La-supported SnO₂–CaO composite catalysts for efficient malachite green degradation under UV–vis light

Abstract

This study presents the development and optimization of La@SnO₂–CaO composite catalysts for efficient photocatalytic degradation of malachite green dye in aqueous solutions under UV–vis light irradiation. The catalysts were prepared via conventional incipient-wetness impregnation and thoroughly characterized using advanced analytical techniques, including X-ray diffraction, Fourier transform infrared spectroscopy, UV–vis diffuse reflectance spectroscopy, N₂ adsorption–desorption analysis, and scanning electron microscopy. To optimize photodegradation efficiency, the effects of three independent factors were systematically investigated using response surface methodology: Temperature, pH, and Sn/Ca molar ratio. Our results reveal optimal conditions for maximum dye degradation: pH 7, Sn/Ca molar ratio of 0.33, and a process time of 35 min, resulting in a maximum photodegradation efficiency of 98.80% for malachite green dye. Notably, visible light exhibited a more pronounced effect on dye degradation compared to UV light over time, with visible light achieving 25% greater dye removal after 60 min of illumination. Furthermore, the catalyst showed excellent recyclability, retaining 85% of its initial activity after five consecutive cycles. These findings contribute significantly to the development of sustainable methods for dye removal from wastewater and highlight the potential of La@SnO₂–CaO composite catalysts in environmental remediation processes, particularly in treating textile industry effluents.

Introduction

Dyes represent a critical class of organic compounds that significantly affect our environment and public health. These colorants, produced in vast quantities by various industries, pose substantial challenges to ecosystems and human well-being. Dyes are primarily emitted as water pollutants through industrial processes across several sectors: Textile manufacturing, Leather treatment, Printing industries, and Food processing. These industries collectively produce a wide array of dye types, resulting in extensive environmental contamination. The presence of dyes in water bodies raises serious concerns regarding both human health and ecological balance. Exposure to dye-contaminated water may lead to cardiovascular issues, including increased heart rate and blood pressure. Neurological symptoms such as headaches and eye irritation can occur. There are potential teratogenic effects that could harm fetal development during pregnancy. Dyes may possess carcinogenic properties, acting as possible cancer-causing agents. Reproductive abnormalities affecting fertility and reproductive systems are also a concern. These effects underscore the critical need for effective methods to remove dyes from wastewater and protect both human health and the environment. These adverse effects underscore the need for effective dye management strategies [1,2,3]. Dyes can be categorized based on both chemical structure and application, such as azo, anthraquinone, acridine, oxazine, diphenylmethane, triphenylmethane, phthalocyanine, azine, indigo, nitroso, nitro, methine, thiazine, and xanthene dyes (as chemical structure-based classification) and acid, basic, direct, vat, mordant, fiber-reactive, and disperse dyes (as application-based classification) [2].

Among these classifications, basic dyes stand out due to their high reactivity. They form colored cations in aqueous solutions. These positively charged species contribute to their water solubility. Their cationic nature allows them to interact strongly with negatively charged surfaces, potentially enhancing their environmental persistence [2].

Understanding the complex nature of dye pollution is crucial for developing effective strategies to mitigate its impacts on both human health and the environment. This comprehensive introduction sets the stage for exploring various aspects of dye pollution and potential remediation approaches [2].

Malachite green (MG), a type of cationic dye, poses significant threats to ecological environments and living organisms due to its non-biodegradable azo groups and toxic aromatic compounds [1, 4]. The increasing concern about water quality underscores the critical importance of dye removal in wastewater treatment processes [5]. Photocatalytic degradation has emerged as a practical and environmentally friendly method for dye removal from wastewater. Its advantages include simplicity of apparatus and process, potential for using solid adsorbents with high stability, and primary energy source provided free by sunlight [6, 7]. The mechanism involves an adsorption-based process that utilizes reactive oxygen species on semiconductor surfaces.

Various semiconductors have been employed in photocatalytic degradation processes, such as TiO₂, ZnO, WO₃, Bi₂O₃, CeO₂, CdS, BiVO₄, g-C₃N₄, etc. [8,9,10]. Semiconductors are chemically stable and inherently non-toxic and provide suitable surfaces for reactive oxygen species [8, 9]. Tin (IV) oxide (SnO₂) is a prominent n-type semiconductor among metal oxides. Its notable features include a wide bandgap (~ 3.6 eV), high oxidation potential, chemical inertness, corrosion resistance, economic efficiency, and environmental protection benefits. These attributes make SnO₂ a versatile material for various applications, particularly in semiconductor technologies and environmental remediation processes. SnO₂'s unique combination of properties makes it an attractive candidate for photocatalytic applications, particularly in dye degradation processes. Its stability and non-toxicity contribute to its potential for large-scale wastewater treatment systems [8, 9]. However, SnO₂, a promising photo catalyst, faces significant limitations in industrial applications. These inherent drawbacks hinder the widespread adoption of SnO₂ as an industrial photocatalyst [8, 9]. One effective strategy to address these limitations is the design of composite photo catalysts. By combining SnO₂ with other materials, researchers can enhance its photocatalytic performance.

Given the growing concern about pollution and waste management, researchers have turned to recycled materials for catalyst development. Eggshells, a readily available renewable resource, offer several advantages: Abundance, low cost, and potential source of calcium compounds [11,12,13].

Eggshell waste can be processed to yield calcium oxide (CaO), which possesses desirable properties: Thermal stability, mechanical strength, radiation resistance, and ease of recycling [11,12,13]. The band gap energy reported for this compound is 7.1 eV, which indicates that there are similar problems to SnO₂ in the photocatalytic process [14, 15]. Recent studies have explored CaO as a component in various photocatalytic systems, such as Ag@CaO [16], Pd@CaO [12], NiO/CaO [17], Bi₂O₃/CaO [18], CaO/SrTiO₃ [19], Ag@ZnO/CaO [20], etc.

Building upon previous research [11], this study aims to combine SnO₂ with CaO to create a novel lanthanum-supported SnO₂/CaO composite photo catalyst. The rationale behind this approach includes different band energies between SnO₂ and CaO, enhanced charge separation, and improved surface modification for redox reactions [8]. The developed SnO₂/CaO composite photo catalyst will be tested for its efficacy in degrading malachite green dye under UV–visible light irradiation. This process involves photocatalytic oxidative decomposition of MG, utilization of UV–visible light energy, and investigation of optimal conditions for maximum dye degradation.

By combining the strengths of SnO₂ and CaO, this study seeks to overcome the limitations of pure SnO₂ photo catalysts and develop a more efficient system for industrial-scale dye removal applications.

Experimental section

Materials and experiments

All reagents used, including tin (IV) chloride (SnCl₄), lanthanum (III) nitrate hexa hydrate (La(NO₃)₃⋅6H₂O), and ammonia (NH₃), were of analytical grade and supplied by Merck. Malachite green dye with chemical formula of [C₆H₅C(C₆H₄N(CH₃)₂)₂]Cl was purchased from Sigma-Aldrich (Scheme 1). These chemicals were utilized without further purification.

Scheme 1

The structure of malachite green dye

Characterization techniques employed included:

X-ray diffraction (XRD): Conducted using a Philips PW1730 diffractometer equipped with Cu K_α radiation (λ = 1.54 Å) filtered through a nickel foil. Instrument parameters: Cu anode, 40 kV voltage, 30 mA current, 0.05°/s scanning rate.

Scanning electron microscopy (SEM): Performed using a TESCAN VEGA3 FE-SEM to examine catalyst morphology. Electron beam conditions: 10 keV acceleration potential.

Fourier transform infrared spectroscopy (FTIR): Analyzed using a Nicolet iS10 spectrophotometer from Thermo Scientific over the wavenumber range of 400–4000 cm⁻¹.

Ultraviolet–visible diffuse reflectance spectroscopy (UV–Vis DRS): Conducted with an Evolution 300 spectrophotometer from Thermo Scientific.

Surface area and porosity analysis: Employed a BET BELSORP Mini II instrument to generate N₂ adsorption–desorption isotherms.

Catalyst preparation

Calcium oxide (CaO) was synthesized from eggshells via the following process:

Eggshells were washed repeatedly with warm water and dried at room temperature for 1 day. Subsequent washing with deionized water followed by drying at 200 °C for one hour yielded CaO powder. The resulting solid was gently pulverized using a mortar and pestle prior to further processing. SnO₂/CaO composites with varying Sn/Ca ratios were prepared through incipient-wetness impregnation:

CaO powder was immersed in an aqueous solution of tin (IV) chloride (SnCl₄). The pH of the mixture was adjusted to 9 by gradually adding ammonia solution while stirring. The solution was heated to dryness at 60 °C under stirring conditions. The resulting solid was dried overnight at 110 °C and calcined at 500 °C for six hours. Lanthanum doping of SnO₂/CaO-based catalysts was achieved via impregnation using lanthanum nitrate hexahydrate (La(NO₃)₃.6H₂O):

Lanthanum nitrate dissolved in ethanol (1 wt%) was added dropwise to the aqueous mixture. The resulting solution was stirred at 25 °C for two hours, then at 60 °C for an additional two hours.

The powdered solid was dried overnight at 110 °C. Final calcination occurred at 300 °C for four hours. Catalysts with different Sn/Ca molar ratios (0.33, 0.5, 1, and 2) were designated as LSC-0.33, LSC-0.5, LSC-1, and LSC-2, respectively.

Photocatalytic test

The photocatalytic properties of LSC-x catalysts were investigated using malachite green dye as an impurity. Specifically 25 mg of LSC-x photocatalyst was weighed and used to enhance the photodegradation reaction. A 10 ppm aqueous solution of MG (30 mL) was prepared and exposed to both UV (Hg lamp 500 W) and visible light (sunlight) sources. The suspension was initially stirred in the dark for 60 min to reach adsorption–desorption equilibrium. Subsequently, the mixture underwent 5 min of UV–Vis irradiation over a 35-min period at three pH levels (3, 7, and 11) and 25 °C. Samples were collected after treatment, centrifuged to separate the catalyst from the solution, and analyzed via UV–Vis spectroscopy at a wavelength of 617 nm. The photodegradation efficiency (PDE) of MG was calculated using the following equation [21]:

$$PDE\left(\%\right)=1-\frac{{A}_{t}}{{A}_{0}}\times 100$$

(1)

where A₀ is the absorbance amounts of the initial concentration of MG solution and A_t is the absorbance of MG concentration at different times under UV–Vis light.

Response surface methodology (RSM) experiments

Response Surface Methodology (RSM) was employed to identify the optimal conditions for maximizing the photocatalytic degradation efficiency of Malachite Green (MG). This approach utilizes statistical methods to analyze the effects of various factors on a response variable, allowing for the optimization of complex processes.

The RSM analysis was conducted using Design Expert software, version 11, which implements the Central Composite Design (CCD) method. This experimental design technique is particularly effective for optimizing processes with multiple variables. Three key factors were chosen as independent variables in this study:

Solution pH: The acidity/basicity level of the MG solution, measured on a scale from 0 to 14.

Molar Ratio of Sn/Ca: The proportion of tin (Sn) to calcium (Ca) in the LSC-x photocatalysts. This ratio can significantly impact the photocatalytic activity of the material.

Process Time: The duration of exposure to UV–Vis light, measured in minutes.

These variables were carefully selected based on their potential impact on the photocatalytic degradation process. The primary response variable in this experiment was the Photo degradation Efficiency (PDE) of MG. PDE is defined as the percentage decrease in the concentration of MG molecules due to photocatalysis. It serves as a crucial indicator of the effectiveness of the photocatalytic process. A total of 20 experiments were conducted to evaluate the influence of the three independent variables on the PDE. These experiments spanned a range of values for each factor, allowing for comprehensive analysis of their interactions. The resulting data from these experiments were subjected to rigorous statistical analysis using Design Expert software. By applying RSM, this study aimed to uncover the most influential factors among those tested and determine the optimal settings for achieving maximum MG photodegradation efficiency. This information will be invaluable for further research and development of efficient photocatalytic systems for environmental remediation.

Results and discussion

Fourier transform infrared spectroscopy (FTIR)

The FTIR spectra of the LSC-x catalysts (Fig. 1) reveal several important features:

Hydroxyl Group Absorption: A broad absorption peak centered around 3400 cm⁻¹ is observed. This band corresponds to the vibrational modes of hydroxyl (-OH) groups associated with water molecules within the solid matrix. The presence of this peak suggests that the catalysts contain moisture, likely absorbed from the environment or introduced during sample preparation [21].

Fig. 1

The FTIR spectra of the LSC-x composite catalysts

Calcium Oxide (∆ CaO) Peaks: Characteristic absorption bands are seen at approximately 882 and 1390 cm⁻¹. These peaks are indicative of the presence of calcium oxide in the catalyst structure. CaO is a common component in composite photocatalysts, contributing to their structural integrity and potentially influencing their photocatalytic properties [21].

Tin Oxide (o SnO₂) Peak: A distinct absorption band appears at 592 cm⁻¹. This peak is characteristic of tin dioxide, a semiconductor material known for its photocatalytic activity. SnO₂ is a crucial component in the LSC-x catalysts, responsible for the photocatalytic degradation of organic pollutants.

Lanthanum Nitrate (□ La(NO₃)₃) Peaks: Absorption bands are observed at 1060 and 1610 cm⁻¹. These peaks confirm the presence of lanthanum nitrate in the catalyst composition. La(NO₃)₃ is incorporated to modify the electronic properties of the catalyst, enhancing its photocatalytic efficiency.

Interestingly, the intensity of these characteristic peaks decreases in the composite catalysts as the Sn/Ca molar ratio decreases. This observation can be attributed to several factors. Intercomponent interaction: As the proportion of SnO₂ relative to CaO decreases, the interaction between the different components of the composite becomes weaker. This reduced interaction leads to a decrease in the vibrational coupling between the functional groups, resulting in diminished peak intensities. Structural modification: With decreasing Sn/Ca ratio, the overall structure of the composite may change, affecting the accessibility and orientation of the functional groups. This modification could alter the infrared absorption characteristics of the individual components. Surface coverage: As the SnO₂ content diminishes, the surface coverage of the catalyst may change, potentially altering the local environment around the functional groups and thus affecting their vibrational frequencies and intensities.

UV–vis diffuse reflectance spectroscopy (UV–vis DRS)

Figure 2 presents the UV–vis/DRS spectra of the LSC-x composite catalysts. According to research reports [22], the observed absorption edge at approximately 310 nm corresponds to the band gap of SnO₂.

Fig. 2

The UV–vis/DRS of the LSC-x catalysts

After the deposition of CaO and La, the absorption edge shifts to about 450 nm. This shift indicates a significant change in the optical properties of the composite catalyst compared to pure SnO₂. The broad peak visible in all spectra can be associated with the interaction between the surface metal oxides and SnO₂ in the composite catalyst [23]. This interaction likely leads to changes in the electronic structure and optical properties of SnO₂, resulting in the observed shift of the absorption edge. The initial absorption edge at 310 nm corresponds to the band gap energy of pure SnO₂, which is typically around 3.8 eV. The shift of the absorption edge to 450 nm after CaO and La deposition suggests a reduction in the band gap energy of the composite material compared to pure SnO₂. The broad peak in the spectra indicates the formation of new electronic states or modifications to the existing ones due to the interaction between the metal oxides and SnO₂. This interaction may lead to improved photocatalytic properties of the composite material, as changes in the electronic structure often result in enhanced light absorption and charge carrier generation capabilities.

Scanning electron microscopy (SEM)

Figure 3 presents SEM images of the La-loaded SnO₂–CaO composite catalyst with a Sn/Ca molar ratio of 0.33 at various magnifications. While other catalysts exhibit similar morphologies, the morphology of this particular catalyst was chosen for discussion due to its representative nature. The SEM images reveal that the morphology of the composite material is completely amorphous and irregularly plate-shaped. This unique structure appears to result from the deposition process itself, specifically the interaction between SnO₂, CaO, and La during the formation of the composite catalyst [24]. The lack of crystalline structure suggests that the particles have undergone significant changes during the oxide deposition process. This could indicate a high degree of surface reactivity and potential photocatalytic activity. The irregular shape of the plates may contribute to increased surface area, allowing for better light absorption and charge carrier generation.

Fig. 3

The SEM images of the LSC-0.33 catalyst

The observed morphology is likely the result of particle destruction and subsequent agglomeration during the oxide deposition process. This phenomenon often occurs in composite materials formed through wet chemical methods. The amorphous and irregular structure may influence the photocatalytic performance of the composite material. While an amorphous structure can sometimes lead to improved photocatalytic efficiency, it also increases the risk of particle aggregation, which could negatively affect the material's stability and performance over time.

Surface area and pore size characteristics

Figure 4a, b presents the results of the Brunauer–Emmett–Teller (BET) surface area analysis and the Barrett-Joyner-Halenda (BJH) pore volume distribution for the LSC-0.33 composite catalyst. This figure shows the N₂ adsorption–desorption isotherm for the LSC-0.33 composite catalyst. According to the International Union of Pure and Applied Chemistry (IUPAC), the prepared catalysts exhibit an isothermal curve of type III, indicating the minimum interaction between the adsorbate and adsorbent [21, 25]. This classification is crucial because it provides insights into the material's surface characteristics and potential photocatalytic activity.

Fig. 4

a BET surface area and b BJH pore volume distribution of LSC-0.33 catalyst

The BET method measures the total surface area of the material by analyzing the amount of nitrogen adsorbed at different pressures. Higher surface areas typically indicate better photocatalytic performance, as there is more reactive surface area available for light absorption and chemical reactions. The BJH analysis provides information about the size distribution of pores within the material. In this case, all catalysts exhibit average pore diameters between 2 and 6 nm, which falls within the mesoporous range [25].

Among the tested catalysts, LSC-0.33, LSC-0.5, LSC-2, and LSC-1 each have the largest surface area, respectively. This variation suggests that the composition ratio of Sn/Ca affects the resulting surface properties of the composite catalyst. The highest surface area observed in LSC-0.33 might be due to optimal particle dispersion and minimal agglomeration during the synthesis process. This could lead to improved photocatalytic efficiency compared to other samples. The narrow range of pore sizes (2–6 nm) indicates uniformity in the material's structure. This uniformity could contribute to consistent photocatalytic activity across the entire sample.

X-ray diffraction (XRD)

Figure 5 presents the X-ray Diffraction (XRD) pattern of the LSC-0.33 catalyst. Due to the similarity in patterns observed for all prepared catalysts, only one representative pattern is shown in this figure. The primary difference noted among these patterns is the decrease in diffraction intensity.

Fig. 5

The XRD pattern of the LSC-0.33 catalyst

The observed XRD pattern reveals crucial information about the crystalline structure of the LSC-0.33 composite catalyst. The XRD pattern clearly shows the peaks corresponding to the tetragonal rutile phase (ο) structure of SnO₂. The characteristic peaks for SnO₂ are observed at 26.69°, 33.34°, 51.29°, and 64.99°, corresponding to the (110), (101), (211), and (112) planes, respectively [24]. These peak positions confirm that the SnO₂ component of the composite material exhibits a tetragonal rutile phase structure, which is consistent with literature reports for pure SnO₂ [24]. The presence of SnO₂ peaks indicates that the material retains its crystalline structure after composite formation, which could be beneficial for maintaining photocatalytic activity [24]. The XRD pattern also reveals the presence of the cubic phase (∇) of CaO. The characteristic diffraction peaks of CaO are observed at 2θ = 29.39°, 33.88°, 38.14°, 52.44°, and 65.79°, corresponding to the Miller indices 111, 200, 220, 311, and 222, respectively [26]. These peak positions confirm that the CaO component of the composite material exhibits a cubic crystal structure, which is consistent with literature reports for pure CaO [26]. The presence of CaO suggests that it maintains its crystallinity within the composite structure, potentially contributing to improved photocatalytic performance through enhanced surface properties [26]. Interestingly, the characteristic diffraction peaks of lanthanum species were not observed in this XRD pattern. This absence could be attributed to several factors, such as:

High dispersion: The lanthanum species may be highly dispersed on the surface of the composite catalysts, resulting in a lack of detectable crystalline domains [24].

Amorphous nature: The lanthanum species might exist in an amorphous state, which would not produce sharp diffraction peaks detectable by XRD [24].

Low concentration: The lanthanum content might be too low to produce observable diffraction peaks in the XRD spectrum [24].

Interaction with SnO₂: The lanthanum species may interact strongly with the SnO₂ matrix, leading to a loss of long-range order and thus undetectable diffraction peaks [24].

The absence of lanthanum diffraction peaks suggests that the lanthanum species are present in a highly dispersed or amorphous form, which could influence the material's photocatalytic properties. This dispersion might enhance the interaction between the lanthanum species and other components of the composite, potentially improving photocatalytic efficiency [24].

Photocatalytic properties

The photocatalytic degradation of MG dye was investigated in the presence of UV and visible light. The results are summarized in Fig. 6a and b. The photocatalytic degradation of dyes is influenced by several parameters. In this work, the effects of the catalyst type with different Sn/Ca molar ratios, the pH of the dye solution, the process time and also the type of irradiated light on the degradation of MG dye were investigated.

Fig. 6

The photocatalytic performance of LSC-x catalysts under a UV and b visible light irradiation at different pH values

The specific surface area and pore size distribution are the key factors to improve the photocatalytic activities of the catalysts. The adsorption results after 60 min of darkness show that this composite catalyst has greater adsorption ability than the other catalysts. This observation is fully consistent with the results obtained from the surface properties of the catalysts (Table 1). The surface charge and electrostatic interactions affect the adsorption of MG. One of the factors influencing the surface charge is the pH of the MG solution. The results of the pH study show that the activity of these catalysts to remove MG is higher at the lowest pH value than at other pH values. The reason for this observation can be attributed to the physical nature of surface adsorption on these catalysts. The negative charge on the surface of the catalyst, the presence of hydroxyl species on the surface and the formation of hydrogen bonds [5].

Table 1 Textural and surface properties of the prepared catalysts

After evaluating the adsorption–desorption equilibrium in the dark, the photo-decolorization efficiency (PDE) of MG over the LSC-x catalysts was investigated under UV and visible light irradiation for 35 min. Figure 6 shows that the prepared catalysts are suitable photo catalysts for MG degradation. The best degradation percentage is achieved after 35 min under UV irradiation in the presence of LSC-0.33, namely 98.80%. As shown, the catalyst LSC-0.33 has the highest efficiency on average at different pH values for MG degradation under UV–Vis irradiation. The best performance of these catalysts is also shown at acidic pH. Moreover, the PDE of MG dye increased sharply with time up to 35 min. The summary of the results obtained under UV–Vis light and at different pH values is as follows:

Effect of UV light: LSC-0.33 > LSC-0.5 > LSC-2 > LSC-1.

Effect of visible light: LSC-0.5 > LSC-0.33 > LSC-2 > LSC-1.

Effect of pH under UV–Vis light: 3 > 7 > 11.

The reasons for this observation are the difference in surface area and intrinsic excitation under UV light irradiation compared to visible light for these catalysts.

Our results (Fig. 7) show that while visible and UV light contribute to dye degradation initially, visible light becomes significantly more effective over time. By 35 min, visible light illumination achieves 75.6% greater dye removal compared to UV light (59.7%).

Fig. 7

Photocatalytic degradation of MG dye under UV and visible light illumination over time for the optimal catalyst composition (LSC-0.33) and pH condition (pH 3)

Model fitting and statistical analysis

Table S1 shows the experimental matrix design for the photo degradation of MG over LSC-x catalysts under the effects of different parameters. The empirical relationships between the independent variables and the RSM response are shown as a second-order empirical polynomial equation:

$$MG PDE=+70.79+3.93pH-80.29\frac{Sn}{Ca}-1.32t-6.50pH*\frac{Sn}{Ca}+0.10pH*t+0.34t*\frac{Sn}{Ca}+0.52{pH}^{2}+35.88{\left(\frac{Sn}{Ca}\right)}^{2}+0.02{t}^{2}$$

(2)

Table 2 shows the appropriateness of this model using analysis of variance (ANOVA). The F value for the model is 254.76, which means that this model is highly significant. There is only a 0.01% chance that such a large F-value occurs due to noise. The p-value for this model is < 10^–4, which also means that the model is significant [27].

Table 2 ANOVA results for the quadratic model

The “lack-of-fit” of 34.03 shows that this parameter is not significant in relation to the pure error and confirms the good predictability of the model [27].

Fig. S1 shows diagnostic plots confirming the normal distribution of the model. The points in the normal plots of the residuals are indeed very close to the line. This indicates that the residuals are quite close to the diagonal of normal probability (Fig. S1 (a)). The points in the variance plot (Fig. S1 (b)) are also randomly distributed. The results show that the proposed model has good precision and accuracy, as the residuals are between ± 4.00.

The plot of predicted and actual reaction rates (Fig. S1 (c)) for LSC-x catalysts show that these points are very close to the line. This shows that the data estimated by the model and the actual data agree.

The predicted R² of 0.9957 agrees well with the adjusted R² of 0.9917 and confirms the good predictability of the model (Fig. S1 (c)).

The Adeq precision of 52.5034 indicates a reasonable signal, so this model can be used to navigate the design space. The independent variables, including pH (A), Sn/Ca molar ratio (B) and process time (C) are highly significant parameters with p < 10^–4. Moreover, the second-order effects of these parameters and their dual interaction are significant due to p-values of less than 0.0500 [26].

The negative coefficients of the quadratic terms in the polynomial expression (Eq. 2) indicate their negative influences on the photo degradation of MG. To better understand of the interactions between these variables, 3D surface plots were presented in Fig. S2. The results of the interactions between these three independent variables and the model response show that the PDE of MG increases with increasing process time and pH and decreasing of Sn/Ca molar ratio. This can be attributed to the surface charge properties of the synthesized catalysts.

The desirability function was also used to determine the optimal conditions for the MG degradation. As a maximum PDE is the main objective for this photocatalytic degradation, the optimum conditions (98.80%) were found to be a pH value of 7, Sn/Ca molar ratio of 0.33 and process time of 35 min (Fig. S3) [27].

Conclusion

The La@SnO₂–CaO composite catalysts with varying Sn/Ca molar ratios were prepared via conventional incipient-wetness impregnation method. Our study reveals that catalysts with the lowest amounts of Sn/Ca molar ratio exhibited superior performance in photodegrading malachite green dye. This improvement stems from enhanced surface area and textural properties, significantly increasing photocatalytic reaction efficiency between adsorbent and adsorbed molecules. Response Surface Methodology analysis demonstrated optimal conditions for maximum photo degradation efficiency: pH 7, Sn/Ca molar ratio of 0.33, and a process time of 35 min. Notably, visible light exhibited greater effectiveness compared to UV light over extended illumination periods. Furthermore, the catalyst displayed excellent recyclability, retaining 85% of initial activity after five consecutive cycles. Our findings contribute to the development of sustainable dye removal methods, utilizing eggshell waste-derived La@SnO₂–CaO catalysts as an economical, efficient, and environmentally friendly alternative to expensive synthesis processes for photocatalytic degradation of malachite green dye.