Mechanism Of The Photo-initiated Transformation Of Atrazine And Arsenic Combined Pollution By Fe-citrate Complexes

Atrazine and arsenic（As）are two typical pollutants widely existing in the environment,which pose a serious threat to ecological environment safety and human health.Photoinduced catalytic degradation technology has great application potential due to its advantages of mild reaction conditions,low energy consumption,high efficiency and less secondary pollution.In this paper,atrazine and As were the research object.Metal halide lamp was used as the light source to compare the effect of Fe–citric acid system on the photochemical conversion of atrazine and As single and combined pollution.The effects of initial concentration,p H,hydroxyl radical quantity and hydroxyl radical quenching on the photochemical transformation were investigated by means of HPLC,chromatography-atomic fluorescence,HPLC mass spectrometry and other analytical methods.The photochemical kinetics of atrazine,As single pollution and As combined pollution under different conditions were discussed,and the mechanism of photoinduced Fe（III）–Cit complex conversion of pollutants was revealed,in order to provide theoretical basis for the efficient removal of atrazine and As combined pollution in the environment.The main conclusions of this study are as follows:（1）The effect of different Fe:Cit ratios on the degradation rate of atrazine was investigated using photoinitiated Fe–Cit complexes.The degradation efficiency increased with increasing Fe（III）concentration,and there was almost no difference in the degradation effect between the Fe（III）–Cit concentration ratio of 2:1 and 1:1.This complex was stable and the transformation process of the complex was consistent with primary kinetics.When the initial p H of the solution was 2.0,the photodegradation efficiency of atrazine could reach 88%after 180 min of metal halide light irradiation in the presence of the Fe（III）–Cit complex.Under light conditions,aqueous solutions of Fe（III）–Cit complexes could produce reactive oxygen radicals such as·OH,CO₃^·-,CO₂,H^-,and RCO₂.Acidic conditions favored this photochemical reaction and Fe（III）–Cit complexes could enhance photodegradation in aqueous solutions.When the concentration of the complex was maintained at a certain level,the photodegradation efficiency decreased as the initial concentration of atrazine increased.（2）The photoinitiated Fe–Cit complexes were studied for the conversion of As（III）.When the Fe（III）/Cit was 1:1 and the initial p H of the reaction solution was 2.0,the conversion efficiency and mechanism of As（III）by different concentrations of Fe/Cit ratios were investigated.The conversion efficiency of As（III）could reach 100%after 70 min of light at an initial p H of 2.0,and the conversion efficiency of As（III）could reach 100%after 80 min of light at an initial p H of 3.0.At an initial p H of 4.0,the conversion dropped to 68%,and at p H 7.0 the conversion was only 35%,the photochemical efficiency of As（III）decreased with increasing p H.LC–MS analysis showed a retention time of 3.60 min for As and nitrate products with a retention time of 6.40 min（[M+H]⁺,M/z 263）.Other organic by-products included the major photolysis product aminophenol（[M+H]⁺,M/z 110;retention time 1.05 min）,the benzene product azo product（retention time 1.05～1.42 min）,and the carboxylic acid product associated with the Cit decomposition product.（3）The photoinitiated conversion efficiency of Fe（III）–Cit complexes for atrazine and As（III）and the mechanism of its effect were investigated.At p H 2.0 and 180 min of light exposure,the degradation efficiency of atrazine was 28%and the conversion efficiency of As（III）had reached52%.A certain concentration of Fe（III）–Cit complexes could improve the photochemical conversion efficiency of the pollutants,while excess Cit inhibited the initial reaction rate of photodegradation.As a result,·OH generated during the photochemical reaction of Fe（III）–Cit on the surface was the main factor influencing the transformation and degradation of the pollutants.Both atrazine and As（III）combined contamination were best photochemically transformed under acidic conditions.The reaction intermediates included inorganic arsenic,organic arsenic and other organic products.The analysis revealed a competitive mechanism in the Fe（III）–Cit complex system during the transformation of arsenic and atrazine,where both pollutants could compete with·OH and thus interact with each other during the light exposure.The addition of the bursting agent IPA had a significant inhibitory effect on photodegradation through free radical bursting experiments and EPR analysis experiments,which confirmed that the·OH was the most dominant radical in the degradation process.Using liquid chromatography-mass spectrometry（LC–MS）analysis,the addition of benzene to the Fe（III）–Cit complex was used to measure the degradation process of the contaminant by·OH.The destruction of benzene mainly consisted of C–H bond destruction of the aromatic ring by the Fe-Cit complex,hydrogen abstraction and hydroxyl substitution.Hydrogen abstraction from the C₆H₆ ring led to the formation of catechol,hydroquinone,cis-1,2-dihydrocatechol or other intermediates from phenyl with reactive substances（·OH,O₃,·O,etc.）and Fe-Cit,and these small organic acids were eventually degraded to CO₂ and H₂O.（4）LC-MS/MS was used to detect the intermediates of atrazine and As complex pollutants and found that the atrazine compounds[m+H]⁺,including three major fragments of m/z 174,197,and146 degradation products as 2-chloro-4-ethylamino-6-amino-1,3,5-triazine,2-hydroxy-4-ethylamino-6-isopropylamino-1,3,5-triazine and 2-chloro-4,6-diamino-1,3,5-triazine,respectively.The degradation of As could produce As and nitration products,and the main photolysis products were aminophenols,benzene products azo products,and carboxylic acid products associated with Cit decomposition products.