Enhanced heavy metal adsorption on functionalized nanotubular halloysite interlayer grafted with aminoalcohols

Nanotubular halloysite mineral with a 1:1 layered structure was interlayer grafted using diethanolamine and triethanolamine. The tailored materials which were stable in aqueous solutions had a constant interlayer gap equal to 3.1 or 3.6 إ, respectively, and showed no swelling properties. The modification significantly improved the halloysite adsorption capacity with respect to Pb(II), Cd(II), Zn(II), and Cu(II). This was due to a two-step gradual diffusion of the metals into the interlayer space and their subsequent binding by amine nitrogen of the grafted aminoalcohol. In particular, the Cu(II) ions were preferentially removed from solution as they readily form complexes with N-donor ligands. The observed competitive protons adsorption on the amine nitrogen increased the pH of solution in contrast to system with raw halloysite where the pH decreased. The proton release in the latter system was due to domination of an ion-exchange and surface complexation removal mechanisms of the cations on the particles surface. The adsorption efficiency was related to the metal hydrolysis constants and in turn the size of the ions was less relevant. The adsorption isotherms and kinetic data in all cases followed the Langmuir and pseudo-second order equations, respectively. The applied desorption procedure illustrated a complex binding of the metals with the raw and modified mineral and showed that the materials cannot be completely recycled.