Abstract:
Engineered nanoparticles (ENPs) including silver (n-Ag) and zinc oxide (n-ZnO) are among rapidly increasing contaminants of emerging concern (CECs) globally. Moreover, pesticides (e.g. atrazine (ATZ)) owing to their extensive application are present in variant ecological systems like ground and surface waters, with unintended threats to aquatic life forms. Further, increasing presence of different classes of CECs and persistent organic pollutants (POPs) has led to their co-existence as mixtures in the environmental systems; but with unknown toxicological outcomes to organisms at different biological levels of organization. Herein, freshwater crustacean Daphnia magna was used as a model organism to assess the biological effects of individual ENPs, their binary mixtures with or without ATZ, and ternary mixtures with ATZ. The organisms were exposed to variant concentrations of ENPs and ATZ in ISO medium (standard freshwater), and immobilization was investigated as the endpoint.
Evaluation of the physicochemical characterization of ENPs was carried out to establish plausible link between their properties, and the observed biological outcomes. Aggregation of n-Ag increased with exposure time; whereas that of n-ZnO significantly decreased over time. For the binary mixtures, results revealed irrespective of ENPs type, aggregation size increased in the presence of ATZ due to hetero-aggregation between the compounds. Results of the ternary mixtures indicated aggregation size was four-fold relative to those of ENPs binary mixtures. Dissolution of individual n-Ag and n-ZnO increased and decreased over exposure period of 48 h, respectively. The presence of ATZ enhanced the dissolution of ENPs in the binary mixtures. However, in the ternary system a reduction of the dissolution of both ENPs in the presence of ATZ was observed.
Studies on the toxicity of individual contaminants based on median effective concentration (EC50) values in decreasing order were n-Ag > n-ZnO > ATZ, largely attributed to high dissolution of n-Ag in the exposure medium. For binary mixtures with n-Ag, higher toxicity was apparent, relative to other mixture systems (binary or ternary). For ternary mixtures, no toxicological outcomes were observed irrespective of the exposure concentrations. These findings demonstrated that the toxic silver ions were the primary driver of toxicity towards D. magna. Moreover, co-existence of both nanoscale and macroscale may lead to the chemical transformation of CECs in the exposure medium with marked reduction of individual contaminants toxic effects.
