Application of a novel definitive screening design to in situ chemical oxidation of acid orange-II dye by a Co2+/PMS system

Abstract

In this work, a novel definitive screening design (DSD) was initially used to investigate the in situ chemical oxidation of acid orange-II (AO II) dye using a homogeneous cobalt-catalyzed peroxymonosulfate (Co²⁺/PMS) system. Experiments were performed in a batch reactor using a synthetic AO II solution containing cobalt ions, humic acid (HA) and five electrolyte salts (NaCl, NaNO₃, NaHCO₃, Na₂SO₄ and NaH₂PO₄). The effects of nine operating variables on AO II degradation: NaCl concentration (0–20 mM), NaH₂PO₄ concentration (0–20 mM), NaHCO₃ concentration (0–20 mM), NaNO₃ concentration (0–20 mM), Na₂SO₄ concentration (0–20 mM), HA concentration (0–40 mg L⁻¹), PMS concentration (2–10 mM), AO II concentration (0–100 mg dm⁻³) and Co²⁺ concentration (0–1.36 mM), were investigated. Analysis of a DSD model revealed that only four variables were statistically significant. Besides, our results indicated that there were no significant interactive effects between the five anions employed. Thereafter, a central composite rotatable design (CCRD) was adopted to further optimize these significant variables. A CCRD model describing AO II conversion as a function of PMS concentration, NaCl concentration and Co²⁺ concentration was then constructed and used to explore the final optimal operating conditions. As a consequence, the combination of DSD and CCRD allowed reducing the number of experiments from over 600 to only 39. Lastly, a plausible reaction sequence concerning the oxidation of AO II in the Co²⁺/PMS/Cl⁻ system was also proposed. The route of AO II oxidation involved hydroxylation and chlorination, signifying the roles of both sulfate radicals and active chlorine species.