Impact mechanisms of polyethylene microplastic on Cd adsorption and passivation by KMnO4-modified biochar in different soils

In the current situation of increasing cadmium (Cd) and microplastic composite pollution, Cd adsorption and passivation in various soils by biochar under microplastic co-contamination conditions require further investigation. In this study, the behaviors and mechanisms of KMnO-modified biochar on Cd adsorption and passivation were examined in 50 different soils (5 soil types, 5 Cd concentrations, and 2 microplastic treatments) with the three-layer mesh method. Moreover, acidified biochar was employed to quantify the contributions of mineral and non-mineral mechanisms to Cd adsorption by biochar in soils. The results revealed that microplastics in the soil impeded the biochar remediation of Cd contamination, which was pronounced in more acidic soils. Under biochar treatment, the relatively stable Cd decreased by 2.49 %, 1.97 %, 0.36 %, 0.84 %, and 0.39 % for five soils (from low to high pH) with 5 % microplastics compared to those without microplastics. The Cd adsorption capacity by biochar was higher in soils with lower pH, and microplastic co-contamination further increased Cd adsorption by biochar. Additionally, higher soil pH correlated with a greater contribution of the mineral mechanism to Cd adsorption, and microplastic co-contamination enhanced the contribution of mineral mechanism. This study aimed to gain a deeper understanding of how microplastics in agricultural soils affect the remediation of Cd pollution by biochar. This study highlighted that biochar-based remediation of heavy metal contamination in soils must consider not only soil pH and heavy metal concentrations, but also the effects of co-existing pollutants, such as microplastics.