Color-Dependent Adsorption Behavior of Tetracycline onto Aged Microplastics

Microplastics have become a significant problem threatening both ecological systems and human health. This study focuses on four colors (red, yellow, blue, and green) of polypropylene (PP) and polymethyl methacrylate (PMMA), and the impact of color differences before and after aging on their environmental behavior. The findings indicate that color is a key factor influencing the aging behavior of microplastics. The red PP surface develops dense wrinkles, while the blue PP exhibits irregular flaking. After aging, the average particle size of PP decreases by 6–30%, with red and yellow PP particles showing even greater reductions. Adsorption studies reveal that microplastics display color-dependent patterns in their adsorption behavior toward tetracycline hydrochloride (TC-HCl). For virgin microplastics, the adsorption capacity order is red > blue > yellow ≈ green. After aging, this order shifts to red > yellow ≈ green > blue, with yellow and green microplastics showing significant improvements in adsorption performance. Both pH and ionic strength influence the adsorption behavior of colored microplastics. The adsorption of TC-HCl by virgin PP reaches its maximum at pH 9, whereas aged PP achieves maximum adsorption at pH 7. For PMMA, maximum adsorption capacity before and after aging occur at pH 5. Low concentrations of NaCl enhance adsorption capacity, but adsorption decreases as NaCl concentration increases. This study provides a theoretical basis for assessing the ecological risks posed by different colored microplastics in complex pollution scenarios.