Leaching characteristics of sulfates and chlorides from fly ash produced by municipal solid waste incineration

Fly ash (FA) produced by municipal solid waste incineration (MSWI) is a hazardous waste material that must be properly treated before disposal in landfills. The aim of the present study was to study the characteristics of MSWI-FA (i.e., morphology, particle size, mineralogical phase, and elemental content) before and after pretreatment, which is stabilization with the use of heavy metal chelating agents or the combination of stabilization and solidification with the use of cement. The results showed that pretreated MSWI-FA particles had more regular shapes with decreased diameters. Further, there was hardly any change to the mineralogical phase, which mainly consisted of calcium, aluminum, and silicon salts. The primary elements included O, Na, S, Cl, K, and Ca, and the weight percentage (wt%) and atomic percentage (At%) of Cl after pretreated was much higher than before. Metal leaching test of pretreated MSWI-FA shown that the concentrations of toxic metals in leachate were below the limits of standard, while the concentrations of alkalinous metals, especially Na and K, were extremely high, ranging from 500 to 900 mg L-1. The results of a two-stage test of the leaching process revealed extremely high contents of the typical water-soluble salts in MSWI-FA. However, the pretreatment process failed to immobilize the sulfates and chlorides in MSWI-FA. In addition, as compared to chlorides, the sulfates in MSWI-FA transferred more quickly into the leachate, while the chlorides will gradually and may completely enter leachate. The development and application of chlorine- and sulfur-free reagents are recommended for the pretreatment of MSWI-FA, and a leachate collection system should be implemented for landfill disposal of MSWI-FA.