A Comprehensive Review on the Aquatic Toxicity of Engineered Nanomaterials

Abstract

As engineered nanomaterials (NMs) are continually introduced into commercial markets, some of them may be eventually released into the aquatic environment during their product life-cycles. This article aims to provide a comprehensive review on the processes which ultimately govern the toxicity of NMs to aquatic organisms. Firstly, their potential entry routes into aquatic ecosystems are identified. Ambient conditions which affect their behavior (i.e., aggregation, sedimentation, dissolution, adsorption, stabilization, degradation and concentration in surface microlayer) and hence alter their characteristics in aqueous medium are also acknowledged. Issues regarding difficulties in their characterization in complex environmental matrices are therefore explicitly considered, while recent efforts to quantify or estimate their concentrations are summarized. Uptake pathways of NMs by aquatic organisms from the water column, sediments and diets are traced, and the resultant toxic mechanisms of four chosen NMs (i.e., nano titanium dioxide (nTiO2), nano zinc oxide (nZnO), nano silver (nAg) and buckminsterfullerene ( C 60)) within the organisms (i.e., algae and bacteria, crustaceans, bivalves and fish) are discussed. Environmental modulations on their toxicities are brought into the picture for further elucidation on their effects. Finally, knowledge gaps are gathered from contemporary studies on aquatic toxicology of NMs, and recommendations are made regarding such investigations in an attempt to improve their clarity, practicality and ecological relevancy.