Adsorption of heavy metals on the surface of nanofiltration membranes: “A curse or blessing”?

Adsorptive membranes, formed by dispersing adsorbent particles in a the polymer matrix of ultrafiltration (UF) membranes, are among a number of promising technologies for removing heavy metal ions from water. They offer much higher water permeability compared to nanofiltration (NF) membranes. However, because of a limited adsorption capacity, adsorptive membranes require frequent regenerations to maintain their selective properties. In contrast, NF membranes rely on size/steric exclusion and electrostatic repulsion. Moreover, by incorporating adsorptive nanoparticles into, for example, the polyamide (PA) layer in thin film nanocomposite (TFN) membranes, NF membranes, in addition to size/steric exclusion and electrostatic repulsion, can also reject heavy metal ions by adsorption. Some TFN-NF membranes for processing feed solutions containing heavy metal ions require regeneration, implying a detrimental effect of heavy metal adsorption on their performance. Still, some do not, which leads to the question stated in the title of this article. In this study, we examined the role of heavy metal ions (Cu2+ and Pb2+) in feed solutions on their rejection and water permeability TFN membranes containing cellulose nanocrystals (CNCs), acetylated CNCs (ACNCs) and CysCNCs in the PA layer. We performed NF experiments of durations ranging from 1 to 16 h. At the end of each experiment, we measured the membranes' heavy metal adsorption, contact angle, and zeta potential and correlated them with time-dependent water permeability and heavy metal rejection. An increase in heavy metal adsorption with time was associated with a desirable simultaneous increase in water permeability and heavy metal rejection. The equilibrium adsorption was reached 8–12 h from the start of the experiments, while membrane performance remained constant. Therefore, membrane regeneration was not only unnecessary but would be undesirable because the adsorbed heavy metals improve membrane performance. In other words, the adsorption of heavy metals on the surface of NF membranes appears to be a “blessing” rather than a “curse."