Reevaluating microplastic‒heavy metal interactions: dominant role of natural inorganic particles over organic matter in adsorption competition

This study compared the adsorption of Cr(VI) and Cd(II) by aged microplastics (MPs), palygorskite (PAL), and humic acid (HA). Aging increased oxygen-containing functional groups on MPs, enhancing their metal adsorption. Kinetic studies showed that PAL adsorbed Cr(VI) and Cd(II) much faster than MPs, with adsorption rates six times higher. In addition, PAL exhibited significantly higher adsorption capacities (23.09 mg/g for Cr(VI) and 24.32 mg/g for Cd(II)) compared to MPs (2.13–3.17 mg/g). This was due to monolayer chemisorption of PAL, while MPs exhibited multilayer adsorption. In addition to a slight increase in Cr(VI) adsorption by PP MPs, HA had a minimal impact on metal adsorption by aged MPs, causing only slight fluctuations. The findings highlight that natural inorganic particles play a major role in the adsorption behavior of aged MPs, while natural organic matter has a limited effect, underscoring the importance of inorganic particulates in assessing ecological impacts.