Synthesis of multi-stimuli-responsive flocculants using reversible addition-fragmentation chain transfer polymerization: Investigating the performance and mechanism of kaolin coagulation

Coumarin-containing amphiphilic block copolymers with different compositions, as a new class of pH- and photo-responsive flocculants, were synthesized through reversible addition-fragmentation chain transfer polymerization of dimethyl aminoethyl methacrylate (DMAEMA), methyl methacrylate, and 7-acryloyloxy 4-methylcoumarin. Proton nuclear magnetic resonance spectroscopy was used to study the structure and composition of the copolymers. Zeta potential results show that the copolymers are applicable as amphoteric flocculants. The optimal flocculant concentration was calculated using the Jar analysis for kaolin flocculation, and the results show that the flocculation dependence on the flocculent concentration decreases with increasing its molecular weight. The dominance of charge neutralization in the flocculants with lower molecular weights and the dominance of adsorption bridging and hydrophobic association in the flocculants with higher molecular weights were indicated. These results are in agreement with the rheological data. To investigate the governing mechanism more precisely, the flocculation rate was calculated using turbidity measurements at different pH values over time. The highest final flocculation rate (99.25 %) corresponds to pH 4.5, which caused by the protonation of the amine groups of the DMAEMA repeating units. The dependence of settling rate on pH indicates the pH-dependent performance of the flocculants. Finally, fluorescence microscopy image of the flocculants shows its proper functioning of in the vicinity of kaolin, which has a promising industrial application.