Influence of alkaline co-contaminants on technetium mobility in vadose zone sediments

• Pertechnetate was slowly reduced in natural arid sediment, and rapidly oxidized.
• Sediment with added Fe2+ rapidly reduced pertechnetate, and was rapidly oxidized.
• In 4 M NaOH, pertechnetate was rapidly reduced by Fe2+ from biotite dissolution.
• In the NaOH-treated sediment, most surface Tc(IV) remained immobilized.

Pertechnetate was slowly reduced in a natural, untreated arid sediment under anaerobic conditions (0.02 nmol g−1 h−1), which could occur in low permeability zones in the field, most of which was quickly oxidized. A small portion of the surface Tc may be incorporated into slowly dissolving surface phases, so was not readily oxidized/remobilized into pore water. In contrast, pertechnetate reduction in an anaerobic sediment containing adsorbed ferrous iron as the reductant was rapid (15–600 nmol g−1 h−1), and nearly all (96–98%) was rapidly oxidized/remobilized (2.6–6.8 nmol g−1 h−1) within hours. Tc reduction in an anaerobic sediment containing 0.5–10 mM sulfide showed a relatively slow reduction rate (0.01–0.03 nmol g−1 h−1) that was similar to observations in the natural sediment. Pertechnetate infiltration into sediment with a highly alkaline water resulted in rapid reduction (0.07–0.2 nmol g−1 h−1) from ferrous iron released during biotite or magnetite dissolution. Oxidation of NaOH-treated sediments resulted in slow Tc oxidation (∼0.05 nmol g−1 h−1) of a small fraction of the surface Tc (13–23%). The Tc remaining on the surface was TcIV (by XANES), and autoradiography and elemental maps of Tc (by electron microprobe) showed Tc was present associated with specific minerals, rather than being evenly distributed on the surface. Dissolution of quartz, montmorillonite, muscovite, and kaolinite also occurred in the alkaline water, resulting in significant aqueous silica and aluminum. Over time, aluminosilicates, cancrinite, zeolite and sodalite were precipitating. These precipitates may be coating surface Tc(IV) phases, limiting reoxidation.