Adsorption Characteristics of WFD Heavy Metal Ions on New Biosourced Polyimide Films Determined by Electrochemical Impedance Spectroscopy

In this work, innovative biosourced polyimide polymers were derived from novel combinations of monomers (Isosorbide, 6-FDA and amine cardo). These polymers were obtained with satisfactory yields by polycondensation of different percentages of isosorbide and of 6-FDA (from 100% to 0%). The synthesized polyimides had number average molecular weight in the 17,600–22,000 g/mol range, and they were stable well above 451 °C (5% weight loss in N2). The glass transition temperature (Tg) were found to be in the range of 318–414 °C, depending on the composition of the copolymers. The adsorption of Water Framework Directive (WFD) heavy metals (Pb2+, Ni2+, Cd2+, Hg2+) on the biosourced polyimide films deposited on a gold electrode was monitored by the measurements of the impedance of the electrolyte/polyimide polymer/electrode interface. The adsorption isotherm of Pb2+on the different polyimide films was modeled according to a Langmuir isotherm with a best fitting. The relative adsorption capacities were higher when the isosorbide content was higher. Comparing the adsorption capacities of the different WFD heavy metals of the isosorbide based polyimide, the sequence is the following one: Pb2+ ≈ Ni2+ > Cd2+ > Hg2+. These results are within the paradigm of Pearson’s HSAB principle about the metal-oxygen interaction. When isosorbide based polyimide is the sensitive part of an impedimetric sensor, the detection limit for all the WFD heavy metals is 50 pM, very lower than the environmental quality standards for these WFD priority hazardous substances.