Surface complexation modeling of uranium(VI) sorbed onto lanthanum monophosphate

• Published in: Journal of colloid and interface science Vol. 263; no. 2; pp. 391 - 399
• Main Authors: Ordoñez-Regil, E., Drot, R., Simoni, E.
• Format: Journal Article
• Language: English
• Published: San Diego, CA Elsevier Inc 15.07.2003; Elsevier
• Subjects: CCM; Chemical Physics; Chemistry; Electrophoretic measurements; Exact sciences and technology; General and physical chemistry; Lanthanum phosphate; Physics; Potentiometric titrations; Solid-liquid interface; Sorption; Surface complexation; Surface physical chemistry; Uranyl ion; Sorption; Lanthanum phosphate; Uranyl ion; CCM; Surface complexation; Potentiometric titrations; Electrophoretic measurements; Acidity constant; Surface complex; Liquid solid interface; Liquid solid adsorption; Zero charge point; Experimental study; Potentiometry; Modeling; Lanthanide compound; Actinide; Electrophoretic mobility; Uranium VI; Thermodynamic properties
• ISSN: 0021-9797; 1095-7103
• DOI: 10.1016/S0021-9797(03)00399-0

Sorption/desorption are basic processes in the field of contaminant transport. In order to develop mechanistically accurate thermodynamic sorption models, the simulation of retention data has to take into account molecular scale informations provided by structural investigations. In this way, the uranyl sorption constants onto lanthanum monophosphate (LaPO 4) were determined on the basis of a previously published structural investigation. The surface complexation modeling of U(VI) retention onto LaPO 4 has been performed using the constant capacitance model included in the FITEQLv3.2 program. The electrical behavior of the solid surface was investigated using electrophoretic measurements and potentiometric titration experiments. The point of zero charge was found to be 3.5 and surface complexation modeling has made it possible to calculate the surface acidity constants. The fitting procedure was done with respect to the spectroscopic results, which have shown that LaPO 4 presents two kinds of reactive surface sites (lanthanum atoms and phosphate groups). The uranyl sorption edges were determined for two surface coverages: 40 and 20% of the surface sites that are occupied, assuming complete sorption. The modeling of these experimental data was realized by considering two uranyl species (“free” uranyl and uranyl nitrate complex) sorbed only onto phosphate surface groups according to the previously published structural investigation. The obtained sorption constants present similar values for both surface complexes and make it possible to fit both sorption edges: log K U=9.4 for P(OH) 2+UO 2 2+↔P(OH) 2UO 2 2+ and log K UN=9.7 for P(OH) 2+UO 2NO 3 +↔P(OH) 2UO 2NO 3 +.