Oxyanion Behavior in Alkaline Environments: Sorption and Desorption of Arsenate in Ettringite

Arsenate sorption by ettringite [Ca 6 Al 2 (SO 4 ) 3 (OH) 12 ·26H 2 O] is examined as adsorption and coprecipitation systems at alkaline pH (10.0−12.5) and for a wide range of As(V) concentration (<1 μM−15 mM). The mode of sorption and sorbate and sorbent concentrations controlled the nature of solid-phase As(V) speciation. Although high pH increased ettringite stability in concentrated As(V) solutions, it did not influence total As(V) sorption. During adsorption, ettringite exposure to concentrated As(V) solutions (>2.0 mM) precipitated new unidentified microcrystalline minerals at the expense of ettringite. Concentrated As(V) solution exposure to coprecipitating ettringite poisoned ettringite crystal growth, with precipitation of some microcrystalline minerals. Sorbed As(V) was also not desorbable in the presence of concentrated sulfate and high ionic strength solutions. Details of As(V) adsorption and coprecipitation systems and inferences on As(V) molecular interactions are proposed.