Sequestration of Cd(II) and Ni(II) ions on fungal manganese oxides associated with Mn(II) oxidase activity

The sequestration of Ni(II) and Cd(II) by biogenic manganese oxides (BMOs) in cultures of an Mn(II)-oxidizing fungus, Acremonium strictum strain KR21-2, was examined. In batch culture experiments, Ni(II) and Cd(II) were sequestered concurrently with the formation of BMOs with initial concentrations of ⩾30 μM Ni(II) and ⩾50 μM Cd(II). The coexistence of Ni(II) and Cd(II) at higher concentrations caused minor Mn oxide precipitation, and most of the metal ions remained in solution. An assay of Mn(II)-oxidizing activity using a concentrated crude solution of Mn(II) oxidase revealed that in association with BMO, enzymatic activity was maintained even at higher concentrations of Ni(II) and Cd(II) (⩾500 μM), while activity was lower in the absence of BMO. BMOs formed in 1 mM Mn(II) efficiently sequestered Ni(II) and Cd(II) at concentrations up to 1 mM at pH 7.0 with maximum sorption capacities of 25.8 mol% for Ni(II) and 25.0 mol% for Cd(II) relative to Mn oxide under aerobic conditions, without observable release of Mn(II). Meanwhile, BMOs heated to 85 °C to inactivate the associated Mn(II) oxidase released significant amounts of Mn(II) and had lower sequestration efficiencies. Thus, the Mn(II) oxidase associated with newly formed BMOs progressively oxidized Mn(II), which was released through ion exchange at the surface and hence reduced competitive sorption. The results of repeated treatments of newly formed and heated BMOs in Ni(II) or Cd(II) solution (∼1 mM) with excess Mn(II) (∼1 mM) demonstrated that the associated enzymatic activity is a prerequisite for continuous sequestration.