The complexation induced by cationic polyelectrolyte (cPE) is considered to be an effective method in conventional water treatment process to deal with negatively charged humic acid (HA), which exists ubiquitously in aquatic environments and negatively affects the quality of drinking water. Accordingly, in order to optimize this process in a rational way, it is essential to understand the interaction between HA and cPE in fundamental physico-chemical terms.

In this study, a series of modified cationic (quaternized) chitosan (qCs) that derived from natural waste are utilized as cPE to develop an environmentally friendly solution. The colloidal structure of the formed HA-qCs complexes are probed by synchrotron-based small-angle X-ray scattering (SAXS) as a function of mixing ratio and pH. In addition, by combining with stopped-flow devices, the kinetics of early-stage complex formation in a timescale of ms can be probed. It was observed that by tailoring the degree of quaternization of qCs, a shift of the phase diagram revealed which correlates with the aggregate structure, which in turn affects their larger scale organization and propensity for flocculation. This insight from colloid science is promising to optimize the water treatment process in industrial fields.