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

•The sorption of As(V), P(V), S(VI), N(V) on modified kaolinites was investigated.•A significant improvement of sorption was observed as compared to raw kaolinites.•The sorption involved an ion-exchange mechanism: anion →→ iodide in the interlayer.•The sorption was best described by Langmuir and pseudo-second order equations.•The kinetic role of external transport and intra-particle diffusion was examined.

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).

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