Influence Of Acidity And Alkalinity Conditions On Norfloxacin Degradation By Aluminum-containing Ferrihydrite In Water

Fluoroquinolone antibiotics exhibit a broad antibacterial spectrum,strong antibacterial activity,and low toxicity effects.Their widespread use and continuous discharge on a global scale maintain a certain concentration level in various environmental media,creating a "pseudo" persistent phenomenon that poses potential threats to human health and the ecological environment.Ferrihydrite is a commonly occurring iron hydroxide oxide found in soil,aquatic sediments,acid mine wastewater,and freshwater lakes.It has a wide distribution in nature,small particle size,large specific surface area,and higher Gibbs free energy,which significantly enhances its surface reactivity.Ferrihydrite can also affect the degradation and transformation of pollutants through catalytic oxidation.However,pure ferrihydrite is nearly absent in the natural environment.Aluminum can coexist with ferrihydrite in the environment through coprecipitation or surface adsorption.The varying aluminum content in ferrihydrite can influence the composition and structure of the mineral,while the acidity and alkalinity conditions of the environment can affect the existing forms of pollutants and minerals.These factors can all influence the transport,transformation,and fate of pollutants.Studies on the degradation effects and mechanisms of antibiotics by ferrihydrite are insufficient.In this study,we focus on the typical fluoroquinolone antibiotic,norfloxacin,and investigate its degradation characteristics and mechanisms in five aluminum-containing ferrihydrite systems under three pH conditions.We explored the effects of aluminum content and acidity and alkalinity conditions on norfloxacin degradation in aluminum-containing ferrihydrite systems,identified degradation products,inferred degradation pathways,revealed degradation mechanisms,and evaluated the toxicity of the products.The main results are as follows:The degradation efficiency of norfloxacin in aluminum-containing ferrihydrite systems was significantly affected by aluminum content and pH.In the same aluminumcontaining ferrihydrite system,the degradation efficiency of norfloxacin exhibited the following trend: pH = 4.0 > pH = 7.5 > pH = 10.0.Under the same pH conditions,15%aluminum-containing ferrihydrite had the best degradation efficiency for norfloxacin in water.At pH = 4.0,pH = 7.5,and pH = 10.0,the degradation rates of norfloxacin in the five aluminum-containing ferrihydrite systems were 58% ～ 88%,44% ～ 72%,and 10%～ 60%,respectively.At pH = 4.0,the degradation rate of norfloxacin was positively correlated with the aluminum content;at pH = 7.5,except for 5% aluminum-containing ferrihydrite,the degradation rate of norfloxacin increased first and then decreased with the increase in aluminum content,reaching the maximum degradation rate in the 15%aluminum-containing ferrihydrite system;at pH = 10.0,the degradation rate of norfloxacin in aluminum-containing ferrihydrite systems exhibited the following trend:15% aluminum-containing ferrihydrite > 5% aluminum-containing ferrihydrite ≈ 20%aluminum-containing ferrihydrite > 10% aluminum-containing ferrihydrite >aluminum-free ferrihydrite.In aluminum-containing ferrihydrite systems,norfloxacin in the zwitterion form was more likely to undergo defluorination than the cationic and anionic forms;the cationic form of norfloxacin was more likely to undergo cleavage oxidation and hydroxylation reactions of the piperazine ring;the anionic form of norfloxacin was more likely to undergo defluorination compared to the cationic form;all three ionic forms of norfloxacin could undergo varying degrees of cleavage oxidation reactions of the quinolone ring;more hydroxylated products were detected at all three pH conditions,while decarboxylation products were fewer.The predicted toxicity of norfloxacin and its degradation products showed that over 80% of the degradation products were harmless substances,and the toxicity of the degradation products was minimal at pH = 7.5,posing less potential risk to the ecological environment.Factors influencing the toxicity of the degradation products included their molecular weight,steric hindrance,and distribution of atomic electronegativity.The findings of this study contribute to a deeper understanding of the interaction mechanisms between aluminum-containing ferrihydrite and fluoroquinolone antibiotics as representative emerging contaminants.The results can serve as a reference for the practical application of iron-aluminum mineral systems to remove such emerging contaminants in future studies.