Co-adsorption enhancement of formaldehyde/carbon dioxide over modified hexagonal boron nitride for whole-surface capture purification.

Simultaneous capture of formaldehyde (HCHO) and carbon dioxide (CO2) in indoor air is promising of achieving indoor-air purification. Of all potential adsorbents, hexagonal boron nitride (h-BN) is one of the most suitable species owing to facile formation of attraction points. Therefore, in this study, performances of HCHO and CO2 being adsorbed over pure/modified h-BN are systematically investigated via density functional theory (DFT) calculations. Minutely speaking, direct interaction between HCHO and CO2, single-point adsorption enhancement of HCHO over modified h-BN, co-adsorption reinforcement of HCHO/CO2 as well as relevant thermodynamic characteristics are major research contents. According to calculation results, there is relatively strong attraction between HCHO and CO2 owing to hydrogen bonds, which is in favor of co-adsorption of HCHO/CO2. As to single-adsorption of HCHO, C-doped h-BN shows better adsorption features than P-doped h-BN and C/P-doped h-BN is slightly weakened in adsorption ability due to surficial deformation caused by P atoms. For co-adsorption of HCHO/CO2, CO2 is the protagonist via formation of quasi-carbonate with the help of delocalized π-orbital electrons. Regarding effects of temperatures on adsorption strengths, they depend on interelectronic interactions among dopant atoms and finally derives from dispersion of π bonds across adsorbents. Overall, this study provides detailed mechanisms for co-capture of HCHO/CO2 to accomplish indoor-air purification.

Research on nonsteady-state adsorption and regulation towards stabler adsorption for benzene over single-wall carbon materials.

With the intention of separating benzene (C6H6) from indoor polluted air and collecting it in a cleaner way, it is promising of getting C6H6 adsorbed on activated carbon materials with outstanding physicochemical properties. In this study, how C6H6 is adsorbed over single-wall carbon materials and relevant adsorption processes are enhanced is thoroughly investigated via density functional theory (DFT). Especially, distinction between partial and whole effects of adsorbents on C6H6 adsorption, features of electron distribution across section of adsorption forms, and regulation mechanism of nonsteady-state adsorption for C6H6 are key points. According to calculation results, C6H6 molecules could be captured by pure single-wall carbon nanotube (CNT) through repulsive forces (quantified as 103.42 kJ/mol) from all quarters, which makes it stay in nonsteady-state adsorption forms and easily run into free state. Therefore, when external temperature increases from 0 to 300 K, molecular movement will be intense enough to help C6H6 break into another random positions instead of statistically remaining immobile. As for this problem, single-wall CNTs are modified through making defects and replacing some C atoms with N atoms, respectively. In this way, surficial electron distribution of modified adsorbents is regulated to tremendously cut down repulsive forces (quantified as 50.30 kJ/mol) and reverse nonsteady-state adsorption into near-equilibrium quasi-steady-state adsorption (single-side attraction near 100 kJ/mol). Therefore, this research would provide useful information for exploiting single-wall carbon materials as effective adsorbents of C6H6 in order to quickly achieve indoor air purification.

Arsenic adsorption enhancement performances of Mn-modified γ-Al2O3 with flue gas constituents involved.

Some flue gas constituents have negative effects on As2O3 adsorption of γ-Al2O3 so promoting arsenic adsorption performances under complicated flue gas conditions is necessary based on previous studies. In this study, γ-Al2O3 is modified with manganous nitrate and then Mn-modified γ-Al2O3 is used as the adsorbents in experiments. Besides, molecular dynamics (MD) simulations and density functional theory (DFT) calculations are performed to explore mechanisms of how loadings of Mn enhance arsenic adsorption features of γ-Al2O3 when being affected by flue gas constituents in microscale and mesoscale, respectively. As for DFT calculations, it is uncovered that electron transfer/interaction among As2O3, flue gas constituents and Mn-modified γ-Al2O3 mostly influences arsenic adsorption. For MD simulations, it is expounded that the collision and aggregation of As2O3 and flue gas constituent molecules have most impact on arsenic adsorption. As far as experiments are concerned, they are conducted to show the macroscopic characterizations of arsenic adsorption performances, corresponding to results of DFT calculations and MD simulations. The understanding of these three different aspects could supply significant references for utilization of Mn-modified γ-Al2O3 in real industries to remove arsenic under complex flue gas conditions.

The distribution and conversion of selenite and selenate with the bubbling of simulated flue gas in simulated WFGD slurry.

Selenium is one of the hazardous trace elements emitted from coal-fired power plants. The distribution of selenium in Wet Flue Gas Desulfurization (WFGD) process is still unclear and even in controversial, impeding the development of selenium removal technologies. This research has found that the selenite in simulated slurry could be reduced by SO2 while selenate has not been affected. Characterization methods including X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were used to provide an evidence that the product of the reduction reaction is amorphous elemental selenium. Meanwhile, the influences of other gaseous components, pH, temperature and S2O82- in simulated slurry has also been considered in this research. It is found that with the increase of SO2 concentration in flue gas, the reduction of selenite increased and the reduction reaction is an exothermic reaction. Meanwhile, the oxidation effect of S2O82- competes with the reduction effect of SO2. This study introduced the influence of flue gas into the research of the conversion of selenium in FGD slurry and indicate the effect of flue gas on the potential emission treatment techniques of selenium in FGD slurry.

Band application of flue gas desulfurization gypsum improves sodic soil amelioration.

Blending flue gas desulfurization (FGD) gypsum with surface sodic soil is a universally recognized method for the rapid amelioration of sodic soils; however, little information is available on whether other application methods (band application) will reclaim sodic soil. Three FGD gypsum application methods (single-band, dual-band and blend applications) and a control treatment (non-FGD gypsum) were carried out using sodic soil in soil bins to investigate the effects of the application method on the wetting front, major cations in the leachate during the process of water infiltration and soluble and exchangeable cations in the soil profile after infiltration. The results showed that the wetting fronts in the band treatments were denser in the horizontal direction than in the vertical direction, but the blend and control treatments only had vertical migration. The main channel of the stream in the band treatment was concentrated below the application site of FGD gypsum. The orders of desalting capacity were blend treatment, dual-band treatment and single-band treatment for the same volume of outlet water. There was no water outflow in the control treatment even after 115 days of leaching. The dual-band treatment significantly decreased the soil sodicity of the 0-40 cm soil profile, while the single-band treatment only effectively reclaimed (horizontally) half of the soil. In the blend treatment, the exchangeable sodium percentages were 21.3 % and 34.7 % at depths of 30-35 cm and 35-40 cm, respectively, and were close to zero at a depth of 0-30 cm. Compared with blend treatment, band application could be a better way to reclaim sodic soil with FGD gypsum due to its advantages of long-term and efficient amelioration with low consumption.

Fate of mercury in flue gas desulfurization gypsum determined by Temperature Programmed Decomposition and Sequential Chemical Extraction.

A considerable amount of Hg is retained in flue gas desulfurization (FGD) gypsum from Wet Flue Gas Desulfurization (WFGD) systems. For this reason, it is important to determine the species of Hg in FGD gypsum not only to understand the mechanism of Hg removal by WFGD systems but also to determine the final fate of Hg when FGD gypsum is disposed. In this study, Temperature Programmed Decomposition (TPD) and Sequential Chemical Extraction (SCE) were applied to FGD gypsum to identify the Hg species in it. The FGD gypsum samples were collected from seven coal-fired power plants in China, with Hg concentrations ranging from 0.19 to 3.27µg/g. A series of pure Hg compounds were used as reference materials in TPD experiments and the results revealed that the decomposition temperatures of different Hg compounds increase in the order of Hg2Cl2

Influence of Flue Gas Desulfurization Gypsum Amendments on Heavy Metal Distribution in Reclaimed Sodic Soils.

Although flue gas desulfurization (FGD) gypsum has become an effective soil amendment for sodic soil reclamation, it carries extra heavy metal contamination into the soil environment. The fate of heavy metals introduced by FGD gypsum in sodic or saline-alkali soils is still unclear. This work aims to investigate the effects of FGD gypsum addition on the heavy metal distributions in a sodic soil. Original soil samples were collected from typical sodic land in north China. Soil column leaching tests were conducted to investigate the influence of FGD gypsum addition on the soil properties, especially on distribution profiles of the heavy metals (Pb, Cd, Cr, As, and Hg) in the soil layers. Results showed that pH, electrical conductivity, and exchangeable sodium percentage in amended soils were significantly reduced from 10.2 to 8.46, 1.8 to 0.2 dS/m, and 18.14% to 1.28%, respectively. As and Hg concentrations in the soils were found to be positively correlated with FGD gypsum added. The amount of Hg in the leachate was positively correlated with FGD gypsum application ratio, whereas a negative correlation was observed between the Pb concentration in the leachate and the FGD gypsum ratio. Results revealed that heavy metal concentrations in soils complied well with Environmental Quality Standard for Soils in China (GB15618-1995). This work helps to understand the fate of FGD gypsum-introduced heavy metals in sodic soils and provides a baseline for further environmental risk assessment associated with applying FGD gypsum for sodic soil remediation.

Simultaneous removal of SO2 and trace As2O3 from flue gas: mechanism, kinetics study, and effect of main gases on arsenic capture.

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.

Simultaneous removal of SO2 and trace SeO2 from flue gas: effect of product layer on mass transfer.

Sulfur dioxide (SO2) and trace elements are all pollutants derived from coal combustion. This study relates to the simultaneous removal of sulfur and trace selenium dioxide (SeO2) by calcium oxide (CaO) adsorption in the medium temperature range, especially the mass transfer effect of sulfate product layer on trace elements. Through experiments on CaO adsorbing different concentrations of SO2 gases, conclusions can be drawn that although the product layer introduces extra mass transfer resistance into the sorbent-gas reaction process, the extent of CaO adsorption ability loss due to this factor decreases with decreasing SO2 concentration. When the gas concentration is at trace level, the loss of CaO adsorption ability can be neglected. Subsequent experiments on CaO adsorbing trace SeO2 gas suggest that the sulfate product layer, whether it is thick or thin, has no obvious effect on the CaO ability to adsorb trace SeO2 gas.

Simultaneous removal of SO2 and trace SeO2 from flue gas: effect of SO2 on selenium capture and kinetics study.

Sulfur dioxide (SO2) and trace elements are all pollutants derived from coal combustion. This study relates to the simultaneous removal of SO2 and trace selenium dioxide (SeO2) from flue gas by calcium oxide (CaO) adsorption in the moderate temperature range, especially the effect of SO2 presence on selenium capture. Experiments performed on a thermogravimetric analyzer (TGA) can reach the following conclusions. When the CaO conversion is relatively low and the reaction rate is controlled by chemical kinetics, the SO2 presence does not affect the selenium capture. When the CaO conversion is very high and the reaction rate is controlled by product layer diffusion, the SO2 presence and the product layer diffusion resistance jointly reduce the selenium capture. On the basis of the kinetics study, a method to estimate the trace selenium removal efficiency using kinetic parameters and the sulfur removal efficiency is developed.