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Abstract
Iron-based sorbent addition is a promising method for arsenic removal from flue gas, but the adsorption process and surface active site that responsible for arsenic adsorption remains unclear. In this work, quantum chemistry methods base on the density functional theory are carried out to explore the mechanism of As2O3 adsorption on Fe2O3(001) Surface. The results indicate that O-top and O-hollow site served as the active site for As2O3 adsorption on α-Fe2O3(001) surface, among these, the activity of O-top is higher. The critical step of As2O3 adsorption lies in the bond breaking of As-O bond of As2O3 molecule, which is confirmed by comparing binding energy of different adsorption sites. The previous experimental studies have proved that O2 and SO2 have a significant impact on arsenic adsorption, and herein, deep insights into arsenic adsorption in the presence of the above gas components are also included. Under the influence of oxygen, the converting of original Fe-top site into O site results in chemisorption between arsenic and α-Fe2O3(001) surface, which is the primary cause for the promoting action of O2. In the presence of SO2, the adsorption activity of the original Fe-top site is enhanced by the new-formed Sads-top site. In addition, the As adsorption capacity of original O-top site had been also promoted because of the SO2 adsorption.
... The lattice It shows that the model is reliable [39]. Referring to previous studies, the Fe-O 3 -Fe termination of (001) surface is the most stable surface termination [10,40,41]. Therefore, on the basis of the optimized α-Fe 2 O 3 cell, a 12-layer p (2 × 2) supercell was constructed and cut out (001) surface. ...
... It shows that the model is reliable [39]. Referring to previous studies, the Fe-O3-Fe termination of (001) surface is the most stable surface termination [10,40,41]. Therefore, on the basis of the optimized α-Fe2O3 cell, a 12-layer p (2 × 2) supercell was constructed and cut out (001) surface. ...
Sulphide gas is an impurity that affects the quality of natural gas, which needs reasonable storage and transportation. In this work, we investigated the adsorption structure and electronic behavior of hydrogen sulfide (H2S), carbonyl sulfur (COS), and methyl mercaptan (CH3SH) on sulphide gas molecules on pure and vacant α-Fe2O3(001) surfaces by density functional theory with geometrical relaxations. The results show that H2S and CH3SH are mainly adsorbed in the form of molecules on the pure Fe2O3(001) surface. On the vacant α-Fe2O3(001) surface, they can be adsorbed on Fe atoms in molecular form and by dissociation. The absolute value of the adsorption energy of H2S and CH3SH on the vacancy defect α-Fe2O3 surface is larger, and the density of states show that the electron orbital hybridization is more significant, and the adsorption is stronger. The charge differential density and Mulliken charge population analysis show that the charge is rearranged and chemical bonds are formed. The affinity of H2S to the vacancy α-Fe2O3(001) surface is slightly higher than that of CH3SH, while COS molecules basically do not adsorb on the α-Fe2O3(001) surface, which may be related to the stable chemical properties of the molecules themselves.
... DFT is a valuable tool to study the mechanisms of interaction at the solid liquid interface. In the study of Zhang and Liu (2019), quantum chemistry methods based on the DFT have been used to explore the mechanism of As(III) adsorption on the surface of Fe 2 O 3 (001). The results show that O-top and O-hollow sites on the α-Fe 2 O 3 (001) surface acted as the active sites for As(III) adsorption, and the O-top activity is higher. ...
... The results show that O-top and O-hollow sites on the α-Fe 2 O 3 (001) surface acted as the active sites for As(III) adsorption, and the O-top activity is higher. The critical step of As(III) adsorption is the breakage of the As−O bond in the As(III) molecule, which is verified by comparing binding energy from various adsorption sites [11]. In the study of Fan et al. (2019), DFT calculations were investigated for the mechanisms of As(III) adsorption on the CaO surface under oxygen atmosphere. ...
Waste recycling and reuse will result in significant material and energy savings. In this research, usage of hospital sludge as a biochar adsorbent for wastewater treatment plants was investigated. Microwave carbonization was used to carbonize the sludge and then chemically activated with ZnCl 2 to increase surface area and porosity. A newly designed iron metal doped sludge biochar carbon (SBC) has effective adsorption of inorganic arsenic (As(III), As 2 O 3 ) in water. The findings clearly demonstrate the viability and utility of using hospital sludge as a source of carbon to generate SBC. The adsorption mechanism of As(III) on SBC’s iron-metal-modified surface has been studied using density functional theory (DFT) to understand the impact of functional complexes on adsorption As(III). Tests showed physical as well as chemical adsorption of As(III) on Fe-SBC surface. Fe’s involvement in functional complexes greatly fostered SBC surface activity and it’s As(III) adsorption ability. The physical adsorption energies of As(III) with Fe functional complexes on the SBC surface were −42.3 KJ mol ⁻¹ . Other hand, the chemical adsorption energies of As(III) on Fe-SBC surface was −325.5 KJ mol ⁻¹ . As(III) is capable of interacting in a bidentate fashion with the dopants through the protonated oxygen atoms and this conformation of the cyclic structure is higher in the adsorption energy than the others.
... DFT is a valuable tool to study the mechanisms of interaction at the solid liquid interface. In the Zhang and Liu [14] research, DFT based quantum chemistry methods were used to explore the mechanism of adsorption of As (III) on the surface of ferric oxide. The results show that Otop and O-hollow sites on the α-ferric oxide acted as the active sites for As (III) adsorption, and the O-top activity is higher. ...
... The results show that Otop and O-hollow sites on the α-ferric oxide acted as the active sites for As (III) adsorption, and the O-top activity is higher. The breakage of the As-O bond in the As (III) is the critical stage of As (III) adsorption, which is checked by comparing binding energy from various adsorption sites [14]. In the study of Fan et al. [15], DFT calculations were investigated for the mechanisms of As (III) adsorption on the CaO surface under oxygen atmosphere. ...
Usages of hospital sludge as a biochar adsorbent for wastewater treatment plants were investigated. Microwave carbonization was used to carbonize the sludge and then chemically activated with ZnCl 2 to increase surface area and porosity. A newly designed amine functional group’s doped Sludge Biochar Carbon (SBC) presents effective inorganic arsenic (As (III)) and organic arsenic (Dimethylarsinic Acid, DMA) adsorption in water. The pore volume, pore size distribution and specific surface area were determined by performing nitrogen adsorption-desorption measurements. The Fourier Transform Infrared of the SBC was recorded to study the functional groups at room temperature. The composition of SBC was further determined by X-ray Photoelectron Spectroscopy. In order to understand the effect of amine functional complexes on arsenic adsorption, the adsorption mechanism of As (III) and DMA on SBC surfaces modified with amine functional complexes was studied using Density Functional Theory (DFT). DFT results showed that both physical and chemical adsorption of As (III) and DMA on SBC surfaces occurred. The participation of amine functional complexes greatly promoted the surface activity of SBC surface and its adsorption capacity on arsenic. The physical adsorption energies of As (III) and DMA on SBC surface with amine functional complexes were − 38.8 and − 32.4 kJ mol − 1 , respectively. The chemical adsorption energies of As (III) and DMA on SBC surface with amine functional complexes were − 92.9 and − 98.5 kJ mol − 1 , respectively.
... The results of the DFT study by Zhang et al. [50] also showed that As 2 O 3 (g) could form eight stable adsorption structures on the (001) surface of Fe 2 O 3 with a minimum adsorption energy of 275.52 kJ mol À1 . One-step hydrothermal preparation was used by Zhao et al. [51] to create a tiny, spherical α-Fe 2 O 3 adsorbent, and the effects of reaction temperature, nitric oxide (NO), oxygen (O 2 ), nitric dioxide (NO), sulfur dioxide (SO 2 ), and inlet arsenic concentration on the adsorption performance were investigated. ...
Arsenic is a highly toxic element in coal and one of the representative toxic trace metals emitted from coal-fired power plants, which is mainly converted into As2O3 vapor during the combustion process of coal. When absorbed by the body, arsenic can cause arsenic poisoning, which not only causes metabolic disorders and subsequent neurotoxicity in the body but also retards growth in young children. Arsenic is of increasing concern due to its bioaccumulation and potential carcinogenicity. This chapter describes the characteristics of arsenic emissions from coal-fired power plants and the various control technologies, including pre-, in-, and post-combustion control technologies. It also provides an outlook on future technological developments and provides theoretical guidance for controlling arsenic in flue gas.
... A quantum computational method was used to investigate the detailed adsorption mechanism of As 2 O 3 on the Fe 2 O 3 (001) surface by Zhang and Liu (2018). Four different sites for possible metal ion adsorption were studied by quantum computational methods and it was found that O-hollow and O-top sites were the most feasible sites for the adsorption of As 2 O 3 on the surface of α-Fe 2 O 3 (001) (Fig. 8). ...
The process of removal of heavy metal ions from contaminated aqueous systems has been studied extensively in terms of mechanisms, influencing factors, thermodynamics and kinetics. However, due to the complex nature of the adsorption process, a thorough understanding related to the adsorption process is still missing. An insight into the adsorption parameters like adsorption energy, preferred adsorption sites, structural dynamics of adsorbents before and after the metal ion removal process, etc. is quite difficult to obtain from experimental studies. Under such conditions, quantum mechanical computations make an important complementary aid in understanding the adsorption process of metal ions effectively, accurately and thoroughly by experimental researchers. Here, in this review paper, we have discussed the importance of quantum computations in elucidating the removal of toxic metal ions and highlighted the mechanistic details of the adsorption process by considering some real-time adsorbents systems for metal ion uptake and recovery from aqueous systems. A comparative analysis is presented throughout to highlight the accuracy and perfection of theoretically supported experimental results in dealing with the heavy metal ion removal process. Finally, optimized geometries, interaction energies, preferable metal ion binding sites and possible mechanism of adsorption on various metal ion adsorbents with special reference to graphene oxide have been discussed in detail.
... Lowsolubility solid materials with large specific surface area and porous structure have mainly served as adsorbents. These adsorbents can provide enough adsorption sites to combine with the pollutants in wastewater (Tian et al. 2017;Zhang and Liu 2019) for achieving sufficient control of the water pollution. Biochar (Ocinski et al. 2016;Ebrahimi et al. 2013;Tian et al. 2011;Cheraghi et al. 2014) has been commonly used as adsorbents for their advantages, such as large surface area, high stability, the low release of contaminants, recyclability, and inexpensive (Setyono and Valiyaveettil 2014). ...
... The adsorption method for treating heavy metals in wastewater and soil can be operated conveniently, and has a wide range of application prospects (Mia et al., 2017). A variety of adsorbents including graphene oxide (Nguyen-Phan et al., 2011), Ca(OH) 2 , Al 2 O 3 , SiO 2 , Fe 2 O 3 , and other mineral oxides (Kuo et al., 2011;Zhang K. et al., 2016;Zhang and Liu, 2019) have been used to remove heavy metals from wastewater. However, most of the currently used materials are inefficient, expensive, and can cause secondary pollution. ...
Lead (Pb)-contaminated wastewater is the most common source of heavy metal ion pollution. In this study, agricultural waste edible fungi residue (EFR) was used to adsorb Pb(II) ions in wastewater as a strategy to reduce environmental pollution and minimize poisoning by Pb. The influence of Pb(II) concentration, solution pH, and EFR concentration on the removal efficiency (R) of Pb(II) was investigated with single factor design and response surface analysis. The maximum predicted R for Pb(II) was 76.34% under optimal conditions of Pb(II) concentration of 483.83 mg/L, EFR concentration of 4.99 g/L, and pH of 5.89. The actual experimental value of R reached 76.97% under these conditions. The competition of Pb(II) ions for the available adsorption sites on EFR limited the maximum R. A comparison of Fourier transform infrared spectroscopy before and after the adsorption of Pb(II), indicated that the functional groups of EFR significantly affected the effect of adsorption of heavy metals, and that the adsorption process was primarily affected by functional groups in the range of wavenumbers from 500 to 2,000 cm⁻¹.
... Lowsolubility solid materials with large specific surface area and porous structure have mainly served as adsorbents. These adsorbents can provide enough adsorption sites to combine with the pollutants in wastewater (Tian et al. 2017;Zhang and Liu 2019) for achieving sufficient control of the water pollution. Biochar (Ocinski et al. 2016;Ebrahimi et al. 2013;Tian et al. 2011;Cheraghi et al. 2014) has been commonly used as adsorbents for their advantages, such as large surface area, high stability, the low release of contaminants, recyclability, and inexpensive (Setyono and Valiyaveettil 2014). ...
Arsenic (As) contamination in groundwater is a major problem in many countries, which causes serious health issues. In this paper, a novel method has been developed for the simultaneous removal of ROX and As(III/V) using the modified sorghum straw biochar (MSSB). The MSSB was characterized by X-ray diffraction, scanning electron microscopy, Fourier Transform Infrared, and Brunauer–Emmet–Teller (BET) surface area. The removal performance of MSSB for ROX, arsenite [As(III)], and arsenate (As(V)) was investigated using batch experiments. At pH of 5, the arsenic concentration of 1.0 mg/L, adsorbent dose of 1.0 g/L, the maximum adsorption capacities of ROX, As(III), and As(V) were 12.4, 5.3, and 23.0 mg/g, respectively. The adsorption behaviors were fit well with the Langmuir and the pseudo-second-order rate model. The results showed that MSSB acted as a highly effective adsorbent to simultaneously remove the composite pollution system consisted of ROX and As(III/V) in aqueous solutions, providing a promising method in environmental restoration applications.
... Therefore, the arsenic level for drinking water has been reduced by the WHO to 10 μg L -1 [16][17][18][19]. In addition, As(III) is more toxic than As(V) since the former binds to single but with higher affinity to nearby groups of sulfhydryls that associate with a variety of proteins and inhibit their activity and given its electronic structure, As(III) is more stable than As(V) [15,20]. ...
Usages of hospital sludge as a biochar adsorbent for wastewater treatment plants were investigated. Microwave carbonization was used to carbonize the sludge and then chemically activated with ZnCl 2 to increase surface area and porosity. A newly designed amine functional group’s (DETA) doped sludge biochar carbon (SBC) presents effective inorganic arsenic (As(III), As 2 O 3 ) and organic arsenic (p-ASA, C 2 H 7 AsO 2 ) adsorption in water. The pore volume, pore size distribution and specific surface area were determined by performing nitrogen adsorption-desorption measurements (BET). The Fourier transform infrared (FTIR) of the SBC was recorded to study the functional groups at room temperature. The composition of SBC was further determined by X-ray photoelectron spectroscopy (XPS). In order to understand the effect of amine functional complexes on arsenic adsorption, the adsorption mechanism of As 2 O 3 and p-ASA on SBC surfaces modified with amine functional complexes was studied using density functional theory (DFT). Results showed that both physical and chemical adsorption of As 2 O 3 and p-ASA on SBC surfaces occurred. The participation of amine functional complexes greatly promoted the surface activity of SBC surface and its adsorption capacity on arsenic. The physical adsorption energies of As 2 O 3 and p-ASA on SBC surface with amine functional complexes were -38.4 and -32.8 KJ mol -1 , respectively. Other hand, the chemical adsorption energies of As 2 O 3 and p-ASA on SBC surface with amine functional complexes were -92.9 KJ mol -1 and -98.5 KJ mol -1 , respectively.
... It means that the molecular interactions characterized by spectroscopic methods do not reflect their real state in the samples. In addition to experimental measurements, computational simulation is another useful tool to study the intermolecular interactions of adsorption [10]. It can provide details that are not observable during spectroscopic characterizations. ...
The adsorption of pollutants on carbonaceous environmental media has been widely studied via batch sorption experiments and spectroscopic characterization. However, the molecular interactions between pollutants and interfacial sites on carbonaceous materials have only been indirectly investigated. To comprehend the adsorption mechanisms in situ, we applied atomic force microscopy force spectroscopy (AFM-FS) to quantitatively determine the molecular interactions between typical amines (methylamines and N-methylaniline) and the surface of highly oriented pyrolytic graphite (HOPG), which was supported by the single molecule interaction derived from density functional theory and batch adsorption experiments. This method achieved direct and in situ characterization of the molecular interactions in the adsorption process. The molecular interactions between the amines and the adsorption sites on the graphite surface were affected by pH and peaked at pH 7 due to strong cation-π interactions. When the pH was 11, the attractions were weak due to a lack of cation-π interaction contributions, whereas, when the pH was 3, the competitive occupation of hydronium ions on the surface reduced the attraction between the amines and HOPG. Based on AFM-FS, the single molecule force of methylamine and N-methylaniline on the graphite surface was estimated to be 0.224 nN and 0.153 nN, respectively, which was consistent with density functional theory (DFT) calculations. This study broadens our comprehension of cation-π interactions between amines and electron-rich aromatic compounds on the micro/nanoscale.
CaSiO3 is highly resistant to sintering and can trap arsenic at high temperatures in the boiler furnace. However, the trapping capacity of CaSiO3 for arsenic does not meet the requirements of practical applications, and it is easy to react with acidic gases, which significantly affects the adsorptive property of arsenic. In this paper, the effect of Al modification on the As2O3 adsorption behaviour on the CaSiO3(001) surface was systematically investigated using a density functional theory. By comparing the magnitude of adsorption energy of different sites, the active site of As2O3 adsorbed on the surface of CaSiO3(001) was determined to be Ca, and the adsorption activity of As2O3 by the silicon oxygen chain composed of [SiO4] tetrahedron is deficient. The Si atoms in the [SiO4] tetrahedral structure are directly replaced by Al atoms, the difference in bond length and bond energy between Al–O bond and Si–O bond is used to promote the redistribution of surface charge and the increase of local structural bond angle of CaSiO3(001), leading to the exposure of new active sites (Si-top and Al-top sites) on the silicon oxygen chain. The new active site can realize the chemical adsorption of As2O3, the higher adsorption energy of the Al-top site is attributed to the stronger s-p orbital hybridization between Al and O atoms after doping, which is more conducive to the charge transfer between As2O3 and the adsorbent surface. In this work, influence of SO2 and HCl gases on the adsorption of As2O3 by modified silicon oxygen chains was also discussed. The results show that SO2 and HCl in the flue gas may occupy the Al-top site on the silicon oxygen chain through chemical adsorption, and reduce the activity of this site, thereby affecting the adsorption of As2O3. However, the exposed Si-top sites owing to Al doping show good acidic gas resistance, which in turn help the surface of Al–CaSiO3(001) can also maintain stable adsorption of As2O3 in SO2 and HCl atmosphere.
Arsenic emitted from coal-fired power plants has attracted widespread attention. Metal oxides are promising adsorbents for arsenic removal, but the influence laws and mechanisms of temperature and flue gas components on their arsenic removal performance are unclear. In this work, systematic research on the effects of adsorption temperature and flue gas components on As2O3 removal by Fe2O3, CaO, and γ-Al2O3 was conducted. The results show that As2O3 removal ability satisfies Fe2O3 > CaO > γ-Al2O3 at 300–900 °C, which is mainly related to their basic site strength. At 500 °C, Fe2O3 has the largest As2O3 removal capacity (adsorption capacity: 158.4 μg/g; removal efficiency: 84.68%). Increasing adsorption temperature is beneficial to the transformation of arsenic to high-valent arsenic (As(V)) on the surface of Fe2O3, CaO, and γ-Al2O3. Low-concentration SO2 will compete with As2O3 for adsorption on the three metal oxides surface, which thereby inhibits their As2O3 removal ability. The presence of NO has an obvious inhibitory effect on the As2O3 removal of Fe2O3, CaO, and γ-Al2O3, which is little affected by the change of NO concentration. Fe2O3 is a metal oxide adsorbent with strong arsenic removal potential. Compared with basic simulated flue gas (BSFG) components, the presence of SO2, NO, and SO2+NO in the simulated flue gas all inhibits the adsorption of As2O3 by Fe2O3. Finally, future work on arsenic removal from coal-fired flue gas adsorbents is presented. This work has important engineering applications and theoretical research guiding significance for the development of high-performance arsenic removal adsorbents.
Chemical looping gasification (CLG) provides a potential way for clean utilization of solid fuels with the innate CO2 separation and capture. Selection of suitable oxygen carrier and operating conditions in reactor are the key for fuels conversion. In this work, Fe2O3–Al2O3, CaSO4–Al2O3 and Fe2O3/CaSO4–Al2O3 oxygen carriers (OCs) were synthesized by impregnation method and innovatively applied in semi-coke chemical looping gasification (SCCLG) for syngas production. The reaction conditions (oxygen carrier types, temperature, steam addition amount) of oxygen carrier for syngas production were tested in a thermogravimetric analyzer (TGA) and a fixed-bed reactor. The experimental results showed that syngas yield of SCCLG with Fe2O3/CaSO4–Al2O3 is more than that with Fe2O3–Al2O3 or CaSO4–Al2O3 OC at 1000 °C when O/C ratio is 1:1. Furthermore, the maximum syngas yield (CO + H2) reached 129.20 mL/g of Fe2O3/CaSO4–Al2O3 at the gasification temperature of 950 °C. Results showed that the optimum operating conditions are steam rate of 0.06 mL/min at 950 °C with O/C of 1:1. Finally, the reaction mechanism of SCCLG was proposed based on experimental methods combing with advanced analytical testing and density functional theory (DFT) calculations results, and it reveals that OC in SCCLG underwent the reduction processes of Fe2O3/CaSO4–Al2O3 to CaFeSO/CaO–Al2O3 and then to Fe/CaS–Al2O3, which can efficiently improve the sulfur escape of CaSO4 OC. The present study provides an efficient way for conversion and utilization of fossil fuels in energy and chemical fields.
The characteristic of metal oxides capture SeO2 was investigated, and the adsorption sites, adsorption energy, and surface transformation were clarified via density functional theory (DFT). The adsorption experiments confirmed the adsorption sequence followed: fine particle Al2O3 > MgO > CaO > Fe2O3, where CaSeO3, MgSeO3, Al2(SeO3)3 and Se²⁻/Se⁰/Se⁴⁺/Fe as the adsorption product. Based on the instability of adsorption products, the adsorption effect of CaO and MgO became better within the range of 400–550 °C, while the adsorption capacity of Fe2O3 and Al2O3 decreased if exceeding 400 °C. Due to the effect of CO2 in the flue gas and the dense pore structure, the retention of SeO2 on CaO surface was inhibited. From the XPS results, Se²⁻ was the stable selenium species on Fe2O3 surface at high temperature. Furthermore, according to the density functional theory calculation, all metal oxides captured SeO2 involved chemical adsorption, where the adsorption sites correspond to the adsorbents: O top sites (CaO and MgO), Al and O top sites (Al2O3), and Fe top sites (Fe2O3). Finally, it was identified Fe top sites were the active adsorption sites for Se⁰, and the transformation of selenium on Fe2O3 surface was also intensively explored as a three-step process: SeO2 adsorption, Se⁴⁺→Se⁰, and Se⁰ → Se²⁻.
In this work, the arsenic (As2O3) amount in colemanite ore was reduced from 4551 to 425 ppm using decrepitation. The crystal and amorphous states of products were determined by XRD measurements and the thermodynamic properties of samples were examined by DSC-TGA analysis. Elemental arsenic was analyzed using an ICP-ES/MS analyzer. As a result of the characterizations, a majority of the arsenic mass was found in the waste material for all temperatures. It was concluded that arsenic is carried on the surface of gangue minerals in the calcination process. Moreover, the decrepitated arsenic compounds tend to accumulate back on solid surfaces thus the amount of arsenic was found to be higher than the amount found in literature which is lower than 100 ppm. Therefore the affinity of arsenic compounds to montmorillonite and colemanite surfaces were further studied comparatively using molecular dynamics (MD) and Monte Carlo (MC) simulations. The resulting energy and density profiles show that the arsenic compounds have a higher affinity towards the montmorillonite surface. The simulation results reflect the decrease of accumulation amount on the surfaces with increasing temperature in parallel with the experimental measurements.
Reducing energy barriers of CO2 being chemisorbed on hexagonal boron nitride (h-BN) is a kernel step to efficiently and massively capture CO2. In this study, aluminum/carbon (Al/C) atoms are used as dopants to alter surface potential fields of h-BN, which aims at lowering energy barriers of adsorption processes. Through theoretical calculations, direct-adsorption structures/properties of CO2, joint-adsorption structures/properties of CO2/H2O, transition state (TS) energy barriers, effects of temperatures on adsorption energies/TS energy barriers and changes of reaction rate constants over different adsorbents are investigated in detail in order to reveal how doping of Al/C atoms promotes CO2 adsorption strength over doped h-BN. According to DFT calculation results, the average adsorption energy of CO2 being directly adsorbed on Al/C-doped h-BN arrives at −59.43 kJ/mol, which is about 5 times as big as that over pure h-BN. As to the average adsorption energy of CO2/H2O and relevant TS energy barrier, they are modified to −118.89 kJ/mol and 40.23 kJ/mol over Al/C-doped h-BN in contrast with −33.91 kJ/mol and 1695.11 kJ/mol over pure h-BN, respectively. What is more, based on thermodynamic analyses and reaction dynamics, the average desorption temperatures of CO2(/H2O) are promoted over doped h-BN and the temperature power exponent is negatively correlated with the activation energy in the Arrhenius equation form. The complete understanding of this study would supply crucial information for applying Al/C-doped h-BN to effectively capturing CO2 in real industries.
In this work, the mechanism of SO2 on CaO capture selenium was deeply investigated combined density functional theory (DFT) with adsorption experiments. The fixed-bed experiments result showed that SO2 inhibited CaO capture selenium from 0 to 1000 while promoted in the range of 1000-2000 ppm, and CaSeO3 was the primary adsorption product at 850 °C. Freundlich equation accurately predicted the adsorption capacity of CaSO4 for SeO2 with the concentration from 0 to 800 ppm, indicating weak adsorption activities for SeO2. Furthermore, the SO2 and selenium oxides adsorption mechanism was studied by DFT. It demonstrated that SO2 and SeO2 were adsorbed on CaO (0 0 1) surface involved chemical adsorption with oxygen atoms as adsorption sites, which led to the competition of SO2 and SeO2 for adsorption sites on the CaO surface. SO2 adsorbed on the CaO surface inhibited the activity of neighboring adsorption sites for SeO2. Moreover, compared with SeO2, SeO and Se⁰ reduced by SO2 were more beneficial adsorbed by CaO, the conversion of SeO2 to selenium and SeO was firstly proposed in selenium adsorption. In detail, SO2 promoted CaO capture selenium through homogeneous selenium conversion. Finally, it was identified that the CaSO4 (0 0 1) surface was inert for selenium adsorption in the flue gas, indicating CaO transition to CaSO4 inhibited selenium adsorption.
In this work, the stable arsenic forms were determined based on density functional theory (DFT) combined thermodynamic system equilibrium calculation. For the stable arsenic species, the capture mechanism by CaO and Fe2O3 was studied by DFT. Trivalent arsenic(III) were the main arsenic forms in the flue gas from thermodynamic system equilibrium analysis. With H2O in the flue gas, arsenic species form was As4O6 from 700 to 1700 K and converted to AsOOH at 900 K. Without H2O in the flue gas, chain As2O3 and AsO2 was generated by As4O6 at 1200 K, and chain As2O3 decomposed to AsO2 as temperature further increased. The conversion pathways between stable arsenic species were studied as the arsenic reaction cycle. The adsorption behavior of stable arsenic species on CaO and Fe2O3 surfaces was investigated via DFT. Oxygen atoms on the CaO surface and ferric, oxygen atoms on the Fe2O3 surface were the main active adsorption sites for arsenic species. The adsorption results implied the strength of CaO and Fe2O3 capture arsenic species: chain As2O3 > AsO2 > AsOOH > As4O6. The band gap of arsenic species also confirmed this conclusion indicating high temperature was beneficial for metal oxide surface to capture arsenic species through arsenic conversion.
To clarify the role of Bi-doped Fe2O3(0 0 1) in the decomposition process of 1,3,5,7-tetranitro-1,3,5,7-tetrazocane(HMX), the effect of Bi-Fe2O3(0 0 1) on the gas products of HMX(NO, NO2, N2O, H2O, CH2O) was emphatically investigated using density functional theory (DFT) calculation. The adsorption configuration, energy, electronic properties and possible reactions of gas products were considered. All of these gases are adsorbed on the Fe-top site of Bi-Fe2O3 surface in molecular form, resulting in the orbital hybridization and electron transfer. NO2 and CH2O interacting with Bi are of higher adsorption energy comparing with gas adsorbed on pure Fe2O3(0 0 1). Since the reaction of NO2 with CH2O and the dissociation of NO2 play an important role in the subsequent decomposition of HMX, the effects of Bi doping on these two reactions were investigated. TS search shows that the activation energy of these two reactions are all decreased on Bi-Fe2O3 surface, which may accelerate the consumption of NO2, indicating that Bi doping can promote the reaction of NO2 and CH2O, thus accelerating the subsequent decomposition of HMX.
Impact of arsenic on the performance of Mg-Ti modified iron-based catalyst during selective catalytic reduction of NOx with ammonia was studied. The results indicated that Mg and Ti modification not only improved the catalytic activity at 100-350°C, but also exhibited remarkable anti-arsenic ability. Mg was dominant in the excellent anti-arsenic performance, and this role was firstly recognized. The anti-arsenic mechanism of Mg-Ti modified iron-based catalyst was ascribed to two aspects. One side, Mg exhibited well reactivity with arsenic which competed with Fe for the adsorption of arsenic and protected active iron sites from arsenic occupation. The other side, presence of Mg promoted the formation of MgFe2O4 in view of the strong interactions between Mg and Fe, suppressing the interactions between arsenic and Fe. Instead, gaseous arsenic turned to bond with Mg-O sites. Consequently, surface acid sites and function Fe-OH/ Fe=O bonds were preserved for de-NOx. Moreover, increase of oxygen content and temperature weakened the deactivation of Mg-Ti modified iron-based catalyst by arsenic. It was due to the oxidation of As2O3 with oxygen content increase and the strengthened reactivity of non-active iron sites with arsenic and Mg with arsenic as temperature elevation, inhibiting the interactions between active iron sites with arsenic.
Adsorption mechanism and competitive adsorption of NH3 and As2O3 molecules on CuO (111) surface were investigated via density functional theory (DFT) simulations. The adsorption configuration, adsorption energy, electronic gains/losses, and projected density of states were thoroughly discussed. Results showed that As2O3 molecule was more likely to adsorb on CuO surface than NH3 molecule. For the adsorption of As2O3 on CuO surface, the "Osuf-Osuf bridge" site was the active site with the chemical adsorption energy of -1.39 eV. In the process of NH3 adsorption on CuO surface, N-Cu bond was formed in chemical adsorption, which was mainly attributed to the charge transfer from NH3 molecule to substrate surface. However, no obvious structural change occurred in physisorption. Thermodynamic analysis indicated that physisorption was the primary form for the adsorption of NH3 and As2O3 molecules on CuO surface under the actual SCR condition.
Layered scale in a steel tube sample, obtained from an oilfield wastewater pipeline in northwestern China, is experimentally examined layer by layer. From the scale layer contacting with steel wall to the center hole (from the 1st to 4th layer), the organic content increases. The majority of 1st-layer is composed with iron oxides (corrosion products), while CaCO3 emerges in the 2nd layer, and its crystal structure transforms from aragonite to calcite as scale grows thicker from the 2nd to 4th layer. Then, FeCO3 and complex carbonate (Ca0.1Mg0.33Fe0.57CO3) are identified in the 3rd and 4th layers. The experiments indicate the corrosion products preferentially form before CaCO3 fouling. So, CaCO3 precipitates are likely to nucleate on the substrate of iron oxides (such as Fe2O3), which provides a newfound impetus to explore the interfacial bonding mechanism of Fe2O3@Fe(110) and CaCO3@Fe2O3(001). By combining MD and DFT approaches, it is demonstrated that two of three O atoms in Fe2O3 molecule have formed four O-Fe bonds with Fe(110), and those bond lengths are shorter than the one in bulk Fe2O3. And, totally seven bonds would form between CaCO3 and Fe2O3(001), in which three O-Ca, one Ca-Fe and three O-Fe bonds are included. For the bonded atoms, the former atom acts as charge acceptor, and the latter tends to be donor.
The conversion of As vapor released from coal combustion to less hazardous solids is an important process to alleviate As pollution especially for high-As coal burning, but the roles of key ash components are still in debate. Here, we used multiple analytical methods across the micro to bulk scale and density functional theory to provide quantitative information on As speciation in fly ash and clarify the roles of ash components on As retention. Fly ash samples derived from the high-As bituminous coal-fired power plants showed a chemical composition of typical Class F fly ash. In-situ electron probe microanalysis (EPMA) was for the first time used to quantify and distinguish the inter-particle As distribution difference within coal fly ash. The spatial distribution of As was consistent with Fe, O, and sometimes with Ca. Grain-scale distribution of As in coal fly ash was quantified and As concentrations in single ash particles followed the order of Fe-oxides > aluminosilicates > unburned carbon > quartz. Sequential extraction and Wagner chemical plot of As confirmed that Fe minerals rather than Al-/Ca-bearing minerals played a vital role in capturing and oxidizing As³⁺ into solid phase (As⁵⁺). Magnetite content in fly ash well-correlated with the increase ratio of As before and after magnetic separation, suggesting magnetite enhanced As enrichment in fly ash. Density functional theory (DFT) indicated that the bridges O sites of octahedral structure on Fe3O4 (111) surface were likely strong active sites for As2O3 adsorption. This study highlights the importance of magnetite on As transformation during bituminous or high-rank coal combustion in power plants and has great implications for developing effective techniques for As removal.
Arsenic is extremely toxic and its release has caused great environmental concerns. Coal combustion is considered to be one of the major anthropogenic emission sources. Arsenic removal technology from coal combustion can be divided into three categories: pre-combustion removal, removal during combustion and post-combustion removal. The post-combustion removal is also called removal from flue gas, which includes several technological developments, namely, the removal using existing air pollutant control devices (APCDs), adsorption, traditional oxidation and advanced oxidation based on removal principle. This review summarizes the latest advances of these arsenic removal technologies. The performance, mechanism and characteristics of arsenic removal technologies were overviewed and analyzed. The merits and drawbacks, and the challenges and prospects of each arsenic removal technologies were discussed. It was found that pre-combustion removal, removal during combustion and removal using APCDs can achieve arsenic removal to a degree, but their removal efficiencies are usually low. Injection of adsorbent into the flue gases can achieve higher arsenic removal efficiency. Calcium-based adsorbents were found to be one of the most efficient ones for arsenic removal. Their shortcoming is the high-temperature sintering and deactivation caused by competitive adsorption of acidic gases. Other adsorbents suffer from low activity, small specific surface area, high cost, or/and little recovery. Further development of advanced adsorbents that are anti-sintering, anti-deactivation, large specific surface area, low-cost, separable, and recyclable should be the main focus in future research. Collaborative control of multiple systems such as removal during combustion, removal using APCDs or/and tail adsorption/oxidation is a promising strategy. Advanced oxidation technologies (AOTs) can achieve high arsenic removal efficiency (90-100%), recovery of arsenic resources and potential simultaneous removal of multi-pollutants, possessing good prospect.
The integrated control of multiple pollutants is a promising approach for efficient and economical pollution reduction. Inspired by the simultaneous removal of SO2 and NOx by the spray-and-scattered-bubble (SSB) technology, this paper further explores gas phase arsenic and selenium removal ability of this new technology. Ammonia concentration, SO2 concentration, liquid/gas ratio and immersion depth, which are the key operating parameters of SSB technology, are evaluated to determine their effect on arsenic and selenium removal. The experimental results indicate that ammonia concentration and SO2 will facilitate the simultaneous removal of arsenic and selenium by SSB technology. However, the excess ammonia concentration and SO2 should avoided to prevent the decrease in removal efficiency caused by the ammonia escape, increased mass transfer resistance, and mechanical carry-over. The maximum removal efficiency for arsenic can be obtained at the liquid-gas ratio of 10 L/m³, and for selenium, the maximum removal efficiency will be reached at 14 L/m³. For the technology of spray-and-scattered-bubble, chemical reaction and mass transfer jointly play the role in contaminant removal. By changing the immersion depth and measuring the corresponding pressure drop, the weight assigned to the effect of chemical reaction and mass transfer effect could be ascertained to a certain degree. It is speculated that chemical reaction will play a more important role for selenium removal in the bubble zone than the mass transfer. Moreover, for arsenic, mass transfer effect will play a more important role than chemical reaction. The sensitivity analysis for simultaneous removal of arsenic and selenium by SSB technology indicating that the variation of operating conditions will lead to a greater change in arsenic removal as compared with selenium.
Carbon-based sorbents have been regarded as the promising candidates for arsenic removal from coal-fired flue gas because of their large surface area and functionality. The effect mechanism of SO2 on As2O3 adsorption over the carbon surfaces was systemically investigated by density functional theory calculations. The results show that the As2O3 molecule can be adsorbed on the carbon surfaces vertically or horizontally. The adsorption energies of As2O3 on the pure carbon surfaces range from -45.47 to -497.74 kJ/mol. When the SO2 concentration is low, the presence of SO2 can facilitate the As2O3 adsorption because of the electronic effects caused by SO2. The adsorbed SO2 can significantly decrease the level of electrostatic potential of neighbor active sites. In addition, the charge distributions of unsaturated carbon atoms are changed after SO2 adsorption, making the carbon surfaces more active for As2O3 adsorption. The enhanced charge transfer between As2O3 and the carbon surfaces with SO2 results in the stronger As2O3 adsorption. However, higher concentration of SO2 could inhibit the As2O3 adsorption because SO2 competes with As2O3 for the active adsorption sites.
Adsorption experiments were carried out to investigate the role of O2 on As2O3 capture by γ-Al2O3. Results showed that As2O3 retention over γ-Al2O3 surface was promoted with the O2 concentration from 0 to 8%. The adsorption process was also enhanced when temperature was increased from 573 K to 1073 K, and As(V) was the primary adsorption products at high temperatures. Furthermore, As2O3 adsorption mechanism was studied through density functional theory (DFT) calculations. Compared to γ-Al2O3 surface, the O2/γ-Al2O3 surface exhibited stronger adsorption ability for As2O3 molecule. In detail, As2O3 adsorption on the ortho-position of adsorbed O2 involved strong chemical adsorption, indicating that the activities of neighboring atoms were strengthened by adsorbed O2, which was consistent with the phenomenon from adsorption experiments. Nevertheless, the As2O3 adsorption on the top site of adsorbed O2 was physical adsorption. Moreover, thermodynamic analysis exhibited that As2O3 adsorption on either γ-Al2O3 surface or the ortho-position of adsorbed O2 were chemical adsorption at 573-1073 K. The stability of adsorption system decreased with increasing temperature, and the adsorption geometry would ultimately convert into more stable structures in next reactions. The experimental and simulation results explicate the role of O2 on As2O3 capture by γ-Al2O3, and provide a guide for arsenic emission control.
In order to reveal the affecting mechanisms of flue gas on As2O3 adsorption by γ-Al2O3 and to enhance the adsorbing capacities of γ-Al2O3, the influences of flue gas constituents on As2O3 adsorption on γ-Al2O3(0 0 1) surface are investigated theoretically via density functional theory (DFT) in this study. The flue gas constituents selected include O2, H2O, SO2 and CO2. O2 converts nearly all of the physisorption structures into chemisorption structures except one structure, in which the O2 electron cloud does not interact with As2O3 molecule and therefore does not enhance the capture of As2O3. For the effects of H2O, SO2 and CO2, they behave almost the same as those of O2, but the physisorption structures vary from different constituents. The difference of stable adsorption structures of O2, H2O, SO2 and CO2 on the surface of γ-Al2O3 and their corresponding properties are the main reason for variance of positions and quantities of As2O3 physisorption structures. Results of this study could provide useful information for enhancing capture capacities of γ-Al2O3 under actual flue gas environments.
Coal-fired power plants have been identified as a major source of selenium emissions that can be controlled by mineral adsorption. The adsorption capacity of CaO for SeO2 adsorption was studied in a simulated flue gas atmosphere, and the amounts of SeO2 absorbed by CaO before and after high-temperature roasting were experimentally evaluated and compared. To explain the effect of CO2 on the adsorption of SeO2, the effect of preadsorbed CO2 on SeO2 adsorption by CaO was investigated by density functional theory (DFT) calculations. The experimental results showed that the adsorption capacity reached the maximum at 700 °C, while the adsorption capacity decreased gradually for the temperatures higher than 700 °C. CO2 in the simulated flue gas caused the carbonation reaction on the surface of CaO, leading to the inhibition of SeO2 adsorption by CaO. The computational results indicated that the SeO2 and CO2 adsorb on the CaO surface by chemisorption, with the O atoms on the CaO (001) surface as the adsorption active sites. Not only did CO2 occupy the active sites on the surface but also the interaction between the Se atom and the O atom of CO2 was found to be weaker than that between the Se atom and the O atom on a clean CaO surface.
In this work, greigite (Fe3S4) and magnetite (Fe3O4) is used as a carrier to study the role of sulfur (S) in the adsorption of antimony (Sb) by greigite. Results show that the synthesized Fe3S4 has stronger adsorption ability to Sb(III)/Sb(V) than its oxide counterpart Fe3O4. The Langmuir maximum adsorption capacities of Fe3S4 to Sb(III) and Sb(V) are 178.87 and 163.52 mg/g, respectively. Change in coordination mode caused by the substitution of S to O is the main reason for the enhancement in Sb(III)/Sb(V) adsorption ability. Sb(III)/Sb(V) adsorb on the Fe-Fe bridge site on (311) plane of Fe3O4, while Fe-S is adsorb at the top site on the (311) plane of Fe3S4; the latter has higher adsorption affinity to Sb than the former. The oxidative conducts of Fe3S4 are only elemental sulfur and lepidocrocite (γ-FeOOH), and no sulfur-containing compounds are identified. This work is expected to provide new understanding to the role of anion in spinel materials.
In order to protect selective catalytic reduction (SCR) catalysts for flue gas denitration in coal-fired power plants, the adsorption of As2O3 on γ-Al2O3(0 0 1) surface is investigated theoretically through density functional theory (DFT) in this study. The adsorption sites, adsorption structures, adsorption energies, electronic clouds, transition processes, and intermediate and transition structures are investigated. The theoretical results indicate that the adsorption of As2O3 molecule on the surface of γ-Al2O3(0 0 1) could be either physical or chemical, depending on the sites the molecule hangs over. Compared with the experimental results from other researchers, this study unveils that, although the apparent adsorption of As2O3 molecule on γ-Al2O3(0 0 1) surface is physical, some of the sites on γ-Al2O3(0 0 1) surface presents strong chemical affinity towards As2O3 adsorption. Further, this study depicts the adsorption process to clarify the reason of the net effect of As2O3 adsorption on γ-Al2O3 being physical. Meanwhile, the study also reveals that apparent physical adsorption of As2O3 on γ-Al2O3(0 0 1) surface is due to the high energy barrier that prohibits the transformation of physical adsorption to chemical adsorption. The research results provide useful information for exploiting γ-Al2O3 as a potential metal oxides sorbent.
The effect of pre-adsorbed O2 on selenium removal by CaO was investigated by density functional theory (DFT) calculations. Adsorption configurations of Se and SeO2 on CaO (0 0 1) surface with pre-adsorbed O2 were optimized. And the conversion of Se to SeO2 on CaO (0 0 1) surface has been studied as well. Results showed that O2/CaO (0 0 1) surface had weaker adsorption ability than CaO (0 0 1) surface for both Se and SeO2, which indicated that pre-adsorbed O2 was not beneficial to selenium capture. When Se atom adsorbed on O2/CaO (0 0 1) surface, two kinds of stable adsorption configurations were formed: O-Se-O group and O-O-Se group on CaO (0 0 1) surface with chemical activities. When SeO2 molecule adsorbed on O2/CaO (0 0 1) surface, some valence electrons in adsorption substrate transferred into the orbits of SeO2 molecule, forming Se-O covalent bond. Additionally, the reaction energy barrier of Se and O2 conversing into SeO2 in heterogeneous was less than that in homogeneous, which indicated that CaO could not only act as an adsorbent, but also promote the conversion of Se to SeO2.
PubChem (https://pubchem.ncbi.nlm.nih.gov) is a public repository for information on chemical substances and their biological activities, launched in 2004 as a component
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Generalized gradient approximations (GGA’s) seek to improve upon the accuracy of the local-spin-density (LSD) approximation in electronic-structure calculations. Perdew and Wang have developed a GGA based on real-space cutoff of the spurious long-range components of the second-order gradient expansion for the exchange-correlation hole. We have found that this density functional performs well in numerical tests for a variety of systems: (1) Total energies of 30 atoms are highly accurate. (2) Ionization energies and electron affinities are improved in a statistical sense, although significant interconfigurational and interterm errors remain. (3) Accurate atomization energies are found for seven hydrocarbon molecules, with a rms error per bond of 0.1 eV, compared with 0.7 eV for the LSD approximation and 2.4 eV for the Hartree-Fock approximation. (4) For atoms and molecules, there is a cancellation of error between density functionals for exchange and correlation, which is most striking whenever the Hartree-Fock result is furthest from experiment. (5) The surprising LSD underestimation of the lattice constants of Li and Na by 3–4 % is corrected, and the magnetic ground state of solid Fe is restored. (6) The work function, surface energy (neglecting the long-range contribution), and curvature energy of a metallic surface are all slightly reduced in comparison with LSD. Taking account of the positive long-range contribution, we find surface and curvature energies in good agreement with experimental or exact values. Finally, a way is found to visualize and understand the nonlocality of exchange and correlation, its origins, and its physical effects.
Oxy-fuel combustion of coal is an advanced technology for CO2 control. The influences of concentrations of CO2, SO2 and H2O on arsenic removal by Fe2O3/γ-Al2O3 sorbent in coal oxy-fuel combustion were investigated. The over high CO2 concentration in oxy-fuel combustion inhibited the adsorption process, while adding O2 or H2O(g) could accelerate this process. With the increase of SO2 concentration, the removal efficiency of gas-phase arsenic firstly increased and then decreased. When SO2 concentration reached 8000 ppm, the arsenic adsorption efficiency was maximal. An adsorption mechanism according to the experiments phenomena was proposed to follow the Mars-Maessen mechanism. XPS characterization results indicated that the arsenic adsorption affected the charge distribution of sorbent. It reveals that electron transfer would occur between substrate and adsorbate in adsorption process.
Fe2O3/γ-Al2O3 sorbents that containing different quantities of iron (III) oxide were prepared by ultrasound-assisted wet impregnation method. Application of these sorbents for arsenic adsorption was tested on a fixed bed reactor and some factors, such as temperature, precursor solution concentration (PSC) and SO2/NO concentration were discussed. The results indicated that more than 65% of gas-phase arsenic was adsorbed at 600 °C, while the efficiency decreased gradually when temperature over 600 °C. The higher precursor solution concentration could increase the amount of active substance on carrier, but also cause structure deterioration of sorbent. SO2 in flue gas exhibited a promotion effect on arsenic adsorption across Fe2O3/γ-Al2O3; however, the effect of NO was not as notable as SO2. The adsorption of arsenic is proposed to follow the Mars–Maessen mechanism. In relative low oxygen condition, the oxidation of arsenic was mainly relied on lattice oxygen of sorbent. Introduction of oxygen in flue gas could reoxidize the reduced oxide and therefore facilitate the arsenic capture.
The existing forms and their inter-transformations are important to study the behavior of arsenic and its capture technology in the flue gas of power plants. In this study, a density functional theory was applied to study the thermodynamic and kinetic aspects of arsenic substances in flue gas. Gibbs free energy comparison was used to evaluate the thermodynamic stability of various arsenic species at four temperatures (1200, 800, 370, and 25 °C), which represent the temperatures of flue gas in the area of the combustion center, horizontal flue, NOx removal reactor, and atmosphere, respectively. The results show that trivalent arsenic molecules are thermodynamically stable at high temperatures and pentavalent species are stable at low temperatures. The arsenic species vary with the temperature. At high temperatures, dehydrated compounds are the major species. These compounds will be hydrated and oxidized by O2 when the temperature declines, as implied by the reaction path study. Arsenic acid becomes the most thermodynamically stable species at 25 °C.
The adsorption of O2 on the perfect and low-coordinated sites of MgO(001) surface has been studied with the finite cluster models embedded in a large array of point charges by density functional method. The point charge value was determined by self-consistence technique. Different kinds of possible models of O2 adsorbed on MgO(001) surface were calculated. The optimization of the geometry, calculation of the adsorption energy, vibrational frequency and analysis of the Mülliken population to those adsorption models were carried out. The results indicate that cationic site in the lowest coordinated corner is the most advantageous position for O2 adsorbed on MgO(001) surface. The O-O bond strength is considerably weakened when O2 lies flatly on the Mg atom at the corner (Mg3c). The calculated adsorption energy of O2 on MgO(001) perfect surface is in good agreement with experimental value. For O2 adsorbed on the perfect surface embedded in nominal ± 2. 0 e point charges and on perfect surface using the bare cluster, the adsorption energies given in this paper show that they have a large deviation from the experimental value. The vibrational frequency of adsorbed O2, which is experimentally difficult to measure due to the existence of isotope exchange, was also calculated.
First-principle calculations based on density functional theory were performed to investigate the micro-mechanism of Hg-0 adsorption on alpha-Fe2O3 (001) surface in the presence of O-2. Considering O-2 is more easily adsorbed on alpha-Fe2O3 than Hg-0, this paper investigated Hg-0 adsorption on O-2 embedded alpha-Fe2O3 (001) surface to clarify the effect of O-2 on the capture of mercury by alpha-Fe2O3. Theoretical calculations indicate that O-2 dissociates in two steps on the surface, leaving one O atom to interact with a surface Fe atom. Hg-0 adsorption on O/alpha-Fe2O3 (001) surface belongs to weak chemisorption, and the potential energy diagram is provided. Additionally, the study of coverage shows that O atom coverage has a huge impact on Hg-0 adsorption. The adsorption mechanism of Hg-0 on the surface changes from weak chemisorption into stronger chemisorption as the O coverage increases from 0.25 to 1 mL, with the largest adsorption energy of -268.1 kJ/mol.
First-principle calculations based on density function theory (DFT) are used to clarify the roles of γ-Fe2O3 in fly ash for removing mercury from coal-fired flue gases. In this study, the structure of key surface of γ-Fe2O3 is modeled and spin-polarized periodic boundary conditions with the partial relaxation of atom positions are employed. Binding energies of Hg on γ-Fe2O3 (001) perfect and defective surfaces are calculated for different adsorption sites and the potential adsorption sites are predicted. Additionally, electronic structure is examined to better understand the binding mechanism. It is found that mercury is preferably adsorbed on the bridge site of γ-Fe2O3 (001) perfect surface, with binding energy of −54.3kJ/mol. The much stronger binding occurs at oxygen vacancy surface with binding energy of −134.6kJ/mol. The calculations also show that the formation of hybridized orbital between Hg and Fe atom of γ-Fe2O3 (001) is responsible for the relatively strong interaction of mercury with the solid surface, which suggests that the presently described processes are all noncatalytic in nature. However, this is a reflection more of mercury's amalgamation ability.
Using spin-density functional theory we investigated various possible structures of the hematite (0001) surface. Depending on the ambient oxygen partial pressure, two geometries are found to be particularly stable under thermal equilibrium: one being terminated by iron and the other by oxygen. Both exhibit huge surface relaxations ( -57% for the Fe and -79% for the O termination) with important consequences for the surface electronic and magnetic properties. With scanning tunneling microscopy we observe two different surface terminations coexisting on single crystalline alpha- Fe2O3 (0001) films, which were prepared in high oxygen pressures.
To facilitate the understanding of the fate of metals during incineration the speciation of metals is critical. Thus, an experimental study to determine the speciation of metals during incineration was performed. Aqueous metal salts of arsenic, cadmium, chromium, and lead were injected into a laboratory reactor, post-flame. Aerosols were captured on a glass fiber filter by isokinetic sampling. Reactor temperature was varied from 600 to 1100°C and the stoichiometric ratio was varied from 0.95 to 1.25. The Reference Intensity Method (RIM) of quantitative X-ray diffraction analysis combined with X-ray transmission was utilized to determine the speciation of aerosols removed from the combustion environment.The results indicate that temperature and stoichiometric ratio affect the formation of crystalline compounds in the combustion environment. Injection of different compounds of the same metal into the furnace resulted in the formation of different products. RIM was used to determine the composition of the crystalline sample fraction when reference intensity ratios were available.
The oxygen coverage, structure, and thermodynamic stability of (0001) surfaces of Fe2O3 (hematite) as a function of temperature and oxygen pressure are investigated by ab initio density functional theory with the generalized gradient approximation. Spin-polarized total energy and force calculations are performed using the projector augmented wave method as implemented in the Vienna ab initio simulation package. At high chemical potentials of oxygen (i.e., high pressure or low temperature), the most stable (0001) surface of hematite is completely covered with oxygen atoms. At low chemical potentials, a structure with one surface iron atom per two-dimensional unit cell is found to be the most stable surface termination. Around 800 K at an oxygen partial pressure of 0.2 bar, this reactive surface iron atom can bind and release an oxygen atom, thus switching between formal oxidation states (III) and (V), i.e., between stoichiometric and ferrate-like states. The fully reduced (iron terminated) surface is found to be thermodynamically unstable and dissociates adsorbed oxygen molecules spontaneously.
This work is the first application of classical atomistic theory to a comprehensive treatment of gamma-Fe2O3 surfaces. The surface energy and attachment energy of several low-index surfaces of gamma-Fe2O3 have been calculated by the classical atomistic simulation methods. A mean-field approximation involving scaling the short-range potentials, charges, and force constants of the iron ions by a fraction corresponding to the partial occupation of octahedral iron sites in the spinel structure was employed. Reconstruction of the polar surfaces was required to remove the dipole perpendicular to the surface and to achieve structural stability. This was accomplished through the addition of vacancies. The two-dimensional periodic calculations were carried out with the MARVIN code. The (112) surface consisting of iron and oxygen ions had the smallest relaxed surface energy of 1.86 J/m(2), while several others including (001), (011), and (012) were within 0.1 J/m(2) of this value. The calculated surface energies of these planes were used in a Wulff plot to predict a polyhedral crystal habit for these crystals at thermodynamic equilibrium. The (111) surfaces terminated with iron ions had the smallest attachment energies, and an octahedral crystal with (001) facets is predicted for the growth morphology of gamma-Fe2O3. Such surfaces possess iron ions in 3-fold coordination sites at 0.5 Angstrom above the plane of oxygen ions, making them accessible to interact with adsorbed molecules. This information is relevant to understanding the growth of nanocrystals of gamma-Fe2O3.
Beginning in 1998, electric power plants burning coal or oil must estimate and report their annual releases of toxic chemicals listed in the Toxics Release Inventory (TRI) published by the U.S. Environmental Protection Agency (EPA). This paper identifies the toxic chemicals of greatest significance for the electric utility sector and develops quantitative estimates of the toxic releases reportable to the TRI for a representative coal-fired power plant. Key factors affecting the magnitude and types of toxic releases for individual power plants also are discussed. A national projection suggests that the magnitude of electric utility industry releases will surpass those of the manufacturing industries which currently report to the TRI. Risk communication activities at the community level will be essential to interpret and provide context for the new TRI results.
The adsorption mechanisms of elemental arsenic and selenium on the CaO (001) surface were studied with periodic slab models based on density functional theory. The results showed that As and Se were strongly adsorbed on O at the top of the CaO surface with maximum binding energies of −2.07 and −2.92 eV, respectively. When the surface coverage was increased from 0.056 to 0.125 monolayer, the binding energies decreased slightly. Analysis of the electronic partial density of states showed that adatom states strongly hybridized with O 2p states to form As–O and Se–O covalent bonds when As and Se were adsorbed on the O top site. In contrast, As–Ca and Se–Ca ionic bonds were formed on the Ca top site as well as the O top site. Population analysis showed that the adsorption of As and Se on the O top site reduced the strength of the substrate Ca–O bonds.
The present work explores the possibility of capturing toxic elements other than sulfur in coal gasification flue gases using metal oxide mixtures. Arsenic and selenium compounds were the elements selected for study because they are toxic species which are present in coal gasification flue gases in different amounts, depending on temperature. Among the regenerable sorbents already developed for hot gas cleaning systems in Integrated Gasification Combined Cycles, metal oxide mixtures based on iron, titanium, or zinc oxides (zinc ferrites and zinc titanates) were tested for arsenic and selenium retention. These sorbents have previously been proved to possess good characteristics for H2S(g) retention. The study was carried out in a laboratory scale reactor, using the sorbent in a fixed bed, at 550 °C. Good retention capacities (56 mg g-1) were obtained in these conditions for selenium in a metal oxide mixture containing zinc titanate. A metal oxide mixture containing zinc ferrite proved to be an appropriate sorbent for both elements, retention capacities being 21 mg g-1 for arsenic and 55 mg g-1 for selenium. The results obtained indicate that arsenic and selenium compounds can be retained together with sulfur compounds in these sorbents and be desorbed in the sorbent regeneration processes.
The effect of porous structure and surface functionality on the mercury capacity of a fly ash carbon and its activated sample has been investigated. The samples were tested for mercury adsorption using a fixed‐bed with a simulated flue gas. The activated fly ash carbon sample has lower mercury capacity than its precursor fly ash carbon (0.23 vs. 1.85 mg/g), although its surface area is around 15 times larger, 863 vs. 53 m2/g. It was found that oxygen functionality and the presence of halogen species on the surface of fly ash carbons may promote mercury adsorption, while the surface area does not seem to have a significant effect on their mercury capacity.
This article describes recent technical developments that have made the total-energy pseudopotential the most powerful ab initio quantum-mechanical modeling method presently available. In addition to presenting technical details of the pseudopotential method, the article aims to heighten awareness of the capabilities of the method in order to stimulate its application to as wide a range of problems in as many scientific disciplines as possible.
This paper is concerned with three trace metals that are sufficiently volatile to occur in the flue gas from coal in the vapor state. These elements are arsenic, selenium, and mercury. Securing their control as particulate matter in an electrostatic precipitator or baghouse may be aided by a process of chemical conversion from a vapor to a solid. This change may not always be necessary because some chemical process may prevent the appearance of each metal in the vapor state or some physical process such as adsorption may obviate the need for chemical conversion. Nevertheless, chemical conversion, if it can be easily achieved, may ensure that a physical deposition process such as electrostatic precipitation or fabric filtration will be an efficient removal process.For arsenic and selenium, the type of chemical process of interest is chemical reduction of the volatile oxides As4O6 and SeO2 to the relative involatile unoxidized elements. Ammonia at the low concentrations found useful for baghouse gas conditioning has been found to be capable of reducing SeO2 to the element, and it also has the potential of reducing As4O6 to the element. For mercury, the oxide is less volatile than the element. Thus, to remove elemental mercury from flue gas, chemical oxidation rather than chemical reduction is conceptually useful. Oxidants of potential value are the elements sulfur and selenium, which may produce the involatile sulfide and selenide.
The emission of potentially toxic compounds of arsenic and selenium present in flue gases from coal combustion and gasification processes has led to the need for gas cleaning systems capable of reducing their content. This work is focused on the capture of these elements in activated carbons which have proven to have good retention capacities for mercury compounds in gas phase. Two commercial activated carbons (Norit RBHG3 and Norit RB3) and a carbon prepared via activation of a pyrolysed coal (CA) were tested in simulated coal combustion and gasification atmospheres in a laboratory scale reactor. Arsenic and selenium compounds were retained to different extents on these carbons, retention efficiency depending mainly on the speciation of the element, which in turn depends on the gas atmosphere. Arsenic retention was similar in both combustion and gasification atmospheres unlike selenium retention. Moreover the retention of arsenic was lower than that of selenium.
A kinetic model for describing the transformations of excluded pyrite (FeS2) particles in a combustion environment is developed by solving the particle heat and mass balances. The model follows the particle composition history, starting from pyrite decomposition to pyrrhotite (Fe1 − xS), and its subsequent oxidation to a molten sulphide-oxide mixture, then magnetite (Fe3O4), and finally haematite (Fe2O3). Particle temperature is found to exceed the gas temperature during this process, due to the exothermic oxidation of iron sulphide. A special feature of the model is that it predicts the duration of the melt phase in the particle. The time spent in the molten state is found to constitute ≈80% of the total time required for oxidation of pyrite to solid magnetite. Calculations also show that coarser pyrite particles stay molten for a longer period, the time being proportional to the square of the particle diameter. Model predictions of particle temperature and sulphur release histories for different oxygen concentrations, gas temperatures, and particle size fractions are compared with existing literature data. These show that oxidation rates are governed predominantly by the resistances to oxygen diffusion through the particle boundary layer and within the particle melt. The mass transfer coefficient in the melt is an adjustable parameter in the model, and has a larger value at higher particle temperatures.
The application of gradient-corrected exchange-correlation functionals in total-energy calculations using a plane-wave basis set is discussed. The usual form of the exchange-correlation potential includes gradients whose calculation requires the use of a high-quality representation of the density which is computationally expensive in both memory and time. These problems may be overcome by defining an exchange-correlation potential for the discrete set of grid points consistent with the discretized form of the exchange-correlation energy that is used in Car-Parrinello-type total-energy calculations. This potential can be calculated exactly on the minimum fast-Fourier-transform grid and gives improved convergence and stability as well as computational efficiency.
In coal combustion systems, the partitioning of arsenic between the vapor and solid phases is determined by the interaction of arsenic vapors with fly ash compounds under post-combustion conditions. This partitioning is affected by gas-solid reactions between the calcium components of the ash particles and arsenic vapors. In this study, bench scale experiments were conducted with calcium compounds typical of coal-derived fly ash to determine product formation, the extent of reaction and reaction rates when contacted by arsenic oxide vapors. Experiments conducted with arsenic trioxide (As(4)O(6(g))) vapors in contact with calcium oxide, di-calcium silicate and mono-calcium silicate over the temperature range 600-1000 degrees C indicated that these solids were capable of reacting with arsenic vapor species in both air and nitrogen. Calcium arsenate was the observed reaction product in all the samples analyzed. Maximum capture of arsenic occurred at 1000 degrees C with calcium oxide being the most effective of the three solids over the range of temperatures studied. Using a shrinking core model for a first order reaction and the results from intrinsic kinetic experiments conducted in air, the reaction rate constants were found to be 1.4 x 10(-3)exp(-2776/T) m/s for calcium oxide particles, 7.2 x 10(-3)exp(-3367/T) m/s for di-calcium silicate particles and 5.5 x 10(-3)exp(-3607/T) m/s for mono-calcium silicate particles. These results therefore suggest that any calcium present in fly ash can react with arsenic vapor and capture the metal in water-insoluble forms of the less hazardous As(V) oxidation state.
Arsenic and selenium compounds may be emitted to the environment during coal conversion processes, although some compounds are retained in the fly ashes, in different proportions depending on the characteristics of the ashes and process conditions. The possibility of optimizing the conditions to achieve better trace element retention appears to be an attractive, economical option for reducing toxic emissions. This approach requires a good knowledge of fly ash characteristics and a thorough understanding of the capture mechanism involved in the retention. In this work the ability of two fly ashes, one produced in pulverized coal combustion and the other in fluidized bed combustion, to retain arsenic and selenium compounds from the gas phase in coal combustion and coal gasification atmospheres was investigated. To explore the possible simultaneous retention of mercury, the influence of the unburned coal particle content was also evaluated. Retention capacities between 2 and 22 mg g(-1) were obtained under different conditions. The unburned coal particle content in the fly ash samples does not significantly modify retention capacities.
Sulfur dioxide (SO2) and trace elements are pollutants derived from coal combustion. This study focuses on the simultaneous removal of S02 and trace arsenic oxide (As2O3) from flue gas by calcium oxide (CaO) adsorption in the moderate temperature range. Experiments have been performed on a thermogravimetric analyzer (TGA). The interaction mechanism between As2O3 and CaO is studied via XRD detection. Calcium arsenate [Ca3(AsO4)2] is found to be the reaction product in the range of 600-1000 degrees C. The ability of CaO to absorb As2O3 increases with the increasing temperature over the range of 400-1000 degrees C. Through kinetics analysis, it has been found that the rate constant of arsenate reaction is much higher than that of sulfate reaction. SO2 presence does not affect the trace arsenic capture either in the initial reaction stage when CaO conversion is relatively low or in the later stage when CaO conversion is very high. The product of sulfate reaction, CaS04, is proven to be able to absorb As2O3. The coexisting CO2 does not weaken the trace arsenic capture either.
Density functional theory study of gas-solid reaction mechanism for mercury and iron oxide
- T Liu
- H Xiao
- L Dong
- X Ning
Liu, T. Density functional theory study of gas-solid reaction
mechanism for mercury and iron oxide. Master's Thesis, Faculty of
Graduate Studies and Research, Huazhong University of Science and
Technology, Wuhan, Hubei, China, 2012.
(17) Xiao, H.; Dong, L.; Ning, X. Heterogeneous catalytic
mechanism of SO 2 oxidation with Fe 2 O 3. Proc. CSEE 2016, 21,
5866−72.
Constants of Diatomic Molecules. NIST Chemistry WebBook
- K Huber
- G Herzberg
Huber, K.; Herzberg, G. Constants of Diatomic Molecules.
NIST Chemistry WebBook; Linstrom, P. J., Mallard, W. G., Eds.;
National Institute of Standards and Technology (NIST): Gaithersburg, MD, 2001; NIST Standard Reference Database Number 69.
Study of the influence of HCl, HBr and SO on Hg adsorption over iron oxide
- L Xue
Xue, L. Study of the influence of HCl, HBr and SO 2 on Hg 0
adsorption over iron oxide. Ph.D. Thesis, Huazhong University of
Science and Technology, Wuhan, Hubei, China, 2015.
Density function study of Fe O synergy with insert in chemical-looping combustion system. Master’s Thesis, Faculty of Graduate Studies and Research
- Q Chen
Density functional theory study of gas-solid reaction mechanism for mercury and iron oxide
- T Liu