Leaching behavior of trace elements from fly ashes of five Chinese coal power plants

Abstract The leaching behavior of trace elements in five fly ashes from five Chinese coal-fired power plants were obtained by conducting the new US EPA leaching test methods (LEAF) and the conventional EPA Toxicity Characteristic Leaching Procedure (TCLP) test method. These were done under a range of pH values and liquid to solid ratios. Despite different elemental and mineralogical compositions of the ashes and different ranks of the respective feed coals, the majority of elements show similar liquid/solid partitioning (LSP) curve trends among the different ash samples in the pH-dependent leaching tests. This indicates that most elements are sensitive to changes in pH. Different LSP curves exist for Cr from pH-dependent tests, probably due to different valence states of this element in the unleached fly ashes. Although most elements show similar LSP curves for different fly ash samples from the liquid/solid (L/S) dependent tests, variation exists for LSP curves of some individual elements among different ash samples. Caution should be taken when generating general L/S dependent leaching characteristics for individual elements. The leachate concentrations of the highly soluble elements such as B and Se, as well as oxyanions under alkaline conditions, are dependent on their initial concentrations in the fly ashes. The conventional EPA method TCLP overestimates the leachability of most hazardous elements, such as Be, B, Co, Ni, Cu, Zn, Cd, Sb, and U, at least in the studied alkaline fly ashes. LEAF tests have the advantage of being a comprehensive evaluation of the leaching characteristics, with a range of pH and L/S allowing for the examination of leachability under various conditions at different disposal or beneficial use scenarios.