Development of selenized magnetite (Fe3O4−xSey) as an efficient and recyclable trap for elemental mercury sequestration from coal combustion flue gas
Graphical abstract
Introduction
Mercury (Hg), an element known for its persistence, transportability and hypertoxicity, is of tremendous environmental concerns as a toxin that causes birth defects and neurological impairments for humans [1], [2], [3]. According to the newly released Global Mercury Assessment, the anthropogenic Hg emission in 2015 reached 2500 tons worldwide [4], far exceeding the Hg emissions from natural processes. Thus, the Minamata Convention entered into force in 2017 among its 128 signatories [5], aiming at giving immediate regulations on Hg emissions from various anthropogenic sources, hence relieving modern world from heavy Hg pollutions. Coal combustion is recognized as the largest single-known source for Hg emissions [6], [7], from which elemental mercury (Hg0), oxidized mercury (Hg2+) and particulate-bound mercury (Hgp) primarily discharge after combusting coals [8]. During the flue gas treatment process, the Hg2+ and Hgp can be effectively degraded by wet flue gas desulfurization (WFGD) and air pollution control devices (APCDs) [9], [10], whereas Hg0 is rarely captured considering its high volatility and insolubility in water [11], [12], [13]. Effectively removing Hg0 from coal combustion flue gas therefore becomes the key challenge to fulfill the obligations imposed by the Minamata Convention.
Despite the development of Hg0 removal technique for decades, activated carbon injection (ACI) remains the most widely commercialized method for Hg0 capture from coal combustion flue gas [14], [15], [16], [17], [18]. Although the injection strategy was proven to be a cost-effective methodology for Hg0 removal as it requires the installation of no extra facility [19], using activated carbons (ACs) as reactive agents suffers critical problems. Specifically, ACs generally exhibited extremely low Hg0 uptake rates, which leads to exceedingly high operation cost, ranging from 14,400 $/lb Hg to 38,200 $/lb Hg for 99% removal efficiency [20]. Moreover, the mixture of Hg-laden ACs with fly ash and/or gypsum not only pollutes the flue gas treatment wastes [21], [22], but also re-emits Hg when the mixture is dumped and landfilled as the physisorption primarily accounts for Hg0 immobilization over ACs [23]. Thus, the development of an alternative Hg0 remediator with high performance and negligible environmental aftermaths is of great urgency for an ideal sorbent to fulfill the Minamata Convention.
To free the fly ash/gypsum from the adverse impact of Hg-laden sorbents, separating the sorbents out of the wastes was demonstrated to be a potential method in previous studies [24], [25], [26]. Specifically, a sorbent with magnetization was adopted in such a scenario, which was hence centrally collected from the sorbent-waste mixture by a magneto. Magnetite (Fe3O4) is an ideal candidate considering its ever strong magnetization among various magnetic materials that are earth-abundant [20], [27]. However, Fe3O4 itself only exhibited very limited Hg0 adsorption performance, which was comparable, even inferior, to that of traditional ACs [20]. This inferiority significantly impairs the usability of Fe3O4 and challenges researchers to conduct proper modifications to make Fe3O4 as an effective Hg0 trap. Although components, like silver (Ag), were adopted as active sites to decorate Fe3O4 surface and combine with Hg0 [28], [29], using noble metals inevitably induces high cost. Moreover, the reaction between Hg0 and Ag forms Hg-Ag amalgam, a mercury species that is relatively susceptible to environmental weathering [30]. Thus, a more advanced modification strategy remains to be proposed to modify earth-abundant and recyclable Fe3O4 into a suitable Hg0 remediator.
The most crucial reason for Fe3O4 exhibited limited Hg0 capture performance was that the affinity between active sites in Fe3O4, i.e., oxygen ligands, and Hg0 is restricted [4], [31]. To solve this problem, a highly active ligand should be introduced into the Fe3O4 system to enhance the affinity between the sorbent and adsorbate. Coincidentally, selenium and Hg were found as one of the best-known examples of a couple of biological antagonism [19]. The binding affinity constant between selenium and mercury reaches 1045, which is the highest value compared to those binding affinity constants between other chalcogens and mercury [32], [33]. Moreover, the introduction of selenium/selenide into Fe3O4 may primarily transfer Hg0 into mercury selenide (HgSe), a mercury species known for its extremely low leachability when environmentally exposed [33]. Thus, it is proper to assume that a proper selenization of Fe3O4 will not only maintain the recyclability of Fe3O4, but also significantly enhance the Hg0 immobilization performance of Fe3O4. The fly ash/gypsum will be free of the detrimental effects imposed by Hg-laden sorbents because the spent sorbents can be effectively retrieved, and the retrieved sorbents may be regenerated and reused to further save the overall cost. The waste sorbents can also be safely dumped and landfilled as Hg0 is permanently sequestrated in the form of stable HgSe.
Accordingly, in this work, the selenized Fe3O4 (Fe3O4−xSey) sorbent was prepared by an simple selenization method. The as-prepared core-shell like Fe3O4−xSey exhibited Hg0 adsorption capacity and uptake rate reaching 8.8 mg g−1 and 3.7 μg g−1 min−1, respectively. This excellent performance was primarily attributed to the porous nature and constructed structure of the Fe3O4−xSey sorbent. The Hg0 was mainly converted into HgSe over the Fe3O4−xSey surface, which indicates that the Hg-laden Fe3O4−xSey can be directly dumped and landfilled with negligible Hg re-emission concerns. Moreover, the spent Fe3O4−xSey can be also replenished and regenerated for reusing purposes, which further saves the overall operation costs. Thus, the Fe3O4−xSey as designed will be a potential alternative to traditional sorbents for Hg0 capture from coal combustion flue gas.
Section snippets
Sorbent preparation
The Fe3O4 nanosphere was synthesized by a hydrothermal method. In a typical procedure, 2.7 g of iron chloride (hexahydrate, FeCl3·6H2O, AR 99.0%, Sinopharm), 7.2 g of sodium acetate (dehydrate, CH3COONa, AR 99.0%, Sinopharm), and 2.0 g of polyethylene glycol (4000, Sinopharm) were added into 80 ml of ethylene glycol (C2H6O2, AR 99.0%, Sinopharm). After the solid was fully dissolved, the mixture was transferred into a 100 ml Teflon stainless-steel autoclave, which was then maintained at 200 °C
Characterizations of the sorbents
As shown in Fig. S1, the XRD pattern of the Fe3O4 as prepared perfectly indexed to its standard card (JCPDs #75-0033), which indicates that the as-synthesized Fe3O4 was in its pure phase with no other iron oxide or precursor detected. After the selenization of the Fe3O4, the XRD pattern of Fe3O4−xSey shows characteristic peaks belonging to both Fe3O4 and FeSe2 (JCPDs #74-0247). The co-existence of these peaks suggests that the Fe3O4 was partially selenized into FeSe2 after the selenization
Conclusion
In this work, a Fe3O4−xSey sorbent was purposefully designed as an efficient and recyclable trap for Hg0 sequestration from coal combustion flue gas. The Fe3O4−xSey synthesized by a simple selenization method based on Fe3O4 exhibited a core-shell like structure with the interior part kept as Fe3O4 that warrants its recyclability, but the exterior part was transferred into FeSe2 that exhibited exceedingly high affinity towards Hg0. Moreover, the selenization process also created more mesopores
Declaration of Competing Interest
The authors declare no conflict of interest.
Acknowledgements
This project was supported by the National Natural Science Foundation of China (51776227), Natural Science Foundation of Hunan Province, China (2018JJ1039, 2018JJ3675), and the Research Council of Hong Kong (17257616 and T21-771/16R).
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