Comparative Studyof As??? Removal Efficiency And Reaction Mechanisms By UV/TiCl₄ And UV/TiO₂

Long-term exposure to high arsenic environments can lead to canceration of human skin and various organs,which seriously threaten human health.In groundwater,arsenic exists mainly as inorganic species,which are aesenite?As?III??and arsenate?As?V??.Compared with As?V?,the activity and biological toxicity of As?III?is greater than As?V?.Under anoxic conditions,inorganic arsenic exists mainly as As?III?.When pH<9.2,As?III?exists as an uncharged neutral molecule and is not easily removed by conventional treatment methods such as adsorption,coagulation,and membrane separation.Under the oxic conditions,inorganic arsenic mainly exists as As?V?.When pH=2.3⁶.9 and pH=6.9¹1,As?V?exists as a negatively charged anion and can be effectively removed by electrostatic adsorption between the hydrolyzed precipitates of the coagulants.The UV light synergistic coagulant can oxidize As?III?into As?V?and then be removed by adsorption,which improved the As?III?removal efficiency in natural water.In this paper,the oxidation removal efficiency of As?III?by TiCl₄ and TiO₂ in synergy with UV light was studied,and the kinetics and isothermal adsorption model of As?III?by UV light synergistic TiCl₄ was analyzed.The results revealed that the removal efficiency of As?III?reached a maximum?73%?at pH=5?near the isoelectric point?by coagulation of TiCl₄.However under UV irradiation conditions,the maximum value of removal efficiency for As?III?was99%at pH=5 by TiCl₄.At pH=5-9,with the pH value increased,the removal efficiency of As?III?decreased by the single coagulation of TiCl₄ and UV/TiCl₄.In the course of UV irradiation,As?III?in solution did not immediately convert to As?V?after light irradiation at pH=5-9,but gradually converted to As?V?with the extension of time.Oxidation and removal of As?III?proceeded at the same time from0 to 1 min.In the course of UV irradiation,TiCl₄ produced hydroxyl radicals to oxidize As?III?in the solution,and the concentration of hydroxyl radicals increased with increasing pH values.The isothermal adsorption of As?III?and As?V?by TiCl₄followed with the Langmuir isotherm adsorption model at pH=5-9,and the affinity of TiCl₄ for As?V?was large and the activation energy between molecules was small under acidic condition.At pH=5,the maximum removal efficiency of As?V?reached99%and the maximum unit adsorption capacity was 99.2?g/mg when the initial concentration of As?V?was 1000?g/L.P25 TiO₂ is an adsorbent,the removal efficiency of As?III?reached a maximum?14%?at pH=5 with P25 TiO₂.Under UV irradiation,TiO₂ is both an adsorbent and an oxidant,and the maximum removal efficiency of As?III?was 20%at pH=5.While at pH=8 and 9,the removal efficiency of As?III?all were 1%.In the process of UV irradiation from 0 to 3 min,As?III?converted rapidly to As?V?.With the increase of the pH value,the fastest oxidation rate was from 1 to 3 minutes at pH=5,which gradually converted the fastest oxidation rate from 0 to 1 min at pH=9.The concentration of hydroxyl radicals generated by TiO₂ increased with increasing pH by UV irradiation.At pH=9,the concentration of hydroxyl radicals was 5.8 times the amount of hydroxyl radicals at pH=5,and TiO₂ could rapidly convert As?III?into solution completely within a short time at pH=9.The isothermal adsorption of As?III?and As?V?by TiO₂ followed the Langmuir isotherm adsorption model.At pH=5-7,the affinity of TiO₂ for As?V?was greater,and with the increase of pH value,the affinity of TiO₂ for As?III?become greater up to a pH=9.