Novel Promotion of Sulfuration for Hg0 Conversion over V2O5-MoO3/TiO2 with HCl at Low Temperatures: Hg0 Adsorption, Hg0 Oxidation, and Hg2+ Adsorption

Different roles of FeS and FeS2 on magnetic FeSx for the selective adsorption of Hg2+ from waste acids in smelters: Reaction mechanism, kinetics, and structure-activity relationship

Magnetic FeSx was developed as a high-performance sorbent for selectively adsorbing Hg2+ from waste acids in smelters. However, further improvement of its ability for Hg2+ adsorption was extremely restricted due to the lack of reaction mechanisms and structure-activity relationships. In this study, the roles of FeS and FeS2 on magnetic FeSx for Hg2+ adsorption were investigated with alternate adsorption of Hg2+ without/with Cl-. The structure-activity relationship of magnetic FeSx for Hg2+ adsorption and the negative effect of acid erosion were elucidated using kinetic analysis. FeS can react with Hg2+ with 1:1 stoichiometric ratio to form HgS, while FeS2 can react with Hg2+ in the presence of Cl- with novel 1:3 stoichiometric ratio to form Hg3S2Cl2. The rate of magnetic FeSx for Hg2+ adsorption was related to the instantaneous amounts of FeS and threefold FeS2 on magnetic FeSx and the amount of Hg2+ adsorbed. Meanwhile, its capacity for Hg2+ adsorption was related to the initial sum of FeS amount and threefold FeS2 amount on the surface and their ratios by acid erosion. Then, magnetic FeSx-400 was devised with adsorption rate of 2.12 mg g-1 min-1 and capacity of 1092 mg g-1 to recover Hg2+ from waste acids for centralized control.

Release and migration of Hg during desulfurization wastewater evaporation process: Whole process evaluation by experimental and theoretical study

Hot flue gas evaporation technology is an effective strategy for zero liquid discharge of desulfurization wastewater. However, there is a potential risk that heavy metals such as Hg may be released from the wastewater during evaporation, disrupting the original balance of the power plant or even exceeding the Hg emission standard. Wastewater evaporation and Hg release behavior were obtained using a single droplet drying system. At an evaporation temperature of 300 °C, approximately 18.5% of Hg was released in the constant wet-bulb temperature period, and the remaining was released in the following evaporation periods. Furthermore, a fixed-bed experiment, in combination with density functional theory calculations, was used to investigate the possible migration mechanisms of released Hg. The results revealed that high HCl concentration, introduced fly ash, and precipitated evaporation products play a crucial role in the fate of Hg, and 85.3% of Hg finally turned into less harmful particulate-bound Hg. This study provides a new and effective strategy for evaluating the migration process of pollutants in wastewater treatment. Moreover, it will serve as an essential reference for advanced wastewater treatment and heavy metals control technologies in the future.

Simultaneously efficient adsorption and highly selective separation of U(VI) and Th(IV) by surface-functionalized lignin nanoparticles: A novel pH-dependent process

The simultaneous removal and selective separation of U(VI) and Th(IV) via adsorption remain challenging due to their strong mobility, reactivity, and similar chemical properties. Thus, a surface-functioned lignin nanoparticle (AL-PEI) was synthesized to adsorb U(VI)/Th(IV) in a unitary system via a pH-dependent process. In alkaline solution, AL-PEI exhibited excellent adsorption performance, and the maximum adsorption capacities for U(VI) and Th(IV) reached 392 and 396 mg/g, respectively. Discrepantly in acidic solution, the adsorption performance of AL-PEI for U(VI) could still reach a high capacity (332 mg/g), whereas highly limited adsorption capacity (less than 40 mg/g) for Th(IV) was obtained, and the separation factor of U(VI) from U(VI)-Th(IV) matrix significantly reached 6662 in 3 M of the HNO₃ medium. The simultaneously efficient adsorption in alkaline solution and highly selective separation performance in acidic solution of AL-PEI also showed excellent anti-ions interference capacities, high reusability, and strong stability. This study is the first to apply lignin fabricating radiation-resistant adsorbent material, and the adsorbent displays good performance for U(VI)/Th(IV) removal and selective separation via a novel pH-dependent process, which is important to the green and sustainable development of nuclear energy and environmental protection.

Hg0 Conversion over Sulfureted HPMo/γ-Fe2O3 with HCl at Low Temperatures: Mechanism, Kinetics, and Application in Hg0 Removal from Coal-Fired Flue Gas

Recently, sulfureted metal oxides have been developed for the catalytic oxidation of Hg⁰ to HgCl₂ using HCl as an oxidant at low temperatures, and they exhibit excellent Hg⁰ removal performance. Owing to the lack of reaction mechanisms and kinetics, further improvement in their performance for Hg⁰ conversion is extremely restricted. In this study, the reaction mechanism of Hg⁰ conversion over sulfureted HPMo/γ-Fe₂O₃ with HCl at low temperatures was investigated using Hg balance analysis and transient reaction. The chemical adsorption of Hg⁰ as HgS and the catalytic oxidation of Hg⁰ to HgCl₂ both contributed to Hg⁰ conversion over sulfureted HPMo/γ-Fe₂O₃. Meanwhile, the formed HgCl₂ can adsorb onto sulfureted HPMo/γ-Fe₂O₃. Then, the kinetics of Hg⁰ conversion, Hg^t adsorption, and HgCl₂ desorption were developed, and the kinetic parameters were gained by fitting the Hg balance curves. Subsequently, the inhibition mechanism of H₂O and SO₂ on Hg⁰ conversion over sulfureted HPMo/γ-Fe₂O₃ was determined by comparing the kinetic parameters. The kinetic model suggested that both HgCl₂ resulting from Hg⁰ oxidation and unoxidized Hg⁰ can be completely adsorbed on sulfureted HPMo/γ-Fe₂O₃ with a moderate mass hourly space velocity. Therefore, sulfureted HPMo/γ-Fe₂O₃ can be developed as a reproducible sorbent for recovering Hg⁰ emitted from coal-fired power plants.

Tunning active oxygen species for boosting Hg0 removal and SO2-resistance of Mn-Fe oxides supported on (NH4)2S2O8 doping activated coke

Active oxygen species (AOS) play an essential role in modulating the activity of activated coke (AC) based samples. In this paper, AC was endowed with abundant AOS by modifying with (NH₄)₂S₂O₈ and MnO_x-FeO_x for Hg⁰ removal. (NH₄)₂S₂O₈ treatment induced abundant micropores and oxygen-containing functional groups, and thus provided more anchoring sites for the dispersion of MnO_x-FeO_x. The synergy of MnO_x-FeO_x and interaction between MnO_x-FeO_x and NAC support contributed to a larger surface area, highly-dispersed active components, stronger reducibility, and more metal ions with high valence of MnFe/NAC. The optimal MnFe/NAC exhibited superior Hg⁰ removal efficiency above 90% at 120∼180 ℃, as well as excellent performance for simultaneous removal of Hg⁰ and NO, and 600 ppm SO₂ and 8 vol.% H₂O addition led to a slight deterioration. XPS and Hg-TPD revealed that mercury adsorbed on MnFe/NAC included phy-Hg, C=O-Hg, COO-Hg, and O_L-HgO. Besides, the priority of AOS for Hg⁰ chemisorption was C=O > COO- > O_L, and Hg²⁺ was also detected in the outlet. Moreover, the SO₂-poisoning effect was ascribed to the sulfation of MnO_x and the occupation of COO- and C=O, and FeO_x incorporation enhanced the SO₂-resistance through weakening SO₂ adsorption on C=O and COO-. The motivation of O₂ mainly contributed to the regeneration of AOS, especially O_L. The excellent regeneration performance and stability further affirmed the application potential of MnFe/NAC for Hg⁰ capture from coal-fired flue gas.

Siloxane-modified MnOx catalyst for oxidation of coal-related o-xylene in presence of water vapor

In coal-combustion energy production, presence of water vapor in flue gas causes catalyst deactivation and leads to the release of large quantities of volatile organic compounds (VOCs). In this study, design of a low-temperature, hydrophobic catalyst for flue gas purification was achieved by modifying support material with inorganic siloxane. Introduction of 5% water vapor into simulated flue gas at 300 °C reduced oxidation efficiency for o-xylene removal by 26% with unmodified MnO_x/γ-Al₂O₃ catalyst, whereas with modified catalyst MnO_x-Si_0.9/γ-Al₂O₃ oxidation efficiency was reduced by only 5%. MnOx-Si_0.9/γ-Al₂O₃ exhibited stable catalytic efficiency for o-xylene gas oxidation containing water vapor for over 200 min. Water-resistance of the catalyst was effective for removal of multi-coal combustion pollutants (Hg⁰ and NO) and moreover, hydrophobicity of the catalyst led to a reduction in surface sulfate deposition, thereby lowering toxicity of SO₂ from simulated flue gas. DRIFTS analysis showed that the hydrophobic catalyst surface not only reduces water adsorption, but also promotes water volatilization. Based on molecular adsorption energies, catalyst support modification with siloxane inhibits water adsorption and promotes organic adsorption and thus provides a new strategy for preparing water-resistant catalysts for flue gas purification.

Simultaneous Adsorption of Gaseous Hg0 and Hg(II) by Regenerable Monolithic FeMoSx/TiO2: Mechanism and its Application in the Centralized Control of Hg Pollution in Coal-Fired Flue Gas

FeMoS_x/TiO₂ was investigated as a regenerable sorbent to simultaneously adsorb Hg⁰ and Hg(II) from coal-fired flue gas for the centralized control of Hg pollution discharged from coal-fired power plants. The performance of FeMoS_x/TiO₂ for Hg(II) and/or Hg⁰ adsorption was evaluated on a fixed-bed reactor at 80 ^oC, and the mutual interference between Hg⁰ adsorption and Hg(II) adsorption was analyzed using individual adsorption, simultaneous adsorption, and two-stage adsorption. FeMoS_x/TiO₂ displayed an excellent capacity for individual Hg⁰ adsorption (41.8 mg g^-1) and a moderate capacity for individual Hg(II) adsorption (0.48 mg g^-1). Two types of adsorption sites were present on FeMoS_x/TiO₂ for gaseous Hg adsorption (S⁰ and FeS₂/MoS₃ sites). X-ray photoelectron spectroscope and kinetic analyses demonstrated that Hg⁰ and Hg(II) could adsorb onto S⁰ sites, whereas only Hg⁰ was adsorbed onto FeS₂/MoS₃ sites. As Hg⁰ competed with Hg(II) for the S⁰ sites, the amount of Hg(II) adsorbed slightly decreased by 16% in the presence of Hg⁰. However, Hg⁰ adsorption onto the FeS₂/MoS₃ sites predominated over the Hg⁰ adsorption onto FeMoS_x/TiO₂ and it was not inhibited in the presence of Hg(II). Therefore, the amount of Hg⁰ adsorbed on FeMoS_x/TiO₂ was only decreased by 2% in the presence of Hg(II).

Structure-Directing Role of Support on Hg0 Oxidation over V2O5/TiO2 Catalyst Revealed for NOx and Hg0 Simultaneous Control in an SCR Reactor

The crystal structure of TiO₂ strongly influences the physiochemical properties of supported active sites and thus the catalytic performance of the as-synthesized catalyst. Herein, we synthesized TiO₂ with different crystal forms (R = rutile, A = anatase, and B = brookite), which were used as supports to prepare vanadium-based catalysts for Hg⁰ oxidation. The Hg⁰ oxidation efficiency over V₂O₅/TiO₂-B was the best, followed by V₂O₅/TiO₂-A and V₂O₅/TiO₂-R. Further experimental and theoretical results indicate that gaseous Hg⁰ reacts with surface-active chlorine species produced by the adsorbed HCl and the reaction orders of Hg⁰ oxidation over V₂O₅/TiO₂ catalyst with respect to HCl and Hg⁰ concentration were approximately 0 and 1, respectively. The excellent Hg⁰ oxidation efficiency over V₂O₅/TiO₂-B can be attributed to lower redox temperature, larger HCl adsorption capacity, and more oxygen vacancies. This work suggests that to achieve the best simultaneous removal of NO_x and Hg⁰ on state-of-the-art V₂O₅/TiO₂ catalyst, a combination of anatase and brookite TiO₂-supported vanadyl tandem catalysts is supposed to be employed in the SCR reactor, and the brookite-type catalyst should be on the downstream of the anatase-based catalyst due to the inhibition of NH₃ on Hg⁰ oxidation.

Complete catalytic reaction of mercury oxidation on CeO2/TiO2 (001) surface: A DFT study

CeO₂/TiO₂ catalyst is a promising material for realizing the integration of denitrification and mercury removal to reduce mercury emissions. Oxidation mechanism of Hg⁰ on CeO₂/TiO₂ (001) surface in the presence of HCl and O₂ was studied by density functional theory (DFT). The results indicated that Hg⁰ was physically adsorbed on CeO₂/TiO₂ (001) surface. As an important intermediate, HgCl was adsorbed on the surface of CeO₂/TiO₂ (001) utilizing enhanced chemisorption, while the adsorption energy of HgCl₂ was only -57.05 kJ/mol. In the absence of HCl, mercury oxidation followed the Mars-Maessen mechanism with a relatively high energy barrier, and the product (HgO) was difficult to desorb, which hindered the reaction process. When HCl existed, reactive chlorine (Cl*) would be produced by the dissociation of HCl, and the mercury oxidation would follow the Langmuir-Hinshelwood mechanism. The co-existence of HCl and O₂ had no significant effect on the adsorption of Hg⁰, but reduced the reaction energy barrier and the final product (HgCl₂) was more easily desorbed from the catalyst surface. In addition, two complete cyclic reaction pathways for catalytic oxidation of Hg⁰ on CeO₂/TiO₂ (001) surface were constructed to clarify the detailed reaction process.

Satisfactory Anti-Interference and High Performance of the 1Co-1Ce/Mn@ZSM-5 Catalyst for Simultaneous Removal of NO and Hg0 in Abominable Flue Gas

The removal performance of NO and Hg⁰, the operating temperature window, and the resistance of SO₂ and H₂O on Mn@ZSM-5 catalyst, which was synthesized by a one-step hydrothermal method with manganese oxide as the active component, were improved by doping different molar ratios of Co/Ce. Co and Ce doping increased the content of Mn⁴⁺ as well as of chemisorbed oxygen and promoted the NO and Hg⁰ removal performance, which reached 96.7 and 98.9%, respectively, in flue gas over the 1Co-1Ce/Mn@ZSM-5 catalyst. Furthermore, with SO₂ and H₂O addition, it decreased slightly to 88.4 and 89.3%, respectively, and then remained stable. The coexistence of SO₂ and H₂O had a synergistic poisoning effect on the activity of the catalyst, while the doping of Co and Ce had a positive influence on the tolerance to SO₂ and H₂O. The excellent anti-interference and high performance of the 1Co-1Ce/Mn@ZSM-5 catalyst in the abominable flue gas were mainly due to the outer surface modification of organosilane and because the sacrificial element Co protected the active sites of Ce and Mn from poisoning, which prevented the redox ability of the catalyst from getting affected.

Novel Counteraction Effect of H2O and SO2 toward HCl on the Chemical Adsorption of Gaseous Hg0 onto Sulfureted HPW/γ-Fe2O3 at Low Temperatures: Mechanism and Its Application in Hg0 Recovery from Coal-Fired Flue Gas

In this work, sulfureted phosphotungstic acid-grafted γ-Fe₂O₃ (HPW/γ-Fe₂O₃) was investigated as a regenerable monolithic sorbent to recover gaseous Hg⁰ upstream of wet flue gas desulfurizations (FGDs), and the effects of HCl, SO₂, and H₂O on the chemical adsorption of Hg⁰ onto sulfureted HPW/γ-Fe₂O₃ were investigated with Hg balance analysis and kinetic analysis. Hg⁰ conversion over sulfureted HPW/γ-Fe₂O₃ was remarkably promoted in the presence of HCl, and most Hg⁰ was catalytically oxidized to HgCl₂. Moreover, the chemical adsorption of Hg⁰ was notably restrained as the key species for Hg⁰ transformation to HgS (i.e., S₂^2-) was rapidly oxidized by Cl*. However, the effect of HCl on Hg⁰ conversion over sulfureted HPW/γ-Fe₂O₃ was almost counteracted by H₂O and SO₂ as they competed with physically adsorbed Hg⁰ and S₂^2- for the consumption of Cl*. Therefore, the chemical adsorption of Hg⁰ onto sulfureted HPW/γ-Fe₂O₃ in the presence of SO₂ and H₂O was slightly inhibited by HCl, and only a small amount of HgCl₂ was formed. Moreover, sulfureted HPW/γ-Fe₂O₃ exhibited a moderate ability for gaseous HgCl₂ adsorption. As a result, sulfureted HPW/γ-Fe₂O₃ showed excellent performance in recovering Hg⁰ from the flue gas upstream of the FGDs for the centralized control of Hg⁰ emitted from coal-fired plants.