MgO modified biochar produced through ball milling: A dual-functional adsorbent for removal of different contaminants

MgO modified biochar produced through ball milling: A dual-functional adsorbent for removal of different contaminants

Journal Pre-proof MgO modified biochar produced through ball milling: A dual-functional adsorbent for removal of different contaminants Yulin Zheng, Y...

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Journal Pre-proof MgO modified biochar produced through ball milling: A dual-functional adsorbent for removal of different contaminants Yulin Zheng, Yongshan Wan, Jianjun Chen, Hao Chen, Bin Gao PII:

S0045-6535(19)32584-6

DOI:

https://doi.org/10.1016/j.chemosphere.2019.125344

Reference:

CHEM 125344

To appear in:

ECSN

Received Date: 30 September 2019 Revised Date:

4 November 2019

Accepted Date: 8 November 2019

Please cite this article as: Zheng, Y., Wan, Y., Chen, J., Chen, H., Gao, B., MgO modified biochar produced through ball milling: A dual-functional adsorbent for removal of different contaminants, Chemosphere (2019), doi: https://doi.org/10.1016/j.chemosphere.2019.125344. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Graphic Abstract

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MgO modified biochar produced through ball milling: A dual-functional adsorbent for

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removal of different contaminants

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Yulin Zhenga, Yongshan Wanb, Jianjun Chenc, Hao Chend, Bin Gaoa,*

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a

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32611, United States

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b

Center for Environmental Measurement and Modeling, US EPA, Gulf Breeze, FL 32561, USA

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c

Mid-Florida Research and Education Center and Department of Environmental Horticulture,

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University of Florida, Apopka, FL 32703, United States

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d

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

Department of Agriculture, University of Arkansas at Pine Bluff, AR 71601, United States

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_______________________

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* Corresponding author, phone: (352) 294-6746, Fax: (352) 392-4092, email: [email protected]

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Abstract A facile ball-milling method was developed to synthesize MgO/biochar nanocomposites as a

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dual-functional adsorbent. The physicochemical properties of the synthesized nanocomposites

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indicated that the composites achieved nano-scaled morphologies and mesoporous structure with

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MgO nanoparticles, which is approximate 20 nm and dispersed uniformly on the surface of the

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biochar matrix. Batch sorption experiments yielded 62.9% removal of phosphate, an anion, and

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87.5% removal of methylene blue, a cationic organic dye, at low adsorbent dosages of 1.0 g L-1

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and 0.2 g L-1, respectively. This work indicates that ball milling, as a facile and promising

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method for synthesis of carbon-metal oxide nanocomposites, lends the advantage of operational

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flexibility and chemical adjustability for targeted remediation of diverse environmental

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pollutants.

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Keywords: Ball milling, nanocomposites, biochar, metal oxide, methylene blue, phosphate

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1. Introduction

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Biochar is porous pyrogenic carbon material, that mainly carbonized from lignocellulosic

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biomass through thermal or hydrothermal process (Meyer et al., 2011). It was initially applied as

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a soil amendment for carbon sequestration (Lehmann, 2007). In environmental remediation,

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biochar has also been extensively investigated as cost-efficient adsorbent to remove both organic

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and inorganic pollutants from aqueous and gaseous environmental media (Creamer and Gao,

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2016; Zhang et al., 2017; Wang et al., 2018a; Yang et al., 2019b). However, the adsorption

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capacity of pristine biochar is limited and its physiochemical properties (active sorption sites,

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surface area and surface charge) vary in feedstock types and production conditions (Li et al.,

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2018). Moreover, the surfaces of most pristine biochars are often negatively charged in

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association with its abundant oxygen-containing function groups, thereby exhibiting specific

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sorption to cations (e.g., heavy metals ions) (Yang et al., 2019b). However, sorption toward

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anionic species (i.e., oxyanion, anionic dyes and organics) is limited (Tan et al., 2016b).

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Therefore, it is crucial to develop engineered methods to control the physiochemical properties

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of biochar to enhance its sorption performance on various environmental contaminants.

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Previous studies have revealed that natural minerals and metal oxides such as periclase

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(MgO), quartz (SiO2) and calcite (CaCO3), play an important role in enhancing surface

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properties and sorption performance of pristine biochar (Yao et al., 2011; Xu et al., 2013).

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Chemical modification by producing metal oxides/hydroxides on the biochar surface can

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enhance its surface properties by enlarging surface areas, adding more sorption sites, and

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adjusting its net surface charges (Li et al., 2017; Creamer et al., 2018; Yang et al., 2019b; Zhang

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et al., 2019). Meanwhile, incorporation of metal oxides/hydroxides can provide synergistic

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effects on sorption performance of biochar composites via various mechanisms, including pore

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filling, intra-particle diffusion, π–π interaction, surface complexation and electrostatic attraction,

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precipitation, redox and photocatalytic degradation, ligand exchange, and ion exchange (Zhang

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et al., 2012b; Yao et al., 2013; Dai et al., 2014; Lyu et al., 2017b). A series of engineered biochar

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products have been developed with incorporation of ultrafine and nano-size metal

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oxides/hydroxides. The most common synthesis approach is through slow pyrolysis of pretreated

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biomass impregnated with metal salt solutions. Metallic and polymetallic oxide/hydroxide

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nanoparticles such as MgO, AlOOH, CaO, MnOx, and ZnO have been successfully fabricated on

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biochar surface through thermal oxidation of their corresponding metal salts (Zhang et al., 2012a;

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Agrafioti et al., 2014; Song et al., 2014; Yu et al., 2018; Zheng et al., 2019a). For example,

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different forms of nanoscale iron or ferric oxide particles can be distributed and stabilized on the

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biochar surface to achieve magnetic properties (Chen et al., 2011; Zhang et al., 2013; Wang et al.,

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2015). Those metal oxides/biochar composites often exhibit higher Brunauer-Emmett-Teller

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(BET) surface area, larger pore volume, and higher sorption capacities toward heavy metals,

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organic and inorganic pollutants than pristine biochars (Tan et al., 2016a). Another synthesis

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approach to manipulate the fabrication of metal oxides/hydroxides on pristine biochar surface is

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through wet-chemistry involving precipitation (e.g., layered double hydroxides) and stabilization

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(e.g., nano-sized metal oxides) followed by evaporation and heat treatment (Wang et al., 2016;

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Wan et al., 2017; Wang et al., 2018b; Yang et al., 2019a). These post-treatment methods are

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beneficial for enhanced controls on synthesis of biochar composited with polymetallic oxides or

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hydroxides in spite of relative reductions of the surface areas (Li et al., 2018). However, both

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above-mentioned synthesis approaches involve solvent treatment and energy consumption for

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drying and dewatering. Substantial amounts of metallic chemicals can be lost in the synthesis

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process and potential chemical release into aqueous environment is highly possible. Furthermore,

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difficulties with quality control under large-scale industrial production can be another limiting

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factor for metal oxides to be effectively formed through thermal or chemical modifications. An

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alternative approach is therefore highly needed.

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In recent years, ball-milling technology has emerged as an economic, environmentally

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friendly, and solvent-free approach for advanced material synthesis of carbon/metal oxides based

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nanomaterials (Lyu et al., 2017a). For example, Hasa et al. utilized ball milling to produce metal

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oxide-carbon composite as anode for electronic energy storage (Hasa et al., 2015). Shan et al.

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investigated ball milling for producing magnetic biochar and activated carbon to adsorb organic

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pollutants (Shan et al., 2016). We postulate that two functionalities can be achieved with ball

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milling of metal oxides/biochar composites. On one hand, ball milling, as a promising top down

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method, can effectively reduce the particle size of metal oxides, thereby producing highly

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functional metal oxides nanoparticles (Wu, 2001). On the other hand, ball milling may modify

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pristine biochar by exposing additional organic functional groups and active sorption sites as

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well as enlarging surface areas and pore volumes (Lyu et al., 2018; Xu et al., 2019). However,

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there are no reports about ball milling of metal oxides/biochar composites in terms of their

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characteristics and adsorption mechanisms.

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The overarching objective of this study is to present a new synthesis of biochar/metal oxide

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composites through a solvent-free ball-milling method. Hickory wood biochar and magnesium

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oxide (MgO) particles were selected as the feedstock material to produce ball-milled

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MgO/biochar composites. The resultant composites then evaluated as a dual-functional adsorbent

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to remove both cationic and anionic pollutants. Specifically, this study consists of the following:

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(1) the pristine biochar was ball milled with varying amount of MgO particles (10%, 25% and

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50%); (2) the resultant composite with 50% MgO was characterized for its physicochemical

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properties; and (3) the sorption performance of the MgO/biochar composites to both methylene

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blue and phosphate (representative cationic and anionic pollutants, respectively) were evaluated.

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2. Experimental Section

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2.1 Materials

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Hickory wood chip was ground, sieved and oven-dried (80 °C) as the feedstock of pristine

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biochar. All the analytical grade chemicals were purchased from Fisher Scientific and used as

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received. Deionized (DI) water (18.2 MΩ) was used for the preparation of experimental solutions.

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2.2 Preparation of MgO-biochar nanocomposites

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Pristine biochar was produced by pyrolyzing the prepared feedstock in a tubular furnace for 1

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h at 600 °C under N2 atmosphere (Zheng et al., 2019a). The resultant sample was washed to

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remove ash and impurities. The biochar sample was then oven-dried and prior stored as HC for

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future use.

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To produce MgO-biochar composites, 1.8 g of the HC and different amount of MgO particles

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(10%, 25% and 50% wt/wt) were ball milled with 500 rpm for 12 h with the rotating direction

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altered every 3 h. The produced MgO-biochar composites were labeled as BMMg10, BMMg25,

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and BMMg50, where the numbers represent the percentages of MgO particles in the composites).

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For the sake of characteristics and comparative studies, HC and pure MgO nanoparticles were

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also processed with the same ball milling conditions and resultant samples were labeled as

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BMHC and BMMgO. The latter two ball-milled samples were simply mixed with a 1:1 ratio and

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the mixture was labeled as BMmix.

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2.3 Characterization of biochar/MgO nanocomposites

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BMMg50 was used as a representative for all the characterizations. BMHC, BMMgO, and

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BMmix were included in some of the analyses for comparison purposes. The morphology and 6

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element composition of the biochar was observed by SEM-EDX (FEI Nova NanoSEM 430). The

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X-ray diffraction (XRD) was employed to determine the crystallographic structure in the

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composite with a scanning rate of 0.2°/s for the 2θ angle range of 10° - 80°. Thermogravimetric

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analysis was performed by TGA/DSC1 analyzer in 25 mL/min air stream with heating rate of

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5 °C/min from 25 °C to 800 °C. XPS (PHI 5100 series ESCA spectrometer, PerkinElmer)

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determine the elemental composition and chemical state of sample surfaces. The BET was used

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to test pore size distribution and pore volume based on N2 adsorption-desorption at 77 K using

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Quantachrome Autosorb-1. Hydrodynamic radii of the composites were determined using a

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Malvern nanosizer Nano Instrument. Fourier transformed infrared spectroscopy (FTIR) (Thermo

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Electron Magna 760, USA) determines the chemical shift and functional groups changes.

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2.4 Adsorption performance of biochar/MgO nanocomposites

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Methylene blue removal capacity by biochar/MgO nanocomposites was determined with 20

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mg of each sample added to 100 mL of 100 mg L-1 MB solution. They were transferred on a

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platform shaker and agitated for 24 h to reach equilibrium (at 25 ± 2 °C). Then, the mixtures

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were immediately filtered through 0.22 µm nylon membrane filters, and MB concentration of the

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filtrates were measured by UV–Vis spectrophotometer at wavelength of 665 nm.

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Similarly, the sorption of phosphate to nanocomposites was determined by 50 mg of each

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sample at 50 mL of 20 mg L-1 solution as total phosphorous (P). After filtration process, the P

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concentrations were immediately measured by inductively coupled plasma optical emission

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spectroscopy (ICP-OES).

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For the adsorption studies, control treatments were conducted only with MB and P under identical conditions. All experiments were performed in replicate and the average values were

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reported. Additional experiments were carried out whenever 5% difference in measurement

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occurred.

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3. Results and Discussion

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3.1 Crystalline and morphological structures of MgO-biochar nanocomposites

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The facile ball-milling synthesis method successfully produced MgO/biochar

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nanocomposites under solvent-free conditions. Fig. 1a presents the XRD patterns of BMMg50

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with several diffraction peaks, which are similar with the peaks of pure MgO nanoparticles,

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attributed to (111), (200), (220), (311) and (222) lattice planes of periclase (MgO). A broad peak

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also displayed in BMMg50 around 2 theta of 23°, represents the typical pattern of the amorphous

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plane of biochar (Li et al., 2017). The appearance of these two patterns further indicates the

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formation of MgO-biochar nanocomposites by the ball-milling method. The XRD results also

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confirm the ball-milling method did not change the chemical state of MgO (Fig. S1). The Debye-

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Scherrer equation was applied to calculate the average crystalline sizes of MgO particles in

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BMMg50 and BMMgO. These results indicate ball milling with biochar reduced the MgO

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particle size from 80 nm to around 20 nm (Table 1). The resultant MgO nano-crystalline in this

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study is similar to that in MgO modified biochar produced from the pyrolysis method (Zhang et

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al., 2012a).

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The surface morphology and elemental content of the MgO-biochar nanocomposites were

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observed by SEM-EDX analysis. Fig. 2a shows a SEM image of BMMg50 in a specific region,

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which clearly displays its nano-sized property. An EDX spectrum (Fig.2 b) of this specific region

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presents the surface elemental content of C, Mg, O (47.7, 29.8, and 21.8%) and trace amount of

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Si (0.7%). A high-resolution SEM shown in Fig.2c was selected for mapping of the elemental 8

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distribution of C, O, and Mg (Fig.2d - f). The results are consistent with the EDX spectra. While

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Mg was closely associated with O distribution, C was distributed more uniformly in the selected

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area. Moreover, EDX analysis and the distribution map also confirm that the distribution of MgO

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nanoparticles in the composite matrix was highly homogeneous.

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3.2 Surface physicochemical analysis of MgO-biochar nanocomposites XPS was used to investigate the surface characteristics of the MgO-biochar nanocomposites.

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The elemental composition from the XPS survey is consistent with the EDS analysis. As it

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shows in Fig. 1b, the high-resolution C1s spectrum of BMMg50 displayed as three typical sp2

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C=C, C–O and C=O bonds, which centered at 284.8, 286.1 and 288.7 eV, respectively. There

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was no obvious binding energy shift on the C1s narrow scan between BMMg50 and BMmix (Fig.

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S2). Fig. 1c presents the Mg 2p spectrum of sample BMMg50, showing a single intense peak at

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50.8 eV corresponding to MgO (Fournier et al., 2002). This Mg 2p spectrum revealed a

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noticeable shift compared with that of BMmix (Fig. S3), indicating the ball-milling process

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could cause intrinsic change on the surface chemistry of the nanocomposite instead of simple

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mixing. The high resolution O 1s spectrum of BMMg50 (Fig. 1d) was separated into two peaks

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at 531.0 and 533.1 eV, corresponding to lattice oxygen of metallic oxide in the form of MgO and

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the surface oxygen containing functional groups, respectively (Ardizzone et al., 1997; Zheng et

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al., 2019b). The slight shifting on the O 1s spectra of BMM50 and BMmix (Fig. S4) further

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confirms ball milling might trigger chemical interactions between MgO and biochar in the

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nanocomposite. During ball milling, material surfaces slide against each other like a frictional

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process, which can transfer a substantial amount of mechanical energy to heat to form hot spots

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at local and bulk scales (James et al., 2012). The high temperature at the hot spots may prompt

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localized changes and formation of bonds between molecules to form composites (Kaupp, 2009).

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The surface area analysis quantitatively demonstrated the porous structure of MgO-biochar

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nanocomposites (Table 1). The BET surface area of biochar increased from 249.7 m2/g to 310.7

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m2/g after ball milling, confirming the ball-milling process can enlarge the biochar surface area

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(Zhang et al., 2019). The BET specific surface area of ball-milled MgO was 4.3 m2/g, much

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lower than that of the pristine or ball-milled biochar. As a result, the specific surface area of

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BMMg50 only reached 140.0 m2/g, which is equivalent to a normalized specific surface area of

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275.7 m2/g of biochar. This suggests that ball milling with MgO particle affected the pore

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structure and surface area of the biochar matrix. Further, it also indicates that the part of the

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MgO nanoparticles might be crushed into the pores of biochar to reduce the overall specific

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surface area of the nanocomposite. During ball milling, both biochar and MgO were broken

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down into smaller particles, resulting in enlarged surface areas of both and newly opened pores

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and cracks in biochar. Nanosized MgO would have strong tendency to penetrate and hot-extrude

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into the pores and cracks of the biochar (Kwon and Leparoux, 2012).

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The characteristic stretches of samples were compared and analyzed by FTIR spectra (Fig.

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S5). BMHC displayed a significant increase in the O-containing function groups comparing with

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pristine biochar, as indicated in the stretching peaks of O-H at 3625 cm-1, breathing of aromatic

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C=C and C=O at 1752 and 1620 cm-1, C-O peak at 1370 cm-1 and bending of C-H at 830 cm-1

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(Zheng et al., 2019b). BMM50 achieved a similar spectrum as BMHC, confirmed that the import

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of MgO nanoparticles in the composites did not have notable effect on biochar’s surface

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functional groups.

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TGA was employed to further identify the thermal characteristics of MgO-biochar

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nanocomposites. As shown in Fig. S6, BMMg50 exhibited less thermal decomposition and lower

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weight loss than BMHC, indicating that incorporation of MgO nanoparticles increased the

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thermal stability of the composite matrix (Yang et al., 2016). Fig. S6 also presents the different

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decomposition pattern between BMMg50 and BMMix, indirectly showing the property change

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on the MgO-biochar nanocomposites produced by the ball-milling method.

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3.4 Dual-functional adsorption performance of MgO-biochar nanocomposites The adsorption performance of MgO/biochar nanocomposites is shown in Fig. 3. The pristine

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biochar HC had limited adsorption for MB, and even negative adsorption for P due to trace

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amount released from itself. The ball-milling process made no difference on P adsorption

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performance of BMHC; however, it enhanced the adsorption of MB for about 8.4 time, likely

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due to increased surface area and pore volume. When MgO was ball milled into the biochar, the

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resultant MgO/biochar nanocomposites exhibited significantly increased adsorption on both P

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and MB. Enhanced P adsorption can be attributed to the strong Mg-P bond and electrostatic

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attraction between anionic P and positively charged MgO surface under tested conditions (Zhang

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et al., 2012a). The P removal capacity also increased with the increasing MgO content in the

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composite. As shown in Fig. S7, SEM image on P post-adsorption sample exhibits the formation

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of surface Mg-P crystal flasks and EDX analysis reveals the appearance of P spectrum. These

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findings also suggest that MgO/biochar nanocomposites possessed positively charged surface

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properties to facilitate adsorption of P anions through electrostatic attraction and surface

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precipitation (Yao et al., 2013). The MB uptake of MgO/biochar nanocomposites was about

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61.5 %– 108.8% higher than that of BMHC. Enhanced MB adsorption on with MgO addition can

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be understood in a context of increased basic buffering capacity of biochar matrix, which is

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favorable to the removal of cationic MB vis electrostatic attractive force under higher pH

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conditions (Zhang et al., 2012a; Li et al., 2016). Among the three MgO/biochar nanocomposites

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samples, MB adsorption first increased and then decreased with the MgO content, suggesting

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there is an optimal ratio between MgO and biochar for contaminant removal. In this study, the

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MgO/biochar nanocomposite at the 1:1 ratio exhibited the highest P removal efficiency along

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with a reasonably high capacity of MB adsorption.

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4. Conclusions

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Dual-functional MgO-biochar nanocomposites were successfully synthesized by a novel and

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facile method that ball milled hickory biochar with MgO particles under a solvent-free condition.

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The MgO-biochar nanocomposites exhibited effective and synergistic removal of methylene blue

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and phosphate from aqueous solutions. This study indicates that the ball-milling method offers

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both simplicity and flexibility in chemical and material synthesis which can be employed for

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strategic environmental remediation of both organic and inorganic pollutants. Compared with

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other synthesis methods, ball milling has the advantage of full incorporation of the synthesis

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ingredients (metal oxide/hydroxides) in the biochar matrix or other carbon-based nanomaterials.

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This can help develop novel adsorbents for target pollutants to suit for specific circumstances.

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Acknowledgments

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The views expressed in this article are those of the authors and do not necessarily reflect the

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views or policies of the U.S. Environmental Protection Agency.

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Tables

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Table 1. Some physicochemical properties of biochar and MgO/biochar nanocomposites before

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and after ball milling. Samples HC

MgO average size (nm) -

BET surface area (m2/g) 249.7

BJH pore volume (cm3/g) 0.112

BMHC

-

310.7

0.140

MgO nanoparticle

80.0

4.3

-

BMMg50

21.2

140.0

0.100

377

19

378

Figures

379 380

Fig. 1. (a) XRD spectra of Mg/biochar nanocomposite (BMMg50) and pure MgO nanoparticles.

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High resolution XPS spectra of C 1s (b), Mg 2p (c) and O 1s (d) regions of Mg/biochar

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nanocomposite, respectively.

383

20

384 385

Fig. 2. (a) SEM image and (b) EDX spectrum pattern of the selected area of BMMg50. (c) SEM

386

image for specific region of MgO/biochar nanocomposite on the analysis of carbon (d),

387

magnesium (e) and oxygen (f) distribution map.

388 389 21

390 391

Fig. 3. Removal of methylene blue (a) and phosphate (b) from aqueous solutions by various

392

biochar-based adsorbents.

22

Research highlights  Solvent-free synthesis of MgO/biochar nanocomposites was achieved through ball milling  MgO nanoparticles (20 nm) dispersed uniformly on the surface of the biochar matrix.  MgO/biochar nanocomposites showed dual functions to effectively adsorb cationic dye and anionic phosphate.  Ball milling method has the advantage of operational flexibility and chemical adjustability

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: