Intensification for the Degradation of Paraphenylenediamine using Hydrodynamic Cavitation Coupled with Advanced Oxidation Processes

2.1 Materials

All the required chemicals, like Paraphenylenediamine (PPD), hydrogen peroxide (H₂O₂- 30% w/v) and tetra-isopropoxide (TTIP) of AR grade, were obtained from S D Fine Chemicals Ltd., Mumbai, India.

2.2 Experimental set-up and analytical method

The experimental set-up of the HC reactor is depicted in Fig. 1. It includes a storage capacity of 15 L, control valves (V1–V3), a cavity-generating system, and a suction discharge pipe with bypass. The plunger pump features a storage suction tube, and the pump discharge is divided into mainline and bypass. To control the flow of liquid through the main tube to the desired pressure, a bypass is used. The pressure gauges P1 and P2 display the upstream and downstream pressures, respectively. The cavity producing device, an orifice plate, is housed in the main tube. The cavitation of different intensities is generated by using several orifice plates. Plate No. 1 has one hole with a diameter of 2 mm, Plate No. 2 and 3 have three holes with a diameter of 2 mm, Plate No. 4 has one hole with a diameter of 3 mm, and Orifice Plate No. 5 has two holes with a diameter of 3 mm which are depicted in Fig. 1(a-b). The details of the flow geometry is summarised in Table 1. To evaluate the hydraulic characteristics of the reactor, the required cavitation number (CV) for each plate was calculated using Eq. (1).

Cv=\(\frac{P2-Pv}{0.5(⍴ \times {v}_{0}^{2})}\) (1)

Where P2, Pv, Vo and ρ is downstream, vapour pressure, velocity and density of the liquid are under consideration. Table 2 summarises the computed cavitation number and geometrical features of the plates. In addition to the HC reactor, the comparison included an Ultrasonic cavitation (UC) reactor. The UV lamp was coupled with UC and HC when treatment was carried out with the help of hydrogen peroxide.

A twin-beam UV/Vis Spectrophotometer(SL210, Elico, India) was used to determine the concentration of PPD samples. The calibration chart was created by plotting concentration vs absorbance. Five samples of 5, 7,10,12, and 15 ppm were utilised in the calibration, and their absorbance was determined by fixing the wavelength at 530 nm(λ _max). For the five samples stated above, the standard curve is displayed. The concentration of samples obtained after treatment from the reactor was determined using the depicted standard curve. Each sample's absorbance was measured at a predetermined wavelength (λ_max = 530nm). The sample concentration is used to compute the % degradation obtained during the process, and Eq. (2) is utilised to calculate per cent degradation.

Where, C is the sampling concentration of PPD at the time of sampling (min) ;

Co is the initial PPD concentration.

For the correlation, the first-order kinetic was employed, and a rate constant was computed for each of the initial PPD concentrations using Eq. (3).

Where, t is the degradation time (min),

C_o is the initial concentration;

C is the final concentration of PPD (ppm) at each interval of time;

k is the rate constant (min-1).

2.3 Synthesis of nano-sized TiO₂ particles

The ultrasonic-assisted sol-gel approach was used to synthesise TiO₂ nanoparticles utilising TTIP as a precursor. For the ultrasonic-assisted sol-gel technique, the water-to-TTIP ratio was 20:50 on a weight basis. The tip-US type Ultrasonicator was utilised, a probe type Ultrasonicator (Dakshin, Mumbai, India) with a frequency of 20 kHz. This sonicator is mainly used to degrade organic pollutants in waste water and synthesise nanoparticles and composites. A tip horn was employed to produce ultrasonic irradiation between the reactor and the sol-gel mixture in this Ultrasonicator, and hydrolysis of titanium isopropoxide continuously took place under ultrasonic irradiation for 1.5h. The prepared nano-catalyst was then held at room temperature for 20 h for slow hydrolysis, followed by 12 h of drying at ambient temperatures. The prepared catalyst was then dried at a suitable temperature in a rotary dryer to eliminate any moisture content and then calcined at 500^oC for 4h in a muffle furnace with a continuous air flow.

2.4 TiO₂ Characterisation

Pure TiO₂ nanoparticles were characterised entirely by using a powder X-ray diffractometer (Phillips PW 1800. 6-80^o). The crystalline structure and morphology (anatase, rutile and brookite) of TiO₂ were obtained. The Cu-Kα radiation (LFF tube 35 kV, 50mA) was selected for the analysis. Figure 2 (a) depicts the standard stick XRD pattern used for the anatase phase. The surface morphology of the nano-catalyst was studied using a scanning electron microscope (ZEISS SIGMA, 6000 Plus, 103671) coupled with energy-dispersive X-ray analysis (EDX). The crystalline size of the prepared TiO₂ nano-catalyst varies with pH, as depicted in Fig. 2 (b). Figure 2(c) depicts the XRD pattern of TiO₂ catalyst generated by the sol-gel method at varied pH. (calcined at 500^oC for 4h with 60 min of irradiation time) A Micromeritics BET analyser (Micromeritics, ASAP 2020, USA) was used to measure the BET surface area, pore-volume, pore size distribution, porosity, and pore diameter. TiO₂ nanoparticles of crystalline size (14nm), pore size (4.42 nm), pore volume (0.0014 cm³/gm), and surface area of 123 (m²/gm) are achieved in 60 min of preparation.

3.1 TiO₂ synthesis and characterisation

The generalised reaction mechanism of hydrolysis of TTIP with water is

Ti(OR)₄+4H₂O → Ti (OH)₄+4R-OH (4)

Ti (OH)₄→ TiO₂xH₂O + (2 − x)H₂O (5)

Where R is i-propyl [1]

The preparation and calcination temperature method for TiO₂ nano-catalyst (pure anatase phase) is crucial. The effect of variation in the pH of a solution is depicted in Fig. 2(c). A substantial fluctuation in crystalline size was observed at various pH levels, as shown in Fig. 2 (b). In a medium acidic solution with a pH of 2, the smallest crystalline size with a uniform suspension was found. Figure 2 (d) depicts SEM images of nano-catalyst prepared at a pH of 2. EDAX analysis, as depicted in Fig. 2(e), confirmed the presence of Ti and O as elemental components of nano-catalyst. The powder, which had been synthesised at a pH of 2, was dried at 500^oC, yielding a pure anatase phase. When the pH of the solution was raised above 3, more precipitants developed, affecting the properties of TiO₂ nanoparticles (Yuvarajan et al., 2017). The morphology of the TiO₂ nano-catalyst is highly influenced by the amount of water used at the start and the temperature at which it is calcined. (Bethi et al., 2015).

3.2 Optimisation of cavitating device (Orifice plates)

The samples for the analysis were obtained under these controlled conditions. After each 20 min interval, the analysis was conducted using a UV-Visible spectrophotometer. For the analysis, a total of five samples were obtained. For each sample obtained after every 30 min, the change in PPD concentration is converted to a percentage degraded. All of the PPD degradation (percentage) data are depicted in Fig. 3(a), and it can be seen from these results that Plate No. 3 has the highest PPD degradation of all the plates. This is because Plate No 3 has the least cavitation number compared to other plates. Orifice plates with several holes can create higher intensity than plates with only one hole. In HC, the degradation of methyl parathion was carried out using a single hole plate (Patil et al., 2012).

In the multiple hole orifice, greater shear is generated, resulting in more cavities (Rajoriya et al., 2017). The degradation of Rhodamine-b in an HC reactor is affected by orifice plates with varied hole geometry (Sivakumar et al., 2002). In PPD treatment, 16.1% degradation is accomplished in the first 20 min, highest for Plate No 3. The same increasing trends were observed for the remaining plates until the 120 min mark. In the HC reactor, the lowest cavitation number favours dye degradation. The lowest cavitation number promotes cavity generation, resulting in more cavities causing more degradation. As a result, operating a reactor at the lowest cavitation number is preferable. In 120 min, Plate No. 3 has resulted in the maximum PPD degradation of 56.1%. Orifice Plate No. 3 is the optimum orifice plate for degrading PPD out of all the orifice plates. At 6 bar pressure, PPD degradation for all plates with the lowest cavitation number is depicted in Fig. 3 (b). It has been observed that the degradation of PPD using plate no 2 and 3 is more than other plates. But increasing trends are observed intern of degradation (%).

3.3 Effect of upstream pressure on % degradation

The effect of increasing the upstream pressure from 2 to 7 bar in HC reactor set-up on PPD degradation was investigated. When the upstream pressure was increased from 2 bar to 7 bar, the degradation of PPD has been increased. The procedure was carried out using orifice plate no 3 at pH 7, PPD concentration of 15 ppm, and operating temperature of 30^oC to determine the effect of increasing upstream pressure on PPD degradation. The obtained results at all the upstream pressures have been compared and are depicted in Fig. 4(a). PPD degradation appears to be aided by increasing upstream pressure in the HC reactor. For every 20 min of treatment, samples were taken and analysed with the use of a spectrophotometer. The rate of degradation was initially increasing at all pressures, but after 80 min, it appears to be slow at 6 bar and 7 bar. In 120 min of treatment at 6 bar upstream pressure, the highest 56.1% degradation was achieved. At 7 bar, similar patterns were observed. As a result, higher upstream pressure favours PPD dye degradation. Higher upstream pressure creates more cavities, which produces more OH ions, resulting in a decrease in PPD concentration.

When analysing the degradation of PPD among all pressures from 2 bar to 7 bar, 6 bar upstream pressure is found to be optimum. During 20 to 120 min of treatment time at 6 bar pressure, an increasing tendency of degradation was observed.

When the operating pressure increases, the velocity increases due to the opening of the orifice, lowering the cavitation number. A decreasing cavitation number indicates the production of more cavities. Higher upstream pressure causes more cavities, resulting in a lower cavitation number. Prominent hydroxyl radicals are produced when cavities are produced finely. At lower cavitation numbers, the quantity of OH* radicals created enhances the level of oxidation or breakdown of the contaminants contained in wastewater (Rajoriya et al., 2016). PPD degradation increases as upstream pressure rise from 2 bar to an optimal value of 6 bar, then remain constant at 7 bar pressure. The degradation rate was accelerated by increasing pressure, which increased cavitational activity at higher working pressures (Jawale et al., 2018). The collapse rate of the cavity will be faster at higher operating pressures, which will accelerate the dissociation of water molecules into hydroxyl radicals.

Because of super cavitating conditions, which disrupt the generation of bubbles downstream of the orifice, the level of degradation will be the same whether the pressure is increased beyond 6 bar. The generation of large vapour pockets originates from the generation of multiple cavities, where the lower formation of OH radicals occurs (Patil et al., 2012). Operating upstream pressure is employed as an energy source for cavitation in HC and provided power is used as an energy source for cavitation in ultrasonic cavitation (Gogate et al., 2013). In an HC reactor, the degradation of PPD has been determined at various operating upstream pressures ranging from 2 bar to 7 bar. The degradation of PPD increases with increasing upstream pressure until it reaches an optimum value of 6 bar, beyond which it remains constant with increasing upstream pressure to 7 bar. As a result, the HC reactor should be operated at an optimum upstream pressure of 6 bar. This 6 bar upstream pressure has been selected as the optimum operating pressure for all additional experimental runs. The greatest 55.2% degradation was achieved in 80 min at this optimal operating pressure.

Gogate et al. (2013) reported a similar trend in the degradation of orange acid II in an HC reactor, achieving 34.2% decolourisation at a pressure of 5 Kg/cm². The degradation of Alachlor was studied by Wang et al. (2009) for various upstream pressures. According to their findings, the degradation of Alachlor is related to the upstream fluid pressures (Wang et al., 2009). At 4 bar operating pressure, Jawale et al. (2018) obtained 18.5% degradation of potassium thiocyanate in an HC reactor with a chemical oxidant. With a hydrodynamic cavitation reactor operating at 3 to 9 bar, Bhagat et al. (2015) found a 12% degradation of 4-Nitrophenol at 5 bar upstream pressure. Patil et al. (2014) tested Imidacloprid degradation at pressures ranging from 1 to 8 bar, with a maximum of 12.8% degradation at 4 bar. Bagal et al. (2013) studied the degradation of 2, 4-dinitrophenol under various upstream pressures ranging from 3 to 6 bar and concluded that 4 bar is the optimum pressure.

3.4 Effect of operating temperature

Operating temperature on PPD degradation is one of the most important elements influencing cavitation and degradation efficiency. Experiments were carried out at 6 bar upstream pressure utilising orifice Plate 3 to determine the effect of temperature on PPD reduction. The temperature in the HC reactor was varied from 25°C to 40°C, with an interval of 5°C at pH of 7 and PPD concentration of 10 ppm. The obtained results are as shown in Fig. 4.(b). It has been observed that the percentage of PPD degradation increases from 30°C to 35°C and then reduces at 40°C. The degradation of 50.1%, 56.1%, 60.2% and 45.1% is achieved at 25°C, 30°C; 35°C and 40^oC respectively. The highest degradation of 60.2% was achieved at 35^oC operating temperature in 120 min of treatment time.

At higher temperatures up to 35°C, the increase in temperature causes more vapour pressure, which leads to produce more cavitation bubbles, which ultimately increases the rate of degradation. The power of collapsing cavities is reduced at temperatures above 35°C, which decreases the degradation rate. As a result, the optimum working temperature for PPD degradation in HC was found to be 35°C. In the HC reactor, comparable results were observed in the degradation of Rhodamine B, where increasing temperature accelerates degradation (Mishra et al., 2010). When utilising HC to degrade Alachlor, it was found that increasing the temperature from 30°C to 40°C boosted degradation, but later increasing the temperature reduced Alachlor degradation (Wang et al., 2009). In the HC reactor, the degradation of Rhodamine-B was performed using hydrogen peroxide, where higher degradation was reported at 50°C, followed by a fall at 60°C (Wang et al., 2009). The degradation of pharmaceutical wastewater was lower when treated at a lower temperature (25°C) compared to temperatures over 25°C, where the degradation was more than 80%. (Petkovsek et al., 2013). With the application of activated carbon, Saheed et al. (2016) reported the maximum degradation of PPD at 30°C, where a lower temperature was more conducive for PPD adsorption on the active surface.

3.5 Effect of pH

The effect of pH on PPD degradation was investigated at an optimum temperature of 35°C. Other parameters such as upstream pressure of 6 bar, dye concentration of 10 ppm, or orifice Plate 3 have opted for the cavitation effect. The required pH varied from 3 to 9, with a two-point difference. The samples were obtained every 20 minutes for the analysis and their concentrations were measured using a UV-Visible spectrophotometer. PPD degradation was reduced by 15% at pH 3, 24.2% at pH 5, 15.2% at pH 9, and 20.1% at pH 7 in the highly acidic medium in the first 20 min of an interval. PPD degradation was increased with treatment time at all the pH values, although the maximum degradation was found to be at pH 5. At pH 5, 24.2% degradation was accomplished in the first 20 min, with increasing trends continuing until 120 min when 62.5% degradation was achieved. In the neutral environment (pH 7), 60.2% reduction is obtained in 120 min of treatment. All the obtained values of the degradation percentage are depicted in Fig. 4(c). The degradation rate was shown to decrease from 80 to 120 min after treatment. PPD degradation reaches 62.5% in this moderate acidic media, i.e., pH 5.

A lower pH appears to be useful for PPD breakdown in the HC reactor. For further investigation, pH 5 opted as an optimum with other parameters. As a result, increasing the pH from 3 to 5 is more beneficial for boosting PPD degradation. The cavitation effect is accelerated when the pH is kept constant. More cavities are produced under these acidic conditions (Wang et al., 2009; Saharan et al., 2013). Because of the more production of OH radicals, produced cavities will be accountable for wastewater degradation (Pradhan et al., 2010). In acidic conditions, produced radicals have a higher oxidation potential. The pH of the wastewater in question is a regulating parameter that must be optimised for successful dye breakdown utilising HC. As a result, it's critical to figure out the molecular structure and functional groups of dye molecules.

The influence of pH on the breakdown of acid red 88 dye was examined by Saharan et al. (2012). They reported that AR-88 dye gets protonated under acidic conditions because the hydrogen ion is connected to the sulphonic group, resulting in more significant degradation. The protonation action causes AR-88 molecules to become hydrophobic, causing them to settle near the cavity-water interface, where they are more vulnerable to assault by •OH radicals. As a result, the maximum degradation occurs at a lower pH. Wang et al. (2009) investigated the effect of pH on Rhodamine-B degradation, finding that at a pH of 3.0, 99% degradation was accomplished in 180 min of treatment using an HC reactor. They observed that an acidic environment promotes Rhodamine-B degradation. •OH radicals have a high oxidation potential at lower pH. Similarly, Patil et al. (2014) investigated the degradation of imidacloprid in an HC set-up at various pH levels, finding that acidic conditions are more beneficial for pollutant breakdown because to the greater oxidation potential and hydroxyl radical production rates.

At a pH of 3.0, the maximum degradation of 23.85% was reported by Joshi and Gogate (2012) in an acidic media, where the maximum degradation of Dichlorvos could be achieved. As a result, the acidic environment is the most suitable for pollutant breakdown; it also reduces the recombination of •OH radicals in water and raises its oxidation potential. Saheed et al. (2016) found a similar trend in eliminating PPD using activated carbon, finding that a lower pH was beneficial for more significant PPD adsorption. During the treatment of natural hair wastewater with AOP, the pH of the solution was reduced (Bessegato et al., 2018). When employing H₂O₂ and O₃ to degrade o-phenylenediamine, greater degradation was accomplished on the higher side of pH, but pH 6.5 was chosen as the optimum due to the cost of maintaining the pH on the higher side (Arowo et al., 2017). The maximum 56% breakdown of PPD was observed at pH 3.5 when Fenton's reagent was used. PPD reduction is most significant at lower pH due to the generation of more Fe (OH)+; however, when pH exceeds 3.5, oxidation efficiency rapidly decreases due to H₂O₂ auto breakdown, which affects the synthesis of OH• radicals (Shridhara et al., 2014). The effect of pH on the photo degradation rate of the o-phenylenediamine using a solar photocatalyst (CuO/Xzeolite) was investigated in the degradation of OPD using a solar photocatalyst (CuO/Xzeolite). From pH 2 to 7, the rate of degradation was initially increased, then decreased as the pH increased to 10. (Nezamzadeh-Ejhie et al., 2011).

3.6 Effect of initial PPD concentration

The influence of initial PPD concentration on its degradation was investigated utilising the HC reactor at 35°C, 6 bar pressure, and pH 5 with orifice plate 3. Experiments were carried out with varying beginning concentrations of PPD (10, 15, 20, and 25 ppm) to see how well they degraded. After a 20 min, all samples were obtained for analysis to determine deterioration. The results of the analysis are depicted in Fig. 4.(d). The extent of PPD degradation is inversely related to its initial concentration. With a rise in the initial concentration, the degradation starts to fall. The % degradation at 10, 15, 20 and 25 ppm was observed to be 62.5%, 60.2%, 45.1%, and 44.2%, respectively in 120 min of treatment time in an HC reactor using an orifice plate. In terms of final degradation, 10 ppm resulted in the maximum degradation, i.e. 62.5%. Rajoriya et al.(2017) observed the same trends in the degradation of Rhodamine-6G using HC, in that the degradation rate was decreased when concentration increased from 10–50 ppm. Even when HC is employed to degrade imidacloprid and methyl parathion, Patil et al.(2012) and Patil et al.(2014) found that the degradation rate reduces as the initial concentration increases.

The initial dye concentration is another crucial component that influences dye breakdown. Following an extensive investigation, it was observed that the degradation of the pollutant will be lowered at higher initial concentrations. The consumption of •OH radicals will be larger at high concentrations of pollutants. Therefore, degradation will be higher as well, though it may be lower due to the higher loading of concentration in terms of percentage. When the concentration of Rhodamine B was increased from 10 to 50 ppm, Wang et al.(2009) found that degradation was decreased from 98.9–63.4%. Wang et al. (2011) reported a similar observation for the degradation of K-2BP using HC, finding that as the concentration was increased from 10 to 50 ppm, the degradation decreased from 94.2–24.7%. The decrease in degradation is due to the increasing concentration of dyes at the same constant formation of hydroxyl free radicals in HC under the same working circumstances (Rajoriya et al., 2016).

The degradation of imidacloprid followed a similar pattern (Patil et al., 2014). OPD degradation in the range of 10–80 ppm initial concentration was examined using a CuO/X zeolite catalyst and solar photocatalytic degradation. The degradation was enhanced up to 25 ppm, but after raising the concentration beyond 25 ppm, the degradation was lowered. This is because the increase in concentration will disrupt the path of the photons entering the solution. Because more OPD molecules will be adsorbed on the photocatalyst, the number of active sites on the catalyst will be reduced, indirectly reducing the formation of hydroxyl radicals (Nezamzadeh-Ejhie et al., 2011). The degradation of PPD using the photo Fenton procedure showed similar tendencies, with a maximum of 61.3% degradation for 30 ppm of PPD.

Degradation will be lowered when the initial concentration of PPD increases. This is because the high dose of PPD results in a comparatively low concentration of OH• radical accessible, but the dosage of H₂O₂ and Fe ²⁺ remains the same, lowering PPD degradation. The number of pollutant molecules grows as the concentration rises while the HO• radical remains constant, reducing degradation (Shridhara et al., 2014). Arowo et al.(2017) found that a lower initial o-PPD concentration leads to increased degradation efficiency but that at higher concentrations, byproducts are formed in large quantities, creating competition for O₃ and OH, resulting in a decrease in degradation efficiency (Arowo et al., 2017).

Figure 5(a) depitcs a plot of ln (Co/C) vs time, with straight lines passing through the origin for 10, 15, 20, and 25 ppm. PPD degradation in HC follows first-order kinetics. When the starting concentration was increased from 15 to 30 ppm, the rate constant was reduced from 18.3 × 10^− 3 to 9.7 × 10^− 3 min^− 1. Due to the limited availability of OH radicals and the growing pollutant content in the reactor, the degradation rate was lowered. When the initial concentration of Alachlor was increased from 10 ppm to 150 ppm, the rate of deterioration fell from 5.22×10^− 2 per minute to 3.87×10^− 2 per minute, indicating first-order degradation [Wang et al., 2009]. The degradation of imidacloprid and Reactive Red 120 dye in HC is of first-order kinetics, according to Raut-Jadhav et al. (2013) and Saharan et al. (2011). Sivakumar et al.(2002) found that the degradation of rhodamine B similarly was first order. Even The rate constant for phenol oxidation in a water jet cavitation reactor was reduced from 16.57 to 4.5 ×10⁴ s^− 1 for 100 to 400 ppm of phenol (Yiyu et al., 2012).

3.7 Effect of Hydrogen Peroxide (H₂O₂) in HC reactor

The rate of degradation of PPD in wastewater is increased when hydrodynamic cavitation is combined with hydrogen peroxide. With a difference of 100 mg/L, the dose of hydrogen peroxide was varied from 100 mg/L to 400 mg/L. The HC with orifice Plate No. 3 was utilised to create cavitation, which caused PPD to degrade. To determine the extent of degradation over time, a upstream pressure of 6 bar, pH of 5, operating temperature of 35^oC, and an initial PPD concentration of 15 ppm were maintained. After a 20-min period, samples were examined to see if the concentration was still present in the effluent.

As the H₂O₂ dose was increased, degradation increased in the first 20 min of the interval. The maximum degradation was 37.6% after 20 min of treatment with a 400 mg/L H₂O₂ dose, followed by 37.4% at 300 mg/L and 36.1% at 200 mg/L of H₂O₂. Out of these three degradations, 36.1% is considered the optimum because it requires fewer H₂O₂ (200 mg/L) than the other two. Similar trends were seen for all dosages, indicating that the rate of degradation increases when the dose. In 120 min of treatment, the dose of 400 mg/L achieved the highest 73.1% degradation. PPD degradation was nearly constant from 80 to 120 min, and this pattern was consistent across all H₂O₂ dosages. Similar trends were seen with a 200 mg/L dosage of H₂O₂, with the greatest 71.3% degradation occurring after 80 min of treatment and then remaining nearly constant until 120 min. When all of the findings of different doses of H₂O₂ were compared, it was observed that a dose of 200 mg/L has resulted in 71.3% optimum degradation. As a result, it has been demonstrated that oxidants such as H2O2 are more effective in degrading PPD than not using H₂O₂. Figure 5 (b) depicts the results obtained with and without H₂O₂

Hydrogen peroxide is more effective in increasing the HC reactor's efficiency in degrading organic pollutants in wastewater. The H₂O₂ dissociates into •OH radicals during the cavitation action, which degrades the pollutants; thus, the effectiveness of pollutant degradation is dependent on the formation of •OH radicals. Individual applications of H₂O₂ will not generate enough •OH radicals. With the help of HC, the intensification for the generation of •OH radicals can be accomplished. Extreme temperature and pressure enhanced the rate of generation of •OH radicals in the HC effect, which indirectly increased the oxidation of PPD in wastewater. HC and H₂O₂ should be combined to improve efficiency, but the H₂O₂ dose should be optimal. The optimal dose of H₂O₂ is responsible for complete utilisation and avoidance of •OH radical scavenging. The rate of reactivity between •OH radicals and pollutant molecules is surpassed by the rate at which •OH radicals are scavenged by H₂O₂ itself beyond the optimal concentration. A dosage of H2O2 should be optimal to avoid excessive scavenging of •OH radicals by H₂O₂. With the application of high dosage hydrogen peroxide in HC, Saharan et al. (2011) reported the same scavenging effect in the degradation of reactive red 120 dye, and Gore et al. (2014) observed the same scavenging effect in the degradation of Orange-4. According to their findings, degradation increased with increasing H₂O₂ dose up to the optimum level of H₂O₂.

Even when Acid red-88 is degraded, similar findings were seen by Saharan et al. (2012). In the degradation of AR88, he reported that the efficiency of HC + H₂O₂ is two times that of individual HC. Raut-Jadhav et al. (2013) investigated the effect of different concentrations of H₂O₂ on imidacloprid degradation using HC and found that the highest degradation was achieved. Jawale et al. (2014) found that by employing HC + H₂O₂ at 1:5 molar ratios, 51.29% cyanide degradation may be achieved in 120 min. Extraneous •OH radicals are produced by H₂O₂ in the HC reactor, which disperses in the reaction solution and accelerates the oxidation rate. Shivakumar et al. (2002) used HC + UV + H₂O₂ with an opening as a cavitation device to completely degrade the dye Rhodamine-B. Orange acid-II and brilliant green dyes were efficiently decolourised in HC with hydrogen peroxide. The degradation rate increased as the H₂O₂ dose was raised up to the optimal level. With the help of powerful oxidants like H₂O₂, the intensity of oxidation of contaminants rises throughout the cavitation process. The use of 300 mg/L H₂O₂ allows for phenol degradation up to 99.12%. The amount of H₂O₂ required is determined by the dye's nature. Excessive H₂O₂ causes the formation of weak radicals, which reduces the oxidation impact (Yiyu et al., 2012).

In the degradation of Methyl parathion in the HC reactor with the orifice plate, Patil et al. (2012) increased the dosages of H₂O₂ from 25 mg/L to 200 mg/L. To oxidise dyes and intermediates produced during degradation, larger molecules with a high molecular weight and a greater molar ratio of pollutants to H₂O₂ are required. As a result, operating parameters such as pH, temperature, upstream pressure, and dye concentration are used to optimise H₂O₂. The use of AOPs such as H₂O₂ in combination with O₃ and photocatalyst in the treatment of genuine hair colour effluent was effective in degrading p-phenylenediamine (Bessegato et al., 2018). The degradation of o-Phenylenediamine in various concentrations of Hydrogen Peroxide was studied using a rotor-stator reactor. The results show that increasing the H₂O₂ dose increases the degradation, with the highest degradation obtained at 30 mg/L and declining with increasing the H₂O₂ concentration. Scavenging impact lowers OH radical generation at greater H₂O₂ concentrations (Arowo et al., 2017). In a photochemical batch reactor, a similar trend was found in the degradation of PPD; 71.2% degradation was obtained at 50 mg/L H₂O₂ in combination with 3 mg/L Fe²⁺ at 10 ppm initial PPD concentration.

3.8 Effect of H₂O₂ + TiO₂ in HC.

When paired with the HC reactor, the photocatalytic method will be an effective wastewater treatment solution. When combined with the UV effect, the HC and photocatalytic oxidation processes boost the efficiency of the reactor. The catalyst in HC will be present inside the water without accumulation, which will promote oxidation indirectly due to the availability of active sites. Additionally, increasing the generation of •OH radicals from oxidants such as hydrogen peroxide, enhances dye degradation. As a result, large-scale treatment of HC and Photocatalyst with oxidants may be beneficial (Gore et al., 2014).

To degrade the PPD in the HC reactor, TiO₂ is employed as a photocatalyst and H₂O₂ as an oxidant. The dissociation of the water molecule by using TiO₂ will produce •OH radicals. At upstream pressure 6 bar, pH 3, initial concentration of 10 ppm, operating temperature 35^oC, and H₂O₂ dose of 200 mg/L, the dose of TiO₂ was increased from 0 mg/L to 400 mg/L with a difference of 100 mg/L. In the previous section, under the action of H₂O₂ alone in HC, these operating conditions were found to be optimal. Figure 6(a) depicts the acquired results. It has been observed that using TiO₂ accelerates the breakdown of PPD in HC.

The calculated optimum % degradation values without TiO₂ are also compared with different TiO₂ dosages. The highest degradation rates were 71.3% without TiO₂ and 77.4% with TiO₂ (300 mg/L). When testing different TiO₂ dosages, 300 mg/L was shown to be the most effective. The same pattern of PPD degradation was obtained at 400 mg/L of TiO₂ for this dose, with 77.4% degradation attained in 80 min of treatment and then remaining constant until the completion of the experiment, i.e. 120 min. A high dosage of TiO₂ produced results almost identical to those obtained with 300 mg/L. As a result, the HC approach with TiO₂ and H₂O₂ has proven to be more effective in degrading PPD present in wastewater. More free radicals were created due to the combination treatment, allowing more PPD to be oxidised in the effluent.

To degrade Reactive Red-2, Wang et al.(2010) adjusted the dosage of TiO₂ in HC from 25 to 500 mg/L. When the TiO₂ dose was increased, degradation was increased, but degradation was decreased when the dose was increased beyond 100 mg/L. Because a higher quantity of TiO₂ was present in the cavitation zone, similar results are reported in our study, with PPD degradation being nearly constant for TiO₂ doses of 300 mg/L and 400 mg/L. As a result, a smaller catalyst dose results in a faster degradation rate. Raut-Jadhav et al. (2013) employed Niobium pentoxide (Nb₂O₅) at a concentration of 200 mg/L for imidacloprid degradation and found that it was two times more effective than using TiO₂. Bagal et al. (2014) similarly found that the effect of TiO₂ dose on diclofenac sodium degradation was minor with a dose of 200mg/L TiO₂. AOPs (O₃ + Photocatalyst + H₂O₂) successfully cleaned a hair dye effluent polluted with p-phenylenediamine (Bessegato et al., 2018). The TiO₂/MMA nanoparticle was used as a novel photocatalyst for the high photocatalytic activity in the degradation of p-phenylenediamine using titanium dioxide-coated by polymethyl methacrylate. So, as the photocatalytic dose is increased, the breakdown of PPD increases considerably (Chen et al., 2009).

The degradation of o-phenylenediamine was also achieved using a CuO/X zeolite photocatalyst. Photocatalyst dose is always a factor in photocatalytic degradation. The optimum dose was found to be 0.1 gm/L because doses greater than 0.1 gm/L reduced degradation effectiveness by increasing the opacity of the solution mixture and increasing light scattering. A shielding effect occurs when too much photocatalyst is in the bulk solution. As a result, the catalyst's surface area and the photocatalyst's capacity to absorb light will decrease. The particles present at the appropriate dose will boost photon absorption and OPD molecule absorption (Nezamzadeh-Ejhieh et al., 2010; Nezamzadeh-Ejhieh et al., 2011). As a result of the foregoing explanation, it can be inferred that combining HC with TiO₂ photocatalyst promotes the degradation of dye or pollutants contained in wastewater by providing more surface area for the production of OH radicals by the photocatalyst. As a result, when paired with H₂O₂, loading in the range of 100–300 mg/L is determined to be more suited for deterioration. As a result, photocatalyst dose and operating conditions must be optimised for greater degrading efficiency, advantageous in large-scale operations.