Structural characterization of polymer-based flocculation of humic acid as a tool for optimizing water treatment

The complexation/flocculation induced by cationic polyelectrolytes (cPE) is regarded as an effective method in conventional water treatment process for addressing negatively charged humic acid (HA), which exists ubiquitously in aquatic environments as a degradation product of plants and negatively affects the quality of drinking water. The interaction between oppositely charged HA and cPE can be regarded as the formation of interpolyelectrolyte complexes (IPEC), where HA serves as the polyanion with high rigidity and polydispersity. IPEC of cationic polydiallyldimethylammonium chloride (PDADMAC) and HA was first studied with a focus on the Ca2+ effects. This was accomplished by determining the phase behavior and characterizing the structures in solution through the use of light scattering and small-angle neutron scattering (SANS). This allowed for the probing of the formation of aggregates with a radius of 120–150 nm and a compaction in the size range of 10−50 nm within these aggregates with the addition of Ca2+. The presence of Ca2+ was found to exert a considerable influence on both the macroscopic phase behavior and the mesoscopic complex structure. Chitosan, a natural polymer derived from waste materials, has been identified as a promising candidate for altering synthetic cationic polyelectrolyte (cPEs) to develop an environmentally friendly flocculant. A library of different modified cationic (quaternized) chitosans (QCSs) with permanent charges was synthesized by the substitution of glycidyl trimethylammonium chloride (GTMAC). A comparison of the HA removal efficiency of QCS and the commercial synthetic PDADMAC revealed that with QCS the precipitation of HA was enhanced and occurred at a lower charge mixing ratio due to the intrinsic hydrophobic domain of QCS. By tailoring the degree of quaternization of QCS, a shift in the phase diagram was observed, which correlated with the aggregate structure. Moreover, the kinetic process was obtained specifically for the initial stage, during which the HA complexes are immediately formed upon mixing. This process may ultimately determine the final flocculation performance and the removal efficiency of HA in water treatment. Time-resolved small-angle X-ray scattering (TR-SAXS) combined with stopped-flow was employed to probe the early-stage complex formation over a timescale of milliseconds. This approach facilitated the elucidation of the rate and efficacy with which HA can interact with diverse cationic polyelectrolytes, including quaternized chitosan with varying degrees of charged groups and PDADMAC, which is 100% charged. By systematically varying the type of cationic polyelectrolyte, the charge ratio between cPEs and humic acid, and additionally by adjusting the pH value, comprehensive insight into the initial formation process and factors controlling the final structures was obtained. This insight from colloid science is promising to optimize the water treatment process in industrial settings.