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Figure4. Effect of the catalyst filling ratio on COD reduction. 3.2. Kinetic Study
The kinetic of COD reduction in ozonation and catalytic ozonation were carried out. The equation of COD reduction in ozonation and catalytic ozonation with respect to time can be written as Equations (1) and (2) [24]. Where [COD 0 ], [COD t ], [O 3 ] [TiO 2 ], [K 1 ], and [K 2 ] are the initial COD, final COD, ozone, catalyst filling rate, and rate constants of ozonation and catalytic ozonation processes, respectively. In Equation (1), the COD concentration is always much higher than the ozone concentration. In Equation (2), the rate of COD reduction is dependent on the reaction between O 3 concentrations and [TiO 2 ]; bothO 3 and [TiO 2 ] were nearly constant, while the COD concentration varies with time. In fact, both ozonation and catalytic ozonation are regarded as pseudo-first-order reactions. Therefore, Equation (2) can be reduced to Equation (3). Finally, Equation (3) was integrated to obtain Equation (4). d ( COD 0 ) dt = − K 1 ( COD t )( O 3 ) (1) d ( COD 0 ) dt = − K 2 ( COD t )( O 3 )( TiO 2 ) (2) d ( COD 0 ) dt = − K ( COD t ) (3) ln ( COD 0 ) ( COD ) = − Kt (4) The solution of Equation (4) is obtained. As shown in Figure 5, K is the quasi-primary kinetic rate constant. In the experiments with different catalyst filling ratios, the rate constants of COD reduction are 0.03202, 0.03986, 0.04572, 0.05045, and 0.05307, and the correlation coefficient is large (R 2 > 0.9), so the quasi-first-order kinetic model fits the experimental data well. With the increase in the catalyst filling ratio, the K value increases, and the reaction rate constant increases from 0.03202 to 0.05307 when the catalyst filling ratio increases from 0 to 7.5%. Therefore, the use of a catalyst affects the initial kinetics of COD reduction.
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