The influence of a washing pretreatment containing phosphate anions on single-mode microwave-based detoxification of fly ash from municipal solid waste incinerators

Abstract

Presently, control of the environmental risk posed by fly ash from municipal solid waste incinerators (MSWIs) urgently requires novel strategies to stabilize targeted heavy metals (HMs), detoxify toxic organic pollutants and separate residue alkali metal chlorides. In this study, a washing pretreatment containing phosphate (P) anions was adopted to stabilize HMs, and the main influencing factors as well as the thermodynamics were investigated. The results show that the adsorption process fits well with the Langmuir model, giving a maximal adsorption capacity of 114.35 mg P g−1 with a preferred liquid-to-solid ratio of 2.5 L g−1; the thermodynamics equation indicated that this was a spontaneous, endothermic and entropy-increasing process. Moreover, wet fly ash absorbed with P anions was further irradiated with single-mode microwave (MW) energy. The results show that under the preferred conditions of a mass ratio of SiC to wet fly of 10:2, temperature of ~480 °C and N2 atmosphere, ~99.1% of the total PCDD/Fs were efficiently degraded, accompanied by a residual toxicity equivalent of 13.0 ng I-TEQ/kg. Notably, the presence of thiourea (TU) shortened the MW irradiation time by 27.8%–61.5% compared with the absence of TU and enhanced the generation of hot spots. Furthermore, the principal parameters of MW-based Hagenmaler reactions apparently demonstrate significant difference with traditional heating-based Hagenmaler reactions mainly including the pyrolysis temperature over the range of 200–450 °C, an inert atmosphere and at least 1 h of residue time. Accordingly, this study provides a promising alternative strategy to stabilize HMs and detoxify organic pollutants, accompanied with a separation of residue alkali metal chlorides.