Oxidation of Orange G by persulfate activated by Fe(II), Fe(III) and zero valent iron (ZVI)

Abstract

Persulfate (PS) was employed in the oxidation of Orange G (OG), an azo dye commonly found in textile wastewaters. Activation of PS was conducted with iron to generate sulfate free radicals (SO4-) with high redox potential capable to oxidize most of the organics in water. Identification of oxidation intermediates was carried out by analyzing at different times organic by-products generated from treatment of a concentrate dye solution (11.6mM) with 153mM of PS and 20mM of Fe(II) at 20°C. Intermediate reaction products (mainly phenol (PH) and benzoquinone (BQ), and in less extent aniline, phenolic compounds and naphthalene type compounds with quinone groups) were identified by GC/MS and HPLC, and an oxidation pathway was proposed for the oxidation of OG with iron activated PS. The effect of iron valence (0, II and III) in the oxidation of an aqueous solution of OG (containing 0.1mM) was studied in a 0.5L batch reactor at 20°C. Initial activator and PS concentrations employed were both 1mM. Complete pollutant removal was achieved within the first 30min when iron II or III were employed as activators. Quinone intermediates generated during pollutant oxidation may act as electron shuttles, allowing the reduction of Fe(III) into Fe(II) in the redox cycling of iron. Therefore, activation of PS by Fe(III) allowed complete OG removal. When zero valent iron (ZVI) was employed (particle diameter size 0.74mm) the limiting step in SO4- generation was the surface reaction between ZVI and the oxidant yielding a lower oxidation rate of the dye. An increase in the oxidant dosage (0.2mM OG, 2mM Fe(III) and 6mM PS) allowed complete pollutant and ecotoxicity removal, as well as mineralization close to 75%.