Bonding properties of humic acid with attapulgite and its influence on U(VI) sorption

Humic acid (HA) can markedly change the surface charge of clay minerals and influences the sorption of uranium(U) on clay minerals in aquatic environments. Furthermore, HA can potentially affect the mobility of U(VI) in the natural environment. The effect of HA on the surface charge, aggregation, sorption and bonding properties of attapulgite [(Mg,Al)₄(Si)₈(O,OH,H₂O)₂₆·nH₂O] was investigated using fluorescence spectroscopy technique in combination with other analytical tools. The UV–vis absorbance spectra and fluorescence peaks obtained in the experiments are attributed to a large number of electron donating substitutes, such as carboxylic acid-like and phenolic acid-like fluorophores, in HA. Electrostatic attraction and ligand exchanges are the primary interaction mechanisms between attapulgite and HA. A significant modification of excitation-emission matrix (EEM) spectra and emission intensity of HA are observed after the interaction, indicating a different binding complex of HA on attapulgite at the molecular level. The sorption kinetics indicate that strong chemical interactions and/or surface complexation primarily contributed to U(VI) sorption. An analysis of U(VI) sorption isotherms and model fitting reveals that the sorption of U(VI) onto attapulgite is a spontaneous and endothermic process. The presence of HA is an important factor that greatly influences the immobilization and removal of U(VI) ions from aqueous solutions.