Characterization of Dissolved Organic Matter from Sewage Sludge using 3D-Fluorescence Spectroscopy and Chemometric Tools

The aim of the present article is to show the possibilities of chemometric tools and the parallel factor analysis (PARAFAC) model, as well as to understand the complexities of the fluorescence emission-excitation matrix (EEM) of water-soluble organic matter (WSOM) extracted from sewage sludge samples obtained with different origins and stabilization procedures. The variation in the composition of WSOM in the different sewage sludge samples could be correlated with the conditions of stabilization; therefore, the use of fluorescence spectroscopy to monitor these changes can be a useful tool to optimize the management of this residual flux, avoiding environmental pollution. However, the "contour density" EEM maps obtained with fluorescence present continuous spectra with different peaks, valleys, and shoulders where manual peak picking can be complicated and subjective, especially in complex matrixes such as sewage sludge. With the PARAFAC algorithm, it is possible to resolve the emission and excitation spectra of the different fluorescence components present in the samples and their contribution to the total fluorescence. Also, the contribution (scores) of the different fluorophors can be coupled with another chemometric tool to provide an effective classification method using the stabilization conditions of sewage sludge as main criterion. In this study, a wide survey of sewage sludge samples (287) was characterized by fluorescence emission-excitation matrix. The final molecular composition of the sewage sludge samples was not related to the stabilization (aerobic vs anaerobic) treatments. However, a sewage sludge classification based on a combined spectroscopic-chemometric approach was obtained, which could be used for the optimization of sewage sludge use in agriculture, minimizing the environmental risks.