Adsorption of ions on zirconium oxide surfaces from aqueous solutions at high temperatures

Surface titrations were carried out on suspensions of monoclinic ZrO ₂ from 25 to 290∈°C slightly above saturation vapor pressure at ionic strengths of 0.03, 0.1 and 1.0 mol · kg^-1(NaCl). A typical increase in surface charge was observed with increasing temperature. There was no correlation between the radius of the cations, Li⁺, Na⁺, K⁺ and (CH₃)₄N⁺, and the magnitude of their association with the surface. The combined results were treated with a 1-pK_a MUSIC model, which yielded association constants for the cations (and chloride ion at low pH) at each temperature. The pH of zero-point-charge, pH_zpc, decreased with increasing temperature as found for other metal oxides, reaching an apparent minimum value of 4.1 by 250∈°C. Batch experiments were performed to monitor the concentration of LiOH in solutions containing suspended ZrO₂ particles from 200 to 360∈°C. At 350 and 360∈°C, Li⁺ and OH^- ions were almost totally adsorbed when the pressure was lowered to near saturation vapor pressure. This reversible trend has implications not only to pressure-water reactor, PWR, operations, but is also of general scientific and other applied interest. Additional experiments probed the feasibility that boric acid/borate ions adsorb reversibly onto ZrO₂ surfaces at near-neutral pH conditions as indicated in earlier publications.