Preparation of KOH and H 3 PO 4 Modiﬁed Biochar and Its Application in Methylene Blue Removal from Aqueous Solution

: Improperly treated or directly discharged into the environment, wastewater containing dyes can destroy the quality of water bodies and pollute the ecological environment. The removal of dye wastewater is urgent and essential. In this study, corn stalk was pyrolyzed to pristine biochar (CSBC) in a limited oxygen atmosphere and modiﬁed using KOH and H 3 PO 4 (KOH-CSBC, H 3 PO 4 -CSBC, respectively). The biochars were characterized by surface area and pore size, X-ray di ﬀ raction (XRD), Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), as well as their behavior in adsorbing methylene blue (MB). Results indicated that the pore structure of CSBC became more developed after modiﬁcation by KOH. Meanwhile, H 3 PO 4 -CSBC contained more functional groups after activation treatment. The pseudo-second-order kinetic and the Langmuir adsorption isotherm represented the adsorption process well. The maximum MB adsorption capacity of CSBC, KOH-CSBC, and H 3 PO 4 -CSBC was 43.14 mg g − 1 , 406.43 mg g − 1 and 230.39 mg g − 1 , respectively. Chemical modiﬁcation signiﬁcantly enhanced the adsorption of MB onto biochar, especially for KOH-CSBC. The adsorption mechanism between MB and biochar involved physical interaction, electrostatic interaction, hydrogen bonding and π – π interaction. Hence, modiﬁed CSBC (especially KOH-CSBC) has the potential for use as an adsorbent to remove dye from textile wastewater.


Introduction
Wastewater from the textile industry typically contains dyes and has a high content of organic toxicants, high color, and strong resistance to biodegradation, photolysis and oxidation, and is potentially carcinogenic. If improperly treated or directly discharged into the environment, wastewater containing dyes can destroy the quality of water bodies and pollute the ecological environment [1]. Methylene blue (MB) is an important synthetic dye and was widely used in the field of chemical indicators, dyes, biological dyes and drugs. Wastewater containing MB can enter a water body through different pathways and cause serious harm to environmental and human health [2]. Therefore, the need is urgent to develop an effective, low cost, easily operated technology for MB wastewater treatment. Adsorption has been an important technology to remove MB from wastewater. The correct choice and preparation of an efficient adsorbent is the priority for effective application of adsorption technology.
Currently, biochar is attracting great attention due to the important function it provides in greenhouse gas emission reduction, soil carbon sequestration, pollutant control and solid waste

Preparation of Biochar and Modified Biochar
First, the corn stalk was washed with deionized water and dried in an oven. After drying, the stalk was crushed and passed through a 100 mesh sieve and then stored for future experimentation. Last, the crushed corn stalk was pyrolyzed to pristine biochar at 500 • C in a muffle furnace under limited oxygen atmosphere. After washing several times with pure water, the biochar was dried in an oven (80 • C for 24 h). This procedure produced pristine corn stalk biochar (CSBC).
A weight of 6.0 g CSBC was put into each of two centrifuge tubes. A weight of 6.0 g KOH and 20 mL pure water was added to one tube. An amount of 6 mL H 3 PO 4 and 20 mL ultrapure was added to the other tube. Then, the two tubes were placed on a shaker to fully oscillate for 24 h, after which Processes 2019, 7, 891 3 of 20 the tubes were removed and the biochars dried at 80 • C for 48 h. Then, each of the treated biochars was placed in an individual ceramic crucible, covered with a lid, and placed in a muffle furnace to activate the biochars. The activation temperature was 700 • C (generally, the biochar was prepared under 700 • C, and thus the activation temperature was chosen as 700 • C), which was attained at a heating rate of 10 • C/min and maintained for 2 h. Then, the crucibles were cooled to room temperature and the modified biochars were removed. Deionized water was used to remove impurities from the biochars by washing them several times until the pH of eluent reached neutrality. The washed biochars were dried in an oven for characterization and use in experiments. Hereafter, the KOH-treated and H 3 PO 4 -treated biochars are referred to as KOH-CSBC and H 3 PO 4 -CSBC, respectively.

Characterization
The pristine and modified biochars were characterized using various techniques. The surface area and pore sized of the biochars were measured using an automatic specific surface area and pore analyzer (Tristar II 3020M, Micromeritics Instrument Corporation, Norcross, GA, USA). The surface morphology of biochars was analyzed using scanning electron microscopy (SEM, S-4800, Hitachi, Japan). Fourier transform infrared spectroscopy (FTIR) was used with the KBr pellet technique to characterize the functional groups of biochars (FTIR, Nicolette is50, Thermo Fisher Scientific, Waltham, MA, USA). The minerals in biochars were detected by X-ray diffraction (XRD, D8 Advance, BrukerAXS GmbH, Karlsruhe, Germany). The surface elements, speciation and relative distribution of elements within the biochars were determined by X-ray photoelectron spectroscopy (XPS, ESCALAB 250, Thermo Scientific-VG Scientific, Waltham, MA, USA).

Influence of Solid-to-Liquid Ratio on Methylene Blue (MB) Adsorption
MB solution (30 mL) with a concentration of 50 mg L −1 was placed in 50 mL centrifuge tubes. A certain mass of pristine or modified biochar was weighed and added into the tubes to adjust the solid-to-liquid ratio at 0.5, 1.0, 2.0, 3.0 and 4.0 g L −1 . Then, the tubes were placed in a constant temperature shaker with a speed of 150 rpm for 24 h at 25 • C. The tubes were centrifuged for 10 min at a speed of 4000 rpm. The resulting supernatant was filtered through a 0.45 µm membrane, and the absorbance of the supernatant at the wavelength of 665 nm was measured using a spectrophotometer.

Influence of Initial pH of Solution on MB Adsorption
MB solution (30 mL) with a concentration of 100 mg L −1 was placed in 50 mL centrifuge tubes. The pH of the solutions was adjusted to 3.0, 5.0, 7.0, 9.0 and 11.0 using either 0.1M HCl or NaOH. A certain mass of pristine or modified biochar was added to the tubes to adjust the solid-to-liquid ratio at 0.5 g L −1 . The tubes were then processed as described in Section 2.4.1 (i.e., oscillation, centrifugation, filtration and absorbance determination).

Adsorption Kinetics
MB solution (100 mL) at a concentration of 50 mg·L −1 , 100 mg·L −1 or 200 mg·L −1 was added to each of three beakers. Then, 0.1 g CSBC, 0.1 g KOH-CSBC and 0.1 g H 3 PO 4 -CSBC was added separately to each beaker. Each beaker was placed on a magnetic stirrer and the temperature held constant at 25 • C. Samples were collected at various times and filtered through a 0.45 µm membrane, after which the absorbance at 665 nm was detected. The concentration of MB was set at 50 mg L −1 and 100 mg L −1 for CSBC, and 100 mg L −1 and 200 mg L −1 for both KOH-CSBC and H 3 PO 4 -CSBC.

Adsorption Isotherm
To develop adsorption isotherms, 30 mL of MB solution was placed in a 50 mL centrifuge tube. The concentration of MB was adjusted in the range 100-500 mg·L −1 . Then, 0.015 g of CSBC, KOH-CSBC and H 3 PO 4 -CSBC was added separately to individual tubes. The tubes were placed in a constant temperature shaker with a speed of 150 rpm for 24 h at 25 • C. Then, the tubes were centrifuged for 10 min at a speed of 4000 rpm. The resulting supernatant was filtered through a 0.45 µm membrane. The absorbance of the supernatant at the wavelength of 665 nm was measured using a spectrophotometer.

Data Analysis
The amount of MB adsorbed by biochar was calculated using Equation (1).
In Equation (1) q t (mg g −1 ) is the adsorption capacity of MB at time t, C 0 (mg·L −1 ) is the initial MB concentration, and C t (mg·L −1 ) is the equilibrium concentration of MB at time t. V is the volume of dye solution, L. The m is the weight of adsorbent used, g.
The kinetic adsorption data were described (fitted) using a pseudo-first-order model and a pseudo-second-order model.
The pseudo-first-order model [25] is described by Equation (2): and the pseudo-second-order model [26] is described by Equation (3): In Equations (2) and (3), q t and q e (mg g −1 ) are the adsorption capacity of MB on biochar at time t and at equilibrium time, respectively; and k 1 (min −1 ) and k 2 (g mg −1 min −1 ) are the rate constants of the pseudo-first-order and pseudo-second-order kinetic reactions, respectively.
The adsorption isotherm data were fitted using the Langmuir and Freundlich models. The Langmuir model [27] is described by Equation (4): The Freundlich model [28] is described by Equation (5): In Equations (4) and (5), C e (mg L −1 ) is the equilibrium concentration of the solution; q e (mg g −1 ) is the equilibrium adsorption capacity; K L is a constant for the Langmuir model; q m (mg g −1 ) is the maximum adsorption capacity; K f (L mg −1 ) is a constant for the Freundlich model; and 1/n is the adsorption affinity constant.

Characterization of Biochars
The scanning electron microscopy (SEM) micrographs of CSBC, KOH-CSBC and H 3 PO 4 -CSBC are shown in Figure 1. The morphology of corn stalk remained in the CSBC. Some fragments that appeared in the CSBC were mainly from the thermal decomposition of cellulose and hemicelluloses. The pyrolysis temperature for CSBC was 500 • C. Thermal decomposition of cellulose, hemicellulose and lignin occurs at 220-315 • C, 315-400 • C and 160-900 • C, respectively [29]. After KOH modification, the biochar pore structure became more developed, and more fragments were observed on the surface of the KOH-CSBC. Some crystal particles also appeared on the surface of KOH-CSBC. The mineral components were formed from the thermal decomposition of KOH. Meanwhile, the modification of CSBC by KOH promoted the formation of a more microporous structure. The modification of biochar using H 3 PO 4 resulted in different characteristics than those resulting from KOH modification. More fragments in biochar bonded together after H 3 PO 4 modification, and the surface of H 3 PO 4 -modified CSBC exhibited a smooth and gelatinous appearance, and the fragments of CSBC bonded together, inhibiting the formation of a microporous structure. are shown in Figure 1. The morphology of corn stalk remained in the CSBC. Some fragments that appeared in the CSBC were mainly from the thermal decomposition of cellulose and hemicelluloses. The pyrolysis temperature for CSBC was 500 °C. Thermal decomposition of cellulose, hemicellulose and lignin occurs at 220-315 °C, 315-400 °C and 160-900 °C, respectively [29]. After KOH modification, the biochar pore structure became more developed, and more fragments were observed on the surface of the KOH-CSBC. Some crystal particles also appeared on the surface of KOH-CSBC. The mineral components were formed from the thermal decomposition of KOH. Meanwhile, the modification of CSBC by KOH promoted the formation of a more microporous structure. The modification of biochar using H3PO4 resulted in different characteristics than those resulting from KOH modification. More fragments in biochar bonded together after H3PO4 modification, and the surface of H3PO4-modified CSBC exhibited a smooth and gelatinous appearance, and the fragments of CSBC bonded together, inhibiting the formation of a microporous structure. The surface area, pore volume and pore size of biochars are displayed in Table 1. After KOH modification, the surface area of biochar significantly increased from 24.2543 to 473.6432 m 2 ·g −1 . The surface area increased approximately 20 times after KOH activation. The pore volume of KOH-CSBC also increased and pore size decreased. However, after H3PO4 modification, the surface area of biochar decreased from 24.2543 to 2.8404 m 2 g −1 . The pore volume of H3PO4-CSBC also decreased and pore size increased. Thus, the measurements of surface area, pore volume and pore size were consistent with the observations of SEM images. KOH modification obviously influenced the surface H3PO4-CSBC H3PO4-CSBC The surface area, pore volume and pore size of biochars are displayed in Table 1. After KOH modification, the surface area of biochar significantly increased from 24.2543 to 473.6432 m 2 ·g −1 . The surface area increased approximately 20 times after KOH activation. The pore volume of KOH-CSBC also increased and pore size decreased. However, after H 3 PO 4 modification, the surface area of biochar decreased from 24.2543 to 2.8404 m 2 g −1 . The pore volume of H 3 PO 4 -CSBC also decreased and pore size increased. Thus, the measurements of surface area, pore volume and pore size were consistent with the observations of SEM images. KOH modification obviously influenced the surface structure of biochar, which suggested the treatment could affect the removal of MB from the solution. Table 1. Surface area, pore volume and pore size of biochars.

Samples
Surface Area (m 2 g −1 ) Pore Volume (cm 3  The XRD spectra of CSBC, KOH-CSBC and H 3 PO 4 -CSBC are shown in Figure 2. Amorphous carbon peaks were obvious in spectra from the CSBC. Some minerals also were detected in the CSBC, including CaCO 3 and KCl. After KOH modification, K and K 2 O were observed in the biochar, suggesting that KOH had reacted with the carbonaceous components in CSBC. Only amorphous carbon was detected in H 3 PO 4 -CSBC, indicating that H 3 PO 4 modification mainly affected the organic components of CSBC. The FTIR spectra of CSBC, KOH-CSBC and H3PO4-CSBC are presented in Figure 3. The functional groups of CSBC, KOH-CSBC and H3PO4-CSBC differed significantly. The specific functional groups for the different types of biochar are shown in Table 2. The main groups in CSBC included OH, aliphatic C, C=O and C=C, carboxyl anions (affected by C=O stretching), Si-O and C-H [30]. The main groups in KOH-CSBC included OH, C=O and C=C, substances in the phenolic O-H band, alkenes (affected by C-H bending), cellulosic ethers (affected by C-O-C stretching), aromatic C-H, and CH-of alkenes and alkane [31]. The modification of KOH led to the disappearance of aliphatic C and carboxyl anions due to the high pyrolytic temperature (700 °C). The appearance of the phenolic O-H band, C-H bending of alkenes, and C-O-C stretching indicated the formation of olefin compounds. The existence of aromatic C-Hand aromatic C-O stretching suggested the further enhancement of the CSBC's aromatic structure. The FTIR spectra of CSBC, KOH-CSBC and H 3 PO 4 -CSBC are presented in Figure 3. The functional groups of CSBC, KOH-CSBC and H 3 PO 4 -CSBC differed significantly. The specific functional groups for the different types of biochar are shown in Table 2. The main groups in CSBC included OH, aliphatic C, C=O and C=C, carboxyl anions (affected by C=O stretching), Si-O and C-H [30]. The main groups in KOH-CSBC included OH, C=O and C=C, substances in the phenolic O-H band, alkenes (affected by C-H bending), cellulosic ethers (affected by C-O-C stretching), aromatic C-H, and CH-of alkenes and alkane [31]. The modification of KOH led to the disappearance of aliphatic C and carboxyl anions due to the high pyrolytic temperature (700 • C). The appearance of the phenolic O-H band, C-H bending of alkenes, and C-O-C stretching indicated the formation of olefin compounds. The existence of aromatic C-Hand aromatic C-O stretching suggested the further enhancement of the CSBC's aromatic structure. C-H, and CH-of alkenes and alkane [31]. The modification of KOH led to the disappearance of aliphatic C and carboxyl anions due to the high pyrolytic temperature (700 °C). The appearance of the phenolic O-H band, C-H bending of alkenes, and C-O-C stretching indicated the formation of olefin compounds. The existence of aromatic C-Hand aromatic C-O stretching suggested the further enhancement of the CSBC's aromatic structure.    [20][21][22]. The modification of H 3 PO 4 caused the disappearance of aliphatic C and carboxyl anions owing to the high pyrolytic temperature (700 • C). Meanwhile, the observation of C-O, -COOH and O-H bending deformation illustrated the enhancement of the CSBC's aromatic structure. Furthermore, the presence of the amino phosphonic acid functional group and P-OH bonds in H 3 PO 4 -CSBC demonstrated that a reaction happened between H 3 PO 4 and the carbonaceous components in biochar. The introduction of phosphorous-containing functional groups on biochar may play an important role for MB removal.
The XPS spectra of CSBC, KOH-CSBC and H 3 PO 4 -CSBC are shown in Figure 4. C, O, N and P were detected in all biochars, but C and O were dominant. The relative content of P increased and the relative content of N decreased after KOH modification. The relative content of C decreased and the relative content of O and P decreased after H 3 PO 4 modification.
containing functional groups on biochar may play an important role for MB removal.
The XPS spectra of CSBC, KOH-CSBC and H3PO4-CSBC are shown in Figure 4. C, O, N and P were detected in all biochars, but C and O were dominant. The relative content of P increased and the relative content of N decreased after KOH modification. The relative content of C decreased and the relative content of O and P decreased after H3PO4 modification.  Results from the XPS peak-differentiating analysis for CSBC, KOH-CSBC and H3PO4-CSBC are displayed in Table 3. The C-1s XPS spectra of biochar exhibited three peaks at 284.77, 285. 58     The N-1s XPS spectra of biochar exhibited two peaks at 398.72 eV and 400.57 eV, corresponding to pyrimidine and pyridine/pyrrole, respectively [37]. The main nitrogen-containing groups were pyrimidine and pyridine/pyrrole in CSBC. The main group was pyridine/pyrrole in KOH-CSBC. The relative content of pyrimidine decreased after KOH modification. The relative content of pyridine/pyrrole increased slightly after H 3 PO 4 modification.

Effect of Solid-to-Liquid Ratio and Initial pHon MB Removal
The effect of solid-to-liquid ratio on MB removal by CSBC, KOH-CSBC and H 3 PO 4 -CSBC is shown in Figure 5a. When the solid-to-liquid ratio increased from 0.5 g L −1 to 4 g L −1 , the efficiency of removing MB by CSBC, KOH-CSBC and H 3 PO 4 -CSBC increased from 13% to 59%, from 99.6% to 100%, and from 93% to 100%, respectively. Higher solid-liquid ratio means a larger amount of adsorbent and a larger number of binding sites for the removal of MB. Compared with pristine biochar (CSBC), the biochar modified by KOH and H 3 PO 4 significantly increased the removal of MB at any given solid-to-liquid ratio. Thus, the modification of biochar using these two activation agents was effective. The efficiency of removing MB by KOH-CSBC and H 3 PO 4 -CSBC was nearly 100% when the solid-to-liquid ratio was equivalent to 1.0 g L −1 .

Adsorption Kinetics of MB on Biochars
The adsorption kinetics of MB and theoretical fitted curves for CSBC, KOH-CSBC and H3PO4-CSBC are presented in Figure 6. As reaction time increased, the amount of MB adsorbed on CSBC, KOH-CSBC and H3PO4-CSBC first increased quickly and then tended to stabilize. Thus, the adsorption process could be divided into a fast stage and a slow stage. In the initial phase of adsorption (fast stage), the MB molecules rapidly occupied the vacant sites on the surface of biochar. After 30 min (i.e., the end of the fast stage and beginning of the slow stage), the decreased number of vacant adsorption sites on the surface of biochar led to the saturation of adsorption capacity. The adsorption process gradually reached equilibrium after approximately 60 min. The effect of initial pH on MB removal by CSBC, KOH-CSBC and H 3 PO 4 -CSBC is shown in Figure 5b. The efficiency of removing MB by CSBC, KOH-CSBC and H 3 PO 4 -CSBC was 41%, 87% and 86% at pH 11.0, respectively. However, the maximum MB removal efficiency achieved by KOH-CSBC was nearly 100% at pH 5.0.
As pH increased, the efficiency of removing MB by CSBC first increased and then tended to stabilize. The removal of MB by CSBC was more favorable under alkaline conditions because MB is a cationic dye. The removal of MB by KOH-CSBC as pH increased first increased and then slightly decreased. The overall removal rate was more than 85% at pH 3.0-11.0. The reduced MB removal rate of KOH-CSBC at higher pH (7.0-11.0) may be related to the loss of binding sites under alkaline conditions. The efficiency of removing MB by H 3 PO 4 -CSBC at pH first increased, then slightly decreased and finally exhibited a slight increase. These changes in removal efficiency may have been caused by the protonation or deprotonation of phosphorus-containing groups in H 3 PO 4 -CSBC at different pH [42].

Adsorption Kinetics of MB on Biochars
The adsorption kinetics of MB and theoretical fitted curves for CSBC, KOH-CSBC and H 3 PO 4 -CSBC are presented in Figure 6. As reaction time increased, the amount of MB adsorbed on CSBC, KOH-CSBC and H 3 PO 4 -CSBC first increased quickly and then tended to stabilize. Thus, the adsorption process could be divided into a fast stage and a slow stage. In the initial phase of adsorption (fast stage), the MB molecules rapidly occupied the vacant sites on the surface of biochar. After 30 min (i.e., the end of the fast stage and beginning of the slow stage), the decreased number of vacant adsorption sites on the surface of biochar led to the saturation of adsorption capacity. The adsorption process gradually reached equilibrium after approximately 60 min.
KOH-CSBC and H3PO4-CSBC first increased quickly and then tended to stabilize. Thus, the adsorption process could be divided into a fast stage and a slow stage. In the initial phase of adsorption (fast stage), the MB molecules rapidly occupied the vacant sites on the surface of biochar. After 30 min (i.e., the end of the fast stage and beginning of the slow stage), the decreased number of vacant adsorption sites on the surface of biochar led to the saturation of adsorption capacity. The adsorption process gradually reached equilibrium after approximately 60 min. The amount of MB adsorbed from the more highly concentrated solution by biochars was larger than that adsorbed from the less highly concentrated solution. When the concentration of MB increased from 50 mg L −1 to 100 mg L −1 , the amount of MB adsorbed on CSBC increased from 25.0 mg g −1 to 28.8 mgg −1 . Likewise, when the concentration of MB increased from 100 mg L −1 to 200 mg L −1 , the amount of MB adsorbed on KOH-CSBC and H3PO4-CSBC increased from 199.8 to 372.3 mg g −1 , and from 91 to 105 mg g −1 , respectively. A higher initial concentration of MB can provide a higher driving force to overcome the mass transfer resistance of dye molecules between the solid phase and the liquid phase, resulting in more contact between dye molecules and the active sites on an adsorbent [43,44]. This phenomenon likely explains the data shown in Figure 6.
The parameters used to model the adsorption kinetics of MB on CSBC, KOH-CSBC and H3PO4-CSBC are shown in Table 4. Compared with predictions using the pseudo-first-order model, the The amount of MB adsorbed from the more highly concentrated solution by biochars was larger than that adsorbed from the less highly concentrated solution. When the concentration of MB increased from 50 mg L −1 to 100 mg L −1 , the amount of MB adsorbed on CSBC increased from 25.0 mg g −1 to 28.8 mgg −1 . Likewise, when the concentration of MB increased from 100 mg L −1 to 200 mg L −1 , the amount of MB adsorbed on KOH-CSBC and H 3 PO 4 -CSBC increased from 199.8 to 372.3 mg g −1 , and from 91 to 105 mg g −1 , respectively. A higher initial concentration of MB can provide a higher driving force to overcome the mass transfer resistance of dye molecules between the solid phase and the liquid phase, resulting in more contact between dye molecules and the active sites on an adsorbent [43,44]. This phenomenon likely explains the data shown in Figure 6.
The parameters used to model the adsorption kinetics of MB on CSBC, KOH-CSBC and H 3 PO 4 -CSBC are shown in Table 4. Compared with predictions using the pseudo-first-order model, the pseudo-second-order model provided a better description of the MB adsorption process, suggesting the adsorption of MB molecules on biochars contained multiple steps (including external liquid film diffusion, surface adsorption and particle diffusion) [45]. Furthermore, chemical interaction played an important role during the MB adsorption process [46]. Table 4. Pseudo-first-order and pseudo-second-order kinetic models adsorption parameters.

Adsorption Isotherms
The adsorption isotherms and fitted prediction curves describing adsorption of MB on CSBC, KOH-CSBC and H 3 PO 4 -CSBC are displayed in Figure 7. As the MB concentration increased, the amount of MB adsorbed on CSBC, KOH-CSBC and H 3 PO 4 -CSBC first increased and then tended to stabilize. A higher initial MB concentration resulted in higher adsorption, which followed the order KOH-CSBC>H 3 PO 4 -CSBC>CSBC.

Adsorption Isotherms
The adsorption isotherms and fitted prediction curves describing adsorption of MB on CSBC, KOH-CSBC and H3PO4-CSBC are displayed in Figure 7. As the MB concentration increased, the amount of MB adsorbed on CSBC, KOH-CSBC and H3PO4-CSBC first increased and then tended to stabilize. A higher initial MB concentration resulted in higher adsorption, which followed the order KOH-CSBC>H3PO4-CSBC>CSBC.
The parameters used to model the adsorption isotherm of MB on CSBC, KOH-CSBC, H3PO4-CSBC are shown in Table 5. Both the Langmuir and Freundlich models described the adsorption process of MB on biochars well, as indicated by correlation coefficients that exceeded 0.85. The Langmuir isotherm better fit the adsorption process of MB on CSBC and KOH-CSBC (with correlation coefficients in the range 0.87-0.95). Technically, the Freundlich model described the adsorption process of MB on H3PO4-CSBC better, exhibiting a correlation coefficient of 0.99 compared to that (0.97) for the Langmuir model.
Based on the fitted Langmuir model, the maximum adsorption capacity of CSBC, KOH-CSBC and H3PO4-CSBC for MB was 45.58 mg·g −1 , 406.43 mg·g −1 and 234.75 mg g −1 , respectively. The adsorption capacities of KOH-CSBC and H3PO4-CSBC were 9.4 and 5.3 times higher than CSBC, respectively. Thus, KOH and H3PO4 modification significantly enhanced the adsorption capacity of biochar for MB, with KOH modification producing nearly twice the increase achieved by H3PO4.
The parameter KL (Equation (4)) reflected the adsorption affinity between biochar and MB molecules [47]. In this study, the KL associated with KOH-CSBC was larger than that for both H3PO4-CSBC and CSBC, indicating that KOH-CSBC had a higher affinity for MB than CSBC and H3PO4-CSBC. Similarly, the parameter 1/n derived for the Freundlich model reflected the difficulty of MB adsorption. Generally, when 1/n is less than 0.5, an adsorbent is easily adsorbed, and when 1/n exceeds 2, the adsorbent is hardly adsorbed [48]. In this research, the value of 1/n for KOH-CSBC and H3PO4-CSBC was less than 0.5, suggesting the adsorption of MB proceeded easily. Meanwhile, the value of 1/n for CSCB was 0.52448, illustrating that adsorption MB could occur, albeit with slightly more difficulty than for KOH-CSBC and H3PO4-CSBC.  The parameters used to model the adsorption isotherm of MB on CSBC, KOH-CSBC, H 3 PO 4 -CSBC are shown in Table 5. Both the Langmuir and Freundlich models described the adsorption process of MB on biochars well, as indicated by correlation coefficients that exceeded 0.85. The Langmuir isotherm better fit the adsorption process of MB on CSBC and KOH-CSBC (with correlation coefficients in the range 0.87-0.95). Technically, the Freundlich model described the adsorption process of MB on H 3 PO 4 -CSBC better, exhibiting a correlation coefficient of 0.99 compared to that (0.97) for the Langmuir model. The parameter K L (Equation (4)) reflected the adsorption affinity between biochar and MB molecules [47]. In this study, the K L associated with KOH-CSBC was larger than that for both H 3 PO 4 -CSBC and CSBC, indicating that KOH-CSBC had a higher affinity for MB than CSBC and H 3 PO 4 -CSBC. Similarly, the parameter 1/n derived for the Freundlich model reflected the difficulty of MB adsorption. Generally, when 1/n is less than 0.5, an adsorbent is easily adsorbed, and when 1/n exceeds 2, the adsorbent is hardly adsorbed [48]. In this research, the value of 1/n for KOH-CSBC and H 3 PO 4 -CSBC was less than 0.5, suggesting the adsorption of MB proceeded easily. Meanwhile, the value of 1/n for CSCB was 0.52448, illustrating that adsorption MB could occur, albeit with slightly more difficulty than for KOH-CSBC and H 3 PO 4 -CSBC.

Activation Mechanism
In this study, modification (activation) of corn stalk biochar using KOH and H 3 PO 4 greatly improved the removal of MB from solution.
KOH activation was an effective method through which to create a porous structure in biochar. Previous studies [49][50][51][52][53] found that the activation mechanism of KOH is as follows. First, KOH melted at 360 • C and then was involved in a reduction reaction with carbon precursors that formed K 2 CO 3 at nearly 400 • C. KOH was gradually consumed at 600 • C and converted entirely to K 2 CO 3 . K 2 CO 3 decomposed into CO 2 and K 2 O when the temperature exceeded 700 • C and disappeared completely at 800 • C. Additionally, the generated CO 2 reacted with other sources of C to form CO at high temperature. Meanwhile, potassium compounds were reduced to metallic potassium by carbon at high temperatures. When the alkaline metal became a highly diffused vapor, it was easily inserted into the carbon lattice and thus generated a more well-developed porous structure in the biochar. The reaction process is described in Equations (6)- (10).
The influence of KOH activation on CSBC was easily observed through surface area analysis and by SEM, XRD, FTIR and XPS characterization. Based on surface area measurements and SEM images, the biochar exhibited a larger surface area and better developed pore structure after KOH modification. XRD analysis detected the existence of K and K 2 O. FTIR and XPS analysis confirmed that biochar had a more developed aromatic structure after KOH high-temperature activation. The structural characteristic of KOH-CSCB resulted in a high ability to remove MB from aqueous solution.
The function of H 3 PO 4 is to first act as an acid catalyst to promote bond-cracking reactions. A chain structure is formed by the reaction of ring and condensation in biochar. Second, the reaction with organic components forms phosphate and polyphosphate bridged and crosslinked biopolymer fragments, which promote the formation of pores [41]. As reported in the literature [41,54], H 3 PO 4 activation proceeded as follows. First, in the pyrolysis process, the oxygen-containing functional group of biochar reacted with H 3 PO 4 to generate water vapor, which further reacted with the carbonaceous components in biochar to form the inner pore structure, as indicated in the following reaction (Equation (11)).
Second, the catalysis of H 3 PO 4 led to the oxidation and fixation of oxygen-containing functional groups on the pore walls, which led to the reduction of pore size or the transformation of mesopores into micropores. Third, H 3 PO 4 promoted the formation of -COOH, -OH, C=C and an aromatic structure, and enhanced the formation of P-O or P-OOH functional groups.
H 3 PO 4 modification of biochar promoted the formation of phosphorus-containing functional groups, -COOH, -OH and a highly aromatic structure, as confirmed by FTIR and XPS analysis. However, surface area and pore analyses showed that H 3 PO 4 modification did not facilitate the formation of a porous structure of biochar, but instead reduced the specific surface area and pore volume. According to SEM images, the pore structure of biochar was agglomerated and fused, an effect that may be related to the material composition, pyrolysis preparation conditions, or the activation process of H 3 PO 4 . Mandal et al. [55] showed that the H 3 PO 4 treatment of rice straw biochar reduced both surface area and pore volume, and concluded these changes might be due to the increase in functional groups inside the pores following H 3 PO 4 treatment. Consequently, the improvement of MB adsorption capacity by H 3 PO 4 treatment of CSBC was independent of the specific surface area and pore structure; rather, other mechanisms determined the high MB adsorption capacity of H 3 PO 4 -CSBC. FTIR and XPS analyses showed that H 3 PO 4 modification obviously affected the oxygen-and phosphorus-containing functional groups of biochar. Torrellas et al. also found that more functional groups existed on the activated carbon after H 3 PO 4 activation [56]. The participation of these functional groups was an important reason for the high MB adsorption capacity of H 3 PO 4 -CSBC.
In this study, corn stalk biochar was prepared at 500 • C. Cellulose and hemicellulose in the corn stalk gradually decomposed thermally at temperatures less than 500 • C. Meanwhile, the carbohydrate, protein and fat compounds also gradually decomposed. Then, the specific surface area became larger and a pore structure gradually formed, both of which would play an important role in MB removal. Furthermore, the aromatic structure of the biochar was gradually improved. The aromatic structure of the biochar could combine with the benzene ring in MB molecules via π-π stacking. CSBC contained abundant functional groups among which -OH and -COOH could bind with nitrogen molecules in MB through hydrogen bonding. The pH experiment indicated that electrostatic interaction also could affect the combination between CSBC and MB. Therefore, the adsorption mechanism between CSBC and MB was the result of electrostatic interaction, hydrogen bonding, π-π stacking and physical interaction (surface contact and pore diffusion).
After KOH modification, the surface area and pore volume of biochar significantly increased. The capacity of KOH-CSBC to absorb MB was greatly increased compared to that of pristine CSBC. Thus, the specific surface area and pore volume of KOH-CSBC played a key role in MB adsorption. The variety of functional groups in KOH-CSBC was limited, especially for oxygen-containing groups. Organic groups in KOH-CSBC disappeared at the pyrolytic temperature of 700 • C, resulting in weak hydrogen bonds and weak electrostatic interaction between KOH-CSBC and MB. However, the aromatic structure and the degree of graphitization of KOH-CSBC became stronger compared to pristine CSBC, and the contribution of π-π stacking to MB removal was relatively more important. Therefore, the main MB removal mechanism of KOH-CSBC involved π-π stacking and physical interaction (surface area and pore diffusion).
After H 3 PO 4 modification, the surface area and pore volume of biochar decreased relative to that of pristine CSBC. However, the capacity of H 3 PO 4 -CSBC to adsorb MB apparently increased, indicating that surface area and pore volume did not play a major role in MB removal by this biochar. FITR analysis showed that H 3 PO 4 -CSBC contained abundant functional groups, including -OH, -COOH, amino phosphonic acid functional group, and P-OH bond. Hydrogen bonding and electrostatic interaction were critical for MB removal by H 3 PO 4 -CSBC. Meanwhile, the aromatic structure and the graphitization degree of biochar were enhanced after H 3 PO 4 modification. Thus, the main MB removal mechanism of H 3 PO 4 -CSBC included electrostatic interaction, hydrogen bonding, and π-π stacking.
The MB removal mechanisms by CSBC, KOH-CSBC and H 3 PO 4 -CSBC are summarized in Figure 8. The MB removal mechanisms by CSBC, KOH-CSBC and H3PO4-CSBC are summarized in Figure  8.

Conclusions
Biochar was prepared from corn stalk and was modified using KOH and H3PO4. KOH modification of corn stalk biochar significantly increases the surface area and pore volume of the biochar. H3PO4 modification enriches the functional groups of biochar. As a result of both types of biochar modification, the maximum adsorption capacity for MB increases approximately 5-10 times (in this study from 43.14 mg g −1 for CSBC to 406.43 mg g −1 for KOH-CSBC and to 230.39 mg g −1 for H3PO4-CSBC). The modification of corn stalk biochar by KOH and H3PO4 also affects the MB adsorption mechanism. Therefore, corn stalk is an ideal natural raw material for preparation of biochar, and biochar production represents an effective way to utilize corn residues. Corn stalk biochar, especially if modified by KOH or to a lesser extent by H3PO4, can be applied to remove MBbased dye from wastewater.

Conflicts of Interest:
The authors have declared no conflict of interest. Statement of Novelty: KOH and H3PO4 were used to modify corn stalk biochar. The order of adsorption capacity was KOH-CSBC>H3PO4-CSBC>CSBC.The interaction mechanism between biochars and methylene blue were different.

Conclusions
Biochar was prepared from corn stalk and was modified using KOH and H 3 PO 4 . KOH modification of corn stalk biochar significantly increases the surface area and pore volume of the biochar. H 3 PO 4 modification enriches the functional groups of biochar. As a result of both types of biochar modification, the maximum adsorption capacity for MB increases approximately 5-10 times (in this study from 43.14 mg g −1 for CSBC to 406.43 mg g −1 for KOH-CSBC and to 230.39 mg g −1 for H 3 PO 4 -CSBC). The modification of corn stalk biochar by KOH and H 3 PO 4 also affects the MB adsorption mechanism. Therefore, corn stalk is an ideal natural raw material for preparation of biochar, and biochar production represents an effective way to utilize corn residues. Corn stalk biochar, especially if modified by KOH or to a lesser extent by H 3 PO 4 , can be applied to remove MB-based dye from wastewater.