Elemental Mercury Removal Performance Research Articles

Investigation of elemental mercury removal performance and mechanism of rice straw biochars from a fluidized bed pyrolysis system impregnated by NH4Br

To remove Hg0 from flue gas efficiently and economically, the rice straw biochars prepared in a fluidized bed pyrolysis system and then modified with NH4Br are employed as the sorbents. The mercury adsorption capacity of Br-modified biochar (1786.85 μg/g) is superior to that of activated carbon (207 μg/g), which indicates Br-modified rice straw biochar sorbent displays higher Hg0 adsorption efficiency and can replace high-cost activated carbon. The sorbents are characterized by Brunauer-Emmett-Teller (BET), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The effects of pyrolysis temperature, pyrolysis atmosphere, reaction temperature, the components of flue gas (O2, NO and SO2) on mercury removal efficiency, kinetics, thermodynamics and mercury removal mechanism are investigated. The results indicate elevating pyrolysis temperature and oxygen content significantly enhance the Hg0 removal efficiency. The Hg0 adsorption process is better fitted by pseudo-second-order kinetic model and proves a spontaneous endothermic process. The presence of O2 and NO in flue gas enhances Hg0 adsorption, while the presence of SO2 restrains it. After Hg0 adsorption, the peak area ratio of C=O (288.5 eV) and C-Br (69.1 eV) by Br-modified rice straw biochars produced under 8 %O2 and 500 °C atmosphere decreases from 17.03 % and 70.14 % to 6.55 % and 50.50 %, respectively, while that of Br− (68.1 eV) increases from 29.86 % to 48.50 %, indicating the surface O* and Br* species are responsible for Hg0 removal, and the products are mainly HgO and HgBr2. This work provides a kind of cost-effective Hg0 removal sorbent and explores the mechanism of Hg0 removal by Br-modified biochars.

Remarkable enhancement of elemental mercury removal performance over natural low-grade pyrolusite through acid-etching

Using natural low-grade pyrolusite as a novel Hg0 absorbent has economic and environmental benefits, but faces the challenge of restricted adsorption and oxidation capacity. Herein, a HNO3-etching strategy is employed to modify the physicochemical property. Characterizations and relevant theoretical calculations reveal the etching’s role: (i) dissolving impurities and adjusting pore structure, which magnify pre-adsorption capacity of reactants and facilitate mutual mass transfer; (ii) creating more oxygen vacancies and promoting electron transfer, which favor the adsorption-dissociation of O2 and the migration of more O-containing species; (iii) increasing reaction rate and lowering energy barrier. The 12-hour experimental results show that the removal efficiency of Hg0 can reach 92.8 % through etching, and the increment of 30.6 % reflects the huge promotion of etching on mercury removal. Moreover, the etching also improves the stability, sulfur/water resistance and regeneration performance. Furthermore, the desorption temperature of HgO diminishes after etching, as the etching enhances the activity of Oads and Olat. By contrasting the preferential and co-adsorption pathways on pristine/defective surfaces, alterations in adsorption mechanism are revealed, further elucidating the effects of increased defects on the enhanced Hg0 oxidation activity. This work offers a practical strategy to synchronously achieve structural optimization and defect construction of pyrolusite, thereby enhancing its removal performance, which holds potential implications for industrial applications in mitigating mercury emissions.

Study on the elemental mercury removal performance of co-pyrolyzed Cl-loading activated carbon and the formation mechanism of C-Cl functional groups

Co-pyrolysis is a convenient method to load chloride on an adsorbent to improve its mercury removal ability. However, as a main reactive species, the formation mechanism of C-Cl functional groups remained unclear. In this study, the coconut shell activated carbon (AC) was co-pyrolyzed with polyvinyl chloride (PVC) to prepare a Cl-loading activated carbon, and it presented the best mercury removal efficiency (greater than 85%) at 140 °C. NO, O2 and HCl promoted the mercury removal ability of Cl-loading activated carbon, while SO2 inhibited. By comparing the performance of Cl-loading adsorbents using different Cl sources and supporters, we found that there were two formation paths for C-Cl functional groups. The first path was that some organic chlorine in PVC was reformed at low temperature (∼300 °C), and adhered to the surface of the adsorbent in the form of C-Cl functional groups. As the co-pyrolysis temperature increased, unstable C-Cl functional groups were decomposed and the mercury removal ability became weak. The second path was that the Cl first released out from PVC as gaseous HCl, and then combined with newly exposed carbon atom sites, which were generated owing to the devolatilization process of AC at high temperature (∼800 °C), to form C-Cl functional groups for mercury removal.

Strengthen the Affinity of Element Mercury on the Carbon-Based Material by Adjusting the Coordination Environment of Single-Site Manganese.

Mercury, as a highly poisonous pollutant, poses a severe threat to the global population. However, the removal of Hg0 can only be carried out at below 100 °C due to the weak binding of the adsorbent. Herein, a series of carbon-based materials with different coordination environments and atomic dispersion of single-site manganese were prepared, and their elemental mercury removal performance was systematically investigated. It was demonstrated that the coordination environment around manganese determines its electronic structure and size, thus affecting its affinity with mercury. The obtained best adsorbents atomically dispersed Mn with atom size near 0.2 nm, achieves high Hg0 removal efficiency and over 13 mg/g Hg0 adsorption capacity at 200 °C. And the SO2 resistance performance of single atoms (∼0.2 nm) is much better than clusters (∼1-2 nm) because of its high selectivity, that the effect of SO2 is only 3%. Density functional theory (DFT) reveals that Mn with four-nitrogen atoms (Mn-N4-C═O) is more active than other number nitrogen coordination materials. Moreover, the presence of carboxyl groups around manganese also promotes affinity for Hg0. This work might shed new light on the enhancement of Hg0 affinity in carbon-based materials and the rational design of the coordination structure of the tunable Hg0 activities.

Tuning dimensionality TiO2/g-C3N4 heterostructure for enhanced elemental mercury removal performance under visible-light

TiO2/g-C3N4 (TCN) heterojunction photocatalysts with dimensionality tuning was prepared, g-C3N4 successfully shifted from 3D (3DBG) to 2D (2DGN). Compared with 2DGN and TiO2, all the as-prepared heterostructure samples exhibited significantly enhanced photoactivities under light irradiation. Through the photocatalytic mercury removal experiment on samples with different doping ratios, the effect of oxygen species such as hydrocarbons or surface-adsorbing molecular H2O and doping amount on the performance of photocatalytic oxidation were investigated. Finally, the photocatalytic reaction mechanism of charge transfer in the dimensional adjustable Z-scheme heterojunction was proposed. This work provides a new strategy for developing efficient photocatalysts to control mercury.

An experimental and DFT study on enhanced elemental mercury removal performance via cerium chloride modified carbon aerogel: A synergistic effect between chemical adsorption and thermal catalysis

The effective capture or conversion of elemental mercury (Hg0), is the key scientific problem for controlling total mercury pollution in coal-fired power plants. In this work, the cerium chloride (CeCl3) modified carbon aerogel (CA) was successfully synthesized via impregnation method. The as-prepared samples maintained the large specific surface area and excellent pore structure of CA, and the CeCl3 was evenly dispersed, which can play a synergistic role in chemical adsorption and thermal catalysis to capture and oxidize the Hg0 effectively, and the Hg0 removal efficiency was greatly improved, up to 98% at 120 °C. The enhanced Hg0 removal performance could be attributed to the surface active chlorine and cerium oxide (CeO2) particle generated in the preparation process. Based on density functional theory (DFT), the mechanism of mercury removal was discussed in detail. This work would give a new insight for mercury and other heavy metal pollution control in coal-fired power plants.

Study of the Effect of Adsorption Temperature on Elemental Mercury Removal Performance of Iron-Based Modified Biochar

An iron-based modified biochar was prepared by the coprecipitation method, including single iron-based modified biochar with FeCl3 and iron-based biochar doped with Cu and Mn from CuSO4 and KMnO4. Combined with the physicochemical properties of sorbents, the effect of flue gas temperature on the mercury adsorption performance of the sorbents was explored within the temperature range of 50–350 °C. The adsorption mechanism was further explored by adsorption kinetics, thermodynamics, and temperature-programmed desorption (TPD) analyses. The results showed that the mercury adsorption capacity of unmodified biochar decreased monotonously with temperature, but the adsorption capacities of Fe/BC, FeCu/BC, and FeMn/BC all enhanced first and then weakened, with 200 °C being the optimum adsorption temperature. FeCu/BC had the strongest adsorption capacity, while Fe/BC had the worst one. C═O and COOH, metal oxides and ions, lattice oxygen, chemisorbed oxygen, and Cl ions were all active sites for oxidative adsorption of Hg0, among which Fe2O3 and CuO or CuFe2O4 exerted a synergistic effect. Excessive temperature caused destruction to the porous structure and deactivation of the active centers, leading to the reduction of physisorption and chemisorption. Mercury adsorption on sorbents needs more activated energy due to the enhancement of chemisorption, and the moderate increase of temperature contributed to the adsorption properties of the sorbents. The Hg0 adsorption on modified biochar was spontaneous and endothermic, and the complexity degree of the adsorption process was promoted. TPD results showed that at 200 °C the removal of Hg0 by modified biochar was mainly strong chemisorption and that HgO was mainly formed.

Inhibitory effects of water vapor on elemental mercury removal performance over cerium-oxide-modified semi-coke

A fixed-bed reactor was used to study the effect of water vapor on the efficiency of Hg0 removal over CeO2-modified semi-coke. Adsorption experiments showed that the oxidation activity, and therefore, the mercury adsorption efficiency decreased with increasing water vapor concentrations. Hydrogen temperature-programmed reduction (H2-TPR) indicated that the oxidation activity of CeO2 decreased after water vapor treatment. Fourier-transform infrared (FT-IR) studies revealed that CeO2-H2O showed new bending vibration peaks at 1110cm−1 ascribed to Ce–OH groups. X-ray photoelectron spectroscopy (XPS) results showed that the content of Ce–OH increased from 20.85% to 39.99%, while the lattice oxygen content decreased from 68.21% to 45.83% after water vapor treatment. Density functional theory (DFT) calculations revealed that H2O can be dissociated on clean and oxygen-defective CeO2(111) surfaces to form H atoms and OH fragments. The H atoms associated with the adjacent oxygen atoms of CeO2, and the OH fragments were bound directly with Ce atoms to form Ce–OH for the clean CeO2 surface; or occupied the oxygen vacancies and were surrounded by three Ce atoms via van der Waals forces, without additional bond formation for the oxygen-defective CeO2 surface. Thus, the formation of Ce–OH groups and the consumption of lattice oxygens reduced the Hg0 removal efficiency.

Effect of water vapor and α-Fe2O3 on elemental mercury removal performance over cerium oxide modified semi coke

Cerium modified semi coke adsorbent (Ce/SC) was prepared by impregnation method and a bench-scale fixed bed reactor was used to study the effect of H2O vapor and α-Fe2O3 on elemental mercury removal efficiency over Ce/SC. Characterizations of X-ray powder diffraction (XRD), hydrogen temperature programmed reduction (H2-TPR) and X-ray photoelectron spectroscopy (XPS) were conducted to investigate the mechanism of elemental mercury removal. The adsorption results showed that H2O had a negative effect on the oxidation activity of the adsorbent. H2O can be dissociated on the surface of CeO2 with partial lattice oxygen transformation into Ce–OH functional groups which led to the decrease of its oxidation activity, and thus resulted in the inhibitory effect of mercury removal efficiency. The addition of α-Fe2O3 had no significant effect on mercury removal over Ce/SC. The mercury removal efficiency of Ce/SC was decreased when the water vapor and α-Fe2O3 existed simultaneously. However, the decrease rate was much lower than that of water vapor conditions alone mainly due to the interaction between water vapor and α-Fe2O3 increased the content of the surface chemical adsorbed oxygen, and thus the oxidation activity and elemental mercury removal performance of Fe2O3 were promoted.

Adsorptive removal of gas-phase mercury by oxygen non-thermal plasma modified activated carbon

The effects of oxygen non-thermal plasma modification on the surface properties and mercury removal performance of activate carbon (AC) were investigated. The raw and modified ACs were characterized by N2 adsorption/desorption, scanning electron microscope (SEM), Boehm titration and Fourier Transform Infrared Spectrometer (FTIR). The results showed that the surface textual properties of the modified ACs were slightly damaged, but the surface chemistries such as the content of surface oxygen-containing groups were significantly changed. The elemental mercury removal performance of raw and modified ACs were also evaluated in a quartz tube fix-bed reactor under N2 atmosphere. The results indicated that the AC treated with oxygen non-thermal plasma had better elemental mercury removal performance compared to the raw AC. This reason was attributed to the oxygen non-thermal plasma treatment increasing the ester groups (CO), carbonyl groups (CO) and adsorption activate sites on AC surface, which played an important role in the adsorption of elemental mercury. The adsorption kinetic of raw and modified ACs could be best described by the Pseudo-first-order model and Pseudo-second-order model, which implied that external diffusion and chemisorption were the control step in the mercury adsorption process of raw and modified ACs.

Removal performance of elemental mercury by low-cost adsorbents prepared through facile methods of carbonisation and activation of coconut husk.

The preparation of chars and activated carbon as low-cost elemental mercury adsorbents was carried out through the carbonisation of coconut husk (pith and fibre) and the activation of chars with potassium hydroxide (KOH), respectively. The synthesised adsorbents were characterised by using scanning electron microscopy, Fourier transform infrared spectroscopy and nitrogen adsorption/desorption analysis. The elemental mercury removal performance was measured using a conventional flow type packed-bed adsorber. The physical and chemical properties of the adsorbents changed as a result of the carbonisation and activation process, hence affecting on the extent of elemental mercury adsorption. The highest elemental mercury (Hg°) adsorption capacity was obtained for the CP-CHAR (3142.57 µg g(-1)), which significantly outperformed the pristine and activated carbon adsorbents, as well as higher than some adsorbents reported in the literature.

Experimental Study on Titanium Based Photocatalyst and its Catalytic Oxidation on Flue Gas Mercury

This paper mainly studied the effects of different iron-doped volume on photo-catalytic oxidation of TiO2 for mercury removal. Through the photocatalytic oxidation system, we evaluated the elemental mercury removal performance of TiO2 with iron-doped mass ratio of 0.5%, 1%, 2% and 3%. While the iron-doped mass ratio were 0.5%, 1%, 2% and 3%, the elemental mercury removal efficiency were 70.83%，52.89%，72.32% and 62.39% respectively, the removal efficiency increased firstly, then declined, and 2% iron-doped was the most appropriate