Fast tracking the molecular weight changes of humic substances in coagulation/flocculation processes via fluorescence...

The removal of a commercial humic acid (HA) and changes in its chemical composition were examined for coagulation/flocculation (C/F) processes based on jar tests using two different coagulants at a wide range of pH. ZrCl₄ showed a better performance in eliminating HA than Al₂SO₄ with the same removal rates at lower dosages. The highest removal rates were found at a neutral pH range (5.0-6.5). The HA was further decomposed into three different humic-like components (C1, C2, and C3) by excitation emission matrix coupled with parallel factor analysis (EEM-PARAFAC). Although the removal rates of all three components generally followed those of dissolved organic carbon, the relative removals of the individual components depended on the coagulant's doses and the solution pH. The fluorescent components of five ultrafiltered size fractions of the HA revealed that the peak with a longer emission wavelength could be associated with larger sized molecules. The C1/C3 ratios of the size fractions exhibited a significant linear relationship with the logarithmic values of the average molecular weight (MW) measured by size exclusion chromatography, which made it possible to predict the HA MW value changes upon the C/F using EEM-PARAFAC alone. Irrespective of the coagulant types and the pH, larger sized HA molecules were removed to a greater extent than smaller sized fractions. The preferential removal was more pronounced for ZrCl₄ versus Al₂SO₄ and at a neutral pH range. Our study suggests a great potential of EEM-PARAFAC in fast tracking the MW of humic substances in conventional C/F processes.