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

ABSTRACT

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 µg·g-1) > PLA-spoon (735 µg·g-1) > PE-bottle (315 µg·g-1) > PET-egg (283 µg·g-1) > PS-spoon (237 µg·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 µg·g-1) > Zn (1267 µg·g-1) > Cd (748 µg·g-1) > Cu (735 µg·g-1) > Ni (722 µg·g-1), and Pb (1520 µg·g-1) > Ni (1412 µg·g-1) > Cu (1408 µg·g-1) > Zn (1118 µg·g-1) > Cd (423 µg·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.