Migration And Transformation Behavior Of Heavy Metals In MSWI Bottom Ash

Municipal soild waste incineration(MSWI), due to volume and mass reduction and energy recovery advantages, becomes an attranctive approach for MSW disposal in China. Bottom ash accounts for approximately 80 % of total MSW residues. According to previous studies, heavy metals enrich in MSWI bottom ash during the incineration process. The leaching of heavy metals from bottom ash is affected by 3 factors: 1. geochemical characteristic, such as acid neutralizing capcity and mineral composition; 2. exterior environment accidents, such as weathing affect; 3. properties of heavy metals in bottom ash.This paper analysed the physical and chemical properties of bottom ash, such as physical compositon, water content, total porosity, mineral form and heavy metal content. The results show that the bottom ash was porous, total porosity at 43.6%, and high alkalinity. The major mineral in bottom ash included quart, calcite, and hydroxide calcium. Simulated landfill experiment of MSWI bottom ash had been conducted for 64 weeks in the laboratory under anoxic and aerobic condition. The characteristics of leachate originated from the simulated experiment were analyzed, such as p H, ORP, TP, TN, COD and TOC. The p H values decreased continually, while ORP increased gradually and then decreased with time. The variation tendencies of the TP, TN, COD and TOC values could be divided into four phases: zooming phase, decreasing rapidly phase, decreasing slowly phase and stabilizing phase. At the end of the simulated experiment, the values of TN, TOC and COD were over the limit values according to Class 1A of Discharge standard of pollutants for municipal wastewater treatment plant(GB18918-2002).The analysises of bottom ash show there was no clear trend in the conent of Cu, Zn and Cr from two columns. Pb conent kept rising in anoxic condition. The Ni content of bottom ash from sampling A kept rising state under anoxic condition while sampling B on the contrary, and it kept decreasing in both samplings under aerobic condition. The content of As kept decreased with time in aerobic condition, but there was no clear trend in anoxic condition. At the initial phase of the simulated experiment, the leachate contained Cd,Pb,Zn,Cr,Ni and Cu, and Cd was under the limited value, but Zn, Cu, Ni and Cr were beyond the limited value of Integrated waste water discharge standard(GB8978-2002).There is a difference in heavy metals mass of bottom ash and leachate, Cu >Cr>Ni>Zn>Pb, and the same element submit to the phenomenon under anoxic and aerobic condition. The leaching ratio was in the order of Ni>Cu>Cr>Pb>Zn, Ni in about 20 %, Cu and Cr in 3 %~8 %, Pb and Zn under 0.12 %.The morphology of Zn from fresh bottom ash was tested by XAFS and fited by Ifeffit software, it finds that the morphology of Zn mainly included Zn2 Si O4, Zn Fe2O4, Zn O and Zn SO4.