Study On The Catalytic Degradation Of Aniline By Sodium Percarbonate:Efficiency And Mechanism

Advanced oxidation processes（AOPs）have attracted considerable attention owing to its strong oxidative capacity for refractory organic contaminants,and the application and research of Fenton process become more popular in recent years.Nevertheless,there are unavoidable disadvantages associated with this technology,including a narrow working pH range,inconvenient storage or transportation for H₂O₂,and ferric precipitation which prevent its full-scale application.Thus,it turned to be appealing to search for more effective and universally applicable oxidant and catalysis for AOPs.In this context,sodium percarbonate（SPC）was applied for the removal of organics from aqueous solution due to its strong oxidizing capacity and stable behavior during storage,transportation,and handling.SPC is more effective over a wide pH range,which is favorable for the removal of persistent organics.To increase the degradation efficiency of contaminant and reveal the reaction mechanism were crucial problems to be solved in the promotion of AOPs based on SPC.In the current study,three catalytic systems of SPC were established with iron（Fe）,vanadium（V）and ultraviolet（UV）utilizing aniline as target.The aniline degradation efficiency was investigated in SPC catalytic system and environmental factors were also examined.Reactive oxidation species generated during the reaction were identified through free radicals scavenging tests,electron paramagnetic resonance（EPR）experiments and analysis of three-dimensional fluorescence spectra characteristic.The degradation pathways were proposed based on the analysis of intermediates,and mechanism for aniline degradation was elucidated accordingly.First,Fe（II）/SPC system was established,by which aniline could be degraded efficiently and rapidly,and the decomposition of aniline increased as the initial Fe（II）or SPC dosage increased.But degradation of aniline would be inhibited when either of the two was excessive and the optimal molar ratio of Fe（II）:SPC:aniline was 8:3:1,with95.68%of aniline removed within 10 min.It was confirmed that·OH was the predominant reactive oxidation species and O₂^·-also participate in the degradation process.Aniline degradation was not significantly affected by initial pH,with only slightly inhibited when p H≥11.0.Furthermore,aniline degradation was not apparently influenced by the addition of Cl^-,with only a little suppression at a high Cl^-concentration.The degradation of aniline showed a similar tendency as Cl^-in the presence of natural organic matters（NOM）,while the influence of SO₄^2-and NO₃^-were negligible.Moreover,HCO₃^-apparently inhibited aniline degradation due to the decrease of Fe（II）in the form of Fe（OH）₃ precipitation.Based on the intermediates detected,two possible pathways for aniline mineralization were proposed with aniline first converted to aminophenol and nitrobenzene,respectively.The two compounds then transformed to phenol and hydroquinone,and converted to quinones through dehydrogenation,which were oxidized to small organic molecules subsequently and ultimately mineralized into CO₂ and H₂O.Chlorobenzene,trichloroethylene and ethylbenzene could be removed by Fe（II）/SPC system from aqueous solution through a similar mechanism as that of aniline.Mn（II）and Co（II）showed certain improvement for aniline degradation by Fe（II）/SPC system and Cu（II）induced the most positive effect with an increment of aniline removal efficiency by 37.92%.In order to further investigate the catalytic performance of other transition metals for SPC,V（IV）was applied for SPC activation.The results showed that aniline could be also effectively degraded by V（IV）/SPC system,and the amount of V（IV）required was relatively less than Fe（II）.As a consequence,an expected result was obtained when the molar ratio of V（IV）:SPC:aniline was 1:8:1,with 90.05%of aniline was removed within 15 min,but further increasing the doses of V（IV）or SPC led to an inhibition.Aniline degradation showed a two-stage process during the reaction time,and the slow stage followed a pseudo-first-order model,with reaction rate constant corresponded well with the variation of V（IV）or SPC doses and aniline degradation performance.The predominant reactive oxygen species was identified to be O₂^·-and CO₃^·-,with·OH also involved in the degradation of aniline.Furthermore,the initial pH and addition of SO₄^2-and NO₃^-did not have a remarkable influence on aniline degradation,while an enhancement was obtained in the presence of HCO₃^-owing to the strong oxidizing ability of·OH.Cl^-caused in a slightly inhibition at high concentration.Based on the intermediates detected in V（IV）/SPC system,three possible pathways for aniline mineralization were proposed with aniline first converted to aminophenol,nitrobenzene and aniline polymer,respectively.These compounds then transformed to small organic molecules subsequently through substitution reaction,dehydrogenation and ring-opening reaction,which were ultimately mineralized into CO₂ and H₂O.UV/SPC system was established to achieve an environmental friendly catalysis.The reaction rate of aniline degradation was relatively lower when SPC was activated by UV compared with that of transition metals.However,1.0 mM aniline could be also eliminated thoroughly within 120 min under the conditions of SPC concentration=4.0mM and UV irradiance=31.50 mw·cm^-2.The reaction process could be divided into two stages accordingly,both of which followed the pseudo-first-order model.The reaction rate and degradation efficiency increased with the increment of SPC dosage and UV irradiance.It was under alkaline condition during the whole reaction process,and·OH,CO₃^·-,and O₂^·-were confirmed to be existed in the system and made contribution to aniline degradation.In addition,the initial pH and SO₄^2-had a negligible influence on the degradation of aniline by UV/SPC system while Cl^-resulted in a little inhibition at high concentration.NO₃^-showed a suppression because of the scavenging effect of radicals although it also induce an enhancement of·OH generation via nitrate photolysis.Similarly,the addition of HCO₃^-and NOM caused a decrease of aniline removal.The intermediates detected in UV/SPC system were similar with that of V（IV）/SPC system,with more aniline polymer generated comparatively,and therefore,an analogous mechanism could be inferred.The degradation efficiency was increased by 26.79%when 2.0 mM Fe（II）was applied in UV/SPC system under the condition of SPC concentration=1.0 mM and UV irradiance=31.50 mw·cm^-2.