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40 mL of dichloromethane was added and shaken to release gas, followed by another shake. The organic phase was extracted for 10 min and then left for 10 min before being collected. The organic phase was extracted for 10 min and left for 10 min, and the organic phase was collected. The aqueous phase was extracted again with 20 mL of dichloromethane, and the organic phase was combined and dehydrated by adding anhydrous sodium sulfate to the organic phase. After that, the organic phase was concentrated to 12 mL using a rotary evaporator at 36 ◦ C for analysis by GC-MS. The heating procedure: 40 ◦ C (hold for 3 min) → 200 ◦ C(5 ◦ C/min hold for 3 min) → 280 ◦ C(10 ◦ C/min, hold for 10 min) with a solvent delay of 4 min.
3. Results and Discussion 3.1. Exploration of Optimum Process Conditions 3.1.1. Effect of Ozone Concentration and Time
Under the experimental conditions of a 10 min reaction time, 25 ◦ C, a pH 7.8, and a catalyst filling rate of 7.5%, the effect of the ozone concentration on the COD reduction was examined. Figure 2 shows the effect of different ozone concentrations and time on COD reduction; it can be seen that the COD reduction increased with time before 8 min. The COD reduction of 16 mg/L and 20 mg/L reached the plateau after 8 min and the COD reductions of 12 mg/L, 16 mg/L, and 20 mg/L decreased after 8 min [19], which proves that the reaction time plays a role in COD reduction. This may be due to the oxidation of the suspended solids in the water with the increase in the reaction time, thus releasing a large number of soluble compounds [20,21]. For this reason, we chose 8 min as the optimal reaction time.
Figure2. Effect of ozone concentration on COD reduction.
Figure 2 also shows that the COD reduction is high at ozone concentrations of 16 mg/L and 20 mg/L, while the COD reduction at an ozone concentration of 4 mg/L is low. It can therefore be assumed that the reduction in COD is proportional to the ozone concentration. The COD reduction increases by 11% at 8 min as the ozone concentration is increased from 4 mg/L to 8 mg/L. However, the COD reduction only increases by 3% at 8 min as the ozone concentration is increased from 16 mg/L to 20 mg/L. Although the COD reduction efficiency is better at 20 mg/L, the best experimental condition should be 16 mg/L ozone concentration considering the limited improvement of reduction efficiency and the treatment cost.
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