Journal of Photochemistry & Photobiology, A: Chemistry 424 (2022) 113628 Available online 29 October 2021 1010-6030/© 2021 Published by Elsevier B.V. The determination of oxidation rates and quantum yields during the photocatalytic oxidation of As(III) over TiO 2 Hany Fathy Heiba a, b, * , Jay C. Bullen a , Andreas Kafzas c, d , Camille Petit e , Stephen J Skinner f , Dominik Weiss a, g, * a Department of Earth Science & Engineering, Imperial College London, London SW7 2AZ, UK b Marine Chemistry Department, Environmental Division, National Institute of Oceanography and Fisheries (NIOF), Egypt c Department of Chemistry, Molecular Science Research Hub, Imperial College London, London W12 0BZ, UK d The Grantham Institute, Imperial College London, London SW7 2AZ, UK e Barrer Centre, Department of Chemical Engineering, Imperial College London, London SW7 2AZ, UK f Department of Materials, Imperial College London, London SW7 2AZ, UK g The Department of Civil and Environmental Engineering, Princeton University, Princeton, NJ 08544, United States A R T I C L E INFO Keywords: Photocatalysis PCO kinetic rates Molar absorptivity Quantum yield Arsenite Oxidation Arsenic species analysis ABSTRACT The determination of reaction rates for the photocatalytic oxidation (PCO) of arsenite (As(III)) using TiO 2 under UV radiation is challenging due to the numerous experimental processes. This includes chemical processes running simultaneously with PCO (e.g. adsorption of arsenic species, direct UV photolysis of As(III)) and the analytical approach used (e.g. whether As(III) or As(V) are measured and used in the calculation of the PCO rate). The various experimental approaches used to date have led to oxidation rates and rate constants which vary by orders of magnitude and contradicting information on rate laws. Here we present the results of a critical ex- amination of possible controls affecting the experimental determination of PCO rates. First, we demonstrate that the choice of analytical technique is not critical, provided that the rate constants are calculated based on the depletion of As(III) after correction of the directly adsorbed As(III). Second, we show the correction of the directly adsorbed As(III) at each time interval is best done by running two parallel experiments (one under UV and the other in dark) instead of running sequential experiment (i.e. running the experiment in the dark then turning on the UV lamp). These fndings are supported by XPS analysis of the oxidation state of TiO 2 -sorbed As. Third, we demonstrate that photolysis by the light source itself, as well as the chemical composition of the so- lution (i.e. the effect of HEPES and the ionic strength), can signifcantly increase As(III) oxidation rates and need to be corrected. Finally, to determine the quantum yield of As(III) oxidation, we measured the photon absorption by the TiO 2 photocatalyst. Our results showed that the quantum yield (Ø) for this oxidation reaction was low, and in the region of 0.1 to 0.2 %. 1. Introduction Arsenic is a potent carcinogen, with tens of millions of people exposed to dangerous levels worldwide [1]. Due to severe toxicity, in 2009 the World Health Organization (WHO) imposed strict regulations lowering the permitted level of arsenic in drinking water from 50 to 10 μg/L [2–4]. The toxicity of arsenic is determined by its speciation: inorganic arsenic is more harmful than organic arsenic [3,4], and inorganic arsenite (As(III)) is 60 times more toxic than inorganic arse- nate (As(V)). The removal of As(III) from contaminated water is more challenging [5] since it predominantly forms non-ionic species at neutral and acidic pH (H 3 AsO 3 ), whereas As(V) exists as anionic species (H 2 AsO 4 - and HAsO 4 2- ) [6]. Therefore, effective water treatment ne- cessitates the oxidation of As(III) to As(V) prior to its removal using strategies such as adsorption, precipitation, or ion exchange [7]. Photocatalytic oxidation (PCO) of As(III) to As(V) using TiO 2 is a promising solution [8] due to the high chemical stability, corrosion resistance, non-toxicity and photocatalytic activity of the material [9]. The accurate determination of oxidation rates is crucial to successfully implement photocatalysts into arsenic treatment plants [10], however, this is challenged by various process that occur during the experimental determination, such as co-occurring adsorption (Fig. S1). To date, * Corresponding authors at: Department of Earth Science & Engineering, Imperial College London, London SW7 2AZ, UK. E-mail addresses: h.heiba17@imperial.ac.uk, hf.heiba@niof.sci.eg (H.F. Heiba), d.weiss@imperial.ac.uk (D. Weiss). Contents lists available at ScienceDirect Journal of Photochemistry & Photobiology, A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem https://doi.org/10.1016/j.jphotochem.2021.113628 Received 20 April 2021; Accepted 23 October 2021