Study on the Impact and Mechanism of Microplastics on the Coagulation-Coupled Ultrafiltration Process in Seawater Desalination Pretreatment

Microplastics (MPs) are recognized as an emerging marine pollutant that poses a challenge to the feed water of desalination plant, but their impact on the desalination pretreatment process remains unclear. In this paper, we systematically analyzed the interaction of 3 μm MPs with inorganic ions and organic matter in seawater in the coagulation-coupled ultrafiltration (UF) pretreatment process, and explored the synergistic membrane fouling of UF membranes by MPs and humic acids (HA). The results showed that the large number of inorganic ions in seawater would promote the removal of MPs. In the presence of both MPs and HA, part of HA combined with MPs. HA was mainly removed when the dosage of Fe3+ was 0–0.1 mMol, whereas HA bound to MPs was mainly removed when the dosage of Fe3+ was 0.1–0.3 mMol, and MPs were removed only when more amount of Fe3+ was added. This indicates that the coagulation priority of HA is higher than that of MPs. MPs and HA caused significant synergistic fouling, with the level of fouling increasing with increasing concentration, but peaking and then mitigating to some extent.This study provides a basis for optimizing the coagulation-coupled UF process in desalination plants in the presence of small-sized MPs.