Enhancement and Mechanism of Rhodamine B Decomposition in Cavitation-Assisted Plasma Treatment Combined with Fenton Reactions

Abstract

In our previous study, a novel method combining underwater high-voltage plasma discharge with acoustic cavitation (ACAP) was developed and implemented using rhodamine B (RhB) as a model organic pollutant. Results revealed that injecting argon gas into the ACAP reactor positively influences RhB decomposition efficiency, but there is still further potential for improvement. The aim of this study was therefore to further improve the efficiency of the ACAP process through Fenton reactions. Two options for ferrous ion supply were considered: the addition of FeCl₂ or the dissolution of iron from ACAP reactor steel parts into the RhB-containing solution. The results revealed that the degradation efficiency is increased by 20% due to the Fenton reactions when the concentration of ferrous ions reaches an optimal value. Lower pH was found to be desirable for the effect of Fenton reactions. Based on measurements using high performance liquid chromatography, a plausible mechanism of RhB degradation by the ACAP process assisted by Fenton reactions is additionally proposed and discussed.

1. Introduction

Clean water is a vital resource for both animal and human life. Growing populations and industrialization have led to a shortage of clean water in many countries. At present, a wide variety of methods exists worldwide to purify water, including chemical, physical, and biological methods of treatment. Recently, much attention has been placed on physical treatment methods to purify water to meet growing demand [1]. The attractiveness of these methods lies in their ability to eliminate or greatly reduce the use of chemicals in comparison with chemical water treatment methods. It should be noted that the use of chemicals in water purification is greatly restricted in areas such as food processing, medical applications, and drinking water. Physical methods of water purification have gained wide acceptance in the treatment of wastewater containing organic pollutants that are difficult to decompose. Physical methods lower wastewater treatment costs and minimize by-products; hence, they are environmentally friendly. Underwater plasma, ultrasound, and UV irradiation, as well as their combinations, have been proposed [2,3,4] for the highly efficient treatment of wastewater.

In the recent past, advanced oxidation processes (AOPs) have been implemented as innovative tools for treating wastewater [5]. Introducing radiative, electrical, or chemical energy and catalysts in the reactors can improve the generation of highly reactive radicals that can chemically attack and degrade pollutant molecules [6]. Electrical plasma discharge has proved to be highly efficient in the removal of organic pollutants, with great environmental compatibility. It is associated with extreme physical conditions capable of generating highly reactive radicals. However, plasma generation in water presents challenges due to the high breakdown voltage required. Additionally, pollutants in water may increase the ionic component of water electrical conductivity, hence making it difficult for breakdown to occur. Ultrasound irradiation generates acoustic cavitation, a physical phenomenon including nucleation, growth, and implosion of gas- or vapor-filled cavities. This cavitation results in the creation of high pressure and temperature hot spots, where a great quantity of highly reactive radicals is generated. However, ultrasonic irradiation-based wastewater treatment has limitations, because the acoustic cavitation zone is restricted in size due to the significant attenuation of ultrasonic wave energy that occurs with the distance traveled from the ultrasonic source to the cavitation zone [7].

In our previous research, we proposed a novel technique for wastewater processing: acoustic cavitation-assisted plasma (ACAP). This technique combines high-voltage pulse discharge and acoustic cavitation [8,9]. In the next step, it was shown that the use of gas injection in the ACAP reactor offers a promising improvement in the process efficiency, which is especially pronounced when argon or oxygen in injected at higher flow rates. This is achieved through the dissolution of gas introduced into the solution, which results in the reduction of the required breakdown voltage and improvement in plasma and cavitation generation [10].

Fenton reagents (Fe²⁺/H₂O₂ combined) have received wide recognition as powerful oxidants with focused applications in dyed wastewater treatment [11]. Reactions between peroxides (commonly hydrogen peroxide (H₂O₂)) and iron ions, generally known as Fenton or Fenton-related reactions, form reactive oxygen species that act to oxidize both organic and inorganic compounds [12]. In recent research, the synergic effect of Fenton reactions in advanced oxidation processes (AOPs) has been explored, aimed at improving organic pollutant removal [13]. Typically, Fenton reagents such as hydrogen peroxide and ferrous salts are added to the reactor. However, when acoustic cavitation and plasma discharge are used for wastewater treatment, hydrogen peroxide is produced in water as a by-product. Ferrous ions can additionally be supplied to the reaction system from metal parts of the equipment used for the treatment. Typical examples of such parts are the ultrasound sonotrode, the electrode, and the container when made of steel or iron. This approach may provide an advantageous, environment-friendly way to enhance treatment efficiency without the use of additional chemicals.

Based on the above, the goal of this study was to investigate the effects of Fenton reactions on the efficiency of rhodamine B degradation when its aqueous solution was treated in the ACAP reactor, which combines acoustic cavitation, high-voltage plasma discharge, and gas (argon) injection. The selection of argon for gas injection was based on the results of our previous studies [9,10], which revealed that argon injection enhances RhB decomposition efficiency significantly. However, even in the current case, it was difficult to achieve adequately high decomposition efficiency in a short treatment time. In the present study, the contribution of Fenton reactions to rhodamine B degradation in the ACAP process was investigated under various concentrations of Fe²⁺ ions and pH values. The improvement in the effect of Fenton reactions is discussed on the basis of our previously research. The following experimental section introduces the equipment and ideas.

2. Experimental

2.1. Experimental Setup

Rhodamine B (Wako Pure Chemical, Osaka, Japan) (hereinafter RhB) was used as a model water pollutant in the present experiments. Ferrous chloride, FeCl₂ (99%, Kojundo Chemical Laboratory, Tokyo, Japan), was used as a source of Fe²⁺ ions. Ammonium molybdate, potassium iodide, and phthalic acid were purchased from Wako Pure Chemical, Japan, and were used for preliminary treatment of H₂O₂-containing samples before measurements of H₂O₂ concentration. Potassium iodide reacts with hydrogen peroxide when ammonium molybdate and vanillin acid are present, yielding iodovanillic acid as detected by UV absorption at 352 nm [14].

Details of the experimental setup have been reported in our previous papers [9,10]. Therefore, only a brief explanation on the setup will be given here. The three main parts of the ACAP reactor were a cylindrical ultrasound sonotrode of 24 mm in radius, one part of which was made of ceramic and the other of titanium or stainless steel (SUS304); the second was a needle-shaped high voltage electrode made of 1 mm tungsten wire; and the third was a cylindrical vessel (diameter 130 mm, height 300 mm) made of acrylic resin. The sonotrode was screwed into a piezoceramic transducer powered by an ultrasonic generator (WS-1200-28, Honda Electronics, Toyohashi, Japan) with adjustable output power in order to transmit ultrasound vibrations into the aqueous RhB solution at a frequency of 20 kHz. The vibration amplitude of the sonotrode tip was set to 36 μm (p-p), significantly exceeding the threshold for the development of cavitation in water, which is 4–5 μm (p-p) [9]. The acoustic power was measured by the calorimetric method, and the results showed that the acoustic power was 120 W for the present experimental condition. The sonotrode served also as a grounded electrode. The tungsten high-voltage electrode was positioned at a distance of 4 mm from the grounded one.

A direct current power source (HAR-40P7.5-LN, Matsusada, Japan) with a maximum voltage of +40 kV and output power of 300 W was applied to charge a 1000 pF ceramic capacitor. The capacitor was then discharged in water through the above electrode system, thereby generating pulses into the solution as shown in Figure 1.

Voltage between the electrodes was measured using an oscilloscope connected to the circuit through a 1000:1 reduction ratio high-voltage probe (PINTEC, Beijing, China), while the current signal was collected using a coil current probe (IWATSU, Tokyo, Japan). Additionally, several experiments were conducted using HPLC-TOFMS (high performance liquid chromatography time-of-flight mass spectrometry) to analyze the possible chemical reagents present in the solutions. The concentrations of RhB and H₂O₂ were measured using a spectrophotometer (UV–VIS) as mentioned above.

Fenton reagents were introduced into the RhB-containing water bath through the following three routes: The first one was the addition of Fe²⁺ ions directly using an FeCl₂ solution, as mentioned above. The second and third routes were the use of steel or iron parts immersed in the water bath. In the first case, a square iron plate (45 mm × 45 mm, thickness: 3 mm) was placed under the sonotrode tip, as shown in Figure 1. In the second case, the titanium tip of the sonotrode was replaced by a stainless-steel tip.

2.2. Experimental Conditions

The volume of treated solution was 2 liters. The solution conductivity was adjusted to 100 µS/cm using the appropriate NaCl solution according to previous research [9]. The initial concentration of RhB was set to 5 mg/L. All solutions were prepared using ion exchanged water. During the experiment, argon gas was injected into the solution at a rate of 8 L/min through a porous plug. The porous plug was placed at the vessel bottom to facilitate the formation of fine bubbles in large quantities.

In order to achieve the optimal reaction conditions for RhB decomposition in the ACAP process assisted by Fenton reactions, experiments were conducted under various values of pH (3.5, 4.0, or 5.0), and concentrations of FeCl₂ ranged from 3.5 to 20.0 mg/L. The duration of RhB decomposition treatment was 12 min in all experiments for purposes of comparing the results with our previous experiments, and the solution samples were taken at 3 min intervals. The vessel was cleaned with purified water before and after each experiment in order to remove residues of RhB that might have remained from previous experiments. To eliminate the possible influence of light on the decomposition efficiency, the experiments were done in total darkness.

The concentration of Fe ion was measured by ICPS-8100 (ICP Emission Spectroscopy). The concentration of RhB in the sample solutions was determined by absorbance spectroscopy at a wavelength of 554 nm using a spectrophotometer (UV–VIS). The efficiency of RhB decomposition (η) was calculated by using the following equation:

$$\eta =\frac{\left\{\left[RhB\right]o-\left[RhB\right]\right\}}{\left[RhB\right]o}\times 100\%$$

(1)

where [RhB]o is the initial concentration of RhB, and the [RhB] is the current concentration of RhB in the sample after treatment.

2.3. Methods for Measuring H₂O₂ Concentration

It is well known that the concentration of H₂O₂ is a key parameter influencing the Fenton and Fenton-like reactions. Thus, in the current experiments, it was of crucial importance to measure the H₂O₂ concentration. The measurements were done by the referenced method [15] with the help of UV–VIS spectrophotometry. Specifically, a mixture of 0.40 M potassium iodide, 0.06 M sodium hydroxide, and 0.10 mM ammonium molybdate was added to 0.10 M phthalic acid for a total volume of 0.75 mL. Then 1.5 mL of RhB solution was added to the mixture, and, after standing for 2 min, the solution absorbance was measured at a wavelength of 352 nm. The calibration curve for H₂O₂ was plotted by measuring the absorbance under the same conditions by mixing 0.75 mL of the detection reagent with 1.5 mL of H₂O₂ at standard concentrations ranging from 0 to 10 mg/L. Furthermore, the calibration data were fitted by the following formula:

$$y=0.382\times x+0.018$$

(2)

where x is the H₂O₂ concentration, and y is the absorbance at 352 nm.

Preliminary experiments showed that the RhB solution had a non-zero absorbance at 352 nm that might have influenced the measurement results, especially when the H₂O₂ concentration was small. Therefore, the following procedure was used to measure the H₂O₂ concentration properly: First, a sample of solution was taken to measure the absorbance at a wavelength of 352 nm, and its value is denoted by y₁ in Equation (3). The detection reagent was then added to the solution, and its absorbance y₂ was measured again. The absorbance of H₂O₂, y, was calculated according to the following equation:

$$y=y_1-1/2\times y_2$$

(3)

The value of y₂ was divided by 2, because the RhB concentration in solution for measuring y₁ was half that for measuring y₂.

3. Results

Using the above experimental methods and equipment, we measured the concentration of RhB under different conditions. Below is an explanation of the experimental results.

3.1. Effect of Temperature and FeCl₂ Addition on Decomposition Efficiency

Temperature can have a significant influence on the kinetics of RhB degradation. However, our preliminary experiments showed that the decomposition efficiency did not depend on whether the water temperature was controlled or not. In these experiments, a water-cooling coil was installed inside the ACAP reactor, and the RhB treatment was carried out with and without water cooling. The conditions and results are presented in

Table 1. Effect of temperature on RhB decomposition efficiency at pH 4 and FeCl₂ 5 mg/L.

	Temperature (°C)	RhB Decomposition Efficiency (%)
With temperature control	20	77.7
Without temperature control	20–40	77.3

. The reason why the temperature control did not have any effect on the treatment efficiency is assumed to be as follows: In the experiments, both the ultrasound irradiation and plasma discharge caused heat evolution into the inter-electrode zone, and then the heat increased throughout the whole bath. However, the temperature was distributed very non-uniformly, being highest near the electrodes and much lower away from them. All reactions involved in the RhB decomposition took place in the inter-electrode zone, the temperature of which can be very high and unaffected by the presence of the water-cooling coil. Based on these findings, all experiments in the present study were performed without controlling the water bath temperature.

Figure 2 shows the effects of Fe²⁺ addition on the RhB decomposition efficiency. Obviously, all Fe²⁺ ions were supplied by FeCl₂. As can be seen, the amount of Fe²⁺ added had a great influence on the decomposition efficiency in the range of 0–5 mg/L FeCl₂ addition, where it increased from approximately 60% to 80%. These results suggest that the Fenton reactions had a great enhancing effect on the RhB decomposition efficiency. However, as the FeCl₂ addition further increased, the decomposition efficiency remained unchanged or even slightly decreased. As will be shown later, the reason for this is a limitation of H₂O₂ generated in the plasma and cavitation zone.

3.2. Effect of Initial pH Level on Decomposition Efficiency

The pH value is an important parameter for the Fenton reactions, and it has been reported that the optimum pH is around 2–5 [16,17]. We conducted experiments using 5 mg/L FeCl₂ solutions in the presence of ultrasonic cavitation and plasma at different initial pH values. The results revealed that the effect of FeCl₂ addition on the decomposition efficiency is pH-dependent. Figure 3 compares time variations of the decomposition efficiency at various pH values. It is clearly seen that the efficiency remained the same at pH = 4.0 and 3.5, reaching 80%. However, it decreased to 60% with increases in the pH value to 5.0. It should be mentioned that 3.5 was the minimum pH value that could be reached under the given conditions [9]. Thus, this result indicates that lower pH levels would be beneficial to achieve better decomposition efficiency. Chang [18] also reported similar results. In his study, the Fenton reactions were used to decolorize the RhB in aqueous solution at pH 2, 3, 4, and 5. The results indicated that higher RhB decomposition efficiency (>90%) could be obtained at pH 2, 3, and 4 compared to 66% at pH 5 after 30 min reaction time. As is well known, RhB is a type of cationic dye, and hence its decomposition rate should be higher at lower pH values. However, according to the results of our previous research [9], a decrease in pH results in an increase in electrical conductivity of the solution, which has a negative effect on the plasma discharge efficiency in the ACAP process. Therefore, based on the above results, pH 4.0 was considered the best value under the given experimental conditions.

3.3. Decomposition Efficiency in the Presence of Steel Parts

The above results thus revealed that the RhB decomposition efficiency became greater in the presence of Fe²⁺ ions in solution due to Fenton reactions. Furthermore, a very small amount of Fe²⁺ ions was needed to enhance the degradation efficiency of RhB in the present conditions. Hence, it will be interesting to examine the possibility of obtaining a similar effect due to Fe²⁺ ions entering into the solution from steel parts of the ACAP reactor. Obviously, such an option would be beneficial in terms of preventing secondary pollution and increasing the economic efficiency. Therefore, we conducted experiments using a square iron plate placed under the sonotrode, as shown in Figure 1. The experiments were performed under exactly the same experimental conditions (vibration amplitude, frequency, reactor design, and so on). Therefore, we assure that the sonochemical activity was also the same. Figure 4 compares the RhB degradation efficiency obtained under different experimental conditions. It can be seen that in the presence of the iron plate, the RhB decomposition efficiency increased to approximately 76% versus 53% without the plate after 12 min of treatment. However, the decomposition efficiency with the iron plate was still lower than the maximum value of 80% obtained at the 5 mg/L concentration of FeCl₂. Additional measurements revealed that the concentration of ferrous ions after 12 min treatment was 1.73 mg/L. This was slightly lower than the concentration of iron ions in 5 mg/L FeCl₂, which was 2.21 mg/L of ferrous ion. This indicated that iron ions dissolved from the iron plates during the ACAP treatment, and that Fenton reactions occurred, promoting the decomposition of RhB. Therefore, placing iron or steel components in the ACAP reactor can also improve the efficiency of RhB decomposition, producing almost the same effect as adding ferrous ions.

Next, the sonotrode with a titanium tip was replaced by a sonotrode with a stainless-steel tip. This stainless steel was composed of 18% Cr and 10% Ni, and the balance was Fe. The experiments were carried out under the same conditions, and the results are presented in Figure 5. It can be seen from this figure that the RhB decomposition efficiency was slightly higher in the experiments with a stainless steel tip as compared to the titanium tip case. This difference was especially noticeable at pH 3.5 and 5. However, the effect of using a stainless steel sonotrode was not as good as that of placing iron plates. At pH 3.5, when a stainless steel sonotrode was used, the RhB decomposition efficiency was highest, at 63%. However, it was lower than the RhB decomposition efficiency of 76% when the iron plate was used. Therefore, it is more effective to place the iron plate under the titanium sonotrode tip than to change it for the stainless steel sonotrode tip.

Thus, it can be concluded from the above results that iron or steel parts of the experimental setup can additionally improve the efficiency of RhB degradation. Details and possible mechanisms will be discussed in the following section.

4. Discussion

The above experimental results show that the efficiency of RhB decomposition is definitely affected by Fenton reactions. Moreover, Fenton reagents can be supplied not only by the addition of iron salts but also from parts of the experimental reactor used in the study. The following section focuses on explanations for possible mechanisms of RhB decomposition

4.1. Possible Chemical Reactions with FeCl₂ Addition

Some basic reactions and mechanisms, which are responsible for the decomposition of rhodamine B, were discussed in our previous papers [8,9,10]. However, as the presence of iron ions in the system makes the relevant chemical reactions more complicated, some typical and important reactions are considered in more detail below.

The first group of reactions involves highly active chemical radicals that are generated due to ultrasonic cavitation and plasma discharge. Ultrasound irradiation in liquids is known to cause acoustic cavitation, whereby the formation, growth, and implosion of bubbles filled with gas and/or vapor takes place. The cavities that are formed oscillate in cycles of ultrasound wave expansion and contraction [19]. The pressure and the temperature in the cavitation bubbles can reach several hundred atmospheres and several thousand degrees K as the bubbles collapse following explosion [20]. Hydroxyl radicals form under these conditions due to a number of phenomena, including water sonolysis, hot particle collision, and formation of a plasma within the cavitation bubbles [21]. The radicals subsequently participate in a variety of reactions inside a gas bubble and/or in the bulk solution itself. Equations (4)–(8) show some typical reactions. [20]:

H₂O +))) → HO•+ H•

(4)

2 HO• → H₂O + •O

(5)

2 HO• → H₂O₂

(6)

HO• + H₂O₂ → HO₂• + •H₂O

(7)

2 H• → H₂

(8)

In plasma-assisted processes, energetic electrons can interact with other molecules such as O₂ and H₂O, thereby producing various reactive species that include HO• and O₂^•−, but also reactive molecules such as O₃ and H₂O₂. Furthermore, photogenerated holes (h⁺) and photo-generated electrons (e⁻) can participate in catalytic redox reactions, producing H₂O₂ and dissolved oxygen [22].

In the case of argon injection, whereas argon is not involved in the chemical reaction, it assists plasma generation by lowering the underwater dielectric breakdown voltage and by raising the temperature in the ultrasonic cavitation zone. As a result of these phenomena, more chemically active hydrogen and oxygen-containing ions and radicals are generated when argon is introduced into the reactor. This was found to further promote RhB decomposition [10].

The second group of reactions comprises the Fenton reactions, which involve hydrogen peroxide and iron ions, Fe²⁺ and Fe³⁺. The commonly accepted mechanism of the Fenton reactions is represented by Equations (9)–(12), and the reaction rate constants, k, can be found in the relevant literature [23]. The Fenton reaction itself is considered to be the reaction represented by Equation (9). This encompasses the oxidation of ferrous to ferric ions that decompose hydrogen peroxide into the hydroxyl radical and the hydroxide ion. The core of Fenton reaction chemistry is typically considered to be reaction (9), but reactions (10)–(12) must also be taken into account as needed for the entire reaction path:

Fe²⁺ + H₂O₂ → Fe³⁺ + HO• + −OH          k = 40 − 80 L M⁻¹s⁻¹

(9)

Fe³⁺ + H₂O₂ → Fe²⁺ + HO₂• + H⁺             k = 9.1 × 10⁻⁷ L M⁻¹s⁻¹

(10)

HO• + Fe²⁺ → Fe³⁺ + −OH                     k = 2.5 − 5 × 10⁸ L M⁻¹s⁻¹

(11)

HO₂• + Fe²⁺ → Fe³⁺ + −HO₂                     k = 0.7 − 1.5 × 10⁶ L M⁻¹s⁻¹

(12)

The ferric ions generated are reduced by reacting with excess hydrogen peroxide, again forming ferrous ion and other radicals based on reaction (10). This reaction is called the Fenton-like reaction, and it allows Fe²⁺ regeneration in an effective cyclic mechanism. It is seen that the reaction rate constant (10) is much lower than that of Fenton reaction (9). In the Fenton-like reaction, apart from ferrous ion regeneration, the hydroperoxyl radical HO₂• is also produced, which can also attack organic contaminants.

Subsequently, highly active radicals HO• and O₂^•– react with RhB molecules to generate a variety of intermediate species, followed by RhB being entirely mineralized, thereby forming CO₂, H₂O, and inorganic nitrogen-containing ions as per Equation (13).

RhB + HO• + O₂^•− → Intermediate products

CO₂ + H₂O + NO₃⁻ + NH₄⁺.     k = 2.5 × 10⁹–9 × 10⁹ L M⁻¹s⁻¹

(13)

Thus, in the ACAP process there are a number of physical and chemical phenomena resulting in the generation of chemically active radicals and free electrons that contribute to the decomposition of rhodamine B in aqueous solution. Among them, the Fenton reaction plays an important auxiliary role in the degradation process, because it allows hydrogen peroxide to convert back into hydroxy radicals. Nevertheless, in the present experimental conditions, the capacity of the Fenton reactions to improve decomposition efficiency was limited, as follows from Figure 2, Figure 3, Figure 4 and Figure 5. Furthermore, due to the limited processing time (12 min) in our experiments, the maximum processing efficiency that RhB can achieve was 80%. If the RhB solution is to be completely decomposed into carbon dioxide and water, the treatment time needs to be increased. Below is one possible explanation of the reasons and underlying mechanisms.

4.2. Production of Hydrogen Peroxide in the ACAP Reactor

The concentration of hydrogen peroxide plays a crucial role in deciding the overall efficiency of the decomposition process. In order to evaluate the hydrogen peroxide production in the ACAP process using Fenton reactions, the H₂O₂ concentration was measured with a UV–VIS spectrophotometer, as mentioned in the previous section. The measurement results are shown in Figure 6. As mentioned above, both the acoustic cavitation and underwater plasma can generate hydroxyl radical HO•, a portion of which can react with each other and thus further generate hydrogen. The reaction details are presented in Section 4.1.

As illustrated in Figure 6, the H₂O₂ concentration and its time variation are significantly dependent on the amount of FeCl₂ added. Without FeCl₂ addition, the H₂O₂ concentration continuously rises. This is because the rate of H₂O₂ self-decomposition is much slower than its production rate [24]. However, when FeCl₂ was added to the solution, the concentration of hydrogen peroxide slowly increased during the first 6–9 min of treatment and then started to decrease with time. In general, this time variation in H₂O₂ concentration becomes smaller as the amount of FeCl₂ addition increases. These results suggest that H₂O₂ is consumed by the Fenton reactions, especially by reaction (9), because it proceeds much faster than reaction (10). As shown in Figure 2, the efficiency of RhB degradation was drastically increased first and then kept almost unchanged when the amount of FeCl₂ added exceeded 5 mg/L. These results, when considered together, indicate that the Fenton reactions were ongoing, and that the production and consumption of hydrogen peroxide were in dynamic equilibrium in the solution. When the concentration of FeCl₂ was increased from 0 to 5 mg/L, the concentration of hydrogen peroxide gradually decreased, but when the concentration of FeCl₂ was higher than 5 mg/L, the concentration of hydrogen peroxide in the solution remained almost constant. This is because the hydrogen peroxide was consumed by Fe²⁺ ions in the Fenton reactions, and the greater the amount of FeCl₂ was added, the lower the concentration of hydrogen peroxide. However, when the amount of FeCl₂ added exceeded 10 mg/L, the differences between the H₂O₂ concentrations became insignificant. This can be readily seen from a comparison of H₂O₂ concentration time variations for FeCl₂ addition of 10 and 20 mg/L. Moreover, the H₂O₂ concentration itself becomes close to the limit of measurement accuracy of the analysis. Thus, results suggest that the efficiency enhancement effect of Fe²⁺ ion addition is limited by the generation rate of H₂O₂ in the cavitation-plasma zone. That is why the RhB decomposition efficiency first increases with the addition of FeCl₂ and then remains almost constant with further addition, as shown in Figure 2.

However, when Fe²⁺ ions are supplied to the RhB solution from the solid surfaces such as the iron plate or the steel sonotrode tip, the additional effect of Fenton reactions is controlled by the dissolution rate of these iron-containing solid surfaces. The results suggest that this dissolution rate is rather slow and depends on the solution pH. Another technique that can potentially enhance the iron dissolution rate is an increase in the surface area of the iron-containing part contacting the liquid bath. This issue should be properly investigated in further studies.

4.3. Mechanisms of RhB Decomposition

Based on the above discussion, a plausible mechanism for rhodamine B decomposition was considered. One mechanism is illustrated in Figure 7. It should be noted that the decomposition process is very complex and involves several parallel and consecutive reactions. First, as mentioned above, active radicals such HO•, H•, and HO₂• are produced according to the following three pathways: (1) sonolysis reaction; (2) water pyrolysis during the plasma discharge; (3) and Fenton and Fenton-like reactions.

The first two mechanisms are considered to be the main contributors to the production of HO• radicals, the greater part of which attacks RhB molecules, causing their decomposition. Thus, the RhB decomposition proceeds mainly according to the first and second pathways. Nevertheless, some proportion of HO• radicals can recombine to produce H₂O₂ according to reaction (7), and this hydrogen peroxide is assumed to serve as a reagent in the Fenton reactions, as discussed above.

Finally, it is important to identify possible intermediate products formed during the reactions leading to RhB decomposition. The identification was made by liquid chromatography, as mentioned above. Samples were taken after 15 min and 30 min treatment times, the results are shown in Appendix A. The proposed decomposition pathway of RhB is shown in Figure 8.

Major intermediates, which form during the degradation process, were thus identified based on the m/z values of the mass spectra, taking into account results in the relevant literature [25,26,27]. As shown in Figure 8, active species such as OH are generated in the ACAP treatment process. The OH radical affects the entire degradation process. Moreover, the intermediates are further decomposed into small molecular intermediates, including organic acids, by cleavage of conjugated structures that are finally oxidized into CO₂ and H₂O.

4.4. Reaction Kinetics and Comparison of Various Treatment Methods

In order to clarify the kinetics of the RhB decomposition reaction in the Fenton-assisted ACAP process, the experimental results of the decomposition of RhB were fitted using a first order kinetics model [28]. The RhB decomposition kinetics can thus be written as follows:

$$ln\frac{\left[C\right]o}{\left[C\right]}=kt$$

(14)

where [C]_o is the initial concentration of RhB, [C] is the concentration of RhB at time t, and k is the first-order decomposition rate constant (min⁻¹).

The fitting kinetic curves and corresponding calculated kinetic constants are presented in Figure 9 for a number of typical measurements. It can be seen that the dots fall more or less on straight lines, suggesting the first order kinetics of RhB decomposition under the given experimental conditions. The first-order kinetic rate constants of RhB decomposition were 0.129 min⁻¹ and 0.115 min⁻¹ in the ACAP process with the 5 mg/L addition of FeCl₂ and with the iron plate, versus 0.060 min⁻¹ when the ACAP treatment was carried out without Fenton reactions. In addition, we compared our results with other data reported for the RhB decomposition in the literature. The results are summarized in Figure 10 and indicate that the ACAP with FeCl₂ added, and with the iron plate in place, both possessed an improved promoting effect on RhB decomposition.

5. Conclusions

In this study, in order to further improve wastewater treatment efficiency, we investigated the promoting effects of Fenton reactions on the decomposition efficiency of rhodamine B in aqueous solution treated by the ACAP process, which was recently proposed by the authors here. Two options for ferrous ion supply were considered: one was addition of FeCl₂, and the other was dissolution of iron from parts of the ACAP reactor into the RhB-containing solution. To investigate the plausible reaction mechanisms, the hydrogen peroxide concentration was measured under different experimental conditions. Additionally, rhodamine B and its decomposition intermediates were analyzed by the HPLC-TOFMS method to predict possible decomposition pathways of RhB. The results of the present study are summarized as follows:

(1)

The Fenton reactions greatly enhanced the efficiency of the acoustic cavitation-assisted plasma decomposition of rhodamine B. Under the present experimental conditions, the decomposition efficiency reached almost 80%, which is 20% greater compared to the case without Fenton reactions.

(2)

When FeCl₂ was added to the solution, the RhB decomposition efficiency was affected by the pH and the concentration of Fe²⁺ ions. At pH = 4, the degradation efficiency increased up to 80% as the amount of added FeCl₂ increased from 0 to 5 mg/L, and then it remained approximately at the same level with further addition of FeCl₂.

(3)

Placing iron or steel components inside the ACAP reactor could also improve the efficiency of RhB decomposition, although the effect was slightly less than in the case of ferrous ion addition, due to the slow dissolution rate of steel parts in the aqueous solution.

(4)

The mechanism of enhanced Fenton-assisted degradation of RhB is suggested to be as follows: ultrasonic cavitation and plasma discharge generate HO• radicals but a portion recombines to produce H₂O₂, which reacts with Fe²⁺ ions to produce HO• radicals again. Under the present experimental conditions, a 5 mg/L addition of FeCl₂ was sufficient to convert all H₂O₂ to the hydroxyl radicals.

(5)

The first order kinetics rate constants of RhB decomposition obtained in our experiments were compared with other studies. The results indicated that the ACAP with FeCl₂ 5 mg/L added to solution directly or with an iron plate installed under the sonotrode tip both had an enhanced promoting effect on RhB decomposition.