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.