Characterisation of metal-complexing membranes prepared by the semi-interpenetrating polymer networks technique. Application to the removal of heavy metal ions from aqueous solutions

Abstract

Novel chelating membranes for heavy metal ions were prepared by the semi-interpenetrated polymer networks technique. The matrix is a crosslinked network of poly(vinyl alcohol) (PVA) that immobilises a commercial or synthetic chelating polymer (CP). The membranes were characterised for the effectiveness of the crosslinking by measurement of swelling ratio and infra-red spectrometry, thermal stability, exchange and sorption capacities. In this paper, the removal of Hg(II), Pb(II), Cd(II) and Cu(II) ions by the most versatile membrane, the PVA/poly(ethyleneimine) (PEI) membrane was studied. The effects of parameters such as temperature, water hardness, the presence of complexing chloride anions and of other metal cations were investigated. The dissolution of the PVA/PEI membrane in water was slow and limited to 5% after two months, showing the efficiency of the crosslinking process. The sorption isotherms obey the Langmuir model and show high retention capacities for Pb(II), Cd(II) and Cu(II) ions. The sorption capacity was greater for metal ions that hydrolyse easily. The maximum sorption capacities of the membrane were 0.729mmolg−1 for Pb(II), 0.692mmolg−1 for Cu(II) and 0.525mmolg−1 for Cd(II), whereas the theoretical and experimental exchange capacities were 9.30mmolg−1 and 2.78mmolg−1, showing that most internal complexing sites of the membrane were not accessible for sorption of ions. Sorption experiments using mixtures of 2, 3 or 4 metal ions showed the selectivity order: Hg(II)>Cu(II)>Pb(II)>Cd(II). The thermodynamical sorption parameters showed a large entropic effect. The retention ratios were remarkably insensitive to the presence of calcium or chloride ions, which suggest possible use for the purification of real wastewaters by filtration.