Separations 2023 , 10 , 148
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3.1.2. Effect of pH The efficiency of the COD reduction also varies significantly at different pH conditions. Figure 3 shows the effect of pH on COD reduction at a reaction time of 8 min, a temperature of 25 ◦ C, and an ozone concentration of 12 mg/min during catalytic ozonation. It can be seen from Figure 3 that the COD reduction increases as the initial pH is increased from 3 to 9 in the catalytic ozonation system. This is mainly due to the fact that the decomposition ratio of ozone in the water body is slow in acidic conditions, so ozone reacts directly with reducing substances in water in molecular form, while under alkaline conditions, it can enhance the decomposition of ozone and promote the generation of hydroxyl radicals [20,21]. It also can be seen from Figure 3 that the COD reduction at pH 11 is higher than pH 9 before 6 min. However, after 6 min, the COD reduction at pH 11 was lower than that at pH 9. This is mainly due to the quenching of hydroxyl radicals caused by too high pH [22]. Therefore, pH 9 was selected as the condition for the treatment of pulp and paper mill wastewater by catalytic ozonation.
Figure3. Effect of initial pH on COD reduction. 3.1.3. Effect of the Catalyst Filling Ratio
The titanium dioxide composite carriers were used as a catalyst, and the catalyst filling ratio was the ratio of the volume of the carrier to the volume of water. Furthermore, the catalyst filling ratio also has a significant effect on the COD reduction [23]. The effect of the catalyst filling ratio on the COD reduction was investigated at a reaction time of 8 min, an ozone concentration of 12 mg/L, and pH 9. As shown in Figure 4, when the catalyst filling ratio is 10%, the COD reduction reaches the highest of 37%. It can therefore be assumed that the catalyst filling ratio of 10% should be selected as the best experimental condition. However, the COD reduction of the 10% catalyst filling ratio was only 1% different from that of the 7.5% catalyst filling ratio. It meant that similar results can be obtained under the condition of 7.5% COD reduction. Therefore, considering the problem of treatment costs, a 7.5% catalyst filling ratio was selected as the optimal experimental condition.
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