Nano-TiO2 both increases and decreases arsenic toxicity: Evidence from different aquatic animal experiments

Titanium dioxide nanoparticles (nano-TiO₂) are usually applied to treat arsenic-polluted drinking water. Additionally, wide application of nano-TiO₂ can enter into the environment inevitably. Furthermore, titanium dioxide nanoparticles (nano-TiO₂) can adsorb ambient pollutants to modify their bioavailability in aquatic organisms, as well as their toxicity of its ambient pollutants. This study investigated arsenic accumulation, subcellular distribution and toxicity on two aquatic animals (Daphnia magna in freshwater and Artemia salina in salt water). Nano-TiO₂ acts as a positive carrier, significantly facilitating D. magna’s ability to uptake As(V) as well as Artemia salina. In Artemia salina, As percentage in biologically active metal (BAM) fractions significantly decreased after the addition of 10 mg L⁻¹ of nano-TiO₂, while the fractional percentage of As increased in cellular debris. These lower As proportions in the sensitive fractions of cells indicate As(V) toxicity inhibition. Higher As(V) EC₅₀ values in nauplii (by a magnitude from 1.97 to 2.76) compared to the control (nano-TiO₂ free) as we increased nano-TiO₂ concentrations from 1 to 1000 mgL⁻¹, indicates that nano-TiO₂ could alleviate As(V) toxicity in A. salina nauplii by enhancing efflux and decreasing the proportion of As in the sensitive fractions of cells. In contrast, in D. magna, even though As accumulation increased with increasing nano-TiO₂ concentrations in D. magna, As(V) toxicity associated with nano-TiO₂ exhibited a dual effect. Compared to the control, the increased As was mainly distributed in BDM (biologically detoxified metal), but Ti was mainly distributed in MSF (metal-sensitive fractions) with increasing nano-TiO₂ levels. Differences in subcellular distribution demonstrated that adsorbed As(V) on nano-TiO₂ could dissociate itself and be transported separately, which results in increased toxicity at higher nano-TiO₂ concentrations. Decreased As(V) toxicity associated with lower nano-TiO₂ concentrations results from unaffected As levels in MSFs (when compared to the control), where several As components continued to be adsorbed on nano-TiO₂. Accordingly, arsenic toxicity on aquatic animals by nano-TiO₂ depends on animal species and the behaviors of the co-contaminants inside, especially their assimilation in the digestive tract. More attention should be paid to the influence of nano-TiO₂on As(V) assimilation in the digestive tract.