Aging amplifies synergistic adsorption and reduction of Cr(VI) by polyamide microplastics

The interaction between new pollutants microplastics(MPs) and toxic heavy metals in a coexisting environment could alter both their environmental behavior and effects. Polyamide (PA) MPs show affinity toward highly toxic Cr(VI), thus the interaction mechanism needs to be further understood. Herein, PA MPs aged with different methods such as ultraviolet irradiation in air/sea water and chemical chlorination, were used to explore their adsorption and reduction performance and mechanism toward Cr(VI) under different solution conditions. The research results show that after aging treatment, the content of amide bonds in PA MPs decreased, while the number of oxygen containing functional groups increased. Under the condition of pH 3.0, the adsorption capacity reaches its maximum value. Higher temperatures facilitate the adsorption process, whereas the presence of salinity and dissolved organic matter (DOM) considerably hampers adsorption. The behavior of PA MPs in adsorption aligns with the pseudo-second-order kinetic model as well as the Langmuir adsorption isotherm model. Further studies show that the binding of Cr(VI) to PA MPs is primarily achieved through electrostatic interactions, complexation, and hydrogen bonding. It is important to mention that PA MPs have the ability to convert Cr(VI) to Cr(III) in solution, even in conditions involving UV light, the presence of DOM, and environments lacking oxygen. UV aging enhanced the adsorption and reduction performance of PA MPs for Cr(VI). This research offers fresh scientific insights into the mechanisms of formation and the environmental interactions related to pollution caused by microplastic-heavy metal composites.