Experimental Studies On Arsenic Synergistic Removal By Wet Flue Gas Desulfurization System And Enhanced Removal By Iron-based Rare Earth Adsorbent

Arsenic,as a heavy metal element,poses great harm to human health and ecological environment due to its high volatility,toxicity,carcinogenicity and teratogenicity.Coal-fired power plants are the substantial source of arsenic pollution in air.As a range of increasingly strict laws on environmental protection are enacted worldwide,it is urgent to strengthen the control of gas-phase arsenic pollution in coal-fired power plants.Currently,heavy metals are mainly removed by using the existing flue gas cleaning unit in power plants and spraying adsorbents in flue gas pass of boilers.The use of flue gas cleaning unit is regarded as the most cost-effective method for arsenic removal,but in fact,arsenic cannot be thoroughly removed in this way.Moreover,the synergistic removal mechanism of arsenic remains unclear.To this end,the wet flue gas desulfurization(WFGD)system was selected as the object of study.The effects of Fe3+concentration and pH value in desulfurization slurry on removal,migration and transformation of arsenic in flue gas were investigated to determine the optimal parameters for achieving the most efficient arsenic removal.The mechanism involving in the removal,migration and transformation of arsenic was also discussed.In order to enhance the removal of gas-phase arsenic,further study on the removal of gas-phase arsenic by adsorbents was conducted.Given that iron-based rare earth adsorbent has good arsenic adsorption activity at medium and low temperature and excellent oxygen storage performance,several types of iron-based rare earth adsorbents were prepared to investigate their efficiency of arsenic removal under different working conditions and reveal the mechanism of arsenic removal by adsorption.The main processes and results are as follows:(1)In a self-designed WFGD simulator,Fe3+was added to the desulfurization slurry.The effects of different concentrations of Fe3+and pH values on the removal efficiency and speciation of gas-phase arsenic were investigated.The migration and speciation distribution of arsenic in desulfurization wastewater and gypsum were further discussed.The surface morphology,composition and Zeta potential of Fe-gypsum particles were analyzed in detail.The results show that:a small amount of Fe3+ reduced the size of gypsum particles and increased specific surface area and Zeta potential,while high concentrations of Fe3+ led to the agglomeration of particles.Reducing the concentration of Fe3+can increase the efficiency of removing gas-phase arsenic.When more Fe3+ was added,the efficiency of arsenic removal was reduced due to the growing resistance of mass transfer.In this sense,the increase of Fe3+is more conducive to the removal of As(Ⅲ).In terms of pH value,the removal efficiency of gas-phase arsenic decreased with the rise of pH value.The ratio of removed gas-phase As(Ⅲ)was always higher than that of As(Ⅴ),regardless of how much the pH value changed.To sum up,increasing the concentration of Fe3+or pH value is conducive to the transfer of arsenic from the liquid phase to the Fe-gypsum solid phase.Furthermore,the Fe-gypsum will have the priority to fasten As(Ⅴ).(2)The Fe-gypsum containing arsenic was leached in acid.The arsenic content in the leachate was measured.The arsenic content of the gypsum before and after acid leaching was also tested,followed by XRD and FTIR analysis.The results show that:in the case of no Fe3+,the arsenic incorporated into the lattice of gypsum accounted for about 12%;but when Fe3+was added,more than 99%of arsenic was absorbed by the Fe-gypsum or formed precipitates like arsenate.The crystal orientation and cell parameters of the arsenic-containing gypsum already changed.The incorporated arsenates in the gypsum structure instead of SO42-were likely to be HAsO42-and AsO43-.(3)The adsorbents including CeFeO,CeLaFeO,CeZrFeO and CeZrLaFeO were prepared together with the reference adsorbent Fe2O3 by using the coprecipitation method,followed by the XRD,BET,H2-TPR,Raman,SEM-EDS and XPS property characterization.The results showed that:the iron-based rare earth adsorbent developed a cubic fluorite solid solution;CeLaFeO,CeZrFeO and CeZrLaFeO had incomplete fluorite structure and dispersed grain distribution,compared with and CeFeO;and a weak peak of Fe2O3 was found in CeFeO and CeLaFeO.The specific surface area of iron-based rare earth adsorbents was much larger than that of the Fe2O3.The iron-based rare earth adsorbents had two reduction peaks on the adsorbed oxygen and lattice oxygen,and CeLaFeO contributed the most surface-adsorbed oxygen.The Raman spectroscopy demonstrates the lattice imperfection in iron-based rare earth adsorbents,among which CeLaFeO had the most serious lattice imperfection.The XPS results further indicate that CeLaFeO had a much higher content of the surface-adsorbed oxygen than Fe2O3,and the content of Ce3+increased after CeLaFeO absorbed arsenic.(4)The impacts of different temperatures and O2 concentrations on the efficiency and speciation of gas-phase arsenic removal by iron-based rare earth adsorbents were investigated on a self-designed fixed-bed reactor.The mechanism of arsenic removal by CeLaFeO was discussed.The results show that:When the O2 concentration was 21%and the temperature range was 200-800℃,the arsenic capture ability of Fe2O3 and iron-based rare earth adsorbents first increased and then decreased with the increase of the adsorption temperature.The optimal adsorption temperature of Fe2O3 was 600℃,and the optimal adsorption temperature of iron-based rare earth adsorbents was 400℃,of which CeLaFeO adsorption efficiency was up to 89%.At the optimal adsorption temperature,the O2 concentration range was 0-21%,the arsenic removal efficiency of Fe2O3 remained basically unchanged with the increase of O2 concentration,while the arsenic removal efficiency of CeLaFeO underwent a decline after increase,up to 98%,and the arsenic removal efficiency of CeZrLaFeO decreased,up to 86%.The iron-based rare earth adsorbents are always more capable of oxidizing As(Ⅲ)than Fe2O3,regardless of the temperature and O2 concentration.When the oxygen concentration in the flue gas was low,reactive oxygen was formed on the oxygen vacancies on the CeLaFeO surface to participate in the oxidation adsorption process,which is the key to CeLaFeO’s strong arsenic adsorption capacity.