Gas-phase Arsenic Adsorption Behavior Of Iron-Based Sorbent And The Reaction Mechanism Study Based On The DFT

In recent years,the prevention and control of arsenic pollution has become a global concern increasingly.Characterized as huge mass emission,long distance spread.,easy to be absorbed by human and highly toxicity,arsenic emission from coal-fired power plant has received great attention.Solid sorbent can not only provide surface area for gaseous arsenic condensation,but also be capable of fixing arsenic by means of chemical and physical sorption.Therefore,it is an effective way to control arsenic content in flue gas and prevent arsenic pollution.At present,the research of gaseous arsenic adsorption from coal-fired flue gas is in the initial stage.The sorbents concerned are mostly concentrated on natural mineral materials and the studies on gaseous arsenic adsorption are limited.In this study,a series of sorbents for gas-phase arsenic were prepared based on the exploration and screening of active material for arsenic adsorption.The working characteristics of adsorbents under conventional flue gas and O2/CO2 flue gas were systematically studied and the adsorption mechanism was discussed.Combined with the calculation method of density functional theory in quantum chemistry,the gas-phase arsenic adsorption of iron-based sorbents was simulated from the perspective of microscopic reaction.The simulation results revealed the effect of adsorption sites and the adsorption energy change for different adsorption structure.From the perspective of arsenic adsorption of fly ash,the arsenic adsorption properties of CaO,Fe2O3 and Al2O3,which are the three main metal oxides in fly ash,was studied.The experimental results revealed that arsenic capture ability of CaO,Fe2O3as well as Al2O3was gradually decreased with the increase of temperature.Among them,arsenic adsorption performance of Fe2O3 showed good temperature adaptability.The arsenic adsorption by CaO and Fe2O3 was mainly relyed on chemical adsorption,and for Al2O3,thephysical adsorption was predominant.Reaction kinetics studies showed that the reaction rate constant of Fe2O3was much higher than that of CaO,which indicatedthat Fe2O3 sorbent has more active sites than CaO sorbent.And,therefore,more efficitive collisions would take place between Fe2O3 and gas-phase arsenic molecules.Combing the arsenic adsorption activity of Fe2O3 and the advantage of ?-Al2O3 as carrier,the Fe2O3/?-Al2O3 sorbent was developed by means of ultrasonic-assisted impregnation method.And the adsorption experiment was carried out by self-made two-stage fixed-bed reactor.The experimental results showed that SO2 in adsorption atmosphere could promote the arsenic adsorption for Fe2O3/?-Al2O3 adsorbents,and its mechanism could be attributed to the catalytic oxidation of arsenic in the gas phase by active sites containing HSO4-/SO42-.However,the affect of NO on gas-phase arsenic adsorption of Fe2O3/?-Al2O3sorbent was not as remarkable as that of SO2.When O2was introduced into the atmosphere,the reduced oxygen sites on sorbent surface could be re-oxidized,the oxidation of As(?)could be promoted by these active sites,andcould be further promoted,and therefore,the fixation of arsenic could be promoted.The high concentration of CO2 in oxy-fuel combustion flue gas inhibited the gas-phase arsenic adsorption capacity of Fe2O3/y-Al2O3.Compared with the air conditions,when the SO2 concentration exceeded 8000×1-6,thecompetition between SO2 and gas-phase arsenic wasobvious.On one hand,the SO2 was oxidized under the catalysis of Fe(?)/Fe(?)on sorbent surface,forming high-boiling sulfuric acid,attaching onto the sorbent surface,and blocking the pores.On the other hand,the lattice oxygen on surbent surface would be consumed continuously by SO2,resultingin the deactivation of active sites for arsenic adsorption,and,therefore,the overhigh SO2 concentration would surpress arsenic adsorption.The hydrogen ions would be generated from the water vapor in atmosphere with the action of Fe2O3/?-Al2O3.The increased concentration of hydrogen ions would dissolve a small part of iron on sorbent surface.And during this process,active oxygen components,such as O2·-,H2O2?·OH,etc.would be formed,which played a key role in oxidation of As(?).The adsorption process of As2O3 on the surface of Fe2O3(001)has been studied by means of density functional theory.By comparing the adsorption energy and bond change of different adsorption sites,the key step of As2O3 adsorption on Fe2O3(001)surface was obtained,and therefore,the adsorption model was simplified.To investigate the impact of SO2 and O2 on arsenic adsorption on Fe2O3(001).the simplified adsorption modle,As on Fe2O3(001),was built up.The results indicated that O top and O hollow are the active sites of arsenic adsorption,among which O top has higher adsorption activity than O hollow and Fe top has the weakest adsorption activity.O2 in adsorption atmosphere made the initialFe top site on Fe2O3(001)surface transform into the O top site,and the arsenic adsorption was therefore facilitated.Except O site on Fe2O3(001)surface would be formed,S(ads)top would be formed from the Fe top site with the action of SO2 in atmosphere.The adsorbed arsenic atom on O top site of S/Fe2O3(001)surface not only built bond with O atom,but also built bond with the adjacent Fe atom and forming a more stable adsorption configuration.