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According to a recent conceptual model for hydration- assisted sorption of organic compounds in natural organic matter (NOM), certain polar moieties o...
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Environ. Sci. Technol. 2002, 36, 4570-4577

Relationship between Strength of Organic Sorbate Interactions in NOM and Hydration Effect on Sorption MIKHAIL BORISOVER* AND ELLEN R. GRABER Institute of Soil, Water and Environmental Sciences, The Volcani Center, A.R.O., P.O.B. 6, Bet Dagan 50250, Israel

According to a recent conceptual model for hydrationassisted sorption of organic compounds in natural organic matter (NOM), certain polar moieties of dry NOM are unavailable for compound sorption due to strong intraand intermolecular NOM interactions. Water molecules solvate these moieties creating new sorption sites at solvated contacts. It is expected that the greater a compound’s ability to undergo specific interactions with NOM, the greater will be the hydration-assisted sorption effect, because penetration of compounds into solvated contacts must involve competition with water at the solvated contact. To test this model, we compare the hydration effect on sorption kinetics and equilibrium for 4 compounds with differing abilities to undergo specific interactions with NOM. Sorption measured on Pahokee peat in aqueous systems was fast compared with n-hexadecane (dry) systems. No concentration effect on attainment of sorption equilibrium was observed. m-Nitrophenol exhibited the greatest hydration-assisted sorption effect, benzyl alcohol showed an intermediate effect, and acetophenone and nitrobenzene showed no hydration-assisted sorption, on an activity scale. The extent of hydration-assisted sorption effect correlates with compound ability to undergo specific interactions. These results support the conceptual model and demonstrate the importance of polar NOM noncovalent links in organizing the NOM phase and in controlling the hydration effect on sorption of organic compounds.

Introduction It is well-known that natural organic matter (NOM) is a very important environmental sorbent for organic compounds under hydrated conditions. As NOM consists of heterogeneous polyelectrolytic macromolecules, hydration of NOM results in swelling, increased flexibility, changes in ionization status of polar functional groups, and conformational reorientation (1-4). As a result of these hydration-related changes in NOM structure, sorbate diffusion in NOM and kinetics of sorption and binding at NOM sites may be affected. Phenomena such as water-sorbate competition, change of sorbate speciation in the NOM phase, direct interaction between complexed water molecules and sorbate molecules in the NOM phase, change in the total volume of the NOM phase, and change in NOM polarity all may contribute to the overall hydration effect on sorbate interactions in NOM. * Corresponding author phone: 972-3-968-3314; fax: 972-3-9604017; e-mail: [email protected]. 4570

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Although certain aspects of NOM hydration appear straightforward, the net effect of NOM hydration on organic compound sorption interactions is far from self-evident due to the scarcity of experimental organic compound sorption data in both dry and wet NOM and the complexity of NOM hydration phenomena (5, 6). Recently, based on experimental data for sorption of organic compounds by dry and hydrated (or solvated by organic solvents) NOM, we suggested a conception of solventdisruptable noncovalent links in NOM (5, 6). According to this idea, certain polar moieties of dry NOM are unavailable for compound sorption due to strong interactions (e.g., hydrogen bonding, proton transfer complexation, bridging via metal cations). By penetrating into the NOM structure, solvent molecules solvate these polar moieties creating new NOM sorption sites for sorbate molecules at the solvated (disrupted) contacts. Since a sorbate molecule is not able to interact with the moiety in the nonsolvated state, it is clear that if the solvent disrupts a polar contact by solvating only one moiety of the contact, the sorbate molecule would not be able to replace the solvent molecule at that moiety. Mechanistically, this means that solvent molecules must solvate both moieties of the polar contact in order for the sorbate molecule to compete with a solvent molecule at one of the solvated moieties. The driving force for solvent-assisted sorption is solvation of the partner of the disrupted polar contact that does not directly interact with the sorbate. There will be a tradeoff between solvent-assisted penetration of organic compound molecules into polar contacts versus competition between sorbate and solvent molecules for new sites at those disrupted contacts. Recently, based on this conception, a new flexible sorption isotherm model was derived accounting for solvent-assisted sorption, sorbate/ solvent competition at disrupted inter- and intramolecular contacts in NOM, and sorption cooperativity (7, 8). At least three scenarios for a hydration (solvation) effect on NOM sorption of organic compounds were suggested and illustrated by experimental data (6). The first scenario is when a sorbate molecule lacks the ability to penetrate into polar contacts in a dehydrated system but may interact with sites not requiring disruption. Upon hydration, water may create new sorption sites via disruption of polar contacts, but sorbate molecules are unable to compete with water for these new sites. No hydration-assisted sorption will be observed. If water also competes for sites that were originally accessible to sorbate molecules in the dehydrated state, a hydration-suppressed sorption effect will be observed. In the second case, the sorbate molecule is not effective at penetrating into a polar contact alone. Penetration occurs together with hydration (solvation) of all the NOM moieties that make up the contact, and the sorbate competes favorably with water for newly created sorption sites. An overall solvation-assisted sorption effect will result. The third scenario is expected when a sorbate is effective at penetrating into dry NOM contacts, and hydration (solvation) results in a decrease of sorption due to sorbate/solvent competition. If this conceptual model is correct, one may expect that the greater a compound’s ability to undergo specific interactions with NOM (i.e., hydrogen bonding, charge complex formation, proton transfer reactions), the greater will be the hydration-assisted sorption effect. This is because penetration of organic compounds into polar contacts solvated by water must involve competition with water molecules. If a sorbate has a small ability to undergo specific interactions, it will not be able to compete with water molecules for a newly solvated NOM site (swollen polar contact). Therefore, a clear indication 10.1021/es0207192 CCC: $22.00

 2002 American Chemical Society Published on Web 09/28/2002

TABLE 1. Properties of Organic Compounds compound

molar vola, cm3/mol

molar refraction,b cm3/mol

aq solubility, mg/L

Log Kow c

-Log Hhd

-Log Hw d

Log Pe

-∆logHocb

acetophenone nitrobenzene benzyl alcohol m-nitrophenol

116.3 102.2 103.4 93.7

36.5 32.9 32.6 35.4

5740c 1949c 40100c 12150h

1.60 1.85 1.10 2.00

4.50 4.56 4.22 5.69

3.36 3.02 4.86 7.06

-0.40f -0.61f -1.02g -3.20i

1.5 2.3 3.4 5.5

a Compound densities from ref 10, 15-20 °C b Reference 9. c Reference 11. d Converted from Ostwald coefficients from ref 12. e Log saturated vapor pressure over the liquid, 25° C; mmHg. f Reference 13. g Extrapolated from 29.5 °C from ref 14. h (610 (20 samples). i Calculated on the basis of reported aqueous solubility, aqueous Henry coefficient, fusion enthalpy (19.2 kJ/mol (15)), and melting point (97 °C (10)).

of the role of swollen NOM polar contacts in hydrationassisted sorption will be found if sorption is assisted by hydration for compounds capable of strong specific interactions and not for compounds of lesser ability. To examine this, we compare the hydration effect on sorption of a series of 4 selected organic compounds whose isotherms are measured from water (hydrated system) and a nonpolar saturated hydrocarbon (dry system). Compounds were selected according to their increasing ability to undergo specific interactions with NOM, based on our earlier classification scheme using sorbate distribution coefficients for transfer from the gas phase to the hydrated NOM phase (HOC (9)). The specific goals of this study are to 1) test sorption kinetics at different concentrations in terms of fractional uptake, thus examining the approach to sorption equilibrium as a function of hydration; 2) determine sorption isotherms of compounds differing in their ability to interact specifically with NOM from a non polar saturated hydrocarbon (dry system) and water (hydrated system); and 3) correlate the hydration effect on sorption with organic sorbate structure. This study is intended to substantiate our mechanistic idea of the hydration effect on sorption of organic compounds by NOM and, through this, to elucidate the structure of NOM sorption domains, NOM aggregation mechanisms and sorption interaction mechanisms, including the role of strong intra-NOM interactions.

Experimental Section Materials. Pahokee peat (83% organic matter) purchased from the International Humic Substances Society was freezedried and used as a model organic matter sorbent. The peat sample contained 49% of organic carbon (OC), 3.3% N, 4.3% H, 0.5-1.2% S on a dry weight basis as determined by elemental analysis (Carlo Erba, EA-1108). Moisture content of the freeze-dried peat was in the range of 2 to 4% w/w as determined by oven-drying at 105 °C. m-Nitrophenol (m-NO2-C6H4-OH; 99%, BDH Chemicals Ltd., Poole England), nitrobenzene (C6H5NO2; 99%, Aldrich), acetophenone (C6H5-CO-CH3; purum, 98% GC, Fluka), and benzyl alcohol (C6H5-CH2OH; ACS reagent, Sigma) were selected as probe sorbing compounds. Compounds were selected according to their similar electronic polarizability (as expressed by similar molar refraction), similar molar volume and increasing ability to undergo specific interactions with NOM. Compound ability to undergo specific interactions with NOM is correlated to the difference between the log gas phase-organic carbon distribution coefficient (log HOC) for a given compound and the log HOC for a nonpolar compound (aromatic hydrocarbon or halogen-substituted hydrocarbon) having the same molar refraction (∆logHOC (9)). ∆logHOC values reported in ref 9 were obtained from aqueous organic compound sorption data averaged for different types of NOM; aqueous sorption data obtained on Pahokee peat were comparable to the values reported in ref 9. Thus small differences between the nature of the average NOM considered in (9) and Pahokee peat will not significantly affect

the values of ∆logHOC used. Properties of organic sorbates are provided in Table 1. Sorption on Pahokee peat was examined from water (Millipore), n-hexadecane (n-C16H34; 99+%, Aldrich), and n-hexane (n-C6H14; HPLC grade, Biolab, Israel). Solubility Measurements. Solubility of m-nitrophenol in water, 0.1 N HCl, n-hexane, and n-hexadecane was measured by the flask method. An excess of chemical was mixed with solvent in glass vials closed by screw caps equipped with Teflon-lined silicone septa, which were shaken at a constant temperature (25 ( 2 °C) on a table shaker in the darkness. After different time intervals, vials were centrifuged and a portion of the supernatant was taken for analysis (see Analytical measurements). Solubility kinetics was followed, and equilibrium solubility was determined. Analytical Measurements. Concentrations of m-nitrophenol, nitrobenzene, acetophenone, and benzyl alcohol in aqueous, aqueous acidic, or acetonitrile-based solutions were determined against external standards by HPLC equipped with autosampler (C8 column, UV diode array detector, acetonitrile-water mobile phase). Concentrations of nitrobenzene, acetophenone, and benzyl alcohol in n-hexadecane were determined by GC-FID equipped with autosampler (J&W DB-5 column). To measure m-nitrophenol concentrations in n-hexane and n-hexadecane, m-nitrophenol was extracted from the hydrocarbon (1 h mixing with aqueous 0.001 N NaOH solution at a sample:extracting solution ratio of 1:6). The aqueous basic extract was neutralized and analyzed by HPLC. Control extraction tests showed an average 99% recovery at different m-nitrophenol concentrations. Sorption Experiments. Sorption was examined in 10 systems: (I) m-nitrophenol from water, (II) m-nitrophenol from 0.1 N HCl, (III) m-nitrophenol from n-hexadecane, (IV) m-nitrophenol from n-hexane, (V) nitrobenzene from water, (VI) nitrobenzene from n-hexadecane, (VII) acetophenone from water, (VIII) acetophenone from n-hexadecane, (IX) benzyl alcohol from water, and (X) benzyl alcohol from n-hexadecane. n-Hexadecane and n-hexane systems are considered dry systems, in contrast to hydrated aqueous systems. Dry inert hydrocarbon systems are preferred compared to sorption from the gas phase, because of the long sorption uptake kinetics involved. For examining sorption kinetics and establishing equilibrium, peat suspensions in prepared solutions were mixed by table shaker continuously at 25 ( 2 °C in the dark in glass vials sealed by screw caps equipped with Teflon-lined silicone septa. All sorption experiments were replicated (mostly triplicates with some duplicates). For analysis, suspensions were centrifuged, and solute concentration in the supernatant solution phase was determined. Sorbate concentrations in the peat phase were determined by difference and are reported on a dry sorbent weight basis. Details of kinetic and equilibrium sorption experiments are summarized in Table 2. In results of equilibrium experiments, blank losses (1%) were neglected, except for system III in which 8% losses were accounted for in calculating sorbed concentrations. pH of aqueous peat VOL. 36, NO. 21, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 2. Experimental Conditions for the Sorption Experiments

series

solute/solvent

solid:liquid ratio, g per mL

I II III IV V VI VII VIII IX X

m-nitrophenol/water m-nitrophenol/0.1 N HCl m-nitrophenol/n-hexadecane m-nitrophenol/n-hexane nitrobenzene/water nitrobenzene/n-hexadecane acetophenone/water acetophenone/n-hexadecane benzyl alcohol/water benzyl alcohol/n-hexadecane

1:40 1:40 1:80 1:80 1:30 1:2 1:20 1:2 1:4.5 1:2

a

initial solute concn range, mg/L

sorption time allowed for the isotherm/sorption time tested in the kinetics series, (h)

initial concns at which sorption kinetics were followed, mg/L

sorbed fraction of total amount, %

10.1-7850 40-1636 22.1-221 18.6-186 22.6-1130 241-12040 26-2025 274-10373 523-31365 103-3076

336/576 576/nda 1584/1800 864/960 168/720 1992/1992 168/264 1032/1032 96/552 1070/1070

20, 7850 nda 22.1, 221 18.6, 186 22.6, 1130 1204 50 519 25000 534

25-85 37-66 27-81 30-80 35-70 12-38 30-60 25-56 29-51 45-77

Not determined.

suspensions was 5.6-5.8 and not dependent on the specific organic solute or its concentration. As such, benzyl alcohol, acetophenone, nitrobenzene, and m-nitrophenol (pK ) 8.36 (16)) are considered to be nonionized in the experiments. Extractability of Sorbed Organic Compounds. After completing aqueous sorption, recovery of sorbed organic compounds was tested in selected samples by extraction into pure water followed by extraction into acetonitrile or by direct extraction into acetonitrile. For this, samples were centrifuged, the aqueous supernatant was replaced by extracting solvent, and suspensions were then mixed continuously by table shaker at 25 ( 2 °C in the dark. Each step of solvent extraction was usually applied for 48 to 72 h. Solute concentrations in extracts were obtained by HPLC (as above). The volume of entrapped solvent in the solid peat phase was accounted for in mass balance calculations at each extraction step. All together, from two to six solvent extractions were applied. Activity Calculation. Comparison of sorption isotherms for organic compounds from different solvents was performed using compound activities referred to the pure compound liquid state. Activities were calculated according to a ) CeH/Cvap sat where a is the compound activity, Ce is the compound equilibrium concentration in solution (mg/L), Cvap sat is the concentration of saturated organic vapor over pure organic liquid (mg/L, calculated on the basis of the saturated vapor pressure data [Log P], see Table 1), and H is the dimensionless compound Henry’s constant (shown in Table 1 for water [log Hw] and n-hexadecane [log Hh]). Activitybased comparison of sorption isotherms of a single compound in different solvents does not depend on the absolute value of the organic compound vapor pressure or on the accuracy of such data.

Results and Discussion Attainment of Sorption Equilibrium in Aqueous and Hydrocarbon Systems. Sorption kinetics was evaluated for all systems listed in Table 2 with the exception of system II. For systems I, III, IV, and V, sorption kinetics was examined at two different initial compound concentrations. Figure 1 shows the attainment of sorption equilibrium for m-nitrophenol from water (I), n-hexadecane (III), n-hexane (IV), and for nitrobenzene from water (V). Approach to sorption equilibrium at two different initial solute concentrations is depicted in terms of relative fractional uptake, computed as ratio of sorbed amount at a given time to sorbed amount at equilibrium (last time point). Sorption of organic compounds by the NOM sorbent from water was found to be fast. For example, aqueous mnitrophenol sorption was completed after about 50 h at both low and high initial solution concentrations (Figure 1A). For 4572

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FIGURE 1. Attainment of sorption equilibrium on Pahokee peat at different initial solute concentrations in terms of relative fractional uptake. Error bars depict the average standard deviation. nitrobenzene (Figure 1D), acetophenone (Figure S1A; Supporting Information), or benzyl alcohol (Figure S1B; Supporting Information), equilibrium was established within 20 h. No slow sorption was observed, although experiments were generally continued for more than 500 h (Table 2). Sorption of organic compounds from hydrocarbon solutions on dried NOM was found to be much slower. Kinetic tests revealed that m-nitrophenol required between 300 and 600 h in n-hexane and approximately 700 h in n-hexadecane (Figure 1B,C) to reach sorption equilibrium. Benzyl alcohol sorption equilibrium in n-hexadecane was reached after 300 h, acetophenone sorption equilibrium after 400 h, and nitrobenzene sorption after 360 h. Experiments were continued for more than 1000 h (Table 2). An increase by about an order of magnitude in the time required for sorption equilibrium in hydrocarbon-based systems as compared with aqueous systems, despite the generally greater solid:liquid ratio in hydrocarbon systems (Table 2), suggests that the

sorbent immersed in the hydrocarbon phase is poorly solvated, and hence, more “rigid”. Concentration Effect on Attainment of Sorption Equilibrium: Parameter Describing Deviation from Equilibrium. When considering rate of attainment of sorption equilibrium as a function of initial solution concentration in systems exhibiting nonlinear isotherms, it is necessary to correctly select the system parameter that describes deviation from sorption equilibrium. Traditionally, kinetics of attaining sorption equilibrium is followed using fractional uptake, defined as the fraction of the compound in the whole system found in the sorbed state at any given time (17). To compare sorption kinetics in systems differing only in initial compound solution concentration, the ratio of fractional uptake to equilibrium fractional uptake, giving relative fractional uptake, is used. Relative fractional uptake is easily calculated as the ratio of sorbed amount at a given time to equilibrium sorbed amount. Fractional uptake is used to assess the extent to which the sorbed phase deviates from equilibrium at a fixed temperature, external pressure and water (or other solvent) activity. If rate-limiting processes are confined to the sorbent phase (i.e., solution phase is well mixed), it is clear that when the sorbed phase reaches equilibrium, the overall system has reached equilibrium. As such, fractional uptake also measures the extent of deviation of the overall system from sorption equilibrium (18). Two other measures for describing the effect of solution concentration on kinetics of attaining sorption equilibrium in systems exhibiting nonlinear isotherms were recently used: (i) the exponent n of the Freundlich model applied to sorption at a given time (e.g., refs 19-21); and (ii) the distribution coefficient (ratio of sorbed concentration to solution concentration; Kd) at a given time (e.g., refs 19 and 20). A decrease in n over time was interpreted to indicate a faster approach to sorption equilibrium in higher concentration solutions (19-21). However, based on sorption data for organic compounds on NOM sorbents, analysis of the Freundlich model, and the traditional fractional uptake approach, we demonstrated that use of the Freundlich exponent n for inferring a solute concentration effect on the rate of attaining sorption equilibrium results in erroneous conclusions (18, 22). The exponent n was found to decrease with increasing time, regardless of the type of concentration effect on rate of attaining sorption equilibrium (i.e., no effect; higher concentration giving higher rate; lower concentration giving higher rate) (18, 22, 23). This temporal n reduction results from the nonproportional shift of a nonlinear sorption isotherm along the solution concentration axis that may dominate over concentration effects on overall sorption kinetics (fractional uptake). Hence, no direct inferences on a solute concentration effect on attainment of sorption equilibrium may be extracted from the analysis of temporal n trends for organic compounds sorbed by soil and sediment materials (18, 22, 23). By virtue of the fact that there is a nonproportional shift along the solution concentration axis for nonlinear sorption isotherms, it should be clear that generally Kd is also not applicable for evaluating solute concentration effect on overall sorption kinetics in a system exhibiting a non linear sorption isotherm. As such, in this study we employ the traditional fractional uptake approach for examining solute concentration effect on attainment of sorption equilibrium. Concentration Effect on Attainment of Sorption Equilibrium: Observations. Considering the relative fractional uptake trends for m-nitrophenol from water at two initial solution concentrations depicted in Figure 1A, there appears to be some small effect of concentration on rate of attainment of sorption equilibrium. In the initially high concentration solution (7850 mg/L), full equilibrium was reached by about 40 h, while after the same 40 h in the initially low concentra-

tion solution (20 mg/L), about 95% of equilibrium was reached. This is a small apparent concentration effect considering that equilibrium m-nitrophenol loading at the highest initial solution concentration reached approximately 90 000 mg/kg, as compared with about 600 mg/kg for the low concentration experiment. Formation of such a concentrated “solution” (9% w/w) of strongly interacting mnitrophenol in NOM may be expected to result in significant changes in NOM structure, contributing to differences in sorbate uptake rate. For nitrobenzene sorption from water (Figure 1D), there was no apparent effect of initial solution concentration on sorption kinetics. Initial solution concentrations differed by about 50 (as compared with 400 for m-nitrophenol). As acetophenone sorption at the lowest initial solution concentration (50 mg/L) was complete within 20 h (Figure S1A; Supporting Information), it is also expected that there will be no concentration effect on attainment of acetophenone sorption equilibrium. Thus in most studied aqueous systems, there was no observable concentration effect on rate of attainment of sorption equilibrium. This result agrees with our previous analysis for sorption of phenanthrene, 2,4dichlorophenol, and 1,3-dichlorobenzene on hydrated NOM sorbents (18, 22, 23). Effect of concentration on rate of attainment of sorption equilibrium was also examined for m-nitrophenol sorption from two hydrocarbon phases (n-hexadecane and n-hexane), where the difference in initial solution concentration was a factor of 10. In n-hexadecane, a 1 order of magnitude difference in solution concentration had no effect on the overall sorption rate (Figure 1B). Due to experimental scattering, it is difficult to comment on a solute concentration effect for kinetics of m-nitrophenol sorption from n-hexane (Figure 1C). Clearly however, sorption equilibrium was reached at the same time (by about 300 h) from both initially high and low concentration solutions. In both aqueous and hydrocarbon systems, it was thus generally found that concentration did not affect rate of attainment of sorption equilibrium, with the possible exception of a single system (m-nitrophenol from water). Recently, based on sorption data, we estimated the full range of possible concentration effect on sorbate intrinsic diffusivity in hydrated NOM materials to be a factor of 2-3 for 1,3dichlorobenzene and atrazine (18). This estimate has been lately supported by Braida et al.’s (24) evaluation of the concentration effect on phenanthrene sorption kinetics in seven NOM-containing sorbents from aqueous solutions. According to their calculations, no more than a 3-fold change in the phenanthrene sorption characteristic time is expected when phenanthrene concentration is changed by 2.5 orders of magnitude. They also found variable effects of concentration on attainment of sorption equilibrium in terms of fractional uptake: small positive effect (higher initial concentration, faster approach to equilibrium), no effect, and negative concentration effect (lower initial concentration, faster approach to equilibrium). In their data, it can be seen that there is no correlation between the type of concentration effect (positive, negative, none) and sorption nonlinearity, such that positive effects were reported for both a linear isotherm (n ) 1.019, Seal Beach soil) and nonlinear isotherm (n ) 0.725, Pahokee peat), and negative effects for a strongly nonlinear isotherm (n ) 0.731, Mount Pleasant soil) and a slighty nonlinear isotherm (n ) 0.91, Port Hueneme soil). These variable results support our earlier conclusion that in typical batch experiments, nonlinear concave down sorption isotherms are not necessarily accompanied by an experimentally significant positive concentration effect on the rate of attainment of sorption equilibrium (18, 22, 23). Equilibrium Sorption. Aqueous and n-hexadecane-phase sorption isotherms for the four compounds are depicted in VOL. 36, NO. 21, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Sorption of acetophenone by Pahokee peat from water and n-hexadecane. (A) Sorbed amount plotted against equilibrium solution concentration. (B) Sorbed amount plotted against compound activity.

FIGURE 3. Sorption of nitrobenzene by Pahokee peat from water and n-hexadecane. (A) Sorbed amount plotted against equilibrium solution concentration. (B) Sorbed amount plotted against compound activity. the A panels of Figures 2-5. For acetophenone (Figure 2A) and nitrobenzene (Figure 3A), there is a large difference between sorption from aqueous and nonaqueous solutions. At a given solution concentration, sorption is generally greater by more than 1 order of magnitude from water than from n-hexadecane (Figures 2A and 3A). In contrast, for benzyl alcohol and m-nitrophenol, differences in sorption uptake between the aqueous and nonaqueous systems are small (Figures 4A and 5A). At a given solution concentration, sorption uptake of m-nitrophenol from n-hexadecane is 4574

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FIGURE 4. Sorption of benzyl alcohol by Pahokee peat from water and n-hexadecane. (A) Sorbed amount plotted against equilibrium solution concentration. (B) Sorbed amount plotted against compound activity. slightly greater than from water, while for benzyl alcohol, there is no apparent difference. These similarities are seen despite the fact that m-nitrophenol is 47 times more soluble in water than in n-hexadecane (Table 1), and benzyl alcohol is miscible in n-hexadecane while its solubility in water is only 40 g/L (Table 1). m-Nitrophenol and nitrobenzene isotherms in both aqueous and nonaqueous systems are strongly nonlinear, as demonstrated by Freundlich n parameters much less than unity (m-nitrophenol-water 0.68; m-nitrophenol-n-hexadecane 0.47; nitrobenzene-water 0.79; nitrobenzene-n-hexadecane 0.79). Sorption kinetics (Figure 1) demonstrates that this strongly nonlinear sorption of both m-nitrophenol and nitrobenzene from water occurs very quickly. To evaluate the hydration effect on sorption of organic compounds by NOM, sorption isotherms for each probe compound in aqueous and hydrocarbon media should be compared on an activity basis to account for compound interactions in the different solution phases (5, 6). Activitybased aqueous and nonaqueous phase sorption isotherms are depicted in the B panels of Figures 2-5. For acetophenone and nitrobenzene, the activity-based isotherms are essentially identical in the two different media (Figures 2B and 3B), thus showing no hydration-assisted sorption. In contrast, at any given solution activity, activity normalized m-nitrophenol sorption from aqueous solution is much greater than from n-hexadecane (Figure 5B), thus demonstrating strong hydration-assisted sorption. Depending on the compound activity region, hydration results in greater activity-based m-nitrophenol sorption by a factor of 4 to 8. Similar extents of hydration-assisted sorption effect have been previously observed for phenol and pyridine (5). For benzyl alcohol, an intermediate hydration assisted effect is observed (Figure 4B), where hydration results in greater sorption by about a factor of 4. In the figures, sorption data are presented per weight of dry peat. If sorption data would be presented per volume of swollen sorbent, the hydration effect would not be eliminated for either m-nitrophenol or benzyl alcohol. For example, following ref 6, sorption normalized to nhexadecane-swollen peat volume would be about 23% greater

FIGURE 5. Sorption of m-nitrophenol by Pahokee peat from aqueous (water, 0.1 N HCl) and saturated hydrocarbon (n-hexadecane, n-hexane) solutions. (A) Sorbed amount plotted against equilibrium solution concentration. (B) Sorbed amount plotted against compound activity. (C) Sorbed amount plotted against reduced concentration. than sorption normalized to weight, while compound sorption normalized to water-swollen peat volume would be about 15% less than sorption normalized to weight. Solute association in solution is not expected to affect the activity-based comparison. Most benzyl alcohol-n-hexadecane solutions are below the association region of alcohols in saturated hydrocarbons (i.e., approximately 0.01 mol/L (25-27)). Aggregation of nitrobenzene and acetophenone should also be insignificant, considering that the more polar acetonitrile (volume normalized dipole moment about twice that of nitrobenzene and acetophenone) was not aggregated in the full range of its solubility in n-hexadecane (up to 9000 uL/L (6)). m-Nitrophenol concentration in hydrocarbons does not exceed 140 mg/L and hence is below the phenol aggregation limit in a nonpolar tetrachloromethane (approximately 400 mg/L (28)) and in cyclohexane (29), and aggregation of substituted phenols is expected to be even weaker (28). For compound solutions in water, association is not expected because water is an effective hydrogen-bonding competitor that prevents aqueous association of polar organic compounds. To prove that m-nitrophenol association in aqueous and hydrocarbon solutions may have only a minor impact on the activity-based comparison, isotherms were also compared in terms of reduced concentration (Figure 5C; concentration normalized by solubility; aqueous solubility reported in Table 1; m-nitrophenol solubility in n-hexane found to be 200 ( 3 mg/L [n ) 12], and m-nitrophenol solubility in n-

hexadecane found to be 261 ( 10 mg/L [n ) 12]). It is clear from Figure 5C that aqueous m-nitrophenol sorption is greater than its sorption from n-hexadecane by a similar factor of 6-10 (depending on the reduced concentration region), thus supporting the comparison made in terms of compound activity (Figure 5B). Importantly, any unaccounted for solute-solute aggregation in aqueous solutions will lead to a shift of aqueous sorption isotherms to higher activities. Any hydration-assisted sorption would thus be underestimated if aggregation of solutes in aqueous solution exists. Effect of Hydrocarbon Phase. To explain the hydration effect on sorption of organic compounds, the influence of the hydrocarbon phase should be considered. Presence of the hydrocarbon phase may affect sorption data by (1) solvent sorption by NOM and (2) solvent competition with sorbate molecules for sorption sites. (1) Since sorption was determined by change in solution phase concentration, solvent sorption may mask the sorption of probe compounds. In such a case, the measured sorbate distribution coefficient Kd (in mL/g) differs from the true Kd by the value of solvent sorption (in mL per g of sorbent (6)). This correction accounts for any masking effect of solvent sorption on Kd (and, therefore, on sorbed concentrations). The hydrocarbon sorption effect on Kd has been previously evaluated to be 10% for Kd 0.5 and negligible for Kd values above unity (6). In this study, the lowest distribution coefficients in hydrocarbon systems were 30 for m-nitrophenol, 1.6 for benzyl alcohol, 0.7 for acetophenone, and 0.4 for nitrobenzene (Kd can be assessed from data in Table 2). Hence, any masking effect of hydrocarbon sorption is considered to be negligible. (2) There is little chance for significant competition between a saturated hydrocarbon and more polar compounds on a dry NOM sorbent. Linear sorption isotherms obtained for nonpolar compounds on dry peat up to activities of 0.6 (30) demonstrate that interactions of nonpolar compounds with dry NOM are quite far from saturating sorption sites (if any). In a previous study, we showed that activity-based sorption of pyridine on Pahokee peat from acetonitrile solution (0.9 vol %) in n-hexadecane with an acetonitrile activity of 0.9 (and n-hexadecane activity of 1) is very similar to activity-based sorption of pyridine by this NOM from pure acetonitrile (unit acetonitrile activity), thus indicating no effect of n-hexadecane on pyridine sorption (6). Also, when comparing sorption of m-nitrophenol from n-hexadecane and n-hexane using reduced concentration (Figure 5C), within scattering, no difference in m-nitrophenol sorption isotherms from the two different hydrocarbons is seen. Due to large differences in molar volume of n-hexane and n-hexadecane (130.6 and 292.8 cm3/mol, respectively), it is expected that they have significantly different diffusivities in dry NOM. The similarity observed for m-nitrophenol reduced concentration-based sorption from n-hexadecane and n-hexane (Figure 5C) suggests that the inert solvent has no effect on m-nitrophenol sorption. Considering all the above, it is not believed that n-hexadecane can affect the sorption of probe compounds through sorption, competition, swelling, or shrinkage of the NOM phase or by altering the ionization status of functional groups. Hydration Effect on Sorption of Organic Compounds with Increasing Ability To Undergo Sorbate-NOM Interactions. Of the four study compounds, m-nitrophenol exhibits the greatest hydration-assisted sorption effect (4-8 times), benzyl alcohol shows an intermediate effect (4 times), and acetophenone and nitrobenzene show no hydrationassisted sorption effect. The extent of the hydration-assisted sorption effect correlates with compound ability to undergo specific interactions (∆log HOC), as defined in ref 9 and tabulated in Table 1. VOL. 36, NO. 21, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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By accounting for compound activity in the solution phase, the difference in sorption of acetophenone and nitrobenzene from aqueous systems versus nonaqueous systems disappears (Figures 2B and 3B). It has also been shown by others that activity-based or reduced concentration-based isotherms for nonpolar compounds on natural organic matter are usually similar in aqueous and water-free systems (e.g., gas phase (30); n-hexane (31)). Therefore, it is worth emphasizing that, for many compounds, the changes in NOM sorbent structure that accompany hydration (e.g., increased flexibility, change in ionization status of polar functional groups; conformational reorientation of macromolecules) do not significantly affect organic compound sorption by NOM. In contrast, m-nitrophenol and benzyl alcohol show clear hydration-assisted sorption (Figures 4B and 5B). Such hydration-assisted sorption conforms to the hydrationassisted sorption observed for other strongly specifically interacting compounds, phenol and pyridine (5), and to solvent-assisted sorption seen for pyridine in acetonitrile, acetonitrile-n-hexadecane mixtures, and acetone-n-hexadecane mixtures (6). Formation of bound residues through creation of covalent bonds between m-nitrophenol (or benzyl alcohol) and NOM functional groups is ruled out by results of recovery experiments (summarized in Table S1, Supporting Information). m-Nitrophenol and benzyl alcohol sorbed by NOM from water at different initial compound concentrations were essentially fully recoverable by extraction into water and acetonitrile. It may be suggested that formation of negatively charged sorption sites in NOM during aqueous ionization of carboxylic groups is the reason for hydration-assisted sorption. However, when we suppressed NOM carboxylic group ionization by sorbing m-nitrophenol from an aqueous solution at pH 1.4 (0.1 N HCl), the reduced concentration-based sorption of m-nitrophenol was found to be the same as sorption from aqueous solutions at pH ) 5.7 (Figure 5C; m-nitrophenol solubility in 0.1 N HCl found to be 15360 ( 1300 mg/L [n ) 4]). Similarly, a change in NOM polarity (i.e., NOM bulk dielectric constant) is not believed to be a major mechanism responsible for the hydration-assisted sorption effect observed for m-nitrophenol or benzyl alcohol. We previously demonstrated that solvent-assisted sorption by NOM (Pahokee peat) may occur for a polar sorbate, pyridine, in organic solvent media where no ionization of NOM carboxylic groups and no significant change of the NOM polarity are expected (i.e., in acetonitrile and n-hexadecane mixtures with acetonitrile and acetone (6)). Activity-normalized pyridine sorption by NOM was found to be comparable in water, acetonitrile, and acetonitrile-saturated (0.9% v/v) n-hexadecane (media with essentially different dielectric constants (6)). According to the conceptual model, if a sorbate has a low ability to undergo specific interactions, it will not be able to compete with water molecules for a newly solvated NOM site (swollen polar contact). Therefore, if sorption of compounds capable of strong specific interactions is assisted by hydration, but not for compounds of lesser interaction ability, this will clearly demonstrate the importance of swollen NOM polar contacts in hydration-assisted sorption. This is indeed what has been found. Hydration-assisted sorption is clearly expressed for compounds capable of strong specific interactions (m-nitrophenol, benzyl alcohol) and not for compounds of lesser ability (acetophenone, nitrobenzene). These results indicate the importance of polar NOM noncovalent links in organization of the NOM phase and in controlling the hydration effect on sorption of organic compounds. Together with our previous studies demonstrating hydration and solvation-assisted sorption for other probe molecules (5, 6), development of a conceptual model for explaining this effect, and derivation of a sorption isotherm based on this model (7, 8), this study helps to relate between organic 4576

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compound structure, compound ability to interact with functional groups in NOM, and the resultant hydration effect on sorption. As NOM is a very important environmental sorbent for organic compounds in hydrated systems, these results, and the conceptual model for NOM hydration effect on organic compound sorption, are relevant for multiple environmental situations.

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 (Institute of Soil, Water and Environmental Sciences, The Volcani Center, Israel) in carrying out the experimental work is greatly appreciated. Comments by the three reviewers are greatly appreciated.

Supporting Information Available Compound recoveries for the aqueous sorption experiments (Table S1) and sorption kinetics for acetophenone and benzyl alcohol from aqueous solutions (Figure S1). This material is available free of charge via the Internet at http://pubs.acs.org.

Literature Cited (1) Hayes, M. H. B. In Humic substances in soil, sediment and water; Aiken, G. R., et al., Eds.; John Wiley & Sons: New York, 1985; pp 329-362. (2) Swift, R.S. In Humic Substances, II in Search of Structure; Hayes, M. H. B., et al., Eds.; A Wiley-Interscience Publication, John Wiley and Sons: Chichester, 1989; pp 450-465. (3) Chen, Y.; Schnitzer, M. In Humic Substances, II in Search of Structure; Hayes, M. H. B., et al., Eds.; A Wiley-Interscience Publication, John Wiley and Sons: Chichester, 1989; pp 621638. (4) Clapp, C. E.; Hayes, M. H. B.; Swift, R. S. In Organic Substances in Soil and Water: Natural Constituents and Their Influences On Contaminant Behavior; Beck, A. J., Jones, K. C., Hayes, M. H. B., Mingelgrin, U., Eds.; Royal Society of Chemistry: London, 1993; pp 33-37. (5) Graber, E. R.; Borisover, M. D. Environ. Sci. Technol. 1998, 32, 258. (6) Borisover, M.; Reddy, M.; Graber, E. R. Environ. Sci. Technol. 2001, 35, 2518. (7) Borisover, M.; Graber, E. R. Langmuir 2002 18, 4775. (8) Borisover, M.; Graber, E. R. Extended Abstracts; Meeting of International Humic Substance Society (IHSS-11), Boston, MA, 2002. (9) Borisover, M. D.; Graber, E. R. Chemosphere 1997, 34, 1761. (10) The Merck Index - An Encyclopedia of Chemicals, Drugs, and Biologicals; Budavari, S., Ed.; Merck and Co., Inc.: Whitehouse Station, NJ, 1996. (11) Gerstl, Z. J. Contam. Hydrol. 1990, 6, 357. (12) Abraham, M. H.; Andonian-Haftvan, J.; Whiting, G. S.; Leo, A.; Taft, R. S. J. Chem. Soc., Perkin Trans. 2 1994, 1777. (13) Daubert, T. E.; Danner, R. P. Data compilation tables of properties of pure compounds; American Institute of Chemical Engineers: 1985. (14) Grayson, B. T.; Fosbraey, L. A. Pestic. Sci. 1982, 13, 269. (15) CRC Handbook of Chemistry and Physics, 76th ed.; Lide, D. R., Ed.; CRC Press Inc.: Boca Raton, FL, 1995-1996. (16) Serjeant, E. P.; Dempsey, B. Ionization constants of organic acids in aqueous solution; IUPAC chemical data series N23; Pergamon Press: New York, 1979; p 989. (17) Crank, J. The Mathematics of Diffusion, 2nd ed.; Clarendon Press: Oxford, 1975. (18) Graber, E. R.; Borisover, M. Environ. Sci. Technol. 1999, 33, 2831. (19) Weber, W. J., Jr.; Huang, W. Environ. Sci. Technol. 1996, 30, 881. (20) Huang, W.; Weber, W. J., Jr. Environ. Sci. Technol. 1998, 32, 3549. (21) Pignatello, J.; Xing, B. Environ. Sci. Technol. 1999, 33, 2837. (22) Borisover, M.; Graber, E. R. Environ. Sci. Technol. 1999, 33, 2839. (23) Graber, E. R.; Borisover, M. Environ. Sci. Technol. 1998, 32, 3286. (24) Braida, W. J.; White, J. C.; Ferrandino, F. J.; Pignatello, J. J. Environ. Sci. Technol. 2001, 35, 2765. (25) Tucker, E. E.; Christian, S. D. J. Phys. Chem. 1977, 81, 1295.

(26) Stokes R. H.; Adamson, M. J. Chem. Soc., Faraday Trans. 1 1977, 73, 1232. (27) Asprion, N.; Hasse, H.; Maurer, G. Fluid Phase Equilib. 2001, 186, 1. (28) Arnett, E. M.; Joris, L.; Mitchell, E.; Murty, T. S. S. R.; Gorrie, T. M.; Schleyer, P. v. R. J. Am. Chem. Soc. 1970, 92, 2365. (29) Lin, L.-N.; Christian, S. D.; Tucker, E. E. J. Phys. Chem. 1978, 82, 1897.

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Received for review May 3, 2002. Revised manuscript received August 5, 2002. Accepted August 21, 2002. ES0207192

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