Structures and Properties of As(OH)₃ Adsorption Complexes on Hydrated Mackinawite (FeS) Surfaces: A DFT-D2 Study

Abstract

Reactive mineral–water interfaces exert control on the bioavailability of contaminant arsenic species in natural aqueous systems. However, the ability to accurately predict As surface complexation is limited by the lack of molecular-level understanding of As–water–mineral interactions. In the present study, we report the structures and properties of the adsorption complexes of arsenous acid (As(OH)3) on hydrated mackinawite (FeS) surfaces, obtained from density functional theory (DFT) calculations. The fundamental aspects of the adsorption, including the registries of the adsorption complexes, adsorption energies, and structural parameters are presented. The FeS surfaces are shown to be stabilized by hydration, as is perhaps to be expected because the adsorbed water molecules stabilize the low-coordinated surface atoms. As(OH)3 adsorbs weakly at the water–FeS(001) interface through a network of hydrogen-bonded interactions with water molecules on the surface, with the lowest-energy structure calculated to be an As–up outer-sphere complex. Compared to the water–FeS(001) interface, stronger adsorption was calculated for As(OH)3 on the water–FeS(011) and water–FeS(111) interfaces, characterized by strong hybridization between the S-p and O-p states of As(OH)3 and the surface Fe-d states. The As(OH)3 molecule displayed a variety of chemisorption geometries on the water–FeS(011) and water–FeS(111) interfaces, where the most stable configuration at the water–FeS(011) interface is a bidentate Fe–AsO–Fe complex, but on the water–FeS(111) interface, a monodentate Fe–O–Fe complex was found. Detailed information regarding the adsorption mechanisms has been obtained via projected density of states (PDOS) and electron density difference iso-surface analyses and vibrational frequency assignments of the adsorbed As(OH)3 molecule.

Reactivity of CO 2 on the surfaces of magnetite (Fe 3 O 4 ), greigite (Fe 3 S 4 ) and mackinawite (FeS)

“…The frozen cores of the Fe, O, S and C atoms were defined up to and including the 3p, 1s, 2p and 1s electrons, respectively. Kohn-Sham valence states were expanded in a planewave basis set with a cutoff for the kinetic energy of 400 eV for Fe 3 O 4 [63] and FeS [51,66,69,72,73], and fixed at 600 eV for Fe 3 S 4 [50,52,[74][75][76][77][78][79], in agreement with previous publications. The Brillouin zone integrations of the surface slabs were performed using a Γ-centred Monkhorst-Pack grid of 5 × 5 × 1 k-points for the simulation of all the materials considered.…”

“…The Brillouin zone integrations of the surface slabs were performed using a Γ-centred Monkhorst-Pack grid of 5 × 5 × 1 k-points for the simulation of all the materials considered. A Hubbard Hamiltonian [80] in the version of Dudarev et al [81] was used for the description of the localized and strongly correlated d Fe electrons of the spinel materials, while we found that it is not needed to describe the properties of FeS correctly [51,66,69,72,73,82]. Following previous works, we have chosen U eff = 3.7 eV for Fe 3 O 4 [63] and 1.0 eV for Fe 3 S 4 [50,75,76,83].…”

“…The anions appear above and below the cation sheets, holding the layers together by vdW forces. We did not consider the spinpolarization effect for the simulation of FeS as we have found it not to have any meaningful effect in the electronic structure of this material [51,66,69,72,73,82].…”

Green Rust: The Simple Organizing &lsquo;Seed&rsquo; of All Life?&nbsp;&nbsp;<strong> </strong>

“…[7,109,110,111,112,113] And the anions required for charge parity in green rust were engaged through ion-dipole interactions with FeIII/FeII cations. [114,115] In strong contrast, nano-confinement of water in mackinawite greatly enhances its self-dissociation, entailing no entropic cost and permits almost frictionless flow between the tetrahedral sheets. [116] Proton flow comparable to that in bulk water in these circumstances would have been via the Grotthuss mechanism at a rate of around 25 nm ns −1 .…”

Hydrazine adsorption on perfect and defective fcc nickel (100), (110) and (111) surfaces: A dispersion corrected DFT-D2 study

“…Such information cannot be obtained directly from experiment and the underlying physical driving forces that control the reactivity of hydrazine with the Ni surfaces are therefore still not fully understood. However, electronic structure calculations based on the density functional theory (DFT) [9] provide an alternative way to gain fundamental insight into these processes, as they are capable of accurately predicting lowest-energy adsorption geometries and identifying charge transfer and other electronic effects [10][11][12].…”