The Efficiency of Mg-Al/Biochar for Methyl Orange and Methyl Red Removal

MgAl-LDH was directly impregnated with biochar to fabricate MgAl-Biochar (MgAl/BC) and applied to remove methyl orange (MO) and methyl red (MR). The XRD, BET, FTIR, TG-DTA and SEM analyses were conducted to characterize the prepared material. The result of XRD characterization diffraction peaks at 11.47, 22.86, 34.69, and 61.6 shows that the precursor was successfully transformed to MgAl-BC. The FT-IR analysis at vibration 1010, 1381,3447 and 1635 cm-1 illustrated that the composite was well formed. The BET analysis showed that the surface area of the MgAl-BC composite was 111.404 m2/g which was larger than that of the precursor, equal to 23.15 m2/g. The kinetic model of the adsorption study both MR and MO were fitted to PSO and followed the Langmuir model with adsorption capacities for MR 142.857 mg/g and MO 128.205 mg/g. The regeneration study of composite indicated higher efficiency than the pristine and show good stability of adsorption process in five cycles.


INTRODUCTION
Dyes disposal was one of the greatest environment issue which endangers the organisms and human beings (Zhao et al., 2018). Although it causes a lot of negative impacts, dyes are still popular in many industries, such as textile, paper , pharmaceutical and paper industries . Many techniques have been studied in order to eliminate dyes from polluted wastewaters such as coagulation (Demissie et al., 2021), microbial ( (Vinsiah et al., 2020). Lately, adsorption became an intriguing technique because of its low cost, ease of work and high efficiency (Bharali & Deka, 2017). Different adsorbents have been researched such as carbon active, graphene (Vinsiah et al., 2020) and layered double hydroxide (LDH) (Oktriyanti et al., 2019).
LDH consists of divalent and trivalent cations which are substituted in the octahedral layer. Diverse kinds of anions, such as carbonate nitrate and chlorin between the two layers which effected the difference in sorption capacities (Lu et al., 2016). LDH is a promising adsorbent which carries balanced cation and negative charge (Santos et al., 2017). The charge balancing gives LDH good cation and anion exchange properties (Tran et al., 2018). There have been many studies applicating LDH as an adsorbent in the adsorption process. Ayawei  MgAl LDH was modified with biochar and used for removing organic pollutants which increased the active site of the adsorbent (Gholami et al., 2020). Another research showed Mg-Al-biochar was used to remove phosphate with the co-pyrolysis method (Lee et al., 2019) Thus, MgAl-BC, as an adsorbent, was widely used for reducing organic pollutants. Methyl red (MR) and methyl orange (MO) were categorized as azo dyes with two aromatic rings. The structure of MR and MO was illustrated in Figure 1.
In this research, MgAl-LDH was modified with biochar (BC) to form MgAl-BC for escalating the LDH adsorption capacity. The adsorbents were applied for removing methyl orange (MO) and methyl red (MR). X-ray diffraction, BET, and FT-IR spectra, TG-DTA and SEM analysis were used to characterize the adsorbents. The variation of time, isotherms, and thermodynamics were studied in the adsorption process.

EXPERIMENT Chemicals and Instrumentations
Mg(NO 3 ) 2 ·6H 2 O, Al(NO 3 ) 2 ·9H 2 O, Na 2 CO 3 , NaOH were acquired from Merck and Sigma-Aldrich. XRD was recorded by a Rigaku Miniflex-600. FTIR spectra were obtained by Shimadzu FTIR Prestige-21. BET was calculated using ASAP Quantachrome apparatus. The thermal analysis of materials were studied by TG-DTA Shimadzu and the morphology was measured by SEM Quanta-650 OXFORD.

Synthesis of Mg/Al LDH
Mg/Al LDH was synthesized using coprecipitation method: 0.75 M Mg(NO 3 ) 2 ·6H 2 O as much as 100 mL and 0.25 M Al(NO 3 ) 3 ·9H 2 O as much as 100 mL were mixed and stirred until homogeneous. 50 mL NaOH solution of 2M and 50 mL Na 2 CO 3 solution of 2 M were dripped, stirred until homogeneous, then slowly dripped a mixture of Mg(NO 3 ) 2 ·6H 2 O and Al(NO 3 ) 3 ·9H 2 O. After being homogeneous, the mixture was maintained for 24 hours at 80 °C to form a precipitate. The final process is to filter and dry the synthesized precipitate at a temperature of 100 °C to dry.

Preparation of Mg/Al-Biochar
MgAl-LDH was fabricated as in previous research (Badri et al., 2021), through co-precipitation. In order to obtain the composite adding Mg(NO 3 ) 2 ·6H 2 O and Al(NO 3 ) 2 ·9H 2 O were mixed with ratio 3 : 1 and stirred, then as much as 3 g biochar was added and stirred pH 10. The heating process was conducted at 60 °C for 3 days and dried for 24 h.

Adsorption study
The dyes removal was investigated by adsorption times variation, dyes concentration, and temperature variation. The pH effect was studied by setting at pH condition at 2-11. MR and MO, in the amount of 0.02 L 100 mg/L with current pH were added into 0.02 g of the adsorbents and stirred for 2 h under 120 rpm. This process was varied from 5 until 150 minutes. The concentrations of dyes were varied by range 60-100 mg/L and temperatures were conducted at 303-333 K.

Regeneration experiment
A certain amount 0.5 g adsorbent was mixed into 50 mL MO/MR solutions (250 ppm) were shaken for and dried for 2 h. After that, 0.01 g dried adsorbent was placed in 20 mL water and mixed in to HCl (0.01 M). The whole process was conducted for five cycles to study the stability of the materials.

RESULTS AND DISCUSSION
The MgAl-LDH, BC, and MgAl-BC diffraction patterns were displayed in Figure 1 and showed that MgAl-BC has a similar diffractogram compared by the pristine and BC. The basal spacing, which was shown from the diffraction peak of MgAl-BC reached 8.31 Å at 11.81° (003), while the pristine -7.71Å. According to Lee et al. (2019) the diffraction peak at 16-20° on BC is indicated as silica in the amorphous state. The curve of the adsorption-desorption N 2 on the materials were displayed in Figure 3.
The curve presented in Figure 2 shows a type IV isotherm pattern with a hysteresis H3. According to IUPAC, the type IV isotherm pattern explains that the material is mesoporous, while the H3 hysteresis pattern indicates that the material has a wide pore distribution (Zhao et al., 2018). Table 1 showed that the surface area of MgAl-BC was higher than MgAl-LDH. On the basis of the above-mentioned phenomenon, the change that occurs from MgAl-LDH to MgAl-BC causes a decrease in agglomeration of LDH, so that the pore size of MgAl-BC is smaller and more regular. Figure 4 showed the spectra of all the materials. According to (Lee et al., 2019), sharp peak at 1380 cm -1 denoted a nitrate ion. A sharp peak around 3400 cm -1 showed the -OH stretching existence of water. Figure 4a indicates the presence of nitrate ions at 1381 cm -1 .
The vibrational peak of BC (    peak at 80 ℃ which indicated the decomposition of water at biochar surface area. An exothermic peak in Figure 5c at 420 ℃ of MgAl-BC showed that the composite process required energy to decompose. Figure 6 illustrated the morphology of the adsorbents. The morphology of MgAl-BC showed the specific character of MgAl-LDH which indicated cubical form and large with shape agglomerated The time variation was carried out to determine the equilibrium state which is presented in Figure 7. On the basis of Figure 7, the equilibrium state of the adsorption process was obtained at 120 minutes. On the other hand, time variation is also used to determine the kinetic parameters which include pseudo-firstorder (PFO) and pseudo-second-order (PSO) kinetic models. The results of the determination of the MR and MO adsorption kinetics parameters can be seen in Table 2. Table 2 shows that the MO adsorption tends to follow the PFO kinetic model, while the MR adsorption tends to follow the PSO kinetic model. Determination of the kinetic model is based on     a linear regression value that is closer to 1. The ability of the adsorbent in the adsorption process is supported by the concentration and temperature variation data in Figure 8. Figures 8 and 9 show that the adsorption process of the MR and MO substances using MgAl-BC has the highest adsorption capacity, this is evidenced by the maximum adsorption capacity data in Table 3 which reaches 107 mg/L. Furthermore, Table 3 also shows the results of determining the isotherm parameters which include the Langmuir and Freundlich adsorption   Table 4. These data showed in this research the adsorption capacity of the adsorbents had higher results than the other. Thus, it concluded that the materials were good adsorbents to remove MO and MR.
Tables 5 and 6 displayed the thermodynamic data from the MO and MR adsorption processes. The adsorption using MgAl-BC and the pristine generated Gibbs energy with negative values which showed spontaneous condition (Normah et al., 2021). The positive enthalpy showed endothermic adsorption  and entropy energy (ΔS°) with signified the increasing of the randomness (Zhao et al., 2018).
The ability of the adsorbent in the adsorption process is also proven by the adsorption selectivity process carried out with time variations. The results of the adsorption selectivity are shown through the UV-Vis spectrum of the mixture of MO and MR in Figure 10. There was a decrease in absorbance at the MO peak compared to MR, which confirmed that the MgAl-LDH, BC, and MgAl-BC adsorbents were easier in the MO adsorption process.
The ability of the adsorbent in the regeneration process in Figure 11 confirms that MgAl-BC, as a composite material, has better structural stability than MgAl-LDH. MgAl-BC is able to go through the regeneration process up to 5 cycles, while MgAl-LDH only lasts up to the 4th cycle. the use of hydrochloric acid as a solvent during the desorption process made it possible that the MgAl-LDH layer was exfoliated due to the use of acid.

CONCLUSIONS
MgAl-BC has been successfully developed as a material that has the potential to overcome dye contamination such as MR and MO. The ability of MgAl-BC in overcoming MR and MO is evidenced by the adsorption capacity data which reached 128,205 mg/g for MO and 142,857 mg/g for MR. The adsorbent regeneration process has also proven that MgAl-BC has a more stable structure than MgAl-LDH, this is shown from the regeneration data of MgAl-BC which lasts up to 5 cycles.