Establishing trends in ion adsorption on the aqueous aluminium hydroxide nanoparticle Al₃₀

Abstract

The fact that chemical reactions at environmental interfaces are becoming accessible to quantum mechanical computational studies provides geochemical researchers with a new means to predict properties that cannot readily be measured and to develop molecular-level understanding of geochemical model systems. Recent computational studies of Cu 2þ and SO 22 4 adsorption onto the Keggin-based aqueous aluminium nanoparticle (Al 30 O 8 ðOHÞ 56 ðH 2 OÞ 18þ 26 ), or Al 30 , revealed opposing trends in adsorption site preference as a function of molecule surface topology. Specifically, the adsorption site favourable for the inner-sphere adsorption of Cu 2þ is on the caps of Al 30 while outer-sphere SO 22 4prefers adsorption in the so-called beltway region of the molecule. When co-adsorbed, it is predicted that both species adsorb in the beltway, consistent with an experimental crystal structure. Here, we discuss results for individual cation and anion adsorption to Al 30 . Our goals are to better understand how the adsorbate properties govern interactions with Al 30 and to assess whether generalisations can be formed. We test the reactivity of cations (Cu 2þ , Pb 2þ , Zn 2þ ) and anions (SO 22 4 , Cl 2 ) to aqueous Al 30 by using density functional theory modelling. It is determined that all the cations favour the adsorption sites on the caps of Al 30 and both anions favour outer-sphere adsorption in the beltway region. The results are discussed in terms of the electrostatic potential of Al 30 and three-dimensional induced charge density mapping.

Ionic adsorption on the brucite (0001) surface: A periodic electrostatic embedded cluster method study

“…Modelling ionic sorption mechanisms on surfaces is key to understanding a wide range of physical phenomena in catalytic chemistry [1][2][3][4][5] , biochemistry 6,7 , energy storage 8,9 and environmental chemistry, where modelling has been used to study the migration of ion contaminants in soils and sediments, [10][11][12][13][14][15][16] and the treatment of polluted water with adsorbent materials [17][18][19] . Computational studies can provide insight into ion/surface interactions at the molecular level by predicting preferred reaction sites, 2,4,5,14,17 calculating the most stable structures during the interactions 1,9,14,19 and allowing comparison of the interaction energies of competing species 15,18,20 .…”

Theoretical discovery of shape reactivity relationships in aluminum nanoclusters

“…Our previous studies have revealed strong outer-sphere ion pair formation for ions in the beltway. 28 Extending this knowledge to the (TBP)2Al30-S structure, we intuit that Step 1 of the EW mechanism (Scheme 7.1) drives the beltway site preference, and subsequent ligand exchange then results in the final inner-sphere ion pair. It is noteworthy that oxygen functional group type and coordination alone cannot explain the greater reactivity of adsorption sites in the Al30 beltway region, even though such arguments have been successfully applied to a variety of mineral surface reactivities.…”

“…25,26 Further details of the computational methods (including benchmarking and convergence studies for Al30) are reported in our previous work. 28,29 The initial isolated molecular structure for Al30 was based on the experimental crystal structure, which has been reported previously. 20,53,66 Based on bond valence analysis, 95 the molecular formula of Al30 is expected to be (Al30O8(OH)56(H2O)26) 18+ , and the molecule has inversion symmetry.…”

“…These trends are also in agreement with the electrostatic potential in that the caps of Al30 are least positive which promotes inner-sphere cation reactivity. 28 Chapter 6 focuses on using DFT calculations to rationalize the crystal structure of an Al30 Keggin species with two surface-bound Cu 2+ , referred to as Cu2Al30-S. The crystallization was done in the presence of disulfonate ions (2,6-NDS), and the structure exhibited Cu 2+ bound to Al30 in a corner-sharing bidentate fashion, while the sulfonate groups had an outer-sphere relationship to the molecule.…”

“…The strength of the Al30-SO4 2-interaction was shown to correlate with topographical variation in the Al30 electrostatic potential. 28,29 Here, we focus on the reactivity of Al30 towards polyoxoanion adsorbates, comparisons of phosphate and arsenate analogues, and ligand substitutions of the oxyanions.…”