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Photocatalytic Oxidation of As(III) to As(V) by TiO2
EPA Grant Number: U915962Title: Photocatalytic Oxidation of As(III) to As(V) by TiO2
Investigators: Ferguson, Megan A.
Institution: California Institute of Technology
EPA Project Officer: Boddie, Georgette
Project Period: January 1, 2001 through January 1, 2003
Project Amount: $102,000
RFA: STAR Graduate Fellowships (2001)
Research Category: Fellowship - Geology , Academic Fellowships , Ecological Indicators/Assessment/Restoration
Description:
Objective:The objective of this research project is to investigate the photocatalytic oxidation of As (III) to As (V) by TiO2.
Approach:The new maximum contaminant level mandated for arsenic in drinking water will require additional arsenic removal procedures in many water distribution systems throughout the United States, particularly in the Southwest. Arsenic is present in groundwater in two inorganic forms: arsenite (AsIII) and arsenate (AsV). The latter is easily removed in common water treatment practices, but arsenite is not effectively removed by such technologies; thus, a preoxidation step is needed to convert all arsenite to arsenate. TiO2 photocatalyzes this reaction in the presence of UV light and oxygen, but the mechanism of action and parameters affecting efficiency are not well understood.
Photochemical studies using TiO2 slurries (Degussa P25, 0.05 g/L) have been conducted to assess the effects of initial [AsIII] (1-50 µM), the presence of competitively adsorbing species (0.5-10 µM phosphate), and length of exposure to UV light on arsenite oxidation. At 1 µM [AsIII], which falls slightly above the upper limit expected in water requiring treatment, photooxidation is complete within 10 minutes.
The mechanism of the AsIII photooxidation reaction has been explored by using a variety of experimental conditions that inhibit reactions involving either hydroxyl radical or superoxide radical, two of the potential oxidizing agents for AsIII. These data suggest that arsenite oxidation by superoxide plays a dominant role in this reaction. The role of arsenite sorption to TiO2 in this reaction also was investigated by comparing sorption isotherm data, initial adsorbed [AsIII] during photocatalyzed experiments, and the effect of adding phosphate to limit available surface sites.
Results indicate that catalyst poisoning by the product AsV or by competitively adsorbing species should not be a major problem. The understanding gained from these experiments will be used to expand to fixed-bed TiO2 systems, which are more practical for water treatment applications.
Supplemental Keywords:fellowship, drinking water, groundwater, common water treatment practices, water distribution systems, TiO2, photocatalytic oxidation, arsenite, AsIII, arsenate, AsV, photooxidation.