Demethanation Trend of Hydrochar Induced by Organic Solvent

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Demethanation Trend of Hydrochar Induced by Organic Solvent Washing and its Influence on Hydrochar Activation Xiangdong Zhu, Yuchen Liu, Feng Qian, Zhongfang Lei, Zhenya Zhang, Shicheng Zhang, Jianmin Chen, and Zhiyong Jason Ren Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b06594 • Publication Date (Web): 20 Aug 2017 Downloaded from http://pubs.acs.org on August 21, 2017

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Environmental Science & Technology

ACS Paragon Plus Environment


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Demethanation Trend of Hydrochar Induced by Organic Solvent

2

Washing and its Influence on Hydrochar Activation

3

Xiangdong Zhu,†,‡ Yuchen Liu,† Feng Qian,† Zhongfang Lei,# Zhenya Zhang,# Shicheng

4

Zhang,†,* Jianmin Chen,† Zhiyong Jason Ren‡

5

†

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Environmental Science and Engineering, Fudan University, Shanghai 200433, China

7

‡

8

Boulder, CO 80309, United States

Shanghai Key Laboratory of Atmospheric Particle Pollution and Prevention (LAP3), Department of

Department of Civil, Environmental, and Architectural Engineering, University of Colorado Boulder,

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# Graduate School of Life and Environmental Sciences, University of Tsukuba, Tsukuba 305-8572, Japan

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* Corresponding author, Tel/fax: +86-21-65642297; E-mail: [email protected] (Shicheng

13

Zhang).

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1 ACS Paragon Plus Environment

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ABSTRACT

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Hydrochar derived from hydrothermal carbonization (HTC) has been recognized as a

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promising carbonaceous material for environmental remediation. Organic solvents are widely

18

used to extract bio-oil from hydrochar after HTC, but the effects of solvent extraction on

19

hydrochar characteristics have not been investigated. This study comprehensively analyzed the

20

effects of different washing times and solvent types on hydrochar properties. Results indicate that

21

the weight loss of hydrochar by tetrahydrofuran washing occurred mainly in the first 90 min, and

22

the loss ratios of elements followed a descending order of H > C > O, resulting in a decrease in

23

H/C atomic ratio while an increase in O/C atomic ratio. The use of various solvents for washing

24

brought about hydrochar loss ratios in a descending order of petroleum ether < dichloromethane

25

< acetone < tetrahydrofuran. Results from the Van Krevelen diagram, FTIR, 13C NMR and XPS

26

further confirmed that demethanation controlled this washing process. Most importantly, the

27

surface area of hydrochar increased after bio-oil removal via washing, which promoted the

28

surface area development for hydrochar-derived magnetic carbon composites, but to some extent

29

decreased the microporosity. Additionally, hydrochar washing by organic solvent has important

30

implications for the global carbon cycle and its remediation application.

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32 33

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INTRODUCTION Compared with the slow pyrolysis for the production of biochar,1,

2

hydrothermal

34

carbonization (HTC) is an emerging process for simultaneous production of renewable bio-oil

35

and hydrochar material from biomass.3 The HTC conversion of biomass demonstrated great

36

potentials for renewable energy generation and environmental remediation.4-6 Hydrochar

37

generated from HTC is an excellent carbonaceous material due to its distinct properties including

38

high carbon (C) content, thermal stability, and moderate porosity.7, 8 Extensive studies have been

39

performed to regulate and optimize the properties of hydrochar so its performance can be

40

improved for targeted applications especially in the field of environmental remediation and

41

agricultural land application.9-12

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It is well characterized that the surface of hydrochar shows strong hydrophobicity due to the

43

presence of bio-oil. Therefore, organic solvents such as acetone (AT) and tetrahydrofuran (THF)

44

are often used to wash hydrochar for bio-oil (also known as heavy oil) collection.13, 14 Previous

45

studies reported that bio-oil has low oxygen (O) content, while

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O-containing groups of char were enhanced after methanol washing.17 These findings imply that

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the loss ratios of C, O and hydrogen (H) contents in hydrochar may be different during washing,

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because if C, H and O have the same loss ratios, O-containing groups of char would not change.

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Therefore, it is reasonable to hypothesize that the properties of hydrochar like ratios of H/C

50

(relating to aromaticity) and O/C (relating to polarity) are would be altered after solvent washing,

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which can greatly affect the char’s property for following applications.9, 17 However, there is little 3 ACS Paragon Plus Environment

15, 16

others reported that the

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information on how solvent-based washing influences hydrochar properties. For example,

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hydrochar is often used as a raw material in the preparation of high-porosity magnetic carbon

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composites (MCs).18 It has been well documented that the thermal stability of hydrochar

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(indicated by its H/C atomic ratio) strongly affects the porosity of hydrochar-derived MCs.9, 19, 20

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Solvent washing can change the H/C atomic ratio as the loss ratios of C and H contents can be

57

different, so the porosity of the hydrochar-derived MCs can vary. On the other hand, the porosity

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of hydrochar will also change after the loss of bio-oil from the hydrochar surface. As a result,

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more chemical activators can be loaded into the pore system, which helps to activate the

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hydrochar material with higher porosity. However, the connection between the properties of

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solvent-washed hydrochar and the porosity of hydrochar-derived MCs has not yet been

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established.

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To investigate this unknown mechanism, the aim of this study is to analyze the effects of

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washing times and solvent types on the changes of hydrochar characteristics, such as elemental

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composition, loss ratios of elemental contents (C, H, O and nitrogen, N), and changes in porosity.

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Specifically, several tasks were performed including (i) the role of washing time on the

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hydrochar washing using THF solvent, (ii) the evaluation of solvent types on hydrochar washing,

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(iii) data analysis and interpretation of the characteristics of hydrochar washing using the Van

69

Krevelen diagram, Fourier transform infrared spectroscopy (FTIR), carbon nuclear magnetic

70

resonance (13C NMR), and X-ray photoelectron spectroscopy (XPS) techniques, and (iv) drawing

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the connection between the properties of washed hydrochar and the properties of



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hydrochar-derived MCs in terms of porosity, magnetic composition, and acid resistance.

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EXPERIMENTAL SECTION

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Chemicals and Materials

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Hydrochar was obtained from the solid residual of Salix psammophila (SP) HTC reaction.

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Typically, 44 L of water and 3.5 kg of SP were reacted in an autoclave, which was heated up to

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280 °C and kept with 60 min. When the reactor was cooled to room temperature by tap water, the

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hydrochar samples were collected by filtration.21 Four analytical grade organic solvents,

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including petroleum ether (PE), dichloromethane (DLM), acetone (AT), and THF, were applied

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for hydrochar washing for bio-oil removal. Zinc chloride (ZnCl2) and iron(III) chloride (FeCl3)

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were applied to modify the washed hydrochar. Reagent grade bisphenol A (BPA) was purchased

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from Aladdin Reagent Corporation.

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Hydrochar Washing Procedure

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An ultrasonic treatment was used in the hydrochar washing with a duration of 30 min each.

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The ratio of hydrochar weight (g) to solvent volume (mL) was kept at 1:10. After washing, the

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hydrochar was filtered and re-washed under the same condition. Considering the practical

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extraction of bio-oil from hydrochar, the hydrochar was washed with 5 times using THF for a

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total duration of 150 min. The washed hydrochar is denoted as H-X, where X represents the

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washing time. To examine the effects of different solvent types, PE, DLM, AE, and THF were

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applied with washing for 3 times each. The hydrochar was also washed with water (WT) as a

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control experiment. The washed hydrochar is denoted as H-Y, where Y represents the solvent 5 ACS Paragon Plus Environment

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used for hydrochar washing. Before characterization measurements, the washed hydrochar was

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heated at 80 °C to constant weight for the removal of residual organic solvent.

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Modification and Characterization of Samples

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Washed hydrochar was modified for MC synthesis using a simultaneous activation and

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magnetization method previously reported.22 Briefly, 6 g of ZnCl2, 1.95 g of FeCl3 and 6 g of

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washed hydrochar were mixed in 30 mL of water. The mixture was then shaken for 24 h and

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air-dried at 80 °C for 4 h. Subsequently, the dried mixture was heat-treated at 600 °C for 90 min

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under a nitrogen gas (N2) flow of 1 L/min. The resultant sample was successively washed with

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0.1 M HCl and water and dried at 100 °C for 4 h. The prepared MCs are henceforth denoted as

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MC-X, where X represents the solvent used for hydrochar washing.

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The elemental composition of samples was analyzed with an elemental analyzer (Vario EL III,

103

Germany). The porosity of samples was determined by N2 adsorption at 77 K using a Quantasorb

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instrument. The BET method was used to calculate the surface area (SBET) based on the partial

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pressure (P/P0) range from 0.04 to 0.2, the total pore volume (Vt) was obtained from the

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adsorbed nitrogen amount at a relative pressure of 0.99. The micropore volume (Vmic) and

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micropore surface area (Smic) was determined by t-plot analysis. CO2 adsorption isotherms were

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performed by using a Quantachrome FL33426 at 273 K and used to determine micropore volume,

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micropore surface area and narrow micropore size distribution (< 1.5 nm) through the fitting of

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DFT model.

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The functional groups were examined using FTIR,

13

C CP/MAS NMR and XPS. Fourier



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transform infrared spectroscopy (FTIR, Nexus470) was performed with a wavenumber range of

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4000-400 cm-1 at a resolution of 2 cm-1. X-ray photoelectron spectroscopy (XPS) was examined

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on a RBD-upgraded PHI-5000C ESCA system (Perkin Elmer). The X-ray anode was carried at

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250 W and the voltage was maintained at 14.0 kV with a detection angle at 54°. Binding energies

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were calibrated by setting C 1s at 284.6 eV. Solid state 13C nuclear magnetic resonance (NMR)

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spectra with cross polarization magic angle spinning (CPMAS) were acquired on a Bruker DSX

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300 NMR spectrometer with 7 mm zirconia rotors. The spinning speed of 4.8 kHz was used, and

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8192 data points were collected. 1H t1 relaxation time and contact time was set to 2 s and 2.5 ms.

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The phase structure of samples was characterized by X-ray diffraction (XRD) using Cu K α

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radiation (λ = 1.5406 Å) at a scan rate of 8°/min and a step size of 0.02° in 2θ. The morphology

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of samples was examined through a scanning electron microscopy (SEM, TS 5136MM). A

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thermogravimetry (TG) analyzer was used to examine the stability of hydrochar, ~ 20 mg

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samples that were heated from 30 to 800 °C in an N2 atmosphere at a rate of 20 °C/min.

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Mössbauer spectroscopy was performed at room temperature with a cobalt isotope

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source using 25 µm α-Fe foil as a reference. The Mössbauer spectroscopy of samples was fitted

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with two components (Fe3O4 and ZnFe2O4) by the standard least square method. The acid

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resistance of MCs (to test Fe leaching) was achieved at an MC concentration of 2000 mg/L under

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a pH of 3.0 with a 24 h contact time. The extracted Fe solution was determined by inductively

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coupled plasma (ICP, P-4010).

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The

composition

of

extracted

bio-oil

was

qualitatively


determined

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Co(Pd)

using

gas

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chromatography/mass spectrometry (GC-MS) (Thermo FOCUS DSQ) with a HP-5 ms column.

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The GC column was programmed to heat at a rate of 30 °C/min from 60 °C (held for 2 min) to

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300°C (held for 5 min).

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The adsorption isotherms of BPA onto MCs were achieved in the range of 10 - 200 mg/L BPA

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(20% methanol) at 25 °C to test the adsorption ability of MCs. The concentration of the MC

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sample was 200 mg/L. The concentrations of BPA were determined by high-performance liquid

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chromatography (HPLC) at 280 nm with methanol and ultrapure water as a mobile phase at a

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volume ratio of 75:25.

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RESULTS AND DISCUSSION

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Dynamic Changes in Hydrochar Characteristics Under Different Washing Times

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As shown in Table 1, the loss of hydrochar mass and C, H, O and N contents mainly occurred

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in the first three THF extractions, while only slight changes were observed thereafter.

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Intraparticle diffuse model confirmed that the bio-oil adsorbed onto the exterior surface of

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hydrochar was first washed off, which was followed by the extraction of bio-oil adsorbed onto

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the interior hydrochar surface (Figure S1a). It is interesting to note that the C and H contents of

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hydrochar decreased, but the O and N contents of hydrochar increased after THF extraction,

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indicating that the loss ratios of C and H contents were notably higher than those of O and N

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contents. As shown in Figure S1b, the loss behaviors of hydrochar due to THF washing can be

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described well by the pseudo-second-order model, suggesting that binuclear desorption

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mechanism regulated on the hydrochar washing by THF. Obviously, the loss of N content had the 8 ACS Paragon Plus Environment


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fastest rate in the hydrochar washing by THF, indicating the N element of hydrochar appeared

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most sensitive to THF extracting. The equation of pseudo-second-order model could be observed

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at note of Figure S1b.

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Small molecules including aromatic phenol, aliphatic acid and aliphatic alcohol were detected

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in the GC-MS chromatograph of the THF-extracted bio-oil (Figure S2 and Table S1).

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Interestingly, these compounds haven’t been detected after the second THF washing, indicating

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that they were completely extracted prior to the first two washings. However, weight loss of

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hydrochar continued. These results suggest that THF-soluble polymers like fragments of biomass

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components and restructured organic matters were undetectable in the GC-MS analysis and

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possessed a lower loss ratio resulted from THF washing.

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Due to the pore blockage of hydrophobic compounds, the hydrochar possessed an extremely

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low surface area (Table S2). After the removal of accumulated substances by washing, the

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surface area of hydrochar increased significantly, especially after the first three THF washings,

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and only negligible changes occurred thereafter. This phenomenon agrees well with the weight

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loss behavior of hydrochar, which is further confirmed by the positive correlation between the

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washing efficiency of hydrochar (loss ratio of hydrochar mass) and the increased surface area of

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hydrochar samples (Figure 1a).

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As shown in Table 1, C content experienced a lower loss ratio than H content while at a higher

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loss ratio than O content. Such loss ratio difference resulted in decreased H/C and increased O/C

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atomic ratios for the THF-washed hydrochar. Thus, THF washing increased the aromaticity and


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polarity of the hydrochar. The derived Van Krevelen diagram suggests that THF-based hydrochar

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washing was mainly followed a demethanation trend (Figure 1b), attributable to higher C and H

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molar ratios than that of O in the extracted bio-oil, such as aromatic phenol, aliphatic acid and

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aliphatic alcohols. This was partially confirmed from the results of GC-MS analysis.

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It has been well documented that increased aromaticity (decreased H/C atomic ratio) can

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enhance the stability of carbonaceous material.23 As shown in Figure 1c and Table S2, THF

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washing introduced great differences in TG characteristics of hydrochar. As expected, THF

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washing enhanced the stability of hydrochar, as indicated by the increases of R50 (novel

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recalcitrance index) and W700 (residual weight at 700 °C in TG curves)24, 25. The detailed R50

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calculation could be seen in the note of Table S2. In addition, these two indices are strongly

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negatively correlated with the H/C ratio of hydrochar (R2 = 0.97 and 0.96, respectively) (Figure

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S3), suggested that the aromaticity of hydrochar also improved by THF washing. A low H/C

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value represented the high aromaticity. The pyrolysis behavior of hydrochar can be divided into

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the following two regions. The first region of the differential thermogravimetry (DTG) curves

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was predominantly due to the decomposition of small molecular compounds (including GC-MS

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detectable matters) with low peak temperatures (Tmax1). After THF washing, the weight loss of

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hydrochar in this region decreased substantially (Table S2). It is clear that few compounds such

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as aromatic phenol, aliphatic acid, and aliphatic alcohol could be detected in the GC-MS analysis

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after the second THF washing (Table S1), which is well corresponded with the Tmax1

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characteristics in DTG profiles of this region (Figure 1c and Table S2). The second region was



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mainly affected by the loss of aromatic species.26 The increase in peak temperature (Tmax2) was

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easily detected along with washing times and negatively controlled by the H/C ratio of hydrochar

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(Figure S3), which also implies the increase in thermal stability of the hydrochar.

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FTIR, NMR, and XPS spectra were used to further confirm the changes of hydrochar

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characteristics after different THF washing times. As shown in Figure 1d, the band intensities at

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~ 2920 and 2845 cm-1 (aliphatic C-H stretching) were obviously weakened with the increase of

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washing times, further confirming the demethanation trend of hydrochar during the washing

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process. Due to the removal of O-containing matters (i.e., aromatic phenol, aliphatic acid, and

200

aliphatic alcohol), the band intensities at ~ 1705 cm-1 (C=O stretching), ~ 1204 and 1120 cm-1

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(C-O stretching) also decreased with the increase of washing times. Smaller changes were

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observed for aromatic C=C (~ 1600 cm-1), lignin C=C (~ 1505 cm-1), and aromatic ring (~ 1450

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cm-1) FTIR bands,27,

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solvent-washing process.

28

indicating that aromatic compounds were more stable during the

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As shown in the 13C NMR spectra of hydrochar, the relative proportion of alkyl C (0-43 ppm)

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decreased with the increase of washing times (Figure 1e and Table S3), resulting in the

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demethanation trend and decreased H/C ratio. These results were confirmed by the positive

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correlation between the relative ratio of alkyl C and H/C atomic ratio (Figure S4). The removal

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of alkyl C resulted in a conspicuous increase of aromatic C (110-145 ppm) and aromatic C-O

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(145-160 ppm), further reflecting an increased hydrochar aromaticity and thermal stability after

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THF washing. Due to a higher retention of O element in the THF-washed hydrochar samples, the


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O-containing groups like O-CH3 (43-60 ppm), O-alkyl C (60-90 ppm), carboxyl/ester (160-190

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ppm) and carbonyl (190-220 ppm) became more evident.

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As presented in Figure 1f, four representative peaks were observed in the hydrochar, which

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could be attributed to C1 (284.6 eV for C-H/C-C, aliphatic/aromatic carbon groups), C2 (285.6

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eV for C-O, hydroxyl/ether groups), C3 (287 eV for C=O, carbonyl/quinone groups), C4 (289 eV

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for O-C=O, carboxylic/ester/lactone groups).29 The XPS results reveal that C-O was the main

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O-containing functional group on the hydrochar surface, corresponding with the higher

219

intensities of aromatic C-O, O-CH3 and O-alkyl than the carboxyl and carbonyl groups in the

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NMR spectra. After THF washing, the relative intensity of C-C/C-H obviously decreased.

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Accordingly, the relative intensity of C-O and C=O obviously increased (Table S4), further

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indicating that demethanation dominated the THF washing of hydrochar.

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Overall, the mass loss of hydrochar was mainly occurred during the first three times of THF

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washing, and demethanation reflected the behavior of hydrochar during THF washings.

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Accordingly, THF washing could gradually bring up O content, aromaticity, and stability while

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bring down C and H contents, as well as the H/C atomic ratio of hydrochar. This is accompanied

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by decreased intensities of aliphatic C-H stretching in the FTIR spectra, alkyl C in the 13C NMR

228

spectra and C1s for C-C/C-H in the XPS spectra.

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Dynamic Changes in Hydrochar Characteristics Under Different Solvents

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As shown in Table S5, the solvents used for hydrochar washing showed significant effects on

231

the loss ratio of hydrochar. The loss ratio of hydrochar using four solvents was in a descending 12 ACS Paragon Plus Environment


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order of PE < DLM < AT < THF. THF exhibited the highest performance during the washing

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process, possibly due to its strong solubility in bio-oil.13 GC-MS analysis further confirmed this

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result (Figure S5 and Table S6). Obviously, THF possessed the highest capability for extracting

235

the GC-MS detectable matters such as aromatic phenol, aliphatic acid, and aliphatic alcohol.

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Accordingly, the surface area of washed hydrochar was increased and positively controlled by

237

their washing efficiency (Figure S6), revealing the pore blockage effect of bio-oil composition on

238

the porosity of hydrochar. SEM spectra indicate that the hydrochar obviously became rough and

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some pores could be observed after AT and THF washings, due to more bio-oil removal (Figure

240

2).

241

As shown in Figure S7, the loss ratio of hydrochar mass is a good indicator for forecasting C,

242

H and O contents and H/C and O/C atomic ratios. A decrease in H/C atomic ratio and an increase

243

in O/C atomic ratio were observed in all solvent-washed hydrochars, probably attributable to the

244

same descending order of loss ratio in elemental composition as H content > C content > O

245

content (Table S5). This might result in the tendency of demethanation of hydrochar washed by

246

PE, DLM, AT, and THF, as confirmed by the typical Van Krevelen diagram (Figure S8).

247

FTIR, NMR and XPS were also used to examine the characteristics of hydrochar washed with

248

the different solvents. Due to PE’s weak extraction ability of aromatic phenol, aliphatic acid and

249

aliphatic alcohol (confirmed in the GC-MS spectrum in Figure S5), PE-washed hydrochar

250

retained stronger signals for C-H stretching (~ 2920 and 2845 cm-1), C=O stretching (~ 1705

251

cm-1), and C-O stretching (~ 1204 and 1120 cm-1) in Figure S9. In contrast, THF-washed


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hydrochar exhibited relatively weak signals in the aforementioned spectrum bands due to its

253

strong extraction ability of bio-oil compositions. Similar trends could also be discerned in the 13C

254

NMR (Figure S10 and Table S7) like decreased alkyl C (0 - 43 ppm), and in the C1s XPS spectra

255

like the decreased C-C/C-H intensity (Figure S11 and Table S8). These three technical

256

characteristics confirmed the effectiveness of PE, DLM, AT and THF in hydrochar washing, and

257

they provide convincing evidence that solvent-washing of hydrochar is regulated by

258

demethanation process.

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Modification of Washed Hydrochar by Simultaneous Activation and Magnetization

260

Different solvent-washed hydrochar samples were further modified for MC preparation. The

261

yields (based on the same weight of washed hydrochar) and elemental composition of MCs

262

remained very similar with respect to the changes in hydrochar properties (Table S9). Little

263

difference in the yields of MCs indicates that the aromaticity of solvent-washed hydrochar is not

264

a key factor in controlling the properties of the derived MCs. But, MCs yields based on the

265

weight of raw un-washed hydrochar (H-WT) indicated the adverse effect of solvent washing, due

266

to the removal of bio-oil in the hydrochar washing process.

267

As shown clearly, solvent washing of hydrochar enhanced the BET surface area and micropore

268

surface area of the derived MCs (Table 2 and Figure S12), which was further confirmed by a

269

positive correlation between the surface area of hydrochar and that of MC (R2 = 0.95, Figure 3a).

270

This result suggests that a hydrochar with higher porosity provides more tunnels for storing

271

ZnCl2 (activator) in the impregnation stage, thereby promoting the activation of hydrochar and 14 ACS Paragon Plus Environment


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producing a MC material with high surface area. The main micropore sizes of samples

273

(concluded from CO2 adsorption isotherms, Figure S12 c) were 0.36, 0.48, 0.52, 0.60 and 0.82

274

nm (Figure 3b). It also should be pointed out that the micropore sizes of MC were not influenced

275

by the solvent washing of hydrochar. Noticeably, solvent washing of hydrochar reduced the

276

microporosity of the derived MCs (indicated by Smic/SBET and Vmic/Vt in Table 2), suggesting that

277

the excess entrance of ZnCl2 into the pores probably caused the collapse of the MCs’ pore walls.

278

Therefore, hydrochar pores play a dual role in the porosity of hydrochar-derived MCs.

279

BPA has toxicity and is widely detected in the environment, so it was selected to examine the

280

adsorption capacity of MCs.30 The adsorption data were fitted to the Langmuir model for the

281

calculation of maximum adsorption capacity (Note of Figure S13). Due to the pronounced

282

pore-filling effect, the MC-THF possessed the highest BPA adsorption capacity (Figure S13a).19

283

This result was further confirmed by the positive correlations between the surface area, the

284

micropore surface area of MCs, and the qm of BPA adsorption (Figure S13b and S13c). BPA

285

possesses a three-dimensional size of 0.383 × 0.587 × 1.068 nm,31 suggesting that MC

286

micropores are better suited for filling BPA molecules. Further confirmation was provided by the

287

stronger correlation between the micropore surface area of MCs and qm than that between the

288

surface area of MCs and qm (Figure S13b and S13c).

289

XRD patterns showed that Fe2O3 (JCPDS 39-1346), Fe3O4 (JCPDS 65-3107), and ZnFe2O4

290

(JCPDS 22-1012) would be the main Fe species (Figure S14). However, XRD analysis cannot

291

effectively differentiate these three species, because they have similar characteristic peaks.


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Therefore, Mössbauer spectroscopy was used to further distinguish the Fe species. As shown in

293

Figure 4, Fe3O4 and ZnFe2O4 were detected as the main Fe species. Based on the following

294

equations (3-5), Fe3O4 is produced from a reduction reaction between Fe2O3 and carbon matrix32,

295

33

296

reaction process between ZnO (acidic oxide) and Fe2O3 (basic oxide).22, 34

; ZnFe2O4 may be formed on the surface of carbon matrix via an aggregation thermochemical

Fe3+ → Fe(OH)3 → FeOOH → Fe2 O3 297

T ≤ 400 o C

(3)

3Fe2 O3 + C → 2Fe3 O 4 + CO

T ≥ 500 o C

(4)

Fe2 O3 + ZnO → ZnFe 2 O4

T ≥ 500 o C

(5)

298

It has been noticed that all MCs derived from organic solvent-washed hydrochar (PE, DLM,

299

AT, and THF) possess a lower ZnFe2O4 mole fraction. Before the activation stage of these

300

samples, more ZnCl2 (a precursor of ZnO) activator would be dispersed into the pores of

301

hydrochar due to its higher porosity (Figure S6). Correspondingly, more ZnO (produced from the

302

decomposition of ZnCl2) is loaded into the pores of MC. Undoubtedly, this will inhibit the

303

evolution of surface reaction between Fe2O3 and ZnO and therefore decrease the content of

304

ZnFe2O4 in MCs derived from organic solvent-washed hydrochar.

305

As shown in Figure S15a, the MCs had similar saturation magnetization values, ranging from

306

19.1 to 25.5 emu/g. However, the solubility of Fe species increased linearly with the increase of

307

MC porosity (Figure S15b), suggesting its decreased acid resistance in the MC derived from

308

organic solvent-washed hydrochar.

309

Overall, because solvent washing promotes porosity formation in hydrochar, the surface area

310

and adsorption capacities of MCs increased accordingly. For the same reason, MCs’



311

microporosity, ZnFe2O4 content, and acid resistance were found decreased.

312

Environmental Implications

Page 18 of 29

313

As discussed above, solvent-washing process significantly altered hydrochar properties in

314

terms of element content, group type and porosity. As elaborated below, the findings from the

315

hydrochar washing mechanism have significant implications for the applications of hydrochar

316

especially in environmental remediation field. Firstly, changes in O-containing group content of

317

will affect the ability of hydrochar on reducing metal ions and organic matters, because carbonyl

318

groups are thought to be responsible for reduction capability of carbonaceous material.35 The

319

content change will also influence the catalytic reactivity of hydrochar in oxidation of organic

320

matters, because O-containing groups act as an efficient activator in persulfate-based

321

oxidations.36 Also, the content change will affect the adsorption of organic matters onto the

322

hydrochar surface through the π-π electron-donor-acceptor interaction.3, 17, 37 In addition, the

323

enhanced porosity of washed hydrochar will likely contribute to the adsorption of organic

324

matters onto hydrochar surface via the pore-filling effect.

325

Lastly, due to the removal of bio-oil, organic solvent washing process could reduce the release

326

of dissolved organic matter from hydrochar and improve the carbon sequestration potential of

327

hydrochar. Thus, organic solvent washing procedure of hydrochar has important implications for

328

the global carbon cycle.

329

ACKNOWLEDGEMENTS

330

This research was funded by the National Key Technology Support Program (No. 17 ACS Paragon Plus Environment

Page 19 of 29


331

2015BAD15B06), the National Natural Science Foundation of China (No. 21407027, 21577025),

332

and the International Postdoctoral Exchange Fellowship Program of China Supported by Fudan

333

University.

334

Supporting Information

335

The pseudo-second-order models, GC-MS spectrum, surface area, Van Krevelen diagram,

336

FTIR spectra, 13C NMR spectra, C 1s XPS spectra, N2 adsorption-desorption isotherms, pore size

337

distribution, adsorption isotherms of BPA onto MCs, XRD patterns, hysteresis loop, correlation

338

analysis of related samples are presented in Figure S1 - S15.

339

GC-MS analysis of bio-oil, BET surface area, TG characteristics, loss characteristics, element

340

compositions, C1s analysis for XPS spectra, C-containing functional groups analysis for NMR

341

spectra of hydrochar, and yields, elemental compositions of MCs are presented in Table S1 - S9.

342

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

343



Page 20 of 29

344

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345

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437 438


a y=0.31x-6.55 R2=0.99, p < 0.01

12

H-3

1.1

H-4 H-2

9 6 3

H-1

o

0.0 H-1 H-0 H-5 Tmax1

60

-0.2

50

200 400 600 Temperature (oC)

40

200

H-5 H-4

H-3

H-4

H-3 H-2 Tmax2

-0.3

e

H-4

aromatic C-O C=O

H-5 H-2 H-1 H-0

800

400 600 Temperature (oC)

1.5

1.0

1.0

Decarboxylation

0.25 0.50 0.75 O/C (atomic ratio)

d H-5

1.00

aromatic aromatic ring C-O C=C

aliphatic C-H

H-4 H-3 H-2 H-1 H-0 C=O

4000

lignin, C=C

3000 2000 1000 Wavenumber (cm-1) H-0

f

H-5 0.29

0.28

0.5

800

aromatic C O-alkyl C O-CH3 alkyl C

H-3

H-4

0.27

Transmittance (arbitrary units)

80 Derivate Weight (%/ C)

Weight (%)

90

-0.1

H-0 H-1

0.0 0.00

30 40 50 60 70 Washing efficiency of hydrochar by THF (%) c 100

70

b H-2

D eh yd ra D tio em n et ha na tio n

15

Page 24 of 29

2.0

H-5 H/C (atomic ratio)

BET surface area increasement (m2/g)


H-1

H-2

C1s (1)

C1s (1)

C1s (1)

C1s (2)

C1s (2)

C1s (2) C1s (3)

C1s (3) C1s (4)

COO

290

288

C1s (3)

C1s (4) 286

282 290

284

C1s (4)

288

286

284

282 290

288

286

284

282

H-3 H-3

H-4

C1s (1)

H-2

C1s (2)

H-1

150 100 50 Chemical shift (ppm)

288

C1s (2) C1s (3)

C1s (3)

C1s (4)

C1s (4)

C1s (4) 290

200

C1s (1)

C1s (2)

C1s (3)

H-0

H-5

C1s (1)

286

284

282 290

0

288

286

284

282 290

288

286

284

Binding Energy (eV)

Figure 1 (a) Correlation between washing efficiency of hydrochar by THF and increase in BET surface area, dynamics in (b) H/C and O/C atomic ratios using Van Krevelen diagram, (c) TG and DTG profiles, (d) FTIR spectra, (e)

13

C NMR spectra, and (f) C 1s XPS spectra for

hydrochar after different THF washing times.


282

Page 25 of 29


H-WT

H-PE

H-AT

H-THF

Figure 2 SEM spectra for the hydrochar washed by different solvents.



1.2 b

920 880 MC-PE

MC-DLM

0.52 nm

y=27.11x+834.8 R2=0.95 p < 0.01 MC-AT

960

1.0 0.8 0.6 0.4

MC-WT MC-AT MC-DLM MC-THF MC-PE

0.48 nm

0.60 nm 0.82 nm

0.36 nm

MC-THF dV(D) (cm3/nm/g)

Surface area of MC (m2/g)

1000 a

Page 26 of 29

0.2 840 0.0

MC-WT 0

1 2 3 4 5 Surface area of Hydrochar (m2/g)

6

0.4

0.6 0.8 1.0 1.2 Pore diameter (nm)

1.4

Figure 3 (a) Positive correlation between surface area of hydrochars and magnetic carbon composites derived from hydrochar being washed by different solvents, (b) narrow micropore size distribution from CO2 adsorption isotherms for magnetic carbon composites derived from hydrochar being washed by different solvents.


Page 27 of 29


ZnFe2O4

Relative Transmission

ZnFe2O4

ZnFe2O4 Fe3O4

Fe3O4

Fe3O4

MC-WT

MC-DLM

MC-PE -10

Relative Transmission

0 5 Velocity (mm/s)

1.0

10

MC-WT

1.5

Fe3O4

Fe3O4

ZnFe2O4/Fe3O4 (mole ratio)

ZnFe2O4

ZnFe2O4

-5

MC-AT MC-DLM MC-PE MC-THF

0.5

MC-THF

MC-AT -10

-5

0 5 Velocity (mm/s)

10

0.0

-10

-5

0 5 Velocity (mm/s)

10

Figure 4 Room-temperature Mössbauer spectra and mole ratio of ZnFe2O4 and Fe3O4 for magnetic carbon composites derived from hydrochar after being washed by different solvents.



Page 28 of 29

Table 1 Loss Characteristics and Elemental Compositions of Hydrochar Samples Washed by THF for 0 ~ 5 Times Loss ratio (%) Sample

a

Ash

C

O

H

N H/C

O/C

0.573

1.59

0.192

8.37

0.619

1.47

0.209

23.6

6.12

0.696

1.12

0.269

63.4

24.7

5.78

0.684

1.09

0.292

5.45

64.9

23.0

5.95

0.697

1.10

0.266

4.46

64.4

25.2

5.21

0.683 0.970 0.294

Massa

Cb

Oc

Hd

Ne

(%)

(%)

(%)

(%)

(%)

H-0

-

-

-

-

-

2.92

69.5

17.8

9.20

H-1

31.7

32.6

26.8

37.8

26.2

3.42

68.5

19.1

H-2

58.7

60.9

45.3

72.5

49.8

3.81

65.8

H-3

66.2

69.1

53.2

78.7

59.6

5.42

H-4

70.9

72.8

62.4

81.1

64.6

H-5

73.4

75.3

62.3

84.9

68.3

Loss ratio of mass = (1-hydrochar weight after washing/raw hydrochar weight)*100%, b loss

ratio of C content = (1-C content of hydrochar after washing/raw C content of hydrochar)*100%, c

loss ratio of O content = (1-O content of hydrochar after washing/raw O content of

hydrochar)*100%, d loss ratio of H content = (1-H content of hydrochar after washing/raw H content of hydrochar)*100%,

e

loss ratio of N content = (1-N content of hydrochar after

washing/raw N content of hydrochar)*100%.


Page 29 of 29


Table 2 Surface Area and Pore Volume Parameters for Magnetic Carbon Composites Derived from Different Solvents Washed Hydrochar SBET

Smic

Smic/SBET

Vt

Vmic

Vmic/Vt

Smic

Vmic

(m2/g)a

(m2/g)a

(%)

(cm3/g)a

(cm3/g)a

(%)a

MC-WT

821.6

785.4

95.6

0.427

0.389

91.0

675.9

0.226

MC-PE

858.9

802.3

93.4

0.454

0.396

87.2

765.0

0.255

MC-DLM

897.3

843.8

94.0

0.471

0.413

87.7

862.9

0.295

MC-AT

912.5

840.5

92.1

0.522

0.417

79.9

777.5

0.262

MC-THF

1001

940.5

94.0

0.559

0.466

83.3

864.5

0.299

Sample

a

Porosity parameters concluded from N2 adsorption isotherms.

b

Porosity parameters concluded from CO2 adsorption isotherms.


(m2/g)b (cm3/g)b

Demethanation Trend of Hydrochar Induced by Organic Solvent

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