Understanding the Adsorption of Heavy Metal Cations at Iron(oxyhydr)oxide-Water Interfaces

Abstract

Heavy metal pollution poses a severe threat to both human well-being and the
health of the ecosystem. An important challenge is understanding how some
of the most problematic metallic pollutants interact with iron-containing minerals, which are widespread in soil and sediments worldwide. Many iron (oxydydr)oxides are recognised for their potential as geo-sorbents and geo-catalysts
that are found to influence the migration, transformation and crystallisation of
various metal elements in nature. Due to their large surface areas, naturallyformed nanoparticles of iron (oxydydr)oxides provides opportunities for adsorption and surface-activated catalysis to happen. An abundance of laboratory studies attempts to understand the adsorption and catalysis mechanisms. However,
to fully understand the chemistry behind the laboratory findings, we decided to
investigate the problem with computer modelling.
Hematite α − Fe2O3 and goethite α − FeOOH were the two minerals discussed
in this thesis. Both of them are ubiquitous and have well defined crystalline
structures. We employed both ab-initio (i.e. density functional theory, abbrev.
DFT) and molecular mechanics methods to establish the surface geometries and
energies. For DFT, we tested a range of exchange-correlation functionals (PBE,
PBEsol, SCAN, TPSS and RTPSS), cut-off energies, k-point densities, spin magnetic moments and suggested to apply Hubbard U correction for the most structurally and electronically robust bulk crystal lattice modelling for both hematite
and goethite. Then, we chose some of the most found surfaces of the two minerals, namely (0001), (0112) and (1014) for hematite, and (021), (110) for goethite
for detailed surface analysis. Hydroxylation was found to stabilise the oxygen termination for the three hematite surfaces studied in chapter three. While for the
two goethite surfaces modelled in this thesis, the results suggested that hydroxylation was not strongly favoured. Following the surface analysis, adsorption
and surface-catalysed oxidation were modelled using molecular mechanics and
ab-initio methods.
In chapter four, a laboratory and modelling-combined approach was applied to
explain the complex relationship between goethite, cadmium ions and (hydrogen)phosphate ions in a simulated environment, with particular emphasis on the
(021) surface. Experimental results suggested that (hydrogen)phosphate ions adsorb at goethite nanocrystals, which may lead to more adsorption sites available
for cadmium ions to approach the positively charged surfaces in an acidic environment. Custom-made grand-canonical Monte Carlo simulation code can help
creating a ”realistic absorption” model with the adsorbent and the adsorbates
”immersed” in a water-filled box.
DFT simulations were performed for the purpose of investigating hematite’s capability to aid Mn(II) to Mn(III) oxidation in chapter five. We constructed and
analysed various possible adsorption mechanisms, comparing the effects on magnetism, charge density, bond lengths and energies. We found that the oxidation
process in the presence of dissolved oxygen is activated by incorporation-type adsorption at hematite surfaces (explained in chapter five). The lack of Mn(II)−→
Mn(III) oxidation found at the hematite (0001) surface may be due to a shortage
of available adsorption sites. Future research is recommended to further investigate the adsorption energies of Mn(II) at different hematite surfaces in various
adsorption sites.
The results presented in this thesis thus provide insights into the interaction
between important pollutants and iron (oxyhydr)oxide surfaces. This thesis also
helps understanding how to approach the modelling of surface adsorption and
offer a way to link laboratory findings with modelling.

Date of Award	15 Nov 2023
Original language	English
Awarding Institution	University of Bath
Supervisor	Steve Parker (Supervisor) & Vera Krewald (Supervisor)