Colloids and Surfaces A: Physicochemical and Engineering Aspects | 2021
Influence of reduction on the fluorescent units and proton binding of humic acids: Synchronous fluorescence spectrum and NICA-Donnan modeling
Abstract
Abstract The effect of reduction on the fluorescent units and proton binding behaviors of two humic acids (HAs) (PPHA and JGHA) was investigated by synchronous fluorescence spectrum (SFS) combined with NICA-Donnan modeling. With variations in pH and reduction time, the fluorescence quenching of the humic-like fraction was much stronger than that of fulvic-like fraction, indicating a greater variation of chemical structure in humic-like fractions. The NICA-Donnan model could satisfactorily describe proton binding, and the optimal NICA-Donnan model parameters indicated that the site density of phenolic groups on reduced HAs increased by 0.95 and 0.72\xa0mmol/g for PPHA and JGHA, respectively. These results were in good agreement with the transferred electrons capacities during the electrochemical reduction (0.90 and 0.76\xa0mmol/gHA for PPHA and JGHA, respectively). A linear negative correlation was established between the peak area of SFS after the peak fitting and the site density of protonated functional groups obtained from the NICA-Donnan model calculation under acidic and neutral conditions. It was then applied to the estimation of the proton binding properties of HAs during the electrochemical reduction. The results showed that reduction had a relatively weak effect on the proton dissociation of carboxylic groups, while the site density of protonated phenolic groups increased significantly with increasing reduction time.