Environmental Pollution 206 (2015) 24e31

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Characterization and phthalate esters sorption of organic matter fractions isolated from soils and sediments Jie Jin a, c, Ke Sun a, *, Ziying Wang a, Lanfang Han a, Zezhen Pan a, Fengchang Wu b, Xitao Liu a, Ye Zhao a, Baoshan Xing c a b c

State Key Laboratory of Water Simulation, School of Environment, Beijing Normal University, Beijing 100875, China 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, MA 01003, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 April 2015 Received in revised form 22 June 2015 Accepted 25 June 2015 Available online xxx

The sorption of two phthalate esters (PAEs) and phenanthrene (PHE) by different natural organic matter fractions (NOMs) was examined. The surface area of the NOMs correlated positively with the starting decomposition temperature (SDT), implying increased number of micropores with the rise of condensation. Sorption of PHE on nonhydrolyzable carbons (NHCs) and other NOMs was respectively dependent on aromatic and aliphatic C contents. Likely physical blocking of the aliphatic moieties and input of black carbon materials led to elevated sorption capacity for PHE of aromatic domains in the NHCs. Sorption of PAEs by NOMs excluding NHCs was jointly regulated by hydrophobic partitioning and H-bonding interactions. The SDT of the NOMs correlated negatively with the Koc when SDT 304  C, likely because the highly condensed domains may impair the availability of amorphous moieties for sorption. This study highlights the influence of domain accessibility of NOMs on sorption of hydrophobic organic contaminants. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Natural organic matter Sorption Phthalate ester Thermal analysis Domain accessibility

1. Introduction Phthalate esters (PAEs) are one of the most widely used manufactured chemicals (Mackintosh et al., 2004). Approximately 5 million tons are produced globally every year (Blair et al., 2009). Because of widespread use of PAEs-containing products, PAEs are frequently detected in almost all types of environmental samples (Shi et al., 2012; Zeng et al., 2008; Huang et al., 2012). Some PAEs, such as di-n-butyl phthalate (DBP) and butyl benzyl phthalate (BBP), are estrogenic (Harris et al., 1997; Sharpe et al., 1995). The United States Environmental Protection Agency (USEPA) has classified several of the most common PAEs as priority pollutants and endocrine-disrupting compounds (EDCs) (Keith and Telliard, 1979). Soil is expected to be a major reservoir for PAEs (Zeng et al., 2009). A main environmental fate of hydrophobic organic compounds (HOCs) is sorption into natural organic matter (NOM) (Mitchell and Simpson, 2013). Cross-polarization and magic-angle spinning 13C nuclear magnetic resonance (CP-MAS 13C NMR) has

* Corresponding author. E-mail address: [email protected] (K. Sun). http://dx.doi.org/10.1016/j.envpol.2015.06.031 0269-7491/© 2015 Elsevier Ltd. All rights reserved.

shown the emphasis on the NOM fractions characteristics in sorption studies (Salloum et al., 2001). With the help of 13C NMR, some studies suggest the importance of aromatic C for organic compound sorption (Ahmad et al., 2001; Kulikova and Perminova, 2002), whereas other studies emphasized the critical roles of aliphatic C (Salloum et al., 2002; Sun et al., 2008). The conflicting conclusions necessitate the study of other important parameters of NOM influencing sorption of HOCs. The accessibility, depending mainly on conformation, has been proposed to play a key role in HOCs sorption (Mitchell and Simpson, 2013). Several studies have demonstrated that soil Oalkyl and lipid components may block high affinity sorption sites in NOM (Ding and Rice, 2011; Mitchell and Simpson, 2013). It was also hypothesized that the crystalline degree of sorption domains could influence partitioning of HOCs to semicrystalline polymers, because the alignment of crystalline subdomains could affect the availability of amorphous domains for HOC sorption (Guo et al., 2012; Hale et al., 2011). However, for NOM, the specific role of condensed domains in the physical blocking of HOCs sorption sites remains poorly understood. Organic fractions, such as humic acid (HA), nonhydrolyzable carbon (NHC), and humins (HM), isolated from soils or sediments exhibit different degrees of condensation

J. Jin et al. / Environmental Pollution 206 (2015) 24e31

(Sun et al., 2013a; Wang et al., 2011). We hypothesize that the condensed domains in these fractions would affect the accessibility of amorphous domains. Recently, thermal analysis is applied frequently to detect the thermal stability of NOM (Cuypers et al., 2002). Moreover, condensed NOM was reported to be more thermostable than amorphous NOM (Cuypers et al., 2002). Hence, thermal analysis of NOM would provide information about the condensation of NOM and better elucidate the role of condensed domains in blocking sorption sites. The specific objectives of this work were therefore to: 1) compare the sorption properties of PAEs among different NOMs and 2) reveal the role of conformation of NOMs in PAEs sorption. DBP and BBP were selected to represent typical PAEs. Sorption of phenanthrene (PHE), a nonpolar compound, to NOM samples was also examined. It is hypothesized that the conformation of NOMs should play different roles in the sorption of PAEs and PHE due to their different chemical properties.

25

energy levels were assigned as follows: 284.9 eV to CeC, 286.5 eV to CeO, 287.9 eV to C]O, and 289.4 eV to COO (Jin et al., 2014). CPMAS 13C NMR analysis was performed on a Bruker Avance 300 NMR spectrometer (Germany). The 13C NMR conditions and chemical shift assignments are detailed elsewhere (Han et al., 2014). Surface area (CO2-SA) was measured by the gas adsorption using CO2 isotherm at 273 K (Quantachrome Instrument Corp., USA), because previous studies demonstrated that N2 at 77 K was unable to detect NOM microporosity, while CO2 at 273 K could enter the micropores (0e1.4 nm) (Sun et al., 2013b). The CO2-SA was calculated using nonlocal density functional theory (NLDFT) and grand canonical Monte Carlo simulation (GCMC) (Braida et al., 2003). NOM fractions were further subjected to thermogravimetry (TG) and differential thermal gravimetric (DTG) analysis. A 5 mg dry sample was scanned using a Thermogravimetric Analyzer (TGA Q50, TA instruments) from 25 to 945  C at 10  C/min under N2. Analysis of the TG and DTG spectra was performed with Universal Analysis 2000 software (Version 4.3A, TA instruments, USA).

2. Materials and methods 2.3. Sorption experiment 2.1. Sorbents and sorbates A total of 33 sorbents were used here. They included 9 original soil/sediment samples (S1, S2, S3, S4, S5, S6, S7, S8, and S9), 8 HAs samples and 3 de-ashed HA samples (S1-HA1, S3-HA1, S4-HA1, S1HA2, S2-HA2, S1-HA3, S2-HA3, S2-HA4, D-S1-HA1, D-S1-HA2, and D-S2-HA2), 7 NHC samples (S1eNHC, S2eNHC, S3eNHC, S4eNHC, S6eNHC, S8eNHC, and S9eNHC), 2 demineralized fractions (DM) (S1-DM and S2-DM), and 4 HM samples (S1-HM1, S1-HM2, S2HM1, and S2-HM2). The original samples consisted of an albic soil and a black soil, three sandy soils, and four river sediments. They were collected from Sanjiang Plain (Heilongjiang Province, China), an area in the vicinity of Tianjin near Bohai Bay, and the rivers in the Tongzhou district of Beijing, respectively (Fig. S1). The specific description of the extraction method for the investigated NOM fractions has been reported in our previous studies (Sun et al., 2010, 2013a). Briefly, HA1 was progressively extracted 7 times with 0.1 M Na4P2O7 from the bulk soils and sediments; then HA2 was obtained from mixing the 7 extractions with 0.1 M NaOH. HA1 and HA2 were further de-ashed by mixing and shaking with HCl/HF (0.1/0.3 M) to get de-ashed HA samples. The bulk samples were also demineralized with 1 M HCl and 10% (v/v) HF to get the DM fractions; then NHC was extracted from the DM fraction using a HCl/HF/CF3CO2H linas et al., 2001). Using (TFA) method described elsewhere (Ge 0.1 M NaOH, HA3 and HA4 were extracted from the DM and NHC fraction, respectively. The residues after HA2 and HA3 extraction were demineralized to provide the HM1 and HM2 fraction, respectively. An aliquot of the NHC was further heated at 375  C for 24 h with sufficient air for the measurement of black carbon (BC). DBP (99þ%) and BBP (97þ%) were purchased from Dr. Ehrenstorfer GmbH (Germany). PHE (98þ%) was purchased from SigmaeAldrich Chemical Co. (USA). Selected properties of the three chemicals are summarized in Table S1. 2.2. Sorbent characterization The bulk C, H, N, and O contents of all samples were determined using an Elementar Vario ELIII elemental analyzer (Germany) via complete combustion. Ash content was measured by heating samples at 750  C for 4 h. Specific elemental composition (e.g. C, O, N, and Si) and functionalities of the top surface layer (depth: 3e5 nm) for the tested samples were examined using Thermo Scientific ESCALAB 250 X-ray photoelectron spectroscopy (XPS) (USA) with a Kratos Axis Ultra electron spectrometer using monochromated Al Ka radiation operated at 225 W. The C1s binding

Batch equilibration techniques were conducted to obtain the sorption isotherms of DBP, BBP, and PHE in 8-mL or 40-mL glass vials at 23 ± 1  C. The sorbates were separately dissolved in methanol as stock solutions. Then, the stock solutions were diluted to 10 concentrations, distributed evenly on a log scale using background solution containing 0.01 M CaCl2 in deionized (DI) water with 200 mg/L NaN3 as a biocide. The methanol concentration in aqueous phase was controlled below 0.1% to minimize the cosolvent effect. The initial concentrations (C0, mg/L) (100e10,000 mg/L for DBP, 100e2500 mg/L for BBP, and 2e1100 mg/L for PHE) were chosen to cover the range between detection limit and aqueous solubility. Finally, the solutions were injected into Teflon-lined screw cap glass vials containing appropriate amounts of sorbents (1e150 mg for DBP, 0.5e75 mg for BBP, and 0.2e50 mg for PHE), which was controlled to achieve 20e80% uptake of initially added sorbates at equilibrium. All vials were filled with the solution up to the minimum headspace to reduce solute vapor loss. On the basis of preliminary tests, the vials were shaken at 110 rpm for 7 d (DBP and BBP) or 10 d (PHE) to reach the apparent sorption equilibrium. Control experiments were concurrently run in the same concentration sequence of solutes without sorbents. All the vials were then placed upright for 24 h. About 2 mL supernatant was withdrawn from each vial and added to separate corresponding 2 mL vials. Then the solution-phase sorbate concentration was detected using HPLC (Dionex U3000, reversed phase C18, 25 cm  4.6 mm  5 mm, Supelco, Bellefonte, PA) with a diode array detector and a fluorescence detector. For DBP and BBP, the mobile phase consisted of 80% methanol and 20% water at a flow rate of 1 mL/min. For PHE, isocratic elution was used at a flow rate of 0.8 mL/min with a mobile phase: 90:10 (v:v) of methanol and water. More detailed HPLC running parameters for detection of the tested solutes were reported previously (Sun et al., 2013a, 2012). All samples including the blanks were run in duplicate. Due to negligible mass loss of sorbates as indicated in the blanks, their uptake by all the sorbents was calculated by mass difference. 2.4. Data analysis The sorption data of DBP, BBP, and PHE by the NOM fractions were fitted with the logarithmic form of Freundlich model (FM): logqe ¼ log KF þ nlog Ce

(1)

Kd ¼ qe/Ce

(2)

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J. Jin et al. / Environmental Pollution 206 (2015) 24e31

Koc ¼ Kd/foc

(3)

where qe (mg/g) is the solid-phase concentration, Ce (mg/L) is the solution phase concentration, and KF ((mg/g)/(mg/L)n) is the sorption affinity-related coefficient. n is a site energy heterogeneity parameter, often used as an indicator of isotherm nonlinearity. Kd is the sorption distributed coefficient and foc is the OC content. The Koc values were calculated at three selected concentrations (Ce ¼ 0.01, 0.1, and 1 Sw, aqueous solubility of solute). The fitting was processed with SigmaPlot 10.0. The correlations among properties of sorbents and their sorption coefficients were investigated using SPSS 18.0 software (SPSS Inc). 3. Results and discussion 3.1. Characteristics of NOM fractions Bulk and surface elemental composition of the 9 soil/sediment samples and their respective extracted fractions (HA, DM, HM, and NHC) is listed in Tables 1, S2 and S3. The surface polarities of the tested NOM fractions were generally lower than their corresponding bulk values with the exception of S4-HA1, S2-HA2, S6eNHC, and S9eNHC (Table 1 and Fig. S2), implying that the majority of hydrophilic functionalities were in their interior part, while the hydrophobic moieties faced outside. Additionally, it was found that the tested NOM fractions contained amorphous (29e30 ppm) and crystalline (32e33 ppm) methylene C (Fig. S3). In particular, the NHC fractions had stronger resonance for the amorphous C compared with other NOM fractions and exhibited no clear rigid alkyl-C peak excluding S1eNHC and S2eNHC (Fig. S3). Also, the positive relationship between the surface carboxyl (COOH) C content of all the NOM fractions and the aromaticity (Fig. S4a) suggests that the surface carboxyl C might be attached to the aromatic domains. It has been demonstrated that the aromatic rings substituted with two or more electron-withdrawing groups,

such as carbonyl (ketone, aldehyde) and carboxyl (Ar-COOH, ArCOOR, Ar-CONR2), could serve as p-acceptor sites in OM (Zhu et al., 2004). Therefore, the enhanced p-acceptor ability of the NOM samples here could be expected. CO2-SA values of the NOM fractions ranged from 21.7 to 100.2 m2/g (Table 1), which were comparable to the CO2-SA values (29.2e133.0 m2/g) of the NOM fractions reported recently (Ran et al., 2013). Studies have shown that OC was a major contributor to the CO2-SA of the organic sorbents in soils and sediments and the CO2-SA of NOM was positively correlated with the bulk OC content (Han et al., 2014; Ran et al., 2013; Xing and Pignatello, 1997), consistent with our current data (Fig. S4b). To clearly investigate the microporous properties of the tested samples, OC-normalized CO2-SA (CO2-SA/OC) was employed. Interestingly, the CO2-SA/OC values of the tested NOMs, excluding NHC samples and S1-HA1, varied inversely with the aromatic C content and directly with the aliphatic C content (Fig. 1a, b), providing strong evidence to support the possibility that microporosity of natural geosorbent could be closely associated with its alkyl matrix. For NHC samples, however, no specific relationship was demonstrated between the two molecular descriptors (i.e., aromatic C and aliphatic C) and the CO2-SA/ OC values. This probably was caused by the “pollution” of NHC by BC material (25.2 ± 14.1%) (Table S3) derived from fire events. Nanopores of BC were mainly contributed by their aromatic moieties (Han et al., 2014). Thus, aromatic C of BC, which coexists with natural sorbents, would interfere in the examination of where (aromatic or aliphatic C) the micropores of NOM originate from. The TG and DTG spectra for all the samples are shown in Fig. S5 and their important thermal parameters are listed in Table 1. The TG data showed that HA (excluding de-ashed HAs), HM, and NHC samples started to decompose at 202e224, 234e266, and 275e550  C, respectively, suggesting that NHC fractions would be more thermally stable than HA and HM fractions. Additionally, condensed NOM was reported to be more thermostable than amorphous NOM (Cuypers et al., 2002). Thus, the high SDT values

Table 1 Elemental composition, surface area, and thermal parameters of various organic matters isolated from soils and sediments. Bulk elemental composition

Surface elemental composition (XPS)

Thermal analysis

Samples

C (%)

H (%)

N (%)

O (%)

(O þ N)/C

C (%)

O (%)

N (%)

(O þ N)/C

CO2-SA (m2/g)

CO2-SA/OC (m2/g)a

SDT ( C)b

MDR (%/ C)c

S1-DM S1-HM1 S1-HM2 S1eNHC S1-HA3 S1-HA1 D-S1-HA1 S1-HA2 D-S1-HA2 S2-DM S2-HM1 S2-HM2 S2eNHC S2-HA3 S2-HA4 S2-HA2 D-S2-HA2 S3eNHC S4eNHC S6eNHC S8eNHC S9eNHC S3-HA1 S4-HA1

30.0 20.5 35.3 42.2 45.9 11.4 60.1 27.3 56.3 50.9 19.3 42.9 50.8 52.0 53.6 27.4 58.7 22.4 12.1 10.7 15.7 16.9 37.0 10.6

3.4 2.6 3.9 4.3 3.4 2.5 4.7 3.5 5.7 5.1 2.4 4.3 4.0 3.3 3.1 2.8 4.9 2.4 1.6 1.6 1.2 1.3 4.1 1.8

1.3 1.0 1.9 0.7 2.3 0.5 2.6 2.3 4.8 2.0 1.3 2.7 1.2 2.7 1.7 2.0 3.5 1.0 0.9 0.6 1.3 0.8 4.1 1.4

23.1 19.8 23.0 19.2 27.1 16.8 29.8 21.7 22.7 28.1 19.9 29.1 25.7 31.0 28.8 17.7 24.9 6.7 4.3 2.6 6.8 4.4 18.9 12.7

0.62 0.77 0.54 0.35 0.49 1.15 0.41 0.67 0.38 0.45 0.83 0.56 0.40 0.49 0.43 0.55 0.37 0.26 0.33 0.23 0.39 0.23 0.48 1.02

60.5 62.2 62.5 65.8 63.5 23.0 67.8 40.41 67.8 57.1 40.6 63.1 60.6 58.3 63.5 43.9 72.4 52.4 39.2 16.0 30.0 13.6 53.1 14.2

21.1 16.6 21.3 20.9 25.9 31.9 28.4 29.50 26.5 22.8 15.7 24.0 17.7 22.8 23.2 36.5 24.0 5.0 5.0 5.0 13.2 10.2 28.7 48.3

2.5 2.1 3.1 4.5 3.2 2.5 3.4 4.05 4.9 3.6 3.4 3.1 2.5 3.8 2.7 3.1 3.5 0.8 0.7 0.6 1.6 1.2 4.0 1.1

0.30 0.23 0.30 0.30 0.35 1.13 0.36 0.63 0.35 0.35 0.36 0.33 0.26 0.35 0.31 0.68 0.29 0.07 0.11 0.27 0.37 0.63 0.47 2.62

51.4 42.5 61.6 31.4 21.7 89.4 89.2 52.5 ndd 65.4 61.0 44.3 100.2 70.8 65.9 96.7 nd 57.0 68.7 27.0 56.6 63.1 nd nd

171.3 207.3 174.5 74.4 47.3 784.2 148.4 192.3 nd 128.5 316.1 103.3 197.2 136.2 122.9 352.9 nd 254.5 567.8 252.3 360.5 373.4 nd nd

249 266 240 275 202 206 275 nd nd 225 234 243 278 202 214 nd nd 305 391 374 504 550 224 213

0.251 0.321 0.298 0.146 0.137 0.202 0.103 nd nd 0.153 0.470 0.242 0.129 0.132 0.106 nd nd 0.262 0.426 0.294 0.388 0.461 0.242 0.319

a b c d

Organic carbon (OC)-normalized surface area. Starting decomposition temperature. The organic matter-normalized maximum decomposition rate. Not detected. Humic acid (HA) as well as de-ashed fraction (D-HA), humin (HM), nonhydrolyzable carbon (NHC) and demineralized fraction (DM).

J. Jin et al. / Environmental Pollution 206 (2015) 24e31

of the NHC fractions here reveal their highly condensed structure. In addition, a positive correlation between the SDT and the CO2-SA/ OC of the NOM fractions, excluding S1-HA1 due to its abnormally high CO2-SA/OC values, was noted (Fig. 1c). This may be attributed to the increasing number of micropores in NOM with the rise of thermal stability and condensation of NOM. Moreover, it was found that the SDT of the NOM fractions increased with decreasing bulk polarity (Fig. S4c), suggesting the high hydrophobicity of condensed domains. The fom-normalized maximum decomposition rate (MDR) of the HMs was generally higher than that of HAs excluding S4-HA1 (Table 1). This might result from the HMs higher aliphatic, but lower aromatic C content (Table S4). The aliphatic components have a lower heat-resistance relative to the aromatic ones (Fig. S4d), which is in line with the higher bond energy of conjugated double bonds than that of single aliphatic or CeO and CeH bonds (Leifeld, 2007).

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3.2. Sorption nonlinearity as a function of NOM properties Sorption isotherms of DBP, BBP, and PHE for the NOM fractions are shown in Fig. S6, S7 and S8, respectively, and the Freundlich parameters are listed in Tables S5, S6 and S7, respectively. The sorption isotherms of DBP, BBP, and PHE were generally nonlinear (Tables S5, S6 and S7). Aromatic moieties were proposed to be the predominant components responsible for nonlinear sorption process (Chefetz and Xing, 2009; Han et al., 2014; Xing and Pignatello, 1997). However, in this study, no specific correlation between the aromatic C content and the n values for DBP, BBP or PHE was exhibited. The lack of correlation demonstrates that: 1) nonlinear sorption probably could take place in other components rather than or together with the aromatic moieties, and/or 2) part of the aromatic domains is inaccessible to PHE adsorption. But interestingly, if only data for NHC fractions were plotted, negative correlations between the aryl C content and the n values for DBP, BBP or PHE as well as positive relationships between the alkyl C content and the n values emerged in Fig. S9. NHC fractions consisted mainly of alkyl and aryl C (Fig. S3 and Table S4). Apparently, owing to a limited variation in chemical compositions (e.g. aromaticity or polar functional groups) of sorbents, a strong correlation between the sorption nonlinearity and the aromatic or aliphatic C content could be expected. Moreover, it was proposed previously that the holefilling process facilitates nonlinear sorption in soils and sediments (Chefetz and Xing, 2009; Cornelissen and Gustafsson, 2005) and that the high-surface area carbonaceous geosorbents were responsible for the substantial nonlinearity of organic chemicals (Gustafsson et al., 1996). Consistently, the present study highlights that the n values for PHE of all the NOM sorbents exhibited a downward trend with increasing CO2-SA (Fig. S9c). Additionally, n values for DBP or BBP increased with increasing bulk polarity index ((O þ N)/C) (Fig. S9d), indicating that the polar groups on the NOM fractions facilitated linear sorption for DBP and BBP. Moreover, it was noted that the sorption nonlinearity of DBP and BBP was enhanced with increasing SDT of NOM fractions (Fig. S9e), hence the thermostable NOM fractions would be relatively more heterogeneous in terms of sorption site energy. 3.3. The role of aromatic and aliphatic C in sorption of solutes by different NOM fractions

Fig. 1. Correlations between the organic carbon (OC)-normalized calculative surface area (CO2-SA/OC) of the natural organic matter fractions (NOMs) excluding nonhydrolyzable carbon (NHC) samples and the aliphatic (a) or aromatic (b) C content; between the CO2-SA/OC of all the NOMs and the initial temperature of starting decomposition (SDT) (c).

Among the 4 types of NOM fractions, NHC fractions had the highest logKoc values (Ce ¼ 0.01 SW) for DBP, BBP, and PHE (Fig. S10). HM, HA, and DM fractions had comparable logKoc values for PHE, while logKoc values for DBP and BBP on HM fractions were generally higher than those on HAs and DMs (Fig. S10). The relative importance of the aromatic moieties of NOM in the overall sorption of HOCs (mainly PAHs) has been highlighted in many studies (Ahmad et al., 2001; Kulikova and Perminova, 2002). However, recently, some published data also emphasized the importance of aliphatic structures in PHE sorption by HA, HM, and NHC (Mao et al., 2002; Ran et al., 2007; Wang et al., 2011). In addition, a stronger correlation was expected between the sorption capacity and the content of mobile amorphous domains rather than the total level of aliphaticity (Chefetz and Xing, 2009; Mao et al., 2002). In our study, for the NOMs excluding NHC fractions, PHE logKoc values (Ce ¼ 0.1 and 1 Sw) correlated positively with aliphatic C content and negatively with aromatic C content (Fig. 2a, b). However, for the NHC fractions, PHE sorption capacity correlated negatively with the alkyl C content and positively with the aromatic C content (Fig. 2c, d), despite the fact that the NHC fractions exhibited a broad peak at ~29 ppm belonging to amorphous alkyl C (Fig. S3). To the best of our knowledge, the finding that aromatic C and aliphatic C are the main sorption domains for PHE by NHC and

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J. Jin et al. / Environmental Pollution 206 (2015) 24e31

other NOM fractions, respectively, has not been reported until now. This result is not in line with a previous study (Ran et al., 2007), which claimed that the logKoc of PHE by the NHC fractions was enhanced with the increased aliphatic C content and H/C ratio values. There are probably two reasons for this inconsistency. 1) The inaccessibility of the high-affinity mobile alkyl domains of the NHC fractions. The alkyl domains may be masked by the condensed domains in the NHC fractions, which will be discussed further below, or be blocked by the O-containing aliphatic groups of NHCs, as indicated by the strikingly negative correlation between the PHE logKd and the methoxyl or carbohydrate C content of the NHC fractions (Fig. S11a). 2) The “pollution” of the NHCs by the BC materials as mentioned above, which could elevate the importance of the aromatic C of NOM in HOCs sorption. This was recently proposed to account for no clear relationship between PHE Koc values by NOM fractions and their aliphaticity (Han et al., 2014). The positive relationship between the logKoc values of PHE by the NHC fractions and the aromatic C content (Fig. 2 d) further demonstrates that p-p electron donor-acceptor (EDA) interactions dominate PHE adsorption. The pep interaction is a specific, noncovalent attractive force that exists between electron-rich (pdonor) and -poor (p-acceptor) arenas (Wang and Xing, 2007). Studies have indicated that the p-electron poor sites of the NOMs may interact with the p-electron rich solutes, for example, PAH compounds, to form p-p EDA complexes (Yang et al., 2011; Zhu et al., 2004). Additionally, the p-acceptor ability of the NOM fractions might be enhanced due to the attachment of carboxylic groups to the aromatic domains (Fig. S4a). The enhanced pacceptor ability would assist strong EDA interactions between the NOM fractions and PHE (p-donor). Consistently, a positive correlation between the PHE sorption capacity (logKd) by the NHC fractions and the content of surface carboxylic C was observed (Fig. S11b). However, EDA interactions between PAEs and the electron-deficient p-acceptor domains in the NOMs are unlikely as aromatic ring in PAEs itself is p-acceptor (Sun et al., 2012).

Correspondingly, the logKoc values of DBP and BBP on all the NOM fractions were correlated negatively with the content of surface carboxylic C and positively with the alkyl C content (Fig. S12a, b). This implies that the sorption of DBP and BBP by the NOMs is dependent on the abundance of alkyl C domains. A similar relationship between the sorption capacity of DBP and BBP by biochars and the ‘soft’ alkyl C content was reported elsewhere (Qiu et al., 2014; Sun et al., 2012). These results further reveal that the spatial arrangement of sorption domains in the NOMs also has a great influence on HOC sorption. Specially, the microenvironment of aromatic C would affect their role in HOC sorption. 3.4. The role of polarity of NOM fractions in sorption of solutes Further analysis on our data shows that the sorption capacity of DBP and BBP by the NHC fractions was negatively correlated with surface polarity (Fig. 3a). This implies that the polar groups could reduce the accessibility of DBP and BBP to sorption domains (alkyl C) of NHCs, thus reducing the Koc values. However, such a mechanism does not account for the positive correlation between the sorption capacity of DBP and BBP on the NOM fractions excluding NHCs and the content of polar groups determined by 13C NMR (Fig. 3b). Dissimilar roles of polarity or polar groups in sorption capacity of NHCs and other NOM fractions reveal their different sorption mechanisms for DBP and BBP, which might be attributed to their different conformation. Sorption of DBP and BBP by the NHCs was mainly governed by hydrophobic interactions, while Hbonding and hydrophobic interactions jointly regulated the sorption of DBP and BBP on other NOM fractions. Our data further shows that the abundance of carbohydrate C components of the NOMs excluding NHC samples was positively correlated with the sorption capacity for DBP and BBP (Fig. 3c). This strongly supports the idea that the carbohydrate C of the NOMs excluding NHC fractions is the principal functionality for H-bonding formation with DBP and BBP. The result, however, contradicts a recent study,

Fig. 2. Correlations between the sorption capacity (logKoc) of phenanthrene (PHE) by the natural organic matter (NOM) fractions excluding nonhydrolyzable carbon (NHC) samples and the aliphatic (a) or aromatic (b) C content; between the logKoc of PHE by the NHC samples excluding S9eNHC and the alkyl (c) or aromatic (d) C content.

J. Jin et al. / Environmental Pollution 206 (2015) 24e31

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Fig. 3. Correlations between the sorption capacity (logKoc) of di-n-butyl phthalate (DBP) (a) and butyl benzyl phthalate (BBP) (b) by the nonhydrolyzable carbon (NHC) samples and the surface polarity; between the logKoc of DBP (c) and BBP (d) by the natural organic matter (NOM) fractions excluding NHC and S2-HA4 (the color legends) samples and the polar C content; between the logKoc of DBP (e) and BBP (f) by the NOM fractions excluding NHC samples and the carbohydrate C content. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

which demonstrated that the carbohydrate components of soil may block high affinity sorption sites for bisphenol A, atrazine, and diuron (Mitchell and Simpson, 2013). The difference between the previous and the present study could be attributed to that the geosorbents used in the previous study were bulk soils rather than NOM fractions. NOM fractions and bulk soil samples have significantly different composition and structure, such as OC content, aromaticity, and degree of condensation, which have been proven to be key factors affecting the sorption behavior of HOCs by the NOMs in previous studies (Kang and Xing, 2005; Sun et al., 2013a; Wang et al., 2011; Yang et al., 2011). Furthermore, the logKoc values of PHE by all the NOM fractions were negatively correlated with the surface polar C content (Fig. S11c). The surface polar groups offered active sites for water clusters formation at their surface through Hbonding, thus reducing the accessibility of PHE molecules to the sorption domains in its matrix. 3.5. The role of thermal stability of NOM fractions in sorption of solutes It was interesting to note that when SDT 304  C (excluding D-

S1-HA1), there was a positive relationship between the SDT values of NOM fractions and the logKoc values of DBP, BBP, and PHE (Fig. 4a, c and e). Whereas, for samples with SDT 304  C, a negative relationship between the SDT values and the logKoc values of DBP and BBP was observed (Fig. 4b, d). The inconsistent relationship between the SDT of the NOM fractions and their sorption for solutes has never been reported before. It was mentioned above that the high SDT values of the NOM fractions reflected their highly condensed structure. Therefore, the inconsistent relationship between the SDT values of the NOM fractions and the sorption capacity of DBP, BBP, and PHE highlight the influence of domain conformation (condensed vs. amorphous) on sorption of organic compounds. It was proposed that the condensation of the sorption domains could influence the partitioning of the HOCs in semicrystalline polymers, because the crystalline subdomains are unavailable or show weak sorption capacity for the HOCs and their alignment could affect the availability of the amorphous domains for HOCs sorption (Hale et al., 2011). In our study, for the 19 samples exhibiting SDT 304  C (17 non-NHC samples and 2 NHC samples), nonspecific interactions play a paramount role in their sorption of DBP, BBP, and PHE (Table 1, Fig. 2a and S12a). In addition, alkyl

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Fig. 4. Correlations between the sorption capacity (logKoc) of di-n-butyl phthalate (DBP) (a and b), butyl benzyl phthalate (BBP) (c and d), and phenanthrene (PHE) (e and f) and the initial temperature of starting decomposition (SDT) of the natural organic matter (NOM) fractions excluding D-S1-HA1 sample (the color legends). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

domains (mainly amorphous alkyl C) are accessible to these three compounds. Thus, the increasing number of high-energy sites in microvoids with the rise of condensation of these samples may enhance the sorption of DBP, BBP, and PHE (Fig. 4a, c and e). For samples demonstrating SDT 304  C, they were highly condensed but still contained some amorphous aliphatic C domains (Fig. S3). The condensed domains may reduce the accessibility of DBP and BBP molecules to amorphous alkyl domains, the latter of which are responsible for the sorption of these compounds (Fig. S12a). Therefore, the sorption capacity of DBP and BBP by the NHC fractions decreased with enhanced SDT values. In addition, it was evident that among all the tested compounds, SDT (304  C) of the geosorbents had the lowest influence on the logKoc values of PHE (Fig. 4). This might be because the condensed aromatic domains in these samples were also highly available for sorption of PHE via p-p interactions (Fig. 2d), which may compensate for the impaired sorption of amorphous domains masked by condensed moieties. Therefore, thermal analysis of the NOM fractions would better elucidate the effect of domain conformation on sorption of HOCs. Our study highlights the role of condensed domains in the physical blocking of HOCs sorption sites.

4. Conclusions This study demonstrated that the micropores of NOM fractions excluding NHCs were closely related to their aliphatic matrices. Moreover, due to the attachment of carboxylic groups, aromatic domains of NHCs acted as p-acceptors, which facilitated elevated sorption of PHE, while it inhibited sorption of DBP and BBP. This shows that the microenvironment of aromatic C would affect its role in the sorption of HOCs. In addition, alkyl C and polar groups functioned differently in the sorption of the NHCs and the other NOM fractions for the tested solutes. Furthermore, inconsistent correlations between the SDT of the NOM fractions and their sorption of DBP, BBP, and PHE highlight the role of domain conformation in the sorption of HOCs by the NOMs. Acknowledgments This research was supported by National Natural Science Foundation of China (41273106), Beijing Higher Education Young Elite Teacher Project (YETP0273), and the Scientific Research

J. Jin et al. / Environmental Pollution 206 (2015) 24e31

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Characterization and phthalate esters sorption of organic matter fractions isolated from soils and sediments.

The sorption of two phthalate esters (PAEs) and phenanthrene (PHE) by different natural organic matter fractions (NOMs) was examined. The surface area...
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