Tracing TiO2 photocatalytic degradation of humic acid in the presence of clay particles by excitation–emission matrix (EEM) fluorescence spectra

• Photocatalytic oxidation of humic acid in the presence of clay minerals.
• Fluorescence properties of oxidized humic acid in the presence of clay minerals.
• Application of excitation–emission matrix fluorescence spectroscopy.
• Correlation between fluorescence and UV–vis spectroscopic properties and DOC.

Excitation–emission matrix (EEM) fluorescence spectral features were evaluated for the elucidation of the photocatalytic degradation of sole humic acid (HA) and HA in the presence of clay minerals. Montmorillonite (Mt) or kaolinite (Kt) was selected to represent two different types of clay minerals. EEM fluorescence signatures displayed irradiation time dependent transformation of humic-like fluorophores to fulvic-like fluorophores in accordance with the photocatalytic mineralization of HA. The role of Ca2+ ions on the photocatalytic degradation of HA could also be visualized through EEM under the specified conditions. Upon photocatalytic treatment, the role of Mt was more pronounced with respect to the effects observed in the presence of Kt in comparison to the results achieved for sole HA. Under prolonged irradiation conditions (tirr = 180 min) almost complete removal of fluorophoric groups was detected in relation to a substantial degree of mineralization (>75% removal of dissolved organic carbon, DOC). As a correlative approach, fluorescence-derived index defined as fluorescence intensity (FI) represented by the ratio of the emission intensity at λemis = 450 nm to that at λemis = 500 nm, following excitation at λexc = 370 nm was successfully employed. FI values were correlated to specified UV parameters (UV254 and UV280) revealing an inverse relationship. Moreover, through normalization of the specified UV parameters to DOC contents, the derived specific UV parameters (SUVA254 and SUVA280) could also be successfully correlated to FI. The obtained results indicated that besides photocatalytic mineralization efficiencies, UV–vis and fluorescence spectroscopic properties could also be employed to gain further information on the humic structural changes attained through non-selective oxidation mechanism in comparison to the data obtained under non-oxidative conditions.