Abstract
Humic acids (HA) extracted from two sewage sludges (one aerobic and the other anaerobic) were characterized by elemental and functional group analyses, visible spectroscopy and coagulation threshold. A comparative study between these HA and HA extracted from leonardite (an oxidized form of lignite coal) was made. Sludge HA, as compared to leonardite HA, had a lower C/H, O/H and C/N ratio, increased N and H content, decreased total acidity, carboxyl and phenolic hydroxyl content and a lower absorptivity. The sludge HA appeared to have more aliphatic, N-rich polymers than the leonardite HA. The type of digestion process (aerobic or anaerobic) underwent by the sludge did not influence to any great extent the functional group content of sludge HA. As compared to the leonardite HA, sludge HA were more resistant to flocculation by the three electrolytes (Ca 2+, Mn 2+, Zn 2+) used in the coagulation-threshold study. Zinc was the most effective electrolyte in precipitating the humic acids.
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