Adsorption characteristics and mechanism of modified biochar for cadmium mediated by microplastics

In this study, iron-modified biochar (IMB) was used as an adsorbent, and PLA microplastics of different particle sizes were selected. The adsorption characteristics and mechanisms of the heavy metal Cd were systematically investigated in systems where PLA and IMB coexisted in ratios of 1:2 and 2:1. Within the range of cadmium concentrations from 20 to 150 mg L^-1, PLA's regulatory effect on IMB's cadmium adsorption is strongly dependent on Cd concentration and PLA particle size: the higher the cadmium concentration (100-150 mg L^-1), the more significant the enhancing effect of PLA on IMB's cadmium adsorption, with a maximum promotion rate of 35.70% at 150 mg L^-1; under low Cd concentration (20 mg L^-1), only 10 μm fine PLA exerts weak synergistic promotion, while 100 μm large PLA exhibits no obvious enhancement or slight adsorption inhibition. When the cadmium concentration was 150 mg L^-1, the IMB adsorption capacity in the optimal matching treatment group with added PLA increased by 35.70% compared to the control group. Further mechanistic investigations preliminarily indicate that the incorporation of PLA not only alters the microstructure of the IMB but also may promote the inner-layer complexation and surface precipitation of Cd²⁺ on the IMB surface by potentially inducing the preferential growth of the Fe₃O₄ phase and redistributing interfacial charges; the crystal phase evolution inference remains to be verified via standard diffraction spectrum matching. SEM, XRD, and FTIR characterization results support the above inference: the introduction of PLA enhances the exposure and active site density of ferrite clusters on the IMB surface, while its ester functional groups may regulate the Cd²⁺ adsorption pathways through coordination competition or steric hindrance effects. This study not only elucidates the mechanism by which PLA enhances Cd adsorption by IMB but also breaks away from the traditional "binary system" research paradigm by establishing an adsorption model for the "biochar-microplastic-heavy metal" ternary system, thereby providing a theoretical basis and mechanistic support for heavy metal remediation in environments where microplastics are present.