Adsorption of Arsenic Ions Transforms Surface Reactivity of Engineered Cerium Oxide Nanoparticles

Cerium oxide (CeO₂) nanoparticles (NPs) are massively used as abrasives in the chemical and mechanical polishing (CMP), an essential process to manufacture semiconductor wafers. The CMP process for arsenide-based semiconductor materials produces wastewater with co-occurring arsenic (As) ions and CeO₂ NPs. We found that CeO₂ NPs adsorbed both arsenite (As(III)) and arsenate (As(V)) ions and the adsorption isotherms suggested different adsorption energies and capacities of the two species. Applying the ferric reducing ability for nanoparticle assay, we revealed that the adsorbed As(III) and As(V) each reduced CeO₂ NP surface reactivity but followed different mechanisms. The adsorbed As(III) ions below a critical coverage (110 mmol/kg) increased occupation of Ce 4f orbitals and thus reduced electron mobility of the original CeO₂ NPs. The adsorbed As(V) ions withdrew electrons from Ce 4f orbitals and likely became oxidizing agents that greatly inhibited the original surface reducing ability. Electron paramagnetic resonance analysis further revealed that adsorbed As(III) and As(V) ions decreased the propensity of CeO₂ NPs to produce reactive oxygen species. This work highlights the importance of examining NPs in their post-use phases in which surface reactivity and hazard potential can be greatly altered by chemical exposure history and NP surface transformations.