Catalytic Oxidation Of As???by H₂O₂ Over Nanoceria:Performance And Mechanism

Arsenic pollution in water environment,especially in groundwater,has attracted worldwide attention.Because of the anoxic nature of groundwater,As is usually present as the electroneutral As(?).Compared with As(?),As(?)has higher toxicity,mobility,and stability.Consequently,the removal of As(?)is usually inefficient by conventional treatment methods such as adsorption and ion exchange.It has been regarded as an effective strategy for As(?)removal to pre-oxidize As(?)into As(?),which could be efficiently removed from water in subsequent processes.Among various oxidants,H2O2 is a clean candidate without any secondary pollution.However,the homogeneous oxidation of As(?)by H2O2 is generally limited to alkaline conditions,and the oxidation efficiency is quite low under circumneutral pH conditions.Therefore,it is of significance to develop efficient techniques for catalytic oxidation of As(?)by H2O2 under circumneutral conditions.Ceria is a widely used catalyst with large oxygen storage capacity and high stability,and is also a promising candidate for efficient catalytic oxidation of As(?)by H2O2 at circumneutral pHs.Nevertheless,the performance and mechanism of such process have never been reported.Meanwhile,nanomaterials have attracted increasing attention in the field of water decontamination due to their high specific surface area and reactivity.Therefore,the objective of this dissertation is mainly to investigate the performance and mechanism of catalytic oxidation of As(?)by H2O2 over nanoceria.Nanoceria(C-CeO2)was prepared by calcination method.The resultant C-CeO2 has cubic fluorite crystal structure and is rich in mesoporous structures with a specific surface area of about 105 m2/g.Efficient catalytic oxidation of As(?)by H2O2 under circumneutral condition was achieved over C-CeO2.As(?)initially at 20.0 mg/L could be completely converted into As(V)in 240 min by dosing 1.33 mM H2O2 at pH 7.0 with 0.2 g/L C-CeO2.The dark control experiment confirmed that the catalytic oxidation of As(?)by H2O2 over CeO2 was not a light-driven process.C-CeO2 exhibited high catalytic activity for the oxidation of As(?)by H2O2 over a wide range of pH(3-9).The transformation of As(?)into As(?)decreased with the increase of initial As(?)concentration,whereas increased with the increase of CeO2 dosage,H2O2 dosage,or initial pH.In addition,C-CeO2 exhibited great potential for sustainable catalytic oxidation of As(?)throughout five consecutive reuse cycles.The catalytic mechanism of the oxidation of As(?)by H2O2 over CeO2 was revealed by the radical quenching experiments,CAT/SOD enzyme inhibition experiments,surface preoxidation experiments,and the comprehensive characterizations with EPR,XPS,UV-vis diffuse reflectance spectroscopy,and Raman spectroscopy.The surface complexation of H2O2 over CeO2 resulted in formation of surface-complexed peroxide(?Ce-OOH),which was proved as the dominant active species for the oxidation of As(?)into As(V).The surface-complexed peroxide was quite stable and could be formed via H2O2-preoxidation.The catalytic performance increased with the amount of the surface-complexed peroxide on CeO2.The superoxide radical(O2-),which chemically equilibrated with surface complexation peroxide species,plays a minor role in the catalytic oxidation of As(?)by H2O2 over CeO2,and the contributions from homogeneous H2O2,dissolved oxygen,and OH were negligible.Furthermore,the catalytic oxidation of As(?)by H2O2 over CeO2 was not mediated by the Ce(?)/Ce(?)redox pair.The utilization of H2O2 during catalytic oxidation of As(?)by H2O2 over C-CeO2 could reach over 99%.Significant nanoeffect was observed for the catalytic oxidation of As(?)by H2O2 over CeO2 from the comparison of catalytic performances with C-CeO2 and three commercially available nanoceria.The nanoceria of different sources were well characterized with various means including TEM,N2 adsorption-desorption,XRD,and XPS.The correlation analysis shows that the specific surface area and crystallite size are the key intrinsic factors affecting the catalytic performance of nanoceria for As(III)oxidation by H2O2.The nano effect was strengthened with the decrease of crystallite size of nanoceria,especially within the sub-10 nm range.The surface effect was evidenced by an increase in the proportion of surface hydroxyl(M-O-H)over total oxygen,which increased the amount of surface-complexed peroxide species.Meanwhile,the size effect led to an increase in the specific surface area and thus the active sites.Both the surface effect and the size effect drive the enhanced catalytic oxidation of As(?)by H2O2 over CeO2.Through the elucidation of the performance and mechanism of catalytic oxidation of As(?)by H2O2 over CeO2 as well as the corresponding nano effect,this dissertation sheds new light upon the development of efficient technologies for As(?)removal from water.