Studies on the preparation of Caro’s acid by ultrasonic enhanced electrochemistry

Ultrasonic cavitation effects can generate hydroxyl radicals and high energy, which is widely applied in the field of oxidation currently. Ultrasound-enhanced electrochemical is used to prepare Caro’s acid, which improves the generate rate of Caro’s acid. In this article, the influences of ultrasonic frequency and ultrasonic power on the electrolysis voltage, electrolyte temperature, electrolyte concentration and the concentration of additive in the process of electrochemical preparation of Caro’s acid was studied. And the optimal production conditions were determined. The research results showed that ultrasonic can significantly improve the production of Caro’s acid and the product can increase by about 20 g/L under the best condition.


Introduction
The study on ultrasonic-induced electrochemical reactions has developed rapidly, which was widely used in water treatment [1][2], ore decomposition [3], electroplating [4] and preparation of nanomaterials [5][6][7][8] and other fields currently. Zhuping [9] and her co-workers had studied treatment of nonleaching refractory gold ores with electrochemical antioxidant under two conditions including ultrasonic and ultrasonic condition. The results showed that ultrasound can reduce the reaction time and increase leaching rate by 35.84%.
Electrolytic oxidation technology [10][11], which is a green and efficient chemical process, is widely used in wastewater treatment [12].with the aid of ultrasonic; the electrochemical process can enhance the mass transfer process, reduce the electrolytic voltage and reduce the energy consumption in ultrasonic environment. Moreover, ultrasonic cavitation can produce a micro environment of high temperature and high pressure, which can produce a lot of hydroxyl radicals with high oxidizing ability and improve oxidation efficiency. Domestic and foreign scholars mainly applied ultrasonic electrochemical oxidation to wastewater treatment. For example, Zhao Guohua [13] from Tongji University combined boron doped electrode with ultrasound to treat phenol and phthalic acid. The results showed that hydroxyl radical produced during electrolytic process under ultrasonic environment. Furthermore, the oxidation capacity increased and the oxidation peak increased by 32%, the degradation efficiency was significantly improved. Meanwhile the energy consumption was greatly reduced.
In this study, ultrasonic chemical and electrochemical were combined together for the preparation of Caro's acid in a low concentration of sulfuric acid. The results indicated that the anode oxidation rate, current efficiency and mass transfer rate were improved greatly with a reduced energy consumption. The problems existing in the preparation of Caro's acid by electrolysis have been solved fundamentally, which lays the foundation for industrial production.  [14]. When the cavitation bubble is been compressed, the vapor in the bubble is decomposed into hydroxyl and hydrogen free radicals. In the absence of oxidized objects, H2O2 is the main product of ultrasonic decomposition. When the air is involved in the reaction, hydrogen free radicals and oxygen were combined rapidly to form hydrogen peroxide free radicals. Hydrogen peroxide produced by ultrasonic cavitation reacted with HSO4-in solution to form Caro's acid, which was shown as formula (9).  Figure  1.

The Influences of Ultrasonic Frequency and Time on Electro oxidation
In ultrasonic environment, the hydrogen free radicals and hydrogen peroxide free radicals can be generated owing to the cavitation effect in water solution, which can enhance the oxidation of the solution and increase the synthesis rate of Caro's acid. Ultrasonic can also improve the ion activity in solution, accelerate ion migration rate and reduce concentration polarization. Therefore, the effects of ultrasonic and ultrasonic time on the electrochemical oxidation process should be investigated. Experimental conditions: the voltage of electrolysis is 6.0V, the concentration of electrolyte is 400g/L, the concentration of ammonium thiocyanate is 0.5g/L, the ultrasonic power is 150W, the temperature of reaction is 15℃ and the proper reaction time is 4 hours while a sample was taken every 15 min for testing. The influences on three conditions for electrolysis process were investigated and the three conditions included absence of ultrasonic, different ultrasonic frequency including 20 kHz and 40 kHz as well as ultrasonic time. The results are shown in As shown in Fig. 2, the concentration of Caro's acid showed a growing trend as time went on. While the generation rate of oxidant decreased when electrolyte time was longer than 3 h, this is because Caro's acid is not stable and hydrolyzed easily and produce sulphuric acid and hydrogen peroxide. While electrolysis process was an exothermic reaction. The longer the time, the higher the temperature, which led to decomposition of Caro's acid. As a result, the optimal time was 3 h. The presence of ultrasonic promoted the generation rate of oxidant. At the same time, oxidant concentration increased when the ultrasonic power was increased. This was mainly because the acoustic cavitation effect helped generate hydrogen free radicals and hydrogen peroxide free radicals, accelerated ion mobility, increased oxidant generation rate. Based on the experiment results, ultrasonic power should be 40 kHz.

The Influence of Ultrasonic Power on Electro oxidation Process
Ultrasonic power is one of the important factors that effect of cavitation effect, which directly affects the reaction temperature and reaction rate. It is very important to investigate the influence of ultrasonic power on electro oxidation process.
Experimental conditions: the voltage of electrolysis is 6.0V, the concentration of electrolyte is 400g/L, the concentration of ammonium thiocyanate is 0.5g/L, the temperature of reaction is 15℃ and the optimal reaction time is 3hours. The experimental results are shown as Fig. 3.  because the increase of ultrasonic power to speed up the rate of the anodic reaction, but if the ultrasonic power was too large then most of the electrical energy was converted into heat, promoting the decomposition of Caro's acid and reducing the rate of oxidation. When the ultrasonic frequency was 20 kHz and 40 kHz, the generation rate of Caro's acid reached its peak value when ultrasonic power was 150W. Therefore, the appropriate ultrasonic power is 150W.

The Influences of Ultrasonic and Electrolysis Voltage
The electrolysis voltage is composed of theoretical voltage and over potential, and the over potential is caused by the polarization of the electrode. The presence of ultrasonic for electrolysis can help reduce concentration polarization and enhance the diffusion mass transfer so as to improve the anode electrode potential and anodic oxygen evolution over potential, inhibited side reactions and improve the formation rate of Caro's acid. The experimental results are shown in Table 1. Experimental conditions: the concentration of electrolyte is 400g/L, the concentration of ammonium thiocyanate is 0.5g/L, the ultrasonic power is 150W, the temperature of reaction is 15℃ and the optimal reaction time is 3 hours. The experimental results are shown in Figure 4.  As shown in Fig. 4, the concentration of the Caro's acid was the highest when the ultrasonic voltage was 7.0V. When the ultrasonic frequency was increased, the peak value of the concentration Caro's acid shifted to the left, which indicated that the presence of ultrasound could reduce the voltage of the cell. After combining with ultrasound, the concentration of oxidant first increases and then decreases with the increase of electrolysis voltage. This is because the voltage is too high, which led to the increasing of anode over-potential and the energy consumption and the decomposition of Caro's acid was accelerated.

The Influences of Ultrasonic Frequency and Additive
After consulting the literature, [15][16] it is impossible to avoid the anode adverse reaction in electrolysis process. Therefore oxygen inhibitors must be added. In this study, ammonium thiocyanate was chosen  Table 2. Experimental conditions: the concentration of electrolyte is 400g/L, the voltage of electrolysis is 6.0V, the temperature of reaction is 15℃, the ultrasonic power is 150W and the optimal reaction time is 3 hours. The experimental results are shown in Fig. 5.   Table 2, additives have great influence on the synthesis of Caro's acid for the reason that the addition of additives led to the formation of SCN-adsorption layer on the anode surface, which greatly improved the oxygen precipitation over potential effectively and reduced electrolytic energy consumption [17]. What we can see from Fig.4 was that the formation rate of Caro's acid firstly increases and then decreases when the additive concentration increases. When the concentration of additive is 0.5g/L, the concentration of Caro's acid reached the maximum value. The concentration of additive shouldn't be too high. The result shows that the anode adsorption layer was too thick when the concentration of additives was too high, which hindered the oxidation of Caro's acid. Ultrasound can reduce the concentration gradient of the solution around the electrode and the concentration polarization of the solution so as to improve the yield of Caro's acid. The higher ultrasonic frequency leads to more Caro's acid, but the overall trend was unchanged. Accordingly, the optimal concentration of additive should be 0.5 g/L.

The Influence of Ultrasonic and Electrolyte Concentration
Ultrasound can enhance diffusion and mass transfer as well as decrease the concentration polarization of solution. Different concentration of Caro's acid solutions have different effects on the generation rate of Caro's acid in the presence of ultrasonic. The experiment results are shown in Table 3.  Experimental conditions: the voltage of electrolysis is 6.0V, the concentration of ammonium thiocyanate is 0.5g/L, the temperature of reaction is about 15℃, the ultrasonic power is 150W and the optimal reaction time is 3 hours. The experimental results are shown in Fig. 6. It can be seen that the concentration of Caro's acid firstly increases and then decreases with the increase of the concentration of sulfuric acid from Fig.6. When the concentration of sulfuric acid is 400g/L, the concentration of Caro's acid reached its maximum value. When the concentration of sulfuric acid was too low, HSO 4 obtained from the decomposition of sulfuric acid was too little and the reaction intensity was low. When the concentration of sulfuric acid was high, the solution viscosity increased and it is not conducive to mass transfer. As a result, the ultrasonic cavitation will be weaken and ultimately lead to a low Caro's acid production rate. The higher the ultrasonic frequency was, the higher the formation rate of Caro's acid was.

The Effects of Ultrasound and Electrolyte Temperature
The addition of electrolytic process will produced a large number of cavitation bubble and cavitation bubble rupture will release heat, which led to the increasing of temperature for the solution. Thereby strengthening the ion transport and electrochemical reaction, but hydrolysis of Caro's acid was also accelerated. On the contrary, reducing temperature can inhibit hydrolysis of Caro's acid. Temperature should be reasonably control after adding ultrasound.
Experimental conditions: the voltage of electrolysis is 6.0V, the concentration of electrolyte is 400g/L, the concentration of ammonium thiocyanate is 0.5g/L, the ultrasonic power is 150W and the proper reaction time is 3 hours, the experimental results are shown in Fig. 7. It can be seen that the temperature had a great influence on the rate of formation of the oxidant from Fig. 7. As the temperature of the electrolyte increases, the concentration of Caro's acid firstly decreased and then increased again and then decreases. This was mainly because the increasing temperature led to increasing hydrolysis of Caro's acid and the decreasing current efficiency. The concentration of Caro's acid increased about 20 g/L, but the overall trend was unchanged. Therefore, the temperature of electrolyte was chosen to be 20℃.

Conclusions
(1) In the process of preparing Caro's acid by electrolysis, adding additives can increase the over potential of oxygen precipitation and restrain the occurrence of oxygen evolution side reaction effectively.
(2) Electrolysis in ultrasonic environment can accelerate the ion migration rate, reduce the tank voltage, increase the anodic oxidation rate and promote the synthesis of over Caro's acid. Therefore the purpose of strengthening production could be achieved.
(3) The experiment results showed that maximum concentration of Caro's acid can reach 104.62g/L under the condition of using 400g/L sulfuric acid as electrolyte, electrolytic voltage control in 6.0V, ammonium thiocyanate concentration is 0.5g/L, the temperature controlled at 20℃, the ultrasonic frequency is 40kHz, ultrasonic power is 150W, optimal time is 3 h.