Effects Of Titanium Dioxide Engineered Nanoparticles On The Bioavailability And Toxicity Of Cd²⁺

The toxicity and environmental risks of engineered nanoparticles have received extensive attention in recent years. However, nanoparticles always co-exist with other pollutants in natural environment. Whether there are any interactions between these conventional pollutants and nanoparticles; and how these interactions may influence the environmental behavior, effects and fate of each other remain largely unclear.In these perspectives, three different types of titanium dioxide particles, the polyacrylate-coated TiO2engineered nanoparticles (Ml), bare titanium dioxide engineered nanoparticles (M2) and their bulk counterparts (M3) were chosen. Then their effects on Cd2+bioavailability and toxicity to a freshwater alga Chlamydomonas reinhardtii were examined. M1was further used to investigate its impact on the uptake, elimination and subcellular distribution of Cd2+in the ciliate Tetrahymena thermophila.M1could be well dispersed in the experimental medium and its pHpzc was approximately2while the pHpzc of the other two materials was around6. There was a rather quick adsorption of Cd2+on all three materials and a steady state was reached within30min. The pseudo-first order kinetics was found for the time-related changes in the amount of Cd2+complexed with M1, M2and M3, respectively. Increase in Cd2+absorption with its ambient concentration at equilibrium followed a single Langmuir isotherm for different concentrations of M2and M3. By contrast, Cd2+adsorption on Ml was dependent on the nanoparticle concentration with the maximum binding capacity31.9,177.1, and242.2mg/g, respectively, when the Ml concentration was1,10, and100mg/l. Furthermore, surface-area-based Cd2+adsorption by M3was higher than that by M2in most Cd2+concentration treatments suggesting that particles size was not the only cause for different adsorption.All the three forms of TiO2could alleviate Cd2+inhibitive effects on C. reinhardtii. Algal growth was less suppressed in treatments with similar total Cd2+concentration but more TiO2. 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 TiO2inside the algal cells. Therefore, it was Cd2+adsorption by TiO2which decreased its ambient free ion concentration and further its intracellular accumulation as well as toxicity.As was different from C.reinhardtii, T.thermophila could take up both Cd2+and M1continuously during a3-h experimental period. The adsorbed Cd2+on M1could also be ingested by T.thermophila. However, the uptake rate of non-adsorbed Cd2+in the presence of M1was higher than that of non-adsorbed Cd2+alone. The proportion of Cd2+in the organelles still increased even though part of Cd2+were desorbed from M1during the process of uptake. Accordingly, M1played as the carrier and subcellular distributor of Cd2+. After entering the cells, little Cd2+and no M1were depurated in5h. Further, M1was toxic to T. thermophile and the toxicity of Cd2+was enhanced in the presence of M1. However, there was no positive relation between the aggravated toxicity and concentrations of M1. The mechanism of combined toxicity needs to be further studied.