Adsorption behavior of nanoplastics from water onto magnetic biochar in single and binary heavy metal systems: A combined experimental and DFT study.

Abstract

Coexisting metals can alter the removal of nanoplastics by magnetic biochar, but comparisons between single- and mixed-metal treatments depend on the concentrations used. We evaluated the apparent uptake of dispersed polystyrene (PS) nanoplastics by magnetic biochar (MBC) under metal-free, Pb(II), Zn(II), and binary Pb(II)/Zn(II) conditions. Each metal was added at 50 mg L^-1 in the corresponding treatment. At an initial PS concentration of 100 mg L^-1, the apparent uptake capacities were 24.25, 19.40, 14.48, and 21.14 mg g^-1, respectively. The binary value was numerically higher than either single-metal value but remained below the metal-free control. Because the binary treatment contained twice the total metal concentration of each single-metal treatment, the observed binary response reflects the combined influence of metal composition and total loading. Control measurements showed metal-associated changes in PS aggregation, while SEM-EDS and XPS analyses detected Pb and Zn on recovered MBC and indicated changes in the surface chemical environment. Selected finite-cluster calculations further illustrated configuration-dependent local metal arrangements and electronic redistribution at the modeled PS-MBC interface. Together, these results establish a treatment-dependent adsorption response under defined batch conditions and indicate that metal identity, total loading, and local interfacial configuration jointly influence dispersed PS removal.