Activated carbon as photocatalyst of reactions in aqueous phase

The main objective of this study was to identify the origin of the photocatalytic behavior of activated carbons in the presence of ultraviolet (UV) light. For this purpose, we selected four commercial activated carbons and sixteen gamma radiation-modified carbons derived from these. Sodium diatrizoate was considered as model compound for the degradation study. The results demonstrate that the direct diatrizoate photodegradation rate is influenced by the solution pH. The presence of activated carbon during diatrizoate degradation markedly accelerates its rate of removal, regardless of the activated carbon used. Witco commercial carbon exerts the highest synergic effect in diatrizoate removal by the UV/activated carbon system, with a synergic contribution >53% after the first minute of treatment. The synergic activity of all of the activated carbon samples is enhanced by gamma radiation treatment. The textural properties of the activated carbons show no clear relationship with their synergic contribution. However, the synergic activity of the activated carbon is more greatly enhanced in the samples with higher percentages of surface oxygen, and among these, the samples with higher percentages of ester/anhydride groups and of carbon atoms with sp2 hybridization. Band gap (Eg) determination of activated carbons revealed that they behave as semiconductor materials and, therefore, as photoactive materials in the presence of UV radiation, given that all Eg values are <4 eV. We also observed that the gamma radiation treatment reduces the band gap values of the activated carbons and that, in a single series of commercial carbons, lower Eg values correspond to higher sodium diatrizoate removal rate values. We highlight that the gamma radiation-modified materials show a higher percentage of carbon atoms with sp2 hybridization, explaining their superior behavior in the process. Finally, we observed that: (i) the activity of the reutilized activated carbons is similar to that of the original carbons, (ii) the presence of dissolved oxygen enhances the rate of diatrizoate removal by UV/activated carbon, and (iii) the UV radiation treatment produces slight chemical modifications in the activated carbons. Based on these results, an action mechanism for the photocatalytic removal of diatrizoate in the presence of activated carbon is proposed, in which the activated carbon acts as a photocatalyst, promoting electrons of the valence band to the conduction band and increasing the generation of HO radicals in the medium.