Metal stabilization mechanism of incorporating lead-bearing sludge in kaolinite-based ceramics

The feasibility and mechanism of incorporating simulated lead-laden sludge into low-cost ceramic products was investigated by observing the reaction of lead with two kaolinite-based precursors under sintering conditions. To investigate the phase transformation process of lead, lead oxide (PbO) mixed with a kaolinite or mullite precursor were fired at 500–950 °C for 3 h. Detailed X-ray diffraction analysis of sintered products revealed that both precursors had crystallochemically incorporated lead into the lead feldspar (PbAl2Si2O8) crystalline structure. By mixing lead oxide with kaolinite, lead feldspar begins to crystallize at 700 °C; maximum incorporation of lead into this structure occurred at 950 °C. However, two intermediate phases, Pb4Al4Si3O16 and a polymorph of lead feldspar, were detected at temperatures between 700 and 900 °C. By sintering lead oxide with the mullite precursor, lead feldspar was detected at temperatures above 750 °C, and an intermediate phase of Pb4Al4Si3O16 was observed in the temperature range of 750–900 °C. This study compared the lead leachabilities of PbO and lead feldspar using a prolonged leaching test (at pH 2.9 for 23 d) modified from the toxicity characteristic leaching procedure. The results indicate the superiority of lead feldspar in stabilizing lead and suggest a promising and reliable strategy to stabilize lead in ceramic products.

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► Formation of lead feldspar as the stabilization mechanism of lead by 3-h sintering.
► Lead feldspar forms at temperatures 700–950 °C from different precursors.
► Pb4Al4Si3O16 and a polymorph of lead feldspar were detected as intermediate phases.
► Lead feldspar is superior to lead oxide in its intrinsic leachability.