Comparative analysis of the photocatalytic reduction of drinking water oxoanions using titanium dioxide

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/photons × 10, respectively. Only bromate and nitrite were reduced at neutral pH, with substantially lower rate constants of 0.034 and 0.0021 cm/photons × 10, 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.

Recent development and future prospects ofTiO2photocatalysis

“…When the redox potential of adsorbed oxyanions lies more positive toward the CB edge of TiO 2 , reduction happens by employing photogenerated electrons (Equation 13). [85][86][87] The thermodynamic feasibility of the oxidative and reductive reactions is based on the redox potentials of the adsorbed species compared to the CB and the VB of TiO 2 . In general, the redox potential of adsorbed donor species must be comparatively negative than the VB of TiO 2 (+2.5 V/ENH at pH 7), whereas acceptor molecules should hold a redox potential that is more positive than that of the CB of TiO 2 (−0.52 V/ ENH at pH 7).…”

The first report of deep eutectic solvent (DES) nano‐photocatalyst (n‐TiO2‐P25@TDI@DES (urea: ZnCl2)) and its application on selective oxidation of benzyl alcohols to benzaldehydes

“…According to the literature 73–75 and based on the above facts, nitrate reduction occurs according to one of the following equations. …”

The first report of covalently grafted semiconductors; n‐TiO2‐P25@ECH@WO3 as a new, efficient, robust and visible‐light‐responsive photocatalyst

“…According to the literature 45–47 and based on the above facts, reduction of nitrate occurs via one of the below equations. …”