The Effect of Natural Chitosan, Humate and Their Complexes on the Rheological Properties of Wet Kaolinite

Control of mechanical properties of natural clays and clayey soils under saturation with water is an important task in agriculture and geoengineering. In this study, kaolinite was modified with cationic chitosan, anionic potassium humate (PHum), and non-stoichiometric chitosan–PHum interpolyelectrolyte complexes (IPCs). The optimal composition of IPCs in terms of the ratio between positively charged chitosan groups and negatively charged potassium humate groups, Q = [NH3+]/[COO−], was determined using laser microelectrophoresis, turbidimetry, and dynamic light scattering and showed Q ≤ 0.3 for anionic IPCs and Q ≥ 3 for cationic IPCs. Amplitude tests showed that introduction of minor amounts of chitosan, not more than 0.5 wt.%, to kaolinite resulted in increased strength characteristics of wet clay by 0.5–1.5 orders of magnitude, which was due to the formation of the cooperative physical network via electrostatic interaction between positively charged chitosan and negatively charged clay particles. Binding of the same amounts of anionic PHum to kaolinite decreased the strength parameters by 0.5–1.0 orders of magnitude because of increased sliding of the clay particles. Addition of 0.25 wt.% of both cationic and anionic IPCs to kaolinite allowed the preservation of the reinforcing effect of chitosan and suppression of the weakening action of PHum. Such behavior of chitosan–PHum IPCs was likely due to redistribution of cationic chitosan between anionic PHum and kaolinite and formation of the kaolinite–chitosan–PHum ternary complex, which minimizes the destabilizing effect of PHum and improves the mechanical properties of the clay.