Gaseous Arsenic Capture in Flue Gas by CuCl2-Modified Halloysite Nanotube Composites with High-Temperature NOx and SOx Resistance.

Gaseous arsenic emitted from coal combustion flue gas (CCFG) causes not only severe contamination of the environment but also the failure of selective catalytic reduction (SCR) catalysts in power plants. Development of inexpensive and effective adsorbents or techniques for the removal of arsenic from high-temperature CCFG is crucial. In this study, halloysite nanotubes (HNTs) at low price were modified with CuCl2 (CuCl2-HNTs) through ultrasound assistance and applied for capturing As2O3(g) in simulated flue gas (SFG). Experiments on arsenic adsorption performance, adsorption mechanism, and adsorption energy based on density functional theory were performed. Modification with CuCl2 clearly enhanced the arsenic uptake capacity (approximately 12.3 mg/g) at 600 °C for SFG. The adsorbent exhibited favorable tolerance to high concentrations of NOx and SOx. The As2O3(III) was oxidized and transformed into As2O5(V) on the CuCl2-HNTs. The Al-O bridge had the highest adsorption energy for the O end of the As-O group (-2.986 eV), and the combination formed between arsenic-containing groups and aluminum was stable. In addition, the captured arsenic could be stabilized in the sorbent at high temperature, making it possible to use the sorbent before the SCR system. This demonstrates that CuCl2-HNTs is a promising sorbent for arsenic oxidation and removal from CCFG.

Synergistic effects of Fe-Mn binary oxide for gaseous arsenic removal in flue gas.

High-efficient and economic sorbents are highly desired for arsenic (As) emission control in flue gas from coal-fired power plant. A series of Fe-Mn binary oxides were prepared by a facile method, and their behaviors for gaseous arsenic removal in flue gas were investigated. The binary oxide exhibited a remarkable synergistic effect for arsenic removal compared with Mn or Fe monometallic oxide. The possible effects of CO2, NO, SO2, and O2 on the removal performance were also studied. The adsorption ability was excellent and stable in simulated flue gas conditions. X-ray photoelectron spectroscopy (XPS) and high-performance liquid chromatography atomic fluorescence spectroscopy (HPLC-AFS) coupling system were applied to analyze the species of surface-adsorbed arsenicals and soluble arsenicals. It was confirmed that the good sorption performance resulted from oxidation of As2O3 (As(III)) to As2O5 (As(V)) by Mn oxide and followed by efficient adsorption of As(V) on Fe oxide. Considering the toxicity of pentavalent arsenicals is lower than trivalent arsenicals, the oxidation of arsenic compounds can not only enhance its removal capacity but also decrease the toxicity of arsenicals after capture.

Accelerated screening of arsenic and selenium fractions and bioavailability in fly ash by microwave assistance.

Huge amounts of fly ash (FA) can be annually produced in power plants. Fly ash always contains high levels of arsenic (As) and selenium (Se) due to the preconcentration of these two elements during coal combustion process. It would be much concerned to screen their fractions and potential environmental behaviors in fly ash for beneficial use and treatment. Fast and practical methods for this purpose are urgently needed. Two fast and effective microwave-assisted sequential extraction schemes (MASE) were developed for fast screening As and Se fractions in fly ash for the first time. The extraction parameters including microwave irradiation time, temperature and power energy were optimized by comparing the results from MASE and the conventional scheme (Wenzel method). The results indicate that the extraction efficiency of As and Se in various fractions can be significantly accelerated by microwave irradiation. The whole procedure operation time can be significantly reduced from 24.5 h to 44 min by microwave assistance compared with the conventional shaking schemes. The recoveries of As and Se in the various extracted fractions were all above 80% with relative standard deviations (RSDs) below 8%. The developed methods were further confirmed by the validation of the certified reference material GBW08401 and fly ash samples from six power plants. The developed MASE methods are practical and effective for fast screening arsenic and selenium fractions in fly ash samples.

Identification of mercury species in coal combustion by-products from power plants using thermal desorption-atomic fluorescence spectrometry on-line coupling system.

Along with the environmental protection policies becoming strict in China, the air pollution control devices (especially selective catalytic reduction (SCR)) are widely equipped in coal-fired power plants. The installation and run of these devices will inevitably affect mercury (Hg) species distribution in coal fired by-products such like fly ash (FA) and gypsum. In this work, a new on-line coupling system based on atomic fluorescence spectrometry (AFS) with a home-made chromatographic workstation was successfully developed to identify Hg species through thermal programmed desorption (TPD). The influences of matrix, furnace temperature, and carrier gas flow on analytical performance were investigated and the parameters were optimized. The FA and gypsum samples from coal-fired power plants equipped with SCR were collected and the mercury species were analyzed by the developed coupling system. HgCl2 and HgO were the main species in FA, while Hg2Cl2 and HgO were the main species in gypsum. All of Hg species in the studied FA and gypsum samples were released below 400 °C. A sequential extraction procedure was applied to further verify the operational Hg species including mobile and non-mobile fractions in FA and gypsum samples. This study demonstrated that AFS coupled with TPD procedure was an effective method to analyze Hg species in coal combustion by-products from power plants.

Fractions of arsenic and selenium in fly ash by ultrasound-assisted sequential extraction.

Sequential extraction has been validated as an effective method to assess the fractions of elements in fly ash. However, the time consumption and high labor costs limit the application of the conventional sequential extraction (CSE) for fast screening of elemental fractions in fly ash. In this study, two ultrasound-assisted sequential extraction (UASE) methods were developed for fast analysis of arsenic (As) and selenium (Se) fractions in fly ash (FA). The parameters of UASE were optimized by comparing the results of As and Se obtained from UASE with those values observed via CSE. The operation time of sequential extraction procedures for As and Se were shortened from 24.5 h to less than 90 min. A certified reference material (CRM, GBW08401) and real fly ash samples were applied to validate the developed UASE. The recoveries of As and Se were found in the range of 82.3% to 114%. For all fractions, the performance of UASE was faster than CSE with the acceptable uncertainties. The analytical results demonstrated that the concentration of As in F3 was found to be higher than other fractions, while the main forms of Se were found to be in F1 and F3 in the fly ash samples. Based on the advantages of high efficiency and easy operation, the developed UASE procedures can be applied for fast screening of the mobility and bioavailability of As and Se in FA from coal fired power plants.