Synergistic Toxic Effect Of Nanoparticles And As(Ⅴ)and Pb(Ⅱ) On Ceriodaphnia Dubia

Nanoparticles （NPs）, which are sized at least one dimension less than100nanometers,exhibit unique physical characteristics such as a large specific surface area and a uniquesurface structure that cause high surface reactivity. NPs are increasingly used forindustrial and commercial purposes as catalysts, semiconductors, cosmetics,microelectronics, drug carriers, etc., which may increase the NP release into theenvironment. Therefore, the potential negative environmental effect of NPs has drawnsignificant attention in recent years. However, most studies merely focused on theenvironmental effect of single nanomaterial. It is common to find the co-existence ofseveral toxins in a same place in the real environment. The aggregation of nanoparticles,the accumulation of nanoparticles in C. dubia, and the synergistic effect of nanopaticleswith the presence of As（Ⅴ） or Pb（Ⅱ） on C. dubia were investigated in this paper. Themain content was as listed below.Aggregation kinetics of TiO₂nanoparticles were investigated under different ionicstrength, pH, and particle concentration conditions. Zeta potential of these particles wasmeasured over the pH range of2-10at three different ionic strengths. Hydrodynamicradius of TiO₂aggregates were plotted as a function of time. Results showed fastaggregations for TiO₂nanoparticles under all conditions. A constant related to theaggregation, Kp, was determined by modeling the aggregation data according to thesecond-order rate law. The highest aggregation rate occurred at the pH close to the pHzpc,where the lowest electrical repulsion between the particles was anticipated. Increasingionic strength and particle concentration enhanced the aggregation rate. The DLVOtheory was used to explain the experimental results. Aggregation of TiO₂nanoparticleshas a significant effect on its number concentration and size, which may lead to a betterunderstanding on the toxicity mechanism. The bioaccumulation of nano-Fe₂O₃（m） with different exposure times, NP concentrations,and pH conditions was investigated using Ceriodaphnia dubia （C. dubia） as the modelorganism. Under natural pH conditions, C. dubia significantly accumulated nano-Fe₂O₃（m） in the gut, with the maximum accumulation being achieved after6hours ofexposure. The concentration of nano-Fe₂O₃also impacted its accumulation, with themaximum uptake occurring at20mg/L or more. In addition, the highest bioaccumulationoccurred in a pH range of7to8where the highest feeding rate was reported, confirmingthat the ingestion of NPs is the main route of nano-Fe₂O₃（m） bioaccumulation. In a cleanenvironment without NPs, depuration of nano-Fe₂O₃（m） occurred, and food additionaccelerated the depuration process.Toxic effect of As （Ⅴ） and n-TiO₂was investigated in present research. The interactionof As （Ⅴ） species with n-TiO₂was examined and found that As （Ⅴ） can accumulate at then-TiO₂surface at high concentrations. To examine the toxic effect, cultured ceriodaphniadubia species were exposed to1) n-titanium dioxide suspensions,2) As （Ⅴ） solutions and3) n-TiO₂suspensions in presence of As （Ⅴ） species. Results show that n-TiO₂is lesstoxic with no median lethal concentration (LC₅₀) found in test concentration range （<400mg/L）. And the median lethal concentration of As （Ⅴ） was found as3.68±0.22mg/L.However, there is high mortality observed if ceriodaphnia dubia exposed to n-TiO₂suspensions in presence of As （Ⅴ） species. The shift of the LC₅₀was found increased firstthen decreased with the increasing of the n-TiO₂concentrations. There is a peak value ofmortality in terms of nanoparticles concentrations （50mg/L）. Preliminary mechanism ofthe synergistic effect was discussed. Hydrodynamic size and sorption capacity werefound the two most important parameters to explain the toxicity results.The toxicity of nano-Al₂O₃, inorganic As（Ⅴ）, and a combination of both was examinedwith C. dubia as the model organisms. Bare nano-Al₂O₃particles exhibited partialmortality at concentrations of greater than200mg/L. When As（Ⅴ） was also present, asignificant amount of As（Ⅴ） was accumulated on the nano-Al₂O₃surface, and thecalculated LC₅₀of As（Ⅴ） in the presence of nano-Al₂O₃was lower than that it waswithout the nano-Al₂O₃. The adsorption of As（Ⅴ） on the nano-Al₂O₃surface and the uptake of nano-Al₂O₃by C. dubia were both verified. Therefore, the uptake of As（Ⅴ）-loaded nano-Al₂O₃was a major reason for the enhanced toxic effect.The effect of two common NPs, nano-CeO2and nano-TiO₂, on the toxicity of Pb wasevaluated using Ceriodaphnia dubia （C. dubia） as the model organism. Standard EPAprocedures were followed in the toxicity evaluation. The toxicity of bare NPs （withoutPb） was first evaluated and safe doses （levels without causing lethal effect） of NPs wereused in the synergistic studies. It was found that the overall toxicity of Pb in the systemcontaining NPs was greater than that of Pb alone, as indicted by the reduced medianlethal concentration (LC₅₀) of soluble Pb. The sorption of Pb onto the NP, and the uptakeof NPs in the gastrointestinal tract of C. dubia were validated. Therefore, the uptake ofPb-loaded NPs increased the exposure of C. dubia to Pb, resulting in the enhancedtoxicity. Reducing the solution pH could shift Pb speciation and enhance the overalltoxicity of Pb, with or without the presence of NPs.Although nano-Fe₂O₃（m） and nano-Al₂O₃alone did not exhibit significant toxicity in theexperimental concentration range, they could significantly enhance the toxicity of As（Ⅴ）.The accumulation of As（Ⅴ） on nano-Fe₂O₃（m） and nano-Al₂O₃, as well as the uptake ofthese NPs in C. dubia, were verified. An inverse relationship between the LC₅₀ of thesoluble As（Ⅴ） and the accumulation of NPs in C. dubia was developed. The uptake ofAs（Ⅴ）-loaded NPs in C. dubia played a key role in enhancing As（Ⅴ） toxicity.