Extraction and characterization of humic acid from Pakistani lignite coals

This study deals with the extraction and characterization of humic acids from low rank coals of Thar, Chakwal, and Quetta. Three respective coal samples were initially oxidized with 2% HNO3 and then treated with 2.5, 3, and 3.5% KOH solutions for the extraction of humic acid. The 3.5% concentration of KOH was found to be the optimum for the extraction of humic acid. The extracted humic acid was investigated for its quantitative assessment through gravimetric and UV spectrophotometer procedures. The gravimetric method showed 24.6, 13.6, and 18.0% of humic acid extraction from Thar, Chakwal, and Quetta coals, respectively, whereas the humic acid determined by UV spectrophotometry was 29.67, 20.1, and 22.6% for the same sample sequence. FTIR analyses of extracted humic acid samples have shown the presence of carboxylic, phenols, alcoholic, and amines functional groups. The 13C NMR spectra of three humic acids have exhibited the broad diversity of aliphatic and aromatic compounds such as carboxylates, carbohydrates, ethylene, phenols, and amines. The results of the study could be useful for selection of suitable coals for humic acid extraction.