Impact Of Borate On Peroxymonosulfate Oxidation Of Organic Compounds:Influence Factors And Mechanisms

Peroxymonosulfate（PMS）-based advanced oxidation processes were widely used for the degradation of organic pollutants.It has been reported that inorganic anions such as H₂PO₄^-/HPO₄^2-,OH^-and HCO₃^-/CO₃^2-could promote the degradation of organic pollutants by PMS.Borate buffer was commonly used for p H control between 7.0 and10.0,thus PMS/borate system was proposed in this work to degrade organics.Addtionally,the heterogeneous or homogeneous catalysis of PMS may suffer the interference from borate buffer solution.This work mainly discussed the efficiency and mechanisms of PMS/borate system in degradation of organic pollutants,and clarified the differences of organics degradation by PMS heterogeneous Cu O(and homogeneous Co²⁺)activation between phosphate and borate buffer.Electron-rich azo dye Acid Orange 7（AO7）and electron-poor dye Rhodamine B（Rh B）were selected as the target organic matter.Effect of oxidants,catalysts and buffers concentrations were studied,as well as p H values.Reactive oxygen species（ROS）generated in three system were determined through free radicals quenching experiments and EPR analysis,and the mechanisms were explored.The main conclusions are as follows:（1）PMS/borate system was used to degrade organic pollutants,and the results showed that the presence of borate significantly enhanced the degradation of AO7 and Rh B by PMS,but it could hardly activate PS and H₂O₂.The reaction rate increased with increasing PMS concentration.The presence of Borate（0.04-0.60 M）accelerated the degradation of AO7 and Rh B.Moreover,the reaction was highly p H-dependent.Although ¹O₂ played a leading role in the degradation of organics by PMS/borate system,a partial contribution was made from SO₄^·-,·OH,and the non-radical pathway.PMS/borate system could effectively remove various organic pollutants,while AO7 and Rh B were mainly converted to small molecules rather than mineralized.（2）The k value of AO7 degradation by PMS/Cu O heterogeneous system in borate buffer(k₂=0.0632 min^-1)was 7.2 times higher than that in phosphate buffer(k₁=0.0087min^-1).Increasing buffer concentration and p H could reduce the difference between phosphate and borate,while the difference would enhanced with the increase of PMS and Cu O doses.PMS/Cu O system in borate buffer produced amouts of SO₄^·-,·OH and ¹O₂than that in phosphate buffer,leading to the difference of k.Phosphate could occupy the hydroxylated active site on the surface of Cu O through ligand exchange,preventing PMS activation by Cu O to form ROS.（3）The k value of AO7 degradation by PMS/Co²⁺homogeneous system in phosphate buffer was greater than that in borate buffer,but the degradation percentage in the first10 seconds was lower than that in borate buffer.The differences were affected from buffer concentration,PMS and Co²⁺dosages,and p H.In phosphate buffer,·OH or SO₄^·-contributed to AO7 degradtion in PMS/Co²⁺system,which was due to the formation of[Co³⁺-HPO₄^2-]⁺complex blocking the reduction of Co³⁺.However,in borate buffer,nonradical pathway（¹O₂）make a critical contribution to AO7 removal in PMS/Co²⁺system,which was owing to the formation of[Co²⁺-B（OH）₄^-]⁺complex stoping PMS activation by Co²⁺.