Efficient Self-Purification of Anionic Surfactants in Bayer Liquor Through a Dense-Ordered Interfacial Structure Generated by Trace Amounts of Cationic Surfactants

The imperative for eco-friendly and cost-efficient removal of anionic organics (surfactants) from Bayer liquor has persisted for decades. This paper proposes a novel and simple self-purification of anionic organics in Bayer liquor through trace amounts of cationic surfactants, synergistically integrated with the security filtration process in the Bayer process. Adding trace amounts of cationic surfactants (≤ 10 μmol·L−1) effectively compressed the cross-sectional of organics, generated the formation of ion pairs (complexes), reduced the critical micelle concentration of organics, and transformed the adsorption layer of organics from loose to dense. In addition, cationic surfactants changed the zeta potential from negative to positive in the electrical double layer of tricalcium aluminate hexahydrate (TCA) through strong physisorption. The physisorption also enhanced the surface dispersion force of TCA. Long- and double-tailed cationic surfactants were more effective in the regulation of molecular interactions than short- and single-tailed cationic surfactants. Ultimately, owing to the reduction of the repulsion among polar groups, the enhanced interaction among hydrophobic groups, as well as the reduced adsorption of OH− and Al(OH)4−, a dense-ordered interfacial structure of organics was subtly generated by trace amounts of cationic surfactants, contributing to the great adsorption capacity of 69.44 mg organic carbon/g. The simple and green self-purification of anionic organics in Bayer liquor is conducive to the sustainable development of the global alumina industry.