Fabrication of Aluminum-Based Hybrid Nanocomposite for Photocatalytic Degradation of Methylene Blue Dye: A Techno-Economic Approach

: Al 2 O 3 –MgO nanocomposite was synthesized using the co-precipitation method for photo-catalytic degradation of methylene blue (MB) dye under UV–Vis light. Box–Behnken design (BBD) in response to surface methodology (RSM) was used for the optimization and modelling of the photocatalytic degradation of MB dye. An analysis of variance (ANOVA) revealed a quadratic model with an R 2 of 0.9880. MB removal followed the ﬁrst-order kinetic model (R 2 = 0.9520, k 1 = 0.007 min − 1 ). Economic feasibility study at optimized conditions showed that the wastewater treatment cost is USD 16.50/m 3 and the payback period is 3.17 years.


Introduction
Methylene blue (MB) is a highly toxic cationic dye widely used in textile, pharmaceutical and printing industries. Exposure to MB dye results in physiological disorders, leading to acute and chronic illnesses such as vomiting, eye irritation and jaundice [1,2]. The discharge of MB-laden wastewater has detrimental effects on aquatic biota. MB changes the aesthetics of aquatic systems, leading to low sunlight available for photosynthesis, resulting in stunted growth and the death of aquatic plants. MB dye forms complex products in aqueous systems, leading to low dissolved oxygen and the death of aquatic organisms, resulting in a loss of biodiversity in marine ecosystems [2].
MB dye can be removed by conventional wastewater treatment methods such as adsorption, coagulation and phytoremediation [1]. However, the major drawback of conventional systems is the formation of secondary pollutants. Additionally, there is a high cost for the regeneration of adsorbents and disposal of pollutant-laden sludge [3].
Currently, advanced oxidation processes (AOPs) have been adopted for the degradation of organic pollutants by utilizing highly reactive hydroxyl radicals [4]. AOPs such as photocatalysis and photo-Fenton reaction can degrade MB dye into smaller and non-toxic molecules [3]. Due to the high stability and efficiency of TiO 2 and ZnO nanosemiconductors, they are widely used in the photodegradation of organic dyes [4]. Al 2 O 3 has high thermal stability, a large surface area and well-defined nanopore structures [5]. It is widely used in sensors, energy storage and pharmaceutical applications [6]. Al 2 O 3 has significant optical properties; hence, it can be used as a photocatalyst [5]. Al 2 O 3 can be synthesized from abundant waste aluminum cans [7], resulting in lower economic costs in comparison to TiO 2 and ZnO.

Photocatalyst Synthesis
The photocatalyst was prepared via a simple co-precipitation technique [8] as shown in Figure S1. Briefly, 0.2 M Al(NO 3 ) 3 ·9H 2 O was mixed with 0.2 M MgCl 2 ·6H 2 O, and 1 M NaOH was added dropwise until pH was 11. A white precipitate formed with constant stirring for 30 min. The precipitate was washed with distilled water, filtered, dried for 12 h at 105 • C and finally calcined at 800 • C for 3 h to obtain Al 2 O 3 -MgO nanocomposite.

Characterization
The optical band gap energy was measured using a Jasco V-570 UV-DRS spectrophotometer (Tokyo, Japan) in the range of 220-2000 nm. The point of zero charge was determined using the batch equilibrium method [4].

Experimental Setup
A stock solution of 50 ppm of methylene blue solution was prepared by dissolving 50 mg of methylene blue dye in 1L of distilled water. The required solutions were prepared using the stock solution. Photodegradation of MB dye was carried out in a photoreactor with a 400 W metal halide lamp. The pH of the MB dye solution was adjusted using 1 M NaOH or 1 M H 2 SO 4 . The experiment was initially run in the dark for 60 min to achieve adsorption-desorption equilibrium. The concentration of MB after adsorption (C o ) was measured at 664 nm using a UV spectrophotometer (Jasco V-630 spectrophotometer (Tokyo, Japan). The light was turned on to initiate photocatalytic degradation. After photodegradation, samples of MB were withdrawn, quenched with methanol and centrifuged at 6000 rpm. The final concentration of MB (C f ) was measured. MB removal was calculated using Equation (1): The experimental conditions in Table 1 are derived from BBD in RSM.

Statistical Analysis
Experimental results were analyzed by response surface methodology (RSM) and one-way ANOVA analysis in Design Expert 13.

Material Characterization
Al 2 O 3 -MgO was characterized using UV-DRS. Figure 1a shows the plot of absorbance and wavelength for Al 2 O 3 -MgO. The maximum absorbance of Al 2 O 3 -MgO was 290 nm.

Statistical Analysis
Experimental results were analyzed by response surface methodology (RSM) a one-way ANOVA analysis in Design Expert 13.

Material Characterization
Al2O3-MgO was characterized using UV-DRS. Figure 1a shows the plot of abso ance and wavelength for Al2O3-MgO. The maximum absorbance of Al2O3-MgO was 2 nm.
where h is Planck's constant, v is the frequency of the photon, B is constant and Eg is band gap energy of Al2O3-MgO; γ is equal to 2 for indirect band gap transition [10]. Fr the Kubelka-Munk plot in Figure 1a, the Eg for Al2O3-MgO is 3.50 eV. The Eg for p Al2O3 is reported to be 5.97 eV [8]. Al2O3-MgO has a lower Eg than pure Al2O3 due to presence of defects and heterojunction in the Al2O3-MgO nanocomposite [6,11]. Figure 2a shows the photodegradation tests of 11 ppm MB dye with pH, time a photocatalyst dosage as 7, 180 min and 500 mg/L, respectively. Pure Al2O3 has a pho degradation efficiency of 43.57%, whereas Al2O3-MgO has 72.72%. Al2O3-MgO ha lower band gap energy than Al2O3 and hence a higher photocatalytic activity.

Initial Photocatalyst Tests and Effect of Operational Parameters
where h is Planck's constant, v is the frequency of the photon, B is constant and E g is the band gap energy of Al 2 O 3 -MgO; γ is equal to 2 for indirect band gap transition [10]. From the Kubelka-Munk plot in Figure 1a, the E g for Al 2 O 3 -MgO is 3.50 eV. The E g for pure Al 2 O 3 is reported to be 5.97 eV [8]. Al 2 O 3 -MgO has a lower E g than pure Al 2 O 3 due to the presence of defects and heterojunction in the Al 2 O 3 -MgO nanocomposite [6,11]. The effect of pH is shown in Figure 2b. As pH increased from 3 to 11, MB removal increased from 0% to 57%. In acidic pH, MB exists as a neutral molecule below its pKa value of 3.8 [12]. The point of zero charge (pH zc ) of Al 2 O 3 -MgO is 10.04, as shown in Figure S2. In acidic conditions, the surface of Al 2 O 3 -MgO is positively charged and MB molecules have low adsorption on the catalyst surface, leading to a low photodegradation efficiency. MB dye exists as cations above the pKa value. As the pH approaches the pH zc , the repulsive force against MB cations is reduced [13]. In alkaline conditions, above the pH zc , the surface The effect of pH is shown in Figure 2b. As pH increased from 3 to 11, MB removal increased from 0% to 57%. In acidic pH, MB exists as a neutral molecule below its pKa value of 3.8 [12]. The point of zero charge (pHzc) of Al2O3-MgO is 10.04, as shown in Figure  S2. In acidic conditions, the surface of Al2O3-MgO is positively charged and MB molecules have low adsorption on the catalyst surface, leading to a low photodegradation efficiency MB dye exists as cations above the pKa value. As the pH approaches the pHzc, the repulsive force against MB cations is reduced [13]. In alkaline conditions, above the pHzc, the surface of Al2O3-MgO is negatively charged and attracts the MB cations. This leads to an uptake of photogenerated holes and electrons, leading to higher photodegradation efficiency. Figure 2c shows the effect of time on the photodegradation of MB dye. MB removal increased from 54% at 60 min to 58% at 180 min. At constant photocatalyst dosage, more electrons and holes are generated by Al2O3-MgO as irradiation time increases. This leads to the production of more reactive radical species, leading to an increment in the photocatalytic degradation efficiency [14].

Initial Photocatalyst Tests and Effect of Operational Parameters
The effect of photocatalyst dosage is shown in Figure 2d. MB removal increased from 48% at 200 mg/L to 58% at 1000 mg/L. An increasing Al2O3-MgO dosage leads to the generation of more active sites and subsequently leads to a higher photodegradation efficiency for MB degradation [11]. However, a high Al2O3-MgO dosage causes agglomeration of particles. This results in low photon uptake by Al2O3-MgO and a low generation of electrons and holes, and hence, a decrease in MB photodegradation efficiency [15].
The effect of the initial MB dye concentration is shown in Figure 2e. MB removal was reduced from 57% to 0% as the initial MB dye increased from 10 to 100 ppm. As the con-  Figure 2c shows the effect of time on the photodegradation of MB dye. MB removal increased from 54% at 60 min to 58% at 180 min. At constant photocatalyst dosage, more electrons and holes are generated by Al 2 O 3 -MgO as irradiation time increases. This leads to the production of more reactive radical species, leading to an increment in the photocatalytic degradation efficiency [14].
The effect of photocatalyst dosage is shown in Figure 2d. MB removal increased from 48% at 200 mg/L to 58% at 1000 mg/L. An increasing Al 2 O 3 -MgO dosage leads to the generation of more active sites and subsequently leads to a higher photodegradation efficiency for MB degradation [11]. However, a high Al 2 O 3 -MgO dosage causes agglomeration of particles. This results in low photon uptake by Al 2 O 3 -MgO and a low generation of electrons and holes, and hence, a decrease in MB photodegradation efficiency [15].
The effect of the initial MB dye concentration is shown in Figure 2e. MB removal was reduced from 57% to 0% as the initial MB dye increased from 10 to 100 ppm. As the concentration of MB dye increases, more MB molecules are adsorbed on the surface of Al 2 O 3 -MgO. This limits the influx of photons on the photocatalyst surface, resulting in a low generation of electrons and holes [11]. At constant Al 2 O 3 -MgO dosage and light radiation, the number of reactive radicals generated is insufficient for MB photodegradation. Intermediates will also compete with the parent MB molecule for radicals, leading to a lower photodegradation efficiency. Figure S3 shows the 3D contour plots for the interaction of parameters. Each parameter contributes to achieving MB photodegradation.

Model Validation and Kinetics
The optimized conditions of pH, time, photocatalyst dosage and initial MB concentration were 11, 115.6 min, 996.846 mg/L and 10 ppm, respectively, resulting in an MB dye removal of 57.82%, as shown in Figure S4a. The verification experiment at optimized conditions revealed that MB removal was 59.20%, as shown in Figure S4b, with an error of 1.68%. The photodegradation of MB dye with Al 2 O 3 -MgO followed first-order kinetics with an R 2 of 0.9520 and a k 1 of 0.007 min −1 , as shown in Figure S5.

Suggested Removal Mechanism
The mechanism for MB degradation by Al 2 O 3 -MgO is shown in Figure S6. It is assumed that MB degradation is due to the attack of generated hydroxyl (OH•) and superoxide (O 2− •) radicals. The radicals are generated when photons of light are incident on Al 2 O 3 -MgO, leading to the excitation of electrons from the valence band (VB) to the conductance band (CB). This leads to the generation of holes (h + VB ) and electrons (e − CB ). Hydroxyl ions from the water will then react with the holes to produce OH•. Oxygen present in the MB solution reacts with electrons to produce O 2− •. The radicals will react with MB dye molecules, resulting in degradation products, carbon dioxide and water. Equations (4)-(8) summarize the degradation mechanism [10,12]:

Economic Evaluation
An economic evaluation for the synthesis of Al 2 O 3 -MgO and its application in the photodegradation of MB dye was conducted. The textile wastewater treatment plant was assumed to be treating 80 m 3 of wastewater per day under optimized conditions. The annual operating time was 300 days. Table S2 summarizes the economic evaluation. The amortization cost (A) was calculated using Equation S5 [15] for a period of 25 years at an interest rate of 6% per annum and was found to be USD 365,765.78. The annual cost (AC) of treating textile wastewater cost per m 3 was evaluated using Equation (S6). For 300 working days, the AC was USD 16.50/m 3 . The revenue generated from selling the Al 2 O 3 -MgO nanocomposite was calculated as USD 3.25 for treating 1 m 3 of wastewater. The estimated cost after wastewater reuse and pollution reduction was USD 0.46/m 3 and USD 0.16/m 3 , respectively. The payback period was 3.17 years, as shown in text S1.

Conclusions
An Al 2 O 3 -MgO nanocomposite was prepared using the co-precipitation method. UV-DRS analysis showed that the band gap energy was 3.50 eV. Al 2 O 3 -MgO nanocomposite was used in the photocatalytic degradation of MB dye. The effect of operating parameters was investigated using RSM. The RSM model had an R 2 of 0.9880, indicating a good Eng. Proc. 2023, 37, 87 6 of 7 prediction of MB removal. MB photocatalytic degradation using Al 2 O 3 -MgO followed first-order kinetics. Economic estimation revealed that the wastewater treatment cost was USD 16.50/m 3 with a payback period of 3.17 years.

Data Availability Statement:
The data presented in this study are available upon request from the corresponding author.