Mercury re-emission, because of the reduction of Hg to form Hg by sulfite, has become a great concern in the desulfurization process. Lowering the concentrations of Hg and sulfite in the desulfurization slurry can retard the Hg formation and, thus, mitigate mercury re-emission. To that end, cobalt-based carbon nanotubes (Co-CNTs) were developed for the simultaneous Hg removal and sulfite oxidation in this work. Furthermore, the thermodynamics and kinetics of the Hg adsorption and effect of Hg adsorption on catalytic activity of Co-CNTs were investigated. Experimental results revealed that the Co-CNTs not only accelerated sulfite oxidation to enable the recovery of desulfurization by-products but also acted as an effective adsorbent of Hg removal. The Hg adsorption rate mainly depended on the structure of the adsorption material regardless of the cobalt loading and morphological distribution. The catalytic activity of the Co-CNTs for sulfite oxidation was not significantly affected due to the Hg adsorption. Additionally, the isothermal adsorption behavior was well-fitted to the Langmuir model with an adsorption capacity of 166.7 mg/g. The mercury mass balance analysis revealed that the Hg re-emission was decreased by 156% by adding 2.0 g/L of Co-CNTs. These results can be used as a reference for the simultaneous removal of multiple pollutants in the wet-desulfurization process.

Sulfite and heavy metals are crucial pollutants in the slurry produced by flue gas desulfurization. In this study, a novel cobalt-based activated carbon fiber (Co-ACFs) catalyst-adsorbent was synthesized using an impregnation method; this bifunctional catalyst-adsorbent was used in wet magnesia desulfurization for the simultaneous catalytic oxidation of magnesium sulfite and uptake of heavy metal (Hg 2+ , Cd 2+ , and Ni 2+ ) ions. The morphology and surface chemistry of ACFs before and after cobalt loading were investigated using various characterization methods. The kinetics on catalytic oxidation of magnesium sulfite was investigated, and the effects of operation conditions on the simultaneous adsorption capacity of heavy metals were examined. Relative to a non-catalysis material, the 40% Co-ACFs material increased the oxidation rate of magnesium sulfite by more than five times. The Langmuir model can describe the adsorption behavior of Co-ACFs on Hg 2+ , Cd 2+ , and Ni 2+ , indicating that the simultaneous uptake of heavy metals is a single-layer adsorption process. The maximum adsorption capacities for Hg 2+ , Cd 2+ , and Ni 2+ are 333.3, 500, and 52.6 mg/g, respectively. A pseudo-second-order model confirmed that the removal of heavy metals is controlled by the chemisorption process.Catalysts 2020, 10, 244 2 of 18 oxidation rate of magnesium sulfite, the oxidation is difficult to accomplish in a real desulfurization process. This problem has become the paramount bottleneck of the oxidation enrichment technology. The excessive sulfite might exhaust the dissolved oxygen in the effluent and decomposed into SO 2 , resulting in further environmental risks. Meanwhile, a large number of heavy metal particles are collected in the desulfurization slurry, which can migrate and transform during the desulfurization process. For example, Hg 2+ present in the desulfurized slurry is easily released into the atmospheric environment because the ion is reduced to Hg 0 by sulfite [9][10][11]. Some co-existing Cd 2+ ions may co-crystallize with the saturated magnesium sulfate in the recovered products to cause secondary pollution. Therefore, the simultaneous removal of heavy metals from desulfurized slurries becomes another technical problem for magnesium sulfate recovery.Studies have shown that transition metal ions such as Co 2+ , Mn 2+ , Fe 2+ , and Cu 2+ can be used for improving the oxidation rate of sulfite. Among these ions, Co 2+ exhibits the highest performance in terms of sulfite oxidation [12]. However, separating the homogeneous catalyst from the desulfurization slurry is difficult and the material must be supplemented continuously, thus resulting in a high operation cost and secondary pollution due to the use of magnesium sulfate products. To address this problem, our research group developed a series of cobalt-based heterogeneous catalysts [13-18] by using impregnation, coprecipitation, and hydrothermal synthesis. These catalysts effectively solved the catalyst separation problem. However, a magnesium desulfurizat...

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