Black Carbon and Kerogen in Soils and Sediments. 2. Their Roles in

Environ. Sci. Technol. 2004, 38, 5842-5852

Black Carbon and Kerogen in Soils and Sediments. 2. Their Roles in Equilibrium Sorption of Less-Polar Organic Pollutants† B A O H U A X I A O , ‡ Z H I Q I A N G Y U , ‡,§ W E I L I N H U A N G , * ,‡ JIANZHONG SONG,§ AND PING’AN PENG§ Department of Environmental Sciences, Cook College, Rutgers University, New Brunswick, New Jersey 08901-8551, and State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Wushan, Guangzhou 510640, People’s Republic of China

The first paper of this series reported that soil/sediment organic matter (SOM) can be fractionated into four fractions with a combined wet chemical procedure and that kerogen and black carbon (BC) are major SOM components in soil/sediment samples collected from the industrialized suburban areas of Guangzhou, China. The goal of this study was to determine the sorptive properties for the four SOM fractions for organic contaminants. Sorption isotherms were measured with a batch technique using phenanthrene and naphthalene as the sorbates and four original and four Soxhlet-extracted soil/sediment samples, 15 isolated SOM fractions, and a char as the sorbents. The results showed that the sorption isotherms measured for all the sorbents were variously nonlinear. The isolated humic acid (HA) exhibited significantly nonlinear sorption, but its contribution to the overall isotherm nonlinearity and sorption capacity of the original soil was insignificant because of its low content in the tested soils and sediments. The particulate kerogen and black carbon (KB) fractions exhibited more nonlinear sorption with much higher organic carbon-normalized capacities for both sorbates. They dominate the observed overall sorption by the tested soils and sediments and are expected to be the most important soil components affecting bioavailability and ultimate fate of hydrophobic organic contaminants (HOCs). The fact that the isolated KB fractions exhibited much higher sorption capacities than when they were associated with soil/sediment matrixes suggested that a large fraction of the particulate kerogen and BC was not accessible to sorbing HOCs. Encapsulation within soil aggregates and surface coverage by inorganic and organic coatings may have caused large variations in the accessibility of fine kerogen and BC particles to HOCs and hence lowered the sorption capacity of the soil. This variability posts an ultimate challenge for precisely predicting HOC sorption by soils from the contents of different types of SOM.

†

This paper is part of the Walter J. Weber Jr. tribute issue. * Corresponding author phone: (732)932-7928; fax: (732)932-8644; e-mail: [email protected]. ‡ Rutgers University. § Chinese Academy of Sciences. 5842

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Introduction Sorption by soils and sediments is among major physicochemical processes governing the fate of relatively hydrophobic organic chemicals (HOCs) in surface aquatic and groundwater systems. Prior studies showed that soil/sediment organic matter (SOM) is the dominant component for HOC sorption (1-3) and that the SOM heterogeneity at the field, particle, and microscopic scales has major impacts on the rates and extents of sorption (4-18). A conceptual dual SOM domain model has been proposed based on a hypothesis that SOM may consist of two physically and chemically different types of domains: a “soft”, rubbery, or amorphous versus a “hard”, glassy, or condensed domain (6-10, 18). The soft SOM domains may consist of fulvic acids (FAs) and humic acids (HAs) in their rubbery states and are characterized by linear sorption isotherms, relatively rapid rates of sorption and desorption, little or no sorption-desorption hysteresis, and greater bioavailability of sorbed HOCs (6-11, 18-23). Conversely, the hard SOM domains may include kerogen, black carbon (BC), and HAs in their glassy states and are possibly responsible for nonlinear sorption isotherms, slow rates of sorption and desorption, sorptiondesorption hysteresis, and significantly reduced bioavailability of sorbed HOCs (6-11, 14-26). Kerogen is operationally defined as particulate organic matter that is not extractable with acids, bases, and mild organic solvents. It is the major organic matter in sedimentary rocks and is the backbone structure of coal (27-29). It can become associated with topsoils and sediments during weathering of sedimentary rocks or coal formations exposed to the earth’s surface. Due to extensive utilization of coal as a major energy source in civil and industrial activities, coal mining and transportation can cause wide geographical spreading of kerogen in soils and sediments (14, 30, 31). BC is characterized by high aromaticity and is generally not an expandable or less expandable matrix when compared to kerogen and HA. It can be broadly defined for a spectrum of combustion-related carbonaceous particles including both charred organic residues (char or charcoal) of organic matter and soot or elemental carbon condensed from organic carbon vapor during combustion processes (32). In addition to direct formation of BC in the environment due to forest fires, crop residue burning, and diesel fuel and coal combustion, extensive use of industrially prepared and commercially available charcoals may introduce large quantities of BC to the environment. Sorptive characteristics of HA, kerogen, and BC have been examined in a number of prior studies (5-22, 24-26, 3350). Sorption of HOCs to HA solids is variably nonlinear, depending upon the chemical compositions and molecular characteristics of HAs, the HOC properties, and temperature conditions (10, 11, 18, 22, 33). HAs that have glass transition temperatures higher than the ambient temperature exhibit nonlinear sorption isotherms for HOCs of relatively large molecular sizes and moderate hydrophobicity such as phenanthrene (10, 18, 22, 33). The organic carbon-normalized sorption capacities or KOC values generally increase with the hydrophobicity of HOCs and depend largely upon the properties of HAs such as polarity and relative contents of different HA structural components such as aromatic versus aliphatic carbons (33, 48-50). Recent studies on HOC-soil interactions have centered on quantification of particulate kerogen and BC and characterization of their potential contributions to the overall sorption by soils and sediments (5-7, 12-17, 25, 26, 30, 31, 10.1021/es049761i CCC: $27.50

 2004 American Chemical Society Published on Web 10/16/2004

34-47). Weber et al. (6) found that kerogen-containing shale particles in glacier tills exhibit more nonlinear isotherms with higher sorption capacities than other SOM in soil matrix. They proposed the concept of soft versus hard carbon SOM for describing the particle-scale SOM heterogeneity and invoked a distributed reactivity model for quantifying distinctly different sorption phenomena exhibited by different types of SOM. Petrographic examinations and less- or nondestructive separation of SOM indicated that a wide spectrum of diagenetically altered organic materials such as coal particles are commonly present in topsoils, river and harbor sediments, and aquifer materials (14, 25, 26, 30, 31, 35, 41, 42). Shales, kerogen isolated from shales, and coal particles identified in sediments and soils were shown to exhibit more nonlinear isotherms and much greater sorption capacities than humic acids (5-7, 11, 14, 26, 35). Several recent studies concluded that BC may play the dominant role in the nonlinear sorptive behavior exhibited by soils and sediments (12, 15, 16, 25, 36, 39-47). Gustafsson et al. (12) employed a high- versus low-temperature thermal oxidation method for quantifying soot versus total organic carbon contents in sediments and used the data to quantitatively explain the measured single-point PAH sorption coefficients for lacustrine sediments. Accardi-Dey and Gschwend (15) found that the pyrene sorption isotherms measured for two sediments thermally treated at 375 °C are significantly more nonlinear than those of their respective original sediments. They attributed the observed nonlinear sorption phenomena to the soot or elemental carbon particles that survived the moderate thermal oxidation. A recent study by Ran et al. (26) showed that even Borden aquifer material, a subsurface material having very low TOC, contained kerogen material that contributes significantly to the overall nonlinear equilibrium sorption of phenanthrene and chlorinated benzenes. Song et al. (30) reported that kerogen and black carbon comprised up to 78% of total organic carbon of the sediments and soils collected from industrialized region. Ghosh et al. (31) reported that coal and charcoal particles accounted for up to 5-7% of the total particles in sediments collected from Harbor Point, NY; Milwaukee Harbor, WI; and Hunters Point, CA, and that more than half of sediment-bound PAHs and PCBs are associated with these carbon particles. Several research groups (17, 36, 40, 43, 46) used laboratory prepared soot and charcoal materials as the sorbents for equilibrium sorption and desorption studies (e.g., refs 36 and 46). Their results showed that freshly prepared BC exhibits organic carbon-normalized sorption capacities an order of magnitude higher than those reported in the literature for soils and sediments. This study characterized sorption properties for physically and chemically different SOM components isolated from soils and sediments. In the first paper of this series (30), we used a wet chemical procedure consisting of acid demineralization, Soxhlet extraction, base extraction, and dichromate oxidation to separate SOM from bulk soil into four fractions: (i) humic acids + kerogen + BC (HKB), (ii) kerogen + BC (KB), (iii) humic acid (HA), and (iv) BC. Contents of the four SOM fractions were quantified for each soil/sediment sample, and the major physical and chemical properties of the SOM isolates were characterized accordingly. Here, we examined the equilibrium sorption properties for both the isolated SOM fractions and their original soil/sediment samples using phenanthrene and naphthalene as the HOC solutes. The isotherm nonlinearity and the Koc values determined in this study indicate very different contributions of humic acids and the particulate kerogen and BC fraction to the overall sorption by the tested soil/sediment samples.

TABLE 1. Selected Physicochemical Properties of the Sorbents (after ref 30) H/C atomic ratio

SSAa (m2/g)

Sandy Soil (SS) ndb nd nd nd 74.8 0.24 59.7 0.20 35.9 0.15 15.0 0.41

nd nd 1.03 0.90 0.73 1.12

2.74 11.17 5.49 5.65 4.56 2.28

3.17 2.84 49.11 49.97 60.96 38.37

River Sediment (RS) nd nd nd nd 85.6 0.25 80.6 0.17 33.0 0.11 5.05 0.38

nd nd 0.83 0.93 0.63 1.34

18.16 24.31 4.15 12.43 9.15 5.60

original extracted HKB KB BC HA

2.10 1.80 54.35 57.45 57.42 39.73

Pond Sediment (PS) nd nd nd nd 77.8 0.18 71.8 0.17 41.0 0.13 6.02 0.42

nd nd 0.75 0.66 0.51 1.09

15.70 18.00 4.36 15.34 6.46 3.62

original extracted HKB KB BC Char

0.87 0.81 17.09 17.53 29.65 67.82

Marine Sediment (MS) nd nd N. D. N. D. 58.0 0.39 57.8 0.37 18.3 0.24 N. D. 0.14

nd N. D. 1.59 1.55 0.72 0.37

19.26 20.83 23.15 15.79 7.24 86.86

sorbent

TOC (wt %)

TOCi/ TOCtotal


1.55 1.40 52.34 48.55 42.60 43.28


a N -BET specific surface area. 2 mass.

b

O/C atomic ratio

nd, not detected due to insufficient

Experimental Section Sorbents and Sorbates. A total of 24 sorbents were used in this study. They include (i) four original soil/sediment samples, (ii) four soil/sediment samples Soxhlet extracted with a mixture of benzene + acetone + methanol, (iii) 15 SOM fractions (HKB, HA, KB, and BC) isolated from the four soil/sediment samples, and (iv) a freshly prepared char sample. The original samples consisted of a sandy soil and three sediments collected from a pond, the Pearl River, and the Wanshan Archipelagos in the suburban areas of Guangzhou, China (30). Detailed information on sampling sites was provided by Song et al. (30). These samples have high contents of coal and char materials due to crop burning in rice fields and extensive domestic utilization of wood, char, and coal as the energy source over the past 2500 years. Meanwhile, due to rapid industrialization in this region over the last two decades, the soils and sediments have been variously contaminated (51). We thus used the Soxhlet extracted soil/sediment samples for better comparison of sorption properties between a given original sample and its isolated SOM fractions that had been Soxhlet extracted. The fresh char sample was used for comparison of sorption properties with the isolated BC and KB samples. This is a char produced from arbor branches in a cooking kiln with limited air entry under uncontrolled temperature condition and is representative of wood-based char materials found in our soil/sediment samples. The amount of HA fraction extracted from the marine sediment was too little to be included in this study. The major properties of these sorbents are summarized in Table 1, and their detailed characteristics were reported in the first paper of this series (30). Phenanthrene and naphthalene were chosen as the two HOC probes in this study because they are among the commonly found HOCs in contaminated sites and their sorption properties have been extensively characterized in VOL. 38, NO. 22, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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prior studies (7, 8, 10, 11, 17, 19-26). The logarithmic octanol-water partitioning coefficients (log KOW) of phenanthrene and naphthalene are 4.57 and 3.30, respectively, and their aqueous solubilities (SW) at 20 °C are about 1.12 and 31.7 mg/L, respectively. Both chemicals in spectrophotometric grades were obtained from Aldrich Chemical Co. and used as received. Solutions. The aqueous solution used in the experiments contained 0.005 M CaCl2 as the major mineral constituent and NaN3 at a level of 100 mg/L as an inhibitor for microorganism growth. The solution pH was adjusted to 2.0 for the sorption experiments involving HAs as the sorbents and to ∼7.0 for all other sorption experiments. The lower pH condition used was to avoid dissolution of HA solids during mixing. The primary stock solutions of both phenanthrene and naphthalene and their initial aqueous solutions were prepared according to a prior study (8). The solute concentrations in the initial aqueous solutions were analyzed with the method described below. Sorption Experiments. Sorption equilibrium experiments were conducted at 22 ( 0.5 °C using flame-sealed glass ampules (20 or 50 mL, Kimble) as batch reactor systems. The experimental procedures detailed in ref 20 were exactly followed. In brief, a series of preliminary and final tests were conducted using the same procedures and experimental setups. Preliminary tests were run to determine an appropriate solid-to-solution ratio for each sorbent-solute system to achieve 40-60% reduction of the initial aqueous phase concentrations. Sorption rate tests spanning from 1 to 60 d were performed, and the results showed that a solidsolution contact time of 21 d was sufficient for attainment of apparent sorption equilibrium for all sorbent-solute systems. The final tests were conducted for acquisition of sorption data reported here. In these tests, glass ampules that contained predetermined amounts of sorbent and appropriate volume of initial aqueous solution and with a headspace of about 0.8 mL were flame-sealed in a natural gas flame. After being checked for leakage and shaken by hand for initial mixing of the contents, the sealed ampules were placed on a shaker set at a speed of 125 rpm for mixing. After being shaken for 21 d, the ampules were set upright for 2 d to allow suspended sorbents to settle. They were then flame-opened, and an aliquot (3 mL) of supernatant was immediately withdrawn from each ampule and transferred to a preprepared 5-mL vial with about 2 mL of HPLC-grade methanol. The amounts of methanol and supernatant were weighed on a balance, and a dilution factor was calculated based on mass ratio and the density data of the mixture (8). The supernatant-methanol mixtures were used for analysis of solute concentrations in the equilibrated solution phase with an HPLC method described below. Control experiments were conducted using reactors containing no sorbent for assessing loss of solutes to reactor components during sorption tests. Results showed that average system losses are consistently less than 4% of initial concentrations for the two HOCs; hence, no correction was made during reduction of sorption data. Solute Analysis. Both phenanthrene and naphthalene concentrations in the initial and the equilibrated supernatants were measured with a reverse-phase HPLC (ODS, 5 µm, 2.1 × 250 mm column on a Hewlett-Packard model 1100) with both diode array UV detector (wavelength at 250 nm for phenanthrene and 220 nm for naphthalene) and fluorescence detector (model HP 1046A, UV excitation/emission wavelengths at 250/332 nm for phenanthrene and at 250/ 364 nm for naphthalene). External methanol phase standards of phenanthrene (0.5-1000 µg/L) and naphthalene (6-20 000 µg/L) were used to establish linear calibration curves for both detectors. The elute solvent used was a 5844

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mixture of HPLC-grade acetonitrile and Milli-Q water at a volume ratio of 90:10 for phenanthrene and 88:12 for naphthalene. Each aqueous-phase solute concentration was calculated from the solute concentration in its respective aqueous methanol mixture determined from HPLC and the dilution factor. The solid-phase sorbate concentrations (qe) at equilibrium condition were computed with

qe )

(Co - Ce)V M

(1)

where Ce is the equilibrium solution-phase expressed in micrograms per liter, and qe is in micrograms per gram. Co, V, and M are the initial solution-phase solute concentration (µg/L), volume (L) of the solution phase, and sorbent mass (g) introduced to each reactor, respectively.

Results and Discussion The Freundlich isotherm model commonly used for quantifying equilibrium sorption of HOCs by soils and sediments has the following forms:

qe ) KFCen

(2)

log qe ) log KF + n log Ce

(3)

and

where KF is the sorption capacity-related parameter (with units of (µg/g)/(µg/L)n) and n is the isotherm linearity index. A linear regression procedure using a statistics software (Systat Version 10.0, SYSTAT Inc.) was utilized to fit each Ce-qe data set of a given sorbent-sorbate system into eq 3. The resulting log KF and n values along with their standard deviations, number of observations, and R 2 values are summarized in Table 2. The table also lists the single-point KOC values () (103 qe/Ce)/fOC) in liters per kilogram of organic carbon calculated at Ce ) 0.005, 0.05, and 0.5SW for phenanthrene and naphthalene, respectively, based on TOC (Table 1) and the isotherm parameters (Table 2). The KOC data included are for direct comparison of sorption capacity among different fractions of SOM and for discussion of the KOC dependence on n and hence Ce. Figures 1 and 2 present respectively the phenanthrene and naphthalene sorption isotherms measured for the original and the Soxhlet-extracted soil/sediment samples and the SOM fractions isolated from the samples. Figure 3 shows the phenanthrene and naphthalene sorption isotherms measured for the KB and BC fractions and the char. In all these three figures, the qe data are normalized based on the TOC content of each sorbent to avoid scattering of qe due to large variations in TOC contents among different sorbents tested. The n values of the isotherms are also marked in the figures for direct comparison. Isotherm Nonlinearity. The data presented in Table 2 and in Figures 1 and 2 indicate that all the sorption isotherms measured for the two solutes and the 24 sorbents are nonlinear, with the n values ranging from 0.469 to 0.784 for phenanthrene and from 0.416 to 0.792 for naphthalene. The sorption isotherms measured for the original soil/sediment samples have n values of 0.586 (MS), 0.638 (PS), 0.707 (SS), and 0.784 (RS) for the phenanthrene isotherms and of 0.680 (MS), 0.686 (PS), 0.698 (SS), and 0.708 (RS) for the naphthalene isotherms. These isotherms are generally more nonlinear than those reported for a set of U.S. EPA reference soils and sediments (52) collected from United States, likely because these Chinese samples contain higher contents of kerogen and BC (Table 1) than the U.S. EPA samples. The soil/

TABLE 2. Freundlich Sorption Isotherm Parameters and Calculated Concentration-Dependent KOC Values phenanthrene

naphthalene single-point KOC (L/kg)

Freundlich isotherm parameters

R2

Ce ) 0.005 SW

Ce ) 0.05 SW

Ce ) 0.5 SW

25 326 32 392 72 062 73 797 24 813 33 518

Sandy Soil (SS) 12 908 14 257 36 793 27 406 12 560 19 489

single-point KOC (L/kg)

Freundlich isotherm parameters

Ce ) 0.005 SW

Ce ) 0.05SW

Ce ) 0.5 SW

0.991 (12)b 0.995 (6) 0.994 (12) 0.994 (12) 0.996 (12) 0.997 (12)

1 944 3 362 9 193 10 617 1 600 1 867

969 1 640 3 547 4 122 861 1 156

483 800 1368 1600 463 716

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samples

n

log KF ((µg/g)/ (µg/L)n)


0.707 (0.0157)a 0.644 (0.0141) 0.708 (0.0112) 0.570 (0.0135) 0.704 (0.0149) 0.764 (0.0108)

0.106 (0.0284)a 0.280 (0.0235) 2.087 (0.0202) 2.306 (0.0201) 1.541 (0.0266) 1.573 (0.0211)

0.993 (18)b 0.991 (20) 0.996 (20) 0.996 (20) 0.992 (20) 0.996 (20)

49691 73594 141140 198719 49022 57645


0.784 (0.0109) 0.603 (0.0104) 0.643 (0.0102) 0.644 (0.0083) 0.722 (0.0148) 0.709 (0.0106)

0.246 (0.0184) 0.913 (0.0154) 2.37 (0.0179) 2.360 (0.0141) 1.656 (0.0268) 1.716 (0.0203)

0.997 (20) 0.995 (20) 0.996 (20) 0.997 (20) 0.993 (20) 0.996 (20)

38309 145419 259337 232121 46034 82073

River Sediment (RS) 23 324 14 201 0.708 (0.0205) 58 329 23 396 0.779 (0.0322) 11 4063 50 168 0.577 (0.0143) 93 349 37 541 0.525 (0.0145) 24 247 12 772 0.666 (0.0125) 41 981 21 473 0.708 (0.0147)

-0.620 (0.0509) -0.688 (0.0864) 1.972 (0.0321) 2.009 (0.0339) 0.787 (0.0309) 0.595 (0.0381)

0.992 (12) 0.993 (6) 0.994 (11) 0.992 (12) 0.996 (12) 0.996 (12)

1 725 2 362 22 405 18 391 1 850 2 335

880 1 420 8 461 6 156 857 1 192

449 854 3195 2061 397 609


0.638 (0.0140) 0.607 (0.0091) 0.545 (0.0092) 0.511 (0.0094) 0.512 (0.0106) 0.704 (0.0145)

0.366 (0.0237) 0.523 (0.0165) 2.496 (0.0166) 2.429 (0.0166) 1.950 (0.0192) 1.680 (0.0283)

0.991 (20) 0.996 (18) 0.995 (20) 0.994 (20) 0.992 (20) 0.992 (20)

59271 90742 263397 201352 66945 72367

Pond Sediment (PS) 25 756 11 192 0.686 (0.0189) 42 064 19 499 0.637 (0.0272) 92 479 32 469 0.568 (0.0179) 65 271 21 158 0.533 (0.0167) 21 783 7 088 0.663 (0.0186) 36 614 18 525 0.783 (0.0151)

-0.674 (0.0486) -0.270 (0.0666) 1.8675 (0.0429) 1.7574 (0.0399) 0.916 (0.0389) 0.343 (0.0390)

0.992 (12) 0.993 (6) 0.990 (12) 0.990 (12) 0.993 (11) 0.996 (12)

2 050 4 730 15 194 9 329 2 606 1 843

994 2 048 5 618 3 180 1 200 1 118

482 887 2077 1084 553 678

original extracted HKB KB BC

0.586 (0.0068) 0.513 (0.0093) 0.615 (0.0102) 0.599 (0.0102) 0.612 (0.0108)

0.106 (0.0124) 0.408 (0.0162) 1.988 (0.0177) 1.967 (0.0178) 1.674 (0.0192)

0.998 (18) 0.994 (20) 0.995 (20) 0.995 (20) 0.994 (20)

71900 136426 293178 264497 81448

Marine Sediment (MS) 27 716 10 684 0.680 (0.0125) 44 473 14 498 ndc 120 790 49 766 0.560 (0.0153) 104 935 41 631 0.630 (0.0189) 33 303 1 3617 0.734 (0.0234)

-0.890 (0.0331) nd 1.262 (0.0389) 1.109 (0.0463) 0.228 (0.0574)

0.997 (12) nd 0.993 (12) 0.991 (12) 0.991 (11)

2 938 nd 11 509 11 179 1 479

1 408 nd 4 178 4 756 801

674 nd 1516 2024 434

char

0.469 (0.009)

3.107 (0.0161)

0.994 (18)

755932

222 578

2.730 (0.0108)

0.998 (20)

41 042

a

One standard deviation.

b

Number of observations.

65 536

n

log KF ((µg/g)/ (µg/L)n)

R2

0.698 (0.0215)a 0.688 (0.0234) 0.586 (0.0147) 0.589 (0.0140) 0.731 (0.0140) 0.792 (0.0144)

-0.856 (0.0542)a -0.641 (0.0571) 1.592 (0.0352) 1.616 (0.0330) 0.426 (0.0335) 0.365 (0.0369)

0.416 (0.0041)

10 694

2786

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FIGURE 1. Phenanthrene sorption isotherms measured for the marine (a), pond (b), and river (c) sediments and the sandy soil (d) and their Soxhlet extracted and isolated SOM samples.

FIGURE 2. Naphthalene sorption isotherms measured for the marine (a), pond (b), and river (c) sediments and the sandy soil (d) and their Soxhlet extracted and isolated SOM samples. sediment samples Soxhlet extracted with the organic solvent mixture exhibit relatively more nonlinear sorption equilibria 5846

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than do the original samples with exception of naphthalene on the river sediment. A possible explanation for the observed

FIGURE 3. Comparison of sorption properties between the KB isolates and freshly prepared char (a and b) and the BC isolates and the char (c and d). discrepancy is that these original samples may contain simple organic chemicals such as fatty acids and organic pollutants such as chlorinated pesticides and PAHs (51). The preloaded organic molecules may have preferentially occupied the high energy “sites” that exhibit more nonlinear sorption. The unoccupied “sites” may exhibit relatively more linear sorption so that the overall sorption isotherms measured for the target sorbates are more linear due to the competitive effect (53). As detailed below, these preexisting simple chemicals in the original samples may have also decreased the sorption capacity for the target solutes. Removal of the chemicals by Soxhlet extraction could increase both the capacity and nonlinearity of the sorption isotherms for the target solutes. Among the four SOM fractions and for a specific HOC solute, HA exhibits more linear sorption isotherm than the HKB and KB fractions. The n values of the phenanthrene sorption isotherms measured for HA, HKB, KB, and BC fractions of the sandy soil are 0.764, 0.708, 0.570, and 0.704, respectively. Similarly, the n values of the naphthalene isotherms measured for the same set of HA, HKB, KB, and BC fractions are 0.792, 0.586, 0.589, and 0.731, respectively. The n value of the isotherm for a given HA is even greater than that of its respective original soil/sediment sample, consistent with prior studies (11, 13). Sorption Capacities. The data summarized in Table 2 indicate that, due to large variations in TOC contents and n values, the log KF values measured for all the sorbents vary from about 0.106 to 3.107 for phenanthrene and from -0.890 to 2.730 for naphthalene. The calculated KOC values exhibit much less variations since the effect of TOC contents was eliminated. In general, Table 2 shows that (i) the KOC value measured for a single sorbent-solute system decreases as a function of Ce, indicating the effect of isotherm nonlinearity; (ii) at lower Ce levels, the original soil/sediment samples exhibit much lower sorption capacities than their respective Soxhlet extracted samples; (iii) among the four SOM fractions

and at a given Ce, KB and HKB fractions have the highest KOC values and BC has the lowest KOC values. At Ce ) 0.5SW, the phenanthrene KOC values of the four original samples fall into a narrow range of 1.07-1.42 × 104, generally consistent with the reported KOC values obtained experimentally or based on log KOC-log KOW correlations (8, 25). At Ce ) 0.005SW, the phenanthrene KOC values of the same set samples scatter in a wider range of 3.83-7.19 × 104. At a given Ce, the Soxhlet extracted soil/sediment samples have KOC value approximately twice as high as their respective original soil/sediment samples. As mentioned above, this is likely due to the preoccupation of the high energy “sites” by the preexisting organic molecules. The KOC values for a specific solute vary dramatically among the four different SOM fractions. The calculated KOC values for phenanthrene at Ce ) 0.005SW are about 1.412.93 × 105 for the HKB and KB fractions, as compared to the phenanthrene KOC values of 5.76-8.21 × 104 for the HAs, 7.36-14.5 × 104 for the Soxhlet-extracted samples, and 3.837.19 × 104 for the original samples at the same Ce level. Similarly, the naphthalene KOC values calculated at Ce ) 0.005SW are 9.19-22.4 × 103, 1.84-2.34 × 103, 2.36-4.73 × 103, and 1.73-2.94 × 103 for the HKB and KB fractions, HAs, Soxhlet extracted, and the original samples, respectively. Sorption on Humic Acids. The phenanthrene and naphthalene isotherms measured for the three HA extracts have n values of 0.704-0.764 and 0.708-0.792, respectively. The phenanthrene and naphthalene KOC values calculated at Ce ) 0.005SW for the HAs are 5.76-8.21 × 104 and 1.87-2.34 × 103, respectively. When compared to the literature data reported for the same sorbates (10, 11, 22, 33), these isotherms are slightly more nonlinear with greater sorption capacities. Further comparison of the elemental compositions of the HAs used in different studies showed that our HA extracts have O/C atomic ratios of 0.38-0.42, lower than the values (0.47-0.52) reported for typical HAs base extracted from soils VOL. 38, NO. 22, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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(27, 33, 54). This is likely because the HAs used in this study were base-extracted after HF/HCl demineralization. They may include HA macromolecules that were either less soluble in base solution when they were bound on mineral surfaces or were formed from hydrolysis of particulate organic matter during HF/HCl treatment. Li et al. (54) showed that eight different HA fractions repetitively extracted from the same batch of Pahokee peat have O/C atomic ratios of 0.36-0.52 and that the HA fractions obtained in the last five cycles of extraction are less polar (O/C e 0.39) and more aliphatic with larger apparent molecular sizes. The preliminary sorption data we collected also showed that these five peat HA fractions had phenanthrene sorption isotherms comparable to the HA sorption data we reported in this study. Another possibility for the observed higher sorption capacities is that the HA extracts might contain very fine BC particles that may not have been removed in several cycles of dissolution/ precipitation/centrifugation procedure. To test this hypothesis, we performed the dichromate oxidation procedure for the three HA extracts and the results showed no measurable BC, suggesting that BC is most likely not present in the HAs. It is well documented in the literature that sorption into the glassy regions of HA matrix follows a dual partitioningadsorption process which exhibit capacity-limiting sorption under equilibrium conditions (9, 10, 18, 22, 33). The nonlinear sorption behavior of HAs observed in this study and also reported by others (10, 18, 22, 33) indicate that HA should not be regarded as a simple linear partitioning medium for HOCs such as PAHs when formulating composite sorption models with a distributed reactivity concept. Sorption on KB Fractions. Compared to the original samples and the HA extracts, the four KB fractions have much higher sorption capacity and significantly more nonlinear isotherms. The sorption isotherms measured for the four KB fractions have n values of 0.511-0.644 for phenanthrene and 0.525-0.630 for naphthalene (Table 2 and Figures 1-3). The KOC values of the four KB fractions at Ce ) 0.005SW are 1.992.64 × 105 and 9.33-18.4 × 103 for phenanthrene and naphthalene, respectively. These KOC values are 2-3 times lower than the values (7.56 × 105 and 4.10 × 104) measured for phenanthrene and naphthalene on the freshly prepared char (Table 2 and Figure 3). It should be pointed out that the phenanthrene isotherm we measured for the char compares favorably with that of a model diesel soot (SRM 2975, NIST) reported by Nguyen et al. (46) and that the phenanthrene KOC value at Ce ) 0.005SW is close to that (5 × 105 at Ce ≈ 0.001SW) of a similar diesel soot reported by Bucheli and Gustafsson (36). These soot materials have N2-gas BET SSAs comparable to our char samples. This suggests that BC of different origins may exhibit similar sorption behavior that is more nonlinear with greater capacity than the KB fractions examined in this study. The KB fractions used in this study were obtained after HF/HCl demineralization, Soxhlet, and base extraction. These fractions contain particulate organic matter such as kerogen, BC, and lignin. In the first paper of this series (30), we used different spectroscopic and microscopic methods for identifying different types of the organic particles. We found that kerogen and black carbon are the predominant components in these fractions and that lignin is at very low contents. The low O/C atomic ratios (Table 1) and high sorption capacity of the KB fractions also indicate low contents of the biopolymer, which is characterized by polar structures with little affinity for phenanthrene (22). Sorption of phenanthrene on BC likely follows both adsorption on external and internal surfaces and pore filling in existing micropores (e.g., refs 15, 17, 43, and 46). Sorption into kerogen matrix may be more complicated because kerogen has both aromatic backbone structures and aliphatic moieties (17). Yang et al. (17) found that the n value of 5848

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phenanthrene sorption isotherm measured for kerogen decreases from 0.64 to 0.39 as the simulated diagenetic condition increases from 200 to 400 °C. The KOC values determined at Ce ) 5 µg/L increase from 5.5 × 105 at 200 °C to 1.2 × 106 at 350 °C and then decrease as kerogen maturation increases further. They proposed three mechanisms for explaining the sorption of phenanthrene on lignite-derived kerogen matrixes: partitioning in aliphatic moieties, adsorption on existing surfaces internal and external to the particle, and adsorption on expandable interlayers of aromatic backbone structures (17). Because its matrix is often compact before being exposed to the surface environment, kerogen has much lower pore volumes than char, and thus the pore filling process is highly unlikely. The results listed in Tables 1 and 2 show that the char with N2-gas BET SSA 5-15 times greater than KB, but it only exhibits KOC values 2-3 times greater at Ce ) 0.005SW. The difference of KOC values between the char and the KB isolates becomes much smaller at higher Ce levels (Figure 3) due to the difference in isotherm nonlinearity. This suggests that adsorption and pore filling of the sorbates in existing pores of the KB isolates is much less important and that absorption within the aliphatic moiety of kerogen and adsorption on expandable aromatic interlayer surfaces may dominate the overall sorption on KB. Impact of Dichromate Treatment on the Sorption Properties of the BC Isolates. As shown in Table 2 and Figures 1-3, the sorption isotherms measured for the four BC isolates have n values ranging from 0.512 to 0.722 for phenanthrene and from 0.630 to 0.731 for naphthalene. They are much greater than the n values of 0.469 and 0.416 respectively for phenanthrene and naphthalene on the char sample. The KOC values calculated at Ce ) 0.005SW for the isolated BC materials range from 4.60 to 8.15 × 104 and 1.48 to 2.61 × 103 for phenanthrene and naphthalene, respectively (Table 2). They are comparably lower than the KOC values of the three HA samples and the four KB isolates. Figure 3c indicates that the phenanthrene isotherms measured for the BC isolates are more linear with much lower capacities that that of the char. Similar trend can also be found in Figure 3d for the naphthalene isotherms. The lower sorption capacity and more linear isotherm exhibited by the BC isolates indicate that the oxidative treatment for removal of kerogen has significantly changed the sorptive properties of the materials. In the isolation procedure, hydrofluoric and hydrochloric acids were used for demineralization of soil/sediment materials, and dichromate and sulfuric acid were used to oxidize other SOM components such as kerogen and HA. BC may be little affected by hydrofluoric and hydrochloric acids, but it can be degraded in hot and acidic chromate solution at a half-life of above 215-410 h (55). As shown in Song et al. (30), the BC isolates have higher contents of carboxylic functional groups on 13C NMR spectra, indicating that the surfaces of BC particles become polar and less favorable for sorption of hydrophobic chemicals after the dichromate treatment. It is apparent that isolation, quantification, and characterization of soot and char materials associated with soils and sediments are still great challenges to researchers. The method we presented in the first paper of this series (30) and other similar oxidation-based procedures (e.g., refs 55 and 56) can be used for quantifying contents of primarily charbased materials. These wet chemical methods are presumably less reliable for recovering submicron soot particles because they can be either lost to decanted supernatant due to their low density or oxidized in dichromate solution because smaller particles have shorter half-lives in the caustic solution. Our data indicate that the BC materials obtained with these procedures are likely not appropriate for HOC sorption studies. The thermal treatment procedure also possesses

FIGURE 4. Calculated contributions of the different SOM isolates to the overall phenanthrene sorption by the soil/sediment samples. unresolved issues as summarized in a recent study (57). When using thermally treated soil samples as sorbents, possible oxidative alterations of the BC surfaces may also cause difference in the measured sorption properties such as increased capacity (47) likely due to charring of organic matter and/or thermal desorption of preloaded organic pollutants or relatively oxidized natural organic matter. To minimize chemical alteration of BC, we recommend physical separation methods such as density-based method using ZnBr2 solution (26), which can be applied after HF/HCl demineralization, Soxhlet, and base extraction. Contributions of Individual SOM Fractions to the Overall Sorption. The differential roles of the SOM fractions in the sorption of phenanthrene and naphthalene by the four soil/ sediment samples are further illustrated in Figures 4 and 5. For better comparison, the isotherms measured for both Soxhlet extracted and original samples and the sum of the contributions from HA and KB are also included in the figures. The isotherms shown in both figures for HA, BC, HKB, and KB were calculated based on the isotherm parameters listed in Table 2 and the mass distribution and TOC content of different SOM fractions given in Table 1. The following equation was used for the calculation:

qei ) f

TOCs i ni K F Ce TOCi

(4)

where TOCs and TOCi are total organic carbon contents of the original soil/sediment sample and the isolated SOM fraction i, respectively; f is the organic carbon-based mass fraction of SOM i; KF and ni are the Freundlich isotherm parameters; and qei is the solid-phase solute concentration contributed from SOM fraction i to the overall sorption of its original sample.

Careful examination of Figures 4 and 5 indicates that, for a given sorbate and for a specific series of SOM samples derived from an original soil or sediment: (i) the HA fraction has the lowest contribution (∼1-5%) to the overall sorption by each sample; (ii) the BC fraction has the second lowest contribution; (iii) both KB and HKB have comparable contributions; (iv) the sum of the sorption capacities of KB and HA fractions is generally lower than that of HKB; and (v) the contributions of HKB and KB fractions even exceed the overall capacity of the Soxhlet extracted sample. It is clear in each figure that the contribution of the HA fraction to the overall sorption by a given soil/sediment is the lowest since its distributed sorption isotherm is located well below the isotherms of all other sorbents of the same sample set. The insignificant role of HA in the overall sorption is apparently due to the low content of HA in each soil/sediment sample tested. As shown in Table 1, the HA contents range from 6.0% to15% of the total SOM. The minor role of HA in the sorption by this particular set of soil/sediment samples is very different from the soils and sediments that have not been impacted by any civil and industrial activity. As described in soil chemistry textbooks, HA should be the major SOM component, and it should dominate the overall HOC sorption by soils that are presumably pristine. FA and lipids, two other fractions of SOM, were not retained in our SOM fractionation procedure (30), and their contributions to the overall sorption by the bulk samples were thus not determined. Due to its high solubility in water of all pH conditions, FA contents in the tested soil/sediment set are expected to be low. The polar nature of FA further limits its contribution to the overall sorption. Lipids, if present in significant quantity in soils/sediments, are an important partitioning phase for HOCs due to their hydrophobic nature. However, when associated with BC and kerogen, lipids may VOL. 38, NO. 22, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 5. Calculated contributions of the different SOM isolates to the overall naphthalene sorption by the soil/sediment samples. block surfaces and pores of organic particles, hence lowering the overall sorption capacity by the bulk samples. The low contribution of BC is primarily due to chemical alteration of its surfaces by dichromate oxidation as described above. Its true contribution is well reflected by the high sorption capacities of the HKB and KB fractions. In fact, the sorption capacities of HKB and KB well exceed those of their respective original and Soxhlet-extracted samples, clearly indicating the dominance of both kerogen and BC particles in the overall sorption of phenanthrene and naphthalene by the bulk soil/sediment samples. Our finding is consistent with the several prior studies that directly or indirectly demonstrated coal and soot particles are very important sorbents in soils and sediments for HOCs (14-16, 26, 35, 36, 44, 45). Another important feature revealed in Figures 4 and 5 is that the isotherms measured for the original and the Soxhletextracted samples are below those of HKB and KB fractions. This clearly indicates that the BC and kerogen associated with soil/sediment aggregates actually exhibit much lower sorption capacities than when they are isolated. We believe this is because these particles are present in soils and sediments as aggregates. Inorganic and organic coating and encapsulation may substantially decrease the exposure of their external and internal surfaces to the solution phase, hence lowering their sorption capacity for HOCs when associated with the bulk soil. Treatment with hydrofluoric and hydrochloric acids had destroyed the inorganic matrixes and exposed the external surfaces of BC and kerogen to solution phase, elevating their capacities for the sorbate molecules. Such a difference in sorption capacity between the isolated and the soil matrix-bound SOM may bear very interesting implication for the rates of sorption and sequestration of HOC in soils and sediments. If the sorption capacity 5850

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of the isolated and clean SOM represented the ultimate HOC sorption capacity for a soil or sediment, the sorption measured in the laboratory could be far from a true equilibrium state. Soils and sediments are in dynamic environmental systems. Inorganic deposits on the surfaces of particulate SOM can dissolve and re-deposit under different physicochemical (e.g., drying-wetting cycle) conditions. Sorption of HOCs by soil/sediment matrixes may continue over extended time periods. The reduced accessibility and hence decreased sorption capacity of kerogen and BC associated with terrestrial soils and sediments may dramatically increase the variability of sorption reactivities for soils and sediments. Such variability can eventually lead to large uncertainties in the prediction of sorption equilibria from the measured contents of kerogen and/or BC along with total organic carbon. It may also challenge the use of model soot or char materials having very large SSAs for precisely quantifying the role of soil/sediment-bound BC in the HOC sorption. This study has revealed that all isolated SOM fractions exhibit nonlinear sorption isotherms and they contribute variously to the overall sorption by the tested soil/sediment samples. Due to its low content in this particular sample set, HA plays an insignificant role in the overall isotherm nonlinearity and sorption capacity. The kerogen and BC particles in the KB isolates exhibit much higher affinities for HOC sorption and greater nonlinear isotherms. They both are the most important active components for the sorption by soils and sediments and are expected to dominate bioavailability and ultimate fate of HOCs in subsurface environments. Our study also indicated that the contributions of these organic particles to the overall sorption capacity of soils and sediments may depend largely on their aggregation with mineral matrixes and their surface coverage by mineral

deposits, natural organic matter, and historically loaded background organic pollutants. These factors should be considered in future studies for mechanistic elucidation and quantitative prediction of HOC sorption by soils.

Acknowledgments The authors thank three anonymous reviewers for their comments and suggestions on this work. This study was funded by the U.S. National Science Foundation (BES-011886) and by CSREES/USDA under Grant 2001-35107-11129. Partial funding was also provided by the Natural Science Foundation of China through the International Young Investigator Program (40128002) to W.H. and the Geochemistry Program (40332019) to P.P.

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Received for review February 16, 2004. Revised manuscript received August 30, 2004. Accepted September 2, 2004. ES049761I

Black Carbon and Kerogen in Soils and Sediments. 2. Their Roles in

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