Adsorption Behavior of Methylene Blue and Rhodamine B on Microplastics Before and after Ultraviolet Irradiation

Abstract

Microplastics (MPs) undergo a series of aging processes and interact with various pollutants in the environment. In this work, we researched and discussed the impact of ultraviolet (UV) aging on the adsorption-desorption behavior of PLA (polylactic acid) and PA66(Polyamide 66) MPs for cationic dyes (Methylene blue (MB) and Rhodamine B (RhB)) were investigated. It was observed that after 30 days of UV irradiation treatment, the increasing amount of oxygen-containing functional groups in MPs, and the more electrical properties tended to be negative. Moreover, the results demonstrated that the aging process significantly enhanced the adsorption capacity of both MPs for RhB and MB, and the adsorption capacity followed in descending order of aged PLA (7.13 mg/g and 14.95 mg/g), > aged PA66 (5.01 mg/g and 1.90 mg/g), > pristine PA66(1.52 mg/g and 1.21 mg/g), > pristine PLA (1.36 mg/g and 1.12mg/g). The fitted results of adsorption kinetics and adsorption isotherms demonstrated that the adsorption was monolayer adsorption process with chemotaxis as the decisive step. Conclusively, as observed through the influencing factor experiments, pH, salinity, surfactant, and humic acid influenced the adsorption of dyes on MPs significantly. In desorption experiments, the desorption efficiency of MB (A-PLA) was reduced from 35.29% to 32.76%, and the desorption efficiency of Rhodamine B (A-PLA) was reduced from 30.35% to 27.42%. In gastric fluid, MPs after loading dyes exhibited a significant desorption efficiency of the desorption to that observed in water and other digestive fluids. Indeed, these findings indicate that aged MPs could have a more pronounced ability to transport dyes, thereby elevating the more serious potential risks to aquatic environments and organisms.

Keywords: Microplastics, UV irradiation, Adsorption-desorption, Methylene blue, Rhodamine B