Study of polymer–metal ion–membrane interactions in liquid-phase polymer-based retention (LPR) by continuous diafiltration

In this work were studied polymer–metal ion–membrane interactions during continuous diafiltration of divalent metal ions (Co2+, Ni2+, Cu2+, Zn2+, Cd2+, and Pb2+) using poly(acrylic acid) (PAA) as water soluble polymer by liquid-phase polymer-based retention (LPR) technique at pH 6. Polyethersulfone (PES) membranes were used and characterized by different methods (solute rejection test, force atomic microscopy (AFM) and contact angle). Changes on fouled membrane roughness (22.91 ± 2.2) with respect to virgin membrane (7.05 ± 4.6) and changes on contact angle measures (76.0 ± 0.9 and 85.0 ± 2.34 with water) suggest an increasing in surface hydrophobicity of PES membrane associated mainly with deposited forming on the surface as results of enhanced in the polymer–membrane interaction throughout proceed filtration. From metal ion concentration data in the permeate throughout filtration experiment polymer–metal interactions were studied and analyzed by equations based on the combination of molar binding ratio and the consideration of chemical equilibrium between the polymer and metal ions. Membrane–metal ion interaction was around 10% for all cations studied, with the following order in the interactions: Ni2+ ≈ Zn2+ > Pb2+ > Co2+ > Cu2+ > Cd2+. A relatively high and specific interaction of the carboxylate groups of PAA with metal ions was observed. This interaction showed the order: Cu2+ > Cd2+ > Co2+ > Ni2+ ≈ Zn2+.