Wool-derived keratin nanofibre membranes for dynamic adsorption of heavy-metal ions from aqueous solutions

Membranes made of randomly oriented wool-derived keratin nanofibers (~240 nm mean diameter), were prepared by electrospinning process, and tested for the Cu(II), Ni(II), and Co(II) metal ion removal from aqueous solutions, through dynamic adsorption tests. The Cu(II) ion adsorption was studied from the isotherm and kinetic point of view, both in non-competitive and competitive conditions. As regards the non-competitive condition, the experimental data had a very good fit with both Langmuir and Freundlich isotherm models. The maximum adsorption capacity obtained from the Langmuir model was about 11 mg/g and the high correlation coefficient of the BET model indicates that the adsorption was a multilayer process. The mean free energy of adsorption, evaluated through the Dubinin-Radushkevich model, was 14.1 kJ/mol, indicating that the adsorption of Cu(II) ions on keratin nanofiber membranes occurred by ion exchange reactions. The process kinetics was evaluated by pseudo-first and pseudo-second order models, the latter showing the highest correlation with the experimental data. The competitive adsorption tests evidenced that the keratin nanofiber membranes maintained a good adsorption capacity for Cu(II) ions and also with the coexistence of Co(II) and Ni(II) metal cations. As regards the selectivity studies, the results showed that the adsorption of metal ions by keratin nanofiber membranes followed the order Cu(II) > Ni(II) > Co(II).