Characterization of the fate and changes of post-irradiance fluorescence signal of filtered anthropogenic effluent dissolved organic matter from wastewater treatment plant in the coastal zone of Gapeau river

Anthropogenic effluent dissolved organic matter (DOM) plays an important role in coastal zone pollution. The objectives of the present study were to characterize the fluorescence signal of anthropogenic effluent DOM from wastewater treatment plant and to evaluate the effect of solar irradiation on the fluorescence signal in the coastal zone. Solar irradiation experiments were conducted to evaluate the effect photochemical degradation using excitation–emission matrix (EEM) method combined with parallel factor analysis (PARAFAC). Results showed high fluorescence of DOM before irradiation and the intensity tends to decrease after 4th and 15th day of irradiation. Rapid photochemical degradation of humic-like fluorophores and appearance of a post-irradiance dominant anthropogenic effluent DOM fluorophores were also observed after irradiation. Our experiments showed a sharp reduction in fluorescence intensity which occurred after 4th day of solar irradiation and the fluorescence signal did not disappeared after 15th day indicating the formation of a specific signal due to solar irradiation. PARAFAC model divided the bulk EEM spectra into three individual fluorescent components with C1 “terrestrial humic-like” and C2 “humic-like of longer wavelength” and C3 is a noisy component with two emission maxima. Multilinear regression of PARAFAC components contribution with mixing composition was most suitable according to the equation C*i = AWWi,0 + AWWi,1.fSW + AWWi,2.fRW, where C*i is the normalized contribution of PARAFAC component number i in a given irradiation day; AWWi,0, AWWi,1, AWWi,2 are the multilinear regression coefficients and contain implicitly the effect of fWW; and WW, SW, and RW are treated wastewater, sea water, and river water respectively. The values of AWWi,0, AWWi,1, and AWWi,2 fitted second-order kinetics with irradiation process with kinetic constant of 9.68, − 987.35, and − 977.67 respectively for C1 equation and the same trend for C2 and no values for C3 due to its noisy character indicating the rapid degradation with increase of fSW and fRW and the predominance of the residual fluorescence coming from fWW which is the content fraction of anthropogenic effluent DOM because AWWi,0 was 100 times less sensitive to photobleaching. A suitable model for predicting the fluorescence EEMs as a function of mixing composition was developed.