Flocculation of polystyrene nanoplastics in water using Mg/Al layered double hydroxides via heteroaggregation

Abstract

Nanoplastics (NPs) nano-scale effects and have strong mobility, posing potential risks to ecosystems and human health. Therefore, it is of great significance to study the interactions of minerals and NPs in water to understand the migration, transformation, and fate of NPs in the environment. This study investigated the flocculation of NPs on layered double hydroxides (LDHs) in water via heteroaggregation under various conditions such as different Mg/Al ratios of LDHs, calcination temperatures, coexisting ions, solution pH, and contact time. We found that the overall reaction achieved high efficiency that almost surpassed 70% and varied with solution pH. The highest flocculation performance (85.87%) was achieved at a Mg:Al ratio of 3:1, followed by 4:1 and 2:1. Moreover, the highest removal rate (80.85%) of NPs on cLDHs was achieved at calcination temperature of 300 °C. The heteroaggregation mechanism, bonding type, and interface reaction of the NPs–LDHs system were analyzed using the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory, spectroscopy, and microscopy. At short separation distance within 5 nm, the total interaction energy was governed by electrostatic repulsion. However, at longer distance, the interparticle forces became dominated by van der Waals and electrostatic attractions, leading to attachment of two types of particles. The NPs could be removed by attachment onto the surface of LDHs without forming other functional groups. However, the calcined LDHs and those prepared by hydrothermal reactions showed different attachment morphologies under scanning electron microscopy, which revealed the mechanisms of surface attachment and encased precipitation. These results provided new insights and theoretical support for controlling NPs pollution in surface water, which could reduce the risks of NPs to ecosystems and human health.