Adsorption behavior of heavy metals onto microplastics derived from conventional and biodegradable commercial plastic products

This study extensively explored the adsorption behavior of heavy metals (Pb+2, Ni+2, Cu+2, Zn+2, and Cd+2) onto microplastics (MPs). The particle sizes of MPs ranged from 0.149 to 0.25 mm. The microplastics were generated from commercial products manufactured from both conventional (polyethylene (PE) bottle, polystyrene (PS) spoon, and polyethylene terephthalate (PET) egg carton) and biodegradable (polylactic acid (PLA) spoon, and polylactic acid (PLA) egg carton) plastics. The study also considered the influence of solution pH on the adsorption capacity of heavy metals. Regarding the adsorption potential for Cu+2, the ranking was as follows: PLA-egg (1408 mu g.g(-1)) > PLA-spoon (735 mu g.g(-1)) > PE-bottle (315 mu g.g(-1)) > PET-egg (283 mu g.g(-1)) > PS-spoon (237 mu g.g(-1)). PLA MPs showed the highest adsorption capacity due to the lower thermal stability and higher presence of surface oxygen functional groups. Moreover, the adsorption capacities of the five metals onto PLA-spoon and PLA-egg decreased in the following order: Pb (1785 mu g.g(-1)) > Zn (1267 mu g.g(-1)) > Cd (748 mu g.g(-1)) > Cu (735 mu g.g(-1)) > Ni (722 mu g.g(-1)), and Pb (1520 mu g.g(-1)) > Ni (1412 mu g.g(-1)) > Cu (1408 mu g.g(-1)) > Zn (1118 mu g.g(-1)) > Cd (423 mu g.g(-1)), respectively. The SEM-EDS, FTIR and XPS results demonstrated that surface oxygen-containing functional groups play an important role during the adsorption process. This study extended its analysis to quantify the metal content of the post-adsorption MPs, revealing uneven adsorption of heavy metals onto the MPs. This implies that the diversity of commercial plastic products may result in significant variations in their ability to adsorb heavy metals, underscoring the importance of effectively managing discarded commercial plastic products.