DEPOSITION KINETICS OF HUMIC MATTER-COATED HEMATITE IN POROUS-MEDIA IN THE PRESENCE OF CA2+

Column filtration experiments were conducted to determine the deposition kinetics of the humic matter-coated hematite (alpha-Fe2O3) colloids on quartz grains in the presence of Ca2+. Three well-characterized humic substances were used. Our results indicate that several mechanisms are responsible for the differences in the attachment efficiencies of these particles. The relative importance of each mechanism varies depending upon the type of humic matter used. Systems coated with the peat humic acid, which is the least acidic, the most hydrophobic and has the highest molecular size of all the samples used here, had the highest stability at approximately [Ca2+] < 0.002 M, despite their lower electrophoretic mobilities. This behavior may be attributed to the greater electrosteric interactions brought about by the adsorbed humic acid. At higher Ca2+ concentrations, however, the stability of the peat humic acid-coated systems was drastically reduced owing to the effect of cation bridging and/or hydrophobic interactions. Our experimental set up does not allow unequivocal determination of the underlying cause of the observed differences between the attachment efficiencies of the particles coated with the two fulvic acids used here. Such differences may be controlled by electrostatic, hydrophobic or electrosteric interactions, or a complex combination of these.