| PhACs (pharmaceutical active compounds), as a kind of new pollutants, have been frequently detected in the aquatic environment and attracted more and more attention. According to incomplete statistics, about 99.5 percent of the water treatment plants are using chlorine disinfection nationwide and therefore, related researches need to be done on the removal of PhACs during chlorination process. In this study, antipyrine (ANT), a widely used anti-inflammatory analgesic and free chlorine were selected as the target compound and the oxidant, respectively. The efficiency of ANT chlorination and the effect of the coexisting substances including humic acid (HA), ammonium (NH4+), and bicarbonate (HCO3-) on the removal of ANT in ultrapure water during free chlorine disinfection were investigated by batch experiments. In the whole process, reaction order and apparent reaction rate constant were calculated. Finally, The surface water and tap water were taken to fit the kinetic model and then analyzed the effect of coexisting substances during chlorination process.The results showed that in the ultrapure water, free chlorine reacted rapidly with ANT under the conditions of pH=7.0±0.2, T=25±1℃, and the degradation efficiency could reach to above 99% in 1 min, which could be expressed by second-order kinetic model. The apparent reaction rate constant (kapp) was calculated with reaction kinetics. When pH<4.5, the neutral antipyrine species were more reactive to HOCl, and the acid catalytic reaction of neutral antipyrine species (kANT=1.08×106 M-1 · s-1) with HOCl was regarded to primary reaction to the elimination of antipyrine, and secondary main reaction was the reaction of cationic antipyrine species with HOCl. When pH was slight acid (>4.5) and neutral, the reaction of neutral antipyrine species with HOCl was mainly attributed to the antipyrine removal. When pH>7, since OCl- (kOCl-=1.35×10-5<10-4 M-1 · s-1) has low reactivity, the reaction rate decreased with the increase of pH.The experimental results of the effects of coexisting substances on the ANT removal showed that both HA and NH4+ had adverse effect on antipyrine chlorination while HCO3- had no obvious effect. Under the conditions of pH=7.0±0.2, T=25±1℃, HA showed significant inhibition to ANT chlorine oxidation with the HA concentration increasing. HA and ANT presented competitive relationship with free chlorine. When NH4+ existed in the solution, hypochlorous acid was the main active oxidant as the ratio of C1:N (mass ratio)>8:1, ANT reacted quickly with hypochlorous acid and therefore was removed in a very short time. When the ratio of Cl:N (mass ratio) was in the range of 1:1-5:1, NH2Cl was the main active oxidant. NH2Cl had a much weaker oxidative capacity than hypochlorous acid and thus ANT could not be removed completely. So the existing forms of chlorine play an important role in ANT removal under NH4+ coexistence.In aquatic environment experiments, second-order kinetic model obtained in ultrapure water can well predict changes in concentration on removal of ANT due to low concentration of coexisting substances. At last all study shows that pH plays a key role in removal of ANT during chlorination process. |
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