The behavior and toxicity of metal oxide nanoparticles in aqueous solution

The dissolution and aggregation of metal oxides nanoparticles (NPs) in aqueous solution not only alter the abundance and toxicology of NPs, but also makes the effective assessment and the correct interpretation of effects of NPs on organisms challenging. The extent to which these processes (dissolution and aggregation) occur largely depend on pH, ionic strength, dissolved natural organic matter (NOM) and NPs characteristics. This study investigated the dissolution and aggregation behavior of the four metal oxide NPs (nZnO, nCuO, nFe₂O₃ and nTiO₂) in aqueous solution as influenced by pH, ionic strength and NOM and examined the toxicity of these NPs to Daphnia magna and interpreted the toxicity in terms of NPs' behavior in aqueous solution.;The dissolution of NPs in distilled and dionized (DDI) water, culture (FETAX) solution, solutions of varying pH, ionic strength and NOM content was investigated. Differences in the dissolution patterns in different solution conditions were observed among the NPs, with some NPs (nTiO₂.) showing low dissolution under all solution conditions. Visual Minteq model showed that the distribution of dissolved metal oxides NPs species was regulated by pH and NOM. The experimental dissolution data were corroborated by Visual Minteq and by the empirical double exponent dissolution rate model.;In the aggregation investigation, the aggregation behavior and fractal dimensions of the metal oxides NPs were studied in solutions such as DDI, FETAX solution, solutions of varying pH, ionic strength and NOM content. The rate of aggregation was found to increase with ionic strength and to decrease with increase in NOM concentration. Increased aggregation corresponded to lower fractal dimensions and increased sedimentation.;Arising from the dissolution and aggregation investigations, the interaction of NOM with NPs in aqueous solution was examined. Specifically, the influence of the NOM on NPs dispersion at different pH values was investigated using TiO₂ NPs selected due to its low dissolution over a wide pH range. With the data suggesting that NPs dispersion is pH dependent, sorption studies of NOM to TiO₂ NPs at different pH values were conducted and the sorption data obtained were used to interpret the dispersion results. The sorption data were also fitted on the nonlinear Langmuir model. This study also examined the possible fractionation of NOM upon sorption to NPs. The results demonstrated that fractionation occurs upon sorption and that both low pH and high ionic strength can enhance fractionation.;The NPs toxicity to Daphnia magna was assessed at two levels of biological organization: organism level with mortality at 48 h (LC ₅₀) used as the endpoint and the cellular level with four biomarkers, glutathione-S-transferases (GST), thiobarbituric acid reacting substances (TBARS), oxidized glutathione (GSH) and metallothionein (MT) used as endpoints. The results suggested that the toxicity observed from NPs in organism and cellular levels is a contribution of both the NPs and the dissolved metal ions. The results further showed that NOM and ionic strength have mitigative effects on NPs toxicity to Daphnia magna through sorption and aggregation processes respectively.