Experimental and modeling study of proton and copper binding properties onto fulvic acid fractions using spectroscopic techniques combined with two-dimensional correlation analysis

Fulvic acid (FA) significantly influences the bioavailability and fate of heavy metals in environments, while its acid-base characters and metal binding processes are still unclear. Here, spectroscopic techniques combined with multiple models (e.g., NICA-Donnan model) and two-dimensional correlation spectroscopy (2D COS) were applied to explore the proton and copper binding properties of FA subfractions (FA(3)-FA(13)). The charge densities, average contents of carboxylic and phenolic groups, average dissociation constants pKa(1) and pKa(2) of sub-fractions ranged 0-16 meq . g . C-1, 5.03-9.58 meq . g . C-1, 2.52-4.67 meq . g . C-1, 4.15-4.33 and 8.52-9.72, respectively. FA sub-fractions had a relatively narrow distribution of carboxyl group and a broad distribution of phenolic group. FA sub-fractions also exhibited roughly two phenolic hydroxyl groups per every 1-3 phenyl rings. Differential absorbance spectra (DAS) derived Gaussian bands were associated to the inter-chromophore interactions, the changes of molecular conformations and functional groups with copper addition. Differential spectra slopes (DSlope(275-295&325-375)) were more significant with higher copper concentration and copper amounts bonded to carboxylic groups. UV-Vis and fluorescence spectra with 2D heterospectral COS revealed the copper binding heterogeneities and sequential orders of chromophores and fluorophores, quantitatively confirming by the order of conditional stability constants (log K-Cu: 4.64-5.56). Salicylic-/polyhydroxyphenolic, hydroxyl and amino groups were strongly associated to the basic units for fluorophores. Sequential changes followed the order of humic-like ->, fulvic-like materials for FA(3)/FA(5), humic-like -> fulvic-like -> tryptophan-like materials for FA(7), and humic-like -> tryptophan-like -> fulvic-like -> tyrosine-like materials for FA(9)/FA(13). Spectroscopic techniques combined with various models (especially for 2D COS) are beneficial to elucidate the binding heterogeneity and sensitivity for metal-organic matters at the functional group level. (C) 2019 Elsevier Ltd. All rights reserved.