Development of Renewable Material Hydrochar-Based CaAl Layered Double Hydroxide to Overcome Methyl Red Dyes Contaminant

The preparation of the CaAl/HC composite was carried out with a supporting material in the form of hydrochar from duku skin. The success of the preparation was demonstrated by XRD, FT-IR, and BET analysis. The diffractogram of CaAl/HC showed diffraction peaks at 2θ = 10.16° (003), 18.0° (002), 20.15° (006) and 65.4° (110). The diffraction showed similarity to diffraction in CaAl and hydrochar double layer hydroxyl. The FT-IR spectrum of CaAl/HC also showed similarity to the CaAl/HC double layer hydroxyl at 3448, 1635, and 1381 cm-1. The characteristic spectrum of the hydrochar also appeared in CaAl/HC at 20.15. BET analysis showed an increase in the surface area of CaAl/HC before modification of 11.842 m2/g and increased to 22.635 m2/g of CaAl/HC. The ability of CaAl/HC as an adsorbent is determined through several parameters including selectivity, regeneration, isotherm, and thermodynamics. The results of dye selectivity showed that CaAl/HC was more likely to absorb MR dyes in a mixture of dyes (DG, MO, PR, MR, CR, and DR). The regeneration results showed the ability of CaAl/ HC which lasted up to 73.26% in the fifth cycle.


INTRODUCTION
Dyes are one of the contaminants the presence of which is often found in the waste that pollutes the environment. They are widely used as the main ingredient in several industries such as paper, cosmetics, food, and textiles. Dyes are included in the group of unsaturated organic compounds with chromophore as a color carrier and auxochrome as a color binder in the fiber. Dyes are divided into cationic, such as malachite green (Palapa, Taher, et al. 2020), methylene blue (Dayanidhi et al. 2020), rhodamine B (Dayanidhi et al. 2020), etc. as well as anionic dyes which include, Congo red , direct green (Hashem, Ahmad, and Badawy 2016), methyl orange (Onder et al. 2020) and methyl red (Yuan et al. 2016). Methyl red (C 13 H 15 N 3 O 2 ) is one of the most anionic dyes widely used dyes in industry. It is one of the dyes that contain azo (N = N) (Bazan-Wozniak and Pietrzak 2020) chromofor groups, making these color substances are difficult to degrade in the environment, toxic and mutagenic if they accumulate in the body (Karri et al. 2018). Increasing the need for color substances makes its use more uncontrollable so that it can cause negative impacts on the environment. For this reason, effective efforts are needed to overcome the problem.
Several studies claim that adsorption is an effective and efficient method of dealing with liquid waste (Milagres et al. 2017). The adsorption method is the process of binding an adsorbate on the surface of the adsorbent to form a layer (Pontes-Neto et al. 2019). The adsorption method was chosen because the process is simple, environmentally friendly, has low energy requirements, low cost, produces high quality effluent, and the adsorbent used has the potential to be regenerated (Oktriyanti et al. 2020). Several factors need to be considered in the adsorption method, such as pH conditions, adsorption time, temperature, and adsorbate concentration. Determination of these parameters needs to be performed to achieve the optimum ability of the adsorbent which produces high adsorption power. The choice of adsorbate is also an important factor in the adsorption process. Types of adsorbate that can be used in adsorption are porous and layered materials such as clay, activated carbon, bentonite, zeolite, hydrochar, and layered double hydroxide.
Layered double hydroxide (LDH) is an adsorbent material belonging to anionic clay with where M 2+ and M 3+ are divalent and trivalent cations, and Am-is charge balancing interlayer anion (Palapa et al. 2021). The advantages of LDH include flexible properties, good thermal stability, and a large enough surface area ). On the basis of these advantages, it is possible for LDH to be modified so that its adsorption ability increases so that it is effective in overcoming contamination. The study of the adsorption ability of LDH has been carried out by (Palapa et al. 2021) using CuCr with a surface area of 4.58 m 2 /g that was applied as an adsorbent of the malachite green color with Qmax reaching 27.585 mg/g. Other studies conducted by (Juleanti et al. 2021) involved the application of LDH CaAl and MgAl to overcome a contamination of Cr(VI) heavy metal ions with adsorption capabilities each reaching 47.02 and 23.15 mg/g. In his study, (Siregar et al. 2021) used LDH NiAl as Congo red adsorbent with adsorption capacity of 61.728 mg /g.
Modification of LDH can be done by intercalation (Palapa et al. 2021) and impregnation (Wahab et al. 2019). Impregnation is a method that can yield products in the form of composites with an increased surface area value from the constituent materials, and allows for an increase in the adsorption ability. The development of LDH into composites can be done with a supporting material, one of which is biomass in the form of hydrochar. The hydrochar produced from the hydrothermal carbonization process has a sufficiently large surface area to become the right choice for use as a supporting material for LDH composites. The research on the use of LDH-hydrochar composites was carried out by (Luo et al. 2020) to prepare MgAl-hydrochar from sewage sludge for Pb(II) adsorption resulting in an adsorption capacity of 62.441 mg/g. The MgAl-hydrochar composite was also prepared by (He et al. 2019) using the main ingredient in the form of tobacco stalk with capacities of 30.69, 31.42, and 41.16 mg/g at different temperatures (25,35, and 45)°C.
In this study, the preparation of the CaAl/HC composites from CaAl layered double hydroxide and supporting material in the form of hydrochar (HC) from duku fruit peel (Lansium domesticum) was carried out. The success of the preparation was supported through XRD, FT-IR, BET, and SEM analysis. The prepared material was applied as a dye adsorbent. The main parameter that was determined was the selectivity of the dye mixture (DG, MO, PR, MR, CR, and DR), which was then followed by the selective dyestuff. Other parameters that are also determined include regeneration, isotherm and adsorption thermodynamics.

Synthesis of layered double hydroxide material
The Ca/Al double layer hydroxy synthesis was carried out in several steps. The first step was to dissolve Ca(NO 3 ) 2 •4H 2 O 0.75 M in a 100 mL volumetric flask with 100 mL 0.25 M Al(NO 3 ) 3 •9H 2 O. The mixture was adjusted to pH 10 with the addition of 2 M NaOH then stirred for 30 minutes. The mixture was stirred for 24 hours at a temperature of 65 °C to produce a white precipitate. The white precipitate was then fi ltered with the help of a vacuum pump and then dried at 100 °C and the Ca/Al double layer hydroxyl was obtained. The resulting layered double hydroxide was characterized using XRD, FT-IR, and BET analysis.

Preparation of composite layered double hydroxide material
The CaAl/HC composite was prepared by making a 30 mL solution of Ca(NO 3 ) 2 •4H 2 O 0.75 M and 0.25 M Al(NO 3 ) 3 •9H 2 O. The mixture was stirred and 3 g of hydrochar was added, then adjusted to pH 10 with the addition of 2 M NaOH. Afterwards, the mixture was kept at 80 °C for 72 hours. The composites obtained were then characterized by XRD, FT-IR, and BET.

Adsorption studies
The adsorption ability of the adsorbent is determined through several parameters including selectivity, regeneration, isotherm, and thermodynamics. The selectivity of the adsorbent was carried out by mixing DG, MO, PR, MR, CR, and DR dyes and stirred for 0-120 minutes and then the wavelength was measured using a UV-Vis spectrophotometer. Selective dye (MR) is followed by a regeneration process to determine the adsorbent ability when used repeatedly. The regeneration process is carried out by adsorption and desorption processes using water with ultrasonic which is carried out for up to 5 cycles. Furthermore, the determination of the thermodynamic and isotherm parameters of adsorption was carried out by varying the adsorption temperature (30, 40, 50, and 60)°C and the concentration of the dye (50, 75, 100, 125, and 150) mg/g. Absorbance was determined using a UV-Vis spectrophotometer.

RESULT AND DISCUSSION
CaAl layered double hydroxide synthesis and CaAl/HC composite preparation were carried out using the coprecipitation method. One of the success parameters was observed based on the characterization of XRD analysis. The results of XRD characterization produce the data in the form of a diff ractogram at a certain angle. The diff raction pattern of the CaAl layered double hydroxide in Figure 1 shows the diff raction at 2θ = 10.18° (003), 20.61° (006), 55.17° (113), and 56.24°( 110). On the basis of these data, the diff raction peak of CaAl was in accordance with JCPDS No. 87-0493 which indicates that the CaAl synthesis has been successfully carried out.
According to (Palapa, Taher, et al. 2020), the diff raction peak around 10° is one of the typical diff raction of materials with a layered structure. In contrast, the diff raction which is around 60°i ndicates that there are anions in the interlayer material. Furthermore, the success of hydrochar preparation was supported by the presence of diff raction peaks at 2θ = 22.47° and 16.02°. This is in accordance with the research of (Normah et al. 2021) which explains that the hydrochar diff raction is around 22° (002) which is a characteristic of cellulose and characteristic of hydrochar is at 15° which is indicated as diff raction of amorphous material.
The FT-IR analysis was also performed as the data to support the success of CaAl/HC composite preparation. The results of the characterization of the CaAl are shown by the spectrum in Figure 2 Determination of the spectrum of the hydrochar presented in Figure 2 BET analysis was performed to determine the N 2 adsorption-desorption pattern, surface area, pore volume and pore diameter. According to Moller and Pich (Siregar et al. 2021) the N 2 adsorption-desorption pattern of CaAl in Figure  3 tends to follow type III. Type III characteristics are characterized by weak interactions between the adsorbate and adsorbent. The space available after the adsorption process on a single layer becomes very low. This causes a larger absorption value at a higher relative pressure. The isotherm pattern of HC resembling type IV indicates a mesoporous material that exhibits non-overlapping adsorption and desorption patterns. The N 2 adsorption-desorption profi le on the CaAl/HC composite indicated the occurrence of H4 hysteresis. The hysteresis occurs in the adsorption-desorption process with a diff erent mechanism on the adsorbent with pores that form gaps and is indicated by the adsorbent having a mesoporous size. Table 1 shows that CaAl has a surface area of 9.621 m 2 /g with a pore volume of 0.027 cm 3 /g and a pore diameter of 3.169 nm. The surface area of HC reaches 11.842 m 2 /g with a pore volume of On the basis of these data, the increase in surface area that occurs in CaAl after being modifi ed to CaAl/HC indicates that the composite preparation process has been successfully carried out. The surface morphology of the adsorbent was analyzed using SEM analysis which is presented in Figure 4. The CaAl layered double hydroxide shows a smooth surface morphology and agglomeration forms. The particle morphology pattern of the hydrochar adsorbent at a temperature of 200 °C which tends to be heterogeneous and has an irregular shape, this is due to the hydrothermal carbonization treatment given, causing the particles to split or commonly referred to as deaggregation. It can be seen that the surface of CaAl/HC still has a smooth surface, but the agglomeration is reduced.
On the basis of the characterization above, CaAl, HC, and CaAl/HC have the potential as adsorbents the ability of which is determined through the parameters of selectivity, regeneration, isotherm, and thermodynamics. Selectivity parameters were carried out by mixing anionic dyes which included DG, MO, PR, MR, CR, and DR and then carried out the adsorption process with time variations of 0, 30, 60, 90, 120 and 150 minutes. The results of the scanning wavelength of the dye mixture are presented in Figure 5. The mixed dye adsorption process used the same concentration of 25 mg/L each.
Determination of adsorbent selectivity parameters CaAl, HC, and CaAl/HC was carried out using a mixture of dyes, including DG, MO, PR, MR, CR, and DR. The adsorption process was carried out with time variations ranging from 0, 30, 60, 90, and 120 minutes. The measurement of the wavelength of the dye mixture was carried out using a UV-Vis spectrophotometer (450-650 nm). Wavelength scan of the dye mixture produced the spectrum are presented in Figure 5. It can be seen that there is a decrease in absorbance for each CaAl, HC, and CaAl/HC adsorbent with increasing time.
The scanned data for the mixed wavelengths of DG, MO, PR, MR, CR, and DR dyes are presented in Figure SL. It can be seen that there is a   The regeneration process was continued with the selective dye, MR, which is presented in Figure 6. regeneration process aims to determine the adsorbent ability that has been used repeatedly. The fi rst stage of regeneration is adsorption on MR 1000 mg/L; then, the desorption process is carried out to break the interaction between the adsorbent and the adsorbate through the water medium using ultrasonic. The working principle of ultrasonic is the propagation of high frequency waves in liquid media. The liquid will be propagated in the form of a sound medium with high-frequency ultrasonic waves that produce microscopic vibrations, so that the adsorbate that is strongly adsorbed can be separated more easily. This adsorption-desorption process was carried out for 5 cycles.
The fi rst cycle of regeneration of CaAl showed an ability that reached 89.75%, the second cycle lasted at 86.66% and decreased in the third cycle of 66.27% and 56.35% in the fourth cycle. The decline occurred until the fi fth cycle which reached 40.11%. Regeneration in HC reached 86.10% in the fi rst cycle and decreased to 71.53%. There was a signifi cant decrease in the third cycle which reached 18.38% and 14.61% in the fourth cycle. The ability of HC continued to decline until the fi fth cycle which reached 9.16%. The regeneration of CaAl/HC in Figure 6 shows the stability of the structure, where the fi rst cycle reaches 97.22% and can last up to the third cycle which reaches 92.12%. The decrease in absorbance ability was seen in the fourth cycle which reached 88.76% and the fi fth cycle 73.26%. The regeneration ability of CaA/ HC remains above 50% so that CaAl/HC has the potential to be used repeatedly.
Further adsorption parameters are determined by the Langmuir Freundlich isotherm model which is presented in Figure 7. The trend of Langmuir Freundlich adsorption isotherm model was determined based on the linear regression value (R 2 ). Figure A shows the tendency of CaAl towards the Freundlich isotherm model. The same thing is also seen in HC and CaAl/HC which have a tendency towards the Freundlich isotherm model. The trend of the isotherm model in Figure 7 is also supported by the R 2 value data in Table 2. CaAl, HC, and CaAl/HC show the R 2 values in the Freundlich isotherm model of 0.999, 0.999, and 0.995, respectively. On the other hand, the R 2 values of the Langmuir CaAl, HC, and CaAl/HC isotherm models are 0.995, 0.967, and 0.973, respectively. (Arabpour, Dan, and Hashemipour 2021) explained that the Freundlich isotherm model indicated that there was a physical adsorption process that occurred due to the weak bond between the adsorbent and the adsorbate involving Van der Waals interactions which allowed the adsorbate to move freely until the adsorption process formed a multilayer. In addition to the value of R 2 , Table 2 also presents the value of adsorption ability (Qmax) of CaAl, HC, and CaAl/HC. The CaAl layered double hydroxide before the modifi cation process had a Qmax of 116.279 mg/g. HC as a composite support material has a Qmax value of 70.423 mg/g. On the other hand, the CaAl/HC composite has the largest Qmax reaching 121.951. The increase in Qmax after the modifi cation process is supported by surface area using BET analysis in Table 1. The surface area of CaAl/HC which reaches 22.635 m 2 /g is the largest Qmax value between CaAl and HC. On the basis of this, the increase in surface area aff ects the adsorption ability of the adsorbent.
After obtaining the isotherm data, the thermodynamic parameters were determined, including enthalpy (∆H), entropy (∆S), and Gibbs free energy (∆G). The results of the determination of the adsorption energy of CaAl, HC, and CaAl/HC on MR are presented in Table 3. The fi rst parameter in thermodynamics in the form of H of CaAl, HC, and CaAl/HC shows positive values in the range of 44.074-86.399 kJ/mol. According to (Juleanti et al. 2021), a positive ∆H indicates that the adsorption process is endothermic, where during the adsorption process it will absorb energy from the environment to the system to assist the interaction between the adsorbent and the adsorbate. According to , the ∆H values which are in the range of 40-120 kJ/mol indicate that the adsorption process takes place by chemisorption, whereas if it is outside this range, the adsorption process takes place by physisorption. On the basis of this statement, the adsorption process on MR using CaAl, HC, and CaAl/HC is in the range of 40-120 kJ/mol; this explains that the adsorption process takes place by chemisorption.   The ∆S value of the MR adsorption process using CaAl, HC, and CaAl/HC in Table 3 shows a positive value. According to (Badri et al. 2021), a positive value of ∆S indicates the adsorption process involves a dissociative mechanism. Table 3 also shows a negative ∆G value, which indicates that the adsorption process takes place spontaneously. In addition, it was seen that the ∆G of CaAl, HC, and CaAl/HC tended to decrease with increasing concentrations. This phenomenon explains that the adsorption process tends to be better carried out at high temperatures.

CONCLUSIONS
The CaAl/HC composites were successfully prepared as evidenced by the main characteristics of CaAl layered double hydroxide and hydrochar through XRD, FT-IR, and BET analysis. CaAl/ HC was applied as adsorbent for methyl red dye. The ability of CaAl/HC is shown through the data on the maximum adsorption capacity which reached 121.951 mg/g. In addition, the adsorption ability of CaAl-HC was also shown through the regeneration results which persisted at 73.26% until the fifth cycle.