Metal impurities are common in low valent manganese (oxyhydr)oxides (hereafter, Mn oxides), yet their effects on the reactivity of the oxides have not been experimentally assessed. In this work, we investigate such effects for the first time by measuring the redox ability of the oxides toward arsenite (As(III)), by varying both the type (redox-inactive nickel vs redox-labile cobalt) and concentration (0–3 wt%) of metal impurities in the mineral structure, as well as by the type of Mn mineral (mixed valence spinel hausmannite vs Mn(III)-only monoclinic manganite). A series of mineral batch reactions were run with As(III) for 8 hrs at pH 5, with regular sampling for total dissolved metal (or metalloid) and [As(V)] by inductively-coupled plasma optical emission spectroscopy and ion chromatography. Mineral characterization was made before and after reaction by X-ray diffraction, Brunauer-Emmett-Teller methods, scanning electron microscopy, transmission electron microscopy, and X-ray absorption spectroscopy (XAS). Especially, XAS analyses were used to detect structural changes by metal impurity substitution, and also examine changes in the oxidation states and coordination chemistry of structural impurities upon As(III) oxidation to As(V). Attenuated total reflectance Fourier transform infrared spectroscopy was also used for <i>in-situ</i> observation of interactions between As species and mineral surfaces. The results of this work demonstrate that Mn oxides with metal impurities exhibit a higher oxidizing ability toward As(III) than pristine ones. After the redox reaction, As(V) was the dominant As species found in the solution and on mineral surfaces. At the equivalent loading, greater impurity effects were found in hausmannite than manganite due to smaller particle sizes and higher surface areas. Ni substitution resulted in more profound impacts in the stability and reactivity of the mineral than Co, with greater impact at higher wt%. During the As(III) oxidation reaction, the release of structural metals occurred, with more Ni released than Co, but overall the majority of the impurities remained in the mineral structure. The present study highlights the importance of considering effects of impurities in mineral reactivity, and thus helps improve our understanding of fate and cycling of transition metals and metalloids that are closely associated with natural Mn oxides in surficial environments.

Reactive metal impurities in low valent manganese oxides: Their effects and significance in surficial geochemical processes

ACS Fall 2021

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