Tunable peroxymonosulfate activation by ( −1 1 1) crystal plane exposed δ-MnO2: Oxidant concentration induced intrinsic mechanisms transformation

Abstract

The dual role of manganese oxides in peroxymonosulfate (PMS) activation for the degradation of organic contaminants makes the mechanism investigation complicated. Herein, a (−111) crystal plane exposed δ-MnO2 (SN-KMO) was synthesized to decipher the debate between direct oxidation process (DOP) and electron transfer process (ETP) mechanisms. Generally, 5 mg/L bisphenol A (BPA) could be efficiently removed in 15 min regardless of in the presence of high (0.5 mM) or low (0.05 mM) PMS concentration. Mechanisms analysis showed that nonradical pathways of DOP and ETP simultaneously contributed to the BPA degradation without the participation of radicals oxidation. Yet, contributions of DOP and ETP at certain PMS dosages differed significantly. Specifically, at high PMS concentration of 0.5 mM, DOP played the dominant role, and the overly protonated manganese active sites could bond with O-O bridged species derived from two PMS molecules to form outer-sphere complexes. Comparatively, at low PMS concentration of 0.05 mM, BPA oxidization was primarily dominated by ETP. The specific ETP was highly dependent on solution pH, and it only existed in the presence of high PMS concentration. The significance of this work lies in synthesizing a (−111) crystal plane exposed δ-MnO2 for efficient BPA degradation with low PMS dosage (0.05 mM) and discovering the unreported outer-sphere complexes for understanding the PMS activation mechanisms by manganese oxides.