Preparation and characterization of a novel Fe3O4-graphene-biochar composite for crystal violet adsorption

Journal Pre-proofs Preparation and characterization of a novel Fe3O4-graphene-biochar composite for crystal violet adsorption Cong Du, Yonghui Song, Shengnan Shi, Bei Jiang, Jiaqi Yang, Shuhu Xiao PII: DOI: Reference:

S0048-9697(19)34653-4 https://doi.org/10.1016/j.scitotenv.2019.134662 STOTEN 134662

To appear in:

Science of the Total Environment

Received Date: Revised Date: Accepted Date:

20 June 2019 5 September 2019 24 September 2019

Please cite this article as: C. Du, Y. Song, S. Shi, B. Jiang, J. Yang, S. Xiao, Preparation and characterization of a novel Fe3O4-graphene-biochar composite for crystal violet adsorption, Science of the Total Environment (2019), doi: https://doi.org/10.1016/j.scitotenv.2019.134662

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Preparation and characterization of a novel Fe3O4-graphene-biochar composite for crystal violet adsorption Cong Dua,b, Yonghui Songa,*, Shengnan Shic, Bei Jiangd, Jiaqi Yanga,b, Shuhu Xiaoa,b,*

a

State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research

Academy of Environmental Sciences, Beijing, 100012, China b

Department of Water Environmental Treatment, Chinese Research Academy of Environmental

Sciences, Beijing 100012, China c School

d

of Life Science, Liaoning Normal University, Dalian 116081, China

Liaoning Key Lab of Marine Fishery Molecular Biology, Liaoning Ocean and Fisheries Science

Research Institute, Dalian, 116023, China *Corresponding

author:

Yonghui

Song:

Tel.+86

10

84928380,

E-mail

address:

[email protected]; Shuhu Xiao: Tel. +86 10 84928381, E-mail address [email protected]

Abstract A novel Fe3O4-graphene-biochar composite (GBC-Fe3O4) was prepared to enhance the adsorption capacity and recollection efficiency of graphene-biochar composites (GBCs). The adsorption characteristics were tested to remove crystal violet (CV), which is a refractory compound in industrial wastewater. Structural and 1

morphological analysis exhibited that a larger surface area, greater thermal stability, and more functional groups were present after Fe3O4 nanoparticles coated the GBC surface. This improved the CV adsorption versus uncoated GBC. The introduction of G and Fe3O4 nanoparticles collectively reduced the zeta potentials of GBC-Fe3O4 to -38.1± 1.1 mV versus -24.3±2.2 mV for GBC and -20.7±1.2 mV for BC. The maximum Qmax values were obtained 436.68 mg/g at 40°C. Fourier transform infrared analysis suggested that the interactions of functional groups, such as aromatic C=C and C=O, -OH, C-C, and π-π played an important role in CV adsorption. The thermodynamic analysis of Langmuir and Freundlich isotherms indicated that the adsorption improved as a spontaneous endothermic process. The saturation magnetization of GBC-Fe3O4 reached 61.48 emu/g, allowing efficient recollection of the material with a magnet. The CV adsorbability of the re-collected GBC-Fe3O4 was 157.31 mg/g, which was slightly lower than freshly prepared GBC-Fe3O4 (199 mg/g). These findings demonstrated that GBC-Fe3O4 was an efficient and reusable multifunctional biochar. Keywords: Biochar; Graphene; Fe3O4 nanoparticles; Adsorption; Crystal violet

2

1. Introduction In recent years, the widespread use of organic dyes in the paper, textile, food, printing, leather, gasoline, and cosmetics industries has increased the release of these dyes into the environment, leading to increasingly serious issues (Kolekar et al., 2008; Mittal et al., 2010; Modi et al., 2010; Saleh and Gupta, 2011; Saravanan et al., 2013; Shi et al., 2014). Crystal violet (CV) is a type of cationic triphenylemethane dye that is widely used as textile colorant, paper dye, and biological stain. It is often found in industrial wastewater and effluent (Kulkarni et al., 2017). The accumulation of CV can cause toxicity to mammalian cells, skin and digestive tract irritation, and respiratory and kidney failure (Mahmoud et al., 2019; Sellaoui et al., 2017). CV has high chemical stability and low biosolvability; thus, various treatment technologies, such as membrane filtration, coagulation, photo-catalysis, and adsorption, have been developed to treat such wastewater (Mittal et al., 2007). Adsorption is an efficient technology for treatment. It uses environmentally friendly and economically competitive materials that can be modified with functional groups for better adsorption behaviors (Fan et al., 2018). The development of high-efficient and cost-effective adsorbents for CV dyes is of utmost importance. Here, carbon materials have great potential for further modification and reusability (Gupta et al., 2014b; Gupta and Saleh, 2013; Mohammadi et al., 2011). Biochar (BC) is an inexpensive, carbon-rich, porous product of the pyrolysis of biomass under an environment with limited oxygen (Lehmann, 2007). It has attracted increasing attention because it has a high sorption capacity for different types of 3

contaminants. Many reports have confirmed that it has superior adsorption ability for organic and inorganic compounds, such as heavy metals, dyes, and polycyclic aromatic hydrocarbons (Bogusz et al., 2015; Li et al., 2017; Tran et al., 2015). To further improve the adsorption capacity of BC, much effort has been expended to increase BC surface areas, such as using BC powder instead of big size biochar and to create BC activity composites by engineering methods (Tang et al., 2015; Zhang et al., 2012). Graphene (G) is a kind of carbonaceous nanomaterial composed of a single, two-dimensional sheet of carbon atoms arranged in a hexagonal shape. It is an efficient adsorbent for various environmental contaminants (Chen et al., 2014; Gupta et al., 2014a; Ma et al., 2015; Wang et al., 2014). Versus uncoated BC, the G-coated BC nanocomposite exhibited an excellent sorption capacity for methylene blue (Zhang et al., 2012). A novel graphene/biochar composite BC (G-coated BC) powder has been tested for phenanthrene and mercury removal (Tang et al., 2015), indicating that an increased maximal adsorption capacity can be achieved due to the increased number of adsorption sites derived from the π-π electron interactions between the pollutants and G-coated BC. Also, the variation of physic-chemical properties of modified biochar powder has been justified for excellent adsorption of atrazine (Zhang et al., 2018b). In practice, large amounts of biochar powder are consumed. In addition, powdered G-coated BC with absorbed contaminants is not easily separated from the aqueous solution for further treatment. Introducing

magnetic

nanoparticles

into

the

adsorbent

by

chemical

co-precipitation can allow the sorbent to be magnetically separated (Zhang et al., 4

2007). Sun et al. reported that coating BC with magnetic nanoparticles could separate the BC and significantly enhance the adsorption capacity for CV (Sun et al., 2015). Recently, magnetic solid-phase extraction has been investigated as a new procedure for environmental treatment (Zhang et al., 2018a). However, only a few Fe3O4 nanoparticle-modified adsorbents have been tested for desorption and reusability (Ghaedi et al., 2015; Nasiri and Alizadeh, 2019; Sun et al., 2015). Functional modification and magnetic combination are also investigated less frequently to create an environmentally sustainable adsorbent using biochar. This study produced a novel Fe3O4-graphene-biochar composite (GBC-Fe3O4) adsorbent and

investigated

its

use

to

treat

CV

wastewater.

The

characteristics,

adsorption/desorption capacities, and reusability of the GBC-Fe3O4 to CV were systematically studied. 2. Materials and methods All of the abbreviations mentioned in this paper are shown in Table 1. 2.1. Chemicals and preparation Graphene was purchased from the Xianfeng Nano-material Company (Nanjing, China). The G solution (1000 mg/L) was prepared as previously reported (Tang et al., 2015). Rice straw was air-dried and milled into 2 mm powder to serve as the feedstock biomass for the production of BC, GBC, and GBC-Fe3O4. All of the other commercial chemicals were of analytical grade. GBC was produced following previously described methods with minor modifications. Rice straw (10 g) was added to the G solution (100 mL), stirred for 2 h, 5

and the mixture was oven-dried for 4 h at 80°C. The pretreated rice straw was then added to a quartz tube and pyrolyzed inside a tube furnace to generate GBC via slow pyrolysis in a N2 environment at 600°C for 1 h. BC was made from rice straw (10 g) without G solution under the same pyrolysis conditions. After being oven dried, the sample was sealed in a container for later detection. The GBC-Fe3O4 was prepared using GBC and Fe3O4 as the raw material nanoparticles and magnetic medium. The Fe3O4 nanoparticles were synthesized by ferrous chloride and ferric chloride via a chemical co-precipitation method (Chen et al., 2011; Sun et al., 2015). After precipitation, the synthesized product was repeatedly washed and dried and studied with Fourier transform infrared spectroscopy (FTIR) (Fig. S1). GBC (1.25 g) was then added into the mixture and stirred continuously for 1 h. This mixture was subsequently separated by centrifugation, and the precipitate was washed in deionized water to a neutral pH and then oven-dried and sealed in a container for further detection. 2.2 Characterization of absorbents The BET adsorption method (ASAP2460, Micromeritics, USA) was used to analyze the surface area and pore volume of the BC, GBC, and GBC-Fe3O4. A Zetasizer Nano 90 (Malvern Instruments, UK) was used to determine the Zeta potential, and the pH was detected using a pH meter. Scanning electron microscopy (SEM) (SU8010, Hitachi, Japan) equipped with an energy-dispersive X-ray analyzer (EDS) was conducted to analyze the structures and surface elemental compositions. Thermogravimetric analysis (TGA) of BC, GBC, and GBC-Fe3O4 was conducted in 6

air stream with 10°C/min heating rate using a Mettler TGA/DTA analyzer (Columbus, OH). The FTIR spectra of BC, GBC, GBC-Fe3O4, and CV-loaded GBC-Fe3O4 were detected, and the magnetic hysteresis loop of the GBC-Fe3O4 was detected with magnetic measurements via a variable field translation balance (MPMS-XL-5, USA). 2.3 Adsorption experiments The CV solution of the 5000 mg/L was used for testing. Here, GBC-Fe3O4 (30 mg) was put in a conical flask (100 mL) and mixed with the CV solution (30 mL) at 200 rpm. The adsorbability of BC, GBC, and GBC-Fe3O4 were detected at 40°C and a pH of 6.0 (initial CV concentration 200 mg/L). The effects of the initial pH (1 to 10) on the CV absorption capacity of GBC-Fe3O4 were tested at 40°C. Adsorption kinetic experiments were conducted with time intervals ranging from 5 to 240 min. Adsorption isotherm experiments were conducted by adding different initial CV concentrations from 100 to 600 mg/L with different temperatures ranging from 20°C to 40°C for 240 min. The control tests were conducted under identical conditions without adsorbents. 2.4 Desorption and reusability of GBC-Fe3O4 To investigate the reusability of the GBC-Fe3O4, 30 mg of the CV-loaded GBC-Fe3O4 was collected with a magnet and added to a conical flask (100 mL) with absolute ethyl alcohol (30 mL) at 30°C and stirred at 200 rpm for 2 h. This desorption process was repeated twice. The GBC-Fe3O4 was recollected and oven dried at 80°C, and the adsorbability of the recollected GBC-Fe3O4 was then detected as mentioned above (200 mg/L CV, pH 6.0, 40°C, 4 h). 7

2.5 Analytical methods The reaction mixture solution was then collected and filtered through a 0.22-μm nylon membrane filter to evaluate adsorption efficiency. The CV concentration was measured using an ultraviolet-visible (UV/vis) spectrophotometer (JASCOV-560, Japan) at 591 nm. The equation below was used to calculate the adsorbability of GBC-Fe3O4, Qe (mg/g): Qe =

(C0 ― Ce)V M

,

where C0 and Ce are the initial CV concentration (mg/mL) and that at equilibrium, respectively. V is the solution volume (mL), and M is the mass of MBC (mg). All of the experiments were carried out in triplicate, and the average values were used in the calculations. All of the data were analyzed using ORIGIN 8. 3. Results and discussion 3.1 Characterization of GBC-Fe3O4 The surface area and pore volume of GBC were both higher than those of BC (Table 2) indicating that the GBC contained some fine-pore structures and the presence of G could potentially increase the adsorbent’s porosity. The surface area and pore volume increased further when the GBC was coated with Fe3O4 nanoparticles (GBC-Fe3O4) suggesting that their presence provided additional surface area and porosity for adsorption. The modified BC by Fe3O4 nanoparticles exhibited better carbonization levels, surface functional groups, and porosity. The Fe3O4 activated sorption sites via electrostatic interactions (Zhang et al., 2018a). The zeta potentials of BC, GBC, and GBC-Fe3O4 were -20.7± 1.25, -24.3± 2.23, and -38.1± 8

1.13 mV, respectively, indicating that the introduction of G could reduce the zeta potential of BC, and the addition of Fe3O4 nanoparticles could reduce it further (Table 2). The decreased Zeta potential suggested that the introduction of G and Fe3O4 nanoparticles collectively enhanced the electrostatic interaction between the negatively charged adsorbent and the cationic CV leading to an improved adsorption capacity. The TGA profiles of BC, GBC, and GBC-Fe3O4 are shown in Fig. 1. In the process of pyrolysis of carbon materials, the surface water loss occurs first (50-100°C). The surface functional groups (such as aromatic C=C groups) then disappeared (100-350°C), and graphitic chars formed beyond 350°C (Inyang et al., 2014). The weight of BC, GBC, and GBC-Fe3O4 were decreased insignificantly below 350°C. Beyond that, the weight loss of BC (~78%) was observed even more than that in GBC (~75%) and GBC-Fe3O4 (~70%) from approximately 350-550°C. When the weight loss was 50%, the degradation temperature of GBC-Fe3O4 was 17 and 32°C, which is higher than the degradation temperature of GBC and BC, respectively. The TGA profiles indicated that GBC-Fe3O4 had the best thermal stability due to the coating of G and Fe3O4 nanoparticles. The SEM and SEM-EDS images of BC, GBC, and GBC-Fe3O4 are shown in Fig. S2 and S3. Versus the porous and rough BC, the typical ripples of G were observed on the surface of GBC (Fig. S2 A, B). Fig. S2 C shows that the typical ripples of G and granular Fe3O4 nanoparticles were present on the surface of GBC-Fe3O4. The EDS spectrum of the surface of BC showed that almost all of the elements 9

on its surface were carbon (Fig. S3 A). The EDS result showed that the surface of GBC not only exhibited a large amount of carbon, but also a small amount of oxygen. The existence of oxygen on the surface of GBC could originate from the few residual functional groups that contained oxygen on the G’s surface or from the BC inside the G sheets (Fig. S3 B). This result confirmed the presence of G in the GBC. The EDS spectrum of the GBC-Fe3O4 surface verified the presence of Fe3O4 nanoparticles coating the GBC via large amounts of oxygen and Fe on the adsorbent’s surface (Fig. S3 C). A magnetic hysteresis curve was described to study the magnetic properties of GBC-Fe3O4. Fig. S4 shows that the saturation magnetization of GBC-Fe3O4 was 61.48 emu/g, which is higher than that from coated cells (14.3 emu/g) (Li et al., 2009). This confirmed that the GBC coated with Fe3O4 nanoparticles was highly magnetic. Moreover, the magnetic separation properties of BC, GBC, and GBC-Fe3O4 were tested for 3 days. The results showed that the GBC-Fe3O4 was stable and easily recollected by a magnet, while BC and GBC both dispersed in the solution and could not be recollected using the magnet (data not shown). This verified the feasibility of using magnetic separation recollecting powdered GBC-Fe3O4 after its initial use. The FTIR spectra of BC, GBC, GBC-Fe3O4, and CV-loaded GBC-Fe3O4 were presented in Fig. S5. The four main adsorption peaks represented the vibrations of the functional groupsin BC were 3431 cm-1 (-OH), 1600 cm-1 (aromatic C=C and C=O), 1383 cm-1 (carboxyl O=C-O), and 1103 (alkoxy C-O due to the vibration of C-H group) (Yang et al., 2017). Four adsorption peaks at 3445 cm-1 (-OH), 1623 cm-1 10

(aromatic C=C and C=O), 1387 cm-1 (carboxyl O=C-O), and 1110 cm-1 (alkoxy C-O) were observed for GBC (Devi and Saroha, 2014). Comparing the peaks of BC and GBC indicated that the aromatic C=C and C=O stretching peaks shifted from 1600 cm-1 to 1623 cm-1. The O =C-O stretching peak in carboxyl shifted from 1383 to 1387 cm-1, and the C-O stretching vibration in alkoxy shifted from 1103 to 1110 cm-1, respectively. Coating BC with G significantly increased the intensity of the vibrations of functional groups versus previously reported biochars (Shen et al., 2018). The variations suggested that the G coated the BC surface via π-π interactions (Ren et al., 2012). When the GBC was coated with Fe3O4 nanoparticles, the adsorption peak at 1110 cm-1 disappeared, and two new peaks appeared at 985 and 572 cm-1. The adsorption peak at 572 cm-1 was attributed to the vibration of the Fe-O in iron oxide, which further confirmed that Fe3O4 nanoparticles were successfully attached to the surface of GBC (Chen et al., 2011). The adsorption peak at 985 cm-1 in GBC-Fe3O4 indicated the ethane stretching vibration of the C-C (Sun et al., 2015). The adsorption peak at 572 cm-1 in the spectrum of the CV-loaded GBC-Fe3O4 indicated that the Fe-O of iron oxide was still present. A comparison of the adsorption peaks of GBC-Fe3O4 and CV-loaded GBC-Fe3O4 indicated that the adsorption of CV on GBC-Fe3O4 caused significant changes to the GBC-Fe3O4 adsorption peaks. A new adsorption peak at 1587 cm-1 was observed for the CV-loaded GBC-Fe3O4. This peak was attributed to the vibration of -NH confirming that the CV was adsorbed onto the GBC-Fe3O4 (Sun et al., 2015). The stretching vibrations of the aromatic C=C and C=O bonds (1623 cm-1), carboxyl O=C-O (1386 cm-1), the C-C (985 cm-1), and the 11

hydroxyl -OH (3445 cm-1) of GBC-Fe3O4 disappeared after the adsorption of CV. These changes suggested that functional groups including π-π interactions; C=C and C=O in the aromatic structure; as well as -OH, C-C, and carboxyl O=C-O could play a significant role in the adsorption of CV(Tang et al., 2015). 3.2 Adsorption capacities of BC, GBC and GBC-Fe3O4 The adsorption capacities of BC, GBC, and GBC-Fe3O4 for CV are shown in Fig. 2. Although all of these three adsorbents could reach adsorption equilibrium in 5 min, their maximum adsorbabilities were significantly different. GBC-Fe3O4 showed the highest adsorption capacity for CV (199 mg/g) followed by GBC (139.5 mg/g) and BC (15.3 mg/g). G coated on BC surface led to an increase in the surface area and pore volume (Table 2) as well as a negatively charged surface. This strengthened the vibration of aromatic C=C bonds and increased the oxygen-containing groups (Fig. S5). G is coated on the BC surface via π-π interactions resulting in a significant increase in functional groups containing oxygen. There were abundant adsorption sites for inorganic pollutants such as CV and heavy metals (Koushkbaghi et al., 2016; Son et al., 2018; Wang et al., 2014). Therefore, GBC was more effective for the adsorption of CV than BC. Moreover, the coating of Fe3O4 nanoparticles on GBC further enhanced its adsorption capacity, because the introduction of Fe3O4 nanoparticles could further increase the surface area and pore volume (Table 2). This might be because GBC contains a large proportion of iron oxide with a small surface areas and abundant transitional pores (Chen et al., 2011). However, the increasing 12

negatively charged surface could enhance the electrostatic interaction between the cationic CV and negatively charged GBC-Fe3O4. Thus, the adsorption capacity of GBC improved. These results indicated that coating Fe3O4 nanoparticles on GBC could not only efficiently increase magnet concentration but could also enhance the GBC adsorption capacity. Compared to various recent adsorbents for CV removal, the modified biochar with graphene and Fe3O4 nanoparticles had improved adsorption capacity and reusability (Table 3). The magnetic property led to the separation of adsorbents from solutions (Nasiri and Alizadeh, 2019), which may combine other adsorbent materials of high adsorption capacity (Mohamed et al., 2018). The absorption capacity of biochar materials varied with different biomasses; however, Fe3O4 modification can substantially improve their interface properties and functional groups. Graphene can be further functionalized to achieve perfect electrical conductivity and strong mechanical capacity. These are important to enhance surface functionalizability under various conditions (Gupta et al., 2014a). Very recently, the Fe3O4-graphene-biochar has testified good absorption properties, which can be used to remove other emerging organic pollutants in water, for example, fluoroquinolone antibiotics ciprofloxacin, and sparfloxacin(Zhou et al., 2019). The adsorption was improved by the π-π electron donor-acceptor interaction, H-bonding, hydrophobic interaction and electrostatic interaction of the Fe3O4-graphene-biochar. Lately, it has been further investigated that Fe3O4 modified biochar (Myriophyllum aquaticum) with graphitized structure successfully catalyzed degradation of some organic pollutants, like p-hydroxybenzoic 13

acid, if peroxymonosulfate was presented(Fu et al., 2019). 3.3 The impacts of pH, temperature, and CV concentration on adsorbability CV adsorption on GBC-Fe3O4 was plotted against pH in Fig. 3. The figure showed that the solution pH did not significantly affect the CV adsorbability of GBC-Fe3O4. Except for the adsorption capacities at solution pH 1 and 2, there were no remarkable differences between the adsorbabilities of GBC-Fe3O4 at pH values of 3 to 10. Among these pH values, pH 6 was the optimal pH for CV adsorption by GBC-Fe3O4 because the highest adsorption capacity was observed at this level. This agreed with Sun et al., who also found that pH of 6 was best for CV adsorption (Sun et al., 2015). They also reported that cationic CV was favorably adsorbed by negatively charged adsorbents. The impacts of temperature and CV concentration on the CV adsorbability of GBC-Fe3O4 were also determined (Fig. 4). As the CV concentration increased from 100 to 500 mg/L, the adsorption capacity of GBC-Fe3O4 increased rapidly and became constant when the CV concentration reached 600 mg/L. The impact of the initial dye concentration was generally related to the direct relationship between the dye concentration and the free binding sites present on the adsorbent surface (Lim et al., 2013). As the initial CV concentration increased, the adsorption capacity of GBC-Fe3O4 decreased. This might be because the adsorption sites were saturated. Fig. 4 also shows that temperature affected the CV adsorption capacity of GBC-Fe3O4. As the increased from 20°C to 40°C, the adsorbability of GBC-Fe3O4 increased at initial CV concentrations from 100 to 600 mg/L. The optimal temperature 14

was 40°C. This result indicates that the adsorption of GBC-Fe3O4 might be endothermic and further suggests that the increased temperature could decrease the viscosity of the CV solution and increase the adsorbability (Yang et al., 2011). The CV adsorbability of GBC-Fe3O4 (430.13 mg/g) was higher than the CV adsorbabilities in previous reports—nearly 23% higher than the highest known CV adsorbability (349.40 mg/g; Table S1). This suggested that GBC-Fe3O4 was an extremely efficient adsorbent for CV. 3.4 Adsorption kinetics of CV Next, the pseudo-second-order (P-S-O) and Weber-Morris intraparticle diffusion models were used to analyze the adsorption kinetics of CV onto GBC-Fe3O4. The integrated and linearized form of the P-S-O kinetics model was as follows: t Qt

1

1

= K Q2 + Qmaxt, (1) 2 max

where Qt and Qmax are the adsorbability (mg/g) at time t (min) and the maximum adsorbability (mg/g), respectively; and K2 is the equilibrium rate constant (g/mg·min) of P-S-O adsorption. The P-S-O kinetic model could not determine the diffusion mechanism, and thus the kinetic results were further analyzed by the Weber-Morris intraparticle diffusion model expressed as: Qt = K wt

12

+I, (2)

where Qt is the adsorbability (mg/g) at time t (min), Kw is the intraparticle diffusion rate constant (mg/(g min1/2)), and the intercept I is obtained via extrapolation of the linear portion of the plot of Qt vs. t1/2. 15

The resulting kinetic data were plotted using both the P-S-O and Weber-Morris intraparticle diffusion models along with the experimental data. As shown in Fig. S6 A, the linear P-S-O equation nicely fitted the experimental data (R2=0.9999) indicating that this model agreed well with the process of CV adsorption onto GBC-Fe3O4. The Qmax was calculated by the P-S-O model to be 199.6008 mg/g, which agreed nicely with the experimental data (199.4 mg/g). These results indicated that boundary layer resistance was not the limiting factor; the adsorption process of CV agreed with the P-S-O model (Yang et al., 2011). The Weber-Morris plot of CV adsorption onto GBC-Fe3O4 was linear (R2=0.9697) and did not pass through the origin indicating that significant intraparticle diffusion occurred during CV adsorption onto GBC-Fe3O4 (Fig. S6 B). The larger intercept represented the thicker boundary layer and had a greater impact (Ozer et al., 2006). In this study, a large intercept was obtained (199.1415) indicating that the impact of the boundary layer on the CV adsorption process could not be ignored. 3.5 Adsorption isotherms of CV The adsorption isotherms of CV onto GBC-Fe3O4 were analyzed using Langmuir and Freundlich isotherms. The equations of these two isotherms could be described as follows: QmaxKLCe

Langmuir isotherm: Qe = 1 + KLCe , (3) 1

Freundlich isotherm: Qe = KFCe

nF

, (4)

The dimensionless constant separation factor or equilibrium parameter (RL) was 16

defined by the equation below: 1

RL = 1 + C0K , (5) L

where Qe is the adsorbability at equilibrium (mg/g), Qmax is the Langmuir maximum adsorbability (mg/g), Ce is the equilibrium CV concentration in solution (mg/L), KL is a constant related to the interaction energies (L/mg), KF is a constant related to the sorption capacity (L/mg), and nF is the heterogeneity factor. The RL value indicated that the adsorption process was favorable when the RL was between 0 and 1, and unfavorable when the RL was > 1 (Inbaraj et al., 2009). The Langmuir and Freundlich plots for CV adsorption onto GBC-Fe3O4 are shown in Fig 5. The Langmuir and Freundlich isotherm parameters for the adsorption of CV onto GBC-Fe3O4 at different temperatures (20-40°C) are listed in Table 4. The Langmuir isotherm assumed that the adsorption of dye molecules was homogeneous at the adsorption site, and no further adsorption could occur at that site (Langmuir, 1918). In this study, the Langmuir isotherm fitted to the CV adsorption process nicely at different temperatures as indicated by the high correlation coefficients (0.9924 for 20°C, 0.9950 for 30°C, and 0.9999 for 40°C). Therefore, the process of CV adsorption on GBC-Fe3O4 could be represented by a monolayer adsorption model, and CV was adsorbed through homogeneous adsorption on the surface of the GBC-Fe3O4. The maximum Qmax and KL values were obtained at 40°C (436.68 mg/g and 0.4589 L/mg), indicating that the CV molecules showed the highest affinity for GBC-Fe3O4 at 40°C. This result was consistent with the data presented in Table 4. Moreover, the RL values were all within 0 to 1 according to equation (5), which 17

confirmed that all CV adsorption processes were benign. Besides, all nF values of the Freundlich isotherm were within 2 to 10. This further confirmed that the adsorption of CV onto GBC-Fe3O4 was favorable (Table 4). High KF values for CV adsorption suggested that GBC-Fe3O4 in the solution had a high dye adsorption capacity. These results were in accordance with those obtained for CV adsorption by Fe3O4-coated BC (Sun et al., 2015). 3.6 Thermodynamic properties of CV adsorption To analyze whether the CV adsorption process was a spontaneous process, the thermodynamic properties were detected at three different temperatures (20°C, 30°C, and 40°C). The thermodynamic parameters, i.e., the variations in free energy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°), for the adsorption of CV onto GBC-Fe3O4 were calculated by the equations below: Kc =

Ce(adsorbent) Ce(solution)

lnKc =

∆S° R

―

∆H° RT

, (6)

, (7)

∆G° = ―RTlnKc, (8) where Kc is the equilibrium constant, Ce (adsorbent) and Ce (solution) are the concentrations of the adsorbed CV on the adsorbent at equilibrium (mg/L) and in the solution at equilibrium (mg/L), respectively, R (8.314 J mol−1 K−1) is the gas constant, and T is the temperature (K). ΔH° and ΔS° were determined from the plot of lnKc versus 1/T (Fig. 6) and presented in Table 5. The ΔG° value of all three temperatures was negative, and ranged within −20<ΔG°<0 kJ/mol, which indicated that the CV adsorption process 18

was feasible and spontaneous (Jalil et al., 2012). In addition, the ΔG° value declined as the temperature increased suggesting that the spontaneity of CV adsorption might decrease with increasing temperature. The value of ΔH° was 45.8549 kJ/mol, which was close to 40 kJ/mol, indicating that the CV adsorption process in this study was endothermic and was induced by dentate exchange and physical interactions (Sun et al., 2015). This result was consistent with the results of the kinetics experiments conducted at these three temperatures discussed above. These results suggest that the number of active sites available on the GBC-Fe3O4 increased with increasing temperature. This resulted in an increase in the adsorbability. The positive ΔS° value (161.7763 J/mol∙K) indicated that the randomness increased at the solid/liquid interface when CV adsorbed onto GBC-Fe3O4 along with the good affinity of the GBC-Fe3O4 for CV. 3.7 Reusability of GBC-Fe3O4 The reusability of the adsorbents was an important factor that determines the effectiveness of adsorption over time. A desorption experiment of GBC-Fe3O4 was conducted before reusing, and the desorption process was found to be instantaneous; the equilibrium was reached within just several seconds. The desorption efficiency was approximately 82.59%. After the desorption experiment, the CV adsorbability of the re-collected GBC-Fe3O4 was 157.31 mg/g. The adsorbability of the re-collected GBC-Fe3O4 was slightly lower than that of the original GBC-Fe3O4 (199 mg/g). The adsorption of CV onto some adsorption sites on GBC-Fe3O4 might be irreversible using absolute ethyl alcohol as applied in desorption experiment. This might explain 19

the decrease in CV adsorption capacity. Although Fe3O4 nanoparticles have been used in other materials with very high adsorption capacity, desorption analysis for reusability is rare (Table 3). Biochar adsorbents modified by Fe3O4 nanoparticles showed good desorption ability, and graphene-Fe3O4 nanoparticles collectively improved the desorption efficiency for reusability here. 4. Conclusions We report a novel Fe3O4-graphene-biochar composite (GBC-Fe3O4). Versus the GBC, the Fe3O4 nanoparticle coating on the surface of GBC not only enhanced the physiochemical properties, i.e., the surface area, pore volume, thermal stability, number of functional groups, and CV adsorption capacity, but it also improved the magnetic properties. Thus, the adsorbent could be recollected with a magnet. The CV adsorption process by GBC-Fe3O4 was favorable, spontaneous, and endothermic. These findings indicated that magnetic GBC is an efficient, economical, and environmentally sustainable multifunctional adsorbent. Acknowledgements The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (No. 51808518, 51878049) and the Major Science and Technology

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24

Table 1 Abbreviations. Abbreviation

Full name

G

Graphene

BC

Biochar

GBC

graphene/biochar composite

GBC-Fe3O4

Fe3O4-graphene-biochar composite

CV

Crystal violet

FTIR

Fourier transform infrared spectroscopy

SEM

Scanning electron microscopy

EDS

Energy-dispersive X-ray analyzer

TGA

Thermogravimetric analysis

UV/vis

Ultraviolet-visible

25

Table 2 Physio-chemical properties of BC, GBC, and GBC-Fe3O4. Sample

pH

SBET (m2/g)

Pore volume (cm3/g)

Zeta

potentials

(mV) BC

6.8

78.009

0.03793

-20.7± 1.25

GBC

7.2

117.595

0.05256

-24.3± 2.23

GBC-Fe3

7.2

145.963

0.06549

-38.1± 1.13

O4

26

Table 3 Comparison of various adsorbents for CV removal. Maximum Desorption%/ adsorption Adsorbent

Magnetic modification

Reusability(mg/g

Reference

capacity ) (mg/g) microwave-synthesiz Macroalgae

ed iron oxide nano-

(Angelova 167

biomass

No test

and

et al., 2016)

microparticles Xanthan gum

(Zheng et none

183±12

No test

(XG)

al., 2019)

hydroxypropyl

(Nasiri and

-b-cyclodextri

Fe3O4 nanoparticles

1269

No test

n

Alizadeh, 2019) (Angelova

Rice husk

none

93.45

No test et al., 2016) (Khan et

Tea dust

none

175.4

No test al., 2016)

Typha latifolia

none

1.0-2.5

27

15-59%/none

(Jayasanth

activated

a Kumari et

carbon

al., 2017) Four

XG/PVI

(Mohamed none

453

adsorption–desor

hydrogel

et al., 2018) ption cycles

Corn stalks

(Sun et al., Fe3O4 nanoparticles

349.40

78%/73.31

biochar Rice straw

2015) Graphene-Fe3O4 436.68

biochar

nanoparticles

28

82%/157.3

This study

Table 4 Adsorption isotherm parameters for the adsorption of CV onto GBC-Fe3O4. T

(°C)

20

30

40

Qexp

Langmuir constants

(mg/g

Qmax

)

(mg/g)

325.8 6 395.7 5 430.1 2

334.4481

392.1569

436.681

KL (L/mg)

0.0397

0.0818

0.4589

29

Freundlich constants

R2

nF

0.992

3.595

4

3

0.995

3.258

0

6

0.999

4.060

9

6

KF (L/mg)

74.7710

96.2491

176.0239

R2

0.971 6 0.941 7 0.879 7

Table 5 Thermodynamic parameters for the adsorption of CV onto GBC-Fe3O4. T (°C)

Kc

△Go (KJ/mol)

△Ho (KJ/mol)

△So (J/mol∙K)

20

1.85

-1.4994

45.8549

161.7763

30

3.76

-3.3381

-

-

40

6.14

-4.7249

-

-

30

Figure Legends. Fig. 1 Thermogravimetric curves for BC, GBC, and GBC-Fe3O4. Fig. 2 Comparison of CV adsorption capacities of BC, GBC, and GBC-Fe3O4 (ORIGIN 8, n=3). Fig. 3 Effect of pH on the CV adsorption capacity of GBC-Fe3O4 (ORIGIN 8, n=3). Fig. 4 Effect of initial dye concentration and temperature on the CV adsorption capacity of GBC-Fe3O4 (ORIGIN 8, n=3). Fig. 5 Langmuir (A) Freundlich (B) isotherms for CV adsorption by GBC-Fe3O4 at different temperatures (ORIGIN 8, n=3). Fig. 6 Plot of lnKc versus 1/T for the estimation of thermodynamic parameters (ORIGIN 8, n=3).

31

Fig. 1

32

Fig. 2

33

Fig. 3

34

Fig. 4

35

(A)

(B)

Fig. 5

36

Fig. 6

Graphical abstract

BC: biochar

GBC: graphene-biochar

Fe3O4-graphene-biochar

Fe3O4-graphene-biochar

 A novel Fe3O4-graphene-biochar composite (GBC- Fe3O4) was prepared.  Fourier transform infrared analysis disclosed interactions of functional groups and crystal violet adsorption. 37

 The adsorption process was spontaneous and endothermic.

38