Coagulative removal of polystyrene nanoplastics using zirconium chloride: performance and mechanism

ABSTRACT

Inorganic metal coagulants have been shown to be a viable option for treating nanoplastics (NPs) in water. However, the current research on inorganic metal coagulants has primarily focused on aluminum, iron, and titanium salts, with relatively insufficient studies on zirconium salts. This study represented the first systematic investigation of polystyrene (PS) NPs removal using zirconium chloride (ZrCl4) as a coagulant. Coagulation experiments revealed that the dosage of ZrCl4 significantly influenced the removal efficiency of PS NPs in water. When the ZrCl4 concentration reached 400 mg/L, the coagulation system achieved a maximum removal efficiency of 98.9% for 25 mg/L of NPs. Results from analytical techniques, including zeta potential measurements, scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy, indicated that at lower ZrCl4 dosages (200 mg/L), charge neutralization and adsorption bridging were the primary coagulation mechanisms for PS NPs. Increasing the coagulant dosage significantly enhanced the sweeping flocculation effect on NPs, thereby substantially improving the removal efficiency. Experiments investigating the influence of coagulation revealed that the optimal dosage system utilizing 400 mg/L ZrCl4 exhibited significant environmental adaptability. Additionally, the presence of Ca2+ improved the coagulation efficiency of low-dose ZrCl4 for PS NPs by strengthening charge neutralization and facilitating sweeping flocculation.