Mercury fractionation in gypsum using temperature desorption and mass spectrometric detection

A quadrupole mass spectrometer was used to study the thermal release of mercury from wet flue gas desulphurization (WFGD) gypsum using temperature-programmed desorption/decomposition (TPD). The inability in direct detection of low concentrations of mercury halogenides in gypsum by mass spectrometry is discussed in detail. The hydrolysis of HgCl2 vapours under specific experimental conditions in the mass spectrometer was considered theoretically and proved experimentally. The mercury concentration in different gypsum fractions varies from 0.22 mg kg-1 (3.27-148 μm, coarse particles) to 20.6 mg kg-1 (0.41-88.0 μm, fine particles). All samples had a similar, symmetrical, single-peak (peak maximum 253–266°C) in the TPD spectra. In the present study, the use of ‘wet’ methods for preparing mercury compounds is introduced in addition to the mercury standards prepared using the ‘dry’ method, as commonly found in TPD. The study showed that selected metals, such as Fe enriched in gypsum samples, significantly influence the shape and the maximum temperature of the Hg TPD curves and that during the mercury compound preparation and the TPD process, Hg species undergo transformations that prevent the identification of their original identity.