Biochar-Modified Layered Double Hydroxide for Highly Efficient on Phenol Adsorption

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Introduction
Water is one of the main needs for carrying out all human activities.Healthy drinking water must meet the chemical, physical, and microbiological requirements.The presence of household waste, industrial waste, and organic waste makes drinking water suitable for con-biodegradation, photochemical, Fenton, and adsorption methods [3][4][5].However, the adsorption method is a method that is very commonly used because it is easy to operate, low cost, and has high efficiency [6].Various kinds of adsorbents are used in phenol adsorption, including biochar [7], clay [8], graphite [9], and layered double hydroxide [10].
Layered double hydroxide (LDH) has a unique property of flexibility, where the interlayer anions can be replaced according to the application of the LDH [11].In addition, LDH has a large surface, so it is very useful in adsorption but has poor structural stability, so it is easy to peel off during application, which results in reduced efficiency in reuse in the adsorption process [12,13].Therefore, it is necessary to add a supporting material in order to improve the structure so that it has better adsorption ability.There are many methods that can be used to synthesize LDH including ion exchange, sol-gel, calcination-hydration, and coprecipitation methods [14,15].However, the coprecipitation method is the most widely used method because it is simple, low cost and does not use high temperatures [16,17].
There are various types of carbon-based materials that can be applied as support materials in improving the structure of LDH, namely graphite, charcoal, chitosan, and biochar [18][19][20][21].Biochar (BC) is a material produced from the decomposition of organic matter which has advantages including being friendly to the environment, easy to manufacture at low cost, abundant availability of raw materials, and has a porous structure which is very helpful in the adsorption process [22,23].
Several studies have investigated the usefulness of LDH composited with carbon-based materials for the adsorption of various types of waste.Research conducted by Normah et al. [24] improved the structure of the NiAl LDH by adding a graphite support material that could increase the adsorption capacity from 29.586 to 72.464 mg/g, and after the composite was performed, the effectiveness of the adsorption process was seen.Up to five reusability cycles did not experience a significant decrease.In addition, the addition of supporting materials to improve the structure of LDH can increase the surface area.This is evidenced by Mahgoub et al. [25] who added cellulose activated carbon to the layered double hydroxide material, increasing its surface area from 50.96 to 303.79 m 2 /g which was very helpful in increasing the adsorption capacity.Hoang et al. [26] used ZnAl LDH material composited with bagasse biochar to remove tetracyclines.The results obtained showed an increase in surface area from 404.05 m 2 /g in bagasse biochar to 456.39 m 2 /g in ZnAl-Biochar bagasse.Another study also used LDH/biochar composite materials to remove cadmium as was done by Liao et al. [27].The results indicated that the surface area of biochar increased from 71.170 m 2 /g to 384.198 m 2 /g, and that its adsorption capacity increased from 44.51 mg/g to 181.53 mg/g.Thus, it is required to do additional research on the creation of waste-removal materials derived from various forms of biochar and LDH.
In this study, NiAl and ZnAl layered double hydroxide materials were composited to Biochar from rice husk using co-precipitation method and then characterized using XRD, FT-IR, SEM, and BET.Determination of the adsorption capacity of phenolic compounds on various materials was observed by carrying out various influences such as pH, time, concentration and adsorption temperature as well as looking at the usability of the material in repeated use.

Chemicals and Instrumentation
In this work, the chemicals used are biochar by Bukata Organic Indonesia, hydrogen chloride (HCl)   The pH of the solution was then adjusted to 10 by adding 2 M NaOH.The resultant mixture was swirled continuously for 17 h at 80 °C.Furthermore, the samples were rinsed with distilled water, filtered and put in an oven with a temperature of 100 °C for drying.

Synthesis of ZnAl LDH
In a beaker glass, 100 mL of 0.25 M Al(NO3)3.9H2Osolution and 100 mL of 0.75 M Zn(NO3)2.6H2Owere added.In addition, 2 M NaOH was added to the mixture in order to lower its pH to 8. The mixture was swirled for 4 h at 80 °C.After being treated, the ZnAl LDH material was then filtered, washed, and dried.
2.4 Preparation of NiAl-BC 30 mL of each solution of 0.75 M Ni(NO3)2.6H2Oand 0.25 M Al(NO3)3.9H2Owere combined in a beaker, 2 M NaOH was slowly added to obtain a pH of 10, and the mixture was agitated for 1 h.In addition, 3 g of biochar are added to the mixture and mixed for three days at 80 °C.Before being heated at 100 °C, the precipitate was filtered and rinsed with deionized water.
2.5 Preparation of ZnAl-BC 30 mL of each of the solutions 0.75 M Zn(NO3)2.6H2Oand 0.25 M Al(NO3)3.9H2Owere combined in a beaker.Slowly adding 2 M NaOH to the mixture until the pH reached 8 and stirring for 1 h brought the pH to 8. Additionally, 3 g of biochar are added to the mixture, which is then agitated at 80°C for 72 h.The precipitate obtained was filtered and rinsed with distilled water.Furthermore, drying is carried out by placing it in the oven at a temperature of 100°C.

Performance of pH point zero charge (pHpzc)
This was done by adding 20 mL of 0.1 M NaCl solution which had been set to a pH of 2, 3, 4, 5, 6, 7, 8, 9, 10, and 11.Then 0.02 g of each adsorbent was added and stirred for 24 h.Furthermore, pH measurements were carried out after the selation stirring process to show the presence of no charge in each adsorbent by making a graph of the relationship between the initial pH and the final pH.

Adsorption of Phenol
The effect of pH, time and the effect of concentration and temperature were observed during the investigation on the phenol adsorption process.In addition, also investigated was the reusability process.The investigation was carried out by inserting 20 mL of phenol solution into a beaker and adding 0.02 g of each adsorbent.Variation of pH is done by setting pH 2-11; contact times used are 0, 5, 10, 20, 30, 40, 50, 60, 70, 90, 120, 150, 180, and 200 min; the effect of concentration and temperature is carried out with concentrations of 15, 20, 25, 30, and 35 mg/L (30-70 °C).
The solution was complexed by combining 1 mL of phenol solution, 1 mL of pH-10 buffer solution, 0.1 mL each of 8% hexacyanoferrate (III) and 2% 4-aminoantipyrine reagent, and 3 mL of distilled water in a beaker before measuring the absorbance with a UV-VIS spectrophotometer.After being homogenized, the mixture was let to stand for 15 min.The concentration was then determined by measuring the absorbance.

Desorption and Regeneration of Adsorbent on Phenol Adsorption
Desorption treatment was carried out after the adsorption process was complete.At first, adsorption was carried out as usual, using 15 mg/L of phenol solution and adding 0.02 g of adsorbent and stirring for up to 2 h.After that, the separation between the filtrate and precipitate was carried out.The precipitate is dried and a desorption process is carried out using 10 mL of distilled water which is put into an ultrasonic device to release the adsorbate attached to the adsorbent.While the filtrate is measured to determine the adsorbed concentration.The regeneration process is carried out by doing the same thing for up to five cycles.

Point of Zero Charge (PZC) of the Adsorbent Materials
The PZC value identifies the state of a material on the surface that has no charge or is zero.A positive charge on the surface is shown by a pH value below the PZC, and a negative charge is indicated by a pH value above the PZC [4].The PZC chart is shown in Figure 1.Based on the PZC values obtained, the PZC values in NiAl LDH are pH 8.3, ZnAl LDH at pH 5.9, BC at pH 5.1, NiAl-BC at pH 9.4, and ZnAl-BC at pH 6.7.This identifies the neutral charge of each material.

Effect of pH
pH has a major influence on the phenol adsorption process, which can affect the nature and structure of the adsorbate and adsorbent, causing differences in adsorption abilities [28].Based on Figure 2, the optimal pH of NiAl LDH and ZnAl LDH is 3, the optimal pH of NiAl-BC and ZnAl-BC is 5, and the pH at BC is 7.This proves that if pH < pHpzc indicates the charge on the surface of the adsorbent is positive.According to Alminderej et al. [29], the presence of a positive surface on the adsorbent will increase the electrostatic force between the phenolic anions so as to increase the ability of the adsorption process.However, when the pH is high or alkaline it will cause the adsorbent to be negatively charged.Therefore, it can experience a decrease in adsorption capacity due to electrostatic repulsion between the negatively charged phenolic anions and the negatively charged adsorbent [30].Furthermore, the optimal pH that has been obtained will be used in the adsorption process.002) is a typical pattern of biochar as a material with an amorphous structure.This is in accordance with research conducted by Prakongkep et al. [32] who performed XRD analysis of biochar produced a diffraction peak at an angle of 2θ = 22.50° (002).When the NiAl and ZnAl LDH were composited on biochar, there was a shift of 2θ = 11.74°(003) in a smaller direction, namely to 2θ = 10.85°(003).This is due to the addition of carbon material (biochar) to LDH [33].In the ZnAl-BC and NiAl-BC composite materials there are diffraction peaks for each constituent material, so that it can be ascertained that the NiAl-BC and ZnAl-BC composite materials were successful.

Characterization of the Adsorbent Materials
The FTIR spectra of NiAl LDH, ZnAl LDH, BC, NiAl-BC, and ZnAl-BC are depicted in Fig- ure 4. The strong absorption band around 1381 cm −1 implies that nitrate anions are present in the LDH interlayer [34].The large absorption band near 3448 cm −1 indicates the presence of -OH stretching vibrations.Ravuru et al. [35] said that the absorption band around 3448 cm −1 indicates that in the LDH interlayer there are O−H vibrations in water molecules.Metal-Figure 1. PZC of adsorbent materials.SEM images of the morphology of NiAl LDH, ZnAl LDH, BC, NiAl-BC, and ZnAl-BC can be seen in Figure 5.The NiAl-BC and ZnAl-BC composites were successfully formed, as shown in Figures 5(d) and (e).This is because the LDH surface of the NiAl-BC and ZnAl-BC composite materials is well distributed by BC particles so as to increase the surface area [37].However, Figure 5(e) demonstrates the existence of an uneven plate-like structure with various diameters that permits BC to deposit on the LDH surface [38].
Figure 6 shows the nitrogen adsorptiondesorption isotherms on NiAl LDH, ZnAl LDH, BC, NiAl-BC, and ZnAl-BC materials.The results obtained showed all type IV hysteresis loop isotherms.This states that all materials have mesoporous characteristics [39].According to Limau Jadam et al. [40], monolayer adsorption occurs at low partial pressures on the type IV nitrogen adsorption-desorption isotherm, followed by multilayer adsorption growth at high partial pressures.
Using BET and BJH methods, the surface area and pore volume of NiAl LDH, ZnAl LDH, BC, NiAl-BC, and ZnAl-BC materials can be Figure 4. Fourier transfer infra-red spectrum of adsorbents determined [40].The surface area of NiAl LDH material increased from 92.683 to 438.942 m 2 /g as a result of the addition of biochar in NiAl-BC composite material.The same phenomenon also occurred in the ZnAl-BC composite material whose surface area increased to 58.461 m 2 /g from 9.621 m 2 /g due to the addition of BC to the ZnAl LDH material.This is in accordance with research conducted by Liao et al. [27] who experienced an increase in the surface area of composite materials with previous precursor materials.Therefore, it can be seen in Table 1 that the identification of the success of LDH modification with Biochar with increased surface area has been successfully carried out.tions going on during the adsorption process [42].

Effect of Isotherms and Thermodynamic Studies
Langmuir and Freundlich isotherm parameters can be seen in Table 3. Determination of isotherm parameters is carried out to determine whether the adsorption process that occurs is more dominant chemically or physically [43].Based on the data in Table 3, all adsorbent materials follow the Freundlich isotherm model, where the value of the correlation coefficient (R 2 ) is closer to one.According to Budnyak et al. [44], the Freundlich isotherm is an adsorption process that occurs in multilayers.The maximum adsorption capacities of NiAl LDH, ZnAl LDH, BC, NiAl-BC, and ZnAl-BC were 45.87, 37.17, 35.33, 74.62, and 52.91 mg/g, respectively.Based on the data in Table 4, it shows the difference in maximum adsorption capacity between this study and other studies.
Table 5 displays adsorption thermodynamic parameters such as H, S, and G.Based on the data obtained, the value of H is positive, which describes endothermic adsorption.The enthalpy values (H) of ZnAl LDH and ZnAl-BC show smaller values than the enthalpy values of BC, NiAl LDH, and NiAl-BC.The small-er enthalpy value indicates a better physisorption adsorption process.A small entropy value (S) indicates a small degree of freedom.G is negative, ensuring that the adsorption process occurs spontaneously.According to [48], a value of G less than 20 kJ/mol indicates adsorption that occurs by physisorption.The types of bonds that occur in physisorption are hydrogen bonds, - interactions, and electrostatic interactions [55,56].

Reusability of Adsorbents
Figure 8 shows the effectiveness of an adsorbent for reuse.The graph obtained showed the reusability process up to five cycles, where NiAl-BC had the highest adsorption percentage with a percentage of 68.10% and experienced an insignificant decrease in the fifth cycle, from 68.10% to 52.23%.The next highest adsorption percentages were ZnAl-BC, NiAl LDH, ZnAl LDH, and BC, which each had adsorption percentages of 64.35, 61.73, 57.73, 53.23%.Also, as the number of reusability cycles went up, the ability to absorb dropped.In the fifth cycle, it dropped by 15.62, 17.50, 14.75, and 24.99%, respectively.

Mechanism of Phenol Adsorption
The interactions that occur in phenol adsorption can be seen in Figure 9 which shows the FT-IR analysis after adsorption.According to Haydari et al. [57] the common mechanisms that occur in phenol adsorption are electrostatic interactions, hydrogen bond interactions, and - electron pair interactions.In Figure 9 it can be seen that there has been a shift in several wavenumbers including the wavenumber 1627 cm −1 which indicates the occurrence of - interactions in C=C aromatic biochar with phenol aromatic rings.At wavenumber 1049 cm −1 there is a shift indicating an electrostatic interaction.The electrostatic interaction is also evidenced by a shift in the metal-oxide wavenumber region where the positive charge on LDH binds to the negative charge on the phenol according to pH<pHpzc.In addition, there is also a hydrogen bond interaction that occurs in phenol adsorption which is marked by a shift at wavenumber 3448 cm −1 .The adsorption mechanism that occurs in phenol adsorption can be seen in Figure 10.

Conclusion
In this study, NiAl LDH and ZnAl LDH composites were successfully prepared using XRD, FTIR, and BET characterization techniques.NiAl LDH and ZnAl LDH have an optimal pH of 3, NiAl-BC and ZnAl-BC have an optimal pH of 5, and biochar has an optimal pH of 7. The kinetics model was PSO-based, whereas the isotherm model was Freundlich-based.NiAl LDH, ZnAl LDH, BC, NiAl-BC, and ZnAl-BC had maximal adsorption capacities of 45.87, 37.17, 35.33, 74.62, and 52.91 mg/g, respective-ly.The recurrent usage of the material demonstrates that NiAl-BC has a superior ability, which is not significantly diminished by the fifth cycle.Then ZnAl-BC, NiAl LDH, ZnAl LDH, and BC follow.

Acknowledgement
Author acknowledges the help and instrumental analysis of the Research Center of Inorganic Materials and Coordination Complexes, Universitas Sriwijaya for assisting in the completion of this study.

Figure 2 .
Figure 2. Effect of pH on adsorption of phenol.Figure 3. X-ray diffractogram of adsorbents.

Figure 3 .
Figure 2. Effect of pH on adsorption of phenol.Figure 3. X-ray diffractogram of adsorbents.

Figure 7
Figure 7 depicts an increase in the concentration of phenol adsorbed on NiAl-BC up to 60 min, after which the concentration tends to remain constant.The BC and ZnAl BC materials have the ability to last up to 70 min, while the ZnAl and NiAl LDH have the ability to last up to 90 min.Using pseudo-first order (PFO) and pseudo-second order (PSO) kinetic models, it is also possible to forecast the adsorption rate based on the influence of time.With the the minimum kinetic rate (k), correlation coefficient (R 2 ) approaching 1, and the similarity between the predicted and observed Qe values, the kinetic model for the adsorption process may be determined.Based on the data in Table

Table 2 .
[41]tic variables models of pseudo-first-and pseudo-second-order.2, it shows that all adsorbents follow the PSO kinetics model.According to Li et al.[41], the PSO kinetic model indicates that the adsorption process proceeds in two stages, with the first fast phase involving physical adsorption or ion exchange on the surface of the adsorbent and the succeeding slow phase involving mechanisms such as microprecipitation.PSO also shows that there are physicochemical interac-

Table 4 .
Maximum adsorption capacity of phenol and comparison with other researches.