Hybrid organic-inorganic nanotubes effectively adsorb some organic pollutants in aqueous phase

Methylimogolite nanotubes (Me-IMO NTs, chemical composition (OH)3Al2O3SiCH3) are characterized by having an inner hydrophobic (fully-methylated) surface. Conversely, the outer surface is hydrophilic and positively charged below the point of zero charge (at pH = 8.6), due to the Al(OH)Al + H+ = Al(OH)2+Al equilibrium occurring at the outer surface of the NTs when they are dispersed in water. In this work, adsorption of MCPA (2-methyl-4-chlorophenoxyacetic acid) and of dichloromethane (DCM) on Me-IMO NTs was studied by means of both adsorption/desorption measurements in batch conditions (at room temperature and at different pH value) and Small Angle X-rays Scattering (SAXS). MCPA, which partially dissociates in water (pKa = 2.90), is mainly adsorbed at the outer surface of the NTs, although measurements in batch conditions show that other types of interaction occur (i.e. H-bond) at higher MCPA concentration. SAXS measurements confirm that MCPA has not entered the NTs inner pores. Interaction of the NTs with DCM, instead, is more complex: SAXS measurements show that DCM molecules interact with the NTs inner surface, whereas batch experiments indicate that additional interactions take place with the outer surface of the NTs.