Performance And Mechanism Of Oxidation Of Microcystin-LR By Cu???or HSO₅^- Assisted With Ascorbic Acid

Microcystins?MCs?are a group of monocyclic heptapeptide toxins.Among more than 80 kinds of MC variants,microcystin-LR?MC-LR?is the most common and toxic one as well as a strong liver tumor promoter.However,MC-LR is chemically stable in aqueous phase,and cannot be effectively removed from drinking water using conventional water treatment processes?such as coagulation&flocculation,sedimentation and filtration?.Therefore,more effective water treatment processes are in urgent need to remove MC-LR from drinking water.In recent decades,the hydroxyl radical?HO^·?-and sulfate radical(SO₄^·-)-based advanced oxidation processes?AOPs?are increasingly popular and widely used to treat the contaminants in water for its high degradation efficiency towards the organic pollutants.In this study,ascorbic acid?Vitamin C,C₆H₈O₈,H₂A?was introduced to establish two new AOPs for the oxidative degradation of MC-LR,with the hope of providing theoretical basis and technical support for removal of MCs,especially MC-LR in algae-laden water.The Cu???/H₂A system consisting of Cu???and H₂A could efficiently degrade MC-LR.A kinetic model was established to precisely predict generation of H₂O₂.Based on the kinetic model,the second-order rate constant between Cu???and ascorbate anion?HA^-,mainly formed at neutral pH?at neutral pH was determined to be 3.0×10³ L/mol·s.Furthermore,quenching tests and electron spin resonance?ESR?analysis were used to identify the primary reactive species,and the results indicated that HO^·rather than Cu???is the primary reactive species and responsible for MC-LR degradation.Besides,batch experiments were further conducted to evaluate the effects of Cu???and H₂A dosages,solution pH and Cl^-concentration on the degradation of MC-LR during the Cu???/H₂A system.The results showed that the degradation efficiency of MC-LR initially increased and then decreased as H₂A dosage increased.And the degradation efficiency of MC-LR increased with increasing Cu???dosage,while decreased with increasing solution pH.Moreover,addition of Cl^-could significantly inhibit the degradation of MC-LR.Meanwhile,12 oxidation products were detected using LC/MS/MS analysis.Based on all the detected oxidation products,a possible degradation pathway of MC-LR in the Cu???/H₂A system was proposed.And the toxicity assessment using protein phosphatase 2A?PP2A?showed that MC-LR was readily degraded and its oxidation products were not hepatotoxic.H₂A was combined with peroxymonosulfate?HSO₅^-,PMS?to produce the H₂A/PMS system,which could also efficiently degrade MC-LR.SO₄^·-,HO^·and ascorbyl radical(A^·-)were found in the H₂A/PMS system using ESR analysis.SO₄^·-and HO^·were identified as the dominant reactive radicals for MC-LR degradation.However,A^·-was negligible for the degradation of MC-LR due to its low reactivity.A first-principles kinetic model was successfully developed to simulate the degradation of MC-LR and predict the radical concentrations,especially the concentrations of A^·-.To further evaluate the kinetic model,the effects of H₂A and PMS dosages,solution pH and concentration of natural organic matter?NOM?on MC-LR degradation were also investigated.The results showed that the degradation efficiency of MC-LR initially increased and then decreased as H₂A dosage increased.And the more PMS dosage,the higher MC-LR degradation efficiency.However,increasing solution pH and NOM concentration had a significant inhibition on MC-LR degradation.Like the Cu???/H₂A system,LC/MS/MS analysis also identified 11 oxidation products,on the basis of which a possible degradation pathway of MC-LR in the H₂A/PMS system was proposed.Of note,the toxicity assessment by PP2A test confirmed that the H₂A/PMS system did degrade MC-LR and the oxidation products of MC-LR were no longer hepatotoxic.