Effects Of Halide Ions On Degradation Of Organic Pollutants In Novel Chemical Oxidation Processes

Chloride ion is one of the major salt components in dyestuff wastewaters.It can greatly affect treatment efficiency of traditional advanced oxidation processes(AOPs)and lead to the generation of chloride products,accumulation of adsorbable organic halogens(AOX)and increase of toxicity.However,the effects of Cl^-on some novel chemical oxidation processes remain at the kinetic level,and the effects of kinetics,chlorinated products,AOX and toxicity have not been comprehensively evaluated.Therefore,there is an urgent need to evaluate the comprehensive applicability of the novel chemical oxidation processes in high-salt wastewater.In addition,bromide is proved to have stronger reducibility than Cl^-and affect efficiency of AOPs to some extent,despite its occurrence as far lower concentrations in most saline waters.In order to further understand the mechanism during treatment of saline wastewater by advanced oxidation processes(AOPs),the degradation efficiency of methylene blue(MB),acid orange 7(AO7),crystal violet(CV)and 2-chlorophenol(2-CP)/3-chlorophenol(3-CP)/4-chlorophenol(4-CP)/2,4,6-trichlorophenol(TCP)in peroxymonosulfate(PMS)/NaOH,heat/persulfate(PS),UV/acetylacetone(AA)and Fe²⁺/PS processes was investigated as a function of a wide range of salinity.In addition,the effects of halogen ions on AOX accumulation,mineralization rate and halogenated products have been studied too.The main conclusions are as follows:(1)PMS/NaOHBoth Cl^-and base can affect PMS activation to produce reactive chlorine or oxygen species,but the overall effects of chloride on this emerging NaOH/PMS technology in saline wastewater treatment are unclear.Here effectiveness of NaOH/PMS,PMS/Cl^-and NaOH/PMS/Cl^-is compared with a gradient concentration of chloride and alkalinity,by probing the degradation of MB.Both NaOH/PMS and PMS/Cl^-systems can rapidly degrade MB due to the generation of singlet oxygen and reactive chlorine,respectively.Interestingly,dye degradation and AOX formation are inhibited in the NaOH/PMS/Cl^-system as high concentrations of Cl^-and base co-exist.Reaction of PMS with chloride diminishes the effective concentration of PMS by base activation,whereas high alkalinity decreases the oxidation capacity of reactive species in return.Therefore,this observation may have significant technical implications for evaluating the applicability of the emerging NaOH/PMS technology and for optimizing the conditions of AOX abatement in PMS-based processes.(2)Heat/PSHalides(X^-)in the industrial wastewater are usually thought to adversely affect the degradation kinetics and mineralization rates in several SO₄^·--based advanced oxidation processes.However,their unfavourable effects might be overestimated,particularly the heat/PS system as tested in the present study.Here the degradation of phenol,benzoic acid,coumarin and AO7 was examined with the presence of chloride or bromide in a heat/PS process.Cl^-was found to have a dual effect(inhibition followed by enhancement)on the decomposition rates of organic pollutants,whereas the effects of Br^-are insignificant within the tested concentration(0-0.2 m M).However,some chlorinated or brominated compounds were identified in this heat/PS system.Unexpectedly,the mineralization rates of AO7,phenol,benzoic acid and coumarin were not apparently inhibited.In addition,the formation of AOX in the heat/PS system was much less than those in the PMS/Cl^-or PMS/Br^-systems.According to the results of kinetic modeling,SO₄^·-was the dominating radical for AO7 degradation without Cl^-or Br^-,but Cl₂^·-was the main oxidant in the presence of Cl^-.SO₄^·-,Br^·and Br₂^·-were responsible for the oxidation of AO7 in the presence of Br^-.The present study assumes that X₂/HOX,rather than halogen radicals,is responsible for the enhanced formation of organohalogens.These findings are meaningful to evaluate the PS-based technologies for the treatment of high-salinity wastewaters and to develop useful strategies for mitigating the negative effects of halides in AOPs.(3)UV/AAFutility of traditional AOPs in saline wastewater treatment has stimulated the quest for novel“halotolerant”chemical oxidation technology.Acetylacetone(AA)has been proven to be a potent photo-activator in the degradation of dyes,but the applicability of UV/AA for saline wastewater treatment needs to be verified.In this study,degradation of crystal violet(CV)was investigated in the UV/AA system in the presence of various concentrations of exogenic Cl^-or Br^-.The results reveal that degradation,mineralization and even accumulation of AOX were not significantly affected by the addition of Cl^-or Br^-.Rates of CV degradation were enhanced by elevating either AA dosage or solution acidity.An apparent kinetic rate equation was developed as r=-d[CV]/dt=k[CV]^a[AA]^b=(7.34×10^-4 m M min^-1)×[CV]^0.16[AA]^0.97.In terms of results of radical quenching experiments,direct electron/energy transfer is considered as the major reaction mechanism,while either singlet oxygen or triplet state(³(AA)*)might be involved.Based on identification of degradation byproducts,a possible degradation pathway of CV in the UV/AA system is proposed.Therefore,the UV/AA process is recommended for treatment of saline dye wastewaters.(4)Fe²⁺/PSTransformation of halogenated organics in AOPs have been extensively investigated.However,there is little information about the fate of halogenated pollutants while exogenic halides(X^-,Cl^-or Br^-)are present.Herein,the degradability and AOX accumulation capacity of chlorophenols(2-chlorophenol(2-CP),3-chlorophenol(3-CP),4-chlorophenol(4-CP)and 2,4,6-trichlorophenol(TCP))were comparatively investigated in the Fe²⁺/PS process with the addition of Cl^-or Br^-.The Cl^-or Br^-was found to have inhibitory effect on the decomposition rates of chlorophenols,and the degradation of chlorophenols proceeded in decreasing order:3-CP>2-CP>4-CP>TCP.The order of AOX formation potential(AOXFP)of chlorophenols is:3-CP>2-CP>4-CP>TCP.In addition,some chlorinated or brominated compounds were identified in the Fe²⁺/PS system.And the mineralization rates of chlorophenols were inhibited by Cl^-or Br^-.In summary,this study demonstrates that both position of the endogenous halogen atom and concentration of the exogenous halogen ion can affect the degradation efficiency,mineralization efficiency and accumulation of adsorbable organic halogens in the advanced oxidation processes.Therefore,the position of endogenous atoms should be comprehensively considered to predict the potential impact of exogenous X^-during high-salt wastewater treatment.