Decolorization of Reactive Black 5 by persulfate oxidation activated by ferrous ion and its optimization

Abstract

We determined the effectiveness of persulfate (PS) activated by Fe2+ to treat Reactive Black 5 (RB5) dye. The experimental variables of persulfate dosage, Fe2+ dosage, initial dye concentration, solution pH, activators, and NaCl were investigated. The complete decolorization of RB5 by 200 mM PS activated by 1 mM Fe2+ was 100% within 12 h but the TOC removal efficiency was only 70% within 24 h. When the PS dosage and Fe2+ dosage increased, the kobs of RB5 decolorization increased. On the other hand, when the initial RB5 concentration increased from 0.01 to 0.05 mM and concentration of PS was fixed at 400 mM, the number of sulfate radicals did not increase proportionally, so the kobs of the RB5 decolorization decreased. Lowering the pH from 11 to 5 increased the kinetic rates of destruction of RB5. At a higher pH, the concentration of Fe2+ could decrease due to the formation of iron oxides which hinder the further reaction of Fe2+ with PS. Five activators (Fe0, Fe2+, Fe3+, Ag0, and Ag+) were chosen to study their capability in activating PS oxidation. At the equivalent mole of activator, the kobs of the treatment of PS activated by Fe0 was the best (2.8609 h−1). Increasing the NaCl concentration from 17.24 mM (1g L−1) to 68.96 mM (4 g L−1) decreased the kinetic removal rates of RB5. The PS oxidation of RB5 was optimized using response surface methodology. The interactive effects of three factors namely, persulfate dosage (X1), Fe2+ dosage (X2), and pH (X3) were investigated. The results indicated the optimum condition to treat 0.01 mM RB5 to be 445 mM PS and 4.91 mM Fe2+ at pH 5.45.