Ionic strength modulates size-controlled ecotoxicity of citrate-coated silver nanoparticles to Daphnia magna in...

Silver nanoparticles (AgNPs) pose significant exposure risks to aquatic organisms, yet how ionic strength (IS) governs their toxicity in complex environments remains unclear. This study examines IS-dependent aggregation, dissolution, and ecotoxicity of citrate-coated AgNPs (Cit-AgNPs) toward Daphnia magna in landfill leachate and domestic sewage. The results show that 0.2 mg/L Cit-AgNPs in 10% domestic sewage, as well as 0.4 mg/L in 5% landfill leachate, inducing 13-100% inhibition rate, where the test concentrations were well below current regulatory limits defined in both Chinese (GB 8978-1996) and U.S. (40 CFR Part 413) standards. It indicates that current environmental regulations may underestimate the risks of AgNPs. Regardless of the matrix type, the increased IS (10-450 mM) significantly (p < 0.01) reduced the Cit-AgNPs' absolute ξ-potential, promoting aggregation (118.43 nm vs. 299.93 nm), reducing the bioavailability and subsequent ecotoxicity effect (100% vs. 13% inhibition). Notably, at ultra-high ionic strengths (450-700 mM), the aggregates underwent a reduction in size and a concomitant increase in toxicity, likely due to surface etching and subsequent disassembly. The ecotoxicity contribution from silver dissolution was found to be minimal, attributing to the dominant formation of less toxic AgCl_x^(x-1)- complexes from the dissolved Ag⁺ ions.