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 Cu²⁺. 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, Cu²⁺–DOM complexation indicated clear differences in DOM compositions. The binding strength increased between Cu²⁺ and fulvic acid-like components in contrast to protein-like components from influent to the effluent, and increasing fluorescence intensity with the addition of Cu²⁺ 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 Cu²⁺–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. Graphical Abstract: [Figure not available: see fulltext.]