Fate of heavy metals during co-disposal of municipal solid waste incineration fly ash and sewage sludge by hydrothermal coupling pyrolysis process

Abstract

In this work, the hydrothermal coupling pyrolysis (HTP) method was used to treat municipal solid waste incineration fly ash (IFA) and municipal sewage sludge (MSS). The regulation of migration mechanism of heavy metals (HMs), which included Cr, Ni, Cu, Zn, Cd, and Pb, were investigated, including the conditional effects of hydrothermal pretreatment (HTT), the pyrolysis temperature, the pyrolysis time, and the heating rate (HR) on the HM distribution. The results indicated that HTT, as a pretreatment method, achieved the redistribution and preliminary immobilization of the HMs, decreasing the potential environmental risk level. After HTP, the HMs (Cr, Ni, and Cu) were more immobilized, and this effect was enhanced when the pyrolysis temperature was increased from 300 to 800 °C. However, Zn, Cd, and Pb evaporated under high temperature. Leaching experiments revealed that all the HMs in the pyro-char from pyrolysis at 800 °C were below the standard (US EPA). The influences of the HR and pyrolysis time on the HM immobilization were slight under a higher temperature. After HTP, the HM environmental risk decreased to a low level. The physico–chemical characteristics of the pyro-char demonstrated that carbon trapping and chemical sedimentation played leading roles in the middle-temperature range, while mineral matrix encapsulation might have been dominant under high temperature.