Soil Chemical Transformations Affect Arsenic Bioaccessibility during Mine Tailings Phytostabilization.

Sulfide-ore derived mine tailings that have undergone weathering under semi-arid conditions at the Iron King Humboldt Mine and Smelter Superfund Site (IKHMSS, Prescott, AZ) comprise As and Pb in the thousands of parts per million and remain devoid of vegetation decades following mining cessation because of extremely low pH, high metal concentrations, low water holding capacity, and other stressors. This makes the tailings susceptible to offsite transport to neighboring communities. As part of a comprehensive field, greenhouse and laboratory study into phytostabilization of the tailings through addition of composted organic matter and seeding with native plants, we are using wet chemistry, microbial genetics and X-ray spectroscopy methods to quantify plant root- and microbial-mediated molecular transformations of arsenic, lead, iron, and sulfur that accompany the stabilization process. Infusion of labile organic matter and irrigation water increases neutrophilic heterotrophic microbial activity and accelerates mineral transformations, particular oxidation of residual sulfides. Two distinct mechanisms of arsenic stabilization and detoxification are observed in the rhizosphere, including incorporation into ferric sulfate root plaques that are collocated with bacterial cells, and binding into arsenic thiol complexes in the root cortex. Implications of metal(loid) transformations are discussed in the context of changes in their gastric and lung bioavailability.