Competitive Sorption between Atrazine and Other Organic

Jul 25, 1996 - No competitive effects were observed with either TCE or PR in rubbery polymers (Figure 5). For Cheshire soil (Figure 4), the co-solutes...
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Environ. Sci. Technol. 1996, 30, 2432-2440

Competitive Sorption between Atrazine and Other Organic Compounds in Soils and Model Sorbents BAOSHAN XING,† JOSEPH J. PIGNATELLO,* AND BARBARA GIGLIOTTI Department of Soil and Water, The Connecticut Agricultural Experiment Station, P.O. Box 1106, New Haven, Connecticut 06504

This is part of a larger study that addresses the question of whether site-specific sorption of organic compounds takes place in soil organic matter (SOM). Competitive sorption is one indication that such interactions may take place. Competitive sorption was tested between atrazine (AT) and other co-solutes in water suspensions of natural and model sorbents. The co-solutes included several s-triazine analogs, a substituted benzene analog (5-chloro-1,3-dimethoxybenzene), and a dissimilar compound, trichloroethene (TCE). The sorbents included a mineral soil (3% OM), a peat soil (93% OM), soil humic acid particles (99% OM), rubbery polymers (polyethylene, cellulose, chitin), a glassy polymer [poly(2,6-diphenyl-p-phenylene oxide)], and a mesoporous silica gel. The rubbery polymers afforded linear single-solute isotherms and no competition, both consistent with ideal (Henry’s law) partition sorption. The other sorbents, including the glassy polymer, gave nonlinear single-solute isotherms and significant competition between AT and its analogs and weak or no competition between AT and TCE. A thermodynamic model, ideal adsorbed solution theory (IAST), was incapable of consistently simulating competition or lack thereof. For the SOM-containing materials, the results indicate that, like glassy polymers, SOM is a dual-mode sorbent. Sorption occurs by a partition mechanism and a holefilling mechanism. The holes are conceptualized as specific sites inside the matrix where complexation follows the Langmuir isotherm and where a degree of specificity is exhibited. In the mineral and peat soils, from one-third to one-half of AT sorption occurs in the hole domain. Combined with previous data, it appears that dual-mode sorption in SOM is applicable to polar and nonpolar compounds alike. For silica, * Corresponding author voice: (203)-789-7237; fax: (203)-789-7232; e-mail address: [email protected]. † Present address, Department of Plant and Soil Sciences, University of Massachusetts, Amherst, MA 01003; e-mail address: bx@pssci. umass.edu.

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the results suggest that the spatial sorption domain of TCE is distinct (possibly further from the surface) than that of the s-triazines.

Introduction The predominant sorbent of neutral organic compounds in soil or sediment is normally the soil organic matter (SOM) fraction when it is present above trace levels (1-3). It is widely held that sorption to SOM occurs by a solid-phase dissolution (partition) mechanism (1, 2) akin to sorption in rubbery polymers (4-7) and analogous to dissolution in organic solvents. In the partition mechanism, the sorbate activity coefficient in the matrix under dilute conditions is concentration-independent, resulting in a linear isotherm and the absence of competition between co-solutes. However, recent studies of sorption of both low- and highpolarity compounds in soils have shown nonlinear isotherms (e.g., refs 8-11) and competitive effects (8, 12). Such results are inconsistent with the partition model if all sorption in the sorbents under investigation can be attributed to SOM. Nonlinearity and competition are often characteristic of sorption processes arising from site-specific interactions. Adsorption at the water-solid interface can vary from linear, when it occurs to an infinite surface of uniform potential, to nonlinear, when there are limitations in the number of available sites or if the surface has a heterogeneous adsorption potential. Nonlinearity occurs because the affinity for solute decreases progressively with increasing solute concentration as sites become filled. The Freundlich expression (eq 1) is often used to model sorption:

S ) KFCN

(1a)

log S ) log KF + Nlog C

(1b)

where S is the sorbed concentration (µg/g), C is the solutionphase concentration (µg/mL), and KF (µg/g)(µg/mL)-N and N (dimensionless) are constants. Although this expression is empirical, N reflects the curvature in the isotherm and may be taken to represent the energy distribution of adsorption sites (13, 14). Competition is the result of overlap in the set of sites that can be occupied by nonidentical solutes. There has been considerable debate over whether soil is a partition medium or an adsorption medium, or both (1, 8, 9, 11, 12, 14-18). Since natural particles usually contain organic and mineral components, partitioning to SOM may occur concurrent with adsorption at the waterorganic or water-mineral interfaces. As an alternative explanation for the anomalies, we are exploring the possibility that specific sorption occurs at preferred “internal sites” (12), analogous to a kind of sorption that takes place in glassy organic polymers (5, 6, 19-25). This idea evolved from experiments showing competitive sorption between halogenated hydrocarbons in a high-organic (93% OM) peat soil (12) and from considering sorption to SOM in the context of synthetic polymer/penetrant interactions (26). In order to explain nonlinearity, competition, and concentration-dependent heat of sorption of hydrophobic compounds in soils and shales, Weber and co-workers (8, 14, 18) introduced the distributed reactivity model, in which

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TABLE 1

Selected Properties of Sorbents and Sorbatesa

FIGURE 1. Structures of the test compounds.

a combination of partitioning and adsorption occurs, depending on the nature of SOM. They correlated nonideal behavior with the fraction of “hard” organic carbon defined as resistant to low temperature persulfate oxidation. They identified particles of ancient shale with higher organic carbon (OC) content and surface area that had greater affinity for the solute than bulk soil (14). In a recent paper (18) published while the present study was being completed, they proposed that sorption varies with the geologic age of OMsfrom partitioning in amorphous, “soft” humic materials to adsorption on increasingly condensed microcrystalline structures formed as diagenesis takes place. They also offered an analogy with crystalline synthetic polymers. In addition, Spurlock and Biggar (11) recently concluded based on the nonlinearity of phenylureas in mineral soils and thermodynamic reasoning that specific interactions in SOM contribute to overall sorption. All of these results are important because they suggest a multiple role for SOM that could have implications for pollutant transport and bioavailability. Nevertheless, the sorbent nature of SOM cannot be determined unambiguously from studies of low-organic soils due to the possible role of the minerals in the observed behavior. It is necessary to include materials that are substantially free of minerals. Additional insight is possible by studying model sorbents that represent the organic solid state. While some information is available on organic matter colloids, sorption in these materials is difficult to quantitate, and it is not clear to what degree colloids represent solid-phase SOM. This report will describe competitive sorption on natural and model sorbents. The natural sorbents include a sandy loam soil (3% OM), a high organic peat soil (93% OM), and a soil humic acid (HA) in particulate form (>98% OM). The model sorbents include rubbery hydrophobic and hydrophilic polymers, a glassy polymer, and a mesoporous amorphous silica as a model for hydroxylated mineral surfaces. The organic sorbents represent a gradation from native SOM in the presence of high and low amounts of minerals, to purified SOM, to homogeneous organic polymers. This study will look at competitive sorption between atrazine (AT) and co-solutes

sorbents

BET surface area (m2/g)

particle diam (mm)

glass transition temp, Tg (°C)

soil peat humic acid cellulose chitin polyethylene Tenaxe silica

5.43b 0.88 0.77 2.4c 5.1c soil (35%). Note in Figures 4-6 that competition generally is greatest at low co-solute concentration and levels off exponentially with increasing co-solute concentration. This rules out attribution of the competitive effect to changes in solute activity coefficient in water or changes in the bulk properties of the sorbent. Note also that a co-solute to solute molar ratio of 5 or 10 is required to give appreciable competition. Wang et al. (35) found no competition between AT and 2-hydroxy-4-ethylamino-6-isopropylamino-s-triazine, a derivative of AT in which OH is substituted for Cl. However, the ratio of ATOH to AT was only 0.5. The IAST model closely simulates competition between AT and PR, CY, or CDB (Figures 3-6). The absence of competition between AT and PR and between AT and TCE in the rubbery organic polymers also is in accord with IAST prediction (Figure 5). However, IAST greatly overpredicts competition between AT and CDsT (on soil) and between AT and TCE (on all sorbents). The meaning of these results is discussed below.

Discussion Silica. Sorption to silica is characterized by nonlinear isotherms and competition between AT and PR but no competition between AT and TCE. The KF values of AT,

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FIGURE 4. Competitive sorption between atrazine and co-solutes in Cheshire soil at constant atrazine concentration and varying co-solute concentration. Each point represents the mean of triplicate vials, and the error bar is the standard deviation.

FIGURE 5. Competitive sorption between atrazine and prometon or trichloroethene in six organic sorbents at constant atrazine concentration and varying co-solute concentration. Each point represents the mean of duplicate vials, and the error bar is the range.

PR, and TCE are comparable in magnitude. The IAST model predicts that both PR and TCE will be competitive. The large overprediction of TCE competition means that silica does not present the same “surface” to the solutessa violation of an underlying assumption of IAST. TCE must sorb at different sites or in different spatial domains than the triazines. We consider possible explanations for this. Although the triazines in contrast to TCE are ionizable, ion-exchange sorption is not likely the basis for differentia-

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tion of sorption domains. The pKa values of the triazines (Table 1) are several units lower than the solution pH of 6. Quinoline and 1-aminonaphthalene sorb predominantly in neutral form to porous silica when pH > pKa + 3 (37). Furthermore, AT sorbs in neutral form to smectites at pH 4.75-6.75 (37). The surface charge on silica can range from slightly attractive toward triazine cations (since pHzpc ∼2; 36) to even slightly repulsive if Ca2+s0.01 M heres complexes with siloxyl groups (as tSisOsCa+) (37). Nor

as competitive, however, since it assumes that they occupy the same sorption domain. Polymers. Polymers can be classified as rubbery or glassy on the basis of segment motion, void space, and cohesive forces. Rubbery polymers have a relatively expanded, flexible structure; while glassy polymers have a more rigid, condensed structure (6). Medium-density PE is rubbery above its glass transition temperature Tg of -125 °C (Table 1). Cellulose is rubbery in water due to extensive hydration (47). Chitin is structurally similar to cellulose (acetamino replaces hydroxyl at the 2-position on each ring) and is presumed to be rubbery in water. For Tenax, Tg ) 227 °C.

FIGURE 6. Competitive sorption between atrazine and prometon or trichloroethene on mesoporous silica at constant atrazine concentration and varying co-solute concentration. Each point represents the mean of duplicate vials, and the error bar is the range.

is pore structure likely to be the basis for differentiation. Although exclusion for steric or electronic reasons is possible in micropores ( 1. It has been shown (14) that a composite of several Langmuir isotherms can result in one that fits the Freundlich equation with N < 1. The same result can be obtained when a partition term is added. There can be little doubt that partitioning of the triazines takes place in SOM. This study demonstrates partitioning of triazines in polar and nonpolar rubbery polymers. The fact that the KF of AT is greater in nonpolar PE than in polar cellulose or chitin indicates that its hydrophobic nature is not a minor factor. SOM is a complex mixture of macromolecules of fulvic, humic, humin, and partially decomposed cellular substances. Almost nothing is known about its tertiery and quaternary structure (49). Judging from the properties of colloidal humic substances, the existence is likely of gel-like regions in SOM where segmental structure is highly solvated, expanded, flexible, and dynamic (51). Partitioning is consistent with the gel concept of SOM. Chiou (1) has drawn an analogy between amorphous SOM and rubbery polymers. Figure 3 shows that high concentrations of PR straighten out the isotherm of AT (i.e., drive N to unity), while shifting it downward (reduce KF). A reasonable explanation for the linearizing effect of PR is that it occupies and blocks AT hole sites, while leaving its partition domain unaffected. TCE had a similar linearizing effect on the isotherm of 1,2-dibromoethane in Pahokee peat (12). A trend in the same direction is apparent in other cases of competition in soil (4). Competition between CO2 and C2H4 or N2O in glassy poly(methyl methacrylate) obeyed the dualmode model with competition in the hole domain only (24). Nevertheless, partitioning cannot be the exclusive mechanism of AT sorption in SOM. Competitive effects are strong evidence for specific sorption. Figure 7 (upper) estimates the contributions of partition and Langmuir terms to the AT isotherm in peat, which is plotted on a linear scale. The data were fitted by nonlinear regresssion to the dual-mode equation (eq 4) using a single Langmuir term used to represent a composite “site”. (Multiple terms would be unjustified for modeling purposes.) An upper limit of Kp may be estimated from Figure 5 by assuming that PR affects

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FIGURE 7. Fit of the linear AT isotherm to the dual-mode model assuming a composite site. Kp was estimated from competitive effects and S° and b were adjustable parameters. Peat: Kp ) 39.4 mL/g; S° ) 161 µg/g; b ) 0.288 mL/µg. Cheshire soil: Kp ) 1.08 mL/g; S° ) 17.2 µg/g; b ) 0.064 mL/µg.

only the hole domain of AT. The equation thus has two adjustable parameters, S° and b. The results (Figure 7) show that hole sorption comprises at least 37-54% of total sorption, depending on concentration. Thus, a substantial fraction of AT sorption is specific, even though its isotherm does not deviate greatly from linearity (N ) 0.916). A reasonable inference is that the nonideal behaviors exhibited by Cheshire soil is due to the dual-mode properties of SOM rather than to adsorption on mineral surfaces. SOM is expected to dominate sorption since it comprises ∼3% of solids. Consistent with this, the surface area of silica is 100 times larger than Cheshire soil but their KF values are within a factor of 2. The single-solute isotherm parameters in soil follow similar trends as in peat and HA: N (PR e AT < TCE); KF (PR > AT > TCE). Fit of the AT isotherm in soil to the dual mode-model (Figure 7, lower) indicates that at least 37-50% of AT is sorbed in the hole domain. This range is almost identical to that of peat. Previous studies of triazine sorption are supportive of a dual-mode mechanism. Isotherms of AT in soil have been reported variously as linear or nonlinear, with N values as low as 0.71 (52; citations in ref 53). Isotherms of AT in humic acid, fulvic acid, and a Laurentian soil (∼6% OC) were Langmuir-shaped, suggesting specific sorption (34 and refs therein). Infrared spectroscopy (54-56) shows H-bond or proton-transfer complexation of s-triazines with humic substances, at least below pH 4. Sullivan and Felbeck (54) refluxed s-triazines with HA in 95% ethanol: a small

residual fraction left after solvent removal of free s-triazine appeared to bond with COOH and phenolic OH groups. In nonpolar solvent, AT forms weak to moderately strong H-bonds with model compounds containing NH2, OH, and COOH groups commonly found in SOM (57). Thus, H-bonding may play an important role in immobilization of AT at hole sites. However, it must be emphasized that formation of H-bonds is not incompatible with a partition mechanism and does not per se prove site-specific sorption. The mechanism of sorption of TCE and other lowpolarity compounds to SOM is usually attributed to partition (1). But certain facts support a dual-mode mechanism: (a) nonlinearity for TCE in Pahokee peat and HA (Table 2) and 1,3-dichlorobenzene in Pahokee peat (9)ssorbents that are practically all OM; (b) bisolute competition among TCE and other halogenated hydrocarbons in Pahokee peat (12) and soils (8, 12, 14). Dual-mode sorption of chlorinated benzenes in SOM will be presented in a separate paper. It follows that SOM acts as a dual-mode sorbent toward both polar (triazine) and apolar (halogenated hydrocarbon) compounds. The SOM sorbents also appear to show selectivity. The hole domains of TCE and the triazines are clearly different since competition between them is weak. This is not surprising given that they differ greatly in size and polarity and given the chemical heterogeneity of SOM (49-51). Their Langmuir domains appear to overlap poorly in Tenax, as well, which likely has few unique kinds of sites. The five triazine analogs varied in their competitive ability toward AT in soil (Figure 4). The relationship between structure and competitive effect is yet unclear from these results. The ability of IAST to predict competition was mixed. Assuming that all sorption takes place in SOM, the results support the idea that SOM contains a limited population of sites spanning a range of steric and electronic characteristics (26). This idea was offered earlier based on phenylurea sorption behavior (11). Each sorbate interacts with a subset of this population, dependent on its structure. Competition is related to the degree of overlap in the subsets. Hence, AT and TCE are only slightly competitive due to their contrasting stereoelectronic properties that cause them to bind to widely different subsets. Conversely, greater competition exists between AT and analogssor between TCE and halogenated hydrocarbons (12)sbecause structurally they are more alike. Although the nature of the Langmuir sites is unknown, it is evident that they are internal to the matrix rather than on the external SOM-bulk liquid interface. The isotherms of 1,3-dichlorobenzene, metolachlor, and 2,4-dichlorophenol on soils and Pahokee peat become increasingly nonlinear with equilibration time, indicating that sites accessible to bulk solution are less abundant (9). This is consistent with the existence of both rubbery and glassy regions in SOM where diffusion through the glassy regions that contain holes is slower. Diffusion is generally slower in glassy than in rubbery polymers (5). Molecular modeling of hypothetical humic macromolecules (50) demonstrates a network of condensed structures and voids that offer possible sites for the trapping of guest molecules. Furthermore, the N2 BET external surface areas of peat and HA are small (Table 1), consistent with the literature (58). In summary, sorption is nonlinear and competitive for a mesoporous silica, a glassy organic polymer, and several SOM sorbents, while sorption is linear and noncompetitive for rubbery polymers. On silica, TCE and the triazines have

sorption domains postulated to lie at different distances from the surface. Sorption in nearly mineral-free SOM materials (peat, HA particles) is consistent with dual-mode behavior previously determined for glassy polymers, in which a partition domain co-exists with an internal domain consisting of a population of holes. The holes are hypothesized to be distributed in energy and to discriminate on the basis of molecule structure. About one-third to onehalf of total AT sorption, depending on concentration, occurs in the hole domain of both the high OM peat and the low OM mineral soil. The similarities in sorption behavior between soil and peat or HA suggest that nonideal effects observed in soil can be attributed to dual mode sorption in SOM. The results also demonstrate the limitations of IAST when the potential exists for sorption of cosolutes in spatially different domains.

Acknowledgments This work was supported by the U.S. Department of Agriculture National Research Initiative. We thank Dominic Grasso for helpful comments. Author-Supplied Registry Numbers: Atrazine, 1912-24-9; CDB, 7051-16-3; CDsT, 3140-73-6; cyanazine, 21725-46-2; prometon, 1610-18-0; TCE, 79-01-6; cellulose, 9004-34-6; chitin, 1398-61-4; PE, 9002-88-4; peat, BS103P.

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Received for review May 24, 1995. Revised manuscript received April 4, 1996. Accepted April 16, 1996.X ES950350Z X

Abstract published in Advance ACS Abstracts, June 1, 1996.