Mechanism Of Manganese(?/?)-mediated Oxidative Removal Of Typical Endocrine Disruptors In Water

Manganese,an important source of essential element in animal and plant,is prevalent and more active in the natural environment.It exists as soluble and solid phase manganese in the natural water.Among them,soluble manganese includes Mn(?),Mn(?),and manganate,while solid phase manganese is mainly present in various minerals,manganese oxide(MnOx).They play an important role in the geochemical redox reaction,which has been paid more and more attention for the environmental transformation of pollutants.In recent decades,endocrine disruptor chemicals(EDCs)have been continuously detected in natural waters.Typical EDCs include industrial chemicals such as BPA,natural estrogens such as estrone(El),17?-estradiol(E2),and estriol(E3),as well as synthetic estrogens such as 17a-ethinyl estradiol(EE2).Therefore,we mainly employed the naturally occurring MnOx and soluble Mn(III)for the conversion of endocrine disrupters in water.It is of great significantce to study the transformation and removal of typical endocrine disruptors in natural waters.(1)Nanostructured manganese oxides,e.g.MnO2,have shown laccase-like catalytic activities,and are thus promising for pollutant oxidation in wastewater treatment.We have systematically compared the laccase-like reactivity of manganese oxide nanomaterials of different crystallinity,including ?-,?-,?-,?-,and E-MnO2,and Mn3O4,with 2,2'-azinobis-(3-ethylbenzthiazoline-6-sulfonate)(ABTS)and 17?-estradiol(E2)as the probing substrates.The reaction rate behaviors were examined with regard to substrate oxidation and oxygen reduction to evaluate the laccase-like catalysis of the materials,among which ?-MnO2 exhibits the best performance.Meanwhile,the product identification showed that there are two pathways for E2 degradation-single electron transfer and two electron oxidation.Moreover,Cyclic voltammetry(CV)was employed to assess the six MnOx nanomaterials,and the results correlate well with their laccase-like catalytic activities.The findings help understand the mechanisms of and the factors controlling the laccase-like reactivity of different manganese oxides nanomaterials,and provide a basis for future design and application of MnO,-based catalysts.(2)The photochemical formation of soluble Mn(?)in the precence of Mn(?)and humic acid was systematically studied under visible-light(>420 nm)irradiation,which is provided by a 500 W xenon lamp with a 420 nm cutoff filter.The effects of different concentrations of Mn(?)and HA at different pH levels was performed to suggest that the photochemical formation of Mn(?)at alkaline and neutral conditions is greater than that at acidic condition.The qunching tests with superoxide dismutase(SOD)showed that Mn(?)formation was completely inhibited,indicating that superoxide ion radicals play an important role in the oxidation of Mn(?)under visible-light irradiation.In the natural water,humic acid(HA)can be considered as an important photosensitizer for effectively converting dissolved oxygen into superoxide radicals,and further oxidized Mn(?)to form reactive Mn(?).Based on these results,it appears that Mn photo-oxidation could constitute a significant,and apparently unrecognized geochemical pathway in natural waters.(3)Based on the above results,the conversion of bisphenol A(BPA)was studied in the Mn(?)/HA/visible light reaction system.The effect of pH values on the removal efficiency of BPA was firstly investigated.It showed that the reaction rate constant of BPA increases with the increasing pH values(pH 4.0 to 5.5)in the coexistence of 500 ?M Mn(?)and 5 mg/L HA,while the pH continues to increase(pH 5.5-7.0),the corresponding reaction rate constant of BPA decreases.All of these reactions followed the pseudo-first order kinetic reaction rate equation.And the quenching tests showed that 10 mM PP and 10 mM BQ could completely inhibit the oxidation of BPA,indicating that both the superoxide radical and Mn(?)play an important role in the oxidation of BPA.In addition,no degradation of BPA was observed in the control group without Mn(?),indicating that reactive oxygen species were not the oxidant for E2,thus,soluble Mn(?)were found to be the oxidant.At the same time,the reaction products of BPA were identified by high performance liquid chromatography-mass spectrometry(HPLC-MS/MS),and the reaction pathway of hydroxylation and polymerization were proposed.These will help to further illustrate the important role of Mn(?)in the conversion of pollutants in natural water and sewage.(4)Steroidal estrogens have been of great concern as emerging contaminants in most near surface waters.Their oxidation by high valence manganese(such as permanganate and manganese dioxide)has been extensively researched,but very few works have employed the naturally occurring Mn(?)as active oxidant for pollutant removal.Irradiation experiments with estrogens and Mn(?)in the presence of humic acid(HA)showed that the oxidation of Mn(?)is started by reactive oxygen species(ROS)to form Mn(?)and then the active soluble Mn(?)oxidize estrogens to generate phenoxyl radicals for the final oligomerization.The oxidation of Mn(?)by ROS in the presence of HA is about 2.0-fold greater enhancement at pH 7.0 and 8.0 than pH 6.0,suggesting that the formation of Mn(?)is strongly pH dependent.The most optimal concentration of Mn(?)and HA for photoproduction of Mn(?)was observed at 500 ?M and 5 ppm,respectively.Under such conditions,the conversion tests of estrone(El),17?-estradiol(E2),estriol(E3),and 17a-ethynylestradiol(EE2)showed that about 60.2%,89.0%,87.6%,and 80.2%were removed by Mn(?)in water after 72 h illumination at pH 6.0,respectively.With regard to E2,the removal rate of E2 in acidic solution(pH 6.0)is more excellent than the neutral and alkaline solution(pH 7.0 and 8.0),indicating that Mn(?)is more active at pH 6.0 than pH 7.0 and 8.0.And the quenching experiments showed that the active Mn(?)is the only oxidant for the conversion of E2,but not superoxide ion.In addition,the product identification and theoretical computation further provides strong evidence for the single-electron conversion of estrogens.These interesting reactions could play an important role for the environmental fate of estrogens in the natural and contaminated water.