Investigating spectroscopic and copper binding characteristics of dissolved organic matter in wastewater using EEMs with two-dimensional Savitzky-Golay second-order differentiation-PARAFAC.

Dissolved organic matter (DOM) in wastewater interacts with heavy metal particles in aquatic environments, which changes their dynamics and bioavailability. For quantifying the DOM, an excitation-emission matrix (EEM) paired alongside parallel factor analysis (PARAFAC) is typically employed. However, a drawback of PARAFAC has been revealed in recent studies, i.e., the rise of overlapping spectra or wavelength shifts in fluorescent components. Here, traditional EEM-PARAFAC and, for the first time, two-dimensional Savitzky-Golay second-order differential-PARAFAC (2D-SG-2nd-df-PARAFAC) were used to study the DOM-heavy metal binding. The samples from four treatment units of a wastewater treatment plant, i.e., influent, anaerobic, aerobic, and effluent, underwent the process of fluorescence titration with Cu. Four components were separated with dominant peaks in regions I, II, and III (proteins and fulvic acid-like) through PARAFAC and 2D-SG-2nd-df-PARAFAC. A single peak was observed in region V (humic acid-like) by PARAFAC. In addition, Cu-DOM complexation indicated clear differences in DOM compositions. The binding strength increased between Cu and fulvic acid-like components in contrast to protein-like components from influent to the effluent, and increasing fluorescence intensity with the addition of Cu in the effluent indicated changes in their structural composition. Moreover, when comparing both methods, the 2D-SG-2nd-df-PARAFAC provided the components without peak shifts and better fitting for Cu-DOM complexation model, demonstrating it to be a more reliable technique compared to only traditional PARAFAC for DOM characterization and quantifying metal-DOM in wastewater.