Unique Adsorption Properties of Malachite Green on Interlayer Space of Cu-Al and Cu-Al-SiW12O40 Layered Double Hydroxides

Cu-Al layered double hydroxide (LDH) was intercalated with Keggin ion of polyoxometalate K4[-SiW12O40] to form Cu-Al-SiW12O40 LDH. The obtained materials were analyzed by X-ray Diffraction (XRD), Fourier Transform Infra Red (FTIR) spectroscopy, and Brunaur-Emmett-Teller (BET) surface area analysis. Furthermore, the materials were used as adsorbents of malachite green from aqueous solution. Some variables for adsorption, such as: effect of adsorption times, malachite green concentration, and also adsorption temperature, were explored. The results showed that diffraction at 11.72° on Cu-Al LDH has interlayer distance of 7.56 Å. The intercalation of that LDH with [-SiW12O40]4− ion resulted increasing interlayer distance to 12.10 Å. The surface area of material was also increased after intercalation from 46.2 m2/g to 89.02 m2/g. The adsorption of malachite green on Cu-Al and Cu-Al-SiW12O40 LDHs followed pseudo second order kinetic and isotherm Langmuir model with adsorption capacity of Cu-Al and Cu-Al-SiW12O40 LDHs was 55.866 mg/g and 149.253 mg/g, respectively. That adsorption capacity is equal with increasing interlayer space and surface area properties of material after intercalation. Thus, the adsorption of malachite green on Cu-Al and Cu-Al-SiW12O40 LDHs is unique and dominantly occurred on interlayer space of LDH as active site adsorption. Copyright © 2020 BCREC Group. All rights reserved


Introduction
Dyes contamination from industrial activities, such as: textile [1], paint, plastic, rubber, and light irradiation [6]. Among these ways, the popular simple method is adsorption due to fast way, easy process, low cost, and also high efficiency to remove dyes from aqueous solution [7]. The main problem for adsorption is to develop adsorbent which has selectivity and high efficiency [8−9]. Certain adsorbents were tested to remove dyes from aqueous phase, such as: zeolite [10], bentonite, montmorillonite, clay [1], chitosan [11], algae [12], cellulose [13], and also layered double hydroxide materials [14].
Layered double hydroxide (LDH) is inorganic layers material with consisted by divalent and trivalent metal cations. The LDH is well-known as hydrotalcite-like materials. The surface layer of materials was balanced with various anions depending on synthetic condition and can be replace and exchanged to obtain unique properties of layer. The general formula of LDH is [M 2+ 1-xM 3+ x(OH)2]+xAx -n mH2O, where M 2+ is divalent metal ion, M 3+ is trivalent metal ion, Ax -n is anion with n valent state, and water of crystallization [15−16]. The common anion on interlayer LDH was found, such as: nitrate, carbonate, hydroxide, and also chloride [17−18]. The existence of these anions results in the small interlayer distance and space on LDH. On the other hand, due to the various applications of LDH as adsorbent of dyes, thus this space is important to create to be larger space to provide active site for adsorbate [19]. The anion should be exchanged with larger anion compound such as polyoxometalate [20]. Polyoxometalate is metal-oxygen inorganic clusters which have various structures, shapes, oxidation state, and also high charges compounds. Polyoxometalate can be used as an intercalant of LDH to increase interlayer distance on LDH [21].
Mg/Al LDH was intercalated with Keggin ion [H2W12O40] 6-to form pillared compound with various ratio of polyoxometalate [22]. Polyoxometalate K3[-PW12O40] has been intercalated on interlayer Zn/Al LDH and the space slightly increase than pure Zn/Al LDH [18]. Keggin type polyoxometalate of K4[-SiW12O40] was also used as intercalant of Ca-Al LDH to form Ca-Al-[-SiW12O40] [23]. According to previous research, the intercalated material can be enhanced its adsorption ability of cationic dye such as malachite green [14]. The malachite green is representative triphenylmethane cationic dye which has seriously bad effect for environment due to covering water surface and high residual [24]. Similar reported by Lesbani et al. [14]. NiAl LDH was intercalated by polyoxometalate has good ability to remove mala-chite green and obtained 80% malachite green removal. In addition, Xu et al. [25] reported that polyoxometalate intercalating ZnAlFe LDH was applied as methylene blue adsorption and obtained adsorption capacity 67.47 mg/g. Polyoxometalate [PW10Mo2O40] 5-with high charges was used to intercalate Zn-Al LDH. The material used as removal agent of methylene blue from aqueous solution with adsorption capacity higher than pure Zn/Al LDH [20].
Herein, Keggin type polyoxometalate K4[-SiW12O40] was used as intercalator of Cu-Al LDH to form Cu-Al-SiW12O40 as adsorbent of malachite green. The materials were characterized by XRD powder analysis, identification of functional group by FTIR spectroscopy, and also measurement of surface area properties by nitrogen adsorption desorption analysis. The malachite green is toxic cationic dye which cannot be degraded by aquatic system [26]. The removal of malachite green from aqueous solution is carried out by batch system adsorption process. The factor that influencing adsorption process was studied such as effect of adsorption time, effect of initial concentration of malachite green, and also effect of adsorption temperature to obtain kinetic and thermodynamic adsorption properties. The unique adsorption properties on Cu-Al and Cu-Al-SiW12O40 LDHs is expected after intercalation process of Cu-Al LDH.

Synthesis of Cu-Al LDH
Synthesis of Cu-Al LDH was conducted using coprecipitation method at pH 10 as previous reported by Palapa et al. [27]. Copper (II) nitrate (0.75 M, 10 mL) was mixed with aluminum (III) nitrate (0.25 M, 10 mL). Reaction was gentle stirred for one hour until dissolution of all starting materials. Sodium hydroxide (4 M) was added to the mixture and pH of solution was adjusted to 10 with this solution. The reaction mixture was kept for 20 hours to form solid material. The solid material was filtered and washed with water several times and dried at 110 °C overnight.

Synthesis of Polyoxometalate K4[-SiW12O40]
The synthesis of polyoxometalate K4[-SiW12O40] was prepared with slightly modification from Lesbani and Co-workers [28]. Solution of hydrochloric acid (4 M, 165 mL) was added to solution of sodium tungstate (182 g in 300 mL water) with gentle stirring to remove solid precipitate of tungstic acid. Solution of sodium meta silicate (11 g in 100 mL water) was added to the reaction mixture following with addition of sodium tungstate (1 M, 80 mL). pH reaction was adjusted to 5 by addition of hydrochloric acid 4 M. The reaction was kept at 80 °C for 1 hour. Reaction was kept at room temperature with constant stirring. Potassium chloride was added to the reaction solution and stirred for 1 hour to form K4[-SiW12O40].

Intercalation of Cu-Al LDH with [-SiW12O40] 4-
Intercalation of Cu-Al LDH with [-SiW12O40] 4-was conducted using ion exchange method. Cu-Al LDH was mixed with solution of 1 M sodium hydroxide (25 mL). Polyoxometalate K4[-SiW12O40] was dissolved with 50 mL water. Solution of polyoxometalate was mixed with solution of Cu-Al LDH with gentle stirring for 5 minutes. Reaction mixture was introduced with nitrogen with slow stirring and kept for 24 hours to form suspension. Suspension was filtered, washed with water several times and dried at 120 °C for 48 hours.

Adsorption Study
Adsorption of malachite green was performed using small batch reactor system equipped with stirring bar and temperature control. Adsorption process was studied by variation of adsorption time, variation of initial concentration of malachite green, and variation of adsorption temperature. Variation of adsorption time was studied in the range of 5-210 minutes. Variation of initial concentration of malachite was studied at 25, 50, 75, and 100 mg/L. Variation of adsorption temperature was studied at 303, 313, 318, and 323 K. Concentration of malachite green after adsorption was analyzed by UV-Visible Spectrophotometer at 619 nm.
Diffraction peak of Cu-Al LDH after intercalation [-SiW12O40] 4-to form Cu-Al-SiW12O40 has diffraction similar with pristine material. The diffraction of 11.72° on Cu-Al LDH was shifted to lower diffraction at 11.34° due to insertion of Keggin ion. These diffractions have interlayer distance 12.10 Å. The increasing interlayer can be calculated as Equation (1) . The intercalation of Cu-Al LDH with [a-SiW12O40] 4-ion will replace the nitrate as anion. Thus our emphasis to identify FTIR spectrum after intercalation is vibration around 1300 cm −1 . The FTIR spectra of Cu-Al-SiW12O40 is shown in Figure 2b. The vibration peak of Cu-Al-SiW12O40 LDH did not found at area 1300 cm -1 thus intercalation of [-SiW12O40] 4ion is conducted. On the other hand, vibration of Keggin ion of [-SiW12O40] 4-was sharply appeared at 1111 cm −1 ( Si−O) and 974 cm −1 ( W=O) [9].
Analysis of adsorption desorption nitrogen on Cu-Al and Cu-Al-SiW12O40 is shown in Figure 3. There is a different adsorption desorption lane for both materials, which was indicated that these materials have hysteresis loop. The profile also showed that type IV isotherm model for Cu-Al LDH. Another hand, the different space between adsorption and desorption of intercalated material is too large, thus this material is not classifying as type I-VI adsorption desorption isotherm model [30]. The BET analysis was obtained from data in Figure  3 as shown in Table 1.
The surface area properties of Cu-Al-SiW12O40 is higher almost two folds than pristine material. On the other hand, opposite results were obtained for dpore and dpore of Cu-Al-SiW12O LDH. The dpore and Vpore of Cu-Al-SiW12O40 LDH was smaller than Cu-Al LDH. Due to opening interlayer distance of intercalated Cu/Al LDH, the dpore and Vpore of Cu-Al-SiW12O40 LDH to be smaller and covered with the large size of anion [-SiW12O40] 4-.
The adsorption of malachite green on Cu-Al and Cu-Al-SiW12O40 was firstly investigated by effect of adsorption time. Figure 4 shows the kinetic adsorption model of malachite green on both LDH adsorbents. The kinetic model was calculated using pseudo-first order (P-FO) kinetic model and pseudo-second order (P-SO) kinetic model by equation below [31].

Materials
Surface area (m 2 /g)  is adsorption time (minute); and k2 is adsorption kinetic rate at pseudo second-order (g.mg -1 .minute -1 ). The kinetic adsorption parameter of PFO and PSO of malachite green on Cu-Al and Cu-Al-SiW12O40 LDH is shown in Table 2.
The kinetic parameter data in Table 2 showed that adsorption of malachite green on Cu-Al and Cu-Al-SiW12O40 LDH was follow PSO kinetic model with R 2 value close to one. The k2 is adsorption rate constant for the adsorption and k2 of Cu-Al LDH is smaller than Cu-Al-SiW12O40 LDH. Thus, of Cu-Al-SiW12O40 LDH is slightly reactive than starting material without intercalation. The second adsorption study is effect of various malachite green concentration and temperature effect as shown in Figures 5a and b.
Malachite green adsorption onto CuAl LDH and CuAl-SiW12O40 was sharply increased by increasing malachite green concentration at 20 mg/L. The optimum adsorption was achieved at malachite green concentration start at 75 mg/L. The adsorption patterns also showed that similar style profile for both Cu-Al LDH and Cu-Al-SiW12O40 LDH. Thus, adsorption type of these adsorbent is similar. The isotherm adsorption of Freundlich and Langmuir was calculated based on data in Figure 5c using equation as below [32]. Langmuir equation: where C is a saturated concentration of adsorbate; m is the amount of adsorbate; b is the PFO kinetic model: (2) where: qe is adsorption capacity at equilibrium (mg.g -1 ); qt is adsorption capacity at t (mg.g -1 ); t is adsorption time (minute); and k1 is adsorption kinetic rate at pseudo first-order (minute -1 ). PSO kinetic model: where qe is adsorption capacity at equilibrium (mg g -1 ); qt is adsorption capacity at t (mg.g -1 ); t Copyright © 2020, BCREC, ISSN 1978-2993 , 15 (3), 2020, 657

Bulletin of Chemical Reaction Engineering & Catalysis
where qe is adsorption capacity at equilibrium (mg.g -1 ); Ce is the concentration of adsorbate at equilibrium (mg.L -1 ), and KF is Freundlich constant. Isotherm Langmuir is appropriate for adsorption of malachite green on both Cu-Al LDH and Cu-Al-SiW12O40 LDH as adsorbents, which was obtained from Table 3. The value of R 2 is closed to one for isotherm Langmuir rather than isotherm Freundlich. The Qmax of Cu-Al-SiW12O40 LDH is higher almost three-fold than Cu-Al LDH. The adsorption capacity of Cu-Al-SiW12O40 is higher than without intercalation can be easily explained due to opening of interlayer distance and also surface area properties of CuAl-SiW12O40. The adsorption capacity is increased by increasing temperature. The adsorption capacity of Cu-Al and CuAl-SiW12O40 LDHs is up to 55.866 mg/g and 149.253 mg/g, respectively. This increasing adsorption capacity is equal with increasing surface area properties. Thus, adsorption of malachite green on Cu-Al and CuAl-SiW12O40 is occurred dominantly on interlayer space of LDH. The several adsorbents for malachite green adsorption were reported by others listed in Table 4. Table  4 showed the adsorption capacity on this work has good adsorbed ability and exhibited good performance to remove malachite green in aqueous solution.
The thermodynamic adsorption parameter was also obtained based on data in Figure 5a and b using equation as follow.
where T is temperature (K); R is the gas constant (8.314 J.mol -1 .K -1 ), and Keq is the reaction on change temperature. Table 5 showed that adsorption of malachite green on all adsorbents were spontaneous occurred in which the negative value of ∆G. The enthalpy is in ten range 7.621-8.916 kJ/mol. These values showed that adsorption of malachite green on both adsorbents is categorized as physical adsorption. The value of ∆S is increased after intercalation means that increasing randomness adsorption process on malachite green on material after intercalation. Thus, opposite in previous results of the isotherm adsorption belong to Langmuir which indicated the adsorption occurs on monolayer surface although the high adsorption capacity might suggest the formation of multiple layers. However, this theory as similar reported by Ribeiro et al. [33] that the Langmuir assumption of dye adsorption is infinite dilution and saturation monolayer occur to gases system, which has a small adsorbed molecule, homogeneous active sites and has very low interaction.

Conclusion
Cu-Al LDH was successfully intercalated with [-SiW12O40] 4-to form Cu-Al-SiW12O40 with increasing interlayer distance from 7.56 Å to 12.10 Å. The surface area properties of material were also increased after intercalation from 46.2 m 2 /g to 89.02 m 2 /g. Adsorption of malachite green on Cu-Al and CuAl-SiW12O40 LDH has adsorption capacity 55.866 mg/g and 149.253 mg/g, respectively. The increasing adsorption capacity is almost three-fold than before intercalation, which was equal with increasing the surface area properties. Thus, adsorption of malachite green in this research was occurred dominantly on interlayer space of LDH and the adsorption process were physisorption due to this adsorption process has low energies from thermodynamics calculated parameters. [3] Palapa, N.R., Taher Table 5. Thermodynamic adsorption parameter.