Effects of Adding Polyamide Microplastics Simulating Natural Aging on Heavy Metal Adsorption in Guizhou Yellow Soil

ObjectiveMicroplastic and heavy metal pollution are global problems that threaten ecosystems and human health. Microplastics in soil can interact with soil particles and affect the physicochemical properties of soil, which in turn affects the behavior of heavy metals in soil.MethodThis study systematically investigates the sorption-desorption behavior of polyamide(PA)microplastics toward cadmium (Cd2+), chromium (Cr3+), and copper (Cu2+) using Guizhou yellow soil as the research object. By simulating PA's natural aging processes through multiple freeze-thaw cycles, high-temperature exposure, and oxidation, the aging mechanisms and adsorption mechanisms are elucidated.ResultThe results showed that the adsorption process of PA on the three heavy metals could be divided into two stages: rapid adsorption (0-90 min), slow adsorption(90-150 min)and equilibrium adsorption (150 min), and the adsorption amount reached more than 99% of the saturation amount. Also, the kinetic behaviors conformed to the pseudo-second-degree model (R2 > 0.999), which indicated that chemosorption was the dominant mechanism. Moreover, the adsorption data fitted better the Langmuir model compared to the Freundlich model (R2 > 0.978), and PA significantly enhanced the adsorption capacity of Cr3+ from the soil (KL value increased by 59.1%). After 21 days of aging, PA surface roughness, crystallinity, specific surface area (+44.49%), and negative charge density (–36.53 mV) increased significantly, resulting in an increase in heavy metal adsorption (P 2+ (18.7%) and the smallest for Cr3+ (4.2%). Furthermore, carboxyl (–COOH) and hydroxyl (–OH)groups generated by oxidizing the PA surface during aging strengthened heavy metal immobil ization through coordination and electrostatic interactions, whereas the increase in the proportion of crystalline regions further inhibited desorption.ConclusionThis study reveals the mechanism of adsorption enhancement of soil heavy metals by aged PA, which provides a theoretical basis for the risk assessment and management of microplastic-heavy metal composite pollution in karst areas.