The materials used in the synthesis of chitosan-Fe₂O₃ composite are FeCl₃ (Merck), demineralized water (pure H₂O), NH₄OH (Merck), chitosan powder (Sigma Aldrich), and NaOH (Merck). For the pH point zero charge and MB degradation procedures, NaNO₃ (Merck), NaOH (Merck), HCl 37% (Merck), and synthetic MB were used as materials.

Fe₂O₃ was synthesized using the co-precipitation method. Initially, a solution of Fe³⁺ precursor was prepared by dissolving 1.46 g of FeCl3 in 100 mL of pure water. Furthermore, it was added with 12% of NH₄OH and constantly stirred until a Fe₂O₃ precipitate was formed at pH 10. The precipitate formed was washed and filtered several times before heating in an oven at 100 °C for 2 h, followed by calcination at 300 °C for 4 h.

Chitosan-Fe₂O₃ was synthesized using the sol-gel method [19]. Initially, 0.24 g of the chitosan powder was dissolved in 30 mL of 2% acetic acid and constantly stirred until a gel was formed. The mixture was added with 30 mL of 0.1 M FeCl3 solution followed by 1 M NaOH until the pH reached 11.2 and stirred at a speed of 520 rpm at 90 °C for 4 h. After forming a reddish-brown product, it was then left for 24 h. The precipitate was decanted, centrifuged, and washed five times with pure water before drying in an oven for 2 h and 30 min at 110 °C. This was followed by calcination at 300 °C for 4 h. The same procedure was conducted on different mass ratios of chitosan and Fe₂O₃, namely (1:2) and (1:3) (w/w). The solids were then characterized using XRD (Shimadzu), SEM-EDX (PhenomProX Desktop), and UV-DRS (Analytic Jena). The best results of the synthesis were used for the photodegradation of MB.

About 30 mL of 0.01 NaNO3 solution was put into each container set at a pH range of 2 to 12 using HCl or NaOH. Furthermore, 0.03 g of the composite was added and stirred using a shaker at 32 °C for 2 h. The mixture was then left for two days, and the final pH was measured using a pH meter from each solution. The pH_pzc value was obtained from the intersection point between the initial and final curves.

The MB degradation study was conducted in batches using a 10-watt UV lamp at pH_pzc conditions. Briefly, 30 mL of MB with concentrations of 1, 2, 3, 4, and 5 ppm were added with 1 mL of 30% (v/v) H₂O₂. About 0.03 g of chitosan-Fe₂O₃ was then added and stirred using a magnetic stirrer at 450 rpm and a temperature of 90 °C into the UV reactor, with an irradiation time of 30, 60, 120, and 180 min. The mixture was centrifuged for 10 min, and the absorbance of the filtrate was measured using a UV-Vis spectrophotometer (Orion™ AquaMate 8000) at a wavelength of 665 nm and analyzed using TOC (Torch Combustion Analyzer, Teledyne tekmar). The percentage (%) of MB degradation can be calculated using the following equation:

where Xo is the initial concentration dye before irradiation and X_t is the concentration dye after irradiation. TOC calculation based on Eq. (1) is as follows:

where C₀ is the initial concentration carbon (before degradation process) and C_t is the final concentration carbon (after degradation process).

Chitosan-Fe₂O₃ composite with various mass ratios of (1:1), (1:2), and (1:3) was characterized using XRD, SEM-EDX, and UV-DRS. The best result was applied to the photodegradation of MB. The photo-catalyst diffractogram of chitosan, Fe₂O₃, and chitosan-Fe₂O₃ composite is shown in figure 1.Figure 1A shows that the typical diffraction peak of chitosan appears at an angle 20 of 19.95°. Foroughnia et al. stated that the typical diffraction peak ranges from 10 to 20° [16], which is consistent with other findings [18]. Furthermore, figure 1B shows the diffraction peak at an angle 20 of 24.13, 33.15, 54.09, 57.59, 62.41, 63.99 and 72.26° with field indexes of (012), (104), (110), (113), (024), (116), (018), (214), (300) and (119), indicating the presence of Fe2O3 compound (JCPDS card No. 33-0664). The diffraction peaks are consistent with previous reports [23].

Figure 1

Figure 1C shows that the diffraction peaks of the chitosan-Fe₂O₃ (1:1) composite appear at an angle 2θ of 24.1, 33.2, 62.2, and 74°. The diffractogram of the chitosan-Fe₂O₃ (1:2) composite in figure 1D shows the presence of a peak angle 2θ at 34.5, 57.4, 64.9 and 75.1°. Furthermore, the peaks at an angle 2θ of 16.9, 32.2, 54.5, and 74.7° appear on the diffractogram of the chitosan-Fe₂O₃ (1:2) composite, as shown in figure 1E. Chitosan-Fe₂O₃ (1:1), (1:2), and (1:3) composite did not specifically indicate the presence of chitosan peaks. This is probably due to a decrease in intermolecular hydrogen bonds since Fe₂O₃ dominates. This condition indicates a good interaction between Fe₂O₃ and the structure of chitosan [24].

The angle 2θ and the intensity peak of each composite are shown in table 1. The composite with a mass ratio of (1:1) showed the highest intensity. Based on the Debye-Scherrer equation, the crystal size of the chitosan-Fe₂O₃ composite with mass ratios of (1:1), (1:2), and (1:3) was 1.13, 2.82 and 8.83 nm, respectively. Therefore, the Fe₂O₃-composite (1:1) was selected for the MB photodegradation application. This is because of its smallest crystal size, high intensity, and the 2θ peak closest to JCPDS card No. 33-0664.

Table 1

Material	2θ (degree)	Peak intensity (cps)	Crystallite size (nm)
Chitosan-Fe2O3 (1:1)	33.2	522	1.13
Chitosan-Fe2O3 (1:2)	34.5	90	2.82
Chitosan-Fe2O3 (1:3)	32.2	173	8.83

Characterization using UV-Vis DRS aims to determine the energy band gap of Fe₂O₃ and chitosan-Fe₂O₃ composite. The bandgap is defined as the distance from the valence to the conduction band of a semiconductor. Table 2 shows the UV-Vis DRS analysis results. The energy band gaps of Fe₂O₃ and chitosan-Fe₂O₃ are 1.93 and 1.41 - 1.64 eV. Vayssieres et al. stated that the energy band gap of pure Fe₂O₃ ranges from 1.93 to 2.20 eV depending on the crystal state and the preparation method [25]. Table 2 shows that the energy band gap of Fe₂O₃ decreases after forming the composite with chitosan. Similar findings have been reported by other studies [26]. According to Fauzi et al., the decrease was associated with a smaller crystal size of the photocatalyst [27]. This leads to an increase in the orbital overlap between the valence and conduction bands. Furthermore, the energy band gap can inversely affect the photocatalytic effectiveness [28].

Table 2

Material	Eg (eV)
Fe2O3	1.93
Chitosan-Fe2O3 (1:1)	1.41
Chitosan-Fe2O3 (1:2)	1.50

The size of the band gap energy value in a composite greatly influences the degradation effect of a dye. The smaller the bandgap energy value, the better the dye degradation effect of the composite because it allows the formation of a large number of charge carriers under light irradiation [29]. A small band gap also provides less relaxation time for recombining electron and hole pairs, so that photodegradation carried out by composites with a small band gap is more favorable for degrading dyes [30]. Based on XRD and UV-DRS characterization, best composite we had is chitosan-Fe₂O₃ (1:1) and was carried out to SEM characterization to see the surface morphology of the composite. The surface morphology of chitosan and chitosan-Fe₂O₃ composite is shown in figure 2.

Figure 2

It shows flakes or sheets that have pores with a magnification of 750x. The surface morphology of the chitosan-Fe₂O₃ composite shows the distribution and aggregation of Fe₂O₃ in chitosan. Many pores and folds are found in the composite, which can support photocatalytic activity. Similar morphology features were reported by Mendis et al. at synthetic material chitosanilmenite sand which was used to photodegradation of MB dye with presenced aggregated spherical nanoparticles [31]. The EDX analysis results of chitosan and chitosan-Fe₂O₃ composite are shown in Table 3. It can be seen that the element Fe increases from 0 to 29.96% after forming a composite with chitosan. This indicates that Fe₂O₃ has successfully modified chitosan.

Table 3

Element	Atomic (wt%)
	Chitosan	Chitosan-Fe2O3
C	42.88	27.19
O	48.68	25.27
Na	8.44	9.03
Cl	-	7.49
Fe	-	29.96
Zn	-	1.06

The pHpzc is a surface state of neutral charge, pH_pzc greatly affects the adsorption of the dye with the catalyst surface on photodegradation [32]. The resulting pH_pzc data will be used in the MB photodegradation process. The pH_pzc of chitosan-Fe₂O₃ (1:1) composite can be seen in figure 3, which shows that the pH_pzc of the chitosan-Fe₂O₃ (1:1) composite is at pH 8. The catalyst's surface will be negative when pH>pH_pzc hence it can degrade cationic dyes and vice versa [33]. In this study, MB dye is cationic, and its pH is set above pH_pzc before the photodegradation process is conducted. Since the composite surface is negative at high pH, it attracts the cationic dyestuff due to electrostatic interaction and the dye is adsorbed onto the composite surface.

Figure 3

MB degradation in this research was done using composite chito-san-Fe₂O₃ (1:1). This chitosan-Fe₂O₃ composite (1:1) was used because the characterization is the best result from the three composite generated materials. According to Zhang et al., as the band gap energy is smaller in semiconductors, they can capture more photons for adsorption, so the theory will give the best result for chitosan-Fe₂O₃ composite on degradation of MB [34]. The effect of irradiation time on the decrease in MB concentration at a dye of 5 ppm is shown in figure 4. It shows that the highest and lowest percentage of decrease in MB concentration was achieved at an irradiation time of 180 and 30 min by 90.00 and 76.84% (gray line). Observations with a similar trend have been reported by Fassi et al. in a comparative study on the photodegradation of bromocresol green using Fe(II)/H₂O₂ and UV [35]. This condition shows that the degradation time is directly related to the contact between the photon rays and the chitosan-Fe₂O₃ composite, leading to a greater decrease in MB concentration. The presence of H₂O₂ acts as a reactant capable of producing hydroxyl radicals («OH) which can degrade MB more effectively [12].

Figure 4

Control studies involving the degradation of MB without UV irradiation and without adding H₂O₂ (comparator I, red line) and without UV irradiation but adding H₂O₂ (comparator II, blue line) were carried out. Based on figure 4, the adsorption process without H₂O₂ (red line) showed the lowest percentage of decrease in MB concentration as compared to the UV irradiation process with H₂O₂ (blue line). The comparator I showed a degradation of 61.07%, while the comparator II was 68.26%. The difference between comparator II and photodegradation in the presence of H₂O₂ was 21.74%. This indicates that the addition of H₂O₂ greatly affects the MB degradation process.

The effect of the initial MB concentration on the decrease in MB concentration at an irradiation time of 180 min is shown in figure 5. It shows that the percentage decrease in MB concentration increased as the initial MB concentration increased. According to Shaban et al., the addition of H₂O₂ is very influential in photodegradation because it is a reactant that produces «OH radicals for degrading dyes [36]. The «OH radicals produced are directly proportional to the amount of degraded MB dye. Furthermore, the combination of H₂O₂ with Fe³⁺, which acts as a homogeneous Fenton catalyst, can form «OH radicals. The Fenton oxidation reaction can be written based on Eq. (3) as follows:

Figure 5

The percentage of decrease in MB concentration was also achieved in comparator I through an adsorption process. The concentration of the dye determines the amount adsorbed on the surface of the composite [37]. A considerable increase in dye drop was also observed in the photodegradation trial without H₂O₂ (comparator II) at the initial MB concentration of 1 to 2 ppm. However, at the initial MB concentration of 3 ppm, the decrease of MB dye did not occur effectively. According to Jesse et al., the higher the dye concentration, the more molecules are adsorbed on the catalyst's surface [38]. Consequently, they will inhibit the interaction between the catalyst and UV light. The formation of hydroxyl radicals on the catalyst's surface becomes constant and tends to act as an adsorbent at high concentrations.

Based on figure 5, the initial MB concentration of 5 ppm shows equilibrium with the adsorption and photodegradation processes without H₂O₂ of 55.28% and 69.60%, respectively. A significant difference in the percentage of decrease was observed with the addition of H₂O₂. The photodegradation process is higher than the adsorption because the UV light used has a role in producing «OH radicals, which can degrade MB. Without UV irradiation, the dye can only undergo the adsorption process [39]. Therefore, the effectiveness of photodegradation will increase with the addition of H₂O₂ to produce «OH radicals. In conclusion, the addition of H₂O₂ can affect the effectiveness of photodegradation under the influence of the initial MB concentration. This is based on the photo-Fenton process, there are two reactions, namely Fenton and photocatalysis, where both reactions produce «OH radicals.

In figure 6 there are two reactions at once, the one where Fe²⁺ ions reacts with H₂O₂ using additional UV light to obtain Fe³⁺ ions and «OH radicals, and the other one where these «OH radicals will degrade MB dye. When there is electron excitation due to the photocatalyst reaction, there is a gap that can be entered by fenton radicals. The photocatalyst process also produces «OH radicals which results in an increase in the number of radicals obtained, so that degraded MB also increases along with increasing the number of radicals. After degradation of MB, the adsorption process occurs by chitosan as an adsorbent.

Figure 6

TOC analysis was conducted to determine the quantitative degradation of organic compounds based on absorbance. Figure 7 shows the test results before and after MB photodegradation. The decrease in absorbance was due to the decomposition process using chitosan-Fe₂O₃ (1:1) composite, directly related to C concentration in MB from 0.6493 to 0.0506 ppm (table 4). Based on these results, it shows that the composite could degrade MB and the percentage of degradation is 92.24%. Fe₂O₃ attach to chitosan improves the degradation activity of MB. Selpiana et al. in their research on the photocatalytic effect of MB degradation using Fe₂O₃ on heterojunction composite, state that Fe₂O₃ increases the capacity of the composite as a dye degrader by synergizing with other composites to reduce the energy band gap, so that photons will be captured more [40].

Before After

Figure 7.

Table 4

Methylene blue	Adjust (Abs)	Concentration (ppm)
Before degradation	48.89	0.6493
After degradation	47.15	0.0506