Constraint of Nanometer-Sized Pyrite Crystals on Oxidation Kinetics and Weathering Products

The oxidation behavior and oxidation products of colloidal pyrite (CPy) from the middle-lower Yangtze River metallogenic belt in China are investigated and compared with normal pyrite (NPy, macrocrystalline pyrite) in order to evaluate the relative environmental impact of CPy. CPy is a cryptocrystalline form of pyrite with nanometer- and submicrometer-sized crystals. Analysis of fresh and oxidized samples by X-ray diffraction (XRD) and scanning electron microscopy (SEM) reveals that the oxidation of CPy proceeds in two stages under ambient conditions without flushing. The first stage is characterized by the formation of Fe(II) oxidation products such as rozenite and melanterite, whereas the second stage is characterized by the formation of Fe(III) sulfates such as rhomboclase and romerite. The oxidation kinetic experiments were designed to investigate the size-controlled oxidation rate of CPy and the results show that the oxidation of CPy in aqueous solution can be described by a zero-order kinetic model, proceeding 36.8 times faster than for NPy. The higher reactivity of CPy is attributed to the nanometer-sized crystals and high specific surface area, which in turn determines the speciation of oxidation products. Due to the nano effect, the oxidation of CPy is more destructive to the mine environment than that of NPy.