Flocculants based on responsive polymers and chitosan for removal of metallic nanoparticles as contaminants of emerging concern present in water

Wastewater containing polluting nanoparticles during purification is an emerging concern that has created the need to develop more efficient methodologies of wastewater treatment. Therefore, this contribution provides a study regarding the removal of gold and silver nanoparticles stabilized with poly(vinylpyrrolidone) (AuNP-PVP and AgNP-PVP) using flocculation at concentrations of 1.5 and 7.5–8 mg/L in kaolin aqueous dispersions. With this goal in mind, chitosan was grafted with poly(N-vinylcaprolactam)(PNVCL) and poly(acrylic acid)(PAAc) (CS-g-PNVCL, CS-g-PNVCL-PAAc), which was verified by characterization using NMR-spectroscopy, thermogravimetric analysis, dynamic light scattering, and zeta potential measurements. At pH 7, the graft copolymers presented a lower zeta potential than chitosan (close to zero), and the PVP-stabilized nanoparticles exhibited a negative surface charge (around −20 mV). The bridging mechanism during the flocculation process was discovered to be responsible for improving the performance of chitosan using chitosan-based graft polymers with low concentration (0.5 mg/L). CS-g-PNVCL-PAAc allowed higher turbidity reduction (82%), while CS-g-PNVCL was the best flocculant in nanoparticle removal (94% from 7.5 mg/L), at room temperature. The flocculants showed higher ability towards the removal of AgNP-PVP, as compared to AuNP-PVP. By increasing the temperature to 45 °C, the turbidity removal increased, while the nanoparticle removal only improved when the CS-g-PNVCL-PAAc flocculant was used. The effectiveness of chitosan-based flocculants for the removal of metallic nanoparticles as emerging contaminants is reported for the first time. Besides, the removal efficiency of metallic nanoparticles by flocculation is superior as compared to adsorption methods.