Mechanistic insight into photocatalytic mineralization of dyes effluent with metal oxides: Parametric optimization and kinetic study

In the last two decades, global water pollution has continuously increased on a very large scale. The increase in water pollution is caused by the industrialization and commercialization of the different types of manufacturing industries, chemical laboratories (Suhan et al. 2020). These industries produce different types of recalcitrant organic pollutants such as dyes (Liu et al. 2024), pesticides (Zhao et al. 2024), insecticides (Wang et al. 2024), pharmaceutical compounds (Li et al. 2024), many personal care compounds (Li et al. 2024). These pollutants are very dangerous for humans and the nature of aquatic life (Singh et al. 2023). Some industries, such as printing, leather, and textile industries, use organic dyes, which require 700,000 tons of dyes each year (Batra et al. 2022) and produce huge amounts of wastewater; therefore, textile dyes are considered a significant pollutant in water (Lanjwani et al. 2024). In the last few years, about 20% of the world's production of water effluents has contained dyes in their discharge from the textile industry. This effluent produces detrimental effects on the kidneys, liver, reproductive functions, and immune systems, and it is a probable carcinogen. Therefore, the degradation of dyes from wastewater is necessary.

Furthermore, dyes are stable, recalcitrant in nature, and non-biodegradable compounds, which are difficult to remove from conventional treatment technology (Waghchaure et al. 2022). The presence of dyes in water affects aquatic life in many ways, such as reducing the penetration of sunlight in water (Patidar & Srivastava 2021), which affects the photosynthesis process and produces several diseases, such as eye irritation and carcinogenic and mutagenic problems (Alavi et al. 2019). Effective treatment of the dyes before the discharge of wastewater is very necessary.

There are several physicochemical methods, such as adsorption (Molla et al. 2019), coagulation (Gan et al. 2023), and biological processes (Türgay et al. 2011), used for the degradation of dyes from wastewater. In comparison to physicochemical methods, advanced oxidation processes (AOPs) such as sonolysis (Dehane & Merouani 2022), electrochemical (Palanisamy et al. 2020; Sarfo et al. 2023), photocatalytic (Bramhaiah et al. 2016; Wen et al. 2022; Sarfo et al. 2023), and electro-Fenton processes (Seror et al. 2022) have gained much attention because most of the AOPs generate the hydroxyl radicals (^•OH) that act as a strong oxidizing agent (E^o = 2.80 V) and are capable of non-selectively degrading the organic pollutants present in wastewater. Alias et al. (2020) synthesized nano flower-like rutile (TiO₂) in different morphologies and used these semiconductors for the methylene blue (MB) dye degradation. The maximum 49.47 mg g⁻¹ absorption capacity and 98.95% MB dye degradation were formed after 180 min of photocatalysis. Selvi et al. (2023) developed bismuth tungstate (Bi₂WO₆) hexagonal ZnO to effectively degrade the MB dye from water using ultraviolet (UV) light, visible light, and sunlight irradiation. About 74% MB was degraded when 50 mg of Bi₂WO₆/ZnO and 3.2 mg L⁻¹ MB solution were treated for 120 min.

The photocatalysis process generally works in UV light, visible light, and sunlight irradiation (Landge et al. 2021). Electrons and hole pair combination are capable of generating electrons that participate in the photolysis of water pollutants. Various activators such as hydrogen peroxides (H₂O₂), peroxymonosulfate (PMS), persulfate (PS), and ozone (O₃) were used as activators to produce more oxidant during photocatalysis (Alias et al. 2020; Seror et al. 2022; Selvi et al. 2023; Sharma et al. 2024). Many studies are available on transition metal oxides (TMOs) based on dye degradation. The TMO-based catalysts are classified into different groups, such as single TMO as a catalyst, doped single TMO and binary TMO compositions, supported catalysts, and suspended or supported TMOs based on their development, composition, advantages and disadvantages, and research trend. Table 1 shows the uses of TMOs for degradation (Kim et al. 2016; Ertis & Boz 2017; Li et al. 2017; Ramachandran & Sivasamy 2018; Senasu et al. 2018; Abdi 2020; Alias et al. 2020; Dawoud et al. 2020; Ullah et al. 2021; Zhang et al. 2021; Singh et al. 2022; Selvi et al. 2023; Sharma et al. 2024). Bismuth-based materials show distinct merits in photocatalysis compared to non-bismuth-based counterparts. Self-doping phenomena of bismuth resulted in the formation of oxygen vacancy (Selvi et al. 2023) which makes them effective catalysts for several chemical reactions driven by light. Out of different bismuth-based materials, bismuth oxide (Bi₂O₃) is easy to synthesize, highly stable, less toxic, and cost-effective. On behalf of the literature review, the present study proposed the synthesis of α-Bi₂O₃ and CuO as a single TMO-based catalyst by using the co-precipitation method. As-synthesised materials are characterized by using various characterization techniques such as X-ray diffraction, scanning electron microscopy, and electron dispersion X-ray analysis. The photocatalytic activity of these materials was studied by using rhodamine-B (RhB) dye as an organic pollutant. During photocatalysis, a parametric study was done by selecting various parameters such as initial dye concentration, effect of pH change, catalyst dose, and temperature. On the basis of intermediate identification, a dye degradation pathway was proposed, along with a reusability analysis of the photocatalyst. The nth-order rate constant (k_n) and the order of reaction (n) were also determined during the parameters optimization of dye degradation.

All the chemicals used are of the analytical grade. RhB dye was purchased from the Yogesh dye stuff product Ltd, India. Reagents copper sulfate pentahydrate (CuSO₄.5H₂O) and ferrous sulfate heptahydrate (FeSO₄.7H₂O) were obtained from S.D. Fine Chemicals, India. Bismuth nitrate pentahydrate [Bi (NO₃)₃.5H₂O] zinc sulfate heptahydrate (ZnSO₄.7H₂O) was obtained from Himedia Laboratories, India. Sodium hydroxide (NaOH), dichloromethane (CH₂Cl₂), and acetone (C₃H₆O) were purchased from Ranbaxy Chemicals Ltd, India. Pure distillates were purchased from Earthman Services Pvt Ltd, India.

The degradation of Rh-B dye is observed under ultraviolet light irradiation. α-Bi₂O₃ particles are used in the photocatalytic reduction of the dyes containing wastewater. The initial concentration of the Rh-B dye was 20 mg L⁻¹. The solution of Rh-B was irradiated using UV irradiation light in a closed chamber in the presence of hydrogen peroxide. A 200 W xenon lamp was used for the degradation study. The solution was kept inside a beaker and stirred using a magnetic stirrer. The magnetic stirrer run per minute (RPM) was maintained at approximately 150–200 rpm for 120 min of treatment time. Samples were withdrawn at regular intervals of time (Singh et al. 2022). The intensity and colour of the solution were measured by using the UV–visible spectrophotometer. The range of the UV–visible spectrophotometer was 200–800 nm. The maximum absorbance of RhB dye was measured at 546 nm. Adsorption–desorption equilibrium was also carried out before the Rh-B degradation experiments.

The precipitation method was used for the preparation of the CuO and α-Bi₂O₃ photocatalyst. In the formation of the copper sulfate pentahydrate, first, we took 150 mL of water in a washed beaker and put the beaker on the magnetic stirrer. The temperature of the system was maintained at 40 °C during the water heating. After that, 4 g reagent was added to the slightly hot water, and the magnetic stirrer speed was maintained at 150–200 rpm. Then, the temperature of the solution was increased from 40 to 80 °C. To maintain the basic pH of the solution, the freshly prepared solution of NaOH (2 g in the 100 mL distilled water) was mixed dropwise. The process of mixing the solution was continued for about 15 − 20 min by maintaining the temperature of the solution at 80 °C. After that, the temperature of the solution was dropped by stopping the process of heating, and the temperature of the mixture was observed to be 30 °C, and the process of stirring was continuous for 90 min. After that, the sample solution was kept in the chamber to cool down and settle down the prepared particles. Prepared particles took 2 h to settle down. After 2 h, a separated layer of prepared particles and water could be seen (Singh et al. 2022). At this stage, the size of the particles was very small so to collect the particles from the solution, we used the centrifuge process. In this process, we took the solution in the centrifuge tubes. These tubes were placed in the centrifuge machine for rotation, and the machine's RPM was set at 5,000 RPM for 15 min. The process of centrifuging proceeded for coagulation of the particles so that we could easily collect the particles and proceed to the further application part. The coagulated particles were collected in the crucible, and then we put this crucible in the oven at 80 °C for 12 h, where the extra moisture was heated up and started to evaporate. Then, this crucible was placed in the muffle furnace at a temperature of 400 °C for about 3 h. Then, to cool down the temperature, it was placed in the designator. As-synthesised material are characterized by using various characterization technique as discussed in next section.

The degradation study of Rh-B dyes was systematically analyzed using a Bi₂O₃ photo catalyst. The effect of different parameters on Rh-B dyes degradation is discussed in the following.

Moreover, at higher dye concentrations, repulsion between particles of dyes takes place, which leads to more dispersion of the pollutant, and therefore, removal of dyes decreases. The effect of the initial concentration of the pollutants on photocatalyst degradation has also been also reported by Shukla et al. (2010) using a TiO₂ catalyst and found that the photodegradation rate of phenol decreases with increases in the pollutant concentration. The dependence of the photodegradation rate has been described in several studies on the initial concentration of different types of dyes by the Langmuir–Hinshelwood (L–H) kinetic model (Shukla et al. 2010).

In this study, an as-synthesized α-Bi₂O₃ catalyst shows the best removal performance (∼95% RhB degradation) within 105 min of treatment time at 10 mg L⁻¹ Rh-B dye concentration. Therefore, for further analysis, a 10 mg L⁻¹ Rh-B concentration was optimized.

Additionally, the work by Singh et al. (2017) demonstrated that the molecules are positively charged and that the breakdown of bromocresol purple dye in acidic to neutral conditions was superior to alkaline media. The maximum RhB removal with α-Bi₂O₃, at different pHs of 3, 5, and 7, was found to be around 48–96% (Figure 5(b)). Therefore, for further analysis, a pH of 7 was selected.

The effect of catalyst dose during photolysis of wastewater is a very important concept (Singh et al. 2017). The initial rates in any reactor system were found to be directly proportional to catalyst concentration, indicating the heterogeneous regime. It is also noted that a certain catalyst concentration must be used for the photodegradation of a specific pollutant in wastewater and that the rate of photocatalysis will even decrease above this limit. It is observed that as the catalyst dose increased, the dye degradation efficiency successively increased. The effect of catalyst dose was tested from 0.2, 0.5, 1, and 1.5 g L⁻¹ for a dye solution of 10 mg L⁻¹ concentration and solution pH of 7 (Figure 5(c)). It was found that dye removal was meaningfully improved by enhancing the α-Bi₂O₃ dose from 0.2 to 1.5 g L⁻¹. As shown in Figure 5(c), 28–42% RhB was degraded at 105 min when each α-Bi₂O₃ dosage was 0.2 gL⁻¹, though a small modification in elimination was attained at the catalyst dose and was enhanced at about 1.5 gL⁻¹. This improvement has happened since increased α-Bi₂O₃ dosages could offer additional active sites for H₂O₂ activation because the number of catalysts increases, and the number of active sites on the photocatalyst surface increases, which in turn increases the number of superoxide and hydroxyl radicals. Besides this, catalyst concentration rises above the ideal value, and the degradation rate falls because the suspension intercepts the light. According to Sun et al. (2008), the effectiveness of the degradation decreased as a result of the excess catalyst preventing the lighting of the ^•OH radical. Additionally, exceeding the ideal catalyst concentration may cause catalyst particles to aggregate, making a portion of the catalyst surface inaccessible for photon absorption and slowing down the rate of degradation. In view of the above, an optimal α-Bi₂O₃ dosage of 1.5 g L⁻¹ was used for further reactions:

Temperature is an important parameter during the catalysis of wastewater. It affects both the bulk properties of the solution and the catalyst properties. It also affects the adsorption/desorption phenomenon of the dye molecules on the photocatalyst surface. The Rh-B degradation was found to be increased with an increase in the temperature from 5 to 20 °C, as shown in Figure 5(d). A maximum of 72% Rh-B degradation after 80 min at 20 °C was found, while the complete Rh-B degradation occurred within 105 min at 20 °C. This might depend upon the activation energy (E_a) of the molecules. For example, Chen et al. (2007) suggested that higher temperatures show higher removal efficiency of the pollutant under photocatalytic activity because it changes the activation energy, adsorption/desorption rate, and electron–hole recombination system. The oxidation rate of the adsorptive capacities also decreases; therefore, the pollutant removal increases at higher temperatures. The dye removal rate increases with the rise of the temperature, whereas no more change was observed after 15–20 °C because the availability of pollutants and catalysts was limited at a fixed dose.

Reusability analysis of photocatalysts is important due to the economics and application of as-synthesized materials. After photocatalytic degradation of the pollutant, centrifugation and filtration took place for the reusability analysis. Five successive Rh-B degradation analyses were carried out using α-Bi₂O₃ for the evaluation of the reusability of a catalyst, as shown in Figure 7(b). After each experiment, the solution was centrifuged and filtered, then washed with ethanol and dried at 90 °C for 120 min. It is observed that photocatalytic degradation was significantly reduced after the 5th run. This may happen during washing, as some loss of the α-Bi₂O₃ from the support surface takes place. Further accumulation of the pollutant on the surface of the photocatalyst reduces the active site available for the interaction of the molecules. Therefore, photocatalytic activity gets reduced after five cycles. After five cycles, the photocatalyst was recovered and calcined at 600 °C for 3 h and then reused. The obtained results prove that thermal regeneration of the photocatalyst is more effective. Therefore, it can be said that thermal treatment is an essential process for the used catalyst to regenerate its activity.

In the present study, the chemical precipitation method was used for the synthesis of α-Bi₂O₃ and CuO semiconductor photocatalysts for the degradation of Rh-B dye. The XRD pattern shows that the α-Bi₂O₃ has a tetragonal and hexagonal structure, whereas CuO shows a monoclinic structure. SEM images of α-Bi₂O₃ show it has spherical particles with an average diameter of 48 nm. The porous CuO morphology showed the collection of uncertain or thread spheres with different shapes and sizes (∼125−175 nm) and average lengths with about 58-nm-thickness. It is found that α-Bi₂O₃ provides higher degradation efficiency for Rh-B degradation compared to CuO. At the optimum conditions of 1.5 g L⁻¹ catalyst dose, pH of 7 and 10 mg L⁻¹ RhB concentration at 20 °C of temperature, more than 95% RhB was removed by the treatment of α-Bi₂O₃. Reusability and stability test analysis of α-Bi₂O₃ confirmed that it is a low-cost and highly effective photocatalyst for the treatment of textile dye industries and other organic pollutant remediation. The dye degradation using as-synthesized photocatalysis with the assistance of prepared samples has been measured and shows its possible use in the treatment of wastewater.

The authors are thankful to the Centre of Research Impact and Outcomes, Chitkara University, Rajpura-140417, Punjab, India, and Uttaranchal University, Dehradun, Uttarakhand, India for providing the financial assistance to analyse the sample and perform this study on a lab scale.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.