Comparison between Soil- and Biochar-Derived Humic Acids

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Comparison between soil-derived and biochar-derived humic acids: composition, conformation, and phenanthrene sorption Jie Jin, Ke Sun, Yan Yang, Ziying Wang, Lanfang Han, Xiangke Wang, Fengchang Wu, and Baoshan Xing Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b04999 • Publication Date (Web): 30 Jan 2018 Downloaded from http://pubs.acs.org on January 30, 2018

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Environmental Science & Technology

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Comparison between soil-derived and biochar-derived humic acids:

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composition, conformation, and phenanthrene sorption

3 †,‡,ǁ

Ke Sun, ‡,* Yan Yang, ‡ Ziying Wang,

4

Jie Jin,

5

Fengchang Wu, § and Baoshan Xing, ǁ,*

‡

Lanfang Han,

‡,ǁ

Xiangke Wang,

†

6 7 8

†

9

University, Beijing 102206, China

College of Environmental Science and Engineering, North China Electric Power

10

‡

11

Normal University, Beijing 100875, China

12

§

13

Academy of Environmental Sciences, Beijing 100012, China

14

ǁ

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

State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research

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

15 16 17 18 19 20 21 22 23 24 25

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

26

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

27

[email protected] (B.S. XING).

28 1

ACS Paragon Plus Environment


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ABSTRACT: Biochar-derived organic matter (BDOM) plays an important role in

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determining biochar’s application potential in soil remediation. However, little is known

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about the physicochemical properties of BDOM and its sorption of hydrophobic organic

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compounds (HOCs). Humic acids (HAs) were extracted from oxidized biochars produced

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from plant straws and animal manures at 450 °C, and their sorption of phenanthrene, a

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representative of HOCs, was investigated. The organic carbon recovery of biochar-derived

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HAs (BDHAs) was 13.9-69.3%. The

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aromatic and carboxylic C, while those of soil-derived HAs (SDHAs) contained abundant

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signals in aliphatic region. BDHAs and SDHAs had comparable CO2 cumulative surface

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areas. BDHAs were found to exhibit higher phenanthrene sorption than SDHAs. After the

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removal of amorphous aromatic components, the logKoc values of BDHAs were

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significantly decreased, implying that amorphous aromatic C regulated phenanthrene

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sorption by BDHAs. In contrast, aliphatic moieties dominated phenanthrene sorption by

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SDHAs, as evidenced by the enhanced sorption after the removal of amorphous aromatics.

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This study clearly demonstrated the contrasting characteristics and sorption behaviors of

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BDHA and SDHA, indicating that biochar addition and subsequent weathering could

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greatly affect native organic matter properties and the fate of HOCs in biochar-amended

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soils.

13

C NMR spectra of BDHAs mainly consisted of

47 48 49 50 2


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INTRODUCTION

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Biochar, produced by the incomplete combustion of biomass, has gained great attention in

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the last decade for its potential role in carbon sequestration and soil remediation.1-4 The

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feasibility of biochar applications can be greatly affected by the characteristics of

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biochar-derived organic matter (BDOM). With weathering, part of the biochar will be

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subject to oxidation and hydration and transformed into a material with operationally

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defined properties similar to humic acid (HA).5 This material, thereafter named

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biochar-derived humic acid (BDHA), is commonly found in soils.5-10 Specifically, in

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Japanese Andosols, the proportion of BDHA to the total HA in the whole soils can reach

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44%.10 Investigating the physicochemical properties of BDHA is thus of great importance

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to better understand the biochar’s application potential in soil quality improvement and

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remediation and predict the effects of biochar addition on native soil organic matter (SOM).

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In previous studies, BDHA was isolated from biochar-received soils using the

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International Humic Substances Society (IHSS) procedure without further purification.11,12

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Soil-derived HA (SDHA) can be concurrently extracted using this procedure. The

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inevitable contamination of BDHA by SDHA in these studies would interfere with the

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results and data interpretation of BDHA. Several recent studies have reported that

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“uncontaminated” BDHA can be isolated from laboratory-produced biochar using a

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chemical agent such as HNO3 to accelerate biochar oxidization.5,13 Therefore, BDHAs

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extracted from oxidized biochars were used in this study. Even using the same fractionation

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method, BDHA and SDHA may differ significantly in composition and conformation due

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to their different parent materials. This difference, however, has seldom been addressed in

3



13

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previous studies. The use of

C nuclear magnetic resonance (NMR) spectroscopy has

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provided spectra of SDHA with varying proportions of alipahtic C, aromatic C, phenolic C,

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and C in carboxylic groups. In contrast, the spectra of BDHA, isolated from biochars13 and

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Amazonian Dark Earth,14 are characterized by predominantly aromatic C and carboxyl

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signals. Furthermore, considering the highly condensed and porous structure of fresh

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biochar,15,16 BDHA may have a higher degree of condensation and a larger surface area

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than SDHA.

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SOM is the dominant sorbent of hydrophobic organic compounds (HOCs).17 The

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ubiquitous presence of biochar in soils18 and the large-scale use of biochar in soil

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remediation and improvement require insights into BDOM-HOC interactions. The

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aforementioned contrasting characteristics of SDHA and BDHA would contribute to their

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different sorption properties. The important role of the aliphatic domains within SDHA in

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the interaction with HOCs has been highlighted in many studies.12,19,20 In contrast, few

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studies have examined the sorption of HOCs by BDHA. We have previously shown that

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the sorption capacity of phenanthrene (PHE) by BDHA was one order of magnitude higher

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than that of SDHA.21 Whereas, the mechanisms underlying the superior sorption capacity

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of BDHA were not established. By simple correlation analysis, we proposed that HOC

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sorption by BDHA was likely regulated by aromatic structures.21 More solid evidence

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needs to be provided. Furthermore, the relative role of the amorphous and condensed

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aromatic domains of BDHAs in HOC sorption remains unknown. To test this, the

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“bleaching” technique was employed to treat the HA samples. This technique has been

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previously used to selectively remove non-condensed aromatic moieties in organic 4


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geosorbents.22,23 If the amorphous aromatic domains play a dominant role in HOC sorption

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by geosorbents, the removal of amorphous aromatics would lower the organic carbon

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(OC)-normalized sorption distribution coefficient (Koc) of HOC. The bleaching treatment

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should otherwise either increase or not affect the Koc values.

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The major objectives of this study were therefore to: 1) explore the difference in

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composition and conformation between SDHA and BDHA, and 2) compare the sorption

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behavior of HOC by SDHA and BDHA and probe the underlying mechanisms accounting

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for the high sorption capacity of BDHA. To achieve these goals, the physicochemical

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properties of SDHA and BDHA fractions were characterized using elemental analysis,

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X-ray photoelectron spectra (XPS), solid-state cross-polarization magic-angle-spinning

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(CP-MAS)

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adsorption; then the sorption of PHE, a representative of HOCs, by both the original and

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bleached HA samples was examined.

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109

13

C NMR, Raman spectra, scanning electron microscopy (SEM), and gas

MATERIALS AND METHODS Sorbates and Sorbents. The

14

C-labeled (98+%) and unlabeled PHE (98+%) were

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purchased from Sigma-Aldrich Chemical Co., and used as sorbates. Five soil samples (S1,

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S2, S3, S4, and S5) were collected to obtain the SDHA fractions. A grassland surface soil

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sample (44°02′27″ N, 81°50′42″ E) was collected from Xinjiang province, Northwest

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China. A peat soil sample (33°04′28″ N, 102°55′42″ E) was collected from Sichuan

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province, Southwest China. Three agriculture soil samples (44°59′12″ N, 127°11′56″ E;

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46°58′22″ N, 132°53′50″ E; 46°58′28″ N, 132°53′15″ E) were collected from the Sanjiang

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Plain, Northeast China. The collected soil samples were extracted with 0.1 M NaOH. The 5



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extracts were acidified with 6 M HCl to pH = 2 and centrifuged to obtain the HA fractions.

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For further purification, the precipitated SDHA fractions were washed with deionized water

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to remove excess acid and soluble HA. The used water was collected for ultraviolet spectra

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analysis, and the HA samples were

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no discernible adsorption peaks. Next, the HA fractions were freeze dried, gently ground to

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pass through a 0.25-µm sieve, and stored for subsequent use. The HA fractions isolated

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from the grassland soil, peat soil, and the three agricultural soils were named SDHA-S1,

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SDHA-S2, SDHA-S3, SDHA-S4, and SDHA-S5, respectively.

washed until the used water showed

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The BDHA fractions used in this study were isolated from oxidized biochars. Grass

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straws of rice, wheat, and maize as well as animal manures of swine, cow, and chicken,

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were collected to produce biochars at 300, 450, and 600 °C for 1 h under N2 condition in a

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muffle furnace. Given that the natural aging process of biochar is quite slow, HNO3

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oxidation was conducted to accelerate biochar aging.5 The biochars were ground and sieved

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(< 0.25 µm) before oxidation at approximately 90 °C for 4 h with 25% HNO3 (~5.5 M) at

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1:30 solid/liquid ratio.5 After the removal of excess acid through washing and

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centrifugation, the BDHA fractions were extracted from the oxidized biochars, similar to

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the process described for the soil. Here, only the BDHAs isolated from biochars produced

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at 450 °C were used in the further characterization and sorption experiments, and the

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reasons are provided below. These BDHA samples were referenced according to feedstocks

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of biochars (rice straw, wheat straw, maize straw, swine manure, cow manure, and chicken

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manure) (i.e., BDHA-RI, BDHA-WH, BDHA-MA, BDHA-SW, BDHA-CW, and

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BDHA-CH). 6


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The bleaching procedure, which was used to selectively remove the non-condensed

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aromatic C moieties (e.g., such as lignin-like and polyphenol units) of samples,22,23 was

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also conducted in this study. The bleaching treatment involved treating 1 g of each HA

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sample three times with 10 g of sodium chlorite, 10 mL of acetic acid, and 100 mL of DI

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water for 7 h each time. After being freeze-dried, all treated HAs (i.e., SDHA-S1-BL,

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SDHA-S2-BL,

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BDHA-WH-BL, BDHA-MA-BL, BDHA-SW-BL, and BDHA-CW-BL) were gently

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ground and stored for further use.

SDHA-S3-BL,

SDHA-S4-BL,

SDHA-S5-BL,

BDHA-RI-BL,

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Sorbent Characterization. The samples were characterized using elemental analysis,

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XPS spectrometer, Raman microspectrometer, CP-MAS 13C NMR, and gas adsorption (273

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K, CO2) to investigate their composition and conformation. More details can be found in

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the Supporting Information.

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Sorption Experiment. The sorption isotherms of PHE by the HA samples were

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achieved using a batch equilibration technique. Non-labeled PHE was dissolved in

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methanol to prepare stock solutions. Test solutions of PHE at various concentrations

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(2-1100 µg/L) were prepared by spiking both 14C labeled and non-labeled stock solutions

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into the background solution (pH 6.5) containing 0.01 M CaCl2 (ionic strength adjuster)

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and 200 mg/L of NaN3 (biocide), with methanol content was kept below 0.1% (v/v) to

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minimize the co-solvent effect. Next, the test solutions were added to the 40-mL glass vials

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with certain quantities of sorbent. The headspace of the vials was kept as small as possible.

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The solid-to-solution ratios were adjusted to ensure that the removal of PHE reached

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20-80% at equilibrium. The vials were then capped with Teflon-lined screw caps, mixed in 7



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the dark, and shaken for 10 d at room temperature (23 ± 1 °C) to reach apparent sorption

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equilibrium based on a preliminary test, which showed that equilibrium was reached within

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8 d. After centrifugation at 3000 rpm for 25 min, the PHE concentration in the supernatant

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was analyzed by scintillation counting.24 The final pH values were in the range of 6.3-6.8.

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All samples were conducted in duplicate. Because PHE sorption by the vials was negligible

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as indicated by the control experiment, the uptake of PHE by the HA fractions was

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calculated by mass balance.

168 169

Data Analysis. The Freundlich and Polanyi-Dubinin (PD) models were used for fitting the sorption data of PHE in this study: log qe = log KF + nlog Ce

(1)

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Kd = qe/Ce

(2)

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Koc = Kd/foc

(3)

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Freundlich model:

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where qe and Ce are the solid (µg/g)- and liquid-phase (µg/L) concentrations of sorbate,

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respectively; KF is the sorption affinity coefficient ((µg/g)/(µg/L)n); n is often applied as an

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indicator of isotherm nonlinearity; and Koc is the OC-normalized sorption distribution

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coefficient (Kd).

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PD model:

log qe = log Q0 + a (∈ sw/Vs)b

(4)

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where Q0 is the saturated adsorption capacity (cm3/kg); ∈ = RTln(Sw/Ce) is the effective

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adsorption potential (kJ/mol); Sw is the water solubility at 20 °C (1120 µg/L for

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PHE); R (8.314 × 10−3 kJ/mol/K) is the universal gas constant; and T (K) is absolute

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temperature; Vs (cm3/mol) is the molar volume of solute; a and b are fitting parameters.

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The fitting was processed with Sigmaplot 10.0 (SPSS, USA). The t-test was used to 8


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analyze the differences in properties and sorption capacity of the tested samples, the

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difference being considered to be signiﬁcant at p < 0.05.

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Characteristics of SDHA and BDHA Samples. The extracted yields of BDHA varied

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with pyrolysis temperatures for biochar production (Table S1). Biochars produced at

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450 °C gave the highest BDHA yields (4.4-63.1%; Table S1), in line with a previous

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study.13 Among the biochars carbonized at 450 °C, chicken manure-derived biochar showed

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the lowest BDHA yield (4.4%), while maize straw-derived biochar gave the highest yield

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(63.1%). The difference in BDHA yields could be attributed to the different OC contents of

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the original biochars, as indicated by the significant positive correlation between the BDHA

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yields and the OC contents of the original biochars (Figure S1). This correlation further

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indicates that after oxidation, the majority of biochar OC was more inclined to be

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associated with BDHA fraction. At 300 °C, the biochars were insufficiently carbonized and

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the HNO3 treatment led to greater oxidation. Therefore, only small quantities of the

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biochars produced at 300 °C were recovered after oxidation, which were insufficient for

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further extraction of BDHA. At 600 °C, the biochars were increasingly graphitized,13,25

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producing negligible amounts of BDHA (Table S1). As a result, only the BDHAs extracted

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from the biochars produced at 450 °C were used in the following experiments. This

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temperature has also been recommended for manufacturing biochar for soil amendment

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purposes.26,27

RESULTS AND DISCUSSION

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The bulk and surface elemental concentration, ash content, CO2-SA, and micropore

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volume of the soils, biochars, and HA samples are shown in Tables S2 and S3. The average 9



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C concentration of the BDHA samples was 49.4%, and the average O and H concentrations

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were 28.9% and 3.1%, respectively (Table S2). After fractionation of the biochars, the

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recovery of OC was 13.9-69.3% for the tested BDHAs (Table S1), implying that the

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addition of substantial amounts of biochar and subsequent weathering would contribute

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greatly to HA in soils. This finding provides a new perspective to explore biochar

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degradation after application and the potential effects on the physicochemical properties of

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SOM. The bulk polarity ((O+N)/C) of the BDHAs was in the range of 0.47-0.60 (Table S2).

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The BDHAs generally contained more C than the SDHAs (Table S2). To better explore the

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elemental composition of SDHA and BDHA, the atomic H/C and O/C ratios of the biochars,

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the BDHA fractions, the SDHA fractions, and the HAs isolated from black and non-black

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soils, as given in the literature, are also provided in Table S4 and presented in a van

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Krevelen diagram (Figure S2). It is clearly showed that the H/C and O/C atomic ratios of

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BDHAs and SDHAs are plotted in distinguishable groupings (Figure S2). In general, the

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position of the SDHAs was concentrated near the lignin/melanoidin zone in the Van

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Krevelen diagram (Figure S2). In contrast, the BDHAs were plotted near the black soil

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HAs (Figure S2). Moreover, the H/C ratios of BDHAs were distinctly lower than those of

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the SDHAs (Figure S2).

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Table S2 also shows the surface elemental compositions of the two types of HAs. The

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XPS-derived surface C concentrations of the HA samples were in the range of

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19.5-1-75.7%, comparable with those observed for peat HA samples.28 The surface C

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enrichment was calculated as the ratio between XPS-based C concentrations and those

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obtained from elemental analysis. The surface enrichment values of all HA samples, except 10


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for SDHA-S3, ranged between 1.03-4.59, implying C enrichment at particle surfaces, or in

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other words, that larger mineral grains are masked by the organic matter. Our observation is

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consistent with previous studies, which have indicated that the minerals in soils and

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sediments are mainly coated with organic matter.28,29 Additionally, the high-resolution XPS

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C1s spectra clearly indicate that BDHAs were characterized by more surface COOH

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groups than the corresponding biochars and SDHAs (Table S5), which imparts to the

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BDHA particles a large cation exchange capacity9,11 and thus a high retention ability of

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plant nutrients, both are favorable features for soil fertility. The abundant surface COOH

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groups of BDHA would also facilitate its interaction with soil minerals,30 which may

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protect BDHA from further degradation. It has been proposed that physical protection and

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interactions with soil minerals play a significant role in biochar persistence in the

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environment.31,32 Furthermore, the sum of all O-containing functional groups (C-O, C=O

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and COOH) of the SDHAs excluding SDHA-S1 were higher than that of the BDHAs

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(Table S5), implying the higher surface hydrophobicity of the BDHAs relative to the

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SDHAs.

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As indicated by the integration results of 13C NMR spectra, the alkyl C was selectively

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oxidized during the oxidation and fractionation process of biochar, leading to the slightly

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higher aromaticity of the BDHAs than that of biochars (Table S6). Furthermore, the signals

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of all types of aliphatic C (alkyl, methoxyl, and carbohydrate C), which were between 0

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and 93 ppm, were quite negligible (no more than 5% of the total spectra area, see Figure 1

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and Table S6), consistent with the low H/C ratios of BDHAs (Table S2). The marginally

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distinguishable alkyl C peak of BDHAs, centered at 20 ppm, is more typical of the 11



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short-chain alkyl substituents, probably methyl groups, on the condensed aromatic rings.13

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In contrast, the alkyl C signal of the SDHAs spanned a wide range of chemical shifts (0-45

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ppm) with the intensity centered at 30 ppm, indicating that long-chain polymethylenic

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structures are important.33 The polar C contents of the SDHA samples were in the range of

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31.4-43.5%, considerably higher than those of the BDHA samples (23.3-26.0%; Table S6).

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Further, it is noted that the carboxyl signals of the SDHAs (Figure 1) were primarily from

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aliphatic carboxyl and amide groups (173 ppm), while as a result of attachment to the

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aromatic structures, those of the BDHAs shifted to 168 ppm.34 Previous studies also

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showed that the carboxyl groups of HA extracts in black soils were mainly associated with

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aromatic rings.9 Emerging on the edge of the aromatic plane of the BDHAs, the

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electron-withdrawing carboxyl groups may increase the π-polarity of the surface aromatic

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rings on the BDHAs, rendering them effective π-acceptors towards PHE (electron donor).

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The Raman spectra of the SDHAs and BDHAs (Figure 2) exhibited two typical peaks,

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namely, amorphous C (A) and a graphite peak (G).35 The A peak of the SDHAs was

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obviously stronger than the G peak, demonstrating that the OC of SDHAs was primarily

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amorphous (Figure 2). In contrast, the G peak of the BDHAs was apparently sharper than

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their A peak (Figure 2), indicating the high crystallinity of OC (mainly aromatics) in the

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BDHAs. The difference in the morphology of the SDHAs and BDHAs was further

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confirmed using SEM. The SEM images in Figures S3 and S4 neatly displayed the

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dramatically different morphological characteristics of the SDHAs and BDHAs. Under the

269

SEM,

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SEM-amorphous forms. The SEM-amorphous particles of SDHA occurred as discrete

the

SDHA

samples

had

coarse

surfaces

and

12


generally

appeared

as

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particles or as sheet forms (Figure S3). A similar morphology was not observed in the

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BDHAs (Figure S4). The BDHA surfaces were smooth and rigid, which was in satisfactory

273

accordance with the condensed aromatic structure of BDHAs suggested by the NMR and

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Raman analysis.

275

The microporosity and surface area (SA) are very important properties of organic

276

materials in soils, as they influence all of the essential functions for fertility, including

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water-holding capacity, nutrient cycling, and the mechanistic evaluation of the sorption

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process.16 The CO2 adsorption isotherms and pore size distribution of the tested samples are

279

shown in Figures S5 and S6, respectively. The CO2-SA values of the SDHA samples ranged

280

from 17.1 to 115.6 m2/g (Table S2), in line with the data of a previous study.36 The BDHA

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samples exhibited similar CO2-SA values (39.2-80.4 m2/g). These values were lower than

282

expected, considering that the pristine biochars, used to obtain the BDHA samples, had

283

high porosity with CO2-SA values in the range of 33.2-388.3 m2/g (Table S3). In this study,

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the SDHA and BDHA fractions had comparable CO2-SA values; thus, the biochar

285

application may have little influence on the specific surfaces of HA fraction in soils.

286

Characteristics of Bleached HA Samples. Substantial amounts of OC were removed

287

by the bleaching treatment (Table S7). With respect to the SDHAs, their OC recovery

288

ranged from 16.5% up to 59.6%, while approximately half of the original OC of BDHAs

289

was recovered (Table S7). The 13C NMR spectra (Figure S7) clearly showed the decrease in

290

the relative concentration of the aromatic structure in SDHAs after bleaching treatment.

291

Furthermore, the Raman spectra illustrated that the bleaching treatment largely decreased

292

the amorphous C peak of the SDHAs and BDHAs (Figure 2). These results revealed that 13



293

the amorphous aromatic C was the main component which was degraded by the bleaching

294

procedure and consistent with the recent research.37

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Along with the removal of amorphous aromatic C, except for SDHA-S4, the micropore

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volume and CO2-SA of SDHAs slightly decreased after the bleaching treatment, whereas

297

the OC-normalized CO2-SA (CO2-SA/OC) values evidently increased (Figure 3 and Table

298

S7). This indicates that the expanded aromatic component of the SDHAs was not a main

299

source of their micropores; otherwise, the removal of the amorphous aromatic C after the

300

bleaching treatment would have resulted in a substantial decrease in the CO2-SA and

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CO2-SA/OC values of the SDHAs. In contrast, a remarkable decline in the micropore

302

volume, CO2-SA, and CO2-SA/OC values of the BDHAs was observed (Figure 3 and Table

303

S7). It was thus clear that the non-condensed aromatic moieties contributed dominantly to

304

the micropores of the BDHAs. Although the SDHA and BDHA samples showed no distinct

305

difference in CO2-SA values, it seems that their micropores were derived from different

306

structural moieties.

307

Comparison of PHE Sorption between the SDHAs and BDHAs. The Freundlich

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sorption isotherms of PHE by the SDHAs and BDHAs are shown in Figure 4. The

309

regression parameters are listed in Table S8. The sorption isotherms of PHE by SDHAs and

310

BDHAs were generally nonlinear, with the respective n values being in the range of

311

0.64-1.01 and 0.45-0.69 (Table S8). The BDHAs exhibited stronger nonlinear sorption than

312

the SDHAs. This could be well interpreted as the result of the higher levels of aromaticity

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of the BDHAs, as evidenced by the inverse relationship between the n values and the

314

aromaticity of all HAs (Figure S8a). Therefore, it can be concluded that the nonlinear PHE 14


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sorption by the SDHA and BDHA fractions was regulated by their aromaticity. PHE can be

316

adsorbed to aromatic regions via π-π electron donor-acceptor (EDA) interactions and this

317

process is expected to generate nonlinear sorption.38 The logKoc (Ce = 0.01Sw) values of

318

PHE for the BDHA fractions ranged from 5.16 to 5.61 mL/g (Table S8). The Freundlich

319

sorption coefficients (logKd and logKoc) of BDHAs (Figure 5 and Table S8) were

320

comparable to those of their corresponding bulk biochar as we have previously shown.39

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Moreover, the t-test analysis showed that the logKd (at Ce = 0.01 Sw, p < 0.001) and logKoc

322

(at Ce = 0.01 Sw, p < 0.05) values of the BDHA samples were significantly higher than

323

those of the SDHA samples. Here, the BDHA samples exhibited excellent sorption capacity

324

for PHE, implying that the high sorption capacity of biochar is long-lasting after being

325

applied into soils. This confirms the feasibility of biochar application in remediating PHE

326

contaminated soil.

327

The excellent sorption properties of the BDHAs may be tied to their distinct structure.

328

It has been proposed that high polarity (usually characterized by O/C or (O+N)/C or total

329

polar functional groups) can reduce the sorption affinity.24,38 The hydrophilic moieties

330

would reduce the accessibility of the HOCs to sorption domains and compete for sorption

331

sites, thereby decreasing HOC sorption by geosorbents.24 Compared with the SDHAs, the

332

BDHAs had lower contents of polar functional groups as indicated by the 13C NMR results

333

(Table S6), which may partly contribute to the higher sorption capacity of PHE by BDHAs.

334

This explanation was further confirmed by the significantly negative relationship between

335

the logKoc values of PHE and the polar C contents of the tested samples (Figure S8b).

336

In addition to polarity, the chemical composition of geosorbents could also influence 15



337

HOC sorption. With respect to this, previous studies have discussed the key role of the

338

aromatic and aliphatic components in the sorption of geosorbents.20,38 However, no

339

significant correlation existed between the sorption capacity of PHE by the SDHAs and

340

their aliphatic C or aromatic C content. It has been proposed that the presence of pyrogenic

341

C within SOM could strengthen the role of aromatic structures in HOC sorption,40 thus

342

resulting in the lack of a clear relationship between the PHE Koc values for the SOM

343

fractions and the aliphatic C content. To examine the dominant sorption domains of PHE

344

by the HA samples, we determined PHE sorption by the bleached SDHAs and BDHAs

345

(Figure 4 and Table S8). After the removal of amorphous aromatic C, the logKoc values (Ce

346

= 0.01 Sw) of PHE for SDHAs except for SDHA-S5 were significantly increased (paired

347

t-test p < 0.05; Figure 5 and Table S8), despite the fact that the bulk polarity ((O+N)/C) of

348

SDHAs generally increased after bleaching treatment (Tables S2 and S7). A previous study

349

also found that bleaching treatment enhanced PHE sorption on the HAs isolated from soils,

350

along with the removal of a significant fraction of aromatic components.22 Therefore, the

351

expanded aromatic components were unlikely to regulate PHE sorption by SDHAs. In good

352

agreement with this conclusion, the logKoc (Ce = 0.01 Sw) of PHE by the bleached SDHAs

353

generally increased with increasing aliphatic C content (Figure S8c), suggesting that PHE

354

sorption by SDHAs was dominated by their aliphatic moieties.

355

With respect to BDHAs, they mainly consisted of aromatic and carboxylic C as

356

illustrated in 13C NMR spectra (Figure 1). The aromatic π-systems in organic materials rich

357

in electron-withdrawing functional groups will tend to be electron-deficient and thus can

358

act as π-acceptors.41 As mentioned above, the electron-withdrawing surface COOH groups, 16


Page 16 of 30

Page 17 of 30


359

located on the edge of the aromatic plain of the BDHAs, may render them effective

360

π-acceptors towards PHE. This, along with the generally positive relationship between the

361

logKoc values of PHE and the aromatic C contents of BDHAs (Figure S8d), indicates that

362

the sorption capacity of BDHAs for PHE may result from the contribution of π-π EDA

363

interactions between the PHE molecules and the aromatic components within BDHAs.

364

Furthermore, the removal of amorphous aromatic C of BDHAs by bleaching treatment was

365

noted to significantly decrease (paired t-test p < 0.01) logKoc of PHE by BDHAs to varying

366

degrees (Figure 5 and Table S8). This indicates that amorphous aromatic C of BDHAs

367

should make a larger contribution to PHE sorption than condensed aromatic C; otherwise,

368

if condensed aromatic C plays a larger role in the high sorption capacity of BDHAs for

369

PHE compared with amorphous aromatic C, the removal of amorphous aromatic C after

370

bleaching treatment should result in the increase of logKoc. Additionally, the bulk polarity

371

((O+N)/C) of the BDHAs significantly increased (paired t-test p < 0.05; Tables S2 and S7)

372

after bleaching, which may also contribute to their decreased PHE sorption.

373

Porosity of sorbents may also affect HOC sorption by geosorbents.40,42 However,

374

despite the fact that the SDHAs and BDHAs had comparable CO2-SA (Table S2), the

375

BDHAs showed much higher sorption capacity for PHE (Figure 5 and Table S8). It is clear

376

that the pore-filling mechanism cannot account for the better sorption capacity of the

377

BDHA samples than the SDHA samples. Consistently, no significant correlation was found

378

between the logKoc of PHE and the CO2-SA values of the HA samples. In particular, the

379

lowest PHE sorption capacity (logKoc) was observed on SDHA-S3, which had the highest

380

CO2-SA (Tables S2 and S8). The influence of micropore filling on the PHE sorption by the 17



381

HA samples was further examined by employing a PD model to fit the sorption data

382

(Figure S9 and Table S9). As expected, no clear trend in the adsorbed volume capacities

383

(Q0, obtained using PD model) of PHE with micropore volume was found for the SDHA

384

and BDHA samples (Figure S10). Therefore, pore-filling was less likely to be the major

385

mechanism of PHE sorption by the SDHA and BDHA samples.

386

Environmental Implications. According to our data, it is noted that after fractionation of

387

the biochars produced at 450 °C, 13.9-69.3% of biochar OC can be recovered in BDHA

388

fraction, indicating that biochar addition and subsequent weathering would contribute

389

greatly to HA in soils. Moreover, the contrasting characteristics and sorption mechanisms

390

of BDHA and SDHA were clearly demonstrated in this study. These results imply that

391

using the alkaline extraction technique to obtain humic substances, pyrogenic organic

392

matter can be concurrently extracted. In other words, the constituents of humic substances

393

were not exclusively formed via humification process; the ubiquitous pyrogenic organic

394

matter from vegetation fires found in most soils5-10 also contributed to the compound

395

mixture extracted in alkaline solution. Therefore, this study extends our knowledge on the

396

nature of humic substances in soil and is beneficial to the development of accurate

397

definition of humic substances. In addition, the stability of OC in soils is crucial for global

398

carbon cycling and climate change mitigation.31,43 As shown in this study, BDHA and

399

SDHA possess different characteristics, which would lead to their different reactivity with

400

soil minerals and biodegradability, and thus different resistance ability against degradation

401

in soils. The investigation of BDOM properties provides a new perspective to explore OC

402

dynamics in biochar-rich soils. Furthermore, we demonstrate that compared with SDHA, 18


Page 18 of 30

Page 19 of 30


403

BDHA showed superior sorption capacity and distinct sorption mechanism for PHE. This

404

finding would enhance the understanding of the fate, transport, bioavailability, and toxicity

405

of HOCs in biochar-amended soils.

406

ASSOCIATED CONTENT

407

Supporting Information. Nine tables and ten figures are included in the Supporting

408

Information. This material is available free of charge via the Internet at http://pubs.acs.org.

409

AUTHOR INFORMATION

410

Corresponding Authors

411

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

412

*(B.X.) Phone: +1 413 545 5212; e-mail: [email protected].

413

Notes

414

The authors declare no competing ﬁnancial interest.

415

ACKNOWLEDGMENTS

416

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

417

Youth Foundation (41522303), the National Key Research and Development Program of

418

China (2017YFA0207002), the National Natural Science Foundation of China (41473087

419

and 41703097), the State Education Ministry, the USDA McIntire-Stennis Program (MAS

420

00028), the China Postdoctoral Science Foundation (2016M600070), and the Fundamental

421

Research Funds for the Central Universities. J. Jin also thanks the China Scholarship

422

Council for supporting her study at the University of Massachusetts, Amherst.

423

REFERENCES 19



424 425

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soil organic matter using structurally modified humic acids. Environ. Sci. Technol. 2003, 37, (5), 852-858.

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atrazine with various humic acid fractions from a single soil sample. Environ. Sci. Technol. 2011, 45, (6),

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(28) Yang, Y.; Shu, L.; Wang, X.; Xing, B.; Tao, S. Impact of de-ashing humic acid and humin on organic

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matter structural properties and sorption mechanisms of phenanthrene. Environ. Sci. Technol. 2011, 45, (9),

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mineralogical soil gradient (0.3-4100 kyr), Hawaiian Islands. Geochim. Cosmochim. Acta 2009, 73, (7),

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self-assembly of organic molecular fragments into zonal structures on mineral surfaces. Biogeochemistry

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Kögel-Knabner, I.; Lehmann, J.; Manning, D. A. Persistence of soil organic matter as an ecosystem

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Song, G.; Nogueira, C. M.; Mangrich, A. S. Lessons from the Terra Preta de Índios of the Amazon region

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for the utilisation of charcoal for soil amendment. J. Braz. Chem. Soc. 2009, 20, (6), 1003-1010.

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

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(37) Han, L.; Ro, K. S.; Sun, K.; Sun, H.; Wang, Z.; Libra, J. A.; Xing, B. New evidence for high sorption

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capacity of hydrochar for hydrophobic organic pollutants. Environ. Sci. Technol. 2016, 50, (24),

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13274-13282.

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(39) Sun, K.; Kang, M.; Zhang, Z.; Jin, J.; Wang, Z.; Pan, Z.; Xu, D.; Wu, F.; Xing, B. Impact of deashing

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treatment on biochar structural properties and potential sorption mechanisms of phenanthrene. Environ.

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Sci. Technol. 2013, 47, (20), 11473-11481.

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(40) Han, L.; Sun, K.; Jin, J.; Wei, X.; Xia, X.; Wu, F.; Gao, B.; Xing, B. Role of structure and microporosity

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in phenanthrene sorption by natural and engineered organic matter. Environ. Sci. Technol. 2014, 48, (19),

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11227-11234.

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(41) Keiluweit, M.; Kleber, M. Molecular-level interactions in soils and sediments: the role of aromatic π-systems. Environ. Sci. Technol. 2009, 43, (10), 3421-3429.

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

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of different organic matter fractions from a same soil source and their phenanthrene sorption. Environ. Sci.

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Technol. 2013, 47, (10), 5138-5145.

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(43) Harden, J. W.; Hugelius, G.; Ahlström, A.; Blankinship, J. C.; Bond-Lamberty, B.; Lawrence, C. R.;

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Loisel, J.; Malhotra, A.; Jackson, R. B.; Ogle, S. Networking our science to characterize the state,

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533

DOI: 10.1111/gcb.13896

534

23



535

Figure Captions:

536

Figure 1. Cross-polarization magic angle spinning

537

samples extracted from various soils (S1, S2, S3, S4, and S5) and biochars produced from

538

rice straw (RI), wheat straw (WH), maize straw (MA), swine manure (SW), and cow

539

manure (CW). “SDHA” and “BDHA” denote humic acids extracted from soils and

540

biochars, respectively.)

541

Figure 2. Raman spectra of original (a and b) and bleached (c and d) humic acids isolated

542

from different soils (S1, S2, S3, S4, and S5) and biochars produced from rice straw (RI),

543

wheat straw (WH), maize straw (MA), swine manure (SW), and cow manure (CW). Note

544

that A and G refer to amorphous and graphite peak, respectively; SDHA and BDHA

545

represent humic acids extracted from soils and biochars, respectively; “BL” denotes the

546

bleached samples.

547

Figure 3. Organic carbon-normalized surface area (CO2-SA/OC) of soil-derived (SDHA)

548

(a) and biochar-derived (BDHA) (b) humic acids before and after bleaching. Note that S1,

549

S2, S3, S4, and S5 represent various soils; RI, WH, MA, SW, and CW represent the

550

biochars produced from straws of rice, wheat, and maize, and manures of swine and cow,

551

respectively.

552

Figure 4. Freundlich sorption isotherms of phenanthrene (PHE) by the humic acids isolated

553

from different soils (S1, S2, S3, S4, and S5) (a, b, and c) and biochars (d, e, and f)

554

produced from rice straw (RI), wheat straw (WH), maize straw (MA), swine manure (SW),

555

cow manure (CW), and chicken manure (CH). “SDHA” and “BDHA” denote humic acids

556

extracted from soils and biochars, respectively; “BL” denotes the bleached samples.

557

Figure 5. Sorption capacity (logKoc, Ce = 0.01 Sw) of phenanthrene (PHE) by soil-derived

558

(SDHA) and biochar-derived (BDHA) humic acids before and after bleaching. Note that S1,

559

S2, S3, S4, and S5 represent various soils; RI, WH, MA, SW, CW, and CH represent the

560

biochars produced from straws of rice, wheat, and maize, and manures of swine, cow, and

561

chicken, respectively.

562

13

C NMR spectra of humic acid (HA)

24


Page 24 of 30

Page 25 of 30

563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588


168 ppm

173 ppm Carboxylic C

Aromatic C

Aromatic C

Carboxylic C

Aliphatic C

SDHA-S5

BDHA-CW

SDHA-S4

BDHA-SW

SDHA-S3

BDHA-MA

SDHA-S2

BDHA-WH

SDHA-S1

BDHA-RI

250

200

150

100

50

0

250

-50

200

13

C chemical shift, ppm

Figure 1.

589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 25


Aliphatic C

150

100

50

0

-50


606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649

a

A

Page 26 of 30

c G

SDHA-S5 SDHA-S4

A SDHA-S3

SDHA-S5-BL

SDHA-S2

SDHA-S4-BL SDHA-S2-BL

SDHA-S1

800

1000 1200 1400 1600 1800 2000 800

A

b

G

1000 1200 1400 1600 1800 2000

d

A

G

BDHA-CW-BL

BDHA-CW

BDHA-SW-BL

BDHA-SW

BDHA-MA

BDHA-MA-BL

BDHA-WH

BDHA-WH-BL BDHA-RI-BL

BDHA-RI 800

G

1000 1200 1400 1600 1800 2000 800

1000 1200 1400 1600 1800 2000

Raman shift, cm-1 Figure 2.

f. 26


Page 27 of 30

Bleached HA

a

1200 1000

g.

800 600 400

676 677

SDHA-S5

SDHA-S4

Figure 3.

679 680 681 682 683 684 685 686 687 27


BDHA-CW

675

SDHA-S3

j. DHA-CH BDHA-RI

e.

i.

BDHA-SW

673

Bleached HA

BDHA-MA

672

Original HA

BDHA-WH

CO2-SA/OC, m /g

d.

180 160 140 120 100 80 60 40 20 0

h.

SDHA-S1

0

c.

SDHA-S2

200

671

678

Original HA

1400

2

b.

670

674

CO2-SA/OC, m /g

1600

2

650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669


b


688 689 690

14000

10000

10000

692

8000

693

6000

a

SDHA-S1 SDHA-S1-BL SDHA-S2 SDHA-S2-BL

12000

691

b

SDHA-S3 SDHA-S3-BL SDHA-S4 SDHA-S4-BL

8000 6000 4000

4000

694

Page 28 of 30

2000

2000

695

0

0 0

696

100 200 300 400 500 600 700 800

6000

697

200

400

600

800

1000

20000 SDHA-S5 SDHA-S5-BL

5000

698

0

c

d

BDHA-RI BDHA-RI-BL BDHA-WH BDHA-WH-BL

16000

4000

700 701

qe, µg/g

12000

699

3000 8000 2000 4000

1000

0

0

702 703

0

100

200

300

400

500

600

25000

700

e

BDHA-MA BDHA-MA-BL BDHA-SW BDHA-SW-BL

0

20000

705

15000

15000

706

10000

10000

707

5000

5000

708

0

0 0

710

200

400

800

1000

0

200

Ce, µg/L

711 712 713

300

400

500

600

Figure 4.

714 715 716 717 718 28


400

700

f

BDHA-CW BDHA-CW-BL BDHA-CH

20000

600

200

25000

704

709

100

600

800

1000

Page 29 of 30


719 720 721 722 723 724 725

747

7.0 Original HA

6.0

Bleached HA

5.0 LogKoc, mL/g

4.0 3.0 2.0

Figure 5.

748

29


BDHA-CH

BDHA-CW

BDHA-SW

BDHA-MA

BDHA-WH

BDHA-RI

SDHA-S5

SDHA-S4

SDHA-S3

0.0

SDHA-S2

1.0 SDHA-S1

726 727 728 729 730 731 732 733 734 735 736 737 738 739 740 741 742 743 744 745 746


749 750 751 752 753 754 755 756 757 758 759 760 761 762

Graphic for manuscript (Abstract Art)

763 764 765 766 767 768 769 770

30


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Comparison between Soil- and Biochar-Derived Humic Acids

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