Coal combustion has caused severe environmental pollution. Mercury is a kind of toxic heavy metal, which widely exists in coal combustion flue gas in the form of mercury vapor, and it has resulted in the extreme negative on environment and humungous damage to human’s health owing to the chemical activity (highly toxic, high stability, bioaccumulation, persistence, and long-distance migration) and toxic Hg-based derivative(Reddy et al., 2012). According to UNEP’s global mercury assessment report in 2018, human activities have increased total atmospheric mercury concentrations up to 450% natural level. Coal combustion flue gas is the main anthropogenic emission source, accounting for 21%, and this number is still increasing(Outridge et al., 2018). In general, there are three types of mercury in flue gas: particle-bound mercury (Hgp), oxidized mercury (Hg2+), and elemental mercury(Hg0)(Yang et al., 2018). Hgp and Hg2+ can be removed by a dust collector and wet desulfurization device in the flue gas treatment facilities, respectively. However, Hg0 is hard to be removed by the general device owing to its high volatility, chemical inertness, and insolubility(Liu & Wang, 2014; Zhou et al., 2019). Thus, there is an urgent need for efficient and cost-effective removal techniques for Hg0.
Currently, catalytic oxidation(Li et al., 2010; Xu et al., 2017), photochemical oxidation(Liu et al., 2014; Zhao et al., 2018), and adsorption(Z. Liu et al., 2019) are common methods to remove Hg0. Adsorption technology has been widely used due to its simple operation, strong stability, and less secondary pollution(Liu et al., 2020; Yang et al., 2007). In conventional adsorptions, activated carbon injection (ACI) is one of the most effective and widely used technology for Hg0 removal owing to its large specific surface and sufficient surface oxygen functional groups(Fan et al., 2016; Krishnan et al., 1994). Nonetheless, raw activated carbon (R-AC) also has some disadvantages, such as high operation cost and limited effect(Sun et al., 2017). At present, increasing the surface active sites is the most effective method of activated carbon (AC) modification, including halide modification(Qu et al., 2016; Tong et al., 2017; Zhong et al., 2017), acid and alkali modification(Zheng et al., 2017), sulfur modification(Sano et al., 2017; Yao et al., 2014), metal and metal oxides modification(S. Yang et al., 2019; Zhu et al., 2018), and plasma modification(Zhang et al., 2016). However, the above methods still have the problem of low Hg0 removal efficiency. Consequently, it is necessary to develop a more efficient and valuable modification method of AC.
Selenium (Se) shows excellent chemical combination ability in removing mercury. The binding affinity constant of mercury and selenide is 1022, which is one million-fold higher than that the binding affinity between mercury and sulfur(Ahmed et al., 2017). Selenium can combine with gaseous mercury to form an extraordinarily stable mercury selenide (HgSe) precipitate, with solubility constant (Ksp, 1.0×10−59) as compared to that of HgSn (Ksp, 1.0×10−52), avoiding the risk of leaching hazardous in subsequent processing(Ahmed et al., 2017; J. Yang et al., 2019). Previous research(J. Yang et al., 2020; J. Yang et al., 2019; Z. Yang et al., 2020) has established that selenium or selenide as an active component plays a key role in the removal of Hg0. Yang et al. selected MIL-101(J. Yang et al., 2019) and copper foam(J. Yang et al., 2020) as the supporters for selenium particles. These two adsorbents show excellent Hg0 adsorption capacity. However, the synthesis process using MIL-101 or copper foam as a carrier is complex and has not been widely used in commercial applications compared with AC. Furthermore, AC is a better material owning to its larger specific surface area, better pore structure and porosity, more active site on the surface, and higher consistency with the current industrial application. Therefore, it is a cost-effective and prospective option to select AC as a carrier of selenium.
In this study, the different proportions of selenium activated carbon adsorbent were synthesized by a simple high temperature selenium impregnation method. The Hg0 removal efficiency was systematically evaluated under different conditions (reaction temperature and gas composition). The new adsorbent has more than ten times the adsorption capacity than R-AC. Meanwhile, according to the DFT calculation and samples characterization analysis, the mechanism of removing Hg0 by the samples is explored. The synergistic effect of physical adsorption caused by AC and chemical adsorption caused by Se provides high Hg adsorption efficiency.