There is growing concern about the threat that nanoplastics (NPs) pose to ecosystems. However, a comprehensive risk assessment of NPs is currently constrained by the paucity of knowledge on the chemical reactivity of NPs, which were previously thought to be chemically inert. This review identifies the chemical reactivity of NPs that have undergone abiotic and biotic weathering, including the formation of free radicals, the increase in the content of oxygen-containing functional groups, and the release of plastic leachates. Their interaction with legacy contaminants, such as heavy metals (HMs), is then examined, highlighting their critical role in the oxidation and reduction of HMs, through free radical-mediated redox processes and electron shuttling by carbonyl groups. This review offers new insights into the risk of NPs, where their interaction with legacy contaminants determines the long-term exposure scenario for ecosystems. The unexpectedly large pool of reactive NPs in nature will not only affect their risks but also impact the biogeochemistry of HMs and other contaminants that could react with free radicals and carbonyl groups.

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