Combined UV–Vis absorbance and fluorescence properties of extracted humic substances-like for characterization of composting evolution of domestic solid wastes

Open-air composting evolution of domestic solid wastes (DSW) collected during 150 days from a neighborhood of Santiago del Estero, Argentina, was investigated using several standard methods of compost analysis, in combination with steady-state UV–Vis absorption and fluorescence measurements of their extracted total humic substance-like (HSL), fulvic acid-like (FAL), and humic acid-like (HAL) molecules. The UV-spectra of the HSL were fitted with a two Gaussian-like bands model, allowing the calculation of the absorption ratio between the electron-transfer and benzenoid band components, Ao,ET/Ao,Bz, which provided information about the type and degree of substitution into the aromatic chromophores. In addition, the variation of the absorbance ratio E4/E6 of the HSL was mainly sensitive to the fraction of FAL substances. Fluorescence quantum yields (ΦF) and full-width at half-maximum (FWHM) for the different humic extracts were also valuable parameters for tracking the evolution of these substances during composting. The excitation–emission matrix (EEM) of the different extracts showed the presence of two fluorescence maxima, namely P1 (λexc  280 nm) respectively. Only the location of the P2 of HAL was sensitive to the evolution of composting, indicating the continuous modification of the fluorophore during the process. The present results indicate the suitability of combined absorption and emission spectroscopical properties in the characterization of both HSL and evolution of composting.