Comparative analysis of the photocatalytic reduction of drinking water oxoanions using titanium dioxide. (1st November 2016)

• Record Type: Journal Article
• Authors: Marks, Randal; Yang, Ting; Westerhoff, Paul; Doudrick, Kyle
• Is Part Of: Water research. Volume 104(2016)
• Page Start: 11
• Page End: 19
• Publication Date: 2016-11-01
• Subjects: Oxoanion -- Photocatalysis -- Titanium dioxide -- Nitrate -- Bromate -- Chlorate; Water -- Pollution -- Research -- Periodicals; 363.7394
• DOI: 10.1016/j.watres.2016.07.052
• Languages: English
• ISSNs: 0043-1354

Abstract

Abstract: Regulated oxidized pollutants in drinking water can have significant health effects, resulting in the need for ancillary treatment processes. Oxoanions ( e.g., nitrate) are one important class of oxidized inorganic ions. Ion exchange and reverse osmosis are often used treatment processes for oxoanions, but these separation processes leave behind a concentrated waste product that still requires treatment or disposal. Photocatalysis has emerged as a sustainable treatment technology capable of catalytically reducing oxoanions directly to innocuous byproducts. Compared with the large volume of knowledge available for photocatalytic oxidation, very little knowledge exists regarding photocatalytic reduction of oxoanion pollutants. This study investigates the reduction of various oxoanions of concern in drinking water (nitrate, nitrite, bromate, perchlorate, chlorate, chlorite, chromate) using a commercial titanium dioxide photocatalyst and a polychromatic light source. Results showed that oxoanions were readily reduced under acidic conditions in the presence of formate, which served as a hole scavenger, with the first-order rate decreasing as follows: bromate > nitrite > chlorate > nitrate > dichromate > perchlorate, corresponding to rate constants of 0.33, 0.080, 0.052, 0.0074, 0.0041, and 0 cm 2 /photons × 10 18, respectively. Only bromate and nitrite were reduced at neutral pH, with substantially lower rate constants of 0.034 and 0.0021 cm 2 /photons × 10 18, respectively. No direct relationship between oxoanion physicochemical properties, including electronegativity of central atom, internal bond strength, and polarizability was discovered. However, observations presented herein suggest the presence of kinetic barriers unique to each oxoanion and provides a framework for investigating photocatalytic reduction mechanisms of oxoanions in order to design better photocatalysts and optimize treatment. Graphical abstract: Highlights: Reduction rate order: bromate > nitrite > chlorate > nitrate > chromate > perchlorate. Oxoanion reduction rates over P90 are governed by unique kinetic barriers. No relationship between oxoanion physicochemical properties and ease of reduction was discovered. Chlorite and nitrite may self-oxidize under photocatalytic conditions. Bromate is the only oxoanion efficiently reduced by P90 at neutral pH.