Mg-Al/Biochar Composite with Stable Structure for Malachite Green Adsorption from Aqueous Solutions

Mg-Al-layered double hydroxide (LDH) was fabricated using a coprecipitation method at pH 10. Thereafter, MgAl-LDH was impregnated with biochar to manufacture a Mg-Al/Biochar composite. The composite was characterized using powder X-ray diffraction (XRD), Fourier-transform infrared (FTIR) spectroscopy, N2 adsorption— desorption, thermogravimetry-differential thermal analysis (TG-DTA), and scanning electron microscopy (SEM) experiments, and was subsequently used for malachite green (MG) adsorption. MG adsorption experiments were performed in a batch system, and the effects of temperature and adsorption kinetic and isotherm parameters on the adsorption process were analyzed. The stability of Mg-Al/Biochar was evaluated using regeneration experiments over three cycles. The peaks at 11.47° (003), 22.86° (002), 34.69° (012), and 61.62° (116), in the XRD profile of Mg-Al/Biochar suggested that Mg-Al/Biochar was successfully fabricated. The surface area of Mg-Al/Biochar was up to five times larger than that of pristine Mg-Al-LDH. The adsorption of MG on Mg-Al/Biochar was dominated by interactions at the surface of the adsorbent and was classified as physical adsorption; moreover the maximum adsorption capacity of Mg-Al/Biochar was 70.922 mg/g. Furthermore, the MG removal of Mg-Al/Biochar during three successive adsorption cycles (i.e. 66.73%, 65.57%, and 65.77% for the first, second, and third adsorption cycle) did not change significantly, which indicated the stable structure of the adsorbent. Copyright © 2021 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0).


Introduction
The use of synthetic dyes in industries, such as: the textile, paint, leather, photography pa-man health and the flora and fauna of ecosystems [5], such dyes should be removed before releasing industrial waste in aquatic systems. Researchers have explored several methods for dye removal from aqueous solutions, including oxidation, photodegradation, membrane separation, biological processes, and adsorption [6][7][8][9]. Among all methods, adsorption is the most suitable owing to its high speed, high efficiency, ease of use, and low cost [3]. However, adsorption efficiency depends on the properties and quality of the adsorbents used for dye removal from aqueous solutions. Adsorbents are typically classified into organic and inorganic. Organic adsorbents, such as agricultural biomass, algae, chitin, chitosan, and cellulose, are commonly found in the environment [10-13]. Adsorption using organic adsorbents primarily depends on the functional groups that act as active sites. Inorganic adsorbents are also commonly found in the environment or can be synthesised, and the most frequently used inorganic adsorbents include bentonite, zeolite, kaolinite, montmorillonite, and layered double hydroxides (LDHs) [14][15][16].
LDHs are typical composites featuring positively charged brucite-like layers and twodimensional nanostructures [17][18]. Moreover, LDHs can be modified to customize and improve their properties for specific applications. LDHs have been used as efficient adsorbents for dye removal from aqueous solutions. Ouassif et al. [19] used ZnAl LDH for tartrazine dye removal and achieved an adsorption capacity of 99.5 mg/g after 60 min at pH 5.8. Sa et al. [20] used CaAl-LDH-NO3 to adsorb Sunset Yellow FCF at a pH of 4 and reported a maximum adsorption capacity (qmax) of 398.41 mg/g. Lesbani et al. [21] used CaAl LDH to adsorb methylene blue and achieved a qmax value of 8.27 mg/g. Calcined MgAl-CO3 LDH was used to remove indigo carmine dye from aqueous solutions by adjusting the pH, temperature, and contact time. The adsorption capacity of calcined MgAl-CO3 LDH was 87.92 mg/g at pH 8.8 after 40 min [22]. Lafi et al. [23] fabricated Mg-Al LDH and used it as adsorbent for Congo red. The adsorption data fit the Langmuir adsorption isotherm model, and the reported qmax value was 111.11 mg/g at pH 7.7.
Previous studies have indicated that the adsorption capacity of LDHs for dyes was limited. Furthermore, LDH modification was required to increase the adsorption capacity of LDHs for dyes. The modification of LDH and carbonbased materials has been increasingly studied, and many researchers have used modified LDHs to remove organic pollutants from wastewater. Hu et al. [24] reported that a NiFe-LDH nanosheet/carbon fibre nanocomposite was effective for removing Congo red and methyl orange from aqueous solutions. Their results demonstrated that modification increased the adsorption capacity of NiFe-LDH for methyl orange and Congo red from 22.2 to 232.6 mg/g and from 103.7 to 448.4 mg/g, respectively. Meili et al. [25] fabricated MgAl-LDH/Biochar composites using pure bovine bone biochar and used them for methylene blue adsorption. The adsorption data fit the Langmuir isotherm and pseudo-second-order (PSO) kinetic models, and the qmax value of the composites at 40 °C ranged between 46.3 and 406.47 mg/g. Zubair et al. [26] reported that the adsorption capacity of MgAl-LDH/Biochar composites for methylene blue (302.75 mg/g after 180 min) was higher than those of pristine biochar (206.61 mg/g after 480 min) and pristine MgAl-LDH (244.47 mg/g after 480 min). Amin et al. [27] reported that the Freundlich qmax value of NiZnFe-LDH composites with date palm biochar and carbon nanotubes for RB5 dye (121 mg/g) was higher than that of pristine NiZnFe-LDH (63.22 mg/g). Palapa et al. [28] fabricated a CuAl-LDH/Biochar composite using rice husk biochar and achieved a qmax of 470.96 mg/g, which was higher than that of pristine CuAl-LDH (59.523 mg/g).
The aforementioned studies indicated that LDH modification can be used to increase the adsorption capacity of LDHs for dyes. Because malachite green (MG), which is a synthetic cationic dye (Figure 1), cannot be degraded by microbes in aquatic systems, its removal using other methods is critical. In this study, Mg-Al-LDH was impregnated with biochar to fabricate a Mg-Al/Biochar composite with high adsorption capacity. Moreover, biochar impregnation increased LDH stability and improved adsorbent reusability. MG adsorption was performed in a batch system, and the effects of contact time and isotherm, desorption, and regeneration parameters on the adsorption process were investigated.

Chemical and Instrumentations
Chemicals were obtained from magnesium nitrate, aluminum nitrate, sodium hydroxide, acetone, and hydrochloric acid which purchased from Merck and Sigma-Aldrich and the rice husk Biochar was acquired by Bukata Organic Indonesia. Material was characterized using X-Ray powder Rigaku Miniflex-6000. IR spectrum was recorded by using FTIR Shimadzu Pestige-21 at wavenumber 400-4000 cm −1 . Surface area, pore diameter, and pore size were measured by BET method by Micromeritic ASAP and sample was degassed using liquid nitrogen and material thermal analysis was studied by TG-DTA Shimadzu was used to study. The morphology of the materials were tested by SEM Quanta-650 OXFORD Instrument. Concentration of MG was analyzed using Biobase BK-UV 1800 PC spectrophotometer at 617 nm.

Preparation of Mg-Al-LDH and Composite Mg-Al/Biochar
Mg-Al-LDH was prepared by dropping a solution 0.75 MMg(NO3)2.6H2O (100 mL) to 0.25 MAl(NO3)3.9H2O (100 mL). To achieve pH 10, NaOH was added in to the mixture and stirred for 30 min. The mixture was heated at 80 ℃ overnight. The obtained precipitate was dried in oven at 100 ºC for 24 hours. The Mg-Al/Biochar was produced by mixing magnesium nitrate and aluminum nitrate (3:1). The mixture was gently stirred for 1 hour. The reaction mixture was poured with 1 g of biochar and reaction was mixed by constant stirring under pH 10. The mixture was heat at 90 °C for 3 days and dried at 110 °C for 5-6 days prior characterization.

Adsorption Process
MG adsorption experiments were performed at adsorption times in the range of 10-200 min, MG concentrations in the range of 10-200 min, and temperatures in the range of 30-60 °C. The concentration of MG in the filtrate after adsorption was analyzed using the aforementioned UV-Vis spectrophotometer.

Desorption and Regeneration Experiments
MG desorption from the Mg-Al/Biochar composite was evaluated using several reagents, namely water, acetone, HCl, and NaOH, and the optimal desorption reagents were subsequently used to regenerate the adsorbent. After desorption, the adsorbent was collected and dried at 100 °C. Thereafter, the regenerated adsorbent was reused for three adsorption cycles. MG dye removal experiments were performed using previously reported optimal time and temperature values.
The N2 adsorption-desorption isotherms of Mg-Al-LDH, biochar, and Mg-Al/Biochar are presented in Figure 3. The adsorption patterns of all samples were different from their desorption patterns. All materials presented H2 type hysteresis loops and, therefore, were classified as mesoporous [33]. The BET surface areas of the adsorbents were calculated using the N2 adsorption-desorption isotherms and the data are summarized in Table 1.
The surface area of Mg-Al/Biochar composite (111.404 m 2 /g) was significantly higher than those of Mg-Al-LDH (23.150 m 2 /g) and biochar (50.936 m 2 /g). Moreover, the pore size and pore volume of the composite were smaller than those of Mg-Al-LDH and biochar because biochar covered the surface of Mg-Al-LDH. These results were consistent with the prediction that when the surface of Mg-Al-LDH was covered by biochar which consists of carbon, adsorption was dominated by physical interactions [34].
The FTIR spectra of the adsorbents are presented in Figure 4. The main vibration at 1381 cm -1 in the FTIR spectrum of Mg-Al-LDH was ascribed to the nitrate ions in the materials used to synthesize Mg-Al-LDH. The stretching vibration at 1635 cm −1 was attributed to the vibration of the OH group of water. The vibration at 2376 cm −1 was ascribed to the C−H bonds of biochar [35]. The vibrations at 3448 and 1635 cm −1 were assigned to the stretching and bending of the O−H bonds of water [28]. The TG-DTA profiles of Mg-Al-LDH and Mg-Al/Biochar are illustrated in Figure 5. The profile of Mg-Al-LDH consisted of three endothermic peaks at 105, 330, and 720 °C, which were assigned to the loss of lattice water, decomposition of nitrates in the interlayer, and destruction of the layered structure, respectively. Conversely, the TG-DTA profile of biochar consisted of two peaks at 110 and 490 °C. The endothermic peak at 110 °C was attributed to the loss of water in the lattice structure, whereas the exothermic peak at 490 °C was attributed to the oxidation of organic compounds in biochar [36]. The TG-DTA profile of Mg-Al/Biochar (Figure 5c) consisted of two endothermic peaks at 110 and 305 °C and one exothermic peak at 490 °C. These data revealed that the composite consisted of Mg-Al-LDH and

Adsorbents
Surface Area (BET) (m 2/ g) Pore Volume (BJH) (cm 3  biochar. The exothermic peak was predominant in the TG-DTA profile of the Mg-Al/Biochar composite, probably because the biochar content of the composite was slightly higher than the Mg-Al-LDH content. The morphologies of the Mg-Al-LDH, biochar, and Mg-Al/biochar are presented in Figure 6. Mg-Al-LDH presented a cubic morpholo-gy with agglomerated particles scattered on the surface, which was in agreement with the findings of Palapa et al. [37]. Conversely, biochar consisted of sharp and large particles with an irregular porous structure. The morphology of Mg-Al/Biochar was a hybrid of the morphologies of Mg-Al-LDH and biochar; Mg-Al/Biochar presented a heterogeneous morphology and the   particles on its surface were large (Figure 7). Mg-Al/Biochar particle size ranged between 0.139-0.341 µm; moreover, its particle size distribution was wider than that of pristine Mg-Al-LDH and, therefore, the surface area of Mg-Al/Biochar composite was higher than that of Mg-Al-LDH. Ahmed et al. [38] suggested that the increase in particle size of Mg-Al/Biochar was caused by the pores of biochar particles being well dispersed on the LDH surface, therefore demonstrating that biochar modification was successful.
MG adsorption on Mg-Al-LDH, biochar, and Mg-Al/Biochar was analysed using kinetic and isotherm adsorption experiments. The adsorption time ranged between 10-210 min and the kinetic parameters were fitted using pseudofirst-order (PFO) and pseudo-second-order PSO kinetic models [24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39]. The experimental data are presented in Figure 8, and the kinetic parameters are summarized in Table 2. The data in Table 2 indicated that MG adsorption on Mg-Al-LDH, biochar, and Mg-Al/Biochar followed the PSO rather than the PFO kinetic model, and the correlation coefficient (R 2 ) for the PSO kinetic model was approximately 1. The PSO reaction constant values suggested that MG adsorption on Mg-Al/Biochar was slower than that on pristine Mg-Al-LDH, probably owing to biochar loading on Mg-Al-LDH. The equilibrium absorption capacity of Mg-    , 16 (1), 2021, 155 Copyright © 2021, ISSN 1978-2993 Al/Biochar was higher than those of Mg-Al-LDH or biochar because adsorption occurred at the surface sites. The isotherm fit parameters for the MG adsorption on Mg-Al-LDH, biochar, and Mg-Al/Biochar are illustrated in Figure 9. The ad-sorption temperature ranged between 30-60 °C and the initial MG concentration ranged between 50-250 mg/L. MG adsorption increased significantly as MG concentration was increased from 50 to 150 mg/L at all temperatures and remained constant after 150 min of adsorption. MG adsorption at high temperatures was higher than at low temperatures for all adsorbents. The data in Figure 9 were used to calculate the isotherm and thermodynamic adsorption parameters, and the results are summarized in Tables 3 and 4, respectively. MG adsorption fit the Freundlich isotherm model better than the Langmuir model. The R 2 values of all adsorbents for the Freundlich model were higher than those for the Langmuir model. These results indicated that MG adsorption on Mg-Al-LDH, biochar, and Mg-Al/Biochar was dominated by physical adsorption and depended on the surface area of the    [40][41][42]. Although chemical interactions probably occurred during MG adsorption, the contribution of chemical adsorption was negligible. The adsorption energies of Mg-Al-LDH, biochar, and Mg-Al/Biochar (Table 3) further confirmed the predominance of physical adsorption in this study [39]. The MG adsorption capacities of several adsorbents reported in the literature and Mg-Al-LDH and Mg-Al/Biochar used in this study are summarized in Table 5. The adsorption capacity of Mg-Al-LDH for MG was similar to that of other LDHs but was slightly lower than those of biomassbased adsorbents. Conversely, the adsorption capacity of Mg-Al/Biochar was higher than those of unmodified LDHs. These results were attributed to the increase in surface area caused by biochar modification and involvement of the active sites of biochar in adsorption.

Bulletin of Chemical Reaction Engineering & Catalysis
The UV-Vis spectrum for MG removal using Mg-Al/Biochar is presented in Figure 10. The adsorption peak was observed at a wavelength of 617 nm. Moreover, the UV-Vis data revealed that adsorption occurred rapidly when the contact time ranged between 0-60 min and reached an equilibrium when the contact time was extended to 90-120 min; MG concentration decreased from 19.905 to 6.168 mg/L. The mechanism of MG adsorption on Mg-Al/Biochar is presented in Figure 11. Mg-Al/Biochar successfully removed MG, probably via electrostatic attractions and hydrogen bonding, because electrostatic attraction occurred between the active sites of biochar and MG.
MG desorption from Mg-Al-LDH, biochar, and Mg-Al/Biochar was performed to determine the predominant type of adsorbentadsorbate interaction. Desorption was performed using water, hot water, acetone, HCl, and NaOH as desorption reagents. The experi-    mental data (Figure 12) revealed that HCl was a suitable reagent for MG desorption from Mg-Al-LDH and Mg-Al/Biochar. However, the best reagent for MG desorption from biochar was acetone [52]. The optimal desorption using HCl indicated that a small degree of ionic interaction was present between MG and Mg-Al-LDH and Mg-Al/Biochar. Although physical interactions were predominant in this study, the electrostatic interactions between the positive charges of MG and the negative charges of Mg-Al-LDH and Mg-Al/Biochar was still observed to a small degree. Mg-Al-LDH, biochar, and Mg-Al/Biochar regeneration was performed using HCl, acetone, and HCl, respectively. The adsorbents were used for three adsorption cycles as presented in Figure 9. Mg-Al-LDH and biochar were unstable; however, Mg-Al/Biochar was stable for three adsorption cycles without a significant decrease in adsorption capacity ( Figure 13). Mg-Al-LDH was exfoliated by HCl and biochar was destroyed by acetone. Conversely, Mg-Al/Biochar was stable in HCl and organic solvents; therefore, the structure of Mg-Al/Biochar was stable and its adsorption capacity did not change significantly over three adsorption cycles.

Conclusions
Mg-Al/Biochar composite was successfully synthesised using Mg-Al-LDH and biochar. The layered structure of Mg-Al/Biochar was confirmed by the peaks at 11.47° (003), 22.86° (002), 34.69° (012), and 61.62° (116) in the XRD profile of Mg-Al/Biochar. The surface area of Mg-Al/Biochar was higher than that of pristine Mg-Al-LDH. Moreover, the adsorption of MG on Mg-Al/Biochar, which was dominated by physical adsorption, followed the PSO kinetic and Freundlich isotherm models. The adsorption energy of Mg-Al/Biochar ranged between 5.531-38.024 kJ/mol, which further confirmed the predominant physical nature of the adsorption process. Furthermore, Mg-Al/Biochar presented a highly stable structure and was reused for three adsorption cycles with negligible changes in adsorption capacity.