Removal behaviors and mechanism of polystyrene microplastics by coagulation/ultrafiltration process: Co-effects of humic acid

Microplastics (MPs) have been detected in drinking water, which could absorb or accumulate humic acid (HA) and threaten the water quality. Coagulation-ultrafiltration (CUF) is a common drinking water treatment technology, but its behavior and mechanism of removing MPs and MPs-HA remain unclear. In this study, the removal mechanism of polystyrene (PS)-MPs coagulated by Al- and Fe-based salts with or without HA was investigated to optimize the CUF process. The results showed that Al-based salt (92.7 %) was better than Fe-based salt (91.2 %) in the removal efficiency of PS or HA, and the optimal coagulants dosage of PS-HA composite system (12 mg center dot L-1) was higher than that of the individual PS system (9 mg center dot L-1). Moreover, the coagulation mechanism was studied by Fourier transform infrared spectroscope (FTIR) and X-ray photoelectron spectroscopy (XPS). The oxygen group in PS and PS-HA was the main binding site of Al and Fe hydrolysate, and the effects of charge neutralization, adsorption bridging, and sweep flocculation became weaker in turn at the optimal dosage. In addition, the cake layer formed by coagulation and the presence of HA alleviated the irreversible membrane fouling by intercepting flow and re-adsorption. This study guides the improvement of the traditional drinking water treatment process to remove MPs.

Automated summary

Microplastics have been found in drinking water, posing a risk to water quality by absorbing humic acid. This study investigated the behavior and mechanism of removing these microplastics and microplastics-humic acid compounds using coagulation-ultrafiltration technology. The results showed that aluminum-based salt was more efficient in removing both polystyrene and humic acid, and the optimal dosage for the composite system was higher than that for individual systems. The study also identified the main binding sites and weakening effects of various mechanisms during coagulation, as well as the alleviating effects of a formed cake layer and the presence of humic acid on membrane fouling. Overall, this research provides guidance for improving traditional drinking water treatment processes to remove microplastics.