Nanostructured metal chalcogenides for the transformation and removal of elemental mercury in industrial flue gases

Abstract

After the implementation of the Minamata Convention in 2017, the contemporary world faces unparalleled pressure in abating Hg0 pollution and recycling Hg0 resources from industrial processes. To achieve these goals, the traditional techniques suffer critical drawbacks and unfeasibility. For a typical example, activated carbons (ACs) are the most commercialized sorbents used for Hg0 removal nowadays. However, the application of ACs not only causes extremely high operation costs due to their limited adsorption capacities and rates, but also makes the separation and recycling of fly ash/Hg0 unpractical. Thus, it is of great urgency to develop Hg0 remediators with excellent performance, cost-effectiveness, and environmental-friendliness simultaneously to fulfill the requirements as imposed by the Minamata Convention.

Firstly, this work aims to develop a tertiary Hg0 catalyst to oxidize Hg0 into Hg2+, i.e., copper-manganese-titanium oxides (CuO-MnOx/TiO2). This typical catalyst solves two critical problems of previously reported catalysts for Hg0 removal. On one hand, the CuO-MnOx/TiO2 could be used in a wide temperature range and avoid the detrimental effects of fly ash (adopted after the ESP system). On the other hand, the CuO-MnOx/TiO2 exhibited decent SO2 resistance, which warrants its durability when using in coal combustion flue gas. With these advantages, it can be expected that the CuO-MnOx/TiO2 may own great recyclability and reusability.

Secondly, this work reports an extremely efficient copper sulfide (CuS) based sorbent for Hg0 immobilization. The anisotropic properties and surface reconstruction of CuS derive its excellent performance. However, as the presence of SO3 exhibited significant detrimental effects on Hg0 adsorption by CuS, further studies need to focus on overcoming this specific research gap to make the CuS suitable in various industrial flue gases.

Thirdly, this thesis for the first time proposed the idea of using metal selenides for Hg0 sequestration in industrial flue gases. Two typical materials, i.e., copper selenide loaded zeolitic imidazolate framework-8 (CuSe/ZIF-8) and amorphous molybdenum triselenide (MoSe3), were synthesized. These two metal selenide based sorbents both exhibited record-high Hg0 immobilization performance. It is believed that there are still large rooms for the development of more efficient selenide based sorbents in the future, and this work will provide valuable guides for their evolutions.

Finally, this work proposed a series of magnetic sorbents based on chalcogenides for Hg0 transformation, removal, and recycling from industrial flue gases. Among these magnetic sorbents, on one hand, the selenide substituted magnetite (Se-Fe3O4) and pyrrhotite (Se-Fe1-xS) merit for their easy preparation and low cost. On the other hand, the rattle-type Fe3O4@CuS and intercalating MoSex(inter)Fe3O4 exhibited extremely high Hg0 removal performances. From these perspectives, they are suitable in different conditions according to specific requirements of different industries.

In a word, this thesis primarily aims at overcoming the research gaps regarding to Hg0 transformation and removal after the implementation of the Minamata Convention. Moreover, another main target is elucidating the intrinsic mechanisms to guide further development of Hg0 removal materials and techniques. It is believed that the remediators and mechanisms as proposed in this thesis are of great importance to promote the progress of related areas.