Photochemistry of Hydrochar: Reactive Oxygen Species Generation

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Photochemistry of Hydrochar: Reactive Oxygen Species Generation and Sulfadimidine Degradation Na Chen, Yahui Huang, Xiaojing Hou, Zhihui Ai, and Lizhi Zhang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b02740 • Publication Date (Web): 11 Sep 2017 Downloaded from http://pubs.acs.org on September 11, 2017

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Environmental Science & Technology

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Photochemistry of Hydrochar: Reactive Oxygen Species

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Generation and Sulfadimidine Degradation

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Na Chen, Yahui Huang, Xiaojing Hou, Zhihui Ai, and Lizhi Zhang*

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Key Laboratory of Pesticide & Chemical Biology of Ministry of Education, Institute of

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Environmental & Applied Chemistry, College of Chemistry, Central China Normal University,

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Wuhan 430079, People’s Republic of China

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* To whom correspondence should be addressed. E-mail: [email protected].

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Phone/Fax: +86-27-6786 7535

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ABSTRACT Biochar, mainly including pyrochar produced via pyrolysis of biomass at moderate

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temperatures of 350−700 oC and hydrochar formed by hydrothermal carbonization in a range of

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150-350 oC, have received increasing attention because of their significant environmental impacts. It

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is known that pyrochar can generate reactive oxygen species even in the dark owing to the presence

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of persistent free radicals, but hydrochar is far less studied. In this study, we systematically

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investigate the photochemistry of hydrochar and check its effects on the sulfadimidine degradation.

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Different from pyrochar derived from the same biomass, hydrochar could generate much more H2O2

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and •OH under daylight irradiation, which could enhance the sulfadimidine degradation rate by 6

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times than in the dark. Raman spectroscopy, Fourier transform infrared spectroscopy, electron

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paramagnetic resonance, and X-ray photoelectron spectroscopy were employed to elucidate this

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interesting phenomenon. Characterization results revealed that the higher reactive oxygen species

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generation ability of hydrochar under solar light irradiation was attributed to its abundant

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photoactive surface oxygenated functional groups. This study clarifies the differences of pyrochar

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and hydrochar on organic pollutants degradation, and also sheds light on environmental effects of

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hydrochar.

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Keywords: Hydrochar; Photochemistry; Reactive oxygen species; Sulfadimidine; Oxygenated

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functional groups

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Introduction

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Biochar, as one of the refractory carbon materials, has received increasing attention because of their

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significant environmental impacts and outstanding agricultural application potential.1-3 For instance,

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the utilization of biochar may increase carbon storage in soil, mitigate soil nitrous oxide emissions,

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and raise agricultural productivity.4-8 Furthermore, biochar is regarded as a “super sorbent” because 2 ACS Paragon Plus Environment

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of its high specific surface areas and unique surface chemistry properties (e.g., enriched

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O-containing groups and highly electronically polarized graphitized carbon surfaces), being used to

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immobilize toxic heavy metal ions and adsorb persistent pollutants, and thus promising for the

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contaminated soil and groundwater remediation.9-12 Generally, pyrolysis and hydrothermal carbonization can be used to convert biomass into biochar.4,

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13

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(pyrochar) in a reactor at moderate temperatures (e.g., 350−700 oC) with limited oxygen.13 Pyrochar

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has been intensively studied as a sorbent to remove heavy metal ions (e.g., Cu(II) and Pb(II)) and

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persistent organic contaminants (e.g., polycyclic aromatic hydrocarbons and polychlorinated

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biphenyls).14-19 Recently, some new properties of pyrochar were explored. For instance, Zhou’s group

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found that pyrochar could catalyze hydrogen peroxide or persulfate to produce reactive oxygen

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species (ROS) for the phenolic compounds degradation, which was ascribed to the presence of

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persistent free radicals (PFRs) generated during the pyrolysis of biomass.20 Meanwhile, Qu and his

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co-workers reported that dissolved fractions of black carbon released from pyrochar could directly

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generate singlet oxygen (1O2) and superoxide (O2-) under sunlight irradiation.21 In contrast, little

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information is available on another kind of biochar produced via hydrothermal carbonization (HTC).

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HTC is often used to mimic the natural coal formation under geothermal conditions,22 and becomes

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a novel thermal chemical technique for converting the wet biomass to solid carbonaceous materials

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(hydrochar) at self-generated pressure and relatively lower temperature with using water as an

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environmental friendly reaction medium.23 In comparison with pyrochar, hydrochar is far less studied.

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Recently, Yu’s group reported that the photoactivity of hydrochar could be improved with iodine

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doping and explained this phenomenon with the semiconductor photocatalysis mechanism.24

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However, the photochemical environmental effects of hydrochar on organic pollutants degradation

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still remain mysterious, and its potential in environmental remediation needs to be explored. 3

Pyrolysis is a traditional process to thermally decompose biomass into carbon-rich solid product

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Meanwhile, HTC process is quite dissimilar from pyrolysis, which might result in the chemical

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structures and properties of hydrochar different from those of pyrochar.4 For example, pyrochar with

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higher stability in soil exhibits a higher potential for carbon sequestration than hydrochar, while

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hydrochar may act as a mid-term fertilizer by slowing the release of plant-available nutrient to soils.25

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Sun and co-workers found that hydrochar could adsorb more bisphenol A, 17a-ethinyl estradiol, and

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phenanthrene than pyrochar.26 In despite of these advances, we still lack a systematic comparison

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between hydrochar and pyrochar derived from the same biomass on the degradation of organic

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contaminants in the dark and under solar light irradiation, and thus fail to evaluate their environmental

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effects and pollutant removal performances.

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In this study, we prepare hydrochar and pyrochar with two common biomass (fallen-leaves and

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woodchips), and then compare their performances on the removal of sulfadimidine (SM2) in the dark

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and under solar light irradiation, aiming to check the differences between hydrochar and pyrochar

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derived from the same biomass for ROS generation and organic pollutants degradation under different

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conditions, especially under solar light irradiation.

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Experimental Section

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Chemicals and Materials. Acetic acid (≥ 99.5%), methanol (≥ 99.5%), ethanol (≥ 99.7%),

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potassium acid phthalate (≥ 99.8%), benzoic acid (≥ 99.5%), tert-butanol (TBA, ≥ 98.0%) and nitric

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acid (65%-68%) were all purchased from Sinopharm Chemical Reagent Co., Ltd. China.

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Sulfadimidine (≥ 98.0%) was purchased from TCI (Shanghai) Development Co., Ltd.

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P-hydroxyphenylacetic acid (PHPA, ≥ 99.0%), horseradish peroxidase (POD, specific activity

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of >100 units mg−1), catalase from bovine liver (CAT, specific activity of 2000-5000 units mg−1),

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superoxide dismutase (SOD, specific activity of ≥ 2500 units mg−1) were purchased from Aladdin

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Chemistry Co., Ltd. China. Acetonitrile and methanol were of HPLC grade (≥ 99.9%) and obtained 4 ACS Paragon Plus Environment

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from Merck KGaA. All chemicals were used as received without any further purification. Deionized

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water was used throughout the experiments.

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Biochar Preparation. Leaves and wood sticks were collected from Platanus acerifolia in the

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campus of Central China Normal University in October. Before the hydrothermal carbonization and

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pyrolysis processes, the collected fallen-leaves and woodchips were washed thoroughly to remove

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the dust and then dried in the air for 24 hours. Then, the dried feedstock was carefully smashed by a

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pulverizer to pass through a 400-mush sieve. The passed powders were called as “fallen-leaves” and

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“woodchips”, respectively. For the synthesis of hydrochar, 7 g of biomass powder was added in 50

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mL of deionized water with magnetic stirring for 2 hours at room temperature. The mixture was

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subsequently transferred into a 80 mL Teflon−lined stainless steel autoclave and heated in an electric

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oven at 200 oC for 5 hours. After cooling to room temperature, the solid was collected, washed with

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deionized water and dried at 60 oC before use. The samples derived from fallen-leaves and

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woodchips were called as HTC-F and HTC-W, respectively. For the pyrolysis of biomass, the tube

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furnace was first heated with a temperature increase of 3 oC/min from 20 oC to 450 oC, and then kept

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at 450 oC for 3 hours. The whole pyrolysis process was continuously purged with high-purity argon.

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After natural cooling to the room temperature, the products were collected and donated as PC-F

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(fallen-leaves) and PC-W (woodchips), respectively. All the biochar samples were stored in the dark

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before use. The preparation procedures were illustrated in supporting information (Scheme S1).

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Materials Characterizations. The element contents of biochar samples were characterized by

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element analysis (vario EL cube, Elementar, Germany) (SI Table S1). The morphology of biochar

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samples was characterized by scanning electron microscopy (SEM, TESCAN MIRA 3, Czech) (SI

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Figure

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(Brunauer-Emmett-Teller) nitrogen adsorption isotherms at 77 K (SI Figure S2 and Table S2). The

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persistent free radicals of biochar powder were detected by an electron paramagnetic resonance 5

S1).

The

specific

surface

area

of

biochar samples was analyzed


by BET


(EPR) spectrometer (Bruker E500, Germany) at room temperature according to previous reports (SI

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Text S1).20,

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USA) equipped with a 532 nm laser excitation source. The Fourier transform infrared (FT-IR)

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spectra of biochar samples were collected by a Nicolet iS50 FT-IR spectrometer (Thermo, USA) in

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the region of 800-4000 cm-1 with the resolution of 2 cm-1. The C and O bonding states on the surface

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of biochar samples were obtained by using an X-ray photoelectron spectroscopy (Thermal scientific,

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ESCALAB 250Xi) (SI Text S2).

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The biochar samples were characterized by DXR Raman Microscope (Thermo,

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Photodegradation of Sulfadimidine. Batch experiments were conducted in 100 mL containers

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with constant magnetic stirring. Briefly, 5 mg of biochar powder were dispersed into 50 mL of SM2

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solution (250 µg L-1). The initial pH value of the solution was adjusted to 7.2 ± 0.1 at 25 °C by dilute

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H2SO4 and NaOH aqueous solutions. The dark reaction was performed in the container covered with

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aluminum foil. For the light reaction, the solutions were irradiated from the top by a daylight lamp (a

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domestic fluorescent lamp, Foshan Electrical And Lighting Co., Ltd., China) with the output energy

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of 40 W at a distance of 0.1 m. The irradiation intensity was 3.82 mW/cm2, which was much weaker

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than that of solar light (66.20 mW/cm2 in May, Wuhan, Hubei Province, China), as detected by the

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optical power meter (PLS-MW2000, Beijing Perfect Light Co., Ltd., China). The wavelength of

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daylight lamp was in the range of 300 -750 nm, as measured by a grating spectrometer (WGD-8/8A)

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(SI Figure S3). The light reaction temperature was maintained at 25 ± 0.1°C with a water circulating

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system (SI Scheme S2). Control experiments without biochar powder were conducted under the

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same conditions. 2 mL of removal solutions were sampled and then filtered through 0.22 µm nylon

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syringe filter for the subsequent analysis. The nylon syringe filter was used to take 10 mL of SM2

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solution (250 µg L-1) for the adsorption saturation before the degradation solution sampling, which

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could prevent the further adsorption of SM2 on the nylon syringe filter. 1 mL of sample solution was

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used to determine the concentration of SM2 by high performance liquid chromatography (HPLC, 6 ACS Paragon Plus Environment

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Ultimate 3000, Thermo), another 1 mL was used to quantify the amount of generated H2O2. For the

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•OH measurement, we replaced the SM2 solution with a benzoic acid aqueous solution (10 mmol L-1,

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the initial pH 7.2 ± 0.1 at 25 °C) and conducted the experiment under the same conditions as those of

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SM2 removal experiments.

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Analytical Methods. The production of hydrogen peroxide (H2O2) was monitored according to a

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modified p-hydroxyphenylacetic acid (POHPAA) emission method.29 Fluorescent dimmer (FD), the

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reaction product of H2O2 with POHPAA fluorescence reagent, emits strong fluorescence at 409 nm

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when excited at 315 nm. The fluorescence reagent was prepared by dissolving 2.7 mg of

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para-hydroxyphenylacetic acid and 1 mg of horseradish peroxidase in 10 mL of potassium hydrogen

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phthalate buffer solution (8.2 g L-1). For the H2O2 concentration detection, 50 µL of fluorescence

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reagent was added in 1 mL of sample and diluted with 1 mL of deionized water, after the next 10 min

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of reaction, 1 mL of NaOH (0.1 mol L-1) was added to quench the reaction. The resulting solutions

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were then measured with the fluorescence spectrophotometer (FL1008M018, Cary, USA).

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Benzoic acid (BA) was used as a probe molecule to detect the production of hydroxyl radical

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(•OH). The products of this reaction are p-hydroxybenzoic acid (p-HBA), m-HBA and o-HBA with

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the ratio of 1.2: 2.3: 1.7.30 The p-HBA could be easily determined by high performance liquid

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chromatography (HPLC, LC-20AT, Shimadzu) with a SB-C18 reverse phase column. The initial

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concentration of BA was 10 mM. The injection volume was 10 µL. The mobile phase contained 30%

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acetonitrile and 70% water. The flow rate was 1 mL min−1 and the detection wavelength was 270 nm

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(SI Text S3).

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The concentration of SM2 was monitored by HPLC. The injection volume was 100 µL. The eluent

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contained 75% of 0.75% (w/w) acetic acid and 25% acetonitrile. The flow rate was 0.8 mL min−1.

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The UV detector was set as 275 nm and the column temperature was maintained at 35 °C.

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The SM2 photodegradation intermediates were identified by liquid chromatography-mass 7 ACS Paragon Plus Environment


spectrometry with tandem mass spectrometry (LC-MS/MS, TSQ Quantum MAX, Thermo, USA).

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The detailed pre-treatment method and analysis procedures were provided in supporting information

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(Text S4).

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Results and discussions

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Sulfadimidine Removal and Reactive Oxygen Species Generation of Hydrochar. We first

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compared the SM2 removal capacities of hydrochar in the dark and under daylight irradiation. As

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shown in Figure 1a, only 19.8% of SM2 was removed by HTC-F within 6 days in the dark,

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suggesting the slight adsorption of SM2 on HTC-F. Interestingly, 72% of SM2 was removed over

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HTC-F within 6 days under daylight irradiation, indicating that hydrochar was daylight responsive.

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Control experimental results revealed that the self-clean of SM2 was negligible either in the dark or

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under daylight irradiation. Both of SM2 removal curves obeyed pseudo-first-order kinetic equation

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(Figure 1b), while the apparent SM2 removal rate constants under daylight (0.2120 d-1) was almost 6

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times that (0.0344 d-1) in the dark. We therefore assumed that hydrochar might be excited by

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daylight to generate ROS for the enhanced SM2 degradation.

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To verify this assumption, we thus determined the generation of H2O2 and •OH in different

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systems, and found that the concentration of H2O2 under daylight irradiation increased to a

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maximum value of 350 µg L−1 in the first three days and then decreased from the fourth day, while

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the generation of H2O2 was negligible in the dark (Figure 2a). Similar phenomena were also

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observed for the •OH generation (Figure 2b). The •OH formation quantum yields (Φ•OH) of

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hydrochar at λ0 = 300 nm was calculated according to a previous report.31 The Φ•OH of hydrochar

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was calculated to be (6.2 ± 0.22) × 10-5, higher than those (Suwannee River humic acid Φ•OH = (3.01

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± 0.12) × 10-5, Suwannee River fulvic acid Φ•OH = (4.29 ± 0.14) × 10-5, and Pony lake fulvic acid

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Φ•OH =(4.56 ± 0.16) × 10-5) of many reported DOM.32 Subsequently, we utilized EPR technique with 8 ACS Paragon Plus Environment

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DMPO as a spin-trapping agent to check the superoxide anion free radical (•O2-) generation in

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different systems (SI Figure S4a). No signal could be observed in the dark, but a strong four-line

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EPR spectrum with the relative intensities of 1 : 1 : 1 : 1, a typical pattern of superoxide anion free

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radical adducted to DMPO, was recorded after 50 min of daylight irradiation. These results

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confirmed that hydrochar could transfer electron to dissolved oxygen under daylight to form •O2-,

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which further reacted with H+ to produce H2O2. The final pH of SM2 solution under daylight

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irradiation increased to 7.9 ± 0.2 at 25 °C, indicative of the H+ consumption during the SM2

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photodegradation with hydrochar. Under daylight irradiation, H2O2 could easily convert to •OH for

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the SM2 degradation. We employed the EPR technique with 2, 2, 6, 6-tetramethylpiperidine (TEMP)

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as a spin trapping agent to check the singlet oxygen (1O2) generation in different systems (SI Figure

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S5), and did not observe any triplet signals in the dark or after 50 min of daylight irradiation, ruling

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out that hydrochar could transfer energy to molecular oxygen to form singlet oxygen. To evaluate the

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contributions of different ROS to the photo-degradation of SM2 on hydrochar, a series of trapping

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experiments were then carried out with adding different kinds of excess scavengers (TBA for •OH

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and CAT for H2O2) everyday.33 As shown in Figure 2c, this daily addition of TBA and CAT inhibited

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80% and 88% of SM2 degradation, respectively. Meanwhile, the degradation rates of SM2 were also

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reduced by the presence of scavengers (Figure 2d). Therefore, we conclude that molecular oxygen

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can be activated by hydrochar under daylight irradiation to generate plenty of ROS for the enhanced

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SM2 degradation.

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To estimate the ROS generation ability of hydrochar in the natural system, we carried out the SM2

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removal experiments under natural solar light in the campus of Central China Normal University

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(8:30-14:30, 11/7/2017). The corresponding solar irradiation intensity was monitored by an optical

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power meter (SI Figure S6). It was interesting to find that 69% of SM2 could be removed with

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HTC-F within 6 hours under solar light irradiation (SI Figure S7a), and the concentration of H2O2 9 ACS Paragon Plus Environment


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reached as high as 433.3 µg L-1 in 6 hours (SI Figure S7b), about 7 times that under daylight lamp.

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Therefore, hydrochar is very promising for the degradation of organic pollutants under solar light

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irradiation. Subsequently, we employed LC-MS/MS to detect the intermediates and products to

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understand the degradation pathway of sulfadimidine (1) in detail. Six degradation intermediates,

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including 4-aminophenol (2), 4,6-dimethylpyrimidin-2-amine (3), N-(4,6-Dimethyl-2-pyrimidinyl)

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-4-(hydroxyamino)benzenesulfonamide

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methylpyrimidine-4-carboxylic acid (6) were identified (SI Figure S8). The mass charge ratios (m/z)

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of all these detected intermediates of SM2 were consistent with those reported in the literatures,34

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which were summarized in Table S3 (Supporting Information). On the basis of these intermediates,

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we proposed a possible degradation pathway of SM2 (SI, Scheme S3).

(4),

2-aminopyrimidine

-4,5,6-triol

(5),

2-amino-6-

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The Role of Hydrochar on the ROS Generation under Daylight. Recently, it was reported that

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dissolved black carbon could generate ROS (e. g. 1O2, O2−) under sunlight.21 As aforementioned,

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hydrochar could induce molecular oxygen activation to generate ROS under daylight irradiation.

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However, the roles of dissolved and undissolved hydrochar on the ROS generation were unclear,

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which were disclosed by a series of separating experiments (SI Text S5 and Scheme S4). As shown

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in Figure 3a, the removal of undissolved hydrochar completely inhibited the degradation of SM2,

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indicating that undissolved hydrochar was responsible for the photo-degradation of SM2.

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Subsequently, we fractionated the dissolved hydrochar form water extracts of bulk hydrochar

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according to the previous reported method, 35 and compared the performances of dissolved and

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undissolved hydrochar on the SM2 degradation and the ROS generation under daylight irradiation

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(SI Text S6 and Scheme S5). As expected, the undissolved hydrochar could efficiently degrade SM2

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within 6 days under daylight irradiation, while the SM2 degradation by the dissolved hydrochar was

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not observed (Figure 3b), confirming that undissolved hydrochar accounted for the SM2

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degradation. It was found that undissolved hydrochar could generate H2O2 and •OH under daylight 10 ACS Paragon Plus Environment

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irradiation, but the generation of ROS by dissolved hydrochar was negligible (Figure 3c and Figure

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3d), confirming that undissolved hydrochar play a dominated role in the SM2 photodegradation and

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the ROS generation.

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Comparative Study of Pyrochar and Hydrochar. For comparison, we examined the ability of

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pyrochar to generate ROS and degrade SM2 in the dark and under daylight irradiation. It was found

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that PC-F could remove 50% of SM2 within 6 days under daylight irradiation, significantly lower

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than that (72%) of HTC-F. In the dark, the SM2 removal efficiency of PC-F was 40% within 6 days,

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which was obviously higher than that (20%) of HTC-F (Figure 4a). All of the removal curves were

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calculated to obey pseudo-first-order kinetics equations (Figure 4b). The apparent SM2 removal rate

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constant (0.2120 d-1) of HTC-F was about 2 times that (0.1026 d-1) of PC-F under daylight

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irradiation. On the contrary, the apparent SM2 dark removal rate constant (0.0848 d-1) of PC-F was

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calculated to be about 2.5 times that (0.0344 d-1) of HTC-F. The rate constants normalized with

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specific surface area were also calculated (SI Table S4), which indicated that the SM2 removal

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efficiency was not only ascribed to their specific surface area. Further separating experiments

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revealed that the SM2 removal in case of PC-F was also attributed to undissolved pyrochar (SI

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Figure S11).

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As the SM2 removal was involved with both adsorption and degradation processes, we first

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compared the adsorption ability of hydrochar and pyrochar with the oxygen trapping experiments by

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continuously purging high-purity argon gas with a flow rate of 1.0 L min-1 for 30 min before the

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reaction and during the whole SM2 removal experiments. As shown in Figure S12, the SM2 removal

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efficiency of PC-F was 40% within 6 days in the presence of molecular oxygen, while the SM2

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removal efficiency decreased to 29% in the absence of molecular oxygen, indicating that 11% of

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SM2 was degraded via molecular oxygen activation induced by PC-F, while 29% of SM2 was

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removed via the adsorption of PC-F. Differently, the absence of molecular oxygen did not 11 ACS Paragon Plus Environment


significantly reduce the SM2 removal efficiency of HTC-F, suggesting that 19.8% of SM2 removal

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was completely attributed to the adsorption of HTC-F. These comparisons revealed that pyrochar

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had a much stronger adsorbing ability than hydrochar, and also confirmed that pyrochar could

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generate ROS for the SM2 degradation in the dark. Subsequently, we compared the generation of

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H2O2 and •OH over HTC-F and PC-F in the dark or under daylight irradiation. As expected, some

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H2O2 was generated by PC-F in the dark (approximately 60 µg L−1 in the first two days), but not

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generated by HTC-F under the same conditions (Figure 4c). While, the maximum concentration of

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H2O2 generated over HTC-F was about 2 times that over PC-F under daylight irradiation (Figure 4c).

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Similar phenomena were observed in the case of •OH (Figure 4d). We therefore conclude that

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hydrochar possess the higher ROS generation and SM2 degradation ability than pyrochar under

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daylight irradiation, but its ROS generation and SM2 degradation abilities were lower than those of

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pyrochar in the dark.

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Many types of biomass, such as plant residual, animal waste and sewage sludge, could be used for

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the biochar preparation.36, 37 Among them, plant residuals (e.g. woodchips, crop straw and corn

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stover), with hemicellulose, cellulose and lignin as the main components, were commonly used to

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produce biochar.13, 23 To check whether the type of plant residuals can affect the ROS generation and

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the SM2 degradation by the resulting hydrochar and pyrochar in the dark and under daylight

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irradiation, we thus prepared different biochar with other feedstock instead of fallen-leaves for

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comparison (SI Figure S13). It was found that biochar derived from woodchips was similar with

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biochar derived from fallen-leaves on the ROS generation and the SM2 degradation under daylight

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irradiation.

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Several studies reported that persistent free radicals (PFRs) generated during pyrolysis of biomass

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could transfer electrons to O2, which further convert to H2O2 and •OH.27, 38, 39 We therefore attributed

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the traces of ROS generated by pyrochar in the dark to the presence of PFRs, and employed electron 12 ACS Paragon Plus Environment

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paramagnetic resonance (EPR) technique to compare the amounts of PFRs generated by pyrochar

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and hydrochar. As expected, obvious singlet EPR signals were observed in the two kinds of

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pyrochars derived from fallen-leaves and woodchips, which were much stronger than those of

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hydrochar (Figure 5). This comparison confirmed that pyrochar contained much more PFRs than

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hydrochar. It was known that the type of PFRs highly depended on the g factor of EPR signals. For

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instance, PFRs with a g factor larger or smaller than 2.0040 were arisen from oxygen-centered such

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as semiquinone radical anions or carbon-centered radicals such as aromatic radicals, respectively. If

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carbon-centered and oxygen-centered radicals coexisted, the corresponding g factor was in the range

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of 2.0030-2.0040.28 We found that the singlet EPR signals generated by pyrochar was similar with

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that of hydrochar, which could be assigned to oxygen-centered radicals, although the EPR signal

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intensity of pyrochar was much stronger than that of hydrochar. To verify whether the ROS

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generation in the dark was dependent on the PFRs, we changed the amount of PFRs in pyrochar by

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an organic solvent extracting technique (SI Text S7),20 and found that the amount of PFRs in PC-F

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and PC-W significantly decreased after methanol extraction (SI Figure S14). The H2O2 generation

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ability of pyrochar after extraction (PC-F-MeOH and PC-W-MeOH) were further examined in the

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dark. As shown in Figure S15 (Supporting information), the PFRs decrease significantly inhibited

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the H2O2 generation, confirming that PFRs was responsible for the ROS generation in the dark.

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It was reported that carboxylated single-walled carbon nanotubes (SWCNT) could more

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efficiently generate ROS (•O2-, 1O2 and •OH) under solar or UVA light irradiation than pristine

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SWCNTs.40,

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(hydrochar and pyrochar) might be related to the amount of oxygenated functional groups (OFGs),

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especially their surface carboxyl groups. To validate this hypothesis, Raman spectroscopy was first

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employed to characterize OFGs on the surface of pyrochar and hydrochar.42-44 As shown in Figure 6a,

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both hydrochar and pyrochar had peaks at D and G bands, indicating the presence of graphitic 13

41

We therefore hypothesized that the photo-generated ROS ability of biochar



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carbon in the two kinds of biochar. The value of ID/IG was in the order of hydrochar > pyrochar,

313

reflecting that hydrochar possessed more abundant defects and functional groups than pyrochar.

314

FTIR was then used to gain more information of functional groups on the surface of pyrochar and

315

hydrochar (SI, Table S5 and Figure 6b).42, 44-50 The peak of hydrochar at 3420 cm-1, being mainly

316

ascribed to OH stretching of H-bonded hydroxyl groups, was much stronger than that of pyrochar.

317

Meanwhile, a more clear characteristic peak at 1710 cm-1, which was ascribed to C=O stretching of

318

carboxyl groups, was observed for hydrochar.42, 48 The broad band of hydrochar at 1160-1030 cm-1,

319

representing the C-O stretching of aliphatic, was more distinct than that of pyrochar.42,

320

Furthermore, hydrochar was of more CH3 carbon, as evidenced by its stronger absorbance at

321

2923-2853 cm-1, whereas pyrochar with weak absorbance at this broad band possessed less CH3

322

moieties, consistent with the relatively higher carbonation degree of pyrochar.45, 50 These above

323

results revealed that hydrochar possessed more oxygenated functional groups including hydroxyl

324

groups, ether groups, and carboxyl groups. XPS was further used to characterize OFGs on the

325

surface of different biochar samples. As shown in Figure 6c-f, C 1s spectra could be deconvoluted

326

into three peaks and the deconvolution of O 1s spectra produced two peaks (Table S6 supporting

327

information).3, 47, 51 Table 1 summarizes the relative percentages of surface oxygen element and

328

OFGs (C-O and O-C=O) on pyrochar and hydrochar. For hydrochar, the oxygen contents related to

329

OFGs were calculated to be 37.8% for HTC-F and 31.3% for HTC-W, respectively. These two

330

values were significantly higher than those (18.8% for PC-F and 26.1% for PC-W) of pyrochar.

331

These obvious differences strongly confirmed that hydrochar possessed more surface OFGs.

332

Moreover, the content of O-C=O in hydrochar was relatively higher, suggesting that there was more

333

carboxyl groups in hydrochar, consistent with the FTIR data (Figure 6b).

48, 49

334

To verify whether the ROS generation under daylight irradiation was dependent on the surface

335

OFGs of hydrochar, we changed the concentration of OFGs on the hydrochar surface with the 14 ACS Paragon Plus Environment

Page 15 of 28


336

oxidation and thermal treatments, respectively (SI Text S7).52 It was found that the surface OFGs,

337

especially carboxyl groups, significantly increased after the oxidation treatment, while disappeared

338

after thermal treatment (SI, Figure S16a and Figure S17a). We thus examined the H2O2 generation

339

ability of hydrochar after the oxidation and thermal treatments under daylight irradiation (SI, Figure

340

S16b and Figure S17b). As expected, the increase of OFGs concentration efficiently promoted the

341

H2O2 generation, while the removal of OFGs completely inhibited the H2O2 generation, validating

342

that OFGs accounted for the ROS generation under daylight irradiation. Therefore, the better ROS

343

generation and SM2 degradation performances of hydrochar under daylight were attributed to its

344

abundant surface OFGs. On the basis of these results, we proposed a possible mechanism of

345

photochemical ROS generation with hydrochar (SI Text S8 and Scheme S6). Similar with dye

346

sensitized semiconductor photocatalysis systems,53 the OFGs on hydrochar surface may serve as a

347

photosensitizer, and hydrochar particles functioned as an electron shuttle.54,

348

photoexcitation of surface OFGs, the excited electrons transfer from the OFGs to the electron

349

acceptor (e.g. O2) adsorbed on the surface of hydrochar particles, generating different ROS.

350

Environmental application. In this study, we have demonstrated that hydrochar particles in aquatic

351

environment could efficiently generate ROS via oxygen molecular activation for the SM2

352

degradation under daylight irradiation. Moreover, hydrochar with more photoactive surface OFGs

353

could generate much more ROS than pyrochar under daylight irradiation, although pyrochar was

354

able to produce trace of ROS even in the dark owing to the presence of PFRs. The higher

355

photochemical ROS generation ability of hydrochar suggests that it is very promising for

356

environmental remediation under solar light irradiation. Therefore, these findings shed light on

357

photochemical environmental effects of hydrochar, and also play a significant role in estimating the

358

performance of different biochar on environmental remediation.

55

Along with the

359



AUTHOR INFORMATION

361

Corresponding Author

362

*Phone/Fax: +86-27-6786 7535; e-mail: [email protected].

363

Notes

364

The authors declare no competing financial interest.

Page 16 of 28

365 366

ACKNOWLEDGEMENTS

367

This work was supported by Natural Science Funds for Distinguished Young Scholars (Grant

368

21425728), National Science Foundation of China (Grant 51472100), the 111 Project (Grant

369

B17019), Self−Determined Research Funds of CCNU from the Colleges’ Basic Research and

370

Operation of MOE (Grant CCNU14Z01001), excellent doctorial dissertation cultivation grant from

371

Central China Normal University (Grant 2016YBZZ037), and the CAS Interdisciplinary Innovation

372

Team of the Chinese Academy of Sciences.

373 374

ASSOCIATED CONTENT

375

Supporting Information

376

Preparation and characterization of biochar samples; EPR and XPS analysis method; quantification

377

of hydroxyl radicals; pre-treatment method and analysis procedure for LC-MS/MS detection;

378

separating, modification and fractionation procedures of biochar samples; possible photochemical

379

mechanism of hydrochar; the diagram of photo-reaction setup; mass spectra and structural formula

380

and m/z values of SM2 and its degradation intermediates; possible sulfadimidine photodegradation

381

pathway; BET surface area of biochar samples; spectrum of daylight lamp; EPR signals of •O2- ,

382

•OH and 1O2; H2O2 generation and SM2 degradation by hydrochar under solar light; O2 trapping

383

experiments; ROS generation and SM2 degradation by biochar derived from woodchips. 16 ACS Paragon Plus Environment

Page 17 of 28


384 385

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524 525 526 527


Page 23 of 28 528


Figure Captions

529 530

Figure 1. (a) Time profiles of the SM2 removal under different conditions. (b) Plots of -ln(C/C0)

531

versus time for SM2 removal under different conditions. The initial concentrations of SM2 and

532

HTC-F were 250 µg L−1 and 100 mg L-1, respectively.

533 534

Figure 2. ROS generation in different systems during the SM2 degradation: (a) H2O2 generation; (b)

535

•OH generation. (c) Profiles of SM2 photodegradation with adding different kinds of scavengers 23 ACS Paragon Plus Environment


Page 24 of 28

536

(TBA for •OH, CAT for H2O2); (d) SM2 photodegradation rate constant with adding different kinds

537

of scavengers. The initial concentrations of SM2, HTC-F and BA were 250 µg L−1, 100 mg L-1 and

538

10 mM, respectively.

539 540

Figure 3. (a) Time profiles of SM2 degradation in hydrochar separating experiments. The dash line

541

was the boundary of hydrochar before and after separation. (b) Profiles of SM2 photodegradation

542

with dissolved, bulk, rest hydrochar, respectively. ROS generation with different fractions of

543

hydrochar during the SM2 degradation: (c) H2O2 generation; (d) •OH generation. The initial

544

concentrations of SM2 and the three kinds of HTC-F were 250 µg L−1 and 100 mg L-1, respectively.


Page 25 of 28


545 546

Figure 4. (a) Time profiles of the SM2 degradation with hydrochar and pyrochar under different

547

conditions. (b) Plots of -ln(C/C0) versus time for SM2 degradation in different systems. ROS

548

generation in different system during the SM2 degradation: (c) H2O2 generation; (d) •OH generation.

549

The initial concentrations of SM2 and the biochar samples were 250 µg L−1 and 100 mg L-1,

550

respectively.

551 552

Figure 5. EPR signals detected in different biochar samples: (a) hydrochar and pyrochar derived

553

from fallen-leaves; (2) hydrochar and pyrochar derived from woodchips. 25 ACS Paragon Plus Environment


Page 26 of 28

554 555

Figure 6. (a) Raman spectra of different biochar samples. (b) FTIR spectra of different biochar

556

sampels. XPS C1s spectra of biochar samples derived from (c) fallen-leaves and (d) woodchips, O1s

557

spectra of biochar samples derived from (e) fallen-leaves and (f) woodchips.

558 559 560


Page 27 of 28


561

Table 1. The relative percentages of surface oxygen element and OFGs (C-O and O-C=O) on

562

pyrochar and hydrochar. C 1s

O 1s O/C (%)

C-C (%)

C-O (%)

O-C=O (%)

C-O (%)

O-C=O (%)

HTC-F

46.7

34.4

18.9

60.4

39.6

37.8

HTC-W

56.8

27.7

15.5

69.9

30.1

31.3

PC-F

75.9

15.7

8.4

85.1

14.9

18.8

PC-W

83.5

11.3

5.2

81.2

18.8

26.1

563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579



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Photochemistry of Hydrochar: Reactive Oxygen Species Generation

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