Data on Sorption behavior of U(IV) on montmorillonite in the presence of carbonic acid

Hamamoto et al. is driven by the need to understand how high carbonate groundwater impact the migration of U(IV), the predominant state of uranium in deep repository environments.

A rigorous batch sorption test was designed to maintain the stability of U(IV), which is highly sensitive to oxidation.
• Solution Preparation: Tetravalent uranium was prepared via electrolytic reduction of an acidic uranyl solution. All experiments were conducted in an argon glove box (O2 < 1 ppm) using deoxygenated and decarboxylated Milli-Q water.
• Test Conditions:
o Solid Phase: High-purity Na-type montmorillonite (Kunipia F).
o Liquid-to-Solid Ratio: 10,000 mL/g.
o pH: 6.0–10.0.
o Carbonate Concentration: 0–100 mmol/L.
o Ionic Strength: 0.05 and 0.5 mol/L adjusted with NaCl.
• Analysis: After 24 hours of shaking, the liquid and solid phases were separated using 10 kDa ultrafiltration or 0.45 µm PTFE filters. Uranium concentrations were measured using ICP-MS.

The data reveals a complex relationship between U(IV) sorption and its chemical environment.
• pH Dependence: Kd values appear to reach a maximum around pH 7. Above pH 9, Kd values vary by approximately three orders of magnitude depending on the carbonate concentration.
• Carbonate Effect: Kd values peak at carbonate concentrations between 10-5 and 10-4 M, then decrease significantly as carbonate concentration increases further.
• Ionic Strength: No clear dependence of Kd on ionic strength was observed.
• Artifact Checks: The study confirmed that while intrinsic U(IV) hydroxide colloids form at 0 mM carbonate, their impact on the final equilibrium parameters was negligible.