Surfactant-assisted synthesis of conducting polymers. Application to the removal of nitrates from water

Three different conducting polymers, polythiophene (PT), polypirrol (PPY) and polyaniline (PANI) have been synthesized via oxidative chemical polymerization in aqueous media, in such a way that the synthesis protocol did not involve any toxic solvents. They have been tested in the abatement of nitrates from an aqueous solution without the need of any metal catalyst. The N-containing polymers (PANI and PPy) were able to remove nitrates to a level that accomplishes the European legislation requirements; however, the nature of each polymer greatly influenced the process mechanism. Whereas ion exchange between Cl^- and SO₄^2- counter-ions in the polymer and NO₃^- from water is the main responsible for the effective nitrate removal in PANI, as assessed by FTIR and XPS analyses, the nitrate removal mechanism on PPy is based in an electron transfer from the polymer to nitrate through N sites located in the pyrrolic ring. On the other hand, PT was not able to exchange nitrate unless it was synthesized with FeCl₃ as oxidant/dopant and an anionic surfactant (sodium dodecyl sulfate -SDS-) is used. In that case, the electrostatic attraction between sulfate (OSO₃^-) groups from the surfactant and Fe³⁺ ions from FeCl₃ produced the anchoring of Cl^- to the oxidized PT growing chain, this favoring ion exchange with nitrate in the aqueous solution, followed by a redox process.