Journal of Environmental Sciences Volume 30 2015 www.jesc.ac.cn Highlight articles 129 Rice: Reducing arsenic content by controlling water irrigation Ashley M. Newbigging, Rebecca E. Paliwoda and X. Chris Le 132 Apportioning aldehydes: Quantifying industrial sources of carbonyls Sarah A. Styler Review articles 30 Application of constructed wetlands for wastewater treatment in tropical and subtropical regions (2000–2013) Dong-Qing Zhang, K.B.S.N. Jinadasa, Richard M. Gersberg, Yu Liu, Soon Keat Tan and Wun Jern Ng 47 Stepwise multiple regression method of greenhouse gas emission modeling in the energy sector in Poland Alicja Kolasa-Wiecek 113 Mini-review on river eutrophication and bottom improvement techniques, with special emphasis on the Nakdong River Andinet Tekile, Ilho Kim and Jisung Kim Regular articles 1 Effects of temperature and composite alumina on pyrolysis of sewage sludge Yu Sun, Baosheng Jin, Wei Wu, Wu Zuo, Ya Zhang, Yong Zhang and Yaji Huang 9 Numerical study of the effects of local atmospheric circulations on a pollution event over Beijing–Tianjin–Hebei, China Yucong Miao, Shuhua Liu, Yijia Zheng, Shu Wang and Bicheng Chen, Hui Zheng and Jingchuan Zhao 21 Removal kinetics of phosphorus from synthetic wastewater using basic oxygen furnace slag Chong Han, Zhen Wang, He Yang and Xiangxin Xue 55 Abatement of SO2–NOx binary gas mixtures using a ferruginous active absorbent: Part I. Synergistic effects and mechanism Yinghui Han, Xiaolei Li, Maohong Fan, Armistead G. Russell, Yi Zhao, Chunmei Cao, Ning Zhang and Genshan Jiang 65 Adsorption of benzene, cyclohexane and hexane on ordered mesoporous carbon Gang Wang, Baojuan Dou, Zhongshen Zhang, Junhui Wang, Haier Liu and Zhengping Hao 74 Flux characteristics of total dissolved iron and its species during extreme rainfall event in the midstream of the Heilongjiang River Jiunian Guan, Baixing Yan, Hui Zhu, Lixia Wang, Duian Lu and Long Cheng 81 Sodium fluoride induces apoptosis through reactive oxygen species-mediated endoplasmic reticulum stress pathway in Sertoli cells Yang Yang, Xinwei Lin, Hui Huang, Demin Feng, Yue Ba, Xuemin Cheng and Liuxin Cui 90 Roles of SO2 oxidation in new particle formation events He Meng, Yujiao Zhu, Greg J. Evans, Cheol-Heon Jeong and Xiaohong Yao 102 Biological treatment of fish processing wastewater: A case study from Sfax City (Southeastern Tunisia) Meryem Jemli, Fatma Karray, Firas Feki, Slim Loukil, Najla Mhiri, Fathi Aloui and Sami Sayadi CONTENTS 122 Bioreduction of vanadium (V) in groundwater by autohydrogentrophic bacteria: Mechanisms and microorganisms Xiaoyin Xu, Siqing Xia, Lijie Zhou, Zhiqiang Zhang and Bruce E. Rittmann 135 Laccase-catalyzed bisphenol A oxidation in the presence of 10-propyl sulfonic acid phenoxazine Rūta Ivanec-Goranina, Juozas Kulys, Irina Bachmatova, Liucija Marcinkevičienė and Rolandas Meškys 140 Spatial heterogeneity of lake eutrophication caused by physiogeographic conditions: An analysis of 143 lakes in China Jingtao Ding, Jinling Cao, Qigong Xu, Beidou Xi, Jing Su, Rutai Gao, Shouliang Huo and Hongliang Liu 148 Anaerobic biodegradation of PAHs in mangrove sediment with amendment of NaHCO3 Chun-Hua Li, Yuk-Shan Wong, Hong-Yuan Wang and Nora Fung-Yee Tam 157 Achieving nitritation at low temperatures using free ammonia inhibition on Nitrobacter and real-time control in an SBR treating landfill leachate Hongwei Sun, Yongzhen Peng, Shuying Wang and Juan Ma 164 Kinetics of Solvent Blue and Reactive Yellow removal using microwave radiation in combination with nanoscale zero-valent iron Yanpeng Mao, Zhenqian Xi, Wenlong Wang, Chunyuan Ma and Qinyan Yue 173 Environmental impacts of a large-scale incinerator with mixed MSW of high water content from a LCA perspective Ziyang Lou, Bernd Bilitewski, Nanwen Zhu, Xiaoli Chai, Bing Li and Youcai Zhao 180 Quantitative structure–biodegradability relationships for biokinetic parameter of polycyclic aromatic hydrocarbons Peng Xu, Wencheng Ma, Hongjun Han, Shengyong Jia and Baolin Hou 191 Chemical composition and physical properties of filter fly ashes from eight grate-fired biomass combustion plants Christof Lanzerstorfer 198 Assessment of the sources and transformations of nitrogen in a plain river network region using a stable isotope approach Jingtao Ding, Beidou Xi, Qigong Xu, Jing Su, Shouliang Huo, Hongliang Liu, Yijun Yu and Yanbo Zhang 207 The performance of a combined nitritation–anammox reactor treating anaerobic digestion supernatant under various C/N ratios Jian Zhao, Jiane Zuo, Jia Lin and Peng Li 215 Coagulation behavior and floc properties of compound bioflocculant–polyaluminum chloride dualcoagulants and polymeric aluminum in low temperature surface water treatment Xin Huang, Shenglei Sun, Baoyu Gao, Qinyan Yue, Yan Wang and Qian Li 223 Accumulation and elimination of iron oxide nanomaterials in zebrafish (Danio rerio ) upon chronic aqueous exposure Yang Zhang, Lin Zhu, Ya Zhou and Jimiao Chen 231 Impact of industrial effluent on growth and yield of rice (Oryza sativa L. ) in silty clay loam soil Mohammad Anwar Hossain, Golum Kibria Muhammad Mustafizur Rahman, Mohammad Mizanur Rahman, Abul Hossain Molla, Mohammad Mostafizur Rahman and Mohammad Khabir Uddin 241 Molecular characterization of microbial communities in bioaerosols of a coal mine by 454 pyrosequencing and real-time PCR Min Wei, Zhisheng Yu and Hongxun Zhang 252 Risk assessment of Giardia from a full scale MBR sewage treatment plant caused by membrane integrity failure Yu Zhang, Zhimin Chen, Wei An, Shumin Xiao, Hongying Yuan, Dongqing Zhang and Min Yang 186 Serious BTEX pollution in rural area of the North China Plain during winter season Kankan Liu, Chenglong Zhang, Ye Cheng, Chengtang Liu, Hongxing Zhang, Gen Zhang, Xu Sun and Yujing Mu J O U RN A L OF E N V I RO N ME N TA L SC IE N CE S 3 0 (2 0 1 5) 5 5– 6 4 Available online at www.sciencedirect.com ScienceDirect www.journals.elsevier.com/journal-of-environmental-sciences Abatement of SO2–NOx binary gas mixtures using a ferruginous active absorbent: Part I. Synergistic effects and mechanism Yinghui Han1,2,3 , Xiaolei Li 4 , Maohong Fan 2,3,⁎, Armistead G. Russell 3 , Yi Zhao1 , Chunmei Cao1 , Ning Zhang1 , Genshan Jiang 1 1. 2. 3. 4. School of Mathematics and Physics, North China Electric Power University, Baoding 071003, China School of Energy Resources and Department of Chemical & Petroleum Engineering, University of Wyoming, Laramie, WY 82071, USA School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA Shanxi Electric Power Exploration & Design Institute, China's State Energy Group Co., Taiyuan 030001, China AR TIC LE I N FO ABS TR ACT Article history: A novel ferruginous active absorbent, prepared by fly ash, industrial lime and the additive Received 20 May 2014 Fe(VI), was introduced for synchronous abatement of binary mixtures of SO2–NOx from Revised 29 September 2014 simulated coal-fired flue gas. The synergistic action of various factors on the absorption of Accepted 11 October 2014 SO2 and NOx was investigated. The results show that a strong synergistic effect exists Available online 21 February 2015 between Fe(VI) dose and reaction temperature for the desulfurization. It was observed that in the denitration process, the synergy of Fe(VI) dose and Ca/(S + N) had the most Keywords: significant impact on the removal of NO, followed by the synergy of Fe(VI) and reaction Fe(VI) temperature, and then the synergy of reaction temperature and flue gas humidity. A Desulfurization scanning electron microscope (SEM) and an accessory X-ray energy spectrometer (EDS) Denitration were used to observe the surface characteristics of the raw and spent absorbent as well as Binary gas fly ash. A reaction mechanism was proposed based on chemical analysis of sulfur and Synergized effect nitrogen species concentrations in the spent absorbent. The Gibbs free energy, equilibrium Thermodynamics constants and partial pressures of the SO2–NOx binary system were determined by thermodynamics. © 2015 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V. Introduction Emissions of sulfur dioxide (SO2) and nitrogen oxides (NOx) from the electric power sector are considered two of the major causes for the deterioration of the atmosphere environment in China (Hu et al., 2010; Lin et al., 2010; Lu et al., 2010). In an effort to abate SO2 and NOx emissions, a number of coal-fired retrofitting units have been investigated, such as flue gas desulfurization (FGD) and wet-scrubbers. There has also been an effort by coal-fired plants to switch to coal of lower sulfur content. These measures alleviate some of the pressure on SO2 air pollution, but the concentration of NOx in the atmosphere has not decreased. In 2011, the total emission of NOx in China was 1206.7 million tons, jumping 6.17% from a year earlier (Han, 2012). Even worse, the area of acid rain never decreased; the main kind of acid rain was converted from simple sulfuric acid to a complex type of sulfuric acid and nitrating acid (Hu et al., 2010). In recent years, some effort has been expended to limit NOx emissions, i.e., selective catalytic reduction (SCR), selective non-catalytic reduction (SNCR), low NOx burners, and so on; however, such denitration ⁎ Corresponding author. E-mail: mfan@uwyo.edu (Maohong Fan). http://dx.doi.org/10.1016/j.jes.2014.10.012 1001-0742/© 2015 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V. 56 J O U RN A L OF E N V I RO N ME N TA L SC IE N CE S 3 0 (2 0 1 5) 55–6 4 equipment installed at the front or the back of the FGD system has some disadvantages such as large footprint and high running cost (Zhao et al., 2012). A great deal of research work aimed at synchronizing abatement of SO2 and NOx has been carried out recently (Xie et al., 2004; Yi et al., 2012; Yuan et al., 2012; Ding and Yazaydin, 2013; Krishnan et al., 2013), but many technical and economic problems have been encountered, impairing the implementation of practicable technologies. Therefore, it is still significant and urgent to develop high efficiency, practical techniques of synchronizing desulfurization and denitration, without introducing secondary pollution, according to the characteristics of coal-fired flue gas, especially those compatible with the existing FGD systems. The linchpin of synchronizing abatement of SO2 and NOx techniques is to oxidize NO into NO2 rapidly in the flue gas, as the latter is readily dissolved in water. There have been several studies on the rapid oxidation of NO (Deshwal et al., 2008; Li et al., 2011; Liu and Zhang, 2011), however, some strong oxidizing absorbents were not only economically infeasible, but also tended to introduce heavy metal secondary pollution during industrial application. In order to solve these practical problems, a new eco-friendly oxidizing additive, Fe(VI), was introduced into a flue gas circulating fluidized bed system in a previous study by the authors (Zhao et al., 2011). Fe(VI) is one of the most promising environmentally benign chemicals and will not produce any harmful byproducts (Sharma, 2010). It has strong oxidizing power and its redox potential ranges from 2.20 V in an acidic environment to 0.72 V in a basic environment. Such outstanding oxidizing performance and environmentally benign nature make Fe(VI) a preferred choice in the pollutant disposal process. The addition of green oxidizing Fe(VI) was effective in improving the synchronization of pollutant removal abilities in flue gas control and purification, which has also recently been confirmed by many researchers (Sharma, 2010; Xia et al., 2011; Dutcher et al., 2011). However, these investigations were limited. As is well known, there are many operational factors that will affect the removal of SO2 and NO in real power plants. In conventional multi-factor experiments, optimization is usually performed by varying a single factor while keeping all other factors fixed under a specific set of conditions. This is not only time-consuming, but also usually incapable of reaching the true optimum due to neglect of the synergistic effects among factors. These shortcomings might restrict the practical application of the promising Fe(VI)-based removal technology. To fill this gap, multi-factor experiments of removing SO2 and NOx were carried out using the developed ferruginous active absorbent, and the synergistic action of various factors was investigated in this work. For the first time, a novel statistical experimental design method, response surface methodology (RSM), was adopted in this work. This is because RSM has many inherent advantages over other experimental design methods, especially in revealing the synergistic effects between multiple variables with fewer experimental trials, optimizing technological operation and guiding industrial management. In addition, the related removal mechanism still remains enigmatic. In this work, the reaction mechanisms were revealed based on chemical analysis and testing techniques such as X-ray diffraction (XRD), scanning electron microscope (SEM) and Energy dispersive spectroscopy (EDS). These results will be very useful for improving and optimizing the flue gas treatment process using Fe(VI) in practical applications. 1. Experimental 1.1. Preparation and characterization of a ferruginous active absorbent In order to meet the demands of industrial applications, the preparation process strove for simple and quick operation, without involving stirring, digesting or drying. The ferruginous active absorbent was prepared by adding an amount of K2FeO4 to a mixture of fly ash and industrial lime with the ratio of 3:1 by weight. The mass percent of the additive, namely, dose of Fe(VI), ranged from 0% to 1.0%. In addition, tiny amounts of water were injected into solid mixtures with the solid/liquid ratio of 10:1 by weight. The fly ash samples were collected from the electrostatic precipitators of the Matou coal-fired plants in Hebei, China; the industrial lime was technical-pure grade (Tianjin Tanggu Chemical Reagent Factory, Tianjin, China); all other reagents were analytical grade, used without further purification and purchased from Beijing Chemical Reagent Co., China. The compositions of fly ash and industrial lime used in the work are summarized in Table 1. The specific surface areas of fly ash, industrial lime, and fresh absorbent were measured (SA-3100 surface area & pore size analyzer, Beckman Coulter Inc., USA) by the BET standard method; the results are as follows: fly ash 4.588 m2/g; industrial lime 10.173 m2/g; and 45.349 m2/g. The additive, K2FeO4, was characterized by an X-ray diffractometer (D8 Advance type, Bruker Corporation, Germany). An scanning electron microscope (SEM, KYKY-2800B type, KYKY Technology Co., LTD., Beijing, China) and an accessory energy-dispersive X-ray spectroscopy (EDS, NORAN System 7 type, Thermo Fisher Scientific Inc., MA, USA) were used to observe the surface properties and to determine relative surface contents of significant elements in the fly ash, lime absorbent, ferruginous highly active absorbent, and its removal products. 1.2. Apparatus and procedure The experiments on synchronous abatement of SO2–NOx binary gas mixtures were performed in a fixed bed reactor, as shown in Fig. 1, with a vertical cylinder with a length of 250 mm and an inner diameter of 20 mm. The fixed bed reactor filled with Table 1 – Composition of fly ash and industrial lime. Components content (wt.%) SiO2 Al2O3 Fe2O3 CaO MgO Ig-loss Others Fly ash Industrial lime 46.6 0.4 17.2 0.15 11.1 0.11 7.3 72.3 6.4 1.3 10 25.2 1.4 0 57 J O U RN A L OF E N V I RO N ME N TA L SC IE N CE S 3 0 (2 0 1 5) 5 5–6 4 Check valve Three-way valve Gaseous products Glass rotameter Glass rotameter Regulator Mixed buffer bottle Gas drying bottle Flue gas analyzer Waste gas scrubber Fixed bed Furnace By-pass Scanning electron microscope Controlling valve SO2 NO N2 Solid products Energy-dispersive X-ray spectroscopy Fig. 1 – Schematic diagram of the experimental procedure for synchronous abatement of binary SO2–NO. ferruginous active absorbent was heated by an outer furnace. The simulated flue gases, SO2 and NO, were supplied from compressed gas steel cylinders. The flow rates of gases were adjusted by pressure reducing valves and glass rotameters. The carrier gas, N2, makes the simulated flue gas well-distributed in the mixed buffer bottle. The simulated flue gas was then passed through the system bypass to the absorption bottles. After the gas steam was steady, the simulated flue gas was introduced to the fixed-bed reactor by turning the three-way valve. The tail gas was discharged to the atmosphere after it was absorbed in absorption bottles, and the three-way valve was then turned again. The removal efficiencies were calculated from the measured concentrations of SO2 and NO at the inlet and outlet as measured by a flue gas analyzer, vario plus industrial type, MRU Intruments Inc., Germany. The flue gas moisture was measured by the flue gas analyzer. The reaction temperature was controlled and measured with a thermocouple built into the furnace. The solid absorbent and solid products were characterized by an X-ray diffractometer, a SEM and an accessory EDS. Concentrations of sulfur (sulfate and sulfite) and nitrogen (nitrate and nitrite) compounds were determined using barium chromate photometry, and N-(1-naphthyl)-ethylenediamine photometry, respectively. 1.3. Experimental design and calculation method To evaluate the degree of significance of various factors and their synergistic effects, and therefore, to determine the optimal operating conditions for the removal of SO2 and NO by the ferruginous active absorbent, a central composite design (CCD) with 4 factors and 5 levels was employed using response surface methodology (RSM), which required 30 runs. The orthogonal factorial design consisted of 16 orthogonal points, 8 star (or axial) points and 6 center points. The variables and their levels selected for the removal of SO2 and NO were as follows: Ca/(S + N) ranged from 0.8 to 1.6; Fe(VI) dose varied from 0.4% to 1.2% (wt); reaction temperature ranged from 40°C to 80°C; and flue gas humidity varied from 2% to 10% (The detailed experimental design and test results are listed in Table S1 and Table S2 in the Additional Information, respectively.). The other related experimental reaction conditions were: the flow rate of the simulated flue gas was 0.12 m3/hr; the packed mass of the absorbent was 10 g. The inlet concentrations of SO2 and NO varied from 1600 to 2000 mg/m3 and from 400 to 800 mg/m3, respectively. All the experimental results were evaluated with the aid of statistical graphics Design-Expert 8.0.5b software (Version 8, Stat-Ease, Inc., Minneapolis, MN, USA) in the data analysis. The removal efficiencies were calculated by η¼ C i;in −C i;out 100% C i;in ðE1Þ where, Ci,in represents the concentration of NO or SO2 at the inlet of the reactor and Ci,out represents the concentration of NO or SO2 at the outlet of the reactor. An important factor, Ca/(S + N), as one of the important indexes effecting the removal efficiencies of SO2 and NO, is usually used to evaluate the utilization of absorbents and is expressed as shown below, m x 90%=74 Ca=ðS þ NÞ ¼ C SO2 =64 þ 1=2ðC NO =30Þ Q 0:001 t ðE2Þ where, m (g) represents the loading weight of the absorbent, x (wt.%) stands for the weight percentage of industrial lime, C SO2 and CNO (mg/m3) are the concentrations of SO2 and NO, respectively, Q (m3/hr) represents the flow rate, t (hr) is the reaction time, the other parameters, “90%”, “74”, “64”, and “30” are the purity of the industrial lime, the molar mass of Ca(OH)2, the molar mass of SO2 and the molar mass of NO, respectively. Correspondingly, the reciprocal of Ca/(S + N) can be estimated as the absorbent utilization (Yang, 1978). 2. Results and discussion 2.1. Synergistic effects of operating factors The absorption behavior of the Fe(VI) absorbent was investigated in the previous studies (Zhao et al., 2011; Zhang et al., 2012) and the independent influences of various factors on the desulfurization and denitration were analyzed. However, single factor 58 J O U RN A L OF E N V I RO N ME N TA L SC IE N CE S 3 0 (2 0 1 5) 55–6 4 analysis cannot fully reflect the inherent law and synergistic action of operating factors on the removal process. As stated above, the experiments of synchronous abatement of SO2– NOx binary gas mixtures were designed by response surface methodology (RSM) to confirm the synergistic effects of four operating factors (Fe(VI) dose, Ca/(S + N), reaction temperature, and flue gas humidity) on desulfurization and denitration efficiencies. The analysis of experimental results indicated that a strong synergistic effect exists between Fe(VI) dose and reaction temperature for the desulfurization process. In the denitration process, the synergy of Fe(VI) dose and Ca/(S + N) had the most significant impact on the removal of NO, followed by the synergy of Fe(VI) and reaction temperature, and then the synergy of reaction temperature and flue gas humidity. These synergistic effects are not a simple addition of various single factors, but a complex nonlinear relationship. Fig. 2 shows the significant synergistic effects. It can be noticed that both the desulfurization efficiency and the denitration efficiency increased rapidly with the Fe(VI) dose, as shown in Fig. 2a, b and c, which indicate that the Fe(VI) additive supplies the “oxidizing-points” on the a absorbent surface. SO2 and NO from flue gas can be removed and converted into high valence state sulfur and nitrogen by these “oxidizing-points”. Fig. 2a, c shows that reaction temperature acts synergistically with Fe(VI) dose for both desulfurization and denitration. Synchronous desulfurization and denitration is a semidry reaction process between porous solids and gases. Reaction temperature has a great effect on adsorption, absorption and diffusion of SO2 and NO on the surface of the ferruginous active absorbent. The optimal temperature was observed to be 60°C in this experiment. The existence of an optimal temperature can be explained by the following: when the temperature is lower than a critical value, adsorption and absorption are predominant and the efficiencies increase with the rise of reaction temperature. When the reaction temperature is higher than a critical value, desorption of gas molecules on the surface of the absorbent increases and the equilibrium adsorptive capacity of gas decreases. The growth trend of removal efficiencies slows with the rise of temperature, particularly for denitration. This critical value of the reaction temperature could be the optimum, and it will also affect the oxidizing properties of Fe(VI). A b ) VI ( Fe e dos t) ,w (% Fe (V I) d ose (% Ca/(S+N) ,w t) d c Fe ( ose )d VI t) ,w (% Flue gas hu mid ity ( %) em nt tio ac Re ) °C e( tur ra pe Fig. 2 – Synergized effects on the desulfurization and denitration, respectively. (a) Synergy of Fe(VI) dose and reaction temperature vs. desulfurization efficiencies, in which the flow rate was 0.12 m3/hr; the concentration of SO2 was 1600 mg/m3; the flue gas humidity was 6%; and Ca/(S + N) was 1.1, respectively; (b) synergy of Fe(VI) dose and Ca/(S + N) vs. denitration efficiencies, in which the flow rate was 0.12 m3/hr; the concentration of NO was 700 mg/m3; the flue gas humidity was 6%; and the reaction temperature was 60°C, respectively; (c) synergy of Fe(VI) dose and reaction temperature vs. denitration efficiencies, in which the flow rate was 0.12 m3/hr; the concentration of NO was 700 mg/m3; the flue gas humidity was 6%; and Ca/(S + N) was 1.1, respectively; (d) synergy of reaction temperature and flue gas humidity vs. denitration efficiencies in which the flow rate was 0.12 m3/hr; the concentration of NO was 700 mg/m3; the reaction temperature was 60°C; and Ca/(S + N) was 1.1, respectively. 59 J O U RN A L OF E N V I RO N ME N TA L SC IE N CE S 3 0 (2 0 1 5) 5 5–6 4 2.2. Structural characteristics and product analysis The SEM analyses of samples (Fig. 4) show that fly ash is relatively smooth because of its small particle size of tens of 1.0 SO2 NO 0.8 Cout/Cin temperature that is too high will destroy the stability of Fe(VI); conversely, a temperature that is too low is detrimental to the removal reaction. A relatively ideal reaction temperature, 60°C, was suggested for future industrial application. Fig. 2a, b, c shows the synergistic effect of Fe(VI) dose together with reaction temperature and Ca/(S + N) for desulfurization and denitration. All peaks of the efficiencies appeared at Fe(VI) dose 0.8% (wt). Generally speaking, the more oxidizing additive is added, the higher the removal efficiencies obtained, especially when the dose of the oxidizing additive is small. However, when the dose of the oxidizing additive gets larger, the efficiency is enhanced more slowly, which can be explained by the saturation of the “oxidizing points” on the active absorbent (Zhao et al., 2011) when the dose gets larger than an optimal value. Considering the synergy of the various effects, the optimal dose of Fe (VI) was determined as 0.8% (wt.) in this work. Generally, Ca/(S + N) is often considered as the main factor for desulfurization, but subsequent analysis of experimental results revealed that the importance of Ca(S + N) is reflected only in the mono-factorial influence on desulfurization. It is seen that Ca/(S + N) has no significant synergistic relationship with the other factors for desulfurization in this work. However, unlike desulfurization, the interaction of Ca/(S + N) and Fe (VI) dose affects denitration to some extent, as shown in Fig. 2b. The data suggest that oxidation of NO is highly dependent on the synergistic effect of Fe (VI) additive and Ca/(S + N), whereby the oxidative product unites with a Ca-base group to form a calcium salt, resulting in the denitration efficiency being influenced by both Fe(VI) dose and Ca/(S + N); however, SO2 reacts with lime easily, even in the absence of Fe(VI), so there is no significant synergy between Ca(S + N) and Fe(VI) for desulfurization. The optimal ratio of Ca/(S + N) is determined as 1.1; correspondingly, the effective utilization of the absorbent might be estimated as 90.9%. In addition to reaction temperature, the flue gas humidity will affect the removal efficiencies, though to a lesser degree. Fig. 2d shows that the denitration efficiencies increased slightly with the flue gas humidity, especially at the lower reaction temperature; but as the flue gas humidity increased, the degree of increase of the removal efficiencies declined little by little. Due to the co-constraint of flue gas humidity and reaction temperature, the 3D surface related to their synergistic effects on the denitration efficiency was relatively flat. Under the obtained optimal experimental conditions, i.e., the dose of Fe(VI) 0.8% (wt), ratio of Ca/(S + N) 1.1, reaction temperature of 60°C, further parallel experiments were performed, and the time-dependence of the removal rates (Cout/Cin) of SO2–NO binary gas mixtures was obtained, as shown in Fig. 3. The results showed the reaction between Fe(VI) and the simulated flue gas occurred very rapidly. The outlet concentrations of SO2 and NO decreased sharply at the beginning of the removal process, and then reached plateaus successively after nearly 10 min; thereafter, the removal rates (Cout/Cin) remained constant, which indicates the stable removal capability of SO2 and NO by Fe(VI). 0.6 0.4 0.2 0 0 15 30 45 60 75 90 Time (min) Fig. 3 – Variation curves of the removal rates of SO2–NO binary gas mixtures with the reaction time under the initial concentrations of SO2 and NO 2000 and 700 mg/m3, respectively. Reaction conditions: the flow rate was 0.12 m3/hr; reaction temperature maintained 60°C, the dose of Fe(VI) was 0.8% (wt), the ratio of Ca/(S + N) was 1.1. microns. The other four solids are much coarser, indicating larger particle size. The pore structure of the fresh absorbent was observed to be clearer than that of fly ash and industrial lime particle. From Table 1, it can be seen that Si is the main component of fly ash. Due to the pozzolanic reaction (Fan and Brown, 2001), Si compounds in fly ash (Fan et al., 2004) might react with lime and the surface of fly ash was corroded by Ca(OH)2, which is the possible reason that the porous structure of the absorbent appeared. The surface of the spent absorbent is evidently different from that of the fresh one, and the white flake layer covered by porous substances might consist of the removal products, such as calcium sulfate, calcium sulfite, calcium nitrate and calcium nitrite. The averaged energy spectrums of the surface of the fresh ferruginous active absorbent and the spent absorbent are shown in Fig. 5, respectively. The rising levels of sulfur content and the appearance of nitrogen content in the spent absorbent demonstrate that chemical reaction took place in the flue gas and ferruginous active absorbent. In order to ascertain the components in the chemical reaction products, the contents of sulfur and nitrogen species in the raw absorbent and the spent absorbent were analyzed by a chemical analysis method, as presented in Table 2. In the raw absorbent, only trace amounts of sulfate were detected, probably from fly ash and industrial lime. It is important to note that no sulfite, nitrate, or nitrite was detected in the raw absorbent, illustrating that the sulfur and nitrogen components in the spent absorbent were derived from the reaction, not from the raw absorbent. The sulfate product came predominantly from desulfurization. The molar ratio of nitrate/nitrite was found to be 2.06, indicating that there are two components in the denitration products, with the nitrite as the main product. This is similar to results obtained in the previous research in a flue gas circulating fluidized bed (Zhao et al., 2011). 60 J O U RN A L OF E N V I RO N ME N TA L SC IE N CE S 3 0 (2 0 1 5) 55–6 4 a b c d Fig. 4 – Scanning electron microscope images of samples. (a) Fly ash particle; (b) industrial lime particle; (c) the spent absorbent; (d) the fresh ferruginous active absorbent, respectively. 2.3. Reaction mechanism The reaction between SO2–NOx binary gas mixtures and the ferruginous active absorbent in a fixed bed reactor may be a complicated physical adsorption and chemical absorption process, involving many reactions and steps. In terms of desulfurization, the presence of lime makes the removal process of SO2 occur quite easily, CaðOHÞ2 þ SO2 →CaSO3 þ H2 O: ð1Þ Here calcium sulfite is regarded as an unexpected product, because it would diminish the quality of gypsum. The production of calcium sulfite is the result of insufficient oxidation. According to an early FT-IR analysis (Li et al., 2002), NO and SO2 can react with each other, as shown below: efficiency of NO was zero using the fly ash or industrial lime alone. The adsorbed NO on the surface of absorbent might be desorbed or replaced by SO2 compounds, likely because calcium compounds interact more strongly with SO2 than NOx. Even if NO could be adsorbed, it is difficult to convert into higher valence nitrogen. Therefore, the involvement of an oxidant is necessary for denitration. In this work, Fe(VI) as an oxidizing additive was dispersed on the surface of the absorbent, which generated many oxidizing sites. These oxidizing sites convert SO2 and NO into SO3 and NO2 rapidly. The fly ash in the absorbent might play a catalytic role in NO oxidation. fly ash NO þ FeðVIÞ → NO2 þ FeðIIIÞ fly ash ð4Þ SO2 þ 2NO→N2 O þ SO3 ð2Þ SO2 þ FeðVIÞ → SO3 þ FeðIIIÞ: 2SO3 þ NO→ðSO3 Þ2 NO: ð3Þ The formed NO2 can react with SO2 or undergo a chain reaction with itself (Li et al., 2002). However, the reaction rate of Reaction (2) would be too slow to produce substantial amounts of high valence sulfur (Armitage and Cullis, 1971). In the presence of lime, Reaction (1) is more likely to occur than Reaction (2). Thus, a strong oxidant is needed to promote the thorough removal of SO2, but this is not the case for denitration. It was found in this experiment that the removal ð5Þ NO2 þ 2SO2 →ðSO3 Þ2 NO ð6Þ 2NO2 →N2 O4 ð7Þ N2 O4 þ 2NO→2N2 O3 ð8Þ J O U RN A L OF E N V I RO N ME N TA L SC IE N CE S 3 0 (2 0 1 5) 5 5–6 4 61 keV Fig. 5 – Energy dispersive spectrums of the ferruginous active absorbent (a) and the spent absorbent (b). These gaseous products could be further adsorbed physically on the surface of the ferruginous active absorbent. When the water vapor in the flue gas was diffused into the surface of absorbent, NO2, N2O4, SO2, and SO3 were dissolved into the water film, 3NO2 þ H2 O→2HNO3 þ NO ð9Þ NO2 þ NO þ H2 O→2HNO2 ð10Þ N2 O4 þ H2 O→HNO3 þ HNO2 ð11Þ HNO2 þ FeðVIÞ→HNO3 þ FeðIIIÞ ð12Þ SO2 þ H2 O→H2 SO3 ð13Þ H2 SO3 þ FeðVIÞ→H2 SO4 þ FeðIIIÞ ð14Þ SO3 þ H2 O→H2 SO4 : ð15Þ CaðOHÞ2 þ H2 SO4 →CaSO4 þ 2H2 O ð17Þ CaðOHÞ2 þ 2HNO2 →CaðNO2 Þ2 þ 2H2 O ð18Þ CaðOHÞ2 þ 2HNO3 →CaðNO3 Þ2 þ 2H2 O ð19Þ CaSO3 þ FeðVIÞ þ CaðNO2 Þ2 →CaSO4 þ CaðNO3 Þ2 þ FeðIIIÞ: ð20Þ The above EDS and chemical analyses demonstrate that the main products of the desulfurization and denitration are calcium sulfate and calcium nitrate, with trace amounts of calcium sulfite and calcium nitrite. On that evidence, we find that Reactions (5), (14) and (17) are the key steps of desulfurization, while Reactions (4), (10), (11) and (19) are the main processes of denitration. In addition, according to the analysis by XRD in the previous work (Zhao et al., 2011), the reduction product of the Fe(VI) additive is calcium ferrite. Therefore, the overall reaction can be expressed as below, fly ash 3SO2 þ 2K2 FeO4 þ 3CaðOHÞ2 → CaSO4 þ Ca2 Fe2 O5 þ 2K2 SO4 þ 3H2 O: ð21Þ These higher valence sulfur and nitrogen oxides react with Ca(OH)2 in the absorbent and form calcium salts, CaðOHÞ2 þ H2 SO3 →CaSO3 þ 2H2 O ð16Þ Under the condition of lack of oxygen, fly ash 6NO þ 2K2 FeO4 þ 3CaðOHÞ2 → CaðNO2 Þ2 þ Ca2 Fe2 O5 þ 4KNO2 þ 3H2 O ð22Þ In an oxygen-enriched environment, fly ash 6NO þ 2K2 FeO4 þ 3CaðOHÞ2 þ 3O2 → CaðNO3 Þ2 þ Ca2 Fe2 O5 þ 4KNO3 þ 3H2 O: Table 2 – Contents of sulfur and nitrogen species in the fresh absorbent and the spent absorbent. Fresh absorbent (mmol/g) Spent absorbent (mmol/g) [SO2− 4 ] [SO2− 3 ] [NO−2] [NO−3] 0.001 0.986 0 0.014 0 0.289 0 0.596 ð23Þ But in practice, there is 6%–8% oxygen in the flue gas, so the above denitration processes, Reactions (22) and (23), might occur concurrently. The product analysis in Table 2 shows that the denitration products are both calcium nitrate and calcium nitrite, 62 J O U RN A L OF E N V I RO N ME N TA L SC IE N CE S 3 0 (2 0 1 5) 55–6 4 but mainly in the form of calcium nitrate. Accordingly, Reaction (23) was inferred as the main reaction. 2.4. Mass balance The fresh and spent samples of ferruginous active absorbent were quantitatively analyzed to examine how much of the absorbent was un-reacted. The formation of sulfur and nitrogen species as inferred above in Reactions (21)–(23) was confirmed by the above mentioned chemical analyses. The comparison between the measured and calculated values of calcium sulfate is presented in Fig. 6a. The experimental data were found to be in good agreement with the calculated values. The mass balance for ferruginous active absorbent was determined as shown in Fig. 6b. Both figures demonstrated a fair mass balance. 2.5. Thermodynamics Whether a chemical reaction is spontaneous or not is associated with the standard-state Gibbs free energy, Δr GΘm ¼ X B γB Δ f GΘm ðB; βÞ ðE3Þ where, ΔγGΘ m (kJ/mol) is the standard-state Gibbs free energy; Δf GΘ m (kJ/mol) is the free energy of the reactant or product; and γB is the stoichiometric ratio in the reaction. According to thermodynamics, the standard-state Gibbs free energies of the main overall reactions (Reactions (21) and (23)) were determined to be −575.57 and − 246.66 kJ/mol, respectively, which means that the desulfurization and denitration using the ferruginous active absorbent are exergonic and spontaneous. The equilibrium constants can be further obtained by ΔGΘ K ¼ e− RT ðE4Þ where, K is the equilibrium constant; T (K) is the reaction temperature; and R (R = 8.314 J/(K·mol)) is the universal gas constant. Under standard conditions (298 K), the equilibrium constants of Reactions (21) and (23) are calculated as 7.8E+100 and 1.73E+43, respectively. Such a large value of the equilibrium constants demonstrates that the desulfurization and denitration using the ferruginous active absorbent are positive and thorough. At a different temperature, the partial pressures are calculated as ΔGΘ þ RT ln J p ¼ 0 ðE5Þ Calculated calcium sulfate (g) 25 Jp ¼ Experimental data Calculated values 20 10 5 a 0 5 10 15 20 25 Measured mass of Calcium sulfate (g) Output exhaut gas plus accumulation and comsumption of absorbent (g/m3) ðE6Þ where, pc and pa are the partial pressures of products and reactants, respectively; Jp is the reaction quotient. The partial pressures of the solid products in the system can be disregarded (ln( pc) = 0). Substituting the values of standard-state Gibbs free energy (−575.57 kJ/mol for desulfurization and −246.66 kJ/mol for denitration), universal gas constant (8.314 J/(K·mol)) into Eqs. (E5) and (E6), the partial pressures of SO2–NOx binary gases can be obtained as shown in Eqs. (E7) and (E8), respectively, without considering the impact of oxygen. 15 0 pc Pa 23:1 103 T ðE7Þ 4:94 103 T ðE8Þ ln pSO2 ¼ − 50 Input Output+accumulation+consumption ln pNO ¼ − 40 Thus, the partial pressures of SO2–NOx binary gases are summarized in Table 3 at the five adopted temperatures. It can be seen that the equilibrium partial pressures of SO2 and NO rise with the reaction temperature, for which the increase is particularly marked in desulfurization, with slight increases in denitration. Such theoretical results are consistent with the above experiments. 30 20 10 b 0 0 10 20 30 40 3. Conclusions 50 Input SO2-NO binary gas and fresh absorbent (g/m3) Fig. 6 – (a) Comparison curves between measured and calculated calcium sulfate; (b) mass balance of input total amount of SO2–NO binary gas and loading fresh absorbent, and output exhaust gas plus accumulation and consumption of absorbent. A novel ferruginous active absorbent was fabricated in this work. The addition of Fe(VI) supplies oxidizing sites in the absorbent, which can convert NO and SO2 into higher valence nitrogen and sulfur compounds synchronously. The porous fly ash might play an adsorptive and catalytic role during the Fe(VI) oxidative removal of SO2 and NO. After the removal reaction, most of the 63 J O U RN A L OF E N V I RO N ME N TA L SC IE N CE S 3 0 (2 0 1 5) 5 5–6 4 Table 3 – Partial pressures of SO2-NOx binary gases. Temperature (K) pSO2 (MPa) pNO (MPa) 323 328 333 338 343 9.383E −32 2.247E −7 2.788E−31 2.837E−7 8.019E−31 3.558E−7 2.235E−30 4.432E−7 6.047E−30 5.486E−7 solid products covered the surface of the absorbent in the form of a “white flake layer”. Experiments of synchronous desulfurization and denitration were carried out using the prepared ferruginous active absorbent in a fixed bed reactor. The study of synergistic effects of operating factors found that strong synergy exists between Fe(VI) dose and reaction temperature for the desulfurization; while in the denitration process, the synergy of Fe(VI) dose and Ca/(S + N) has the most significant impact on the removal of NO, followed by the synergy of Fe(VI) and reaction temperature, and then the synergy of reaction temperature and flue gas humidity. The appearance of nitrogen and the increased level of sulfur content in the energy spectrum of the spent absorbent indicates that the removal reactions are mainly chemical absorption between SO2, NO and the ferruginous active absorbent, in other words, NO and SO2 were oxidized first and then absorbed chemically. The stepwise reaction paths were analyzed. Calcium sulfate and calcium nitrate were the main removal products of desulfurization and denitration, respectively. Acknowledgments The research was supported by the National Natural Science Foundation of China (Nos. 51308212, 10974053), the National Key Technology R&D Program (No. 2011BAI02B03), the Fundamental Research Funds for the Central Universities (No. 13ZD18), and the State Scholarship Fund (No. 201206735009). 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