Enhanced vector transport of microplastics-bound lead ions in organic matter rich water

Microplastics act as a vector for toxic trace metals and their surface complexation mechanisms under different environmental conditions are poorly understood. The present study evaluates the interaction mechanisms of pristine and aged polyethylene (PE) microplastics with Pb2+ in water environments in the presence of NaNO3 and humic acid (HA). The influence of pH, contact time, and concentration of Pb2+ on the sorption performances of PE microplastics have also been examined at different ionic strengths (0.001-0.1 M NaNO3), pH (2-9), reaction time (48 h), Pb2+ loading concentrations (1-25 mg L-1), and HA concentrations (0.5-2.5 mg L-1). The Pb2+ adsorption onto both pristine and aged microplastic showed a gradual increase with increasing pH, reaching maximum adsorption at around pH 5-6. Adsorption of Pb2+ onto both PE microplastics decreased at higher ionic strengths and increased at higher HA concentrations suggesting the possible hydrophobic and electrostatic interactions between microplastics and Pb2+ ions. Adsorption kinetic data for pristine PE microplastics were well described by fractional power model indicating time-dependent adsorption, whereas aged PE showed rate-limiting chemisorption by fitting with a pseudo-second-order kinetic model. Isotherm equilibrium data for pristine PE microplastics fitted well for the Freundlich model implying favourable physisorption on the heterogeneous surfaces. Both Hill and Freundlich models were the best-fitted models for aged PE microplastics suggesting the involvement of cooperative multilayer physisorption. Desorption of PE microplastics-bound Pb2+ was greatly influenced by the solution pH. The ascertaining facts elucidated the vector potential of PE microplastics for Pb2+, impacting their migration and destination in water systems where the adsorption could be influenced by the pH, ionic strength, and dissolved organic matter of the water system.