Investigation of eluted characteristics of fulvic acids using differential spectroscopy combined with Gaussian deconvolution and spectral indices.

The characteristics of subfractions of soil fulvic acid (FA3, FA5, FA7, FA9, and FA13) using stepwise elution from XAD-8 resin with pyrophosphate buffers were investigated by differential absorption spectroscopy (DAS) and differential fluorescence spectroscopy (DFS) combined with mathematical deconvolution and spectral indices. The log-transformed absorbance spectra (LTAS) exhibited three regions for both acidic-buffer-eluted subfractions (AESF) and neutral-buffer-eluted subfraction (NESF) and four regions for basic-buffer-eluted subfractions (BESF) according to the differences in spectral slopes. The DAS spectra of FA subfractions were closely fitted with seven Gaussian bands with maxima location at 199.66, 216.18 ± 1.50, 246.20 ± 3.85, 285.22 ± 7.26, 345.64 ± 5.30, 389.27, and 307.37 nm, respectively (R2 > 0.993). The content of salicylic-like and carboxyl groups in FA subfractions decreased, while the phenolic chromophore increased with elution sequence. From the 11 spectral indices, AESF had greater molecular weight, condensation, polymerization, hydroxyl radical production, humification degree, and terrigenous contribution, as well as contained more conjugated aromatic structures and less N-containing groups than NESF and BESF. The humification degree and humic characters of FA subfractions were closely associated to the aromaticity, molecular condensation, and DOM-metal-bound functional groups. The proper separation of FA into subfractions is beneficial for reducing its complexity and heterogeneity, which helps us to further explore its chemical properties and interactions with various contaminants in soil environments. Graphical abstract.