title: Roles of manganese oxides in degradation of phenol under UV-Vis irradiation: adsorption, oxidation, and photocatalysis.
abstract: Manganese oxides are known as one type of semiconductors, but their photocatalysis characteristics have not been deeply explored. In this study, photocatalytic degradation of phenol using several synthesized manganese oxides, i.e, acidic birnessite (BIR-H), alkaline birnessite (BIR-OH), cryptomelane (CRY) and todorokite (TOD), were comparatively investigated. To elucidate phenol degradation mechanisms, X-ray diffraction (XRD), ICP-AES (inductively coupled plasma-atomic emission spectroscopy), TEM (transmission electronic microscope), N2 physisorption at 77 K and UV-visible diffuse reflectance spectroscopy (UV-Vis DRS) were employed to characterize the structural, compositional, morphological, specific surface area and optical absorption properties of the manganese oxides. After 12 hr of UV-Vis irradiation, the total organic carbon (TOC) removal rate reached 62.1%, 43.1%, 25.4%, and 22.5% for cryptomelane, acidic birnessite, todorokite and alkaline birnessite, respectively. Compared to the reactions in the dark condition, UV-Vis exposure improved the TOC removal rates by 55.8%, 31.9%, 23.4% and 17.9%. This suggests a weak ability of manganese oxides to degrade phenol in the dark condition, while UV-Vis light irradiation could significantly enhance phenol degradation. The manganese minerals exhibited photocatalytic activities in the order of: CRY > BIR-H > TOD > BIR-OH. There may be three possible mechanisms for photochemical degradation: (1) direct photolysis of phenol; (2) direct oxidation of phenol by manganese oxides; (3) photocatalytic oxidation of phenol by manganese oxides. Photocatalytic oxidation of phenol appeared to be the dominant mechanism.
authors.affiliation: Key Laboratory of Subtropical Agriculture and Environment, Ministry of Agriculture, College of Resources & Environment, Huazhong Agricultural University, Wuhan 430070, China.
authors.forename: Qin
authors.initials: Q
authors.lastname: Zhang
authors.forename: Xiaodi
authors.initials: X
authors.lastname: Cheng
authors.forename: Chen
authors.initials: C
authors.lastname: Zheng
authors.forename: Xionghan
authors.initials: X
authors.lastname: Feng
authors.forename: Guohong
authors.initials: G
authors.lastname: Qiu
authors.forename: Wenfeng
authors.initials: W
authors.lastname: Tan
authors.forename: Fan
authors.initials: F
authors.lastname: Liu
mesh_terms: D004785:Environmental Pollutants / Q000737:chemistry* / Q000528:radiation effects; D017895:Manganese Compounds / Q000737:chemistry*; D010084:Oxidation-Reduction; D010087:Oxides / Q000737:chemistry*; D019800:Phenol / Q000737:chemistry* / Q000528:radiation effects; D010782:Photolysis; D014466:Ultraviolet Rays
chemical_list: D004785:Environmental Pollutants; D017895:Manganese Compounds; D010087:Oxides; D019800:Phenol; C027424:manganese oxide