Equilibrium and kinetics of calcium-uranyl-carbonate adsorption on silica nanoparticles

The present work focuses on studying U(VI) adsorption to silica nanoparticles at neutral to weakly alkaline pH and in the presence of calcium and carbonate ions, i.e. when calcium-uranyl-carbonate complexes dominate aqueous U(VI) speciation. The adsorption behavior was studied in batch experiments under various experimental conditions such as solid:liquid ratio, contact time, pH, and varying concentrations of U(VI). The U(VI) distribution ratio (R_d) decreases from 32.2 to 3.2 L g^-1 as the pH increases from 7.1 to 8.6, due to the increase in concentration of Ca(UO₂)(CO₃)₃^2- and UO₂(CO₃)₃^4- complexes in solution. Under the experimental conditions, the adsorption data are best fitted to the Langmuir isotherm and the adsorption capacity (q_m) is 51.39 ± 2.89 mg g^-1. On the other side, the U(VI) adsorption follows the pseudo-second-order rate equation which suggests that the adsorption is of chemisorption type. (author)