Ecotoxicity of CdTe and its functional groups on Enchytraeus albidus
Abstract
Laboratory toxicity tests are used worldwide to assess the acute and chronic toxicity of specific pollutants to contribute towards the calculation of environmentally safe concentrations for nanomaterials (NMs). The field of nanotechnology is rapidly growing and the use of manufactured NMs in commercial products is increasing, however very little is known about the environmental effects of these materials. Soils are the end depository so it is essential to have an understanding of how the materials will affect soil organisms. The aim of this study was to assess the acute and chronic ecotoxicity of cadmium/tellerium (Cd/Te) Quantum Dots (QDs) utilizing Enchytraeus albidus as a test organism. The first objective was to determine the lethality of the NMs on Enchytraeus albidus, determine the reproductive success using sub lethal concentrations, assess avoidance behaviour and biomarker responses, the approach was to compare the toxicity of the three functional groups (Polyethylene glycol (PEG), Carboxylic acid (COOH) and Ammonia (NH3)) and lastly determine the uptake and distribution of the nanomaterial within the test organism. The nanomaterial was characterized by measuring the hydrodynamic size distribution by using Dynamic Light Scattering. Transmission electron microscopy was used to measure the diameter of QDs. Low dissolution rates (<23.7%) were found for the QDs coated with three different functional groups. The mean particle size showed that the NH3 group exhibited the smallest particle size of the three functional groups. Range-finding exposures were used to determine the concentration range for the definitive test using a standard Organization for Economic Co-operation and Development (OECD) guideline (220). The prepared exposure soil was characterised by oven-drying the soil and digesting with 10 mL of HNO3 for metal content following a three-week exposure and compared to the corresponding bulk metals CdCl2 and TeCl4. In comparison with the bulk metals after a threeweek exposure, the QDs displayed considerably less Cd and Te of the same nominal concentration. Transport of NMs in soil was conducted utilizing a flow-through system. Results indicated that when a stock concentration of QDs in Milli-Q water was added to the top of soil, the highest metal content was found in the eluted water and eluted clay fraction with the third highest in the top layer of the soil. When the NMs were homogenously mixed into the soil the highest metal content was again found in the eluted water and eluted clay fraction, but the metal content in the soil column increased towards the lower level of the soil. CytoViva Dark field imaging illustrated internalization of QDs in the intestine and ICP-MS analysis of whole worm tissue indicated metal uptake. The Lethal Concentration (LC50) values could not be calculated from the acute toxicity test because no mortality was observed. Only QD-COOH could determine the Effective Concentration (ECx) value for reproduction, with the EC10 and EC20 for COOH, calculated as 124.1 mg/kg and 720.6 mg/kg respectively. For QD-PEG and QD-NH3 no ECx values could not be determined. Avoidance behaviour of E. albidus was assessed and no significant behaviour was observed compared to the control, whereas the corresponding bulk metals caused avoidance at the highest concentrations (500 mg/kg). A series of biomarkers (Catalase (CAT), superoxide dismutase (SOD), protein carbonyl (PC), Malondialdehyde (MDA), Acethylcholinesterase (AChE), lipid fractions and protein content) were utilized to determine sub-lethal effects on the enchytraeid. Biomarker responses indicated that oxidative damage occurred after a three-week exposure. Inhibition of CAT and SOD occurred indicating no defence mechanisms could be activated to counter the stress of QDs, which can be explained by the low lipid fraction and high MDA content. Protein carbonyl content compared to the control, indicate protein damage has occurred. AChE displayed significant inhibition of QD-PEG 100 mg/kg is related to the high avoidance response to QD-PEG 100 mg/kg. In conclusion, the acute lethality tests showed no toxicity on the survival of the enchytraeids. However, chronic toxicity tests indicate that NM internalisation does occur and the QDs do have a sub-lethal biomarker response on the worms although survival, reproduction and avoidance response were not affected.