Cation Exchange on Vadose Zone Research Park Subsurface Sediment, Idaho National Laboratory

Abstract

The exchange of Sr2+, Mg2+, Na+, Li+, and NH4+ for Ca2+ on a sediment from the Vadose Zone Research Park at Idaho National Laboratory was characterized for use in modeling transport and remediation of the 90Sr‐contaminated vadose zone at the nearby Idaho Nuclear Technology and Engineering Center. Cation exchange was measured at ionic strengths of 0.1 and 0.005 and a range of cation compositions. The Sr–Ca exchange on this sediment is nonpreferential (Vanselow selectivity coefficient KV = 1), whereas in NH4–Ca exchange NH4 is preferentially adsorbed (KV ≫ 1). The data were modeled using the Vanselow, Rothmund–Kornfeld, and Gapon (KG) exchange coefficients. The Gapon coefficient was the least dependent on exchanger‐phase composition. The KG values were incorporated into a geochemical software program that includes aqueous‐ and solid‐phase reactions. Modeled NH4+ and Ca2+ exchanger phase and solution composition were in good agreement with the experimental data at both ionic strengths.