Multi-metal competitive adsorption on plastic polymers: Evidence from single and binary metal exposure experiments

This study investigates the adsorption behaviour of eight heavy metals (Fe, Cu, Zn, Cd, Mg, Mn, Co, Ni) on four common plastic polymers (PVC, PP, PET, and HDPE) under single-metal and binary-metal exposure conditions. To evaluate interaction mechanisms under controlled conditions, experiments were conducted using 1000 ppm metal solutions as standardized laboratory exposures designed to assess maximum binding potential rather than environmentally representative concentrations. Under these standardized conditions, Fe²⁺, Cu²⁺, and Mg²⁺ exhibited strong adsorption affinity, particularly on PVC and HDPE. Binary-metal combinations showed enhanced or suppressed uptake patterns. Principal Component Analysis (PCA) further revealed clear clustering patterns consistent with metal-specific and polymer-specific interactions demonstrating that adsorption behaviour is influenced by both polymer type and metal combinations. To complement these controlled experiments, plastic debris collected from coastal environments was analysed independently. Confocal Raman spectroscopy identified nine polymer types among the field samples, and ICP-OES analysis detected adhered metals-primarily Fe, Mg, Mn, and Zn-on environmentally weathered plastics. X-ray Photoelectron Spectroscopy (XPS) was further consistent with the presence of surface-bound metals on selected debris samples. Although environmental concentrations were substantially lower than laboratory exposures, the observed affinity patterns provide indicative support for polymer-dependent metal accumulation on plastic debris. However, given the controlled laboratory conditions and limited environmental sampling, these findings should be interpreted as mechanistic insights rather than direct representation of environmental behaviour.