Characterization of pH-fractionated humic acids derived from Chinese weathered coal

To reduce the compositional and structural heterogeneity of humic acids (HAs) and achieve better use of HA resources, in this study, we report a new sequential dissolution method for HAs derived from Chinese weathered coal. This method was used to separate HAs into seven fractions by adjusting the pH (3-10) of the extraction solution. The results showed that the HA fractions derived from Chinese weathered coal were concentrated up to 90.31% in the lower pH solutions (3-7). The compositional and structural characteristics of the HA fractions were determined by elemental analysis; ultraviolet-visible (UV-Vis), Fourier transform infrared (FTIR), and solid-state ¹³C-nuclear magnetic resonance (NMR) spectroscopies; and other techniques. The results showed significant differences among the HA fractions. The concentrations of the total acidic groups and the carboxyl groups decreased with the increasing pH of the extraction solution. However, the HA fractions derived from extraction solutions with pH 3-4 had relatively lower aromaticity but a higher protonated carbon content. The HA fractions derived from extraction solutions with pH 6-7 had the highest aromaticity and the greatest abundance of COO/N-C=O. This study demonstrated that adjusting the pH of the extraction solution is one way to fractionate HAs from Chinese weathered coal and to obtain HA fractions with compositions and structures that could serve as useful material for study and utilization.