Arsenate, orthophosphate, sulfate, and nitrate sorption equilibria and kinetics for halloysite and kaolinites with an induced positive charge

Abstract

Mineral-based sorbents, such as raw and modified clay minerals and zeolites, are widely used in pollution control. Sorbents capable of immobilizing anionic pollutants are rare and usually based on hydrotalcite-like minerals. Grafted kaolinite derivatives have been shown to effectively remove aqueous Cr(VI). Therefore, the sorption equilibrium and kinetics of arsenate (H₂AsO₄⁻, HAsO₄²⁻), orthophosphate (H₂PO₄⁻, HPO₄²⁻), sulfate (SO₄²⁻), and nitrate (NO₃⁻) were investigated in this study. Triethanolamine was grafted in the interlayer space of well-ordered kaolinite, poorly-ordered kaolinite, and halloysite and the amine group was subsequently quaternized using iodomethane. The formed organic iodide controlled the interlayer gallery height and the mobile iodide ions could be ion-exchanged. Arsenate, orthophosphate, sulfate, and nitrate adsorption capacities were significantly improved, particularly for the well-ordered kaolinite. This was due to its higher reactivity in modification processes and subsequently higher content of grafted molecules. The calculated Dubinin–Radushkevich adsorption energies suggested that ion-exchange dominated for all anions. Higher pH values affected the anion species and increased OH⁻ competition which resulted in decreased sorption. The sorption isotherms and kinetics were most accurately modeled using Langmuir and pseudo-second order equations, respectively. The investigations including Weber–Morris and Boyd kinetic models helped to identify the sorption rate limiting step which was external mass transfer or intra-particle diffusion. The latter was connected with a fraction of fine particles (∼0.3 μm) and micropores (<2 nm).