Characterising the interactions between Cu(II) and fulvic acid subcomponents using differential fluorescence spectroscopy combined with parallel factor analysis

Fulvic acid (FA) consists of various organic compounds that interact with metals in the aquatic environment. Interactions between subcomponents (FA(3), FA(5), FA(7), FA(9) and FA(13)) of FA and Cu(II) were investigated using fluorescence quenching titration by coupling differential fluorescence spectroscopy (DFS) and parallel factor analysis (PARAFAC). The material fluorescence intensities derived from FA subcomponents decreased with Cu(II) concentration increase, and stronger quenching was experienced at low Cu(II) concentrations. Humic-like materials of FA(3) and protein-like materials of FA(9) have relatively higher presence of carboxylic groups with greater molecular polarity, and preferential interaction of Cu(II) occurs. Fluorescence DFSs can be successfully broken down into five components as follows: fulvic-like components of a major compound containing carboxylic-like and phenolic-like groups, fulvic-like components of a major compound containing groups of other components, humic-like components of a major compound containing carboxylic-like and phenolic-like groups, humic-like components of a major compound containing carboxylic-like groups, and protein-like components. The fulvic-like components of a major compound containing carboxylic-like and phenolic-like groups exhibited stronger quenching with Cu(II) in five FA subcomponents. Static quenching is the dominant mechanism for the binding of Cu(II) by the FA subcomponent. The log K of quenching constants fitted using the modified Ryan-Weber equation (R-2 = 0.9368-0.9985) ranged from 4.32 to 6.14 for five components derived from FA subcomponents. Subcomponents that were eluted earlier exhibited stronger binding affinity with Cu(II).

Automated summary

The study revealed that the subcomponents of FA exhibit fluorescence quenching when interacting with Cu(II), with FA(3) and FA(9) materials showing higher carboxylic group content and stronger affinity with Cu(II).