Published in Soil Sci. Soc. Am. J. 67:1740-1745 (2003).
© 2003 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
DIVISION S-2—SOIL CHEMISTRY
Time Dependence of Chlorobenzene Sorption/Desorption by Soils
Michael Sharera,
Jeong-Hun Parka,b,
Thomas C. Voiceb and
Stephen A. Boyd*,a
a Dep. of Crop and Soil Science, Michigan State Univ., East Lansing, MI 48824
b Dep. of Civil and Environmental Engineering, Michigan State Univ., East Lansing, MI 48824
* Corresponding author (boyds{at}msu.edu).
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ABSTRACT
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Soil contact time with organic chemicals (aging) has been found to cause increased sorption coefficients and reduced isotherm linearity as well as increased environmental sequestration and persistence. In this study, the sorption/desorption behavior of chlorobenzene (CB) on four soils was evaluated after aging periods of 24 h and 14 mo. The four soils ranged in organic matter content from 0.69 to 13.4%. Sorption isotherms were performed after each aging period to observe changes in CB uptake. Desorption kinetic profiles were generated after each aging period to observe changes in CB release from soil, with desorption followed for a period of four months. These data demonstrated no increase in CB uptake by three soils between the 24-h and 14-mo sorption periods and only a slight increase (
22%) by the fourth soil. The sorption isotherms were linear (r2 > 0.99) and did not display increased non-linearity with aging. A three-site desorption model was used to evaluate desorption parameters for both aging periods. Sorption/desorption was not fully reversible; non-desorbable fractions (fnd) were observable after only 24 h of aging, and ranged from 0.17 to 0.27. Non-desorbable fraction increased for all four soils with longer aging time; after 14 mo aging fnd ranged between 0.28 and 0.45. The CB sorption data were difficult to reconcile with current dual mode models that have been proposed to account for the sorption of organic contaminants and pesticides by soil organic matter.
Abbreviations: CB, chlorobenzene • DCP, dichlorophenol • HPLC, high-pressure liquid chromatograph • OC, organic C • SOM, soil organic matter
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INTRODUCTION
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MANY STUDIES HAVE INDICATED that sorption and desorption of organic chemicals in soils are not rapid, reversible processes, despite past assumptions to the contrary. Deviations from this assumption could have major effects on contaminant transport and fate. A study by Ball and Roberts (1991) indicated that sorption of tetrachlorobenzene was a slow process that could take hundreds of days to reach equilibrium. They also observed that sorption coefficients on a low organic C (OC) aquifer material were an order of magnitude greater than those predicted by partition theory (Chiou et al., 1979). Similarly, Xing and Pignatello (1996) showed that equilibrium sorption coefficients may increase 1.3 to 10 times between short and long soil-chemical contact times (aging), and that sorption isotherms may become more non-linear with increased aging. Data from field studies have found that certain compounds (e.g., ethylene dibromide, simazine [6-Cloro-N,N'-diethyl-1,3,5-triazine-2,4-diamine]) demonstrate unexpected persistence in soils compared with laboratory studies of binding and degradability in freshly spiked samples (Scribner et al., 1992; Steinberg et al., 1987). The formation of such a protected fraction was attributed to long pesticide-soil contact times. Numerous other investigators have found that the irreversibly sorbed fraction of a chemical increased with increased aging (McCall and Agin, 1985; Pavlostathis, 1992; Pignatello, 1990). From these examples, it seems apparent that soil-chemical contact time does have an effect on sorption/desorption behavior, hence fate, and that these processes are not always rapid and reversible. The specific effects of aging time on contaminant behavior, however, are not well defined or understood.
Despite empirical evidence that aging can affect sorption/desorption processes, there are relatively few long-term, laboratory-controlled aging studies that explicitly measure sorption and desorption parameters. Most long-term aging studies have utilized field-contaminated soils where residues have been subjected to long-term weathering including fluctuating temperature and moisture content. In many of these studies, the chemical under study is 
spiked
into non-contaminated soil and parameters such as extractability (Hatzinger and Alexander, 1995), biodegradability (Hatzinger and Alexander, 1995; Steinberg et al., 1987), bioavailability (Scribner et al., 1992), or desorption (Pavlostathis, 1992; Pavlostathis and Jaglal, 1991; Scribner et al., 1992; Steinberg et al., 1987) are measured and compared with the corresponding parameters for field-aged chemicals. Conclusions from these types of studies have been consistent in that field-aged chemicals all demonstrate reduced extractability, bioavailability, biodegradability, and desorption extent. However, information from field studies can be limited because of uncertainties in the total chemical mass involved and soil-chemical contact times (Harmon and Roberts, 1994). Furthermore, most aging studies have not explicitly measured desorption rate parameters.
There are also very few studies that have systematically investigated the role of soil organic matter (SOM) in the aging process. Given the dominant role of SOM as a sorptive phase for organic contaminants in soils (Chiou, 2002; Chiou et al., 1979), it seems reasonable to expect that the effects of contaminant aging may be different in soils with different SOM contents. Although no studies have examined directly the role of SOM and aging, scrutiny of the literature demonstrates that there may be some relationship between these parameters. In a review paper on contaminant aging in soils, Loehr and Webster (1996) suggested that the effect of aging time on sorption/desorption processes might be greater in soils with high OC contents. Conversely, it was observed that for dichlorophenol (DCP), the largest relative increase in the sorption coefficient (K) between 1 and 180 d occurred in a mineral soil (1.74% OC) rather than in peat (54.1% OC) (Xing and Pignatello, 1996). For desorption, some investigators have noted a relationship between a larger resistant fraction and increased SOM for aged chemicals (Carroll et al., 1994; Pignatello, 1990). In one of the most comprehensive studies on aging (McCall and Agin, 1985), there appeared to be a greater effect on Kappd (the apparent soil-water equilibrium distribution coefficient) of picloram aged in low OC soils (0.45%) than in high OC soils (2.9%). The Kappd was measured by a 2-min desorption protocol after aging picloram on these soils for 0 and 300 d. In the low OC soil, the Kappd increased 5.4 times between 0 and 300 d, but only 1.6 times in the high OC soil. Also measured were Kappd values after sorption for 100 d and for desorption (after 100 d of sorption); the difference in these values represents the degree of hysteresis between a single sorption and desorption equilibrium point after 100 d. The difference in the values for the high OC soil was only 1.08 times (effectively the same; no hysteresis) whereas in the low OC soil the difference was 11.3 times. These changes suggest that the effect of aging may be greater in low OC soils than in high OC soils. This may be in contrast to what some researchers have expected, but would agree with the results found by Xing and Pignatello (1996) in their study of DCP sorption on peat and mineral soil.
In this study, long-term, laboratory-controlled aging studies were conducted with the objective of explicitly measuring changes in sorption and desorption parameters with aging, and to evaluate the role of SOM content in the aging process. Sorption isotherms of CB on four soils with SOM contents that ranged from 0.69 to 13.4%, were measured after 24 h and 14 mo of aging. Then, desorption kinetics were measured for CB on each soil after sorption, with desorption being followed for up to four months.
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MATERIALS AND METHODS
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The soil samples used in this study were from the Colwood A horizon (fine-loamy, mixed, active, mesic Typic Endoaquoll), Schoolcraft A horizon (fine-loamy, mixed, superactive, mesic Typic Argiudoll), and the Capac A and B horizons (fine-loamy, mixed, active, mesic Aquic Glossudalf). Soils were collected from areas located in southern Michigan, air-dried, ground, and sieved (2 mm). Sand, silt, clay (hydrometer method), and SOM content (loss on ignition) were measured by the Soil and Plant Nutrient Laboratory at Michigan State University and are reported in Table 1. All soils were 
irradiated with 5 x 104 Gy at the University of Michigan Ford Nuclear Reactor Laboratory using a cobalt-60 irradiator. Soils were contained in 500-mL polypropylene bottles during irradiation and remained sealed until use.
The amount of a particular soil and solution used to develop sorption isotherms was based on preliminary sorption experiments. A linear isotherm equation was employed to estimate the amount of soil and solution needed to sorb 50% of the added solute while also minimizing headspace volume. Sorption vessels were 20-mL glass ampules that were autoclaved before use. A mass of soil was weighed and transferred to the ampule using a metal spatula. A 0.005 M CaCl2 solution, which had been autoclaved for 25 min and amended with 200 mg L-1 NaN3, was added to the ampules using a sterilized repipet. All additions of soil and solution to the ampules, and subsequent procedures were performed in a sterilized laminar-flow hood using sterilized equipment.
Additions of the 14C-CB (Sigma Chemical Co., radiochemical purity >97%) stock solution (in acetone) and flame sealing of the ampules were performed as follows. An aliquot of CB stock solution was added to the ampule using a 10- or 25-µL syringe to achieve initial liquid concentrations ranging from 0 to 25 mg L-1. The ampule was immediately flame sealed using an Ampulmatic flame sealer unit. Triplicate ampules were set up for each of the seven sorption isotherm points. After flame sealing, the ampules were equilibrated for either 24 h or 14 mo. They were first shaken on a reciprocating shaker for either 24 h (for 24-h aging period) or 72 h (for 14-mo aging) then either analyzed immediately (24 h aging) or stored in the dark with periodic shaking (14 mo aging). For analysis, each ampule was centrifuged at 635 x g for 8 min, the ampule broken at the neck, and a 1-mL aliquot of the aqueous phase was added to 8 mL of scintillation fluid. The samples were shaken and the next day analyzed by liquid scintillation counting.
For the desorption experiments, sorption equilibrium was established as described above at a single initial liquid-phase concentration (15 µg L-1). The ampules were aged for the appropriate time period (24 h or 14 mo) then shaken and centrifuged as described above. Then, a quadruplicate set of ampules was opened by cutting the stem (1.6– 2.0 cm from the top of the ampule) using a 6-mm (1/4-in) glass drill bit, and the solution phase (200 µL) sampled using a 200-µL pipette. This sample was analyzed using liquid scintillation counting. A larger aliquot of the solution phase was immediately removed (
75–80% of the total volume of solution) using a 25-mL gas-tight syringe, and the same volume of CB-free solution (autoclaved 0.005 M CaCl2 with 200 mg L-1 NaN3) added to the ampule to initiate desorption. The ampule was resealed and shaken on a reciprocating shaker. At the appropriate time interval, the quadruplicate set of tubes was centrifuged at 635 x g for 8 min. The remainder of the stem was broken at the neck and 1 mL of the aqueous phase was sampled and analyzed by liquid scintillation counting. This gave the concentration of CB that desorbed into solution after the given desorption time (up to 4 mo).
Chlorobenzene was analyzed using a high-pressure liquid chromatograph (HPLC) connected in series to a Waters 480 UV-VIS detector (Waters, Milford, MA) and an INUS Beta Ram 14C detector (IN/US System, Inc., Tampa, FL). Samples were chosen at random at several time points during 14 mo of aging and analyzed by HPLC for possible degradation products. No degradation of CB during the aging regime was observed.
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Mathematical Model for Desorption
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In this study, a recently developed three-site desorption model was used to analyze desorption data, assuming (i) the sorbent is composed of equilibrium, nonequilibrium, and nondesorption sites, (ii) sorption equilibrium can be described by a linear isotherm, and (iii) that the rate of release from non-equilibrium sites is proportional to the concentration gradient between these sites and the liquid phase (Park et al., 2001, 2002, 2003). The nondesorption sites have been defined in this work and others (Park et al., 2001, 2002, 2003; Sharer et al., 2003) as those containing sorbate that cannot be released to aqueous solutions during the experimental desorption period.
Mathematically, equilibrium and non-desorption partitioning are described by:
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while the release from non-equilibrium sites follows the first-order expression:
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where, Seq, Sneq and Snd (µg kg-1) are the sorbed concentrations in equilibrium sites, nonequilibrium sites, and nondesorption sites, respectively. These three terms are related to the total sorbed-phase concentration (S):
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Kp (L kg-1) is the sorption coefficient, C (µg L-1) is liquid phase concentration, Ce is liquid phase concentration at sorption equilibrium. t (h) is the desorption time, 
(h-1) is the first-order desorption rate coefficient for non-equilibrium sites, feq is the equilibrium site fraction, fneq is nonequilibrium site fraction, and fnd is nondesorption site fraction. Kp was calculated from the sorption isotherm, fnd corresponds to the plateau of the desorption rate profile, and feq, fneq, and were estimated by nonlinear regression analysis of desorption data, with the constraint that the sum of three fractions, feq, fneq and fnd, equals one.
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RESULTS
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Isotherm data were fitted to both the linear sorption model (S = Kp Ce) and the Freundlich model (S = KF Cne), where KF is the Freundlich sorption coefficient ([mg kg-1] [mg L-1]-n) and n describes curvature of the Freundlich sorption isotherm. Comparing Kp values from each aging period, it is evident that apparent sorption equilibrium is reached quickly on most soils tested (Fig. 1)
. For Schoolcraft A, Capac A, and Colwood A, the Kp after 24 h is nearly identical to Kp after 14 mo (Fig. 1 and Table 2). The slopes of these isotherms are not statistically different (at the 0.005 level) between each aging period. The exception is CB sorption on Capac B, where there was a statistical difference between isotherm slopes. For this soil, which has the lowest OC content (0.4%), Kp increased by 22% from 24 h to 14 mo (Table 2). When the isotherm data were fit using the linear isotherm model, r2 values for all isotherms fell between 0.997 and 0.999. Parameters for the linear isotherm model are listed in Table 2. Organic matter normalized sorption coefficients (Kom values) on the four soils appear to agree with literature values for CB (Table 2). The Freundlich equation n values, which are an indicator of isotherm curvature, are listed in Table 2.
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Fig. 1. Sorption isotherms of chlorobenzene (CB) on four soils after 24 h (closed circles) and 14 mo (open squares) of CB-soil contact time.
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Table 2. Linear sorption model (S = KpCe ) parameters for chlorobenzene (CB) sorption by four soils after 24 h and 14 mo of aging, and organic matter normalized sorption values (Kom).
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Desorption is presented as the percentage of the amount expected assuming complete reversibility of the sorption isotherm (Fig. 2) . Of the total amount of CB that desorbed, most was released during the first 10 h. Slow desorption occurred after this period for some soils, but the amount was <20% of the total desorbable material in all cases. The three-site desorption model was used to fit desorption rate data and produce parameter estimates (Table 3). The coefficients of determination (r2) were higher than 0.7 except for the Capac A (24 h) and Capac B (24 h and 14 mo), where the individual desorption datum points were more variable. The equilibrium site fraction ranged from 0.53 to 0.74 for the 24 h aging samples and from 0.43 to 0.59 for the 14-mo aging samples. The non-equilibrium site fraction for the 24-h samples was similar for all sorbents, ranging from 0.15 to 0.19. For all soils, a non-desorption fraction (fnd) was evident after only 24 h aging. The magnitude of fnd increased with aging of samples from 24 h to 14 mo, and the desorption-rate coefficients ranged from 0.0017 to 0.95 h-1 (Table 3).
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Fig. 2. Desorption of chlorobenzene (CB) from four soils after 24 h and 14 mo of CB-soil contact time. The inside plots present desorption of CB during the first 80 h. The amount desorbed is expressed as the percentage of the expected amount, assuming complete sorption-desorption reversibility.
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Table 3. Three-site desorption model parameters for chlorobenzene (CB) desorption from four soils after 24 h and 14 mo of CB-soil contact time and subsequent four-month desorption time.
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For CB sorption/desorption on the four soils, equilibrium and non-equilibrium site fractions tended to decrease, and non-desorbable fractions increase at the longer aging time. The largest non-desorbable fraction was observed for the Schoolcraft soil where 73% of sorbed CB was desorbed after for 24 h, but only 55% desorbed after 14 mo of aging (Fig. 2 and Table 3).
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DISCUSSION
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There seems to be little or no effect of aging on the overall extent of sorption for CB in the four soils studied. Although there was a statistically significant increase in Kp for the Capac B soil, the increase was relatively small. It appears that for CB, equilibrium is reached quickly in soils of moderate or high SOM content. The parameters presented in Table 2 demonstrate good agreement between calculated Kom (based on Kow) (Chiou et al., 1983) and experimental Kom, which can be expected to vary up to 2 to 3 times among different soils (Kile et al., 1995). This information provides supporting evidence that SOM is the dominant sorptive phase for CB in the soils tested.
Results from these experiments did not show any consistent increase or decrease in non-linearity with aging time. This is evidenced by the lack of any decrease in r2 values associated with the linear model, or any systematic change (increasing or decreasing) in the Freundlich n parameters (Table 2). This is in contrast to the observations of Xing and Pignatello (1996) and Weber and Huang (1996). For example, Xing and Pignatello (1996) observed increases in non-linearity of isotherms for dichlorobenzene (DCB) and DCP between 1 and 180 d. In the present study of CB, isotherm linearity (as evidenced by changes in Freundlich n values) decreased in two cases (Colwood A and Capac B), increased in another (Capac A), and did not change significantly in a third (Schoolcraft A). The lack of an increase in CB sorption coefficient or isotherm non-linearity with aging brings into question the general applicability of dual-mode sorption models which involve the existence of a glassy phase and/or internal pores in SOM (Weber and Huang, 1996; Xing and Pignatello, 1996). According to the dual-mode models, these adsorption domains, which manifest increases in sorption and isotherm non-linearity, are reached only over long soil-sorbate contact times since they are conceived as being internal to the SOM matrix.
The statistically significant increase in Kp on Capac B with aging time is consistent with evidence from other studies indicating that aging has a greater effect on sorption in relatively low OM soils (McCall and Agin, 1985; Xing and Pignatello, 1996). However, the magnitude of the increase observed here is relatively small compared with that observed by Ball and Roberts (1991). They found an order of magnitude increase in the long-term sorption of tetrachlorobenzene in an aquifer material (0.02% OC) over the amount of sorption expected based on organic matter partitioning (using the equation logKom = 0.904logKow - 0.779) (Chiou et al., 1983). Here, the long-term sorption of CB on Capac B is approximately the same (42 L kg-1) as the amount of sorption expected based on organic matter partitioning (73 L kg-1). It is possible that the discrepancy in the observations between these studies could be accounted for by the differences in the chemicals used and the OC content of the sorbents. As mentioned earlier, it was observed that for DCP the largest relative increase in the sorption coefficient between 1 and 180 d occurred in a mineral soil (1.74% OC) rather than in peat (54.1% OC) (Xing and Pignatello, 1996). The magnitude of the increase for DCP due to aging was 175%; in this study, the increase for CB is only 22%. Results from McCall and Agin (1985) also demonstrated an increased effect of aging in low OM soils. From these examples, it would appear that there is an increased effect of aging in soils of low OC content, but the magnitude of this effect does not appear to be consistent.
The desorption profiles show that three different regions of desorption behavior exist (Fig. 2). Equilibrium release (instantaneous desorption) was evidenced by the high solution concentrations at the first time point (20 min), and is defined as the region where desorption rates are faster than our ability to measure them. Non-desorption sites have been defined as those containing sorbate that cannot be released to aqueous solution during the experimental desorption period, but may be extractable by organic solvents (Park et al., 2001). On a practical level, nondesorption fraction is defined when the change in amount released to solution is less than the error in subsequent data points. We observed that the desorption process was essentially complete within 24 to 80 h and there was little or no increase in desorption for the remainder of the desorption period, up to 4 mo. The rate-limited release region is therefore defined by the continued increase in aqueous phase concentrations that occur in the period from the first data point to the point beginning of the plateau. In reality, during this period we are likely observing the effect of desorption from sites having a range of desorption rates, and the rate coefficients estimated mathematically are average values.
The data show a significant non-desorbable fraction of CB for 24 h of aging, and an increase in this pool for 14 mo of aging. This observation that the amount of nondesorbable material increase with time is consistent with observations from other studies (Pavlostathis, 1992; Pignatello, 1990). Previous work has suggested that sorption and desorption of organic chemicals in soils is reversible in accordance with a retarded diffusion mechanism (Wu and Gschwend, 1986). The data presented here, along with data from other studies on the irreversibility of PAH sorption/desorption (Fu et al., 1994; Kan et al., 1994; Kan et al., 2000), are difficult to reconcile with the retarded diffusion model. The size of the desorption-resistant fraction observed for PAH's of 30 to 50% is similar in magnitude to that observed here for CB (Fu et al., 1994). The rapid formation of a desorption-resistant fraction suggests that a specific physical or chemical sorption interaction may be responsible, rather than slow diffusion. According to Xie et al. (1997) irreversible sorption of organic chemicals occurred in the humin fraction of soil. In their experiments, most irreversibly sorbed atrazine was found in the bound-humic acid and mineral fractions.
The findings discussed herein show that prolonged (up to 14 mo) aging of CB in soils, performed in a controlled laboratory setting, had little or no effect on the extent of sorption. Likewise, prolonged aging did not manifest reductions in isotherm linearity. Sorption equilibrium appeared to be reached quickly (<24 h) in three of the soils studied. A fourth soil with lower SOM content showed only a slight (22%) increase in sorption after 14 mo equilibration. A significant non-desorbable fraction (0.17 to 0.27) formed rapidly (within 24 h) and increased with aging (0.28 to 0.45). The sorption characteristics of CB in four soils described here seem inconsistent with current dual mode models proposed to account for the sorption of organic contaminants by SOM including the glassy/rubbery polymer model (Weber and Huang, 1996; Young and Weber, 1995) and the internal holes model (Pignatello and Xing, 1996; Xing and Pignatello, 1996; Xing and Pignatello, 1997). It may be prudent to show restraint in describing detailed molecular scale sorption mechanisms in the absence of any molecular scale information.
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ACKNOWLEDGMENTS
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This research was supported by the USDA National Research Initiative Competitive Grants Program.
Received for publication January 22, 2003.
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TOP
ABSTRACT
INTRODUCTION
