Improving Membrane Filtration for Copper Speciation: Optimal Salt Pretreatments of Polyethersulfone Membranes to Prevent Analyte Retention

[Image: see text] Membrane filtration has been increasingly used to separate dissolved metal ions from dispersed particles, commonly using ultrafiltration membranes, for example, polyethersulfone (PES) membranes with a molecular weight cut-off of 3 kDa. The disadvantage of this technique is an undesired retention of ions, resulting from Coulomb interactions with sulfonic acid groups of the membrane. Therefore, such a membrane acts similar to a cation exchanger column. We solved this drawback by a pretreatment of the PES membrane by other cations. Using CuSO(4) as a model compound, we compared the effectiveness of five cations using their salt solutions (Ca(2+), Mg(2+), Fe(2+), Ag(+), Ba(2+)) as pretreatment agents and identified the most effective pretreatment component for a high recovery of copper ions. After membrane filtration without pretreatment, only 52 ± 10%, 64 ± 5%, 75 ± 8%, and 89 ± 7% of nominal Cu concentrations were obtained using initial concentrations of 0.2, 0.5, 1.0, and 4.0 mg L(–1), respectively. The efficiency of the investigated cations increased in the order Fe < Ag < Mg < Ca < Ba. Furthermore, we analyzed the most efficient concentration of the pretreatment agent. The best performance was achieved using 0.1 mol L(–1) CaCl(2) which increased copper recovery to slightly below 100%, even at the lowest tested Cu concentration (recovery 93 ± 10% at 0.2 mg L(–1)). In the environmentally relevant Cu concentration range of 0.2 mg L(-1), 0.1 mol L(–1) BaCl(2) was identified as the most efficient pretreatment (103 ± 11%).