Density functional theory modeling of chromate adsorption onto ferrihydrite nanoparticles

James D Kubicki,Nefeli Bompoti,Nadine Kabengi,Maria Chrysochoou

doi:10.1186/s12932-018-0053-8

Abstract

Density functional theory (DFT) calculations were performed on a model of a ferrihydrite nanoparticle interacting with chromate ( {text{CrO}}_{4}^{2 - } ) in water. Two configurations each of monodentate and bidentate adsorbed chromate as well as an outer-sphere and a dissolved bichromate ( {text{HCrO}}_{4}^{ - } ) were simulated. In addition to the 3-D periodic planewave DFT models, molecular clusters were extracted from the energy-minimized structures. Calculated interatomic distances from the periodic and cluster models compare favorably with Extended X-ray Absorption Fine Structure spectroscopy values, with larger discrepancies seen for the clusters due to over-relaxation of the model substrate. Relative potential energies were derived from the periodic models and Gibbs free energies from the cluster models. A key result is that the bidentate binuclear configuration is the lowest in potential energy in the periodic models followed by the outer-sphere complex. This result is consistent with observations of the predominance of bidentate chromate adsorption on ferrihydrite under conditions of high surface coverage (Johnston Environ Sci Technol 46:5851–5858, 2012). Cluster models were also used to perform frequency analyses for comparison with observed ATR FTIR spectra. Calculated frequencies on monodentate, bidentate binuclear, and outer-sphere complexes each have infrared (IR)-active modes consistent with experiment. Inconsistencies between the thermodynamic predictions and the IR-frequency analysis suggest that the 3-D periodic models are not capturing key components of the system that influence the adsorption equilibria under varying conditions of pH, ionic strength and electrolyte composition. Model equilibration via molecular dynamics (MD) simulations is necessary to escape metastable states created during DFT energy minimizations based on the initial classical force field MD-derived starting configurations.

Highlights

Adsorption is a critical process in environmental chemistry that can control the fate and transport of aqueous species [1]
Energy minimizations using central valence force field (CVFF) typically lowered the potential energy of the model systems on the order of 5 kJ/mol from the randomized structure of H2O molecules initially provided by Maestro
Energy minimizations with the Density functional theory (DFT) method described above could decrease the potential energy on the order of 1000 kJ suggesting that the H-bond network from CVFF was limiting the accuracy of the model structure. (Note that the ferrihydrite nanoparticle and chromate ion structures were previously approximated via DFT calculations, so this error could have been larger because 1000 kJ pertains predominantly to H-bonding and the H2O configuration only.) CVFF likely underestimates H-bonding, DFT methods such as those used here can overestimate H-bonding [47], so the reader is cautioned about the significant inaccuracies in the DFT results reported

Summary

Introduction

Adsorption is a critical process in environmental chemistry that can control the fate and transport of aqueous species [1]. The most common experimental method to study environmental adsorption chemistry has been to perform adsorption isotherm experiments involving selected solid phases and varying concentrations of an adsorbent. Kubicki et al Geochem Trans (2018) 19:8 element or compound for a particular solid; Villalobos and coworkers have clearly shown that the adsorption isotherm can vary significantly depending on the crystal habit of the substrate involved [4, 5] In these papers, adsorption of species such as P b2+ and chromate onto goethite was inversely proportional to goethite specific surface area—a result contrary to expectation. A similar effect may be present in a study that observed changes in relative fractions of adsorbing species of chromate, selenite, and sulfate onto ferrihydrite as a function of Al-substitution [8] In this case, Al may change the habit of the solid as well as changing the pKas of the surface metal-OH groups [9]. In order to understand adsorption reactions relevant to environmental chemistry, it is necessary to model these reactions on all possible adsorbing surfaces [10]

Methods

Results

Conclusion