(510n) Leaching and Fixation of Heavy Metals in Municipal Solid Waste Incineration Fly Ash (MSWI FA) Based on CO2 Uptake Process

Due to the massive production by development in industries and economy, lots of municipal solid waste (MSW) have been produced daily and most of them are treated by utilization, incineration and landfill. For some developed nations in Europe, researches to utilize MSW incineration (MSWI) ash as a construction materials have been conducted since their physical properties are similar to that of aggregate. Except for the utilization, most of the produced MSW is treated by incineration and landfill. However, for the nations whose land area is confined, landfill is not preferred since it requires large area due to the volume of MSW itself. To decrease the volume of MSW, incineration can be considered, however, the problem occurs in the treatment of MSWI ashes. When MSWI ashes are buried, hazardous components such as heavy metals and other metal components can be leached out resulting in the pollution of underground water. Since underground water can flow to the far distance, environmental and biological problem is not limited to the land close to the landfill area.

To prevent negative impact on the environment, stabilizing method of MSW ash have been studied and applied. Representative ways are thermal treatment, solidification and washing treatment. In thermal treatment method, MSWI ash is heated to the high temperature in the range of 1500 – 1700 K resulting in the stabilization of the heavy metal and salts whose vapor pressure is relatively high. One of the merit of this method is that the hazardous organic component can be treated in the same time by the thermal oxidation in that temperature range. However, it costs a lot to maintain high temperature condition. For solidification process, some components in MSWI ash are fixed by the reaction with waste cement or the slag. The merit of this method is the prevention of hazardous component leaching by forming the compounds whose binding force is higher than that in the ash. Also, due to the reaction, the mass after the solidification is increased make it harder to transport to the landfill site. In the washing treatment, hazardous materials in soluble phase are dissolved into the solvent, mainly water. However, it requires the further treatment of wastewater containing dissolved components. In this study, we discussed a new ways to treat MSWI fly ashes which contains shows higher heavy metal contents compared to the MSWI bottom ashes.

Among various kinds of methods for the stabilization of heavy metals in municipal solid waste incineration fly ash (MSWI FA), acid leaching is the most prevalent. However, the process of heavy metals leaching in MSWI FA by various acids must be compared. In this study, we extracted the heavy metals in MSWI FA using 0.5 M solutions of strong acids (HCl, H₂SO₄ and HNO₃), as well as weak acids (NH₄Cl, (NH₄)₂SO₄ and NH₄NO₃). Solidification by CO₂ injection occurred in all the leaching solutions. The leaching test was performed for solids such as treated MSWI FA and the precipitates produced in the solidification process, compared with untreated MSWI FA. The solids were analyzed by using X-Ray fluorescence spectroscopy, X-Ray diffraction, and scanning electron microscopy. The leaching solutions were analyzed by inductively coupled plasma-optical emission spectroscopy. Using weak acid solutions, Zn, Pb, and Cu were leached 15 - 70 times, two times, and 20 - 150 times, respectively, more than those leached strong acids. Except H₂SO₄ and (NH₄)₂SO₄ solutions, all the acids reduced the mass of MSWI FA and extracted heavy metals. The crystalline precipitates were the carbonates. HNO₃ solution was found to be the most appropriate solvent for the proposed method.