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:
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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.
MgO modified biochar produced through ball milling: A dual-functional adsorbent for
removal of different contaminants
Yulin Zhenga, Yongshan Wanb, Jianjun Chenc, Hao Chend, Bin Gaoa,*
32611, United States
Center for Environmental Measurement and Modeling, US EPA, Gulf Breeze, FL 32561, USA
Mid-Florida Research and Education Center and Department of Environmental Horticulture,
University of Florida, Apopka, FL 32703, United States
Department of Agricultural and Biological Engineering, University of Florida, Gainesville, FL
Department of Agriculture, University of Arkansas at Pine Bluff, AR 71601, United States
* Corresponding author, phone: (352) 294-6746, Fax: (352) 392-4092, email: [email protected]
Abstract A facile ball-milling method was developed to synthesize MgO/biochar nanocomposites as a
dual-functional adsorbent. The physicochemical properties of the synthesized nanocomposites
indicated that the composites achieved nano-scaled morphologies and mesoporous structure with
MgO nanoparticles, which is approximate 20 nm and dispersed uniformly on the surface of the
biochar matrix. Batch sorption experiments yielded 62.9% removal of phosphate, an anion, and
87.5% removal of methylene blue, a cationic organic dye, at low adsorbent dosages of 1.0 g L-1
and 0.2 g L-1, respectively. This work indicates that ball milling, as a facile and promising
method for synthesis of carbon-metal oxide nanocomposites, lends the advantage of operational
flexibility and chemical adjustability for targeted remediation of diverse environmental
Keywords: Ball milling, nanocomposites, biochar, metal oxide, methylene blue, phosphate
Biochar is porous pyrogenic carbon material, that mainly carbonized from lignocellulosic
biomass through thermal or hydrothermal process (Meyer et al., 2011). It was initially applied as
a soil amendment for carbon sequestration (Lehmann, 2007). In environmental remediation,
biochar has also been extensively investigated as cost-efficient adsorbent to remove both organic
and inorganic pollutants from aqueous and gaseous environmental media (Creamer and Gao,
2016; Zhang et al., 2017; Wang et al., 2018a; Yang et al., 2019b). However, the adsorption
capacity of pristine biochar is limited and its physiochemical properties (active sorption sites,
surface area and surface charge) vary in feedstock types and production conditions (Li et al.,
2018). Moreover, the surfaces of most pristine biochars are often negatively charged in
association with its abundant oxygen-containing function groups, thereby exhibiting specific
sorption to cations (e.g., heavy metals ions) (Yang et al., 2019b). However, sorption toward
anionic species (i.e., oxyanion, anionic dyes and organics) is limited (Tan et al., 2016b).
Therefore, it is crucial to develop engineered methods to control the physiochemical properties
of biochar to enhance its sorption performance on various environmental contaminants.
Previous studies have revealed that natural minerals and metal oxides such as periclase
(MgO), quartz (SiO2) and calcite (CaCO3), play an important role in enhancing surface
properties and sorption performance of pristine biochar (Yao et al., 2011; Xu et al., 2013).
Chemical modification by producing metal oxides/hydroxides on the biochar surface can
enhance its surface properties by enlarging surface areas, adding more sorption sites, and
adjusting its net surface charges (Li et al., 2017; Creamer et al., 2018; Yang et al., 2019b; Zhang
et al., 2019). Meanwhile, incorporation of metal oxides/hydroxides can provide synergistic
effects on sorption performance of biochar composites via various mechanisms, including pore
filling, intra-particle diffusion, π–π interaction, surface complexation and electrostatic attraction,
precipitation, redox and photocatalytic degradation, ligand exchange, and ion exchange (Zhang
et al., 2012b; Yao et al., 2013; Dai et al., 2014; Lyu et al., 2017b). A series of engineered biochar
products have been developed with incorporation of ultrafine and nano-size metal
oxides/hydroxides. The most common synthesis approach is through slow pyrolysis of pretreated
biomass impregnated with metal salt solutions. Metallic and polymetallic oxide/hydroxide
nanoparticles such as MgO, AlOOH, CaO, MnOx, and ZnO have been successfully fabricated on
biochar surface through thermal oxidation of their corresponding metal salts (Zhang et al., 2012a;
Agrafioti et al., 2014; Song et al., 2014; Yu et al., 2018; Zheng et al., 2019a). For example,
different forms of nanoscale iron or ferric oxide particles can be distributed and stabilized on the
biochar surface to achieve magnetic properties (Chen et al., 2011; Zhang et al., 2013; Wang et al.,
2015). Those metal oxides/biochar composites often exhibit higher Brunauer-Emmett-Teller
(BET) surface area, larger pore volume, and higher sorption capacities toward heavy metals,
organic and inorganic pollutants than pristine biochars (Tan et al., 2016a). Another synthesis
approach to manipulate the fabrication of metal oxides/hydroxides on pristine biochar surface is
through wet-chemistry involving precipitation (e.g., layered double hydroxides) and stabilization
(e.g., nano-sized metal oxides) followed by evaporation and heat treatment (Wang et al., 2016;
Wan et al., 2017; Wang et al., 2018b; Yang et al., 2019a). These post-treatment methods are
beneficial for enhanced controls on synthesis of biochar composited with polymetallic oxides or
hydroxides in spite of relative reductions of the surface areas (Li et al., 2018). However, both
above-mentioned synthesis approaches involve solvent treatment and energy consumption for
drying and dewatering. Substantial amounts of metallic chemicals can be lost in the synthesis
process and potential chemical release into aqueous environment is highly possible. Furthermore,
difficulties with quality control under large-scale industrial production can be another limiting
factor for metal oxides to be effectively formed through thermal or chemical modifications. An
alternative approach is therefore highly needed.
In recent years, ball-milling technology has emerged as an economic, environmentally
friendly, and solvent-free approach for advanced material synthesis of carbon/metal oxides based
nanomaterials (Lyu et al., 2017a). For example, Hasa et al. utilized ball milling to produce metal
oxide-carbon composite as anode for electronic energy storage (Hasa et al., 2015). Shan et al.
investigated ball milling for producing magnetic biochar and activated carbon to adsorb organic
pollutants (Shan et al., 2016). We postulate that two functionalities can be achieved with ball
milling of metal oxides/biochar composites. On one hand, ball milling, as a promising top down
method, can effectively reduce the particle size of metal oxides, thereby producing highly
functional metal oxides nanoparticles (Wu, 2001). On the other hand, ball milling may modify
pristine biochar by exposing additional organic functional groups and active sorption sites as
well as enlarging surface areas and pore volumes (Lyu et al., 2018; Xu et al., 2019). However,
there are no reports about ball milling of metal oxides/biochar composites in terms of their
characteristics and adsorption mechanisms.
The overarching objective of this study is to present a new synthesis of biochar/metal oxide
composites through a solvent-free ball-milling method. Hickory wood biochar and magnesium
oxide (MgO) particles were selected as the feedstock material to produce ball-milled
MgO/biochar composites. The resultant composites then evaluated as a dual-functional adsorbent
to remove both cationic and anionic pollutants. Specifically, this study consists of the following:
(1) the pristine biochar was ball milled with varying amount of MgO particles (10%, 25% and
50%); (2) the resultant composite with 50% MgO was characterized for its physicochemical
properties; and (3) the sorption performance of the MgO/biochar composites to both methylene
blue and phosphate (representative cationic and anionic pollutants, respectively) were evaluated.
2. Experimental Section
Hickory wood chip was ground, sieved and oven-dried (80 °C) as the feedstock of pristine
biochar. All the analytical grade chemicals were purchased from Fisher Scientific and used as
received. Deionized (DI) water (18.2 MΩ) was used for the preparation of experimental solutions.
2.2 Preparation of MgO-biochar nanocomposites
Pristine biochar was produced by pyrolyzing the prepared feedstock in a tubular furnace for 1
h at 600 °C under N2 atmosphere (Zheng et al., 2019a). The resultant sample was washed to
remove ash and impurities. The biochar sample was then oven-dried and prior stored as HC for
To produce MgO-biochar composites, 1.8 g of the HC and different amount of MgO particles
(10%, 25% and 50% wt/wt) were ball milled with 500 rpm for 12 h with the rotating direction
altered every 3 h. The produced MgO-biochar composites were labeled as BMMg10, BMMg25,
and BMMg50, where the numbers represent the percentages of MgO particles in the composites).
For the sake of characteristics and comparative studies, HC and pure MgO nanoparticles were
also processed with the same ball milling conditions and resultant samples were labeled as
BMHC and BMMgO. The latter two ball-milled samples were simply mixed with a 1:1 ratio and
the mixture was labeled as BMmix.
2.3 Characterization of biochar/MgO nanocomposites
BMMg50 was used as a representative for all the characterizations. BMHC, BMMgO, and
BMmix were included in some of the analyses for comparison purposes. The morphology and 6
element composition of the biochar was observed by SEM-EDX (FEI Nova NanoSEM 430). The
X-ray diffraction (XRD) was employed to determine the crystallographic structure in the
composite with a scanning rate of 0.2°/s for the 2θ angle range of 10° - 80°. Thermogravimetric
analysis was performed by TGA/DSC1 analyzer in 25 mL/min air stream with heating rate of
5 °C/min from 25 °C to 800 °C. XPS (PHI 5100 series ESCA spectrometer, PerkinElmer)
determine the elemental composition and chemical state of sample surfaces. The BET was used
to test pore size distribution and pore volume based on N2 adsorption-desorption at 77 K using
Quantachrome Autosorb-1. Hydrodynamic radii of the composites were determined using a
Malvern nanosizer Nano Instrument. Fourier transformed infrared spectroscopy (FTIR) (Thermo
Electron Magna 760, USA) determines the chemical shift and functional groups changes.
2.4 Adsorption performance of biochar/MgO nanocomposites
Methylene blue removal capacity by biochar/MgO nanocomposites was determined with 20
mg of each sample added to 100 mL of 100 mg L-1 MB solution. They were transferred on a
platform shaker and agitated for 24 h to reach equilibrium (at 25 ± 2 °C). Then, the mixtures
were immediately filtered through 0.22 µm nylon membrane filters, and MB concentration of the
filtrates were measured by UV–Vis spectrophotometer at wavelength of 665 nm.
Similarly, the sorption of phosphate to nanocomposites was determined by 50 mg of each
sample at 50 mL of 20 mg L-1 solution as total phosphorous (P). After filtration process, the P
concentrations were immediately measured by inductively coupled plasma optical emission
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
reported. Additional experiments were carried out whenever 5% difference in measurement
3. Results and Discussion
3.1 Crystalline and morphological structures of MgO-biochar nanocomposites
The facile ball-milling synthesis method successfully produced MgO/biochar
nanocomposites under solvent-free conditions. Fig. 1a presents the XRD patterns of BMMg50
with several diffraction peaks, which are similar with the peaks of pure MgO nanoparticles,
attributed to (111), (200), (220), (311) and (222) lattice planes of periclase (MgO). A broad peak
also displayed in BMMg50 around 2 theta of 23°, represents the typical pattern of the amorphous
plane of biochar (Li et al., 2017). The appearance of these two patterns further indicates the
formation of MgO-biochar nanocomposites by the ball-milling method. The XRD results also
confirm the ball-milling method did not change the chemical state of MgO (Fig. S1). The Debye-
Scherrer equation was applied to calculate the average crystalline sizes of MgO particles in
BMMg50 and BMMgO. These results indicate ball milling with biochar reduced the MgO
particle size from 80 nm to around 20 nm (Table 1). The resultant MgO nano-crystalline in this
study is similar to that in MgO modified biochar produced from the pyrolysis method (Zhang et
The surface morphology and elemental content of the MgO-biochar nanocomposites were
observed by SEM-EDX analysis. Fig. 2a shows a SEM image of BMMg50 in a specific region,
which clearly displays its nano-sized property. An EDX spectrum (Fig.2 b) of this specific region
presents the surface elemental content of C, Mg, O (47.7, 29.8, and 21.8%) and trace amount of
Si (0.7%). A high-resolution SEM shown in Fig.2c was selected for mapping of the elemental 8
distribution of C, O, and Mg (Fig.2d - f). The results are consistent with the EDX spectra. While
Mg was closely associated with O distribution, C was distributed more uniformly in the selected
area. Moreover, EDX analysis and the distribution map also confirm that the distribution of MgO
nanoparticles in the composite matrix was highly homogeneous.
168 169 170
3.2 Surface physicochemical analysis of MgO-biochar nanocomposites XPS was used to investigate the surface characteristics of the MgO-biochar nanocomposites.
The elemental composition from the XPS survey is consistent with the EDS analysis. As it
shows in Fig. 1b, the high-resolution C1s spectrum of BMMg50 displayed as three typical sp2
C=C, C–O and C=O bonds, which centered at 284.8, 286.1 and 288.7 eV, respectively. There
was no obvious binding energy shift on the C1s narrow scan between BMMg50 and BMmix (Fig.
S2). Fig. 1c presents the Mg 2p spectrum of sample BMMg50, showing a single intense peak at
50.8 eV corresponding to MgO (Fournier et al., 2002). This Mg 2p spectrum revealed a
noticeable shift compared with that of BMmix (Fig. S3), indicating the ball-milling process
could cause intrinsic change on the surface chemistry of the nanocomposite instead of simple
mixing. The high resolution O 1s spectrum of BMMg50 (Fig. 1d) was separated into two peaks
at 531.0 and 533.1 eV, corresponding to lattice oxygen of metallic oxide in the form of MgO and
the surface oxygen containing functional groups, respectively (Ardizzone et al., 1997; Zheng et
al., 2019b). The slight shifting on the O 1s spectra of BMM50 and BMmix (Fig. S4) further
confirms ball milling might trigger chemical interactions between MgO and biochar in the
nanocomposite. During ball milling, material surfaces slide against each other like a frictional
process, which can transfer a substantial amount of mechanical energy to heat to form hot spots
at local and bulk scales (James et al., 2012). The high temperature at the hot spots may prompt
localized changes and formation of bonds between molecules to form composites (Kaupp, 2009).
The surface area analysis quantitatively demonstrated the porous structure of MgO-biochar
nanocomposites (Table 1). The BET surface area of biochar increased from 249.7 m2/g to 310.7
m2/g after ball milling, confirming the ball-milling process can enlarge the biochar surface area
(Zhang et al., 2019). The BET specific surface area of ball-milled MgO was 4.3 m2/g, much
lower than that of the pristine or ball-milled biochar. As a result, the specific surface area of
BMMg50 only reached 140.0 m2/g, which is equivalent to a normalized specific surface area of
275.7 m2/g of biochar. This suggests that ball milling with MgO particle affected the pore
structure and surface area of the biochar matrix. Further, it also indicates that the part of the
MgO nanoparticles might be crushed into the pores of biochar to reduce the overall specific
surface area of the nanocomposite. During ball milling, both biochar and MgO were broken
down into smaller particles, resulting in enlarged surface areas of both and newly opened pores
and cracks in biochar. Nanosized MgO would have strong tendency to penetrate and hot-extrude
into the pores and cracks of the biochar (Kwon and Leparoux, 2012).
The characteristic stretches of samples were compared and analyzed by FTIR spectra (Fig.
S5). BMHC displayed a significant increase in the O-containing function groups comparing with
pristine biochar, as indicated in the stretching peaks of O-H at 3625 cm-1, breathing of aromatic
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
(Zheng et al., 2019b). BMM50 achieved a similar spectrum as BMHC, confirmed that the import
of MgO nanoparticles in the composites did not have notable effect on biochar’s surface
TGA was employed to further identify the thermal characteristics of MgO-biochar
nanocomposites. As shown in Fig. S6, BMMg50 exhibited less thermal decomposition and lower
weight loss than BMHC, indicating that incorporation of MgO nanoparticles increased the
thermal stability of the composite matrix (Yang et al., 2016). Fig. S6 also presents the different
decomposition pattern between BMMg50 and BMMix, indirectly showing the property change
on the MgO-biochar nanocomposites produced by the ball-milling method.
214 215 216
3.4 Dual-functional adsorption performance of MgO-biochar nanocomposites The adsorption performance of MgO/biochar nanocomposites is shown in Fig. 3. The pristine
biochar HC had limited adsorption for MB, and even negative adsorption for P due to trace
amount released from itself. The ball-milling process made no difference on P adsorption
performance of BMHC; however, it enhanced the adsorption of MB for about 8.4 time, likely
due to increased surface area and pore volume. When MgO was ball milled into the biochar, the
resultant MgO/biochar nanocomposites exhibited significantly increased adsorption on both P
and MB. Enhanced P adsorption can be attributed to the strong Mg-P bond and electrostatic
attraction between anionic P and positively charged MgO surface under tested conditions (Zhang
et al., 2012a). The P removal capacity also increased with the increasing MgO content in the
composite. As shown in Fig. S7, SEM image on P post-adsorption sample exhibits the formation
of surface Mg-P crystal flasks and EDX analysis reveals the appearance of P spectrum. These
findings also suggest that MgO/biochar nanocomposites possessed positively charged surface
properties to facilitate adsorption of P anions through electrostatic attraction and surface
precipitation (Yao et al., 2013). The MB uptake of MgO/biochar nanocomposites was about
61.5 %– 108.8% higher than that of BMHC. Enhanced MB adsorption on with MgO addition can
be understood in a context of increased basic buffering capacity of biochar matrix, which is
favorable to the removal of cationic MB vis electrostatic attractive force under higher pH
conditions (Zhang et al., 2012a; Li et al., 2016). Among the three MgO/biochar nanocomposites
samples, MB adsorption first increased and then decreased with the MgO content, suggesting
there is an optimal ratio between MgO and biochar for contaminant removal. In this study, the
MgO/biochar nanocomposite at the 1:1 ratio exhibited the highest P removal efficiency along
with a reasonably high capacity of MB adsorption.
Dual-functional MgO-biochar nanocomposites were successfully synthesized by a novel and
facile method that ball milled hickory biochar with MgO particles under a solvent-free condition.
The MgO-biochar nanocomposites exhibited effective and synergistic removal of methylene blue
and phosphate from aqueous solutions. This study indicates that the ball-milling method offers
both simplicity and flexibility in chemical and material synthesis which can be employed for
strategic environmental remediation of both organic and inorganic pollutants. Compared with
other synthesis methods, ball milling has the advantage of full incorporation of the synthesis
ingredients (metal oxide/hydroxides) in the biochar matrix or other carbon-based nanomaterials.
This can help develop novel adsorbents for target pollutants to suit for specific circumstances.
The views expressed in this article are those of the authors and do not necessarily reflect the
views or policies of the U.S. Environmental Protection Agency.
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synthesis of graphitic carbon nitride-modified biochar for the removal of reactive red 120
through adsorption and photocatalytic degradation. Biochar 1-8.
Table 1. Some physicochemical properties of biochar and MgO/biochar nanocomposites before
and after ball milling. Samples HC
MgO average size (nm) -
BET surface area (m2/g) 249.7
BJH pore volume (cm3/g) 0.112
Fig. 1. (a) XRD spectra of Mg/biochar nanocomposite (BMMg50) and pure MgO nanoparticles.
High resolution XPS spectra of C 1s (b), Mg 2p (c) and O 1s (d) regions of Mg/biochar
Fig. 2. (a) SEM image and (b) EDX spectrum pattern of the selected area of BMMg50. (c) SEM
image for specific region of MgO/biochar nanocomposite on the analysis of carbon (d),
magnesium (e) and oxygen (f) distribution map.
388 389 21
Fig. 3. Removal of methylene blue (a) and phosphate (b) from aqueous solutions by various
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: