Forward Osmosis Membranes Modified with Lignosulfonate Coated ZnO Nanoparticles for Efficient Heavy Metal Wastewater Treatment

In the present study, the ZnO nanoparticle surface is coated with lignosulfonate (ZnO-lignin) via a novel in-situ method using industrial lignosulfonate as raw materials to obtain a hydrophilic nanomaterial for incorporation into the polyamide layer of a thin-film nanocomposite (TFN) forward osmosis membrane through interfacial polymerization. Incorporation of hydrophilic ZnO-lignin into the polyamide layer induces the formation of nanochannels around the nanoparticles since water molecules absorbed on hydrophilic nanoparticles can terminate the interfacial polymerization by hydrolysis of trimesoyl chloride monomers. In addition, ZnO-lignin significantly impacts the polyamide layer’s properties in the modified TFN membranes, which had measurably more hydrophilic, thinner and smoother surfaces than the bare thin film composite (TFC) membrane. The covalent bonding of hydroxyl groups with trimesoyl chloride enables the synthesis of a thin polyamide film with high stability and improved performance. At the same time, the sulfonic groups endive membrane surfaces with a negative charge, hence immensely enhancing the membrane selectivity toward NaCl and heavy metal ions. These changes in the properties of the polyamide layer are nearly twice the water flux and raise the selectivity for the optimal membrane. With the assistance of 400 ppm of ZnO-lignin, the water flux of the TFN-ZLS.2 membranes were augmented up to 22.5 LMH, corresponding to 95% of the water flux enhancement compared to the control TFC membrane. In addition, the TFN-ZLS.2 membrane presented higher rejection toward Cr⁺³ and Cu⁺² than control TFC membranes, verifying enhancement of the selectivity of the polyamide layer by incorporating ZnO-lignin. Our results indicate that hydrophilic shells in ZnO-lignin nanoparticles significantly develop TFN membranes with high separation performance.