Comparative Toxicity of Nanoparticulate ZnO, Bulk ZnO, and ZnCl₂ to a Freshwater Microalga (Pseudokirchneriella subcapitata): The Importance of Particle Solubility

TLDR

Metal oxide nanoparticles are increasingly used in commercial products, raising concerns about their environmental fate and potential toxicity, especially since they are assumed to remain as small, highly bioavailable particles in aquatic systems. The study examined nanoparticulate ZnO (≈30 nm) in freshwater to assess its dissolution and toxicity relative to bulk ZnO and ZnCl₂. The results showed that ZnO nanoparticles rapidly dissolved and aggregated into flocs, and that the toxicity to *Pseudokirchneriella subcapitata* was comparable for nanoparticulate ZnO, bulk ZnO, and ZnCl₂, with a 72‑h IC₅₀ of ~60 µg Zn L⁻¹ driven solely by dissolved zinc, underscoring the importance of solubility in toxicity assessments.

Abstract

Metal oxide nanoparticles are finding increasing application in various commercial products, leading to concerns for their environmental fate and potential toxicity. It is generally assumed that nanoparticles will persist as small particles in aquatic systems and that their bioavailability could be significantly greater than that of larger particles. The current study using nanoparticulate ZnO (ca. 30 nm) has shown that this is not always so. Particle characterization using transmission electron microscopy and dynamic light scattering techniques showed that particle aggregation is significant in a freshwater system, resulting in flocs ranging from several hundred nanometers to several microns. Chemical investigations using equilibrium dialysis demonstrated rapid dissolution of ZnO nanoparticles in a freshwater medium (pH 7.6), with a saturation solubility in the milligram per liter range, similar to that of bulk ZnO. Toxicity experiments using the freshwater alga Pseudokirchneriella subcapitata revealed comparable toxicity for nanoparticulate ZnO, bulk ZnO, and ZnCl2, with a 72-h IC50 value near 60 µg Zn/L, attributable solely to dissolved zinc. Care therefore needs to be taken in toxicity testing in ascribing toxicity to nanoparticles per se when the effects may be related, at least in part, to simple solubility.