Degradation Kinetics And Mechanisms Of Intermediates Of Dyes In The Treatment Of High Salinity Wastewater

It is well known that a series of refractory organic pollutants will be produced during the wastewater treatment of dyes in advanced oxidation processes,such as polycyclic aromatic hydrocarbons,chlorobenzene,and quinone compounds.In addition to the typical characteristics of high organic content,low degradation efficiency,high chromaticity,and high biological toxicity,dyes and their intermediates wastewater also contain a high content of salts,especially sodium chloride(Na Cl).It indicates that the excess Cl^-may not only affect the degradation of dyes and the secondary oxidation of dye degradation intermediates,but also lead to the accumulation of adsorbable organic halogens(AOX)in wastewater and the enhancement of biological toxicity.Therefore,it is of significant importance to investigate the degradation kinetics and mechanism of dye degradation intermediates in highly saline wastewater.Several typical degradation intermediates of dyes,including phthalic acid(PA),coumarin(COU),maleic acid(MA)and fumaric acid(FA),were selected as model contaminants in this paper.The degradation kinetics,reaction mechanism,AOX accumulation,and acute toxicity transformation were investigated in the presence of chloride by using Co/PMS,UV/PMS and Fe/PMS systems based on SO₄^·-,respectively.Our attention has also been focused on the formation and transformation mechanism of chlorinated organic by-products in highly saline systems.The main results are shown as follows:(1)The degradation efficiency of phthalic acid(PA)was investigated in the Co/PMS system.The degradation rate of PA enhanced when PMS concentration,Co²⁺concentration,and p H increased.An inhibiting effect of Cl^-on PA degradation process by Co/PMS/Cl process was obtained,whereas a dual(inhibitory then accelerating)impact of Cl^-on the decomposition of PA?s potential parent compounds AO7 and Rh B was observed.Besides,the presence of chloride had no apparent effect on PA degradation in PMS/Cl system.Based on the results,the formation of chlorinated by-products,AOX value,toxicity,and mineralization decreased with the concentration of chloride increasing.In addition,decarboxylation,halogenations,and hydrolyzation were the primary reactions during PA degradation process.Moreover,the presence of NH₄⁺and Cl^-had a dual effect on the degradation of AO7,a parent compound of PA.The[NH₄⁺]/[Cl^-]concentration ratio had a significant effect on AO7 degradation efficiency.(2)The impact of Cl^-contents(0?300 mM)on coumarin(COU)oxidation in UV/PMS process was discussed.A dual effect:an inhibiting impact with[Cl^-]<10 m M,whereas an accelerating effect with[Cl^-]>10 m M was obtained.According to the results of TOC removal,the mineralization of COU increased with the reaction time,whereas reduced by enhancing the chloride dosage.Based on the intermediates indentified by GC-MS,some more toxic chlorinated organic compounds were produced during the COU degradation process,which led to the accumulation of AOX and enhancement of acute toxicity.Besides,the fluorescence intensity reduced on the addition of Cl^-.Due to the transformation of SO₄^·-to^·OH,the fluorescence intensity gradually enhanced with p H varying from 2.0 to 7.0.(3)In order to systematically investigate the performance of chloride on the depletion of short-chain carboxylates of dye decomposition intermediates,maleic acid(MA)and fumaric acid(FA)were chosen as model compounds in the Fe/PMS system.It indicated that the oxidation rate of MA increased first and then decreased when the molar ratio of[Fe²⁺]/[PMS]varied from 3:1 to 1:3.The maximum oxidation rate was obtained at[Fe²⁺]/[PMS]=1:1.Besides,the presence of Cl^-(0?300m M)restrained the decomposition of MA and FA by Fe/PMS process.The addition order of reagents had a significant impact on MA or FA depletion and substrate-Fe²⁺-PMS addition accelerated the oxidation process of the substrate.Using UHPLC-QTOF-MS techniques,the production of potentially toxic chlorinated by-products was confirmed in the presence of chloride ions.The kinds of chlorinated by-products during MA oxidation were more than that in the FA system.Moreover,the presence of high contents of Cl^-led to a formation and accumulation of AOX and acute toxicity by Fe/PMS process,whereas the mineralization decreased.According to the results of kinetic modeling,the inhibiting impact of chloride on MA oxidation could be attributed to the scavenging effect of Cl^-to SO₄^·-and the formation of less reactive chlorine radicals.Moreover,the effect of various radicals was dependent on the levels of Cl^-.SO₄^·-was the primary radical during MA oxidation process at a low chloride concentration,while Cl₂^·-was the dominant radical responsible for the depletion of MA in high levels of chloride.The results of sensitivity analysis revealed the consumption of SO₄^·-by Cl^-had the most important impact on MA depletion in the presence of5m M Cl^-.In addition,the addition order of reagents had a significant impact on the degradation of AO7,a parent compound of butenedioic acid.It indicated that the degradation efficiency of AO7was higher under Fe-PMS addition order.