In this paper, new information on physical and chemical properties of the widely used nanostructured Halloysite mineral are reported. Given that the Halloysite has a tubular structure formed by a variable number of wrapped layers containing Si-OH and Al-OH groups, their proton binding affinity was measured at different ionic strengths and ionic media by means of potentiometric measurements in heterogeneous phase. One protonation constant for the Si-OH groups and two for the Al-OH groups were determined. The protonation constant values increase with increasing of the ionic strength in all the ionic media. This suggests that the presence of a background electrolyte stabilizes the protonated species through the formation of weak complexes between ions of the supporting electrolytes and the protonated species. Ten weak species were determined with different stoichiometry. It was shown that the interactions do not depend on the nature of the supporting electrolytes but on the charge. The surface charge of Halloysite was estimated by ζ potential measurements as a function of pH, and the values obtained are consistent with the nanotubes ionization predicted by using the protonation constants for the Si-OH and Al-OH groups. The total solubility of the Halloysite nanotubes, was also determined in NaCl aqueous solution. These measurements showed that the solubility slightly increases with increasing ionic strength and contact time between Halloysite and NaCl solution. Goodness-of-fit (GOF) criteria were used to test the application of these models with good results. The obtained results confirm that the behavior of Halloysite in water is strictly correlated to the experimental conditions of the aqueous suspension (e.g., pH, ionic strength, and ionic media). The thermodynamic data here reported are of main importance in the several applications where is exploited the charge separation between the inner and outer surfaces of this nanotubular material

In this study, environmental friendly halloysite nanotubes and their amino derivatives were used as adsorbent materials for lead(II) ions. The adsorption ability of both nanomaterials towards Pb 2+ ions has been studied in NaCl aq , at I = 0.1 mol L -1 , in the pH range 3 -6. Moreover, the effect of ionic strength on the adsorption process was evaluated at the pH of maximum efficiency of the adsorbent materials. Kinetic and equilibrium experiments were carried out by using the Differential Pulse Anodic Stripping Voltammetry (DP-ASV) technique to check the metal ion concentration in solution after contact with the two adsorbents. Different isotherm and kinetic equations were used to fit the experimental data. The speciation of metal ion and the characterization of the adsorbents with different techniques were considered in order to establish the suitable experimental conditions for the metal ion removal. The collected data showed that the functionalization of halloysite enhances the adsorption ability of the clay mineral and it makes the nanoclay a good candidate for metal removal from aqueous solutions.

Halloysite nanotubes were functionalized with bis-vinyl imidazolium salts and PdNPs to obtain an efficient catalyst for fine chemical synthesis.

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