Degradation Of Azo Dyes(Methyl Orange And Congo Red) By Sodium Persulfate Activated With Different Particle Sizes Of Zero-valent Zinc

With the rapid development of economy,lots of environmental pollution problems have been more and more serious,particularly for water pollution.Azo dyes,as the main sources of pollution,have been attracted much attentions.Azo dye is a kind of typical dyes which has complex structure,strong toxicity,and difficult for biochemical degradation.The development of advanced oxidation technologies,such as free radical technology,have provided possibility for its complete degradation.There have been a lot of papers reporting the degradation mechanism based on hydroxyl radicals.While,sulfate radicals was discovered to have the more stronger oxidation ability than the hydroxyl radicals.Sulfate radicals could be obtained by the activation of persulfate by zero-valent metals.Different sizes of zero-valent metals have different effect on the degradation,discoloration and mineralization.Therefore,in this paper,different sizes of zero-valent metal zinc was chosen to activate sodium persulfate.The optimal conditions of degradation of two azo dyes including methyl orange and congo red and their reaction mechanism were proposed.This paper is divided into the following two parts:Part Ⅰ:Sodium persulfate（PS）activated by zero-valent zinc（Zn⁰）with different particle sizes was conducted in order to degrade MO.Results showed that Zn⁰ with different particle sizes has different effects on MO degradation.Zn⁰ with smaller particle sizes has larger specific surface area,which can more effectively degrade MO.Strong oxidizing free radical of sulfate radical(SO₄^·-)and hydroxyl radical（·OH）were generated during the reaction.The reaction rate was fast at the beginning,and then slowed down,finally increased again.Concentrations of the initial PS and Zn⁰ as well as pH have obvious effects on the degradation of MO.Eventually,the optimal condition obtained for degrading MO was:pH=4,concentrations of the initial PS and Zn⁰ being 10 mg/L and 1.5 g/L,respectively and particle size of Zn⁰ being 150 μm.In the case,decolorization of MO was 87%,and the removal of TOC and COD was 37%and 55%,respectively.Part Ⅱ:A series sizes of Zn⁰,including particle sizes<150 μm,150 μm～300 μm and≥300 μm was performe to catalyze PS to degrade CR.Results indicated that the decrease of zero-valent zinc particle sizes led to the increase of decolorization,mineralization and COD of CR.The initial concentrations of PS,Zn⁰ and pH exhibited obvious effects on the degradation of CR.Finally,the optimal conditions for CR degradation was obtained,which included pH 6,concentration of PS = 150 mg/L,initial concentration of Zn⁰ =1.5 g/L,and particle size of Zn⁰ =150 μm.In the case,the decolorization of CR,the removal of TOC and COD in the solution was 86%,55%and 72%,respectively.