Catalytic Oxidation Of Arsenite And Manganese(?) And Their Coupling On The Surface Of CuO Nanoparticles

Recently,the wide application of CuO nanoparticles?NPs?in engineering field inevitably leads to its release into various geologic settings,which has aroused great concern about the geochemical behaviors of CuO NPs due to its high surface reactivity and impact on the fate of co-existing contaminants.However,the redox transformation of pollutants mediated by CuO NPs and the underlying mechanism still remain poorly understood.Nano metal oxides have small size and high surface activity.It is rarely reported whether they can promote oxidation of As??? and Mn???and thus influence their environmental behavior after being discharged into the environment.Here,we studied the interaction of CuO NPs with As???and Mn???,and explored the reaction pathways using batch experiments and multiple spectroscopic techniques.The primary results are shown as follows:?1?In situ quick scanning X-ray absorption spectroscopy?Q-XAS?analysis verified that CuO NPs is capable of catalytically oxidizing As???under dark conditions efficiently at a wide range of pHs.As???was firstly adsorbed on CuO NPs surface and then gradually oxidized to As?V?with dissolved O₂ as the terminal electron acceptor.As???adsorption increased to the maximum at pH close to PZC of CuO NPs?^pH 9.2?,and then sharply decreased with increasing pH,while the oxidation capacity monotonically increased with pH.X-ray photoelectron spectroscopy?XPS?and electron paramagnetic resonance?EPR?characterization of samples from batch experiments indicated that two pathways may be involved in As???catalytic oxidation:?1?direct electron transfer from As???to Cu?II?,followed by concomitant re-oxidation of the produced Cu?I?by dissolved O₂ back to Cu?II?on CuO NPs surface,and?2?As???oxidation by reactive oxygen species?ROS?produced from the above Cu???oxygenation process.These observations facilitate a better understanding of the surface catalytic property of CuO NPs and its interaction with As???and other elements with variable valence in geochemical environments.?2?Using batch experiments multiple spectra techniques,we compared the oxidation amount of Mn???by different types of metal?including CuO,?-Fe₂O₃,ZnO and?-Al₂O₃ NPs?at different pH levels and then explored the catalytic oxidation pathway of Mn???at the semiconductor interface.The results of batch experiments show that CuO and?-Fe₂O₃ can efficiently catalyze the oxidation of Mn???under the condition of pH?7.Moreover,results of oxidation kinetics and removal kinetics of Mn???in the presence of CuO and?-Fe₂O₃ show that the removal of Mn???in solution is mainly caused by its oxidation.By comparing the difference in the oxidation amount of Mn???under air and O₂ conditions,it is revealed that O₂ is the terminal electron acceptor of Mn???oxidation on the surface of metal oxides NPs.The distribution of elements determined by HR-TEM-EDS indicates that there is a significant coupling relationship between Cu and Mn.The results of XPS and XAS analysis indicate that the catalytic oxidation products of Mn???by CuO NPs is mainly Mn????OH?O_x.Results of electrochemical calculations show that the indirect electron transfer pathway from Mn???through the mineral interface?CuO and?-Fe₂O₃?to O₂ occurs,which is the main electron transfer process that can efficiently catalyze Mn???oxidation.?3?Through batch experiments and multiple spectral analysis,the reaction pathways and mechanisms of As???and Mn???absorption and oxidation in the presence of CuO NPs were investigated.The results of oxidation kinetics of As???show that the binary composite system of CuO-Mn???can promote As???oxidation at pH 8 compared with the absence of Mn???.Moreover,the oxidation of As???increases with the molar ratio of Mn???/As???.When Mn???/As???=6,the effect of promotion does not increase which might be due to the fact that the surface active sites of NPs are completely occupied at this molar ratio.The oxidation kinetics of As???under N₂ and O₂ conditions were compared,results showing that O₂ can promote the oxidation of As???by Mn???.The XPS results show that the Mn???oxide,not Mn?IV?,is the main Mn specie responsible for As???oxidation.The XAS results confirm that Mn???acts as an electron shuttle during the oxidation of As???.The new formed Mn???promotes the oxidation of As???and is then rapidly reduced to Mn???returning back to the solution.At the same time,CuO NPs can preferentially adsorb As???and As?V?.When the surface site is occupied by As?V?,Mn???in the solution will not be further oxidized,which may be attributed to that CuO NPs have larger PZC,and the newly formed Mn???has strong oxidation ability which can be reduced to Mn???and return to the solution.To sum up,above results are helpful to understand the reaction characteristics of CuO NPs interface and its interaction mechanism with variable-valence elements,and also provide a basis for seeking efficient and economical As???pollution treatment technology.