Environ. Sci. Technol. 2003, 37, 5657-5664
Classifying NOM-Organic Sorbate Interactions Using Compound Transfer from an Inert Solvent to the Hydrated Sorbent MIKHAIL BORISOVER* AND ELLEN R. GRABER Institute of Soil, Water and Environmental Sciences, The Volcani Center, ARO, Bet Dagan 50250 Israel
Interactions of a wide set of organic compounds with model natural organic matter (NOM, Pahokee peat) were examined using a new approach that converts aqueous sorption to compound transfer from n-hexadecane to the hydrated NOM. This conversion accounts for solute-water interactions and applies the same inert reference medium for all compounds of interest, making it possible to classify sorbates according to the strength of sorbateNOM interactions. Differences in strength of organic compound interactions in the sorbed phase as great as 4-5 orders of magnitude are demonstrated. The strongest interactions were observed for compounds with wellestablished H-bonding potentials. Considering hydrocarbons and Cl-substituted hydrocarbons, aliphatic compounds gain more upon distribution from the n-hexadecane medium to NOM than do aromatic compounds. Sorption nonlinearity was tested by comparing the change in n-hexadecanehydrated NOM distribution coefficient (Kd,i) versus sorbed concentration for the different compounds. Only those compounds that interact most strongly with NOM demonstrated significant sorption nonlinearity, expressed by a strong reduction in Kd,i as a function of sorbed concentration. The relationship between compound ability to interact with NOM and reduction in Kd,i as a function of sorbed concentration can be used to characterize compound distribution among different sorption domains.
Introduction It is well-known that sorption interactions between organic compounds and natural organic matter (NOM) are important in controlling their environmental behavior. An often-used approach for studying organic sorbate-NOM interactions involves measuring sorption of a single compound by a set of different NOM sorbents. Generally, the physical phase status and chemical nature of a given NOM sorbent are not easily definable. An alternative approach involving sorption of multiple compound probes on a single NOM sorbent has the advantage that sorbate-sorbent interactions can be related to the better-defined chemical structure or properties of the relatively simple sorbing compounds. By judicious choice of probe compounds, it is possible to test how sorption is affected by different compound characteristics (for example, presence of aliphatic vs aromatic moieties, existence of isomers or conformers, aqueous solubility, 1-octanol* Corresponding author email:
[email protected]; telephone: +972-3-968-3314; fax: +972-3-960-4017. 10.1021/es034640o CCC: $25.00 Published on Web 11/13/2003
2003 American Chemical Society
water distribution, H-bonding potential, molar volume, electronic polarizability, etc.). There are several established approaches relating organic sorbate-NOM interactions to sorbate structure or properties. These include correlations of compound NOM-water distribution coefficients (Kd) with 1-octanol-water distribution coefficients (Kow), aqueous solubilities (Sw), molecular connectivities, etc. (1-6). However, a Kd-based comparison of sorbate-NOM interactions for different organic compounds may be obscured by solute-water interactions that contribute directly to Kd values (Kd is the ratio of sorbed concentration to equilibrium solution concentration Cw; Kd may be concentration-dependent). Solute-water interactions may be accounted for to a certain extent by correlating Kd (or sorbent organic carbon-normalized distribution coefficient Koc) with Sw or Kow or by plotting sorbed concentration against solubility-normalized solution concentration (Cw/Sw). However, solubility normalization entails a distinct surrounding for each compound in its reference state, while 1-octanol correlations involve multiple complex solute interactions with the reference 1-octanol solvent, possibly rendering comparisons problematic. Recently, we demonstrated a different approach to account for organic solute-water interactions and used it to develop a classification scheme for sorption of organic compounds by NOM (7, 8). Gas phase-hydrated NOM sorbent distribution coefficients Hx were calculated from experimentally determined aqueous distribution coefficients Kd (or Koc) and aqueous Henry’s coefficients. Dispersion interactions were accounted for by comparing Hx values for different compounds of the same electronic polarizability. Differences in Hx for organic compounds with the same electronic polarizability were then found to correlate with organic compound ability to undergo specific interactions with NOM (7). This was apparently the first application of such an approach for studying sorption of organic compounds by fully hydrated NOM. Recently, we also used the analysis of Hx values to examine sorption interactions of organic compounds at hydrated mineral surfaces (8). This gas phase transfer analysis (7, 8) is sensitive to differences in electronic polarizability of comparable organic compounds, cannot be applied in a straightforward manner to sorption data with concentration-dependent Kd, and requires aqueous Henry’s coefficients that are not always available. Considering these drawbacks, we modified the gas phase transfer analysis by calculating and comparing distribution coefficients between hydrated NOM and an inert solvent (Kd,i) for different organic compounds (8, 9). An inert solvent in this context is understood to be incapable of specific interactions. Comparing sorption interactions of different sorbates with hydrated NOM using an inert solvent (nhexadecane) as a reference medium is much less sensitive to differences in electronic polarizability of compounds in contrast to the earlier gas phase transfer analysis (7). This is because the effect of compound electronic polarizability on gas phase transfer to organic media is very similar for different organic solvents and NOM (8). For example, it was found that linear dependencies of molar refraction (MR) versus distribution coefficients (log Hx) between the air phase and the supercooled liquid, 1-octanol, n-hexadecane, and NOM have essentially the same slopes (8). Thus, when considering compound transfer from an inert solvent to an NOM sorbent, electronic polarizability differences for compared compounds become relatively unimportant. The modified method may be also applied to compounds for which gas phase-hydrated NOM sorbent distribution coefficients Hx cannot be calcuVOL. 37, NO. 24, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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lated from aqueous distribution coefficients Kd due to the lack of aqueous Henry’s coefficients (8). In the current work, this inert solvent-based analysis is applied for examination of sorption interactions for a wide set of organic compounds on one model natural organic matter (Pahokee peat from the International Humics Substances Society, IHSS). The inert solvent reference state, as opposed to any other kind of solvent, eliminates or minimizes differences in solute-solvent interactions for different solutes, enabling a comparison between sorbates in terms of their interactions in NOM. The evaluation is not limited by the shape and curvature of sorption isotherms. By applying the inert solvent approach, this research pursues three major goals: (i) to classify sorbate compounds according to the strength of their interactions with NOM; (ii) to test the effect of compound structure on sorbate-NOM interactions (i.e., effect of H-bond forming groups, effect of aliphatic moieties vs aromatic moieties); and (iii) to examine the shape of sorption isotherms using a model-free criterion. The new classification scheme is used to test the relationship between sorption nonlinearity and the ability of organic compounds to undergo strong interactions with NOM. Sorption nonlinearity is examined over a certain sorbed concentration range rather than over a certain solute concentration range.
Theoretical Approach Concept. Sorption of compounds by a sorbent from a medium is described by the distribution coefficient Kd (ratio of sorbed concentration to solution concentration). If the solvent medium is water, contributions from solute interactions with the bulk water phase that are inherent in Kd can be eliminated by converting Kd to the gas phase-hydrated sorbent distribution coefficient (Hx) using the aqueous Henry’s coefficient (Hw; 7, 8):
Hx )
Hw [vapor concentration] ) Kd [sorbed concentration]
(1)
Equation 1 represents a thermodynamic cycle. Log Hx is proportional to the sum of the change of chemical potential during compound transfer from the gas phase standard state to the hydrated NOM standard state and an additional term involving the sorbate activity coefficient at a specific sorbed concentration. It should be understood that Henry’s coefficient terminology is applied for distribution between the gas phase and any other phase, including other solvents or the NOM sorbent. We define Kd,i as the coefficient of compound distribution between an inert solvent and the hydrated sorbent according to the following:
Kd,i )
Hi Hx
(2)
where Hi is the Henry’s coefficient for compound distribution between air and an inert solvent at infinite dilution. Log Kd,i is proportional to the sum of the change of chemical potential during compound transfer from the infinitely dilute inert solvent standard state to the hydrated NOM standard state and an additional term involving the sorbate activity coefficient at a specific sorbed concentration. By substituting eq 1 into eq 2, one obtains
Kd,i )
KdHi Hw
(3)
such that, for compounds with readily available or measurable Hi and Hw values, Kd,i is easily computed. For those compounds for which Henry coefficients are not accessible, 5658
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the ratio (Hi/Hw) may be replaced by the inverted ratio of solubilities of the compound in water (Sw) and inert solvent (Si), such that
Kd,i )
KdSw Si
(4)
Equation 4 is based on the assumption that compound gas phase-solvent distribution coefficients are concentrationindependent up to the solubility limit and that the compound phase is the same in saturated aqueous solution and saturated inert solvent solution. To examine interactions between organic compounds and hydrated NOM without the complications of compound interactions in the bulk water phase, the proposed approach suggests comparing Kd,i of compounds using either eq 3 or eq 4. Additionally, it is often instructive to reconstruct sorption isotherm data on the basis of solution concentrations in the inert solvent (Ci). Ci is converted from equilibrium aqueous solution concentrations Cw according to
Ci ) Cw
Hw Si ) Cw Hi Sw
(5)
Compound activities in the aqueous and inert solvent phases are identical. Ratios (Hw/Hi) and (Si/Sw) are related to the change in chemical potential of a compound upon transfer from the infinitely dilute standard state in water to the infinitely dilute standard state in n-hexadecane. Differences between inert solvent-normalized sorption isotherms for different compounds can be correlated with compound structure or sorbent structure. When comparing sorption by NOM for sorbents with different organic matter contents, Kd, Kd,i, and sorbed concentrations should be normalized to organic matter or organic carbon content. Inert Solvent Selection. n-Hexadecane is selected as the reference inert solvent for several reasons. (i) n-Hexadecane is a saturated hydrocarbon incapable of specific interactions (H-bonding, electron donor-electron acceptor complex formation), such that it serves as a single inert medium for a wide spectrum of organic compounds. (ii) Log Hi values were found to be the same for numerous compounds having the same electronic polarizability (7, 10). For these compounds, comparing n-hexadecane-based Kd,i values is equivalent to comparing Hx (8) that are, by definition, free from any contributions of compound-solvent interactions. For certain compounds that may have differing Hi values at the same electronic polarizability (R) or molar refraction (MR ) 4/3πNR; N is Avogadro’s number), comparing Kd,i values specifically includes the difference in the ability of the given compounds to undergo dispersion and induction interactions in n-hexadecane as compared with organic matter. As such, the Kd,i comparison can reveal the complexity of the hydrated NOM phase as compared with n-hexadecane and thus serve as the basis for classifying both sorbate-NOM interactions and the sorbent itself. (iii) According to eq 2, the Kd,i-based analysis should be much less sensitive to differences in sorbate electronic polarizability as compared with the Hxbased analysis. This is because the effect of MR of many organic substances on the gas phase distribution coefficient was found to be virtually the same in NOM and in nhexadecane (slope of -log Hx vs MR dependence ) 0.13 mol/ cm3, and slope of -log Hi vs MR dependence ) 0.12 mol/cm3, respectively, 7). (iv) Selection of an alternative saturated hydrocarbon as a reference medium should not significantly affect the comparison of strength of interactions between the compound and NOM. This is because the slope of the log Hi versus MR regression line in n-hexadecane was identical to that of the single regression line obtained for gas phasesaturated hydrocarbon distribution for 46 different com-
TABLE 1. Physical-Chemical Properties of Studied Organic Compounds chemical compd 3-nitrophenol phenol pyridine benzyl alcohol atrazine 2,4-dichlorophenol acetophenone nitrobenzene 2-chloronitrobenzene anisole trichloromethane 1,3-dibromopropane cyclohexane trans-1,2-dichlorocyclohexane phenanthrene naphthalene toluene benzene 1,2-dichlorobenzene 1,3-dichlorobenzene 1,4-dichlorobenzene 1,2,4-trichlorobenzene 1,3,5-trichlorobenzene 1,2,3-trichlorobenzene
abbrev
molar vola (cm3/mol)
molar refraction (cm3/mol)
log [Hw/Hi]
Sw solid (mmol/L)
Sw (supercooled) liquid (mmol/L)
log Kow
simulation model usedb
NPh Phe Py BA Atr DCP AP NB CNB Ani TCM DBP CH DCCH Phen Napht Tol Benz 1,2-DCB 1,3-DCB 1,4-DCB 1,2,4-TCB 1,3,5-TCB 1,2,3-TCB
93.7 89.0 80.6 103.4 181.7 117.9 116.3 102.2 115.7 108.6 80.5 101.9 108.1 129.3 181.9 110.3 106.4 88.9 112.7 114.1 117.8 124.8 125 125
35.4c 28.4c 24.1c 32.6c 65i 37.4i 36.5c 32.9c 37.9i 32.9c 21.5c 31.1n 27.7n 37.4n 62.2c 44.0c 31.1c 26.2c 36.0c 36.1c 36.3c 41.0c 41.0c 41.0c
-1.37d -1.08d -0.42d -0.64d 0.47j 1.51k 1.14d 1.54d 2.30k 2.09d 1.69d 2.07k 3.86d 2.45k 4.78d 3.4d 2.68d 2.16d 3.52d 3.69d 3.7d 4.43d 4.48d 4.51d
87.3e 870g na na 0.166g 19.95g na na 5.08l na na na na na 0.0051g 0.195g na na na na 0.51g na 0.050g 0.129g
514 f 1380g na 372g 6.17g 27.54g 47.9g 15.85g 6.4m 14.1g 69.2g 7.48l 0.71o 1.57l 0.033g 0.78g 6.17g 18.6g 0.87g 0.89g 1.12g 0.22g 0.14g 0.28g
2.00g 1.46g 0.9g 1.1g 2.47g 3.09g 1.6g 1.85g 2.4l 2.11g 1.95g 2.38l 3.44o 3.1l 4.55g 3.34g 2.53g 2.00g 3.46g 3.49g 3.45g 4.22g 4.26g 4.09g
nah na na na na DMM na na DMM na PMM DMM FM DMM na na na na PMM FM FM PMM PMM PMM
a Molar volumes are calculated from compound densities: Atr density is from ref 14; DCP density is from ref 15; all others are from refs 16 and 17. b PMM, Polanyi-Manes model; DMM, dual-mode model; FM, Freundlich model. Model parameters are from ref 18. c Ref 7. d Hw and Hi values are converted from Ostwald coefficients of organic compounds in water and in n-hexadecane compiled in ref 19. e Ref 13. f From aqueous solubility reported in ref 13 and melting correction obtained from ref 2. g Ref 2. h na, nonapplicable. i Estimated by fragments. j Log (Si/Sw) where solid atrazine solubility in n-hexadecane is 0.45 ( 0.01 mmol/L (measured by bottle-shake method in this work; concentration at saturation was determined by GC-FID after following solubility kinetics for 312 h). k Estimated using additive scheme (details in Supporting Information). l Ref 18. m Melting correction calculated using heat of fusion of chloronitrobenzenes as 4.7 kcal/mol (17). n By Lorenz-Lorentz equation MR ) V(n2 - 1)/(n2 + 2) where V and n are molar volume and refractive index of the substance in its liquid state for the sodium D-line at 293 K, respectively (17). o Ref 3.
pounds and solvents such as n-pentane, n-hexane, n-heptane, and cyclohexane (slope of -log Hi vs MR dependence calculated to be 0.12 mol/cm3 from reported Gibbs free energy regression data; 11).
Experimental Section General. Aqueous sorption data for 24 compounds on IHSS Pahokee peat generated in our laboratory and reported by others were analyzed using the described approach with n-hexadecane as the reference inert solvent system. For the purposes of this paper, the terms “nonpolar” and “low-polar” will be used to describe hydrocarbons and Cl-substituted hydrocarbons, and the term “polar” will be used to delineate all other compounds. Materials. The IHSS standard Pahokee peat was reported by the IHSS to contain 85% organic matter consisting of 45.7% C, 3.13% N, 4.74% H, and 15% ash w/w (12). The peat used in our in-house sorption experiments was determined to contain 83% organic matter with a composition of 49% C, 3.3% N, 4.3% H, and 0.5-1.2% S on a dry weight basis (elemental analysis; Carlo Erba, EA-1108; 13). Peat was γ-irradiated to minimize microbial degradation of compounds in sorption experiments, freeze-dried to about 3% water content, and stored in a closed container in a desiccator with silica gel. Dry weight basis was determined by drying at 105 °C for 24 h. Studied organic compounds, their abbreviations, and relevant properties (molar volume, molar refraction, aqueous solid and liquid solubility, log Kow) are listed in Table 1. Organic compounds examined in this work are considered to be generally nonionized in the aqueous phase at pH values reported for aqueous suspensions of this peat NOM (pH values specified in refs 13 and 20). In-House Sorption Experiments. New aqueous sorption data is reported for five compounds: anisole (Ani; Aldrich
Chemical Co., 99%), phenanthrene (Phen; Aldrich Chemical Co., >98%), naphthalene (Napht; Aldrich Chemical Co., >98%), benzene (Benz; Bio Lab. Ltd, Israel, >99.5%) and toluene (Tol; Aldrich Chemical Co., 99.8%). Samples were obtained in triplicate or duplicate, and blanks were obtained in duplicate at 23 ( 2 °C. In all sorption experiments, sorbed concentration per dry sorbent weight was found by the difference between initial and equilibrium solution concentrations. Sorption of anisole, phenanthrene, and naphthalene from water (Millipore) was measured using procedures similar to those described elsewhere in detail (13, 20). Briefly, anisole sorption kinetics and sorption equilibrium were followed in aqueous batch experiments where the solid:liquid ratio was 1:20 (fraction sorbed 50-70%, with fast kinetics established after 24 h and followed until 256 h). In phenanthrene sorption experiments (solid:liquid ratio 1:3100; fraction sorbed 6085%), equilibrium was reached after 40 h and followed until 600 h at two concentration levels. In naphthalene sorption experiments (solid:liquid ratio 1:500; fraction sorbed 2560%), sorption equilibrium was reached after 60 h and followed until 250 h. After reaching equilibrium and being centrifuged, solution concentrations of sorbing compounds were measured by HPLC equipped with an autosampler (Shimadzu; C8 column, UV diode array detector, acetonitrilewater mobile phase). Prior to measuring phenanthrene concentration, phenanthrene-containing solutions were diluted quantitatively in water-acetonitrile mixtures to minimize adsorption losses. Losses of anisole (1%) were neglected. Losses of phenanthrene (12%) and naphthalene (10%) were considered in calculating sorbed concentrations. Benzene and toluene sorption experiments were conducted using procedures similar to those reported for volatile compounds in ref 21. Experiments were carried out in 2-mL VOL. 37, NO. 24, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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glass vials fitted with Teflon-lined screw caps. The peat was weighed into the vials, and varying amounts of 200 mg/L HgCl2 solution were added to the peat, followed by varying amounts of saturated benzene or toluene solution in 200 mg/L HgCl2 to make up 1 mL of total solution volume. The solid:liquid ratio (1:25) was determined in preliminary experiments to give approximately 50% sorption. After equilibrium was reached (72 h), benzene or toluene was measured in the headspace by GC-FID (Varian 3800 GC equipped with a Varian 8200 autosampler) and compared with blanks. Peak areas obtained from headspace analyses were compared with external standards prepared in 200 mg/L HgCl2, and all aqueous concentrations (standards, initial solution, and equilibrium solution phase concentrations) were computed using Henry coefficients. Literature Data Sources. Previously reported experimental aqueous sorption data from refs 13 and 20 for phenol (Phe), pyridine (Py), atrazine (Atr), 3-nitrophenol (NPh), nitrobenzene (NB), acetophenone (AP), and benzyl alcohol (BA) were used in the current analysis. Additionally, sorption data for 2,4-dichlorophenol (DCP), 2-chloronitrobenzene (CNB), trichloromethane (TCM), 1,3dibromopropane (DBP), cyclohexane (CH), trans-1,2-dichlorocyclohexane (DCCH), and isomeric di- and trichlorobenzenes (DCBs, TCBs) from ref 18 were simulated using the best-fit model parameters from the original paper. Models used for simulation are listed in Table 1. Equilibrium solution concentration ranges were evaluated on the basis of Figures 3-8 and Supporting Information Figure S3 in ref 18. For the current analysis, best data simulation was the major criterion for choosing the interpolating equation, and no specific sorption mechanism or model interpretation is implied. Small deviations of measured data from model-provided values (18) are not important for the aim of the current work, in which sorption data are compared on a log scale. Phase Distribution Data and Activities. For most compounds, Hi and Hw used in calculations were obtained from data compiled in ref 19 and presented in Table 1 as log [Hw/Hi]. Atrazine distribution between hydrated sorbent and n-hexadecane was calculated using atrazine solubilities (eqs 4 and 5) in water and n-hexadecane (Table 1). Values of log [Hw/Hi] for DCP, CNB, DBP, and DCCH were calculated from the log [Hw/Hi] values for related compounds using the additive scheme (details in Supporting Information). Activities (a) of organic compounds used for correlation of sorption data are related to the pure (supercooled) liquid state. Activity-related sorption data for 3-nitrophenol (NPh), phenol (Phe), pyridine (Py), acetophenone (AP), benzyl alcohol (BA), and nitrobenzene (NB) were taken from refs 13 and 20. Activities of all other compounds were determined by normalizing the equilibrium solution concentration (Cw) by the aqueous (supercooled) liquid solubility (Sw; a ) Cw/Sw).
Results and Discussion Aqueous Sorption Isotherms. Experimental data for aqueous sorption of different organic compounds on Pahokee peat are shown in Figure 1, with sorbed concentration (mg/kg) on the ordinate plotted against aqueous solution concentration (mg/L) on the abscissa. With the exception of phenanthrene, the total difference in sorbed concentration for this large set of compounds is only approximately 1-1.5 log units at any given solution concentration (Cw), and the total difference in Cw for a given sorbed-phase concentration is about 1.5-2 log units. Thus the possibility to differentiate between different compounds and between essentially nonpolar compounds as contrasted with polar compounds on the basis of their aqueous sorption isotherms is very limited. This is because, for many compounds, interactions in the bulk aqueous phase compensate for those in the 5660
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FIGURE 1. Aqueous sorption of organic compounds on Pahokee peat. Sorbed concentrations (mg/kg) plotted against aqueous solution concentration (mg/L). Data sources are given in the Experimental Section and Table 1. sorbent. A more extensive data set, including larger lowpolar compounds, would presumably show a greater total difference in sorbed concentration range at any given solution concentration. n-Hexadecane-Based Sorption Isotherms. Rebuilt isotherms following the inert solvent approach (eq 5) are shown in Figure 2. When plotted in this way, the total range of sorbed concentration at a given solution concentration in nhexadecane (Ci) is as great as 4-5 orders of magnitude for the different organic compounds. Polar compounds demonstrate the greatest range in sorbed concentration at any Ci. Recalling that the aqueous sorption isotherm is affected by both solute-bulk water interactions and sorbate-NOM interactions and that the conversion of Cw to Ci eliminates the solute-bulk water component thus laying bare the sorbate-sorbent contribution, it is clear that the inert solvent approach enables the detection of those compounds that interact very strongly with NOM (i.e., polar compounds). This effect is not apparent in the traditional aqueous isotherm representation (Figure 1), and it paves the way for classifying compounds on the basis of the strength of their interactions with the NOM sorbent. Using this representation as a classification scheme, it is seen from Figure 2 that the overall trend in increasing strength of compound interactions with NOM is as follows: aromatic and Cl-substituted aromatic hydrocarbons e aliphatic hydrocarbons and Cl-substituted aliphatic hydrocarbons ≈ Ani ≈ CNB < NB < AP < DCP ≈ Atr ≈ BA < Py < Phe < NPh. The strongest sorption interactions were observed for pyridine, phenol, and 3-nitrophenol, a reasonable result considering the well-known potential of these compounds to participate in H-bonding and, possibly, in partial proton transfer. When considering hydrocarbons and Cl-substituted hydrocarbons, it may be seen in Figure 2 that aliphatic
FIGURE 2. Sorbed concentrations of organic compounds (mg/kg) on Pahokee peat plotted against compound concentration in n-hexadecane (mg/L) calculated using eq 5. Dashed lines show sorption trends for aromatic hydrocarbons and their Cl-substituted derivatives, and solid lines depict sorption trends for aliphatic hydrocarbons and their Cl-substituted derivatives. compounds (solid lines) generally interact more strongly with the NOM sorbent than aromatic compounds (dashed lines). This means that aliphatic compounds gain more upon distribution between a purely aliphatic medium, n-hexadecane, and the fully hydrated, much more polar and essentially aromatic Pahokee peat, than do aromatic compounds. The difference cannot be related to electronic polarizability because aliphatic compounds generally have comparable or lower MR values than aromatic compounds (Table 1). Among the aromatic compounds, there is no correlation between molar volume of the compound and sorption; likewise, for aliphatic compounds, there is no clear correlation between molar volume and sorption. Therefore, the differentiation between aliphatic and aromatic hydrocarbons and Cl-substituted derivatives in terms of sorbed concentration is not expected to be related to molar volume. Higher inert solvent-based sorption for the aliphatic compound, 1,2-dichloroethane, as compared with the aromatic compounds m-xylene and phenanthrene on chalk organic matter was also reported recently (21). Although it is not yet clear what mechanism contributes to the increased potential of aliphatic compounds to interact with NOM as compared with aromatic compounds, in part, this differentiation may result from the well-known potential of some aliphatic Cl-substituted compounds (e.g., TCM) to form H-bonds with functional groups (22). Also, it may be due to the conformational heterogeneity of certain aliphatic compounds as compared with rigid aromatic molecules (21). For example, it was suggested earlier that the different conformers of 1,2-dichloroethane might have different interactions with humic substances (23). Greater NOM sorption interactions of aliphatic hydrocarbons and Cl-substituted aliphatic hydrocarbons as compared with aromatic hydrocarbons and Cl-substituted aromatic hydrocarbons suggest that the possible formation of electron donor-electron acceptor complexes between the studied aromatic compounds and the NOM sorbent does not play an important role in sorption
of these compounds. This suggestion is in accord with a recent report (24) that the binding of polynuclear aromatic hydrocarbons (PAH), chlorinated compounds, and alkanes by soluble humic substances may be correlated using the simple Flory-Huggins concept without considering a role for electron donor-electron acceptor interactions. The relative distribution of nonpolar (low-polar) saturated aliphatic compounds as compared with aromatic compounds to NOM and possible mechanistic interpretations should be tested with larger sorbate and sorbent data sets before arriving at any firm conclusions. The introduction of functional groups into the molecule structure does not always result in strong changes in sorption interactions with NOM. Such compounds as Ani, CNB, and NB, with relatively reactive functional groups, show small or no increase in distribution as compared with low polar compounds (Figure 2). For example, anisole (Ani) sorption is the same as that of TCM, DBP, and DCCH as is CNB sorption in a mid to high concentration range. Sorption of NB is only somewhat greater than that of these other compounds. Therefore, while certain compounds may be thought of as “reactive” due their functional groups, they behave in this specific NOM sorbent (IHSS Pahokee peat), as do more classically nonpolar and low-polar compounds. Figure 2 also shows that 2,4-dichlorophenol (DCP) interacts with NOM much more weakly than other studied phenols, apparently due to the intramolecular H-bonding that is typical for o-substituted phenols (22). Borisover and Graber (7) previously reported other examples of decreased sorption interactions with NOM for o-substituted phenols as compared with m- and p-substituted phenols. This is a result of intramolecular H-bonding in the o-isomers, which reduces their H-atom donor activity in intermolecular H-bonding. Strong interactions are observed for benzyl alcohol as compared with its isomer, anisole (Figure 2), thus illustrating the aptitude of the sorbate hydroxyl group for interacting with NOM. Likewise, comparing sorption of phenol with that of anisole (a pair routinely used to detect the H-bonding potential of solvents; 25), the much greater interactions of phenol demonstrate the great propensity of the phenolic hydroxyl for H-bond interactions with NOM. Activity (Solubility-Normalized Concentration) and 1-Octanol-Based Sorption Isotherms. For comparison with other popular established approaches to relating organic sorbate-NOM interactions to sorbate structure or properties, sorption data are plotted in Figure 3 against compound activity (solubility-normalized concentration; Figure 3A) and against solution concentration in 1-octanol (by using eq 5 with octanol-water distribution coefficients Kow instead of (Hw/Hi); Figure 3B). It may be seen from Figure 3 that although the trends observed in the n-hexadecane-based representation (Figure 2) are retained in the activity and 1-octanol-based plots, the extent of differentiation between the different compounds is substantially reduced. In particular, the marked differences in sorption of polar compounds are significantly lower on both the activity scale (Figure 3A) and 1-octanol-based scale (Figure 3B), such that it would be difficult to comment on the H-bonding ability of NOM on the basis of these representations. For the activity normalization (Figure 3A), compression of the overall pattern is due to a compensation of the differences in sorbate-NOM interactions by the different multiple (and specific) interactions occurring in the pure liquid states of the different compounds. Similarly, sorbed concentration plotted against 1-octanol-based solution concentration (Figure 3B) also results in a strong collapse of many isotherms, an understandable result considering the well-recognized use of 1-octanol as an NOM surrogate for modeling and correlation purposes. Like the compression VOL. 37, NO. 24, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Sorption of organic compounds by Pahokee peat. (A) Sorbed concentrations (mg/kg) plotted against compound activity. (B) Sorbed concentrations (mg/kg) plotted against compound concentration in 1-octanol (mg/L). Dashed lines show sorption trends for aromatic hydrocarbons and their Cl-substituted derivatives, and solid lines depict sorption trends for aliphatic hydrocarbons and their Cl-substituted derivatives. resulting from the activity normalization, the compression for 1-octanol normalization is due to multiple complex solute interactions with 1-octanol, which compensates for similar complex sorbate interactions with NOM. It is worth reiterating that the collapse of isotherms in Figure 3B demonstrates that interpretation of sorbate interactions with NOM based on 1-octanol-water distribution for compounds with functional groups may be strongly erroneous due to underestimating the role of different solute-1-octanol interactions (7). The fact that less useful information is obtained from the solubility-normalized (activity) approach (Figure 3A) or 1-octanol approach (Figure 3B) as compared with the n-hexadecane inert solvent approach (Figure 2) demonstrates our major point: that instead of using a complex reference state (1-octanol approach) or multiple reference states (solubility-normalization approach), using a single inert solvent as a reference state may be valuable for delineating the differences in organic compound interactions with NOM. Forcing sorption isotherms to coincide by using Koc-Kow correlations or sorbed concentration versus solubilitynormalized concentration plots, which may be useful for data correlations or predictions, is less informative for understanding the strength of sorbate interactions with NOM. Specifically, discrimination of sorbates on the basis of their interactions with NOM can be better examined by converting aqueous sorption data into inert solvent-based sorption. This approach can be particularly constructive and important when considering sorption behavior of many compounds that include functional groups (pesticides, other pollutants) 5662
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and whose potential to interact specifically with NOM should not be underestimated. The classification can be used for other goals, for example, to examine the relationship between sorption isotherm shape (linearity, sigmoidality) and strength of compound interaction. This is demonstrated in the following section. Nonlinearity and Shape of Sorption Isotherms. A number of recent studies have reported nonlinear aqueous sorption isotherms for different hydrophobic organic compounds (mainly hydrocarbons and halogen-substituted derivatives), and, to a lesser extent, for compounds with functional groups. The mechanism(s) responsible for such nonlinear sorption behavior of nonpolar and low polar organic compounds are still disputed. Likewise, the relationship between the presence of functional groups in the sorbate molecular structure and isotherm nonlinearity is unclear. While some studies reported a connection between sorbate-NOM specific interactions and sorption nonlinearity (26, 27), others found no relationship (18). Considering this, we apply our classification to test whether strong sorbate-NOM interactions are associated with nonlinear sorption isotherms. For such an analysis, two important issues should be resolved: (i) how should sorption isotherm nonlinearity be evaluated and (ii) how should isotherm nonlinearity for different sorbates in different systems be compared. (i) The Freundlich exponent n is frequently used to measure and compare isotherm nonlinearity in the literature. However, we have previously shown that the Freundlich model often does not describe sorption data sufficiently well to use model parameters in such a way (28-30). Inadequate data description by the Freundlich model may result from an uptake-related increase in sorption sites (28), or from the presence of high surface area impurities whose sorption properties may be better described by a pore-filling model (27). We showed how even an apparently good data description (R2 ) 0.999) may be nevertheless inadequate (30), leading to erroneous conclusions on the effect of sorbed concentration on rate of attainment of sorption equilibrium, as discussed in refs 13, 29, and 30. Recently, these observations were confirmed in a study (18) demonstrating large changes in Freundlich exponent n as a function of compound concentration range. Therefore, we consider that a model-free criterion for examining isotherm nonlinearity is essential. This model-free criterion is the distribution coefficient (Kd or Kd,i). (ii) Sorption nonlinearity is expected to result from interactions in the sorbed phase rather than in the solvent phase. Hence, when comparing different systems, it is meaningful to compare variation in the distribution coefficient Kd or Kd,i over the same sorbed concentration range rather than the same solution concentration or solubilitynormalized concentration range. Such a comparison avoids complications or masking effects that could arise due to different sorbate concentrations in the sorbent phase. Note, for example, that differences in sorbed concentration of different compounds at the same activity (Cw/Sw) may reach 1-3 orders of magnitude (Figure 3A). Since comparing Kd at the same activity (or solubility-normalized concentration) can be misleading, it should be avoided in favor of a comparison based on sorbed uptake. Comparing sorbatesorbent interactions at the same sorbed concentration is, indeed, a customary approach for polymer sorbents and has been applied to NOM when described as a polymer phase (26). Considering the foregoing, we plot Kd,i against sorbed concentration to examine isotherm nonlinearity as a function of sorbate interaction strength (Figure 4). Kd,i is used rather than Kd because when plotted against sorbed concentration, different organic compounds can be distinguished on the basis of the strength of their interactions with NOM.
FIGURE 4. Sorption of organic compounds by Pahokee peat. loglog plot of Kd,i values against sorbed concentration (mg/kg). Simulated isotherms were not considered, as each model will dictate a different shape depending on the assumptions implicit in the model. In Figure 4 it is seen that for many compounds considered polar (BA, Atr, NB, AP, Ani) and nonpolar (Benz, Tol, Napht, Phen), it is generally not possible to relate, within scattering, the nonlinearity of isotherms to the ability of compounds to interact with NOM. Sorption of Atr, BA, Ani, Benz, and Phen is characterized by essentially constant Kd,i over the sorbed concentration range studied, while weakly expressed isotherm nonlinearity as depicted by changes in Kd,i for Napht, NB, and AP cannot be discriminated on the basis of either organic compound “polarity” or the magnitude of Kd,i. Only those compounds that interact most strongly with NOM (NPh, Py, and, to a lesser extent, Phe) show a significant reduction in Kd,i as a function of sorbed concentration. Such strong interactions may include, among others, H-bonding or proton-transfer phenomena. These results suggest that for organic compounds of intermediate ability to undergo interactions with NOM (in terms of Kd,i magnitude), the presence of sorbate functional groups may not induce isotherm nonlinearity, as seen by variability in Kd or Kd,i. This follows from consideration of sorption domains for polar compounds, which may have many additional sorption domains as compared with nonpolar compounds, including polar NOM sites and/or NOM links (13, 31, 32) or NOM-complexed water (7). These additional sorption domains result in the overall greater interactions for polar compounds observed in the sorbed phase (greater Kd,i, Figure 4). The interplay between the relative abundance of these additional domains and their capacities and affinities for sorbate molecules will dictate whether isotherm nonlinearity will be expressed. For example, as polar compounds may have more sorption domains, at any given sorbed concentration the extent of site occupation in any given domain may be less for polar compounds than for nonpolar compounds, resulting in more linear isotherms. In contrast, a certain high affinity domain may be much more densely populated than other less specific domains, which may result in an increase in isotherm nonlinearity as well as an increase in overall strength of sorbate-sorbent interactions. Thus there will be no single relationship between nonlinearity of sorption isotherms and
ability of organic compounds to undergo strong specific interactions. Relating the degree of isotherm nonlinearity for different compounds to their ability to interact with a sorbent may be helpful in characterizing compound distribution among different sorption domains. For example, moderately interacting polar compounds like BA, Atr, and Ani have no important specific sorption domain with high occupancy, thus displaying linear isotherms. In contrast, NPh and Py, which are capable of strong NOM interactions, experience relatively high occupancy of at least one important specific sorption domain and thus demonstrate strong isotherm nonlinearity. Figure 4 also shows that for certain compounds (e.g., Phe, NB, and AP), the plot of Kd,i against sorbed concentration has a complicated shape, involving an increase in Kd,i after reaching a certain minimum. This behavior would correspond to an S-shaped region of the isotherm plotted as sorbed concentration against solution concentration. For phenol, such an S-shaped isotherm was construed to result from uptake-related creation of sorption sites (20). We consider that such an extreme or flattening in a Kd,i or Kd dependence versus sorbed concentration is the proper tool for identifying and comparing sorption features (i.e., sigmoid isotherm shape) for different sorbing organic compounds.
Acknowledgments This research was supported by a grant from the Israel Science Foundation (Grant 400/00-1) and a grant from the Israel Ministry of Environmental Quality (No. 901). Help from Lyudmila Chekhansky and Nadezhda Bukhanovski in carrying out the experimental work is greatly appreciated. The constructive comments of the three reviewers were greatly appreciated.
Supporting Information Available Additive scheme calculations of log [Hw/Hi] values for DCP, CNB, DBP, and DCCH. This material is available free of charge via the Internet at http://pubs.acs.org.
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Received for review June 22, 2003. Revised manuscript received September 10, 2003. Accepted October 2, 2003. ES034640O
