Increasing applications of zinc oxide nanoparticles (ZnO NPs) in commercial products causes concerns regarding the effects of these NPs on ecological systems and human health. This study investigated the removal effect of ZnO NPs by coagulation from the aqueous system by measuring turbidity and solubility under different pH values and concentrations of humic acid (HA). Results showed that pH has significant impact on coagulating ZnO NPs by introducing FeCl3 (0, 0.03, and 0.3 mM) in aqueous phase. At pH 3–6, more than 53% of ZnO NPs were dissolved as Zn2+. At pH 6–8, removal of ZnO NPs resulted from combined effects of coagulant enmeshment and charge neutralization. When pH ranged from 8 to 11, average removal efficiency of turbidity of ZnO NPs by FeCl3 (0.3 mM) was above 97% and the solubility of ZnO NPs was lower than 5%. Removal of ZnO NPs was mainly due to the effect of enmeshment of coagulants at pH 8–11. Results also suggested that HA could impede aggregation of NPs when concentrations of FeCl3 were 0 and 0.03 mM. While the concentration of FeCl3 increased to 0.3 mM, the effect of enmeshment of coagulants was greater than the impediment of HA on the aggregation of ZnO NPs at pH 8–11. Therefore, coagulation served as an effective method to remove ZnO NPs from the aqueous system with pH ranging from 8 to 11.
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