Effects of Ni incorporation on the reactivity and stability of hausmannite (Mn3O4): Environmental implications for Mn, Ni, and As solubility and cycling

Trace metal structural impurities are common in Mn (II/III) oxides, yet their effects on the oxides' reactivity and stability have not been experimentally assessed. The present investigation quantifies such effects for the first time by measuring the extent of mineral dissolution of pristine and Ni-substituted hausmannite (MnIIMnIII2 O4) (at 1 and 2 wt% Ni) in 8-h batch reactions at pH 5 with/without arsenite (As(III)). Ni substitution occurred at Mn(III) octahedral sites, causing noticeable structural modification in lattice parameters with a decrease in Jahn-Teller distortion, particularly at 2 wt%. In both acidic and reductive dissolution (with As(III)), the Ni-substituted hausmannite exhibited enhanced Mn release relative to the pristine mineral, with concurrent release of structural Ni increasing with substitution percentage. When As(V) release was normalized by surface area, Ni-substituted hausmannite showed a higher As(III) oxidation percentage than the pristine phase. Further, higher ratios of Mn (II):As(V) were observed in Ni-substituted hausmannite. As K-edge X-ray absorption spectroscopy and attenuated total reflectance-Fourier transform infrared spectroscopy analyses indicated that As(III) oxidation lead to the formation of binuclear bidentate As(V) surface complexes. Enhanced reactivity of Ni-substituted hausmannite may be attributed to lowered mineral stability, which promotes accelerated mineral dissolution and increased structural Mn release, resulting in formation/exposure of highly reactive sites. Thus, structural impurities dictate the properties, reactivity, and stability of the Mn(II/III) oxides, affecting the level of dissolution and the extent of redox reactions, which together impact the fate and cycling of transition metals and metalloids in surface environments.