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Open AccessFeature PaperArticle

A Model Study of the Photochemical Fate of As(III) in Paddy-Water

by 1 and 1,2,*
Dipartimento di Chimica, Università di Torino, Via Pietro Giuria 5, 10125 Torino, Italy
Centro Interdipartimentale NatRisk, Università di Torino, Largo Paolo Braccini 2, 10095 Grugliasco (TO), Italy
Author to whom correspondence should be addressed.
Academic Editor: Pierre Pichat
Molecules 2017, 22(3), 445;
Received: 27 January 2017 / Revised: 3 March 2017 / Accepted: 6 March 2017 / Published: 11 March 2017
(This article belongs to the Special Issue Photon-involving Purification of Water and Air)
The APEX (Aqueous Photochemistry of Environmentally-occurring Xenobiotics) software previously developed by one of us was used to model the photochemistry of As(III) in paddy-field water, allowing a comparison with biotic processes. The model included key paddy-water variables, such as the shielding effect of the rice canopy on incident sunlight and its monthly variations, water pH, and the photochemical parameters of the chromophoric dissolved organic matter (CDOM) occurring in paddy fields. The half-life times (t1/2) of As(III) photooxidation to As(V) would be ~20–30 days in May. In contrast, the photochemical oxidation of As(III) would be much slower in June and July due to rice-canopy shading of radiation because of plant growth, despite higher sunlight irradiance. At pH < 8 the photooxidation of As(III) would mainly be accounted for by reaction with transient species produced by irradiated CDOM (here represented by the excited triplet states 3CDOM*, neglecting the possibly more important reactions with poorly known species such as the phenoxy radicals) and, to a lesser extent, with the hydroxyl radicals (HO). However, the carbonate radicals (CO3•−) could be key photooxidants at pH > 8.5 provided that the paddy-water 3CDOM* is sufficiently reactive toward the oxidation of CO32−. In particular, if paddy-water 3CDOM* oxidizes the carbonate anion with a second-order reaction rate constant near (or higher than) 106 M−1·s−1, the photooxidation of As(III) could be quite fast at pH > 8.5. Such pH conditions can be produced by elevated photosynthetic activity that consumes dissolved CO2. View Full-Text
Keywords: arsenic contamination; paddy-field floodwater; sunlight-induced reactions arsenic contamination; paddy-field floodwater; sunlight-induced reactions
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Carena, L.; Vione, D. A Model Study of the Photochemical Fate of As(III) in Paddy-Water. Molecules 2017, 22, 445.

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