DENSITY FUNCTIONAL THEORY MODELING OF FERRIHYDRITE NANOPARTICLE OXYANION ADSORPTION

Ferrihydrite is a critical substrate for adsorption of oxyanion species in the environment¹. The nanoparticulate nature of ferrihydrite is inherent to its formation, and hence it has been called a “nano-mineral”². The nano-scale size and unusual composition of ferrihydrite has made structural determination of this phase problematic. Michel et al.³ have proposed an atomic structure for ferrihydrite, but this model has been controversial^4,5. Recent work has shown that the Michel et al.3 model structure may be reasonably accurate despite some deficiencies^6-8. An alternative model has been proposed by Manceau⁹.

This work utilizes density functional theory (DFT) calculations to model both the structure of ferrihydrite nanoparticles based on the Michel et al. ³ model as refined in Hiemstra⁸ and the modified akdalaite model of Manceau⁹. Adsorption energies of carbonate, phosphate, sulfate, chromate, arsenite and arsenate are calculated. Periodic projector-augmented planewave calculations were performed with the Vienna Ab-initio Simulation Package (VASP¹⁰) on an approximately 1.7 nm diameter Michel nanoparticle (Fe₃₈O₁₁₂H₁₁₀) and on a 2 nm Manceau nanoparticle (Fe₃₈O₉₅H₇₆). After energy minimization of the surface H and O atoms. The model will be used to assess the possible configurations of adsorbed oxyanions on the model nanoparticles.

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