Comparison of adsorption and desorption of triclosan between microplastics and soil particles

Microplastic (MP) pollution in soil has been becoming an emerging environmental hot spot, but little is known about the interaction between MPs and chemical contaminants in soil. In this study, batch experiments were performed to study adsorption-desorption behavior and mechanism of triclosan (TCS) on MPs, polyethylene (PE) and polystyrene (PS), and soil particles. PE showed the highest adsorption rate (29.3 mg μg^-1 h^-1) and equilibrium capacity (1248 μg g^-1), while the similar profiles between PS (0.27 mg μg^-1 h^-1 and 1033 μg g^-1, respectively) and soil (0.60 mg μg^-1 h^-1 and 961 μg g^-1, respectively). Two adsorption stages, representing liquid-film and intra-particle diffusion were observed obviously for PE. Adsorption isotherm results revealed that the interaction between MPs and TCS was relatively weak. The sorption potential of soil was lower than that of MPs especially at high concentrations. PE addition induced TCS sorption increase in soil, while PS had no significant (P > 0.05) influence. For MP-soil systems, TCS preferred to adsorb on MPs, which was more pronounced for PE than PS. The desorption rate of TCS was the highest for soil, followed by PE and PS, while equilibrium release amount ranked: PE > PS > soil. Moreover, soil solution better facilitated the desorption, with the amount increasing by 38% for PE compared with 0.01 M CaCl₂ solution. Therefore, MPs, especially PE with high adsorption and desorption potentials may serve as a source and carrier to TCS, and its amendment can change TCS environmental behavior and further risk in soil.