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 epsilon-MnO2 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 Pb2+ (at an initial concentration of 400 mg/L), 65.64 mg/g for Fe3+ (at 300 mg/L) and 8.5 mg/g for Cd2+ (at 10 mg/L). Mechanistic studies revealed that ion exchange was the main adsorption mechanism for removal of Pb2+, while electrostatic interaction was dominant for Fe3+ and Cd2+ removal. In the case of Pb2+ adsorption, double corner sharing (DCS) complexes were formed at the edges, which promoted the dissolution of structural Mn(III), thereby generating Mn2+ and vacancies which promoted Pb2+ adsorption via an ion exchange process. In the case of Fe3+ and Cd2+ adsorption, along with electrostatic adsorption, triangle corner sharing (TCS) dominated complex formation at the intermediate vacancies, and less Mn2+ was released. Note that the Mn2+ was in dynamic equilibrium with structural Mn(III) and Mn(IV). The actual AMD was retreated in 5 cycles by epsilon-MnO2, the removal rates of 100% were achieved for Pb2+, Fe(total) and Mn2+, together with 98% for Cu2+ and 95.1% for Cd2+, all of which reached the national emission standards. This approach is a highly suitable option for AMD treatment that avoids toxic sludge formation.