| The toxicity, bioaccumulation, and persistence of mercury make it an urgent task to find efficient methods to reduce mercury emission from coal-fired power plants. The elemental mercury (Hg0) is hardly removed from flue gas due to its characteristic of high volatility and low solubility in water. Thus, numerous researchers have focused on efficient technologies to promote Hg0 adsorption and oxidation. Traditional sorbent injection method and catalytic oxidation have been found serious defects, such as low stability and hardly recycling of sorbents and catalysts. Heterogeneous Fenton-like reaction is one of the most efficient advanced oxidation processes (AOPs), which has been widely used in wastewater treatment field. This success due to its advantage that it can generate high-active’OH radicals, reduce H2O2 consumption, and has wide pH window. Hg0 removal studies over heterogeneous Fenton-like reaction would be of great scientific and practical values.In this work, the α-Fe2O3, γ-Fe2O3, and Fe3O4 were prepared as heterogeneous Fenton-like catalysts, and characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), Brunauer-Emmett-Teller (BET), and X-ray photoelectron spectroscopy (XPS). The performance of heterogeneous Fenton-like reactions over Hg0 removal was investigated in a bench-scale bubbling reactor. Some main parameters such as H2O2 concentration, solution pH value, catalyst dosage, and reaction temperature were systematically studied to optimize reaction conditions. The result characterized through electron paramagnetic resonance (EPR) showed that Fe3O4 improve ’OH yield during Fenton-like reaction. XPS method detected the existence of Fe2+/Fe3+ redox pairs on Fe3O4 surface.Hg0 removal efficiency was only 39% when used Fe3O4 as the heterogeneous catalyst. Therefore, to enhance Hg0 removal efficiency, Fe3.xTix04, Fe3-xCoxO4 and Fe3-xCux04 catalysts were prepared using chemical coprecipitation method. Characterization results showed that the crystallinity and crystal constant of Fe3-xCux04 decreased, but the crystal constant of Fe3-xTixO4 and Fe3-xCoxO4 increased. The partical size of Fe3.xTixO4 was 1～2 μm, whereas Fe3-xCoxO4 was 1～5μm. The surface area of Fe3-xTix04 and Fe3-xCoxO4 was about 21～69 m2/g and 10～16 m2/g, respectively. However, the surface area of Fe3-xCuxO4 was about 112～139 m2/g, which was much larger than Ti and Co doped catalysts. A series of Hg0 removal experiments were carried out, the results showed that Hg0 removal efficiency over Fe3-xTixO4 and Fe3-xCuxO4 catalysts reached to 95% and 97%, respectively. While Hg0 removal efficiency over Fe3-xCoxO4 remained only 60% after 120 min. H2O2 concentration and Ti/Cu content in Fe3-xTixO4 and Fe3-xCuxO4 both had significant influence on Hg0 removal in the self-designed reactor system. Hg0 removal by heterogeneous Fenton-like reaction could obtain a higher efficiency at neutral or week acid medium. The optimum catalyst dosage and reaction temperature was 0.6 g/L and 50℃, respectively. The presence of SO2 in the simulated flue gases had little effect on Hg0 removal over Fe3-xTixO4, while inhibited effect was found over Fe3-xCoxO4 and Fe3-xCuxO4 catalysts. NO significantly improved Hg0 removal during the reactions. The stability of Fe3-xTixO4 and Fe3-xCuxO4 was poor during heterogeneous Fenton-like reaction. ICP analysis showed that the leached Ti and Cu ions were found to be approximately 4.9 mg/L and 3.6 mg/L, and the leached Fe ions were found to be approximately 0.699 mg/L and 0.361 mg/L when pH was 3.2. The consecutive experiments showed that the average Hg0 removal efficiency had a slightly decrease after three runs.Furthermore, Ti and Cu were simultaneously introduced into the Fe3O4 structure to improve ·OH generation during Fenton-like reaction. As well as, the stability and recyclability of catalyst was trying to improving after Ti and Cu doped. XPS analysis showed the existence of Fe2+/Fe3+, Cu+/Cu2+, and Ti3+/Ti4+ redox paris on Cu0.3Fe2.7-xTixO4 surface. Indeed, Cu and Ti introduction into the catalyst greatly increased the surface areas and active sites on the catalyst surface to produce ·OH radicals. XPS analysis also confirmed the reversibly oxidized and reduced Cu-Fe-Ti active site by comparing bonding energies before and after Fenton-like reaction. Results suggested Ti and Cu enrichment at the catalyst surface, which led to a relatively large difference in lattice parameter between the stoichiometric bulk and surface of catalyst particles. Hg0 removal efficiency could obtain above 96% in 180 min under the optimum conditions. The introduction of Cu and Ti widen the pH window during the heterogeneous Fenton-like reaction. The presence of SO2 in the simulated flue gases had little effect on Hg0 removal efficiency, while NO significantly improved Hg0 removal by Hg(NO3)2 generation during Fenton-like reaction. Considering its magnetism, stability, and recyclability, the Cu0.3Fe2.7-xTixO4 catalyst makes it economically viable, since the spent catalyst could be easily separated from Fenton-like solution by magnetic separation to reuse and the catalyst activity had not affected by magnetic separation. However, the saturation magnetization (Ms) values of decreased with the increase of Cu and Ti content in the catalyst structure.Finally, the density functional theory (DFT) was used to study the mechanism of Hg0 removal in the process of Fe3O4/H2O2 system. The crystal structure model of Fe3O4 catalyst was established by Material Studio using the Cambridge Sequential Total Energy package (CASTEP). The adsorption and dissociation of H2O2 molecule were investigated on the Fe3O4 (111) surface over two different Fe terminal (Fetet1 and Feoct2). The results showed that the Feoct2-terminated surface was more favored for H2O2 dissociation, and H2O2 was easier to decompose and generate two hydroxyls than Fetet1-terminated surface. In addition, the oxidation reaction of Hg0 was studied on OH-Fe3O4 (111) pre-adsorption surface. Through the discussion of Fetet1 and Feoct2 terminal mechanism and Mulliken charge population, we found that hydroxyls had different reaction activity generated on different Fe-terminal. The oxidation of Hg0 could form stable oxidized mercury species on Fe-terminated surface and most of the lost electron from Hg0 transfered to unbonded hydroxyl during Hg0 oxidation. The result showed that the combine of Hg and hydroxyl was exothermic reaction, which was favorable to spontaneous processes of Hg oxidation. |
