Hydrothermal Synthesis of Porous Birnessite Nanospheres for Heavy Metal Removal from Simulated Acid Mine Drainage: Synergistic Roles of Ion Exchange and Vacancies
摘要
A one-step hydrothermal method is proposed to prepare porous birnessite nanospheres, and their adsorption behavior and mechanism for heavy metal ions in acid mine drainage (AMD) were investigated. The results showed that, with the increase of synthesis temperature, the crystallinity, grain size, and oxidation degree of birnessite improved, and the adsorption capacity increased with the rise in solution pH. Under a solution pH of 5, the maximum adsorption capacities of birnessite for Cu2+, Zn2+, Pb2+, and Cd2+ were 122.6 mg/g, 155.0 mg/g, 619.6 mg/g, and 188.0 mg/g, respectively. After three adsorption–desorption cycles, the adsorption capacities for Cu2+, Zn2+, Pb2+, and Cd2+ remained at 60.8 mg/g, 54.9 mg/g, 286.5 mg/g, and 78.0 mg/g, respectively. These heavy metal ions are adsorbed via active oxygen species on the birnessite surface, and through ion exchange with K+ ions in the birnessite interlayers. Adsorption isotherm and kinetics analyses indicated that the date fitted the Langmuir model for all four heavy metal ions. The adsorption of Pb2+, Zn2+, and Cd2+ involved chemical adsorption whereas Cu2+ adsorption was dominated by physical interactions. The findings demonstrate that the birnessite-type MnO2 (δ-MnO2) has great potential for treating heavy metal ion pollution in AMD.