Degradation Of Nitroimidazoles By In-situ Generated Persulfate Activation

Nitroimidazole antibiotics are a class of synthetic drugs that widely appear in municipal sewage and natural water.Nitroimidazoles are found in pharmaceuticals and personal care products(PPCPs)and,when discharged into the environment,have adverse effects on water environment,biology and ecology.Due to its carcinogenic,teratogenic and genetic toxicity,it may eventually pose a threat to human health and survival.Advanced oxidation technologies(AOTs)based on persulfate(PS)has attracted much attention for years because it can generate strong oxidizing free radicals to degrade organic pollutants quickly and efficiently under certain activation conditions.In this thesis,the typical drugs ronidazole(RNZ),metronidazole(MNZ),and tinidazole(TNZ)were selected as target pollutants.By electrochemically generating persulfate(PS)and ferrous ions(Fe2+)in situ,we built a new sulfate radical oxidation system that simultaneously generated oxidants and activators in high concentration.Sulfate radicals can be generated continuously by regulating addition flow rate of Fe2+,and then oxidative degradation of nitroimidazole antibiotics in water can be achieved.We explored the effect of electrolytic conditions such as anode additives,temperature,current density,and anolyte composition in the electrochemical system;the effects of PS concentration,n(Fe2+)/n(PS),the dosage mode of Fe2+ on degradation,as well as the degradation mechanism and proposed pathway of nitroimidazole.The specific research results are as follows:1.Two-compartment electrolytic flow cell could produce S2O82-and its activator Fe2+ online at the same time by electrolysis.The anode additive ammonium polyphosphate significantly improved the current efficiency of S2O82-at the initial stage of electrolysis(30 min)and could be maintained to 180 minutes.At the same time,the anode additive did not affect the Fe2+ formation at cathode.Low temperature was beneficial to the formation of anode S2O82-,making it difficult to hydrolyze.Properly increasing the temperature increased the current efficiency of the Fe2+.But when the temperature was higher than 15 °C,it had little effect on cathode electrolysis.The current efficiency of the anode S2O82-increased as the current density Since the cathode was a material control process based on reducing H+,the current efficiency of Fe2+ decreased as the current density increased.The optimal electrolysis conditions were:(1)anolyte solution was a mixture of 4 mol/L(NH4)2SO4 and 0.15% ammonium polyphosphate,and the catholyte was 0.1 mol/L Fe Cl3 mixed with 0.5 mol/L H2SO4(2)at the current density of 150 m A/cm2(3)kept the temperature at 15 ° C.2.PS was difficult to degrade nitroimidazole pollutants,but Fe2+/PS system had a strong ability to degrade RNZ,MNZ,and TNZ.Properly increasing the dosage of PS improved the degradation efficiency.Considering the cost and environmental protection requirements,the optimal n(Fe2+)/n(PS)of RNZ,MNZ,TNZ were controlled at 1: 2.Controlling the flow rate of the homogeneous activator Fe2+ by the peristaltic pump could effectively adjust the release rate of Fe2+ in the solution.The optimal rates of Fe2+ for RNZ,MNZ and TNZ were 0.3 m L/min,0.3 m L/min,and 0.15 m L/min,respectively.The degradation rates were 96%,95%,and 84%,respectively.The degradation could be characterized by a pseudo-first-order pattern.3.During the degradation of RNZ,MNZ,and TNZ,both SO4·-and ·OH were predominant reactive species in the Fe2+/PS system,of which SO 4·-contributed more.The TOC removal rates were 17%,23%,and 14%,respectively.Fe2+/PS system destroied the structure of nitroimidazole,but it is difficult to completely mineralize.Therfore,most of the pollutants were oxidized to intermediate products.The degradation path for nitroimidazole was proposed: ·OH attacked the nitro group on the side chain,followed by the substitution reaction of Cl-in the solution.SO 4·-mainly acted on the ring-opening reaction of imidazole,mainly attacked the C = N,C = C,C(5)-N(6)bonds to break them into small molecular compounds,and then continued to mineralize into CO2,H2 O,NO3-.ect.