Alpha-hydroxy ketones and sunlight, a novel effluent treatment?

Abstract

There is a need for cheap and simple methods to destroy recalcitrant pollutants such as dyes in waste water. It is shown that the radicals produced from the UV A and B photolysis of alpha-hydroxy ketones are capable of completely bleaching a wide range of azo dyes (Orange II, Amaranth, Evans Blue, Direct Green 26 and Direct Red 80). It is found that complete bleaching occurs when the ratio of [ketone]/[azo groups] is greater than 2.6–7.6 dependent on dye structure and occurs over a wide pH range (2–11 investigated). Sunlight may be used as a light source and bleaching occurs in minutes. Mechanistic investigation shows that in the presence of air the bleach reaction is predominately a 1-electron oxidation induced by the benzoyl peroxy radical. From reaction with phenols of known electropotential, the reduction potential of the benzoyl peroxy radical is estimated as ∼1.9V v NHE. It is also shown that the benzoyl peroxy radical can undergo oxygen atom transfer reactions. In the absence of air a 1-electron reduction by the alpha-hydroxy radical [E(acetone, H⁺/2-hydroxy-2-propyl) = −1.4V v NHE] is the main reaction. Thus the system may be switched from oxidative to reductive by removal of oxygen. Initial products are given for both oxidative and reductive routes. First cost estimates look attractive.