TiO₂-photocatalytic Oxidation Mechanism Of As（Ⅲ） And Its Dhoto-electrochemical Anti-corrosion Of Carbon Steel

Arsenic is one of the most poisonous elements in environmental pollutants. Studies on long-term human exposure reported that arsenic in drinking water was associated with liver, lung, kidney, bladder, and skin cancers. Arsenic in groundwater occurs primarily in inorganic form, either as arsenite (As(Ⅲ)), or arsenate (As(Ⅴ)). As(V) could be efficiently removed by common water treatment methods. However, the removal of As(Ⅲ) by such processes is often substantially less efficient. The oxidation of As(Ⅲ) to As(V), is required for the efficient removal of arsenic.TiO2photo-catalysis has been shown to be effective in oxidizing As(Ⅲ) to As(V) in the presence of oxygen. However, the As(Ⅲ) photo-oxidation mechanism is complex, has been an issue of considerable controversy. Lines of evidence supporting As(Ⅲ) oxidation by super-oxide, hydroxyl radicals, valence band holes have been reported.To unequivocally answer who, among the photo-hole, super-oxide, and its photo-induced derivates, is the main oxidant in the normal aerated aqueous solutions and at open circuit conditions, their respective contribution to As(Ⅲ) oxidation was studied. The contribution of photo-hole to As(Ⅲ) oxidation was investigated under conditions of anodic bias potentials and under illumination. The role of super-oxide in the dark and its photo-induced derivates under illumination were studied respectively under the conditions of negative bias potentials and air-saturated which conditions no photo-holes would be involved in the oxidation of As(Ⅲ). The results show the sum contribution of super-oxide and its derivates is considerably high (up to43%), it is not more than that of photo-hole (57%).In order to Distinguish between the direct and indirect interfacial hole transfer, the experiments of electrochemical measurements and As(Ⅲ) oxidation were conducted in the conditions of anodic potential and N2saturated when no super-oxide was involved in the oxidation of As(Ⅲ). An Kinetics model was established and used to reveal the photocatalytic oxidation of As(Ⅲ). The measurements of photocurrent and expeneriments of As(Ⅲ) oxidation were carrird out in different light intensities and As(Ⅲ) concentrations. Based on the Kinetic model, the results showed hydroxyl radicals were mainly responsible for the photocatalytic oxidation of As(Ⅲ). Fluorinated TiO2, Hydroxyl radical and valence band hole scavenger were also used to to seek for clearer evidences for the role of hydroxyl radicals and valence band holes in the course of As(Ⅲ) oxidation. The results also confirmed that hydroxyl radicals is the main oxidant for the oxidation of As(Ⅲ).The effect of fluorinated TiO2, the addition of As(Ⅲ) and the romovel of O2on the anticorrosion performances of TiO2electrodes have been investigated by the electrochemical methods. The results showed that fluorinated TiO2, the addition of As(Ⅲ) and the romovel of O2could obviously enhanced photocurrent and decrease the potential under illumination, thus improve the anticorrosion performances for carbon steel. Fluorinated TiO2, could produce an efficient photocathodic protection for carbon steel under the condition of N2saturated and the solution with As(Ⅲ).