Effects of adsorbed arsenate on the rate of transformation of 2-line ferrihydrite at pH 10.

2-Line ferrihydrite, a form of iron in uranium mine tailings, is a dominant adsorbent for elements of concern (EOC), such as arsenic. As ferrihydrite is unstable under oxic conditions and can undergo dissolution and subsequent transformation to hematite and goethite over time, the impact of transformation on the long-term stability of EOC within tailings is of importance from an environmental standpoint. Here, studies were undertaken to assess the rate of 2-line ferrihydrite transformation at varying As/Fe ratios (0.500-0.010) to simulate tailings conditions at the Deilmann Tailings Management Facility of Cameco Corporation, Canada. Kinetics were evaluated under relevant physical (~1 °C) and chemical conditions (pH ~10). As the As/Fe ratio increased from 0.010 to 0.018, the rate of ferrihydrite transformation decreased by 2 orders of magnitude. No transformation of ferrihydrite was observed at higher As/Fe ratios (0.050, 0.100, and 0.500). Arsenic was found to retard ferrihydrite dissolution and transformation as well as goethite formation.