Environ. Sci. Technol. 2001, 35, 84-94
Detailed Sorption Isotherms of Polar and Apolar Compounds in a High-Organic Soil GUOSHOU XIA AND JOSEPH J. PIGNATELLO* Department of Soil and Water, The Connecticut Agricultural Experiment Station, 123 Huntington Street, P.O. Box 1106, New Haven, Connecticut 06504
Sorption isotherms of 13 apolar liquids and solids and polar solidsssix in unprecedented detailsare used to evaluate a polymer-based model for natural organic matter. While all isotherms are nonlinear, the “running” Freundlich exponent n varies markedly with concentration. The isotherms show linear-scale inflection consistent with the presence of flexible (deformable) porosity as predicted by the glassy polymer-based Extended Dual-Mode Model (EDMM). The EDMM assumes dissolution and holefilling domains in the organic solid, with provision for sorbatecaused plasticization of the solid and “melting” of the holes. Features of the EDMM are illustrated for chlorinated benzenes in poly(vinyl chloride). The solutes fall into categories of “hard” (aliphatics and 2,4-dichlorophenol) and “soft” (chlorinated benzenes, 2-chloronitrobenzene) according to their ability to plasticize organic matter. Comparison of domain coefficients at infinite dilution reveals that organic solutes have a modestly greater affinity for holes than dissolution sites (by 0.1-0.6 log unit), as expected by the polymer model. Sorption of CHCl3 shows timedependent hysteresis diminished at high concentrations by the plasticizing effect. Sorption of CHCl3 also shows a type of hysteresis for glassy solids known as the “conditioning effect” in which high loading of sorbate increases hole population upon its removal and thus leads to enhanced uptake and nonlinearity when a second sorption is performed. A Polanyi-based, fixed-pore filling model applied to the adsorption component of the isotherms gave widely variant volumetric pore capacity, contrary to its own stipulations, and could not explain the hysteresis.
Introduction Sorption of hydrophobic organic compounds (HOCs) to “natural organic matter” (NOM) is often assumed to occur by partition. The partition model regards NOM as a threedimensional macromolecular phase and attributes sorption to solid-phase dissolution, analogous to dissolution in liquids (1-3). Although useful in many contexts as a simplification, the partition model is undergoing re-evaluation prompted by findings of contrary (nonideal) behaviors, such as nonlinearity (e.g., refs 4-8), competitive effects (e.g., refs 4, 5, and 9-11), sorption-desorption hysteresis (e.g., refs 8 and 12-18), and recognition (19-21) that soils may contain various carbonaceous materials that may behave differently * Corresponding author phone: (203)974-8518; fax: (203)974-8502; e-mail:
[email protected]. 84
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as sorbents: ordinary humic substances, diagenic substances such as kerogen and low-rank coals, catagenic substances such as hard coals, and black carbon (soot and charcoal). Nonideal behaviors signify the presence of pores that act as condensation sites. The question is, what is the nature of these pores? Considering the possible components making up the organic carbon fraction, such pores can be fixed or flexible and potentially distinguishable on this basis. Black carbon and hard coals are low H/C materials composed of disordered (crumpled) micrographitic (polyaromatic) sheets (22, 23). Solute association occurs by adsorption and pore-filling. Since altering the pore structure of charcoal requires breakage of covalent bonds (24), such materials must be regarded as fixed-pore solids whose pores are little affected by temperature and undeformed by the adsorbate. Several studies (7, 19, 20, 25, 26) have attributed nonlinearity and competitive sorption in soil to the presence of small amounts of high surface area carbonaceous materials such as these. By contrast, humic substances (fulvic acid, humic acid, and humin, often bound to remnant biomolecules) (27, 28), unstructured kerogens (29, 30), and low rank coals (29, 31, 32) are macromolecular solids with flexible pores. Their macromolecules contain highly functionalized aliphatic, alicyclic, aromatic, and heteroaromatic backbones crosslinked by metal ions or covalent bonds. Ring clusters tend to be smaller than tricyclic (27, 28, 31). While coals are highly cross-linked, NMR studies show they contain an abundance of mobile nuclei (33). Natural macromolecular solids are capable of absorbing small molecules into their matrix and swelling as a result (31, 32, 34, 35). Amorphous macromolecular solids characteristically undergo phase transitions between “glassy” and “rubbery” states at a specific glass transition temperature Tg. These states are distinguished by macromolecular mobility. Humic molecules have features that favor high Tg in polymers, especially crosslinking, rings, and unsaturated bonds that promote chain stiffness. Measured Tg values of some relevant natural substances are wet and dry humic acid, 43-72 °C (36, 37); wet and dry forest soils, 77-79 °C (38); coals, >300 °C (39); and wet lignin (a major precursor of terrestrial humics), 6090 °C (40). While the rubbery state behaves like a partition sorbent, the glassy state, due to its rigidity, possesses free volume not filled by macromolecules. These semi-permanent internal nanoscale voids (holes) can serve as adsorption sites (4, 5, 41-46). In the context of NOM, the holes may be regarded as voids within the folds of single macromolecules, between macromolecules, or between the organic phase and the anchored mineral surface. Polymers (46), soil organic matter (5, 47), and coals (35, 48) have high internal surface area associated with the holes. The holes in glassy solids are considered flexible because the penetrant may deform the pore it interacts with and because the solid is capable of relaxing with heat or through plasticization by the penetrant. Fixed-pore micrographitic and flexible-pore macromolecular substances may coexist in some soils. One way to distinguish them is by isothermal penetrant-induced changes in sorption behavior. We show here evidence for the presence of flexible porosity in NOM due to the glassy character of some components. This study examined sorption of HOCs to Pahokee soil, a reference standard from the International Humic Substances Society that sorbs various HOCs nonideally (4, 5, 10). The soil contains 93% NOM (6.9% ash). Its elemental composition is humic substance-like (H/C, 1.27; O/C, ∼0.6-0.7; N/C, 0.059) (5). Gas isotherms (5) show low 10.1021/es001320l CCC: $20.00
2001 American Chemical Society Published on Web 11/14/2000
external surface area (0.90 m2/g) but high internal microporosity, reflecting its glassy character. We first attempted to see whether sorption was consistent with fixed porosity in the sample. Finding it was not, we then attempted to determine whether features of a polymer model, including hysteresis, could be seen in the isotherms.
Experimental Section Materials. The Florida Pahokee soil (6% moisture) is described elsewhere (5). Solutes were from Aldrich Chemical Co. and were >99% pure. [14C]Benzene was from Sigma Chemical Co. Solutes and their abbreviations are listed in Table 1. They are grouped as apolar liquids, apolar solids, and polar solids. The feature distinguishing “polar” compounds is an ability to form a H-bond competitive with water. CNB is an H-bond acceptor, while DCP is amphoteric. Relevant solute properties are given in Table S1 in the Supporting Information. Poly(vinyl choride) (PVC) of nominal particle size 200 µm was from Aldrich. It was ground to 1-5 µm in a cryogenic vibratory ball mill at 77 K. Sorption Isotherms. Vessels were 10-mL glass centrifuge tubes with aluminum foil layered between the contents and the PTFE liner (7). About 20 mg (for TCB) to 2 g (for Benz) of soil on a dry-weight basis was added, followed by distilleddeionized water containing 0.005 M CaCl2 and 200 mg/L NaN3 (hereafter, synthetic groundwater) to almost fill the tube (12 mL). The soil to water ratio was adjusted so that 30-70% sorption was achieved. After 12-16 h of prewetting, solutes were added via methanol carrier, keeping the methanol mole fraction in the range of 0.00009-0.0004. Although methanol content varied slightly over an isotherm, no effect on bulk solution properties, NOM structure, and magnitude of solute sorption is expected at such low levels. The tubes were mixed end-over-end at 6 rpm for 10 days (except 7 or 21 day for TCM) in an incubator at 22 ( 1 °C. TCM isotherms at 7 and 21 day were almost identical (vide infra). TCBs isotherms at 10, 30, and 210 day were little different. While equilibrium is always difficult to guarantee, we believe continued sorption beyond 10 day would not have affected isotherm shape. A similar protocol as the soil was followed for PVC except sorption time was 14 day for 1,2-DCB (isotherm was unchanged after 60 day) and 18 day for 1,2,4-TCB. After being equilibrated, the tubes were centrifuged at 350g for 30 min. For Benz, 2 mL of supernatant was assayed for radioactivity. For the others, an aliquot was extracted with CH2Cl2 or hexane, using (except for DBP) internal standards of naphthalene or DBP, respectively. The CH2Cl2 extracts were analyzed by GC/FID and a DB-5 capillary column; the hexane extracts were analyzed by GC/ECD and a DB-624 capillary column (J&W Scientific, Folsom, CA). Each experiment included 12-36 calibration standards over the test concentration range. Since the range covered many orders of magnitude, the ECD response was generally nonlinear, and thus calibration curves had to be fitted to a power law. Adsorbed mass was calculated by difference between total and solution-phase mass, correcting for solute loss to headspace and vessel. Solute loss, always
