Metal sorption to dolomite surfaces

Potential human intrusion into the Waste Isolation Pilot Plant (WIPP) might release actinides into the Culebra Dolomite where sorption reactions will affect of radiotoxicity from the repository. Using a limited residence time reactor the authors have measured Ca, Mg, Nd adsorption/exchange as a func tion of ionic strength, P-CO2, and pH at 25 degrees C. By the same approach , but using as input radioactive tracers, adsorption/exchange of Am, Pu, U, and Np on dolomite were measured as a function of ionic strength, P-CO2 an d pH at 25 degrees C. Metal adsorption is typically favored at high pH. Cal cium and ME adsorb in near-stoichiometric proportions except at high pH. Ad sorption of Ca and Mg is diminished at high ionic strengths (e.g., 0.5M NaC l) pointing to association of Na+ with the dolomite surface, and the possib ility that Ca and Mg sorb as hydrated, outer-sphere complexes. Sulfate ampl ifies sorption of Ca and Mg, and possibly Nd as well. Exchange of Nd for su rface Ca is favored at high pH, and when Ca levels are low. Exchange for Ca appears to control attachment of actinides to dolomite as well, and high l evels of Ca2+ in solution will decrease K(d)s. At the same time, to the ext ent that high P(CO2)s increase Ca2+ levels, K(d)s Will decrease with CO2 le vels as well, but only if sorbing actinide-carbonate complexes are not obse rved to form (Am-carbonate complexes appear to sorb; Pu-complexes might sor b as well. U-carbonate complexation leads to desorption). This indirect CO2 effect is observed primarily at, and above, neutral pH. High NaCl levels d o not appear to affect to actinide K(d)s. (C) 1999 Elsevier Science Ltd. Al l rights reserved.