Over one hundred million people worldwide are exposed to arsenic contaminated groundwaters. In oxygenated environments arsenic is found in the +5 oxidation state and is readily removed through adsorption onto iron oxides. However, in South Asia, where arsenic exposure is most severe, arsenic is found in the +3 oxidation state. This species is both more toxic and more difficult to remove than As(V). The current generation of arsenic treatment plants often fail to meet the World Health Organisation’s 10 μg L-1 guideline limit, and the oxidation of As(III) to As(V) is therefore necessary to achieve effective removal of arsenic in South Asia.
Multifunctional photocatalyst-sorbents are an attractive solution, as a single material can be used to both oxidise As(III) and adsorb As(V). Whilst multifunctional behaviour is often achieved by coupling different materials into new composite forms, our understanding of how material coupling influences the behaviour of each component (i.e. photocatalytic oxidation and adsorption) remains limited.
This thesis investigated a mesoporous TiO2/Fe2O3 composite. This work found that monolayer adsorption of As(V) is component additive, with TiO2 and Fe2O3 surface components behaving independently of one another. Multilayer adsorption of As(III) is not component additive, due to differences between the surface morphology of meso-TiO2/Fe2O3 and single-phase reference samples. Coupling TiO2 with Fe2O3 significantly reduced photocatalytic activity, due to parasitic absorption of ultraviolet light by the Fe2O3 phase. The kinetic mechanism also changed after coupling, from a reactive oxygen species (ROS) mediated pathway to direct surface oxidation. This work considered the balance between the high sorbent concentrations needed for long filter device life-times, and the low photocatalyst concentrations needed to achieve effective light penetration through TiO2 suspensions. Kinetic adsorption modelling predicted that at least 10 g L-1 material is required for effective arsenic removal, presenting constraints on how this technology can be engineered.