Interactions between stepwise-eluted sub-fractions of fulvic acids and protons revealed by fluorescence titration combined with EEM-PARAFAC.

In aquatic environments, pH can control environmental behaviors of fulvic acid (FA) via regulating hydrolysis of functional groups. Sub-fractions of FA, eluted using pyrophosphate buffers with initial pHs of 3.0 (FA3 ), 5.0 (FA5 ), 7.0 (FA7 ), 9.0 (FA9 ) and 13.0 (FA13 ), were used to explore interactions between the various, operationally defined, FA fractions and protons, by use of EEM-PARAFAC analysis. Splitting of peaks (FA3 and FA13 ), merging of peaks (FA7 ), disappearance of peaks (FA9 and FA13 ), and red/blue-shifting of peaks were observed during fluorescence titration. Fulvic-like components were identified from FA3 -FA13 , and protein-like components were observed in fractions FA9 and FA13 . There primary compounds (carboxylic-like, phenolic-like, and protein-like chromophores) in PARAFAC components were distinguished based on acid-base properties. Dissociation constants (pKa ) for fulvic-like components with proton ranged from 2.43 to 4.13 in an acidic pH and from 9.95 to 11.27 at basic pH. These results might be due to protonation of di-carboxylate and phenolic functional groups. At basic pH, pKa values of protein-like components (9.77-10.13) were similar to those of amino acids. However, at acidic pH, pKa values of protein-like components, which ranged from 3.33 to 4.22, were 1-2units greater than those of amino acids. Results presented here, will benefit understanding of environmental behaviors of FA, as well as interactions of FA with environmental contaminants.