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Catalytic Wet Peroxide Oxidation of Phenol : H2O2 Gradual Addition Study

Asghar Molaei Dehkordi, Amir Ali Ebrahimi, and Seyedehmansoureh Shahalami. Chemical Engineering, Sharif University of Technology, Tehran, Iran

The catalytic wet peroxide oxidation (CWPO) of phenol was studied in a batch system using a commercial activated carbon supplied by Jacobi Co. (Sweden, type of AquaSorb 101). The influences of various operating parameters including the initial pH of solution, temperature, catalyst loading, and the addition modes of hydrogen peroxide on the fractional conversion of phenol were investigated. The stability of hydrogen peroxide and consequently its effect on phenol conversion is dependent upon the pH of solution. To investigate this effect it was decided to measure the concentration of hydrogen peroxide and phenol vs. time at different pH of solution (2-10). The least consumption of H2O2 was observed at pH values of 2 - 4. Moreover, H2O2 has been decomposed rapidly at pH of around 10. The low consumption rate of H2O2 will result in the high phenol conversion; therefore, the high phenol conversion was obtained at pH around 4. Increasing the temperature over the range of 70-80 C does not have profound effects on the conversion of phenol; therefore, the appropriate temperature could be 70 C. It could be seen in all the experiments that the phenol concentration increased after a great initial reduction. This was happened during the beginning of the reaction (10 30 min) and proposed a reversibility mechanism in phenol degradation process. This reversibility was significantly higher in non catalytic reaction (figure 1). Two different modes of addition of H2O2, i.e. rapid and gradual addition with different flow rates were examined. The same amounts of H2O2 (77.65 mmol/L) were added to the reaction mixture with two different modes of operation to investigate the effect of H2O2 addition modes on the fractional conversion of phenol. The addition rate of H2O2 was adjusted at 0.3235, 0.3883 and 0.647 mmol/min. As can be seen from figure 1, the final conversion of phenol in the case of rapid addition of H2O2 is higher than that obtained with gradual addition of H2O2 which is contrary to what we expected. Furthermore, in the case of gradual addition of oxidant, high fractional conversions of phenol followed by the progressive decrease can be observed. This behavior is same as what we observed in the case of rapid addition in last experiments and may be attributed to the reversible mechanism of phenol degradation because of the lack of OH. There are two reasons which cause the lack of OH, (1) low catalyst loading, (2) low concentration of hydrogen peroxide. The first reason could be confirmed by considering the results of the catalyst loading variations which shows the reversibility was higher in the non catalytic reaction. The second explanation could be confirmed by considering the effect of increasing the addition rate of the oxidant on the phenol conversion which decreases the reversibility due to the increase in the OH concentration. But there is still a problem why the phenol conversion is lower than that of rapid addition mode of the oxidant. To study whether such a trend could be obtained for other catalyst loading or not, a number of CWPO experiments with other catalyst loading were carried out while other operating conditions were kept unchanged. Figure 3 shows the variations of the phenol conversion with time as a function of catalyst loading and modes of H2O2 addition on the CWPO of phenol. As can be noticed from this figure, the catalyst loading plays an important role in the CWPO process. According to these experimental results for the catalyst loadings larger than 2 g/L, the aforementioned-progressive decrease in the fractional conversion of phenol could not be observed. Moreover, the fractional conversions of phenol for the catalyst loadings larger than 2 g/L were much higher than those of lower catalyst loadings. This behavior can be explained by two explanations, (1) an increase in the catalyst surface area available to the reactants which causes an increase in OH concentration and (2) an increase in the catalyst surface area which increases the adsorption capacity and consequently hides the reversibility effect. Therefore, it can be concluded that there is a minimum value of catalyst loading which above the performance of mode of gradual addition is much better than that of rapid addition mode under identical conditions particularly the total hydrogen peroxide consumed.

Figure 1. Effect of catalyst loading on the phenol conversion.

T = 70 C, CHP = 77.65 mmol/L, pH = 5.5, Cph = 500 mg/L.

Figure 2. Effect of addition modes of hydrogen peroxide on the phenol conversion.

Cph = 500 mg/L, catalyst loading = 1 g/L, pH = 5.5, T = 70 C.

Figure 3. Effect of catalyst loading and addition modes of H2O2 on the phenol conversion.

Cph = 500 mg/L, pH = 5.5, T = 70 C.

Keywords:

Phenol oxidation; CWPO; Activated carbon; Wastewater treatment; Phenol