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

Abstract

Uranyl carbonate (UC) and silica colloids (cSiO2) are widely distributed in carbonate-rich subsurface environments associated with uranium pollution. Mobile colloids such as cSiO2 can affect uranium's transport efficiency in the groundwater environment. Therefore, elucidating the mechanism of UC and cSiO2 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), cSiO2 is more prone to be deposited on the surfaces of quartz sand (QS) than UC, resulting in cSiO2 preventing UC transport. Compared to pH 7 and 9, at pH 5 the adsorption of uranium [in the form of 81.5% UO2CO3(aq), 8.6% UO22+, and 5.2% UO2OH+] on cSiO2 renders cSiO2 more prone to aggregate, causing smaller amounts of cSiO2 (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 cSiO2/QS cause the pH of the suspension to varying, which in turn causes changes in the zeta potential and particle size of cSiO2. Eventually, the recovery rates of cSiO2 and UC are changed, depending upon the colloid particle size. Changes in ionic strength can seriously affect the stability of cSiO2 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 cSiO2 and UC in the column. This study provides a strong basis for investigating UC pollution control in porous media.