Co-transport of uranyl carbonate and silica colloids in saturated quartz sand under different hydrochemical conditions

Uranyl carbonate (UC) and silica colloids (cSiO 2 ) are widely distributed in carbonate-rich subsurface environments associated with uranium pollution. Mobile colloids such as cSiO 2 can affect uranium's transport efficiency in the groundwater environment. Therefore, elucidating the mechanism of UC and cSiO 2 co-transport in a saturated porous medium with different ionic strength ( IS ), pH, and UC concentration is essential for the prevention and control of groundwater radioactive pollution. At low UC concentrations (<2.1 × 10 −5  M), cSiO 2 is more prone to be deposited on the surfaces of quartz sand (QS) than UC, resulting in cSiO 2 preventing UC transport. Compared to pH 7 and 9, at pH 5 the adsorption of uranium [in the form of 81.5% UO 2 CO 3 (aq), 8.6% UO 2 2+ , and 5.2% UO 2 OH + ] on cSiO 2 renders cSiO 2 more prone to aggregate, causing smaller amounts of cSiO 2 (86.6%) and UC (55.8%) to be recovered. Mechanisms responsible for the evolution of the pH and zeta potential in effluents have been proposed. Chemical reactions (ligand-exchange reactions and deprotonation) that occur in the QS column between UC and cSiO 2 /QS cause the pH of the suspension to varying, which in turn causes changes in the zeta potential and particle size of cSiO 2 . Eventually, the recovery rates of cSiO 2 and UC are changed, depending upon the colloid particle size. Changes in ionic strength can seriously affect the stability of cSiO 2 particles, and that effect is more significant when UC is present. Moreover, colloidal filtration theory, a non-equilibrium two-site model, and the Derjaguin–Landau–Verwey–Overbeek theory successfully describe the individual-transport and co-transport of cSiO 2 and UC in the column. This study provides a strong basis for investigating UC pollution control in porous media.