Critical Review
Mechanisms of Slow Sorption of Organic Chemicals to Natural Particles JOSEPH J. PIGNATELLO* AND BAOSHAN XING Department of Soil and Water, The Connecticut Agricultural Experiment Station, P.O. Box 1106, New Haven, Connecticut 06504-1106
The use of equilibrium expressions for sorption to natural particles in fate and transport models is often invalid due to slow kinetics. This paper reviews recent research into the causes of slow sorption and desorption rates at the intraparticle level and how this phenomenon relates to contaminant transport, bioavailability, and remediation. Sorption kinetics are complex and poorly predictable at present. Diffusion limitations appear to play a major role. Contending mechanisms include diffusion through natural organic matter matrices and diffusion through intraparticle nanopores. These mechanisms probably operate simultaneously, but the relative importance of each in a given system is indeterminate. Sorption shows anomalous behaviors that are presently not well explained by the simple diffusion models, including concentration dependence of the slow fraction, distributed rate constants, and kinetic hysteresis. Research is needed to determine whether adsorption/desorption bond energies may play a role along with molecular diffusion in slow kinetics. The possible existence of high-energy adsorption sites both within the internal matrix of organic matter and in nanopores is discussed. Sorption can be rate-limiting to biodegradation, bioavailablity, and subsurface transport of contaminants. Characterization of mechanism is thus critical for fate and risk assessment. Studies are needed to measure desorption kinetics under digestive and respiratory conditions in receptor organisms. Conditions under which the constraint of slow desorption may be overcome are discussed, including the addition of biological or chemical agents, the application of heat, and the physical alteration of the soil.
Introduction Sorption to natural solids is an underlying process affecting the transport, degradation, and biological activity of organic compounds in the environment. Although often regarded as instantaneous for modeling purposes, sorption may in fact require weeks to many months to reach equilibrium. It was not until the mid to late 1980s that serious study of sorption kinetics in soils and sediments began, despite early circumstantial evidence going back to the 1960s that the natural degradation of certain pesticides in the field slowed or stopped after a while (1, 2). Sorption kinetics of contaminants on airborne particles has just recently received attention (3). Fate, transport, and risk assessment models all contain terms for sorption; therefore, an understanding of the dynamics of sorption is crucial to their success. Ignoring slow kinetics can lead to an underestimation of the true extent of sorption, false predictions about the mobility and bioavailability of contaminants, and perhaps the wrong choice of cleanup technology. Kinetics can also be an important mechanistic tool for understanding sorption itself. In this paper, we focus on updating our knowledge of the causes of slow sorption and desorption. In addition, we discuss its significance to bioavailability and the remediation of organic pollutants. Much of the research in this area has been carried out in batch systems where particles are suspended in a well-mixed aqueous solvent. Thus, we restrict discussion to phenomena occurring on the intraparticle scale, that is, within individual soil grains or within aggregates that are stable in water. We shall exclude transport-related nonequilibrium behavior (“physical nonequilibrium”), which may also play an important role in nonideal solute transport in the field and in some experimental column systems. Physical nonequilibrium is due to slow exchange of solute between mobile and less mobile water, such as may exist between particles or between zones of different hydraulic conductivities in the soil column, and occurs for sorbing and nonsorbing molecules alike. It can give rise to transport behavior (plume spreading, “tailing” of the solute curve, etc.) that looks much like sorption nonequilibrium. It is irrelevant * Corresponding author telephone: (203) 789-7237; fax: (203) 7897232; e-mail address:
[email protected].
0013-936X/96/0930-0001$12.00/0
1995 American Chemical Society
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TABLE 1
Recent Examples of Observed Slow Sorption or Desorption in Natural Sorbentsa Uptake contact period (d)
PCE in aquifer sand material TeCB in aquifer sand material pyrene in lake sediments phenanthrene in lake sediments picloram in various soils lindane in subsurface fine sand (corrected for abiotic hydrolysis) atrazine in soil metolachlor in peat metolachlor in soil 1,3-dichlorobenzene in peat 1,3-dichlorobenzene in soil
long
short
approx ratiob Kdapp(long)/Kdapp(short)
slow fractionb,c
ref
10 100 180 180 300 167
1 1 3 3 7 4.2
3 10 2 2 1.5-3.9 4
0.67 0.90 0.50 0.50 0.33-0.74 0.74
33 33 107 107 126 38
up to ∼0.3 0.22-0.33d 0.31-0.37d 0.14-0.39d 0.19-0.48d
127 55 55 55 55
22 30 30 30 30
1 1 1 1 1
1.4 1.6 1.3 1.4 Release sparging or leaching time
PCB-contaminated river sediments TCE-contaminated subsoil TCE-, PCE-, toluene-, xylene-contaminated soils atrazine-contaminated soil metolachlor-contaminated soil naphthalene-contaminated soils EDB-contaminated soil naphthalene-spiked soil (3-90 d contact) simazine-spiked soil naphthalene-spiked soil (1-, 7-, 30-d contact) phenanthrene-spiked soil (7-20-d contact) TCE-spiked soil (2.5-, 5.5-, 15.5-mo contact) PAHs on urban aerosols atrazine on soil (4-, 12-, or 24-d contact)
7-d continuous removal seven 1-d washings or 24 000 column PVe 14 washings over 7 d 70-d leaching at 1 PVe/d 70-d leaching at 1 PVe/d 3-d gas purge 10-d batch desorption 3-d gas purge 35-d in the field many 2-h to 7-d washings 10 washings over 178 d five 1-d washings 28 d (130 m3 of moist N2) six 6-d batch desorptions
remaining slow fractionb
ref
0.17-0.45 0.25-0.27 0.48-0.94 0.56 0.59 0.1-0.5 0.96 0.1-0.2 0.9 g0.6 0.62 0.10, 0.25,0.45 0.4-0.6 0.35-0.55
28 34 128 20 20 47 25 47 27 15 15 34 3 127
a PCE, tetrachloroethene; TeCB, 1,2,4,5-tetrachlorobenzene; picloram, 4-amino-3,5,6-trichloropicolinic acid; lindane, γ-1,2,3,4,5,6-hexachlorocyclohexane; PCB, polychlorobiphenyl congeners; EDB, 1,2-dibromoethane; TCE, trichloroethene; atrazine, 2-chloro-4-ethylamino-6-isopropylamino1,3,5-triazine; metolachlor, 2-chloro-N-[2-ethyl-6-methylphenyl]-N-[2-methoxyethyl]acetamide; simazine, 2-chloro-4,6-bis(ethylamino)-1,3,5-triazine]. b Listed as estimates from graphs and tables in original work and may be rounded. c Slow fraction ) 1 - K app(short)/K app(long). d Concentration d d dependent. e PV, column pore (void) volumes.
to bioavailability per se, except that microbial populations and/or activity may vary within the flow regime. Recent papers discussing physical nonequilibrium are available (4-9). We shall also exclude chemisorption involving covalent bonds as well as “bound residue” formation, which is defined as any organic carbon remaining after exhaustive extraction that results from degradation of the parent molecule. It is safe to say that the mechanisms governing sorption rates are not fully established. Thus, this paper is partly speculative.
Slow Sorption and the Sorption Distribution Coefficient Research over the last decade or so has made it clear that (1) the solid-phase to solution-phase distribution coefficients (Kd) routinely are not measured at true equilibrium; (2) the use of equilibrium rather than kinetic expressions for sorption in many fate and effects models is questionable; and (3) the kinetics of sorption are complex and poorly predictable. In most cases, the uptake or release of organics by natural particles is bimodal in that it occurs in fast and slow stages. The division between them is rather arbitrary, but in many cases it occurs at a few hours to a few days. Hereafter, the term slow will be used to describe the fraction sorbed or desorbed in the slow stage. Adjectives such as resistant, recalcitrant, rate-limiting, slowly reversible, and nonequilibrium are also used in the literature.
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The magnitude of the slow fraction is not trivial, as many long-term studies testify. Some recent examples appear in Table 1. During uptake, the apparent sorption distribution coefficient (Kdapp) can increase by 30% to as much as 10fold between short contact (1-3 d) and long contact times. The values listed in Table 1 should not be construed as predictive nor necessarily representative. Data are sparse, and our level of understanding is insufficient to make predictions. During the slow uptake stage, experimentally observed changes in solution-phase concentration can be small over periods of many hours and are easily masked by random analytical errors. Consequently, it has been common in many routine sorption experiments to falsely conclude that the system has come to equilibrium after 1 or 2 days. Desorption likewise often reveals a major slow fraction (10-96%) following a comparatively rapid release. Historically contaminated (aged) samples, where contact times may have been months or years, can be enriched in the slow fraction owing to partial dissipation or degradation of more labile fractions before collection. The slow fraction of some pesticides was found to increase with contact time in the environment (10). When the total contaminant present must be determined by extractionssuch as in field samples or in spiked samples where uncertain losses occurred during an experimentsthe choice of extraction conditions is important to ensure complete recovery of the analyte. Extraction methods are
commonly validated with freshly spiked soil samples. Unfortunately, validation is seldom performed on aged samples that are enriched in resistant fractions. Hot extraction with water-miscible solvents has been shown to be superior for extracting resistant fractions compared to traditional methods like solvent-shake at room temperature, purge-and-trap, and Soxhlet techniques (11-15). It is well known that recovery by nonmiscible solvents (e.g., hexane) decreases with aging (16, 17). Supercritical CO2 extraction has not been fully investigated; but some reports indicate that, even in the presence of organic solvent modifiers which increase its solvation power, it is inferior to hot solvent for extracting resistant fractions (18, 19). In some studies, the slow fraction is likely to have been underestimated due to incomplete recovery. This can lead to erroneous conclusions when some process of interest is being measured against the mass of contaminant believed to be present. For example, one may deem that biodegradation is successful when actually loss of only the labile fraction has been evaluated. Since Kd is time-dependent on a scale well beyond that of most laboratory sorption experiments, the true extent of sorption is known for just a few systems. Many reported Kd values represent principally the fast component rather than overall sorption (20). Free energy correlations involving Kd are thus brought into question. For example, molecular structure-Kd relationships rest on the assumption of equilibrium or at least that all compounds have attained the same fractional equilibrium. However, sorption rates can depend greatly on molecular geometry and electronic properties. This is clearly evident in regard to diffusion through a viscous medium such as organic matter or a pore structure (see below). Moreover, Brusseau and co-workers (21, 22) showed that a mass transfer coefficient determined from soil column elution was inverse loglinearly related to the octanol-water partition coefficient for closely related compounds and that polarity in the molecule caused an additional decline in the mass transfer coefficient. Further research is needed to determine to what degree nonequilibrium can influence free energy relationships of sorption. In general, the sorption equilibrium assumption in fate and effects models is invalid when the fate/transport process of interest occurs over comparable or shorter time scales than sorption. Given that, one can imagine many processes that might be more sensitive to kinetic than thermodynamic sorption behavior; for example, uptake by an animal that comes into brief or intermittant contact with the soil. The equilibrium assumption has been found to fail in a growing number of cases. There are numerous examples of longterm persistence in soils of intrinsically biodegradable compounds even when other environmental factors are not limiting for microbial growth (2, 23-25). These are backed by a laboratory study showing that aging of the soil-contaminant mixture prior to the addition of microbes reduced bioavailability (26) and by a field study showing that aging reduced herbicidal activity (27). Also, the fact that bioremediation of soil often levels off after an initial rapid decline [e.g., PCBs (28) and hydrocarbons (29)] is believed to be due mostly, if not solely, to the unavailability of a resistant fraction. Finally, nonequilibrium sorption affects the hydrodynamic transport of contaminants by causing asymmetrical concentration vs time (elution) curves. In relatively homogeneous soil columns, this asymmetry is exhibited
by early breakthrough, a decrease in peak breakthrough concentration, breakthrough front tailing, and elution-front tailing (5); whereas, nonsorbing solutes like 3H2O or Cltypically show little or no evidence of asymmetry. In more heterogeneous media as exists in the field, the effect of nonequilibrium sorption on transport is less distinct. Vadose (30) and saturated zone (4) studies reveal a decrease in velocity and aqueous-phase mass of the contaminant plume, relative to a nonsorbing tracer, with increasing travel time or distance. While this is consistent with a timedependent increase in Kdapp due to rate-limiting sorption, an interpretation is complicated by permeability variations in the flow field (physical nonequilibrium) as well as variability in Kd itself within the substrata (7, 8). Both of these can lead to tailing via plume spreading. The relative importance of sorption nonequilibrium and physical nonequilibrium is likely to depend greatly on the heterogeneity of the flow field and the type of particles that make it up.
Mechanisms Possible Rate-Limiting Steps. The potential causes of slow sorption are activation energy of sorptive bonds and masstransfer limitations (molecular diffusion). Sorption can occur by physical adsorption on a surface or by partitioning (dissolution) into a phase such as natural organic matter (NOM). The intermolecular interactions potentially available to neutral organic compoundssvan der Waals (dispersion), dipole-dipole, dipole-induced dipole, and hydrogen bondingsare common to both adsorption and partitioning. In solution these forces are fleeting. For example, the mean lifetime of the H2O‚‚‚NH3 hydrogen bond is 2 × 10-12 s (31). Adsorption to a flat, unhindered, and rigid surface is ordinarily unactivated or only slightly activated and so should be practically instantaneous (32). Desorption, however, is generally activated. The kinetic energy of desorption (Edes*) is the sum of the thermodynamic energy of adsorption (Q)si.e., the depth of the potential energy wellsand the activation energy of adsorption (Ead*) (32). A physisorbed molecule where Ead* ) 0 and Q e 40 kJ mol-1 will have a lifetime on the surface of e∼10-6 s (32). For these reasons, most small compounds might be expected to adsorb and desorb practically instantaneously at the microscale. However, there may be situations in which Ead* or Edes* is much greater. Large or long molecules that can interact simultaneously at multiple points can be more difficult to desorb. There may be steric hinderance to desorption or adsorptionsan ink bottleshaped pore is an example. Lastly, there may be a cooperative change in the sorbent induced by the sorbate that makes Q larger, as occurs in substrate binding to enzymes. We must be open to these possibilities for pollutant molecules in highly heterogeneous systems like soil particles. It is noteworthy that even small, weakly polar molecules like halogenated methanes, ethanes, and ethenes exhibit slow sorption/desorption in soils (25, 33, 34). The thermodynamic driving force for their sorption is hydrophobic expulsion from water, but their main interaction with the surface is only by dispersion and weak dipolar forces. Most researchers, nevertheless, attribute slow kinetics to some sort of diffusion limitation. This is almost certainly true because sorbing molecules are subject to diffusive constraints throughout almost the entire sorption/desorption time course because of the porous nature of particles. Diffusion is random movement under the influence of a
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FIGURE 1. Schematic of a soil particle aggregate showing the different diffusion processes. Natural “particles” are usually aggregates of smaller grains cemented together by organic or inorganic materials. Porosity is due to spaces between grains and fissures in individual grains.
concentration gradient (35). Particles are porous by virtue of their aggregated nature and because the lattice of individual grains in the aggregate may be fractured. Figure 1 is a conceptualization of a soil or sediment particle aggregate showing possible diffusion processes. To reach all sorption sites, diffusing molecules must traverse bulk liquid, the relatively stagnant liquid “film” extending from the solid surface (film diffusion), pores within the particle (pore diffusion), and penetrable solid phases (matrix diffusion). Diffusion coefficients of organic molecules can be expected to decrease along that same order, but except for bulk aqueous diffusion, few data are available for relevant natural particle systems. The observed kinetics in any region of the sorption vs time curve will reflect one or more of these diffusive constraints, which may act in series or parallel. The mixing that takes place in most experiments ensures that bulk liquid or vapor diffusion is not rate-limiting. Likewise, film diffusion is probably not rate-limiting. Film diffusion of inorganic ions is reduced or eliminated with vigorous mixing (36). Weber and Miller (37) and later Miller and Pedit (38) concluded that in well-mixed batch systems film resistance of lindane and nitrobenzene on subsurface materials was insignificant compared to intraparticle diffusion, but may have been significant for nitrobenzene in columns (39). Film diffusion is potentially rate-limiting for the initial fast stage of sorption; but it is not likely to be important in the long-term phenomena we have been considering. This leaves pore diffusion and matrix diffusion as likely rate-limiting steps in slow processes. Diffusion in pores can occur in pore liquids or along pore wall surfaces. Liquid
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and surface diffusion may act concurrently and are difficult to distinguish (40, 41). A model of hydrophobic sorption to mineral surfaces (42) postulates that sorption occurs on or in “vicinal” watersthe interfacial region consisting of relatively ordered sorbed water moleculessrather than on the bare surface itself. If this model is correct, liquid and surface diffusion practically merge. Surface diffusion is expected to increase in relative importance: (i) in very small pores where fluids are more ordered and viscous, and where the sorbate spends a greater percentage of time on the surface; (ii) at high surface concentrations. Surface diffusion was invoked for porous resins (43) and activated carbon (44, 45) because intraparticle transport appeared to be faster than could be accounted for by liquid diffusion. A surface diffusion model was used to simulate sorptiondesorption of lindane with some success (38). However, it has been argued that surface diffusion is insignificant on soil particles because of the discontinuity of the adsorbing surface (33), if not the low mobility of the sorbate itself (46). Kinetic Behavior. Proposed mathematical kinetic models include first-order, multiple first-order, Langmuir-type second-order (i.e., first-order each in solute and “site”), and various diffusion rate laws. The equations and their incorporation into the advection-dispersion model for solute transport are available in several good reviews (5, 6, 40). All except the diffusion models conceptualize specific “sites” to/from which molecules may sorb in a first-order fashion. Most sorption kinetic models fit the data better by including an instantaneous, nonkinetic fraction described by an equilibrium sorption constant . None of the models are perfect, although diffusion models are more successful than first-order models when they have been compared (20, 41). First-order kinetics are easier to apply to transport and degradation models because they do not require knowledge about particle geometry. Fit to a particular rate law does not by itself constitute proof of mechanism. Nonmechanistic models have been employed also. Pedit and Miller (41), on considering the inter- and intraparticle heterogeneity of soil, modeled the monthslong uptake of diuron by a stochastic model, which treated sorbate concentration (Kd) and first-order rate constant as continuously distributed random variables. We call attention to three features of slow sorption kinetics that, if fully explained, could lead to a deeper understanding of the causes of slow sorption. First, a single rate constant often does not apply over the entire kinetic part of the curve (20, 46-48). In the elution of field-aged residues of atrazine and metolachlor from a soil column, a model with a single diffusion parameter underestimated desorption at early times and overestimated desorption at late times (20). Mass transfer coefficients obtained by modeling elution curves depend on the contaminant residence time in the columnsi.e., the flow rate (49). In desorption studies, plots of the logarithm of fraction remaining vs time tend to show a progressive decrease in slope, indicating greater and greater resistance to desorption (47). Hence, desorption in natural particles seems to be, kinetically speaking, a continuum. On considering that soil may be a continuum of compartments ordered by their desorption rate constants, Connaughton et al. (47) modeled the increasing desorption resistance of naphthalene by assuming that the rate constant is distributed according to a statistical Γ density function, itself having two parameters. The intrinsic heterogeneity of soils on many levelsse.g., polydisperse primary and secondary particles, a wide range
of pore sizes, and spatial variations of mineral and organic components on the micro-scale, etc.sis fully compatible with continuous kinetics. An underlying problem in studying slow sorption is that we are never dealing with a homogeneous sorption/diffusion medium. Second, the slow fraction (Ssl) is inversely dependent, often markedly, on the initial applied concentration, Co (14, 46, 50-52), meaning that it assumes greater importance at lower concentration. Equilibrium considerations alone may partly explain this: when the sorption isotherm is nonlinearsthat is, when N in the Freundlich equation (Cs ) KFCaN, where Cs and Ca are the sorbed and aqueous concentrations), is less than unitysintraparticle retardation will increase as the concentration inside the particle declines (38, 46, 48). However, in some studies it appears that the concentration dependence is steeper than expected based on equilibrium nonlinearity. In studies of TCE vapor sorption to various porous particles at 100% relative humidity, Farrell and Reinhard (46) showed that the slow fraction remaining after N2 gas desorption was highly concentration-dependent and not well simulated by considering only equilibrium nonlinearity. In batch experiments of a soil containing 1.26% OC (50), an empirical nonlinear expression was used to relate “slow fraction” (amount remaining after desorption to infinite dilution for 5 d) to initial concentration (Ssl ∝ Con). The exponent n was found to be 0.90 for PCE, 0.73 for 1,2-dibromo-3chloropropane, and 0.49 for TCE. The isotherm of TCE in the same soil was linear (N ) 1.01) (53). While the Freundlich parameters were not measured for the other two compounds, experience shows (25, 53, 54) that such compounds give linear or slightly nonlinear isotherms (N > ∼0.9) in soils that have a substantial amount of NOM. Thus, for TCE at least, the concentration dependence of the slow fraction is greater than the fast fraction. In a study of metolachlor and 1,3-dichlorobenzene in two soils (55), N was greater for sorption of a fast fraction (1 d contact time) than a slower fraction (the difference between 30 and 1 d contact times). This means that the slower fraction becomes increasingly dominant as the total concentration declines. Third, sorption is often kinetically hysteretic, meaning that the slow state appears to fill faster than it empties. Further research must be done to validate this. Many examples exist of apparent “irreversible” sorption of some fractionsor at least exceedingly long times to achieve desorptionsfollowing relatively short contact times (1, 5, 15, 56-59). Hysteresis may be caused by experimental artifacts or degradation (1, 5, 56). Also, to fairly assess hysteresis from the desorptive direction requires that samples be at true equilibrium. Kan and co-workers (15) sorbed naphthalene and phenanthrene to a sediment (0.27% NOM). While uptake appeared to reach equilibrium in a few days, successive desorption stepssusually lasting 1-7 d and totalling as long as 178 dsreleased less than 40% of chemical, even from samples sorbed for only 1 d (Table 1). Good mass balance was obtained upon soil extraction with CH2Cl2 at 45 °C. Miller and Pedit (38) examined sorption of lindane to a subsurface soil corrected for dehydrohalogenation reactions. They found that an intraparticle diffusion model, whose parameters were obtained from uptake, could account for most but not all of the hysteresis observed upon sequential desorption. We note that the sorbed concentrations declined by only 2- or 3-fold after the three-step
desorption, and the model fit seems to worsen with step. Had further steps been performed to uncover more resistant fractions, it is possible that even less of the hysteresis would have been accounted for. Harmon and Roberts (48) found the effective diffusion coefficient of PCE in aquifer sediment to be 2-4 times smaller in the desorptive direction. They cautioned that the sorptive diffusion coefficients were obtained by others using a different technique. Inspection of their data reveals that the tail end of the desorption curves tends to flatten out, indicating a substantial fraction of PCE (∼20% of initital) that is overpredicted by the model, i.e., desorbs at a much slower rate. The above three features of slow sorption suggest but do not prove a departure from regular Fickian diffusion. Fickian diffusion is symmetrical with respect to sorption and desorption, and the diffusion coefficient is concentration-independent provided the sorbate does not alter the sorbent properties (35). Further careful experiments are needed to confirm whether sorption in soils truly deviates from Fickian diffusion. If it does, one implication is that the making/breaking of bonds may play a role along with molecular diffusion in sorption/desorption rate limitations, even for classically “noninteracting” compounds like aromatic hydrocarbons and chlorinated solvents. The behaviors above are in large measure a signature of sorption to sites having a distribution of energies. If interaction with an array of sites is responsible for sorption in the slow state the following might be expected: (i) a distribution of desorption rate constants corresponding to a distribution of activation energies; (ii) inverse concentration dependence of the slow fractionsat low applied concentration, the higher energy sites (which are more important relative to the fast state) are populated preferentially; and (iii) kinetic hysteresis since the activation energy of desorption is normally greater than that of sorption from/to a specific site. We may better understand the meaning of these observations in the context of the two models that have been put forth as the most likely causes of slow sorption in natural particles: the organic matter diffusion model (OMD) and the sorption-retarded pore diffusion model (SRPD). They are shown pictorially in Figure 2 and are discussed below. Organic Matter Diffusion. The OMD model postulates diffusion through NOM solids as the rate-limiting step (5, 21). This is intuitively satisfying given the abundant thermodynamic evidence that partitioning (dissolution) in NOM is the primary mechanism of sorption when NOM and water are sufficiently abundant (54). NOM can exist as surface coatings or discreet particles. Supporting the OMD mechanism are the following: (1) inverse correlations between mass transfer parameters and NOM content (20, 28, 50, 60, 61); (2) organic cosolvents increase the rate in accord with their ability to ‘swell’ NOM (62); (3) inverse linear free energy correlations between rate constant and Kd or the octanol-water partition coefficient Kow (3, 21, 22); and (4) a decrease in rate for polar molecules capable of hydrogen bonding to acceptor groups within NOM (22). Yet these results are also consistent with SRPD if the active sorbent material in pores is taken to be NOM coatings on pore walls. Moreover, OMD is at odds with the observation of slow sorption in zero or extremely low NOM materials (33, 46, 63). We may ask: Are diffusion length scales and diffusion coefficients (D) in natural particles consistent with NOM
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FIGURE 2. Schematic of two models for slow sorption. (a) Organic matter diffusion (OMD), illustrating diffusion through a rubbery phase A, diffusion through a more condensed glassy phase B, and adsorption in a “Langmuir site” C (see text). (b) Sorption-retarded pore diffusion (SRPD). Retardation by rapid-reversible sorption to pore walls, and “enhanced adsorption” in pores of very small diameter due to interaction with more than one surface.
as the diffusive medium? Desorption of resistant fieldaged pesticides (EDB, atrazine, metolachlor) in soil show little particle size dependence down to the clay-size fraction (20, 25), suggesting that the upper limit diffusion length scale is on the order of the clay particles (103-102 nm). If a single effective diffusivity (Deff) applies over this radial length, Deff would equal ∼10-17 cm2/s or less. Desorption of PCBs from river sediments (28) also showed no particle size effects and indicated diffusion length scales of ∼30 nm, corresponding to Deff of 10-20-10-21 cm2/s. The true dimensions of NOM are essentially unknown, but thicknesses of 30-1000 nm are not unreasonable for NOM coatings or discreet NOM particles. In regard to Deff values, the obvious analogy to NOM is synthetic organic polymers. The polymer-phase concept of humics is replete in the literature. Diffusion in polymers occurs by either a place change mechanism, in which movement is accomplished by cooperative interchange of position of polymer segments and the penetrating molecule,
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or by a defect mechanism where the penetrant may jump between lattice defects, voids, pores, etc. (64). Polymers are said to have glassy (condensed, rigid) or rubbery (expanded, flexible) structures with respect to the order and cohesive forces of the polymer chains. Likewise, humic substances are described as having condensed and expanded regions (65). Choosing a polymer to model NOM is difficult because NOM in situ is expected to be highly variable in its properties, even within the same contiguous material. Furthermore, structure of and sorption to NOM can be strongly affected by soil minerals (66, 67). Attempts have been made to estimate the cohesive forces holding the humic polymer chains together in relation to their effects on the diffusivity and solubility of sorbate molecules (28). The true valuesswere it possible to determine themsare likely to cover a wide range. Reported D values in polymers at 25-30 °C for a molecule like CCl4 having a diameter of 0.55 nm range over many orders of magnitude, from 10-7 cm2/s in rubbery polymers (polyethylene) to 10-17 cm2/s in glassy polymers (polyvinyl chloride) (64, 68). Diffusivity is sensitive to the size and shape of the penetrant, much more so for glassy than rubbery polymers. One might expect a molecule to experience large changes in diffusivity as it moves between expanded and condensed regions of NOM. Accordingly, Carroll et al. (28) suggest that the bimodal desorption vs time curves of PCBs from sediments are due to desorption from these two types of phases. Future work is needed on determining organic compound diffusivities in NOM particles and on finding appropriate polymer models. Diffusion kinetics in polymers is widely variable depending on polymer structure, particle size distribution, diffusant structure, diffusant concentration, temperature, and the history of exposure (64, 69, 70). Mixtures of polymer sphere sizes can lead to bimodal diffusion curves (71, 72). Since the diffusion rate is inversely related to the square of the radius, the proportion of fast and slow phases of the uptake or release curve depends on the size distribution. Obviously, the dimensions of NOM in a given soil will be truly diverse. Relatively high diffusant concentrations can cause polymer swelling or crazing as the diffusant front advances (69). These changes affect both the compound’s solubility (partition coefficient) and diffusivity, which in turn dictate the shape of the kinetic curve. Bimodal curves can result. Pollutant concentrations in the environment may sometimes be high enough (e.g., a chemical spill) to swell or soften NOM. Cosolvents can do the same thing. Methanol cosolvent increased the desorption rate constant of diuron and several PAHs (62). Usually though, we are dealing with dilute contaminant and no cosolvent. Sorption under dilute conditions in rubbery polymers generally is linear and obeys Fick’s lawsthat is, D is concentration-independent, and mass transfer is symmetrical with respect to the forward and reverse directions and proportional to the square root of time (64, 69, 73). Sorption in glassy polymers on the other hand is anomalous in that it is typified by nonlinear (N < 1) isotherms, concentration-dependent D, and a tendency toward bimodal kinetics and sorption-desorption hysteresis (64, 68, 70, 72). This is reminiscent of sorption/diffusion behavior of many compounds in soils. Anomalous behavior in glassy polymers has been attributed to dual-mode sorption. This was first proposed
for gaseous molecules like CO2 and CH4 (70) and later for small organic molecules (68, 71, 72). Dual-mode sorption is the sum of (a) normal linear partitioning taking place in the bulk of the polymer and (b) a hole-filling mechanism in which the incoming molecules undergo Langmuir-like adsorption in voids internal to the polymer matrix. The latter is the cause of isotherm nonlinearity and non-Fickian tendencies. Linear sorption can be restored by conversion of the glassy state to the rubbery state by increasing the temperature above the glass transition point (Tg) or by softening with organic solvents. The exact nature of the voids is presently unknown. Solid-state 31P-nuclear magnetic resonance spectroscopy showed that mobile and immobile sorbed forms of tri-n-butyl phosphate exist in glassy polystyrene (74). The relative mobility of the immobile forms appeared to span a wide range. Dual-mode sorption in NOM may rationalize qualitatively some behaviors of contaminants in natural particlessnamely, nonlinear isotherms, competitive sorption, and kinetic hysteresis. Isotherms are often nonlinear when a sufficiently wide solute concentration range is used (37, 39, 55, 75-77). Examples include hydrophobic compounds in a peat soil composed almost entirely (93%) of NOM (55). It might be expected that isotherms would linearize at high concentrations as the adsorption sites became filled, but this would depend on how the sites were distributed in energy. Investigators have also shown competitive sorption between nonpolar compounds in suspensions of soils (53, 76), the mentioned peat (53), and humic-coated clay (66). Competitive sorption clearly indicates some measure of site specificity (53, 76). As shown in Figure 2, we may envision NOM as a bulk partition medium consisting of rubbery (A) and glassy (B) regions. Dispersed in the glassy regions are adsorption sites (C) of various energies, analogous to the voids of glassy polymers. In agreement with the dual-mode model, phenanthrene isotherms became more linear with increasing temperature in soil and shale samples where NOM was believed to be the predominant sorbent (75). This is consistent with a transition to a more rubbery state. The nature of the adsorption sites is speculative. They could be some type of inclusion complex between the guest pollutant molecule and host subunit(s) on the NOM macromolecule. Soil humic acid has condensed polyaromatic regions, even after extraction and reconstitution to a particulate form (78, 79). It has been suggested that polyaromatic structures provide adsorption sites (75). Although it is far from certain at this time that the dualmode mechanism plays a role in slow kinetics, the aforementioned results of ours (55), showing a decrease in the Freundlich exponent N with time in NOM, are at least consistent with it. The existence of high-energy adsorption sites could account for kinetic hysteresis. It might be expected that such sites would fill faster than they would empty. Thus, it is plausible that desorption becomes at some point rate limited by release from these sites, while sorption is principally rate limited by diffusion through bulk NOM. The nonlinear relationship between Ssl and Co discussed above is also plausibly attributed to the presence of sites. Sorption-Retarded Pore Diffusion. The SRPD model (Figure 2) postulates the rate-limiting process to be molecular diffusion in pore water that is retarded, chromatographic-like, by local sorption on pore walls (80) (Figure 2). Walls may or may not be composed of NOM. Assumptions
by most modelers are that local sorption is instantaneous, particles are uniformly porous, and sorption parameters Kd and Deff in the pore are constant. According to the SRPD model, rates are expected to be inversely dependent on the square of the particle radius, on the tortuosity of pores (bending and twisting, interconnectivity, presence of deadend pores), on the constrictivity (steric hindrance) in the pores, and on the Kd. The inverse dependence on Kd does not distinguish SRPD from OMD. For natural particles, observations that point to SRPD include faster rates after particle pulverization, which reduces pore path length (25, 33, 50), and after acidification, which was suggested to disagregate grains by dissolving the inorganic oxide cements that hold the aggregates together (50). Correlation of rate with particle size is only qualitative at best. In one case where a rough correlation was found (80), experiments took place over a few hours at most. In another case (33) where coarse aquifer sand particles equilibrated PCE and TeCB generally faster than fine particles, the particles were calcite-cemented aggregates that had considerable internal porosity and surface area (81). In many systems, the particle size dependence of desorption is altogether absent (20, 25, 28, 46, 82). For example, desorption of field-aged pesticides in soil (20, 25) and PCBs in river sediments (28) was not related to the nominal particle radius down to the clay size fraction, suggesting that the length scale of diffusion is proabably less than 100 nm. The absence of size dependence might be rationalized by assuming that most of the porosity exists in an outer shell that is of similar thickness among size fractions. Another possible reason is that sorption capacity may not be uniformly distributed within the aggregate. Ball and Roberts (33), for example, found that Kd of PCE and TeCB varied markedly among different size fractions and a magnetically separated fraction of an aquifer material. But in a study of desorption of TCE from silica with monodisperse particle sizes and narrow pore size distributions, investigators found no particle size dependence (46). Tortuosity and constrictivity are difficult to evaluate. Both are expected to vary inversely with pore size. However, in a silica pore, diameters ranging from 6 to 30 nm had little effect on TCE desorption (46). It is possible that this effect shows up only in pores that are smaller than 6 nm. The analytical tools for measuring nanopore characteristics in natural materials are undeveloped, and the theoretical foundations are too weak to incorporate them into the SRPD model (33, 46, 48). Thus, research is needed on characterizing the geometry and spatial distribution of pores. The kinetic continuum discussed earlier could be rationalized by a heterogeneous pore structure in which there is a distribution of diffusivities within the particle. Thus, we may envision pores that fill and empty quickly, along with those that do so slowly. A potentially important influence on constrictivity in the pore is the viscosity of water. Polar minerals have one or more layers of water strongly sorbed on their surfaces (83). Viscosity measurements of colloidal silica particles in water indicate there is a monolayer more or less immobilized on the surface (84). The water contained in a pore of a few Angstroms in diameter may be ice-like and therefore greatly restrict solute diffusion. Molecular modeling can potentially give insight on the structure of water in nanopores. The long-term desorption of EDB from soil to water could not be modeled by SRPD without invoking enormous
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tortuosity or constriction in pores (25). Likewise, Deff’s for desorption of alkanes and PAHs from urban atmospheric particles were 106 times smaller than expected for sorption retarded gaseous diffusion in pores (3). One rationale is afforded by rejecting the SRPD assumption of instantaneous local equilibrium in the pore. For instance, a PCB required hours to desorb from dissolved commercial humic acid (85). From the standpoint of solutes, dissolved humic acid macromolecules may well represent the smallest or most penetrable humic materials existing on wall surfaces. Also, substituted benzenes required hours to desorb from surfaces of alkyl-modified, nonporous silica gel particles (86). Such slow stepwise desorption rates could strongly retard transport through a pore compared to the instantaneous case. The mathematics of diffusion in systems containing one diffusive medium in another have been discussed (35). An alternative explanation for the extraordinarily small Deff has been offerred (46, 87). According to this enhanced adsorption hypothesis, as the pore size decreases to roughly the adsorbate diameter, the calculated interaction potential increases up to 5-fold compared to the single surface case owing to multipoint interaction of the adsorbate with pore walls (Figure 2). Furthermore, since the pore volume is small compared to the wall surface area, the adsorbate spends less and less time in pore solution, ultimately being restricted to diffusion along the surface, which may be intrinsically slower. Farrell and Reinhard (46, 87) equilibrated TCE vapors with a column of porous silica particles at 100% relative humidity where all micropores are expected to be filled with water. Desorption with a stream of humidified N2 proceeded in two distinct phases and was incomplete after purging times lasting weeks. The silicas behaved similarly to natural sorbents in that (1) the diffusion length scale was not the nominal particle radius and (2) the slow fraction was less than linearly related to initial TCE concentration, which was attributed to a limited sorption capacity in high-energy micropore sites. The authors suggest that microporosity gives rise to both isotherm nonlinearitysindicative of a distribution of site energiessand slow desorption. Here again, we see the consequences of energetic heterogeneity in the sorbent that was referred to earlier in a general sense and in the context of OMD. Caution is certainly called for in interpreting these experiments. Condensation of TCE in pores during the equilibration period cannot be ruled out since vapor concentrations were close to saturation. Removal of TCE in a condensed phase from a pore may be slower. In apparent contradiction of enhanced adsorption is the fact that diffusion of small molecules through the micropores (5-7 Å) of synthetic aluminosilicate zeolites is remarkably faststhe time to reach equilibrium of small molecules like hexane (88) and TCE (89) in micron-size particles being on the order of 102 min (D ∼ 10-12 cm2/s). A form of pore diffusion that deserves more attention is clay interlayer diffusion. Hydrated metal ion-exchanged clays (e.g., with Ca2+) do not extensively sorb hydrophobic molecules, but neither are such compounds excluded from hydrated interlayer spaces. Na-montmorillonite in water exhibited uptake of TCE lasting over 25 d (90). Desorption of atrazine from some Ca2+-smectites revealed formation of a tightly bound fraction (91). Smectites exchanged with tetraorganoammonium cations have a much higher affinity for hydrophobic compounds (ref 92 and references therein), but their kinetics have not been studied. Some evidence
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suggests that clay interlayers are not important. Montmorillonite formed a much smaller fraction of slowly released TCE than either silica or microporous glass beads (46). Steinberg et al. (25) observed the lowest field residues of EDB in the clay size fraction. Concluding Remarks Regarding Mechanism. It is quite likely that both OMD and SRPD mechanisms operate in the environment, often probably together in the same particle. OMD may predominate in soils that are high in NOM and low in aggregation, while SRPD may predominate in soils where the opposite conditions exist. But this has not been established. Slow desorption from an organicfree silica, a substance so closely related to soil minerals, is strong evidence that the mineral fraction is important. Resolving the individual contributions of OMD and SRPD in natural materials constitutes a challenge to future investigators. We have seen that both mechanisms offer the potential for high-energy adsorption sites to play a role. These sites may be more rate-limiting in the desorptive direction than the sorptive direction. Further research is critical in this area. Evidence indicates that a decrease in the rate constant occurs with increasing molecular size and hydrophobicity. However, this is consistent with all of the mechanisms discussed.
Significance of Slow Sorption Mechanism to Bioavailability and Remediation The bioavailability of chemicals in soil to microbes, plants, and animals is important from the perspective of remediation and risk assessment. Ex situ or in situ cleanup of soil requires mass transport of contaminants through the materials, which in turn depends on sorption kinetics. Microbes take up substrates far more readily from the fluid than the sorbed states (89, 93-96). Thus, it is no surprise that aged chemicals are resistant to degradation compared to freshly added chemicals (25, 27, 97, 98) and that degradation of freshly added chemicals often tails off to leave a resistant fraction (26, 98-100). Bioavailability has been called a major limitation to complete bioremediation of contaminated soils (29, 101). The soilcontaminant-degrader system is dynamic and interdependent. A mechanistic-based biodegradation model must be built on the mechanism(s) governing sorption/desorption, in addition to the biological mechanisms governing cell growth and substrate utilization in the matrix. A number of groups are now developing sorption-degradation kinetic models (26, 102-106). Both diffusion and twobox (equilibrium and first-order kinetic compartments) sorption concepts have been explored. The bioavailability of pollutants to wildlife and humans is also an area of critical importance. Pollutants can be taken up in pore water, by dermal contact, by particle ingestion, or by particle inhalation. The dynamics of sorption are not currently incorporated into exposure and risk models for organics. Availability in most cases is assumed to be 100% (107). Recently, the following have been demonstrated: (1) the time between spiking and testing affects bioavailability (2, 108); (2) the kinetics of desorption control bioaccumulation of historical contamination (e.g., PAHs in benthic animals; 109); and (3) historically contaminated soils are less toxic and/or lead to lower body burdens than equivalent amounts of spiked soils (110, 111). In order to model bioavailability, it is crucial that we understand sorption kinetics and the factors that influence
rates under the conditions of exposure. Take particle ingestion, for instance. The intestines of warm-blooded animals are often at higher temperature than the soil being ingested. Molecular diffusion through a viscous medium like NOM and desorption from a surface are activated processes and hence temperature sensitive. For example, the apparent activation enthalpy for desorption of historical residues of EDB from soil into water was 66 kJ/mol, corresponding to a 7-fold rate increase from 25 to 40 °C (25). The application of heat increased the rate of desorption of PCBs from river sediment and reduced the resistant fraction (28). Also, there is evidence that pH is important. Acidification of a soil suspension to pH
