Mercury fractions transformation during sludge thermal treatment: implications for mercury release and stabilization in municipal sludge

Purpose As the only global pollutant among heavy metals that can be transported over long distances in gaseous form, mercury (Hg) and its forms are of global concern. More than 80% of the Hg in wastewater can be transported into sewage sludge, which becomes an important part of the global Hg emission inventory, and it is therefore crucial to analyze potential Hg emissions and transformation rules during sludge treatment/disposal. Materials and methods Sludge samples were collected from wastewater treatment plants (WWTPs) located in eight cities in China. The Hg fractionation in the sludge samples was assessed using a seven-step sequential chemical extraction (SCE) method. The relationship between the transformation of chemical constituents and the release of Hg during heating was analyzed by Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). Results and discussion We found that the proportion of oxidizable Hg and strongly acid-soluble Hg in sludge gradually decreased as the temperature increased, ultimately resulting in about 90% of residual Hg forms. Similar transformation characteristics were observed in sludge samples with different Hg concentrations. At temperatures between 100–200℃, the decomposition of organic components in sludge resulted in the breakdown of organic complex Hg. Subsequently, in the range of 200–400℃, the decomposition of silicate minerals may be the cause for the disappearance of HgS. Conclusion This study provides insights into the transformation of Hg fractions during sludge heating, shedding light on the roles of organic matter and mineral composition in this process. Understanding these dynamics is crucial for developing effective strategies to manage and mitigate Hg pollution during sludge disposal. Graphical Abstract