Degradation Of Chlorophenol By Hydrogen Peroxide Catalyzed By Molybdate

Chlorophenols are extensively found in the wastewater from industries of paper,pesticide,dye and pharmaceuticals.Given the high toxicity and refractory biodegradation,they show carcinogenic,teratogenic and mutagenic effects to human beings.As early as 1987,2-chlorophenol?2-CP?has been listed as a priority pollutant by the U.S.Environmental Protection Agency and its concentration in drinking water should be controlled below 0.3?g L^-1.Conventionally,oxidaiton of 2-CP by generating high concentrations of hydroxyl radicals?·OH?can effectively mineralize2-CP into CO₂ and H₂O.In the recent years,selective oxidation of organic contaminants on the basis of ¹O₂ and O₂^·-radicals generated from H₂O₂ activation is attracting more attention.In this study,we will explore the efficiency and mechanism of catalytic degradation of 2-CP by H₂O₂ and MoO₄^2-under alkaline conditions,and then investigate the synergistic catalysis of H₂O₂ by MoO₄^2-and Cu²⁺,and their contributions to the degradation of 2-CP under acidic conditions.In this study,we found that the MoO₄^2-/H₂O₂ homogeneous system could effectively degrade 2-CP under alkaline conditions.The degradation rate of 2-CP was increased with increasing pH values?3.0-11.0?under the conditions of 1 mM MoO₄^2-,500 mg L^-1 H₂O₂ and 100 mg L^-1 2-CP.At pH 11.0,the degradation of 2-CP reached68%in 5 h with the reaction rate constant of 3.38×10^-3 min^-1.When the pH values were increased to 12.0 and 13.0,the reaction rate constants decreased to 2.75×10^-3and 0.83×10^-3 min^-1,respectively.When the concentration of H₂O₂ was increased to2000 mg L^-1,the degradation rate at pH 12.0 was higher than that at pH 11.0,and the degradation of 2-CP could reach 100%.This suggests that the degradation efficiencies and mechanisms depend on pH and H₂O₂ conditions.Our experiments further point out that ¹O₂ is the predominant oxidative species at pH 11.0,whereas O₂^·-plays the predominant role at pH 12.0.Differences in the distributions of the reaction products in GC-MS analysis further indicate that polymerization,hydroxylation and epoxidation of 2-CP is more favored by ¹O₂,whereas dechlorination of 2-CP is more likely by O₂^·-.We also investigated the catalytic effect of MoO₄^2-/Cu²⁺on the decomposition of H₂O₂ under acidic conditions.Actually,the catalytic efficiency of H₂O₂ by MoO₄^2-is low under acidic conditions.At pH 3.0,only 9%of 500 mg L^-1 H₂O₂ was decomposed by 1 mM MoO₄^2-catalyzed after 24 h.However,when 1 mM Cu²⁺was added into the system,the decomposition efficiency increased to 96%.In the pH range of 3.0-6.0with the Mo:Cu of 1:1(1 mM MoO₄^2-),2-CP showed the highest degradation efficiency at pH 5.0,and 68%of 500 mg L^-1 2-CP could be degraded within 6 h with a complete decomposition of H₂O₂ in 30 min.We further confirm that radicals such as HO₂^·and·OH are not the primary oxidative species in the degradation of 2-CP under these conditions,whereas a non-free radical mechanism is involved in this process.MoO₄^2-can react with H₂O₂ to form Mo₂O₃?O₂?₄?H₂O?₂^2-and HO₂^·,the latter of which can be rapidly disproportionated to H₂O and O₂ by Cu²⁺.This increases the generation Mo₂O₃?O₂?₄?H₂O?₂^2-,the primary oxidant for 2-CP.This study extends the application of MoO₄^2-/H₂O₂ in the degradation of organic contaminants in aqueous solutions,providing an opportunity to investigate the selective oxidation of organic contaminants under alkaline conditions.Moreover,it enriches our understanding on the decomposition pathways of H₂O₂ in the advanced oxidation processes as well as the transformation pathways of different oxidative radicals in these processes.