Background: Zinc oxide nanoparticles (ZnO-NPs) have been shown to exert a non-negligible impact on the environment.
Objective: Kaolin and humic acid were used in the aqueous environment to study their effects on the removal of ZnO-NPs.
Methods: In this work, polyaluminum ferric chloride (PAFC)/cationic polyacrylamide (CPAM) coagulants were used together with kaolin and humic acid to study their effects on the removal of ZnO-NPs and analyze their mechanism of action.
Results: The results showed that the removal rate of ZnO-NPs in the humic acid system decreased by about 30% compared to that in the pure water system, and increasing ionic strength and humic acid concentration were not conducive to removing ZnO-NPs. On the other hand, the ZnO-NPs removal rate in the kaolin system was up to 96.28%, and increasing ionic strength and kaolin concentration contributed to the removal of ZnO-NPs. In the humic acid and kaolin systems, the effects of coagulant dosage and pH on the removal of ZnO-NPs were about the same as in the pure water system. Moreover, 5 mg/L humic acid inhibited floc growth during removal of ZnO-NPs by coagulation with PAFC/CPAM. In contrast, 5 mg/L kaolin promoted flocs growth, resulting in stronger and more stable flocs and a 5.25% increase in the fractal dimension compared to the pure water system.
Conclusion: These results suggested that suspended solids and natural organic matter in the water could directly affect the effectiveness of coagulation to remove ZnO-NPs.
Keywords: Metal oxide, kaolin, coagulation, suspended solids, natural organic matter, suspension.