Synthesis, Optical Characterization, and Adsorption of Novel Hexavalent Chromium and Total Chromium Sorbent: A Fabrication of Mulberry Stem Biochar/Mn-Fe Binary Oxide Composite via Response Surface Methodology

In this study, a new generation chromium sorbent, mulberry stem biochar/Mn-Fe binary oxide composite (MBC-MFC), was fabricated by chemical precipitation on carbonized mulberry stem according to response surface methodology (RSM) results. RSM was more convenient to figure out the optimized preparation condition of MBC-MFC theoretically for achieving a maximum removal efficiency of Cr(VI) and total chromium (TCr), compared to labor-intensive orthogonal experiments. The RSM results showed that Fe/Mn concentration (CFe; CMn), MBC activation temperature after soaking in KOH solution (T), and pH during precipitation of Fe-Mn oxide were three main factors to significantly affect the efficiency of MBC-MFC (p < 0.05) in Cr(VI) and TCr removal. With the selected condition (C Fe = 0.28 mol/L; C Mn = 0.14 mol/L; T = 790°C; pH = 9.0), MBC-MFC was synthesized with a large surface area (318.53 m2/g), and the point of zero charge values of MBC-MFC was 5.64. The fabricated MBC-MFC showed excellent adsorption performance of Cr(VI) and TCr in an aqueous solution. The maximum Cr(VI) and TCr removal capacity of MBC-MFC was 56.18 and 54.97 mg/g (T = 25°C, pH = 3.0, t = 48 h, and dosage = 0.10 g/50 ml), respectively, and the maximum Cr(VI) adsorption of MBC-MFC was 4.16 times that of bare MBC, suggesting the synergistic effects of Fe/Mn oxides and MB on the performance of MBC-MFC in Cr(VI) and TCr removal. The adsorption mechanism of MBC-MFC on chromium was mainly contributed by surface complexation and electrostatic attraction. Our study offers valuable outlooks to develop high-performance biochar-based sorbents for heavy metal removal and sustainable environmental remediation.


INTRODUCTION
Toxic heavy metal ions are very harmful to human health and environmental ecosystem Liu et al., 2021), among which chromium-containing compounds are among the most ubiquitous contaminants (Kretschmer et al., 2019;Pei et al., 2020;Xu et al., 2021). Chromium commonly exists in the industrial waste and natural environment in the form of Cr(III) and Cr(VI) (Andrade et al., 2019;Qiu et al., 2020;Zhang et al., 2020). Cr(III) is comparatively stable in an environment with low mobility and solubility, while Cr(VI) primarily exists in the form of oxyanions such as CrO 4 2− , Cr 2 O 7 2− , and HCrO 4 − , which are highly soluble and mobile . The toxicity of Cr(VI) is much greater than that of Cr(III) and would cause carcinogenesis in living creatures. Therefore, Cr(VI) has been identified as the top priority hazardous pollutant by the United States Environmental Protection Agency (EPA) (Lv et al., 2020;Park, 2020). The Ministry of Ecology and Environment of the People's Republic of China has set the maximum concentration of Cr(VI) for the legal discharge of surface water as 50 ug/L (Lv et al., 2020).
In order to remove the chromium from industrial wastewater, various methods have been investigated (Ma and Tsai, 2017;An et al., 2018;Zhao et al., 2018;Li et al., 2019a;Breytus et al., 2019;Ye et al., 2019;Choi et al., 2020;Tang et al., 2020), such as membrane separation, electrochemistry, chemical precipitation, photocatalysis, ion exchange, constructed wetlands, and adsorption. Among these available approaches, the adsorption method is one of the most commonly used methods based on its good adsorption capacity, recyclability, low-cost, environmentalfriendly materials, and operation simplicity (Qin et al., 2020). Biochar is a kind of widely used material for chromium adsorption due to its high specific surface area and hierarchical pore structures. Biochar could be easily obtained via thermochemical pyrolysis of various waste biomass under an oxygen-free atmosphere, such as rice straw, algae, walnut shell, energy crops, bagasse, forest residue, and even activated sludge. Thus, biochar is regarded as a well-produced material based on widespread natural material resource with high specific surface area and hierarchical pore structures, which could prevent the agglomeration of Fe-Mn binary oxide nanoparticles and provide adsorption sites. In addition, the biochar surface contains a lot of oxygen-containing functional groups, which allowed the organic pollutants and heavy metal pollutants to be adsorbed to the surface of biochar and reduced Luo et al., 2020). Biochar has been demonstrated as a promising sorbent for heavy metal adsorption . Guangxi Zhuang Autonomous Region is one of the provinces in China with the largest number of mulberry trees used for silkworm cultivation . In China, about six million tons of mulberry stem is harvested each year and conventionally used for burning, which not only wastes resources but also causes environmental pollution (Liao et al., 2018).
Iron and manganese oxides have been demonstrated to have a high adsorption affinity towards heavy metals. Besides (Aryal et al., 2011;Granados-Correa and Bulbulian, 2012), Fe-Mn mixed metal oxide nanocomposites also showed good regeneration and stability activity for Cr(VI) removal (Wang and Fu, 2013). The granular Fe-Mn binary oxide (GFMO) was prepared and the adsorption capacity of GFMO for Cr(VI) was 16.79 mg/g . Fe-Mn bimetal oxide (IMBO) nanospheres were synthesized by template-free method and the adsorption capacity of IMBO towards Cr(VI) was 105.96 mg/g . Iron and manganese oxides are easy to aggregate, which would have a significant influence on their adsorption performance. Thus, they are normally loaded on biochar to inhibit their aggregation.
HA/Fe-Mn oxides-loaded biochar (HFMB) was synthesized and used to adsorb cadmium (Cd) and arsenic (As) that the maximum adsorption capacity of HFMB was 67.11 mg/g for Cd(II) and 35.59 mg/g for As(V), which is much higher than pristine biochar (11.06 mg/g and 0 mg/g for Cd(II) and As(V), respectively) . Liang et al. (2021) used Mnincorporated ferrihydrite to adsorb Cr(VI) and the maximum adsorption capacity was 48.5 mg/g, which was 30% higher than pure ferrihydrite. Luo et al. (2013) also used manganese dioxide/ iron oxides combined with acid oxidized multiwalled carbon to adsorb Cr(VI) and showed excellent adsorption performance for Cr(VI). Sand media simultaneously coated with iron and manganese were applied to treat synthetic wastewater contaminated with both Cr(VI) and As(V) (Chang et al., 2012). Our group  discovered that Fe-Mn binary oxide/mulberry stem biochar could synergistically improve the sorption of Cr(VI). In summary, the composite adsorbent consisting of manganese/iron oxides and scaffold structure with high specific surface area is beneficial to its adsorbing performance of heavy metals, and biochar has been demonstrated as a typical scaffold to load both manganese oxides and iron oxides for heavy metal adsorption. "Green" iron oxide combined with mulberry acted as a sorbent to the removal of Cr(VI) from aqueous solutions (Poguberovi´et al., 2016). It has shown great potential to remove Cr(VI) by using the biochar and Fe-Mn binary oxide composite.
Response surface methodology (RSM) is a collection of mathematical and statistical techniques based on the fit of a polynomial equation to the experimental data, which is a useful tool to study the interactions of two or more parameters. The Plackett-Burman (RSM-PB) and the Box-Behnken (RSM-BBD) are two designs in RSM (Hanrahan and Lu, 2006), which have been used for designing and optimizing the environmental experiments. RSM-PB was designed as a two-level experimental design that requires fewer runs than a comparable fractional design and can be used to identify the more important independent variables from a long list of candidate factors and select them to realize a complete factorial design (He and Tan, 2006). RSM-BBD is a secondorder multivariate technique based on three-level incomplete factorial designs that received a wide application for assessment of critical experimental conditions, that is, the maximum or minimum of response function (MourabetRhilassi et al., 2012). However, to the best of our knowledge, there are limited studies on the application of RSM in optimizing the oxidation of iron and manganese on mulberry stem biochar (MBC) for Cr(VI) removal.
In this study, the optimized preparation condition of MBC-MFC to maximize its removal rate of Cr(VI) and T Cr was figured out based on RSM theoretically. Accordingly, MBC-MFC was fabricated based on the optimized condition, its performance in T Cr and Cr(VI) removal was studied, and the corresponding removal mechanisms were investigated.

Materials
All chemical reagents (viz., KMnO 4 , Fe(NO 3 ) 3 •9H 2 O, KOH, NH 4 HCO 3 , K 2 Cr 2 O 7 , H 3 PO 4 , H 2 SO 4 , HCl, C 2 H 5 OH, etc.) used in this study were of analytical grade and purchased from Shanghai Guoyao Group Chemical Reagent Co. Ltd., China. Stock solution [1,000 mg/L Cr(VI)] was prepared with K 2 Cr 2 O 7 and ultrapure water and was used to prepare diluted chromium-containing solutions for adsorption experiments. 1,000 mg/L of Cr(VI) standard solution was purchased from the National Nonferrous Metals and Electronic Materials Analysis and Testing Center, China, and stored at 4°C. Ultrapure water was used to prepare all solutions.

Preparation of Mulberry Stem Biochar Composite and Mulberry Stem Biochar/ Mn-Fe Binary Oxide Composite
The preparation methods of MBC and MBC-MFC were based on Chinese invention patents (Liang et al., 2019).
Preparation of MBC: the peeled mulberry was crushed into particles of less than 2 mm and finally dried in an oven (80°C) until it reached the constant weight. The dry mulberry was pyrolyzed at 450-650°C in a muffle furnace at the heating rate of 5°C/min until it reached a set temperature and maintained specified temperature for 3-6 h. The final products were denoted as MBC, which was stored in airtight glass bottles.
Preparation of MBC-MFC: some MBC was added to a conical flask containing 100 ml of 9.0-15.0 mol/L NaOH solution and stirred for 12-36 h and then filtered and dried; the dried MBC was activated at 700-900°C for 1 h. After activation, MBC was washed with ultrapure water 3-5 times and then filtered, and the filter cake was placed in a porcelain dish and dried at 110°C for 6 h to obtain the MBC intermediate. 3-6 g MBC intermediate was added to a 500 ml conical flask and oscillated with an ultrasonic oscillator at a frequency of 25-45 kHz for 30 min; then, the conical flask was put into a magnetic constant temperature water bath at 25°C. Under the magnetic stirring condition, 50 ml of 0.05-0.15 mol/L KMnO 4 solution and 50 ml of 1.0-1.5 mol/L NH 4 HCO 3 solution were added to the conical flask and stirred for 15 min; immediately after, 50 ml of 0.1-0.3 mol/L Fe(NO 3 ) 3 solution was added to the conical flask, and then the pH of mixture suspension was adjusted to 7.5-10.5 with 1.0 mol/L NaOH solution. Next, the conical flask was put into a water bath at 95°C reacted for 4-8 h, after being cooled to room temperature, filtered, and washed three times with ultrapure water and one time with absolute ethanol. The filter cake was placed in a porcelain dish and dried at 85°C for constant weight to get the MBC-MFC; then, the MBC-MFC was ground and sifted by a 200-mesh sieve.

Optimization of Preparation Conditions Through RSM
RSM is employed to optimize the preparation conditions in this work to study the effects of pH, temperature, concentration, and time on T Cr and Cr(VI) removal efficiency of MBC-MFC. RSM-PB was used to figure out the main factors from various factors that affect the removal rate of sorbent from lots of variables (He and Tan, 2006), and RSM-BBD was employed to optimize those main factors to obtain the best conditions for fabricating MBC-MFC with excellent performance to adsorb T Cr and Cr(VI) (Liang et al., 2019).

RSM-PB in RSM
The thermochemical pyrolysis temperature , residency time (Rogovska et al., 2012), heating rate (Rogovska et al., 2012), and heating method (Liang et al., 2017) of biochar have a great influence on the number of functional groups on biochar. Moreover, nanocomposite sorbents with nanosized metal oxyhydrates are attached to carbonaceous surfaces within biochar and increase the sorption of contaminants (Ahmed et al., 2016). In this paper, eight parameters for MBC-MFC preparation (Supplementary Table 1) have been evaluated for their effects on the removal rate of T Cr and Cr(VI). All preparation factors in Supplementary  Table 1 were set on the basis of previous single factor experiment to make the obtained results under critical conditions, such as the chosen variables ranges, batch experiment conditions, and the applied concentrations.

RSM-BBD in RSM
Based on the results of RSM-PB, we selected three main factors for MBC-MFC preparation and their effects on the removal rate of T Cr and Cr(VI), which was described as Eq.1 (Grady and Kenneth, 2006). The three selected factors were defined as X 1 , X 2 , and X 3 , respectively. They were coded at three levels, −1 (minimum), 0 (central), and +1 (maximum), covering the whole range of interest (Supplementary Table 2). The data in Supplementary Table 2 represent the factors and their level ranges in RSM-BBD.

Adsorbent Characterization
The morphological structures of MBC and MBC-MFC (under the optimal conditions' preparation) were analyzed by scanning electron microscopy combined with energy dispersive spectrum analysis (SEM-EDS) (Hitachi, JSM-7900F, Japan). X-ray diffractometer (XRD) (PANalytical, X'Pert PROX, Netherlands) and X-ray photoelectron spectroscopy (XPS) (Thermo Fisher Scientific, ESCALAB 250Xi, United States) analyses were used to identify the adsorbent constituents. A Fourier transform infrared spectrometer (FT-IR, PerkinElmer, Llantrisant, United Kingdom) was employed to collect spectra in the 400 and 4,000 cm −1 regions to identify the surface functional groups. The surface zeta potentials of MBC-MFC and MBC were measured by a Nano-ZS90 apparatus (Malvern Panalytical, Nano-ZS90, United Kingdom). Specific surface area, pore size distribution, and pore volume were performed by a JW-BK200C apparatus (BET-N 2 ) (Beijing Jingwei Gaobo, JW-BK200C, Beijing, China).

Batch Adsorption Experiments
The batch adsorption experiments were investigated by adding 0.10 g MBC-MFC to a series of 100 ml polyethylene plastic centrifuge tubes containing 50 ml of 45 mg/L Cr(VI) solutions, which were adjusted to a certain pH with 0.1 mol/L NaOH or 1.0 mol/L HCl. The centrifuge tubes were sealed by paraffin.

Desorption Studies
After Cr(Ⅵ) adsorption reached equilibrium, 0.1 g of MBC-MFC was added to a 100 ml plastic centrifuge tube; then, 50 ml of the regeneration solution of HCl or NaOH solution (0.1, 0.3, or 0.5 mol/L) was added and shaken for 48 h at 25°C. After 4 h, the concentrations of desorbed Cr(VI) and T Cr in the solution were measured.

Regeneration and Reuse of Adsorbent
In order to test the reusability of MBC-MFC, three consecutive adsorption-regeneration cycles were performed in triplicate. During the adsorption process, 0.10 g of MBC-MFC was added to 50 ml Cr(VI) solution and shaken for 48 h. The regeneration method was carried out as follows: the used adsorbent was collected and gently washed with deionized water, dispersed into 50 ml of a regenerative solution (0.1, 0.3, and 0.5 mol/L HCl or NaOH), and shaken for 48 h. The concentration of desorbed Cr(VI) and T Cr in the solution was also measured. Lastly, the adsorbent after desorption was collected and washed with deionized water about pH 7.0 for the next cycle.

RESULTS AND DISCUSSION
where F, E, H, and C were the most significant effect factors (p < 0.01), demonstrating that the regression equation provided strong applicability.
In the T Cr response model, the R 2 value was 0.9710 and the adjusted R 2 Adj value was 0.8938, suggesting a high correlation between the actual results and the predicted values (Abdulhameed et al., 2019). The order of the influence of various factors on T Cr was F > E > H > G > C > A > B > D, where F, E, and H were the most significant influence factors (p < 0.05), indicating that the regression equation provided strong applicability.
The main factors (p < 0.05) of standardized Pareto graphs were shown in Supplementary Figure 1 (Vasiee et al., 2016;Ma et al., 2016). Factors such as F, E, H, and C were significant factors affecting the removal rate of Cr(VI), as shown in Supplementary Figure 1A, while F, E, and H were significant factors affecting the removal rate of T Cr , shown in Supplementary Figure 1B. Thus, we propose that F, E, and H are the most significant factors to affect the removal rate of T Cr and Cr(VI). Therefore, F, E, and H factors were used as the main factors for RSM-BBD (Vasiee et al., 2016). Due to the fact that the other influence factors were not significant, the optimal preparation conditions of the previous single factor experiment were adopted. Thus, A, B, C, D, and G were set at 5.70 mol/L, 450°C, and 3, 12, and 4 h, respectively.

Optimization by RSM-BBD
According to the results of Supplementary Figure 1, the factors in RSM-BBD of X 1 ′, X 2 ′, and X 3 ′ were E, F, and H, respectively. The experimental scheme in Supplementary Table 5 was obtained through the RSM-BBD, and 17 kinds of MBC-MFC were prepared under preparation conditions described in Supplementary Table 5. Except that the Cr(VI) concentration was 60 mg/L, other experimental conditions and methods were the same as RSM-PB. The observed and predicted mean removal rates of T Cr and Cr(VI) are shown in Supplementary Table 5.
In the Cr(VI) and T Cr response model, Design-Expert 8.0.6 software was employed to perform quadratic regression fitting the data in Supplementary One independent variable factor of F, E, and H was kept at a fixed level (coded value was 0) to analyze the influence of interaction between other variables on the adsorption capacity of Cr(VI) and T Cr on MBC-MFC. Accordingly, a 3D (three-dimensional response surfaces) surface map and 2D (plotting twodimensional contour lines) profile could be obtained (Jang and Lee, 2018), as shown in Figures 1A-C and Figures 1D-F. Figure 1A showed the interaction between E (700-840°C) and H (pH 7.6-9.2), while F was kept at a fixed level. Figure 1B showed the interaction between F (C Fe 0.10-0.24 mol/L; C Mn 0.05-0.12 mol/L) and H (pH 7.6-9.4), while E was kept at a fixed level. Figure 1C showed the interaction between E (740-820°C) and F (C Fe 0.10-0.18 mol/L; C Mn 0.05-0.09 mol/L).
Using the same analysis method as Figures 1D-F, the condition of E was selected at 700-850°C; F was C Fe 0.10-0.24 mol/L, C Mn 0.05-0.12 mol/L, and H was 7.5-9.4. According to analysis results of RSM, the best optimized synthesis conditions of MBC-MFC were determined: E, F, and H were set at 790°C, 0.14 mol/L, and 9.0, respectively. Through Figure 1, we can intuitively see that the preparation conditions have great effects on the adsorption capacity of Cr(VI) and T Cr on MBC-MFC.

Validation of the Model
The optimal values of the selected variables were got by solving the equation (Eqs. 2, 3) using Design-Expert 8.0.6 software: A, B, C, D, E, F, G, and H were set at 5.70 mol/L and 9 mol/L, 450°C, 3 and 12 h, 790°C, C Fe 0.28 mol/L, C Mn 0.14 mol/L, 4 h, and 9.0, respectively. The theoretical removal rates predicted under the above conditions were 95.13 and 87.2% to Cr(VI) and T Cr , respectively. However, the experiment mean removal rates were 96.47 and 88.63% to Cr(VI) and T Cr , respectively. The relative deviation between the predicted value and the experimental value was 1.40 and 1.64%, respectively, proving the validity of our model.

Characterization of Mulberry Stem Biochar Composite and Mulberry Stem Biochar/ Mn-Fe Binary Oxide Composite
According to the results measured by BET-N 2 , the surface area of MBC and MBC-MFC was 179.29 and 318.53 m 2 /g, respectively, and the total pore volume of MBC-MFC (0.337 cm 3 /g) was 2.6 times that of MBC (0.132 cm 3 /g). Increasing surface area and total volume are considered as one of the favorable conditions for improving the adsorption capacity of MBC-MFC. The SEM-EDS images ( Figure 2) revealed that there were lots of pores with different sizes on the surfaces of MBC and MBC-MFC. Some Fe-Mn oxides were loaded on the surface of MBC-MFC with the porous structure not being disrupted ( Figure 2B), which will be of great benefit to Cr(VI) and T Cr adsorption. The chemical composition of MBC-MFC showed that the elemental molar ratio of Fe to Mn was close to 2:1, the same as that of the preparation precursor. The EDS analysis showed that the content of O (17.75%) in MBC-MFC was significantly higher than that of MBC (13.46%), suggesting that MBC-MFC owned more adsorption sites for heavy metal complex adsorption (Liu et al., 2020a).
XRD patterns of MBC and MBC-MFC are illustrated in Figure 3A. 26.60°was corresponding to amorphous carbon (Liu et al., 2020b). The XPS spectra of MBC and MBC-MFC are shown in Figure 3B. The distinct peaks of Fe and Mn could only be observed on MBC-MFC, which confirmed the successful fabrication of Fe 2 O 3 and Mn 3 O 4 on MBC.
The FT-IR spectra of MBC and MBC-MFC were as shown in Figure 3C. The peak appearing at 3,391-3,422 cm −1 and 1,569-1,639 cm −1 was related to the stretching of -OH groups and C O, respectively, indicating that both MBC and MBC-MFC contained C O and -OH groups. The bending vibration at about 1,383 cm −1 was mainly contributed by the stretching vibration of Fe-O-Mn, and the bands at 1,118 cm −1 were associated with the bending vibration of hydroxyl groups of metal oxides (M-OH) (Du et al., 2017). MBC-MFC showed additional broadband at 595 cm −1 , relating to the stretching vibration of Fe-O-Fe (Luo et al., 2013). Therefore, the SEM-EDS, FT-IR, and XRD spectra confirmed that iron oxide and manganese oxide were successfully coated on the surface of MBC-MFC.  Figure 2, the pH ZPC of MBC-MFC was 5.64. When pH < pH ZPC (5.64), the functional groups of absorbent were protonated and positively charged, promoting the adsorption of chromium ions onto the MBC-MFC surface. When pH > pH ZPC (5.64), the surface of the adsorbent was negatively charged, which weakens the attraction for Cr(VI). In addition, a higher pH would tend to generate more OH − , which would occupy some of the adsorption sites, thereby reducing the amount of Cr(VI) adsorption on MBC-MFC .
The effects of pH on the adsorption of Cr(VI) by MBC-MFC are shown in Figure 4A. In Figure 4A, when the initial Cr(VI) concentration was set at 30 and 60 mg/L, the adsorption capacity of Cr(VI) increased slowly as the pH raised from 2.0 to 3.0. The uptake of cationic metal ions by biochar at pH 2.0 was lower than that at pH 3.0 (Senthilkumar et al., 2019). Further enhancement of equilibrium pH negatively influenced the adsorption capacity of MBC-MFC (3.0 < pH < 9.0). It shows that ion exchange and electronic interaction may be involved in the adsorption mechanism of Cr(VI) (Choudhary and Paul, 2018). The optimal pH value for MBC-MFC to adsorb Cr(VI) varies with the initial Cr(VI) concentration. When the initial Cr(VI) concentration was 30 mg/L, the best initial pH range was 2.0-5.0. When the initial Cr(VI) concentration was 60 mg/L, the best initial pH range was 2.0-3.0. When the pH value was in the range of 8-11, the adsorption capacity of Cr(VI) was relatively low and basically remains unchanged, which was similar to the results of Chen et al. (2018). The pH of the subsequent experiments was 3.0.
The effects of contact time on adsorption are shown in Figure 4B. It can be seen from Figure 4B that, with the increase of the adsorption time, the adsorption capacity of Cr(VI) increases, but with the extension of time, the adsorption capacity gradually reaches equilibrium. In the initial stage of adsorption, due to the large number of pores on the surface of MBC-MFC, it can provide more binding sites, and it was easy to adhere to the surface of MBC-MFC after adsorption. As the contact time increases, the number of adsorption binding sites decreases, making it difficult for the adsorption reaction to continue, thus reaching adsorption equilibrium (Zhang et al., 2018). When the initial concentrations were 30, 45, and 60 mg/L, the adsorption equilibrium was reached after 420, 960, and 1,440 min, and the removal rates were 99.2, 99.7, and 99.3%, respectively. With the increase of Cr(VI) concentration, the electrostatic repulsion between ions increases, so the adsorption equilibrium time was prolonged (Mondal et al., 2007;Park, 2020). After reaching the adsorption equilibrium, the equilibrium adsorption capacity of initial concentrations of 30, 45, and 60 mg/L was about 14.97, 22.46, and 29.97 mg/g, respectively.
The effects of temperature on adsorption are shown in Figure 4C. As shown in Figure 4C, the maximum adsorption capacity of MBC and MBC-MFC for Cr(VI) was 12.84 mg/g and 54.31 mg/g at 25°C, respectively. Compared with MBC, the adsorption capacity of MBC-MFC on Cr(VI) has been greatly improved, which was 4.16 times that before modification. At 25, 35, and 45°C, the equilibrium adsorption capacity of MBC-MFC for Cr(VI) was about 54.31, 58.57, and 62.60 mg/g, respectively.
The effects of MBC-MFC dosage on chromium adsorption are shown in Figure 4D. In Figure 4D, when the initial concentration of Cr(VI) was set at 30 mg/L, with the increase of the dosage from 0.02 g/50 ml to 0.40 g/50 ml, the adsorption capacity decreased from  46.97 to 3.75 mg/g, and when the initial concentration of Cr(VI) was set at 60 mg/L, the adsorption capacity decreased from 68.50 to 7.45 mg/g. The adsorption capacity of Cr(VI) decreased with the increase of the dosage, while the removal rate of Cr(VI) increases with the increase of the dosage. When the initial concentration of Cr(VI) was set at 60 mg/L, the removal rate increased from 45.66 to 99.98%. When the dosage was greater than 0.10 g/50 ml, the removal rate did not increase. The reason may be that, with the increase of the dosage, the total reaction sites on the sorbent surface were increased . Therefore, the effective dosage for Cr(VI) was selected at 0.10 g/50 ml for the latter experiments.
In this study, we employed a pseudo-first-order kinetic model and pseudo-second-order kinetic model to study the Cr(VI) adsorption kinetic onto MBC-MFC and MBC. The fitting plots are shown in Figures 5A,B, respectively, and the kinetic parameters are summarized in Supplementary Table 7. The pseudo-second-order kinetic model better fitted the experimental data of the Cr(VI) and T Cr removal by the MBC-MFC and MBC (R 2 0.957-0.989) and the Cr(VI) equilibrium adsorption capacities (14.97, 14.96, 9.61, and 9.50 mg/g) calculated by the pseudo-second-order kinetic model was approximately equal to the Cr(VI) experimental adsorption capacities data (14.98, 14.97, 8.90, and 8.82 mg/g) (Pap et al., 2018). Figure 6 shows the application of Langmuir and Freundlich isotherms to the Cr(VI) adsorption on MBC-MFC and MBC. Supplementary Table 8 lists the isotherms parameters. The Langmuir model better fitted the experimental data of the MBC-MFC and MBC (R 2 0.909-0.998) than the Freundlich model, which may imply monolayer adsorption of Cr(VI) and T Cr (Wen et al., 2018). The MBC-MFC exhibited superior T Cr and Cr(VI) adsorption capacity than MBC. The maximum T Cr and Cr(VI) adsorption capacities (q m ) of the MBC-MFC reached 54.97 and 56.18 mg/g, respectively. It was about 4.11 times that of MBC, which is 5.61 times that of dolomite adsorbent (Albadarin et al., 2012), 2.64 times that of Eucalyptus globulus bark biochar (Choudhary and Paul, 2018), 8.03 times that of biochar derived from municipal sludge (Chen et al., 2015), slightly higher than 2.64 times that of Eucalyptus globulus bark biochar (Zhang et al., 2018), and 2.14 times of Fe/Mn metal oxide nanocomposites (Yacou et al., 2018;Supplementary Table 9). The adsorption capacity of Cr(VI) was calculated based on the remaining amount of Cr(VI) in solution, while the adsorption capacity of T Cr was calculated based on the remaining amount of chromium in solution. According to our result, the maximum adsorption capacity of Cr(VI) was 2.20% higher than that of T Cr , indicating the existence of a little Cr(III) in the solution. The Cr(III) in solution after adsorption was originated from the redox reaction of Cr(VI) (Lv et al., 2012).

Regeneration and Reusability Study
Strong acids and base are the most common agents used to elute heavy metals, and the choice of eluent depends on the adsorption mechanism of heavy metals and the nature of the adsorbents (Markovski et al., 2014). The results of desorption experiments are shown in Figure 7A. It can be seen from Figure 7A that 0.1, 0.3, and 0.5 mol/L HCl were used as the desorption solvent, and the adsorption capacity of Cr(VI) by the adsorbent after desorption was 23.03, 26.17, and 27.67 mg/g, respectively. However, 0.1, 0.3, and 0.5 mol/L NaOH were used as the desorption solvent, and the adsorption capacity of Cr(VI) by the adsorbent after desorption was 28.79, 29.43, and 29.43 mg/g, respectively. Therefore, NaOH solution performed a better desorption effect than HCl solution as the regeneration reagents. Considering the cost factor, a 0.3 mol/L NaOH solution was selected as the desorption solution for the desorption regeneration of MBC-MFC.
The MBC-MFC desorption and resorption experiment is shown in Figure 7B. It can be seen from Figure 7B that as the number of desorption cycles increases, the adsorption amount of Cr(VI) also decreases. It can be attributed to the following reasons: ① The specific surface area (effective adsorption point) of the adsorbent was gradually decreasing during the adsorption-desorption cycle. ② During the adsorption-desorption cycle, the functional groups on the surface of the adsorbent were gradually weakened, but there were still a certain number of functional groups that play a role in adsorption (Li et al., 2016). After three cycles of adsorption/desorption experiment, the adsorption capacity of Cr(VI) showed little effect on Cr(VI) adsorption, which was only slightly reduced from 29.95 to 26.81 mg/g.
It shows that MBC-MFC can be regenerated and reused by NaOH solution, which also fully shows that the interaction between adsorbent and adsorbate mainly depends on electrostatic adsorption, which was consistent with the aforementioned characterization analysis and kinetic experiment analysis results . Mechanisms for Cr(VI) and T Cr Adsorption Figure 8A presents the survey XPS spectra of MBC-MFC before and after Cr(VI) adsorption. The element of Cr (576.2; 578.4 eV) was absent in the MBC-MFC result. However, it appeared on the XPS spectra of MBC-MFC after Cr(VI) adsorption, demonstrating the successful adsorption of Cr onto MBC-MFC. To further understand the Cr(VI) adsorption on MBC-MFC, high-resolution Cr2p spectra were measured, shown in Figure 8B. Obviously, no chromium peak was found on MBC-MFC before Cr(VI) adsorption. The broad peak of Cr2p 3/2 was found on the XPS result of MBC-MFC, which could be deconvoluted into peaks at binding energies of 578.4 and 576.2 eV, corresponding to the characteristic peaks of Cr(VI) and Cr(III) Zhong et al., 2020). The areas of the two peaks indicate that Cr(VI) is partially reduced to Cr(III) . The highresolution Cr2p spectra demonstrated the coexistence of Cr(III) and Cr(VI) on the microspheres of MBC-MFC. MBC-MFC can adsorb both Cr(III) and Cr(VI). We believe that there was a reduction of Cr(VI) to Cr(III) in the experiments. In order to understand the adsorption mechanism of Cr(VI) on MBC-MFC, Fe2p and Mn2p peaks of MBC-MFC and after Cr(VI) adsorption were measured ( Figures 8C-F). The observed two asymmetric peaks located at 710.3 and 724.3 eV could be ascribed to Fe2p 3/2 and Fe2p 1/2 , respectively. For Fe2p 3/2 , the peak at 710.3, 712.4, and 717.5 eV was attributed to Fe(II) and two forms of Fe(III), respectively (Li et al., 2019b). The binding energy of 710.0 eV (Fe2p 3/2 ) of Fe(II) peaks was shifted to 709.70 eV after Cr(VI) adsorption ( Figures 8C,E), implying that the Cr(VI) adsorption occurred through bidentate ligand or monodentate interaction with iron oxide (Peng et al., 2019). The observed two asymmetric peaks located at 641.4 and 640.3 eV could be ascribed to Mn2p 3/2 , respectively ( Figure 8D). The peak at 641.4 eV was ascribed to Mn(IV), and another peak at 640.3 eV was related to Mn(II) and Mn(III) in different forms (Li et al., 2019b;Lin et al., 2017). The binding energy of 641.4 eV (Mn2p 3/2 ) of Mn2p peaks was reduced by 0.3 eV after Cr(VI) adsorption ( Figure 8F), suggesting that the Cr(VI) adsorption occurred through surface complexation with Mn (Xiong et al., 2017).
In addition, chromium (CrO 4 2− , Cr 2 O 7 2− , and HCrO 4 − ) could be also adsorbed on the surface of MBC-MFC by electrostatic interaction or forming outer-sphere surface complexes. As illustrated in Supplementary Figure 2, by regulating the solution pH to be lower than the pH zpc value of MBC-MFC (5.64), the zeta potential of MBC-MFC was positive and desirable for adsorption of chromium anions by electrostatic attraction.

CONCLUSION
The MBC-MFC was successfully fabricated by chemical precipitation on the carbonized mulberry stem, acting as a novel adsorbent to remove T Cr and Cr(VI). RSM was employed to optimize the fabrication condition of MBC-MFC to realize the optimal removal of T Cr and Cr(VI). The predicted removal rates of Cr(VI) and T Cr by RSM agreed well with that of the experiment data. The MBC-MFC was demonstrated to own a high surface area (318.53 m 2 /g) containing large oxygen functional groups, which provided abundant sites for T Cr and Cr(VI) adsorption. In addition, the pH zpc of MBC-MFC was 5.64, which makes it more suitable for adsorption of anions chromium under lower pH (such as pH 2.0). Its maximum adsorption capacities of Cr(VI) and T Cr were 56.18 and 54.97 mg/g, respectively (T 25°C, pH 3.0, t 48 h, and dosage 0.10 g/50 ml). The adsorption kinetics and isotherms demonstrated that a higher Cr(VI) removal efficiency could be achieved using our developed MBC-MFC than other biochar adsorbents due to the synergistic effects between biochar and Fe-Mn binary oxides. The adsorption mechanism of MBC-MFC on chromium was mainly contributed by electrostatic attraction and surface complexation. It should be noted that the MBC-MFC material prepared based on RSM optimization showed superior adsorption capacity of hexavalent chromium compared to MBC-MFC prepared by traditional methods. Our study offers a valuable outlook to develop high-performance biochar-based heavy metal sorbents for sustainable environmental remediation. Last but not least, taking advantage of mulberry stems as adsorbent not only saves production costs and reduces air pollution caused by burning mulberry stems but also solves the problem of resource utilization of agricultural and forestry wastes. This paper only studies the adsorption performance of MBC-MFC under indoor experiment conditions; there would be still a long way to go before its practical engineering applications due to the fact that the actual adsorption of industrial wastewater would be affected by many conditions, such as complex pollutants system and temperature variation of wastewater during different processes.

DATA AVAILABILITY STATEMENT
The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.