A Comparison Between Alkaline and Decomplexing Reagents to Extract Humic Acids from Low Rank Coals

Humic acids (HAs) were obtained from two low rank coals (lignite and leonardite) by using either alkali extractants (0.1 M NaOH, 0.1 M KOH or 0.25 M KOH) or solutions containing Na4P2O7 (0.1 M Na4P2O7 or 0.1 M NaOH/Na4P2O7). In both coals, the greatest yields were obtained with 0.25 M KOH and the lowest with the 0.1 M alkalis, whereas the extractants based on Na4P2O7 yielded intermediate values and were more effective on the lignite.Chemical analysis showed that the leonardite HAs consisted of molecules that were less oxidized and had fewer functional groups than the HAs released from the lignite. Moreover, the HAs extracted by reagents containing Na4P2O7 exhibited more functional groups than those extracted with alkali, this effect being more apparent in lignite because of its greater cation exchange capacity. Gel permeation chromatography indicated that the leonardite HAs contained a greater proportion of higher molecular size compounds than the lignite HAs, and that both solutions containing Na4P2O7 released HAs with a greater proportion of smaller molecular compounds from the lignite than did the alkali extractants.