Application of thermal desorption for the identification of mercury species in solids derived from coal utilization

Abstract

The speciation of mercury is currently attracting widespread interest because the emission, transport, deposition and behaviour of toxic mercury species depend on its chemical form. The identification of these species in low concentrations is no easy task and it is even more complex in coal combustion products due to the fact that these products contain organic and mineral matter that give rise to broad peaks and make it difficult to carry out qualitative and quantitative analysis. In this work, a solution to this problem is proposed using a method based on thermal desorption. A sequential extraction procedure was employed for the comparison and validation of the method developed. Samples of fly ashes and soils were analyzed by both of these methods, and thermal desorption was found to be an appropriate technique for mercury speciation. Even in the case of low mercury contents, recovery percentages were close to 100%. The main mercury species identified in the samples studied were HgS and, to a lesser extent, HgO and HgSO4. In addition, although the presence of mercury complexes cannot be demonstrated, the desorption behaviour and sequential extraction results suggest that this element might be associated with the mineral matrix or with carbon particles in some of the solids.

Mercury pollution in vegetables, grains and soils from areas surrounding coal-fired power plants

“…Also as shown in Table 1, it is reasonable to predicate that the mercury concentrations of surrounding soils will increase, if these two power plants continue to operate. The average mercury concentrations of soil samples collected 1 km away from Power Plant Baoji (606 μg/kg)33, Power Plant “A Horizon” (19,900 μg/kg)34, and Power Plant Chengdu (24,546 μg/kg)35 were much higher than those of Power Plants A (305 μg/kg) and B (383 μg/kg) in this research (Table 1). The main reason for this discrepancy should be that these three power plants have been operated for much longer than the two for this study.…”

A comparison of devices using thermal desorption for mercury speciation in solids

“…Discussion of the different mercury compounds identified in each type of sample (Table 3) is not a matter for this work and has already been described in previous studies carried out in HgTPD-1 by the authors [18,23]. Nevertheless, it is worth mentioning that in those studies red HgS and mercury bound to unburned particles (namely either Hg-OM or Hg-HA), were the main species present in the fly ashes.…”

“…The fly ashes, namely CTL, CTA and CTP, were obtained from two pulverized coal power plants and a fluidized bed combustion plant, respectively. The characteristics of these fly ashes have been described elsewhere [18]. Two samples of gypsum (Gypsum A and Gypsum Z) were collected from the Wet Flue Gas Desulfuration Plants (WFGD) of two 1200 MW power stations.…”

“…Table 1 shows desorption temperatures for the main mercury compounds as described in the literature, obtained by using different experimental devices and conditions. The technique has been tested on a wide variety of solids, which include; sediments and soils [15][16][17][18], waste lamps [19], fly ashes [18,20], gypsum [21][22][23] and sorbents from Hg retention [24,25]. However, the validity of the method must remain open to doubt until the influence of several instrumental parameters upon the results has been clearly established.…”

Overview and challenges of mercury fractionation and speciation in soils

“…The extensive work by Biester et al [10,35,[68][69][70][71] demonstrated the adaptation of an atomic absorption spectrometer, by means of an in-house apparatus consisting of an electronically controlled heating unit and a mercury detection unit [69]. An alternative method for mercury speciation by thermodesorption consists of the use of direct mercury analysers, such as the LECO ® AMA-254 [23,26] or Lumex ® RA-915 + PYRO-915 [72,73], by simply adjusting combustion temperature and the heating programme. Thermo-desorption methods present some advantages over conventional chemical extraction methods and x-ray absorption methods.…”