Traditional MPs and Degradable MPs Differences in Arsenic Adsorption and Their Mechanisms

Abstract

To investigate the differences and mechanisms in the adsorption behavior of As(V) between degradable MPs (PCL, PLA) and conventional non-degradable MPs (PE, PP), this study conducted batch adsorption experiments to compare the adsorption kinetics and thermodynamics of the two types of MPs under three particle sizes (100 μm, 150 μm, 200 μm). Isotherm modeling was conducted by applying the Langmuir and Freundlich theories to interpret the collected experimental results,The reaction dynamics were evaluated using first-order and second-order pseudo-kinetic approaches, complemented by structural and compositional analyses through FTIR spectroscopy and XRD patterns. The results showed that the specific surface area and total pore volume of degradable MPs were approximately 5 and 10 times higher than those of conventional MPs, respectively. The surfaces of degradable MPs were rich in oxygen-containing polar functional groups such as C=O and C-O-C, whereas conventional MPs were dominated by non-polar C-H groups. The adsorption capacity for As(V) followed the order: PLA-MPs (135.93 mg·kg⁻¹) > PCL-MPs (98.94 mg·kg⁻¹) > PE-MPs (87.18 mg·kg⁻¹) > PP-MPs (79.12 mg·kg⁻¹). The adsorption process of degradable MPs was better described by the pseudo-second-order kinetic model, indicating a dominant role of chemisorption. In contrast, conventional MPs exhibited comparable fitting degrees for both pseudo-first-order and pseudo-second-order models, suggesting that their adsorption was jointly controlled by physical interactions and weak chemical processes. The C=O and C-O-C groups in degradable MPs served as key sites for coordinative adsorption of As(V), while conventional MPs only participated in weak physical adsorption via C-H groups. Decreasing particle size enhanced the adsorption performance of all MPs, with this effect being particularly pronounced for conventional MPs, implying that their adsorption relies more heavily on specific surface area. The enhanced As(V) removal efficiency and accelerated sorption rates observed in degradable microplastics can be attributed to their porous architecture coupled with polar functional moieties present on the surface. thereby potentially serving as more efficient vectors for arsenic transport within environmental systems. This study provides a theoretical basis for scientifically assessing the composite pollution risks of the two types of microplastics and formulating differentiated environmental management strategies.