Effect of anion concentrations on Hg2+ reduction from simulated desulphurization aqueous solutions

Abstract

The influence of different anions on the stability of mercury in absorbing solutions containing sulphite ions was investigated. The re-emission of gaseous mercury in the presence of fluoride, chloride, bromide, nitrate, carbonate and sulphate ions from the scrubbing solution was determined. Fluoride, chloride and bromide ions contribute to mercury stabilization, bromide being the most efficient anion for retaining mercury in sulphite aqueous solutions. High carbonate concentrations prevent the reduction of mercury by sulphite ions due to the stabilization of the complexes formed between oxidised mercury, hydroxide and sulphite ions or through the formation of mercury carbonate species. However, nitrate and sulphate ions do not influence the equilibrium between mercury and sulphite ions and, as a consequence, the re-emission of mercury is not modified. The results of this study suggest that the pH is one of the most important parameters for achieving an efficient control of mercury pollution in desulphurization systems. The re-emission of mercury was observed to increase when the pH value of the slurries was higher than 6 due to the stabilization of the mercury in the form of . Since the pH depends on the concentration of SO 2 in the flue gas and the type of limestone, the alkalinity that comes from the sorbent used in the scrubber may play an important role in mercury capture in such systems.

Temperature programmed desorption as a tool for the identification of mercury fate in wet-desulphurization systems

“…The behaviour and distribution of the mercury in the experiments summarised above, as well as the proportion of mercury reemitted, have been described and discussed elsewhere [8,9,22,23]. In the present paper these gypsum samples were characterised for their mercury composition in order to assess the potential of the HgTPD method to confirm and improve upon previously discussed results.…”

“…The first-series of slurries was prepared using different limestone samples (D-G) [23]. The second series was obtained from experiments in which different additives and ions were added (H-P) [8,9,22]. The limestone that yielded gypsum B was used in the second series.…”

“…Four of them are FGD gypsum samples collected from industrial coal combustion power plants in Spain. The others were obtained using a laboratory-scale device that simulates mercury retention in wet scrubbers [8,9,22,23]. The lab-scale experiments to produce gypsum samples were carried out at a pH in a range of 4.0-8.0.…”

Study of mercury re-emission in a simulated WFGD solution containing thiocyanate and sulfide ions

“…Thus, it could be proposed that the inhibition effect due to SCN À might have similar features as halogen ions, as it was reported in many studies [16,[19][20][21][22][23][24]34], had evident inhibition effects on bivalent mercury reduction, where the formation of stable complex (ClHgSO 3 À and Cl 2 HgSO 3 2À ) (this had been assumed in our previous work [21] and other studies [23]) could be the main cause of mercury reduction inhibition in the presence of Cl À ions. For the purpose of revealing the inhibitory mechanism of SCN À ions, a UV-visible spectrum was used to identify the interactions among Hg 2+ , sulfite ion and SCN À in the following section.…”

“…Nevertheless, other liquid-phase ions in WFGD solution (including halogen ions, nitrate ion, sulfate ion, carbonate ions, hydroxyl ion and metal ions) were also reported, which were able to affect the oxidized mercury reduction process [16][17][18][19][20][21][22][23][24] via the interactions between oxidized mercury and sulfite ions. For instance, bromide ions could react with Hg 2+ to generate HgBr n À(n-2) (n = 1-4), which were very stable and hence limited the reduction and re-emission of mercury in the presence of sulfites ions [16,21].…”