Rapid Toxicity Screening of Gasification Ashes

Waste gasification technology is considered a sustainable solution to reduce waste volumes and recover energy by producing syngas. Meanwhile, it also produces solid residues including bottom ashes and fly ashes that contain metals or organic compounds. The solid residues can be reused as secondary raw materials. However, due to the diverse sources of solid waste and uncertain composition of bottom ashes and fly ashes, it is difficult to establish a standard chemical method to evaluate the quality of the ashes, and hence poses challenges for its applications. A lot of studies have demonstrated that the major restriction of the ashes applications is associated with the toxicity of ashes. Therefore, toxicity screening of ash is the primary condition for reusing the ash. In this study, we established a standard protocol for rapid screening of gasification ashes on the basis of in vitro and in vivo testing, and thus guide the proper disposal of the ashes.  We used three different test methods consisting of in vitro human cell lines (liver and lung cells), and in vivo tests on Drosophila melanogaster and Daphnia magna to examine the toxicity of six different types of ashes. For each ash, different leachate concentrations were used to examine the toxicity, with C_o being the original extracted leachate concentration, while C/C_o being subsequent diluted concentrations. The IC50 for each leachate was also quantified for use as an index to classify toxicity levels. The results demonstrated that the toxicity evaluation of all types of ashes using different models is consistent with each other. As the different models showed consistent qualitative results, we can choose one of the models, for example liver cell line or lung cell line, as the standard for rapid toxicity screening of gasification ashes. We may classify the gasification ashes into three categories according to the IC50, 24h value on liver cells or lung cells models, namely “toxic level I” (IC50, 24h > C/C₀ = 0.5), “toxic level II” (C/C₀ =0.05 < IC50, 24h < C/C₀ = 0.5) and “toxic level III” (IC50, 24h < C/C₀ = 0.05). Such a simple yet informative approach can help to determine the toxic effects of various types of ashes generated in common practice of gasification plants. Subsequently, appropriate disposal methods can be recommended for each toxicity category.