Preparation of MnOx-Modified Biochar and Its Removal Mechanism for Cr(VI) in Aqueous Solution

Abstract

Biochar (BC) is considered to be a great potential adsorbent to remove various contaminants, but the sorption capacity for chromium (Cr) is predominantly limited for the net negative charge. In this study, BC from rice husk was impregnated with Mn(NO₃)₂ solution to synthesize MnOx-modified BC (MnOBCs) for enhancing Cr(VI) removal in an aqueous solution. MnOBCs were characterized, and MnOx (manganese ore) was found to be the dominant crystal in MnOBCs. Batch sorption and kinetic experiments combined with spectral analysis were carried out to elucidate the sorption capacity and mechanisms of Cr(VI) sorption onto BC and MnOBCs. Results showed that the sorption kinetic process fitted to the Elovich model, and the modification enhanced the sorption capacity of Cr(VI) on BC. Compared to ion strength, pH is the main control factor for Cr(VI) fixed on BC and MnOBCs, and the sorption amount decreased with the pH value increasing. Moreover, X-ray photoelectron spectroscopy results showed that the proportion of Mn(II) decreased from 33.59% to 8.33%, and that of Mn(VI) increased from 30.58% to 52.72% after Cr(VI) sorption on MnOBCs. Meanwhile, the reduction reaction of Cr(VI) occurred during the sorption process on the BC and MnOBCs surface, and the reduction reaction was more obvious on the MnOBCs surface. MnOx loaded in BC can enhance the electrostatic attraction and redox capacity, which can improve the removal of Cr(VI) in an aqueous solution. This study provides information on the sorption and redox of Cr(VI) on BC, and allows us to better understand the mechanism of Cr(VI) removal in solutions by MnOBCs.

1. Introduction

Chromium (Cr) is a strong carcinogen and frequently detected in water and soil environments owing to natural and anthropogenic activities, including tanning, dyeing, chemical manufacturing, and metal smelting [1]. Cr(VI) and Cr(III) are dominant forms of Cr in water, and they can interconvert with each other in the environment [2]. Cr(III) is an important component of the glucose tolerance factor and participates in lipid metabolism regulation [3]. However, Cr(VI) compounds have attracted attention because of their potential toxic effects on humans, animals, plants, and microorganisms [4]. Thus, it is essential to further explore the removal method of Cr(VI) in the environment by its transformation and immobilization.

Currently, many Cr(VI) removal methods have been studied, such as biological reduction [5], chemical precipitation [6], sorption [7,8], complexation [9], and photocatalytic removal [10]. In addition, sorption is considered a practical method for Cr removal from wastewater due to its wide applicability and high efficiency [11]. Many carbon-based materials and their derivatives (such as activated carbon [12], carbon nanotubes [13], graphene [14], and BC [15]) have been used used to remove Cr(VI).

Biochar (BC) is a product of high-temperature pyrolysis of carbon-rich biomass, which is obtained from the waste of agriculture [16], forestry [17], animal husbandry [18], and fishery [19], under anaerobic conditions. Meanwhile, BC has been considered as a new way to increase the “carbon sink” [20]. In recent years, BC was used as an excellent adsorbent to remove hazardous chemical compounds because of its special physicochemical properties [21,22,23]. For example, BC showed good sorption for Pb and Cd, and maximum sorption capacities reached 359 mg g⁻¹ and 135.7 mg g⁻¹, respectively [24,25]. Biochar from freshwater hornwort and macroalgae showed the same good sorption capacity for Cr(III), with maximum sorption of 163.9 mg g⁻¹ and 60.2 mg g⁻¹, respectively [26]. It was also shown that the sorption of Cr(III) by freshwater macroalga biochar increased with the increase of pyrolysis temperature [27]. Meanwhile, the maximum adsorption capacity of Cr(III) by biochar pyrolyzed from waste tomato leaves and stems at 800 °C was 169.5 mg g⁻¹ [28]. However, BC is not good at removing anionic negative ions, such as Cr(VI), As, Se, and Sb. For instance, pyrolytic BC from bean straw was difficult to adsorb Cr(VI), and sorption capacity was only 6.48 mg g⁻¹ [29]. The maximum sorption capacity of As on BC prepared from pine wood was only 0.265 mg g⁻¹ [30]. BC pyrolyzed from pine birch and bamboo at 600 °C for 1 h showed a sorption capacity of 2.59 and 3.35 mg g⁻¹ for Cr(VI), respectively [31].

Nevertheless, some studies expressed that BC could be modified by other materials to improve its removal capacity for contaminants in an aqueous solution. The physical, chemical, and biological methods used in BC modification can enhance its sorption efficiency [32]. Unlike other physical and chemical modification methods, metal oxide loading onto BC can increase the positive charge and its affinity for negatively charged metals [33]. Modified magnetic biochar (BC) was prepared from the litchi shell for removing Cr(VI) from wastewater, and this adsorbent is not only reusable but also easy to recycle [34]. For example, Fe-Mn-Ce oxide-modified corn stover BC composites enhanced the electrostatic attraction between the composites and As(III) [35]. Similarly, the Cr(VI) anion interacts electrostatically on the surface of the positively charged Fe-Mn oxide-modified BC [36]. On the other hand, copper-modified activated bamboo charcoal (MAC) can have more effective removal rates for Cd, Pb, As, and Cr over a wider pH range than commercial activated charcoal (CAC) [37]. Meanwhile, Cu-modified activated bamboo charcoal (10BCKOH) was used to remove Ru dyes from an aqueous solution mainly by the ion-exchange mechanism [38]. As shown in

Table 1. Study on Cr(VI) removal by different biochar.

Biochar Feedstock	Main Chemicals Used	Reported Mechanisms	qmax (mg g−1)	Reference
Phoenix tree leaves	FeCl3·6H2O	electrostatic attraction, reduction, and chelation	27.2	[44]
Sewage sludge	FeCl3·6H2O	electrostatic attraction, complexation, and reduction reaction	31.53	[45]
Rice husk	FeCl3·6H2O ZnCl2	electrostatic attraction and ion exchange	9.97	[46]
Corn stover	ZnSO4	electrostatic attraction and complexation	24.5	[47]
Wheat straw	Bi2O3	electrostatic attraction and reduction reaction	12.23	[48]
Sea buckthorn stones	ZnCl2	electrostatic attraction, complexation, and reduction reaction	19.3	[49]
Rice husk	FeSO4·7H2O	electrostatic attraction and reduction reaction	23.25	[50]
Corncob	FeCl3·6H2O	electrostatic attraction, ion exchange and adsorption coupled-reduction	25.94	[51]
Melia azedarach wood	Fe(NO3)3·9H2O	reduction reaction	25.27	[52]
Corncobs	Fe(NO3)3, FeCl3·6H2O and pyrrole	ion exchange, chelation, complexation, and reduction reaction	19.23	[53]
Rice husk	Mn(NO3)2	electrostatic attraction, complexation, and reduction reaction	28.58	This study

, the maximum adsorption capacity of MnOBCs for Cr(VI) was higher than that of most other modified biochars in the literature reported in recent years. Meanwhile, most studies have focused on Fe-modified biochar. Fewer studies have been conducted on Cr(VI) removal by Mn-modified biochar.

Manganese oxides are important minerals in soils with many advantages, including variability, abundant hydroxyl groups, and often applied to remove contamination [39]. Nevertheless, the high cost of MnOx limits its utilization as a sorbent. The combination of BC and nanomaterials promotes its efficiency in environmental remediation [40]. Therefore, MnOx-modified BC was used for pollutant removal with favorable performance [41]. For example, MnOx-modified BC was prepared by subjecting BC to a series of reactions with KMnO₄ and exhibited excellent sorption capacity for CuCA [42]. In addition, manganese oxide-modified macro-mesoporous BC was prepared and showed a strong preference for heavy metal sorption when high concentrations of the substances coexisted [43]. On the other hand, using the high reduction capacity of MnOx, the manganese iron oxide-modified corn stover BC reduced Cr(VI) to Cr(III) effectively, and reduced the toxicity of Cr [36].

Considering these findings, MnOx plays a very important role in BC modification and Cr(VI) reduction, but it remains unclear what the valence state of Mn exerts on such an electron transfer process with BC. Therefore, in this study, (1) BC was prepared from rice husk and modified by MnOx using chemical impregnation; (2) The physicochemical property of BC and modification of BC were determined and characterized; (3) Sorption kinetic and isothermal sorption experiments of Cr(VI) were carried out to obtain the sorption capacity, and the effects of pH and ion strength on the removal of Cr(VI) were investigated. (4) Energy-dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) were analyzed to elucidate the removal mechanism of Cr(VI) by BC and modified BC.

2. Materials and Methods

2.1. Chemical and Reagents

All chemicals were analytical grade or better. Potassium dichromate (K₂Cr₂O₇), phosphoric acid (H₃PO₄), and manganous nitrate (Mn(NO₃)₂) were from Chengdu Kolon Chemical Co., Chengdu, China. Sodium hydroxide (NaOH), sulfuric acid (H₂SO₄), hydrochloric acid (HCl), Nitric acid(HNO₃), and acetone(C₃H₆O) were from Chongqing Chuandong Chemical Group Co. Chengdu, China. Diphenylcarbazide(C₁₃H₁₄N₄O) was from Shanghai Aladdin Biochemical Technology Co., Ltd. Shanghai, China. The solvent used in experiments was deionized water.

2.2. The Modification of BC

Rice husk (a typical biomass waste) collected from a farmland in Chongqing, China, was used to prepare BC at 600 °C. Briefly, rice husk was dried and ground to less than 2.0 mm and then carbonized in a tube furnace at 600 °C with 5.0 °C min⁻¹ under N₂ protection for 2.0 h. N₂ flow rate of 200 mL min⁻¹. The prepared BC was ground to pass through a 0.15 mm sieve size, and then placed in a ziplock bag and set aside.

Then, BC was immersed in HNO₃ solution with a mass fraction of 10% for 1.5 h, and washed with deionized water to neutral and finally dried in an air oven. The preparation process of MnOx-modified BC (MnOBCs) is shown in Figure 1. First, BC (after acid-washed) was impregnated with 0.05 or 0.1 mol L⁻¹ Mn(NO₃)₂ solution for 6 h according to a solid–liquid ratio of 1:10. Next, the water was evaporated in a constant temperature water bath and then dried in a vacuum drying oven at 90 °C for 24 h. Finally, the dried BC was placed into a tube furnace under N₂ protection and baked at 600 °C for 3 h. It was processed twice following the process described above. MnOBCs was then obtained. The samples impregnated with 0.05 mol L⁻¹ Mn(NO₃)₂ were named MnOBC-1. The samples impregnated with 0.1 mol L⁻¹ Mn(NO₃)₂ were named MnOBC-2, whereas the untreated BC was labeled as BC.

2.3. Physicochemical Characterization

Fourier transform infrared spectrometer (Nicolet-iS10, Thermo Fisher Scientific, Waltham, MA, USA) was used to obtain the surface functional groups of BC and MnOBCs. The surface morphology was examined by scanning electron microscope (SEM, Quanta 450 FEG, FEI, Hillsboro, OR, USA) and transmission electron microscope (TEM, Talos F200X, Thermo Fisher Scientific, MA, USA). The elemental composition in the specific area was examined by Energy Dispersive Spectrometer (EDS, SuperX, FEI, OR, USA). The specific surface area and pore space characterization were analyzed by the multi-channel specific surface area and pore size analyzer (MicroActive for TriStar II Plus 2.02, Micromeritics Instrument Corporation, Norcross, GA, USA). The crystal structure was analyzed by X-ray diffractometry (Bruker D8, Bruker Corporation, Billerica, MA, USA). The chemical state and surface composition of the MnOBCs were determined by X-ray photoelectron spectrometer (XPS, Escalab 250Xi, Thermo Fisher Scientific, MA, USA). Cr(VI) concentration was measured by spectrophotometry with diphenylcarbazide. In the diphenylcarbazide spectrophotometric method, the color developer was prepared by dissolving 0.2 g of diphenylcarbazide in 50 mL of acetone solvent and diluting it to 100 mL with water. To determine the concentration of Cr(VI) in the solution, 0.5 mL (1 + 1) sulfuric acid solution, 0.5 mL (1 + 1) phosphoric acid, and 2 mL color developer were added sequentially to 50 mL of the sample solution. After the color development reaction at 5–10 min, the resulting mixture was measured with a UV-Vis spectrophotometer (UV-1800, Aoyi Instruments (Shanghai) Co. Shanghai, China) at the wavelength of 540 nm.

2.4. Sorption Experiments

K₂Cr₂O₇ was dissolved in deionized water to gain the Cr(VI) stock solution, and the pH was adjusted by 0.1 mol L⁻¹ HCl and 0.1 mol L⁻¹ NaOH. The influence of the initial pH on Cr(VI) removal was investigated by mixing 0.025 g of BC or MnOBCs and 25 mL Cr(VI) solution (50 mg L⁻¹), and the initial pH ranged from 2.0 to 10.0. Then, suspensions were shaken 24 h at 150 rpm.

The influence of ionic strength on the removal of Cr(VI) by BC and MnOBCs was investigated. BC or MnOBCs (1 g L⁻¹) was mixed with 50 mg L⁻¹ Cr(VI) solution, and the concentration of NaCl was designed to 0, 0.1, 1, and 10 mmol L⁻¹, and the pH was adjusted to 2. Then, it was shaken at 25 °C and 150 rpm for 24 h.

Kinetic sorption experiments were performed in batch tests with the 50 mg L⁻¹ Cr(VI) solution. First, 0.1 g of BC or MnOBCs was weighed in a conical flask and added to the 100 mL of Cr(VI) solution. Then, the pH value was adjusted to 2, and the samples were continuously shaken in a water bath with an oscillation frequency of 150 rpm for 48 h. At the designed time, a certain amount of suspension was taken out. The Cr(VI) concentration in the filter liquid was measured. Batch sorption experiments were carried out, and initial concentration of Cr(VI) was from 0 mg L⁻¹ to 200 mg L⁻¹, and the pH value was adjusted to 2. The suspension was shaken for 48 h.

All samples were filtered through a 0.45 μm filter membrane, and the Cr(VI) concentration in the filter liquor was tested. In addition, all experiments were repeated twice, and the average value of the results from the analysis was taken.

2.5. Data Analysis

The sorption capacity of Cr(VI) on BC or MnOBCs was calculated by the following Equation (1),

$${\mathrm{q}}_{\mathrm{e}}=\mathrm{V} \frac{\left({\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{e}}\right)}{\mathrm{M}}$$

(1)

where q_e (mg g⁻¹) is the sorption capacity, V (L) is the volume, C₀ and C_e (mg L⁻¹) are the initial and equilibrium Cr(VI) concentrations, respectively, and M (g) is the adsorbent weight.

To evaluate the performance of different adsorbent materials, three different models, including the pseudo-first-order model (Equation (2)), pseudo-second-order model (Equation (3)), and Elovich model (Equation (4)) [54], were applied to describe the sorption kinetics of different sorbent systems.

In(q_e − q_t) = Inq_e − k₁t

(2)

$$\frac{\mathrm{t}}{{\mathrm{q}}_{\mathrm{t}}}=\frac{1}{{\mathrm{k}}_2{\mathrm{q}}_{\mathrm{e}}^2}+\frac{\mathrm{t}}{{\mathrm{q}}_{\mathrm{e}}}$$

(3)

$${\mathrm{q}}_{\mathrm{t}}=\frac{1}{\mathsf{\beta }}\mathrm{In}(\mathsf{\alpha }\mathsf{\beta })+\frac{1}{\mathsf{\beta }}\mathrm{In}(\mathrm{t})$$

(4)

where q_t and q_e (mg g⁻¹) were the sorption capacity at time t and equilibrium, respectively. k₁ (1 min⁻¹) and k₂ (g mg⁻¹ min⁻¹) were the sorption rate constant, α (mg g⁻¹ min⁻¹) and β (g mg⁻¹) are Elovich constants, which denotes the initial sorption rate constant and desorption rate constant, respectively.

The Langmuir and Freundlich isotherm models are widely used to study the sorption mechanism, and the equations are given as follow.

$${\mathrm{q}}_{\mathrm{e}}=\frac{{\mathrm{q}}_{\max }{\mathrm{K}}_{\mathrm{L}}{\mathrm{C}}_{\mathrm{e}}}{1+{\mathrm{K}}_{\mathrm{L}}{\mathrm{C}}_{\mathrm{e}}}$$

(5)

$${\mathrm{q}}_{\mathrm{e}}={\mathrm{K}}_{\mathrm{F}}{\mathrm{C}}_{\mathrm{e}}^{1/n}$$

(6)

where C_e (mg L⁻¹) is the equilibrium concentration; q_max (mg g⁻¹) is the maximum sorption capacity; K_L (L mg⁻¹), K_F (mg^1−1/n L^1/n g⁻¹), and n (dimensionless) are the isotherm constants.

Experimental data were processed with Excel software, and curves were drawn with Origin 2021 software. Solution spectrum analysis and material crystallography were performed using XPSPEAK and Jade.

3. Results and Discussion

3.1. The Physiochemical Properties of BC and MnOBCs

3.1.1. Specific Surface Area and Pore Structure

N₂ adsorption–desorption isotherms of BC and MnOBCs samples are given in Figure S1. The isotherms of all specimens belonged to type IV, and the isotherms had hysteresis loops (H3 for BC and H4 for MnOBCs), indicating that they presented mesoporous structures [55]. The pore size distribution (Figure S1) of MnOBCs ranged from 2 nm to 10 nm, suggesting that they are mesoporous materials [56]. Meanwhile, the specific surface areas (

Table 2. Pore structures of BC and MnOBCs.

	BC	MnOBC-1	MnOBC-2
BET specific area (m2 g−1)	33.67	302.02	299.56
Total pore volume (cm3 g−1)	0.025	0.130	0.134
Average pore diameter (nm)	3.02	1.72	1.79

) of MnOBCs were nearly 10 times larger than those of BC, which may be due to the dispersed attachment of MnOx particles loaded on MnOBCs. In addition, the average pore size of MnOBCs decreased with the enrichment of MnOx, which may be due to the formation of MnOx and oxygen-containing functional groups blocking the mesopores and micropores of MnOBCs.

3.1.2. Surface Morphology Characteristics

Figure 2 presents the morphological characteristics of BC and MnOBCs samples. We can see that the BC had clear pores, a honeycomb network, and different shapes of ash particles that are attached. The surface morphology of MnOBCs was rough, and nanoparticles were adsorbed on their surface and pores. As the concentration of Mn(NO₃)₂ increased, the coverage area of the particles attached to the surface increased (Figure 2b,c). These results confirmed that MnOx loaded on the BC successfully. Of note, the substrate surface of MnOBC-2 was mostly covered by MnOx.

3.1.3. Analysis of Functional Groups and Mineralogical Characterization

The FT-IR spectra (Figure 3a) showed that BC and MnOBCs were rich in functional groups. For instance, there was an intense peak at 3429 cm⁻¹, due to the stretching vibration of the O-H bond in the hydroxyl group [57]. There was a spectral band at about 1574 cm⁻¹, due to C=O and C=C bonds [58]. Moreover, the high-intensity peak around 1094 cm⁻¹ corresponded to C-O stretching and C-OH bending, whereas the peak at 797 cm⁻¹ represented the asymmetric bending vibration of Si-O-Si [59,60]. The peak at 1417 cm⁻¹ represented the stretching vibration of the carboxyl group (-COOH). However, the characteristic peaks of MnOBCs at 3429 cm⁻¹ (hydroxyl vibration peak) became weaker; thus, the absorption peak at 797 cm⁻¹ also became weaker, indicating that the ash on the BC was cleaned during the pre-preparation process. Abundant functional groups in BC can provide reaction or interaction sites for Cr(VI) transformation and sorption on the material surface [61,62].

XRD was performed to characterize the crystallinity of the composites. As seen from the XRD spectrum (Figure 3b), BC had poor crystallinity, and both BC and MnOBCs showed broad diffraction peaks at 2θ = (20–30)°, which were diffraction peaks of an amorphous structure. With the help of Jade 6 software, the characteristic peaks of MnOBCs were located at 2θ = 35.1°, 40.7°, 58.9°, 70.4°, 74.0°, and 88.1°, which are in agreement with the standard card spectra (PDF#-07-0230), corresponding to the 111, 200, 220, 311, 222, and 400 crystal planes of crystalline manganese ore, respectively [63]. Combined with the SEM images of MnOBCs (Figure 2), it was confirmed that MnOx was successfully loaded onto the BC, and the impregnation of MnOx on the surface of BC was directly related to the Mn(NO₃)₂ concentration.

3.2. Cr(VI) Sorption Performance of BC and MnOBCs

3.2.1. Effect of Initial pH on Cr(VI) Sorption

In general, the Cr(VI) form and surface charge properties of the material should be influenced by pH. In this study, five pH gradients from acidic to basic (pH = 2, 4, 6, 8, 10) were selected to investigate the effect of pH on the sorption amount of Cr(VI) on BC and MnOBCs. Results indicated that sorption amount of Cr(VI) decreased with increasing pH (Figure 4a). This phenomenon is attributed to the formation of Cr(VI) changed with pH, and Cr(VI) in the solution exists in the form of HCrO₄⁻ at pH ≤ 3. HCrO₄⁻ can be converted to Cr₂O₇²⁻ and CrO₄²⁻ when the pH increases, and CrO₄²⁻ plays a dominant role if the pH is greater than 6.0 [2]. In addition, the sorption free energy of HCrO₄⁻ is lower than that of Cr₂O₇²⁻ and CrO₄²⁻ [64], which may lead to the higher sorption amount of Cr(VI) in a more acidic condition. Meanwhile, MnOBCs usually exhibit a high zero charge point (pH_pzc) [65]. At lower pH values, MnOBCs have positively charged surfaces and interact more readily with negatively charged ions. Meanwhile, the protonation on the BC and MnOBCs surface gradually decreases with pH increasing, and the sorption capacity is weakened [66]. In addition, the reduction reaction of Cr(VI) to Cr(III) is a proton-consuming process, which could be inhibited with pH increasing [67]. In one word, both of the surface charge of MnOBCs and the intensity of Cr(VI) reduction reaction varied with pH, which can lead to the different sorption capacity at different pH [68,69].

3.2.2. Effect of Ion Strength on Cr(VI) Sorption

The ions concentration in the sorption system may affect the properties of sorption [70]. The ions produced during the hydrolysis of these salts may reduce the sorption efficiency of metal ions through competing sorption sites [71]. To better simulate the actual water conditions, the effect of NaCl concentrations on the removal of Cr(VI) by BC and MnOBCs was investigated at pH = 2.0. As shown in Figure 4b, the sorption amount of Cr(VI) on BC and MnOBCs did not change clearly when the ionic strength increased. It was suggested that Cr(VI) removal by BC and MnOBCs cannot be affected by the ion strength.

3.2.3. Sorption Kinetics

To further investigate the sorption mechanism, sorption kinetic experiments of Cr(VI) sorption on BC and MnOBCs were conducted. As shown in Figure 5, the amount of Cr(VI) sorption on BC and MnOBCs increased with prolonged reaction time. Sorption processes seemed quick at the beginning, and slowed down after 6 h, because of most sorption sites occupied and the resistance enhanced [72]. The pseudo-first-order and pseudo-second-order model were employed to fit the sorption kinetic data, and the correlation coefficients (

Table 3. Regressed kinetic parameters for Cr(VI) sorption onto BC and MnOBCs.

	The Pseudo-First-Order Model			The Pseudo-Second-Order Model			Elovich Model
	qe (mg g−1)	k1	R2	qe (mg g−1)	k2	R2	α	β	R2
BC	5.12	0.14	0.8570	5.37	0.037	0.9334	182.83	2.30	0.9799
MnOBC-1	13.84	0.018	0.7503	15.20	0.0016	0.8594	2.14	0.47	0.9802
MnOBC-2	22.04	0.020	0.7281	23.76	0.0013	0.8429	5.02	0.31	0.9821

) changed in the range of 0.7281–0.8570 and 0.8429–0.9334, respectively. It was indicated that the experimental data did not conform to the pseudo-first-order and pseudo-second-order kinetic models. However, the Elovich equation fitted for sorption process properly, and the correlation coefficients were 0.9799 to 0.9821. The Elovich equation is often used to describe the chemisorption kinetics on highly heterogeneous adsorbents [73]. It indicates that the sorption process of MnOBCs with respect to Cr(VI) was mainly via chemisorption. The higher α values than β values may also reflect the rapid sorption in the early stages [74].

3.2.4. Sorption Isotherms

The sorption isotherms of Cr(VI) on BC and MnOBCs are shown in Figure 6. The Langmuir and Freundlich models were used to understand the sorption mechanism. The Langmuir model was derived from the hypothesis of monolayer sorption, and the Freundlich model was used for multilayer sorption [75]. The correlation coefficient values (

Table 4. Langmuir and Freundlich equation parameters calculated using nonlinear curve-fitting for sorption isotherms for BC and MnOBCs.

	Langmuir			Freundlich
	qmax (mg g−1)	KL	R2	KF	1/n	R2
BC	10.10	0.082	0.8582	2.71	0.23	0.9821
MnOBC-1	20.69	0.57	0.9079	10.25	0.17	0.9886
MnOBC-2	28.58	0.30	0.8863	11.54	0.21	0.9868

) obtained from the Langmuir and Freundlich equilibrium isotherm models show that the Freundlich model could better interpret the sorption process of Cr(VI) by MnOBCs, indicating that the sorption of Cr(VI) on MnOBCs was a heterogeneous chemisorption process. The sorption capacity of BC for Cr(VI) was very weak, and the maximum sorption capacity fitted by the Langmuir model was only 10.10 mg g⁻¹. The maximum sorption capacities of MnOBC-1 and MnOBC-2 for Cr(VI) were 20.69 and 28.58 mg g⁻¹, which are 2.04 and 2.83 times higher than those of BC, respectively. Combined with the SEM plots (Figure 2), it further illustrates that the higher the loaded MnO_x content, the higher the sorption amount.

For the Freundlich equation, the value of 1/n usually ranges from 0 to 1, representing the exchange strength or surface inhomogeneity [76]. The larger the value of the sorption equilibrium constant K_F, the greater the sorption capacity [77]. As shown in Table 4, MnOBC-2 had the largest K_F value, indicating that it had the largest sorption capacity. The sorption capacity of BC was minimal. The maximum sorption capacity of MnOBC-2 for Cr(VI) might be related to electrostatic interaction and the higher specific surface area for the largest amount of MnOx loaded. In addition, the Mn-O-Cr complexation might lead to more sorption capacity in MnOBCs.

3.3. Formation and Distribution of Cr in BC and MnOBCs

In general, Cr(III) is less toxic than Cr(VI) and less mobile due to the limited solubility of the hydroxide [66]. Therefore, the immobilization mechanism of Cr(VI) on BC and MnOBCs was determined by XPS, EDS.

3.3.1. XPS Spectra Analysis

In this work, Cr species in BC and MnOBCs was determined by XPS, which can be applied to analyze the surface chemical composition and oxidation states. Typically, the binding energy of the Cr2p_3/2 at 576.9 eV represents Cr(III), and the binding energy at 578.0 eV represents Cr(VI) [78]. As shown in Figure 7, Cr(VI) and Cr(III) were found on the surface of BC and MnOBCs. This may be because Cr(VI) was immobilized on the surface of the material due to the texture and highly porous surface of BC, and then the free radical groups on the surface of BC reduced some of the adsorbed Cr(VI) to Cr(III) [79]. In addition, the proportion of Cr(III) on the surfaces of MnOBC-1 and MnOBC-2 was 32.45% and 44.61%, respectively, which were larger than that of Cr(III) on the surface of BC (28.45%). The difference in the reduction capacity of Cr(VI) indicated the difference between the reduction components of BC and MnOBCs, further suggesting that Mn elements in MnOBCs were also involved in the Cr(VI) reduction reaction.

To provide insights into the Cr(VI) reduction and sorption mechanisms, XPS analysis was performed on the MnOBC-2 with and without Cr(VI) sorption, and shown in Figure 8. As Figure 8a shown, elemental Mn peaks were found in XPS spectroscopy of MnOBCs, and the Mn2p_3/2 spectrum (Figure 8b) had three peaks at binding energies of 641.4, 642.3, and 643.5 eV, which were attributed to Mn(II), Mn(III), and Mn(IV), respectively [80]. The valence of Mn elements on the surface of MnOBC-2 was mainly composed of Mn(II), Mn(III), and Mn(IV), accounting for 33.59%, 35.83%, and 30.58%, respectively. However, in the MnOBC-2 with Cr(VI) sorption, the percentage of Mn(II) decreased to 8.33%, whereas the percentage of Mn(III) and Mn(IV) increased to 38.95% and 52.72%, respectively. This indicates that during the sorption of Cr(VI), the Mn on the surface of MnOBC-2 was oxidized, as Mn(II) was mainly oxidized to Mn(III) and Mn(IV). Therefore, the removal mechanism of Cr(VI) also includes redox reactions. The negatively charged Cr(VI) migrates to the positively charged MnOBCs surface under electrostatic interaction. The fixed portion of Cr(VI) was reduced to Cr(III) by BC-loaded MnOx and free radicals and their reduction, whereas Mn(II) in MnOx was oxidized and its valence state was elevated.

To further elucidate the mechanism of Cr(VI) sorption, Figure 8c shows the C-functional group distribution on MnOBC-2. As shown in Figure 8c, the C1s spectra of MnOBC-2 represented C-C, C-O, and O=C-O characteristic peaks at 284.8, 286.1, and 288.7 eV, respectively [81]. The area percentages of C-O and O=C-O of MnOBC-2 were 38.01% and 12.45%, respectively. In contract, after the sorption of Cr(VI), these percentages decreased to 19.40% and 10.64%, respectively. It is indicated that the O=C-OH functional group plays a key role in the removal of Cr(VI), which can be attributed to the complexation reaction between the carboxyl group (COOH) and Cr(VI). The O1s spectra (Figure 8d) showed that the surface of MnOBC-2 contained not only O from the BC itself but also O from MnOx. The binding energies 530.1, 531.4, 532.2, and 533.4 eV were chosen as the splitting peak positions corresponding to Mn-O (the lattice oxygen), Mn-OH, C-OH, and O-C=O [82,83]. Compared to MnOBC-2, the percentage of the peak area of Mn-OH decreased from 32.05% to 12.71% when Cr(VI) was sorption on MnOBC-2, indicating that the hydroxyl group was involved in the Cr(VI) sorption. On the contrary, the area of the peak assigned to Mn-O increased from 2.59% to 9.23%, probably forming Mn-O-Cr on the surface of MnOBC-2.

3.3.2. TEM-EDS Analysis

TEM-EDS analysis was performed for BC and MnOBCs with Cr(VI) sorption to understand the composition and distribution of elements (i.e., C, O, Ca, Si, Mn, and Cr) (Figure 9). Therefore, the immobilization mechanism of Cr(VI) on BC and MnOBCs was deter-mined by XPS and TEM-EDS. As shown in Figure 9, the EDS indicated that Cr was detected in BC and MnOBCs. In addition, the presence of Mn elements was detected in MnOBC-2, which further indicates that MnOx was successfully loaded on BC. Meanwhile, the surface of MnOBCs contains not only C, O, and Mn but also K, Ca, Na, Mg, S, and P, which were present in BC as essential elements for growth and affected the fixation of Cr(VI) in the aqueous solution by BC.

3.4. Cr(VI) Immobilization Mechanisms on BC and MnOBCs

The impregnation of BC with MnOx could improve its surface morphology, functional group modification, and elemental composition, and results in the sorption capacity of the Cr(VI) increase [84]. In addition, the MnOBCs exhibited a higher reduction potential and ability to fix Cr(VI) [85]. There are several possible pathways for the immobilization mechanism of Cr(VI) on BC and MnOBCs: (1) Compared with BC, MnOBCs have a larger specific surface area and more sorption sites, which could fix more Cr(VI). (2) Hydroxyl (-OH) and carboxyl (-COOH) groups could be involved in the immobilization of Cr(VI), and Mn-O-Cr was formed on the surface of MnOBCs, which is given in the XPS analysis results. The surface of BC was rich in carboxyl and hydroxyl groups, which had strong affinity for Cr(VI) ions [86]. (3) With the loading of MnOx, the positive charge on the BC surface increases, facilitating the fixation of Cr(VI) by electrostatic attraction. Previous studies demonstrated that manganese oxides had a positively charged mineral surface [87]. (4) The loading of MnOx increases the reduction of Cr, whereas Cr(III) is more easily immobilized. Meanwhile, previous studies demonstrated that when Mn(II) and Cr(VI) are adsorbed in ferrihydrite at the same time, Mn(II) and Cr(VI) undergo redox reactions [88]. Therefore, BC-loaded MnOx particles may play an important role in the immobilization of Cr(VI) in the solution.

4. Conclusions

In this work, MnOBCs were successfully prepared by the chemical impregnation method to improve the sorption of Cr(VI). The specific surface area of MnOBCs increased by nearly 10 times than that of BC. The sorption kinetic process of Cr(VI) could be fitted by the Elovich kinetic model, and sorption isotherm data could be fitted by the Freundlich equation. In addition, the sorption capacity of Cr(VI) decreased sharply with the increase in pH and could not be influenced by ionic strength. Moreover, Cr(VI) and Cr(III) were found on the surface of BC and MnOBCs, and Mn(II) on the surface of MnOBCs was oxidized to Mn(III) and Mn(IV) during the Cr(VI) sorption process. Meanwhile, the functional groups (-COOH and -OH) on BC substrates were also involved in the Cr(VI) sorption by complexation reaction. In short, the sorption mechanism of Cr(VI) on MnOBCs includes the electrostatic attraction, complexation, and reduction reaction of Cr(VI). This study sheds light on the change of Mn valence in MnOBCs during Cr(VI) immobilization, and MnOBCs are potentially promising for environmental applications in the removal of Cr(VI) from an aqueous solution due to their low cost and effective sorption capacity.