Oxidative degradation of aqueous organic contaminants over shape-tunable MnO2 nanomaterials via peroxymonosulfate activation

Cryptomelane-type MnO2 nanomaterials with a-crystalline phase were prepared by heat treatment of the mixture of MnCO3 and potassium oxalate followed by acid treatment. The evolution of the crystalline phase and morphology from MnCO3 to cryptomelane was investigated by XRD, BET, SEM and TEM. Moreover, MnO2 in nanoparticle, nanorod and nanofiber shapes could be controlled by adjusting the reaction time and temperature in the acid-treatment step. MnO2 nanofibers demonstrated promoted catalytic activity and recyclability for the oxidative degradation of various organic dyes, tetracyclines and bisphenol A via peroxymonosulfate (PMS) activation. About 99% of RhB, AO7, MB, red 2 and bisphenol A at a concentration of 20-50 mg/L could be degraded over MnO2 nanofibers within 12 min. Furthermore, antibiotics, like tetracycline, chlortetracycline and oxytetracycline, and bisphenol A could be mineralized efficiently, and 69-81% TOC removal was obtained. Compared to MnO2 nanowires synthesized by the traditional method with KMnO4, MnO2 nanofibers showed enhanced surface area (102 m(2)/g), more labile lattice oxygen species and higher oxygen vacancy content, which might be responsible for the superior catalytic performances. The radical quenching test and EPR analysis proved that radical (O-2(center dot-), SO4 center dot- and OH center dot) and non-radical (O-1(2)) processes both existed, and the latter dominantly contributed to the degradation. Based on kinetic study, the activation energy of MnO2 nanofibers/PMS for RhB degradation was only 10.55 kJ/mol, which was lower than that of MnO2 nanowires (16.80 kJ/mol). This study not only offered a new, simple and economical protocol for the scaled-up synthesis of nano-scale MnO2 materials, but also demonstrated its potential in water remediation.