Role of the surface composition of the polyethersulfone–TiiP–H2T4 fibers on lead removal: from electrostatic to coordinative binding

The removal of toxic metal ions from water remains an environmental challenge. Promising processes involve formation of metal complexes with appropriate ligands and their successive separation using filtration over membranes. Water-soluble macromolecular ligands are, therefore, important to improve this technology. In this investigation, we have shown that insertion of a Ti-acac-based precursor on non-water-soluble thermoplastic polyethersulfone membranes allows an effective immobilization of the meso-tetra(N-methyl-4-pyridyl)porphine which, in turn, can safely remove Pb2+ ions from water. In particular, it has been demonstrated that the porphyrin readily coordinates Pb2+ thus forming a sitting-atop metal derivative. Moreover, X-ray photoelectron spectroscopy has favored a complete characterization of surfaces of fiber membranes, thus providing evidences of Pb2+ coordination. The efficiency of Pb2+ removal from water has been discussed on the basis of competitive porphyrin deprotonation versus metal complex stability in water, and these data shed light on the role of porphyrin concentration on the fiber surface that can switch the metal absorption from a pure electrostatic interaction to coordinative complexation.