Degradation of Procion Golden Yellow H-R Dye Using Ultrasound Combined with Advanced Oxidation Process

Abstract

The current study aims to degrade Procion Golden Yellow H-R through ultrasound-induced cavitation coupled with various oxidants. A comprehensive investigation was conducted to examine the impact of parameters, specifically pH, power, and frequency, on the extent of degradation. The primary aim was to optimize degradation by solely utilizing a cavitation reactor where only 23.8% degradation was observed under the established optimum conditions of pH 2.5, frequency of 22 kHz, and power of 200 W. The investigation of the combined process of cavitation with H₂O₂, Fenton reagent (H₂O₂/Fe²⁺), NaOCl, and potassium persulphate (KPS) was subsequently conducted under optimized conditions. The combined operations greatly enhanced degradation with the use of H₂O₂ loading of 0.1 g/L leading to 53.3% degradation and the H₂O₂/Fe²⁺ ratio of 1:0.25 resulting in 94.6% degradation, while the NaOCl quantum of 0.075 g/L yielded 90% degradation and the KPS quantity of 2 g/L resulted in 97.5% degradation in the specific combinations. A toxicity test on two bacterial strains, Staphylococcus aureus and Escherichia coli, was carried out using the original dye solution and after treatment. The various individual and combination processes were compared using the parameters of cavitational yield and total treatment cost. The study elucidates that combining ultrasonic cavitation with KPS is an effective method for treating wastewater containing Procion Golden Yellow H-R dye, especially when implemented at a larger scale of operation.

1. Introduction

The carcinogenic, mutagenic, and environmentally hazardous nature of pigmented dyes creates significant concerns for the quality of discharge of effluents from textile industries. Dyes are chemical substances that provide color through chromophores, whereas auxochrome functional groups intensify the effect by strengthening the contact between dye molecules and the substrate, such as garment fibers [1]. Various techniques, such as coagulation, ozonation, catalytic oxidation, membrane filtration, solid-phase separation, and reverse osmosis, have been extensively used to eliminate organic contaminants from water. However, most of these technologies are economically unfavorable, lack specificity, and can result in the production of secondary effluents [2].

The wastewater produced during dyeing processes has a low ratio of biochemical oxygen demand (BOD) to chemical oxygen demand (COD), along with a high concentration of color. Hence, advanced treatment is required to yield a greater quantity of biodegradable chemicals [3] as the use of traditional methods such as chemical, biological, and physical treatment technologies for dye wastewater is restricted due to cost inefficiency and disposal issues [4].

Advanced oxidation processes (AOPs) offer the possibility of high efficacy in breaking down azo dyes [5] based on the use of different oxidizing agents, such as hydrogen peroxide, Fenton’s reagent, or ozone. AOPs often break down pollutants by attack of the in-situ generated OH radicals [6]. Ultrasound-induced cavitation has also shown significant promise mainly as a laboratory-scale technique for the destruction of contaminants. The cavitation generates localized areas where high temperature-driven radical reactions, such as the production of hydroxyl radicals can take place based on generated hot spots [7]. There have been reports in the literature dealing with the effective application of ultrasound. For example, Siddique et al. [8] elucidated that ultrasound along with oxidants like ozone, persulfate, H₂O₂, and Fenton’s reagent result in the improved breakdown of various constituents and effective management of real wastewater from the industry.

Azo dyes have the potential to be detrimental to the environment, and releasing them untreated into streams might result in substantial environmental repercussions [8,9]. Reactive azo dyes are often used in dyeing because they produce bright colors, do not fade easily in water, and are easy to use [10]. Reactive dyes have the special function of forming chemical bonds with cellulosic fibers. Due to their high reactivity, they are extensively used in the textile sector [11,12]. Procion Golden Yellow H-R is an important hydrophilic dye often used for dyeing cotton textiles and hence, considered in the current work. The analysis of the literature revealed that there is still no concrete evidence on how well ultrasound can decolorize Procion Golden Yellow H-R, and hence, the current research aims to investigate the efficiency of ultrasonic cavitation-based technologies in degrading Procion Golden Yellow H-R. The study centers on the use of an ultrasonic reactor that employs dual frequencies. For establishing the importance of using various oxidants in combination with ultrasound, we present a brief overview of the literature on advanced oxidation applied for the degradation of dyes. Hassaan et al. [13] studied the oxidation of acid yellow-11 using O₃ and UV-assisted ozonation. The research evaluated the effectiveness of various procedures for decolorization and detoxification. Ozonation decolorization was reported to be very efficient, with over 99% decolorization within 20 min at 100 μg/mL AY-11 dye concentration and pH 9. Using O₃ and UV in a combined mode resulted in a somewhat decreased decolorization efficiency for AY-11, suggesting that trends of any combination cannot be generalized. Modirshahla et al. [14] studied the degradation of Acid Yellow 23 using the Fenton and photo-Fenton methods. The ideal pH for both systems was reported as 3, with photo-Fenton showing a higher efficiency than Fenton and resulting in a higher rate of oxidation. Azzaz et al. [15] optimized the photocatalytic degradation of MB and reported that the degradation of MB was affected by concentration, ionic zinc, NaCl, and flow velocity. To achieve an 85.91% photocatalytic degradation and a desirability of 1.0, an ideal flowrate of 500 mL/min and concentrations of [MB], [Zn²⁺], and [NaCl] of 75 mg/L, 45 mg/L, and 0.125 M, respectively, were considered as being the best. Jaafarzadeh et al. [16] studied the degradation of azo dye as Reactive Orange 107 utilizing magnetite nanoparticles (MNPs) (Fe₃O₄) in a sono-Fenton-like process. Under optimal circumstances (300 W/L ultrasonic power, pH 5, H₂O₂ concentration of 10 mM, and MPN loading of 0.8 g/L), the azo dye was destroyed in 25 min. The TOC study revealed 87% mineralization at 180 min at 100 mg/L dye concentration. Behin et al. [17] utilized response surface methodology (RSM) to optimize the process of ultrasound/ultraviolet-assisted oxidative desulfurization in an airlift reactor. The combination of US/UV/O₃/H₂O₂ was applied for the desulfurization of non-hydrotreated kerosene-containing sulfur and aromatic compounds. The combination yielded the highest desulfurization and de-aromatization yields, with actual extents of 91.7% and 48%, respectively. A desirability function and graphical multi-response optimization process were employed to optimize the operating variables of desulfurization. The most effective conditions were a superficial gas velocity of 0.05 cm/s applied for a duration of 15 min. In another study, Behin et al. [18] examined the degradation of trifluralin, a commonly used pesticide, through the application of an advanced oxidation process in a concentric tube airlift photoreactor. The study investigated the impact of pH, superficial gas velocity, and time on the removal efficiency. It was reported that the airlift photoreactor significantly improved the efficiency and availability of UV light, resulting in complete degradation within 60 min under the optimal conditions of pH 9 and superficial gas velocity of 0.15 cm/s.

The review of the literature proves that the current attempt is a completely new study that looks into using ultrasonic reactors with oxidants like H₂O₂, Fenton’s reagent, NaOCl, and KPS for the degradation of the Procion Golden Yellow H-R dye for the first time. The main goal is to come up with the best cavitation-based treatment method operational at a pilot scale with the usage of oxidants in optimal circumstances. The study is commercially significant because of its innovative character and objective to evaluate various combinations to assess their benefits for Procion Golden Yellow H-R degradation. The study also analyzes the influence of operational factors on the effectiveness of the decolorization and reduction of COD using individual ultrasonic cavitation and also using different amounts of oxidants combined with ultrasonic cavitation. Different processes are compared in terms of the treatment cost, cavitational yield, and kinetic rate constant under the most favorable operational conditions.

2. Materials and Methods

2.1. Materials

The study utilized Procion Golden Yellow H-R dye as the pollutant.

Table 1. Physicochemical properties of Procion Golden Yellow H-R dye.

Parameter	Values
Synonyms	Reactive Orange 12, Reactive Golden Yellow H-R
Molecular weight	739
Molecular formula	C21H14ClN8Na3O10S3
Absorption maxima	420 nm

presents a summary of the physicochemical properties of Procion Golden Yellow H-R, and Figure 1 provides a representation of the molecular structure of the dye. All the chemicals were acquired from Thomas Baker, including H₂O₂ (30% W/V), FeSO₄.7H₂O (98% purity), NaOCl, H₂SO₄ (98%), KPS (98%), NaOH (97%), HgSO₄ (99%), Ag₂SO₄ (99%), and ammonium ferrous sulfates. A solution of Procion Golden Yellow H-R dye was prepared at an initial concentration of 1000 ppm, using tap water and then diluted in accordance with the experimental technique to the necessary volume and required concentrations. Using recently made solutions of 1 M H₂SO₄ and 1 M NaOH, the pH of the solution was adjusted. A Hanna digital pH meter that was calibrated using the standard solutions was used to measure the pH.

2.2. Experimental Methodology

In this study, a dual-frequency ultrasonic reactor was utilized to decolorize Procion Golden Yellow H-R. The schematic of the ultrasonic reactor is shown in Figure 2. The reactor works with either 22 kHz or 44 kHz alone or their combination with variable power between 100 and 300 W. The power was adjusted using a control mechanism on the sonicator, allowing for precise power level changes while keeping the frequencies constant at 22, 44, and 22 + 44 kHz. The sonicator utilized in this study was provided by M/S Dakshin, Mumbai, and is constructed entirely from stainless steel.

The reactor can function at various volumes with a maximum capacity of 10 L [19]. The treatment of dye was initially optimized in the study by adjusting various parameters: the pH varied from 2.5 to 10; frequencies at 22, 44, and 22 + 44 kHz; power in the range from 100 to 250 W; H₂O₂ (0.05 to 0.5 g/L); Fenton reagent ratio in range from 1:05 to 1:1.5 as H₂O₂/Fe²⁺ proportion; NaOCl loading (0.025 to 0.1 g/L); and KPS loading (0.1 to 2 g/L). Each experiment used a 4 L mixture at a fixed Procion Golden Yellow H-R concentration of 25 ppm [19]. Since the objective of the work was to compare different processes and equipment operating conditions, a fixed dye concentration was considered in the current study based on the results of earlier work. A thorough reactor cleaning preceded each experiment. The experiments were performed at a stable temperature of 30 ± 2 °C, utilizing a submersible pump for cooling. Thermocouples were utilized for temperature measurement.

To evaluate the efficacy of conventional methods (without the use of ultrasound), a treatment volume of 4000 mL was utilized for the H₂O₂, Fenton, NaOCl, and KPS treatments. Agitation was achieved using a pitched blade impeller, working at 200 revolutions per minute (rpm). The experiments were carried out for a total of 180 min, during which samples were collected at 30 min intervals for analysis. Different experiments performed over 180 min duration involved various approaches, including US, H₂O₂, NaOCl, and KPS, individually and in combination. The Fenton and US + Fenton studies were performed for 30 min. Data were collected at regular 5 min intervals for analysis in this set of experiments. Through initial experiments, the duration for approaches utilizing oxidants was determined, revealing the highest degradation level during this specific timeframe. The trials were conducted three times with the mean results used for analysis and associated discussion. The errors were within a tolerance of ±2% of the reported mean value.

2.3. Analysis

The dye concentration was measured using a UV-1900 UV–vis Spectrophotometer (Shimadzu, Kyoto, Japan) at a wavelength of 420 nm to assess the degree of decolorization [20]. The COD analysis was conducted using a COD digester obtained from Hanna Equipment Pvt. Ltd., India. The COD levels were calculated by conducting the titration of the digested samples using a (NH₄)₂Fe(SO₄)₂·6H₂O solution (0.03 N) [21].

For the combined approaches, based on the obtained values of kinetic constants, the synergistic index was estimated as follows:

$$f={\displaystyle \frac{{\mathrm{k}}_{\mathrm{cavitation}+\mathrm{additive}}}{{\mathrm{k}}_{\mathrm{cavitation}}+{\mathrm{k}}_{\mathrm{additive}}}}$$

(1)

where

• ${\mathrm{k}}_{\mathrm{cavitation}+\mathrm{additive}}$ = kinetic rate constant of ultrasonic cavitation in conjunction with the additive;
• ${\mathrm{k}}_{\mathrm{cavitation}}$ = kinetic rate constant for only ultrasonic cavitation;
• ${\mathrm{k}}_{\mathrm{additive}}$ = kinetic rate constant of only additive.

The National Facility for Biopharmaceuticals (NFB) conducted toxicity testing using two bacterial strains, Staphylococcus aureus and Escherichia coli, for the samples both before and after treatment. To generate a bacterial lawn, microorganisms were inoculated individually onto Mueller–Hinton agar plates and incubated at 37 °C for 24 h.

3. Results and Discussion

3.1. Effect of pH

The decolorization of Procion Golden Yellow H-R was investigated at different pH levels ranging from 2.5 to 10. The experiment employed a fixed volume of 4 L, 32 °C as temperature, 200 W as ultrasonic power and a period of 3 h. The initial concentration of 25 ppm was kept constant. Figure 3 illustrates that the decolorization was more pronounced in the acidic medium than the basic conditions. The decolorization percentages obtained at different pH levels of 2.5, 3, 4, 7.5, and 10 were 23.8%, 19.6%, 18.5%, 16.7%, and 14.5%, respectively. It was clearly seen that the decolorization achieved its highest level at pH 2.5, which was identified as the most suitable pH for further analysis. The degradation of Procion Golden Yellow H-R E6G is more pronounced at lower pH levels due to its easy penetration into the active zone of cavitation. In addition, the hydroxyl radicals offer a higher oxidation potential at lower pH levels compared to higher pH levels. It has been also reported that, at a higher pH range, the recombination rate of ·OH radicals is higher, resulting in a lower availability for the desired oxidation [22]. The findings are consistent with previous studies that examined various dyes. According to Wang et al. [23], the degradation of methyl orange was found to be higher at a lower pH of 3.5, with a maximum of 10% degradation. In a study conducted by Zhang et al. [24], it was demonstrated that the decolorization of C.I. acid orange was influenced by the pH level, with the highest decolorization observed at pH 2 and with the other conditions set as hydrogen peroxide at a concentration of 5 mM, iron powder as 0.5 g, and ultrasonic power of 201 W. The observation was attributed to the fact that hydroxyl radicals were generated quickly in the acidic environment. Saharan et al. [25] conducted a study on the decolorization of Reactive Red 120 using cavitation and found that a decolorization extent of 60% was achieved at pH 2 after a treatment time of 3 h. The comparison of the various results demonstrates the effectiveness of the current study on the degradation of Procion Golden Yellow H-R, as indicated by the quantitative differences in the degradation extents.

3.2. Effect of Frequency

The current study explored the influence of frequency on Procion Golden Yellow H-R degradation using various operating frequenices as 22 kHz, 44 kHz, and 22 + 44 kHz under the conditions of 4 L as the working volume, 25 ppm as Procion Golden Yellow H-R loading, 32 °C as temperature, power of 200 W, a 3 h treatment time, and an optimum pH of 2.5. The highest decolorization was found at 22 kHz with an actual value of 23.8%, followed by 15.6% at a frequency of 44 kHz (Figure 4). The combined frequency operation of 22 + 44 kHz resulted in a 14.3% decolorization. The investigation results show that the best frequency for Procion Golden Yellow H-R decolorization is 22 kHz. The use of low frequency increases the degree of physical effects and the number of cavitation events, which, in turn, facilitates the degradation reaction. The effectiveness of low-frequency ultrasound, such as 22 kHz, in enhancing the decolorization of Procion Golden Yellow H-R is attributed to its ability to generate more intense cavitation. These intense cavitation events result in stronger physical effects and the production of more reactive species, which are crucial for the breakdown of the dye. In addition, it results in increased cavitation events and improved energy distribution within the solution. In contrast, higher frequencies tend to induce bubble coalescence, resulting in a decrease in the number of active cavitation sites and overall degradation efficiency. The findings of this study are consistent with previous research, emphasizing the importance of frequency in optimizing the degradation process.

Momin et al. [19] also reported that 22 kHz was the most effective frequency for a decolorization of 65.9%, followed by 44 kHz with a decolorization of 50.4%, and finally, the operation of 22 + 44 kHz resulted in a minimum decolorization of 47%. In another study by Xu et al. [26], the degradation of rhodamine B was examined under different frequency conditions (25, 40, 60, and 80 kHz). It was elucidated that exposure to ultrasonic irradiation at an optimum frequency of 40 kHz resulted in a degradation of 97%, whereas a lower degradation of 91% after 30 min of exposure was seen at a frequency of 25 kHz. Rhodamine B degradation of 45.65% and 36.13% after 30 min was seen after exposure to 60 and 80 kHz, respectively. It is thus illustrated that utilizing the optimal frequency is the most effective for degradation and the value is specific to the dye, clearly elucidating the importance of the first study for Procion Golden Yellow H-R degradation.

3.3. Effect of Power

An investigation was also conducted to examine the effect of different ultrasonic power levels (ranging from 100 to 250 W) on the process of decolorizing Procion Golden Yellow H-R. The treatment lasted for 3 h at a pH of 2.5, a temperature of 32 °C, an operating volume of 4 L, an optimum frequency of 22 kHz, and 25 ppm as the Procion Golden Yellow H-R loading. Figure 5 illustrates the correlation between ultrasonic power and dye decolorization, where it is seen that a power of 200 W achieved the highest decolorization of 23.8% after 180 min. Lower decolorization extents of 8.2%, 16.1%, and 22.6% were achieved with ultrasonic powers of 100 W, 150 W, and 250 W, respectively. The utilization of higher power levels of ultrasound typically leads to an increased frequency of cavitation events, until the system’s optimal power limit is reached. Following this stage, the degradation stabilizes or even decreases as a result of a reduction in the intensity of cavitation collapse [27]. In a similar study conducted by Gote et al. [28], comparable results regarding the impact of power on the degradation of Chrysoidine R dye were presented. An investigation utilizing a power range from 80 W to 120 W revealed that the degradation was the lowest (27.03%) when the power level was set at 80 W. As the power level increased, the degradation also increased, with degradation percentages of 42.5% and 45.6% seen at 100 W and 120 W, respectively. The higher degradation at a higher power was attributed to the enhanced generation of more cavities and radicals. Momin et al. [20] used ultrasonic cavitation and oxidants to decolorize Procion Brilliant Yellow H-E6G. Power levels from 75 to 150 W were used for the degradation study, and it was reported that the decolorization increased from 75 to 140 W, with actual values of 3.6%, 7.5%, 8.1%, and 9.88% observed at 75, 100, 130, and 140 W, respectively. Cavitational effects promoted turbulence and free radicals, enhancing decolorization. It was also reported that, from 140 to 150 W, decolorization did not alter considerably, which was explained by the cushioning effects induced by several bubbles combining to create larger ones at much higher powers.

In summary, the studies investigating the effects of operating parameters in the individual ultrasound treatment indicated that the most favorable conditions were pH level of 2.5, frequency of 22 kHz, and a power output of 200 W. The use of only ultrasound resulted in a lower degradation (around 20%), mainly due to the fact that only ultrasound-induced cavitation produces a limited quantum of free radicals. With the objective of enhancing degradation, various additives were applied that can act as a source of oxidizing species based on the dissociation of additives under the cavitating conditions. The best conditions established for only ultrasound were further used in combination with oxidation processes so as to achieve synergistic effects that can also reduce the process costs and energy consumption.

3.4. Effect of H₂O₂ Loading in the Combination of Ultrasound with H₂O₂

Experiments were conducted at varying H₂O₂ loading values of 0.05–0.5 g/L to evaluate the effectiveness of using H₂O₂ in conjunction with an ultrasonic reactor to decolorize Procion Golden Yellow H-R. Figure 6 shows the decolorization results achieved with different H₂O₂ loadings over a 180 min treatment. The decolorization percentages were 46.6%, 53.3%, 42.5%, and 39.8% for H₂O₂ dosages of 0.05, 0.1, 0.2, and 0.5 g/L, respectively. According to the findings, a concentration of 0.1 g/L of H₂O₂ was found to be the most effective, resulting in a decolorization of 53.3% with a kinetic rate constant of 4.00 × 10⁻³ min⁻¹. A separate experiment was conducted using the optimized concentration of H₂O₂ (0.1 g/L) without the use of an ultrasonic cavitation reactor, where a decolorization value of 36.1% was observed at a rate constant of 2.40 × 10⁻⁴ min⁻¹ (

Table 2. Results of kinetic fitting using the first-order kinetic model at a 22 kHz frequency.

Processes	Extent of Decolorization (%)	Extent of Mineralization (COD Reduction %)	First Order
			k (min−1)	R2
Only US	23.8	7.8	1.40 × 10−3	0.969
US + H2O2	53.3	17.8	4.00 × 10−3	0.98
US + Fenton	94.6	29.6	8.97 × 10−2	0.982
US + KPS	97.5	34.4	2.06 × 10−2	0.991
US + NaOCl	90	25.8	1.21 × 10−2	0.988

). The results confirm that the US + H₂O₂ approach has a higher efficiency in terms of higher decolorization (53.3%) compared to the H₂O₂ process alone (36.1%). The improvement can be attributed to the enhanced production of hydroxyl radicals by the decomposition of hydrogen peroxide induced by ultrasound [29], when the optimum loading of H₂O₂ was applied. The decolorization decreased to 39.8% when the H₂O₂ loading was increased from 0.1 to 0.5 g/L, attributed to the scavenging effect of the residual hydrogen peroxide at high loadings, as also observed in the literature [30].

The calculation of the synergistic index was also performed as per the following relationship:

$$f={\displaystyle \frac{4.00\times {10}^{-3}}{1.40\times {10}^{-3}+2.40\times {10}^{-4}}}=2.44$$

(2)

The combined approach of US and H₂O₂ showed a synergetic index of 2.44, confirming the effectiveness of the combined approach compared to the individual processes of US and H₂O₂.

Similar results regarding the ideal H₂O₂ loading and synergism for combined operation have also been documented in academic sources. Lu et al. [31] explored the breakdown of phenol using both cavitation and H₂O₂. The research showed that 300 mg/L of H₂O₂ was the best concentration, leading to a maximum breakdown of 99.12%. The study conducted by Merouani et al. [32] evaluated the impact of hydrogen peroxide, at concentrations ranging from 50 to 1000 mg/L, on the degradation of rhodamine B. It was reported that the rate of degradation rose as the concentration of H₂O₂ increased, peaking at 100 mg/L, beyond which the degradation decreased. It can be thus said that it is necessary to conduct studies to determine the best H₂O₂ loading for the degradation of any contaminant using a combination of ultrasound and H₂O₂, thereby illustrating the importance of the current study for Procion Golden Yellow H-R degradation.

3.5. Effect of the Fenton (H₂O₂/Fe²⁺) Reagent Ratio in the Combination of US + Fenton

In the case of the combined Fenton method and ultrasound, the ratio of the oxidant (H₂O₂/Fe²⁺) was varied across a range of values from 1:0.25 to 1:1.5 maintaining a constant H₂O₂ loading of 0.1 g/L, which was determined as the optimal condition. The experiments were carried out using specific parameters of 25 ppm as the dye concentration and a working volume of 4 L. The ultrasonic power was adjusted to 200 W, the frequency set to 22 kHz, and the pH adjusted to 2.5. The duration of the treatment was set to 30 min [33]. The effectiveness of decolorization using this combination showed variations depending on the ratios of the oxidants of (H₂O₂/Fe²⁺). The actual values of decolorization were 94.6%, 92.5%, 91.3%, and 88% for ratios of 1:0.25, 1:0.5, 1:1, and 1:1.5, respectively, as shown in Figure 7. The US + Fenton method, utilizing a 1:0.25 H₂O₂/Fe²⁺ oxidant ratio, demonstrated exceptional efficacy in decolorizing the dye solution. An impressive decolorization of 94.6% was achieved in just 30 min with a rate constant of 8.97 × 10⁻² min⁻¹. Using the correct loadings of Fenton’s reagent is essential to prevent adverse impacts on decolorization and sludge production [34]. There have been reports indicating that the utilization of high concentrations of H₂O₂ and Fe²⁺ can result in the reduction in the availability of •OH radicals, which, in turn, can decrease the rate at which pollutants are broken down [35].

Another experiment was conducted under optimal Fenton loading conditions without the use of an ultrasonic reactor that yielded 75.6% decolorization with a rate constant of 4.26 × 10⁻² min⁻¹. The rate constants obtained were used to calculate the synergistic index:

$$f={\displaystyle \frac{8.97\times {10}^{-2}}{1.40\times {10}^{-3}+4.26\times {10}^{-2}}}=2.04$$

(3)

The synergetic index value of 2.04 suggests that the combined US + Fenton operation is more preferable compared to the distinct operations of US and Fenton. The research indeed elucidated that US + Fenton was more efficient (94.6%) than Fenton oxidation alone (75.6%).

Harichandran et al. [36] employed the Sono-Fenton method to assess the effect of Fe²⁺ on the degradation of DR81 under fixed conditions of dye concentration, H₂O₂ loading, and ultrasonic frequency. The US + Fenton approach with a concentration of 0.20 g/L Fe²⁺ was found to provide the best decolorization of 98% after 120 min, whereas no increase in degradation was further observed beyond the concentration of Fe²⁺ as 0.2 g/L. In another study conducted by Song et al. [37], the combination of US and Fenton chemistry was examined for its effectiveness in breaking down acid red 88 dye. The researchers found that the optimal ratio of H₂O₂/Fe²⁺ was 18, resulting in a remarkable 98.6% degradation of the dye achieved at a pH of 3 and a treatment time of 135 min. The variations in the obtained optimum oxidant ratio emphasize the necessity of a thorough investigation and the significance of the current research. The current investigation coupled with a comparison with the literature revealed that the particular system influences the ideal oxidant ratio and the obtained intensification due to ultrasonic waves clearly emphasizing the relevance of this study for Procion Golden Yellow H-R degradation.

3.6. Effect of NaOCl Loading in the Combination of US + NaOCl

NaOCl is an important oxidant known for its reliability, convenience, and safety. NaOCl is a frequently used chemical for water disinfection, as well as for wastewater treatment. It is important to note that NaOCl is prone to deterioration with time, particularly when subjected to light, heat, or certain pH conditions. To ensure its stability, the current work ensured that NaOCl was kept in a cold and dark environment while maintaining an alkaline pH.

The current research investigated the effects of combining NaOCl with US at various concentrations on the obtained degradation. The results show degradation extents of 81.7%, 84.8%, 90%, and 85.7% for various loadings of NaOCl of 25, 50, 75, and 100 ppm, respectively. The experiments related to effect of NaOCl were conducted at 200 W, 22 kHz, pH of 2.5, and 25 ppm as the dye loading. The duration of the treatment was 3 h. Figure 8 provides a clear illustration of the highest degradation of 90% observed at a 75 ppm NaOCl concentration, with a kinetic rate constant of 1.21 × 10⁻² min⁻¹. The degradation decreased after the NaOCl concentration surpassed the optimal value of 75 ppm. An additional study was performed to assess the effectiveness of NaOCl alone in decolorization at 75 ppm loading, where the decolorization efficiency of 48% was observed with a kinetic rate constant of 3.50 × 10⁻³ min⁻¹. The combination of US and NaOCl demonstrated a notably higher efficiency of 90% in comparison to the NaOCl process used alone. The synergistic index was estimated using the following equation:

$$f={\displaystyle \frac{1.21\times {10}^{-2}}{1.40\times {10}^{-3}+3.50\times {10}^{-3}}}=2.47$$

(4)

The combined operation of US and NaOCl was indeed more effective than each operation carried out separately, as confirmed by the synergetic index value of 2.47. The main contribution of cavitational effects is the promotion of micromixing and turbulence, leading to increased interaction between the oxidant and the pollutant. Moreover, the improved decolorization and predicted synergistic effects are attributed to the production of hypochlorous acid as an additional oxidant via the interaction of OCl⁻¹ ion with water and its subsequent breakdown into hydroxyl radicals under cavitation.

In a similar study conducted by Tiong et al. [38], the researchers examined the effectiveness of NaOCl combined with ultrasound (US) for decolorizing RB dye. It was reported that the US + NaOCl method demonstrated remarkable efficacy in degrading dyes, with a significant 95% degradation at 1 wt% determined to be the most effective NaOCl loading. Gogate et al. [39] studied the effectiveness of cavitation combined with NaOCl, and it was reported that the combined approach with NaOCl loading of 1.25 g/L resulted in a total effluent decolorization within 20 min.

The results of the current study, along with a comparison to the existing literature, demonstrate the variations in the optimal treatment efficacy and the need for elucidating specific NaOCl loading in the system being studied. Based on the current findings, a concentration of sodium hypochlorite (NaOCl) of 75 ppm was shown to be the most effective for the breakdown of Procion Golden Yellow H-R dye.

3.7. Effect of KPS Loading in the Combination of US + KPS

The study also investigated the impact of KPS addition in the presence of ultrasound at different loadings as 0.1, 0.5, 1, 1.5, and 2 g/L, for which the decolorization values achieved were 24.6%, 36.3%, 50.2%, 74.3%, and 97.5%, respectively. Figure 9 shows that the optimal KPS loading for obtaining a 97.5% decolorization was 2 g/L, with a kinetic rate constant of 2.06 × 10⁻² min⁻¹. Research conducted under ideal circumstances using only KPS without the use of an ultrasonic reactor resulted in a decolorization efficiency of 59.6% and a kinetic rate constant of 4.70 × 10⁻³ min⁻¹. The US + KPS technique produced a greater efficiency of 97.5% than the KPS procedure alone, which reached 59.6% efficiency. The synergistic index was estimated as follows:

$$f={\displaystyle \frac{2.06\times {10}^{-2}}{1.40\times {10}^{-3}+4.70\times {10}^{-3}}}=3.38$$

(5)

The combined US + KPS operation results in a much better efficacy compared to the separate US and KPS operations as provided by the synergetic index value of 3.38.

Patil et al. [40] examined the effects of different potassium persulfate (KPS) concentrations on dye degradation and reported that degradation increased steadily between 0.7 and 3.7 mM. The pattern shown in this research may be explained by the fact that, when KPS loadings rise till the optimal level, sulfate and hydroxyl radical concentrations rise as well in the desired proportions, encouraging an attack on the azo bond, which accelerates the process of deterioration. As per the research, there was a significant decrease in dye degradation when the KPS dose was raised beyond the optimum to 4.4 mM. After analyzing the results of the current experiment, it is recommended to select a concentration of 2 g/L as the optimal level of KPS. The significance of this research is underscored by the fact that the results illustrate the dependency of decolorization and the optimal KPS loading on the specific contaminant.

3.8. Comparison of Mineralization Using Different Approaches

To compare the mineralization attained by using various AOPs combined with a US reactor, change in COD was measured. For every technique, the ideal oxidant loadings were used at constant pH of 2.5, 32 °C as temperature, 22 kHz of frequency, and 200 W of power. The treatment duration for only US, US + H₂O₂, US + KPS, and US + NaOCl was 180 min, while for US + Fenton, it was 30 min. Accurate measurements of COD levels were obtained using a dye concentration of 400 ppm. A strong relationship is shown in Figure 10 between the decolorization and the mineralization quantified by COD reduction, which implies that, under the right circumstances, the original chemicals decompose into intermediates that undergo further oxidation throughout the treatment process, producing the end products, which include water and carbon dioxide.

The US + KPS technique achieved the highest level of mineralization at 34.4%, surpassing other treatment methods, like only US, US + H₂O₂, US + Fenton, and US + NaOCl, which resulted in mineralization values of 7.8%, 17.8%, 29.6%, and 25.8%, respectively. Based on this study, the US + KPS treatment proved to be the most efficient in removing color and breaking down Procion Golden Yellow H-R. The decolorization achieved was 97.5%, with the maximum mineralization observed at 34.4%, under optimal operating conditions. It can be thus said that The US + KPS process efficiently degrades Procion Golden Yellow H-R into mineralized compounds, surpassing alternative methods, because of its increased production of hydroxyl radicals and presence of sulphate-based oxidizing agents as well.

3.9. Toxicity Analysis

The toxicity of both samples against the bacterial strains of Escherichia coli and Staphylococcus aureus was assessed for the cases of the different treatment methods, including US, US + H₂O₂, US + Fenton, US + KPS, and US + NaOCl. Figure 11 displays cultured plates with positive and negative controls labeled as PC (+ve) and NC (−ve), respectively. A positive control was employed to illustrate chloramphenicol’s detrimental effects on bacteria. On the other hand, distilled water served as a negative control, confirming any absence of toxicity and the maximum growth that can occur. The results for the toxicity of the treated and untreated samples revealed that no sample revealed any inhibition. The findings in

Table 3. Toxicity analysis of the untreated and treated samples (only US, US + H₂O₂, US + Fenton, US + NaOCl, and US + KPS).

	Zone of Inhibition in mm
Sr No.	Organism	Untreated	Only US	US + H2O2	US + Fenton	US + NaOCl	US + KPS	Positive Control	Negative Control
1.	E. coli	-	-	-	-	-	-	40	-
2.	S. aureus	-	-	-	-	-	-	28	-

show that the samples contain no hazardous chemicals that might inhibit the development of the applied test microorganisms. The study’s results also imply that the intermediates generated by treatment processes, such as only US, US + H₂O₂, US + Fenton, US + KPS, and US + NaOCl, are non-toxic, clearly supporting the possible application of ultrasonic cavitation as a pretreatment method for biological oxidation.

3.10. Cavitational Yield and Operating Cost for Different Treatment Approaches

The cavitational yield was calculated by dividing the level of decolorization by the power dissipated per unit volume (50 W/L).

Table 4. Results for treatment cost and cavitational yield of various approaches at 22 kHz frequency.

Scheme	Extent of Decolorization (%)	Cavitational Yield (mg/J)	Energy Required (kWh)	Total Operational Cost (INR/L)
Only US	23.8	1.10 × 10−5	6.30 × 10−1	5.34
US + H2O2	53.3	2.47 × 10−5	2.81 × 10−1	2.89
US + Fenton	94.6	2.63 × 10−4	2.64 × 10−2	0.75
US + KPS	97.5	4.51 × 10−5	1.54 × 10−1	4.68
US + NaOCl	90	4.17 × 10−5	1.67 × 10−1	1.44

shows the statistics for cavitational yield. The cost of treating Procion Golden Yellow H-R degradation was estimated based on the energy requirements and the cost of chemicals involved in the process, as per the details explained in our earlier work [19]. The US + Fenton process revealed a cost of 0.75 INR/L, which was much cheaper than the other procedures. The US + Fenton approach has the greatest cavitation efficiency, estimated at 2.63 × 10⁻⁴ mg/J. It is important to note that Fenton process also generates sludge whose disposal requires additional costs. The cavitational yield attained with solely US was 1.1 × 10⁻⁵ mg/J, with a treatment cost of 5.34 INR/L. The treatment costs for the combined technique are generally lower than the independent ultrasonic cavitation-based devices. Additional improvements to cavitating devices and the utilization of affordable oxidants have the potential to decrease the overall treatment expenses, thus providing opportunities for future research. Momin et al. [41] also investigated the efficacy of combining cavitation and oxidants for breaking down reactive violet 1. The report observed that the combination treatment method of cavitation and Fenton was shown to be the most successful in degrading reactive violet 1, which is consistent with the results of this investigation. The observed association between treatment costs and particular effluent contributes to the study’s importance with general inference that combined techniques under optimized conditions would help to reduce treatment costs.

4. Conclusions

The study illustrates the effectiveness of using an ultrasonic reactor along with various oxidants, including Fenton, KPS, H₂O₂, and NaOCl, to treat wastewater containing Procion Golden Yellow H-R. The use of US alone resulted in a degradation of 23.8% within 180 min; however, combining the KPS technique with the US resulted in a much greater efficiency of 97.5% in only 180 min. The kinetics of Procion Golden Yellow H-R degradation followed a first-order mechanism for each method used. The highest mineralization (34.4% COD reduction) was observed with the US + KPS process, which also minimized sludge formation compared to the Fenton method. Although the US + Fenton process was the most cost-effective at 0.75 INR/L with 94.6% decolorization, it did produce sludge, which can incur additional disposal costs. In contrast, the US + KPS method, though more expensive at 4.68 INR/L, offered superior decolorization without sludge issues. Toxicity tests confirmed that all treated samples were free of harmful intermediates. In conclusion, the US + KPS process demonstrated its superiority in degrading Procion Golden Yellow H-R, particularly when applied on a larger scale. The method also produces less sludge compared to the Fenton process and achieves higher levels of decolorization. It is also important to understand that the treatment costs are specific to the best conditions applied for the degradation and can vary depending on the type of cavitation reactor and oxidants used for the effluent treatment.