Environ. Sci. Technol. 2007, 41, 3952-3958
Strong Sorption of Phenanthrene by Condensed Organic Matter in Soils and Sediments Y O N G R A N , * ,† K E S U N , †,§ Y U Y A N G , † B A O S H A N X I N G , * ,‡ A N D E D D Y Z E N G † State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China, Department of Plant, Soil, and Insect Sciences, University of Massachusetts, Amherst, Massachusetts 01003, and Graduate School, Chinese Academy of Sciences, Beijing 100049, China
The nonhydrolyzable carbon (NHC) and black carbon (BC) in three contaminated soils and seven sediments from the Pearl River Delta and Estuary, China, were isolated upon treatments with an acid hydrolysis method and with a combustion method at 375 °C, respectively, and their sorption isotherms for phenanthrene (Phen) were established. It was found that NHC is chemically and structurally different from the biopolymer and humic substances and consists mainly of aliphatic and aromatic carbon using elemental analysis, 13C nuclear magnetic resonance spectroscopy (13C NMR), and Fourier transformed infrared spectroscopy (FTIR). All the sorption isotherms are nonlinear and are well fitted by the Freundlich model. The single-point organic carbon-normalized distribution coefficient (Koc) measured for the isolated NHC is 1.3-7.7 times higher than that for the bulk samples at the same aqueous concentration of Phen. The NHC fractions play a dominant role to the overall sorption in the bulk samples. The bulk soils and their NHC fractions have lower sorption capacity than the bulk sediments and their NHC fractions, relating to the different source of organic matter between soils and sediments. The Phen sorption capacity in the NHC samples is related significantly to H/C ratios and aliphatic carbon, but negatively to aromatic carbon, demonstrating the important role of aliphatic carbon to the Phen sorption and the fate in the investigated soils and sediments.
Introduction Sorption of hydrophobic organic compounds (HOCs) is an important process because it governs the fate, transport, bioavailability, and toxicity of these compounds in soils and sediments. Sorption of HOCs is directly related to natural organic matter (NOM) present in the soils and sediments. It was reported that NOM comprises two important heterogeneous sorption domains: a “rubbery”, soft, or amorphous domain and a “glass”, hard, or condensed domain (1-3). Several recent studies revealed that kerogen and black carbon materials can be important NOM components in soils and * Address correspondence to either author. Phone: 86-2085290263 (Y.R.); (413)545-5212 (B.X.). E-mail:
[email protected] (Y.R.);
[email protected] (B.X.). † Guangzhou Institute of Geochemistry, Chinese Academy of Sciences. ‡ University of Massachusetts. § Graduate School, Chinese Academy of Sciences. 3952
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sediments (4-9) and that these materials, instead of humic acids, may dominate the overall HOC sorption by soils and sediments (6-7, 10-11). Many investigators reported that the structural components of NOM affect the sorption of HOCs. A positive correlation between HOC sorption capacity and aromaticity of humic and fulvic acids was reported (12-14). Sorption capacity was negatively correlated with certain polarity indices of NOM derived from elemental ratios (15-16). Kile et al. (17) used an index of the polar functional groups in NOM to explain differences in the carbon tetrachloride sorption capacity and affinity in 19 soils and 9 freshwater sediments. However, other investigators (18-20) revealed that aliphatic carbon of NOM could significantly contribute to sorption of HOCs. The above investigations clearly show the importance of identifying the structural components of NOM in soils and sediments. In the previous paper, it was found that NHC including kerogen carbon (KC), black carbon (BC), and aged organic matter is important for the extraction and distribution of native polycyclic aromatic hydrocarbons (PAHs) in the soils and sediments (21). KC is organic carbon disseminated in sediments not soluble in acid, base, and organic solvents (coal particles in sediments now qualify as kerogen). In this paper, NHC is considered to be approximate to, but not equal to, the molecularly uncharacterized OC that constitutes a significant component of resistant organic matter preserved in soils and sediments (22-23). Hydrolyzable organic matter (HC), which includes young organic matter (hydrolyzable sugars and amino acids) (23), is operationally defined as amorphous OC. This study investigates the effect of condensed NOM (i.e., NHC) in soils and recent sediments on sorption and fate of HOCs. We hypothesize that NHC in soils and sediments is very important for the sorption and fate of PAHs. We examine the sorption behavior of Phen on the original soils/sediments and their isolated organic matter fractions to relate the sorption behaviors of Phen to the composition and properties of the organic matter fractions.
Materials and Methods Samples and Isolation of NOM Fractions. Three surface soil samples (HP04, HP05, and HP06) were collected to a depth of 20 cm in July 2002 from Hangpu district of Guangzhou city, China. The six surface sediment samples (0-20 cm) were collected from the Pearl River Estuary (PRE) using a box sampler in August 2002. The water depth was 25 m for C01, 16 m for C02, 25 m for C03, 29 m for C04, 39 m for C05, and 69 m for C08. One river sediment (WR) at water depth of 10 m was collected from the surface layer (0-10 cm) from the Xijiang River of the Pearl River Delta (PRD) in November 2003. The sampling sites are presented in Figure S1 (Supporting Information). NHC and BC were measured upon treatment with a revised HCl/HF/trifluoroacetic acid method on the basis of that of Ge´linas et al. (24) and with a combustion method at 375 °C for 24 h with sufficient air. Specifically, carbonates were first dissolved in 1 N HCl for 24 h. The residual fraction was treated with 1 N HCl and 10% HF for 5 days, which was repeated four times. Polysaccharides were released by trifluoroacetic acid (TFA) hydrolysis. Finally, the residual hydrolyzable organic matter was removed with 6 N HCl at 110 °C for 24 h. The residues (NHC) were obtained. An aliquot of the NHC was heated at 375 °C for 24 h with sufficient air for the BC measurement. A demineralized size fraction (230 µm, WR4DM) for the WR sediment was collected using 10.1021/es062928i CCC: $37.00
2007 American Chemical Society Published on Web 05/04/2007
FIGURE 1. Cross-polarization/magic-angle spinning (CP/MAS) 13C nuclear magnetic resonance (NMR) spectra (a) and Fourier transform infrared spectra (FTIR) (b) of the demineralized WR size fractions (WR4DM) and the NHC samples isolated from the sediments and soils.
TABLE 1. Physicochemical Properties of OC in the Bulk, NHC, BC, and DM Samples samples
OC(bulk) (wt%)
OC(NHC) (wt%)
OC(DM) (wt %)
OC(BC) (wt %)
C01 C02 C03 C04 C05 C08 HP04 HP05 HP06 WR4-DM
1.020 ( 0.079a 0.336 ( 0.031 0.618 ( 0.020 0.680 ( 0.036 0.536 ( 0.027 0.568 ( 0.017 1.770 ( 0.140 3.350 ( 0.177 2.470 ( 0.047 0.557 ( 0.003
36.9 30.7 23.2 4.97 23.6 34.4 50.7 52.3 55.8 25.6
ndb nd nd nd nd nd 20.7 25.5 29.2 23.8
5.20 0.30 3.62 1.25 0.56 nd 16.4 17.3 34.1 nd
a
NHC/OC %
NHC wt%
BC/OC %
BC wt %
aromatic C/NHC(%)
aliphatic C/NHC(%)
H/C (NHC)
O/C (NHC)
H/C (BC)
O/C (BC)
35.4 31.4 34.3 34.9 25.6 45.4 70.5 64.8 55.3 37.1
0.361 0.106 0.212 0.237 0.137 0.258 1.248 2.171 1.349 0.193
15.3 9.15 17.3 4.14 4.73 nd 12.5 10.3 16.2 nd
0.133 0.032 0.049 0.012 0.032 nd 0.221 0.345 0.400 nd
nd 52.7 48.1 nd nd 43.5 64.6 54.2 nd 39.5
nd 40.0 42.1 nd nd 52.1 23.2 38.2 nd 47.2
0.81 0.94 0.94 0.91 0.92 0.94 0.58 0.85 0.56 0.83
0.19 0.27 0.24 0.34 0.28 0.19 0.20 0.24 0.17 0.16
nd nd nd nd nd nd 0.09 0.12 0.09 nd
nd nd nd nd nd nd 0.08 0.13 0.12 nd
Represents averages and standard deviations of total organic carbon.
wet sieving and sedimentation and was treated with 1 M HCl/10% HF. Characterization of NOM Fractions. The NOM fractions were then analyzed for C, H, N, and O using an Elementar Vario ELIII or a Heraeus CHN-O-RAPID elemental analyzer. The cross-polarization (CP) and magic-angel spinning (MAS) 13C NMR spectra of the isolated samples were obtained with a Bruker DRX-400 NMR spectrometer (5). A Midac 2100 infrared spectrophotometer (Irvine, CA) was used to generate the Fourier transformed infrared (FTIR) spectra. The NMR and FTIR spectra for the samples are presented in Figure 1, and the functional group assignments for 13C NMR are listed in Table S1 (Supporting Information). Some of the physicochemical properties for the investigated samples are summarized in Table 1. Sorption Experiments. 14C-radiolabled and unlabeled Phen (>98%) was purchased from Sigma-Aldrich. Its physicochemical property is introduced in a previous paper (25). All sorption isotherms were obtained using a batch equilibration technique in 8 mL or 40 mL glass vials at 25 ( 1 °C. Background solution (pH ) 7) contained 0.01 mol/L CaCl2 and 200 mg/L NaN3 as a biocide. The vials were sealed with Teflon lined screw caps and then with Parafilm (Chicago, IL) to prevent any Phen vapor loss. Preliminary tests showed that the apparent sorption equilibrium was reached at 14 days for the original samples and their NOM fractions, respectively. After the equilibrium, the vials were centrifuged at l000g force for 20 min, and 1 mL of the supernatant was added to 8 mL of Scintiverse cocktail (Fisher Scientific, PA)
b
nd: not determined.
for liquid scintillation counting (Bechman LS6500, Fullerton, CA). Sorption Model. The Freundlich isotherm model has the following form:
log qe ) log K F + n log Ce
(1)
where qe is the solid-phase concentration (µg/g) and Ce is the liquid-phase equilibrium concentration (µg/L). KF is the sorption capacity-related parameter ((µg/g)/(µg/L)n) and n is the isotherm nonlinearity index. The modeling results are summarized in Table 2. The table also lists the single-point organic carbon-normalized distribution coefficients Koc (Koc ) qe/(Cefoc)) in units of mL/g OC calculated at Ce/Sw ) 0.005, 0.05, and 0.5 for Phen, in which foc and Sw are organic carbon content and Phen solubility, respetively. The Phen sorption isotherms and their fitting to the Freundlich equation for the original samples and NOM fractions are presented in Figure S2 (Supporting Information).
Results and Discussions Properties of NOM Fractions. NHC accounts for 25.6-35.4% and 64.8-73.8% of the total organic carbon in the estuary sediments and in the urban soils, respectively (Table 1). BC accounts for 4.14-17.3% of the OC with an average of 11.2% and a standard deviation of 5.03% in the nine soil/sediment samples (Table 1). This value is close to the median value of 9% (quartile range 5-18%) reported for around 300 sediments globally (25). However, different “BC” methods isolate a different part of the BC continuum. The BC isolation method used here isolates the graphitic BC (24). The O/C atomic VOL. 41, NO. 11, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Freundlich Isotherm Parameters and Concentration-Dependent Distribution Coefficients (Koc) for the Samples Koc (mL/g) Ce ) 0.05 Sw
Ce ) 0.5S w
59 800 64 000 74 300 64 700 64 400 48 200 34 600 32 100 33 500
33 900 34 900 37 900 35 500 35 300 28 800 18 900 19 800 18 400
19 200 19 000 19 400 19 500 19 300 17 200 10 300 12 200 10 100
DM (Demineralized Sample) 20 0.994 1.86 22 0.996 1.67
45 900 35 000
25 700 23 600
14 400 15 900
NHC (Nonhydrolyzable Carbon) 24 0.996 2.37 24 0.997 2.42 24 0.999 2.46 24 0.998 2.64 24 0.998 2.66 24 0.995 2.61 24 0.995 2.39 22 0.993 2.42 24 0.994 2.18
145 000 172 000 187 000 272 000 278 000 248 000 135 000 154 000 90 700
75 500 97 200 97 500 144 000 141 000 129 000 60 400 75 800 45 900
39 200 54 800 50 700 75 900 71 900 67 600 27 100 37 400 23 200
349 000 817 000 1629 000
120 000 263 000 583 000
41 500 84 400 208 000
samples
log KF
n
Na
R2
log KFocb
C01 C02 C03 C04 C05 C08 HP04 HP05 HP06
-0.06 ( 0.015c -0.44 ( 0.016 -0.07 ( 0.009 -0.18 ( 0.009 -0.31 ( 0.016 -0.32 ( 0.013 0.03 ( 0.017 0.09 ( 0.008 0.06 ( 0.014
0.754 ( 0.009c 0.737 ( 0.009 0.708 ( 0.005 0.739 ( 0.005 0.739 ( 0.008 0.776 ( 0.006 0.737 ( 0.009 0.790 ( 0.004 0.739 ( 0.007
22 23 22 23 24 24 24 24 24
Bulk 0.997 0.997 0.999 0.999 0.997 0.998 0.997 0.999 0.998
1.96 2.00 2.09 2.01 2.00 1.85 1.74 1.66 1.72
HP04 HP05
1.16 ( 0.023 1.08 ( 0.021
0.748 ( 0.013 0.828 ( 0.011
C01 C02 C03 C04 C05 C08 HP04 HP05 HP06
1.94 ( 0.017. 1.91 ( 0.017 1.85 ( 0.010 1.34 ( 0.013 2.04 ( 0.012 2.14 ( 0.018 2.10 ( 0.016 2.10 ( 0.012 1.93 ( 0.020
0.716 ( 0.009 0.751 ( 0.009 0.716 ( 0.005 0.723 ( 0.007 0.708 ( 0.007 0.718 ( 0.010 0.652 ( 0.009 0.707 ( 0.006 0.704 ( 0.011
HP04 HP05 HP06
2.10 ( 0.018 2.52 ( 0.017 3.08 ( 0.023
0.537 ( 0.011 0.507 ( 0.010 0.554 ( 0.013
a
Number of data.
b
20 22 21
BC (Black Carbon) 0.992 2.89 0.993 3.28 0.99 3.55
KFoc is the OC-normalized sorption capacity coefficient with unit of µg/g-OC/(µg/L)n. c Standard deviation.
ratios of the isolated NHC in eight of the nine samples range from 0.17 to 0.28, and the H/C atomic ratios in the nine samples range from 0.56 to 0.94 (Table 1). As NHC in C04 is as low as 4.97%, high ash content may lead to overestimation of O/C ratio. The degree of NOM maturation increases as the O/C and H/C ratios decrease. The O/C ratios for NHC are lower than the average O/C ratios (0.53) for typical humic substance extracted from soils and sediments (26). The BC samples combusted at 375 °C from the three soil samples have atomic O/C and H/C ratios of 0.078-0.13 and 0.090.12, respectively, suggesting that BC has higher degree of maturation than KC. The NHC percents are close to values of the acid (6 M HCl) insoluble carbon (43-69%) reported for other coastal and marine sediments (23, 27). They further indicated that the acid insoluble carbon contained considerable ancient carbon using carbon isotope techniques. This ancient organic matter includes bitumen or kerogen found in sedimentary rocks that have been thermally matured, combustion products of fossil fuels (BC and soot), and aged terrigeneous organic matter as a result of long residence times within the drainage basin. The terrigeneous organic matter also contains degraded and altered lignin (28). The 13C NMR spectra for the selected NHC samples reveal a large contribution from alkyl carbon (0-45 ppm) and the aryl carbon (93-148 ppm) but a small contribution from methoxyl carbon (45-63 ppm), carbohydrate (63-93 ppm), O-aryl (148-165 ppm), and carboxyl (165-187 ppm) carbon (Figure 1a, Table S1 in Supporting Information). The aliphatic carbon (0-93 ppm) and aromatic carbon (93-165 ppm) range from 23.2 to 52.1% and from 39.5 to 64.6%, respectively (Table 1). The carboxyl and carbonyl carbon ranges from 7.3 to 12.1% for the five samples. In contrast, the 13C NMR spectra for the demineralized sample (WR-DM) reveal a larger contribution from methoxyl carbon (45-63 ppm), carbohydrate (63-93 ppm), O-aryl (148-165 ppm), and carboxyl (165-187 ppm) carbon (Figure 1a). 3954
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The 13C NMR spectrum of the isolated NHC in Figure 1a is very similar to that of the kerogen isolate from the Borden sand (6). Matured and unweathered kerogen often contains only alkyl and aromatic carbon, but less matured amorphous kerogen may have other functional groups indicative of its precursor materials such as humic substances. However, humic substances generally have relatively large contents of the methoxyl, carbonhydrate, alkyl-substituted aromatic, oxygen-substituted aromatic/phenolic, and carboxylic carbon (5, 26). Finally, considerable percentages of aromatic carbon in the isolated NHC from the soils and sediments (Table 1) indicate that they may originate from different thermally matured bitumen or kerogen, aged terrigeneous organic matter, black carbon, or lignin. For infrared (FTIR) analysis (Figure 1b), the peak assignments are methyl C-H stretching compounds (2920 cm-1), methylene C-H stretching (2850 cm-1), C-H bending in methyl groups (1373 cm-1), CdC hydrogen bonded (1511 cm-1), aromatic carbonyl/carboxyl CdO groups (1704 cm-1), aromatic CdC (near 1600 cm-1), and C-O stretching (1200 and 1040 cm-1) (26). On the basis of Figure 1b, the aliphatic groups (2920 cm-1, 2850 cm-1) and aromatic groups (near 1600 cm-1) are obviously stronger than the other groups, consistent with 13C NMR spectrum of these NHC. The CdO band is aromatic carbonyl/carboxyl groups rather than aliphatic CdO (28), demonstrating that the isolated NHC has some aromatic acids and aldehydes. However, the absorption band of CdO group is weaker than that of CdC group. Sorption Isotherms. The data presented in Table 2 and in Figure S2 (Supporting Information) indicate that all of the sorption isotherms for Phen on the 23 samples are nonlinear and well fitted by the Freundlich equation. The n values in the original samples range from 0.71 to 0.79, and those in the NHC samples range from 0.65 to 0.75. The n value for each of the three BC samples in the soils ranges from 0.51 to 0.55, much lower than any one of the other organic fractions. The
FIGURE 2. Correlation analysis of the sorption parameters with NHC properties.
nonlinearity factor n is related to sorption site energy distribution and has been related to heterogeneous glass, hard, or condensed NOM domain and to maturation degree of NOM (1-6, 10). The lower the n value is, the more heterogeneous is the sorption site energy distribution or the higher is the degree of NOM maturation. The OC-normalized Freundlich sorption capacity values log KFoc range from 1.66 to 2.09, from 2.18 to 2.66, and from 2.89 to 3.55 (µg/g OC)/(µg/L)n for the bulk samples, NHC fractions, and three BC fractions, respectively (Table 2). Previous investigations showed that the KFoc values range from 2.4 to 4.1, from 1.7 to 3.8, and from 2.5 to 3.5 (µg/g OC)/(µg/L)n for the sorption of Phen on kerogen, coal, and BC, respectively (25). High KFoc value means high sorption
capacity of Phen on NOM. More discussion on its meaning is detailed in the following section. The Koc value measured for Phen decreases as a function of Ce because of isotherm nonlinearity. Regardless of Ce levels, the original soil/sediment samples exhibit much lower sorption capacities than their NHC isolates (Table 2). Moreover, sorption capacity parameters log KFoc and Koc for the bulk sediment samples and for four of the six NHC isolates are significantly higher than those of the bulk soil samples and their NHC isolates, respectively. The average Koc values for Phen at Ce ) 0.005Sw is 62 600 ( 8500 mL/g and 217 000 ( 5620 mL/g for the six original sediments and their NHC isolates, respectively, as compared to 33 400 ( 1250 mL/g and 126 000 ( 3250 mL/g for the three original soils and VOL. 41, NO. 11, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Sorption isotherms of Phen (circles) and the Freundlich modeling (lines) on the bulk samples, and the contribution of their NOM fractions to the total Phen sorption on the bulk (dashed and dotted lines). their NHC isolates, respectively. It is obvious that the organic matter of the sediments has stronger sorption capacity for Phen than that of the soils whether the polar organic matter/ functional groups are removed or not. As the distance from the Pearl River Estuary increases from C01 to C08, log KFoc and Koc for NHC at three levels of relative solubility significantly increase. From the following section, it is found that the different NOM type, rather than the different polar functional group in the soil and sediment NOM, leads to the observed difference in the sorption capacity between soils and sediments. Among the four NOM fractions (DM, HC, NHC, and BC) and at a given Ce, NHC and BC fractions have the highest Koc values and HC has the lowest Koc values. At Ce ) 0.5SW, the Koc values of the nine original samples fall into a range of 10 100-19 500 mL/g. At Ce ) 0.005SW, the Koc values of the same set of samples range from 32 100 mL/g to 74 300 mL/g. At a given Ce, the two demineralized (DM) soil samples have similar Koc value to their respective original soil samples, suggesting that the NOM property was not greatly changed by the dilute acid treatment (27). The Phen Koc values for three BC at Ce ) 0.005SW are close to those (500 000-1 000 000 mL/g at Ce ) 0.001SW) of the soot samples reported in the literature (29-30). As the method used to get the BC fraction was designed to isolate soot (and also graphitic carbon) (24), this result is therefore not surprising. This suggests that BC of different origins may exhibit similar sorption behavior to the BC fractions examined in this study. Correlation between Sorption Isotherm Parameters and NOM Properties. Figure 2 shows that the sorption nonlinearity factors (n) of Phen on the NHC fractions are significantly related to the aliphatic carbon and are inversely correlated with the aromatic carbon of the NHC fractions (p < 0.01). These correlations strongly suggest that structurally more aromatic NHC exhibits greater isotherm nonlinearity. As the aromatic carbon is usually associated with degree of NOM condensation (1, 4-5, 10), the lower n value may suggest that the degree of NOM condensation is higher. Moreover, significant correlations between log Koc at each of the three Phen concentrations and aliphatic carbon or H/C ratios for the original samples and the NHC fractions are shown (p < 0.01, Figure 2). However, there is negative correlation between log Koc at the three Phen concentrations and aromatic carbon, respectively (p < 0.01), and there is no correlation between the sorption parameters and O/C rations (not shown). Hence, it is concluded that the aquatic organic matter rich in aliphatic carbon and high in H/C in the sediments has stronger sorption capacity for Phen than does terrestrial organic matter rich in aromaticity and low in H/C in the soils. It is controversial whether aromatic or aliphatic groups play a dominant role in the sorption of HOCs. For example, old or relatively aromatic organic matter in humic substances, 3956
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shale, or coal may have higher KOC values than younger organic matter in surface soil/sediment (12-15). In contrast, a significant positive correlation was reported between Phen sorption capacity and content of nonpolar aliphatic poly(methylene)-rich domains and between Phen Koc values and paraffinic carbon content (18-20). Kang and Xing (31) revealed a positive trend between aliphaticity in soil humic substance and log Koc values for Phen. As the NHC fractions isolated in this study for the aquatic sediments contain higher polymethylene carbon (algenan, lipid, cutan, and cutin) and lower aged soil NOM and lignin than those for the soils, they exhibit higher sorption capacity. The glassy and resistant aliphatic carbon might be more nonpolar than the corresponding aromatic carbon including carboxyl-containing lignin, leading to stronger sorption for Phen. Contribution of Each of the NOM Fractions to the Overall Sorption of Phen. Each contribution of the NOM fractions to total sorption (Figures 3 and S3, Supporting Information) was normalized to bulk sample mass by using the equation qe,i ) [Kf,i(Ce)nφoc,ifoc,bulk]/foc,i, where Kf,i and n are the Phen Freundlich sorption coefficients listed in Table 2, φoc,i is the OC mass fraction (NHC/OC and BC/OC), foc,bulk is the fraction organic carbon content of the bulk sample, and foc,i is the fraction organic carbon content in the NOM fraction in Table 1. The contributions of NHC and BC fractions even exceed the overall capacity of the original extracted or DM sample (Figures 3 and S3 in Supporting Information), clearly indicating the dominance of both NHC and BC particles in the overall sorption of Phen by the bulk soil/sediment samples. Our finding is consistent with several prior studies that directly or indirectly demonstrated that kerogen, coal, polymethylene carbon, and BC particles are very important sorbents in soils and sediments for HOCs (4, 11, 18-19, 29-30). Recent investigations also indicated that the condensed NOM fractions including kerogen, coal, and BC in fact can explain >90% of the total sorption (6, 21, 25). It is clear in Figures 3 and S3 in the Supporting Information that the contribution of the HC fraction to the overall sorption by a given soil/sediment is the lowest (
