Characterization of the composition of water DOM in a surface flow constructed wetland using fluorescence spectroscopy coupled with derivative and PARAFAC

This study was conducted to discuss fluorescence spectroscopic properties of water dissolved organic matter (DOM) in a large-scale constructed wetland and to reveal the composition and humification degree. Four humification indices (HIXs) deduced from emission spectra (A ₄/A ₁), excitation spectra (I ₄₂₆/I ₃₆₆), synchronous fluorescence spectra (I ₃₇₀/I ₃₃₀) and derivative (I ₃₆₀/I ₃₂₀) were used for the description of composition and humification degree of DOM. Three components (C1–C3) were derived from excitation-emission matrices with parallel factor analysis (PARAFAC). The results showed that dissolved organic carbon (DOC), total nitrogen (TN) and total phosphorus (TP) concentrations in tributaries water decreased when they flowed through the constructed wetland. The values of A ₄/A ₁, I ₄₂₆/I ₃₆₆, I ₃₇₀/I ₃₃₀ and I ₃₆₀/I ₃₂₀ suggested that the DOM consisted of organic components with low molecular weight and low aromatic contents in the tributary. I ₃₆₀/I ₃₂₀ could differentiate humification degree more clearly than the other three HIXs. The fluorescence index indicated a more microbially derived composition of DOM in the constructed wetland. The DOM fluorescence was dominated by the tryptophan-like fluorescence material (C3, more than 40 %), followed by humic-like substance (C1, about 30 %). Non-humic component (C2, about 20 %) was the least component. The percentage of C1 declined from upstream to downstream along the wetland, while C3 showed inverse tendency. The proportion of C2 remained relatively constant in all sampling points. Principal component analysis suggested that the humification degree of the DOM in the slowly flowing region was the highest among the wetlands, followed by those from the regions disturbed by tributaries, the sampling sites in mainstream and the outlet zone of wetland. The humification degree also showed a good correlation mainly with TN, TP and Chlorophyll a. Therefore, fluorescence spectroscopy technique coupled with derivative and PARAFAC might be a useful method to investigate composition and to evaluate the humification degree of DOM in aquatic environment.