Characteristics of molecular size fractions of humic substances derived from oxidized coal

Humic substances extracted from oxidized coal were fractionated by ultrafiltration in a MW sequence ranging from 500 to 50OK. Ultrafiltered fractions were analyzed by HPSEC and IR. Results showed that ultrafiltration can efficently fractionate an heterogeneous polydisperse humic acid (HA) into more homogeneous molecular size fractions. HPSEC provides a rapid and precise means of evaluating nominal molecular sizes of HAs. As for natural HAs, coal-derived products were found to show molecular sizes depending on both ionic strength of the medium and their properties as polyelectrolytes. Fractions of lower size range present more than one chromatographic peak whereas the higher size range fractions revealed mainly a single broad intense peak. Compositional data and IR spectra suggest that simple S and hydroxyl-rich aromatic and aliphatic compounds concentrate in the lower size fractions. Conversely, fractions of higher size range appear more complex and composed of more highly condensed structures as it is also inferred by the high C/H ratio.