Speciation, Characterization, and Mobility of As, Se, and Hg in Flue Gas Desulphurization Residues

Flue gas from coal combustion contains significant amounts of volatile elements such arsenic (As), selenium (Se) and mercury (Hg), which could lead to serious environmental health risks. The capture of these toxic elements in the scrubber with a flue gas desulphurization (FGD) technique has resulted in generation of a metal-laden residue. Since FGD residues are being increasingly used in a variety of beneficial reuse applications, it is important to determine metal speciation and mobility to understand the environmental impact of its reuse. In this paper, we report the solid phase speciation of As, Se and Hg in FGD residues using X-ray absorption spectroscopy (XAS), X-ray fluorescence spectroscopy (XRF), and sequential chemical extraction (SCE) techniques. The SCE results indicated a strong possibility of As association with iron oxides, while Se was distributed among all geochemical phases. Hg appeared to be mainly distributed in the strong-complexed phase. XAS analysis indicated that As existed in its oxidized state (As (V)) while Se and Hg was observed in primarily reduced states as selenite (Se (IV)) and Hg(I), respectively. XRF images suggest a strong association of Hg and As with Fe oxide materials within FGD residues. The results from the SCE tests and leaching studies at different pH conditions indicated that the labile fractions of As, Se and Hg were fairly low and thus suggestive of their stability in the FGD residues. However, the presence of a fine fraction enriched in metal content in the FGD suggested that size fractionation of the FGD residues is important in assessing the environmental risks associated with their reuse.