MSWI Bottom Ash Co-disposed With MSW In Landfill: Heavy Metal Leaching Behavior And Biological Risk Assessment

Due to the scarity of the land resource, incineration became a popular way for the treatment of the municipal solid waste (MSW), which resulted in the large production of municipal solid waste incinerator (MSWI) bottom ash. However, MSWI bottom ash contains high level of heavy metals. The leaching of heavy metals may inhibit its potential beneficial reuse as a secondary construction material and cause serious environmental pollution after landfill disposal. In this study, bottom ash from six typical MSW incinerators located in different cities of Zhejiang province was sampled. The content and speciation distribution of the heavy metal were analysed. Based on the results, Cu, Zn and Pb were chosen to study the long-term leaching behavior and the leaching mechanism of the heavy metals, by the combination analysis with acid neutralizing capacity (ANC), heavy metal speciation distribution, pH depending leaching test and Visual MINTEQ model. According to the leaching test. Cu and Zn were chosen to study the effect of weathering pre-treatment on the speciation distribution and leaching behavior of heavy metals in MSWI bottom ash. Based on above results, three sets of simulated landfills were designed to study the effect of different mass proportion of MSWI bottom ash co-disposal with MSW on the leaching behavior of Cu and Zn as well as the biological risk. Furthermore, two sets of simulated landfills were designed to study the effect of leachate recalculation on the stabilization process, heavy metal leaching behavior and biological risk of the MSWI and MSW co-disposal landfill. The main conclusions of this study were listed below.(1) MSWI bottom ash contains high level of heavy metals, and the average heavy metal content in the bottom ash followed the sequence of Zn> Mn> Cu> Pb> Cr> As> Ni> Co> Mo> Cd> Hg. Zn, Cu, Cr, Mn and Pb were the major heavy metals whose contents were above 300 mg/kg. Considerable amounts of heavy metals were presented as the exchangeable fraction and carbonate bound fraction. According to the calculation, there were 95.3 mg of Cu,19.4 mg of Pb and 363.8 mg of Zn presented as exchangeable fraction and carbonate bound fraction in 1 kg MSWI bottom ash. which had the potential to leach out and cause serious pollution to the surrounding environment.(2) The leaching of Cu, Zn, Pb was relatively low in alkine condition. However. they would be greatly leached when the pH was below 6.0,6.7 and 7.0, respectively. Moreover, the leached heavy metals had higher bioavailability in the acidic environment. The leaching of Cu and Zn was limited by their speciation distribution in MSWI bottom ash, while Pb leaching might be controlled by the precipitation and dissolution equilibrium of specific minerals, which resulted in the relatively low level of Pb leaching compared with Cu and Zn. In this study, the leaching ratio could be up to 72.4% and 45.0 for Cu and Zn. while the leaching ratio of Pb was only 1.9%. Generally. decreasing the bottom ash pH to the acid condition by acid precipitation would be a long process because of the high ANC of MSWI bottom ash. suggesting the environmental impact was negative when MSWI bottom ash was reused. However, if the MSWI bottom ash was disposed in the MSW landfill. the heavy metal might be greatly leached due to the high level of organic acid in the MSW landfill.(3) Weathering treatment could change the fractionation of Cu and Zn in MSWI bottom ash. After weathering treatment, the exchangeable fraction, carbonate bound fraction and Fe-Mn oxides bound fraction of Zn increased from 0.3%,17.1%,19.6% to 0.7%,22.7% and 36.7%, respectively. The organic matter bound fraction kept relatively steady, while the residual fraction of Zn reduced from 55.1% to 30.9%. The organic matter bound fraction of Cu decreased from 68.7% to 36.8% while the exchangeable fraction, carbonate bound fraction and Fe-Mn oxides bound fraction of Cu increased from 1.8%.2.5%,2.8%,24.1% to 2.7%,3.7%,3.8%,53.7%, respectively. It indicated the weathering treatment could increase the potential leaching ability of the heavy metals. The leaching of Cu and Zn decreased in SPLP procedure, which was probably caused by the increase of the aluminum (hydr)-oxides during the weathering treatment. However, their leaching increased in TCLP procedure, in which the leachate pH was acidic. It might be attributed to the fact that the adsorption capacity of the aluminum (hydr)-oxides was restricted in the acidic condition, and the increasing of leaching ability of Cu and Zn after the weathering treatment was therefore expressed.(4) MSWI bottom ash co-disposal with MSW had little impact on the Cu concentration in leachate. However, it had some influence on the Zn concentration in the leachate, which was depended on the mass proportion of MSWI bottom ash disposed in the landfill. If 10% mass proportion of MSWI bottom ash was disposed in MSW landfill, Zn concentration in the leachate could be notably increased and the total leaching amount of Zn could be increased by 30%. However, if 20% mass proportion of MSWI bottom ash was disposed in MSW landfill, Zn concentration could be decreased, constrastly. It might be ascribed to the fact that 20% mass proportion of MSWI bottom ash would significantly increase the pH of landfill, which restricted the mobility of Zn. Cu was mainly presented as organic matter bound fraction in the leachate. The co-disposal of MSWI bottom ash with MSW in landfill had little impact on the fractionation of Cu in the leachate. However, it could facilitate the transformation of Zn fractionation from the small molecular to big molecular, which decreased the bio-availability of the leached Zn.10% and 20% mass proportion of MSWI bottom ash co-disposed with MSW would decrease EC50 of the leachate from 530 mL/L to 294 mL/L and 314 mL/L at the beginning of landfilling, respectively, indicating the co-disposal could increase the biotoxicity of the leachate. However, the EC50 of the leachate was increased from 258 mL/L to 294 mL/L and 314 mL/L at the late phase of the landfilling, indicating the co-disposal of MSWI bottom ash and MSW could decrease the biotoxicity of the leachate with the process of landfilling.Besides, Cu and Zn content of MSW from the bottom layer was higher in MSWI bottom ash and MSW co-disposed landfill than that of MSW landfill, which increased the potential threat of the landfill. Corresponding pollution prevention measures should be enhanced to prevent the potential pollution by the abnormal heavy metals releasing from the MSWI bottom ash and MSW co-disposed landfill.(5) The recirculation of leachate could accelerate the stabilization process of MSWI bottom ash and MSW co-disposed landfill. It had little impact on the Cu concentration in leachate. However, it could notably reduce the Zn concentration in the leachate. The recirculation of leachate significantly reduced the accumulated leaching amounts of Cu and Zn from 11.47mg,144.01mg to 4.71mg and 14.69mg. respectively. Due to the recirculation, the bioavailability of the leached Cu and Zn was also reduced, which meant the environmental risk by the discharge of heavy metal from landfill was greatly migrated. The bioreactor operation with leachate recirculation of MSWI bottom ash and MSW co-disposed landfill not only accerated the stabilization process of landfill, but also reduced the release of pollutant and the bio-toxicity of the leachate. providing a new idea for the designing of the landfill. However, leachate recirculation treatment would increase the heavy metal contents of MSW in the bottom layer of MSWI bottom ash and MSW co-disposed landfill, which may probably lead to the increasing of the potential environmental risk. To prevent the potential pollution by the abnormal heavy metals releasing from the landfill, relevant pollution prevention measures should be enhanced.