Poorly Crystalline Nanoparticles Formed in Arsenic-Bearing Mine Tailings in Zimapan, Mexico.

The heavy metalloid arsenic is ubiquitous to natural soils and occurs in multiple oxidation states, generally As(III) and As(V), capable of forming organic oxyanions and the atypical inorganic form. The study site in Zimapán, Mexico is abundant with limestone and should in theory not have a AMD problem. This region in Hidalgo state, Mexico has seen mining since the late 1500’s and has historically been mined for Pb, Zn, and Ag. The mine tailings Zimapan, Mexico offer an excellent model to systematically investigate the geochemical processes of the heavy metals.

The general objectives of this study are: 1) to characterize these nanoparticles and colloids on the atomic scale and their transformations in a limestone environment, 2) to characterize As speciation in each associated solid phase and the mobilization and transformation of As associated with the mineral phase conversions, and 3) to model the reactivity of the nanoparticles and the As geochemistry in the limestone environment.

The samples exhibited a pH near 2, and indicated high mineral heterogeneity among one another. X-ray diffraction, along with other methods, confirmed samples to contain an abundance of iron oxides such as ferrihydrite, goethite, lepidocrocite, and plumbojarosite in the oxidized tailings. In addition, pyrite, gypsum, orthoclase, muscovite and quartz were also present as residues. One sample of almost pure scorodite contained over 100,000 ppm of As. The XANES experiments unanimously indicated that arsenic occurred as arsenopyrite form in the original tailing, but was oxidized to form arsenate in an intimate association with nanometer-sized iron oxides. It was determined that the majority of the As in the samples persist as As(V). There is also evidence of samples having both forms of Arsenic (As(III) and As(V)) together.