Organosilicon quaternary ammonium compound-modified kaolin: A novel approach for the preparation of an algal bloom eliminator with excellent stability and high algae removal efficiency

Quaternary ammonium compound-modified clay (QAC-MC) has the advantages of a low dose and high efficiency in terms of algae removal and has high application potential for controlling harmful algal blooms (HABs) in various scenarios. However, the poor stability and secondary pollution risks of conventional QAC-MC limit its widespread adoption. In this study, molecular simulations revealed that physical adsorption of QACs containing a single quaternary ammonium active centre (-N⁺R₃) on clay surfaces led to poor stability. Therefore, a novel approach for enhancing QAC-MC stability by targeted augmentation of Si-OH active sites on clay surfaces and the introduction of a secondary active centre—siloxane groups (-Si(OR)₃)—into QAC molecules was proposed. In this approach, kaolin served as the substrate and was sequentially treated with tetraethyl orthosilicate and trimethyl [3-(trimethoxysilyl) propyl] ammonium chloride (SiQAC), ultimately yielding the novel SiQAC-MC. The results indicated that this novel composite not only significantly improved stability but also forced the charged active -N⁺R₃ groups to protrude outwards from the composite surface, which enabled efficient capture of algal cells. Compared with conventional QAC-MC, the novel SiQAC-MC exhibited greater binding strength and surface positive charge, with the binding energy changing from -0.28 eV to -3.71 eV and the surface potential increasing from -3.24 mV to +8.47 mV. As a result, the algae removal efficiency for Heterosigma akashiwo increased from 38% to 97% at 0.1 g/L. Overall, SiQAC-MC prepared by the new method showed high stability and excellent algae removal, providing theoretical and technical support for developing safe and effective HAB control agents.