Colloidal silica nanomaterials reduce the toxicity of pesticides to algae, depending on charge and surface area

Abstract

Colloidal silica nanomaterials are promising adsorbents for aquatic pollutants. The present study quantifies the toxicity reduction of differently charged pesticides (paraquat (cationic), pentachlorophenol (anionic) and diflufenican (uncharged)) by co-exposure to weakly anionic, strongly anionic and cationic silica nanomaterials in growth tests with freshwater green algae. The hypothesis was that the silica nanomaterials would preferentially adsorb the oppositely charged pesticide and reduce algal toxicity accordingly. Three different concentrations of each spherical nanomaterial (10, 20 and 50 mg L⁻¹) were tested in mixtures with a fixed pesticide concentration (4 μM paraquat (EC60), 0.002 μM diflufenican (EC80) and 0.2 μM pentachlorophenol (EC90)). In addition, we investigated the role of the specific surface area by comparing the performance of the anionic spherical silica nanomaterial with the anionic elongated silica nanomaterial (0.4–16 mg L⁻¹). Adsorption of pesticides onto the various nanomaterials was confirmed by chemical analysis of the supernatants after removing the nanomaterial with ultracentrifugation. The results show that a concentration of 16 mg L⁻¹ and 50 mg L⁻¹ of the spherical and elongated strongly anionic silica, respectively, completely annulled paraquat toxicity. The cationic nanomaterial reduced pentachlorophenol toxicity by 30–50% at all test concentrations, but the effect seems to be influenced by pH and phosphate concentration of the test medium. The cationic silica also reduced the toxicity of diflufenican by 10–20%, likely due to non-electrostatic interactions. The study further indicates that the presence of algae affects the NM–pesticide equilibrium since the pesticide is taken up by the organism instead of being bound to the NM.