Cd2+ toxicity as affected by bare TiO2 nanoparticles and their bulk counterpart

Toxicity of engineered nanoparticles has received extensive attention in recent years. However, nanoparticles always co-exist with other pollutants in natural environment. Whether there are any interactions between these classical pollutants and nanoparticles; and how these interactions may influence the environmental behavior, effects and fate of each other remain largely unclear. For this purpose, effects of bare titanium dioxide engineered nanoparticles (TiO2-NP) and their bulk counterpart (TiO2-BC) on Cd2+ bioavailability and toxicity to the green alga Chlamydomonas reinhardtii were examined in the present study. We first investigated the kinetics and equilibrium isotherm of Cd2+ adsorption on both particles in the algal culture medium. Pseudo-first-order adsorption kinetics was observed with equilibrium rate constant ranging from 0.19 to 0.33 min−1. Increase in Cd2+ adsorption with its ambient concentration at equilibrium followed a single Langmuir isotherm for different concentrations of TiO2. Furthermore, surface-area-based Cd2+ adsorption by TiO2-BC was higher than that by TiO2-NP in most Cd2+ concentration treatments suggesting that particle size was not the only cause for different adsorption. Both forms of TiO2 could alleviate Cd2+ inhibitive effects on C. reinhardtii. However, Cd2+ toxicity and its bioaccumulation were comparable as long as its free ion concentration in ambient toxicity media was similar regardless the particle size and concentration of TiO2. There was no TiO2 inside the algal cells either. Therefore, it was Cd2+ adsorption by TiO2 which decreased its ambient free ion concentration and further its intracellular accumulation as well as toxicity.

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► Pseudo-first-order adsorption of Cd2+ on both nano- and bulk-sized TiO2 was found.
► Increase in Cd2+ adsorption with ambient concentration followed Langmuir isotherm.
► TiO2 had no inhibitive effects on Chlamydomonas reinhardtii.
► TiO2 reduced the ambient free Cd2+ concentration and its bioavailability.
► Cd2+ toxicity was alleviated in the presence of TiO2.