Geo-fluid modeling of the natural purification mechanism of Ningyo-toge mill tailings pond

Kawasaki, Yohei*; Granata, G.*; Tokoro, Chiharu*; Ohara, Yoshiyuki

Arsenic in mine wastewater is an important issue in environmental research. In this study, to better understand the phenomena involved in the removal of arsenic from wastewater, the dynamic behavior of arsenic in Ningyo-toge Mill Tailings Pond was investigated. By utilizing geosphere fluid modeling software (GETFLOWS), we first reproduced the overall area flow through the advection-dispersion of non-reactive silicate ion with assuming the groundwater inlet. Following this, the total concentrations of arsenic and iron were reproduced by including in the advection-dispersion model, the first order kinetics for the oxidation of As(III) and of Fe(II) by the dissolved oxygen, whose concentration was set according to data previously obtained by sampling and analysis. Based on the natural oxidation of Fe(II) to Fe(III) and subsequent precipitation of Fe(III) as ferrihydrite, the adsorption of As(V) or As(III) on ferrihydrite was also included in the simulation by considering a surface complexation model with Langmuir mechanism. The profile concentrations of As(III) and Fe(II) determined by simulations were found to be in good agreement with the values measured on the field. The results also highlighted that the natural purification of Ningyo-toge Mill Tailing Pond is natural oxidation of iron and arsenic with consequent adsorption on ferrihydrite.