Investigation on the removal performances of heavy metal ions with the layer-by-layer assembled forward osmosis membranes

- Removal of heavy metal ions by LbL assembled FO membranes was investigated.
- The five heavy metal ion rejection during the FO process exceeded 99.31%.
- Heavy metal ion adsorption on FO membranes played an important role in the high rejection.

The removal performances of heavy metal ions with the layer-by-layer (LbL) assembled forward osmosis (FO) membranes were investigated in this study. The prepared FO membrane with the multi-layer polymer network was fabricated by assembling multiple polyethylenimine (PEI) and sodium alginate (SA) bilayers on a polydopamine-functionalized polyvinylidene fluoride (PVDF) supporting membrane. The prepared FO membrane possessed high rejection of five heavy metal ions, i.e., Cu2+, Ni2+, Pb2+, Zn2+ and Cd2+. The influences of different osmosis modes, number of bilayers, pH, time, temperature, regeneration as well as the concentration of feed and draw solutions on the heavy metal ion removal performances of the FO membrane were studied. Results showed that all the heavy metal ion rejections with the 3-layer assembled FO membranes exceeded 99.31% in AL-FS (the active layer facing the feed solution) mode when 1 M MgCl2 was employed as the draw solution, and the rejection in AL-FS mode was higher than that in AL-DS (active layer facing the draw solution) mode. The experimental results revealed that the number of bilayers of the polyelectrolyte pairs, the hydrated radii of metal ions and the heavy metal ion adsorption all contributed to the high rejection of the heavy metal ions.