NANOSIZED MANGANESE (HYRD)OXIDE, HAUSMANNITE (MN3O4) AND MANGANITE (MNOOH) AND THEIR REDOX ABILITY ON AS(III) OXIDATION

Hausmannite (Mn(II)Mn(III)₂O₄) and manganite(γ-MnOOH) are the most abundant low-valence manganese (hydr)oxides (Mn-(hyrd)oxides) in the environment. Both minerals are often co-present in natural settings and intermediate Mn(III) in the structure can involve in electron transfer reactions with redox-sensitive elements, controlling their fate and environmental behaviors. While mixed mineral phases are prevalent, virtually all the laboratory studies have utilized a single mineral system to evaluate the redox ability of a mineral. Therefore, in the present study, we aim to investigate both single and mixed mineral systems to adequately evaluate their redox ability to arsenic (As) in nature. The nano-sized hausmannite and manganite were synthesized and their structures were confirmed with x-ray diffraction (XRD) and surface areas (SA) measured by Brunauer-Emmett-Teller (BET) and recorded at 136 and 28.5 m²/g, respectively. Transmission electron microscopy (TEM) analysis shows that hausmannite has an octahedral shape whereas manganite has a needle shape. Then, the As(III) oxidation by hausmannite and manganite were examined in both the single system and the binary systems by conducting a series of batch experiments at pH 5 for 8 hours with 0.2 g/L of particles. For the mixed system, three different ratios (weight percent, w.t.%) of hausmannite to manganite were prepared at 1:4, 4:1 and 1:1. The aliquots were collected and analyzed with ion chromatography (IC) for As(V) concentration. When the concentration of As(V) was normalized by mineral’s SA, hausmannite, as a single system, produced 7.6 µmoles/m², whereas manganite of 4.1 µmoles/m². However, in the binary system, the oxidation efficiency of As(III) increased as increasing the ratio of manganite in the system. This observation indicates that the heteroaggregation of two particles and their aggregation structure may have a significant impact upon As(III) oxidation reaction, altering the redox behaviors of minerals. Thus, our work demonstrates that the mixed mineral phases behave differently from individual minerals for the redox reaction with As(III), and therefore, laboratory batch experiments with individual minerals only may underestimate the ability of Mn-(hyrd)oxides to perform redox reactions in the environment.