Environ. Sci. Technol. 2002, 36, 3625-3634
Distributed Reactivity Model for Sorption by Soils and Sediments. 15. High-Concentration Co-Contaminant Effects on Phenanthrene Sorption and Desorption WALTER J. WEBER, JR.,* SUNG HO KIM, AND MARTIN D. JOHNSON Environmental and Water Resources Engineering, Departments of Chemical Engineering and Civil and Environmental Engineering, The University of Michigan, Ann Arbor, Michigan 48109-2125
Soil and sediment materials having organic matter matrixes of different geochemical character were examined with respect to their sorption and desorption of phenanthrene in the presence of order-of-magnitude larger concentrations of trichloroethylene (TCE) and dichlorobenzene (DCB). These co-contaminants depressed phenanthrene sorption in the lowest residual solution phase concentration ranges of that target solute investigated, whereas in its highest residual concentration regions phenanthrene sorption was either not affected or was actually enhanced. In both concentration ranges, the effects observed varied with the hydrophobicity and relative concentration of the cocontaminant and with the geological maturity and associated degree of condensation and aromatization of the soil/ sediment organic matter (SOM). Desorption isotherms for phenanthrene indicate the occurrence of increased hysteresis in the presence of high concentrations of DCB and TCE, the effect increasing with increased degree of associated organic condensation. Tests in which high concentrations of DCB and TCE were added after completion of the phenanthrene desorption experiments show clear evidence of partial displacement of sorbed phenanthrene to the solution phase. The results of the work support the concept of SOM glass-transition concentrations, above which matrix deformation occurs and so-called “conditioning effects” are observed.
Introduction Hydrophobic organic contaminants (e.g., polycyclic aromatic hydrocarbons) are commonly present in aqueous systems as minor components of mixed nonaqueous phase liquids (NAPLs) that include substantially higher levels of less hydrophobic and more soluble organic contaminants (e.g., chlorinated solvents, chlorinated aromatics, and aromatic fuel hydrocarbons). Under such conditions, the local dissolved-phase concentrations of the principal NAPL components often approach their respective solubility limits. * Corresponding author phone: 734-763-2274; fax: 734-936-4391; e-mail:
[email protected]. 10.1021/es020557+ CCC: $22.00 Published on Web 07/04/2002
2002 American Chemical Society
Depending upon circumstances, including the characteristics of the NAPL mixtures and the soils or sediments present, the sorption and desorption behaviors of dissolved HOCs are likely to reflect the interactive effects of organic solutes present in “dominant” solution-phase concentrations (i.e., concentrations that are orders of magnitude greater than those of the dissolved HOCs). Previous solute competition and displacement studies in multicomponent systems have yielded variable results, with competition between simultaneously sorbing cosolutes or displacement of one sorbed solute by another being reported in some cases (1-5) and no such effects being observed in others (6, 7). Careful analyses of these seemingly disparate results, however, reveal that the existence or absence of observed effects can depend on several factors, including (i) whether the sorption process is primarily one of absorption or adsorption, (ii) the relative chemical characters of the solutes involved, (iii) the sequence of solute loading events, and (iv) the nature of the soil/sediment materials involved, particularly with respect to whether their associated SOM matrixes are of highly amorphous or chemically reduced nature. The underlying reasons and cause-and-effect relationships associated with the first two of these factors are reasonably understood, but those associated with the third and fourth are still somewhat ambiguous. The broad purpose of the work described here was to examine these factors under varied but well-defined experimental conditions (e.g., soil and solute types and sorbate loading orders). The more focused specific objective was to quantify the effects of dominant concentrations of two common chlorinated organic compounds on the sorption and desorption behaviors of a target HOC with respect to geosorbents having SOM matrixes of different physicochemical characters. An even more specific goal within the context of that objective was to determine whether the behaviors observed empirically were consistent with, and interpretable in terms of, theoretical premises underlying the concept of distributed sorption reactivity (8). The theoretical premises assumed as a basis for the studies described herein are well-supported by prior work (8-20) and are summarized as follows: (i) soil/sediment organic matter (SOM) is polymeric in character (8-15); (ii) HOC sorption by soil/sediment organic matter is similar in behavior to HOC sorption by synthetic polymers (12, 1618); (iii) HOC sorption by synthetic polymers leads to polymer swelling (15, 19, 20); and (iv) HOC sorption by soil/sediment organic matter leads to swelling of SOM matrixes (9, 11, 13, 14). Our purpose was to determine whether these premises are also applicable to system conditions that are remarkably different from those in the investigations from which they derive. Consistent with terminology used throughout much of the literature on this subject, the general term sorption includes both absorption and adsorption, the former being defined fundamentally as a linear-free-energy bulk-phase partitioning process and the latter as a nonlinear-free-energy surface phenomenon (21).
Experimental Section Figure 1 presents a schematic outline of the overall experimental program pursued. Details regarding the geosorbents and sorbates used and the specific operations involved in the sorption, desorption, and displacement investigations are briefly summarized in the following discussion. Sorbents. Canadian peat, Chelsea soil, and Lachine shale were selected as representatives of different classes of organic VOL. 36, NO. 16, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Schematic outline of experimental matrix. rich natural geosorbents. Experiments conducted in earlier studies with these three materials have established that associated inorganic materials play no significant role with respect to their sorption of phenanthrene (i.e., phenanthrene sorption behavior is dominated by their respective organic SOM matrixes). Each has been characterized extensively in terms of the chemistry of its associated organic matter (e.g., elemental atomic analysis and 13C NMR) and each was used in investigations described earlier in this series (11-15). Their respective organic matrixes have undergone different degrees of geochemical diagenesis and are, thus, of different physicochemical character. Canadian peat is a brownish fiberdominated material having the geochemically least mature and most extensively oxidized and highly amorphous SOM of the three geosorbents, with an oxygen/carbon atomic ratio of 0.70. It also has the highest organic carbon content at 47.53 wt %. Chelsea II soil, which contains 5.45 wt % of lessamorphous and less-oxidized organic carbon, is a geochemically more mature topsoil collected near the town of Chelsea in southeastern Michigan. The oxygen/carbon atomic ratio of the base-extracted organic matter from this soil is 0.58. Finally, the geochemically most mature material tested was Lachine shale, collected from Paxton Quarry west of Alpena in the northern part of Michigan. The Lachine shale’s SOM of 8.27 wt % as carbon is comprised by macromolecules that have undergone significant chemical reduction and is, thus, the least amorphous (i.e., most condensed) of the three geosorbents studied. The kerogen extracted from the Lachine shale has an oxygen/carbon atomic ratio of only 0.13. Sorbates. Representatives from each of three different classes of chemical compounds common to contaminated aqueous systems were selected for study. Phenanthrene, a PAH of midrange hydrophobicity, was selected as the probe or “target” solute. This compound is commonly present at sites contaminated by coal tars and petroleum products and derivatives. The “dominant” solutes selected were trichloroethylene (TCE), a chlorinated solvent constituting one of the most common groundwater contaminants, and 1,4dichlorobenzene (DCB), commonly used as a space deodor3626
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TABLE 1. Selected Properties of Target and Dominant Solutes at 25 °C (21) solute property chemical formula molecular weight log KOW Henry’s constant KH ((atm‚m3)/mol) × 103 aqueous solubility CS (mg/L)
1,4-dichloro- trichlorophenanthrene benzene ethylene C14H10 178.23 4.568 4.0 × 10-2
C6H4Cl2 147.01 3.38 2.24
C2HCl3 131.40 2.29 11.7
1.1
83.1
1100
ant for toilets and refuge containers and as a fumigant for moth, mold, and mildew control. The physical and chemical characteristics of these compounds are summarized in Table 1. Spectrophotometric-grade phenanthrene was obtained from Aldrich Chemical Co. Inc. (St. Louis, MO), the TCE in certified ACS grade from Fisher Scientific (Pittsburgh, PA), and the DCB in 99+% pure grade from Aldrich Chemical Co. Background Solution. The background solution employed throughout the study contained calcium chloride (CaCl2, 0.555 g/L) as a mineral constituent, sodium bicarbonate (NaHCO3, 0.005 g/L) to maintain the experimental solutions at approximately neutral pH, and sodium azide (NaN3, 0.100 g/L) to inhibit microbial activity. All aqueous solutions were prepared using water that had been doubledistilled, ion-exchanged, and membrane-filtered (Milli-Q, Fisher Scientific). Sorption Isotherms. The isotherm experiments were conducted in completely mixed batch reactors (CMBRs). For experiments involving Canadian peat (3.46 ∼ 3.54 mg/reactor, for a solid-solution ratio of =0.0945 mg/mL) and Chelsea soil (21.0 ∼ 21.9 mg/reactor, for a solid-solution ratio of =0.581 mg/mL), these reactors consisted of 35-mL screwtop glass centrifuge tubes with Teflon-lined silicon septa. For experiments with Lachine shale (2.57 ∼ 2.66 mg/reactor,
FIGURE 2. Isotherm confidence intervals (95%) and data points for phenanthrene sorption from single-solute systems (]) and bi-solute systems (O). for a solid-solution ratio of =0.0416 mg/mL), the CMBRs were 50-mL flame-sealed glass ampules (Wheaton, Millville, NJ). Huang and Weber have determined that 3 weeks of sorbent-solution contact time is adequate to reach apparent equilibrium between aqueous phenanthrene solutions and Canadian peat and Chelsea soil, while a 2-month contact period suffices for Lachine shale (14, 15). These investigators reported that approximately 95% or more of equilibrium phenanthrene sorption occurs in such systems over the stipulated time periods, yielding fitted Freundlich model parameters that do not change significantly over substantial additional periods of contact. Nine different initial aqueousphase concentrations of phenanthrene were prepared for each single-solute isotherm experiment. This was done by introducing carefully measured quantities of a highconcentration stock solution into appropriate (i.e., centrifuge
tube or ampule) CMBRs containing background solution and sorbent. The phenanthrene stock solutions were prepared using methanol as a solvent, taking care to ensure that the mole fraction of methanol in each reactor was no more than 10-3 to prevent solvent effects on phenanthrene sorption (14). The bi-solute isotherm experiments involved a similar approach except that a fixed amount of either TCE or DCB was introduced to the reactors immediately after the phenanthrene. Independent tests with crystalline phenanthrene were conducted to demonstrate that TCE and DCB did not affect the solubility of phenanthrene in the amounts used in the bi-solute experiments. DCB was injected as a stock solution prepared in methanol, and TCE was injected directly. The introductions were done in a matter of minutes while the equilibrium experiments were run for either 3 weeks VOL. 36, NO. 16, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Freundlich Parameters for Phenanthrene Sorption and Desorption Isotherms Freundlich sorption parameters sorbent Canadian peat
log KFa
sorbates phenc
1.38 0.81 0.99/36 (1.34-1.42)d (0.78-0.83) 1.11 0.91 1.00/18 (1.05-1.16) (0.88-0.94) 1.25 0.91 0.99/17 (1.18-1.32) (0.87-0.95) 0.52 0.78 0.99/36 (0.47-0.57) (0.75-0.80) 0.29 0.90 0.99/34 (0.23-0.35) (0.86-0.93) 0.52 0.86 0.99/18 (0.45-0.58) (0.82-0.90) As above, with desorption into DCB solution rather than plain buffer solution
phen with DCB phen with TCE Chelsea soil
phen phen with DCB phen with TCE phen with DCB
Lachine shale
phen with TCE
As above, with desorption into TCE solution rather than plain buffer solution
phen
2.48 (2.46-2.50) 1.45 (1.32-1.56) 1.28 (1.15-1.42)
phen with DCB phen with TCE a
Freundlich desorption parameters
R2/Nb
n
Units of KF ) (µg/g)/(µg/L)n.
b
0.54 (0.53-0.55) 0.93 (0.86-1.01) 1.03 (0.95-1.11)
Number of observations. c Phenanthrene.
d
1.00/35 0.98/17 0.98/17
log KF
n
R2/N
1.36 (1.30-1.41) 1.30 (1.21-1.38) 1.43 (1.35-1.52) 0.62 (0.56-0.68) 0.71 (0.61-0.81) 0.98 (0.91-1.05) 0.29
0.84 (0.80-0.87) 0.89 (0.83-0.95) 0.86 (0.79-0.92) 0.79 (0.75-0.83) 0.86 (0.78-0.93) 0.76 (0.71-0.81) 0.99
0.99/34
0.99/17
(0.21-0.38) 0.54
(0.93-1.05) 0.91
1.00/18
(0.49-0.59) 2.40 (2.36-2.44) 2.33 (2.24-2.41) 2.52 (2.46-2.58)
(0.87-0.94) 0.67 (0.64-0.70) 0.80 (0.73-0.87) 0.73 (0.68-0.77)
0.99/18 0.99/17 0.98/36 0.97/18 0.99/18
1.00/10 0.99/11 0.99/12
Confidence intervals (95%).
TABLE 3. Pseudo KOC Values for Phenanthrene Sorption and Desorption sorption sorbent (wt % OC)a
sorbates
Canadian peat (47.5)
phenanthrene phen with DCB phen with TCE
Chelsea soil (5.5)
phenanthrene phen with DCB phen with TCE phen with DCB phen with TCE
Lachine shale (8.3)
phenanthrene phen with DCB phen with TCE
a
Weight percent of organic carbon.
b
KOC,Ce)1µg/Lb
50.4 13.5 (45.8 ∼ 55.5) (12.0 ∼ 15.1) 26.8 14.3 (22.8 ∼ 31.5) (12.1 ∼ 17.1) 37.4 20.0 (31.7 ∼ 44.1) (15.6 ∼ 25.7) 60.5 12.9 (54.3 ∼ 67.5) (11.6 ∼ 14.4) 35.9 17.4 (30.8 ∼ 51.0) (15.0 ∼20.1) 60.3 22.9 (51.6 ∼ 70.5) (18.6 ∼ 28.2) desorption into DCB solution desorption into TCE solution 3634.9 (3386.9 ∼ 3903.3) 341.6 (259.4 ∼ 450.7) 230.4 (166.3 ∼ 319.8)
9
149.4 (139.1 ∼ 160.2) 215.0 (152.6 ∼ 301.5) 289.4 (199.1 ∼ 420.2)
KOC,Ce)1µg/L
KOC,Ce)1000µg/L
47.9 (42.5 ∼ 54.1) 41.8 (35.6 ∼ 49.2) 56.9 (48.0 ∼ 67.5) 76.7 (67.5 ∼ 86.8) 94.8 (77.1 ∼ 117.4) 175.6 (152.3 ∼ 201.8) 35.9 (31.2 ∼ 42.2) 63.5 (56.9 ∼ 69.7) 3044.3 (2607.0 ∼ 4660.5) 2555.6 (1844.0 ∼ 3536.9) 3967.3 (3297.7 ∼ 4783.3)
15.3 (12.4 ∼ 19) 19.8 (12.5 ∼ 31.7) 20.9 (13.0 ∼ 33.4) 18.0 (13.9 ∼23.3) 35.3 (21.1 ∼ 58.9) 33.7 (21.4 ∼ 53.0) 34.2 (18.7 ∼ 63.3) 32.9 (20.0 ∼ 54.0) 313.7 (124.8 ∼ 591.4) 624.4 (222.7 ∼ 1759) 597.7 (232.9 ∼ 1546.8)
µg/g of organic carbon.
or 2 months, and the order of solute introduction did not therefore affect the sorption and desorption results significantly. No attempt was made to quantitatively characterize sorption of the dominant solutes. The initial concentrations of TCE and DCB in the bi-solute experiments in which those solutes were employed were approximately 900 and 40 mg/ L, respectively. These values were selected to represent nearsaturation solubility conditions for these dominant solutes, and the results thus reflect nearly maximum potential effects on phenanthrene sorption. CMBRs containing either Canadian peat or Chelsea soil were tumbled end-over-end for 3 weeks to allow their contents to reach apparent equilibrium, while those containing Lachine shale were tumbled continuously for 2 months. To separate the sorbents and solutions at the end of the experiments, the tube-type reactors were centrifuged 3628
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KOC,Ce)1000µg/L
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for 15 min at 1500 rpm and the ampule-type reactors set upright for 3 days to allow solids to settle. For phenanthrene and DCB analysis, supernatant samples were mixed with 2-mL aliquots of methanol and the analyses performed by reversed-phase HPLC (Hewlett-Packard model 1090, ODS, 5 µm 2.1 × 250 mm column). A diode array detector was employed for phenanthrene concentrations between 30 and 1000 µg/L and a fluorescence detector for concentrations between 0.5 and 30 µg/L. A UV detector was employed for determining DCB concentrations between 0.4 and 40 mg/L, and a florescence detector was periodically employed for confirmative analysis. Solid-phase concentrations were calculated by mass balance on each sample. For TCE analysis, an aliquot of supernatant was mixed with hexane and the hexane phase analyzed by GC. For low TCE concentration ranges (below 1 ppm), we used an HPGC 5890 with an FID
FIGURE 3. Isotherm confidence intervals (95%) and data points for phenanthrene sorption from single-solute systems (]) and desorption into fresh buffer solutions (O). detector, and for high-concentration ranges (1-1000 ppm), we used an HPGC 6890 with an ECD detector. It was assumed that all residual aqueous-phase TCE partitioned quantitatively to the hexane phase. This method of extraction for TCE quantitation has been used with success throughout the research described in earlier papers in this series and by other investigators as well (e.g., refs 22 and 23). Desorption and Displacement Experiments. Desorption isotherm measurements were performed immediately after completion of the sorption isotherm experiments and in the same reactor systems. Samples for analysis of residual aqueous-phase solute concentrations from the sorption experiments were withdrawn, and the remaining supernatant was decanted. So as not to lose any solids, the solution was decanted very carefully until approximately less than 1 or 2% of it remained in each reactor. Either a fresh background solution or one that also contained either DCB or TCE was then introduced to each centrifuge tube or ampule-type CMBR, and desorption was measured in a manner and over time periods identical to those employed for the associated sorption experiments. Desorption of target solute in singlesolute systems was allowed to proceed for 3 weeks in the case of Canadian peat and Chelsea soil and for 2 months for Lachine shale. The individual desorption experiments were intended to each quantify one well-defined equilibrium condition for the target solutes and the effects of TCE and DCB on this specific condition. They were thus designed and conducted in the some manner as were the sorption experiments, as single-step experiments rather than as multiple-step water “extraction” experiments. Upon completion of the sorption or desorption experiments, residual solids were used to conduct subsequent TCE and DCB displacement experiments. The term displacement as used here implies the forced desorption into solution phase of one or more previously sorbed substances as a result of the sorption of another solute from solution phase. The
Canadian peat and Chelsea soil displacement experiments were conducted immediately after completion of the phenanthrene single-solute desorption experiments. In the case of Lachine shale, however, the displacement experiment was performed after the phenanthrene single-solute sorption experiment. Lachine shale requires 2 months to reach equilibrium, whereas 3 weeks is sufficient for Canadian peat and Chelsea soil equilibrium. Had the same procedure been used for Lachine shale as was for Canadian peat and Chelsea soil, at least 6 months would have been needed to complete a series of sorption, desorption, and displacement experiments. Immediately after completion of sampling for analysis of residual phenanthrene concentrations from the sorption or desorption runs, either TCE or DCB was introduced to the solution remaining in each centrifuge tube. Solute losses in the displacement test CMBRs were quantified using sorbentfree control reactors identical to and treated in the same manner as the sorbent-containing reactors. The control experiments revealed that percentage losses of primary solute were closely grouped around their respective average values for all reactors in each set of reactor types. The average values were less than 10% of the final equilibrium concentration for the centrifuge tube reactors and 5% for the ampule reactors. It was therefore concluded that these losses were sufficiently small to be discounted in interpretation of the experimental data. The Freundlich isotherm model was used to describe and interpret all sorption, desorption, and displacement isotherm data. This model has the form
qe ) KFCen
(1)
log qe ) log KF + n log Ce
(2)
or, in linearized form,
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FIGURE 4. Isotherm confidence intervals (95%) and data points for phenanthrene sorption in the presence of a second solute in dominant concentration (]) and desorption into fresh buffer solutions (O). where qe and Ce represent solid-phase and liquid-phase equilibrium concentrations, respectively. The parameter KF is the Freundlich unit-capacity coefficient, and n is a joint measure of the relative magnitude and diversity of energies associated with a particular sorption process. The Freundlich model is a logical choice for description and interpretation of overall isotherms for heterogeneous sorbents in that it derives rigorously from summation of multiple Langmuirtype models representing different energy and limiting capacity sorption states (21).
Results and Discussions Sorption Experiments. The phenanthrene single-solute and bi-solute sorption isotherm data for the three geosorbents are presented in Figure 2. The lines in these and all other 3630
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isotherm plots presented herein represent the 95% confidence bounds for the associated best-fit Freundlich isotherm model for each data set, and the symbols represent individual data points. Parameters for the best-fit model calibrations shown in Figure 2, as well as for those of all other isotherm conditions given in Figures 3-6, are summarized in Table 2, along with numerical values defining the respective 95% confidence intervals. For purposes of comparison, Table 3 summarizes organic-carbon normalized concentration-specific sorption capacities calculated for all isotherm conditions tested. These “pseudo” KOC values (i.e., qe/Ce values at specific values of Ce) are essentially concentration-dependent single points on the nonlinear isotherms at arbitrary Ce values of 1 and 1000 µg/L. The nonlinearity of all sorption equilibria examined is readily evident in the fact that the KOC values given
FIGURE 5. Isotherm confidence intervals (95%) and data points for phenanthrene sorption by Chelsea soil in the presence of a second solute in dominant concentration (]) and desorption into buffer solutions containing the second solute (O). in Table 3 for each geosorbent at the two different Ce values are remarkably different. Parts A, C, and E of Figure 2 indicate that DCB effectively competes as a dominant co-contaminant with phenanthrene for sorption by each of the solids in the lowest aqueousphase residual concentration ranges of target solute studied. Parts B and D of Figure 2 suggest that TCE has a lesser effect than DCB on phenanthrene sorption by Canadian peat and Chelsea soil in this concentration range. As noted in part F of the figure, however, TCE is observed to exhibit a competitive effect similar in magnitude to that of DCB on phenanthrene sorption by Lachine shale. In the high residual concentration ranges of target solute, there appear to be little if any effects of TCE on the sorption of phenanthrene by Lachine shale (part F of Figure 2) but an apparent enhancement of phenanthrene sorption by the Canadian Peat and Chelsea soil (parts B and D of Figure 2). In the context of distributed sorption reactivity, these seemingly disparate results are in fact consistent with the effects observed in the low ranges of residual phenanthrene concentration. Many natural geosorbents comprise extensive arrays of sorption sites having different functional types and different potential energies of association with any given solute. This is the fundamental premise underlying the distributed reactivity model (8). It has been shown by a number of investigators that for operational purposes it is often possible and practical to lump these sites into two broad categories, variously referred to as high and low energy sites, hard and soft carbon sites, and glassy and rubbery SOM matrix sites. These terms converge in the polymer-based dual-domain concept and model for geosorbents referenced in the Introduction to this paper (12, 16-18), and we use that concept to interpret and explain the results observed in this work. We assume that the operative sorption processes in the cases studied are principally entropy driven (i.e., no significant electrostatic or covalent sorbate-sorbent interactions are involved). On the basis of the relative log KOW and CS values given in Table 1, we thus expect phenanthrene to sorb preferentially on a molecule per molecule basis and DCB to be a stronger sorbate than TCE. For SOMs having different energy sites, the net driving force for a clearly dominant cosolute in a bi-solute system may well result in pre-emptive occupation of a large fraction of the highest energy sites. This can occur even though individual molecules of the target solute are more “adsorbable” than those of the cosolute if there are sufficiently more of the latter present to generate a larger total entropic energy driving them from solution phase. In such cases, larger percentages of the sorbed target solute would be forced to occupy lower energy sites in the bi-solute system than they
would elect to do in a comparable single-solute system. This would be increasingly the case with increasing adsorbability of the dominant cosolute (e.g., more so for DCB than for TCE). The effect would also be most readily evident in the lower regions of residual target solute concentration because driving forces for further sorption are lowest in these regionss. In other words, there are proportionately lower densities of high-energy sites available to a target solute in single-solute systems of very high residual concentration and, thus, proportionately larger percentages of that sorbate must sorb at lower energy sites. High-energy sites are generally more abundant in glassy polymeric matrixes than they are in rubbery matrixes. However, while Canadian peat and Chelsea soil may therefore have lower numbers of high-energy sites per unit mass than Lachine Shale, a greater percentage of them are likely to be more readily accessible to sorbates. This is because the SOM matrixes of the two younger materials are more highly amorphous and, thus, present less resistance to diffusive transport of sorbate molecules to such reactive sites. To explain the high phenanthrene aqueous-phase concentration range observations, we suggest that DCB and TCE, by virtue of their very high solution-phase concentrations, induce swelling of the condensed and less amorphous SOM matrixes of the shale, thus enhancing diffusive transport of phenanthrene molecules to a larger percentage of the total number of sites available. Similar effects have been observed and discussed by Xing and Pignatello (3, 20). As shown in Table 2, Freundlich n values for sorption of phenanthrene were increased in the presence of the dominant second solutes for all three geosorbents (i.e., the apparent diversity of energy sites was reduced in each case). We attribute this effect, like that of enhanced phenanthrene uptake at high residual concentrations, to the swelling of SOM matrixes in the presence of the TCE and DCB (i.e., the SOMs are transformed temporarily (reversibly) to more rubbery states), thus resulting in enhanced partitioning-type linear sorption. This explanation is consistent with the observation by Xia and Ball (7) that partitioning becomes a more significant contributor to the overall sorption behavior of a specific target sorbate when cosorbates are present. We suggest that the swelling of SOM matrixes in the presence of massive concentrations of organic sorbate has an effect similar to that at high temperatures. In both cases, energy input (respectively chemical and thermal in character) is increased, leading to increased disordering of SOM structure. On this basis, we hypothesize two related concentration equivalents to the glass-transition temperature, Tg; that is, a solution-phase “glass transition concentration”, Cg, or a sorbed-phase “glass-transition loading”, Qg°, corresponding to Cg). This concept would serve to explain why the overwhelmingly dominant concentrations of TCE and VOL. 36, NO. 16, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 6. Isotherm confidence intervals (95%) and data points for phenanthrene desorption into fresh buffer solutions (]) and phenanthrene displacement into buffer solutions containing a dominant concentration of a second solute (O). DCB used in the present work do not completely “outcompete” the phenanthrene for sorption. For all other conditions, constant solute competition for sorption sites decreases with the increased “rubberyness” of an SOM matrix (i.e., as n f 1). The effect of this increasing rubberyness on the heterogeneity of the sorption process is particularly dramatic in the case of Lachine shale, for which the value of n increases from 0.54 for single-solute phenanthrene sorption to 0.93 and 1.03 in the presence of dominant concentrations of DCB and TCE, respectively. This is consistent with the notion that the high-energy sorption sites that exist in an SOM are less readily accessible in the glassy state of that SOM than in its rubbery state. More of the high3632
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energy sites are thus made accessible by matrix swelling and disordering, which can be induced either by increasing temperatures or by the higher sorbate/sorbent mass loading ratios that generally correspond to high residual solutionphase concentrations. Sorption-Desorption Hysteresis. Figure 3 shows no apparent hysteresis for single-solute phenanthrene sorption and desorption by Canadian peat, but some is evident for Chelsea soil and slightly more for Lachine shale. Conversely, marked hysteresis for phenanthrene is evident for all three sorbents when dominant concentrations of co-contaminants are present during the sorption stage, as shown in Figure 4. This is an important observation from a mechanistic point
of view, particularly regarding anticipated field behaviors in different contaminant scenarios. It is also one that does not necessarily follow from the theoretical developments referenced to this point. To explain this observation, we introduce the concept of a matrix “trapping” effect. That is, if an SOM matrix is in a swollen state during the sorption of a target solute, a greater number of the total sites available within that matrix will become occupied. If the solution-phase condition is then abruptly changed (i.e., temperatures or dissolved solute concentrations are sharply decreased), the matrix will collapse to a less swollen state before the sorbed solute can escape, thus entrapping it within the collapsed matrix. This effect will be particularly pronounced for sorbing components of solute mixtures that are either thermodynamically favored or mass transfer impaired by the polymer matrixes of the SOM (i.e., are either more strongly sorbed than other solutes or are stearically more hindered in terms of intramatrix diffusion). This concept, originally associated with our earlier explanations of desorption hysteresis in single-solute systems (24), explains the results shown in Figure 3 and embraces the effects illustrated in Figure 4 (i.e., dominant cosolute effects on target solute hysteresis increase with increased degree of ambient matrix condensation). The trapping-effect concept was tested further in additional phenanthrene desorption experiments with Chelsea soil. In these experiments, dominant solution-phase concentrations of TCE or DCB were maintained in the fresh background solutions used to replace those remaining at the end of the sorption phases of the bi-solute isotherm tests. The results of these additional experiments are displayed in Figure 5A and B for DCB and TCE, respectively. It is clear from comparisons of parts A and B of Figure 5 with parts C and D of Figure 4 that the presence of the dominant cosolutes during the desorption step mitigated hysteresis and prevented the trapping effect observed in Figure 4, this presumably by maintaining the sorbent matrix in an “expanded” state. Indeed, the results shown in Figure 5A and B compare favorably to those presented in Figure 3B for phenanthrene sorption and desorption in single-solute systems. The results of the foregoing tests shed light on what has been reported as a “conditioning effect” associated with the exposure of a sorbent to high concentrations of solute. As described above, this phenomenon can be explained in terms of the deformation of geosorbent organic matrixes by high solid-phase loadings of sorbate. The effect has been claimed by some to result in irreversible deformations and “fixed” desorption resistant fractions (e.g., refs 26-30). The results obtained here with very high concentrations and loadings of dominant sorbates confirm the effects of deformation on conditioning, but they contradict the assumption of irreversible deformation leading to fixed desorption resistant capacities; at least for the conditions studied. If we accept the notion of a glass-transition concentration (i.e., Cg or Qg°) as an equivalent to the glass-transition temperature, as suggested in the previous discussion, then conceptually we should not expect the sorbent-matrix deformation to be irreversible under normal environmental conditions. The results also suggest that the more rapid elution of solutes such as TCE and DCB expected in sequential desorption steps would act to increase the difficulty of water extraction of target solute in each of a series of sequential steps. Other evidence that supports our trapping-effect hypothesis is the fact that the Freundlich model n values for the desorption steps of the isotherms presented in Figure 5A and B exceed those for the desorption steps shown in Figure 4C and D. These n values, as well as those from the other sorption and desorption tests are arrayed in Table 2. It may be noted from the values given in Table 2 that the differences
cited are statistically significant at the 95% confidence interval level. Displacement Experiments. The results of experiments in which sorption and desorption isotherms for the target solute were first developed in single-solute systems and dominant concentrations of the second solute then added to the residual solutions are illustrated in Figure 6. The figure displays both the displacement isotherms and the corresponding single-solute desorption isotherms. It may be observed first that the displacement effects generally increase as a function of geosorbent character in the order Canadian peat through Lachine shale, essentially the same order in which the effects of direct competition are shown to increase in Figure 2. In other words, the effects of direct competition and those of displacement each correlate with the degree of SOM diagenesis (i.e., extent of condensation/aromaticity). It is similarly evident in the lowest residual primary solute concentration regions of the isotherms that the effects in both cases essentially increase the corresponding Freundlich n values and shift overall process character in the general direction of partitioning or absorption. The explanations advanced earlier to explain the direct competition effects of the dominant solutes are thus applicable as well to their observed displacement effects, as predicted by the DRM theory. The combined results of these two additional types of experimental tests of the underlying theory of this model further supports its general applicability for describing HOC sorption/desorption behavior in soil/sediment systems.
Acknowledgments We thank Tom Yavaraski, EWRE Program Laboratory Manager, for his assistance in HPLC and GC analytical methods development. Funding for the research was provided in part by the National Institute of Environmental Health Sciences (NIEHS) Superfund Grant No. C040133. Partial support was also provided in the form of an EPA STAR graduate environmental education fellowship award to M.D.J. The content of this publication does not necessarily represent the views of either agency.
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Received for review January 25, 2002. Revised manuscript received May 14, 2002. Accepted May 29, 2002. ES020557+
