Morphological, chemical surface and electrical characterizations of lignosulfonate-modified membranes

Asymmetric sulfonated-polysulfone/polyamide membranes were modified by incorporating different amounts of lignosulfonate (LS) into the network (5, 10, 20 and 40 wt.%) in order to increase membrane conductivity due to the presence of sulfonic acid groups in the LS structure. LS-modified membranes were geometrically characterized by scanning electron microscopy (SEM) micrographs, which show two different substructures: a dense top layer (around 2.7 μm thickness) and a thick porous support (thickness between 95 and 102 μm, and porosity ranging between 37 and 42%). Chemical top layer characterization was carried out by XPS analysis, which shows the complete covering of the PS/PA membrane by LS and it is correlated with the increase of LS in the casting solution. Electrical characterization was carried out by impedance spectroscopy with the membranes in contact with HCl solution at different concentrations by using equivalent circuits as models. These results allow the separate determination of the electrical contribution associated to the top layer and the porous support; modified membranes and top layers conductivities were calculated from electrical resistance and geometrical values previously determined from SEM analysis. According to these results, 10 wt.% of LS seems to be the more appropriated figure to increase both membrane and top layer conductivity (σm ∼ 0.06 (Ω m)−1 for 1 M HCl solution).