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

The exchange of Sr 2+ , Mg 2+ , Na + , Li + , and NH 4 + for Ca 2+ on a sediment from the Vadose Zone Research Park at Idaho National Laboratory was characterized for use in modeling transport and remediation of the 90 Sr‐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 K V = 1), whereas in NH 4 –Ca exchange NH 4 is preferentially adsorbed ( K V ≫ 1). The data were modeled using the Vanselow, Rothmund–Kornfeld, and Gapon ( K G ) exchange coefficients. The Gapon coefficient was the least dependent on exchanger‐phase composition. The K G values were incorporated into a geochemical software program that includes aqueous‐ and solid‐phase reactions. Modeled NH 4 + and Ca 2+ exchanger phase and solution composition were in good agreement with the experimental data at both ionic strengths.