Isolation and Characterization of Different Organic Matter Fractions

Apr 17, 2013 - on the surface of SOM.20 Yang et al.21 highlighted the importance of surface ...... Prentice Hall: Upper Saddle River, NJ, 2008. (27) P...
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Isolation and Characterization of Different Organic Matter Fractions from a Same Soil Source and Their Phenanthrene Sorption Ke Sun,*,† Jie Jin,† Mingjie Kang,† Zheyun Zhang,‡ Zezhen Pan,† Ziying Wang,† Fengchang Wu,§ and Baoshan Xing*,∥ †

State Key Laboratory of Water Simulation, School of Environment, Beijing Normal University, Beijing 100875, China Department of Civil and Environmental Engineering, Princeton University, Princeton, New Jersey 08544, United States § State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 100012, China ∥ Stockbridge School of Agriculture, University of Massachusetts, Amherst, Massachusetts 01003, United States ‡

S Supporting Information *

ABSTRACT: Four humic acids (HAs) including de-ashed HAs (D-HAs), two humins (HMs), nonhydrolyzable carbons, and demineralized fraction (DM) were isolated separately from two soils and characterized detailedly; then their sorption of phenanthrene (Phen) was examined. The sequence of removal of HAs and minerals affected molecular composition of HMs. After de-ashing, thermal stability of HAs was improved; however, sorption (logKoc) also decreased due to removal of amorphous alkyl-C. Significant correlations between CO2 surface area of HAs with their sorption coefficients (n and Koc) suggested that pore filling could dominate Phen sorption. Alkyl-C could facilitate elevated thermal stability of OM and Phen sorption, supporting that thermal stability of OM was correlated with Phen sorption. The OM fraction composed of aromatic moieties (AMs) did not produce the highest logKoc, providing strong evidence to dispute the dominant role of AMs in Phen sorption. No correlations between the Koc values of Phen by all tested sorbents and their bulk or surface polarity were observed, suggesting that the role of bulk or surface polarity of OM fractions in regulating Phen sorption was dependent on soil sources. This work shows the major influence of bulk and surface composition of OM and amorphous alkyl-C isolated from a soil sample on hydrophobic organic compounds sorption.

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INTRODUCTION Soil organic matter (SOM) has been identified as a predominant sorbent for hydrophobic organic compounds (HOCs) in soils and sediments as long as the total organic carbon is >0.1%.1 Moreover, physicochemical characteristics of SOM have been correlated with sorption capacity to better identify structures controlling sorption of HOCs in soils and sediments.2−8 With respect to the effect of composition of SOM on sorption of HOCs, current literature still debates on the relative importance of aromatic and aliphatic carbon components. For instance, some works suggested the dominant importance of aromatic carbons of SOM in HOCs sorption,5,6 whereas several other studies recently emphasized the potential role of aliphatic carbons of SOM for HOCs sorption.8−12 Little consensus on which domain of SOM is more important could be partly explained by the fact that most previous studies used SOM sorbents from diverse sources. In this study, a series of SOM were isolated from a single soil; then their sorption of HOCs was conducted to investigate the role of chemical composition (aromatic and aliphatic moieties). Thus, by using a series of SOMs isolated from a same source material, the effect of various minerals, precursors of SOMs, and other factors on © 2013 American Chemical Society

their sorption of HOCs caused by different sources of soil should be eliminated as much as possible. Besides chemical characteristics, organic matter conformation and polarity could influence the sorption of HOCs.13−15 Minerals associated with the SOM were reported to change the conformation of SOM.16 Additionally, alkyl-C structures of SOM consist of the two types of sorption domains, crystalline and amorphous, and amorphous methylene carbons were proposed to be mainly responsible for sorption of HOCs.17 The polarity of SOM can reduce the sorption domain accessibility of SOM and decrease the sorption affinity.18,19 Functional groups (or polarity) could be distributed within and on the surface of SOM.20 Yang et al.21 highlighted the importance of surface polarity for HOCs sorption. However, to date, the influence of conformation and surface polarity of the SOM from a single soil sample on sorption of HOCs has rarely been investigated and is poorly understood. It has been Received: Revised: Accepted: Published: 5138

December 20, 2012 April 9, 2013 April 17, 2013 April 17, 2013 dx.doi.org/10.1021/es3052087 | Environ. Sci. Technol. 2013, 47, 5138−5145

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Article

Figure 1. Flowchart summarizing the major steps including extraction of humic acids (HAs: HA1, HA2, HA3, and HA4), de-ashed HAs (D-HA), humins (HM1 and HM2), nonhydrolyzable carbon (NHC), and demineralized fraction (DM).

mainly composed of quartz and montmorillonite, whereas quartz and hydrous mica were major minerals of albic soil accompanied with a small amount of kaolinite, montmorillonite, feldspar, and chlorite. The extraction steps of various SOM fractions including HAs, HMs, NHC, and demineralized fraction (DM) are shown in the flowchart (Figure 1). HA fraction 1 (HA1) and fraction 2 (HA2) were progressively extracted. HA1 was obtained from mixing extractions with 0.1 M Na4P2O7 seven times; then HA2 was produced from mixing the seven extractions with 0.1 M NaOH seven times.18 The soil residue after HAs extraction was demineralized with 1 M HCl and 10% (v/v) HF at 1:5 solid/liquid ratio and shaking at 40 °C for 5 d continuously. Finally, the supernatant was removed by centrifugation at 4500 rpm for 30 min. The same treatment was repeated six times in order to get HM fraction 1 (HM1) containing an adequate amount of organic carbon (OC) and low mineral content. HA fraction 3 (HA3) was obtained by further extraction with 0.1 M NaOH from soil residue (DM) (i.e., after the above demineralization). The DM residue after HA3 extraction was collected as HM fraction 2 (HM2). HA fraction 4 (HA4) was isolated from the NHC fraction with 0.1 M NaOH after NHC was extracted from the whole soil using a HCl/HF/trifluoroacetic acid (TFA) method described elsewhere.2,29 All SOM fractions were freeze dried prior to analysis and sorption experiments. Various SOM fractions extracted from two soils (A and B) were prefixed with A- and B-, respectively, and labeled as A- or B-SOMs to distinguish the two soils. Phenanthrene (Phen) was purchased from SigmaAldrich Chemical Co and used as a sorbate. Sorbent Characterization. The C, H, N, and O contents of the two soils and their respective SOM fractions were determined using an Elementar Vario ELIII elemental analyzer. Solid-state cross-polarization magic angle spinning 13C NMR spectroscopy analysis was performed on a Bruker Avance 300 NMR spectrometer (Karlsruhe, Germany) operated at 13C frequency of 75 MHz. The NMR running parameters were as follows: spinning rate of 12 kHz, contact time of 3.5 ms, recycle delay time of 5 s, and line broadening of 100 Hz. The chemical shift assignments are depicted elsewhere.2 Surface area (SA-

reported that SOM can have different elemental composition, functionalities, and conformations, depending on origin, age, and other environmental factors22,23 and that these properties of SOM influence their sorption of HOCs.17,24,25 However, Yang et al.21 used only one peat soil to examine the properties of its different SOM fractions and their interactions with HOCs. Considering the variability in sorption characteristics of SOMs and vast differences between peat (i.e., organic) and mineral soils,26 a wide range of soils need to be used. Therefore, in this study two mineral soils, which distribute abundantly in China, were selected to isolate SOM fractions. Various SOM fractions including different forms of humic acids (HAs), humins (HMs), and nonhydrolyzable carbon (NHC) were obtained and characterized to investigate their properties as well as their HOCs sorption. Thermal analysis of SOM fractions has recently gained increasing interest due to its inexpensiveness, little sample preparation, and rapid output of reproducible results27 and thus is applied frequently to detect the thermal stability and recalcitrance of SOM in soils.13,21,28 We hypothesize that the parameters of thermal analysis for SOM could be related to the chemical composition of SOM and sorption capacity of HOCs. Hence, the findings of thermal analysis for SOM would aid to better elucidate the major factors controlling the sorption of HOCs by the SOM. The major objectives of this study were therefore to (1) isolate and characterize different SOM factions, (2) distinguish the relative importance of structural properties (e.g., aromaticity vs aliphaticity), conformation (e.g., amorphous vs condensed, and minerals), and polarity (surface and bulk) of SOM fractions extracted from a same mineral soil for HOCs sorption, and (3) gain insight into the relationships between the parameters of thermal analysis for SOM fractions and their chemical composition as well as their HOCs sorption.

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MATERIALS AND METHODS Soil Organic Matter Fractions and Sorbate. Albic (A) and black (B) soils were collected from Sanjiang Plain, Heilongjiang province, China. Minerals of the black soil were 5139

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Table 1. Bulk and Surface Elemental Composition of the Albic (A) andBlack (B) Soils and Theirs Various Organic Matter Fractions bulk elemental composition (elemental analysis) samples

surface elemental composition (XPS)

(O+N) /C

C (%)

O (%)

N (%)

Si (%)

F (%)

Na (%)

Al (%)

(O+N) /C

CO2−SA (m2/g)

ash (%)

fomb

ratio of polarityd

its SOM fractions 1.00 0.13 nda 3.36 1.32 23.1 2.55 1.03 19.8 3.85 1.88 23.0 4.28 0.69 19.2

nd 0.62 0.77 0.54 0.35

21.6 60.5 62.2 62.5 65.8

37.3 21.1 16.6 21.3 20.9

2.0 2.5 2.1 3.1 4.5

24.8 6.8 5.9 6.4 5.6

nd 6.4 13.2 3.6 UL

nd 2.7 UL 3.1 3.2

14.3 ULc UL UL UL

1.37 0.30 0.23 0.30 0.30

41.0 51.4 42.5 61.6 31.4

nd 42.2 56.1 36.0 33.6

nd 57.8 43.9 64.0 66.4

nd 0.48 0.30 0.56 0.86

45.9 51.8 11.4 60.1

3.35 3.30 2.46 4.73

2.26 1.51 0.54 2.59

27.1 25.9 16.8 29.8

0.49 0.40 1.15 0.41

63.5 67.9 23.0 67.8

25.9 25.8 31.9 28.4

3.2 2.5 2.5 3.4

2.3 1.7 21.0 0.4

5.0 2.2 4.7 UL

UL UL 1.7 UL

UL UL 10.7 UL

0.35 0.32 1.13 0.36

21.7 46.4 89.4 89.2

21.4 17.5 68.8 2.8

78.6 82.5 31.2 97.2

0.71 0.80 0.98 0.88

C (%)

H (%)

N (%)

O (%)

albic (A) A-S0 A-DM A-HM1 A-HM2 ANHC A-HA3 A-HA4 A-HA1 D-AHA1 A-HA2 D-AHA2

soil and 2.7 30.0 20.5 35.3 42.2

27.3 56.3

3.49 5.69

2.29 4.80

21.7 22.7

0.67 0.38

nd 67.8

nd 26.5

nd 4.9

nd 0.6

nd 0.2

nd UL

nd UL

nd 0.35

52.5 nd

45.2 10.5

54.8 89.5

nd 0.92

black (B) B−S0 B-DM B-HM1 B-HM2 BNHC B-HA3 B-HA4 B-HA1 D-BHA1 B-HA2 D-BHA2

soil and 4.4 50.9 19.3 42.9 50.8

its SOMs 1.26 0.30 5.14 2.01 2.42 1.33 4.34 2.65 4.00 1.17

nd 28.1 19.9 29.1 25.7

nd 0.45 0.83 0.56 0.40

25.5 57.1 40.6 63.1 60.6

44.5 22.8 15.7 24.0 17.7

2.7 3.6 3.4 3.1 2.5

27.3 6.8 5.7 4.0 3.6

nd 7.3 26.0 2.9 UL

nd 2.3 5.6 2.9 2.6

nd UL UL UL UL

1.40 0.35 0.36 0.33 0.26

60.9 65.4 61.0 44.3 100.2

nd 13.9 57.1 21.0 18.3

nd 86.2 43.0 79.0 81.7

nd 0.78 0.43 0.59 0.65

52.0 53.6 13.6 53.2

3.34 3.05 2.49 4.06

2.71 1.71 0.53 2.05

31.0 28.8 20.1 30.9

0.49 0.43 1.14 0.47

58.3 63.5 22.5 63.9

22.8 23.2 38.1 27.7

3.8 2.7 2.4 4.7

3.4 3.8 28.4 3.7

8.2 4.5 6.5 UL

3.5 2.3 2.1 UL

UL UL UL UL

0.35 0.31 1.36 0.39

70.8 65.9 131.2 nd

11.0 12.8 63.3 9.8

89.1 87.2 36.7 90.2

0.71 0.72 1.19 0.83

27.4 58.7

2.84 4.87

2.02 3.45

17.7 24.9

0.55 0.37

43.9 72.4

36.5 24.0

3.1 3.5

16.0 UL

0.5 UL

UL UL

UL UL

0.68 0.29

96.7 nd

50.0 8.1

50.0 91.9

1.24 0.78

a

Not detected. bOrganic matter content. cUnder limitation of detection. dRatio of surface polarity index ((O+N)/C) to bulk polarity index ((O +N)/C). Soil (S0), humic acids (HA) as well as de-ashed fractions (D-HA), humins (HM), nonhydrolyzable carbons (NHC), and demineralized fraction (DM). Polarity index ((O+N)/C) of organic matter in the bulk and at the surface of individual samples was calculated from the atomic ratio of (O+N) and C.

SOM fractions (0.1−40 mg) were weighed into in 40 mL glass vials, which resulted in 20−80% uptake of initially added Phen. The initial aqueous-phase Phen concentrations (C0, μg/L) were selected to yield a set of isotherm data for each sample that distributed evenly on a log−log scale plot and to cover approximately 3 orders of magnitude in aqueous-phase solute equilibrium concentrations (Ce, μg/L). Therefore, Phen solutions with different concentrations (5−1100 μg/L according to their detection limits and solubility in water, 1.12 mg/L) were added into the vials and shaken for 14 d. Preliminary tests indicated that the apparent sorption equilibrium was reached before 7 d. The blanks consisted of Phen solution without sorbents. Headspace was kept at a minimum to reduce solute vapor loss. After 14 d, the vials were centrifuged at l000 g force for 20 min, and the supernatant was then withdrawn from each vial for HPLC analysis of the solution-phase sorbate concentration. All experiments including the blanks were run in duplicate and performed at room temperature (23 ± 1 °C). Supernatant was transferred to a 2 mL vial and analyzed using HPLC (HP model 1100, reversed phase C18, 15 cm ×4.6 mm ×4.6 μm, Supelco, Bellefonte, PA) with a diode array detector at 254 nm for concentrations ranging from 50 to 1100 μg/L and a fluorescence detector at excitation and emission

CO2) using CO2 isotherms at 273 K (Quantachrome Instrument Corp., Boynton Beach, FL) were calculated using nonlocal density functional theory (NLDFT) and grand canonical Monte Carlo simulation (GCMC).30 Surface elemental composition and carbon-based functionalities were determined using X-ray photoelectron spectroscopy (XPS) with a Kratos Axis Ultra electron spectrometer using a monochromated Al Ka source operated at 225 W. Spectrum processing was accomplished with the Xpspeak41 software. Thermal gravimetric analysis (TGA) was performed to examine the thermal stability of various SOM fractions. Approximately 5 mg of sample were heated using a thermo-gravimetric analyzer (TGA Q50, TA Instrument) from 25 to 945 °C at a heating rate of 10 °C/min under N2. It has been reported that ash would not decompose during the TGA experiments.23 Therefore, the remaining weight percentages of organic matter at various temperatures were calculated by the exclusion of the ash content from the original values. Sorption Experiment. A batch technique was employed for all sorption experiments in this study. Background solution consisted of 0.01 M CaCl2 in deionized water with 200 mg/L NaN3 to minimize biological activity. A series of the original bulk soil samples (without acid treatment) and their isolated 5140

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wavelengths of 250 and 364 nm, respectively, for concentrations from approximately 0.2 to 50 μg/L. Isocratic elution was used at a flow rate of 0.8 mL/min with a mobile phase of 90:10 (v:v) of methanol and deionized water. Data Analysis. The sorption data for Phen on the original soils and their SOM fractions were fitted to the logarithmic form of Freundlich isotherm model (FM) log qe = log KF + n log Ce

(1)

where qe [μg/g] is the equilibrium sorbed concentration; Ce [μg/L] is the equilibrium aqueous concentration; KF [(μg/g)/ (μg/L)n] is the Freundlich affinity coefficient; and parameter n is a site energy heterogeneity factor, which is often applied as an indicator of isotherm nonlinearity. Here, “n” was calculated using the least-squares regression method. We described the concentration dependence of the affinity of the sorbate to the sorbent by calculating organic carbon (OC)-normalized solid/ liquid distribution coefficients (Koc) at concentrations (Ce) equal to aqueous solubility (Sw) of solute (at 0.01Sw, 0.1Sw and 1Sw) (Table 2).

Figure 2. Cross-polarization magic angle spinning 13C NMR spectra of humic acids (HAs) (HA1, HA2, HA3, and HA4), humins (HM1 and HM2), nonhydrolyzable carbon (NHC), and demineralized fraction (DM) of albic (A) and black (B) soils. “D” denotes de-ashing.

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changed (Table S1, Supporting Information). Also, HM2 of both soils exhibited higher aliphatic C and lower aromatic C than their precursor (DMs) after HA3 extraction (Table S1, Supporting Information and Figure 2), implying that aromatic carbons of DMs might not be bound to soil minerals tightly relative to the aliphatic carbons of DMs and/or the solubility of aromatic moieties of DMs was likely higher than the aliphatic counterparts. Finally, the surface polarity of most of SOM fractions was lower than their corresponding bulk values (Table 1), indicating that part of hydrophilic moieties would be in their interior, indicating an enrichment of hydrophobic components on the surface after drying. It was noted that the SOM fractions contained amorphous (29−30 ppm) and crystalline (32−33 ppm) methylene carbon (Figure 2). The two NHC fractions had strong resonance for the amorphous carbon compared to other SOM fractions (Figure 2). Thus, it can be assumed that NHC fractions possibly have relatively high sorption capacity. It was also noted that HM1 and HM2 contained considerable amounts of carbohydrate after the removal of HA1 and HA2 (Figure 2). Relationship between Thermal Stability of SOM Fractions and Their Chemical Composition and Sorption of Phen. The TGA data showed that HA1 of two soils started to decompose at 147.7 and 145.4 °C, respectively. After de-ashing, the two values correspondingly changed to 205.6 and 202.8 °C (Figure S1, Supporting Information). Hence, deashing treatment could strongly improve the thermal stability of HAs, probably due to chemical composition and conformation changes of organic matter.33 The positive correlation of alkyl carbon contents of the SOM fractions isolated for both A and B soils to their initial temperature of starting decomposition was observed (Figure 3a), suggesting alkyl carbons increase the thermal stability of the SOM. Similarly, it was found that the starting decomposition temperature of the HA fractions increased with their increasing alkyl carbon contents.13 Moreover, it has been reported that alkyl carbon of SOM benefits Phen sorption.2,11,12 Therefore, it can be concluded that the effect of alkyl carbon of SOMs on both their thermal stability and Phen sorption likely together supports the positive relationship between the initial temperature of starting decomposition and sorption capacity of Phen (logKoc) observed (Figure 3b). Therefore, understanding the thermal

RESULTS AND DISCUSSION Characteristics of SOM Fractions. Elemental compositions of two soils and their respective different HA, DM, HM, and NHC fractions are listed in Table 1. Their HA1 fractions had the highest ash content at 68.8% and 63.3%, respectively. After de-ashing treatment with HF and HCl, the ash content of HA1 and HA2 fractions decreased. De-ashing treatment, to a large extent, altered the surface chemical composition of HA1 and HA2 (Table 1). The C and O contents on the surfaces of HA1 and HA2 were enriched, and their surface polarity ((O +N)/C) decreased (Table 1). Moreover, the ratios of the bulk O/C and H/C and bulk polarity ((O+N)/C) also declined (Table 1). Furthermore, considerable amounts of silicon (15.3−28.4%) (Table 1) were observed on the surface of the two original soils, HA1 and HA2. Our observation was inconsistent with a previous report that minerals in soil and sediment particles were mainly covered by organic matter;31 but the result of this study was supported by abundant evidence that substantial parts of mineral surfaces are not covered by organic matter.32 Appreciable difference in distribution of carbon functionalities among these SOM fractions indicated their heterogeneous structures (Figure 2 and Table S1, Supporting Information). Furthermore, among four different HA fractions, the content of aliphatic carbons ranked in an order of D-HA2 > D-HA1 > HA3 > HA4 (D-HA1−2 fractions were used for comparison because NMR data of HA1−2 fractions could not be obtained correctly due to the interference by the abundant minerals in them), and the order of aromatic carbons was reverse (Figure 2 and Table S1, Supporting Information). In addition, HA2 was partly composed of alkyl, methoxyl, carbohydrate, aryl, and carboxyl C, while HA3 consisted of aromatic and carboxyl C mainly (Figure 1), although HA2 and HA3 were extracted from the original soil and its DM, respectively, using the same extraction method, revealing demineralization of the investigated soils altered the functional groups of the HAs extracted from these soils. However, for HM1 and HM2, they had similar functional groups (Figure 2), suggesting the sequence of HA extraction and de-ashing had no obvious effect on the type of functional groups of organic matter in HMs, but the relative abundance of functional groups 5141

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their SOM fractions with exception of A-HA2 and D-A-HA2 were nonlinear and well fitted with the Freundlich equation. Nonlinear coefficients (n) of the A soil and its various SOM fractions were in the range of 0.72−1.01, and those of the B soil were in the range of 0.46−0.86. It was proposed previously that as more molecules entered into micropores, the isotherms became more nonlinear,34 and high-surface area carbonaceous geosorbents were responsible for the substantial nonlinearity of organic chemicals.35,36 It is interesting to observe that the SOM fractions from the B soil exhibited larger SA-CO2 than those from the A soil (Table 1). Moreover, the negative correlation of SA-CO2 of the sorbents to their sorption nonlinearity coefficient (n) and positive correlation between SA-CO2 and sorption capacity (logKoc) at low Phen concentration were observed (Figure S3a,b, Supporting Information ). Therefore, it was concluded that the pores within SOM possibly regulated the isotherm nonlinearity of Phen sorption and may account for the higher sorption capacity (logKoc: 4.66−5.65) of the soil B and its SOM fractions than those (4.44−5.16) of the soil A and its SOM fractions. However, pore filling is not the only one sorption mechanism of SOM fractions because the highest Phen sorption capacity (logKoc) was observed on the NHCs that did not have the highest SA-CO2 (Tables 1 and 2). While for HA1 and HA2 fractions, the significant relationships between the SA-CO2 and Phen sorption coefficients (n and logKoc) (Figure 4) supported that pores within HA1 and HA2 samples were very important for Phen sorption. According to their NMR spectra (Figure 2), the most obvious difference between HA1−2 (HA1 and HA2) and HA3−4 (HA3 and HA4) was that the latter was mainly composed of aromatic moieties, which benefit the π−π interaction. Thus, pore filling would regulate the overall sorption of HA1−2, while the π−π interaction should also be important in addition to pore filling for HA3−4 fractions, HM, and NHC sorbents. Finally, a negative relationship between sorption capacity (logKoc) of SOM fractions and nonlinearity n values (Figure S3c, Supporting Information ) implied that nonlinear sorption relative to the overall sorption might control Phen sorption capacity. The difference in Phen Koc values among the soils and their SOM fractions was not obvious with the exception of NHCs, but their Koc values were in the same order of magnitude as reported previously.2 The highest Koc values of Phen were observed in NHC fractions (Table 2). HM fractions had Phen logKoc values comparable to HAs, while higher than D-HAs. Recently, there were some reports emphasizing the importance of aliphatic moieties in Phen sorption to HAs and HMs.2,11 Moreover, a better correlation was expected between sorption capacity and the level of amorphous domains rather than the total level of aliphaticity. Many studies also reported positive correlations between the aromatic carbons of sorbents and sorption coefficients.37 Furthermore, the above-mentioned conclusions were made based on the fact that the sorbents (i.e., SOM) in their experiments were mostly from different soils, peats, or sediments. In this study, the various SOM fractions were extracted from a same soil to investigate the roles of aliphatic and aromatic structures of SOM in HOCs sorption. In our work, there was no obvious correlation between Phen Koc values of various SOM fractions and their aliphaticity and aromaticity (Table S1, Supporting Information ), although these sorbents were from a same source material. The result implies that the other factors such as conformation and accessibility of domains in the tested sorbents could also

Figure 3. Correlations between the temperatures of starting decomposition of soil organic matter (SOM) fractions extracted from albic (A) and black (B) soils with their alkyl-C contents (a) or sorption capacity of phenanthrene (logKoc) (b), between their organic matter contents (fom) or aromaticity of SOM fractions and final residual weight percentage of organic matter within SOM fractions obtained from thermal gravimetric (TG) spectra (i.e., remained weight percentage of organic matter when heating reached the highest temperature) (c), between ash contents or aromaticity of various SOM fractions from two soils and maximum decomposition rate based on their the differential thermal gravimetric (DTG) spectra (i.e., the largest values of the y-axis in Figure S1b of the Supporting Information) (d).

stability of SOM fractions may be helpful to gain insight into their chemical structure as well as their sorption of HOCs. Additionally, the residual weight percentage of organic matter for SOM fractions enhanced with the elevated organic matter content ( fom) (Figure 3c) and their decomposition rate increased generally with increasing ash content (Figure 3d). It was reported that organic matter was distributed on the surface of minerals in soils and sediments.31 Thus, scattered SOM on the surface of minerals was possibly sensitive and accessible to thermal treatment and easily decomposed compared to the aggregated SOM. Finally, the positive correlation between final residual weight percentage of organic matter according to the thermal gravimetric (TG) spectra of SOM fractions and their aromaticity (Figure 3c) as well as the negative correlation between the maximum of decomposition rate of organic matter based on their differential thermal gravimetric (DTG) spectra and aromaticity (Figure 3d) suggest that aromatic moieties of SOM would be more recalcitrant relative to the lower heat resistance of aliphatic carbon components.21,28 Comparison of Phenanthrene Sorption with Different Organic Matter Fractions. Because of the nonlinearity of the sorption isotherm, the Koc values were calculated at three solute concentrations (Ce) (at Ce = 0.01 SW, 0.1 SW, and 1 SW). The data presented in Table 2 and Figure S2 of the Supporting Information indicate that all sorption isotherms in the soils and 5142

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Table 2. Freundlich Isotherm Parameters and Calculated Concentration-Dependent Distribution Coefficients (LogKd and LogKoc) log Kd

logKoc (mL/g)

Na

R2

Ce = 0.01Sw

Ce = 0.01Sw

Ce = 0.1Sw

Ce = 1Sw

albic (A) soil and its SOM fractions A-S0 1.4 0.85 A-DM 22.3 0.85 A-HM1 22.2 0.88 A-HM2 34.8 0.83 A-NHC 107.3 0.76 A-HA3 16.0 0.90 A-HA4 49.9 0.79 A-HA1 16.5 0.73 D-A-HA1 73.2 0.72 A-HA2 10.5 1.01 D-A-HA2 23.7 0.94

18 18 18 18 18 18 18 18 18 18 18

1.00 1.00 1.00 1.00 1.00 0.99 1.00 0.98 0.99 0.99 1.00

2.99 4.19 4.22 4.36 4.78 4.10 4.48 3.93 4.57 4.03 4.31

4.55 4.71 4.91 4.82 5.16 4.44 4.76 4.87 4.79 4.59 4.56

4.40 4.56 4.80 4.65 4.92 4.34 4.55 4.60 4.50 4.61 4.50

4.25 4.41 4.68 4.48 4.68 4.25 4.34 4.33 4.22 4.62 4.43

black (B) soil and its SOMs B−S0 14.6 B-DM 144.6 B-HM1 159.9 B-HM2 279.2 B-NHC 701.2 B-HA3 136.9 B-HA4 268.2 B-HA1 148.8 D-B-HA1 34.0 B-HA2 83.4 D-B-HA2 43.6

20 20 18 18 18 18 18 20 16 18 20

0.99 0.99 0.98 0.97 0.98 0.99 0.98 1.00 1.00 0.98 1.00

3.65 4.74 4.75 4.97 5.32 4.67 4.93 4.68 4.38 4.54 4.48

5.00 5.03 5.46 5.33 5.62 4.95 5.20 5.55 4.66 5.10 4.71

4.51 4.63 5.03 4.88 5.12 4.51 4.72 5.08 4.52 4.74 4.57

4.01 4.23 4.59 4.42 4.62 4.07 4.24 4.61 4.38 4.38 4.42

samples

KF

n

0.51 0.60 0.57 0.54 0.50 0.56 0.52 0.53 0.86 0.64 0.85

a

Number of data. Koc is the organic carbon (OC) normalized sorption distributed coefficient (Kd); Sw: aqueous solubility of phenanthrene; Soil (S0), humic acids (HA) as well as de-ashed fractions (D-HA), Humins (HM), nonhydrolyzable carbons (NHC) and Demineralized fraction (DM).

Figure 4. Relationships between surface area (SA, determined using CO2 adsorption) of HA1 and HA2 isolated from albic (A) and black (B) soils, respectively, and their sorption coefficients (sorption capacity (logKoc) (a) and nonlinearity n indexes (b)).

influence the Phen sorption. Especially, HA3 and HA4 fractions had the highest abundance of aromatic carbons (83.3−92.5%) among all SOM fractions but did not have the highest Phen Koc values (Table 2 and Table S1, Supporting Information ), demonstrating that aromatic moieties of SOM from a same soil were not a crucial factor to regulate HOCs sorption. In addition, the NHC fractions were composed mainly of alkyl-C and aromatic carbons, unlike HA4 being mainly composed of aromatic carbons. Moreover, both NHC and HA4 contained negligible hydrophilic groups such as carbohydrate and methoxyl. However, NHC fractions of the two soils had higher Phen Koc values than HA4 fractions (Table 2). From their 13C NMR spectra (Figure 2), the remarkable difference between HA4 and NHC was that NHC contained substantial amounts of amorphous alkyl-C in addition to aromatic C compared to

HA4. Therefore, this result further concluded that the amorphous alkyl-C fractions of NHCs in the A and B soils possibly regulated Phen sorption. The above result provided the strong evidence to support that aromatic moieties of SOM were not necessarily related to high sorption capacity of HOCs. Impact of Organic Matter−Mineral Interactions on Phenanthrene Sorption. LogKoc values of Phen (at Ce = 0.01 SW) by HA1 and HA2 slightly decreased after de-ashing (Table 2). Conversely, de-ashing treatment increased Phen Koc values by the two bulk soils (Table 2). De-ashing exerted little effect on isotherm nonlinearity of Phen by HA1 and HA2 of the A soil while greatly weakened the nonlinearity of HA1 and HA2 of the B soil, which was possibly attributed to the difference in the soil source material of HA. In addition, de-ashing increased the total O of HA1 from 16.8% to 29.8% for the A soil and 5143

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that the surface polarity of the SOM fractions isolated from the mineral soil may not necessarily be important, which was inconsistent with the previous result from an organic soil (peat),21 indicating that the influence of the surface polarity of SOM fractions on the sorption of HOCs depends on source materials (mineral vs organic soils in this case). Finally, it was recently reported that old and stable SOM was not necessarily related to molecular structure or thermodynamic stability.28 However, the conclusion was based on the comparison of three different soils; the effect of the molecular structure and/or minerals associated with SOM isolated from a same source on the SOM age was not clear. The extraction of SOM fractions from a same soil will be helpful in investigating the relationship of OM age with its molecular structure and/or mineral association. This study will help to understand the role of the molecular structure and conformation of SOM isolated from a same soil in the sorption of HOCs.

from 20.1% to 30.9% for the B soil (Table 1), but the surface O and Si contents of HA1 and HA2 decreased after de-ashing treatment (Table 1). O-containing functional groups in SOM were reported to serve as ligands to form complexes with metals in minerals.38 These surface O atoms bound to minerals could be removed by de-ashing along with reduction of the surface Si content (Table 1). Also, the change in elemental composition and polarity ((O+N)/C) of HA1 and HA2 after de-ashing shows that such a treatment changed their chemical composition. De-ashing made the surface polarity of HA1 and HA2 become low, which was not in agreement with the previous result. 21 Moreover, it was noted that the surface polarity of all the de-ashed HAs including D-HA1 and D-HA2 was lower than their total polarity, while the surface polarity of the HAs (not de-ashed) was generally higher than their respective total polarity (Table 1), implying that the polar functional groups were relatively concentrated on the surfaces of the HAs compared to their respective de-ashed HAs. It is hypothesized that the geosorbents of higher polarity would have lower sorption for hydrophobic organic compounds because they have substantial hydrophilic moieties, which can provide sites for water cluster formation at their surface through H-bonding. Water clusters reduce the surface hydrophobicity of sorbents and the accessibility of organic compound molecules to sorption domains, as well as compete with sorbate for sorption sites, thus reducing their sorption.13 It is interesting to see that the sorption capacity of HA1 and HA2 was reduced, even though their bulk and surface polarities ((O+N)/C) became lower after de-ashing (Tables 1 and 2). Such a mechanism could not explain the above result. Also there were no significant correlations between the bulk polarity or surface polarity of all tested SOM fractions and their sorption of Phen. Therefore, the polarity of HAs including surface or total polarity in this study possibly did not affect Phen sorption as in other work that emphasized the importance of surface polarity of SOM fractions isolated from an organic soil.21 The difference in the role of surface polarity of SOM fractions in their sorption may be attributed to the difference of source materials between the investigated soils (organic vs mineral soils). As a result, the influence of surface polarity of SOM fractions on sorption of HOCs should be further investigated in the future using more soil types. Impact of De-ashing Sequence on Phenanthrene Sorption. HM1 and HM2 of the soils were extracted by removal of both HAs and minerals, but the sequence was different. HM2 was obtained after removal of minerals first, then HA, while HM1 was done after first removal of HA, then minerals. HM1 had lower OC content than HM2 with their range being 19.3−20.5% and 35.3−42.9%, respectively (Table 1). Therefore, to obtain OC-rich HMs, the removal sequence of minerals and HAs could be selected to target specific geosorbents needed in the future. HM1 had higher sorption capacity (Koc) than their respective HM2 (Table 2). On the basis of their 13C NMR data, HM2 contained higher aromatic carbons and lower alkyl-C content than HM1 (Figure 2 and Table S1, Supporting Information ), suggesting that the first removal of minerals from soil could enhance the aromaticity of HM2 and reduce the sorption of Phen. Environmental Implications. This study provided direct evidence to support that aromatic moieties of SOM from a same mineral soil were not a major factor to govern Phen sorption, while alkyl-C and conformation of SOM could play an important role in their sorption of HOCs. Our results suggest

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ASSOCIATED CONTENT

S Supporting Information *

Table of functional groups from the 13C NMR spectra; figure of thermal gravimetric (TG) and differential thermal gravimetric (DTG) spectra of the humic acids (HA1) as well as de-ashed HA1, humins (HM1 and HM2), nonhydrolyzable carbons (NHC), and demineralized fraction (DM) of the albic and black soils; figure of relationships between between surface area (SA, determined using CO2 sorption) of the various soil organic matter (SOMs) isolated from albic and black soils and their sorption nonlinearity n and sorption capacity (logKoc) and between the nonlinearity n indexes and sorption capacity (logKoc) of the sorbents. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: (86)-10-58807493(K.S.); (1)-413-545-5212 (B.S.X.). Fax: (86)-10-58807493 (K.S.); (1)-413-545-3958 (B.S.X.). Email: [email protected] (K.S.); [email protected] (B.S.X.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was in part supported the National Natural Science Foundation of China (41273106), Program for New Century Excellent Talents in University (NCET-09-0233), Fundamental Research Funds for the Central Universities (2009SD-8), NSF (DEB-1146184), and Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry. The authors thank the three anonymous reviewers for their comprehensive reading of the manuscript and constructive comments.

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REFERENCES

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