Reverse Trojan-horse effect decreased wastewater toxicity in the presence of inorganic nanoparticles

We studied the toxicological interaction of 46 micropollutants from a biologically treated wastewater effluent in mixtures with silica, amine-modified silica, titanium dioxide and magnetite nanoparticles. The pollutants tracked in this work were polar pharmaceuticals belonging to different therapeutic groups, some of their metabolites and artificial sweeteners, the concentrations of which were mostly in the tens to hundreds of ng L-1 range. The results showed particularly high adsorption for furosemide, gemfibrozil and the aminopyrine metabolite 4FAA. There was preferential adsorption of the less polar compounds on the less polar nanoparticles. The total amounts of compounds adsorbed and quantified were 13.4, 4.8, 10.8 and 7.1 mu g g(-1) for SiO2, SiO2-NH2, TiO2 and Fe3O4, respectively. The toxicity of wastewater-nanoparticle mixtures was assessed using the bioluminescent cyanobacterium Anabaena sp. PCC 7120 CPB4337. The interactions were quantified by means of the combination index (CI)-isobologram method. The binary mixtures of wastewater with SiO2, SiO2-NH2 and TiO2 displayed antagonism for the lower affected fractions, which corresponded to the lower concentrations. For higher effects and for the Fe3O4 nanoparticles over the whole tested range, the mixtures were additive leading to synergism for the higher affected fractions. No internalization was observed. The results showed that the reduced toxicant bioavailability due to the interaction with nanoparticles is relevant for micropollutants at environmental concentrations. The amount of anthropogenic pollutants retained by metal oxide nanoparticles has significant toxicological effects.