Insight into the adsorption of humic acid/Gd3+ complex on the surface of Al2O3 studied in situ by QCM-D and ex situ by ellipsometry and XPS

Abstract

The adsorption of humic acid (HA) on an Al₂O₃ surface in the presence of Gd³⁺ was studied in situ by quartz crystal microbalance with dissipation (QCM-D) and ex situ by ellipsometry and XPS. Adsorption of a HA/Gd³⁺ complex yielded an adlayer of a mass of 354 ng cm⁻², the adlayer is characterized by a relatively high dissipation indicating a significant viscoelastic character. The viscoelasticity is enhanced by the salt and water molecules penetrating the adlayer causing it to stretch. The dry state average thickness of the adlayer is about 23 Å as measured by ellipsometry. Ellipsometry revealed a slower kinetics in the case of HA/Gd³⁺ adsorption than that of HA alone. XPS spectra proves the adsorption and demonstrates quantitatively that the adsorption of HA/Gd³⁺ and HA show the same trend, except for the higher adsorption in the case of HA/Gd³⁺ compared with HA. The structure of the HA/Gd³⁺ adlayer may be assumed according to the results of the three techniques to consist of a monolayer of HA bound directly to the surface of Al₂O₃ and connected via a Gd³⁺ bridge to a HA upper layer.