Mechanism, kinetics, and pathways of self-sensitized sunlight photodegradation of phenylarsonic compounds

• Phenylarsonic compounds from animal wastes easily accumulate in surface water bodies.
• They undergo 1O2-based self-sensitized photolysis in water under sunlight irradiation.
• The -AsO(OH)2 group is cleaved from the aromatic ring during their photodegradation.
• The photodegradation can be described by a 1O2-based “heterogeneous” kinetic model.
• Natural organic matter significantly inhibits photodegradation of these compounds.

Being highly water-soluble, phenylarsonic feed additives discharged in animal wastes can easily accumulate in surface water bodies. The photodegradation mechanism, kinetics, and pathways of p-arsanilic acid (p-ASA), 4-hydrophenylarsonic acid (4-HPAA), and phenylarsonic acid (PAA) in water under simulated and natural sunlight irradiation were investigated. The -AsO(OH)2 group was cleaved from the aromatic ring during photodegradation, and p-benzoquinone and p-hydroquinone were formed as the major organic degradation intermediates. Experimental results did not indicate any significant direct photolysis of the phenylarsonic compounds under simulated and natural sunlight irradiation, but consistently showed that they sensitized the formation of singlet oxygen, which was responsible for their photodegradation and oxidation of the As(III) released. A simple 1O2-based “heterogeneous” model was developed, which could well describe the kinetics of 1O2 formation and phenylarsonic compound photodegradation under various conditions. Indirect photolysis caused by inorganic ions commonly present in natural waters was negligible, while natural organic matter could significantly inhibit their photodegradation. The half-lives of p-ASA, 4-HPAA, and PAA photodegradation under simulated sunlight irradiation (765 W m−2, 25 °C) were 11.82 ± 0.19, 20.06 ± 0.10, and 135 ± 6.0 min, respectively, while their degradation rates under natural sunlight in the Pearl River Delta of southern China were 5 times slower due to lower irradiation intensity and water temperatures (19–23 °C).

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