It can be seen from Figure S6 (Supporting Information) that the leaching contents of Cd, Cr, Cu, and Pb increase at the 24th h or on the 30th day as MgO contents decrease from 1.6 to 4%. The results indicate that the sintering of ceramsite with higher contents of MgO leads to a chemically restructured matrix that may have the potential to stabilize heavy metals by decreasing their availability for leaching (29, 30). This also implies that the change in metals binding over time in aquatic environments is weak and that the toxic metals bound in the ceramsite pose no harmful impact on the environment. It is, therefore, concluded from the above results that as the MgO contents increase, stronger chemical bonds are formed between these heavy metals and the silicates or aluminosilicates in the ceramsite, making it difficult for them to be leached even over a long period.
The heavy metal compounds in ceramsite with Fe2O3 contents of 7% (CaO of 4% and MgO of 3%) are selected for analyses. The evidence of chemical immobilization of heavy metals can be gained from the mineralogical characterization, and the heavy metal compounds in ceramsite can be identified by XRD analyses (as shown in Figure S7, Supporting Information). The main compounds in the three ceramsites (Fe2O3of 7%, CaO of 4%, and MgO of 3%) are crocoite, chrome oxide, cadmium silicate, and copper oxide. The transformation of these heavy metals to the crystalline state is advantageous for the long-term stability of the metals, and the crystalline solids are likely to have improved binding capacity. Moreover, it is possible that the heat-induced transformation of crystallization causes most of the heavy metal ions to transfer from the surface to the interior of the particles.
It can be seen from Figure S6 (Supporting Information) that the leaching contents of Cd, Cr, Cu, and Pb increase at the 24th h or on the 30th day as MgO contents decrease from 1.6 to 4%. The results indicate that the sintering of ceramsite with higher contents of MgO leads to a chemically restructured matrix that may have the potential to stabilize heavy metals by decreasing their availability for leaching (29, 30). This also implies that the change in metals binding over time in aquatic environments is weak and that the toxic metals bound in the ceramsite pose no harmful impact on the environment. It is, therefore, concluded from the above results that as the MgO contents increase, stronger chemical bonds are formed between these heavy metals and the silicates or aluminosilicates in the ceramsite, making it difficult for them to be leached even over a long period.The heavy metal compounds in ceramsite with Fe2O3 contents of 7% (CaO of 4% and MgO of 3%) are selected for analyses. The evidence of chemical immobilization of heavy metals can be gained from the mineralogical characterization, and the heavy metal compounds in ceramsite can be identified by XRD analyses (as shown in Figure S7, Supporting Information). The main compounds in the three ceramsites (Fe2O3of 7%, CaO of 4%, and MgO of 3%) are crocoite, chrome oxide, cadmium silicate, and copper oxide. The transformation of these heavy metals to the crystalline state is advantageous for the long-term stability of the metals, and the crystalline solids are likely to have improved binding capacity. Moreover, it is possible that the heat-induced transformation of crystallization causes most of the heavy metal ions to transfer from the surface to the interior of the particles.
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