Synergistic Removal Of Norfloxacin By Nanoscale Zero-valent Iron-potassium Monopersulfate

Norfloxacin（NOR）is biotoxic and can pose a threat to human health when released into the environment.The common methods are biodegradation and adsorption,but the efficiency is low.The advanced oxidation technology based on persulfate should degrade NOR,which has attracted extensive attention recently.At room temperature persulfate has steady chemical property,so researchers often use transition metals(such as Fe²⁺)to activate it.However,Fe²⁺is consumed too quickly and needs to be added multiple times.In this study,based on the property that Fe²⁺can be released continuously when iron is corroded in water,potassium monopersulfate（PMS）is activated by nano-zero-valent iron（nZVI）,which not only overcomes the limitation of Fe²⁺activation,but also the iron oxide formed by Fe²⁺corrosion can adsorb residual NOR and degradation products,so as to play a synergic role in NOR removal,and avoid the secondary pollution of degradation products.In this paper,the preparation conditions of nZVI were optimized from the aspects of iron source,mole ratio of NaBH₄ and FeSO₄,surface modifier,bimetallic composite of Ni,attapulgite loading.The effects of nZVI dosage,temperature,common anions in natural water on NOR removal rate were investigated.The physical and chemical properties of nZVI before and after its reaction were characterized by X-ray diffractometer,transmission electron microscope and specific surface area analyzer.The free radicals types,kinetics,thermodynamics,process and mechanism by nZVI activated PMS（nZVI-PMS）in synergic removing NOR were discussed,and the degradation pathways and products were analyzed.The main conclusions were as follows:（1）The best conditions for preparation of nZVI:FeSO₄ is used as iron source,the mole ratio of NaBH₄ to FeSO₄ is 2,no recombination with Ni,no surface modifier added and no load with attapulgite.Under this prepared condition when nZVI activated PMS,the removal efficiency of NOR reached 97.4%after reaction in 2 h.（2）When the dosage of nZVI was 1 g/L,the concentration of PMS was 0.4 mM,the temperature range was 10-55℃,and the pH value was 3-9,the removal efficiency of NOR was all above 97%by nZVI-PMS.The common anions Cl^-、HCO₃^-and SO₄^2-slightly promote the removal of NOR,while NO₃^-has a weak inhibition.When humic acid concentration was 0-10 mg/L,the the removal efficiency of NOR was not significant.（3）By means of electron paramagnetic resonance and free radical quenching experiments,it is determined that the main free radicals in nZVI-PMS system are:SO₄^-·,followed by OH·.The process and mechanism of synergic removing NOR by nZVI-PMS were as follows:Fe⁰corrosion in nZVI produced Fe²⁺,activated PMS by Fe²⁺to produce SO₄^-·and OH·free radicals,and the removal of NOR was completed within 5 min.The residual NOR and its degradation products can be removed by adsorption after nZVI is converted to lepidocrocitr through a series of reactions with water,OH^-,and dissolved oxygen.（4）By using mass spectrometry to detect the degradation products,it is speculated that the degradation path and products of NOR were as follows:free radical attacked causes piperazine ring breaking,benzene ring breaking and defluorination,and finally degradation into short chain small molecular acids.