Characterizing the fluorescent properties and copper complexation of dissolved organic matter in saline-alkali soils using fluorescence excitation-emission matrix and parallel factor analysis

The goals of this study are to identify the individual fluorophore component in dissolved organic matter (DOM) of saline-alkali soils, to determine the binding capacities and stability constants of Cu (II) with fluorescent DOM components, and to analyze the potential impact on the complexation between Cu (II) and DOM in different soil environment. DOM extracted from five saline-alkali soil samples were studied by fluorescence quenching titrations combined with three-dimensional excitation-emission matrix (3D-EEM) spectra and parallel factor analysis (PARAFAC). Modified Stern-Volmer model was used to calculate the conditional stability constant (K) and the percent of fluorophores (f %) participating in the complexation reaction. Three main fluorescent components in the DOM, e.g., fulvic-like (component 1), humic-like (component 2), and protein-like (component 3) were identified and characterized by fluorescence EEM-PARAFAC. Both the components 1 and 2 showed fluorescence quenching by gradual addition of Cu (II), while component 3 had no marked change in fluorescence intensity. The higher log K of DOM-Cu (II) complexes suggests that the DOM has a high Cu binding affinity. Meanwhile, humic-like components showed higher in the proportion of organic ligands than those of fulvic-like components in the DOM of saline-alkali soils. The fluorescence quenching titration using EEM-PARAFAC analysis allows for the assessment of metal ion interactions with specific fluorophores, and is a good approach to study the geochemical behavior of metal ions in saline-alkali soils and to provide a support for the management of saline-alkali soil environment.