The influence of particle size and natural organic matter on U(VI) retention by natural sand: Parameterization and mechanism study.

Contamination caused by radionuclides such as uranium (U) has become an increasingly serious environmental problem. The unique and diverse features of uranyl ions (U(VI)) remarkably dominate their mobility and environmental impact on the ecosystem. Understanding the sorption behavior and fate of aqueous U(VI) ions on natural mineral(s) such as quartz sand (a typical type of crystalline silica (SiO2)) particles is essential for unraveling many environmental issues. In this work, the sorption of uranyl ions by various particle size quartz sands under different reaction conditions was thoroughly investigated. The quartz sand with an average particle size of 3.588 μm exhibited an excellent sorption performance for the removal of aqueous U(VI) ions at pH 5.0. The sorption rate increased as the dosage of sorbent increased. The sorption rate descends with the rise of the initial U(VI) concentration while its sorption amount is reversed. The elevation of temperature impeded the U(VI) sorption. Humic acid (a typical natural organic matter) showed significant impacts on U(VI) removal. Ions of Ca2+, CO32− and K+ remarkably inhibited the U(VI) sorption, while PO43− ions significantly promoted the U(VI) sorption. The pseudo second-order kinetic model could fit well with the experimental sorption data. The U(VI) sorption is mainly chemisorption and it is an exothermic process. After sorption, the surface of used quartz sand became much smooth and XPS signals of U(VI) were detected, evidencing the success of the removal of aqueous U(VI). The outcomes of this study highlighted both solution pH and natural organic matters played critical roles on U(VI) removal by sand particles. This study further enhances our comprehension from the molecular-scale process manipulating U(VI) sorption behavior at the mineral-aqueous interface.