Metal-free photocatalyst based on highly porous activated carbon obtained from agro-industrial residues. Characterization and photocatalytic evaluation

The chemical activation of sunflower seed shells (SSS) with H₃PO₄ led to highly porous activated carbon (AC_SSS^H₃^PO₄) containing carbon-centered environmentally persistent free radicals (EPFRs), quinone functional groups, among others, and a slight graphitization degree. With high diclofenac (DFC) and ibuprofen (IBP) dark adsorption capacity (∼98 % after 1 h), AC_SSS^H₃^PO₄ was evaluated as a metal-free photocatalyst using quantitative and qualitative approaches under UV and visible light irradiation. However, due to the high adsorption of both pollutants onto the solid, monitoring photocatalytic activity was impossible. For this reason, a quantitative and qualitative strategy was developed to evaluate the existence of pollutant degradation in highly porous activated carbon. On the one hand, the qualitative approach was based on the use of ATR-FTIR spectroscopy, detecting IR bands of both molecules adsorbed on the activated carbon and evaluating their intensity reduction or the apparition of new IR bands assigned to oxidized byproducts after 5 h of light irradiation. On the other hand, the quantitative approach was based on a desorption procedure to remove DFC and IBP from the AC_SSS^H₃^PO₄ after 5 h of light irradiation and quantifying their concentrations by UHPLC. Thus, it was found that under irradiation for 5 h at λ > 260 nm, AC_SSS^H₃^PO₄ showed 62 and 30 % degradation for adsorbed DFC and IBP, respectively, while under visible irradiation, pollutants degradation was 42–29 % and 20 %. AC_SSS^H₃^PO₄ thus demonstrated significant photoinduced activity, probably due to EPFRs, quinone, and graphitic functional groups.