Email: anastasia.ilgen@alaska.edu Anastasia Ilgen CV

This study investigates the sorption and redox behavior of toxic elements As and Sb in clay-rich natural and laboratory systems. Aluminosilicates and Al-oxides are ubiquitous and can influence the speciation and, therefore, the transport and bio-availability of toxic elements such as arsenic (As) and antimony (Sb). Since Fe(II) or Fe(III) is often found substituting for Al(III) in clay mineral structures, the ability of the structural Fe to reduce or oxidize As and Sb was also investigated.

Laboratory studies indicate that As aqueous speciation in nontronite NAu-1 clay suspension is controlled by two simultaneous reactions: 1) fast oxidation of As(III) by dissolved O2 and structural Fe(III) (aerobic) or structural Fe(III) (anaerobic) and 2) the slow reduction of As(V) by dissolved Fe(II) (Figure 1). The oxidation capacity of the nontronite NAu-1 is greatly enhanced when ~ 20% of its structural Fe(III) is reduced to Fe(II). This partially reduced clay also has a significantly higher ability to immobilize Sb(V) compared to the fully oxidized NAu-1. Analysis of the extended x-ray absorption fine structure (EXAFS) spectra shows that both Sb(III) and Sb(V) form inner-sphere sorption complexes (primarily bi-dentate corner-sharing) on the surfaces of hydrous Al oxide (HAO), and the clay minerals kaolinite KGa-1b and nontronite NAu-1.

In looking at natural systems, we examined arsenic from a geothermal source (Dachny geothermal area, Kamchatka, Russia) (Figure 2). The arsenic is in reduced As(III) form and is oxidized to As(V) after mixing with the Falshivaia river water downstream from the Mutnovsky geothermal power plant (Figure 3). Both As(III) and As(V) are found in aqueous and adsorbed forms. EXAFS data analysis shows that sediment phase arsenic is adsorbed onto both Al- and Fe-rich phases with a bi-dentate corner-sharing local geometry.

Figure 2. Dachny geothermal area

Figure 3. Mutnovsky geothermal power plant

These research findings indicate that As and Sb can be effectively immobilized by Al-rich mineral phases, while the substrate nature and oxidation state of structural Fe, along with the presence of dissolved Fe(II), can greatly affect the fate and transport of As and Sb.