Colloidal Chemistry in Water Treatment: The Effect of Ca2+ on the Interaction between Humic Acid and Poly(diallyldimethylammonium chloride) (PDADMAC)

The complexation of humic acid (HA), as a major component of natural organic matter (NOM) in raw water, with polycations is a key step in the water treatment process. At sufficiently high addition of a polycation, it leads to neutralization of the formed complexes and precipitation. In this work, we studied the effect of the presence of Ca2+ ions on this process, with poly(diallyldimethylammonium chloride) (PDADMAC) as a polycation. This was done by determining the phase behavior and characterizing the structures in solution by light scattering and small-angle neutron scattering (SANS). We observe that with increasing Ca2+ concentration, the phase boundaries of the precipitation region shift to a lower PDADMAC concentration, which coincides well with a shift of the ζ-potential of the aggregates in solution. Light scattering shows the formation of aggregates of a 120–150 nm radius, and SANS shows that Ca2+ addition promotes a compaction in the size range of 10–50 nm within these aggregates. This agrees well with the observation of more densely packed precipitates by confocal microscopy in the presence of Ca2+. Following the precipitation kinetics by turbidimetry shows a marked speeding up of the process already in the presence of rather small Ca2+ concentrations of 1 mg/L. It can be stated that the presence of Ca2+ during the complexation process of HA with a polycation has a marked effect on phase behavior and precipitation kinetics of the formed aggregates. In general, the presence of Ca2+ facilitates the process largely already at rather low concentrations, and this appears to be linked to a compaction of the formed structures in the mesoscopic size range of about 10–50 nm. These findings should be of significant importance for tailoring the flocculation process in water treatment, which is a highly important process in delivering drinking water of sufficient quality to humans.