Simultaneous leaching of arsenite, arsenate, selenite and selenate, and their migration in tunnel-excavated sedimentary rocks: II. Kinetic and reactive transport modeling.

Predicting the fates of arsenic (As) and selenium (Se) in natural geologic media like rocks and soils necessitates the understanding of how their various oxyanionic species behave and migrate under dynamic conditions. In this study, geochemical factors and processes crucial in the leaching and transport of arsenite (AsIII), arsenate (AsV), selenite (SeIV) and selenate (SeVI) in tunnel-excavated rocks of marine origin were investigated using microscopic/extraction techniques, column experiments, dissolution-precipitation kinetics and one-dimensional reactive transport modeling. The results showed that evaporite salts were important because aside from containing As and Se, they played crucial roles in the evolution of pH and concentrations of coexisting ions, both of which had strong effects on adsorption-desorption reactions of As and Se species with iron oxyhydroxide minerals/phases. The observed leaching trends of AsV, AsIII, SeIV and SeVI were satisfactorily simulated by one-dimensional reactive transport models, which predict that preferential adsorptions of AsV and SeIV were magnified by geochemical changes in the columns due to water flow. Moreover, our results showed that migrations of AsIII, SeIV and SeVI could be predicted adequately by 1D solute transport with simple activity-K'd approach, but surface complexation was more reliable to simulate adsorption-desorption behavior of AsV.