| The trace toxic elements such as arsenic(As)and selenium(Se)in coal can be released into flue gas during combustion process,and finally transferred into the environment.The released As and Se are harmful to human health.Therefore,it is urgent to control the toxic elements(especially high toxic gaseous arsenic)in coal-fired flue gas.Understanding the speciation transformation process of arsenic and selenium during coal combustion is important for researchers to develop control techniques.At present,speciation analysis of heavy metals mainly includes sequential extraction fractions and chemical species analysis.Due to the complex matrix and huge production,conventional sequential extraction procedure is always time-consuming,which is difficult to meet the requirements of rapid analysis for a large number of samples.Meanwhile,the sample pretreatment methods for chemical species analysis for coal and its combustion products are still lacking.In this study,microwave and ultrasound were applied to establish the rapid sequential extraction schemes for arsenic and selenium fractions,and to develop the efficient pretreatment methods for arsenic speciation.Based on the developed arsenic fractions and speciation methods,the speciation migration process and transformation mechanism of gaseous arsenic on the surface of metal oxides were studied.Ultrasound-assisted sequential extraction(UASE)and microwave-assistedsequential extraction(MASE)were successfully established to rapidly analyze the arsenic and selenium fractions in fly ash and coal samples.Compared with the conventional methods,the sequential extraction procedure of arsenic and selenium by UASE scheme was greatly shortened(from about 24.5 h to about 70 min and 90 min,respectively).The operation time of the whole process could be significantly shortened from 24.5 h to within 45 min using MASE scheme.The recoveries of the proposed extraction schemes were higher than 80%with good stability.The actual sample analysis results showed that selenium in fly ash was mainly in non-specific fraction(F1)and weak Fe-Al crystalline fraction(F3),selenium in coal mainly existed in weak Fe-Al crystalline fraction.For arsenic in fly ash or coal,the proportion of weak Fe-Al crystalline fraction was apparently higher than other fractions.Effective ultrasonic/microwave-assisted pretreatment methods for arsenic speciation in coal and its by-products were established.Microwave-assisted(2000 W,80?,40 min)and ultrasound-assisted(40 kHz,20?,40 min)schemes were the optimal extraction methods for coal/FA and gypsum samples,respectively.The distribution of arsenic species in coal,fly ash,and gypsum from power plants was successfully determined by high performance liquid chromatography hydride generation atomic fluorescence spectrometry(HPLC-HG-AFS).The results showed that arsenic in coal mainly existed in As(?),and the proportion of As(? was low,while only As(?)was detected in fly ash and gypsum.Considering that the fractions and chemical species of arsenic were related to its environmental migration ability and toxicity,the speciation analysis of arsenic was feasible to develop the pollution control technologies of arsenic in flue gas based on species transformation.Sequential extraction and speciation analysis results demonstrated that arsenic mainly bounded to Fe-Al oxide fraction(F3),and the oxidation of arsenic from low valence state to high valence state(As(?)-As(?))was the key step to achieve efficient adsorption and immobilization of gaseous arsenic on the surface of particles or metal oxides.Therefore,a series of Fe-Mn binary oxides(FMBO)with different molar ratios were prepared and their removal performances for gaseous arsenic were studied.Mn-Fe binary metal oxides exhibited excellent synergistic adsorption performance for gaseous arsenic compared with Mn and Fe single metal oxides.The adsorbent had good and stable adsorption performance,and the adsorption capacity could reach 24.23±2.47 mg/g under the condition of simulated flue gas at 600?.X-ray photoelectron spectroscopy(XPS)and HPLC-HG-AFS were used to analyze the adsorbed arsenic and soluble arsenic on the adsorbents' surface.Mn oxide could oxidize As2O3 to As2O5,which promoted its effective adsorption on Fe oxide.Considering that the toxicity of As(?)was lower than that of As(?),the synergistic effect of Fe and Mn not only effectively improved the adsorption capacity of gaseous arsenic but also achieved the detoxification of arsenic after capture.Hence,a novel Fe-Mn-Ti oxides(FMTO)was further synthesized.Titanium could effectively improve the electron transfer ability of composite metal oxides at low temperature,which was conducive to the oxidation of low valent arsenic and effective adsorption of arsenic on the adsorbents' surface.Under simulated flue gas conditions,the arsenic adsorption capacity of FMTO was 37.0±4.8 mg/g,and the oxidation rate of As(?)could reach up to 100%at 350?.The developed FMTO showed excellent tolerance to high concentration of flue gas components.The immobilization and detoxification performance of FMTO on gaseous arsenic was subsequently evaluated by leaching test and HPLC-HG-AFS.The characterization of electron spin resonance(ESR)and XPS results suggested that titanium and manganese mainly played an oxidation role,while iron mainly played the role of adsorption.Aiming at the limitation of high cost for traditional metal oxides,a series of manganese modified attapulgite(ATP)composites were successfully synthesized.The adsorbents showed good immobilization and removal performance for gaseous arsenic.Both SO2 and NO in flue gas showed promoting effect for arsenic capture over Mn-ATP.SO2 mainly depending on the generation of sulfate and then combined with arsenic to improve the adsorption efficiency,while NO accelerated the oxidation process of As(?)to As(?).The results of toxicity characteristic leaching procedure and sequential extraction test demonstrated that Mn modified ATP had an excellent immobilization effect on gaseous arsenic owing to hydration and oxidation effect,which significantly reduced the second exposure risk of arsenic in the adsorbents.In this study,the effective analytical methods for arsenic and selenium sequential extraction fractions and chemical species in coal and its products were established.A series of gaseous arsenic adsorption and solidification materials were synthesized.Combined with speciation analysis results,the adsorption and solidification process and mechanism were deeply discussed,which is significant for the development of coal pollution monitoring and control technology. |
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