Arsenic　(As)　and　Selenium　(Se)　contaminations　present　an　urgent　environmental　concern,　whereas　its　removal　remains　a　significant　challenge　due　to　the　lack　of　fundamental　knowledge　of　the　adsorption　mechanism　and　effective　adsorbents.　Herein,　{201}　TiO2　exhibited　the　application　potential　for　As(III)　and　Se(IV)　removal,　and　the　antagonistic　effect　was　examined　at　the　molecular　level　using　X-ray　absorption　spectroscopy　and　density　functional　theory　calculations.　The　Langmuir　adsorption　capacity　of　As(III)　and　Se(IV)　on　{201}　TiO2　was　0.32　mmol/g　and　0.25　mmol/g,　respectively.　The　rate　constant　k　for　Se(IV)　adsorption　was　5.46　g/(mmol/h),　which　was　10.2　times　higher　than　that　of　As(III)　(0.48　g/(mmol/h)).　EXAFS　and　CD　-MUSIC　results　demonstrated　that　both　As(III)　and　Se(IV)　formed　bidentate　binuclear　inner-sphere　complexes　at　the　bridge-Ti4C　of　{2　0　1}　TiO2.　The　pd　hybrid　orbital　energy　of　Ti　-O　bonds　in　Ti2O2SeO　complex　(-2.42　eV)　was　lower　than　that　in　Ti2O2AsO-　complex　(-2.26　eV),　indicating　the　better　adsorption　stability　and　stronger　affinity　of　Se(IV)　adsorbed　on　{201}　TiO2.　The　coexisting　Cd(II)　formed　ternary　complexes　with　As(III)/Se(IV)　on　{201}　TiO2,　enhancing　the　Freundlich　adsorption　of　Se(IV)　to　0.69　mmol/g　and　As(III)　to　0.43　mmol/g.　The　introduction　of　Cd(II)　altered　the　orbital　hybridization　interaction,　and　the　charge　transfer　from　Ti　-3d　to　Cd　-5s　orbitals　improved　the　adsorption　energies　of　As(III)/Se(IV)　on　{2　0　1}　TiO2.　The　ICOHP　value　of　the　Se　-O　-Cd　bond　was　higher　than　that　of　the　As　-O　-Cd　bond,　indicating　the　stronger　affinity　of　Cd(II)　and　Se　-O　dangling　bond.　Gaining　insights　into　the　surface　complexation　modeling　at　the　molecular　and　electronic　level　reveals　the　nature　of　coexisting　ions　adsorption　behaviors.