Hexavalent chromium release from lignite fly ash and related ecotoxic effects

Hexavalent chromium Cr(VI) is a pollutant of immense concern due to its high mobility to water sources and highly toxic properties. In most cases, Cr(VI) could be released from lignite fly ash in aquatic environment when fly ash comes into contact with water. In this study, the contribution of the leaching patterns and bioavailability of Cr(VI) from lignite fly ash to the overall ecotoxic properties of fly ash leachates was originally examined and leaching procedures were evaluated in this context. A series of customized leaching tests were conducted and a battery of ecotoxicity tests including the crustacean Daphnia magna and the photobacterium Vibrio fischeri was applied. The leaching of Cr(VI) was pH and liquid to solid (L/S) ratio dependent, exhibiting the highest releases at pH values between 7 and 8. At the liquid to solid ratio (L/S) equal to 100 L/kg, the (CrVI) release reached a plateau, implying the presence of diffusion constrains and/or solubility hindrances. The toxic effect of the leachates obtained under leaching at pH 7 towards D. magna was relatively high (TU = 28.6 (23.8-35.7) at L/S = 10 L/kg). Interestingly, the toxicity of the leachates towards D. magna not only was significantly correlated to Cr(VI) (r = 0.961, P < 0.01), but the toxicity of the leachates (in absolute values) was matching the toxicity of the Cr(VI) revealing its remarkable contribution to the overall effect. In addition, the lower sensitivity of the bacteria V. fischeri when exposed to the leachates, along with the time dependence of the toxicity profiles supported the interpretation of the results obtained in this study

Assessment of vulnerable freshwater ecosystems and various aquatic effluents by means of ecotoxicological assays

A battery of ecotoxicological assays of different trophic chain levels has been performed for water and sediment samples of two Greek vulnerable freshwater ecosystems (Karla Lake in Thessaly Region and Koronia Lake in Central Macedonia Region, respectively). These lakes, despite remediation programs, are still characterized by varied water quality. Furthermore, the quality of aquatic reservoirs aimed for human consumption as well as this of treated and untreated wastewater samples was also assessed through these bioassays. The outcome of the present research shows that condition of the studied lake ecosystems is still poor and intervention is needed for rectification of their continuous decline. Samples from treated municipal wastewater from the same regions were of medium to satisfactory quality, as measured by ecotoxicological assays. Despite causative relationships between burdened physicochemical parameters and increased toxicity, this toxicity could not always be attributed to these parameters. Results of bioassays in combination with standard physicochemical measurements of water samples may aid the integrated assessment of environmental risk arising from presence or release of these water samples in the ecosystem