模拟日光照射下土霉素的复合光化学转化动力学及环境归趋

Aqueous multivariate photochemical transformation kinetics and fate of oxytetracycline under simulated solar irradiation

环境水体中存在众多可解离的有机污染物,而不同的解离形态表现出相异的环境行为,如光化学反应活性和归趋.本文以土霉素(OTC)为例,研究了水中可解离化合物的表观光降解、光氧化等复合光化学转化动力学,并揭示了其相应的环境光化学归趋和贡献.模拟日光(λ>290nm)照射下,OTC的光降解遵循准一级反应动力学,光解速率常数(k)对pH表现出明显的依赖性.这是由于随pH值升高,解离形态(i)从OTCH_2~0、OTCH^-到OTC^(2-)渐变,相应的k_i和量子产率(Φ_i)显著增大.竞争动力学实验进一步表明,·OH和~1O_2均可以氧化降解OTC,且降解速率随着pH的增大而加快.这与3种解离形态不同的去质子化程度和氧化反应活性有关.去质子化增加化合物的电子密度,利于活性氧物种的亲电攻击,从而促进光氧化反应.基于以上OTC解离形态的不同光化学转化活性,评估了环境表层水体(pH=6~9)中的复合光化学转化半减期(t_(1/2,E)).t_(1/2,E)值依赖于基质pH和季节,北纬45°晴天正午,t_(1/2,E)在0.131 h(仲夏,pH=9)到3.63 h(仲冬,pH=6)之间.多数情况下,OTC的表观光解是其最主要的光化学转化途径,~1O_2氧化次之,而·OH氧化的贡献非常小.以上结果证明了OTC不同解离形态具有不同的光化学转化动力学.因此,为了准确评价水环境中抗生素等可解离有机污染物的光化学归趋和风险,有必要综合考虑所有解离形态的复合光化学行为.

Many ionizable organic pollutants are present in the aqueous environment.They are of great concern because their environmental behavior（ e.g.,photochemical reactivities and fate） is highly dependent on different dissociation forms. In this study,we adopted oxytetracycline（ OTC） as a case to investigate the kinetics of multivariate photochemical reactions,apparent photolysis and photooxidation,and to reveal the corresponding environmental photochemical fate.It was observed that the photodegradation followed the pseudo-first-order kinetics under simulated solar irradiation（ λ 〉 290 nm）. The photolytic rate constants（ k） obviously depended on the solution pH,due to the changes of dissociation forms（ from OTCH20 to OTCH- and OTC2-） with the increasing pH. OTC2-photolysis is the fastest and had the highest photolytic efficiency,followed by OTCH-and OTCH20. Competition kineticexperiments indicated that OTC was oxidized by ·OH and 1O2,and the reaction was facilitated by the increasing pH.This pH dependence is attributed to the diverse deprotonation degrees and photooxidation reactivities of the 3 dissociation forms. Deprotonation can enhance the electron donating ability,decrease the steric hinerance,and hence facilitate the reactions of OTC with reactive oxygen species. Furthermore,the environmental multivariate phototransformation half-lives（ t1/2,E） were dependent on matrix pH and seasons,ranging from 0.131 h at pH 9 in summer to 3.63 h at pH 6 in winter at 45° N latitude.Among the multivariate photochemical processes,apparent photolysis is the most dominant,followed by 1O2 and ·OH oxidation. This study confirmes that different dissociation species of OTC have distinct phototransformation kinetics.Thus,to better understand the photochemical fate and risk of ionizable organic pollutants（ e.g.,antibiotics） in surface waters,the effects of acid-base dissociation and multiple photoreactions should be considered.