Adsorption mechanism of arsenic(V) on aged polyethylene microplastics: Isotherms, kinetics and effect of environmental factors

Abstract

Microplastics gradually age when they enter the environment, and the adsorption mechanism and factors affecting the adsorption of arsenic (As), which is widely present in water, by aged microplastics remain unclear. This study is concerned with the kinetics and thermodynamics of As(V) adsorption by unaged, acid-aged, alka-aged and UV-aged polyethylene (PE) microplastics. In addition, it seeks to ascertain the influence of environmental factors (pH, dissolved organic matter, and NO₃^-) on As(V) adsorption by the aforementioned microplastics. The results showed that all three aging methods caused the surface of PE microplastics to become roughened and wrinkled, more oxygen-containing functional groups appeared, and the crystallinity increased. The adsorption process of As(V) on aged-PE microplastics can be better fitted with the pseudo-second-order kinetic model than the pseudo-first-order and Elovich models, suggesting that the adsorption of As(V) on aged-PE microplastics is dominated by chemisorption. Compared with unaged microplastics, the adsorption of As(V) by aged-PE microplastics increased significantly in the following order: alka-PE > acid-PE > UV-PE. The Freundlich model is more suitable for describing the isothermal adsorption of As(V) on aged PE microplastics than the Temkin model. In addition, the Langmuir model is also suitable for describing the isothermal adsorption of As(V) on unaged-PE, UV-PE and acid-PE microplastics. This finding suggests that the adsorption of As(V) by aged PE microplastics is predominantly facilitated via surface complexation and van der Waals forces. Furthermore, the adsorption of As(V) on aged-PE microplastics exhibited a tendency to be promoted and then inhibited with increasing pH, NO₃^- and dissolved organic matter concentrations. The findings of this study could offer significant theoretical insights into the biogeochemical cycling of As in the presence of aged microplastics.