The Utilization of Mg-Al/Cu as Selective Adsorbent for Cationic Synthetic Dyes

Mg-Al-LDH is a chemical compound produced through co-precipitation technique and modified with Cu(NO3)2.6H2O to form Mg-Al/Cu. However, the research on the capability of these compounds for adsorbing mixtures of cationic dyes as well as malachite green (MG), methylene blue (MB), and Rodhamine-B (Rh-B) has not been carried out. Therefore, this research aims to determine the performance of Mg-Al-LDH and Mg-Al/Cu for removing cationic dyes. The materials used were characterized by using XRD powder, FT-IR, and N2 adsorption desorption. The Adsorption process was conducted by batch system and several effects were investigated, such as kinetic parameter, isotherm, and the temperature condition. The stability feature of Mg-Al-LDH and Mg-Al/Cu was obtained from the regeneration process in the five cycles. The results presented that Mg-Al/Cu was effectively produced, which was indicated by the formation of layer at 10.792° (003), 22.94° (006), 35.53° (112), 55.78° (110), and 56.59° (116). Mg-Al-LDH and Mg-Al/Cu were found to adsorbed MG than the other cationic dyes with adsorption capacity of 68.996 mg/g and 104.167 mg/g, respectively. The unique properties of Mg-Al/Cu includes, structural stability towards the reuse of adsorbent subsequently for five times, without significant decrease of adsorption ca-


Introduction
In recent years, dyes are widely utilized for coloring agent in diverse industries, as well as in plastics, rubber, paper, and electroplating manufactory [1][2][3]. They are also used as additives in food, cosmetics, petroleum products and solution, such as: oxidation [10], membrane separation [11], photodegradation [12], biological process [13], and adsorption [14]. Among all the methods, adsorption is the most effective for removing dyes, because of its easy operation, swift process, and high efficiency [15,16]. Layered double hydroxides (LDHs), generally known as hydrotalcite-like is an inorganic anion-exchangeable adsorbent, used for adsorbing toxic materials from polluted water [17,18].
Research showed that the use of LDH as an adsorbent removes dyes effectively. Shan et al. [19] used Mg-Al LDH to remove red dyes with maximum adsorption capacity of 37.16 mg/g for Congo Red and 59.49 mg/g for Reactive Red with optimal adsorbent dosage of 1.0 g and contact time of 60 min. Ouassif et al. [20] applied ZnAl LDH for tartrazine dye with adsorption capacity of 99.5 mg/g at pH 5.8 after 60 mins. Ali et al. [21] used Ni-Al to remove congo red from aqueous solutions with maximum adsorption capacity of 80.94 mg/g. Grover et al. [22] fabricated Zn-Al LDH for brilliant green removal with maximum adsorption capacity of 87.0 mg/g at 15 to 35 °C. Based on these reports, it was observed that the dyes adsorption capacity is still limited, therefore, the modification of LDH need to be conducted widely.
Recently, the LDH modification open a new window research for faster and high rates of removing dyes. In this study, Mg-Al LDH was chosen as the precursor material and was modified with Cu(NO3)2.6H2O, in order to enhance the cationic adsorption capacity. MG, MB, and Rh-B are synthetic cationic dyes which are undegradable by microbial in aquatic system, therefore, their removal in these dye are vital. In this research, Copper (Cu) was used as a supporting materials due to its enormous versatility, allowing the expansion of multifunctional materials with some applications, such as: in adsorption, antimicrobial compounds, catalyst, and energy storage [23]. However, the stability of LDH also increased after modification toward reusability adsorbent. The adsorption was conducted by batch system and investigated through the effect of contact time, isotherm fit parameter, and regeneration studies. The structure of MG, MB and Rh-B were shown in Figure 1.

Chemical and Characterization
( 3 75 . 1 3 g / mol ) , a n d Cu(NO3)2.6H2O (290.79 g/mol) were obtained from Sigma-Aldrich. The samples were tested by using X-ray diffraction performed with a Rigaku Miniflex-6000 diffractometer. The surface area where the materials were obtained was measured with a Quantachrome adsorption-desorption. The FTIR spectrum was determined using a Shimadzu Prestige-21 spectrophotometer and the dyes concentration were recorded by using UV-Visible spectrophotometer Biobase BK 1800 at 554 nm for Rh-B, 617 for MG, and 664 for MB.

Synthesis of Mg-Al-LDH and Mg-Al/Cu
Mg-Al-LDH was produced through coprecipitation method following the procedure in previous research [24]. To obtained Mg/Al-Cu, a mixture of Mg(NO3)2.6H2O with Al(NO3).9H2O in the proportion 3:1 of Mg 2+ and Al 3+ , and 25 mL Cu(NO3)2.6H2O (0.75 M) were added. The preparation was mixed under vigorous stirring at pH 10. The resulting suspension was heated for 24 h at 80° and dried in oven for 6 h at 250 °C.

Selectivity of Dyes Mixture
The selectivity was conducted on the adsorbents. As much as 2 mg with 20 mL of dyes (MG, MB, Rh-B) were stirred for several minutes and the contact times was investigated from 10 to 150 min. The dye concentrations w e r e r e c o rd ed b y u s i n g U V -V i s spectrophotometer.

Adsorption Process
After obtaining the most adsorbed dye in the selectivity, the process continued with adsorption by varying the time, concentration, and temperature. The adsorption was conducted for 10-200 min, and the concentration was tested at range of 20-100 ppm and temperature of 303-333 K. The concentration of dye in the filtrate after adsorption was analyzed by using UV-Vis spectrophotometer.

Regeneration Experiment
As much as 5 g adsorbent were add with 50 mL MG in glass bottle and stirred for 120 min, then dried for 2 h. Then 0.01 g of the residue was added with water and HCl, and stirred for 120 min. This process was repeated in five cycles.

Materials and Characterization
The phase structures of the adsorbents were determined from X-ray diffraction pattern and shown in Figure 2 and 56.59° (116). Cu oxides at Figure 2b appeared at 29.51 Å and 32 Å with high diffraction peaks. The presence of (003), (006), (112), (110) and (116) planes indicated that the materials were successfully fabricated. Figure 3 depicts the nitrogen desorption of Mg-Al-LDH and Mg-Al/Cu. Obviously, the two samples presented type IV isotherms, which was classified as H3 hysteresis loop. The material sizes were indicated in the mesoporous nature. The surface area of Mg-Al-LDH after modification was slightly increased from 21.511 m 2 /g to 23.139 m 2 /g, and was shown in Table 1. Furthermore, diameter pore of Mg-Al/Cu was smaller than the pristine. It was assumed that there were more active sites formed in the modification process.
The spectra obtained in the IR of Mg-Al-LDH in Figure 4 at 3430 cm −1 , indicated intense and broad band, which was attributed to stretching vibrations of OH groups of water molecules and the layer. The stretching vibration was board because it was associated with vibration of hydroxyl group in water molecule at 1635 cm −1 . The peak at 1381 cm −1 was a consequence of the nitrate ion used as a starting material. The vibration of metaloxygen was observed at 883.25, 671.23, 447.49 and 354.9 cm −1 . As shown in FT-IR spectrum of Mg-Al/Cu, the stretching vibration at 447.49 cm −1 was ascribed to the bond of Cu−O, which

Adsorbents
Surface Area (m 2/ g) Volume Pore ( confirmed the presence of Mg-Al/Cu. The increasing of the band intensity at 700-500 cm −1 , was due to the presence of Cu oxides on the surface of LDH. The capability of the obtained material as selective adsorbents in a mixture of cationic dyes (MG, MB, and Rh-B), with varied pH were shown in Figure 5. Comparing with MB and Rh-B, Mg-Al/Cu removed MG at contact time range of 10-30 min, and attained an equilibrium at 60-150 min. This is because the size of MG is smaller than MB and Rh-B. Therefore, Mg-Al/Cu showed good adsorption performance and selectivity for removing MG. The dyes concentrations in adsorption selectivity of MG, MB, and Rh-B using Mg-Al/Cu adsorbent was shown on Table 2. The data showed that Mg-Al/Cu was selective on MG, compared to MB and Rh-B with optimum condition without adjustment of pH. MG concentration decreased from 10 mg/L to 5.633 mg/L in 150 minutes. However, at acidic condition (pH 3) and alkaline (pH 9), the concentration of MG decreased from 10 mg/L to 7.132 and 5.840 mg/L, respectively. In previous research, Xu et al. [25] reported that pH has an important role in adsorption selectivity. In acidic pH, the dye molecule was protonated and at alkaline it was deprotonated. In previous research of using Zn-Al LDH as an adsorbent, methyl orange was deprotonated at high pH values [26]. In acidic condition, H + ion was increased due to decrease in the active adsorption spots on the surface of C-Zn-Al LDH [27].   (PFO) (Equation (1)) and Pseudo Second Order (PSO) (Equation (2)), and the equations were described as follows:

MG Adsorption onto
(1) (2) qe is dye adsorbed capacity (mg/g), and k is rate constant of PFO and PSO. The influence of contact time was shown in Figure 6, and the result were summarized in Table 3. The adsorption process was increasing rapidly from the initial process, and finally the equilibrium was reached after 120 min. The data showed that MG adsorption followed PSO model because the qe values and R2 from PSO were higher than the PFO. As reported by Zhou et al. [28], the adsorption followed PSO than PFO, which indicated that the diffusion of intraparticle was the rate limiting step. Figure 7 showed the effect of concentration and temperature variation on MG adsorption. The temperature of MG adsorption was conducted from 303-333 K, with initial concentration of MG at 50-100 mg/L. The MG adsorption was increased sharply from 60 mg/L to 70 mg/L at all temperature, and tend to be stable from 80 mg/L to 100 mg/L after 120 min adsorption. The equations of Langmuir (Equation (3)) and Freundlich (Equation (4)) were presented as follows: (3) where qmax and KL are Langmuir constant, KF and n are Freundlich constant, with n giving an indication of favorable adsorption process and verifying the type of adsorption process. The MG Adsorption was favorable at high temperature, which was summarized in Table  Figure

Materials
Qeexp (  4 and 5, in order to obtain isotherm and thermodynamic parameters. These data showed that the adsorption of MG on Mg-Al-LDH and Mg-Al/Cu were fitted into Langmuir isotherm model than the Freundlich. The R 2 value for both adsorbents was closer to the Langmuir than Freundlich model. Based on the Langmuir model, the adsorption form a monolayer of dye molecules, because when dyes were adsorbed, the site becomes saturated and unavailable for other molecules [29]. The adsorption capacity of Mg-Al/Cu is higher than the precursor. According to Maziarz et al. [30], the phenomena due to the adsorption of the undissolved metal oxide and the mechanism of MG removal, was effected by anion exchange in the interlayer space. It was also affected by the amount of surface area, where Mg/Al-Cu has bigger surface area than Mg/Al The thermodynamic parameter, such as enthalpy (ΔH), entropy (ΔS), and Gibbs energy (ΔG) were calculated using this equation, where T is temperature and R is the gas constant [31]. (5) The thermodynamic data as shown in Table  5 was also correlated with the adsorption energy, which was dominated by physical adsorption. The adsorption energy for Mg-Al LDH (15.993 kJ/mol) and Mg-Al/Cu (26.715 kJ/mol) were in the range of physical adsorption process and the differences of IR and XRD characterization before and after adsorption process at Figure 8 and Figure 9 indicated of chemisorption processes. The adsorption of MG using Mg/Al-Cu involves both physisorption and chemisorption processes. As the previous research Elkhider et al. [32] reported that MO re-

Adsorbent
Adsorption isotherm moval onto SF-B-CoNiAl composite was physical adsorption and chemisorption processes, the physisorption was observed form the amount of ΔH (2-40 kJ/mol) and chemisorption was indicated from the kinetic data which followed by pseudo-second order (R 2 >0.99). According to Chebli et al. [33] the enthalpy values (14.49 kJ/mol) of Biebrich Scarlet (BS) adsorption ascribed that the interaction type is a physical adsorption and the BS adsorption was also followed by chemical adsorption due to the the dye interaction in the interlayer space of the LDHs which shown from the FTIR characterization.
The positive value ∆H reflected that the MG adsorption was endothermic. Moreover, the negative amount of ΔG denoted that MG adsorption was running spontaneous. The endothermic process with negative amount of ΔG indicated that the adsorption was running spon-taneous at higher temperatures [34]. The same phenomena also occurred by other research, by using Zn/Al LDH removing MG with ∆H= 14.503 kJ/mol indicated endothermic process and ΔG= −13.513 kJ/mol denoted the adsorption was running spontaneous [35]. According to Palapa et al. [36], ΔH and ΔG with the liquid phase were affected by various conditions such as concentration, temperature, and interaction between solid and liquid interfaces. Table 6 showed the comparison of the other adsorbents on MG adsorption.
The IR spectra of Mg-Al/Cu adsorbent after the adsorbed MG was showed on Figure 8. The strong bond of hydroxyl group of water was detected at 3441 cm −1 before and 3444 cm −1 after MG adsorption. The peak at 1381 cm −1 before adsorption process was due to the presence of nitrate ion and was detected at 1361 cm −1 after adsorption. The vibration of metal-oxygen was    , 16 (4), 2021, 703 Copyright © 2021, ISSN 1978-2993 observed at 883.25, 671.23, 447.49 and 354.9 cm −1 before adsorption. And also at 748, 647, 551 and 590 cm −1 after adsorption. The XRD of Mg-Al/Cu before and after adsorption was showed in Figure 9. There was visible differences in the diffraction pattern of Mg-Al/Cu before and after MG adsorption. The XRD pattern of Mg-Al/Cu before adsorption showed peak at 10.79° (003), 22.94° (006), 35.53° (112), 55.78° (110), and 56.59° (116). After adsorption, the presence of the diffraction peaks appeared at 12.78°, 24.52°, 38.21°, 66.36°, and 68.30°, which corresponded to the (003), (006), (112), (110), and (116) planes. Mg-Al/Cu characterization for IR and XRD after adsorption showed differences, compared to before the process, which indicated the reaction between adsorbate and adsorbent. As reported in previous research, Mg-Al LDH successfully removed methyl orange, because of the hydrogen bonding that formed and the electrostatic interaction between the reacting entities [47] The regeneration process was conducted to determine the stability structure of the adsorbents. The regeneration of MG on Mg-Al-LDH and Mg-Al/Cu were shown in Figure 10. At the 1 st , 2 nd , 3 rd , 4 th , and 5 th cycle, the MG adsorption of Mg-Al LDH was observed at 66.07%, 63.02%, 37.66%, 21.47%, and 7.46%, respectively. Meanwhile, for Mg-Al/Cu, the adsorption of MG reached 82.55%, 76.26%, 64.92%, 54.51%, and 47.28%, at 1 st , 2 nd , 3 rd , 4 th , and 5 th cycle, respectively. The data showed that MG adsorption process of Mg-Al/Cu was more stable than the pristine compound, after the five cycles. This was because the Mg-Al/Cu surface area and interlayer space was higher than that of Mg-Al LDH. Therefore, the structure of Mg-Al/Cu was stable and the adsorption capacity did not change significantly over the five adsorption cycles.

Conclusion
In conclusion, Mg-Al/Cu was successfully synthesized with well-known layer structure as represented in the XRD, IR and BET characterization. The adsorption of MG on both adsorbents followed PSO kinetic model. The energy of 15.993 kJ/mol for Mg-Al LDH and 26.715 kJ/mol for Mg-Al/Cu was indicated in the physical adsorption process. After five cycles adsorption process, Mg-Al/Cu was found to be more stable than Mg-Al/LDH.