Novel magnetite/persulphate/ozone hybrid system for catalytic degradation/ozonation of sunset yellow dye from wastewater

Abstract

Food dyes with a diverse set of colorants stimulate appetite and enhance aesthetic charm of food on table but at the same time these dyes contaminate the aquatic and biological ecosystems due to their cytotoxic and carcinogenic potentials. Herein, we report magnetite catalyzed removal of sunset yellow dye from water through catalytic degradation and ozonation. The magnetite catalyst revealed about 83% catalytic degradation and 92% catalytic ozonation performance toward sunset yellow dye at 100 and 25 min, respectively. Furthermore, the persulphate/magnetite/O₃ hybrid system revealed superior performance compared to the persulphate/magnetite under identical conditions. Kinetic studies revealed that the dye degradation data followed second-order kinetics, suggesting that the dye removal process is physicochemical in nature. This study further demonstrates that the persulfate/magnetite/O₃ hybrid system can efficiently decompose sunset yellow dye in aqueous solution compared to the Fenton’s reagent and simple catalytic decomposition processes which is attributed to its unique structural features.

Graphical abstract

Keywords:

• Catalysis
• degradation
• fenton
• kinetics
• ozonation
• persulphate
• second order
• sunset yellow dye

1. Introduction

The global water consumption increased with the rapid growth in human population and industrial development. Thus, it is necessary to manage the wastewater treatment for survival of water resources [1]. Dyes, whether natural or synthetic, can have a variety of impacts on the substances and substrates with which they come into contact [2]. The dyes lead to contamination of substances and also affect the substrate physically and chemically [3]. Particularly, the human life and their communities are badly affected via morphological, physiological, anthropological, psychological, aesthetic and financially [4]. In 2010, it was estimated that the production of organic dyes in whole world is 2.1 metric tons, costing 14.4 billion dollars [5], and its preparation setup is increasing with time scale [6]. Based on latest survey, 7 × 10⁵ tons of dyes are produced in the world and with the passage of time its demand increased, especially in 2008, it crossed the cost of US $11 billion. These dyes are not consumed totally but only ∼1–2% are used as a dyes setup and ∼10–15% are wasted during the excessive releasing when the dying process is under the action [7]. For coloring the substances, sunset yellow (SSY) dye is also used. Due to the treatment technologies of sewage water, a huge amount of SSY dye has become part of water bodies [8]. The uncontrolled degrading ability of SSY can be treated by traditional ways which increases the demand of advanced oxidation processes. Activated persulphate (PS) can be used for the production of sulphate radical (SO₄^•−) which is an important oxidizing agent and has the ability to oxidize pollutants. Further, it’s pH dependent and impotent due to its selectivity and prolong half-life [9]. Nowadays the PS are activated for the degradation of dyes to generate the ${\mathit{\text{SO}}}_4^{*-}$ and other intermediates by many ways such as transition metals, microwave, heat and ultraviolet light. Different metal oxides such as Fe₂O₃, Co₃O₄, Fe₃O₄, MnO₂ etc., are used for the activation of PS in order to degrade the organic pollutants [10].

Similarly, the degradation technology for ultra-low amount of particular contaminants can be investigated [11]. Therefore, the health risks of sunset yellow dye can be minimized by using environmental-friendly, cost-effective and practically treating technologies. For this purpose, the iron oxides are mostly used due to their widespread nature of iron oxides and zero toxicity. Similarly, iron oxides have low cost and have the ability of activating others. Magnetite (Fe₃O₄) is the best option among all iron oxides due to its easy separation method after its utilization in dyes degradation [12].

It has been reported that the degradation of PAHs by PS activated magnetite reveal better results compared to the bare PS [13]. Similarly, the sulfamonomethoxine decomposition via the Fe₃O₄/PS catalyst was also investigated and its decomposition performance was compared for 60 molar solution. It was observed that the utilization of magnetite for the activation of PS reveal high performance i.e. 90% for the removal of sulfamonomethoxine [14]. Similarly, ozone is also a strong oxidizing agent due to its E₀¼ 2.08 V value and is used for various wastewater treatment in order to obtain clean water. It plays a key role in the production of sulfate radicals from PS and as a result, the dyes degradation performance increases by using ozone [15]. Advanced oxidation process coupled with ozonation is an important strategy for the treatment of sewage water which possesses refractory organics and is considered one of the best option for solving the main problem of water pollution [16]. Due to the degradation ability of ozone, it can be used for various kinds of organic contaminated water treatment and as a result, it provides various potential oxidants and hydroxyl radicals, which have oxidation potential of E₀¼ 2.7 V. It has been applied extensively for organic wastewater treatment in nearly past few years and revealed exceptional performance in homogeneous catalysis, which made dyes degradation very easy [17]. The magnetite like catalyst can promote the ability of ozone for oxidation and as a result, the dyes degradation can be driven. Besides, it has been reported that the combination of ozone along with PS can reveal high performance for removal of COD colors and NH₃–N, compared to the bare PS or ozone [18].

Based on the above-mentioned literature and to the best of our understanding, it can be said that there is a lack of research addressing the issues related to wastewater treatment using the synergistic approach of magnetite nanoparticles, persulphate, and ozone in a hybrid system. Hence, it was expected that the degradation of dyes by the PS/magnetite/O₃ hybrid system may give better results as compared to traditional methods. The activation of persulfate (PS) by both magnetite catalyst and an oxidizing agent such as ozone can enhance dye degradation, resulting in rapid purification of water. Therefore, a new PS/magnetite/O₃ hybrid system was applied for the degradation of Sunset Yellow (SSY) dye, potentially offering high degradation efficiency during ozonation as well as catalytic degradation. This study investigated the effects of various experimental parameters including dye concentration, catalyst dosage, pH, catalyst-to-PS ratio, catalyst-to-ozone ratio, and their corresponding ratios in the PS/magnetite/O3 ternary hybrid system on degradation efficiency and physicochemical parameters. In addition to providing a simpler and cost-effective method, this research may significantly contribute to advancing academic knowledge in the field.

2. Materials and methods

2.1. Materials and apparatus

Sodium Persulphate (Na₂S₂O₈), Ozone (O₃), Iron Sulfate (FeSO₄), Iron Chloride (FeCl₃), Ammonia (NH₃), Sodium hydroxide (NaOH), Perchloric acid (HClO₄), Hydrochloric acid (HCl), Iodine, starch, Sodium thiosulfate (Na₂S₂O₃) and Potassium iodide (KI) were purchased from Sigma-Aldrich in analytical grade. A food grade dye Sunset Yellow (C₁₆H₁₀N₂Na₂O₇S₂) was purchased from a local market. The morphological study was done with the help of scanning electron microscope equipped with a field emission gun (Model of FEGSEM/EDS is 354900-7673-000-08 en, Germany, 30kv). Similarly, the composition and crystal nature were studied via the Energy dispersive X-ray (EDX) spectroscopy, X-ray diffractometer (Model no: AXRD-LPD System USA, 220 V, 20 A, 50 Hz) and FTIR (Fourier Transformed Infrared Spectrophotometer, with attenuated total reflection, ATR, accessory model IRSpirit, produced by Shimadzu, 220-240 V). The peaks observed in the XRD pattern provide valuable information regarding the arrangement of atoms in a crystal which allows for identification of the crystal structure of material. The 250 mL volume glass reactor was used for performing the experiments while the water bath (Model: BKD-0506, BIOBASE BIODUSTRY, Shandong CO LTD, China; Power supply 220 V/50 Hz) was used to maintain constant temperature. The concentration of SSY dye was studied before and after the degradation by using UV-Visible spectrophotometer (UV1800ENG240V, SOFT, Shimadzu Corporation, Japan, model UV-1800, 220–240 V-50–60Hz 140VA). The wavelength range of UV-Visible spectrophotometer was 200–800 nm. Similarly, Ozone was produced from atmospheric air by using a common Ozonator (Sky zone 3 ozone generator (local, Karachi, Pakistan). The chemical structure, molecular weight and λ_max of sunset yellow food dye are shown in Scheme 1.

Scheme 1. Properties and chemical structures of Sunset Yellow dye.

2.2. Preparation of magnetite nanoparticles (Fe₃O₄)

The magnetite nanoparticles (MNPs) were synthesized according to the reported method [19]. About, 6.30 g of FeCl_3·6H₂O (a source of Fe[III]) and 4.2 g of Fe₂SO_4·7H₂O (source of Fe[II]) were mixed in 50 ml of deionized water and stirred continuously at 80 °C while adjusting the pH by adding aqueous ammonia (30%) and monitored continuously by using pH meter (Model: HI 5521, powered by a 12 Vdc power source). The solution was continuously stirred at 500 rpm for two hours after the addition of aqueous ammonia. The resulting products were filtered through Whatman filter paper and repeatedly washed with ethanol and water to preserve the stability, quality, and purity of Fe₃O₄ nanoparticles for characterization and future use as a catalytic materials for dyes degradation. Finally the products were dried at 60 °C in oven (The Zhengzhou Brothers France CO., Ltd, drying oven, type: BR-DR-110, Voltage 220 V 50 Hz, power 1 kW and ID No. 2020194220) for 6 h [15, 20].

2.3. Degradation experiments

For the degradation of SSY dye in aqueous solution, two types of experiments were conducted. Firstly, SSY dye was degraded by activating persulphate with magnetite, and then by coupling it with ozone for a synergistic effect. The as-prepared magnetite nanoparticles were used for PS activation in order to remove the sunset yellow dye from aqueous solution using Na₂S₂O₈, Fe₃O₄ and O₃ as a source of PS, catalyst and ozonation respectively. Iodometric method was used to determine the concentration of ozone in gas phase. For this goal, the gaseous ozone was bubbled into two sets of glass bottles by a ceramic sparger. Each bottle was filled with 200 mL of a 2% potassium iodide (KI) solution, purchased from Sigma-Aldrich in analytical grade. To make the solution acidic, 10 mL of 1 N HCl was added, and the released iodine was titrated with 0.005 N standard solution of Na₂S₂O₃ using starch as an indicator [21]. For the catalytic degradation and catalytic ozonation, the effect of magnetite, PS, pH, ozone, and dye concentration was investigated. The experiments were conducted in a 250 mL volume glass reactor at a constant temperature of 25 °C using a water bath (model: BKD-0506, BIOBASE BIODUSTRY, Shandong CO LTD, China; Power supply 220 V/50 Hz). Degradation of the dye was performed at various pH levels including 3, 7, and 11 to assess its impact under acidic, neutral, and basic conditions during both catalysis and catalytic ozonation processes. The effects of various parameters such as PS, magnetite, and ozone concentrations were investigated individually for the catalytic degradation of sunset yellow dye. Each parameter was kept constant while the others were varied [22], as shown in Scheme 2.

Scheme 2. Experimental set up for catalysis/ozonation.

Similarly, kinetics studies involved determining the rate constant for the reaction order. The dye concentration before and after each degradation experiment was analyzed using a UV-Visible spectrometer [23]. Dye removal percentages during the experiments were calculated using Equation (1)(1) $$\mathrm{\%}\mathrm{ }\mathit{\text{dye removal}}=100-\left[\left(\frac{C}{C_0}\right)x\mathrm{ }100\right]$$(1) [20,21]. (1) $$\mathrm{\%}\mathrm{ }\mathit{\text{dye removal}}=100-\left[\left(\frac{C}{C_0}\right)x\mathrm{ }100\right]$$(1)

In the given equation, ‘C₀’ denotes the initial concentration of the sample solution, which refers to the dye concentration (ppm) before the commencement of catalytic degradation. Also, ‘C’ represents the concentration of solution (ppm) at any given time ‘t’ (min) during the catalytic degradation of the dye solution.

3. Results and discussion

3.1. Characterization

The XRD analysis was carried out through X-ray diffractometer to check the crystallinity of the synthesized nanoparticles. The X-ray diffractometer (Model no: AXRD-LPD System USA, 220 V, 20 A, 50 Hz) has a Cu-Kα radiation source with wavelength (α) equals to 0.154 nm was operated. The measurement was performed in a 2θ values range of 10–80° (Figure 1(a)) before and after the use of Fe₃O₄. The results clearly reveal that the as-synthesized Fe₃O₄ nanoparticles are crystalline in nature. There is no change observed before, during, or after use, even after thermal heating (200 °C), indicating the stability and consistent nature of these nanoparticles throughout the dye degradation process. The diffraction peaks observed at 30.35°, 35.71°, 43.51°, 53.59°, 57.31 and 63.11°, correspond to the Six characteristic peaks for Fe₃O₄ are shown by their indices (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1) and (4 4 0) planes, respectively. The peaks observed in the XRD pattern provide valuable information regarding the arrangement of atoms in a crystal, which allows for identification of the crystal structure of material. The results obtained from the XRD analysis are found to be consistent with previously reported studies, confirming the crystalline nature of the as-synthesized Fe₃O₄ nanoparticles [24].

Figure 1. (a) XRD pattern, (b) FTIR spectra, and (c) EDX spectrum of the as-synthesized magnetite catalyst.

The FTIR spectra of magnetite nanoparticles (Fe₃O₄), before and after use, are shown in Figure 1(b). The characteristics peak at 606 cm⁻¹ showed the formation of Fe₃O₄. Similarly, the characteristic peak of Fe–O–Fe showed the shifts to 581 cm⁻¹. The broad band at 3100–3600 cm⁻¹ presented the vibration of O–H bond. Additionally, the band at 1620 cm^-1 corresponds to water molecules absorbed by the surface of the magnetite (Fe₃O₄) nanoparticles [24]. The FTIR results indicate that the nature of the magnetite nanoparticles remains unchanged before use, after use, and after thermal heating at 200 °C, as shown in Figure 1(b).

From the EDX spectrum, it is clear that the sample exhibit an intense peak at 0.7 kev corresponding to the oxygen atom. This indicate that the catalyst has a high amount of oxygen (Figure 1(c)). Similarly, three other peaks were observed at 0.6, 6.3, and 7.0 keV, which correspond to the binding energies of iron and indicate the formation of magnetite nanoparticles. This confirms the formation of magnetite nanoparticles and consistent with the previous report [25]. The atomic percentage composition of iron and oxygen is approximately 72.41 and 27.59%, respectively. The experimental result not only reflect the presence of iron and oxygen peaks in the sample but also gives the information about the elemental composition of magnetite.

The surface morphology of the catalyst was investigated by SEM analysis at different magnifications, as shown in Figure 2(a–d). Clear nanoparticle agglomeration can be observed, possibly originating during the essential drying processes. The agglomerates exhibit spherical shapes with nanometric sizes, consistent with Fe₃O₄. During catalytic degradation, the surface of these nanoparticles acts as active sites for PS interaction, enhancing dye degradation. Metal oxides with various morphological structures exhibit catalytic stability and reactivity for pollutant degradation [26]. Previous reports on similar type of nanoparticles have shown that the spheres possess uniform flower-like features, which is built from several nano-sheets and the entire sphere surface was covered by flower-like architecture with some voids on the surface of sheets/materials [27].

Figure 2. SEM images (a–d) of the as-synthesized magnetite catalyst.

3.2. Sunset yellow (SSY) dye degradation study

3.2.1. Effect of the amount of catalyst at different time intervals

The results revealing the effect of the magnetite catalyst amount on the percentage degradation of SSY through catalytic degradation and catalytic ozonation along with their relevant kinetics are displayed in Figure 3(a–d). In such experiments, the amount of catalyst was varied from 0.125 to 0.5 g, and degradation experiments were conducted at different time intervals until the dye solution was nearly decolorized. The initial concentrations of the SSY dye and persulphate (PS) were 20 ppm and 8 mM, respectively at pH 3 and temperature was 25 °C. The results showed that performance of dye degradation enhanced with increasing the dosage of magnetite nanoparticles. After 110 min, the percentage removal of dye was observed to be 32.63, 62.32, and 82.98% at the catalyst dosage of 0.125, 0.25 and 0.5 g, respectively.

Figure 3. Effect of catalyst dose on the (a) catalytic degradation (b) catalytic ozonation, (c) second order kinetics for catalytic degradation, and (d) second order kinetics for catalytic ozonation.

Based on these observations, it was concluded that 0.5 g of catalyst dosage was an optimum value for the activation of PS as it has the highest degradation rate for SSY dye [28]. This indicates that the activation of PS towards the SSY dye degradation is remarkably increased with the increase in dosage of catalyst [29]. As expected, the magnetite catalyst could absorb the dye molecules on its surface and led to the large surface area for modulation of PS ions. Similarly, the magnetite catalyst could also acts as an electron acceptor and facilitate the electron transfer from SSY dye to PS ions. As a result, highly reactive sulphate radicals can be produced [30]. The sulphate radicals originated from PS activation then attacks on the molecules of dye and convert them into smaller and less harmful molecules [31]. The magnetite catalyst for PS activation can led to the faster and more efficient degradation of dye with less wastage and fewer byproducts [32]. During the Fenton process, PS ions (S₂$O_8^{-2}$) and Fe(II) react through electron transfer mechanism and released the S $O_4^{-}$ as mentioned in Equation (2)(2) $${\mathrm{S}}_{2}2O_8^{-2}\to \mathrm{S}{O}_4^{\mathrm{*}}+\mathrm{S}{O}_4^{-}\mathrm{ }$$(2) . Based on the advanced oxidation processes for scavenging effects of S $O_4^{-},$ the optimum dosage of Fe(II) for activation of PS played a key role as mentioned in Equation (3)(3) $${\mathrm{S}}_2{O}_8^{-2}+{\mathrm{\text{Fe}}}^{+2\mathrm{ }}\to {\mathrm{\text{Fe}}}^{+3}+\mathrm{S}{O}_4^{\mathrm{*}}+\mathrm{S}{O}_4^{-}\mathrm{ }$$(3) . (2) $${\mathrm{S}}_{2}2O_8^{-2}\to \mathrm{S}{O}_4^{\mathrm{*}}+\mathrm{S}{O}_4^{-}\mathrm{ }$$(2) (3) $${\mathrm{S}}_2{O}_8^{-2}+{\mathrm{\text{Fe}}}^{+2\mathrm{ }}\to {\mathrm{\text{Fe}}}^{+3}+\mathrm{S}{O}_4^{\mathrm{*}}+\mathrm{S}{O}_4^{-}\mathrm{ }$$(3)

From Figure 3(a), it is clear that the dye degradation performance is lesser at the low dosage of the catalyst. However, as the magnetite dosage increases up to 0.5 g, the dye removal increases due to the high capability of catalyst to activate the PS and produce maximum S $O_4^{-}$ for degradation of dye. However, the over excess amount of magnetite catalyst further reduces the performance of PS for dye degradation. Consequently, 0.5 g of the magnetite dosage is the optimum level for superior dye degradation. As obvious, at 0.5 g magnetite dosage, the catalyst performance for dye degradation is 72.22 and 82.98%, at 20 and 110 min, respectively. Notably, the dye degradation performance decreased when the amount of magnetite catalyst dosage increased up to 0.75 g due to the scavenging interaction of ferrous ions and sulphate ions as mentioned in Equation (4)(4) $${\mathit{\text{Fe}}}^{+2}+{\mathit{\text{SO}}}_4^{\mathrm{*}}\to {\mathit{\text{Fe}}}^{+3}+{\mathit{\text{SO}}}_4^{\_}\mathrm{ }$$(4) . This is identical to the Fenton process [33]. (4) $${\mathit{\text{Fe}}}^{+2}+{\mathit{\text{SO}}}_4^{\mathrm{*}}\to {\mathit{\text{Fe}}}^{+3}+{\mathit{\text{SO}}}_4^{\_}\mathrm{ }$$(4)

Similarly, Figure 3(b) reveals the catalytic ozonation of SSY dye in the presence of different amount of magnetite catalyst and various time interval till the decolorization of dye solution. It can be concluded that the catalytic ozonation process of SSY dye removal also depends upon the magnetite dosage [34]. Keeping all the parameters consistent, by varying the magnetite dosages to 0.125, 0.25, and 0.5 g at a specific time of 25 min, the dye degradation is observed to be 63.70, 71.760, and 80.140% respectively. From these results, it is concluded that 0.5 g of magnetite exhibit exceptional ability for the catalytic ozonation of dye [34].

For wastewater treatment, the activation of PS by magnetite and ozone is very crucial. This process is also called magnetite/ozone/PS (MOPS)-assisted oxidation, which gives more fruitful results as compared to that without ozonation. The main reason for its high performance is the generation of sulphate radicals from PS which can efficiently decompose various dyes [35]. Similarly, Ozone also reduces magnetite into Fe⁺² which is the main cause of hydroxyl radical generation and attacks on the PS and generate SO₄^•− which further proceed dye degradation. Similarly, magnetite can decompose Ozone into additional reactive species such as superoxide radicals (O₂^•−) and singlet oxygen (¹O₂). These reactive species can improve the potential of MOPS systems which leads to the dyes degradation. The combined effect of magnetite and Ozone can cause the formation of strong oxidants such as peroxymonosulphate which generate SO₄^•− and degrade dyes into various fragments [36].

Notably, both types of dye degradation experiments were conducted under identical conditions but the only difference was the addition of Ozone during the catalytic ozonation process. As obvious, the dye degradation after 20 min for 0.125, 0.25, and 0.5 g of catalyst in the absence of Ozone was 27.92, 57.99 and 72.22%. While, in the presence of Ozone, the dye degradation was 64.39, 71.62, and 79.46%, respectively. These results reveal that the catalytic ozonation is more fruitful compared to that of the catalytic degradation [37]. As discussed in the literature, the use of PS and Ozone with Fe⁺² could make the maximum removal of color, COD and NH₃-N, compared to the Ozone or PS alone. The magnetite was found to be physiochemically most stable and having both of the Fe²⁺ and Fe³⁺ ions which can initiate the Fenton reaction based on the mechanism of classical Haber-Weiss [36].

For SSY dye catalytic degradation, different kinetics models such as zero order, first order, pseudo first order, pseudo second order, and second order kinetics models were applied and their summary is presented in Table 1. It can be observed that the second order kinetics model has relatively higher Regression factor (R²) and less RMSE values as compared to other kinetic models as shown in Table 1. Therefore, it is suggested that the second-order kinetics model could be a better choice for analyzing the kinetics-related data. For 0.125, 0.25, and 0.5 g magnetite dosage, the Regression factor (R²) was 0.58, 0.62 and 0.79, respectively in the presence of 10, 20, 30, and 40 ppm dye concentration, 8 mM PS, pH 3, at temperature 25 °C. This confirms that the data follow second order kinetics model to some extent while it can’t be fitted properly to the equation of other orders. For different dosage of magnetite toward removal of SSY dye, a linear relationship is obtained between the reaction time and inverse of changes in the dye concentration as revealed in Figure 3(c). This indicates that it obeyed the second order kinetics model in presence of modified dosage of magnetite such as 0.125, 0.25 and 0.5 g. The rate constant (k) for the magnetite catalyst dosage of 0.125, 0.25 and 0.5 g was predicted to be 9.420 × 10⁻⁵, 3.6 × 10⁻⁴ and 1.39 × 10⁻³ (M⁻¹min⁻¹), respectively, as provided in Table 2. This trend also shows that 0.5 g magnetite has the maximum capability for higher rate constant. However, the values of half-life were found to be 479.5, 188.0 and 81.4 min for 0.125, 0.25, and 0.5 g of the catalyst, respectively. These kinetics investigations are in closer agreement with the previous report [38]. The rate constant (k) is determined from the data of slope of Equation (5)(5) $$\frac{\mathrm{d}\left[\mathrm{C}\right]}{\mathrm{\text{dt}}}=-{\mathrm{k}\left[\mathrm{C}\right]}^2$$(5) , while half-life is determined from Equation (6)(6) $$t_{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}=\frac{1}{k\left[C_0\right]}$$(6) [39]. (5) $$\frac{\mathrm{d}\left[\mathrm{C}\right]}{\mathrm{\text{dt}}}=-{\mathrm{k}\left[\mathrm{C}\right]}^2$$(5) (6) $$t_{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}=\frac{1}{k\left[C_0\right]}$$(6)

Table 1. Summary of different kinetics models regarding the catalytic degradation/ozonation for the removal of Sunset Yellow dye under variable experimental conditions.

S.No	Parameters	Zero order		Pseudo first order		First order		Pseudo second order		Second order
		Regression factor (R^2)	RMSE values	Regression factor (R^2)	RMSE values	Regression factor (R^2)	RMSE values	Regression factor (R^2)	RMSE values	Regression factor (R^2)	RMSE values
1	SSY 40 ppm	0.610	0.007	0.439	8.55E^−4	0.424	4.23E^−4	0.005	0.493	0.90	5.23E^−6
2	PS 8 mM	0.559	0.007	0.817	0.001	0.504	5.13E^−4	0.001	0.597	0.96	2.96E^−5
3	Magnetite 0.5 g	0.613	0.002	0.499	0.002	0.547	6.43E^−4	6.45E^−4	0.498	0.79	2.78E^−5
4	pH = 3	0.459	0.008	0.633	0.001	0.439	8.55E^−4	4.851	0.378	0.71	2.67E^−4
5	SSY-O3 40 ppm	0.470	0.008	0.876	0.012	0.001	0.404	0.485	0.005	0.90	2.42E^−4
6	PS-O3 8 mM	0.656	0.002	0.770	0.009	6.5E^−4	0.440	0.543	0.097	0.98	4.68E^−4
7	Magnetite-O3 (0.5 and 0.5 g/L)	0.493	0.008	0.731	0.010	0.002	0.504	0.55	0.001	0.97	2.60E^−4
8	PH = 3-O3	0.447	0.064	0.712	0.011	0.001	0.817	0.46	0.041	0.95	8.59E^−4
9	O3 0.5 g/L	0.649	0.036	0.674	0.007	9.0E^−4	0.519	0.52	0.053	0.96	9.20E^−6

Table 2. Kinetic parameters for catalytic degradation for the removal of Sunset Yellow dye under variable experimental conditions.

S.No	Parameters	Parameters value	Constant parameter	k (M^−1min^−1)	t1/2 (min)	Regression factor (R^2)	RMSE values
1	Magnetite	0.125 g	SSY 20 ppm, PS 8 mM, pH 3 and temperature 25 °C	9.42 × 10^−5	479	0.58	2.71E^−4
2		0.25 g		3.60 × 10^−4	188	0.62	9.95E^−5
3		0.50 g		3.90 × 10^−4	81.4	0.79	2.78E^−5
4	Persulphate	2 mM	Magnetite 0.5 g, SSY 20 ppm, pH 3 and Temperature 25 °C	6.24 × 10^−4	33.1	0.98	1.93E^−4
5		4 mM		6.13 × 10^−4	53.9	0.93	5.54E^−5
6		8 mM		2.70 × 10^−3	23.5	0.96	2.96E^−5
7	pH	3	Magnetite 0.5 g, SSY 20 ppm, PS 8 mM and Temperature 25 °C	4.30 × 10^−4	78.7	0.71	2.67E^−4
8		7		3.66 × 10^−4	176	0.74	6.33E^−5
9		11		1.22 × 10^−4	514	0.53	3.28E^−5
11	Sunset Yellow dye	10 ppm	Magnetite 0.5 g, PS 8 mM, pH 3 and temperature 25 °C	2.3 × 10^−2	3.73	0.60	1.08E^−5
12		20 ppm		1.1 × 10^−2	5.04	0.70	1.21E^−5
13		30 ppm		7.3 × 10^−3	5.77	0.81	1.03E^−5
14		40ppm		8.2 × 10^−3	4.37	0.90	5.23E^−6

Where C₀ and C are the concentrations (ppm) of dye in the bulk solution before degradation and during degradation process at desired time ‘t’ (in min). The t_1/2, k and R² values for SSY dye along with variable parameters are provided in Tables 1–3. The linearity of the plots for second order kinetics is also affected by the dosage of the catalyst, which indicate that the degradation process is chemical in nature.

Table 3. Kinetic parameters for catalytic ozonation for the removal of Sunset Yellow dye under variable experimental conditions and their comparison with literature.

S. No	Parameters	Parameters value	Constant parameter	k (M^−1min^−1)	t1/2 (min)	Regression factor (R^2)	RMSE values	Followed order of kinetics	Reference
1	Magnetite	0.125 g	SSY 20 ppm, PS 8 mM, pH 3, ozone inlet 2.5 g/L and temperature 25 °C	5.6 × 10^−3	8.23	0.98	6.80E^−4	2nd	Present work
2		0.25 g		7.5 × 10^−3	6.77	0.98	2.78E^−4	2nd	Present work
3		0.5 g		1.1 × 10^−2	5.57	0.97	2.60E^−4	2nd	Present work
4	Persulphate	2 Mm	Magnetite 0.5 g, SSY 20 ppm, pH 3, ozone inlet 2.5 g/L and Temperature 25 °C	5.0 × 10^−3	9.60	0.93	4.06E^−4	2nd	Present work
5		4 Mm		6.7 × 10^−3	8.80	0.94	5.78E^−4	2nd	Present work
6		8 Mm		9.7 × 10^−3	5.97	0.98	4.68E^−4	2nd	Present work
7	pH	3	Magnetite 0.5 g, SSY 20 ppm, PS 8 mM, ozone inlet 2.5 g/L and Temperature 25 °C	1.1 × 10^−2	5.77	0.95	8.59E^−4	2nd	Present work
8		7		8.3 × 10^−3	6.68	0.91	8.74E^−4	2nd	Present work
9		11		4.8 × 10^−3	11.7	0.89	5.93E^−4	2nd	Present work
10	Ozone	1g/L	Magnetite 0.5 g, SSY 20 ppm, PS 8 mM, pH 3 and temperature 25 °C	1.5 × 10^−2	6.26	0.82	0.0024	2nd	Present work
11		2.5g/L		4.0 × 10^−3	11.8	0.90	5.39E^−4	2nd	Present work
12		5g/L		1.38 × 10^−4	339	0.96	9.20E^−6	2nd	Present work
13	Sunset Yellow	10 ppm	Magnetite 0.5 g, PS 8 mM, pH 3, ozone inlet 5 g/L and temperature 25 °C	4.1 × 10^−3	19.0	0.60	3.41E^−4	2nd	Present work
14		20 ppm		4.0 × 10^−3	10.5	0.70	2.27E^−4	2nd	Present work
15		30 ppm		4.3 × 10^−3	7.67	0.81	2.39E^−4	2nd	Present work
16		40ppm		4.0 × 10^−3	6.00	0.90	2.42E^−4	2nd	Present work
17	Acid Red73	10ppm	Reaction time = 60 min, Fe3O4@HAP = 1 g/L, pH = 5.0, UV intensity = 9.3 mW/cm^2)	0.055/0.052	–	0.967/0.825	–	1st/2nd	[40]
18	Methylene blue	30ppm	Reaction time = 120 min, Fe3O4 N$\stackrel{’}{P}$s (cynara cardunculus leaf) 0.03 g	3.63/0.0061	–	0.90796/0.96435	–	1st/Pseudo 2nd	[41]
19	Orange II dye	10ppm	50 Watts LED Light, 0.03 g of Fe2O3–TiO2 photocatalys, 0.025 mL of H2O2, 60 min	0.1 × 10^−2	–	0.99	–	2nd	[42]
20	Methylene blue	10ppm	300 min, 0.05 g Polyanaline, arch lamp housing 500 W,	0.0505		0.9604		Pseudo 1st	[43]

Similarly, the kinetics models were applied on catalytic ozonation and it is found that the Regression factor is highest in case of second order kinetics model under constant parameters of PS 8 mM, pH 3, 2.5 g/L of Ozone dosage and 10, 20, 30, and 40 ppm SSY dye with variation of magnetite dosage such as 0.125, 0.25, and 0.5 g. Their respective Regression factor (R²) values are 0.98, 0.98 and 0.97 and the corresponding ‘k’ values are 0.0056, 0.0075 and 0.011 (M⁻¹min⁻¹), respectively (Table 3). The k value is found to be the highest at 0.5 g of magnetite dosage. For determination of half-life time for second order kinetics model, Equation 6(6) $$t_{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}=\frac{1}{k\left[C_0\right]}$$(6) was followed which gives the values of 8.23, 6.77 and 5.57 min for 0.125, 0.25, and 0.5 g, respectively. A linear relationship is observed between reaction time and inverse of change in the dye dosage from the dye removal plots for different amount of catalyst during catalytic ozonation as depicted in Figure 3(d). This again signifies that the degradation data follow second order kinetic model at all concentrations of catalyst [44]. A similar trend for the removal of textile dyes was also observed with an average value of second order rate constant 1.99 × 10⁻² L/(mg·min) [45]. In addition, it was also reported that the degradation of azo dyes by MPSO and MPS follows the second order kinetics model which has the rate constant in the range of 1.99 × 10⁻² to 2.34 × 10⁻² L/(mg·min) [46].

3.2.2. Effect of amount of persulphate (PS) at different time intervals

For determining the effect of the amount of PS ions on the catalytic degradation of SSY and to assess their optimal dose, the amount of PS was varied from 2 mM to 8 mM over different intervals of time while keeping other parameters fixed. The results for catalytic degradation and catalytic ozonation and their corresponding kinetics results are shown in Figure 4(a–d). We observed that the degradation extent of dye increased with the increase in PS dosage up to 8 mM and approximately 68.99 and 83.09% degradation was noticed at 10 and 110 min, respectively (Figure 4(a)). Further increase in PS dosage reduced the degradation performance due to the interference of surplus S $O_4^{-}$ anions in system [47]. Similarly, at high dosage of PS the S $O_4^{-}$ consumption given by S₂$O_8^{-2}$ and the excessive amount of S $O_4^{-}$ led to the coupling among themselves as shown in Equations (7)(7) $${\mathit{\text{SO}}}_4^{\mathrm{*}}+S_2{O}_8^{-2}\to {\mathit{\text{SO}}}_4^{-}+S_2{O}_8^{-2}$$(7) and (8)(8) $${\mathit{\text{SO}}}_4^{\mathrm{*}}+{\mathit{\text{SO}}}_4^{\mathrm{*}}\to S_2{O}_8^{-2}$$(8) , respectively [48]. (7) $${\mathit{\text{SO}}}_4^{\mathrm{*}}+S_2{O}_8^{-2}\to {\mathit{\text{SO}}}_4^{-}+S_2{O}_8^{-2}$$(7) (8) $${\mathit{\text{SO}}}_4^{\mathrm{*}}+{\mathit{\text{SO}}}_4^{\mathrm{*}}\to S_2{O}_8^{-2}$$(8)

Figure 4. Effect of persulphate dosage on the (a) catalytic degradation (b) catalytic ozonation, (c) second order kinetics for catalytic degradation, and (d) second order kinetics for catalytic ozonation.

The effect of PS on catalytic ozonation was also studied by varying the PS dosage while keeping other parameters such as the amount of magnetite, ozone concentration, and dye concentration constant at 0.5 g, 2.5 g/L, 20 ppm, and pH 3, respectively, at 25 °C, as shown in Figure 4(b). It was found from that the removal performance was 59.61, 66.77, and 78.09% when the PS dosages were 2, 4, and 8 mM, respectively. Thus, 8 mM PS was found to be the suitable amount under the present conditions. In another study for methylene blue dye, it was observed that the use of 5% by weight of CoO-meso-CN also effect the degradation of dye with the change in PS dosage [49]. In the absence of PS dosage, the removal performance was 24% while in the presence of catalyst, the degradation was increased within 15 min when combined with 5 to 20 mM of PS due to the formation of extra reactive radicals in the mixed system [49]. It was also noted that an increase in PS dosage from 20 to 30 mM could result in the decrease of dye degradation due to the self-quenching effect of extra radical formation [48].

Similar procedures were followed for conducting both the catalytic degradation and catalytic ozonation except that ozone was utilized in the latter case in addition to other variables. During the catalytic degradation process, the experiment was performed for 20 min and about 34.98, 48.31, and 72.22% of SSY dye was degraded over the catalysts. However, in case of catalytic ozonation process, about 59.11, 66.21, and 76.89% of the dye was degraded in the presence of 2, 4, and 8 mM PS, respectively. From these observations, it is evident that catalytic ozonation yields better results compared to the catalytic degradation process for SSY dye. Our findings align well with a previous study on the degradation of azo dyes, where the authors compared the outcomes of catalytic degradation with and without the presence of ozone [50].

Regarding the kinetics studies, various models were tested again but the second order kinetics model gave acceptable results. For this purpose, the inverse of dye concentration (1/[C]) was plotted against time intervals (t) in the presence of various concentration of PS such as 2, 4, and 8 mM as shown in Figure 4(c). Straight lines were obtained when 1/C versus time were plotted which revealed that the data follow the order kinetics where the rate constant were calculated from the slopes of these fitting lines [51]. The obtained data fitting, or regression factor (R²) values, were found to be 0.98, 0.93, and 0.96, respectively, for a dye concentration of 20 ppm, a catalyst dosage of 0.5 g, a temperature of 25 °C, and a pH of 3. The values of rate constant (k), R², and half-life are given in Table 2. The rate constant (k = 0.0027) is greater for 8 mM of PS, compared to the other concentrations of PS in a solution. Similarly, the half-life values for PS dosages such as 2, 4, and 8 mM are found to be 33.12, 53.97 and 23.55 min, respectively, which reveal that the least value of half-life is obtained for 8 mM of PS utilization. Similarly, the second-order kinetics equation also best fits the data of catalytic ozonation at different ozone concentrations, such as 1, 2.5, and 5 g/L (Figure 4(d)). The Regression factor (R²) and rate constants (k) values for experiments conducted with 0.5 g of catalyst, 2.5 g/L of ozone, 20 ppm dye concentration, and at pH 3 are noted to be 0.931, 0.942, 0.98, while the observed rate constant values are 0.005, 0.0067 and 0.0097 (M⁻¹min⁻¹) at 2, 4, and 8 mM PS dosages, respectively. Highest value of R² and k while the lowest value of half-life (t_1/2) are observed for 8 mM PS dosage employed in catalytic ozonation as well (Table 3). Similarly, the Orange II dye also followed second-order kinetics, with an R² value of 0.99 and a rate constant of 0.1 × 10⁻². The 10 ppm dye solution was degraded in one hour in the presence of a 50 watts LED Light, 0.03 g of Fe₂O₃–TiO₂ photocatalyst and 0.025 mL of H₂O₂ as presented in Table 3. Again, our results are in good agreement with the previous report on other dyes [52].

3.2.3. Effect of solution pH at different time intervals

The effect of solution pH (acidic, neutral, and basic) was also studied on the catalytic degradation of SSY, to report the optimum pH conditions for the desirable results. The solution pH values were 3, 7 and 11, while the other parameters such as dye concentration, amount of catalyst, and concentration of PS, were kept constant. The results for catalytic degradation and catalytic ozonation at different pH values are given in Figure 5(a–d). We observed that after 20 and 110 min, the catalytic degradation was 72.22 and 83.13%, respectively at pH 3, 43.08, and 53.6% respectively at pH 7, and 22.64, and 29.2% at pH 11 (Figure 5(a)). The percentage degradation pattern followed the order of pH 3 > pH 7 > pH 11 similar to the trend of Fenton reaction [28]. For degradation of dye at pH 3, the availability of ferrous ions was maximum but at neutral and basic medium, the ferrous ions made some insoluble complexes such as Fe(H₂O)₅(OH)⁺ and Fe₂(H₂O)₄(OH)₂. These complexes have less affinity for PS activation and finally the complexes such as [Fe₂(H₂O)₇(OH)₃]³⁺_, [Fe(H₂O)₈(OH)₂]⁴⁺ and [Fe₂(H₂O)₇(OH)₅]⁵⁺ could be transformed to Fe(OH)₃ which can leads further condensation and stabilization of contaminants, thereby decreasing the degradation rate [53]. More fruitful results obtained at pH 3 in comparison to the other pH values may be due to some reasons. It could enhance the reactivity of magnetite, which can led to the surface charge to become more positive and as a result, the PS reactivity of the surface site could increase. This results in the production of more sulfate radicals, which can react with SSY dye. Similarly, at low pH value, the PS are also stable and exhibit long half-life and gain extra time to react with magnetite surface and produce sulphate radicles which can then react with dye to degrade it into fragments. Similar observations for methyl orange dye degradation were reported in which 90% of dye was degraded at pH 3, but a slight decrease was observed when the pH value increased above this value. The acidic medium might provide extra conductivity to the solution for the better dissolution of iron [54]. Thus, extra Fe²⁺ would emerge at the lower pH medium solutions, which would react with PS to generate extra ${\mathrm{\text{SO}}}_4^{-2}$ ions, as a result, the decolorization of dye is enhanced. It is supposed that the activation of PS is due to formation of hydroxyl radicals not due to the ferrous ions as presented in Equations (9)(9) $${\mathrm{\text{SO}}}_4^{\mathrm{*}}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{\text{OH}}}^{\mathrm{*}}+{\mathrm{\text{HSO}}}_{4\mathrm{ }}^{-}\mathrm{ }$$(9) and (10)(10) $${\mathrm{\text{SO}}}_4^{-}+{\mathrm{\text{OH}}}^{\mathrm{*}}\to {\mathrm{\text{OH}}}^{\mathrm{*}}+{\mathrm{\text{SO}}}_4^{-2}$$(10) . Most of the other reports also concluded best dye degradation results in acidic medium [55]. (9) $${\mathrm{\text{SO}}}_4^{\mathrm{*}}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{\text{OH}}}^{\mathrm{*}}+{\mathrm{\text{HSO}}}_{4\mathrm{ }}^{-}\mathrm{ }$$(9) (10) $${\mathrm{\text{SO}}}_4^{-}+{\mathrm{\text{OH}}}^{\mathrm{*}}\to {\mathrm{\text{OH}}}^{\mathrm{*}}+{\mathrm{\text{SO}}}_4^{-2}$$(10)

Figure 5. Effect of solution pH on the (a) catalytic degradation (b) catalytic ozonation, (c) Second order kinetics for catalytic degradation, and (d) second order kinetics for catalytic ozonation.

Likewise, a similar approach was adopted for the effect of solution pH on the catalytic ozonation while using similar experimental conditions except that ozone at the concentration of 2.5 g/L was used in this case. The SSY dye degradation efficiency was noticed as 80.14, 70, and 62.67% at pH 3, 7, and 11 respectively while performing the reaction for 25 min. Again, higher degradation efficiency was observed at pH 3 in catalytic ozonation as well (Figure 5(b)). At the same pH values (pH = 3), the degradation efficiency was 80.14% for catalytic ozonation which is higher in comparison to the catalytic degradation (72.22%). The effect of pH is more pronounced in acidic media compared to neutral and basic media, as discussed in the catalytic degradation section.

Comparatively, the dye removal performance for O₃/PS/Fe⁺² is greater, compared to that of the O₃/Fe⁺² at pH 3–4, but it decreases with increasing pH above this range. In acidic medium, hydrates are made by Fe⁺² at pH 1–4. However, beyond this pH range, no hydrate formation occurs; thus, additional PS could be activated by Fe⁺² for organic dyes. An earlier report [56] also shows that when the solution pH is raised from 4 to 10, the Fe⁺² tends to form precipitates of iron hydroxide, leading to reduced Fe⁺² availability in the presence of PS for S $O_4^{-*};$ the dye removal process is supposed to be less effective under these conditions. The breakdown of the ozone molecule into HO* radicals by a basic media also increases the removal of organic dye. In previous studies, these two factors had been balanced out, resulting in a nearly constant rate of removal of methyl orange dye at pH above 4 [56]. At low pH, the ozone is more stable and has longer half-life which could lead to the production of hydroxyl radicals and those radicals can react with dye and convert it into smaller fragments. The activation of PS by magnetite only at pH 3 for the removal of SSY dye follows reactions mentioned in Equations (11)(11) $${\mathit{\text{Fe}}}_3{O}_4+S_2{O}_8^{-2}\to {\mathit{\text{Fe}}}^{+2}+S{O}_4^{\text{−*}}+S{O}_4^{\mathrm{-}2}$$(11) and (12)(12) $$S{O}_4^{\text{−*}}+\mathit{\text{SSY}}\to \mathit{\text{Products}}\mathrm{ }$$(12) . (11) $${\mathit{\text{Fe}}}_3{O}_4+S_2{O}_8^{-2}\to {\mathit{\text{Fe}}}^{+2}+S{O}_4^{\text{−*}}+S{O}_4^{\mathrm{-}2}$$(11) (12) $$S{O}_4^{\text{−*}}+\mathit{\text{SSY}}\to \mathit{\text{Products}}\mathrm{ }$$(12)

The $S{O}_4^{-*}$ radicals, which are formed from decomposition of PS, react with dye and convert it into fragments, while magnetite acts as a catalyst during this process. Similarly, the activation of PS by ozone and magnetite follows the reactions mentioned in Equations (13)–(17). (13) $$O_{3\mathrm{ }}+H_{2}O\to O{H}^{\mathrm{*}}+O_2^{‐\mathrm{*}}$$(13) (14) $$O_2^{‐\mathrm{*}}+{\mathit{\text{Fe}}}_3{O}_4\to {\mathit{\text{Fe}}}^{+2}+O_2$$(14) (15) $$O_{3\mathrm{ }}+{\mathit{\text{SO}}}_4^{-2}\to O{H}^{\mathrm{*}}+S{O}_4^{‐\mathrm{*}}$$(15) (16) $${\mathit{\text{Fe}}}^{+2}+S_2{O}_8^{-2}\to {\mathit{\text{Fe}}}^{+3}+S{O}_4^{‐\mathrm{*}}+S{O}_4^{-2\mathrm{ }}$$(16) (17) $$S{O}_4^{‐\mathrm{*}}+\mathit{\text{SSY}}\to \mathit{\text{Products}}\mathrm{ }$$(17)

When ozone is added to the dye solution along with PS and magnetite, it produces the hydroxyl ($O{H}^{*}$) and superoxide ($O_2^{-*}$) radicals. These radicals react with dye and produce less harmful products. Similarly, the magnetite produces sulfates from PS and also removes the superoxide radicals. Therefore, due to the production of hydroxyl radicals, the ozonation is better than catalysis.

The kinetics models were also applied to the data related to the effect of solution pH on the catalytic degradation and catalytic ozonation of SSY dye and we observed that the data fit to the second order kinetics model, compared to the other models. These results for such fitting can be seen in Figure 5(c,d). For the catalytic degradation of SSY dye, the Regression factor values of second order kinetics models are 0.710, 0.740 and 0.530 at pH 3, 7, and 11, respectively. Similarly, k values of second order are 1.43 × 10⁻³, 3.660 × 10⁻⁴ and 1.220 × 10⁻⁴ (M⁻¹min⁻¹) at pH 3, 7, and 11, respectively. This reveals that the higher rate constant (faster degradation) is observed at pH 3, compared to the other pH values. The corresponding half-life values are found to be 78.71, 176.63 and 514.46 min at pH 3, 7, and 11, respectively (Figure 5(c)). Our results for catalytic degradation, under various pH values are in good agreement with the previous report [57]. Correspondingly, the data for the effect of catalytic ozonation also follow second order kinetics model based on their high regression factor (R²) values as shown in Figure 5(d). The values of rate constant (k) are found to be 0.011, 0.0083 and 0.0048 (M⁻¹min⁻¹) at pH 3, 7, and 11 respectively. The corresponding half-life values are observed to be 5.77, 6.68 and 11.77 min at pH 3, 7, and 11, respectively. A similar trend in kinetics under various pH conditions was also noted for the removal of other pH-sensitive pollutants [58].

3.2.4. Effect of ozone concentration in degradation mixture at different time intervals

The effect of ozone concentration in the reaction mixture toward degradation of SSY dye solution was also studied at different interval of time while using 0.5 g of catalyst, 8 mM PS and 20 ppm concentration of dye. The results for catalytic ozonation under these conditions are given in Figure 6(a,b). We observed that the extent of degradation increases with the increase in concentration of ozone from the inlet, and 19.92, 60.44 and 79.72% of SSY degradation occurred at the ozone inlet of 1, 2.5, and 5 g/L, respectively during experiments for 25 min. This shows that 5 g/L of ozone inlet has maximum capacity for degradation (Figure 6(a)). Similar results were also observed for ozone dosage in case of degradation study for methyl orange dye [59].

Figure 6. Effect of ozone concentration on the (a) catalytic ozonation, (b) second order kinetics for catalytic ozonation.

Figure 7. Effect of initial concentration of SSY dye on the (a) catalytic degradation (b) catalytic ozonation, (c) Second order kinetics for catalytic degradation, and (d) second order kinetics for catalytic ozonation.

Regarding the kinetics investigation, it was observed that the data well-fit to the second order kinetics model at higher ozone dosage such as the Regression factor (R²) values are 0.82, 0.90 and 0.96 at 1, 2.5, and 5 g/L ozone dosages, respectively as revealed in Figure 6(b). Similarly, the observed rate constant (k) values are 1.5 × 10⁻², 4 × 10⁻³ and 1.38 × 10⁻⁴ (M⁻¹min⁻¹) for 1, 2.5, and 5 g/L of ozone dosage, respectively. This reveals that the degradation rate is faster at 1 g/L of zone. Similarly, the half-life values for ozone inlet of 1, 2.5, and 5 g/L are 6.26, 11.8 and 339.84 min, respectively. A similar trend, such as an increase in the % removal and decrease in the rate constant with the ozone inlet dosage was reported in the previous study for other dyes as well [60].

3.2.5. Effect of initial concentration of SSY in degradation mixture at different time intervals

The effect of varying the initial concentration of SSY dye for catalytic degradation and catalytic ozonation was also investigated. Various dye concentrations in the range of 10–40 ppm were used at 0.5 g of magnetite, 8 mM PS concentration at pH 3, at room temperature (25 °C) and the results are shown in Figure 7(a–d). The catalytic degradation percentage of SSY dye for its initial concentrations such as 10, 20, 30, and 40 ppm are 17.32, 20.57, 25.17, and 32.79%, respectively after 20 min of the experiment. However, after 110 min of the activation of PS by magnetite, the degradation percentages for 10, 20, 30, and 40 ppm are 20.5, 25.57, 33.63 and 38.79%, respectively as can be seen from Figure 7(a). However, beyond 40 ppm dosage of dye, the % removal decreased. Similar outcomes were reported for 2,4-DCP removal with the help of CeO₂, TiO₂ and MIL₅₃Fe MOF [61].

Similarly, regarding the effect of initial dye concentration on the catalytic ozonation, the observed % degradation for 10, 20, 30 and 40 ppm was 50.32, 63.41, 72.63 and 77.47%, respectively after 25 min of experiment and the results are shown in Figure 7(b). Again, a decrease in the percentage removal of dye with an increase in dye dosage after 40 ppm may be due to the non-selective nature of the mixture, which may drop the net decolorization rate of dye substantially [49]. Likewise, we observed 38 and 92% degradation of a 40 ppm dye solution after 110 and 25 min of experimentation for catalytic degradation and catalytic ozonation, respectively. This reveals that the ozonation based catalytic degradation is more efficient that simple catalytic degradation. Notably, a complex type of oxidative mechanism is involved in the degradation of organic dyes by using magnetite/PS and magnetite/PS/ozone. This is because, during this process, the reactive oxygen species and secondary oxidants are produced [62]. In addition, sulfate radicals (SO₄^•−) are also formed, which can break down the azo bonds, leading to simpler and uncolored fragments.

Analysis of the kinetics revealed a good fit of the data to the second-order kinetics model. When tested with the second-order kinetics model, R² values for catalytic degradation were found to be 0.60, 0.70, 0.81, and 0.90 for dye concentrations of 10, 20, 30, and 40 ppm, respectively (Figure 7(c)). Similarly, the rate constant values for 10, 20, 30 and 40 ppm are observed to be 0.023, 0.0108, 0.0073 and 0.0082 (M⁻¹min⁻¹), respectively. Likewise, the observed half-life (t_1/2) values are 3.735, 5.0460, 5.77 and 4.37 min for 10, 20, 30 and 40 ppm, respectively during the catalytic degradation. The obtained results are in good agreement with the previous kinetics study [63]. In case of the catalytic ozonation, the Regression factor (R²) values for the second order kinetic model are obtained to be 0.94, 0.97, 0.97 and 0.96 for 10, 20, 30 and 40 ppm of SSY dye, respectively (Figure 7(d)). The rate constant and half-life values are 0.041, 0.0040, 0.0043, 0.0040 (M⁻¹min⁻¹) and 19.02, 10.5, 7.67, 6.0 min for 10, 20, 30 and 40 ppm, respectively (Table 3).

4. Conclusions

In summary, we have fabricated magnetite nanoparticles via co-precipitation method and applied the activation of persulphate for the degradation of SSY dye. The synthesized nanoparticles were characterized by using SEM, EDX, FTIR and XRD for their crystallinity, phases, surface analysis and morphology. The magnetite acts as a catalyst for activation of persulphate toward catalytic degradation and catalytic ozonation of SSY dye. Magnetite demonstrates a maximum degradation of 83 and 92% for catalytic degradation and catalytic ozonation of SSY dye after 100 and 25 min of experiment, respectively. This demonstrate that the catalytic ozonation can achieve the desired goal in a shorter period of time, compared to the catalytic degradation under the same experimental conditions. Kinetics investigation indicate that the dye degradation follows a second order kinetics and reflect that the dye removal process is physicochemical in nature. It is found that the increase in concentration leads to the increase in dye degradation of SSY dye in short time. Furthermore, comparison of the present hybrid system with the previous literature and the different sections of the present investigations reflects that the difference in the degradation efficiencies of various materials is affected by the chemical composition, texture and morphological behaviors, such as different crystallinity, phases, surface morphology, solvent conditions, pH, temperature, amount of catalyst, amount of ozone, and also the amount of PS addition. Similarly, it is speculated that inducing charges on the surface of PS create additional active sites for the efficient dye degradation. These investigations demonstrate that the PS/magnetite/O₃ hybrid system could degrade the SSY in aqueous solutions to a higher degree in comparison to the Fenton’s reagent and simple catalytic degradation process. The materials/methods followed in this work can be carried out in any less-advanced setup and does not need highly skilled scientific personals. In addition to the useful applications of the present system for the removal of many other toxic pollutants through an inexpensive approach, this study may be equally useful from academic point of view as well.

Acknowledgments

The authors are grateful to Abdul Wali Khan University Mardan, Pakistan, and Prince Sultan University, Riyadh, Saudi Arabia for the overall support during this work. Authors would like to thank Prince Sultan University for paying the APC.

Disclosure statement

No potential conflict of interest was reported by the author(s).