Arsenic(As)poses a great health threat to human.Developing effective As removal technologies is of paramount importance.Adsorption using hybrid materials is an effective method to remove As in groundwater,due to its low cost and high efficiency.Recently,many works about As adsorption using rare-earth oxides modified materials(REMM)have been reported.However,the understanding of adsorption processes remains challenging due to the lack of mechanistic study on the molecular-level.The motivation of our study is therefore to investigate the adsorption mechanism of arsenite(As(Ⅲ))and arsenate(As(Ⅴ))on lanthanum impregnated activated alumina(LAA).In our study,batch adsorption experiments and multiple complementary molecular-level techniques including EXAFS,SERS,in-situ ATR-FTIR,and quantum chemical simulation have been used to study the interactions of As and LAA.LAA exhibited about 2~3 times higher As(Ⅲ)and As(Ⅴ)adsorption capacity than activated alumina.The results of EXAFS demonstrated that As were adsorbed on LAA primarily as the monodentate complexes ?LaO-AsO2 for As(Ⅲ)and ?LaO-AsO3 for As(Ⅴ).This conclusion was further confirmed by our SERS studies.Moreover,in-situ ATR-FTIR results illustrated that HAsO4 and H2AsO4 were the dominated species of As(Ⅴ)adsorbed on LAA at pH 5~9.In addition,density functional theory(DFT)calculation was employed to calculate the IR spectra of ?LaO-AsO2 and ?LaO-AsO3 clusters using Gaussian 09 package.The calculated vibrational frequencies from those clusters were consistent with the observed IR bands.Our work improves the understanding of As interaction with REMs on the molecular-level,and also helps to understand the adsorption behavior of other oxyanions such as phosphate,chromate,and selenite on hybrid metal oxides.