Silver nanoparticles separation from the water using nanofiltration membranes: The role of mono- divalent salts and NOM

• Nanofiltration removes dissolved Ag released by AgNPs from the water.
• The increase of ionic strength reduces rejection of dissolved Ag.
• The removal of dissolved Ag is controlled by surface chemistry.
• Dissolved Ag complexed with NOM or NOM-Ag complexes un-dissolved the Ag in solution.

This study investigates the use of membrane processes to remove silver nanoparticles (AgNPs) from waters and, at the same time, the dissolved silver (Ag) released from AgNPs. The effect of ionic strength and natural organic matter (NOM) on membrane performance is evaluated. Ionic strength has a significant effect on the permeate flux more important in the presence of AgNPs, decreasing flux. The total dissolved silver in the permeate increases with ionic strength, because the surface charge potential decreases which promoted a reduction of the double layer thickness resulting in lower rejections. In addition, dissolved Ag is complexed with chlorine. Therefore, both AgNPs and dissolved Ag released from NPs were practically removed from water (removals higher than 92%). For NOM with AgNPs water, no significant differences are obtained in the permeate fluxes, varying NOM concentration. These results are due to the AgNPs charge and zeta potential, since NPs are negative at the studied pH and approaches zero with ionic strength, increasing aggregation. The main mechanism involved in the AgNPs removal is size exclusion, while dissolved Ag (resulted for NPs dissolution) is removed by size exclusion and electrostatic interactions due to complexation with salts and NOM.