Modified Layered Double Hydroxide Mg/M 3+ (M 3+ = Al and Cr) Using Metal Oxide (Cu) as Adsorbent for Methyl Orange and Methyl Red Dyes

Mg/Cr-layered double hydroxide (Mg/Cr-LDH) and Mg/Al-layered double hydroxide (Mg/Al-LDH) intercalated metal oxide (Mg/Cr-Cu and Mg/Al-Cu) were synthesized by the co-precipitation method which is indicated by the X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and Brunauer Emmett Teller (BET) analysis. Mg/Cr-LDH intercalated metal oxide increased its surface area from 21.5 to 38.9 m 2 /g, while the surface area of Mg/Al-LDH from 23.2 to 30.5 m 2 /g. The adsorption capacity of Mg/Cr-Cu is 64.156 mg/g for methyl orange (MO) and 78.740 mg/g for methyl red (MR), and the adsorption capacity of Mg/Al-Cu is 97.087 mg/g for MO and 108.696 mg/g for MR. Equilibrium time on the adsorption process occurred at 90 minutes with adsorption kinetics followed by pseudo-second-order (PSO). The adsorption isotherm followed the Langmuir isotherm equation. Data of thermodynamic parameters indicate that the adsorption process in this study occurs spontaneously and endothermically. The regeneration results show that Mg/Cr-Cu and Mg/Al-Cu can be used for the 5 cycles regeneration process of MO and MR adsorption process. Interactions that occur between adsorbents and adsorbate include physical interactions, interactions with the involvement of hydrogen bonds, and electrostatic interactions.


Introduction
The industrial sector is the fastest-growing development sector in Indonesia. Developments in this sector will, of course be followed by an increase in environmental pollution caused by waste, especially pollution in the aquatic environment, because in general industrial liquid waste is discharged directly into ditches or rivers. Industrial liquid waste is waste generated from various production processes in the industry. This liquid waste contains pollutants that cause the water to become colored [1][2][3].
The dyes in the textile industry that are widely used are methyl red and methyl orange which are types of anionic dyes [4]. The presence of dyes as waste is very disturbing to aesthetics and the environment. Water pollution due to dye waste will have a negative impact on the ecosystem and the surrounding environment, but this impact will be visible in the long term. Therefore, it is necessary to treat the dye waste [5].
One way of processing dye waste that is quite optimal is using the adsorption method [6][7][8][9]. The process of handling liquid waste that has been contaminated with dyes through adsorption has many advantages, namely easy [10], simple [11], inexpensive [12], and effective [13]. The selection of the type of adsorbent in addition to being viewed from the side of effectiveness and selectivity in adsorption is also expected to be cheap and easy to manufacture. Layered double hydroxide (LDH) can be used as an alternative to a dye waste treatment [14].
Recently, LDHs have gained great attention from scientific researchers. LDHs have been used in various fields such as biomedicine, energy storage, photochemistry, environment, etc. LDHs have advantages including low cost, high surface area, and high adsorption capacity [15,16]. In improving the performance of LDH materials, it is necessary to modify the material so as to improve its physical performance and application. Materials that can be modified in LDH are metal oxides.
Research conducted by Taher et al. [17], a composite material based on montmorillonite-mixed metal oxides derived from ZnAl-LDH resulted in an adsorption capacity of 40 mg/g on the Congo Red dyes adsorption. Kumar et al. [18] said that ZnO and SnO 2 can remove malachite green oxalate dye with a maximum adsorption capacity of 310.50 mg/g and 216.90 mg/g. Research conducted by Budiman and Zuas [19] said that cerium dioxide nanoparticles (CeO 2 -NP) can be used as an adsorbent to remove Acid Orange-10 dye with a maximum adsorption capacity of 33.33 mg/g. Nickel (II) oxide can be used as an adsorbent in the adsorption process of acid orange 7 (AO7) and indigo disulfonate (ID) with a maximum adsorption capacity of 178.57 mg/g and 227.27 mg/g [20].
In this study, modification of LDH materials, namely Mg/Al-LDH and Mg/Cr-LDH with metal oxide (Cu), was applied to the removal of anionic dyes including methyl orange (MO) and methyl red (MR). The use of two types of LDH material aims to see which modified material produces better performance on dye adsorption. The manufacture of metal oxides Mg/Al-Cu and Mg/Cr-Cu was carried out with the aim of increasing adsorption capacity; besides that, it was also expected that the interaction of functional groups on Mg/Al-Cu and Mg/Cr-Cu would have good stability. This research is expected to provide benefits for the handling of dye waste, thus environmental pollution can be handled as well as possible. The adsorption characteristics of MO and MR by adsorbents in terms of adsorption kinetics, adsorption isotherms, and adsorption thermodynamics, as well as the stability of the adsorbent structure were studied through regeneration studies.

Materials and methods 2.1 Materials
The materials used in this study such as Mg( Precipitate was dried at 353 K for 24 hours, and then the process of calcination at a temperature of 523 K for 6 hours.

Adsorption studies
The effect of contact time on anionic dyes can be studied by varying the contact time (0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 120, and 150 minutes). 0.025 g adsorbent was added to a 50 mL Erlenmeyer containing 25 mL of dye solution with a concentration of 100 mg/L. The mixture was stirred, and the filtrate was measured using a UV-Visible spectrophotometer. The effect of concentration and temperature of adsorption was studied by varying the concentration (60, 70, 80, 90, and 100 mg/L) and temperature (303, 313, 323, and 333 K). 0.02 g adsorbent was added to an Erlenmeyer 50 mL containing 20 mL of MO and MR, stirring for 1 hour, and then the filtrate was measured using a UV-Visible spectrophotometer.

Regeneration process
Regeneration of adsorbent is carried out by adsorption and desorption processes first. MO and MR 100 mg/L were added with 0.1 g of adsorbent. The mixture was stirred for 2 hours, and the absorbance of the filtrate was measured using a UV-Visible spectrophotometer. Adsorbents that have been used are carried out the desorption process using the water-assisted ultrasonic system, then dried, and carried out for the next regeneration cycle by the same method.

Result and discussion
Materials of Mg/Cr-layered double hydroxide (Mg/Cr-LDH) and Mg/Al-layered double hydroxide (Mg/Al-LDH) intercalated metal oxide (Mg/Cr-Cu and Mg/Al-Cu) characterized using the X-ray diffraction (XRD), Fouriertransform infrared spectroscopy (FTIR), and Brunauer Emmett Teller (BET) analysis. The result of the XRD analysis is shown in Fig. 1. According to Badri et al. [21] diffraction pattern of Mg/Cr-LDH at angles 11° (003), 22° (006), 36° (115), and 60° (110). Fig. 1  Based on the diffraction peaks of Mg/Cr-Cu and Mg/ Al-Cu, the material has decreased crystallinity caused by the calcination process. Calcination of samples at 773 K or fewer leads to the collapse of the lamellar structure and new diffraction lines originating from metal oxides that exhibit lower crystallinity. Calcination at higher temperatures (>773 K) will result in increased crystallinity of the metal oxide [23,24]. In this study, the calcination process was carried out at 523 K. This aims to produce metal oxide (Cu) without eliminating the material characteristics of LDH in the composite material.
The results of the FT-IR characterization of materials are shown in Fig. 2 The graphs of nitrogen adsorption-desorption isotherms of materials shown in Fig. 3 indicate that the material has hysteresis and follows type IV isotherms. According to Ahmad et al. [25], type IV isotherm shows hysteresis of mesoporous-sized materials with strong hysteresis activity on the adsorbent-adsorbate interaction. Based on data BET analysis in Table 1, Mg/Cr-LDH and Mg/Al-LDH modified with metal oxide (Mg/Cr-Cu and Mg/Al-Cu) through the co-precipitation method showed increased surface area. Mg/Cr-LDH intercalated metal oxide increased its surface area from 21.5 to 38.9 m 2 /g, while the surface area of Mg/Al-LDH from 23.1 to 30.5 m 2 /g. The surface area of Mg/Cr-Cu is larger than the others because the material produces a smaller pore size and pore volume, so the formation of more functional groups is produced. More functional groups will affect the surface area of the material. Formation of a smaller surface area, it is caused by the agglomeration process (increase in particle size) in the material. The pore sizes of the materials were  [25], and pore volume of the materials were 3.2-6.6 cm 3 /g. Mg/Cr-LDH has a larger pore diameter and pore volume.
Materials are applied as adsorbents in the MO and MR adsorption process, with varying contact times and the initial concentration of dyes on adsorption temperature, desorption process, and regeneration process. Based on the results of the adsorption contact time shown in Fig. 4, the equilibrium adsorption of MO and MR occurred at 90 minutes, with an insignificant increase in adsorption concentration at a contact time of more than 90 minutes.
Based on data Table 2 shows that the adsorption kinetics followed pseudo-second-order (PSO), with the value of the linear regression coefficient (R 2 ), which is close to the value 1 [12]. It revealed that the adsorption process indicates the involvement of chemisorption between adsorbent and adsorbate [26]. The kinetic model of PSO also shows that the process occurs influenced by adsorbents and adsorbates [27]. PFO and PSO equations are described in Eqs. (1) and (2) where Q e is the adsorption capacity at equilibrium (mg/g), Q t is the adsorption capacity at time t (mg/g), t is contact time (min), k 1 is equilibrium rate constant for the PFO model (min −1 ), and k 2 is equilibrium rate constant for the PSO model (g/mg min).
Based on data of effect the initial concentration of MO and MR in Figs. 5 and 6 indicates that increasing temperature causes an increase in concentration adsorption. Table 3 shows the large adsorption capacity that occurred at an adsorption temperature of 333 K. The adsorption capacity of Mg/Cr-Cu is 64.5 mg/g for MO and 78.8 for MR, and the adsorption capacity of Mg/Al-Cu is   Table 3. The Langmuir model is better than the Freundlich model in the adsorption process in this study, with the value of R 2 closer to the value of 1 [28,29]. Langmuir and Freundlich isotherm equations are described in Eqs. (3) and (4) respectively: where C e is the concentration of the dye solution at the equilibrium (mg/L), Q e is the adsorption capacity at the equilibrium (mg/g), Q m is the maximum adsorption capacity (mg/g), 1/n is the empirical parameter associated with surface heterogeneity, k L is the adsorption constant of the Langmuir model (L/g), and k F is the adsorption constant of the Freundlich model (mg/g (L/g) −1/n ). In general, the Langmuir isotherm model assumes that a monolayer adsorption process can occur on the adsorbent surface without mutual interaction between adsorbed molecules (chemisorption), while the Freundlich isotherm model involves not only adsorption on adsorbents with inhomogeneous surfaces but also adsorption between adsorbed molecules, or the adsorption process occurs in a multilayer (physisorption) [30,31]. This study tends to follow the Langmuir isotherm model, which assumes the adsorption process occurs in a monolayer manner (chemisorption). The thermodynamic parameters of materials are shown in Tables 4-7. A positive ∆H value indicates that    the adsorption is an endothermic process [32,33], while a ∆S value indicates that the degree of irregularity in the adsorption process is small in large concentrations, and the overall ∆G value is negative, indicating a spontaneous adsorption process [34,35]. The thermodynamic parameters were calculated by Eqs. (5) and (6): where T is the temperature (K), R is the molar gas constant (8.314), ΔH is the enthalpy (kJ/mol), ΔS is the entropy (J mol/K), and ΔG is Gibbs free energy (kJ/mol). Table 8 shows the maximum adsorption capacity of methyl orange and methyl red using various types of adsorbents. Table 8 confirms that the maximum adsorption capacity of MO and MR using Mg/Cr-Cu and Mg/Al-Cu adsorbents in this study is greater than the adsorbents shown in Table 8 [36][37][38][39][40][41][42][43][44][45][46][47].
Based on data of the regeneration process in Fig. 7, materials of Mg/Cr-Cu and Mg/Al-Cu can be used in the five cycles regeneration process of MO and MR adsorption process. In the last cycle, the percentage adsorption of MO using Mg/Cr-Cu and Mg/Al-Cu is about 30%, while in MR about 40% for Mg/Cr-Cu and 80% for Mg/Al-Cu. This indicates that the Mg/Cr-LDH and Mg/Al-LDH after intercalated metal oxide (Cu) improved performance in process regeneration. The MR adsorption process produces a greater adsorption capacity than MO, it can occur assumed due to the difference in molecular weight (MO > MR) and the chemical interactions that occur. It also affects the results of adsorbent regeneration, where the regeneration process has better efficiency for MR removal compared to MO when using Mg-Cr/Cu and Mg-Al/Cu. A schematic illustration of the MO and MR adsorption mechanism using Mg/Cr-Cu and Mg/Al-Cu is presented in Fig. 8, the possible interactions that occur between adsorbents and adsorbate include physical interactions, interactions with the involvement of hydrogen bonds, and electrostatic interactions.

Conclusion
Mg/Cr-LDH intercalated metal oxide increased its surface area from 21.5 to 38.9 m 2 /g, while the surface area of Mg/ Al-LDH from 23.2 to 30.5 m 2 /g. Based on data obtained from this study, it can be concluded that the largest adsorption capacity occurred at a temperature of 333 K, where the adsorption capacity of Mg/Cr-Cu is 64.5 mg/g for MO and 78.8 mg/g for MR, and the adsorption capacity of Mg/Al-Cu is 97.1 mg/g for MO and 108.7 mg/g for MR. The adsorption kinetics followed PSO for MO and MR. The adsorption isotherm followed the Langmuir isotherm with a value of R 2 closer to the value of 1. Adsorption in this study includes an endothermic process and occurs spontaneously. Based on data, the regeneration results show that Mg/Cr-Cu and Mg/Al-Cu can be used for 5 cycles regeneration process of MO and MR adsorption process.