Electrochemical detection of heavy metal ions adsorbed on microplastics with varying surface charges

Microplastics (MPs) are global pollutants found in various environmental compartments, including oceans, freshwater bodies, soils, and air. Their persistence and potential to adsorb heavy metals raise significant concerns regarding water quality and ecosystem health. Understanding the interaction between MPs and heavy metals is crucial for assessing environmental risks and developing remediation strategies. In this study, we use electroanalytical techniques to investigate the adsorption of heavy metal ions— Cr³⁺, Zn²⁺, and Ni²⁺—by polystyrene MPs with different surface functionalizations: carboxyl, amino, and unfunctionalized. We conducted electrodeposition of metal ions on a gold ultramicroelectrode both before and after mixing with MPs. Following this, we used anodic stripping voltammetry to measure the change in the electrodeposited charge (ΔQ). This charge difference, observed before and after the interaction of ions with MPs, was analyzed to understand the adsorption kinetics and dynamics. Our experiments revealed that carboxyl-functionalized MPs exhibited the highest ΔQ due to strong electrostatic attraction with the metal ions, with values 1.67 ×, 1.50 ×, and 1.22 × greater than those for amino-MPs for Cr³⁺, Zn²⁺, and Ni²⁺, respectively. Amino-MPs displayed considerable ion adsorption, suggesting poor electrostatic repulsion between two positively charged entities. Interestingly, unfunctionalized MPs, which had a negative surface charge similar to carboxyl-MPs, showed different adsorption characteristics, with lower ΔQ values for all metal ions. This study underscores the significant role of surface functionalization on the adsorption efficiency and kinetics of heavy metal ions by MPs. It also demonstrates the utility of electroanalytical techniques in understanding metal ion-MP interactions, offering insights into potential environmental impacts and remediation strategies, as well as in developing electrochemical sensors for detecting heavy metal ions adsorbed on MPs.