Sorption Kinetics of Organic Contaminants by Sandy Aquifer and Its

Feb 1, 2005 - The results showed that the sandy aquifer material has low TOC (∼0.021 wt %) and low specific surface area (∼0.4 m2/g), which are ...
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Environ. Sci. Technol. 2005, 39, 1649-1657

Sorption Kinetics of Organic Contaminants by Sandy Aquifer and Its Kerogen Isolate Y O N G R A N , * ,† B A O S H A N X I N G , ‡ P. SURESH C. RAO,§ GUOYING SHENG,† AND JIAMO FU† State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Wushan, Guangzhou 510640, People’s Republic of 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

Long-term sorption behaviors of phenanthrene (Phen) on the Borden sand from 1 min to 365 days and of Phen and 1,2dichlorobenzene (DCB) on the isolated kerogen from 1 to 120 days were characterized by examining the time dependence of solute phase distribution relationships (PDRs) and compared with the prior reported sorption of tetrachlorobenzene (TeCB) and tetrachloroethene (PCE) on the pulverized and/or acid-treated bulk sand and size fractions. The sorption kinetics for Phen on the bulk sand and its kerogen isolate can well be described by the fractional power kinetics equation (qe ) ktb). The similar rate parameter b for qe(t) vs t at 5-7 levels of initial concentrations of Phen on the two sorbents, respectively, ranging from 0.077 to 0.099 and from 0.072 to 0.086, indicates the similar sorption kinetics rate. The modifiedFreundlich parameters of TeCB on the pulverized or acidtreated 0.3-mm size fraction match those of Phen on the isolated kerogen, suggesting the same natural organic matter (NOM) property of the two sorbents. As the prior investigation underestimated the KOC value for the TeCB sorption on the acid-treated 0.3-mm size fraction by a factor of 1.76, the estimated time to reach 95% of sorption equilibrium is much longer than the prior estimation (over 10 years vs about 2.5 years). The estimated times to reach 95% of sorption equilibrium at three levels of relative solubility for Phen on the bulk sand and its isolated kerogen are, respectively, longer than one decade, demonstrating the similar diffusion length for Phen on the two sorbents. The observed slow sorption kinetics is related to nanometer-pore diffusion within kerogen matrix. The investigation supplies new clues for explaining the often observed much longer persistence of organic contaminants in soils and sediments than the prediction based on the short-term laboratory experiment.

Introduction Sorption and desorption kinetics for hydrophobic organic contaminants (HOCs) in geosorbents suggest that these * Corresponding author phone: 86-20-85290263, fax: 86-2085290706; e-mail: [email protected]. † Chinese Academy of Sciences. ‡ University of Massachusetts. § Purdue University. 10.1021/es049114r CCC: $30.25 Published on Web 02/01/2005

 2005 American Chemical Society

processes occur over a wide range of time scales, fast time scales occurring on the order of minutes to days and slow time scales occurring on the order of weeks or even years. The slow sorption/desorption rates are frequently rate limiting for biodegradation, bioremediation, and subsurface transport. Recent work has attributed these rates to intraorganic matter diffusion and/or intra-aggregate diffusion (15). Organic matter diffusion models with shallow penetration depths on the order of submicrometers to tens of nanometers have also been proposed (2,6-8). Molecular diffusion in hydrophobic microporous materials, such as zeolites, glasses, and carbon molecular sieves, occurs by a series of activated jumps and is governed primarily by steric energy barriers (5,9-10). Diffusion and adsorption in micropores are suggested to cause slow desorption of VOCs and pesticides from soil and sediment samples (11-14). When natural organic matter (NOM) is coated with soil/sediment aggregate particles, diffusion of hydrophobic organic solute through inorganic barriers into isolated NOM domains can be very slow (15). Some researchers suggest that micropore constrictions connect larger cavities of mesopore size (16). NOM associated with soils and sediments is the dominant constituent for sorption, sequestration and attenuation of relatively hydrophobic organic chemicals (4-5,17). The chemical, structural, and surface heterogeneity of NOM has strong influences on the rates and equilibria of sorption and desorption of HOCs in soils and sediments (18-28). A concept of “soft carbon” (or expanded, rubbery state) vs “hard carbon” (or condensed, glassy state) NOM has been invoked to operationally delineate chemical and structural heterogeneity of NOM and to elucidate the mechanisms for sorption by soils and sediments (22-25,29-30). In the context of this conceptual model, the expanded NOM domains follow a partitioning process and the condensed NOM domains follow external/internal surface adsorption processes. It is assumed in prior studies that the condensed NOM dominates isotherm nonlinearity, sorption-desorption hysteresis, slow rates of sorption and desorption, and competition among coexisting HOCs exhibited by soils and sediments (31-33). The Borden site was well characterized and used for in situ demonstration of several remediation technologies and for field-scale tracer and contaminant transport tests (3437). Its aquifer material was utilized as a sorbent in several HOC sorption equilibrium and rate studies. The prior sorption studies showed that the rates of HOC uptake by the material were slow and that the measured sorption distribution coefficients (Kd), the ratios of the sorbent-phase concentration (qe) to the aqueous phase concentration (Ce) of several HOC solutes, were greater than their respective Kd values predicted from hydrophobicity (i.e., KOW) of the HOCs and the organic carbon fraction (fOC) based on empirical log KOC-log KOW correlations. The observed overall slow rates of sorption were attributed to retarded intraparticle diffusion (37) and the higher-than-predicted sorption coefficients were attributed to interaction of HOC solutes with mineral surfaces (34) or unknown high-affinity NOM (1). This study investigates the effect of condensed NOM at particle scale on the overall sorption rate. Our recent study demonstrated that Borden aquifer material contains reworked kerogen having relatively high sorption affinities for HOCs even though its total organic carbon (TOC) is lower than 0.1%, and the adsorption (hole filling) mechanism is dominating in sorption of hydrophobic organic contaminants by the isolated kerogen (26-27). In this study, timedependent sorption kinetics of two HOC solutes were measured for both the isolated kerogen and the original sand. VOL. 39, NO. 6, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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The measured sorption over extended periods of time and the interpretation of very slow sorption rate are presented here.

Experimental Section Sorbents. According to Ball et al. (36), the aquifer material consists mainly of mineral grains and rock fragments of sizes ranging from about ∼850 to ∼75 µm. Carbonate rock fragments are about ∼13-17 wt % of the bulk material, which contain rock fragments of biological origins (i.e., biomicrite and biosparite). The major characteristics of the material can be found in the paper (36). In the prior study, the TOC content was analyzed following the procedure described by Ball et al. (36), and the N2-BET specific surface area and microporosities were measured using techniques suggested by Gregg and Sing (38) and 90-point N2 adsorptiondesorption isotherm data collected at liquid nitrogen temperature (ASAP-1000, Micromeritics). The results showed that the sandy aquifer material has low TOC (∼0.021 wt %) and low specific surface area (∼0.4 m2/g), which are consistent with the data reported by Ball et al. (36). NOM was isolated using a simple demineralization procedure. Borden aquifer material (16.6 kg) was added into a series of glass flasks, and the carbonate rock fragments were dissolved using 1 M HCl. After completion of decarbonation, fine particles of the treated material were transferred to Teflon beakers, and concentrated HCl/HF acid mixture at a 1:1 volumetric ratio was added to dissolve silicates at ∼60 °C for 48 h. After three cycles of HF/HCl treatment, the solid residual was washed with Milli-Q water until the pH of the supernatant reached ∼7. After removal of the supernatant, the solid was oven dried at 65 °C and divided into two portions; one (kerogen isolate I) was stored in a glass bottle for use in sorption experiments of this study, and the other was further treated with hydrochloric acid to partially remove fluorides. The latter concentrated fraction (kerogen isolate II) was only used for NOM characterization (26). The kerogen isolate I is a gray-colored powder and contains only ∼0.736 wt % of organic carbon. Scanning electronic microscopy and petrographical examinations showed that fluorite formed during HF/HCl treatment is the dominant component of the isolated material. The kerogen isolate II concentrated for both 13C NMR and petrographical studies has TOC of 5.56 wt % and N and H contents of, respectively, 0.338 and 1.30 wt %. The 13C NMR spectrum has two major peaks at ∼33 and ∼137 ppm, representing aliphatic (0-60 ppm) and aromatic carbons (100-160 ppm) (53 and 47%, respectively) (26). The spectrum is very similar to the kerogen materials isolated from Altrium shales collected from Paxton Quarry of Michigan (31). Examinations under fluorescence microscopy (blue light excitation) and optical microscopy in both transmitted and reflected modes showed that the isolated NOM is highly heterogeneous at the particle scale. The organic particles were irregularly shaped and have sizes ranging from submicrometers 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 (26). The 13C NMR spectrum and organic petrology examinations indicated that the isolated organic matter consists of kerogen. Sorbates, Background Solutions, and Analytical Techniques. Phen and DCB, obtained in spectrophotometric grades (>98%) from Aldrich Chemical Co., Inc., were used as the HOC probe solutes for this study. The octanol-water partition coefficients (log Kow), aqueous solubility (Sw) or supercooled solubility (Sscl), molar molecular volume (Vm), density (F), and the KOC values estimated from empirical correlations are listed in Table S1 (Supporting Information) (39-43). For the comparison with the prior investigation (1), 1650

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the same physicochemical property is also listed for TeCB and PCE. The procedures employed for preparation of background solutions (0.005 M CaCl2) and initial solute solutions and analytical techniques are detailed elsewhere (26,31-32). Sorption Experiment. Sorption kinetics at different solidsolution contact times was measured using batch systems described in the prior studies (26,29). The experiments consisted of both preliminary and final tests. The preliminary test was designed to determine appropriate sorbent-tosolution ratio for each sorbent-solution system that yields a 40-70% reduction in aqueous-phase solute concentration in each reactor at the completion of sorption experiment. The final test was run to collect final sorption data. The completely mixed batch reactors consisted of glass centrifuge bottles (25 mL, Corex) sealed with screw caps, Teflon-lined silicone septa, and silver septa for short-term measurements (t < 4 days) and flame-sealed glass ampules (10-, 20-, 50-mL, Kimble) for long-term measurements. The solid-solution contact times for sorption ranged from 1 min to 365 days for the bulk sand and from 1 to 120 days for the kerogen isolate I. Reactors used for solute phase distribution relationships (PDRs) measurements at t > 4 days were mixed continuously in a rotary shaker set at 125 rpm on a horizontal mode and were also disturbed intermittently by hand 3-4 times a day in the first two weeks and once a day in the rest of solidsolution contact times to ensure complete mixing condition. After mixing, the ampules were set upright for 2 days to allow suspended solids to settle; our preliminary test showed that such a time period was sufficient to separate solids from solution. Reactors used for PDR at t < 4 days were filled with a predetermined amount of aqueous sorbate solution and mixed using a benchtop tumbler operated at 12 rpm. After centrifugation or settling, each reactor was opened, weighed, and ∼3 mL of the supernatant was withdrawn and mixed with ∼2 mL of methanol in a 5-mL vial capped with a Teflon top. Details of the experimental procedures employed have been reported previously (26,29,32). Sorption Isotherms. The time-dependent sorption isotherms obtained for both the bulk sand and the kerogen isolate I (TOC ) 0.735 wt %) were fitted to the Freundlich model having a form of

log qe(t) ) log KF(t) + n(t) log Ce(t)

(1)

and the 6-week sorption data of Phen and DCB on the bulk sand and the kerogen isolate, and the reported PCE and TeCB sorption isotherms in ref 1 were fitted to the modifiedFreundlich equation in the two forms

log qe ) log K′F + n log Cr

(2)

log (qe′) ) log (Qo′) + n′ log(Cr)/Vm

(3)

where qe(t) or qe and Ce(t) or Ce are the time-dependent or apparent equilibrium solid-phase and aqueous-phase solute concentrations expressed as µg/g and µg/L, respectively; KF(t) or KF and n(t) or n are the time dependent or apparent equilibrium Freundlich capacity parameter and isotherm nonlinearity factor, respectively; K′F is the modified-Freundlich isotherm capacity coefficient; and Cr is the dimensionless aqueous phase concentration. For less polar and sparely soluble compounds, Cr is related to solute activity (a) in water phase referenced to their respective pure liquid or supercooled liquid state at a given temperature condition (40,44). qe′ is the sorbed solute volume per unit mass of sorbent (cm3/ kg), Qo′ is the maximum volume sorbed (cm3/kg), n′ is a fitting parameter, and Vm is the molar volume (cm3/mol) of solute, respectively. The values of K′F and Q′o can be further normalized to OC (K′foc ) K′F/fOC and Q′foc ) Q′o/fOC).

TABLE 1. Parameters for Temporal PDRs KOC (L/g-OC)

Freundlich model t

log KF(t)a

n (t)

R2

Nb

Ce/ Sw ) 0.001

Ce/ Sw ) 0.05

Ce/ Sw ) 0.75

1 min 5 min 10 min 30 min 1h 3h 12 h 1 day 2 days 4 days 7 days 14 days 21 days 30 days 42 days 45 days 60 days 90 days 130 days 180 days 256 days 365 days 1 day 2 days 4 days 7 days 7 days (rept) 14 days 21 days 42 days 90 days 120 days

-2.902 ( -2.832 ( 0.041 -2.814 ( 0.036 -2.666 ( 0.029 -2.623 ( 0.044 -2.578 ( 0.048 -2.367 ( 0.022 -2.237 ( 0.014 -2.207 ( 0.030 -2.178 ( 0.025 -2.115 ( 0.033 -2.088 ( 0.029 -1.915 ( 0.015 -1.891 ( 0.035 -1.953 ( 0.028 -1.949 ( 0.025 -1.949 ( 0.028 -1.827 ( 0.036 -1.791 ( 0.060 -1.831 ( 0.017 -1.829 ( 0.022 -1.827 ( 0.032 0.442 ( 0.030 0.552 ( 0.013 0.569 ( 0.022 0.630 ( 0.014 0.594 ( 0.007 0.644 ( 0.014 0.665 ( 0.026 0.642 ( 0.014 0.646 ( 0.013 0.663 ( 0.017

0.960 ( 0.966 ( 0.021 0.981 ( 0.018 0.944 ( 0.014 0.967 ( 0.023 0.921 ( 0.025 0.882 ( 0.011 0.888 ( 0.014 0.889 ( 0.016 0.894 ( 0.014 0.901 ( 0.018 0.874 ( 0.016 0.813 ( 0.010 0.812 ( 0.019 0.812 ( 0.015 0.817 ( 0.015 0.820 ( 0.021 0.819 ( 0.021 0.822 ( 0.032 0.805 ( 0.010 0.814 ( 0.013 0.804 ( 0.018 0.745 ( 0.017 0.710 ( 0.007 0.708 ( 0.012 0.703 ( 0.007 0.703 ( 0.003 0.695 ( 0.008 0.697 ( 0.014 0.694 ( 0.007 0.706 ( 0.006 0.709 ( 0.008

0.998 0.995 0.996 0.998 0.993 0.992 0.998 0.997 0.996 0.997 0.995 0.996 0.997 0.995 0.994 0.997 0.995 0.996 0.993 0.997 0.997 0.992 0.996 0.999 0.998 0.999 1.000 0.999 0.997 0.998 0.999 0.998

14 14 14 12 14 14 14 14 14 14 14 14 26 12 21 10 10 8 7 20 14 19 10 10 10 10 10 8 8 24 16 14

5.94 6.98 7.29 10.2 11.3 12.5 20.2 27.3 29.2 31.2 36.1 38.3 56.8 59.9 51.9 52.5 52.5 69.6 75.5 68.7 69.2 69.4 365 469 487 560 516 534 515 576 582 604

5.08 6.10 6.76 8.20 9.91 9.17 12.7 17.6 18.9 20.6 24.5 23.4 27.3 28.7 24.9 25.6 25.9 34.3 37.7 32.1 33.4 32.2 135 150 155 175 161 172 179 174 186 192

4.56 5.56 6.42 7.04 9.06 7.40 9.21 13.0 14.0 15.5 18.7 16.6 16.4 17.3 15.0 15.6 15.9 21.0 23.3 18.9 20.2 18.9 67.4 68.5 70.2 78.3 72.2 78.7 86.3 75.9 84.8 86.4

1 day 2 days 4 days 7 days 23 days 42 days

-0.683 ( 0.050 -0.658 ( 0.062 -0.749 ( 0.106 -0.710 ( 0.057 -0.742 ( 0.037 -0.720 ( 0.040

0.782 ( 0.012 0.786 ( 0.015 0.806 ( 0.027 0.795 ( 0.016 0.805 ( 0.010 0.830 ( 0.024

0.999 0.997 0.989 0.994 0.998 0.997

8 10 12 16 14 20

10.5 11.3 10.1 10.5 10.2 12.0

4.48 4.90 4.71 4.69 4.74 6.16

2.48 2.75 2.78 2.69 2.80 3.89

sorbate/ sorbent Phen/bulk sand

kerogen isolate I

DCB/kerogen isolate I

aUnits of K are (µg/g)/(µg/L)n. F of n.

b

0.028c

0.014d

Number of observations. c ( represents one standard deviation of log KF.

Equation 3 is equivalent to the Dubinin-Polanyi adsorption model when the exponent parameter b′ is equal to 1 in the latter (27). Linear regression procedures using SYSTAT software (Version 8.0, SYSTAT Inc.) were utilized respectively for fitting eqs 1-3 to the sorption data. Resulting Freundlich equation parameters along with their standard deviations, and/or K′foc and Q′foc parameters, number of observations, and R2 values, are given in Tables 1 and 2. Representative Phen sorption data for one of the two samples, the original Borden sand, are shown in Figure 1. Temporal PDR parameters (log Kd(t) and n(t)) for the two sorbate-sorbent systems are plotted against logarithmic time in Figure 2. Single-point organic carbon normalized sorption coefficients (KOC in L/g-OC) were calculated using the TOC contents of the sorbents and the best-fit Freundlich isotherm parameters in Tables 1 and 2 for three different Ce levels representing 0.1, 5, and 75% of the aqueous solubility for each solute. The results are also listed in Tables 1 and 2. Sorption Kinetics Model. The time-dependent sorption data were also fitted to the fractional power model in the following form

qe(t) ) ktb

(4)

where k and b are the empirical constant and rate parameter,

d

( represents one standard deviation

respectively. Many investigators used this equation, especially for P sorption/desorption in soils and sediments (45-47). Aharoni and Sparks (45) provided the mathematical basis for illustrating that this empirical kinetics model reflects a solution of the diffusion equation. Wells et al. (48) also showed that this equation derives directly from the transient equation for Fickian diffusion.

Results and Discussion Temporal PDRs, Kd(t), and KOC(t) for the Samples. Temporal PDRs measured for Phen sorption by the Borden sand and the isolated NOM samples were found to become increasingly nonlinear as sorption increased over time. To avoid crowding, only 5 of the 22 PDRs measured for the original sand are shown in Figure 1. The log Kd(t) at 5% of the relative solubility and n(t) for the original sand and the isolated NOM are shown in Figure 2 as a function of log t. As shown in Figure 2a, the nonlinearity for the original sand increases as a function of observation time and corresponds to an increase in the log Kd(t) values. Close inspection of this figure reveals initiation stages (t < 1 h), logarithmic stages (1 h < t < 90 days), and apparent equilibrium stages (t > 90 days) for n(t), patterns similar to those reported earlier for several soils, sediments, and shales (29). However, the log Kd(t) appears to be at the VOL. 39, NO. 6, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 2. Freundlich and Modified-Freundlich Sorption Parameters of Phen, TeCB, and PCE on the Various Sorbents KOC (L/g-OC )

Freundlich and modified-Freundlich models sorbate/ sorbent kerogen isolate I

log Kfa 0.64 ( 0.03c

acid-treatede pulverizedf

-0.59 ( 0.16 -0.87 ( 0.19

pulverizedf

-1.82 ( 0.17

original sandf pulverizedf (Ce < 50 µg/ L)

-2.89 ( 0.11 -3.00 ( 0.11

K′foc (mg/g-oc)

Q′foc(µL/g-oc) Phen

225 264 378 38.7 121 275

193

n

R2

Nb

Ce/Sw ) 0.00 1

0.69 ( 0.02d

0.998

24

576

174

76

0.989

5 22

574 288

173 131

75 77

22

127

60

36

TeCB/-20+40 mesh fraction 142 0.69 ( 0.13 203 0.80 ( 0.04 TeCB/-80+120 mesh fraction 20.8 0.79 ( 0.03 PCE 74.8 170

0.83 ( 0.06 0.90 ( 0.07

29 15

2.62 2.89

Ce/ Sw ) 0.0 5

1.38 1.95

Ce/ Sw ) 0.75

0.86 1.49

aUnits of K are (µg/g)/(µg/L)n. b Number of observations. c ( represents 95% confidence interval on log K . d ( represents 95% confidence F F interval on n. e The sorption data was recalculated based in Table 5 and Figure 6 in ref 1. f The Freundlich isotherm parameters are cited from ref 1.

they attributed this difference to experimental error, recalculation using the Freundlich or modified-Freundlich equations and the five-point sorption data (qe and Ce) of TeCB at 101 days on the acid-treated 0.3-mm size fraction in Table 5 and Figure 6 in ref 1 illustrates that the Freundlich parameter n is much lower than that of the pulverized sample at the same contact time (Table 2, Figure 4).

FIGURE 1. Phase distribution relationships measured for Phen on the original Borden sand at time periods of 1 min, 1 h, 1 day, 90 days, and 180 days. logarithmic stage. In the logarithmic stage, n(t) values decrease and log Kd(t) values increase as functions of log t. For the isolated NOM, Phen sorption does not reach an apparent equilibrium stage (Figure 2b). The DCB sorption exhibits apparent equilibrium stage (t ∼ 1 day) (Figure 2c), suggesting that DCB with a smaller molecular size and lower KOW value (Table S1, Supporting Information) appears to reach sorption equilibrium faster. The data listed in Table 1 clearly indicate: (1) that the KOC values for the kerogen isolate are ∼2-90 times greater than their respective values (Table S1, Supporting Information) predicted from log KOC-log KOW correlations (41-43); (2) that, for two solutes and at fixed Ce, the isolated NOM fraction has 3-7 times greater KOC value than the original sand (26). Figure 3 illustrates that KOC(t) values for Phen change exponentially with sorbate-sorbent contact times at three levels of relative solubility. Comparison with the Reported TeCB and PCE Sorption. Our investigation is compared with the prior reported sorption of TeCB and PCE by Ball and Roberts (1). The pulverized Borden sand and its size fractions have particle diameter of 8 and 5-50 µm, respectively (1,37). Ball and Roberts (1) observed that the 0.3-mm size fraction treated three times by phosphate acid had much higher Kd values for TeCB than the pulverized Borden sand of the same size fraction (363 ( 97 vs 112 ( 2 mL/g, respectively). Although 1652

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Another feature of the modified-Freundlich equation is that the solubility normalized sorption capacities (K′foc) and the sorption volumes (Q′foc) for DCB and Phen on the kerogen isolate and TeCB on the pulverized or acid-treated 0.3-mm size fraction, and for PCE and DCB on the bulk sand are respectively very close, and the K′foc and Q′foc values of DCB and Phen on the kerogen isolate are respectively higher than those of PCE and DCB on the bulk sand (Table 2, Figure 4). The similar K′foc values indicate that maximum sorption capacity is similar for DCB, TeCB, and Phen on the kerogen isolate and the 0.3-mm size fraction and for PCE and DCB on the bulk sand. The similar K′foc values for several HOCs on a given sorbent were also recently observed by others (32-33,44). Carmo et al. (44) reported that the values of log K′foc for naphthalene and Phen were very similar for several soils with different physicochemical properties. From Table 2, the Freundlich parameter n and KOC values at three levels of relative solubility for the TeCB sorption by the acid-treated 0.3-mm size fraction match those of Phen by the isolated kerogen. Moreover, the n value of TeCB on the pulverized sand of two size fractions is not different from that of Phen at t >21 days on the original sand in Table 1. This is not unexpected as the two solutes have nearly the same KOW (Table S1, Supporting Information) and similar molecular size (TeCB 0.79 nm and Phen 1 nm) (31,37). The Freundlich sorption parameters and KOC values for PCE on the original sand and pulverized sand are also similar. The Kd values of TeCB at 0.1% of relative solubility or at low concentrations on the pulverized bulk sand, -20+40, -80+120, and -120+200 mesh fractions, and on the acidtreated -20+40 mesh fraction, cited from ref 1 or recalculated in Table 2, are compared with those of Phen on the original sand and the kerogen isolate in this study (Figure 5). Figure 5 illustrates that KOC lines for TeCB on the acid-treated 0.3mm size fraction and for Phen on the isolated kerogen overlap. Each of the lines is much higher than that of TeCB on the pulverized original sand or different size fractions. The Kd value for the Phen sorption on the original sand is the lowest. The Kd-fOC correlation is very good for sorption of TeCB on the pulverized Borden sand and size fractions with the correlation coefficient R2 higher than 0.99. Therefore,

FIGURE 2. Changes in n(t) and logKd(t) at Ce/Sw ) 0.05 as a function of log time. Error bars for n(t) represent one standard deviation. (a) Sorption of Phen by the original Borden sand, (b) sorption of Phen by the isolate kerogen I, and (c) sorption of DCB by the isolated kerogen I.

FIGURE 3. Changes in the KOC(t) values as a function of t at Ce/Sw ) 0.001, 0.05, and 0.75. The lines for Phen are the best fit to the fractional power kinetics model. (a) Sorption of Phen by the original Borden sand, (b) sorption of Phen by the isolated kerogen I, and (c) sorption of DCB by the isolated kerogen I.

it appears that the pulverization is only partially effective for expediting sorption equilibrium of hydrophobic organic solutes on the original sand and its various size fractions. As kerogen is highly condensed NOM defined operationally by its insolubility in stringent solvents such as hydrofluoric acid and pyridine, standard demineralization using HCl and HF acids was reported not to greatly change kerogen structure and composition (49). This is confirmed by the same KOC values and similar modified-Freundlich parameters for the sorption of Phen on the isolated kerogen and TeCB on the acid-treated or pulverized size fraction (Table 2, Figure 4). The sorption volume of PCE also partially overlaps with

that of Phen on the kerogen isolate and that of TeCB on the pulverized size fraction. If the PCE isotherm at low concentrations (Ce < 50 µg/L) reported by Ball and Roberts (1) is used, the sorption isotherm based on the volume overlaps with that of Phen on the kerogen isolate (Figure S2, Supporting Information). Moreover, Ball and Roberts (1) observed that the low sorption for TeCB and PCE was associated with the small size fractions. In our extraction procedure, the fine particles were adequately extracted. And the 13C NMR spectrum of the kerogen isolate II showed no oxygencontaining functional groups (26). Thus, the extracted VOL. 39, NO. 6, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. Sorption isotherms of Phen and DCB at 6 weeks on the bulk sand and the isolated kerogen I cited from refs 27 and 33, PCE on the pulverized sand, and TeCB on the pulverized or acid-treated -20+40 mesh size sand cited from ref 1. The lines are the best fit to the modified-Freundlich model in the two forms.

kerogen accounting for 19% of the organic carbon could represent the NOM property in the bulk sand. Slow Sorption Kinetics of Phen and Its Mechanism. As NOM in soils and sediments is the dominant constituent for sorption of HOCs (4-5, 17), we use organic-carbon normalized constants (qe-OC and KOC) in the bulk sand and the kerogen isolate to define qe(t)/qe(∞) ) KOC(t)/KOC(∞) at a given Ce/Sw for the two sorbents with the same NOM property, taking account of their different TOC contents. The fractional power eq 4 can well describe the time-dependent KOC values of Phen at Ce/Sw ) 0.001, 0.05, and 0.75 in Figure 3 with R2 ranging from 0.87 to 0.94 on the bulk sand and from 0.78 to 0.94 on the kerogen isolate, and the original time-dependent sorption data of Phen (qe(t), Ce(t), or log KOC(t)) with R2 ranging from 0.81 to 0.98 on the bulk sand and from 0.77 to 0.87 on the kerogen isolate (Figure S1, Supporting Information). The equation exponent b for qe(t) vs t on the bulk sand and the kerogen isolate ranges from 0.077 to 0.099 and from 0.077 to 0.087, respectively (Figure S1, Supporting Information), suggesting the similar sorption kinetics rate. The sorption equilibrium is not reached even for Phen on the kerogen isolate. From Figure 3b, the KOC values at three Ce/Sw levels increase exponentially as a function of sorbentsorbate contact times. Moreover, the small molecules DCB and naphthalene have higher adsorption (hole-filling) volumes than the larger molecules TCB and Phen, suggesting that accessibility to the micropore of kerogen by large HOC molecules is reduced (27). Assuming that Phen has the same sorption capacity (K′foc) as DCB on the kerogen isolate, the estimated KOC values for Phen using the modified-Freundlich parameters (33) are 881, 266, and 116 L/g at Ce/Sw ) 0.001, 0.05, and 0.75, respectively. By use of the kinetics modeling equations listed in Figure 3b, the estimated times to reach 95% of the sorption equilibrium are longer than one decade at the three levels of relative solubility of Phen on the kerogen isolate. If the above estimated KOC values or the KOC values at 120 days in Table 1 for Phen on the kerogen isolate are assumed as the equilibrium KOC values on the original sand, the equilibrium times are predicted to be over thousand yearsbased on the kinetics equations in Figure 3a. This seems to be unreasonable. Although the NOM property of the kerogen

FIGURE 5. Kd values at 0.1% of solubility or low concentrations vs fOC for the Phen sorption on the bulk sand and the kerogen isolate I in this study and for the TeCB sorption cited from ref 1 or recalculated in Table 2 on the pulverized samples and the acid-treated fraction. The dotted and dashed lines respectively represent the Phen KOC on the kerogen isolate I and the TeCB KOC on the acid-treated 0.3-mm size fraction. 1654

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isolate is shown to be the same as that of the 0.3-mm size fraction, sorption sites of NOM in the bulk sand may be blocked by inorganic minerals, and Phen may be occluded by inorganic minerals and hence would not access all of the coated NOM. Recently Cornelissen et al. (50) showed that the clay minerals affect the micropore size distribution of black carbon in sediments. Assuming that Phen has the same sorption capacity (K′foc) as DCB on the bulk sand, the estimated KOC values for Phen using the modified-Freundlich parameters (33) are 147, 65, and 36 L/g at Ce/Sw ) 0.001, 0.05, and 0.75, respectively, the highest of which is also very close to the KOC values (143 ( 45 L/g) of TeCB at the similar relative solubility on the pulverized bulk sand (1). The estimated times to reach 95% of the equilibrium are longer than one decade using the kinetics equation in Figure 3a, which are similar to those of Phen in the kerogen isolate. However Ball and Roberts (37) reported that equilibrium time for sorption of TeCB on a 0.3-mm size fraction is about 2.5 years. The large discrepancy for equilibrium time between Phen and TeCB could attribute to the underestimation of Kd. On the basis of the acid-treated Kd value (Kd ) 196 mL/g) at 0.4% of relative solubility (the TeCB concentration range used in ref 1), the equilibrium time for TeCB on the 0.3-mm size fraction is estimated to be over 10 years from the timedependent sorption rate data in Figure 3 of the reference (37). The very long time to attain sorption equilibrium for Phen is also consistent with other investigations. If the diffusion coefficients (1.3-5.8 × 10-17 cm2/s) estimated by Ghosh et al. (51) for Phen and anthracene on coal particles are used to calculate equilibrium times for the isolated NOM, a simple approximation using the Fick’s first law shows that equilibrium times for the organic particles with spherical diameters of 1-5 µm in the isolated kerogen range from 0.68 to 76.2 years. Steinberg et al. (11) reported that the halfequilibration times for the residual 1,2-dibromoethane in the soils were about 2-3 decades. Moreover, direct and indirect observations support shallow penetration depths of HOCs. A PAH microanalysis by microprobe laser desorption/ laser ionization mass spectrometry indicated near surface (penetration depth perhaps < 5 µm) sorption mechanisms for most of PAHs on coal particles (8). Investigation on desorption rates of field-aged pesticides and PCBs from soil and sediment showed little particle-size dependence down to the clay-size fraction, suggesting that diffusion lengths were on the order of 1 µm or less (2,6-7). Similarly, it was found that the dark limestones and dark sandstone fragments of 2-4-mm size containing allochthonous charcoal and bituminous coal particles with a maximum diameter of 5-150 µm showed much slow diffusion for Phen and were far from equilibrium even after 700-1 000 days (21). The very slow approach to equilibrium is related to nanometer-pore diffusion of organic solutes into condensed NOM matrixes. The CO2 and Ar sorption indicates that NOM has appreciable internal microporosity and majority of the NOM surface area is formed by nanometer pores with maximum restrictions of approximately 0.3-0.5 nm (23,5253). Kerogen has a three-dimensional structure, with aromatic nuclei cross-linked by aliphatic chainlike bridges (49,54). The nuclei appear to be formed mainly from clusters (10-100 nm in diameter) of 2-4 or more parallel aromatic sheets with the interlayer spacings ranging from 0.34 to more than 0.8 nm (55). The aromatic sheets contain up to 10 condensed aromatic homo- or heterocyclic rings. Bridges that crosslink nuclei together are linear or branched aliphatic chains, and/or oxygen- or sulfur-containing functional groups. Both nuclei and bridges may have various functional groups, but the total number is limited. Hence, the kerogen matrix also has considerable internal nanometer pores.

The diffusion of liquid molecules in NOM may be analogous to that of synthetic organic polymers (4). Reported diffusion coefficients (D values) in polymers at 25-30 °C for a molecule such as CCl4 having a diameter of 0.55 nm range over many order of magnitude, from 10-7 cm2/s in rubbery polymers to 10-17 cm2/s in glassy polymers (56). Diffusivity is sensitive to the size and shape of the penetrant, much more so for glassy than rubbery polymers. Some investigators (2,3) showed that adsorbate structure and KOW strongly influenced diffusion rates of HOCs in soil organic matter. Since large differences in the experimental and predicted KOC values were observed between the kerogen isolate and the bulk sand in this study and among the mineral separates of similar organic carbon content and among the various size fractions (1), these differences may suggest that certain mineral phase (e.g., clay, Fe and Al oxides) blocks and retards the HOC solute diffusion into inorganic mineral-organic matter aggregate. On the contrary, the organic materials in the coarse fractions, localized within intragranular porous carbonate minerals, may be more effectively utilized for sorption (1). However, the sorption volumes of DCB or PCE on the bulk sand partially or completely overlap with those of Phen on the kerogen isolate or those of TeCB on the pulverized size fraction (Figure 4b and Figure S2, Supporting Information). Obviously, the experimental precision for the two small-size molecules, DCB in this study and PCE in ref 1, is not adequate to evaluate the effect of minerals on the coated NOM. Whether all of the sorption sites of NOM in the bulk sand could be fully accessed by HOCs needs further experimental investigation. The study shows that the sorption equilibrium time is very long for Phen on the kerogen isolate (over one decade) and on the bulk Borden sand (over one decade) as well for the prior reported sorption of TeCB on the 0.3-mm size fraction (over one decade), which was underestimated in the prior investigation (37). The very slow sorption rate is associated with the nanometer-pore diffusion in the kerogen matrixes.

Acknowledgments We thank Dr. Weilin Huang of Rutgers University for kindly providing the phenanthrene sorption rate data by the original sand. We thank three anonymous reviewers for their constructive comments. This study was funded by the National Natural Science Foundation of China (40172105), the Excellent Youth Fund of the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, and the Chinese Academy of Sciences (ZKCX3-SW-121).

Supporting Information Available Solute physicochemical properties, the original time-dependent sorption data and the fractional power kinetics modeling, and the solubility normalized Freundlich sorption isotherms in the two forms for PCE (Ce < 50 µg/L), DCB, TeCB, and Phen. This material is available free of charge via the Internet at http://pubs.acs.org.

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Received for review June 11, 2004. Revised manuscript received November 25, 2004. Accepted December 3, 2004. ES049114R

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