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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