Competitive Removal of Cationic Dye Using NiAl-LDH Modified with Hydrochar

In this study, NiAl-LDH was modified with hydrochar using the NiAl-Hydrochar composite coprecipitation method. Materials were characterized by XRD and FT-IR analysis. XRD diffractogram and FT-IR spectra show that the NiAl-Hydrochar composite material has the characteristics of the precursors. NiAlHydrochar composite materials have a large adsorption capacity to adsorb cationic dyes. The adsorption follows the Langmuir adsorption isotherm model with the maximum capacity (Qmax) of the NiAl-Hydrochar composite material reaching 256.410 mg/g for malachite green and the adsorption process takes place spontaneously and endothermically. The regeneration process of NiAl-Hydrochar composites was more stable and the decrease was not significant (>70%). The selectivity of the dye mixture showed that the adsorbent was more selective for malachite green dye compared to methylene blue and rhodamine-B. Keyword: layered double hydroxide, composite, hydrochar, cationic dye, stability, selectivity Received: 2021.04.22 Accepted: 2021.05.15 Published: 2021.06.07 Ecological Engineering & Environmental Technology 2021, 22(4), 124–135 https://doi.org/10.12912/27197050/138120 ISSN 2719-7050, License CC-BY 4.0 ECOLOGICAL ENGINEERING & ENVIRONMENTAL TECHNOLOGY


INTRODUCTION
Among the environmental problems that often occur, the handling of dye wastewater is still one of the most discussed and is of concern to the world (  , ion exchange (Mikif et al., 2018), manipulation and photodegradation (Saghir et al., 2020). Technologies for removing pollutants have improved (Stawiński et al., 2018).
Adsorption-based industrial wastewater treatment has received considerable attention during Competitive Removal of Cationic Dye Using NiAl-LDH Modified with Hydrochar Normah 1 , Neza Rahayu Palapa 3 , Tarmizi Taher 2,5 , Risfidian Mohadi 5 , Fitri Suryani Arsyad 4 , Aldi Priambodo 5 , Aldes Lesbani 3,5* the last two decades (Mrózek et al., 2019). Adsorption is considered one of the best industrial wastewater treatment methods because it is more efficient, broad application, ease of operation (Kaykhaii et al., 2018), insensibility to toxicants (Stawiński et al., 2018) and adjustable adsorption selectivity (Mrózek et al., 2019). Thus, finding new adsorbents with sufficient affinity to address dye waste is a scientific and industrial challenge (Mrózek et al., 2019) Layered double hydroxide (LDHs) known as anionic clays, are 2D structured layered materials with controlled structure and composition and have excellent absorbent properties  The application of LDH as an adsorbent has disadvantages such as in the adsorption regeneration process because LDH can peel off the layer during the application process so that it can be used repeatedly . Therefore, it is more effective if it is modified to form a composite with the supporting material to increase the integrity of the coating. Supporting materials that can be used are carbon-based materials such as activated carbon (Hu et al., 2020), graphite (Lv et al., 2019), biochar  and hydrochar (He et al., 2019).
Hydrochar has a porous structure and functional groups which can be ideal support material for LDH. LDH composites with carbonbased support materials can effectively prevent agglomeration, increase the surface area to provide more active sites to improve the adsorption performance (Hu et al., 2020 Alagha et al. (2020) reported that the biochar-Mg/Al composite has the potential to be used repeatedly in the phosphate adsorption process and the regeneration process has decreased which is not too significant after five cycles of 92%. Ahmed et al. (2020) reported that MgFe-LDH/ waste foundry sand composites have work stability with six times recycling with an efficiency exceeding 80% to remove congo red.
In this study, NiAl-LDH was synthesized using the coprecipitation method modified with hydrochar (HR) to produce a NiAl-HR composite. The material is used as an adsorbent to remove contaminants from malachite green (MG), methylene blue (MB) and rhodamine-B (Rh-B). The materials were analyzed using XRD, FT-IR characterization. Application as an adsorbent, this research carries out an adsorption process with isotherm and thermodynamic parameters, studies of regeneration and adsorption selectivity.

EXPERIMENT Chemicals and instrumentations
All reagent and chemicals used were analytical grades supplied by Merck and Sigma-Aldrich including Ni(NO 3 ) 2 ·6H 2 O (99%) and Al(NO 3 ) 3 ·9H 2 O (99%) and used as the precursors for NiAl-LDH, NaOH (99%), and HCl (37%) were used without further purification. Deionized (DI) water was used for all synthesis and treatment processes. In this experiment, the biomass of rambutan fruit peel (Nephelium lappaceum L.) was collected from household waste as a precursor for the hydrochar manufacturing process using the hydrothermal carbonization method. Before being carried out for the hydrothermal carbonization process, the samples were dried at a temperature of 100°C to remove moisture.

Synthesis of NiAl-LDH
NiAl-LDH was prepared by the coprecipitation method . The molar ratio used for the 3:1 LDH synthesis from a mixed solution with Ni(NO 3 ) 2 ·6H 2 O 3 M and Al(NO 3 ) 3 ·9H 2 O 1 M is added to the beaker. The mixture was stirred for 30 minutes and then slowly dripped with 2M NaOH to pH 10. Then, stirred for 12 hours at a temperature of 60°C. After that, the samples were dried at 100°C in an oven for 12 hours.

Preparation hydrochar
Hydrothermal carbonization was performed using a 100 mL hydrothermal apparatus. 2.5 grams of rambutan (Nephelium lappaceum L.) rind powder then put into Tefl on and added with 50 mL of deionized water which will be used for the carbonization process. Then, the Tefl on was put into the autoclave, tightly closed, and heated in an oven with a temperature of 200°C for 10 hours. The solid product is separated from the liquid product by fi ltration and the solid is dried for 12 hours at a temperature of 150°C. black solids as a result of the hydrothermal carbonization process are called hydrocarbons.

Preparation composite NiAl-HR
Synthesis of NiAl-HC composites was carried out using the coprecipitation method. 10 mL of Ni(NO 3 ) 2 ·6H 2 O 3 M was mixed with 10 mL of Al(NO 3 ) 3 ·9H 2 O 0.25 M and stirred. Then, 1 g of solid hydrochar is added (Palapa, Taher, Rahayu, et al., 2020). The mixture is dripped with 2 M NaOH until it reaches pH 10 slowly. The mixture was stirred for 3 days at 80°C. The resulting solid was fi ltered, washed, and dried at 60°C for 1 day.

Characterization methods
X-ray diff raction (XRD) data from NiAl-LDH, HC and Composite NiAl-HC materials between 5 and 80 using XRD Rigaku minifl ex-6000 using Cu K radiation at a scanning rate of 1 s -1 was used to determine the phase structure of LDH samples. Observation of functional groups of the material used analysis with the FT-IR Shimadzu Prestige-2 which was measured with a wavenumber range of 400-4000 cm -1 and using KBr pellets. Morphological observations were carried out with a scanning electron microscope (SEM) Quanta-650 Oxford instrument at an acceleration voltage of 10 kV. Analyzing the concentration of adsorbed dye using the BK-UV Biobase BK-UV 1800 PC spectrophotometer with long wavelengths of rhodamine-B (560 nm), malachite green (617 nm), and methylene blue (664 nm).

Adsorption isotherm
The adsorption equilibrium experiment was carried out in a 100 mL beaker, in which 50 mL of dye solution with diff erent initial concentration variations (20, 40, 60, 80 and 100 mg/L) were added to the beaker. 0.05 g of adsorbent was added to the dye solution and stirred for 2 hours with various adsorption temperatures (303, 313, 323 and 333K) to achieve the adsorption equilibrium. then the fi nal concentration of the dye in the solution was analyzed using a spectrophotometer at each wavelength. The sample should be separated between the fi ltrate and the residue before analysis to minimize the fi ne disturbance of the adsorbent during analysis.
The isotherm parameters are obtained from the Langmuir and Freundlich equations. Langmuir and Freundlich isotherm equations are as follows: Langmuir: Synthesis of NiAl-HC composites was carried out using the coprecipitat Ni(NO3)2.6H2O 3 M was mixed with 10 mL of Al(NO3)3.9H2O 0.25 M and sti hydrochar is added (Palapa, Taher, Rahayu, et al., 2020). The mixture is dripped reaches pH 10 slowly. The mixture was stirred for 3 days at 80°C. The resulting so and dried at 60°C for 1 day.

Characterization methods
X-ray diffraction (XRD) data from NiAl-LDH, HC and Composite NiAl-H and 80 using XRD Rigaku miniflex-6000 using Cu K radiation at a scanning r determine the phase structure of LDH samples. Observation of functional grou analysis with the FT-IR Shimadzu Prestige-2 which was measured with a wavenum cm -1 and using KBr pellets. Morphological observations were carried out wi microscope (SEM) Quanta-650 Oxford instrument at an acceleration voltage of concentration of adsorbed dye using the BK-UV Biobase BK-UV 1800 PC spect wavelengths of rhodamine-B (560 nm), malachite green (617 nm), and methylene

Adsorption Isotherm
The adsorption equilibrium experiment was carried out in a 100 mL be dye solution with different initial concentration variations (20, 40, 60, 80 and 100 m beaker. 0.05 g of adsorbent was added to the dye solution and stirred for 2 hours temperatures (303, 313, 323 and 333K) to achieve the adsorption equilibrium. then of the dye in the solution was analyzed using a spectrophotometer at each wavelen be separated between the filtrate and the residue before analysis to minimize the adsorbent during analysis.
The isotherm parameters are obtained from the Langmuir and Freundl and Freundlich isotherm equations are as follows: Langmuir : Freundlich: Log q e = log K F + 1/n log C e (2) where: C is a saturated concentration of adsorbate; m is the amount of adsorbate; b is the maximum adsorption capacity (mg.g -1 ); K ML is the Langmuir constant (L.mg -1 ). q e is adsorption capacity at equilibrium (mg.g -1 ); C e is a concentration of adsorbate at equilibrium (mg.L -1 ); K F is Freundlich constant.
Thermodynamic parameters are calculated by equations 3 and 4: (4) where: T is temperature (K), R is the gas constant (8.314 J.mol -1 .K -1 ), K eq is the reaction on charge temperature, ∆G is the change in Gibbs free energy (kJ/mol), ∆S is the change in entropy (kJ/mol), and ∆H is the change in entropy (kJ/mol).

Regeneration experiment
Regeneration was carried out using an ultrasonic instrument which was carried out by adsorbing 50 mL of the dye solution with a concentration of 100 mg/L, plus 0.5 g of adsorbent and stirring for 4 hours. After the adsorption process is carried out, the desorption process is carried out using water. The desorption process is carried out with the adsorbent that has been used for adsorption and taken as much as 0.01 g, each adding 10 mL of water and stirring for 2 hours. The regeneration process uses used adsorbents. The solution was measured and analyzed using a UV-Vis spectrophotometer. Before being analyzed with a UV-Vis spectrophotometer, the mixture must be separated between the fi ltrate and the residue. The regeneration process is carried out in seven cycles with the same procedure as before.

Selectivity adsorption
The selectivity was done by mixing three dyes, namely malachite green, methylene blue and rhodamine-B using a concentration of 10 mg/L as much as 100 mL. The mixture is then added 0.05 g of adsorbent in 50 mL of the dye mixture and stirred with time variations of 0, 15, 30, 60 and 120 minutes. Then separate the fi ltrate and residue, the fi ltrate was measured for wavelength with a UV-Vis spectrophotometer.

RESULTS AND DISCUSSION
XRD analysis aims to analyze the structure of the material that has been synthesized. The NiAl-LDH, HC, NiAl-HC diff raction patterns are presented in Figure 1. The NiAl-LDH diff raction pattern in Figure 1 Figure 1(b) shows that HC with a peak of 2θ = 22° with a diff raction plane (002) represents cellulose I and a peak between 15° to cellulose II with a diff raction plane (101) which has an amorphous form. Figure 1(c) shows the NiAl-HC composite pattern which has the same diff raction pattern as NiAl-LDH and HC with a diff raction angle of 11.03° and HC at a diff raction angle of 22°. The diff raction pattern of NiAl-HC composites decreased in intensity due to the amorphous HC. Figure 2(a) shows the FT-IR spectrum of Ni-Al-LDH. The vibratory band at 3483 cm -1 corresponds to the strain vibrational mode of the water molecules which are fi ssured by hydrogen bonds. Likewise, bands approaching 1630 cm -1 correspond to the bending vibrations of O-H from the water interlayer. The intense band at 1384 cm -1 is caused by the vibration mode of the nitrate ion in the interlayer space. The band characteristics in the metal-oxygen bond stretching appear at 664 cm -1 and 540 cm -1 due to the various lattice vibrations associated with the sheet metal hydroxide (Tcheumi et al., 2020). Figure 2(b). shows the main infrared absorption band of HC. The FT-IR spectrum is derived from cellulose and hemicellulose. It has been observed that the band between 3425 cm -1 is associated with the strain of the O-H groups forming hydrogen bonds and the types of intramolecular and intermolecular bonds that occur in polymer compounds, such as phenols, alcohols, carboxylates. acids, such as cellulose, hemicellulose, and lignin. The weak band at 2924 cm -1 , in HC, is a bond related to the -CH strain indicating the presence of lignin in HC. The 1620 cm -1 band indicates the presence of an aromatic  Figure 2(c) shows the spectrum of the NiAl-HR composite. The peaks appeared at 3483 cm -1 , 1630 cm -1 , 1384 cm -1 and 617 cm -1 . All the peaks that appeared in the NiAl-HR composites were NiAl-LDH and HR vibrations. The thermodynamic parameters were studied by varying the initial concentrations of malachite green, methylene blue, and rhodamine-b with the adsorption temperature as shown in Figure 3, Figure 4 and Figure 5. The fi gure shows that the adsorption capacity increases with increasing the adsorption temperature used. It is very visible that temperature greatly aff ects the adsorption capacity of the dye, this is because when the temperature increases it can cause an increase in the number of active sites and there will be an increase in the number of interactions between adsorbates. and active sites. by increasing the number of interactions between the adsorbate and the active site, it will increase the amount of adsorption capacity. Table 1 shows that the highest adsorption capacity (Q max ) in NiAl-HR composite material reached 256.410 mg/g at 333 K for malachite green, methylene blue of 144.928 mg/g, and Rhodamine-B of 71.429 mg/g. So that the LDH material which is composed with Hidrochar is considered as an eff ective adsorbent to remove cationic dyes. Adsorption Capacity NiAl-HR has a greater adsorption capacity compared to pure materials. The adsorption capacity of NiAl-HR to adsorb MG was greater than that of MB and Rh-B.
The determination of the adsorption isotherm model is seen from the linear regression value which is close to the value 1. From the data in Table 1, it can be seen that the isotherm model for each adsorbent and adsorbate tends to approach the Langmuir isotherm model with linear  . Langmuir model shows adsorption with a uniform energy distribution at the active site and assumes that there are no further interactions between adsorbed molecules. Therefore, Langmuir is considered once the active site is occupied by adsorbate, then no further interactions occur on that site. Table 2 assumes that the adsorption occurs endothermically because it has a positive H value (10.861-35.221 kJ/mol) and the adsorption process which has an H value <84 kJ/mol, it is assumed that the adsorption process occurs physically. S is positive, assuming the degree of irregularity in the solid-liquid phase during the adsorption process of the dye onto the adsorbent surface (Dang et al., 2020). With this, a positive S value indicates that the dye has a high affinity for the adsorbent because in the adsorption process the energy required to form a bond with the adsorbent is lower than the energy required to break the same bond. According to Neolaka et al. (2020) G is negative, it is assumed that the spontaneity of the adsorption process and the adsorbate has a high affinity for the adsorbent. Decreasing the value of G with increasing adsorption temperature indicates that the adsorption process is better at higher temperatures. It can also be seen from the data that with increasing adsorption temperature, the adsorption concentration of the dye also increases.
The adsorption capacities of several adsorbents for MG, MB and RH-B dyes and the synthesized adsorbents in the form of NiAl-LDH, HR and Ni-Al-HR used in this study are presented in Table 3. The adsorption capacities of MG, MB and RHB for NiAl-LDH and hydochar have a larger capacity compared to other adsorbents. NiAl-HR composites have a greater adsorption capacity compared to LDH and hydrochar for the adsorption of MG, MB, and Rh-B. This results in the interaction of the hydrochar active site on the composites.
The results of the adsorption regeneration study using NiAl-LDH, HR, and NiAl-HR are shown in Figure 6. The results showed that the NiAl-HR composite material had a greater adsorption capacity and a more stable regeneration process than pure material. The adsorption of malachite green, methylene blue and Rhodamine-B using NiAl-HR material was stable for five regeneration cycles and experienced an insignificant decrease. This is because the process of modifying LDH with carbonbased materials can increase the stability of the structure so that it can be used repeatedly as an adsorbent. Meanwhile, the adsorption of NiAl-LDH and HR experienced a significant decrease in the regeneration process, this was because the material had an unstable structure and peeling and structural damage occurred. So it can be concluded that modification of LDH with hydrochar can increase the stability of the LDH structure for the adsorption regeneration process.
The maximum wavelength of a mixture of malachite green, methylene blue and rhodamine-B dyes was measured in a neutral pH using a UV-Vis spectrophotometer in the wavelength range of 500-700 nm shown in Figure 7. The selectivity of adsorption to the dye mixture can be seen from changes in wavelength the maximum absorbance during adsorption. In neutral conditions, it can be seen that for NiAl-LDH, HR and NiAl-HR adsorbents there is a decrease in absorbance after adsorption. Figure 7 shows that among the mixed dyes, MG dyes were adsorbed more than MB and Rh-B. This can be seen from the decrease in the absorbance value with the adsorption contact time and at 120 minutes the adsorbed concentration of MG reached 50.465 mg/g. The maximum wavelength of a mixture of malachite green, methylene blue and rhodamine-B dyes was measured in base pH using a UV-Vis spectrophotometer in the 500-700nm wavelength range shown in Figure 8. The selectivity of adsorption to the dye mixture can be seen from changes in wavelength during adsorption. In alkaline conditions, it can be seen that for NiAl-LDH, HR and NiAl-HR adsorbents there is a decrease in absorbance after adsorption. Figure 8 shows that among the mixed dyes, more adsorbed than MB and Rh-B. This can be seen from the decrease in the absorbance value with the adsorption contact time and the adsorbed MG concentration at 120 minutes reached 84.263 mg/g. compared to neutral pH, the adsorbed concentration of alkaline pH is greater.  The maximum wavelength of a mixture of malachite green, methylene blue and rhodamine-B dyes was measured in an acid pH using a UV-Vis spectrophotometer in the 500-700 nm wavelength range shown in Figure 9. The selectivity of adsorption to the dye mixture can be seen from changes in wavelength during adsorption. Under acidic conditions, it can be seen that for NiAl-LDH, HR and NiAl-HR adsorbents there is a decrease in absorbance after adsorption. Figure  9 shows that among the mixed dyes, more MG adsorbed as seen from the decrease in the absorbance value with the adsorption contact time and the adsorption concentration of MG at 120 minutes reached 98.263 mg/g for the NiAl-HR composite adsorbent.

CONCLUSIONS
In this study, the NiAl-HR composite material was successfully synthesized using the coprecipitation method and applied as an adsorbent to remove cationic dyes. NiAl-HR composite materials have a large adsorption capacity to adsorb cationic dyes. The adsorption follows the Langmuir adsorption isotherm model with the maximum capacity (Q max ) of the NiAl-HR composite material of 256.410 mg/g for MG and the adsorption process takes place spontaneously and endothermically. The regeneration process of NiAl-HR composites was more stable and the decrease was not signifi cant (>70%). The selectivity of the dye mixture showed that the adsorbent was more selective for MG dye compared to MB and Rh-B.