the partitioning between selected hydrophobic compounds and macromolecules and their breakthrough curves in saturated soil columns. Theoretical D e v e l o p m e n t
The movement of a contaminant through the soil in the presence of macromolecules or other nonreactive organic substances can be described by dividing the soil system into three phases: a liquid or aqueous phase, a solid phase-the soil, and a mobile organic phase-the macromolecule. The following equations can be written to describe the individual phases:
* Associate Professor, Dept. of Ecological Chemistry, University
of Lund. 8 Graduate student, Dept. of Ecological Chemistry, University of
Lund. 1278
Environ. Sci. Technol., Vol. 23, No. 10, 1989
0013-936X/89/0923-1278$01.50/0
0 1989 American Chemical Society
kao v* = v,+ ,vo
--
.
at
.
Equation 8 becomes
-
- vo -a(4poco) koa~poCo + kaoBpaCa(3) ax2 ax where C, is the concentration in aqueous phase (gg-'), C, the concentration in mobile organic phase (gg-'), C, the concentration in solid phase (gg-'), D, the dispersion in aqueous phase (m2.day-'), Dothe dispersion in mobile organic phase (m2-day-'), k,, the first-order transfer coefficient aqueous to mobile organic phase (day-'), k, the first-order transfer coefficient aqueous to solid phase (day-l), koathe first-order transfer coefficient mobile organic to aqueous phase (day-'), k, the first-order transfer coefficient solid to aqueous phase (day-'), n the porosity of the soil (m3-m-3),V, the interstitial velocity of aqueous phase (mday-'), Vothe interstitial velocity of mobile organic phase (mday-'), pa the density of aqueous phase ( m g m 3 , po the density of mobile organic phase ( m ~ m - ~ ) , pa the particle density of the soil (mgm-3), 4 the volume fraction occupied by the mobile organic phase, and 0 the volume fraction occupied by the aqueous phase. In the above equations, the aqueous phase was assumed to totally cover both the solid phase and the mobile organic phase or macromolecule such that transfer of the contaminant from the mobile organic phase to the solid phase requires passage through the aqueous phase. The total change can be written as the sum of eq 1-3 or DO
a2(4poco)
When local equilibrium exists kaoepaCa = koa4poCo or = -kaoBpa - -C o
k~
(5)
ca
koa4Po
and k&'paCa = ksa(1 - n)psCs
(6)
or kdpa = -c, km(1 - n ) ~ , Ca Defining a new variable C* as the total mass concentration of the mobile contaminant over all phases (7) C* = BPaCa + $poco then the total change can be shown to follow the equation kd
=
ac* at
(Da
+
30)5 (
- Va + z V o )
(8)
By defining the variables (9)
D* = D, + -Do kao koa
and
Equation 12 has been solved for a variety of boundary conditions. Several one-dimensional solutions have been summarized by van Genuchten and Alves (35). Experimental Section Sorption Isotherms. Three different methods were used to obtain partition coefficients. Soikwater, dextran:water, and humic acid:water partition coefficients were measured by equilibrating varying concentrations of 14Clabeled hexachlorobenzene (HCB*) with soil and several concentrations of the macromolecule (dextran, a polysaccharide, or synthetic humic acid). The soil used for the soil-dextran sorption experiment was obtained from the current surface horizon of an infiltration basin at the Vomb water treatment plant in southern Sweden. The treatment plant produces the drinking water for the cities of Lund and Malmo from lake water. The soil was a fine sand with organic carbon fraction, determined with a Leco Model CR 12 total carbon analyzer, equal to 0.002 by weight. The partition coefficients were determined by equilibrating, for 24 h in an end-over-end shaker, 2.0 g of soil in 30 mL of groundwater from the infiltration site [total organic carbon (TOC) of the groundwater was less than 2 mgL-'I. Varying concentrations of dextran (125, 250, 500, and 1000 mgL-') and HCB* (10, 20, 40, and 80 pL of a 1 pCi-mL-' solution in acetone) were added to the water. Test tubes were centrifuged at 3000g for a period of 20 min, and 1 mL of the supernatant was added to 5 mL of Beckman CP scintillation cocktail and measured for 60 min on a Beckman Model 1801 scintillation spectrophotometer. HCB*, dissolved in a small amount of acetone, was initially added, in excess of its water solubility, to each test tube such that sufficient activity would remain in the aqueous phase, after equilibration, for detection. Blank isotherms (no soil) were measured to determine activity in stock solution and quenching due to the addition of the dextran to the solution phase. Sorption isotherms with the humic acid were performed in a similar manner except that it was necessary to acidify the soil (pH 3) to remove the calcium carbonate and to use distilled water rather than groundwater to prevent the humic acid from forming a visible floc and precipitating out of solution during the sorption experiment. The humic acid was prepared by mixing 50 mg of humic acid (Fluka) in 1.0 L of distilled water and stirring continuously for 3 days. The fluid was then filtered through an ashless, very fine filter paper (Munktell OOH) to remove particulate matter. The total organic carbon of the filtrate was measured to be 24 mgL-' as carbon with a Beckman Model 915B total organic carbon analyzer. Equilibrating solutions in the sorption studies were prepared from this stock, and the stock was diluted to 16 and 8 mg.L-'. Corrections to the amount of humic acid carbon remaining after equilibration were based on a soikwater partition coefficient measured for the humic acid in a column experiment discussed later. Natural dissolved organic carbon (DOC) concentrate was prepared by concentrating groundwater DOC with an Amicon DC-2 hollow fiber cartridge fiiter, in a concentrate mode. The filter had a nominal cutoff molecular weight of 3000. The groundwater with an initial concentration of 1-2 mg.L-' DOC was concentrated to 28 mg.L-'. The Environ. Sci. Technol., Vol. 23, No. 10, 1989
1279
f
AIR SUPPLY FEED SoLUTloN SATURATED WITH HEXACHLOROBENZENE ANTHRACENE
SINTERED G L A S S GRADUATED
PUMP
- N O T TO SCALE-
i-1 -/
Figure 1. Diagrammatic representation of experimental configuration.
majority of the carbon found in this groundwater sample passed through the filter, and it is estimated that less than 10% of the carbon was larger than MW 3000 based on the observed concentration factor. The groundwater DOC: water partition coefficient (kp)was determined by placing the DOC concentrate in dialysis bags with a nominal molecular weight exclusion size of lo00 and equilibrating groundwater with added HCB*. The original groundwater was used outside the bag to avoid any influence due to differences in ionic strength and composition. However, this made it necessary to assume all of the carbon in solution behaved the same way, since there was carbon outside the dialysis bag as well as inside. Samples were equilibrated for 4 days in closed Erlenmeyer flasks according to the experimental procedures of McCarthy and Jimenez (27). The partition coefficient was determined based on the concentration of HCB* in both solutions at equilibrium. Dynamic Studies. The experimental arrangement for the column studies is shown in Figure 1. The stainless steel experimental columns were 5 cm in diameter and 3 cm in length. The top and bottom of the soil samples were supported with sintered stainless steel plates. A feed solution was stored in a 10-L glass carboy, and 3-mm glass tubings were used to carry the fluid from the carboy to the stainless steel column. The top of the carboy was sealed with a Teflon-covered stopper, and air pressure was equalized by allowing room air to pass through a flask saturated with unlabeled hexachlorobenzene (HCB), anthracene, and pyrene (see Figure 1). Short pieces of silicone tubing were used to make the joints in the glass tubing, thus minimizing the contact between the silicone and the feed solution. The effluent from the column f i s t passed through an XAD-2 column in those studies where the analyses were made by gas chromatograph (GC). The XAD-2 traps were made in glass with screw adaptors and 1280
Environ. Sci. Technol., Vol. 23,No. 10, 1989
Teflon gaskets for direct connection to the stainless steel soil column. Approximately 1.5 cm3 of XAD-2 was supported on sintered glass frits. Otherwise XAD-2 was not in the flow system and the samples passed through Teflon tubings to a Fractovap fraction collector where they were collected in glass test tubes. The time period used for collection of a sample varied according to the volume of sample needed for the analysis. Flow was controlled by an Ismatec MV-GE peristaltic pump with 0.51 mm i.d. silicone tubings and set to give a Darcy velocity of approximately 0.5 mday-' (the average Darcy velocity for water infiitration at Vomb). Corrections were made in the sampling time to account for the volume of fluid in the lines from the column to the fraction collector. Four columns and two carboys were used so that one carboy fed two columns with the hydrophobic compounds mixed in groundwater and the other carboy fed two columns with the hydrophobic compounds mixed in groundwater with the macromolecule. The concentration of the compounds in the feed solutions was determined from samples collected in separate XAD-2 columns. The experiments were run at a constant temperature of 11 "C. Interstitial velocities for water (V,) and macromolecule (VJ or the macromo1ecule:soil partition coefficientb) were determined by measuring the Darcy flux through the columns and simultaneously the retention time for 3Hz0 and the macromolecule. Breakthrough curves for 3Hz0 were obtained from groundwater spiked with 6 pCi of 3Hz0 L-l. Spiked groundwater was pumped through the soil columns and collected in 2-mL portions by the sample collector for 2 h. One-milliliter aliquots were added to 5 mL of Beckman CP cocktail, and the activity was determined by liquid scintillation. Breakthrough curves for dextran, humic acid, and natural DOC were produced with the same procedure, and column effluent concentrations were determined with a Beckman DB spectrophotometer at 630,220, and 260 nm, respectively. It was assumed that the void volume occupied by the water and macromolecule remained constant throughout the experiment, and changes in the interstitial velocities could be estimated by measuring the Darcy velocity as a function of time. 4SCa was used as a model tracer to back up the assumption of local equilibrium in the mathematical model described, since 45Ca binds to soil particles and an instantaneous reversible equilibrium is established. The breakthrough curve was measured from labeled 45Ca(4pCi-L-l) in the same way as for 3Hz0. To obtain breakthrough curves for hydrophobic compounds in the presence and absence of 500 mgL-l dextran, a mixture of 0.02% NaN (to minimize microbial activity), 7.2 pgL-' hexachlorobenzene, 6.1 pgL-' anthracene, and 8.15 pgL-' pyrene was prepared in each of the two 10-L carboys by stirring for at least 12 h. The concentration in the water phase was determined after -500 mL of solution had been pumped through separate XAD-2 traps. The carboys ran out of feed every fourth or fifth day and were then replaced by a new feed solution. Excess water was removed from the XAD-2 resin by a water vacuum applied for 15 s. The resins were extracted with four successive 1-mL applications of toluene followed by 1 mL of methylene chloride. Subsequently, all of the extracting solvent was passed through a drying column of approximately 1cm3 of anhydrous Na2S04. The XAD-2 resins were washed with 5 X 1 mL of methylene chloride, 5 X 1 mL of methanol, and approximately 500 mL of distilled water before being reused. To determine the trapping efficiency of the XAD-2 resin, a 30 nCi solution of HCB* was added to 100 mL of groundwater with 500
mg-L-' dextran in a 250-mL Erlenmeyer flask and stoppered with a rubber stopper wrapped in aluminum foil. The mixture was stirred for 55 min and sampled immediately to obtain the feed concentration. Eight milliliters of this solution was pumped through each of six XAD-2 columns at a rate of 0.7 mL.min-' and the effluent was discarded. An additional 2 mL was pumped through the XAD-2 columns and trapped in glass test tubes. One milliliter of this eluant was added to 5 mL of Beckman CP cocktail and counted. The fraction not recovered in the eluant was assumed sorbed by the XAD-2. The trapping efficiency was found to be 79% (SD 0.96). Trapping efficiency without dextran was determined in a similar manner and found to be 81% (SD 0.1). Extraction efficiency was calculated from the same columns as used to determine trapping efficiency. Three milliliters of toluene was slowly passed through the resin connected to the anhydrous Na2S04trap and then collected in a glass test tube. One milliliter of the eluant was added to 5 mL of Packard Instafluor. A fourth milliliter of toluene was passed through the XAD-2 column with no sulfate trap and collected in 5 mL of cocktail. Following the fourth and final milliliter of toluene, the procedure was repeated with 1 mL of methylene chloride. The total extraction efficiency with 4 mL of toluene and 1 mL of methylene chloride was 86% (SD 3.7). For the samples collected by XAD-2 traps from the soil columns, the volume of the eluant was measured and 10 pL of phenylphenanthrene (560 pgmL-' acetone) was added as an internal standard prior to concentration with a rotary evaporator to a final volume of approximately 300 pL. The exact volume was measured with a 1000-pL syringe to the nearest 5 pL. Some samples collected by XAD-2 did not contain dextran. The experimental results in the dynamic studies were normalized to the concentration in the feed solution as measured by trapping on XAD-2. Where the trapping efficiency remains constant as reported above (SD 0.96)) the normalizing procedure minimizes the error associated with losses due to trapping efficiency as well as losses due to extraction efficiency. Sample volumes through the XAD-2 remained approximately constant throughout the experiment. Therefore, the trapping efficiency should remain consistent throughout the experiment. The extracts were analyzed by a Varian Model 3700 gas chromatograph equipped with a 25 m X 0.25 mm i.d. fused silica capillary column coated with SE-54. One microliter of the sample was injected by using split/splitless injection. The split was opened after 30 s. Temperature programming was used. Initial temperature was 55 "C, and the temperature increase was 15 "C.min-l. The final temperature of 240 "C was maintained for 10 min. Nitrogen was used as a carrier gas at 1.5 mL.min-' flow and as a makeup gas for the flame ionization detector maintained at 300 "C. The three compounds were identified and quantified by using retention time and area count with a Hewlett-Packard Model 3390A reporting integrator against the internal standard. The coefficients of variation of repeated injections of a standard solution were 0.06,0.05 and 0.04, n = 7 for hexachlorobenzene, anthracene, and pyrene, respectively. Results and Discussion There has been debate in the literature regarding the most appropriate mathematical boundary conditions for describing a physical problem (36, 37). The boundary conditions used to solve eq 12 were selected on the basis of which conditions best fit an experimental breakthrough curve for &Ca as shown in Figure 2, since with the physical
CICO
30 000 1
1'ooo
20000 Y
v 0 0
c measured
- C,=
6.8 C,
0
2000
0
4000
SOI conc. ( cpm , m1-l)
0.44 0.3 0.2 0.1
1
measured
- fit
d -0.37 cm
-
V - 115.7 cm I d 8 0.45
,
0.4
0.2
t
1.o
0.0
0.6
Time ( d a y s )
Flgure 2. Experlmentai breakthrough curve and sorption isotherm for %a for study sdl. The regesskw i b forthe isotherm yields a pattition coemcisnt (k.,) of 6.8. The arrow on the breakthrough cuve indicates when the addition of '%a was discontinued.
arrangement used in this study it was not possible to determine a priori the appropriate mathematical boundary conditions. Both concentration and flux type boundaries were considered, but mixed boundaries were not considered. Once the boundary conditions were established based on a qualitative comparison of the shape of the calculated breakthrough curve versus the experimental data, the same boundary conditions were used throughout the rest of the analysis. The boundary conditions selected were C*(x,O) = 0
ac*
-(m,t) ax
= finite
The solution adapted from van Genuchten and Alves (35) as applied in this study is
-c -* k 1) c,
[ AA(x,( x
t) 1) - A b , t
- to)
O < t S t O
t >
(14)
6
where 1 A(x,t) = - erfc 2 [ s I 1 l 2 exp[
-(R*x - V*t)'
]
-
4D*R*t v*x
V*2t
v*x
R*x
+ V*t
To determine how appropriate the proposed mechanisms of facilitated transport are, several model coefficients were determined. Interstitial velocities and dispersivities were evaluated for the different phases. Experimental data along with regression function fits to eq 14 are shown in Figure 3 for individual columns in a water-dextran-soil system and a water-groundwater DOC-soil system. The Environ. Sci. Technol., Vol. 23, No. 10, 1989
1281
"'I
6000
0.90
'
,/'/
--,
0.70
P
-
4000 -
Y
d
-.-8
3000
-
2000
-
A
0
v)
K
-
,
