The Levels, Speciation, and Influencing Factors of Mercury in MSWI Fly Ashes of a High Geological Background Area

Fly ash (FA) captures most mercury (Hg) in the waste stream during municipal solid waste incineration (MSWI), and the content and speciation of Hg in MSWI fly ash (MSWI-FA) have a significant impact on the ecological environment. In this study, 245 fly ash samples were collected from 18 MSWI facilities in Guizhou Province, a fragile karst area with a high Hg background in Southwest China. The results indicate that total Hg ranged from 0.52 to 444 mg/kg among samples, while the geometric means varied from 0.85 to 223.33 mg/kg for different incinerators, with a weighted geometric mean of 22.14 mg/kg, more than double the national average. Substantial temporal variability in Hg content suggested intermittent inputs of Hg-rich waste into the MSW stream. While positive correlations (p < 0.05) were observed between Hg and chlorine, potassium, and cadmium, the moderate to low R2 values indicate that Hg enrichment is a complex multi-factor system influenced by heterogeneous waste compositions and transient thermochemical conditions. Speciation analysis revealed that most of the Hg exists in its elemental form (F4), constituting over 80% of the total Hg. However, the water-soluble fraction (F1) and the ion-exchangeable fraction (F2) each reached up to 26–29% in some samples, indicating substantial environmental mobility. These findings demonstrate that MSWI-FA in Guizhou contains elevated Hg levels, posing considerable ecological and environmental risks. Enhancing the classification and management of Hg-containing waste in MSW is critical to mitigating the environmental risks of fly ash, particularly in karst ecological areas.