Environ. Sci. Technol. 2004, 38, 4340-4348
Importance of Adsorption (Hole-Filling) Mechanism for Hydrophobic Organic Contaminants on an Aquifer Kerogen Isolate Y O N G R A N , * ,† B A O S H A N X I N G , ‡ P. SURESH C. RAO,§ AND JIAMO FU† State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Wushan, Guangzhou 510640, PR China, Department of Plant and Soil Science, University of Massachusetts, Amherst, Massachusetts 01003-0960, and School of Civil Engineering, Purdue University, West Lafayette, Indiana 47907-1284
Sorption and desorption behaviors of four hydrophobic organic compounds (HOCs) were investigated for an isolated kerogen material from Borden aquifer material with total organic carbon of 0.021%. The solubility-normalized modified Freundlich equation and the combined linear and PolanyiDubinin (PD) equation can quite well describe the sorption or desorption isotherms. The partition component is estimated and compared using desorption data, dualmode modeling, and the reported partition coefficients. The result suggests that the dual-mode modeling and the combined linear and PD modeling may overestimate the partitioning component. The partition component is not so important as assumed before in sorption of HOCs for the studied sorbent. As the fitted PD equation has an exponent parameter b′ approaching 1, it is equivalent to the modified Freundlich equation. The small molecules 1,2dichlorobenzene (DCB) and naphthalene (Naph) have higher adsorption volumes. The lower adsorption volumes for 1,3,5-trichlorobenzene (TCB) and phenanthrene (Phen) suggest that accessibility to the holes of kerogen by large HOC molecules is reduced. The desorption hysteresis is approximately constant for DCB when the relative aqueous concentration ranges from 0.0007 to 0.6, but for Phen is only obvious at higher relative aqueous concentrations. The varied sorption and desorption behaviors for DCB and Phen are satisfactorily explained by an adsorption/ hole filling mechanism and entrapment of some adsorbates in the kerogen matrix and by possible pore deformation mechanism at high concentrations.
1. Introduction Sorption and desorption by soils and sediments affect chemical and biological reactivity, transport, and risks of toxic chemicals in aquatic and terrestrial environments. Natural organic matter (NOM) associated with soils and sediments is the dominant constituent for sorption, seques* Corresponding author phone: 86-20-85290263; fax: 86-2085290706; e-mail:
[email protected]. † Chinese Academy of Sciences. ‡ University of Massachusetts. § Purdue University. 4340
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 16, 2004
tration, and attenuation of relatively hydrophobic organic chemicals (HOCs) (1-5). The chemical heterogeneity of NOM also affects the rates and equilibria of sorption and desorption of HOCs in soils and sediments (4, 6-7). Depending on parental sources and diagenetic alteration histories, NOM may comprise different types of organic materials ranging from biopolymer, humus, kerogen and coal materials, and black carbon. The stepwise disappearance of CdO and the formation of water and carbon dioxide result in a decrease in O/C during dia- and catagenesis. As alteration continues, the disappearance of saturated H-C functions causes a decrease in aliphatic content and a rise in aromaticity (8, 9). After primary deposition, kerogen may be eroded and redeposited within more recent, geologically younger sediments. Some investigators proved this reworked kerogen to be the dominant form of NOM in several sediments and an aquifer (2, 10-12). A concept of “soft carbon” (or expanded, rubbery state) vs “hard carbon” (or condensed, glassy state) NOM has been invoked to operationally delineate chemical heterogeneity of NOM and to elucidate the mechanisms for sorption by soils and sediments (2, 3, 13-17). For humic acid, alkylaromatics form the basic skeleton that are linked covalently to aliphatic chains. So, it may have a spongelike or membraneand micelle-like structure (18, 19). The highly oxidized carbons in humic acid should be easily hydrated in aqueous solution and therefore most likely to exist in a highly expanded or rubbery state. The reduced aromatic and aliphatic carbons are less likely hydrated in aqueous solution and hence remain in a relatively condensed or even glassy state. During diaand catagenesis, the NOM condensation (cross-linkage) increases along with increases in the corresponding glass transition temperature (Tg) of associated NOMs. In analogy to glassy polymer, the hard carbon or condensed organic matter domain would exhibit some combination of sorption behaviors involving linear partitioning and nonlinear intramatrix, hole-filling retention. In contrast, the soft carbon or expanded organic matter domain would exhibit partitioning behavior associated with linear local isotherms, rapid diffusion, no competition for sorption, and sorption reversibility (3, 4, 20). NOM may also exhibit transition from a condensed state to a loosely knit rubbery state as temperature or solute concentration is elevated to a given level. Prior investigations reveal the importance of microporosity and geometric heterogeneity in NOM and their role in the HOCs sorption. For a polymer, holes and pores can be interexchangeable. The CO2 and Ar sorption indicates that NOM has appreciable internal microporosity, and the majority of the NOM surface area is formed by micropores and submicropores with maximum restrictions of approximately 0.3-0.5 nm (21-23). The hole-filling mechanism can account for at least 45-68% and 37-50% of total sorption at up to 0.17 and 0.1-0.35 of Ce/Sw, respectively, for 1,3-DCB and atrazine on the Pahokee peat and a mineral soil (16, 17). The sorption volumes of the small solutes DCB and Naph on the Pahokee peat are respectively the same as the CO2 determined micropore volume and higher than those of the large solutes Phen and TCB (24). Xia and Ball (25) recently introduced the combination of partitioning (linear isotherm) and pore-filling (adsorption) to fit sorption isotherms of nine HOCs in an aquifer. They suggested that the adsorption contribution was only important at low relative concentration. It was noted that their model fits might not provide a sensitive measure of maximum adsorption capacity, especially for HOCs with the Freundlich parameter n approaching to 1. Moreover, Kleineidam et al. (26) assumed that the organic 10.1021/es035168+ CCC: $27.50
2004 American Chemical Society Published on Web 07/09/2004
carbons of a peat and three types of coals deducted from soot carbon remained at 375 °C are available for partitioning and found that the adsorption volumes of several HOCs are related to the microporosity measured by nitrogen gas. Furthermore, surface (geometric) heterogeneity has an important influence on HOC adsorption on fractal surfaces such as NOM (27, 28). The Langmuir-derived model for HOC sorption onto/into fractal sorbents may provide a consistent and mechanistic explanation for both adsorption and partitioning processes. Hence, accurate estimation of adsorption and partition components is critical to delineating different sorption mechanisms. A different maturation degree of kerogen and different maceral groups in condensed NOM may have various microporosities. Transmission electric microscopy illustrated that pore diameters in exinite and inertinite of coal ranged from 2 to 100 nm, with most of the pores varying from 2 to 50 nm, but those in vitrinite were smaller than 2 nm (29). In kerogen samples at low and medium degree of catagenesis, the interlayer spacings are widely spread (0.34-more than 0.8 nm), probably due to the fixation of nonaromatic groups on the graphitic layer which prevent them from getting closer. As catagenesis progresses, the nonaromatic groups begin to disappear and the interlayer space spreading decreases (30). We hypothesized that the condensed NOM with varied micropore size distribution may exhibit different sorption and desorption behaviors for different molecular-size HOCs. We used the dispersive kerogen isolated from the Borden aquifer as sorbent and four HOCs as sorbates and the solubility-normalized Freundlich equation and the DubininPolanyi equation to test this hypothesis.
2. Experimental Section 2.1. Sorbents. The dispersive kerogen isolate containing 0.736% OC was separated from the Borden sand using an acid (HCl-HF) digestion procedure. The 13C NMR spectrum indicated that its concentrated fraction (5.56% OC) consists of aliphatic and aromatic carbons (53.4% and 46.6%, respectively), typical of diagenetically altered and chemically reduced kerogen material (11). Examinations under fluorescence microscopy and optical microscopy showed that the isolated kerogen is highly heterogeneous at the particle scale (11). The organic particles are irregularly shaped and have sizes ranging from submicron to about 40 µm; the majority of the particles have sizes of 1-5 µm. Both vitrinite and bituminite or liptinite are the dominant macerals, and fusinite is a minor component. Vitrinite reflectance (Ro) for the vitrinite and bitumen in the isolated NOM ranges from 0.66% to 0.82% and from 0.1 to 0.23%, indicating a relatively low-maturation kerogen. 2.2. Sorption and Desorption Experiments. Phen and DCB, obtained in spectrophotometric grade (>98%) from Aldrich Chemical Co., were used as the HOC probe solutes for this study. Some of the physicochemical properties for DCB, Phen, TCB, and Naph were reported in the prior publications (11, 12) and listed in Table S1 (Supporting Information). Primary stock solutions, stock solutions, background electrolyte solutions, and initial aqueous solutions were prepared following the procedure of Weber and Huang (7). CaCl2 at a level of 0.005 M was the major mineral constituent of the background solution, and 100 mg/L of NaN3 was added to inhibit biological activity. To adjust the solution pH between 7 and 8, NaHCO3 (5 mg/L) was added. Solute concentrations of the initial aqueous and equilibrated aqueous solutions were analyzed on reverse phase HPLC (Hewlett-Packard model 1100; ODS, 5 µm, 2.1 × 250 mm C-18 column) with both diode array UV and fluorescence detector following procedures described in ref 24 for the four HOC solutes. The detection limits of the methods for
Phen, Naph, TCB, and DCB are 0.5, 5, 2, and 20 µg/L, respectively. Sorption and desorption isotherms were measured at 23 ( 2 °C using batch systems and following a withdraw-refill procedure described in prior studies (12, 20). Two different experimental protocols (the method of consecutive desorption, MCD, and the method of repeated addition, MRA) were used to investigate sorption and desorption behaviors of Phen and DCB (31, 32). The completely mixed batch reactors (CMBRs) consisted of flame-sealed glass ampules (10 and 20 mL). Each reactor, containing a given amount of sorbent, was filled with initial aqueous solution up to the shoulder and immediately flame-sealed. The reactors were mixed continuously in a rotary set at 125 rpm on a horizontal mode. The long-term experiments over 4 months for the sorption and over 2 months for the desorption showed that solidsolution contact times beyond 7 days were sufficient for attainment of sorption and desorption equilibrium. The desorption procedure employed nearly constant dilution factors (98.5 ( 0.68%) for all of the sorbate-sorbent systems. The sorbed concentration errors caused by the residual solutions were deducted according to the residual solution weights and the solute concentrations in the residual solutions. Volatilization losses during sorption and desorption were minimized by performing the operation quickly (
