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Characterization and Phenanthrene Sorption of Natural and Pyrogenic Organic Matter Fractions Jie Jin, Ke Sun, Ziying Wang, Yan Yang, Lanfang Han, and Baoshan Xing Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b04573 • Publication Date (Web): 30 Jan 2017 Downloaded from http://pubs.acs.org on January 31, 2017

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Characterization and Phenanthrene Sorption of Natural and

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Pyrogenic Organic Matter Fractions

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Jie Jin, †,‡,§ Ke Sun, ‡,* Ziying Wang, ‡ Yan Yang, ‡ Lanfang Han, ‡,§ and Baoshan Xing, §,*

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Beijing 102206, China

School of Environment and Chemical Engineering, North China Electric Power University,

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Normal University, Beijing 100875, China

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§

State Key Laboratory of Water Environment Simulation, School of Environment, Beijing

Stockbridge School of Agriculture, University of Massachusetts, Amherst, MA 01003, USA

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**Corresponding authors: Phone: +86 10 58807493; fax: +86 10 58807493; e-mail:

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[email protected] (K. SUN). Phone: +1 413 545 5212; fax: +1 413 577 0242; e-mail:

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[email protected] (B.S. XING).

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ABSTRACTS: Pyrogenic humic acid (HA) is released into the environment during the

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large-scale application of biochar. However, the biogeochemistry of pyrogenic organic matter

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(PyOM) fractions and their sorption of hydrophobic organic compounds (HOCs) are poorly

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understood in comparison with natural organic matter (NOM) fractions. HA and humin (HM)

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fractions isolated from soils and the oxidized biochars were characterized. Sorption of

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phenanthrene (PHE) by these fractions was also examined. The characterization results

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demonstrate that pyrogenic HAs are different from natural HAs, with the former having

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lower atomic H/C ratios, more abundant aromatic C, and higher concentrations of surface

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carboxylic groups. Compared with the fresh biochars, the Koc of PHE on their oxidized

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biochars, pyrogenic HA, and HM fractions were undiminished, which is encouraging for the

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use of biochar in soil remediation. The PyOM fractions exhibited stronger nonlinear sorption

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than the NOM fractions. In addition, the PyOM fractions had higher sorption capacity than

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the NOM fractions due to their low polar C content and high aryl C content. The results

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obtained from this work will shed new light for the impact of the addition of biochar on the

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biogeochemistry of soil organic matter and on the fate of HOCs in biochar-amended soil.

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INTRODUCTION

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Soil organic matter (SOM) is composed of various organic components differing in

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molecule size and chemical composition, 80% of which can be humic substances (HS).1

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According to the protocol of classic chemical fractionation, HS are extracted from soils as

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fulvic acid (FA), humic acid (HA), and humin (HM). Because of the large amounts of HS in

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soils, small deviations in the composition of the above different fractions may have a

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significant effect on soil properties. Vegetation fires can modify the previously existing C as

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well as produce a considerable amount of newly formed C in the ecosystem. Globally,

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vegetation fires produce 40 to 250 million tons of pyrogenic C per year.2 The global

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pyrogenic C stored in sediments, soils, and waters reaches 300 to 500 giga-metric tons.3

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Specifically, pyrogenic C can comprise up to 40% of the total SOM in grasslands and boreal

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forests.4 Moreover, the practical applications of pyrogenic organic matter (PyOM) in the form

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of biochar have received great attention in light of its excellent performance in soil

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remediation and C sequestration.2 It is expected that the ubiquitous presence of PyOM, also

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called char, charcoal, biochar or black carbon (BC), will alter the physical and chemical

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properties of soil.

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Specifically, humic-fractions isolated from soils, such as Amazonian Dark Earth soils and

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volcanic ash soils in Japan, have been suggested to be enriched by PyOM-derived HA.5-9

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These studies suggested that, with weathering, part of the biochar was subjected to oxidation

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and hydration and transformed into FA and pyrogenic HA with graphite-like structures

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consisting of highly condensed aromatic rings, contributing considerably to the high humus

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accumulation in these soils. Though the chemical and structural properties of PyOM fractions 3

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have been detailed in these studies, it is noted that most characteristic studies for PyOM

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fractions collected PyOM samples directly from fire-impacted soils without further

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purification to remove the coating of natural organic matter (NOM) fractions. Even though

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biochar shows strong sorption for natural HA,10 resulting in the contamination of PyOM by

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natural HA,11 the interference of natural HA has been sparsely considered. Moreover, the

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extraction of HA is an essential step in the pretreatment of charcoal for 14C measurement.12,13

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It is reasonable to assume that the existence of natural HA will affect characteristic studies of

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PyOM fractions. Several previous studies reported that substances consistent with HA and FA

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have been extracted from fresh laboratory-produced biochar with the use of oxidizing agents,

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such as HNO3.14,15 Using elemental analysis, thermogravimetry, and

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resonance (NMR) spectra, they concluded that the HA isolated from biochars and

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pyrogenic-derived HA extracted from fire-impacted soils had similar characteristics.14

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Therefore, in this study, biochar-derived HA was used as the model pyrogenic HA in

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comparison with natural HA.

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C nuclear magnetic

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The fate of hydrophobic organic compounds (HOCs) in soils/sediments is mainly

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regulated by their sorption to solid phases.16 The sorption of HOCs in soils/sediments is often

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dominated by BC (i.e., PyOM),17-19 which exhibits as high as 1000 times higher capacity for

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HOCs than other organic matter fractions.19,20 Therefore, biochar amendment is of interest to

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immobilize HOCs in soils.21,22 A requirement for the successful use of biochar to restore

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contaminated sites is that the excellent sorption capacity of HOCs by biochar can be

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maintained over a long period of time. However, current literature still debates the effects of

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aging on the HOC sorption of biochar. For instance, some works found that the sorption of 4

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HOCs by biochar can be weakened by aging,23,24 whereas several other studies reported that

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biochar had a good sorption capacity for HOCs that was sustained during harsh aging.21,25

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The conflicting conclusions necessitate further study of the effect of biochar oxidation on

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HOC sorption. In most previous studies, the sorption capacity of pyrogenic HA was sparsely

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considered. As mentioned above, with weathering, biochar will release poly-aromatic HA

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molecules, which may affect the strength of contaminant sorption. Thus, to probe the

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influence of oxidation on biochar sorption and accurately predict the fate of HOCs in

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biochar-amended soils, the sorption properties of different PyOM fractions, including

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pyrogenic HA, should be explored. Pyrogenic HA features an aromatic structure,5-9 distinct

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from natural HA.1 In addition, it was reported that aromatic C plays an important role in HOC

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sorption.26 We therefore hypothesize that pyrogenic fractions extracted from biochars will

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demonstrate different sorption properties from NOM fractions. The objectives of this study

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were to (1) provide quantitative chemical and structural information concerning PyOM

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fractions extracted from biochar, (2) distinguish differences in composition between the

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NOM and PyOM fractions, and (3) compare the sorption difference of phenanthrene (PHE)

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between the PyOM and NOM fractions.

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MATERIALS AND METHODS

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Sorbents and Sorbates. A soil sample (AG) under slash-and-burn agricultural practices

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(BC% = 14.5%, quantified by chemo-thermal oxidation method at 375 °C for 24 h) was

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collected from the Sanjiang Plain, Northeast China. A soil sample (GR) with a low BC

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content (2.2%) was collected from a grassland in Xinjiang province, Northwest China. The

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HA and HM isolation methods have been described elsewhere.27,28 Briefly, the soil samples 5

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were extracted with 0.1 M NaOH for 7 times. Next, the mixture of the 7 extractions was

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acidified to pH = 2 to separate the HA precipitate from the soluble FA fraction. The HA

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fraction was then de-ashed by mixing and shaking with 0.1 M HCl and 0.3 M HF. The

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precipitated residue after HA extractions was de-ashed by 1 M HCl and 10% (v/v) HF to get

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the HM fraction. After centrifugation, the isolated HA and HM were washed with deionized

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water, freeze dried, and ground to fine powders (< 0.25-µm) for further characterization and

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sorption experiment.

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Wheat straw and swine manure were carbonized at various heat treatment temperatures

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(HTT = 300, 450, and 600 °C) to produce different biochars. After being ground and sieved

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(< 0.25 µm), the biochars were oxidized at approximately 90 °C for 4 h using 25% HNO3

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(~5.5 M) at 1:30 solid/liquid ratio.29 After the removal of excess acid, the HA and HM of

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biochar were extracted similarly as described for the soil. The extracted yields of HA after the

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acid treatment of the biochars varied depending upon the different HTT (Table S1 and S2).

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Biochars produced at 450 °C gave the highest HA yields, consistent with a previous study.14

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At 300 °C, the biochars were not sufficiently pyrolyzed and the HNO3 treatment resulted in a

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larger degree of oxidative degradation, producing small quantities of HA. At 600 °C, the

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biochars were increasingly graphitized, decreasing the yield of HA.14,29 The amount of

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pyrogenic HA obtained from biochars produced at 300 °C and 600 °C was not sufficient for

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subsequent use. Thus, the pyrogenic HAs and HMs, extracted from the two biochars

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produced at HTT = 450 °C, were used in this study. The temperature used was also that

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recommended for manufacturing biochar for soil amendment purposes.30,31 Here, these

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biochar samples were referenced according to feedstock source materials (maize straw and 6

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swine manure) (i.e., MA and SW). The PyOM samples were named as MA-AO, MA-HA,

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MA-HM, SW-AO, SW-HA, and SW-HM. AO represents acid oxidation.

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The

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C-labeled and unlabeled PHE were used as sorbates. The tested chemicals used in

this study were purchased from Sigma-Aldrich at the highest purity available.

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Sorbent Characterization. The bulk elemental (C, H, O, and N) analysis of all sorbents

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were determined using dry combustion with an Elementar Vario ELIII elemental analyzer

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(Germany). The ash content was measured by heating the sorbents at 750 °C for 4 h. The

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surface elemental composition and functionalities were examined with an X-ray

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photoelectron spectrometer (XPS) (Thermo Scientific ESCALAB 250 XPS, USA). The XPS

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running parameters are detailed in a previous study.32 The C1s fine spectrum was

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deconvoluted into the following regions: C-C at 284.9 eV, C-O at 286.5 eV, C=O at 287.9 eV,

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and COO at 289.4 eV.32 Solid-state cross-polarization magic angle spinning 13C NMR spectra

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were obtained on a Bruker Avance 300 NMR spectrometer (Germany) at 75 MHz. The pore

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and specific surface properties of all samples were determined by gas (CO2 and N2)

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adsorption performed on a Quantachrome Autosorb-iQ gas analyzer (USA).

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Sorption Experiment. Batch PHE sorption experiments were conducted as described in

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Han et al.26 Briefly, test solutions (2-1100 µg/L) of PHE were prepared by diluting the

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labeled and nonlabeled stock solutions using the background solution containing 0.01 M

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CaCl2 (ionic strength adjuster) and 200 mg/L of NaN3 (biocide), and adjusted to pH 6.5 with

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0.1 M HCl or 0.1 M NaOH. The solute solutions were slowly added into the 40-mL glass

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vials with an amount of sorbents, ensuring that the headspace was kept to minimum. The

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amount of sorbents was adjusted to result in 20-80% uptake of PHE. The methanol 7

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concentration in the test solutions was always less than 0.1% (v/v) to avoid cosolvent effects.

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The glass vials were capped with Teflon-lined screw and shaken in the dark at 23 ± 1 °C for

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10 d. The preliminary tests showed that sorption equilibrium was reached before this time.

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Next, after centrifugation, 1.5 mL of supernatant was withdrawn from each vial and added to

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separate corresponding vials containing 4.0 mL of scintiverse cocktail (Fisher Scientific,

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USA) and analyzed by liquid scintillation counting. The final pH values were in the range of

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6.4-6.8. For each concentration point, two blanks without sorbents were run at the same time.

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In view of the negligible mass loss of PHE in controls, sorbed concentrations of PHE by

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sorbents were calculated by mass difference. Data Analysis. Three nonlinear models were used for fitting sorption data of PHE in this

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study. They are the Freundlich (FM), Polanyi-Dubinin (PD), and Dubinin-Radushkevich (DR)

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models. More details can be found in the Supporting Information.

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RESULTS AND DISCUSSION

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Characteristics of Natural and Pyrogenic Organic Matters. HNO3 oxidation of MA

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and SW resulted in an obvious increase in the bulk O and N concentrations (Table 1 and

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Figure 1). The high O content of the oxidized biochar is probably due to a rise in

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O-containing functional groups, such as carboxylic, phenolic, and nitro groups, introduced by

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HNO3 treatment, whereas the elevated N concentrations might be caused by the formation of

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the nitro groups from HNO3.14 By contrast, from the OC recovery (%) of the oxidized

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biochars (Table S2), both MA and SW suffered OC loss after HNO3 oxidation, consistent

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with previous studies.11,33

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After fractionation of the oxidized biochars, the pyrogenic HA was found to be the major 8

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component, comprising 63.1 wt% and 45.0 wt% of MA and SW, respectively; meanwhile,

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OC recovery (%) was 45.5% and 69.3% for MA-HA and SW-HA, respectively (Table S2).

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The proportions of HA released from the biochars were comparable to those reported in the

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literature.14 The elemental compositions of HA fractions from different sources were also

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analyzed. Whereas the HA extracted from AG soil subject to frequent burning gave an OC

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content of 49.0%, the biochar HAs showed slightly higher OC contents (51.9% and 53.6%),

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both of which were much higher than GR-HA (16.5%) with little BC input (Table 1 and

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Figure 1). A high OC content was also found in the HAs obtained from other pyrogenic

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C-rich soils, which was in the range of 57-63%.34 AG-HA, MA-HA, and SW-HA possessed

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comparable bulk polarity, lower than GR-HA (Figure 1). To better compare the elemental

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composition of pyrogenic HA and natural HA, the atomic H/C and O/C ratios of biomass,

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pristine biochar produced at 450 °C, and HA isolated from biochars, pyrogenic C-rich (black)

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soils, pyrogenic C-deficient (non-black) soils, and litters from the literature were also

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tabulated in Table S3 and presented in the van Krevelen diagram (Figure 2). The oxidative

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degradation of biochar with dilute HNO3 and subsequently NaOH extraction resulted in

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overall oxidation and dehydrogenation of the samples, as shown in the change in atomic H/C

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and O/C ratios (Figure 2). In general, the biochar HAs were plotted near the black soil HAs in

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the Van Krevelen diagram (Figure 2). By contrast, the position of non-black soil HAs is

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concentrated near the melanoidin/lignin zone (Figure 2). Additionally, both biochar HAs and

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black soil HAs had lower H/C atomic ratios than the non-black soil HA samples, indicating

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that the formers had higher aromaticity and higher degree of condensation.

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The surface elemental compositions as indicated by the XPS analysis had a similar but 9

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more obvious trend compared to the bulk elemental analysis (Table 1 and Figure 1). For

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instance, the bulk O contents slightly increased from 10.2-11.8% in the fresh biochars to

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16.3-18.3% in the oxidized biochars, whereas the surface O contents increased dramatically

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from 16.0-25.7% to 51.1-51.9% (Table 1). The XPS C1s spectra clearly showed that the

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increments of O were attributed to the incorporation of O-containing functional groups,

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mainly carboxylic groups (Table S4). Unlike bulk OC, the surface C consistently decreased

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after acid oxidation (Figure 1). These results suggested that the external surface of biochar

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particles suffered a higher degree of oxidation than the interior. In addition, pyrogenic and

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natural HAs had similar surface polarities (Figure 1f). However, pyrogenic HAs were

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characterized by more surface COO(H) groups than natural HAs, which could account for the

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higher CEC of black soils in comparison to the non-black soils.7

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The 13C NMR spectra also illustrated that remarkable chemical modifications of biochars

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were detected after HNO3 treatment, such as increments in polar groups and the formation of

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carboxylic groups (Table S5 and Figure S1), consistent with the elemental analysis and XPS

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results. The polar C content of the PyOM fractions ranged from 21.0% to 26.1%, lower than

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those of the tested NOM fractions (35.9-43.5%) (Table S5). The aromaticity of MA-AO and

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SW-AO were slightly higher than their corresponding pristine biochar samples (Table S5).

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This might be because aliphatic groups can be selectively lost during oxidation.35 In addition,

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the

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Amazonian Dark Earth soil36 were replotted in Figure 3 so that comparisons among HAs

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from different sources can be better visualized. As expected, the 13C NMR spectra of the HAs

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extracted from MA and SW are similar to that of HA from the Amazonian Dark Earth soil,

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C NMR spectra of the natural and pyrogenic HA as well as the HA extracted from an

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confirming that PyOM may be a possible source of the HA fraction in soils. In addition,

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pyrogenic HAs primarily consisted of aromatic and carboxyl C, whereas natural HAs clearly

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showed the aliphatic signals (0-93 ppm) (Figure 3). Moreover, the aromaticity of pyrogenic

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HAs was approximately twice that of natural HAs (Table S5).

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The gas adsorption isotherms and DFT pore size distribution of the tested samples are

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shown in Figure S2 and S3. The micropore volume of organic matters was found to be

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positively correlated with their OC content,26,37 consistent with our data (Figure S4). Thus, in

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comparison with fresh biochar, the micropore volume of MA-AO declined due to the loss of

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OC after acid oxidation, whereas that of SW-AO increased with an elevated OC content

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(Table 1 and S6). The N2 surface area (N2-SA) values of biochars were consistently decreased

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after HNO3 treatment (Table S6). N2 can probe the outer surface of minerals;38 therefore the

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obvious lower N2-SA of SW-AO than that of SW could be explained by the removal of some

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mineral within SW by HNO3 treatment. The micropore volumes were 0.020 cm3/g and 0.025

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cm3/g for MA-HA and SW-HA, respectively, much lower than those of the pristine biochars

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(0.111 cm3/g for MA and 0.046 cm3/g for SW). Additionally, the micropore volumes of the

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pyrogenic HAs were slightly lower than those of natural HAs (Table S6), despite the fact that

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biochar is very porous.26,39 Moreover, natural HAs had 1-2 magnitude higher N2-SA than

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pyrogenic HAs (Table S6), likely caused by the high ash content of natural HAs. To test this,

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the ash component of GR-HA was obtained after combustion at 750 °C for 4 h and its N2-SA

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was measured, which was 132.6 m2/g. The high N2-SA of the ash component of GR-HA

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implies that the ash component greatly contributed to the N2-SA of GR-HA. A previous study

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also found that N2-SA of organic sorbents increased with increasing ash content.38 11

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Impact of Acid Oxidation on PHE Sorption. The sorption isotherms of PHE by the

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original biochars were highly nonlinear (Figure S5 and Table S7). The acid treatment caused

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the increase of n values (Table S7), implying that a more expanded sorbent was produced by

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HNO3 oxidation. The acid treatment exerted dissimilar influences on the logKd and logKoc

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values of the tested biochars (Figure 4). A statistical comparison of the logKd and logKoc

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values before and after HNO3 oxidation was conducted by taking three selected isotherm

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points at Ce = 0.01Sw, 0.1Sw, and 1Sw as individual measurements, giving three values for the

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statistical analysis (Table S7). The results show that influence of HNO3 oxidation on PHE

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sorption by SW was statistically nonsignificant (paired t-test p = 0.08 for logKd and p = 0.79

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for logKoc). A previous study also found that the fresh biochar produced from hardwood and

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the correspond aged biochar had comparable sorption capacity for simazine.21 By contrast,

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the sorption of PHE by MA-AO was unexpectedly elevated compared with the untreated

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sample (paired t-test p < 0.05 for logKd and p < 0.01 for logKoc).

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From the XPS and

13

C NMR analysis, it was observed that acid treatment caused the

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development of surface O-containing functional groups, mainly carboxyl groups (Table S4

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and S3). The hydrophilic moieties of biochars can facilitate the coating of water films at the

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surface of the sorbents through H-bonding. The water films can weaken the sorption of HOC

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molecules by decreasing the accessibility of target solutes to sorption domains as well as

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occupying the sorption sites. Apparently, the surface hydrophobic mechanism cannot

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dominate the sorption of PHE to the oxidized biochars. The undiminished PHE sorption on

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the oxidized biochars may be attributed to the rise in aromaticity (Table S5), which benefits

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the π-π electron donor-acceptor (EDA) interactions between PHE molecules and the aromatic 12

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domains within the oxidized biochars. The role of biochar aromaticity in PHE sorption has

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been documented in previous studies.26,37 Moreover, compared with the fresh biochars, the

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oxidized biochars may have greater π-acceptor strength as a result of the attachment of

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carboxylic groups, introduced by HNO3 oxidation, to the aromatic rings. This will aid strong

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EDA interactions between the oxidized biochars and PHE molecules (π-donor).

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Additionally, it has been shown that the pore-filling mechanism dominate the PHE

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sorption by biochars.26,39 PD and DR models were therefore employed to determine the role

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of micropore filling in PHE sorption. Sorption isotherms fitted using PD and DR models are

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shown in Figure S6. The fitted data (Table S8 and Table S9) indicate that better fitting was

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obtained for the PD model, as indicated by the higher R2 values. Therefore, the adsorption

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parameters of PHE from the PD model were used in the following discussion. The adsorbed

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volume capacities (Q0) were approximately 0.74 and 0.91 cm3/kg for MA and SW,

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respectively (Table S8), both of which were reduced after oxidation. In addition, there is a

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positive correlation between the Q0 of PHE on the tested PyOMs (except for MA, MA-HA,

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and SW-HA) and their micropore volume (Figure S7), indicating that the pore-filling

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mechanism also contributed to PHE sorption by the samples.

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Sorption of PHE to Pyrogenic Humic Acid and Humin Fractions. The nonlinear

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coefficients (n) of the pyrogenic HA and HM fractions were in the range of 0.45-0.64 (Table

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S7). The Koc values of MA-HA and MA-HM were obviously higher than those of the fresh

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biochars (Figure 4). This might arise from the increased aromaticity (Table S5) after a series

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of chemical treatments as mentioned above. By contrast, the Koc values of SW-HA and

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SW-HM were comparable to that of SW. These results demonstrate that the feedstock of 13

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biochars will affect the sorption properties of their PyOM fractions. Additionally, as shown in

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Figure S7, the pore-filling mechanism may also contribute to PHE sorption by pyrogenic HM

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fractions. The sorption affinity (Kd and Koc) for PHE by MA-HM is higher than that by

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SW-HM (Table S7), which might be partly explained by the larger micropore volume of

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MA-HM (Table S6). For the pyrogenic HA fractions, their Q0 for PHE deviated from the

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relationship between the Q0 and the micropore volume (Figure S7). In addition, although

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their micropore volumes were much lower than those of the fresh biochars (Table S6), they

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still demonstrated higher (MA-HA) or comparable (SW-HA) Koc values compared with the

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fresh biochars (Table S7). Therefore, the pore-filling mechanism may not regulate the

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sorption of PHE by the pyrogenic HA fractions.

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Here, the oxidized biochars and biochar-derived pyrogenic HA and HM fractions

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maintained excellent sorption for PHE, implying that the high sorption capacity of PHE by

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biochars may be long-lasting in the environment. This confirms the promising future of

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biochar application in the context of contaminated site remediation. It should be noted that

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the coating of the biochar surface by microbially produced organic matter11 or native HS,22

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which was a general phenomenon in the natural environment and might weaken the sorption

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capacity of biochar, cannot be simulated by simple HNO3 oxidation. It was found that the

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sorption capacity of hydroquione was substantially reduced from 9.61 mg/g by the fresh

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biochar to 4.28 mg/g by the field aged biochar.40

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Comparison of PHE Sorption between the NOM and PyOM Fractions. The nonlinear

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coefficients (n) of the NOM fractions are in the range of 0.58-0.89. The natural HA and HM

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fractions exhibit higher n values than the corresponding PyOM fractions (Table S7), which 14

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might be attributed to their different aromatic C contents. PHE could be adsorbed to aromatic

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domains via π-π interactions.26,41 This process is expected to generate nonlinear sorption.26,41

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Consistently, the n values of PHE for all the natural and pyrogenic HA and HM fractions are

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correlated negatively with the aromatic C content (Figure S8a). Therefore, it was concluded

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that the nonlinear PHE sorption by the NOM and PyOM fractions was regulated by their

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aromatic C content.

314

It is noted that the sorption affinity (Kd and Koc) of pyrogenic HAs was an order of

315

magnitude higher than that of natural HAs (Table S7). The pyrogenic HM fractions also

316

displayed larger Kd and Koc values for PHE than the natural HM fractions (Table S7). As

317

shown in Figure S8, the logKoc of PHE of all the HAs is inversely correlated with the bulk

318

polar C content and positively correlated with the aromatic C content, implying that

319

hydrophobic partitioning and π-π EDA interactions between PHE and the aromatic moieties

320

contributed to the overall PHE sorption by the HA fractions, supported by other studies.26,27,32

321

In addition, the logKoc of PHE by the HA fractions is generally negatively correlated with the

322

surface C-O and C=O content of the HA fractions, but positively correlated with the surface

323

COO C content (Figure S8). As mentioned above, the formation of carboxylic groups on the

324

edge of aromatic components may lead to the elevated sorption of PHE via enhanced π-π

325

EDA interactions. Besides COO, C=O can also attract electrons from the benzene rings.

326

However, the functional groups of COO and C=O exerted opposite effects on PHE sorption.

327

The reason for this observation remains unclear and further study is needed. For the HM

328

samples, the logKoc of PHE is negatively correlated with the alkyl C content, carbohydrate C

329

content, bulk and surface polar C content, and estimated surface polarity, and positively 15

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330

correlated with the aryl C content (Figure S9). Thus, PHE sorption by the natural and

331

pyrogenic HM may also be regulated by the hydrophobic mechanism and π-π EDA

332

interactions. Taken together, in comparison with NOMs, PyOMs showed superior sorption

333

capacities due to their low polar C content and high aryl C content.

334

Environmental Implications. The use of large amounts of biochar and subsequent aging

335

may release pyrogenic HA into the environment, which in turn will change the composition

336

of the organic matter fractions in soils, affecting HOC sorption. The results in this study

337

demonstrated that pyrogenic HAs were characterized by lower atomic H/C ratios, more

338

surface –COO(H) groups, and higher aromaticity compared to natural HAs. In addition,

339

pyrogenic HA had a much larger sorption capacity for PHE. Elucidating the distinct

340

composition and HOC sorption properties of pyrogenic HA makes it possible to better

341

understand the effect of biochar addition on the biogeochemistry of SOM and more

342

accurately assess the mobility and bioavailability of HOCs in biochar-amended soils. This

343

study also implies that using the protocol of classic chemical fraction method, not only

344

geologically formed HA, but also pyrogenic-derived HA can be isolated from soils. Therefore,

345

the “pollution” of HA by the pyrogenic HA should be taken into consideration in HA studies.

346



347

Supporting Information

348

The Supporting Information is available free of charge on the ACS Publications website. Nine figures and nine tables are provided (PDF).

349

350

ASSOCIATED CONTENT



AUTHOR INFORMATION 16

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Corresponding Author

352

* Phone: +86 10 58807493; fax: +86 10 58807493; e-mail: [email protected] (K.

353

SUN). Phone: +1 413 545 5212; fax: +1 413 577 0242; e-mail: [email protected] (B.S.

354

XING).

355

Notes

356

The authors declare no competing financial interest.

357



358

This research was supported by the National Natural Science Foundation--Outstanding

359

Youth Foundation (41522303), the National Natural Science Foundation of China

360

(41473087), and the USDA NIFA McIntire-Stennis Program (MAS 00028). J.J. also

361

thanks the China Scholarship Council for supporting her study at the University of

362

Massachusetts, Amherst.

363



364

(1) Steelink, C., Peer reviewed: investigating humic acids in soils. Anal. Chem. 2002, 74 (11), 326 A-333

365

A.

366

(2) Lehmann, J., A handful of carbon. Nature 2007, 447 (7141), 143-144.

367

(3) Hockaday, W. C.; Grannas, A. M.; Kim, S.; Hatcher, P. G., The transformation and mobility of

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charcoal in a fire-impacted watershed. Geochim. Cosmochim. Acta. 2007, 71 (14), 3432-3445.

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(4) Preston, C.; Schmidt, M., Black (pyrogenic) carbon: a synthesis of current knowledge and

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uncertainties with special consideration of boreal regions. Biogeosciences 2006, 3 (4), 397-420.

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(5) Mao, J.-D.; Johnson, R.; Lehmann, J.; Olk, D.; Neves, E.; Thompson, M.; Schmidt-Rohr, K.,

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(20) Cornelissen, G.; Gustafsson, Ö.; Bucheli, T. D.; Jonker, M. T.; Koelmans, A. A.; van Noort, P. C.,

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leaching in soil. Soil Biol. Biochem. 2011, 43 (4), 804-813.

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contaminant adsorption in black carbon (biochar)-amended soil for the veterinary antimicrobial

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biochar. J. Soil. Sediment. 2011, 11 (1), 62-71.

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(25) Hale, S. E.; Hanley, K.; Lehmann, J.; Zimmerman, A.; Cornelissen, G., Effects of chemical, biological,

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and physical aging as well as soil addition on the sorption of pyrene to activated carbon and biochar.

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microporosity in phenanthrene sorption by natural and engineered organic matter. Environ. Sci. Technol.

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2014, 48 (19), 11227-11234.

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(27) Sun, K.; Jin, J.; Kang, M.; Zhang, Z.; Pan, Z.; Wang, Z.; Wu, F.; Xing, B., Isolation and

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characterization of different organic matter fractions from a same soil source and their phenanthrene

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sorption. Environ. Sci. Technol. 2013, 47 (10), 5138-5145.

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(28) Kang, S.; Xing, B., Phenanthrene sorption to sequentially extracted soil humic acids and humins.

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in soil quality and plant growth in a three year field trial. Soil Biol. Biochem. 2012, 45, 113-124.

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biochar as a soil amendment. Soil Res. 2007, 45 (8), 629-634.

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

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feedstock and pyrolysis temperature. Environ. Sci. Technol. 2012, 46 (21), 11770-11778.

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(34) de Melo Benites, V.; de Sá Mendonça, E.; Schaefer, C. E. G.; Novotny, E. H.; Reis, E. L.; Ker, J. C.,

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Properties of black soil humic acids from high altitude rocky complexes in Brazil. Geoderma 2005, 127 (1),

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matter from different carbonaceous chondrite groups. Geochim. Cosmochim. Acta. 2005, 69 (4),

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(36) Araujo, J. R.; Archanjo, B. S.; de Souza, K. R.; Kwapinski, W.; Falcão, N. P.; Novotny, E. H.; Achete,

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C. A., Selective extraction of humic acids from an anthropogenic Amazonian dark earth and from a

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chemically oxidized charcoal. Biol. Fert. soils 2014, 50 (8), 1223-1232.

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(37) Jin, J.; Sun, K.; Wu, F.; Gao, B.; Wang, Z.; Kang, M.; Bai, Y.; Zhao, Y.; Liu, X.; Xing, B.,

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Single-solute and bi-solute sorption of phenanthrene and dibutyl phthalate by plant-and manure-derived

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biochars. Sci. Total Environ. 2014, 473, 308-316. 21

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(38) Ran, Y.; Yang, Y.; Xing, B.; Pignatello, J. J.; Kwon, S.; Su, W.; Zhou, L., Evidence of micropore

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(39) Xiao, F.; Pignatello, J. J., Interactions of triazine herbicides with biochar: steric and electronic effects.

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Water Res. 2015, 80, 179-188.

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(40) Cheng, C.-H.; Lehmann, J., Ageing of black carbon along a temperature gradient. Chemosphere 2009,

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469 470 471 472

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Figure Captions:

477 478

Figure 1. Bulk (a, b, and c) and surface (d, e, and f) elemental composition for the two

479

biochars produced from maize straw (MA) and swine manure (SW), the oxidized biochars,

480

and humic acids (HA) as well as humins (HM) extracted from the two biochars, an

481

agriculture soil (AG), and a grassland soil (GR).

482 483

Figure 2. Atomic H/C and O/C ratios in biomass (plants before burning), biochars produced

484

at 450 °C, humic acids (HA) extracted from biochars, black and non-black soils, and litters,

485

and separate plant components (carbohydrate, lignin, protein, and lipids) (IHSS peat HA was

486

presented as reference) (the H/C and O/C data of the samples are cited from

487

literatures7,9,14,28,34). Linear regressions for biochars and corresponding pyrogenic HAs

488

extracted from biochars are shown.

489 490

Figure 3. Cross-polarization magic angle spinning 13C NMR spectra of humic acids (HAs)

491

isolated from an agriculture soil (AG), a grassland soil (GR), biochars produced from swine

492

manure (SW) and maize straw (MA), and an Amazonian Dark Earth (ADE) soil (the NMR

493

spectrum of ADE-HA is cited from the report by Araujo et al.36).

494 495

Figure 4. Concentration-dependent distribution coefficients (Kd and Koc) of fresh biochars

496

produced from maize straw (MA) and swine manure (SW), oxidized biochars (MA-AO and

497

SW-AO), and humic acids (HA) and humins (HM) isolated from biochars and soils. “GR”

498

and “AG” denote grassland soil and agriculture soil, respectively.

499 500 501 502 23

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Environmental Science & Technology

503 504 505 506

Table Caption:

507 508

Table 1.Bulk and Surface Elemental Composition of the Biochars, Soils, and Pyrogenic and

509

Natural Organic Matter Fractions

510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 24

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526

100

Biochar Oxidized biochar

100

HM HA

a 80 Surface C, %

Bulk C, %

80 60 40 20

SW

GR

60 40

AG

HM HA

Biochar Oxidized biochar

MA

SW

GR

AG

70

b

60 Surface O, %

30 Bulk O, %

d

0 MA

40

HM HA

Biochar Oxidized biochar

20

0

20 10

Biochar Oxidized biochar

HM HA

e

50 40 30 20 10

0

0 MA

SW

GR

AG

MA 0.8

1.0

Biochar or soil Oxidized biochar

HM HA

0.8 0.6 0.4 0.2

c

Estimated surface (O+N)/C

1.2

Bulk (O+N)/C

527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560

0.0

SW

GR

AG

Biochar

HM

Oxidized biochar

HA

f

0.6 0.4 0.2 0.0

MA

SW

GR

AG

MA

SW

GR

AG

561 562

Figure 1. Bulk (a, b, and c) and surface (d, e, and f) elemental composition for the two

563

biochars produced from maize straw (MA) and swine manure (SW), the oxidized biochars,

564

and humic acids (HA) as well as humins (HM) extracted from the two biochars, an

565

agriculture soil (AG), and a grassland soil (GR).

566 25

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Environmental Science & Technology

2.5

Decarboxylation Oxidation Carbonization

2.0

Hydration

Lipid Protein

Biomass

Cellulose

1.5 Melanoidin/lignin

H/C

567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591

Page 26 of 30

1.0

Biochar Biochar HA Black soil HA Non black soil HA Biomass IHSS peat HA Litter HA

Coal

0.5

0.0 0.0

0.2

0.4

0.6

0.8

1.0

O/C

592

Figure 2. Atomic H/C and O/C ratios in biomass (plants before burning), biochars produced

593

at 450 °C, humic acids (HA) extracted from biochars, black and non-black soils, and litters,

594

and separate plant components (carbohydrate, lignin, protein, and lipids) (IHSS peat HA was

595

presented as reference) (the H/C and O/C data of the samples are cited from

596

literatures7,9,14,28,34). Linear regressions for biochars and corresponding pyrogenic HAs

597

extracted from biochars are shown.

598 599

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600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619

Environmental Science & Technology

ADE-HA MA-HA

SW-HA

GR-HA

AG-HA

250

200

150

100

50

0

-50

13

C chemical shift, ppm

620 621

Figure 3. Cross-polarization magic angle spinning 13C NMR spectra of humic acids (HAs)

622

isolated from an agriculture soil (AG), a grassland soil (GR), biochars produced from swine

623

manure (SW) and maize straw (MA), and an Amazonian Dark Earth (ADE) soil (the NMR

624

spectrum of ADE-HA is cited from the report by Araujo et al.36).

625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 27

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642 643 644 645

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106

106

Koc

Kd

646 105

647 105

648

104

649 103 10-1

650

MA MA-AO MA-HM MA-HA

MA MA-AO MA-HM MA-HA

100

101

102

103

104

104 10-1

651

656

Kd or Koc, mL/g

655

102

103

104

Koc

Kd

652

654

101

107

106

653

100

105

106

104 3

10

657

102 10-1

658

104

105

SW SW-AO SW-HM SW-HA 0

10

1

10

2

10

3

4

10

10

104 10-1

SW SW-AO SW-HA SW-HM

100

101

102

103

104

106 Kd

659

Koc 105

660 103

104

661 GR-HM GR-HA AG-HM AG-HA

662 663

102 10-1

100

103

101

102

103

104

664 665

102 10-1

GR-HM GR-HA AG-HM AG-HA

100

101

102

103

104

Ce, µg/L

666 667

Figure 4. Concentration-dependent sorption coefficients (Kd and Koc) of fresh biochars

668

produced from maize straw (MA) and swine manure (SW), oxidized biochars (MA-AO and

669

SW-AO), and humic acids (HA) and humins (HM) isolated from biochars and soils. “GR”

670

and “AG” denote grassland soil and agriculture soil, respectively.

671 672

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Table 1. .Bulk and Surface Elemental Composition of the Biochars, Soils, and Pyrogenic and Natural Organic Matter Fractions

Bulk elemental composition Samples

MA

C

O

N

H

C/N

(%)

(%)

(%)

(%)

74.4

11.8

1.0

3.8

85.9

Surface elemental composition (XPS) O/C

0.12

H/C

0.61

(O+N)/C

0.13

(O+N) /Cc

Ash

C

O

N

Si

(%)

(%)

(%)

(%)

(%)

9.1

73.7

16.0

2.1

8.3

0.31

SW

33.7

10.2

2.6

2.6

15.3

0.23

0.91

0.29

50.9

48.5

25.7

4.6

12.3

0.33

MA-AO

54.6

18.3

3.4

2.9

18.5

0.25

0.64

0.31

20.8

41.3

51.9

4.6

2.2

0.40

SW-AO

48.7

16.3

6.3

2.8

9.0

0.25

0.68

0.36

25.9

42.8

51.1

0.0

2.0

0.43

MA-HA

53.6

31.5

3.5

3.1

17.8

0.44

0.69

0.50

8.2

64.7

28.5

4.0

1.2

0.41

SW-HA

51.9

30.0

6.6

3.3

9.1

0.43

0.76

0.54

8.2

73.2

25.7

0.0

1.1

0.39

MA-HM

52.1

29.0

3.0

3.2

20.2

0.42

0.74

0.47

12.7

65.6

26.0

3.5

0.8

0.16

SW-HM

4.7

1.4

0.4

0.3

13.0

0.22

0.90

0.30

93.2

10.1

45.8

0.0

30.0

0.23

GR

3.6

nd a

0.4

0.8

11.1

nd

2.53

nd

nd

nd

nd

nd

nd

nd

b

b

b

b

b

GR-HM

5.5

16.1

0.5

0.8

13.2

2.19

1.70

2.27

77.1

69.9

28.2

1.9

0.0

0.45

GR-HA

16.5

17.6

1.7

3.0

11.5

0.80

2.17

0.88

61.2

75.7

22.5

1.8

0.0

0.40

AG

1.4

nd

0.1

0.5

12.1

nd

4.40

nd

nd

nd

nd

nd

nd

nd

b

b

96.1

18.4

17.6

0.0

0.0

0.24

11.4

19.5

42.0

2.0

24.3

0.58

b

b

b

AG-HM

1.1

2.7

0.0

0.2

85.5

1.86

2.02

1.87

AG-HA

49.0

29.7

4.2

5.7

13.5

0.46

1.39

0.53

a

Not detected. b It is noted that GR-HM and AG-HM showed abnormally high O/C and (O+N)/C ratios, indicating that the measurements of O and H of GR-HM and AG-HM may be not correct.

The C contents of these two samples were too low; therefore, the measurements of O and H may be affected by mineral-bound water. cThe surface (O+N)/C ratios were estimated using the XPS data of surface functional groups listed in Table S4. Maize straw-derived biochcar (MA), swine manure-derived biochar (SW), oxidized biochar (AO), humic acids (HA), humins (HM), grassland soil (GR), and agriculture soil (AG).

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