Environ. Sci. Technol. 1997, 31, 792-799
Dual-Mode Sorption of Low-Polarity Compounds in Glassy Poly(Vinyl Chloride) and Soil Organic Matter BAOSHAN XING† AND JOSEPH J. PIGNATELLO* Department of Soil and Water, The Connecticut Agricultural Experiment Station, P.O. Box 1106, New Haven, Connecticut 06504
The widely accepted dissolution (partition) model of sorption to soil organic matter (SOM) has been challenged by evidence that SOM has a non-uniform sorption potential. This study presents data supporting a previously suggested alternative dual-mode model of sorption in which dissolution and hole-filling mechanisms take place concurrently, as in glassy organic polymers. The holes are postulated to be nanometer-size voids within the organic matrix that provide complexation sites. The main focus was on sorption of chlorobenzene, 1,2-dichlorobenzene, and 1,3-dichlorobenzene, but some experiments were carried out also on 2,4-dichlorophenol and the herbicide metolachlor. Sorption from water to high-organic soils, humic acid particles, and poly(vinyl chloride) is nonlinear, competitive, and predictably responsive to conditions that affect hole populations such as temperature and cosolvent addition. Sorption to a peat soil and its components became progressively nonlinear and competitive in the order humic acid, native peat, humin; this order reflects the increasing “glassy”si.e., rigid, condensedsnature of organic matter according to modern concepts of humic structure. Gas adsorption isotherms (N2 at 77 K and CO2 at 273 K) reveal the presence of internal microvoids accessible only by diffusion through the solid phase. The degree of nonlinearity and competition correlate with the CO2-measured microvoid volumes of the sorbents. The hole-filling mechanism is more important for the kinetically slow fraction.
Introduction Soil organic matter (SOM) consists of humic substances and partially decomposed biomass. When present above trace levels in particles, SOM is the predominant sorbent of hydrophobic organic compounds (HOCs). The mechanism of sorption to SOM has received a high level of attention because of its fundamental importance to HOC transport, bioavailability, and toxicity. In the early literature, the mechanism was often attributed to adsorption on the solid organic surface (1-3). However, there is ample reason to believe that penetration of the organic phase takes place and that a surface-adsorbed complex, if it forms at all, is merely transient to an absorbed state. Polymers, for example, characteristically are penetrable by small molecules (4, 5). Humic colloids have a Gaussian distribution of organic mass from the edge to the center due to high solvation of charged * 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.
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and polar groups at the interface (6), suggesting that under moist conditions the “surface” of SOM may, in fact, be indistinct. In the dry state, the external surface area of SOM available to N2 at 77 K is only ∼1 m2/g, which many believe is too low to account for the high affinity of organic matter for HOCs (7, 8). The modern concept of sorption to SOM is a solid-phase dissolution (partition) mechanism. The partition model has become well-entrenched in the literature and appears in textbooks (e.g., ref 9). The partition model employs the gelpolymer concept of SOM in which the material is viewed as a three-dimensional tangle of macromolecules that offers an organo-lipophilic phase, analogous to organic liquids, for escape of HOCs from the lipophobic environment of water (9-12). Sorption to a material of this nature is expected to be concentration-independent (linear) and noncompetitive. Evidence has been presented recently, however, suggesting that the partition model is simplistic. This evidence includes the following: (i) nonlinear isotherms, requiring that the activity coefficient of the “dissolved” compound inside SOM be concentration-dependent; (ii) competition in multisolute systems, implying site specificity; (iii) concentration-dependent heat of sorption, indicating that the sorption potential is inconstant; and (iv) evidence that SOM possesses an “internal surface” (13), as discussed later in the paper. To explain the results, we have proposed the dual-mode model (14,15), which has been used to describe sorption to glassy polymers (5, 16-20). Polymers are characterized on the basis of their internal structure as either glassy or rubbery. The glassy state is more condensed, and the polymer segments have higher cohesive forces than the rubbery state. Sorption of gases and organic molecules to the rubbery state occurs by dissolution, while sorption to the glassy state occurs by concurrent dissolution and hole-filling mechanisms. The holes are postulated to be local regions of unrelaxed free volume (voids) having molecular dimensions where one or a few sorbate molecules undergo an adsorption-like interaction with the internal surface. Specificity of sorption exists even in cases where directed forces between functional groups does not take placesfor example, vinyl chloride in poly(vinyl chloride) (PVC) (17), hexafluorobenzene in polystyrene (18), and gases like CO2 and C2H2 in various polymers (5, 20). Solidstate nuclear magnetic resonance (NMR) has shown the coexistence of relatively mobile and relatively immobile sorbed molecules in glassy polymers (18, 19). While SOM is not strictly a polymer because it has no repeating units, the polymer analogy is appropriate because of the macromolecular nature of SOM and has been used often in the literature. Humic substances have been described as having expanded and condensed regions (8, 21, 22), which may be analogous to rubbery and glassy states of polymers. According to the dual-mode model (14), total sorption to SOM is the sum of sorption in the dissolution domain S(D) and sorption in the hole-filling domain S(H) (eq 1): n
S ) S(D) + S(H) ) KpC +
Soi biC
∑1+bC i)1
(1)
i
Bearing in mind that SOM is a heterogeneous material, S(D) is given by a linear term in which C is the solute concentration and Kp is a lumped sorption coefficient representing all available dissolution regions; S(H) is given by a sum of Langmuir expressions representing multiple sites, where bi and Soi are the affinity and capacity constants, respectively, for each unique site. Despite widely-held beliefs, isotherms in soil that span several orders in concentration are often appreciable non-
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TABLE 1. Composition of Sorbentsa sorbent
C
H
N
ash
SiO2
Fe2O3
F
Pahokee peat soil whole soil (PPS) HF/HCl-treated whole soil (T-PPS) humic acid (PPS-HA) humin (PPS-humin) HF/HCl-treated humin (PPS-T-humin) Alberta humic acid (A-HA) Windsor humic acid (W-HA)
44.6 52.7 51.8 43.8 50.1 53.3 51.1
4.70 4.04 3.22 4.10 5.07 3.96 4.66
3.09 3.61 3.25 3.06 3.57 3.54 6.35
6.9 1.24 5.6 25.6 12.4 0.6 8.7
1.6 0.014 -
0.14 -
1.01 -
a
Percent of dry weight; (-) means not determined.
linear and obey the Freundlich equation:
S ) KFCN
(2)
where KF and N are constants and where N < 1. We have noted (14, this work) that some such curves fit the Freundlich and dual-mode models about equally well. This is not surprising, as it can be seen that each Langmuir term in eq 1 imparts curvature, and since the Freundlich isotherm and one consisting of multiple Langmuir terms are theoretically related (23, 24). The significance of this is that the Freundlich isotherm can be used as a surrogate for the dual-mode isotherm, which, despite being more mechanistic, is rather less practical because of the number of parameters. In particular, the value of N can be taken as an index of site energy distribution (24), as has been shown mathematically; i.e., the smaller the N, the broader the energy distribution and, by inference, the greater the contribution of the holefilling mechanism. The present study tests the dual-mode sorption model for SOM by comparing observed with expected behavior according to polymer theory and modern concepts of SOM structure. The compounds of main focus are chlorinated benzenes. Because such compounds are low polarity and thus limited in their intermolecular interactions to nonspecific dipolar/dispersion forces, they provide the most meaningful test of the dual-mode model vis a´ vis the partition model. The SOM sorbents all have >92% organic matter content.
Experimental Section Materials. Sorbent elemental composition performed by Galbraith Laboratories, Knoxville, TN, appears in Table 1. Florida Pahokee peat soil (PPS) is an International Humic Substance Society reference material (Department of Chemistry, Colorado School of Mines, Golden, CO). It is highly humified (25) and has no visible undecomposed plant matter. A subsample (T-PPS) had been de-ashed with 2.88 M HF/1.6 M HCl for 60 d, washed with deionized distilled water to remove the acids, and then freeze-dried (26). Pahokee peat humic acid (PPS-HA) was extracted from PPS with Na4P5O7 solution (27). Alberta humic acid (A-HA) was extracted by Na4P5O7 from a Mollisol from Alberta, Canada (27). Windsor humic acid (W-HA) was extracted by NaOH solution from a Merrimac sandy loam from Conecticut (28). The extracts were high-speed centrifuged at 8600g to remove undissolved material, and the HA was precipitated with HCl. In the case of W-HA, the precipitate was carried through two additional dissolution-centrifugation-precipitation steps. The precipitates were then washed repeatedly with distilled water (PPS-HA and A-HA) or dialyzed (W-HA) to remove chloride, freeze-dried, and crushed into e500 µm particles. The insoluble solid remaining after extraction of PPS is referred to as PPS-humin. A subsample of PPS-humin was de-ashed as above with HF/HCl and is referred to as PPS-T-humin. PVC microspheres were purchased from Aldrich and have a median diameter of 110 µm, a range of 60-150 µm, and a
specific surface area (N2 BET, 77 K) of 0.36 m2/g (characterized by Quantachrome Corp., Boynton Beach, FL). 1,2- Dichlorobenzene (1,2-DCB), 1,3-dichlorobenzene (1,3DCB), chlorobenzene (CB), and 2,4-dichlorophenol (DCP) were from Aldrich Chemical Co. [ring-UL-14C]-1,2-DCB and [ring-UL-14C]-DCP were from Sigma Chemical Co. [ring-UL14 C]Metolachlor (2-chloro-N-[2-ethyl-6-methylphenyl]-N-[2methoxy-1-methylethyl] acetamide) was a gift from CibaGeigy Corporation. Unlabeled metolachlor was obtained from Crescent Chemical Co. General Methodology for Solution-Solid Sorption Isotherms. Sorption was carried out in suspensions of particles in water containing 0.01 M CaCl2 and 200 µg/mL of either NaN3 or HgCl2 as bio-inhibitor. The suspensions were prepared in either 8-mL Teflon-lined screw cap vials or (for experiments longer than 4 d) in flame-sealed ampules. Details are given elsewhere (14, 15). Briefly, the solids concentration was adjusted to achieve 25-85% uptake of the principal solute. All samples in a series were compositionally identical except for solute concentration. Shaking was continuous for shortterm experiments and twice per day for long-term experiments. Prior to sampling, the vials were centrifuged at 850 g for 20 min unless noted otherwise. The biocide HgCl2 had no effect on 24-h isotherm parameters of metolachlor in PPS. Sorption to non-settling colloids was anticipated to have negligible effect on apparent aqueous concentrations of the test solutes due to their relatively low Kow values, PPS-humin (different at p < 0.01 all cases). Figure 2b shows that the competitive effect between 1,2- and 1,3-DCB follows the expected order: PPS-HA < PPS < PPS-humin.
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Determination of Internal Microvoid Volume of SOM and Its Correlation with Nonideal Sorption Behavior. The surface area of organic matter in soils customarily has been determined by calculating monolayer coverage of N2 from adsorption isotherms at 77 K using the Brunauer-EmmettTeller (BET) equation (7, 8). This method indicates that SOM has a very low apparent surface area. However, studies of coals, molecular sieves, and recently soils indicate that N2 diffusion through nanopores (2 nm wide) (35), and afforded BET surface areas
