Degradation Of Fluoroquinolones Using SO₄^-·-Advanced Oxidation Processes

Fluoroquinolones,as one of the most widely used antibiotics in the world,are detected in surface water,groundwater and even drinking water.The persistence of antibiotics in the environment will lead to the generation of bacterial resistance,which poses a serious threat to human and ecological environment.Therefore,it is of great significance to develop fluoroquinolone antibiotics for efficient removal in water.In this paper,quinolone antibiotics levofloxacin and ciprofloxacin were selected as the main research objects,and the degradation of target pollutants were studied by ultrasonic/K₂S₂O₈ and heat-activated K₂S₂O₈ systems.（1）the degradation of levofloxacin aqueous solution in Ultrasonic（US）system,K₂S₂O₈ system and US/K₂S₂O₈ system were discussed.Production processes and change rule of active free radicals in US/K₂S₂O₈ system were studied based on the experiments of K₂S₂O₈ concentration,ultrasonic power,p H value and the influence of adding isopropanol or tert butanol.The results showed that under the conditions of levofloxacin concentration 30 mg·L^-1,the ultrasonic power 360 W,K₂S₂O₈ concentration 20 mmol·L^-1and p H=9,the removal rate of levofloxacin in US/K₂S₂O₈ system,K₂S₂O₈system and US system after 180min was 90.00%,5.82%and 13.89%,respectively.The synergistic effect in US/K₂S₂O₈ system facilitated the production of more SO₄^-·and·OH free radicals.Moreover,both ultrasonic and chain reaction were capable to produce·OH in US/K₂S₂O₈ system,and the chain reaction was the major contributor.The existence of K₂S₂O₈ was also the necessary condition for·OH production.The chain reaction would be quenched when the concentration of free radicals at high level.The concentration of K₂S₂O₈ had an important effect on the production of SO₄^-·,but the high concentration of K₂S₂O₈ would not benefit the production of SO₄^-·.In the processes of levofloxacin degradation in US/K₂S₂O₈ system SO₄^-·and·OH free radicals were increasing and decreasing together by chain reaction,and·OH played a dominant role in the reaction.It showed that·OH played a major role in the early stage of the reaction,and gradually weakened with the extension of the reaction time.SO₄^-·had little contribution in the early stage of the reaction,and gradually increased with the proceeding of the reaction.（2）Ciprofloxacin（CIP）removal efficiency in aqueous solutions in the ultrasonic（US）,K₂S₂O₈,and US/K₂S₂O₈ systems was investigated.The free radical generation and action ratio were studied based on variations of K₂S₂O₈ concentration,ultrasonic power,p H,and the addition of isopropanol（ISP）or tert-butyl alcohol（TBA）in the US/K₂S₂O₈ system.The results showed that under conditions of 20 mg·L^-1 CIP concentration,20 mmol·L^-1 K₂S₂O₈ concentration,an ultrasonic power of 360 W and p H=7,CIP removal efficiency in the US/K₂S₂O₈ system was 92.20%after 180 min.The reaction in the US/K₂S₂O₈ system was explicitly divided into two stages:free radical generation and pollutants degradation.The ultrasonic and chain reaction facilitated enhanced generation of SO₄^-·and·OH.The presence of K₂S₂O₈ can promote·OH generation and K₂S₂O₈ concentration also exerted a significant effect on SO₄^-·generation,however,high concentrations were not beneficial to the reaction.Quenching reactions occurred under high concentrations of·OH and SO₄^-·.During the initial stage of the reaction,·OH played a more prominent role than SO₄^-·,however,the role of SO₄^-·gradually increased as the reaction proceeded and eventually surpassed·OH.（3）We also investigated the effect of heat-activated K₂S₂O₈ system on the degradation of ciprofloxacin.The results showed that at a temperature of 60℃,a K₂S₂O₈ concentration of 20 mmol·L^-1 and p H=7,the removal rate of ciprofloxacin reached 92.21%,the mineralization rate reached 74.42%,and the degradation rate constant kobs was 0.0591 min^-1.The kobs of ciprofloxacin had a quadratic relationship with the concentration of K₂S₂O₈ and the fitting formula is:k_obs=-3.16×10^-2+7.56×10^-3[K₂S₂O₈]+1.66×10^-4[K₂S₂O₈]~2.Arrhenius equation showed that the degradation rate constant kobs of ciprofloxacin increased with higher temperature.In addition,the morphological distribution of ciprofloxacin and dominant free radicals under different p H conditions led to the difference of degradation rate,and kobs is the largest at p H=7.Cl^-,HCO₃^-and humic acid can inhibit the degradation rate,but Fe³⁺and Ca²⁺can promote the degradation of ciprofloxacin.The quencher experiment showed that the degradation of ciprofloxacin was mainly the result of the combined effect of·OH and SO₄^-·.Also,the rule of free radical action was put forward,both SO₄^-·and·OH increased and decreased simultaneously.The action of·OH on CIP degradation was basically unchanged,whereas the SO₄^-·action on CIP degradation gradually increased over time.Before40 min,the action of·OH on CIP removal was higher than that of SO₄^-·.After 40 min,SO₄^-·occupied a higher proportion of CIP removal than·OH.