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 mechanical polishing (CMP), an essential process to manufacture semiconductor wafers. The CMP process for arsenide-based semiconductor materials produces a wastewater with co-occurring ionic arsenic (As) ions and CeO2 NPs. We found 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 nanoparticles (FRAN) 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 the critical coverage (110 mmol/kg) increased occupation of Ce 4f orbitals and thus reduced electron mobility of the original CeO2 NPs. 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 (EPR) analysis further revealed that adsorbed As(III) and As(V) ions decreased the propensity of CeO2 NPs to produce reduced reactive oxygen species (ROS). 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.