Cyclodextrin nanosponges as o-toluidine sorbents

Hydrophilic cyclodextrin (CD) nanosponges in which the glucopyranoside macrocycles were covalently linked by connecting arms deriving from 1,4-bisacryloyilpiperazine or 2,2-bisacrylamidoacetic acid (BAC) were synthesized from a-, b- and g-cyclodextrins by eco-friendly procedures and tested as sorbents of o-toluidine, a carcinogenic wastewater contaminant [1]. The sorption ability of CD nanosponges and that of parent a-, b-, and g-CDs was determined by monitoring the depletion of o-toluidine from 10-4 M (10 ppm) aqueous solutions in batch reactors under mild magnetic stirring. The absorption kinetics were registered by monitoring the decrease of the o-toluidine voltammetric peak currents in linear sweep voltammetry experiments using multi-walled carbon nanotubes-modified glassy carbon electrodes [2,3]. The experimental curves fitted a mono-exponential kinetic model, indicating a good homogeneity of the sorbent materials. In all cases, the analytical technique revealed o-toluidine abatement down to 0.16 ppm, lower by one order of magnitude than the figures from all other sorbents so far reported [4]. Under the adopted conditions, that corresponded to those usually found on the field in o-toluidine polluted aquifers, the absorption capacities ranged from 88 to 199 µmol g-1 (10-21.3 mg g-1), mostly depending on the type and content of the cyclodextrin moieties and, to a minor extent, to the type of connecting arms. The observed capacities in terms of µmol o-toluidine/µmol cyclodextrin were very close to those of parent CD in case of BAC-g-CD, higher of a 1.3-1.6:1 factor for b-CDand a 3.2:1 factor for a-CD-based nanosponges, indicating the presence, in these nanosponges, of cooperative effects among adjacent CD units in the complexation of o-toluidine. All nanosponges were completely regenerated by extracting with methanol. After regeneration, the absorption capacity slightly improved, suggesting a rearrangement of the nanosponge network. Overall, it may be reasonably concluded that the CD nanosponges reported in this paper warrant potential as o-toluidine exhaustive sorbents.