Mapping National-to-Global Heterogeneities of Metal-Containing Nanoparticles in Municipal Solid Waste Leachates

The occurrence of metal-containing nanoparticles (Fe-, Pb-, Ti-, Cu-, Mn-, Cr-, Hg-, and Ag-NPs) in leachates from 74 municipal solid waste (MSW) disposal sites, spanning 7500 latitudinal kilometers and serving an urban population of 39.6 million, was quantified by single-particle inductively coupled plasma mass spectrometry. All leachate samples contained more than three metal-containing NPs, with the summed number concentrations and sizes ranging from 10⁹ to 10¹⁴ particles L^-1 and from 12 to 100 nm, respectively. Transmission electron microscopy with energy-dispersive X-ray spectroscopy analysis of representative subsamples further revealed the colocalization of multiple elements within individual NPs, indicating heteroaggregation and/or complexation with metal ions. MSW leachates serve as long-term sources of metal-containing NPs to groundwater because the summed number concentrations of metal-containing NPs in the downgradient groundwater were 2.2-515.7 times higher than their counterparts in the upgradient groundwater. Random forest models further identified leachate total organic carbon (TOC) and total metal concentrations as the primary drivers of NP spatial heterogeneity, enabling global NP mapping and hotspots identification. Under effective TOC mitigation (≤30.3 mg L^-1), 84% of leachates showed NP reductions of 6-99.4%. These findings provide a prioritized framework for NP monitoring and MSW sites management, supporting the Basel Convention's goals for sustainable waste management.