Effect of titanium dioxide nanoparticles on copper toxicity to Daphnia magna in water: Role of organic matter

Inevitably released into natural water, titanium dioxide nanoparticles (nano-TiO2) may affect the toxicity of other contaminants. Ubiquitous organic matter (OM) may influence their combined toxicity, which has been rarely reported. This study investigated the effect of nano-TiO2 on Cu toxicity to Daphnia magna and the role of OM (dissolved or particle surface bound) in inducing combined effects. The effect of nano-TiO2 on heavy metal accumulation depended on the adsorption capacity for heavy metals of nano-TiO2 and the uptake of nano-TiO2-metal complexes by organisms. Nano-TiO2 significantly decreased Cu accumulation in D. magna, but the reducing effect of nano-TiO2 was eliminated in the presence of humic acid (HA, a model OM). In the Cu and HA solution, nano-TiO2 slightly affected the bioavailability of Cu2+ and Cu-HA complexes and thus slightly influenced Cu toxicity. The nanoparticle surface-bound HA reduced the effect of nano-TiO2 on the speciation of the accumulated Cu; therefore, the combined effects of nano-TiO2 and Cu on biomarkers similarly weakened. HA-altered Cu speciation may be the main factor responsible for the influence of HA on the combined effects of nano-TiO2 and Cu. This study provides insights into the combined effects of nano-TiO2 and heavy metals in natural water.