Toxicokinetics and Particle Number-Based Trophic Transfer of a Metallic Nanoparticle Mixture in a Terrestrial Food Chain

Herein, we investigated to which extent metallic nanoparticles (MNPs) affect the trophic transfer of other coexisting MNPs from lettuce to terrestrial snails and the associated tissue-specific distribution using toxicokinetic (TK) modeling and single-particle inductively coupled plasma mass spectrometry. During a period of 22 days, snails were fed with lettuce leaves that were root exposed to AgNO₃ (0.05 mg/L), AgNPs (0.75 mg/L), TiO₂NPs (200 mg/L), and a mixture of AgNPs and TiO₂NPs (equivalent doses as for single NPs). The uptake rate constants (k_u) were 0.08 and 0.11 kg leaves/kg snail/d for Ag and 1.63 and 1.79 kg leaves/kg snail/d for Ti in snails fed with NPs single- and mixture-exposed lettuce, respectively. The elimination rate constants (k_e) of Ag in snails exposed to single AgNPs and mixed AgNPs were comparable to the corresponding k_u, while the k_e for Ti were lower than the corresponding k_u. As a result, single TiO₂NP treatments as well as exposure to mixtures containing TiO₂NPs induced significant biomagnification from lettuce to snails with kinetic trophic transfer factors (TTF_k) of 7.99 and 6.46. The TTF_k of Ag in the single AgNPs treatment (1.15 kg leaves/kg snail) was significantly greater than the TTF_k in the mixture treatment (0.85 kg leaves/kg snail), while the fraction of Ag remaining in the body of snails after AgNPs exposure (36%) was lower than the Ag fraction remaining after mixture exposure (50%). These results indicated that the presence of TiO₂NPs inhibited the trophic transfer of AgNPs from lettuce to snails but enhanced the retention of AgNPs in snails. Biomagnification of AgNPs from lettuce to snails was observed in an AgNPs single treatment using AgNPs number as the dose metric, which was reflected by the particle number-based TTFs of AgNPs in snails (1.67, i.e., higher than 1). The size distribution of AgNPs was shifted across the lettuce-snail food chain. By making use of particle-specific measurements and fitting TK processes, this research provides important implications for potential risks associated with the trophic transfer of MNP mixtures.