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

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 200mM PS activated by 1mM Fe2+ was 100% within 12h but the TOC removal efficiency was only 70% within 24h. When the PS dosage and Fe2+ dosage increased, the k(obs) of RB5 decolorization increased. On the other hand, when the initial RB5 concentration increased from 0.01 to 0.05mM and concentration of PS was fixed at 400mM, the number of sulfate radicals did not increase proportionally, so the k(obs) 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 (Fe-0, Fe2+, Fe3+, Ag-0, and Ag+) were chosen to study their capability in activating PS oxidation. At the equivalent mole of activator, the k(obs) of the treatment of PS activated by Fe-0 was the best (2.8609h(-1)). Increasing the NaCl concentration from 17.24mM (1gL(-1)) to 68.96mM (4 gL(-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 (X-1), Fe2+ dosage (X-2), and pH (X-3) were investigated. The results indicated the optimum condition to treat 0.01mM RB5 to be 445mM PS and 4.91mM Fe2+ at pH 5.45.