Understanding mercury binding on activated carbon

Understanding the mechanism by which mercury adsorbs on activated carbon is crucial to the design and fabrication of effective capture technologies. In this study, the possible binding mechanism of mercury (Hg) and its species, i.e., HgCl and HgCl₂ on activated carbon is investigated using ab initio-based energetic calculations. The activated carbon surface is modeled by a single graphene layer in which the edge atoms on the upper side are unsaturated in order to simulate the active sites. In some cases, chlorine atoms are placed at the edge sites to examine the effect of chlorine on the binding of Hg, HgCl and HgCl₂. It has been concluded that both HgCl and HgCl₂ can be adsorbed dissociatively or non-dissociatively. In the case of dissociative adsorption, it is energetically favorable for atomic Hg to desorb and energetically favorable for it to remain on the surface in the Hg¹⁺ state, HgCl. The Hg²⁺ oxidized compound, HgCl₂ was not found to be stable on the surface. The most probable mercury species on the surface was found to be HgCl.     

Characteristics of commercial selective catalytic reduction catalyst for the oxidation of gaseous elemental mercury with respect to reaction conditions

The performance of V₂O₅/TiO₂-based commercial SCR catalyst for the oxidation of gaseous elemental mercury (Hg⁰) with respect to reaction conditions was examined to understand the mechanism of Hg⁰ oxidation on SCR catalyst. It was observed that a much larger amount of Hg⁰ adsorbed on the catalyst surface under oxidation condition than under SCR condition. The activity of commercial SCR catalyst for Hg⁰ oxidation was negligible in the absence of HCl, regardless of reaction conditions. The presence of HCl in the reactant gases greatly increased the activity of SCR catalyst for the oxidation of Hg⁰ to oxidized mercury (Hg²⁺) such as HgCl₂ under oxidation condition. However, the effect of HCl on the oxidation of Hg⁰ was much less under SCR condition than oxidation condition. The activity for Hg⁰ oxidation increased with the decrease of NH₃/NO ratio under SCR condition. This might be attributed to the strong adsorption of NH₃ prohibiting the adsorption of HCl which was vital species promoting the oxidation of Hg⁰ on the catalyst surface under SCR condition.     

The effect of activated carbon surface moisture on low temperature mercury adsorption

Experiments with elemental mercury (Hg⁰) adsorption by activated carbons were performed using a bench-scale fixed-bed reactor at room temperature (27°C) to determine the role of surface moisture in capturing Hg⁰. A bituminous-coal-based activated carbon (BPL) and an activated carbon fiber (ACN) were tested for Hg⁰ adsorption capacity. About 75-85% reduction in Hg⁰ adsorption was observed when both carbon samples’ moisture (∼2 wt.% as received) was removed by heating at 110°C prior to the Hg⁰ adsorption experiments. These observations strongly suggest that the moisture contained in activated carbons plays a critical role in retaining Hg⁰ under these conditions. The common effect of moisture on Hg⁰ adsorption was observed for both carbons, despite extreme differences in their ash contents. Temperature programmed desorption (TPD) experiments performed on the two carbons after adsorption indicated that chemisorption of Hg⁰ is a dominant process over physisorption for the moisture-containing samples. The nature of the mercury bonding on carbon surface was examined by X-ray absorption fine structure (XAFS) spectroscopy. XAFS results provide evidence that mercury bonding on the carbon surface was associated with oxygen. The results of this study suggest that surface oxygen complexes provide the active sites for mercury bonding. The adsorbed H₂O is closely associated with surface oxygen complexes and the removal of the H₂O from the carbon surface by low-temperature heat treatment reduces the number of active sites that can chemically bond Hg⁰ or eliminates the reactive surface conditions that favor Hg⁰ adsorption.     

Importance of activated carbon's oxygen surface functional groups on elemental mercury adsorption

The effect of varying physical and chemical properties of activated carbons on adsorption of elemental mercury (Hg⁰) was studied by treating two activated carbons to modify their surface functional groups and pore structures. Heat treatment (1200 K) in nitrogen (N₂), air oxidation (693 K), and nitric acid (6N HNO₃) treatment of two activated carbons (BPL, WPL) were conducted to vary their surface oxygen functional groups. Adsorption experiments of Hg⁰ by the activated carbons were conducted using a fixed-bed reactor at a temperature of 398 K and under N₂ atmosphere. The pore structures of the samples were characterized by N₂ and carbon dioxide (CO₂) adsorption. Temperature-programmed desorption (TPD) and base-acid titration experiments were conducted to determine the chemical characteristics of the carbon samples. Characterization of the physical and chemical properties of activated carbons in relation to their Hg⁰ adsorption capacity provides important mechanistic information on Hg⁰ adsorption. Results suggest that oxygen surface complexes, possibly lactone and carbonyl groups, are the active sites for Hg⁰ capture. The carbons that have a lower carbon monoxide (CO)/CO₂ ratio and a low phenol group concentration tend to have a higher Hg⁰ adsorption capacity, suggesting that phenol groups may inhibit Hg⁰ adsorption. The high Hg⁰ adsorption capacity of a carbon sample is also found to be associated with a low ratio of the phenol/carbonyl groups. A possible Hg⁰ adsorption mechanism, which is likely to involve an electron transfer process during Hg⁰ adsorption in which the carbon surfaces may act as an electrode for Hg⁰ oxidation, is also discussed.     

Removal of elemental mercury from coal combustion flue gas by chloride-impregnated activated carbon

In this work, adsorption of vapour-phase elemental mercury (Hg⁰) from pulverised-coal combustion flue gas by commercially available granular activated carbons treated with zinc chloride (ZnCl₂) impregnation was investigated. The experiment results showed that ZnCl₂ impregnation significantly enhanced the adsorptive capacity for mercury vapour, but decreased the specific surface area of the activated carbon. This could be explained by the occurrence of chemisorption, which was confirmed by adsorption tests over a wide range of temperatures. The influence of ZnCl₂ solution concentration on the mercury removal performance was also studied. Mechanisms of mercury adsorption onto the Cl-impregnated activated carbon were proposed.     

Effect of chemical activation of an activated carbon using zinc chloride on elemental mercury adsorption

Hg⁰ adsorption experiments with different kinds of activated carbons (ACs) were conducted in nitrogen environment to study the effects of ACs with different surface characteristics on adsorption. Three kinds of ACs prepared by steam activation did not adsorb elemental mercury (Hg⁰) in nitrogen environment, while another kind of AC prepared by chemical activation using zinc chloride (ZnCl₂) showed significant Hg⁰ adsorption capability in the same experimental conditions because Hg⁰ was oxidized by the oxidative elements on the surface of AC. ACs without oxidative elements did not adsorb Hg⁰ through physical adsorption in nitrogen environment. It was therefore concluded that Hg⁰ adsorption by AC was a chemical adsorption process.     

活性焦汞吸附特性及动力学机理分析

通过固态吸附剂汞吸附效能测定系统进行了太西活性焦吸附单质汞的实验,考察单质汞Hg⁰入口浓度和温度对太西活性焦脱汞性能的影响并探讨了吸附机理。结果表明:在CO₂/N₂/O₂/NO/Hg⁰体系中,不同Hg⁰入口浓度获得的汞脱除效率曲线相似,脱除效率随Hg⁰入口浓度增大而增加;变入口浓度工况下,汞的吸附全过程遵循准二级动力学反应模型,该吸附过程以化学吸附为主;在反应温度从403 K升高到423 K的过程中,系统发生了化学吸附或者化学反应,423 K可作为活性焦的最佳除汞温度;Bangham方程可对变温工况下活性焦脱除单质汞的吸附过程进行准确描述,并获得不同温度下活性焦脱汞的吸附速率常数存在k_423K>k_463K>k_403K的关系。     

Computational insights into interactions between Hg species and α-Fe2O3 (0 0 1)

First-principle calculations based on Density Functional Theory were performed to investigate the binding mechanisms of mercury species on α-Fe₂O₃ (0 0 1) surface. This is crucial in demonstrating the contribution of α-Fe₂O₃ existing in fly ash for mercury removal. It has been determined that Hg⁰ is adsorbed on the α-Fe₂O₃ (0 0 1) surface with physisorption mechanism. The oxidized forms of HgCl and HgCl₂ can be adsorbed on α-Fe₂O₃ (0 0 1) dissociatively or non-dissociatively. In the case of dissociative adsorption, a close examination of the energy diagram indicates that HgCl may be favorable for the adsorption of Cl and desorption of Hg. The dissociation of HgCl₂ with the binding of Cl and HgCl on the surface is possibly the dominant interaction pathway.     

改性桑树枝焦对模拟烟气中汞的吸附性能

采用固定床热解、蒸汽活化和改性剂(H₂O₂、ZnCl₂和NaCl)浸渍等方法制得不同的桑树枝焦。在固定床吸附实验台上,研究了蒸汽活化、改性剂、吸附温度和烟气组分等对改性桑树枝焦汞吸附性能的影响。结果表明:蒸汽活化显著提高了桑树枝热解焦的比表面积,H₂O₂改性可进一步提高桑树枝蒸汽活化焦比表面积并改善其孔隙结构参数,ZnCl₂和NaCl改性则降低了桑树枝蒸汽活化焦的比表面积、D-R微孔容积和总孔容。10%H₂O₂和30%H₂O₂浸渍改性桑树枝焦的单位汞吸附量分别是蒸汽活化焦的2.02倍和1.77倍;相同改性剂浓度下,ZnCl₂改性焦的单位汞吸附量比NaCl改性焦稍好;随着ZnCl₂浓度增大,改性桑树枝焦的汞吸附性能增强,MT600-A-ZnCl₂(5%)桑树枝焦的单位汞吸附量达到29.55 μg·g^-1,是蒸汽活化焦的3.37倍。在吸附温度为60～120℃范围内,H₂O₂改性焦的汞吸附效率及单位汞吸附量随着吸附温度的升高而下降,而ZnCl₂改性焦的单位汞吸附量则随着吸附温度提高呈现先增大后减小的趋势,最佳吸附温度为90℃。烟气中SO₂和NO组分对汞吸附性能有一定的抑制作用,随着SO₂和NO浓度的增加,汞吸附效率和单位汞吸附量均稍有下降。     

Removal of Hg2+ from aqueous solution using a novel composite carbon adsorbent

A novel composite carbon adsorbent (GCA) has been prepared by immobilizing activated carbon and genipin‐crosslinked chitosan into calcium alginate gel beads via entrapment and applied to the removal of mercury (Hg²⁺) ions from aqueous solution (e.g., drinking water). Two bead sizes and two mixing ratios of components were obtained and characterized. Batch experiments were performed to study the uptake equilibrium and kinetics of Hg²⁺ ions by the GCA. The Hg²⁺ adsorption capacity of GCA was found to be dependent of pH and independent of size of the adsorbent. The Hg²⁺ adsorption rate of GCA increases with decreasing its bead size. However, both adsorption capacity and rate of GCA for Hg²⁺ increase with increasing its chitosan content. Otherwise, it was shown that the GCA has higher Hg²⁺ adsorption capacity and rate than activated carbon, which might be caused by the incorporation of chitosan into the GCA. The maximum Hg²⁺ adsorption capacity of GCA was found to be 576 mg/g, which is over seven times higher than that of activated carbon. Our results reveal the uniform distribution of activated carbon and chitosan within the alginate gel bead and that Hg²⁺ ions can diffuse inside the bead. It also demonstrated the feasibility of using this GCA for Hg²⁺ removal at low pH values. The Hg²⁺ absorbed beads of the GCA can be effectively regenerated and reused using H₂SO₄. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Mercury oxidation and adsorption characteristics of chemically promoted activated carbon sorbents

Previous entrained-flow tests conducted under elemental mercury (Hg⁰)-laden air found that significant amounts of oxidized mercury (Hg²⁺) are not adsorbed onto cupric chloride-impregnated carbon (CuCl₂-AC) and brominated activated carbon (DARCO Hg-LH), but entrained to the gas phase. In this study, these sorbents were tested in a fixed-bed system and a filter-added entrained-flow system to further investigate Hg⁰ oxidation and adsorption characteristics of CuCl₂-AC and DARCO Hg-LH. These test results suggested that CuCl₂-AC has different sites available for Hg⁰ oxidation and Hg adsorption, and the resultant oxidized mercury generated from the reaction between Hg⁰ and CuCl₂ is re-adsorbed at the site of CuCl₂-AC available for adsorption. The resultant oxidized mercury was also found to be easily re-adsorbed onto CuCl₂-AC and DARCO Hg-LH in the filter connected to the entrained-flow reactor.     

Gas-phase elemental mercury removal by novel carbon-based sorbents

To develop cost-effective carbon-based sorbents used in gas-phase elemental mercury removal, the performance of commercial bamboo charcoal (BC) produced from renewable bioresource of bamboo and modified BC was investigated with a bench-scale fixed-bed reactor. The simple impregnation method was used to modify the BC using ZnCl₂, NH₃·H₂O and HNO₃ separately. BET, XPS, elemental analysis and FT-IR were used to determine the pore structure and surface chemistry of the sorbents. The characterization of the physical and chemical properties of sorbents in relation to Hg⁰ adsorption capacity provided important information on the Hg⁰ adsorption mechanism. This suggested that the modified BCs have excellent adsorption potential for elemental mercury at a relatively higher temperature of 140 °C except for the NH₃·H₂O impregnated BC. The amount of Hg⁰ absorbed depends on the concentration of impregnant used. The impregnation has increased the sorbents’ active sites for mercury adsorption and the specific reaction mechanism has been further analyzed.     

Comprehensive Methodology for the Investigation of Mercury Adsorption on Activated Carbons

In mercury adsorption on activated carbons both physisorptive and chemisorptive mechanisms play a role. The systematic investigation of Hg⁰ chemisorption is difficult because equilibrium capacities cannot be determined due to slow adsorption mechanisms. Therefore, the present publication suggests a three‐step approach: 1) Breakthrough curves are used to assess the dynamics of Hg⁰ adsorption. 2) The contributions of physisorption and chemisorption can be distinguished by coupled adsorption and desorption experiments. 3) Temperature programmed desorption (TPD) experiments are performed to get information about specific chemisorptive binding sites on the surface. This approach was tested on four characteristic examples of impregnated and non‐impregnated activated carbons.     

Analysis of mercury species present during coal combustion by thermal desorption

Mercury in coal and its emissions from coal-fired boilers is a topic of primary environmental concern in the United States and Europe. The predominant forms of mercury in coal-fired flue gas are elemental (Hg⁰) and oxidized (Hg²⁺, primarily as HgCl₂). Because Hg²⁺ is more condensable and far more water soluble than Hg⁰, the wide variability in mercury speciation in coal-fired flue gases undermines the total mercury removal efficiency of most mercury emission control technologies. It is important therefore to have an understanding of the behaviour of mercury during coal combustion and the mechanisms of mercury oxidation along the flue gas path. In this study, a temperature programmed decomposition technique was applied in order to acquire an understanding of the mode of decomposition of mercury species during coal combustion. A series of mercury model compounds were used for qualitative calibration. The temperature appearance range of the main mercury species can be arranged in increasing order as HgCl₂ < HgS < HgO < HgSO₄. Different fly ashes with certified and reference values for mercury concentration were used to evaluate the method. This study has shown that the thermal decomposition test is a newly developed efficient method for identifying and quantifying mercury species from coal combustion products.     

Preparation of Activated Carbons from Tobacco Stems by Potassium Hydroxide Activation and Phosphine Adsorption

Activated carbon preparation from tobacco stems by KOH activation at different activation temperatures and KOH/char mass ratios were investigated in this study. The effects of preparation parameters on activated carbon pore structure, morphometrics, microcrystallinities, and surface functional groups were characterized by N₂ adsorption, SEM, XRD, and FTIR technologies, respectively. The optimum preparation condition of activated carbon was activation temperature of 850°C, and KOH/char mass ratio of 2. Under this condition, the BET surface area of 2215 m²/g, and the pore volume of 1.343 cm³/g can be obtained. Prepared activated carbon showed clearly honeycomb holes, and a predominated amorphous structure. With increase of activation temperature and KOH/char mass ratio, decrease of surface oxygen functional group, and aromatization of the carbon structure was found. The activated carbon was subject to PH₃ purification, and the maximum PH₃ adsorption capacity of 253 mg/g can be realized based on well prepared KOH-AC with modification of 2.5% Cu. It seems that the activated carbon produced from chemical activation of tobacco stem would be an effective and alternative adsorbent for PH₃ adsorption because of its high surface area, adsorption capacity, and low cost.     

Kinetic modeling of H2O2-enhanced oxidation of flue gas elemental mercury

Mercury emissions from coal-fired power plants account for 40% of the anthropogenic mercury emissions in the U.S. The speciation of mercury largely determines the amount of mercury capture in control equipments. Conversion of insoluble Hg⁰ into more soluble Hg²⁺ facilitates its removal in scrubbers. Past studies suggest that an added supply of OH radicals possibly enhance the mercury oxidation process. This study demonstrates that the application of H₂O₂, as source of OH radicals, accelerates the oxidation of Hg⁰ into Hg²⁺. A detailed kinetic reaction mechanism was compiled and the reaction pathways were established to analyze the effect of H₂O₂ addition. The optimum temperature range for the oxidation was 480–490 °C. The sensitivity analysis of the reaction mechanism indicates that the supply OH radicals increase the formation of atomic Cl, which accelerates the formation of HgCl₂ enhancing the oxidation process. Also, the pathway through HOCl radical, generated by the interactions between chlorine and H₂O₂ was prominent in the oxidation of Hg⁰. The flue gas NO was found to be inhibiting the Hg⁰ oxidation, since it competed for the supplied H₂O₂. Studying the interactions with the other flue gas components and the surface chemistry with particles in the flue gas could be important and may improve the insight into the post combustion transformation of mercury in a comprehensive way.     

Selective Hydrogenation of Cinnamaldehyde to Hydrocinnamaldehyde over SiO2 Supported Nickel Phosphide Catalysts

The heterogeneous mercury reaction mechanism, reactions among elemental mercury (Hg⁰) and simulated flue gas across laboratory-scale selective catalytic reduction (SCR) reactor system was studied. The surface of SCR catalysts used in this study was analyzed to verify the proposed reaction pathways using transmission electron microscopy with energy dispersive X-ray analyses (TEM-EDX) and X-ray photoelectron spectroscopy (XPS). The Langmuir–Hinshelwood mechanism was proven to be most suitable explaining first-layer reaction of Hg⁰ and HCl on the SCR catalyst. Once the first layer is formed, successive layers of oxidized mercury (HgCl₂) are formed, making a multi-layer structure.     

Separation of Mercury from Aqueous Mercuric Chloride Solutions by Onion Skins

Abstract

The separation of mercury from aqueous HgCl₂ solutions by onion skins (outermost coat) was studied both experimentally and theoretically. The distribution equilibria were measured by the batchwise method. The experimental results revealed that onion skin is a useful material for separating mercury from aqueous systems. The distribution data obtained at 25°C were analyzed by using the theory based on the law of mass action. The separation of dissolved mercury by onion skins was found to be a process accompanied by an ion-exchange reaction of the cationic complex HgCl⁺ and an adsorption of the neutral complex HgCl₂. The equilibrium constants of the ion-exchange and adsorption processes at 25°C and the mercury-binding capacity of onion skins were determined. Further, it was found that the distribution equilibrium of mercury is comparatively insensitive to temperature.     

Catalytic performance of CuCl<Subscript>2</Subscript>-modified V<Subscript>2</Subscript>O<Subscript>5</Subscript>-WO<Subscript>3</Subscript>/TiO<Subscript>2</Subscript> catalyst for Hg<Superscript>0</Superscript> oxidation in simulated flue gas

CuCl₂-SCR catalysts prepared by an improved impregnation method were studied to evaluate the catalytic performance for gaseous elemental mercury (Hg⁰) oxidation in simulated flue gas. Hg⁰ oxidation activity of commercial SCR catalyst was significantly improved by the introduction of CuCl₂. Nitrogen adsorption, XRD, XRF and XPS were used to characterize the catalysts. The results indicated that CuCl₂ was well loaded and highly dispersed on the catalyst surface, and that CuCl₂ played an important role for Hg⁰ catalytic oxidation. The effects of individual flue gas components on Hg⁰ oxidation were also investigated over CuCl₂-SCR catalyst at 350 oC. The co-presence of NO and NH₃ remarkably inhibited Hg⁰ oxidation, while this inhibiting effect was gradually scavenged with the decrease of GHSV. Further study revealed the possibility of simultaneous removal of Hg⁰ and NO over CuCl₂-SCR catalyst in simulated flue gas. The mechanism of Hg⁰ oxidation was also investigated.     

Elimination of inorganic mercury from waste waters using crandallite‐type compounds

The removal of inorganic mercury from waste water streams arising from mines, using an artificial amorphous compound of the crandallite type synthesized in our laboratory, Ca_0.5Sr_0.5Al₃(OH)₆(HPO₄) (PO₄), has been investigated. This compound exhibits an extremely wide range of ionic substitutions: Ca²⁺ and Sr²⁺ were interchanged with Hg²⁺, so the mercury content of the waste water, ranging from 70 to 90 ppm, was reduced to less than 0.1 ppm. The process has been studied under batch conditions. The crandallite showed a high capacity for the exchange of mercury from mercuric nitrate solutions, 1.555 meq g^?1. The ion‐exchange equilibrium isotherms for Hg²⁺ were correlated by the Langmuir equation. The recovery of mercury from Hg‐crandallite using HCl solutions and thermal treatment was also studied. Optimum recuperation of mercury is achieved by chemical reaction with HCl solution (pH 2.25). At these conditions, 75% of the mercury is recovered as the HgCl₄^2? complex in a simple batch process, and the crandallite (in the protonic form) can be reused. © 2003 Society of Chemical Industry