ES&T Wei, Xin

Role of Structure and Microporosity in Phenanthrene Sorption byNatural and Engineered Organic MatterLanfang Han,† Ke Sun,*,† Jie Jin,† Xin Wei,† Xinghui Xia,† Fengchang Wu,‡ Bo Gao,§ and Baoshan Xing||

†State Key Laboratory of Water Environment 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§State Key Laboratory of Simulation and Regulation of Water Cycle in River Basin, China Institute of Water Resources andHydropower Research, Beijing, 100038, China||Stockbridge School of Agriculture, University of Massachusetts, Amherst, Massachusetts 01003, United States

*S Supporting Information

ABSTRACT: Natural sorbents including one humic acid(HA), humins (HMs), nonhydrolyzable carbons (NHCs), andengineered sorbents (biochars) were subject to bleaching toselectively remove a fraction of aromatic C. The structuralproperties and sorption isotherm data of phenanthrene (Phen)by original and bleached sorbents were obtained. Significantcorrelations between Phen Koc values by all sorbents and theirorganic carbon (OC)-normalized CO2 cumulative surface area(CO2−SA/OC) suggested that nanopore-filling mechanismcould dominate Phen sorption. After bleaching, naturalsorbents still contained large amounts of aromatic C, whichare resistant to bleaching, suggesting that they are derived fromcondensed or nonbiodegradable organic matter (OM). Aftereliminating the effect of aromatic C remaining in the bleached samples, a general trend of increasing CO2−SA/OC of naturalsorbents with increasing aliphaticity was observed, suggesting that nanopores of natural sorbents are partially derived from theiraliphatic moieties. Conversely, positive relationships between CO2−SA/OC or Phen logKoc of engineered sorbents and theiraromaticity indicated the aromatic structures of engineered sorbents primarily contribute to their nanopores and dominate theirsorption of HOCs. Therefore, this study clearly demonstrated that the role of structure and microporosity in Phen sorption isdependent on the sources of sorbents.

■ INTRODUCTION

Sorption of hydrophobic organic compounds (HOCs) to soil/sediment organic matter (SOM) is a crucial factor governingtheir fate in the environment.1 Thus, the sorption behavior ofHOCs in soils/sediments is of growing concern in the vastresearch.2 Numerous findings on sorption mechanisms betweenHOCs and SOM have been documented. It has been proposedthat chemical composition, physical conformation, and polarityof SOM affect HOCs sorption.3 The application of cutting-edgenuclear magnetic resonance (NMR) spectroscopy has empha-sized the importance of chemical composition at the molecularlevel in investigating the sorption mechanism of HOCs bygeosorbents.2−9 Among them, the relative role of aliphatic andaromatic carbon (C) domains within SOM in HOCs sorptionhas drawn particularly great research attention from environ-mental scientists in the past few decades.2 Much work indicatedthe significant contribution of the aromatic moieties of SOM tothe overall sorption of HOCs and the positive correlationsbetween sorption affinities and aromaticity were highlighted inthose studies.4,6,10−13 However, sorption potential of aliphatic

domains has been demonstrated to be largely ignored insorption interactions of HOCs with SOM.14−16 Chefetz et al.17

observed a positive trend between Koc values and thealiphaticity of a series of sorbents with different levels ofaromaticity and aliphaticity. A similar trend was also exhibitedwith humic substances that humins (HMs) were observed tohave higher sorption affinity of phenanthrene (Phen) thanhumic acids (HAs), even though HAs had higher aromaticitythan HMs.18 More recently, Ran et al.19 and Sun et al.20

proposed that Phen sorption was strongly correlated to thecontent of aliphatic moieties of nonhydrolyzable carbon(NHC) and coal samples. These divergent findings suggestthat a consensus on the relative role of aliphatic and aromatic Cwithin SOM in affecting sorption process of HOCs is urgentlyneeded. Recently, to elucidate the role of aliphatic and aromatic

Received: May 5, 2014Revised: July 31, 2014Accepted: September 3, 2014Published: September 3, 2014

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© 2014 American Chemical Society 11227 dx.doi.org/10.1021/es5022087 | Environ. Sci. Technol. 2014, 48, 11227−11234

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C as sorption domains for HOCs, Chefetz and Xing2 collecteda large and diverse set of published data on Phen Koc values,aromaticity and aliphaticity of organic sorbents covering naturaland engineered sorbents. They found that when a large data setwas plotted, no specific correlation was presented betweenPhen Koc values and aromaticity of natural sorbents, includinghumic substances from different sources, biopolymers (such ascellulose, chitin, lignin, cutin, and cutan), diagenesized sampleslike kerogen, and biological samples such as algae, cuticles, andleaves. Interestingly, when the data for engineered sorbents wasadded to this data set, a general trend of increasing Koc withincreasing aromaticity was recorded although a significant linearrelationship between them was not obtained. Conversely, onlyfor natural sorbents, a general trend of increasing phanthreneKoc values with increasing aliphaticity was displayed. If data forengineered sorbents was included, no relationship was exhibitedbetween binding coefficients and aliphaticity.The contribution of pore-filling mechanism to the sorption

of HOCs by SOM has been previously identified.21−23

Especially, it has been showed that the nanopore-filling is thedominant mechanism for sorption of Phen and benzene byNHC and coals.20 It has been mentioned above that a generaltrend of increasing Phen Koc values with increasing aliphaticityof only natural sorbents and a similar trend between Phen Kocvalues and aromaticity of engineered samples were reported byChefetz and Xing.2 If pore-filling mechanism governs thesorption of HOCs by sorbents, it is very reasonable tohypothesized that microporosity of natural sorbents andengineered sorbents should, respectively, be derived fromtheir aliphatic and aromatic moieties. However, how thestructure and microporosity of natural and engineered sorbentsare related to sorption of HOCs is not well understood.One chemical degradation technique, referred to as

“bleaching”, has been previously employed to selectivelyremove noncondensed aromatic moieties such as lignin-likeand polyphenols units in SOM, and simultaneously retain char-derived aromatic C.24 Based on our above hypotheses,bleaching treatment would influence nanopore characteristics,in turn, affect sorption properties of natural and engineeredsorbents in a different pattern. Thus, this technique would aidto test our hypothesis.The major works of this study were therefore to (1) remove

the aromatic components of natural and engineered sorbents bybleaching treatment; (2) determine the nanopore properties oforiginal natural and engineered samples (OR) as well as theircorresponding bleached samples (BL) using CO2 isotherms at273 K; (3) obtain the aliphatic and aromatic C characteristics ofthese OR and BL using cross-polarization magic angle spinningC-13 nuclear magnetic resonance (CPMAS 13C NMR); (4)quantify the sorption affinity of HOCs to these OR and BL. Inthis study, natural organic matter fractions (NOM), includingHA, HM, and NHC, were selected as natural sorbents; biocharsproduced from rice straw and pine wood were used asengineered samples.

■ MATERIALS AND METHODSSorbate and Sorbents. Phen was used as a sorbate and

purchased from Sigma-Aldrich Chemical Co. One riversediment sample (bulk 1) was collected using a stainless steelgrab sampler in July 2008 from a river in the Tongzhou districtof Beijing.25 Three soil samples (bulk 5, bulk 7, and bulk 8)were also collected to a depth of 20 cm in July 2007 from thesurface soils in the vicinity area of Tianjin near Bohai Bay,

China.25 Albic (A) and black (B) soils were sampled fromSanjiang Plain, Heilongjiang province, China.26 The collectedsamples were subjected to a series of treatment to obtaindifferent organic matter fractions including HA, HM, andNHC, whose extraction along with their purification andhomogenization methods were described elsewhere.25,26 Briefly,HA1 fraction was obtained from mixing extractions with 0.1 MNa4P2O7 seven times.27 The soil residue after HAs extractionwas demineralized with 1 M HCl and 10% (v/v) HF at 1:5solid/liquid ratio and shaking at 40 °C for 5 days continuously.Finally the supernatant was removed by centrifugation at 4500rpm for 30 min. The same treatment was repeated for six timesin order to get HM fraction containing adequate amount oforganic carbon (OC) and low mineral content. NHC fractionwas extracted from the whole soil using a HCl/HF/trifluoro-acetic acid (TFA) method described elsewhere.19,28 Blackcarbon (BC) in this study was obtained by heating an aliquot ofthe NHC sample at 375 °C for 24 h with sufficient air.29 Thesix biochars were produced from two kinds of feedstockmaterials, rice straw, and pine wood, respectively. After washingand grinding to obtain a particle size of less than 1.5 mm, thesefeedstocks were charred at 300, 450, and 600 °C, respectively,for 1 h in a closed container under oxygen-limited conditions ina muffle furnace. Then the biochars were washed with 0.1 MHCl followed by deionized (DI) water flushing until neutralpH,30 subsequently oven-dried at 105 °C, and gently milled topass a 0.25 mm sieve (60 mesh) prior to further analysis. Thesebiochar samples were hereafter abbreviated and referred as totheir individual two initial capitals of feedstock source (ricestraw and pine wood) (i.e., RI and PI) and heat treatmenttemperatures (HTT) (300, 450, and 600 °C) (i.e., RI300,RI450, RI600, PI300, PI450, and PI600)The details of bleaching procedures were described else-

where.24 Briefly, bleaching involved treating 10 g of eachsorbent (HA1, NHC1, NHC5, NHC7, NHC8, A-HM, A-NHC,B-HM, B-NHC, RI300, RI450, RI600, PI300, PI450, andPI600) three times with 100 g of sodium chlorite (NaClO2),100 mL of acetic acid (CH3COOH), and 1000 mL of DI waterfor 7 h for each time. All BL were freeze-dried, ground, andstored for their characterization and sorption work.

Sorbent Characterization. The C, H, N, and O contentsof all samples were measured using an Elementar Vario ELIIIelemental analyzer (Germany). Solid-state cross-polarizationmagic-angle-spinning 13C NMR spectroscopy analysis wasperformed on a Bruker Avance 300 NMR spectrometer(Karlsruhe, Germany) operated at 13C frequency of 75 MHzto get structural information on all studied samples. The NMRrunning parameters are available in the Supporting Information(SI) and the chemical shift assignments are depicted else-where.31 Surface area (CO2−SA) was calculated using nonlocaldensity functional theory (NLDFT) and grand canonicalMonte Carlo simulation (GCMC) using CO2 isotherms at273 K (Quantachrome Instrument Corp, Boynton Beach, FL)(SI Figure S1) because previous studies show that N2 at 77 Kwas unable to detect BC microporosity while CO2 at 273 K canenter the micropores (0−1.4 nm).26,32

Sorption Experiment. All sorption isotherms wereobtained using a batch equilibration technique at 23 ± 1 °C.Appropriate amount of investigated samples (0.1−8.0 mg) wereadded to the background solution containing 0.01 M CaCl2 inDI water with 200 mg/L NaN3 to minimize biodegradation.The amount of sorbents was controlled to result in 20−80%uptake of initially added Phen. The initial aqueous-phase Phen

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concentrations (C0, 2−1000 μg/L), which was chosen to coverthe range between detection limit and aqueous solubility (1.12mg/L), were added into the vials and shaken for 10 days.Preliminary tests showed that the apparent sorption equili-brium was reached before 10 days. The blanks consisted ofPhen solution without sorbents. Headspace was kept minimalto reduce solute vapor loss. After being shaken on the rotaryshaker for 10 days, all vials were placed upright for 24 h.30 Thesupernatant was then withdrawn from each vial and wastransferred to a 2 mL vial for analyzing solution-phase sorbateconcentration with HPLC (HP model 1100, reversed phaseC18, 15 cm × 4.6 mm × 4.6 μm, Supelco, PA) with a diodearray detector for concentrations ranging from 2 to 1000 μg/Land a fluorescence detector for concentrations from approx-imately 0.2 to 50 μg/L.27 Isocratic elution was used at a flowrate of 0.8 mL/min with a mobile phase: 90:10 (v:v) ofmethanol and DI water. All samples, along with blanks, weremeasured in duplicate.Data Analysis. The sorption data were fitted to the

logarithmic form of Freundlich isotherm model:

= +q K n CLog log loge F e (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 nis the Freundlich exponential coefficient. The investigatedcorrelations among properties of sorbents as well as theirsorption coefficients of Phen (Pearson correlation coefficients:r, and significant level: p) were obtained from the Pearsoncorrelation analysis by SPSS 16.0 software (SPSS Inc.).

■ RESULTS AND DISCUSSIONCharacteristics of NOM Fractions and Biochars. The

elemental composition, atomic ratio, ash content, and surfacearea of original and bleached samples (NOM fractions andbiochars) are shown in Table 1. The appreciable differences inbulk compositions among various original NOM fractionsrevealed their heterogeneous structures. Moreover, obviouslydifferent chemical compositions detected in NHCs fromdifferent soil/sediment sources (Table 1) were consistentwith the previous literature which postulated that the

Table 1. Yields by Bleaching Treatment, Elemental Compositions and Surface Area Analysis of NOM Fractions and Biochars

mass OC

recovery recovery C H N O (O+N) CO2−SA CO2−SA/OC Ash

samples (%)a (%)b (%) (%) (%) (%) /C (m2/g) (m2/g) (%)

NOM Fractions (Natural Sorbents)HA1 54.2 4.0 2.9 26.5 0.41 24.9 45.9 12.4NHC1 22.4 2.3 1.0 3.6 0.15 57.0 254.7 70.7NHC5 15.7 1.2 1.2 6.7 0.39 44.4 282.8 75.1NHC7 21.4 1.5 1.0 4.8 0.21 40.7 190.4 71.3NHC8 12.1 1.6 0.9 4.4 0.13 9.5 78.1 81.1A-NHC 42.2 4.3 0.7 19.2 0.36 31.4 74.4 33.6A-HM 20.5 2.6 1.0 19.8 0.77 42.5 207.3 56.1B-NHC 50.8 4.0 1.2 25.7 0.40 100.2 197.2 18.3B-HM 19.3 2.4 1.3 19.9 0.83 61.0 316.1 57.1HA1-BL 25.3 9.3 19.9 2.1 1.1 17.7 0.70 17.1 85.8 59.2NHC1-BL 42.5 52.2 27.5 3.3 0.4 16.4 0.46 45.0 163.4 52.4NHC5-BL 18.3 27.3 23.4 1.9 0.8 16.7 0.56 36.2 154.5 57.2NHC7-BL 51.6 49.2 20.4 2.3 0.2 15.5 0.58 25.6 125.6 61.6NHC8-BL 43.2 26.4 7.4 0.9 0.2 13.4 1.38 15.0 203.7 78.2A-NHC-BL 29.9 19.1 27.0 2.1 0.6 15.0 0.43 8.0 29.7 55.3A-HM-BL 30.2 41.7 28.3 2.9 1.7 20.0 0.57 27.1 95.9 47.1B-NHC-BL 20.5 17.2 42.6 3.1 0.9 18.5 0.34 175.9 413.1 34.9B-HM-BL 71.8 43.9 11.8 1.8 1.0 16.8 1.13 33.4 282.2 68.5

Biochars (Engineered Sorbents)RI300 53.2 3.9 1.1 24.2 0.36 188.5 354.3 17.6RI450 57.0 2.6 1.2 15.6 0.22 293.4 514.5 23.6RI600 60.4 1.7 1.1 8.9 0.13 390.6 647.1 27.9PI300 64.7 4.8 0.0 28.6 0.33 155.0 239.6 1.9PI450 73.1 2.8 0.1 20.1 0.21 408.1 558.3 3.9PI600 81.4 2.3 0.1 11.7 0.11 544.6 668.7 4.4RI300-BL 24.4 12.3 26.9 3.0 0.4 24.9 0.70 85.2 316.3 44.9RI450-BL 52.6 36.2 39.2 2.2 0.7 27.8 0.55 130.7 333.5 30.1RI600-BL 66.1 54.9 50.2 1.6 0.8 19.6 0.31 257.2 512.8 27.8PI300-BL 12.4 8.3 43.1 5.0 0.1 45.4 0.79 16.6 38.6 6.4PI450-BL 57.1 40.3 51.6 2.4 0.0 36.6 0.53 110.9 215.0 9.4PI600-BL 69.2 55.4 65.1 2.2 0.0 25.8 0.30 402.1 617.5 6.8

aMass Recovery (%) = M(BL)/M(OR) × 100, bOC Recovery (%) = OC(BL) × M(BL)/[ OC(OR) × M(OR)] × 100, where M is the weight of original orbleached sample (HA, HM, NHC, biochars); Mass Recovery denotes the bleaching treatment yields humic acids (HA), humins (HM),nonhydrolyzable carbons (NHC), pine wood (PI), rice straw (RI), Original samples:OR; Bleached samples: BL.



physicochemical nature of SOM can vary greatly as a functionof the origin, age, weathering, maturation, and soil depth.11,33,34

As for biochars, with increasing HTT, C content increased,while H and O contents as well as bulk polarity decreased asreported elsewhere (Table 1).35 The removal of aromatic C bybleaching greatly altered bulk composition of all samples,including natural and engineered sorbents (Figure 1 and Table

1). From the OC recovery (%) of the tested samples afterbleaching (Table 1 and Figure 1a), most of the C of HA wasremoved because its recovery of OC was very low (9.3%),suggesting that the HA contained small amounts of BC, whichis resistant to bleaching. Additionally, the OC recovery ofbiochars reduced with the increasing HTT (Figure 1b),indicating that the high-temperature biochars contained moreresistant C compared to the low-temperature biochars. Afterbleaching, the C content of investigated samples generallydeclined except for three NOM fractions covering NHC1,NHC5, and A-HM (Table 1), which had high abundance of ashcontents (>55%). The ash contents of these three fractionsconsistently decreased (Table 1), indicating that the increase ofbulk C contents in these three samples after bleaching could be

partly explained by the fact that NaClO2 used in bleachingtreatment can remove a portion of minerals under acidicconditions.24 Furthermore, such a treatment led to the generalincrease in the polarity (e.g., (N+O)/C) except for A-HM andB-NHC (Figure 1c and d) as a portion of aromatic C and theirfunctional groups had been oxidized during the treatment,suggesting that a fraction of hydrophobic aromatic componentswas successfully removed.The 13C NMR spectra also illustrated that bleaching caused

structural modification (SI Figure S2 and Table S1). Accordingto the distribution of C functional groups, the reduction of therelative content of aromatic C was noted in both NOMfractions and biochars after bleaching (SI Table S1). Amongthem, a regular alternation was observed in aromatic C contentof biochars after bleaching that the decreased content ofaromatic C after bleaching declined with the increasing of HTT(SI Table S1 and Figure 2b), indicating that more condensed

aromatic C of the biochars produced at high HTT possibly ismore difficult to be bleached compared to the biochars at lowHTTs. As a result of the reduction of the aromatic C, therelative intensity of aliphatic C (0−108 ppm) was enhanced(Figure 2c and d). Nonetheless, it should be mentioned thatmost bleached samples still contained a considerable portion ofaromatic C (SI Table S1). For instance, NHC1 still had 25.8%of aromatic C after bleaching, which could be attributed to thatlarge percentages of aromatic moieties of the tested sampleswere resistant to bleaching. The percentage of the remainingaromatic C after bleaching to the total aromatic C of theiruntreated counterparts was further calculated (SI Table S1). Itwas found that regarding NOM fractions, the contribution ofbleaching-resistant aromatic C accounted for 6.8%, 25.8−40.6%, and 10.3−49.9% to the total aromatic C of HA, HM,and NHC fractions, respectively. Chefetz et al.24 demonstratedclearly that bleaching, in the case of aromatic substrates, iseffective for decomposing noncondensed aromatic structuressuch as lignin-like and polyphenols units detected in HAs, whilecondensed moieties were not susceptible to be bleached. Theyalso proposed that the residual aromatic C after bleaching likelyoriginated from charcoal and/or charred plant materials,collectively referred to as BC. Moreover, as shown in SI

Figure 1. Organic carbon (OC) recovery % (a and b) of naturalorganic matter (NOM) fractions (left) and biochars (right) afterbleaching; comparison of bulk polarity (c and d), CO2-derivdedcalculative surface area (CO2−SA) (e and f) and OC-normalizedCO2−SA (CO2−SA/OC) (g and h) between NOM fractions (left) orbiochars (right) and their corresponding bleached fractions.

Figure 2. Comparison of aromatic C (a and b) and aliphatic C (c andd) between natural organic matter (NOM) fractions (left) or biochars(right) and their corresponding bleached fractions.



Table S2, the contribution of BC obtained from combustion at375 °C of each NHC sample represented more than 10% to theNHC fractions except of BC5 (7.6%). Therefore, it could beconcluded that the NOM fractions contained a certain amountof BC, which could be also supported by ubiquitous occurrenceof BC in soils/sediments: median BC contents as a fraction oftotal OC are 4% for 90 soils, 9% for 300 sediments and are upto 30−45% in fire-impacted soils.36

The Relationship between Micropore Properties ofNOM Fractions and Biochars and Their Aromatic andAliphatic C. The microporosity and surface characteristics oforganic sorbents in soils/sediments are pivotal for themechanistic evaluation of sorption. It has been shown thatthe traditionally recommended N2 sorption techniques wouldunderestimate the SA of OM with pores less than 0.5 nm.37−40

Since CO2 at 273 K can enter the micropores (0−1.4 nm),41

the application of CO2−SA helps us to gain a better insight intonanoporosity and SA of SOM. The CO2−SA of the naturalsorbents ranged from 9.5 to 100.2 m2/g and CO2−SA values ofthe tested biochars was in the range of 155.0−544.6 m2/g,which was comparable to the CO2−SA of a temperature seriesof wood biochars reported recently.40 Obviously, biocharsexhibit higher CO2−SA than natural sorbents (Table 1). TheCO2−SA values of the NOM fractions obtained in this studywere lower than those of the eight American Argonne Premiumcoals (113−225 m2/g)42−44 and comparable or lower than theCO2−SA values of SOM and coals reported by Ran et al.44 Inthis study, the CO2−SA of all samples generally decreased afterbleaching, except for NHC8, B-NHC (Figure 1e and f). It wasreported that CO2−SA of biochars is positively correlated withtheir OC contents30 and the similar linear correlations werealso observed for NOM fractions in other investiga-tion,39,41,44,45 which was consistent with our data (Figure 3a).This suggests that OC is very likely a major contributor toCO2−SA of sorbents. Therefore, to better compare the impactof the removal of aromatic C on the SA of samples, OC-

normalized CO2−SA (CO2−SA/OC) was employed instead ofCO2−SA. The range of CO2−SA/OC values of the NOMfractions and biochars investigated in this study was 45.9−316.1m2/g and 239.6−668.7 m2/g (Table 1), respectively, suggestingthat besides C content of sorbents, other properties of OMwithin these investigated sorbents, such as chemical composi-tions, molecular structure, configuration and maturation as wellas geochemical alteration, should exert an influence on themicroporosity and SA. It was noted that the CO2−SA/OCvalues of HA, NHC8, and A-NHC were less than 100 m2/g(Table 1), which is different from the previous results that therange of CO2−SA/OC values (113.3−610.5 m2/g) for a widerange of NOM fractions and their average CO2−SA/OC is 185m2/g.39,41,44,45 As presented in Figure 1g and h, the bleachingtreatment, to a dissimilar extent, exerted an influence on CO2−SA/OC of NOM fractions and biochars. With respect to NOMfractions, CO2−SA/OC of six samples (NHC1, NHC5, NHC7,A-NHC, A-HM, and B-HM) decreased after treatment,whereas that of 1HA, NHC8, and B-NHC increased; incontrast, CO2−SA/OC of biochars consistently declined afterthe removal of aromatic C, implying that the micropores ofengineered sorbents were probably derived from aromaticmatrix, while those of natural sorbents were not necessarilyderived from aromatic moieties. In order to further examine themolecular structure of NOM and its relationship with themicropores of OM within natural and engineered sorbents, thecorrelations between CO2−SA/OC and the contents offunctional groups as indicated by 13C NMR were conducted(Figure 4 and SI Figure S3). It was noted that CO2−SA/OC

values of both original and bleached biochars were significantlyand positively correlated with their aromaticity (Figure 4a) andnegative relationships between CO2−SA/OC values of biocharsand their aliphaticity were also detected (Figure 4b), providingthe robust evidence to support that nanopores of engineeredsorbents were majorly contributed by their aromatic moieties.On the other hand, no specific correlations were obtainedbetween aromaticity as well as aliphaticity of only originalNOM fractions and their CO2−SA/OC values (SI Figure S3a

Figure 3. Correlations between CO2-derivded calculative surface area(CO2−SA) of original and bleached natural organic matter (NOM)fractions and biochars and their bulk C content (a); correlationsbetween logKoc values (mL/g) of Phen by original and bleached NOMfractions and biochars and their organic carbon (OC)-normalizedCO2−SA (CO2−SA/OC) (b); correlations between logKoc values(mL/g) of Phen by original and bleached biochars and theiraromaticity (c) and aliphaticity (d).

Figure 4. Correlations between CO2−SA/OC of original and bleachedbiochars and their aromaticity (a) or aliphaticity (b); correlationsbetween calibrated organic carbon (OC)-normalized calculativesurface area (SA) (CO2−SA/OC) of original natural organic matter(NOM) fractions excluding HA1, A-NHC, and NHC8 and theircalibrated aliphaticity (c) or calibrated aromaticity (d).



and b). However, it was interesting to find that when the datafor bleached NOM samples, whose aromatic C was mainlyderived from BC, were added to the data set of SI Figure S3aand b, the general trend of CO2−SA/OC values witharomaticity or aliphaticity was changed (SI Figure S3c andd). Although this change was not very remarkable, it wasassumed to be of significance since it seems to indicate thataromatic C of BC which coexists with natural sorbents would,to some degree, affect the relationship between aromaticity oraliphaticity of natural samples and their CO2−SA/OC.Consequently, to eliminate the effect of BC-derived aromaticC as much as possible, the contents of both natural aromaticand aliphatic C of original NOM fractions were obtained bydeducting contribution of bleaching-resistant aromatic C to thetotal OC of each NOM fraction. Additionally, CO2−SA/OCvalues of original natural samples were also calibrated bydeducting the contribution of CO2−SA/OC of bleachedcounterparts. The calibrated aromaticity, aliphaticity, andCO2−SA/OC values are listed in SI Table S3. Interestingly,the calibrated CO2−SA/OC values of natural sorbents wereclosely related to the calibrated aliphaticity, but negativelyrelated to the calibrated aromaticity excluding HA, A-NHC, andNHC8 because their abnormally low CO2−SA/OC values(Figure 4c and d). The above findings not only suggest thataromatic moieties of BC, which coexists with NOM, couldaffect the structure and microporosity but also demonstrate thatthe microporosity of NOM was closely associated with theiraliphatic matrix, as we hypothesized.The Role of Nanopores, Aromatic and Aliphatic C in

Sorption of Phen by Both NOM Fractions and Biochars.The Freundlich isotherms are shown in SI Figure S4 and S5,and the fitting parameters are listed in SI Table S4. Thesorption isotherms of Phen by original NOM and biocharswere nonlinear with n values being in the range of 0.50−0.89and 0.38−0.71, respectively, and well fitted with the Freundlichmodel (SI Table S4). The isotherms for biochars were all highlynonlinear (n < 0.71), similar results were reported by Lattao etal.,40 reflecting the predominance of adsorption/pore-fillingmechanisms. The removal of certain aromatic moieties bybleaching resulted in the rise of n values as compared to that ofthe untreated samples except for RI600. Especially for NOM,the bleached samples nearly exhibited a linear and partition-type sorption behavior (SI Table S4), which implies that amore expanded sorbent was produced due to the removal ofaromatic moieties and also supports that aromatic moietiesshould be the predominant components responsible fornonlinear sorption process as reviewed by Chefetz and Xing.2

Bleaching exercised a great effect upon Koc (OC content-normalized sorption coefficient) (SI Table S4). Except forNHC8, the Koc of NOM fractions and biochars all decreasedcompared with their untreated samples (SI Table S4), whichwas similar to the results presented by Huang et al.46

Additionally, after removal of aromatic moieties, bulk polarity(e.g., (O+N)/C) of the NOM fractrions and biochars generallyincreased, which may be responsible for their decreasing PhenKoc values because the polarity of SOMs can significantly affectsorption capacity of HOCs and the SOMs with relatively lowpolarity show the higher sorption capacity than those with highploarity.27,47,48 The significant and negative correlation oflogKoc values of Phen by the original and bleached biochars totheir bulk polarity (e.g., (N+O)/C) (SI Figure S6) supportsour hypothesis. However, recently, Lattao et al.40 found that nosimple relationship stands out between logKoc values and O/C

ratio, surface area (N2 and CO2), and porosity and theydemonstrated that sorption is a complex function of biocharproperties and solute molecular structure, and not verypredictable on the basis of readily determined char properties.It has been widely documented that pore-filling mechanismplays a key role in HOCs sorption by microporous solids ofSOM.21,22 For example, Ran et al.21 reported that sorptionbehaviors of Phen and dichlorobenzene (DCB) by kerogenwere satisfactorily explained by hole-filling mechanism. Likethese studies, the significantly positive correlation between Kocvalues of Phen by all original and bleached sorbents and theirCO2−SA/OC obtained in our case (Figure 3b) implied thatpore-filling could be a major mechanism regulating sorptioninteractions of HOCs-SOM. Moreover, the slope of the linearregression line for the NOM fractions was higher than that ofthe biochars (Figure 3b), implying that although the biocharsgenerally have higher CO2−SA per unit mass of their OC thanthe NOM fractions (Table 1), the sorption capacity of CO2−SA per unit mass of OC within NOM fractions could beremarkably higher than that within the biochars in this study.Therefore, it can be assumed that the sorption capacity ofsorbents depends on not only their CO2−SA per unit mass ofOC but also on other factors such as the chemical composition,structure and configuration of the contributor to CO2−SA.Meanwhile, as we demonstrated before, nanopores of naturalsorbents and biochars were perhaps mainly derived from theiraliphatic and aromatic moieties, respectively. Thus, CO2−SAassociated with the aliphatic moieties within NOM fractionsshould have higher sorption capacity compared to the CO2−SAderived from the aromatic matrix within the biochars. As aresult, we must not think only of how much CO2−SA a sorbenthas, but also of its chemical composition (e.g., aliphatic andaromatic moieties) to evaluate its sorption capacity for HOCs.Furthermore, our data showed that the Phen Koc by bothoriginal and bleached biochars was strikingly and positivelyrelated to their aromaticity but negatively correlated to theiraliphaticity (Figure 3c and d). This was exactly the same as thefindings by Chefetz and Xing,2 who observed a general trend ofincreasing Phen Koc values with increasing aromaticity ofengineered samples. However, in our work, there was nosignificant correlation between Phen Koc of these tested NOMfractions and their aromaticity or aliphaticity (SI Figure S3e andf). Similar conclusions were previously reported by Yang et al.49

They performed experiments with sorption of Phen by HA andHM fractions isolated from a single soil sample and showedthat neither aromatic nor aliphatic components of HAs andHMs could serve as predictors of the soil’s ability to sorb Phen.It has been above-mentioned that the aromatic C in NOMfractions might partly originate from BC materials, which wouldinterfere in exploring where (aromatic or aliphatic C) thenanopores of NOM originate from. Additionally, it was notedthat BC appeared particularly higher sorption affinity to Phenwith logKoc (Ce = 0.01Sw) ranging from 5.67 to 6.51 than NHCbecause of high CO2−SA/OC (150.0−887.7 m2/g) resulted byubiquitous micropores (SI Table S2). As long as BC materialsenter into soils and sediments, they would therefore influencethe sorption properties of HOCs by NOM and strengthen theimportance of aromatic C of NOM in HOCs sorption by soilsand sediments contaminated by BC, thus, the role of aliphaticC within NOM in HOCs sorption could be correspondinglymasked. Therefore, we propose that the “pollution” of NOM byBC materials could, to a large degree, account for no clearrelationship between Phen Koc values by NOM fractions and



their aliphaticity, consequently, influence on investigating therole of aliphatic moieties within NOM fractions.Environmental Implications. This study demonstrated

that the nanopores of natural (NOM) and engineered sorbents(biochars) are closely related to their aliphatic and aromaticmatrices, respectively. Significant and positive correlationsbetween Phen Koc values by the NOM fractions or biocharsand their CO2−SA/OC in this study suggest that nanopore-filling mechanism plays a dominant role in the sorption ofHOCs by these sorbents, which are found to be microporoussolids. In addition, aliphatic C of the NOM fractions andaromatic C of the investigated biochars, respectively, weredemonstrated to be key factors affecting their microporosityand sorption behaviors of HOCs. Moreover, BC is almostcomposed of aromatic moieties and is characterized bystructural stability and high sorption capacity. It inevitablychanges the structures of NOM. Hence, the importance ofaliphatic C within NOM in the sorption of HOCs has oftenbeen masked. We used a novel approach by combiningfractionation, bleaching, and 13C NMR to estimate the effect ofBC. The findings of this work can explain the ongoing debateon the relative role of aromatic and aliphatic C in the sorptionof HOCs by SOM and uncover that how the aliphatic andaromatic C within both natural and engineered sorbents playthe role in the sorption of HOCs, which is important forcorrectly predicting the fate of HOCs in soils and sediments.The results described in this study provide importantimplications for the interpretation of sorption mechanisms oforganic contaminants in SOM.

■ ASSOCIATED CONTENT

*S Supporting InformationFigure of Carbon dioxide (CO2) adsorption isotherm on thevarious NOM factions and biochars, figure of 13C NMR spectraof original and bleached NOM fractions and biochars, figure ofcorrelations between CO2−SA/OC of original NOM fractionsand their aromaticity and aliphaticity, between CO2−SA/OC oforiginal and bleached NOM fractions and their aromaticity andaliphaticity as well as between logKoc values of Phen by originalNOM fractions and their aromaticity and aliphaticity, figure ofsorption isotherms of Phen by NOM fractions; figure ofsorption isotherms of Phen by biochars, figure of correlation oflogKoc values of Phen by sorbent to their bulk polarity; table ofFunctional Groups from the 13C NMR Spectra, table ofproperties of BC obtained from combustion of NHC at 375 °C,table of the calibrated aromaticity, aliphaticity and CO2−SA/OC values of NOM fractions, table of Freundlich isothermparameters. This material is available free of charge via theInternet at http://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Author*Phone: 86-10-58807493; fax: 86-10-58807493; e-mail:[email protected].

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This research was supported by National Natural ScienceFoundation of China (41273106), Beijing Higher EducationYoung Elite Teacher Project (YETP0273), and the Scientific

Research Foundation for the Returned Overseas ChineseScholars, State Education Ministry.

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