Sequestration of La3+ by fungal manganese oxides and the effect of Mn(II) oxidase activity

Abstract

This study examined the sequestration of La3+ by biogenic manganese oxides (BMOs), formed by the Mn(II)-oxidizing fungus Acremonium strictum strain KR21-2. When newly formed BMOs were added to La(NO3)3 solutions under aerobic conditions at pH 6.0, effective sequestration of La3+ (23.5 mol% relative to Mn oxide) was observed without any release of Mn2+. On the other hand, under anaerobic conditions the BMOs released significant amounts of Mn2+ and exhibited lower sequestration efficiencies, especially at low La3+ concentrations. The reason is that La3+ sequestration by BMOs is accompanied by the release of Mn2+ due to ion exchange, and the released Mn2+ competes with La3+ for sorption on the surface. The Mn(II) oxidase in the newly formed BMOs can progressively oxidize Mn2+ and thereby enhance La3+ sequestration. Repeated treatments of newly formed BMOs demonstrated that enzymatic Mn(II) oxidation readily converted exogenous Mn2+ to Mn oxide, providing new sites for sequestering La3+. The addition of NaN3 deactivated the associated enzyme and subsequently released La3+ from the BMO surface due to sorption competition by exogenous Mn2+. Therefore, the Mn(II) oxidase activity is important for maintaining high La3+ sequestration efficiency of the BMOs. Experiments with mixed solutions indicated that the BMOs preferentially sequester La3+ and Cd2+ over Zn2+ and Ni2+. These results provide new insights on the recovery of La3+ (and possibly other trivalent rare earth ions) through the BMOs.