Polyoxometalate Intercalated MgAl-Layered Double Hydroxide for Degradation of Malachite Green

Improving the selectivity of photocatalysis of LDH pristine (MgAl-LDH) and LDH composite (MgAl-SiW 12 O 40 ] and MgAl-[PW 12 O 40 ]) was synthesized and used for degraded malachite green (MG). The effects of the amount of catalyst, pH value, and reaction times on degradation performance were discussed. MG degraded better composites than LDH pristine. The results indicated that MgAl-LDH was successfully synthesized by showing the peak dif-fractions at angles 10.39°(003), 20.17°(006), and 34.8°(009). Both kinds of attained MgAl-[SiW 12 O 40 ] and MgAl-[PW 12 O 40 ] had the typical structure of LDH that proved by appeared diffraction at 2θ angles 7.73°, 28.6°, 35.6° for MgAl-[PW 12 O 40 ] and at 2θ angles 8.61°, 24.27°, 34.96° and 66.34° for MgAl-[SiW 12 O 40 ]. The FTIR result indicates materials used for fifth regeneration, which confirmed the LDH composite structure. The photodegradation activity of MG for pristine MgAl LDH (56.1%), composites MgAl-Pw (84.6%) and MgAl-Si (87.8%), respectively. The successful ability of photodegradation process by the percentage of degradation on material LDH-polyoxometalate composite showed the increasing of photodegradation catalytic and the regeneration ability of LDH pristine.


INTRODUCTION
Malachite green (MG) is the primary contaminant in water which are included organic pollutants and an organic dye used in industrial applications, especially in the aquaculture industry, due to as an antibacterial agent.Also used as dye material for wool, paper, leather, etc. on the other side, it has excellent outcomes with adverse effects such as mutagenesis, carcinogenicity also malformation especially for mammals (Salamat et al., 2019).Some contaminants of malachite green with high motilities and toxicities, mainly appear contaminant in aquatic organisms and accumulate in plants, which poses a potential risk to the ecosystem and human health (Patel et al., 2019) as a basic dye, malachite green is a toxic cationic dye, which is cannot efficiently biodegradable, thus BOD 5 in MG is almost to zero (Yong et al., 2014).Thus, its unbiodegradable removal from wastewater should be considered on the environment.Recently, various treatment practices to the removal of MG are adsorption (Burakov et al., 2018), extraction, biological degradation (Nasir et al., 2020) and application on photodegradation (Shahzad et al., 2016) have been used to reduce contaminants from wastewater (Gupta et al., 2018).Photocatalysis, a green treatment technology, is considered one of the most promising technologies because of its low cost, satisfactory performance, and high efficiency (Shi et al., 2019 andLu et al., 2020).The photodegradation process is still considered as most favorable to remove dyes from water, thus of its high efficient

ECOLOGICAL ENGINEERING & ENVIRONMENTAL TECHNOLOGY
and simply eliminate (Nazir et al., 2020).The photocatalyst have been applicated to eliminate dyes from aqueous such as: industrial, activated carbon, biological waste materials, and porous carbon (Zhang et al., 2018).
Layered double hydroxide (LDH) consisting of brucite-like layers with positively charged multimetal cations and interlayer compensating anion, have been applied to eliminate pollutants.LDH has general formula M 1-x 2+ M x 3+ (OH) 2 ] x+ (A n ) x/n •mH 2 O where M 2+ and M 3+ are divalent metal cations (Ca, Mg, Co, Fe, Cu, Ni, Mn) and trivalent metals (Al, Fe, Cr).The value of x is equal to the molar ratio of M 3+ /(M 2+ +M 3+ ), where the value is between 0.20 and 0.33 ; and A is the anion that is between the layers of valence n (Cl -, NO 3-, ClO 4-, CO 3-, SO 4-and so on) (Malherbe & Besse, 2000).Recently, LDH research has been related to adsorbents to remove pollutants from wastewater (organic and inorganic), energy storage and heavy metal ion (Yoshida et al., 2015;Behbahani et al., 2020).Single LDH is not considered an efficiently catalyst due to its morphology (Kaul et al., 2016).The structural characteristics of LDHs direct to the manipulation of morphology and active centers (such as electronic states, crystal faces, and defects) via various control synthesis techniques such as intercalated, immobilized etc, thereby significantly gaining performance of catalytic (Tonda et al In the present study, MgAl pristine and MgAl composite which was composited with two different polyoxometalate (POM) type Keggin K 3 [α-PW 12 O 40 ] and K 4 [α-SiW 12 O 40 ] were synthesized using co-precipitation method and their photocatalytic performances were investigated such as the effect of pH, catalyst loading, contact time and regeneration of material.We select POM as intercalate to composite with MgAl-LDH due to POM anions have high negative charge for increasing the performance capability for cationic dye, that can endow the function of photodegradation malachite green (MG) dye.The material is applied as a photocatalyst to degrade MG dye.The prepared material were characterized using FTIR, XRD, SEM and UV-DRS.Furthermore, application as a photocatalyst in this study carried out the photodegradation process such as the effect of pH, catalyst loading, contact time and regeneration of material.

EXPERIMENTAL Chemical and instrumentation
Magnesium nitrate, zinc nitrate, aluminium nitrate, sodium phosphate, sodium tungstate, sodium carbonate, sodium hydroxide and hydrogen chloride were used in this research.Malachite green (MG) is one of the synthetic dyes which has formula C 23 H 25 CIN 2 (4-[(4-dimethylaminophenyl)-phenyl-methyl]-N) and maximum absorbance at λmax 616 nm.Characterization of materials was carried using Shimadzu FTIR Prestige-21.Analysis of XRD was conducted out by Rigaku XRD Miniflex-6000 diffractometer.The degradation of MG was analyzed by UV-Vis Biobase BK-UV 1800 PC spectrophotometer at 617 nm.UV-Vis Diffuse Reflectance Spectrometer for band gap analysis was carried out using JASCO V-760 and for SEM analysis was conducted by using SEM FEI Quanta 650.Polyoxometalate K

Synthesis of MgAl/LDH
An amount of 18.75 g Zinc Nitrate 0.75 M mixed in water (100 mL) and 9.3 g aluminum nitrate 0.25 M dissolved in water (100 mL) in a stirrer for 2 hours.MgAl-LDH was synthesized by a modified co-precipitation method by Lesbani et al. (2021).The pH of the mixture was adjusted to pH 10 by adding sodium hydroxide (2M).This mixture was stirred for 6 hours at 85 o C. The resulting product was washed and dried using an oven at a temperature of 110 o C then the MgAl LDH was weighted.

Preparation of composite
The composite were prepared by adding a solution A (1 g of polyoxometalate compound on 50 mL of distilled water) and solution B (2 g of LDH with 25 mL of 1 M NaOH).The suspension is mixed rapidly under conditions of N 2 gas for 24 hours and then a product washed and dried.Characterization of the results of the intercalation material will be carried out using XRD analysis, FT-IR spectrophotometer, SEM analysis and UV-DRS.

Photocatalytic study
The composite was applied to degrade dye malachite green solutions.Firstly, using optimum catalyst weight with 20 mg/L malachite green was placed in the dark condition and magnetically stirred for 30 achieve a desorption equilibrium.For the photocatalytic test applied at a certain time interval, 20 mL of the suspension with 20 mg/L malachite green.Optimization degradation includes variations in pH at range (1, 3, 5, 7 and 9), for catalysts loading at 0.075, 0.1, 0.25, 0.5, and 0.75 g respectively and degradation contact time at 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, and 120 minutes.This degradation process is implemented using UV light.The percentage of degradation is defined based on the following equation formula: Percentage of Degradation (%) = (Co -Ct)/Together x 100, where Co is the initial dye concentration and Ct is the dye concentration after degradation (Hadnadjev-Kostic et al., 2017).

Regeneration experiment
In order to eximine the reproducibility of the photocatalyst, the suspension after first reaction (2.4) was centrifuged to collect a solid powder.The suspension was precipitate and separate from the solution.Then, the precipitate powder was dried at 70 o C for 24 h.The solid powder was subjected to the next photocatalytic reaction.The above procedure was repeated fifth time to confirm the repeatability of both LDH pristine and LDH composite photoacatalyst.

LDH-based materials
Figure 2 shows the stepwise preparation synthesized catalyst.FT-IR analysis was utilized to determine the surficial groups of LDH.The composites were successfully synthesized.The characterization by FT-IR was also carried out on LDH pristine and LDH composite, it is all described on Figure 2. The FTIR spectrum showed a band between 3420-3500 cm -1 , which was indicated from brucite-like layers and interlayer water molecules.Interlayer water also gives medium-intensity (H 2 O) absorption close to 1630 cm -1 .The 1381 cm -1 band represents CO 3 vibrations (Zhao et al., 2018).The W-O and W-O-W bands are in the band 983-870 cm -1 and 804.84 cm -1 respectively (Wang et al., 2008).Figure 2 shows that these bands are still present in their typical LDH on catalyst LDH-polyoxometalate.
The MgAl-LDH and LDH composites were estimated by XRD on Figure 3.The result shows that the main components of LDH structure is that indicated angles 2θ at 10.39°(003), 20.17°(006), and 34.8°(009).The morphology of as-prepared MgAl-LDH was investigated by SEM analysis, as shown in Figure 4. Morphology can be observed for all

Effect of optimization variables on malacite green degradation
The presence of polyoxometalate on Mg degradation on pH in both composite MgAl-[SiW 12 O 40 ] and MgAl-[PW 12 O 40 ] can affect the stability of LDH structure.Enhancing the abundant of anionic can give the ability to form positively charged electron holes especially media on pH less than 7 that are effective in creating OH• radicals which has the capability to degrade dye stuff but also cause photodecomposition of polyoxometalate into ions.There is photocatalytic reaction UV irradiated catalyst MgAl-POM will produces a hole (h vb + ) and electrons (e cb ) H 2 O will form a hysroxyl radical (•OH) which able to degrade malachite green became simply intermediate.Process mechanism photocatalysis using MgAl-POM is shown at the following stage: This causes in this study MgAl-LDH was obtained at pH 7 while pH 9 for K 3   6 shows the effect pH on degrading malachite green by catalysts both LDH pristine and LDH composite.Catalysts loading on the degradation process also were examined in this study by implementing at the optimum pH in 120 minutes.Figure 7 show the results of various catalysts loading on degrading malachite green

Regeneration of catalyst
The use of the catalyst is expected to be efficient so that it can be used repeatedly to degrade organic pollutants.This ability is known from the decrease in degradation percentage after several cycles of repeated use. Figure 8 shows a slight decrease in the degradation ability after the reuse of composite as a photocatalyst.After 5 cycles of  Figure 10 shows that these bands are still present for the catalyst that has degraded MG.

Figure 1 .
Figure 1.The structure of the malachite green (MG) Figure2shows the stepwise preparation synthesized catalyst.FT-IR analysis was utilized to determine the surficial groups of LDH.The composites were successfully synthesized.The characterization by FT-IR was also carried out on LDH pristine and LDH composite, it is all described on Figure2.The FTIR spectrum showed a band between 3420-3500 cm -1 , which was indicated from brucite-like layers and interlayer water molecules.Interlayer water also gives medium-intensity (H 2 O) absorption close to 1630 cm -1 .The 1381 cm -1 band represents CO 3 vibrations (Zhao et al., 2018).The W-O and W-O-W bands are in the band 983-870 cm -1 and 804.84 cm -1 respectively(Wang et al., 2008).Figure2shows that these bands are still present in their typical LDH on catalyst LDH-polyoxometalate.The MgAl-LDH and LDH composites were estimated by XRD on Figure3.The result shows that the main components of LDH structure is that indicated angles 2θ at 10.39°(003), 20.17°(006), and 34.8°(009).Liu et al. (2022) when XRD patterns show the reflection of (006), (009), (012), (110) and (113), which can be indexed typically

Figure 5 .Figure 6 .
Figure 5. Process mechanism photocatalysis using MgAl-POM using the catalyst there was a decrease %degradation of MgAl-LDH 46.1%.The decrease in %degradation of LDH composite after 5 cycles of use by MgAl-[PW 12 O 40 ] and MgAl-[SiW 12 O 40 ] were 78.4% and 80.4%, respectively.The reusability of LDH pristine and LDH composite corresponded to the %degradation and apparent rate value.From the results of reusability, it can be seen that the MgAl-[SiW 12 O 40 ] was better than the MgAl-[PW 12 O 40 ].

Figure 7 .
Figure 7. Effect of a) times and b) catalysts weight on MG degradation by catalysts

Figure 8 .Figure 9 .
Figure 8. Regeneration of catalyst in the fifth cycle ., 2018 and Wang et al., 2019).LDHs have diverse morphological advantages and adjustable compositions, such as a interlayered arrangement of mortal ions, and broad chemical compositions and ability to exchange anions on these layers (Hu et al., 2019) and LDH materials have high tunable band gap and adsorption capacity which are capable for cation-anionexchange and provide spatially separated redoks reaction sites (Mohapatra et al., 2016).

Table 1 .
EDX of catalysts

Table 2 .
The value of %degradation malachite green