Adsorption Study of Malachite Green Removal from Aqueous Solution Using Cu/M (M=Al, Cr) Layered Double Hydroxide

Layered double hydroxide (LDH) Cu/Al and Cu/Cr had been used as adsorbent of malachite green (MG) in aqueous solution. The properties of Cu/Al and Cu/Cr LDHs were analyzed by X-ray diffraction, surface area analysis (BET) and FTIR spectroscopy. Adsorption study of MG was achieved at pH 9. Adsorption of MG follows the pseudo-second-order kinetic model. Langmuir isotherm was suitable for adsorption of MG on both LDH with a maximum adsorption capacity of 59.52 mg/g. The thermodynamic study indicated that the adsorption process is physisorption, spontaneous, and endothermic process. Adsorption of MG onto LDHs involve the acid-base interaction between adsorbent and adsorbate.


Introduction
The use of reactive dye in the textile industry for ethnic fabric and clothes increases sharply in this decade. The textile industry contributes a significant number of liquid effluent pollutants 1 , due to the high quantities of dyes and water used in the coloring processes 2 . Textile wastewater is a complex mixture and contains a highly polluting substance, i.e., heavy metal cations, and dye 3 . Most of dyes substance contained aromatic structure, which makes them toxic 4 , non-biodegradable, and also carcinogenic for humans and the environment 5,6 . In the environment, these colored effluent's was covering the surface of waters and reduced the oxygen 7 . In order to remove pollutants in wastewater, several techniques have been developed, such as adsorption 8 , filtration 9 , coagulation 10 and photocatalysis 11 . Among these methods, adsorption is a suitable method to remove dye from the aqueous solution because it is an easy technique, fast, cheap, and has no pollutants 12,13 . The adsorption of several dyes has been reported by using various sorbents, i.e. activated carbon 14 , algae 15 , alumina-MOF 16 , sawdust modification 17,18 , kaolin 5 , bentonite 19,20 and layered double hydroxide 21,22 .
Layered double hydroxide was studied as sorbent because of their adsorption ability due to exchangeable anion in the interlayer, high surface area and accessible to synthesized [23][24][25] . Recently, layered double hydroxide was used as a sorbent of dyes such as Ca/Al after being modified through the solvothermal method. The modified LDH applied for adsorption congo red and showed adsorption capacity 228 mg/g 26 . Mg/Al-NO3 LDH was reported in adsorption of methyl orange with adsorption capacity 144 mg/g 27 . Mg/Al LDH also reported in adsorption of methylene blue with adsorption capacity 80 mg/g 28 whereas Ni/Fe LDH was reported in adsorption of methyl orange having adsorption capacity 205 mg/g 29 . Layered double hydroxide, also known as hydrotalcite, has cations substituted by other divalent and a trivalent metal cation. The general formula of layered double hydroxide is [M1−x 2+ Mx 3+ (OH)2] x+ Ax/n n− ·mH2O, with A n as exchangeable anion interlayer [30][31][32][33] . Various methods of synthesis of layered double hydroxide by using a different combination of cations were reported such as coprecipitation 34 , sol-gel 35 , hydrothermal 36 and hydrolysis 37 . The mixed metal salts solutions dissolved followed by aging with hydrothermal treatment 33 or without high temperature for more extended times 23 .
In present work, Cu/Al and Cu/Cr layered double hydroxide was synthesis by co-precipitation method. The physicochemical of sorbent were characterized by X-ray diffraction, surface area analysis carried out by adsorption-desorption N2 BET method and the functional groups evaluated using FTIR. The malachite green dye is selected as a model pollutant in this study. Adsorption study also was aimed to determine kinetic and isotherm adsorption parameters models by varying adsorption times, dyes initial concentration, pH, and temperature of adsorption. The desorption study was examined in several solvents to determine the optimum conditions for the desorption of MG.

Methods
Synthesis of Cu/Al and Cu/Cr LDHs 38 The synthesis of Cu/Al-LDH carried out by adding 0.75 M copper nitrate solution into 0.25 M aluminum nitrate followed by vigorous stirring for an hour. In a separate glass, 2 M sodium hydroxide added simultaneously into 1 M sodium carbonate, and then after an hour, added drop wisely at room temperature with vigorous stirring for six hours. The pH was adjusted at 10 by adding aqueous NaOH. After six hours, the mixing solution aged at 80 o C overnight. The Cu/Al-LDH was washed and dried in room temperature to obtained Cu/Al LDH. The synthesis of Cu/Cr-LDH was carried out in a similar way to Cu/Al-LDH. The divalent and trivalent metal cation ratio was unchanged. 0.75 M of copper nitrate and 0.25 M of chromium nitrate.

Adsorbent Characterization
Cu/Al and Cu/Cr-LDHs were successfully synthesized by co-precipitation method using the mixture of a metal cation and base solution (Na2CO3 and NaOH). The molar ratio adjusted to 3:1 and the pH mixture solution were setup around 9-10 at 80 o C. After precipitation solidly aged for overnight, the mixture gel was dried at room temperature. According to Bukhtiyarova (2019), the pure LDH was obtained by kept pH 10-11 as long as the synthesis process 23 . The Powder X-ray diffraction results are shown in  The d-spacing value (003) by Bragg's equation was calculated 0.754 nm from 2 value at 11.7 o for Cu/Al-LDH and 0.758 nm Å from 2 value at 11.6 o for Cu/Cr-LDH. Powder XRD pattern of Cu/Cr sample shows the noise peaks at 30 o , which indicate the crystal impurities (the presences of Na[Al(OH)4]). This impurity usually formed because of high concentration of NaOH, which reacted with aluminum at high pH. Another impurity shows by 2 value at 34 o indicates the presence of copper oxide.
The diffraction pattern of Cu/Al and Cu/Cr-LDH diffractions represent a rhombohedral structure and a 3R polytypism of LDH. according to Zhitova the unitcell parameters were determined as follows: 'a' = 2d(110) and 'c' = 3d(003) 39 . The calculated lattice parameters are shown in Table 1. The values of 'c' parameter of both LDH are similar because of the presence of nitrate anion in the interlayer. The difference in the 'a' parameter caused by the difference in ionic radii of lower Al 3+ (0.50 Å) compared to Cr 3+ (0.61 Å). The nitrogen adsorption-desorption isotherms of Cu/Al and Cu/Cr-LDHs showed in Fig.2. The curve of adsorption-desorption indicates type IV, which belongs to mesoporous materials. The hysteresis loop of these materials is H3-type, which consistent with cavitation-induced evaporation, which is the cavitation threshold, is exceeded hysteresis loop (P/Po) > 0.42. The H3 hysteresis explained that the binding strength of anion located in the interlayer. Table 1 shows the BET surface areas, pore volume and pore size of Cu/Al and Cu/Cr-LDH. According to IUPAC classification, the pore size of both LDH is a mesopore type. The calculated BET surface areas for Cu/Al-LDH (46.2 m 2 /g) shows that it is tenfold bigger compared to Cu/Cr-LDH (4.67 m 2 /g).   The morphology of Cu/Al and Cu/Cr LDH was investigated by SEM Quanta-650 OXFORD Instrument and the result is shown on Fig.4. The morphology of Cu/Al and Cu/Cr LDH present the heterogeneity with round shape and aggregated particles on the surface. The similar result reported by Mao et al. 2018, the aggregated morphologies caused by high temperature during synthesis 40 . According to Bukhtiyarova 2019; Parida and Mohapatra 2012, LDHs has morphology "sand Rose" which is irregular layer shape with agglomerated particles in the surface of materials 23,41 .

Effect of pH on Adsorption Malachite Green by Cu/Al and Cu/Cr LDH
The adsorption process was conducted by mixing LDHs with MG solution in various pH condition to obtain maximum removal of MG. The pH condition of adsorption was adjusted within range 4-11 (Fig.5). The optimum pH obtained at pH 9 and then decreases at 10-11. The MG removal achieves at 21.471 % and 20.473 % from 50 mg/L initial solution, respectively. The result is by what had been reported by Das et al. (2018). This author was using Mg/(Al+Fe) Ternary LDH to remove MG from its aqueous solution 42 . The increasing amount of adsorption occurs due to the change of active sites of the LDH, which is rich in OHand accumulated on the LDH surface to form negatively charged. At a higher pH, the surface site was deprotonated, and through the electrostatic bond, interaction occurs between the surface site and cationic dye molecules. The maximum wavelength (max) of selected dyes measured in two different pH condition (4 and 9) by UV-Vis spectrophotometer at 400-800 nm is displayed on Fig.6. The adsorption selectivity of LDHs against selected dyes can be traced from changes of max measurement during adsorption time. The dyes mixture comprised of malachite green (MG), methylene blue (MB) and rhodamine-B (RhB). The UV-Visible spectra shown in Fig.6. In both conditions of acid and base, max remains unchanged for all dyes. The adsorption selectivity of LDH for cationic dyes removal best condition is at pH 9 as indicated by well-separated peaks. Fig.6a and 6d show that for Cu/Al and Cu/Cr LDH has efficiency remove MG in base condition at 10 ppm but not significant at 25 ppm ( Fig.6b and 6e). Among the cationic dyes, MG was being adsorbed more then methylene blue (MB) and rhodamine-B (RhB) at the base condition. The decreases in absorbance line with a contact time of adsorption confirm that sorbentadsorbate interactions will continue to occur at pH 9 until the condition of equilibrium reached. In acid conditions (pH 4), the cationic dyes mixture does not interact in both LDHs. Low pH affected the LDH surface and turned it into positively charged and repels cationic dye molecules.

Effect of contact time
For several initial concentrations, the contact time was adjusting for determining the relationship between MG removal to contact time. The adsorption condition was adjusted at pH 9 (50 mg and 50 mL MG solution in several concentrations). Fig.7 shows that the dye uptake increased rapidly with time and after that reach equilibrium. For Cu/Al-LDH (Fig.7a), the equilibrium was reached at 50 min for 10-50 mg/L, but for MG solution with a higher concentration (70-90 mg/L), the equilibrium was reached at 70 min. Furthermore, the increase of adsorption capacity was caused by the driving force of the concentration gradient from increases initial concentration. Fig.7b shows the effect of time from MG removal by Cu/Cr-LDH in several initial concentrations. The equilibrium was reached at 50 min for lower concentration and higher concentration at 120 min. Fig.7b was shown too that an increasing initial concentration of MG causes the increase the MG removal uptake.
Adsorption kinetics was studied by pseudo-first-order and pseudo-second-order to predicted the mechanism process of Cu/Al and Cu/Cr-LDH sorption. The pseudo-first-order (PFO) equation was followed: Moreover, pseudo-second-order (PSO) can be expressed to: With qe denotes adsorption capacity at equilibrium, k1 is the rate constant of PFO, k2 is the rate constant for PSO. The rate constant was obtained from plotting the equation to get correlation coefficients and listed in Table 2.
The parameters and correlation coefficients (R 2 ) obtained from linear plotting log(qe-qt) vs t (PFO) and t/qt vs t (PSO) was listed in Table 2. The result showed based on R 2 , Cu/Al and Cu/Cr-LDH were >0.977 for PSO then PFO. This also supported by the qe(calc) PSO was close to the qe(experiment). This phenom indicated that the adsorption was following PSO kinetic model, which represents that electrostatic attraction from sorbent-sorbate. Additionally, the decreasing k2 values over the increasing MG initial concentrations reflected in the shorter time needed by the respective systems to achieve equilibrium at lower initial concentrations 43 . In other words, the higher the k2 value indicates, the adsorbate molecule is reactive condition.

Effect of temperature and isotherm modeling
The effect of increasing temperature in several concentrations has affected increasing dye uptake for Cu/Al and Cu/Cr-LDH. The effect of temperature was investigated over the temperature range 303-323 K. Fig.8 represents the MG uptake in several concentrations for Cu/Al-LDH. The higher MG removal was shown at 323 K amount of 27.80 mg/g in higher concentration (100 mg/L). Fig.8b shows the higher MG uptake at 323 K amount 25.217 mg/g. According to Das (2018), the increasing dye uptake while increases temperature caused the decreases solution's viscosity, increase porosity or interlayer, resulting in the enhancement of active sites form adsorbent 40 . Adsorption isotherm was studied using Freundlich and Langmuir model to analyzed the adsorption process. The Freundlich isotherm adsorption model has usually described multilayer process in heterogeneous surface, and the Langmuir isotherm is also popular to describe interact of adsorbentadsorbate which extent in one layer or one site occupied in the homogeneous surface. The linear form of Langmuir isotherm follows: Freundlich isotherm model is given as: Ln qe = ln kF + 1 ln Ce (4) Where, qmax and kL are Langmuir constants, kF and n are Freundlich constants with n giving an indication of favorable the adsorption process and verify the type of adsorption process. The isotherm adsorption parameters were shown in  Table 4.

Thermodynamic study
The thermodynamic parameters, including enthalpy (ΔH), entropy (ΔS) and Gibbs energy (ΔG) was calculated in this research. The result of thermodynamic was calculated by The Van't Hoff equation, which written as followed: with R is constant gases, and Kd is adsorption distribution coefficient. The parameters of thermodynamic determined by calculation of Van't Hoff linear regression as 1/T vs ln (Kd). Figure 9 shows the linearity of interaction between adsorbate and adsorbent on the effect of temperature. Gibbs energy determines the spontaneity and feasibility of the adsorption process. If the value of Gibbs energy is negative, the adsorption process occurs spontaneously. The decreases in Gibbs energy suggests the adsorption process is more spontaneous at high temperature. The positive value of entropy indicates the high affinity of the adsorbent for dye molecules. It also identified the randomness at the solid-solution interface during the adsorption process.
Enthalpy value resulted from calculation confirmed that the interaction between adsorbent-adsorbate is physisorption having range 2-40 kJ/mol. The adsorption enthalpy calculated at initial concentrations of 50 mg/L indicates the adsorption process leads to chemisorption. The chemisorption process is likely to occur in low concentration with highly selective adsorbate adsorption site 51 . The adsorption process tested on both adsorbents proof that it effectively removes malachite green. Figure 10 presented the XRD pattern of samples after MG adsorption. The diffraction at 11 o and 23 o ° occurred in the adsorbents adsorbing MG, suggests that adsorption process did not alter the LDH structure. The interlayer space of Cu/Al-LDH after adsorption process increases to 7.582 Å from 7.540 Å whereas, for CuCr, it decreases to 7.584 Å from initial value 7.664 Å, respectively. However, these small discrepancies indicated that MG adsorbed in interlayer does not direct perpendicularly to host layer 52 .

Desorption Study
The desorption study was evaluated by contacting the adsorbed dye on the solid surface with water assuming that the interaction between adsorbent-adsorbate is weak. In other cases of adsorbent-adsorbate interaction, the adsorbed dye can be desorbed by using acid or alkaline water. The desorption, in this case, is driven by ion-exchange 53 . In this work, various commonly used desorption solutions were tested within the batch system in order to find the most suitable desorbing solution. The percentage of the dye desorption from the anion exchanger was calculated by: The various solutions used in dye desorption were HCl, NaOH, NaCl, Na-EDTA, diethyl ether and HONH3Cl in the same concentration (0.01 M) (see Fig.11). The higher percent of desorption in acid solution (HCl and HONH3Cl) possibly due to surface site of LDH becomes positively charged so that the electrostatic attraction of adsorbent-adsorbate and H-bonding are weakened. Furthermore, in the base condition, the desorption is also higher because of hydrophobic interaction and OHions are found to have a higher affinity for the anion exchange. However, the desorption efficiency using diethyl ether, NaCl and also water shows low effectiveness on dye desorption because it desorbed less than 25% of dye adsorbed.

Recycled adsorbent effectivity
The maximum desorption of MG from Cu/Al and Cu/Cr LDH achieved by using HCl (Fig.11), the desorption effectiveness of both LDHs are 95% and 90%. These LDHs furthermore used for regeneration purpose. The materials were washed by water for 30 minutes. The reusing cycle was conducted using ratio 0.05 g material against MG 0.05 L for two hours. The similar procedure is repeated 3 times. Figure 12 presents the three-cycle for Cu/Al and Cu/Cr LDHs. The first cycle shows that the effectivity of both LDHs are 96% and 93%, respectively. However, the effectiveness in the adsorption of MG in the second and third cycles markedly decreased. The occurrence of structural damage after the desorption process might be responsible for this lower capacity. A similar finding was reported by Leon et al. (2018). The author concluded that when the desorption process in an acidic medium 54 ; the hydrogen bonding at the interlayer was damaged. This causes the effectiveness of Cu/Al and Cu/Cr adsorption to decrease.

Conclusion
In the present research, Cu/Al and Cu/Cr LDHs have been used as an efficient adsorbent to remove MG from aqueous solution. The physicochemical properties of adsorbent showed that the interlayer space of Cu/Al and Cu/Cr LDHs was equivalent to surface area properties. The optimum values of pH adsorption of MG on both LDHs were achieved at pH 9. Contact time for the adsorption of MG was obtained at 70 min for Cu/Al and 120 min Cu/Cr, respectively. Findings indicate that the pseudosecond-order kinetic model showed the best-fitted with data of MG removal by both adsorbents with coefficients correlation above 0.983 for all concentrations. Isotherm study was followed Langmuir model shows the monolayer adsorption process with maximum capacity up to 59.523 mg/g. Furthermore, adsorption of MG on LDHs involves the acid-base interaction together with physical bonding between adsorbent and adsorbate.

Acknowledgement
The