Combined experimental and theoretical studies on adsorption mechanisms of gaseous mercury(II) by calcium-based sorbents: The effect of unsaturated oxygen sites.

Accurate mercury speciation measurements are critical for developing methods for mercury removal from flue gas, but the lack of reliable adsorbents has made Hg2+ selective retention challenging. Calcium oxide (CaO) loaded on porous support is promising for HgCl2 selective adsorption because of its porosity and alkaline nature. The main hypothesis investigated in this paper is if the capacity of CaO sorbent for HgCl2 selective adsorption is attributed to its basic sites, then this will be drastically impacted by the calcium precursors. We synthesized a suite of CaO/SiO2 sorbents from different precursors, including hydrated calcium oxide (CaO-HS), calcium nitrate tetrahydrate (CaO-NS), and calcium acetate monohydrate (CaO-AcS), to investigate their performance on HgCl2 selective adsorption in a fixed-bed reactor. Compared with CaO-HS and CaO-NS, CaO-AcS was demonstrated to have the strongest affinity for HgCl2 and almost complete breakthrough for Hg0. Advanced porosity and surface basicity of CaO-AcS were confirmed by characterization analysis. CaO (001) and CaO (011) facet as well as surface defects that have different unsaturated O sites were observed using the high resolution transmission electron microscope (HRTEM). Combined theoretical and experimental methods were used to study the interaction mechanisms between HgCl2 and basic sites on CaO-AcS surfaces. Density functional theory (DFT) calculations indicated all CaO surfaces weakly interact with Hg0, while four robust bonding states of HgCl2 were predicted on different basic sites with the intensity in increasing order: Monodentate < Tridendate < Bidentate < Bridging. This was consistent with HgCl2-TPD experiments that demonstrated that the four HgCl2 adsorption configurations on CaO-AcS were attributed to different unsaturated O sites. The findings in this work highlight the application potential of CaO-AcS for gaseous Hg2+ sampling and measurement from coal-fired flue gas.