Complete reaction mechanisms of mercury oxidation on halogenated activated carbon

Ultra-high adsorption of Hg0 using impregnated activated carbon by selenium

Activated carbon was one of the main adsorptions utilized in elemental mercury (Hg⁰) removal from coal combustion flue gas. However, the high cost and low physical adsorption efficiency of activated carbon injection (ACI) limited its application. In this study, an ultra-high efficiency (nearly 100%) catalyst sorbent-Se_x/Activated carbon (Se_x/AC) was synthesized and applied to remove Hg⁰ in the simulated flue gas, which exhibited 120 times outstanding adsorption performance versus the conventional activated carbon. The Se_x/AC reached 17.98 mg/g Hg⁰ adsorption capacity at 160 °C under the pure nitrogen atmosphere. Moreover, it maintained an excellent mercury adsorption tolerance, reaching the efficiency of Hg⁰ removal above 85% at the NO and SO₂ conditions in a bench-scale fixed-bed reactor. Characterized by the multiple methods, including BET, XRD, XPS, kinetic and thermodynamic analysis, and the DFT calculation, we demonstrated that the ultrahigh mercury removal performance originated from the activated Se species in Se_x/AC. Chemical adsorption plays a dominant role in Hg⁰ removal: Selenium anchored on the surface of AC would capture Hg⁰ in the flue gas to form an extremely stable substance-HgSe, avoiding subsequent Hg⁰ released. Additionally, the oxygen-containing functional groups in AC and the higher BET areas promote the conversion of Hg⁰ to HgO. This work provided a novel and highly efficient carbon-based sorbent -Se_x/AC to capture the mercury in coal combustion flue gas. Graphical abstract Selenium-modified porous activated carbon and the interface functional group promotes the synergistic effect of physical adsorption and chemical adsorption to promote the adsorption capacity of Hg⁰.

[Formula: see text] modified AC for removing gaseous elemental mercury from flue gas

Mercury pollution from coal-fired power plants has always been an environmental concern, and adsorption technology is an effective method for Hg⁰ removal. In this study, copper-iron binary metal sulfide modified activated carbon ([Formula: see text]) adsorbent was synthesized. The performance and mechanism of mercury removal were tested by adsorption experiments and characterization methods. The results showed that the mercury removal efficiency of [Formula: see text] under simulated flue gas (SFG) conditions at 150 °C could reach 91%, which was higher than [Formula: see text] and [Formula: see text]. Compared with [Formula: see text], the introduction of Fe increased the proportion of [Formula: see text], which was usually linked to [Formula: see text], resulting in the generation of more [Formula: see text](Cu^IS^I) species. Meanwhile, the generation of active sulfur sites such as [Formula: see text] was generated, which had a facilitative effect on the oxidation of Hg⁰. Stable [Formula: see text] was the predominant product on the adsorbent surface. The 2% sulfur loadings had significantly improved the sulfur resistance of conventional AC.

The Effects of Physical-Chemical Evolution of High-Sulfur Petroleum Coke on Hg0 Removal from Coal-Fired Flue Gas and Exploration of Its Micro-Scale Mechanism

As the solid waste by-product from the delayed coking process, high-sulfur petroleum coke (HSPC), which is hardly used for green utilization, becomes a promising raw material for Hg⁰ removal from coal-fired flue gas. The effects of the physical-chemical evolution of HSPC on Hg⁰ removal are discussed. The improved micropores created by pyrolysis and KOH activation could lead to over 50% of Hg⁰ removal efficiency with the loss of inherent sulfur. Additional S-containing and Br-containing additives are usually introduced to enhance active surface functional groups for Hg⁰ oxidation, where the main product are HgS, HgBr, and HgBr₂. The chemical-mechanical activation method can make additives well loaded on the surface for Hg⁰ removal. The DFT method is used to sufficiently explain the micro-scale reaction mechanism of Hg⁰ oxidation on the surface of revised-HSPC. ReaxFF is usually employed for the simulation of the pyrolysis of HSPC. However, the developed mesoporous structure would be a better choice for Hg⁰ removal in that the coupled influence of pore structure and functional groups plays a comprehensive role in both adsorption and oxidation of Hg⁰. Thus, the optimal porous structure should be further explored. On the other hand, both internal and surface sulfur in HSPC should be enhanced to be exposed to saving sulfur additives or obtaining higher Hg⁰ removal capacity. For it, controllable pyrolysis with different pyrolysis parameters and the chemical-mechanical activation method is recommended to both improve pore structure and increase functional groups for Hg⁰ removal. For simulation methods, ReaxFF and DFT theory are expected to explain the micro-scale mechanisms of controllable pyrolysis, the chemical-mechanical activation of HSPC, and further Hg⁰ removal. This review work aims to provide both experimental and simulational guidance to promote the development of industrial application of Hg⁰ adsorbent based on HSPC.

Mechanochemical bromination of unburned carbon in fly ash and its mercury removal mechanism: DFT study

The mechanochemical (MC) brominated fly ash is a cost-effective mercury removal adsorbent, in which unburned carbon (UBC) plays an important role. The MC bromination mechanism of UBC and its mercury removal mechanism were completely studied through the density functional theory (DFT) method. Various defects on zigzag and armchair edge models were constructed at the micro-scale to simulate the MC effect on UBC at the macro-scale. The results reveal that the intact surface of zigzag and armchair can be constructed into abundant defective structures by MC action. Compared with the complete surface, bromine is more favorable to bind on the defective surface, resulting in more and stronger C-Br covalent bonds and more active sites. These defective structures also have a promoting effect on mercury adsorption. For the bromine-embedded structure, although the appropriate defective structure accounts for less, it not only can promote the adsorption and oxidation of mercury by improving adsorption ability or decreasing the oxidation energy barrier but is also easier to generate. Due to defect types formed by MC interaction on the UBC surface are much more diverse and complex, this study provides the theoretical basis for further research.

Coordinatively Unsaturated Selenides over CuFeSe2 toward Highly Efficient Mercury Immobilization

Metal selenides have been demonstrated as promising Hg⁰ remediators, while their inadequate adsorption rate primarily impedes their application feasibility. Based on the critical role of coordinatively unsaturated selenide ligands in immobilizing Hg⁰, this work proposed a novel strategy to enhance the Hg⁰ adsorption rate of metal selenides by magnitudes by purposefully adjusting the selenide saturation. Copper iron diselenide (CuFeSe₂), in which the surface reconstruction tended to occur at ambient temperature, was adopted as the concentrator of unsaturated selenides. The adsorption rate of CuFeSe₂ reached as high as 900.71 μg·g^-1·min^-1, far exceeding those of the previously reported metal selenides by at least 1 magnitude. The excellent resistance of CuFeSe₂ to flue gas interference and temperature fluctuation warrants its applicability in real-world conditions. The theoretical investigations and mechanistic interpretations based on density functional theory (DFT) calculation further confirmed the indispensable role of unsaturated selenides in Hg⁰ adsorption. This work aims not only to develop a Hg⁰ remediator with extensive applicability in coal combustion flue gas but also to take a step toward the rational design of selenide-based sorbents for diverse environmental remediation by the facile surface functionalization of coordinatively adjustable ligands.

Molecular-level insights into the immobilization of vapor-phase mercury on Fe/Co/Ni-doped hierarchical molybdenum selenide

A novel and efficient adsorbent (TM-MoSe₂, TM = Fe, Co, Ni) for mercury removal was developed and studied. The adsorption of mercury species (Hg⁰, HgCl, and HgCl₂) and the oxidation of Hg⁰ by HCl on TM-MoSe₂ (001) surface were explored at molecular level by density functional theory (DFT). The results shown that the Hg⁰ adsorption capacity of MoSe₂ was improved by the doping of Fe/Co/Ni, which was also confirmed by experiments. The initial Hg⁰ removal efficiency of MoSe₂-based adsorbents reached 96.4-100.0%. In addition, HgCl was mainly adsorbed on TM-MoSe₂ (001) surface in the form of dissociation. The escape of Hg atom from HgCl resulted in the release of Hg⁰ again. However, HgCl₂ could be fixed well on the surface of adsorbent through molecular adsorption or dissociative adsorption. For the oxidation process of Hg⁰ by HCl, it abided with the Langmuir-Hinshelwood mechanism. In comparison with direct oxidation (Hg → HgCl₂), two-step pathway (Hg → HgCl → HgCl₂) was an achievable reaction route with lower energy. Furthermore, the Hg → HgCl process was the rate-limiting step of the two-step pathway. The proposed adsorption and oxidation mechanism of mercury species on TM-MoSe₂ (001) provide advanced strategies on the development of adsorbents for industrial mercury removal.

Spherical-shaped CuS modified carbon nitride nanosheet for efficient capture of elemental mercury from flue gas at low temperature

Mercury (Hg⁰) pollution poses a huge threat to human health and the environment due to its high toxicity, long persistence and bioaccumulation in the environment. Most of the traditional Hg⁰ adsorbents have a low reaction rate, high operating cost, especially poor resistance to SO₂, which limited their practical application. In this work, nanosheet g-C₃N₄ was used as the support and modified by CuS to capture flue gas mercury. Take advantage of the large specific surface area of g-C₃N₄ to increase the BET of the composite and decrease the use of CuS. The effects of CuS loading, reaction temperature, and common components in the coal-fired flue gas on the mercury removal performance were studied respectively. The experimental outcomes showed that the 10CuS/g-C₃N₄ (10CuS/CN) reaches as high as almost 100% Hg⁰ removal efficiency under the temperature of 40-120 ℃. Meanwhile the common components like SO₂, NO, HCl and H₂O have no obvious inhibition effects on Hg⁰ removal efficiency of the 10CuS/CN adsorbent. S_x^2- and Cu²⁺ as the primary bonding sites shows a synergy effect on Hg⁰ removal. 10CuS/CN is a promising material for Hg⁰ removal under various flue gas conditions, which is expected to be a substitute for traditional adsorbents.

Understanding the effect of thiophene sulfur on brominated petroleum coke for elemental mercury capture from flue gases

Developing highly efficient and inexpensive adsorbent is a critical technology for elemental Hg removal from the coal combustion flue gases worldwide. Here, we present a novel approach that a waste by-product of petroleum coke containing organic sulfur enhanced bromine binding during the bromine modification process and the brominated petroleum coke increased its mercury adsorption. Experiments and density functional theory reveal that the mercury adsorption capacity directly correlated with the surface organic sulfur and the binding bromine content. Our direct observations and theoretical modeling demonstrate that HgBr and Hg(Br) Br are the primary chemical forms chemisorbed on the surface of this new carbon-based sorbent, which is approaching to bind on the carbon site next to the S atom. The synergetic effect of the inherent thiophene sulfur and loaded bromine enhanced the Hg removal efficiency of the adsorbent.

Insight into the effect of facet-dependent surface and oxygen vacancies of CeO2 for Hg removal: From theoretical and experimental studies

The reaction mechanisms of Hg oxidation on CeO₂(111) and (110) surface are clarified by a group of designed experiments and density functional theory (DFT) calculations. CeO₂ nanorods and nanoparticles with exposure (110) and (111) faces were prepared by hydrothermal methods, and their morphological properties were investigated using XRD, XPS and HRTEM. Combined experimental and DFT results, the nanorods show better activity than nanoparticles. The total oxidation of Hg can be partially prohibited by the high barriers for the incorporated chlorine activation at reduced surfaces, due to the strong electronic repulsion of heavily accumulated charges. The energy barrier profiles suggest Hg oxidation is much more favorable on CeO₂(110) surface than that on CeO₂(111) surface. In the Hg oxidation via HCl and O₂, the role of O₂ is not only replenishment of lattice oxygen, but also could generate surface oxygen as active center for HCl active. The complete catalytic cycle can be identified as four parts: (i) HCl activated by lattice oxygen, (ii) Hg oxidation on defect surface, (iii) HCl activated by adsorbed oxygen and (iv) Hg oxidation on stoichiometric surface. The results of this study provide deep insights into the effects of CeO₂ nanocatalyst morphology on the Hg oxidation.

Effects of temperature and SO3 on re-emission of mercury from activated carbon under flue gas conditions

Mercury (Hg) is a toxic and bio-accumulating heavy metal that is predominantly released via the combustion of coal. Due to its toxicity, the Environmental Protection Agency (EPA) has introduced Mercury and Air Toxics Standards (MATS) Rule regarding allowable Hg emissions. In order to reduce emissions, power plants have widely adopted activated carbon (AC) injection. AC injection has proven to be an effective method to reduce Hg emissions, but the re-emission of previously adsorbed Hg during unit operation, likely due to changing temperature or flue gas composition, could be problematic. This study specifically examined the effects of temperature and sulfur trioxide (SO₃) concentration, by ramping temperature and SO₃ concentration independently and simultaneously, on AC samples that are already exposed to flue gas and saturated in presence of Hg, sulfur dioxide (SO₂) and nitric oxide (NO). Of these two suspected factors to cause re-emission, temperature had the greater impact and resulted in re-emission of both elemental and oxidized Hg with a greater fraction of oxidized Hg, which can be attributed to elemental Hg being more strongly bonded to the AC surface. Surprisingly, exposing the samples to increasing concentrations of SO₃ had nearly no effect under the conditions examined in this study, possibly as a result of the samples being already saturated with sulfur prior to the SO₃ ramp tests to simulate transient conditions in the plant.

Structural defects in 2D MoS2 nanosheets and their roles in the adsorption of airborne elemental mercury

In this research, ab initio calculations and experimental approach were adopted to reveal the mechanism of Hg⁰ adsorption on MoS₂ nanosheets that contain various types of defects. The ab initio calculation showed that, among different structural defects, S vacancies (Vs) in the MoS₂ nanosheets exhibited outstanding potential to strongly adsorb Hg⁰. The MoS₂ material was then prepared in a controlled manner under conditions, such as temperature, concentration of precursors, etc., that were determined by adopting the new method developed in this study. Characterisation confirmed that the MoS₂ material is of graphene-like layered structure with abundant structural defects. The integrated dynamic and steady state (IDSS) testing demonstrated that the Vs-rich nanosheets showed excellent Hg⁰ adsorption capability. In addition, ab initial calculation on charge density difference, PDOS, and adsorption pathways revealed that the adsorption of Hg⁰ on the Vs-rich MoS₂ surface is non-activated chemisorption.

Density Functional Theory Study on Mechanism of Mercury Removal by CeO2 Modified Activated Carbon

Doping of CeO2 on activated carbon (AC) can promote its performance for mercury abatement in flue gas, while the Hg0 removal mechanism on the AC surface has been rarely reported. In this research, density functional theory (DFT) calculations were implemented to unveil the mechanism of mercury removal on plain AC and CeO2 modified AC (CeO2-AC) sorbents. Calculation results indicate that Hg0, HCl, HgCl and HgCl2 are all chemisorbed on the adsorbent. Strong interaction and charge transfer are shown by partial density of states (PDOS) analysis of the Hg0 adsorption configuration. HCl, HgCl and HgCl2 can be dissociatively adsorbed on the AC model and subsequently generate HgCl or HgCl2 released to the gas phase. The adsorption energies of HgCl and HgCl2 on the CeO2-AC model are relatively high, indicating a great capacity for removing HgCl and HgCl2 in flue gas. DFT calculations suggest that AC sorbents exhibit a certain catalytic effect on mercury oxidation, the doping of CeO2 enhances the catalytic ability of Hg0 oxidation on the AC surface and the reactions follow the Langmuir–Hinshelwood mechanism.

Effects of oxygen functional complexes on arsenic adsorption over carbonaceous surface

The adsorption mechanism of As₂O₃ on carbonaceous surface modified with oxygen functional complexes was studied using density functional theory to understand the effect of oxygen functional complexes on arsenic adsorption. Full-parameter geometrical optimization and single point energy were calculated on B3LYP/def2-SVP and B3LYP/def2-TZVP level. Results showed that As₂O₃ adsorption on bare carbonaceous surface took place in physical as well as chemical way. The adsorption energies were between -2.07 kJ/mol to -480.20 kJ/mol. Compared to armchair model, zigzag model was more suitable as a carbonaceous sorbent. The participation of oxygen functional complexes greatly promoted the surface activity of carbonaceous surface and its adsorption capacity on arsenic. The adsorption energies of arsenic on carbonaceous surface with oxygen functional complexes were between -111.56 kJ/mol to -669.46 kJ/mol. The promotion order of oxygen functional complexes on surface activity was: phenol > lactone > carbonyl > semiquinone > carboxyl. Oxygen functional complexes promoted adsorption capacity of carbonaceous surface through enhancing the activities of neighboring carbon atoms rather than directly providing active sites. Mayer bond order was a reliable way to understand the adsorption process of arsenic on carbonaceous surface. This study provides a new idea for using modified carbonaceous sorbent to remove arsenic pollution from power stations.