Effects of Fulvic Acid on Uranium(VI) Sorption Kinetics

Abstract

This study focuses on the effects\nof fulvic acid (FA) on uranium­(VI)\nsorption kinetics to a silica sand. Using a tritium-labeled FA in\nbatch experiments made it possible to investigate sorption rates over\na wide range of environmentally relevant FA concentrations (0.37–23\nmg L^–1 TOC). Equilibrium speciation calculations\nwere coupled with an evaluation of U­(VI) and FA sorption rates based\non characteristic times. This allowed us to suggest plausible sorption\nmechanisms as a function of solution conditions (e.g., pH, U­(VI)/FA/surface\nsite ratios). Our results indicate that U­(VI) sorption onto silica\nsand can be either slower or faster in the presence of FA compared\nto a ligand-free system. This suggests a shift in the underlying mechanisms\nof FA effects on U­(VI) sorption, from competitive sorption to influences\nof U­(VI)-FA complexes, in the same system. Changes in metal sorption\nrates depend on the relative concentrations of metals, organic ligands,\nand mineral surface sites. Hence, these results elucidate the sometimes\nconflicting information in the literature about the influence of organic\nmatter on metal sorption rates. Furthermore, they provide guidance\nfor the selection of appropriate sorption equilibration times for\nexperiments that are designed to determine metal distribution coefficients\n(<i>K</i>_d values) under equilibrium conditions.