Study on speciation of HgCl₂ supported over activated carbon in simulated flue gases by temperature-programmed decomposition desorption mass spectrometry

HgCl₂ that was homogenously dispersed over activated carbon was prepared by an HCl solution method (HgCl₂(HCl)/AC), in the present study. The speciation of HgCl₂(HCl)/AC in simulated flue gases was further investigated by a temperature-programmed decomposition desorption mass spectroscopy method, to clarify its transformation mechanism. It was found that HgCl₂ sublimation occurred after Hg-H₂O pretreatment, evidenced by the disappearance of the desorption peak of HgCl₂ at 80 °C. After Hg-H₂O pretreatment, new species of HgCl₂ were formed, as confirmed by similar peak intensities of HgCl₂ at mass numbers 270 and 272 at 335 °C. The SO₂-H₂O or H₂S-H₂O pretreatment helped stabilize the HgCl₂ species supported over the AC, as evidenced by the weaker desorption peak of HgCl₂ at around 80 °C. For the HgCl₂(HCl)/AC pretreated with SO₂-H₂O, four desorption peaks of mercury were found, at 280, 335, 380 and 480 °C, indicating that new mercury compounds were formed during pretreatment or the TPDD process. The mercury desorbed at 280 °C was derived from a mercury complex of [HgCl]₂SO₄⋯A-C, while the mercury desorbed at 480 °C resulted from HgSO₄. Some HgO species might be formed and stabilized over the AC during the TPDD process, corresponding to two mercury desorption peaks at 335 and 380 °C. After the pretreatment of HgCl₂(HCl)/AC with H₂S-H₂O, some HgCl₂ were transformed to HgS and HgO species, as evidenced by the mercury desorption peaks at 280, 305 and 380 °C.