Migration Behavior Of Heavy Metals In MSWI Bottom Ash As Leachate

The incineration technology has gradually become an important methods ofdealing with municipal solid waste (MSW). Incineration residues are composed with80%bottom ash and20%fly ash in weigh. Bottom ash contains much more heavymetals than MSW. In the landfill site, the heavy metals in bottom ash can dissolve in theprecipitation and runout with the leachate, which may cause secondary pollution to thegroundwater. So studying on the migration behavior of heavy metals in bottom ash hasimportant theoretical significance and practical application value.The physical and chemical properties were determined on bottom ash. Thelaboratory simulated leaching experiment was carried out by aerobic column andanaerobic column. The properties of the bottom ash were determination, such as density,total porosity ratio, as well as the moisture content. pH, COD, TOC, TN and TP ofleachate were determined. The results showed that the total porosity of bottom ash was43.6%, and the bottom ash was kind of porous medium, leachate pH value was as highas high12.7, COD and TOC of the leachate reached about16000mg/L and4000mg/L,respectively. With the time, COD and TOC concentration gradually reduced. The TPconcentration of leachate was at around10mg/L, and gradually deceased with time. Atthe beginning of the TN was about110mg/L and increased to130mg/L at8th week thenbegan to decrease.The leachate of bottom ash contained heavy metals, such as Cd, Pb, Zn, Cr, Ni, Cu.The content of Cd, Pb, Zn were lower than the limit of Integrated Waste waterDischarge Standard(GB8978-2002). And the content of Cr, Ni, Cu were much higherthan the limit of Integrated Waste water Discharge Standard(GB8978-2002). In the sameenvironment, the changes of heavy metal concentration with time in the leachate weredifferent. Cd, Cr, Ni, Cu presented primarily increases and then decrease, while thechange trends of Zn and Pb were adverse.According to the previous study, heavy metals in bottom ash mainly existed ashydroxide in strong alkaline condition (pH>12). The initial concentration of heavymetals were calculated according to the solubility of metal hydroxide. One-dimensionalconvection-diffusion model was applied. The permeability coefficients and dispersioncoefficients were obtained by calculation and computer simulation. The initial concentration and the model parameters were adopted to the model and concentration ofheavy metal was obtained by differential calculation. The calculated and determinedvalues of heavy metal concentrations were analysed and compared, and the correlationsof Cu, Pb, Cd, Ni were above0.9.