Elucidating Aqueous Cerium(IV) Adsorption Dynamics on Tin Oxide Surfaces Using Electrochemistry

Understanding the interactions of aqueous f-element ions in the presence of solid surfaces is vital in many applications, including predicting fate and transport of f-elements in the environment, developing extraction methods for remediation and recycling of these elements, and developing f-element sensing technologies. In this report, we elucidate some mechanistic details of Ce4+ adsorption to tin oxide surfaces from aqueous media using primarily cyclic voltammetry on fluorine-doped tin oxide (FTO) electrodes. Electrochemistry is a low-cost, in situ, facile, and sensitive method that can provide both quantitative and qualitative details about surface reactions. Herein, we show that Ce4+ spontaneously adsorbed from pH 4 aqueous sulfate solutions with a binding constant of 2.18 × 104 L/mol. We also show that when adsorbed Ce4+ was reduced to Ce3+, it underwent a dramatic and irreversible change in speciation. Surface speciation and this irreversible transformation were strongly influenced by both the electrolyte anion and the pH of the solution. A permanent (or long-lasting) pH effect was observed on the initial adsorption reaction of Ce4+ to the surface, which was maintained even after exposure to a different pH solution. The data highlight the important roles that both pH and ligands in the solution played in cerium adsorption to tin oxide. Furthermore, we present data that suggest that Ce3+ may have incorporated into the tin oxide lattice. This work demonstrates the utility of electrochemistry in understanding surface adsorption reactions, especially when paired with other surface-sensitive techniques.