Nanosponges by the oxo-Michael polyaddition of cyclodextrins as sorbents of water pollutants: the o-toluidine case

Abstract

Hydrophilic cyclodextrin nanosponges were prepared by the oxo-Michael polyaddition in an aqueous solution at pH > 10 of α-, β-, and γ-cyclodextrin with 1,4-bisacryloylpiperazine or 2,2-bisacrylamidoacetic acid. These nanosponges and, for comparison purposes, their precursor cyclodextrins were tested as sorbents of o-toluidine, a carcinogenic wastewater contaminant, by monitoring the depletion of o-toluidine from a 10−4 M (10 ppm) aqueous solutions. To this aim, an innovative analytical procedure was used: The voltammetric peak currents of o-toluidine in linear sweep voltammetry experiments were registered using multi-walled carbon nanotubes-modified glassy carbon electrodes. The experimental sorption curves fitted a mono-exponential kinetic model, and the residual o-toluidine was 0.16 ppm, one order of magnitude lower than those of all other sorbents reported so far. The sorption capacities ranged from 88 to 199 µmol g−1 (10–21.3 mg g−1), equal to or higher than those of the parent cyclodextrins. All nanosponges were completely regenerated by extracting with methanol. After regeneration, the sorption capacity slightly improved, suggesting a rearrangement of the nanosponge network. Overall, it may be reasonably concluded that the cyclodextrin nanosponges reported in this paper warrant potential as o-toluidine exhaustive sorbents.