One-pot synthesis of siliceous ferrihydrite - coated halloysite nanorods in alkaline medium: Structure, properties and cadmium adsorption performance.

The application of amorphous ferrihydrite (Fh) for Cd(II) removal is restricted by its unstable and easily transformable nature. Although doping with silicates stabilized ferrihydrite, its product siliceous ferrihydrite (SiFh) again suffered from the disadvantage of spontaneous agglomeration. Herein, ferrihydrite was hybridized with halloysite nanotubes (HNTs) to prepare a novel siliceous ferrihydrite - coated halloysite nanorods (SiFh@HNTs) in alkaline medium, to break through the current barriers. The characterization results showed that SiFh@HNTs could simultaneously overcome the defects of easy phase transformation of ferrihydrite and easy aggregation of SiFh nanoparticles (NPs). Meanwhile, the optimal SiFh@HNT ₄₀ with halloysite content of 40 % formed a well-developed mesoporous structure and exhibited the desired surface properties: a high specific surface area of 303.4 m ² /g, an isoelectric point as low as pH _iep  = 4.5, and rich functional Fe - OH groups. The formation mechanism of such excellent sturcture-properties of SiFh@HNT ₄₀ were mainly attributed to two factors: the generation of smaller (∼5 nm) SiFh NPs induced by the integration of halloysite-derived SiO ₄ ^4- into ferrihydrite, and the dispersion of SiFh NPs on clay nanotubes. Furthermore, the adsorption capacity of SiFh@HNT ₄₀ for Cd(II) was up to 137.8 mg/g at 30 °C and pH 6, which was much higher than that of aggregated ferrihydrite (11.2 mg/g), halloysite (18.8 mg/g) and goethite (49.4 mg/g). The adsorption thermodynamics study revealed the adsorption of Cd(II) on SiFh@HNT ₄₀ was clearly chemisorption with a (ΔH _ads ) _q of 43.3 kJ/mol. Characterization results of XPS and FTIR confirmed that the rich Fe - OH groups on SiFh@HNT ₄₀ was the main adsorption sites, and Cd(II) was specifically adsorbed by inner-sphere surface complexation. In addition, SiFh@HNT ₄₀ had application potential in the mixed-metal wastewaters treatment. Cyclic regeneration experiments showed that SiFh@HNT ₄₀ had good regeneration performance and could be reused many times.
Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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