Study on adsorption performance and mechanism of peanut hull-derived magnetic biochar for removal of malachite green from water

Magnetic biochar (MBC) has the advantages including wide source of raw materials and low cost, and has become a potential adsorbent for water treatment, overcoming the shortcomings of biochar (BC) with the hard separation of solid and liquid. Peanut hull-derived magnetic biochar loaded with Fe3O4 (Fe3O4/BC) was prepared by co-precipitation method. By means of material characterization and batch processing experiments, material properties and environmental factors affecting adsorption performance were investigated. The adsorption mechanism of Fe3O4/BC on malachite green (MG) was revealed using adsorption isotherms, adsorption kinetics and thermodynamics. The results showed that Fe3O4 particles were uniformly loaded, the total pore volume was increased, surface oxygen-containing functional groups were formed, and the maximum adsorption capacity of the biochar reached 175.4 mg g−1, 1.6 times of that before modification. In a wide PH range, Fe3O4/BC showed high adsorption performance for MG, and significant influence from ionic strength wasn’t found. Chemical adsorption was the main adsorption mechanism, including electrostatic interaction, cation exchange, hydrogen bonding and π-π interaction. The study of adsorption mechanism will promote the application of MBC in the removal of organic pollutants from water.


Introduction
Malachite green (MG) is widely used as a biocide in the aquaculture industry and as an industrial dye in the cotton, silk, leather, and other industries [1].As a typical organic pollutant in water, MG has attracted extensive attention because of its carcinogenicity and a potential threat to ecosystem [2][3][4][5].Adsorption method has become an effective method to remove organic pollutants such as MG from water because of its advantages of simple operation, wide source of adsorbent and fast adsorption process [6,7].At present, the commonly used adsorbents are zeolite, metal organic framework, molecular sieve, activated carbon and biochar (BC).Among them, BC is obtained by pyrolysis of waste biomass generated in agriculture, forestry, light industry and sewage treatment and other industries.BC has become one of the most attractive adsorbents owing to its advantages of low cost, wide source, developed pore structure and abundant surface functional groups [8][9][10][11][12][13].
However, due to its poor solid-liquid separation performance, the BC adsorbed with pollutants is difficult to be recovered from water for reuse, and the effluent is also at risk of secondary pollution, which restricts its further application [14,15].Magnetic biochar (MBC) is a class of magnetic composite materials obtained by loading magnetic precursor substances (transition metal salt solution, natural iron minerals and iron oxides) on the basis of BC or biomass [16,17].Compared with cobalt, nickel and other magnetic precursor materials, iron has the characteristics of strong magnetism, low cost and non-toxic, and is widely used in the present [18,19].Under the action of the external magnetic field, MBC can be quickly separated from the water phase, and then realize recycling, regeneration and reuse [20].There are some common techniques employed toproduce magnetic material such as hydrothermal, co-precipitation, pyrolysis, and calcination method [21,22].These methods are utilized by many researches to synthesize magnetic biochar for water treatment.Bharath et al dispersed Fe 3 O 4 nanoparticles onto the surface of porous graphene sheets by hydrothermal method at 180 °C to prepare magnetic graphene composite materials, which can achieve efficient removal of organic dye methyl violet within 5 min [23].Peng et al prepared rice residue-based MBC loaded with Fe and Zr bimetallic oxides by co-precipitation method, and the material showed high adsorption performances to As(III) and As(V) with the maximum adsorption quantity of 107.6 mg g −1 and 40.8 mg g −1 at pH 6.5 and 8.5, respectively [24].Yang et al pyrolyzed corn stalks and ZIF-67 (a metal organic frameworks synthesized from cobalt nitrate and 2-methylimidazole) under nitrogen atmosphere to produce magnetic porous carbon whose adsorption capacities for imidacloprid and thiamethoxam can reach as high as 189 and 133 mg g −1 , respectively [25].Sun et al used industrial lignin and FeCl 3 •6H 2 O as raw materials to prepare MBC by calcination at 800 °C.The maximum adsorption capacity of the material for methylene blue from solution reaches 200 mg g −1 , and it can selectively adsorb methylene blue in various dye systems [26].
Peanuts are an important oil crop.The global peanut production in 2019 was 48.8 million tons, and the peanut hull production reached 15 million tons [27].Except for a small amount of feed and chemical raw materials, most of them are directly burned or discarded in farmland, not only polluting the environment, but also wasting the resources.As we all know, peanut shells are composed of many microfibers, rich in cellulose and hemicellulose, and low ash content [28].In recent years, peanut shell had been used as the substrates for the synthesis of adsorbents for wastewater treatment.Bharath et al prepared biochar composites loaded with Fe 3 O 4 nanoparticles using peanut shell carbon, ferric salt and ferric salt at 180 °C by hydrothermal method, and the material shows a high electrosorption capacity (maximum adsorption capacity 24.5 mg g −1 at 1.2 V) for Cr(VI) [29].Renita et al prepared MBC by hydrothermal method with peanut shell biochar powder, FeCl 3 and FeSO 4 .The adsorption capacity of acid fuchsin dye was 27.69 mg g −1 after adding 1 g MBC at room temperature for 120 min [30].Kumar et al pyrolized peanut shell at 923 K in N 2 atmosphere to produce biochar.Under the optimal conditions of pH 2.0, temperature 303 K, and dosage of adsorbent 2.5 g l −1 , the maximum adsorption capacity of this material for Cr(VI) is 29.38 mg g −1 , and the adsorption mechanism is characterized by electrostatic attraction, reduction and complexation [31].Zhang et al synthesized magnetic biochar by pyrolysis of peanut shell and iron-rich red mud at 900 °C.The results show that the material has a high specific surface area (164.43 m 2 g −1 ) and adsorption capacity of phosphorus (142.6 mg g −1 ), whose mechanisms include electrostatic effect, Fe-O-P bonding and surface precipitation [32].In the above research, magnetic activated carbon was synthesized by various magnetic precursor materials with different preparation methods, which was used for the removal of pollutants such as dyes, heavy metals and nutrients.
The factors affecting the adsorption behavior of pollutants are complicated, which are related to the characteristics of materials, as well as environmental factors [6,8].In addition, the adsorption mechanism between materials and various pollutants is also different.How to make full use of material properties, achieve efficient removal effect, and promote practical application through the exploration of factors affecting adsorption properties and the revelation of adsorption mechanism is still a challenge.Herein, magnetic biochar loaded with Fe 3 O 4 was prepared from peanut Hull-derived biochar and ferric salt solution by co-precipitation method.With various characterizations, the material properties conducive to adsorption were discussed.The effects of pH, temperature, ionic strength and amount of adsorbent on MG adsorption were investigated, and the solid-liquid separation performance and reuse performance of Fe 3 O 4 /BC were evaluated.Based on adsorption kinetics, isotherm and thermodynamic analysis, the adsorption mechanism of Fe 3 O 4 /BC to MG was discussed, which provided a reference for the optimization of the process conditions for removing organic pollutants from water by MBC.

Materials
The drugs and reagents used in the experiment mainly included: hydrochloric acid, sodium hydroxide, absolute ethanol, MG, ferric chloride, ferric sulfate and ammonia.All reagents were analytically pure.Among them, FeCl 3 •6H 2 O and FeSO 4 were the magnetizing substances, MG was the target organic pollutant, and HCl and NaOH were used to adjust the pH value of the solution.

Preparation of Fe 3 O 4 /BC
The material (Fe 3 O 4 /BC) was prepared by co-precipitation method.After rinsing the peanut hull with tap water, it was soaked in deionized water for 30 min to remove the ash.And then it was dried in an oven at 80 °C for 6 h.Next, it was crushed thrice with a pulverizer, passed through 200 screens, and was bagged for use.Peanut hull powder was placed in Muffle furnace and pyrolyzed at 500 °C for 3 h under air-limitation conditions to obtain BC. 1 g BC via a 200 mesh screen and 0.05 g of polyethylene glycol was added in water.The mixture was stirred for 20 min, and then allowed to stand for 30 min.Then 2 g of FeCl 3 •6H 2 O and FeSO 4 were added at a 2:1 ratio and continued stirring for 30 min.Then ammonia aqueous solution was added dropwise, and the mixture was heated and stirred at 55 °C for 30 min.After the end of the reaction, the product was separated with a magnet.Absolute ethanol was washed and dried at 80 °C for 4 h to obtain Fe 3 O 4 /BC.

Characterization of Fe 3 O 4 /BC
The adsorbent Fe 3 O 4 /BC was analyzed using various characterization methods.The morphology of the Fe 3 O 4 /BC samples was characterized by field emission scanning electron microscopy (SEM).The crystal structure was analyzed by x-ray diffraction (XRD) mode (Bruker D8 Advanced Diffractometer).The Cu Kα radiation was between 20°and 70°.High-resolution x-ray photoelectron spectrometer (XPS; AXIS Ultra DLD, UK) was used to analyze the chemical composition of samples of Fe 3 O 4 /BC as well as the changes in chemical valence and surface element content.The charge was corrected by using the energy standard C 1 s = 284.80eV.Chemical structure in the 400 ∼ 4,000 cm −1 regions was determined by Fourier transform infrared (FT-IR) spectroscopy (IS5).The specific surface area, pore size distribution and specific pore volume of Fe 3 O 4 /BC were analyzed by Micromeritics ASAP2020 HD88 specific surface area analyzer.Before testing, samples were pretreated at 80 °C.The elemental composition of biochar was determined by ICP-OES.

Batch experiments
To optimize the adsorption time of the sample, the solution was placed on a stirrer by adding 20 mg of the sample to 50 ml of MG solution (20 mg l −1 , 30 mg l −1 ) for adsorption kinetics.Sample 6 ml of the mixture at 10minute intervals and centrifuge at 8,000 rpm for 10 min.At the same time, to determine the saturated adsorption capacity of each adsorbent, 20 mg of each sample was placed in a 100 ml Erlenmeyer flask, 50 ml of MG solution (different concentrations) was added, and the Erlenmeyer flask was placed in constant temperature water for isothermal adsorption experiments.Bath shaker at 30, 25 and 20 °C for 2 h.After 2 h, centrifuge 1 ml of the solution at 8,000 rpm for 10 min.The concentration of MG residues in the resulting supernatant was measured by UV-vis spectrophotometry at λ = 617 nm.222), (400), (422), (511), ( 440) and (533) crystal planes, which belong to Fe 3 O 4 (JCPDS No.88-0866) [33,34].The material is a composite of Fe 3 O 4 and BC. Figure 1(b) shows the FT-IR plot of Fe 3 O 4 /BC.It can be seen that the absorption peak at 3436 cm −1 is caused by the tensile vibration of the O-H group in the water molecules absorbed on the sample surface, and the telescopic vibration frequency of the aromatic ring C=C is low, and a weak shoulder peak appeared at 1,588 cm −1 .The absorption peaks at 1,631 cm −1 and 1,380 cm −1 belong to C=O and C-O telescopic vibrations, respectively.These three absorption peaks are typical aromatic meridian characteristic peaks [35].The broad peaks generated by the telescopic vibrations of aryl ethers C-O-C and Fe-O occurred around 1,100 cm −1 and 570 cm −1 [36].After loading iron, the vibration of characteristic peaks corresponding to C=O and C-O on BC surface is obvious.This indicates that the surface of the material contains hydroxyl group, carboxyl group, ether bond and Fe-O bond, which is advantageous for the removal of pollutants as adsorbent [12].Figures 2(a)-(b) show the SEM images of the apparent morphology of biochar materials at a calcination temperature of 500 °C.It can be seen that the surface of the pristine biochar is smooth and has more pores.On the surface of biochar loaded with Fe, many small white spherical particles can be clearly seen on the surface of the material.It can be seen from the elemental analysis in table 1 that the mass fraction of Fe and O elements of the material is significantly increased.The results show that the loaded Fe is mainly in the form of iron oxide, and there are a large number of Fe-O bonds on the surface of the material, which is consistent with the FT-IR analysis results.In addition, EDS showed that the content of C and Fe in the material was high, and the number of light spots was large, indicating that Fe was evenly distributed in the material.Combined with XRD analysis, it can be inferred that Fe 3 O 4 crystal particles are successfully loaded on the surface of biochar.

Results and discussion
The N 2 adsorption-desorption isotherms of materials BC and Fe 3 O 4 /BC are shown in figure 3(a).The results show that the isotherms of biochar all belong to type IV isotherms, and there is capillary condensation in the high pressure range, which is an obvious H4 type lagging rings.The presence and shape of the hysteresis ring at a relative pressure of 0.1-0.9indicate that the adsorbent was micro-mesoporous and is a kind of fractured pore material [37].After modification, the adsorption capacity of N 2 is significantly enhanced in the later stage, indicating that the specific surface area of the material is increased in the later stage.Combined table 2 and figure 3, it is found that the specific surface area of Fe 3 O 4 /BC decreases and the average pore size increases, which may be due to the fact that some of the micropores are occupied by iron nanoparticles.Fe 3 O 4 /BC has fewer micropores, but more mesoporous, and the increase of mesoporous can promote the diffusion of MG into pores, which is conducive to the occurrence of adsorption [38].The increase of pore volume of Fe 3 O 4 /BC is also conducive to the improvement of mass transfer efficiency of MG between solution and adsorbent [39].The adsorption experiment showed that the removal rate of BC to MG was only 40%, while Fe 3 O 4 /BC reached more than 90% within 30 min.The adsorption performance of biochar for pollutants is jointly affected by the pore structure and surface chemical properties of the adsorbent [14,16].
The comparison of XPS spectra (figures 4(a), (b)) confirms that the only new element introduced is iron.The C1s peak can be decomposed into a number of peaks with different binding energies, as shown in figures 4(c)-(d).Due to the effective in-situ activation of Fe 3 O 4 , some new oxygen-containing functional groups (C=O and Fe-O) can be formed.By analyzing the types of chemical bonds bonded with C, it can be seen that the material contains abundant oxygen functional groups, and the proportion of carbon-oxygen bonds increases to 32.09%, which may affect the charge distribution on the surface of the material in solution [40,41].In addition, these groups improve the wettability of the adsorbent and prevent the aggregation of BC, thereby favoring adsorption [42][43][44].The characteristic peaks of the Fe element can be detected in figure 4(e), confirming the presence of the element.The combination of iron oxides with peaks corresponding to 711 and 725 eV indicates the successful loading of Fe 3 O 4 [45,46].Obviously, this result is consistent with the FT-IR result.preparation of Fe 3 O 4 /BC material was successful and has application potential.In order to guide the process design of MG removal, according to the characteristics of textile printing and dyeing wastewater, the effects of adsorbent dosage, initial pH of the solution, temperature and ionic strength on the removal rate were investigated.

Effect of Fe 3 O 4 /BC dosage
The concentration of MG was 30 mg l −1 , and the effect of Fe 3 O 4 /BC material on MG removal performance was investigated.The results are shown in figure 6(a).It can be seen from figure 6(a) that when the amount of Fe 3 O 4 /BC was increased from 5 mg to 25 mg, the removal rate of MG gradually increased with the increasing material dosage.However, when the dosage of adsorbent reached 20 mg, the MG removal rate didn't increase significantly.

Effect of initial pH of solution
The pH value of solution is one of the important factors affecting the adsorption performance.As a cationic dye, MG has a pKa value of 6.90.When the pH of the solution is lower than 6.90, MG exists mainly in molecular form, and as the pH of the solution further increases, MG gradually dissociates into cations.Not only does pH affect the dissociation forms of pollutants in the solution, but also the surface chemistry of the adsorbent.Tsai et al [47] and Mohan et al [48] discussed the effect of solution pH on the adsorption behavior of biochar, and confirmed that the change of pH would cause the dissociation of functional groups on the active site of the adsorbent which ultimately results in the adsorption performance.Therefore, this study use 0.1 M NaOH or 0.1 M HCl to adjust the pH of the solution to a range of 3-11 to investage the effect of pH on the removal of Malachite green pollutants.Figure 6(b) shows a pH point-of-zerocharge (PZC) of 9.01 for Fe 3 O 4 /BC.When the solution is acidic, the surface of Fe 3 O 4 /BC is positively charged.The main reason was that the surface of Fe 3 O 4 /BC contains several oxygen-containing functional groups.These groups are bonding H + in acidic solution on Fe 3 O 4 /BC and tuned positively charged surface, and many hydrogen ions compete with MG pollutant cations for adsorption sites.There is electrostatic repulsion between the positively charged surface and the protonated MG molecules, hence the reactive groups cannot bind MG [49].As shown in figure 6(c), the material has poor removal effect on MG in water under acidic environment.
However, in the alkaline ranges (pH 9-11), the material exhibits high removal efficiency, which may be due to the negative functional groups on the surface of the material binding to MG cations.The carboxyl and hydroxyl ions on the surface of biochar in an alkaline environment generate negative charges and are therefore very favorable for binding cationic dyes to achieve high removal efficiency [50].The attraction between positive and negative charges was enhanced with a further increase in the pH of the solution, which gradually became the main interaction force to promote the adsorption of MG by Fe 3 O 4 /BC, so that Fe 3 O 4 /BC obtains a stable high adsorption capacity for MG under high pH conditions.In addition, it is worth noting in figure 6(c) that in weakly acidic or neutral states (pH 5-7), the material still has a certain adsorption capacity for MG, which indicates that other interactions are involved in addition to electrostatic adsorption.

Effect of temperature
The effect of temperature on MG removal rate was investigated in the range of 5 °C-35 °C.It can be seen from figure 6(d) that the removal rate of MG was increased with the increase in temperature.Thermodynamic studies show that ΔH > 0 of the adsorption process is an endothermic process, and the increase of temperature is conducive to adsorption.In addition, according to the solvent displacement theory in liquid phase adsorption, temperature rise accelerates the migration of dyes in solution and the desorption of water molecules on the surface of adsorbent [51].The above factors work together to form the phenomenon that the removal rate increases with the increase of temperature.This is consistent with the research results of Eltaweil et al on the adsorption behavior of MG by corn stalk-derived magnetic biochar [52].Figure 6(d) also shows that at 25 °C, 30 °C and 35 °C, the MG removal rate reached more than 95% after 5 min.Considering the actual temperature of the waste water, the economic cost and the practical application, 25 °C is appropriate.

Effect of ionic strength
Textile printing and dyeing wastewater contains a lot of inorganic salts, such as NaCl, Na 2 SO 4 , Na 2 CO 3 and NaHCO 3 , etc, which mainly come from the residue of printing and dyeing additives, the neutralization treatment of desizing and mercerizing wastewater [53,54].Therefore, it is necessary to study the effect of ionic strength on adsorption of pollutants.Under the conditions of 30 mg l −1 MG , 20 mg Fe 3 O 4 /BC, pH 9, 25 °C and 0 ∼ 0.1 mol/L NaCl concentration, the effects of ions on adsorption were investigated.As can be seen in figure 6(e), the fluctuation of adsorption rates is small, all of which are less than 2.5%.On the one hand, the reason may be that Na + neutralizes the negative charge on the surface of the adsorbent, reducing the adsorption effect of MG.On the other hand, ions can promote the protonation of MG, prevent the agglomeration of dye molecules, and promote adsorption [55,56].The adsorption rate did not change significantly, and the change of ion concentration had little effect on the adsorption effect, which was consistent with the results of Elgarahy et al [57,58].The study investigated the effects of four inorganic salts (NaCl, Na 2 SO 4 , Na 2 CO 3 and NaHCO 3 ) of 0.1 mol L −1 on the adsorption of MG by MBC. Figure 6(f) shows that Na 2 CO 3 and NaHCO 3 have slightly stronger influence on adsorption than NaCl and Na 2 SO 4 , and show a certain synergistic effect.It may be that the hydrolysis of Na 2 CO 3 and NaHCO 3 to hydroxyl ion promotes the electrostatic adsorption between the negatively charged Fe 3 O 4 /BC and MG cation.It can be seen that the MG removal rate of the material is not significantly affected by salt, indicating that the material has potential in the treatment of printing and dyeing wastewater with high salinity.

Solid-liquid separation performance and reuse performance
The hysteresis loop and related magnetic performance parameters of Fe 3 O 4 /BC are shown in figure 7(a).The magnetization of Fe 3 O 4 /BC positively increased and reached saturation with the enhancement of the applied magnetic field, showing typical ferromagnetic characteristics.The saturation magnetization value of Fe 3 O 4 /BC (17.22 emu g −1 ) ensures that the material quickly achieves magnetic separation under the condition of external magnetic field.As shown in figure 7(a), Fe 3 O 4 /BC can be separated from MG solution within 30 s with the help of 1.2 T magnetic rod.Cyclic adsorption tests were carried out to evaluate the reuse performance of the materials.
The regeneration and reuse of adsorbent materials is of great practical significance for reducing the cost of pollutant treatment and avoiding secondary pollution [59,60].Herein, the desorption and regeneration experiment of Fe 3 O 4 /BC was carried out by ultrasonic regeneration method.Firstly, the fully adsorbed Fe 3 O 4 /BC was separated from the solution (MG concentration 30 mg l −1 , reaction temperature 25 °C, Fe 3 O 4 /BC dosage 0.02 g, adsorption 24 h) by magnetic rod and placed in a tapered bottle with plug.Then 100 ml of distilled water was added for 2 h ultrasonic shock.Finally, the adsorbent is washed 3 times in distilled water and dried for next use.As shown in figure 7(b), after four consecutive uses of the material, the removal rate of MG was 78%.The decrease of adsorption performance of Fe 3 O 4 /BC after repeated use may be related to the desorption regeneration method.Ultrasonic regeneration can desorbate adsorbent through acoustic cavitation, which has the advantages of low energy consumption and recovery of useful substances, but it also has the characteristics of unstable desorption efficiency [61].The solvent regeneration method breaks the phase balance between adsorbent materials and pollutants through the addition of solvents, so that pollutants are desorbed [62].Zhu et al [63] used ethanol to desorption malachite blue from sludge-based magnetic biochar, and Yang et al [25] used acetone to desorption nicotinic insecticides from corn stalk magnetic biochar, both of which achieved good results.In the future, the reuse performance of the material can be improved by optimizing the desorption regeneration method.

Adsorption isotherm
The concentration of MG solution in this experiment was added according to the gradient.The isothermal adsorption results of Fe 3 O 4 /BC for MG, fitted with the equations Langmuir and Freundlich equations, are shown in figure 8 and table 3. The R 2 value of Langmuir isotherm is higher than that of Freundilch isotherm, indicating that the Langmuir isotherm is more suitable for describing the adsorption characteristics of Fe 3 O 4 /BC to MG at this temperature.It is generally believed that the Langmuir model is obtained by the adsorption of adsorbent on an open surface with a single molecular layer, while the Freundlich model is suitable for the simulation of the adsorption equilibrium of multimolecular layers on an inhomogeneous surface of the adsorbent [64,65].It can be inferred that the adsorption process of Fe 3 O 4 /BC on MG is mainly monolayer adsorption.Langmuir's equation is expressed in the following mathematical formula: Freundlich's equation is expressed in the following mathematical formula: C e -solution concentration after equilibrium of adsorption reaction, mg/l; Q e -adsorption capacity of magnetic biomass to malachite green during adsorption equilibrium, mg/g; Q m -theoretical adsorption capacity of magnetic biomass to adsorbate, mg/g; k L , k F and n are all constants;

Kinetics of adsorption
In the adsorption kinetics test of Fe 3 O 4 /BC on MG, the adsorption fitting pattern is shown in figure 9(a).There were two stages of fast adsorption and slow adsorption throughout the whole adsorption process.Further, the pseudo-first-order, pseudo-second-order, and intraparticle diffusion models were used to fit the experimental data (figure 9(a)).pseudo-first-order model equations:   pseudo-second-order model equations:

= +
t-contact time, min; Q t -adsorption capacity of MBC at t time, mg/g; Q e -adsorption capacity at the moment when adsorption reaches equilibrium, mg/g; k 1 , k 2 -are all constants; As shown in table 4 that the correlation coefficients R 2 of the pseudo-second-order kinetic equation were 0.9884 and 0.9993, respectively, and the fitted adsorption capacity was close to the actual adsorption capacity.This can be used to describe the adsorption process of Fe  [66,67].As shown in figure 9(b), the first stage represents membrane diffusion with the curve rising sharply, the second stage represents intraparticle diffusion, and the third stage represents the adsorption-desorption equilibrium.The first stage of rapid increase is due to the adsorption site that can be used on the outer surface of the adsorbent, which is easy to contact with MG molecules in the aqueous solution, after which the rate of Fe 3 O 4 /BC surface gradually decreases and adsorption occurs in the sorbent pores.In conclusion, the process of Fe 3 O 4 /BC adsorption of MG is more complicated, and more than one control step is involved in adsorption.
The adsorption properties of similar magnetic biochar are shown in table 5.It can be seen that the adsorption properties of magnetic biochar composites are greatly affected by many factors such as biomass raw materials, magnetic precursor types, preparation methods and adsorption conditions.The adsorption capacity of the synthesized material in this study is at a high level, but attention should also be paid to the relatively low specific surface area of the material, which can be prioritized to improve the adsorption performance in the future.

Thermodynamic studies
From table 6, it can be seen that the thermodynamic influence and mechanism of Fe 3 O 4 /BC adsorption on MG.ΔH > 0 in the adsorption of MG indicates that the adsorption capacity of Fe 3 O 4 /BC for MG changes positively and the adsorption capacity increases with the increasing reaction temperature.Ambient temperature is

Adsorption mechanism
The adsorption of pollutants by biochar is mainly attributed to electrostatic adsorption, ion exchange and chemisorption [21,73].The electrostatic adsorption mechanism has been discussed in detail in section 3.2.2above.Ion exchange behavior includes anion exchange and cation exchange.The adsorption behavior of MG on biochar was attributed to cation exchange.In the Fe 3 O 4 /BC material, it can be found that there is abundant Fe 3+ /Fe 2+ on the surface of Fe 3 O 4 /BC.The iron ions dissociated from the surface of biochar have a strong ion exchange ability, and the ion exchange occurs with MG, so that MG is adsorbed to the surface of the material.FT-IR analysis was used to investigate the MG adsorption mechanism.Figure 10(a) shows the FT-IR spectra of Fe 3 O 4 /BC and Fe 3 O 4 /BC-MG.New peaks appeared at 1,210, 1,199, 1,087 and 902 cm −1 , indicating that MG was absorbed by Fe 3 O 4 /BC.The absorption band of the hydroxyl group of Fe 3 O 4 /BC was shifted at 3,415 cm −1 indicating that there was a hydrogen bond between Fe 3 O 4 /BC and MG molecules.The C=C telescopic vibration peak at 1,510 cm −1 was enhanced, manifesting that the π-π interaction might be involved in MG adsorption [74].
XPS analysis showed that the sample consisted mainly of Fe, O, and C (figure 10(b)).The C1s spectra of Fe 3 O 4 /BC showed that the binding energy of three peaks after adsorption of MG at 283.9 eV (C-C/C-H), 285.3 eV (C-O) and 288.2 eV (C=O) changed, as shown in figure 10(c).Some new oxygen-containing functional groups (C-O and C=O) were formed.This indicates that these groups may interact with Fe 3 O 4 /BC via hydrogen bonds.In the Fe2p spectrum of Fe 3 O 4 /BC-MG (figure 10(d)), the binding energy peaks of 710.7 and 724.6 eV are attributed to Fe 2p3/2 and 2p1/2, with peak-to-peak spacing of approximately 13.7 eV, which is consistent with Liang's report [75].Satellite signals of 716.9 eV confirm the presence of Fe(III) [76].Based on

Conclusion
Fe 3 O 4 /BC was prepared from peanut hull by co-precipitation method, which was used to adsorb and remove MG from water.Fe 3 O 4 particles were uniformly loaded on BC.The pore structure of the material changed and the total pore volume increased.The surface of the material contained abundant oxygen-containing functional groups whose number increases.The adsorption of MG by Fe 3 O 4 /BC was adapted to a wide range of pH (5)(6)(7)(8)(9)(10)(11).With an increase in the initial pH, the electrostatic attraction between Fe 3 O 4 /BC and MG was enhanced, and the adsorption effect is improved.MG removal rate was not significantly affected by ionic strength.The removal rate of MG can reach more than 95% under the conditions of initial concentration of MG of 30 mg l −1 , Fe 3 O 4 /BC dosage of 20 mg, pH of 9, and temperature of 25 °C.Under the action of 1.2 T magnetic rod, Fe 3 O 4 /BC can be separated from MG solution in 30 s, showing strong solid-liquid separation performance.
The adsorption of MG on Fe 3 O 4 /BC was mainly chemical adsorption, which was a spontaneous endothermic process, and the adsorption can be promoted by rising temperature.The adsorption mechanism can be attributed to electrostatic adsorption, cation exchange, hydrogen bonding, and π-π interaction.The revelation of adsorption mechanism and the discussion of the influencing factors of adsorption performance will be beneficial to exert the adsorption performance of materials and improve the removal effect of organic pollutants in water.
Figures2(a)-(b)show the SEM images of the apparent morphology of biochar materials at a calcination temperature of 500 °C.It can be seen that the surface of the pristine biochar is smooth and has more pores.On the surface of biochar loaded with Fe, many small white spherical particles can be clearly seen on the surface of the material.It can be seen from the elemental analysis in table 1 that the mass fraction of Fe and O elements of the material is significantly increased.The results show that the loaded Fe is mainly in the form of iron oxide, and there are a large number of Fe-O bonds on the surface of the material, which is consistent with the FT-IR analysis results.In addition, EDS showed that the content of C and Fe in the material was high, and the number of light spots was large, indicating that Fe was evenly distributed in the material.Combined with XRD analysis, it can be inferred that Fe 3 O 4 crystal particles are successfully loaded on the surface of biochar.The N 2 adsorption-desorption isotherms of materials BC and Fe 3 O 4 /BC are shown in figure3(a).The results show that the isotherms of biochar all belong to type IV isotherms, and there is capillary condensation in the high pressure range, which is an obvious H4 type lagging rings.The presence and shape of the hysteresis ring at a relative pressure of 0.1-0.9indicate that the adsorbent was micro-mesoporous and is a kind of fractured pore material[37].After modification, the adsorption capacity of N 2 is significantly enhanced in the later stage, indicating that the specific surface area of the material is increased in the later stage.Figure3(b)shows that Fe 3 O 4 /BC has a strong force with nitrogen, and the initial adsorption curve is type I, which is due to the strong adsorption potential exhibited by the numerous micropores and mesopores.
Fe 3 O 4 /BC for MG As shown in figure 5, the adsorption performance of Fe 3 O 4 /BC increases rapidly within 5 min.Compared to BC, the adsorption performance of Fe 3 O 4 /BC on MG was significantly improved.It can be seen that the

Figure 6 .
Figure 6.(a) Effect of Fe 3 O 4 /BC material dosage on MG adsorption; (b) Zeta potential-pH plot for Fe 3 O 4 /BC; (c) Percentage adsorption of MG as function of pH by Fe 3 O 4 /BC; (d) Effect of temperature on MG adsorption; (e)Effect of ionic strength; (f) Effect of ionic type.

3 O 4 /
BC to MG, indicating that there is chemisorption occurs in this adsorption behavior.The analysis results of the intraparticle diffusion model are shown in figure 9(b).The intraparticle diffusion model of Fe 3 O 4 /BC adsorption on MG showed a multiple co-linear relationship.During the adsorption of MG by Fe 3 O 4 /BC, MG molecules quickly transferred to the surface of Fe 3 O 4 /BC, and then slowly entered the internal pores of biochar.With increasing of adsorption duration, the diffusion rate of MG in Fe 3 O 4 /BC pores gradually decreased, and the adsorption process of Fe 3 O 4 /BC to MG gradually reached equilibrium.The intraparticle diffusion model for Fe 3 O 4 /BC adsorption of MG was segmented.However, intraparticle diffusion was not the main controlling step in the adsorption process

Table 1 .
Elemental analysis of biochar samples by ICP-OES.

Table 2 .
Pore structure parameters of BC and Fe 3 O 4 /BC.

Table 3 .
Isotherm parameters of adsorption of MG by Fe 3 O 4 /BC.

Table 4 .
Parameters of kinetic model for adsorption of MG.

Table 5 .
[71,72]ion capacities comparison with other magnetic adsorbents.conducive to promoting the adsorption reaction and Fe 3 O 4 /BC to remove MG from water.The experimental results of the effect of temperature on the removal rate also confirm the inference.ΔG is negative and ΔS is positive, indicating that the adsorption process is spontaneous and irreversible[71,72].