Degradation of rhodamine B in dilute aqueous solution with Ultrasonic Cavitation

Received Nov. 19, 2004; Support by the National Natural Science Foundation of China (20377019)

Abstract This paper presented the research result of degradation of rhodamine B as a model of basic dye in aqueous solution with Ultrasonic Cavitation. The ultrasonic degradation kinetics of rhodamine B in water was found to be first-order and the degradation rate coefficient is 3.37×10^-3 min^-1 (R= 0.997, n=8) at 25±1℃. The influence of the initial concentrations, reaction temperature, the pH of medium and Fenton reagent on the ultrasonic decomposition of rhodamine B in water were also investigated.
Keywords Rhodamine B, degradation, ultrasonic cavitation, sonochemistry

Ultrasound is widely used for medical imaging, cleaning and emulsification. Now ultrasound has different synthetic applications^[1,2] and has received as an advanced oxidation process for the decomposition of organic pollutants in aqueous solution. The sonochemical reaction is regarded to originate from acoustic cavitation. The cavitation processes consist of the creation, growth and implosive collapse of gas vacuoles in a solution. According to the "hot spot" theory ^[3], extreme temperature(-5000K) and high pressure(-1000atm) occur within the bubbles during cavitational collapse. Under these extreme conditions, most of organic compounds decomposed in the cavitation bubbles as well as compounds present in the surrounding condensed layer undergo reactions comparable to those found in high temperature combustion.
    The ultrasonic degradation of a large number of chemical compounds of environ- mental interest such as 1,1,1-trichloroethane ^[4], chlorinated hydrocarbon ^[5,6], phenols ^[7,8] and surface active regents ^[9] in aqueous solution have been studied. Dye is an important kind of environmental pollutants in rivers or lakes. But there is a few reports on the sonolysis of dye in water before ^[10,11]. In this paper we have studied the sonication of aqueous solution of rhodamine B that was chosen as a model of basic dye.

1 EXPERIMENTAL             
Aqueous solutions of rhodamine B (5.0 mgL^-1 by mass) were prepared by adding 5.0 mg rhodamine B to 1.0 L deionized water and stirring for 1 h. The sonication of 100 ml aqueous solutions of rhodamine B in a 150ml glass reactor cell were performed with a 20 KHz Model JCS-204 Ultrasonic Reactor. The aqueous solution was saturated with pure air before and during the sonication. The ultrasonic power was 50 w. The reaction temperature was controlled with the help of condensation water surrounding the reactor cell. The quantifications of rhodamine B were carried out with model UV-3000 photometer (Shimadzu, Japan) and Model 244 HPLC (Backman, USA).

2 RESULTS AND DISCUSSION
2.1 Degradation kinetics                           
The investigation of the sonochemical degradation kenetics of rhodamine B were carried out with 100 ml of air saturated aqueous solution (pH 5.20±0.2) at 25^oC. During the sonication, the concentrations of rhodamine B were determined every 30 min. The result was showed in Figure 1. From the result it can be seen that the concentrations of rhodamine B in aqueous solutions decreased exponentially with sonication time, indicating first-order kinetics. Regression on the experiment data delivered the first- order reaction rate coefficients is 3.37×10^-3 min^-1 ( r=0.997, n=8).

Fig. 1 Ultrasonic degradation kinetics of rhodamine B at 25^oC

2.2 Effect of initial concentrations
The investigation of the effect of rhodamine B initial concentrations on the degradation rate coefficients was carried out with 5-50 mgL^-1 rhodamine B aqueous solution at 25^oC. It was found that the ultrasonic degradation rates of rhodamine B varied at different initial concentrations. Table 1 showed the degradation rate coefficients and the percentage of rhodamine B removed for 4 h at different initial concentrations. It was observed that the degradation rate coefficients was decreased with increasing of the initial concentration of rhodamine B. This could be explained by surmising that the degradation intermediates of rhodamine B compete for the ·OH radicals with rhodamine B itself, and some of intermediates could diffuse into the cavitation bubbles changing the condition of bubbles collapse^[7]. As a result, the more intermediates that were formed and then oxidized by ·OH radicals, the greater is the decrease in rate coefficients of rhodamine B degradation.

Table 1 The degradation rate coefficients (k₁) and the percentage of rhodamine B removed for 4 h at different initial concentrations (25^oC, pH=5.2)

2.3 Effect of temperature
The influence of the temperature of the reaction solution on the ultrasonic degradation of rhodamine B was investigated and the result was showed in Figure 3. It was found that the ultrasonic degradation rate coefficients of rhodamine B were 3.37×10^-3, 2.72×10^-3 , 2.20×10^-3 and 1.41×10^-3 min^-1 at temperature 25±1^oC, 35±1^oC, 45±1^oC and 60±2^oC respectively. The result showed that with increasing the reaction temperature, the rate of degradation was decreased in the range of temperature in this study. It has been reported that three different regions are formed in the aqueous sonochemical process ^[12]: (1) The gas phase within the cavitation bubble where elevated temperature and high pressure are produced, (2) The interfacial zone between the bubble and the bulk solution where the temperature is lower than that inside the bubble but still high enough for a sonochemical reaction. (3) The bulk solution at ambient temperature where reaction still takes place. Of the aforementioned three regions, we prefer the interfacial zone as the region where rhodamine B was destructured because of the low vapor pressure of the compound. As the bulk temperature of water increased, the vapor pressure of water and volatile solutes inside the cavitation bubbles is increased. The collapse of cavity is thus cushioned more than that at a lower bulk temperature, this results in more moderate conditions and a lower sonochemical degradation rate.

Fig. 3 Ultrosonic degradation kinetics of rhodamine B at different temperature

2.4 Effect of the acidity of the medium
The acidity of the medium, which results in modification of the physical properties of molecules with ionisable functional groups, plays an important role in sonochemical degradation of chemical pollutants. The influence of the acidity of the reaction medium on the ultrasonic degradation of rhodamine B was also studied and the results was showed in Table 2. During sonication the buffers limited the pH-variation to less than 0.2 pH-unit. So there is an influence of pH on the degradation rate of rhodamine B. It was found that the ultrasonic degradation of rhodamine B was strongly dependent on pH. The degradation rate coefficients and rhodamine B removed for 4 h in acidic water(pH 2-4) are higher than those obtained in neutral aqueous solution (pH 5-10), and higher than also obtained at basic medium.

Table 2 The degradation rate coefficients (k₁)and the percentage of rhodamine B removed for 4 h at different pH (5 mg L^-1 , 25^oC)

2.5 Effect of free radicals accelerator
It is well known that in water ultrasound can cause the formation of OH radicals which are the very strong and nonspecific oxidizing species. Since OH radicals are major free radical and important precursors for many products formed in sonolysis, the production rate of HO· strongly influence the sonolysis efficiency. However, a lot of HO radicals generated in cavitation bubbles would combine each other to form H₂O₂ in water and lead to the decreasing of the sonolysis efficiency. Many efforts have therefore been devoted to improve the efficiency of sonochemical degradation^[13-15], particularly in view of the fact that a substantial amount of energy employed in generating the radicals is not effectively converted into an optimum yield of the desired products. A possible means of overcoming the generation of H₂O₂ is to use it to produce additional ·OH radicals. According to Fenton reaction , Fe²⁺ can react with H₂O₂ to produce ·OH,
Fe²⁺ + H₂O₂  --->   FeOH²⁺ + HO·
    Fe³⁺ can be reduced by H₂O₂ via the formation of a Fe³⁺-hydroperoxy complex,
FeOH²⁺ + H₂O₂  --->  Fe(HO₂)²⁺
Fe(HO₂)²⁺   --->   Fe²⁺ + HO₂·
    So Fe²⁺ can produce additional HO radicals and accelerate the sonolysis of pollutants. The effects of Fe²⁺ on the ultrasonic degradation rate of rhodamine B was investigated with sonication of 5 mgL^-1 rhodamine B aqueous solution at pH 4.0 (25^oC) and the results were showed in Figure 3. It is found that the concentration of Fe²⁺ significantly affects the degradation rate coefficients of rhodamine B. The degradation rate coefficients of rhodamine B increase with increasing initial concentration of Fe²⁺ in the range of concentration in this study.

Fig. 4 Effect of Fe²⁺ concentration on degradation rate of rhodamine B at 25℃(pH= 4)

3 CONCLUSION
It was found that the ultrasonic degradation kinetics of rhodamine B in aqueous solution is first-order and the degradation rate coefficient is 3.37×10^-3 min^-1 at 25^oC. It was observed that the degradation rate coefficients were decreased with increasing of initial concentration of rhodamine B and reaction temperature. It was found that the ultrasonic degradation of rhodamine B was strongly dependent on pH. The degradation rate coefficients of rhodamine B in acidic or basic water are higher than those obtained in neutral aqueous solution. The result also showed that Fe²⁺ can produce additional OH radicals and accelerate the sonolysis of rhodamine B in water.