Thiol-modified biochar synthesized by a facile ball-milling method for enhanced sorption of inorganic Hg2+ and organic CH3Hg+

Journal Pre-proof Thiol-modified biochar synthesized by a facile ball-milling method for enhanced sorption of inorganic Hg2+ and organic CH3 Hg+ Honghong Lyu, Siyu Xia, Jingchun Tang, Yaru Zhang, Bin Gao, Boxiong Shen

PII:

S0304-3894(19)31311-1

DOI:

https://doi.org/10.1016/j.jhazmat.2019.121357

Reference:

HAZMAT 121357

To appear in:

Journal of Hazardous Materials

Received Date:

4 August 2019

Revised Date:

13 September 2019

Accepted Date:

28 September 2019

Please cite this article as: Lyu H, Xia S, Tang J, Zhang Y, Gao B, Shen B, Thiol-modified biochar synthesized by a facile ball-milling method for enhanced sorption of inorganic Hg2+ and organic CH3 Hg+ , Journal of Hazardous Materials (2019), doi: https://doi.org/10.1016/j.jhazmat.2019.121357

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Thiol-modified biochar synthesized by a facile ball-milling method for enhanced sorption of inorganic Hg2+ and organic CH3Hg+

Honghong Lyua, Siyu Xiab, Jingchun Tangb,* Yaru Zhangb, Bin Gaoc, Boxiong Shena,*

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Tianjin Key Laboratory of Clean Energy and pollution control, School of Energy and

b

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Environmental Engineering, Hebei University of Technology, Tianjin 300401, China

Key Laboratory of Pollution Processes and Environmental Criteria (Ministry of Education),

Tianjin Engineering Research Center of Environmental Diagnosis and Contamination

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Remediation, College of Environmental Science and Engineering, Nankai University, Tianjin

Department of Agricultural and Biological Engineering, University of Florida, Gainesville,

FL 32611, United States

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* Corresponding author:

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c

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300350, China

College of Environmental Science and Engineering, Nankai University, Tianjin 300350, E-mail

addresses:

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China

Jingchun

Tang

([email protected]);

Telephone:

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+8613682055616

School of Energy and Environmental Engineering, Hebei University of Technology, Tianjin 300401, China E-mail addresses: Boxiong Shen ([email protected]); Telephone: +8618622132754

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Graphical abstract

3-MPTS

Open Pore

Blocked Pore

Hg2+

C

O

Larger surface area More functional groups

C

O

Ball milling

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More negatively charged surface

CH3Hg+ 3-MPTS solution

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Pristine biochar

BMS-biochar

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Highlights

Thiol-modified biochar was synthesized by simply ball milling biochar with 3-MPTS.

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BMS-biochar showed enhanced sorption for Hg2+ and CH3Hg+ compared to CIS-biochar.

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Mercury was removed by electrostatic attraction, ligand exchange, and complexation.

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Surface diffusion was the rate-limiting adsorption step for BMS-biochar.

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Hg2+ and CH3Hg+ adsorption was a monolayer adsorption on heterogeneous surfaces.

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

Abstract: Modification of thiol on biochar often demands complex synthetic procedures and

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chemicals. In this work, a simple and environment friendly thiol-modified biochar

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(BMS-biochar) was successfully synthesized by ball milling pristine biochar with 3-mercaptopropyltrimethoxysilane (3-MPTS). The resultant BMS-biochar was characterized and tested for aqueous inorganic Hg2+ and organic CH3Hg+ removal. Characterization results showed that 3-MPTS was loaded on the surface of biochar through oxygen-containing functional groups (i.e., –OH and C–O) and π–π bond. Ball milling method improved the properties of BMS-biochar, namely, more efficient –SH load, a larger surface area, more 2

functional groups, more negatively charged surface, which resulted in higher removal efficiency of Hg2+ and CH3Hg+ (320.1 mg/g for Hg2+ and 104.9 mg/g for CH3Hg+) compared to the pristine biochar (105.7 mg/g for Hg2+ and 8.21 mg/g for CH3Hg+) and thiol-modified biochar through chemical impregnation (CIS-biochar) (175.6 mg/g for Hg2+ and 58.0 mg/g for CH3Hg+). Ball milling increased the sorption capacities of Hg2+ and CH3Hg+ through surface adsorption, electrostatic attraction, ligand exchange, and surface complexation.

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Modeling results suggested that the surface diffusion was the rate-limiting adsorption step for BMS-biochar. This work gave prominence to the potential of ball milling for the preparation

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of thiol-modified biochar to remove mercury especially organic CH3Hg+ by adsorption.

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Keywords: Thiol; Biochar; Ball mill; Mercury; Methylmercury

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1. Introduction

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Mercury (Hg), a commonly occurring heavy metal in water environment, is often derived from a lot of industrial processes such as oil refining, batteries, thermometers, fluorescent

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light tubes, pesticides, cosmetics, and pharmaceuticals [1, 2]. It poses high risk to public health due to the characteristics of carcinogenicity, persistence, and bioaccumulation [3]. In

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general, Hg is present as inorganic mercury (Hg+ and Hg2+) and organic mercury (CH3Hg+ and CH3CH2Hg+) in the natural environment [4]. Compared to the inorganic mercury, which mainly damages the immune system and kidneys [5], organic mercury is hyper-toxic, extremely fat-soluble, accumulating, poses a high risk to the cardiovascular system and the brain, and is easy to be formed by inorganic mercury conversion [6]. The United States 3

Environmental Protection Agency (USEPA) recommends a maximum CH3Hg+ intake of 0.1 µg/kg per day [7]. Thus, it is of great importance to develop practically reactive materials which can remove both Hg2+ and CH3Hg+ to minimize the toxicity of mercury. Adsorption is an effective method for the removal of both organic and inorganic contaminants [8, 9]. Many carbon adsorbents, such as graphene oxide/Fe-Mn [10], thiol functionalized magnetic carbon nanotubes (CNTs-SH@Fe3O4) [11], and magnetic carbon

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nanotubes composite (MWCNTs-Fe3O4) [12], have been developed to remove Hg2+ from aqueous water and soils [13]. However, rare carbon adsorbent was reported for CH3Hg+ removal due to its high toxicity and low affinity for adsorbents [14]. Biochar, a porous carbon

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material that can be derived from a wide range of biomass, has been proposed as effective

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adsorbents for environmental remediation [15, 16]. Huang et al. [14] reported that biochar was effective for control of inorganic Hg2+ but less effective for CH3Hg+ sorption. Liu et al.

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[13] suggested that the addition of biochars might be a remediation method for reducing the

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release of Hg2+ and CH3Hg+from sediment, but potential for Hg methylation under some conditions requires consideration. Therefore, surface modification has attracted attention to

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improve the binding affinity of biochar for both Hg2+ and CH3Hg+. Thiol groups (–SH) are the commonly used functional groups [17]. Biochar modified by

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thiol can increase the adsorption sites for the sorption of Hg through ligand exchange and complexation, and introduce more negative charges to the surface of biochar to enhance the electrostatic adsorption for the positively charged contaminants such as heavy metals (e.g., Hg2+, Pb2+, and Cd2+) and methylmercury (CH3Hg+) [14]. Chemical impregnation has been developed to introduce the thiol to biochar surface. Huang et al. [18] synthesized 4

thiol-modified biochar via an ammonium hydroxide method. However, the sorption capacity of thiol-modifed biochar for Hg2+ (126 mg/g) was slightly higher than that of biochar (105 mg/g), and modifation of biochar by thiol with this method requires complex synthetic procedures and chemicals. In comparison with the traditional chemical impregnation, ball milling has been desired to produce carbon materials in an environmentally friendly way because of the advantages of

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low cost, simple process, and high efficiency [19]. Ball milling can break chemical bonds, increase the specific surface area, and create new surfaces by the high energy generated in the process [20]. Our previous study reported that ball milling enhanced the maximum sorption

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capacities of ball-milled biochar for Ni2+ and methylene blue from 211 to 1949 mmol/kg and

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from 17.2 to 354 mg/g, respectively [21, 22]. However, no studies were reported about the preparation of thiol-modified biochar through a ball milling method (BMS-biochar), and the

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effectiveness of BMS-biochar on Hg2+ and CH3Hg+ removal is still unknown until now.

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The overall goal of this research was to synthesize a thiol-modified biochar through ball milling and investigate its ability for adsorption of aqueous Hg2+ and CH3Hg+. The specific

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objectives were as follows: (1) compare physicochemical properties of thiol-modified biochar prepared by impregnation (CIS-biochar) and ball milling (BMS-biochar); (2) determine the

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sorption kinetics and isotherms of aqueous Hg2+ and CH3Hg+ by CIS-biochar and BMS-biochar; and (3) study the mechanisms that govern adsorption of Hg2+ and CH3Hg+ by BMS-biochar.

2. Materials and methods 5

2.1. Materials All chemicals used in this study were of analytical grade. Deionized (DI) water was used to prepare all the solution. Mercury nitrate monohydrate (Hg(NO3)2·H2O) (98%), methylmercury chloride (CH3HgCl) (97%), and ethanol (HPLC grade) were purchased from Anpel Laboratory Technology (Shanghai, China). 3-MPTS (97%) was purchased from Beijing Bailingwei Technology Co., Ltd. (Beijing, China). Potassium hydroxide (KOH),

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nitric acid (HNO3), potassium borohydride (KBH4), and ammonium hydroxide solution (NH4OH) (25% NH3 in H2O) were procured from Tianjin Sanjiang Technology Co., Ltd.

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(Tianjin, China). Feedstock biomasses for pristine biochar, poplar wood chips were air-dried

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and sieved to 0.5–1 mm particles.

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2.2. Preparation of pristine biochar, CIS-biochar, and BMS-biochar The poplar wood chips were oven dried at 80 °C for 12 h and placed in a muffle furnace

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(SX-G07102, Zhonghuan Furnace Co., Ltd., Tianjin, China) to produce biochar at 300 °C for 3 h (the preliminary experiment results showed that 300 °C biochar had the greatest sorption

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capacities of Hg2+ and CH3Hg+, data not shown). The products that pass through a 100 mesh

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were rinsed with distilled water repeatedly to achieve constant pH. Then, the biochar was oven-dried at 80 °C for 24 h, and stored in vials for further use. CIS-biochar was prepared following an method reported in a previous work [18]. In brief,

3 g biochar was added into a mixed solution of water and ethanol (3.6 mL water and 114 mL ethanol). With strong magnetic stirring and nitrogen purging, 2.4 mL of 3-MPTS was added 6

into the mixture dropwise. After stirring for 6 h, the solution pH was adjusted to 10 with NH4OH. To ensure complete reaction, the suspension was stirred for 24 h. The mixture was then washed with ethanol and DI water for 3 times, and freeze-dried for subsequent uses. The BMS-biochar was prepared by ball milling 3 g biochar samples with 120 mL mixed solution of water, ethanol, and 3-MPTS (3.6 mL water, 114 mL ethanol, and 2.4 mL 3-MPTS). It should be note that 3-MPTS was added into the agate jars (500 mL) dropwise. 300 g agate

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balls (diameter of 15 mm-to diameter of 5 mm-to diameter of 3 mm mass ration of 2:5:3) were added into the mixture, and then, the agate jars were placed in a ball mill machine (F-P4000, Fukasi Instrument Co., Ltd., Hunan, China) and operated at 300 rpm for 12 h with

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rotation direction altered every 6 h. After the ball milling, the resultant product was washed

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with ethanol and DI water for 3 times, freeze-dried for 48 h, and labeled as BMS-biochar. To test biochar-to-ball mass ratio and ball milling time effect, various amounts of biochar (3 g, 6

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g, and 15 g) were mixed with 300 g agate balls (i.e., biochar-to-ball mass ratio = 1:100, 1:50,

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and 1:20) for 3, 6, 12, 30, and 48 h, respectively. To investigate the effect of rotation speed, 3 g biochar was mixed with 300 g agate balls and operated at 300, 400, and 600 rpm,

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respectively, for 12 h.

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2.3. Characterization

The specific surface area, pore volume, and pore size of biochar, CIS-biochar, and

BMS-biochar were measured by the multipoint N2 adsorption BET method (ASAP2460, Micromeritics, Atlanta, USA). The scanning electron microscopy (SEM) was used to characterize the surface morphology (JEOL, Tokyo, Japan). The thermal stability was determined by thermal gravimetric analysis (TGA) (NETZSCH, Freistaat Bayern, Germany). 7

Fourier transform infrared spectroscopy (FTIR) was applied to analyze the surface functional groups (FTS6000, Bio-rad, Beijing, China). Zeta potential was tested by a Malvern Zeta sizer Nano ZEN3690 Instrument (Malvern Instruments, Worcestershire, UK). X-ray photoelectron spectroscopy (XPS) was performed to analyze the surface atomic ratio and binding energy C1s and S2p.

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2.4. Batch sorption Batch sorption experiments were conducted in sealed 30 mL PTFE vials in triplicate to investigate the sorption ability of the BMS-biochar. 0.01 M NaNO3 was added into the Hg2+

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or CH3Hg+ solution to maintain a constant ionic strength. The initial solution pH was adjusted to 7.0 ± 0.2 with HNO3 (1 M and 0.1 M) and NaOH (1 M and 0.1 M). For Hg2+, 1.12 mg of

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each sorbent was mixed with 28 mL (a dosage of 40 mg/L) of 5 mg/L Hg2+ solution (28 mL

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reaction system was maintained to ensure minimal headspace and ease of operation). For CH3Hg+, 1.12 mg of each sorbent was mixed with 28 mL (a dosage of 40 mg/L) of CH 3Hg+

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solution, the initial CH3Hg+ concentration was 1 mg/L. The vials were then sealed and rotated on an end-over-end rotator operated at 40 rpm at room temperature (25 ± 2 °C) for 3 d. After

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the experiments, the mixtures were filtered through PTFE filters (0.22 µm pore size, Jin Teng

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Experimental Equipment Co., Ltd, Tianjin, China), and the filtrate was collected to determine the aqueous Hg2+ or CH3Hg+ concentrations. Control tests (i.e., aqueous solution without the adsorbent) showed that the loss of Hg2+ or CH3Hg+ from solutions (including the adsorption of vials and filters) was negligible (< 4%). The kinetics and isotherm of Hg2+ or CH3Hg+ adsorption onto BMS-biochar were carried out according to the same procedure as above mentioned. For Hg2+, sorption kinetics testes 8

were carried out with an Hg2+ concentration of 5 mg/L, and the adsorbent-to-solution ratio was 1.12 mg per 28 mL solution. For CH3Hg+, the initial concentration was 1 mg/L, and the sorption kinetics was examined by mixing 1.12 mg BMS-biochar with 28 mL CH3Hg+ solution. At predetermined times (0.5, 1, 2, 4, 6, 9, 24, and 48 h), triplicate PTFE vials were sampled and tested for aqueous Hg2+ or CH3Hg+ concentration. Sorption isotherms were performed according to the same conditions described above.

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Hg2+ sorption isotherms were carried out by mixing 1.12 mg of BMS-biochar with 28 mL of Hg2+ solutions containing different concentrations (1.0–25 mg/L) for 48 h. For CH3Hg+, 1.12 mg of adsorbent was mixed with 28 mL of CH3Hg+ solution. Initial CH3Hg+ concentration

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range was from 0.1 to 8.0 mg/L and equilibrium time was 48 h.

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2.5. Analysis methods

An AFS-830 atomic fluorescence spectrometer (Titan Instruments, Beijing, China) was

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used to determine the aqueous Hg2+ or CH3Hg+ concentrations following the Environmental Protection Standards of People’s Republic of China (HJ694-2014). The detection limit was

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0.04 µg Hg/L.

3. Results and discussion 3.1. Characterization The particle morphology of the pristine biochar, CIS-biochar, and BMS-biochar are compared using the SEM images (Fig. 1). The surface of the pristine biochar was rough and 9

porous (Fig. 1a). The morphology of the CIS-biochar remained similar to that of the pristine biochar except that it became a bit smoother. This might be ascribed to the immersion of ethanol during the modification process (Fig. 1b) [18]. Compared with the biochar and CIS-biochar, BMS-biochar was irregular granular-like and the diameter of particles were about below 1 µm (Fig. 1c), which suggested that ball milling technology can break the biochar particles into fine grains.

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The surface area, pore size, and pore volume of biochar was 1.83 m2/g, 9.76 nm, and 0.003 m3/g, respectively (Table 1). After thiol-modification via chemical impregnation, the specific surface area and pore volume of CIS-biochar increased slightly from 1.83 to 5.21

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m2/g and from 0.003 to 0.006 m3/g, respectively. These changes might be ascribed to that the

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tar particles which block the pores of biochar were cleaned and the pore of biochar increased during the chemical modification process. For BMS-biochar, the surface area, pore volume,

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and pore size of BMS-biochar increased by ball milling significantly. Compared to the

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pristine biochar and CIS-biochar, BMS-biochar had a larger specific surface area (61.34 m2/g), pore size (19.05 nm), and pore volume (0.291 m3/g) (44-fold increase on average). Lyu

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et al. [21] investigated the effects of ball milling on biochar’s surface area. They reported that ball milling in ambient air increased the surface area of 300 oC biochar from 2.00 to 8.30

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m2/g. The results suggested that the improvement of the specific surface area was due to the increase of external surface area rather than an opening of pores. In this study, the surface area of the BMS-biochar increased significantly from 1.83 to 61.3 m2/g, which suggested that ball milling not only increased the external surface area, but also opened the internal pore of biochar with the help of chemical modification. These enhanced properties favored the Hg2+ 10

or CH3Hg+ adsorption on the surface of BMS-biochar. The pore size and pore volume of BMS-biochar were 2.45 and 97 times greater than that of the pristine biochar, respectively, which further proved that ball milling enlarged or opened the pores of the biochar. The elemental analysis showed that the C, N, O, S, and Si contents of the pristine biochar were 77.47%, 0.54%, 20.96%, 0.17%, and 0.86%, respectively (Table 1). After thiol-modification, the C content decreased while O and N contents increased, and thus, the

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O/C and (O+N)/C ratios increased for CIS-biochar (0.29 and 0.30, respectively) and BMS-biochar (0.43 and 0.44, respectively). The BMS-biochar had the highest O/C and (O+N)/C ratios, suggesting that BMS-biochar had the lowest hydrophobicity and highest

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polarity [23]. These results might be attributed to the introduction of functional groups during

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the ball milling process, which was consistent with our previous study [21]. Moreover, the S and Si contents of BMS-biochar (0.79% and 3.13%, respectively) were higher than that of the

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pristine biochar (0.17% and 0.90%, respectively) and CIS-biochar (0.62% and 1.63%,

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respectively), indicating that more 3-MPTS was loaded on the surface of biochar during the ball milling process, which made them better adsorbents for the Hg2+ or CH3Hg+ adsorption.

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The TG curves of the pristine biochar, CIS-biochar, and BMS-biochar are compared to investigate the stability of the samples (Fig. 1d). The TG curves can be divided into three

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stages including the mass loss of water below 120 °C, the degradation of functional groups and cellulose (120–600 °C), and the pyrolysis of the carbon skeleton above 600 °C [14, 24]. The total mass loss of biochar, CIS-biochar, and BMS-biochar were 55.2%, 62.1%, and 88.8%, respectively. The highest weight loss observed in BMS-biochar from about 300 to 600 °C was the ascribed to the decomposition of functional groups, suggesting that ball 11

milling introduced more functional groups on the surface of biochar. The results were consistent with the elemental analysis (Table 1) and FTIR results (Fig. 1e). FTIR results showed that six peaks were observed for biochar at 3389, 2927, 1724, 1602, 1442, and 1040 cm-1, respectively, corresponding to the vibrations of –OH, C–H, C=O, and C=C groups (Fig. 1e). After thiol modification, the peak intensity of functional groups decreased slightly for CIS-biochar (e.g., –OH at 3389 and 1442 cm-1, C=C at 1602 cm-1, and

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C–O at 1040 cm-1). These changes suggested that oxygen-containing functional groups (i.e., – OH and C–O) and π– π bond played an important role in 3-MPTS soldering onto the surface of biochar. However, compared to the pristine biochar and CIS-biochar, the intensity of

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functional groups increased significantly for BMS-biochar, for instance, the –OH at 3389 and

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1442 cm-1, C=C at 1602 cm-1, C–H at 2927 cm-1, and C–O at 1040 cm-1. Moreover, three new peaks were observed for BMS-biochar at 2595, 799 and 442 cm-1, which was corresponded to

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the vibrations of –SH (2595 cm-1), and Si–O (799 and 442 cm-1), respectively [25-27]. The

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results further proved that the 3-MPTS was attached onto the surface of biochar successfully and ball milling introduced functional groups onto the surface of BMS-biochar.

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The surface potential of the BMS-biochar may also be altered by the increase of the acidic functional groups. Zeta potential results showing a surface charge of 10.8 to -37.9 mV

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for biochar, from -5.0 to -41.4 mV for CIS-biochar, and from -14.8 to -44.2 mV for BMS-biochar over the pH range of 2.0-9.1 (Fig. 1f). Moreover, the point of zero charge (PZC) was lowered from about 3.5 to < 2.0. The results were consistent with the FTIR results suggesting that 3-MPTS and surface functional groups were introduced onto the surface of biochar through ball milling method. 12

XPS analysis was performed for the pristine biochar, CIS-biochar, and BMS-biochar (Fig. 2). For the pristine biochar, binding energies of C 1s spectra in Fig. 2a at 288.5, 286.1, and 284.6 eV were ascribed to O−C=O/C=O, C−O, and C−C/C=C, respectively [28]. For CIS-biochar, the proportion of C−C/C=C increased from 54.3% to 71.2%, and the proportion of O−C=O/C=O and C−O decreased (Fig. 2b). The results were consistent with our FTIR result. Ball milling increased the contents of oxygen-containing functional groups of

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BMS-biochar, for instance, the proportion of O−C=O/C=O increased from 8.6% to 19.9% significantly (Fig. 2c). The S2p peaks of the pristine biochar were weak (Fig. 2d), which suggested the low content of S in the biochar. For CIS-biochar, the S2p spectra (Fig. 2e) was

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deconvoluted into three peaks at 168.1, 164.3, and 162.9 eV, which were assigned to S For BMS-biochar, the

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oxidation products, C−S/C=S, and S2p3/2 (−SH), respectively [29].

peak of S oxidation products at 168.1 eV disappeared, and the proportions of C−S/C=S and

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S2p3/2 (−SH) increased from 21.4% to 30.4%, and from 61.4% to 69.7%, respectively (Fig.

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2f). Ball milling process avoided the generation of S oxidation products and facilitated the loading of –SH, which played an important role in the adsorption of Hg2+ and CH3Hg+ [3].

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3.2. Effects of ball milling on Hg2+ and CH3Hg+ sorption

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Hg2+ and CH3Hg+ removal rate increased with increasing ball milling time and with decreasing biochar-to-ball mass ratio (Figs. 3a and b). At biochar-to-ball mass ratio of 1:20, Hg2+ and CH3Hg+ removal rate increased from 18.7% to 68.2% and from 8.7% to 53.2%, respectively, as ball milling time was increased from 3 h to 48 h. When the biochar-to-ball mass ratio was 1:100, Hg2+ removal rate increased from 68.2% to 97.5% and peak removal rate was reached at 12-48 h (98.2%). Similarly, the removal rate of CH3Hg+ increased from 13

39.5% to 82.2% with increasing ball milling time from 3 to 12 h and reached a peak at 12-48 h. At a milling time of 12 h, decreasing biochar-to-ball mass ratio from 1:20 to 1:100 increased the Hg2+ and CH3Hg+ removal rate from 45.9% to 97.5% and from 29.8% to 82.2%, respectively. Moreover, the removal rate of Hg2+ and CH3Hg+ remained constant at 97.1%-97.7% and 82.6%-89.4%, respectively, over the rotation speed range of 300-600 rpm (Fig. S1). Based on these results, all subsequent BMS-biochars were prepared under the

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optimal conditions of 12 h ball-milling time, biochar-to-ball mass ratio 1:100, and 300 rpm rotation speed.

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Compared to the pristine biochar, CIS-biochar offered 1.74 times greater Hg2+ removal rate (from 39.9% to 69.5%) (Fig. 3c). Ball milling enhanced Hg2+ adsorption on the

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BMS-biochar with a removal rate of 97.5%. The results were positively related to the surface

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area of the adsorbents (from 1.83 for biochar to 61.34 for BMS-biochar, Fig. 3c). The sorption of inorganic Hg2+ onto biochars is mainly controlled by the following mechanisms:

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surface adsorption, electrostatic attraction, ligand exchange, surface complexation, and chemical precipitation [11, 30]. Ball milling method resulted in enhanced physiochemical

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properties of the BMS-biochar, namely, larger surface area (Table 1) and more functional groups (e.g., −SH, −OH, and −COOH) (Fig. 1e and Fig. 2). Therefore, BMS-biochar was

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more effective to adsorb Hg2+. For instance, −SH on the surface of biochar had a strong binding affinity for Hg2+ due to the soft Lewis acid-base interaction [31]. They can adsorb Hg2+ via ligand exchange and surface complexation (discussed in section 3.3). The results suggested that ball milling method is an efficient method to synthesize thiol-modified biochars. 14

Similar trends were observed for the CH3Hg+ (Fig. 3d). The pristine biochar had a weak affinity to CH3Hg+ with a removal rate of 13.5%. The modification of biochar by thiol increased the CH3Hg+ removal rate to 63.2% for CIS-biochar. Ball milling further enhanced the thiol-modified biochar removal efficiency for CH3Hg+ (82.2%). These were positively related to the specific surface area of the sorbents as well (the correlation was not significant), which proved the important role of the surface adsorption to the adsorption of CH3Hg+ on the

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BMS-biochar. The results were consistent with the report by Wahby et al. [30], who found that mercury sorption on activated carbon increased with increasing specific surface area. The contents of S and Si of BMS-biochar (0.79% and 3.13%, respectively) were higher than that

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of CIS-biochar (0.62% and 1.63%, respectively), which might contributed to the adsorption

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of CH3Hg+ through surface complexation between the functional groups (e.g., −SH and Si−O) and CH3Hg+. In addition, CIS-biochar with low specific area (5.21 m2/g) presented a high

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Hg2+ and CH3Hg+ removal rate (69.5% and 63.2%, respectively), further suggesting the role

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of −SH in Hg2+ and CH3Hg+ removal. 3.3. Adsorption kinetics

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Kinetic tests were performed to investigate the removal rate of Hg2+ and CH3Hg+ (Figs.

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4a and b). For Hg2+, the sorption displayed a rapid initial rate within one hour (from 0 to 7.9%, 14.0% and 54.7% for the pristine biochar, CIS-biochar, and BMS-biochar, respectively) and slowed down till equilibrium at ~ 9 h for CIS-biochar and BMS-biochar (69.1% and 92.2%) and ~ 24 h for biochar (38.9%). Therefore, 48 h was used to make sure Hg2+ reaction equilibrium. For CH3Hg+, the sorption increased rapidly during the first two hours for biochar and CIS-biochar (from 0 to 6.0% and 19.1% for biochar and CIS-biochar, respectively) and 15

reached equilibrium within 9 h for CIS-biochar and 24 h for biochar. It should be noted that BMS-biochar displayed faster sorption kinetics than the other sorbents. For instance, the sorption displayed a rapid initial rate within 0.5 min (from 0 to 49.7%) and then slowed down till equilibrium at ~ 4 h for BMS-biochar. After equilibrium, higher mercury removal rate was observed for BMS-biochar (82.2%) compared to the pristine biochar (13.5%) and CIS-biochar (65.7%).

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The pseudo first-order, pseudo second-order, and external mass transfer models were applied to imitate the kinetic data (Figs. 4c-f). The resultant parameters are given in SI Table S1. For Hg2+ and CH3Hg+, the pseudo second-order kinetic model (R2 ≥ 0.995) had a better fit

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than the pseudo first-order model (R2 =0.689-0.984), indicating that chemisorption was the

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rate-limiting step for Hg2+ and CH3Hg+ adsorption [32]. The key controlling factors of the sorption process were determined by the external mass transfer model (SI Table S1). All R2

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values obtained for biochar, CIS-biochar, and BMS-biochar on Hg2+ and CH3Hg+ sorption

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were > 0.990. For biochar, CIS-biochar, and BMS-biochar, the mass transfer coefficient kf increased from (1.03±0.26) × 10-5 to (1.20±0.02) × 10-4 cm/s for Hg2+ and from (0.07±0.01) ×

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10-5 to (1.64±0.05) × 10-4 cm/s for CH3Hg+, indicating BMS-biochar was favorable for Hg2+ and CH3Hg+ removal. The results indicated that sorption kinetics of Hg2+ and CH3Hg+ by

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biochar and CIS-biochar were the combination of chemisorption and external mass transfer. kf values of Hg2+ and CH3Hg+ in the BMS-biochar were 1.20×10-4 and 1.64×10-4 cm/s, respectively, which might be ascribed to that surface diffusion was the rate-limiting adsorption step for BMS-biochar due to its ultrafine particle size [22]. 3.4. Adsorption isotherms 16

Adsorption isotherms of Hg2+ and CH3Hg+ on the pristine biochar, CIS-biochar, and BMS-biochar are presented in Fig. 5 and the resultant fitting parameters are provided in SI Table S2. Both Langmuir and Freundlich model fitted the adsorption isotherms of Hg2+ and CH3Hg+ well (i.e., all R2 >0.904). The Langmuir isotherm describes monolayer sorption [33]. The Freundlich isotherm presumes non-ideal adsorption occurs on a heterogeneous surface [34], it is used for describing chemisorption process. The results indicated that the adsorption

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of Hg2+ and CH3Hg+ on the sorbents was a monolayer adsorption on heterogeneous surfaces. The Freundlich linearity constants (n) were below 1, indicating that the adsorption of Hg2+ and CH3Hg+ on the sorbents were affected by chemisorption under the experimental

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conditions [35]. The results were consistent with the kinetics fitting results. Moreover, for

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Hg2+, n values were arranged in the order biochar (0.57) > CIS-biochar (0.40) > BMS-biochar (0.30). Similar trend was observed for CH3Hg+. These findings suggested that Hg2+ and

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CH3Hg+ adsorption on the BMS-biochar was controlled by multiple mechanisms including

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surface adsorption, electrostatic attraction, ligand exchange, and surface complexation. For the pristine biochar, the maximum sorption capacity (qm) gained from the Langmuir

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model was 105.7 mg/g for Hg2+ and 8.21 mg/g for CH3Hg+. After thiol-modification by chemical impregnation, the qm values of CIS-biochar increased to 175.6 mg/g for Hg2+ and

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58.0 mg/g for CH3Hg+. Ball milling method increased the sorption capacity of Hg2+ and CH3Hg+ significantly. Compared with the pristine biochar and CIS-biochar, BMS-biochar had the largest sorption capacity of Hg2+ (320.1 mg/g) and CH3Hg+ (104.9 mg/g). Thus, ball milling method resulted in a 3-fold increase in aqueous Hg2+ removal ability and a 13-fold increase in aqueous CH3Hg+ removal ability. Moreover, BMS-biochar offered higher Hg2+ 17

and CH3Hg+ removal capacity than other reported adsorbents [1, 36-42]. For instance, thiol-functionalized graphene oxide/Fe-Mn composite (233.2 mg/g for Hg2+ and 36.7 mg/g for CH3Hg+) [14], Na2S modified biochar (5.71 mg/g for Hg2+), KOH modified biochar (4.26 mg/g for Hg2+), and activated carbon (4.57 mg/g for Hg2+) [1], and 5 wt.% NH4Cl loading-biochar (0.157 mg/g for Hg2+) (Table 2). Therefore, thiol-functionalized ball milling is as an efficient method to prepare modified biochar with improved physicochemical and

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sorption properties. 3.5. Sorption mechanisms

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Based on the batch adsorption experiments and modeling results, the mechanisms of the sorption of Hg2+ and CH3Hg+ onto BMS-biochar are summarized (Fig. 6). 1) Firstly, ball

re

milling not only improved the external surface area, but also opened the internal pore of

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biochar with the help of chemical modification. These changes enhanced the Hg2+ and CH3Hg+ sorption by surface adsorption. 2) Ball milling method increased the amount of

na

functional groups in biochar and facilitated more 3-MPTS loading onto the surface of biochar (i.e., more –SH), and thus, enhanced the Hg2+ and CH3Hg+ sorption via ligand exchange and

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surface complexation. 3) The increase in functional groups resulted in more negatively

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charged surface of BMS-biochar and increased the Hg2+ and CH3Hg+ sorption through electrostatic attraction. 4. Conclusions

Biochar from poplar wood chips was pyrolyzed at 300 °C, modified by thiol group through chemical impregnation and ball milling method, respectively, and applied for the 18

inorganic Hg2+ and organic CH3Hg+ removal. Thiol-modified biochar with improved physicochemical properties was successfully synthesized by the ball milling method. In specific, ball milling method improved the surface area of BMS-biochar through both increasing the external surface area and opening the internal pore of biochar. The functional groups were increased significantly during the ball milling process and facilitated the 3-MPTS soldering onto the surface of biochar (e.g., through –OH, C–O and π–π bond). These

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changes have positive impacts on the interaction between BMS-biochar and mercury. Compared to the pristine biochar and CIS-biochar, BMS-biochar offered higher Hg2+ and CH3Hg+ adsorption ability (from 105.7 to 320.1 mg/g for Hg2+ and from 8.21 to 104.9 mg/g

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for CH3Hg+). Sorption kinetic and isotherm modeling results showed that the adsorption of

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Hg2+ and CH3Hg+ on the BMS-biochar was a monolayer adsorption on heterogeneous surfaces, and surface diffusion was the rate-limiting adsorption step for BMS-biochar.

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Characterization results suggested that ball milling mainly increased Hg2+ and CH3Hg+

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adsorption via enhancing surface adsorption, electrostatic attraction, ligand exchange, and surface complexation. Ball milling has been widely used in large scale production processes

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due to its low cost, flexibility, and simplicity. Cost analysis results showed that BMS-biochar offered lower cost than the pristine biochar and CIS-biochar (supporting information, section

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1). Moreover, BMS-biochar can be regenerated by 5% thiourea and 2 M KI solution. Therefore, ball milling technology may introduce promising and innovative technological breakthroughs to benefit society, especially with respect to improving energy and environmental sustainability. Further work should be carried out to study the effects of natural organic matter and the optimization conditions for BMS-biochar reuse. This work 19

provides a dexterous method for the synthesis of thiol-modified carbon materials.

Acknowledgements This work was supported by the Hebei Outstanding Youth Science Foundation [D2019202453]; National Natural Science Foundation of China [41807363, U1806216]; Key

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Laboratory of Original Agro-Environmental Pollution Prevention and Control, Ministry of Agriculture/Tianjin Key Laboratory of Agro-environment and Safe product [18nybcdhj-1, 18nybcdhj-5]; the National Key Research and Development Program of China

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[2018YFC1802002]; 111 program, Ministry of Education of China [T2017002]; and Key

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na

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Project Natural Science Foundation of Tianjin [18JCZDJC39800].

20

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(PFOA) using starch-stabilized magnetite nanoparticles, Sci. Total Environ. 562 (2016) 191-200.

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[35] B. Chen, Z. Chen, Sorption of naphthalene and 1-naphthol by biochars of orange peels

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with different pyrolytic temperatures, Chemosphere 76 (2009) 127-133. [36] R. Peng, G. Chen, F. Zhou, R. Man, J. Huang, Catalyst-free synthesis of triazine-based porous organic polymers for Hg2+ adsorptive removal from aqueous solution, Chem. Eng.

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[37] W. Wang, M. Chen, X. Chen, J. Wang, Thiol-rich polyhedral oligomeric silsesquioxane

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as a novel adsorbent for mercury adsorption and speciation, Chem. Eng. J. 242 (2014) 62-68.

[38] X. Xe, Y. Lu, H. Meng, C. Li, Structure and adsorptive property of carbon materials derived from thermal and mechanochemical reaction of CaC2 and chlorinated polymers, Chem. Eng. J. 372 (2019) 181-190.

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[39] N. Saman, K. Johari, S.S. Tien, H. Mat, Removal of Hg(II) and CH3Hg(I) Using Rasped Pith Sago Residue Biosorbent, Clean-Soil Air Water 42 (2014) 1541-1548. [40] G. Li, M. Liu, Z. Zhang, C. Geng, Z. Wu, X. Zhao, Extraction of methylmercury and ethylmercury from aqueous solution using surface sulfhydryl-functionalized magnetic mesoporous silica nanoparticles, J. Colloid Interf. Sci. 424 (2014) 124-131. [41] T.A. Saleh, M. Tuzen, A. Sari, Polyamide magnetic palygorskite for the simultaneous removal of Hg(II) and methyl mercury; with factorial design analysis, J. Environ. Manage. 211 (2018) 323-333.

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[42] G. Shao, H. Yao, A. Liu, Z. Zhang, J. Huang, S. Yuan, Polyphenylene sulfide-based adsorption resins: synthesis, characterization and adsorption performance for Hg(II) and

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-p

As(V), Polym. Advan. Technol. 28 (2017) 1735-1742.

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26

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-p

re

lP

na

ur

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Figure captions Fig. 1. SEM images (a-c), TG curves (d), FTIR sprctra (e), and zeta potential (f) of biochar, CIS-biochar, and BMS-biochar. Fig. 2. XPS spectra of C1s (a-c) and S2p (d-f) for biochar, CIS-biochar, and BMS-biochar. Fig. 3. Effects of ball milling conditions (time and mass ratio) on Hg2+ (a) and CH3Hg+ (b) adsorption by BMS-biochar. (c and d) Hg2+ and CH3Hg+ removal efficiencies from

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aqueous solution by the pristine biochar, CIS-biochar, and BMS-biochar.

Fig. 4. Sorption kinetics (a and b), pseudo-first order kinetic model fittings (c and d), and pseudo-second order kinetic model fittings (e and f) of Hg2+ and CH3Hg+.

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Fig. 5. Isotherms of Hg2+ and CH3Hg+ adsorption onto biochar, CIS-biochar, and

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BMS-biochar.

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na

BMS-biochar.

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Fig. 6. Illustration of governing mechanisms of Hg2+ and CH3Hg+ adsorption on

27

Figures (b) CIS-biochar

(c) BMS-biochar

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(a) Biochar

(d)

100

Biochar CIS-biochar BMS-biochar

-p

60

40

re

Weight (%)

80

20

0

lP

0

100

200

300

400

500

600

700

800

(e)

4000

na

1724 C=O

3500

Zeta potential (mV)

BMS-biochar 2927 2595 C-H -SH 1700

1442 -OH

C=O

3000

2500

1513 -OH

2000

1500 -1

Wavenumber (cm )

799 Si-O

1040 C-O

442 Si-O

1000

Biochar CIS-biochar BMS-biochar

0

1602 C=C

ur

3389 -OH

(f)

10

CIS-biochar

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Transmittance(a.u.)

Biochar

500

900

Temperature (C)

-10 -20 -30 -40

1

2

3

4

5

6

7

8

9

10

pH

Fig. 1. SEM images (a-c), TG curves (d), FTIR sprctra (e), and zeta potential (f) of biochar, CIS-biochar, and BMS-biochar.

28

900

70000

(d) Biochar S2p

(a) Biochar C1s 880

60000

C-C/C=C 284.6 eV 54.3%

40000

860

Intensity (a.u.)

Intensity (a.u.)

50000

C-O 286.1 eV 37.1%

30000

O-C=O/C=O 288.5 eV 8.6%

20000 10000

S oxidation products 168.9 eV 14.2%

C-S/C=S 164.3 eV 85.8%

840 820 800 780 760

0

294

292

290

288 286 284 Binding energy (eV)

282

740 174

280

168

166

164

162

160

1600

50000

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(e) CIS-biochar S2p

(b) CIS-biochar C1s 1400

40000

C-O 286.0 eV 25.6%

20000

O-C=O/C=O 288.0 eV 3.2%

10000

1200

1000

S2p3/2 (-SH) 162.9 eV 61.4%

C-S/C=S 164.3eV S oxidation products 21.4% 168.1 eV 17.2%

800

-p

30000

C-C/C=C 284.3 eV 71.2%

Intensity (a.u.)

Intensity (a.u.)

170

Binding energy (eV)

60000

600

0

292

290

288

286

284

282

110000

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C-O 286.1 eV 34.8% O-C=O/C=O 287.4 eV 19.9%

ur

60000

294

C-C/C=C 284.5 eV 45.3%

na

70000

292

290

168

166

164

162

160

158

Bingding energy (eV)

(f) BMS-biochar S2p

100000

80000

170

3400

(c) BMS-biochar C1s

90000

172

280

re

Binding energy (eV)

Intensity (a.u.)

294

Intensity (a.u.)

172

288

286

284

3000

S2p3/2 (-SH)

2800

163.5 eV 69.7%

C-S/C=S 165.2eV 30.3%

2600 2400 2200 2000 1800

282

280

172

170

168

166

164

162

160

Bingding energy (eV)

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Bingding energy (eV)

3200

Fig. 2. XPS spectra of C1s (a-c) and S2p (d-f) for biochar, CIS-biochar, and BMS-biochar.

29

(a)

(b)

100

CH3Hg+ removal rate (%)

80

60

40

Biochar:balls = 1:20 Biochar:balls = 1:50 Biochar:balls = 1:100

20

80

60

40 Biochar:balls = 1:20 Biochar:balls = 1:50 Biochar:balls = 1:100

20

0

0

6

12

18

24

30

36

42

48

0

6

12

Ball milling time (h)

18

24

30

36

42

50 60

40 30

40

Hg

20 20

10

0

70

30

0

20

Biochar

BMS-biochar

CIS-biochar

10 0

BMS-biochar

re

CIS-biochar

30

40

10

50 40

50

0 Biochar

Surface area

60

20

60

+

CH3Hg removal rate

ro of

Surface area

CH3Hg removal rate (%)

2

60

2+

removal rate (%)

80

80

-p

2+

Hg removal rate

70

(d)

+

100

70

Surface area of the adsorbents (m /g)

90

(c)

48

Ball milling time (h)

2

0

Surface area of the adsorbents (m /g)

Hg2+ removal rate (%)

100

Fig. 3. Effects of ball milling conditions (time and mass ratio) on Hg2+ (a) and CH3Hg+ (b)

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adsorption by BMS-biochar. (c and d) Hg2+ and CH3Hg+ removal efficiencies from aqueous

Jo

ur

na

solution by the pristine biochar, CIS-biochar, and BMS-biochar.

30

100

100

(b)

(a) 80 CH3Hg removal (%)

2+

CIS-biocar

60

+

Hg removal (%)

BMS-biochar 80

40

Biochar

BMS-biochar

60

CIS-biocar

40

Biochar

20

20

0

0 0

6

12

18

24 30 Time (h)

36

42

0

48

6

4

+

2+

3

ln(qe-q)

2

2

1

0 0 24

30

36

42

Time (h) 1.2 2+

42

48

6

12

18

24

30

36

42

48

Time (h)

12

+

Pseudo-second order

1.0

(f) CH3Hg Pseudo-second order

na

.6

t/q (h/(mg/g))

10

.8

t/q (h/(mg/g))

0

lP

(e) Hg

48

re

18

36

Biochar CIS-biochar BMS-biochar

1

12

30

Pseudo-first order

-p

ln(qe-q)

3

6

24

ro of

(d) CH3Hg

Biochar CIS-biochar BMS-biochar

4

0

18

Time (h)

(c) Hg Pseudo-first order

5

12

.4

Biochar CIS-biochar BMS-biochar

.2

-.2 0

ur

0.0

6

12

18

24

Biochar CIS-biochar BMS-biochar

6

4

2

0

36

42

48

0

6

12

18

24

30

36

42

48

Time (h)

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Time (h)

30

8

Fig. 4. Sorption kinetics (a and b), pseudo-first order kinetic model fittings (c and d), and pseudo-second order kinetic model fittings (e and f) of Hg2+ and CH3Hg+.

31

100 400

Langmuir model Freundlich model

(a) Hg2+

(b) CH3Hg

350

+

Langmuir model Freundlich model

80

BMS-biochar

BMS-biochar

200

CIS-biochar

60

CIS-biochar

e

250

q (mg/g)

qe (mg/g)

300

40

150 100

Biochar

20

50

Biochar 0

0 0

2

4

6

8

10

0.0

.5

1.0

1.5

2.0

2.5

3.0

Ce (mg/L)

Ce (mg/L)

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Fig. 5. Isotherms of Hg2+ and CH3Hg+ adsorption onto biochar, CIS-biochar, and

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lP

re

-p

BMS-biochar.

32

Larger surface area 3-MPTS

Hg2+

More functional groups

Open Pore

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More negatively charged surface

Blocked Pore

CH3Hg+

re

-p

BMS-biochar

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Fig. 6. Illustration of governing mechanisms of Hg2+ and CH3Hg+ adsorption onto

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BMS-biochar.

33

Table 1 Physiochemical properties of biochar, CIS-biochar, and BMS-biochar.

Sample

BET specific surface area (m2/g)

BJH pore size (nm)

BJH pore volume (cm3/g)

C%

N%

O%

S%

Si%

O/C

(O+N) /C

1.83

7.76

0.003

77.47 ±2.11

0.50±0.0 01

20.96± 0.43

0.17±0 .001

0.90±0 .001

0.27±0 .002

0.28±0 .002

CIS-bio char

5.21

5.01

0.006

75.36 ±1.02

0.51±0.0 00

21.88± 0.02

0.62±0 .001

1.63±0 .001

0.29±0 .001

0.30±0 .001

BMS-bi ochar

61.34

19.05

0.291

66.53 ±1.59

0.77±0.0 01

28.79± 0.19

0.79±0 .002

3.13±0 .001

0.43±0 .002

0.44±0 .002

Jo

ur

na

lP

re

-p

ro of

Biochar

34

Table 2 Reported Hg2+ and CH3Hg+ adsorption capacities of various sorbents. Contamina qm nts (mg/g)

Initial concentration (mg/L)

pH

Dosage (g/L)

Equilibrium time (h)

Reference

BMS-biochar

Hg

320.1

1.0-25

7.0

0.04

48

This work

CH3Hg+

104.9

0.1-8.0

7.0

0.04

48

Na2S modified biochar

Hg

5.71

0.01-0.50

6.0

0.3

24

[1]

calcium carbide-derived carbon materials

Hg

291.2

50-250

5.0

2.0

5

[36]

triazine-based porous organic polymers

Hg

229.9

6-529

4.0

0.4

0.67

[37]

PPS‐ based mercapto resin

Hg

210.6

500

2.0

12

[38]

Polyamide magnetic palygorskite

Hg

211.93

10-400

7.0

20.0

0.5

[39]

CH3Hg+

159.7

10-400

7.0

20.0

0.45

Na-montmorillon ite

Hg

57.7

200

7.0

1.0

48

49.1

230

4.0

1.0

48

14.4

0.1-1

7.0

0.03

0.03

[41]

[42]

-p

lP

re

/

[40]

Jo

ur

surface CH3Hg+ sulfhydryl-functi onalized magnetic mesoporous silica nanoparticles

na

CH3Hg+

ro of

Adsorbent

Thiol-rich polyhedral oligomeric silsesquioxane

Hg

12.9

0.0005

6.0

2.0

0.33

MeHg

46.73

0.0005

6.0

2.0

0.33

35