Parasitic Light Absorption, Rate Laws and Heterojunctions in the Photocatalytic Oxidation of Arsenic(III) Using Composite TiO₂/Fe₂O₃

Abstract

Composite photocatalyst-adsorbents such as TiO 2 / Fe 2 O 3 are promising materials for the one-step treatment of arsenite contaminated water. However, no previous study has investigated how coupling TiO 2 with Fe 2 O 3 influences the photocatalytic oxidation of arsenic(III). Herein, we develop new hybrid experiment/modelling approaches to study light absorption, charge carrier behaviour and changes in the rate law of the TiO 2 /Fe 2 O 3 system, using UV-Vis spectroscopy, transient absorption spectroscopy (TAS), and kinetic analysis. Whilst coupling TiO 2 with Fe 2 O 3 improves total arsenic removal by adsorption, oxidation rates significantly decrease (up to a factor of 60), primarily due to the parasitic absorption of light by Fe 2 O 3 (88 % of photons at 368 nm) and secondly due to changes in the rate law from disguised zero-order kinetics to first-order kinetics. Charge transfer across this TiO 2 -Fe 2 O 3 heterojunction is not observed. Our study demonstrates the first application of a multi-adsorbate surface complexation model (SCM) towards describing As(III) oxidation kinetics which, unlike Langmuir-Hinshelwood kinetics, includes the competitive adsorption of As(V). We further highlight the importance of parasitic light absorption and catalyst fouling when designing heterogeneous photocatalysts for As(III) remediation.

Teaching Adsorption Chemistry by Constructing Surface Complexation Models (SCM) in PHREEQC

“…18 Ongoing research to enhance adsorption-based water treatment systems includes nanomaterial adsorbents with high adsorption capacities and tunable surface properties, 19 magnetic materials that are easily recovered from treated water, 20 and multifunctional materials that introduce photocatalytic capabilities. 21 We characterize adsorption systems using wet lab experiments to measure adsorption capacities and reaction rates. With limited time and resources, however, we can only study a finite number of experimental conditions.…”

“…gold from electronics waste and phosphate for fertilizer production . Ongoing research to enhance adsorption-based water treatment systems includes nanomaterial adsorbents with high adsorption capacities and tunable surface properties, magnetic materials that are easily recovered from treated water, and multifunctional materials that introduce photocatalytic capabilities …”