Mixture interactions of two metal oxides engineered nanoparticles and an organic pollutant in river water systems

Abstract

Widespread use of engineered nanoparticles (ENPs) in consumer products and industrial applications has increased rapidly in recent years. Among the commonly used metal oxide ENPs are aluminium oxide (nAl2O3) and copper oxide nanoparticles (nCuO). These ENPs are commonly applied in consumer products such as personal care products (PCPs), and therefore are likely to co-exist in the aquatic systems. Hence, it is critical to consider the fate and transformation of ENPs and other pollutants as mixtures in the aquatic systems. The influence of water chemistry parameters such as ionic strength (IS), pH and natural organic matter (NOM) in synthetic and natural water on the fate of ENPs were investigated. The transformation processes considered, were aggregation and dissolution. The interplay of these processes may have diverse pollutant implications for the aquatic organisms. To date, numerous studies have reported on the fate and transformation of individual ENPs in aqueous media. However, typically high exposure dosages were used that are unlikely to be found in the natural aquatic environments. In addition, only a few studies have considered mixtures of ENPs with organic pollutants such as triclosan (TCS). To address this knowledge gap, the fate and transformation of individual, binary and ternary mixtures of nAl2O3, nCuO, and TCS were investigated in deionised water (DIW) and natural river water. The latter was sourced from the Elands river (ER) and Bloubank river (BR) both in South Africa. All aggregation and dissolution kinetics were performed at low concentrations of ENPs. This was to reflect the likely situation to be found in the environment. In DIW, humic acid (HA) (a NOM surrogate) showed a concentration-dependent stabilization effect on aggregation for both individual and mixtures of ENPs. High IS induced higher aggregation of ENPs in DIW. Hence, NOM inhibits aggregation of ENPs whereas IS enhances aggregation. The pH inhibits aggregation of ENPs to a lesser extent compared to NOM, but it enhances aggregation around the point of zero charge. Both ENPs were found to be more highly stabilized in river water compared to DIW. Broadly speaking, ENPs exhibited concentration-dependent aggregation, it being lower in ER compared to BR. The differences are attributed to variations in water chemistry, e.g. NOM and the presence of electrolytes. Dissolution of ENPs was higher in ER than in BR water, and higher at lower concentrations of ENPs. Dissolution of nCuO was enhanced in the presence of nAl2O3. Binary mixtures of ENPs had higher tendency to aggregate compared to the individual components. Combinations of nCuO or nAl2O3 with TCS were less likely to aggregate in river water. This implies that the TCS acted as a stabiliser for the nanoparticles. However, this stabilisation effect was compromised when both ENPs were present. Since the ENPs do form agglomerates, they may undergo sedimentation. Consequently, this could lead to interaction with benthic organisms. However, if they are stabilized in natural waters, they might interact with pelagic organisms. Both cases imply the possibility of inducing deleterious consequences. The most important finding of the present study is that the behaviour of ENPs in water is very complicated. In DIW, HA strongly inhibits aggregation of ENPs whereas IS promotes their aggregation. However, in freshwater systems the aggregation of ENPs is uniquely influenced by source-specific water chemistry factors that counteract with each other. This makes it difficult to predict the fate and transformation of ENPs in natural aquatic systems. Hence, it may not be possible to generalize on ENPs transformation outcomes, even of the same ENPs type, in different water matrixes.