Application of Quaternized Chitosan in Enhancing Natural Organic Matter (NOM) Removal from Water by Flocculation

Humic acid (HA), which is a ubiquitous natural organic compound in surface waters to be removed during water treatment, can be removed by complexation with an oppositely charged polyelectrolyte. This process is typically done with commercial polycations such as poly(diallyldimethylammonium chloride) (PDAD) but should equally be possible with bioderived polymers like quaternized chitosan (QCS), easily obtained with different degrees of quaternization by reaction with glycidyl trimethylammonium chloride (GTMAC). A comprehensive phase study at different pHs was conducted on solutions of HA and QCS of varying degrees of quaternization, which was compared to the behavior with PDAD. Complexation was monitored by ζ-potential measurements, HA flocculation, and floc growth by laser light diffraction, providing a deeper understanding of the flocculation mechanisms. The composition of the biphasic regions was determined by total organic carbon (TOC) measurements. Our findings reveal that the structural properties of polycations significantly influence the flocculation efficiency and that the charge density on the QCS and hydrophobic interactions play an important role in that process, as they shift the phase behavior as well as the kinetics of flocculation. In general, QCS performs similarly well in HA precipitation as PDAD and, in addition, its properties can become optimized by the degree of quaternization, where an intermediate value optimizes the properties in terms of HA removal efficiency and flocculation kinetics.