Sorption of arsenate on cerium oxide: a simulated infrared and Raman spectroscopic identification

Abstract

Ceria (CeO₂) is a candidate for arsenic removal, and characterizing its surface speciation is crucial for controlling its removal ability. Here, we focus on arsenates and exploit ab initio calculations to study their interaction with the three most stable surfaces of CeO₂. The adsorption of arsenate is stronger on the {100} surface followed by the {110} and {111} surfaces. We find that arsenate can potentially adsorb to CeO₂ surfaces, with a range of binding configurations. Interestingly, we discovered a 5-fold coordinated As(V) species in a trigonal bipyramidal coordination, which is stable and displays a strong interaction with the surfaces, pulling oxygen out of the surfaces, which should be a valuable model to address in As adsorption experiments such as EXAFS. We then predict the infrared (IR) and Raman spectral signatures, finding that adsorbed arsenates have a characteristic spectral fingerprint between 200 and 1200 cm⁻¹. Characteristic peaks compared with experiments gives confidence in the modelling. The 5-fold coordinated As species in particular shows potential diagnostic As–O stretching modes between 635–756 cm⁻¹ in IR spectra and 387–521 cm⁻¹ in Raman spectra. While all binding modes for arsenate adsorption on ceria provide IR active modes, interestingly this is not the case for Raman active modes. Here, we provide a set of reference spectra and binding modes for arsenates on CeO₂ that can further experimental characterization of arsenate speciation, and provide control of its impact on the removal performance of cerium dioxide.