Efficient removal of heavy metals from acid mine drainage by ε-MnO2 adsorption

The clean-up of acid mine drainage (AMD) is challenging, as classical treatment including a pH neutralization step produces large amounts of toxic sludge . Here we demonstrated that removal of heavy metals through adsorption at low pH was achievable with the use of ε-MnO 2 adsorbent, which could be produced in a simple one-step process. The material, which was negatively charged even at pH 2, exhibited outstanding potential for the removal of heavy metals at strongly acidic pH. At pH 2, adsorption capacities were achieved of 172.14 mg/g towards Pb 2+ (at an initial concentration of 400 mg/L), 65.64 mg/g for Fe 3+ (at 300 mg/L) and 8.5 mg/g for Cd 2+ (at 10 mg/L). Mechanistic studies revealed that ion exchange was the main adsorption mechanism for removal of Pb 2+ , while electrostatic interaction was dominant for Fe 3+ and Cd 2+ removal. In the case of Pb 2+ adsorption, double corner sharing (DCS) complexes were formed at the edges, which promoted the dissolution of structural Mn(III), thereby generating Mn 2+ and vacancies which promoted Pb 2+ adsorption via an ion exchange process. In the case of Fe 3+ and Cd 2+ adsorption, along with electrostatic adsorption, triangle corner sharing (TCS) dominated complex formation at the intermediate vacancies, and less Mn 2+ was released. Note that the Mn 2+ was in dynamic equilibrium with structural Mn(Ⅲ) and Mn(Ⅳ). The actual AMD was retreated in 5 cycles by ε-MnO 2 , the removal rates of 100% were achieved for Pb 2+ , Fe (total) and Mn 2+ , together with 98% for Cu 2+ and 95.1% for Cd 2+ , all of which reached the national emission standards. This approach is a highly suitable option for AMD treatment that avoids toxic sludge formation.