Solar-induced heterogeneous photocatalytic degradation of methyl-paraben

• The benchmark UV-activated Aeroxide TiO2 P-25 exhibited high efficacy.
• Superoxide radicals (O2−) were the dominant reactive species.
• H2O2 addition was detrimental to process efficiency.
• Initial pH and substrate concentration highly affect the initial reaction rate.

A solar-induced heterogeneous photocatalytic study on the treatment of methyl-paraben in water has been performed. A solar simulator equipped with a 150 W xenon ozone-free lamp was employed for the experimental runs. Various commercial TiO2 catalysts have been assessed and the effect of catalyst loading has been extensively studied. Titanium dioxide Aeroxide P-25 was determined as the optimum catalytic material, even at low catalyst loadings. MeP abatement increased with increasing P-25 loading up to 0.5 g L−1, above which further increase brings no actual improvement on the initial rate of the reaction. The addition of an electron acceptor (i.e. hydrogen peroxide) inhibited the degradation rate of MeP, whereas superoxide radicals were found to be the dominant reactive species in the heterogeneous solar-induced photocatalytic degradation of MeP. Experimental design methodology was applied to assess the significance of variables such as initial pH0, MeP concentration and catalyst dosage and to evaluate their effect on the pseudo-first order reaction rate constant (kapp) of the photocatalytic reaction. Complete elimination of 1 mg L−1 MeP was achieved after 35 min at inherent pH 5.2 and 0.5 g L−1 Aeroxide P-25, whereas for the respective run with 10 mg L−1 MeP 240 min reaction time was required. Under the latter conditions, 42% of mineralization was obtained and six intermediates were identified by GC–MS, namely propyl acetate, 2-(2-butoxyethoxy) ethanol, 2,5-dihydroxy-methyl benzoate, hydroquinone, diethyl phthalate and 1,2-benzenedicarboxylic acid bis-(2-methylpropyl) ester.

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