Role of acid gases in Hg0 removal from flue gas over a novel cobalt-containing biochar prepared from harvested cobalt-enriched phytoremediation plant
Introduction
Mercury (Hg), as the highly toxic element, can be transported globally in the atmosphere and transferred in the food chain, posing a serious threat to the ecosystem and human health [[1], [2], [3]]. Mercury pollution has become one of the important environmental issues and attracted worldwide attentions. In order to control the mercury emissions, the global treaty “Minamata Convention on Mercury” has come into force in August 16th, 2017. Coal-fired power plants are known as the largest anthropogenic source of mercury emission [2,4]. In coal combustion flue gas, mercury is mainly present in three forms: elemental mercury (Hg0), oxidized mercury (Hg2+) and particulate-bound mercury (Hgp). Hg2+ and Hgp can be effectively captured by wet desulfurization and dust control devices, respectively [[5], [6], [7], [8]]. As the dominant mercury species in flue gas, high-volatile and water-insoluble Hg0 is difficult to be removed by the aforementioned devices [[9], [10], [11]]. Thus, cost-effective technologies to remove Hg0 from flue gas are urgently needed.
In recent years, biochar has been extensively researched and used for mercury removal from flue gas, due to its high surface area, low costs and extensive sources [12,13]. To further improve the mercury removal performance, biochar is usually modified by halogens [[14], [15], [16]], sulfur [[17], [18], [19], [20]] and metal oxides [13,21,22] to increase the surface activity sites for mercury adsorption and oxidation. Among them, metal oxides modification (e.g. Co, Cu and Mn) can give biochar the ability to catalytically oxidize Hg0 into easily captured Hg2+ and thus achieving excellent mercury removal performance. However, metal oxides modification also means an increase in the cost of catalysts/sorbents, which would detract from their practical application values. On the other hand, phytoremediation has been extensively applied for ecological restoration of water and soils contaminated by heavy metal [23]. The resultant heavy metal enriched plants after harvesting remains to be treated and utilized properly. Siberian Iris is a plant that has been widely used for phytoremediation of Co-contaminated water and soils. In our work, by one-step pyrolysis, the Co-enriched Siberian Iris was transformed into cobalt-containing biochar, which showed good performance on mercury removal. By this promising method, the performance of biochar on mercury removal could be improved without need of additional metal oxides modification and the Co-enriched phytoremediation plant could be sufficiently and properly utilized. This work might also shed light on the utilization of other heavy metal (V, Cr, Cu, etc.) enriched phytoremediation plants for mercury removal from flue gas, since the species such as V2O5, Cr2O3 and CuO are generally considered to be active for Hg0 oxidation removal [[24], [25], [26]].
Coal-combustion flue gas contains typical acid gas components including SO2, NO and HCl, which have significant effects on Hg0 removal efficiency. Inhibitory influence of SO2 on Hg0 removal has been reported in previous studies. The inhibitory influence might come from the competitive adsorption between SO2 and Hg0 or the deactivation of catalyst by forming surface sulfate species [[27], [28], [29], [30]]. In contrast, some other research works revealed that SO2 could facilitate Hg0 removal by generating SO3 and sulfated sites over catalyst surface, which served as the new active sites for Hg0 adsorption and oxidation [[31], [32], [33]]. For the effect of NO, it was found that NO could form active sites (NO2, NO+) to enhance Hg0 removal [[34], [35], [36]], while another study showed that the generated nonactive species (NO2−, NO3−) might cover the active sites, thus obstructing Hg0 removal [37]. HCl can greatly enhance Hg0 removal by the reaction between Hg0 and formed active chlorine species. Different mechanisms have been proposed to interpret the heterogeneous Hg0 oxidation by HCl over various catalysts such as Deacon reaction [38], Eley-Rideal [39,40] and Langmuir-Hinshelwood [41,42] mechanisms. Although extensive researches regarding Hg0 removal under different flue gas compositions were carried out, the effect and mechanism of acid gases on Hg0 removal are still controversial and not well understood, especially for the Hg0 removal over cobalt-containing biochar prepared from cobalt-enriched Siberian Iris.
The above situation drives us to systematically study the effects of acid gases (SO2, NO and HCl) on Hg0 removal over such novel cobalt-containing biochar. The role of acid gases in Hg0 removal was investigated via the results of experiments and characterizations. The probable mechanism of acid gas influence was deduced and can provide some basic insights into the application of such material for mercury removal from flue gas.
Section snippets
Sample preparation
Cobalt-enriched Siberian Iris was obtained by the means of nutritional cultivation. The detailed culture method was described in the Supplementary Material. By one-step pyrolysis process, the Co-enriched Siberian Iris was transformed into cobalt-containing biochar. Briefly, the acquired plant biomass including stems and rhizomes after harvesting was dried at 105 °C and then crushed and sieved into a 50 mesh size. Subsequently, the obtained sample was pyrolyzed at 650 °C (with a ramp rate of
Physicochemical properties of the biochar
The chemical component of the cobalt-containing biochar was examined by ICP-MS and shown in Table 1. Cobalt is generally regarded as the effective active component for mercury oxidation removal [43,44]. The content of cobalt in the biochar was 10.65%, much higher than that of other metal elements, suggestive of its dominant role in mercury removal. The surface element composition was further measured by EDS analysis and the results are displayed in Fig. 1(a) and Table 1. It could be found that
Conclusions
In this work, the Co-enriched Siberian Iris was transformed into the biochar containing highly-dispersed cobalt oxides by one-step pyrolysis process, and the obtained biochar was used for elemental mercury removal from flue gas. It was found that NO and HCl both facilitated Hg0 removal over the biochar while SO2 suppressed it. The role of acid gases (SO2, NO and HCl) in Hg0 removal over such biochar was investigated via the results of experiments and characterizations. The adsorbed NO (HCl)
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was financially supported by the Natural Science Foundation of Hubei Province (2019CFB324), the Research Startup Foundation for Introducing Talent of ZUEL (31721911305) and the Fundamental Research Funds for the Central Universities (2722019JCT036).
Credit author statement
Honghu Li: Writing - original draft, Conceptualization, Data curation. Jingdong Zhang: Supervision, Writing - review & editing. Yanxiao Cao: Investigation, Writing - review & editing. Chaoyang Liu: Software. Fei Li: Formal analysis. Yongwei Song: Methodology. Jiangjun Hu: Writing - review & editing. Yuan Wang: Conceptualization, Resources.
References (64)
- et al.
Investigation of subbituminous coal and lignite combustion processes in terms of mercury and arsenic removal
Fuel
(2019) - et al.
Status review of mercury control options for coal fired power plants
Fuel Process. Technol.
(2003) - et al.
Mercury adsorption and oxidation in coal combustion and gasification processes
Int. J. Coal Geol.
(2012) - et al.
Mercury emissions from coal combustion: modeling and comparison of Hg capture in a fabric filter versus an electrostatic precipitator
J. Hazard. Mater.
(2008) - et al.
Recent advances in mercury removal technology at the National Energy Technology Laboratory
Fuel Process. Technol.
(2004) - et al.
Removal of gaseous Hg0 using novel seaweed biomass-based activated carbon
Chem. Eng. J.
(2019) - et al.
Removal of elemental mercury from flue gas using wheat straw chars modified by Mn-Ce mixed oxides with ultrasonic-assisted impregnation
Chem. Eng. J.
(2017) - et al.
Efficient removal of elemental mercury by magnetic chlorinated biochars derived from co-pyrolysis of Fe(NO3)3-laden wood and polyvinyl chloride waste
Fuel
(2019) - et al.
Use of a non-thermal plasma technique to increase the number of chlorine active sites on biochar for improved mercury removal
Chem. Eng. J.
(2018) - et al.
Vapor-phase elemental mercury adsorption by activated carbon co-impregnated with sulfur and chlorine
Chem. Eng. J.
(2017)
Enhanced mercury removal by transplanting sulfur-containing functional groups to biochar through plasma
Fuel
Mercury removal using metal sulfide porous carbon complex
Process. Saf. Environ.
ZnS/AC sorbent derived from the high sulfur petroleum coke for mercury removal
Fuel Process. Technol.
Superior performance and resistance to SO2 and H2O over CoOX-modifed MnOX/biomass activated carbons for simultaneous Hg0 and NO removal
Chem. Eng. J.
Removal of elemental mercury from flue gas using CuOX and CeO2 modified rice straw chars enhanced by ultrasound
Fuel Process. Technol.
Cadmium removal capability and growth characteristics of Iris sibirica in subsurface vertical flow constructed wetlands
Ecol. Eng.
Mercury oxidation by hydrochloric acid over TiO2 supported metal oxide catalysts in coal combustion flue gas
Fuel Process. Technol.
Significance of Fe2O3 modified SCR catalyst for gas-phase elemental mercury oxidation in coal-fired flue gas
Fuel Process. Technol.
Research of mercury removal from sintering flue gas of iron and steel by the open metal site of Mil-101(Cr)
J. Hazard. Mater.
Role of flue gas components in Hg0 oxidation over La0.8Ce0.2MnO3 perovskite catalyst in coal combustion flue gas
Chem. Eng. J.
Exploration of reaction mechanism between acid gases and elemental mercury on the CeO2-WO3/TiO2 catalyst via in situ DRIFTS
Fuel
Effects of flue gas components on removal of elemental mercury over Ce-MnOX/Ti-PILCs
J. Hazard. Mater.
The influence of flue gas components and activated carbon injection on mercury capture of municipal solid waste incineration in China
Chem. Eng. J.
Reaction mechanism for the influence of SO2 on Hg0 adsorption and oxidation with Ce0.1-Zr-MnO2
Fuel
Impact of SO2 on elemental mercury oxidation over CeO2-TiO2 catalyst
Chem. Eng. J.
NH3 inhibits mercury oxidation over low-temperature MnOX/TiO2 SCR catalyst
Fuel Process. Technol.
Mercury capture by native fly ash carbons in coal-fired power plants
Prog. Energy Combust. Sci.
CuO/TiO2 catalysts for gas-phase Hg0 catalytic oxidation
Chem. Eng. J.
Promotional effect of Mo addition on CoOX/Ti-Ce catalyst for oxidation removal of elemental mercury in flue gas
Fuel
Impact of HCl and O2 on removal of elemental mercury by heat-treated activated carbon: Integrated X-ray analysis
Fuel Process. Technol.
Superior activity of MnOX-CeO2/TiO2 catalyst for catalytic oxidation of elemental mercury at low flue gas temperatures
Appl. Catal. B Environ.
Catalytic oxidation of Hg0 by MnOX-CeO2/γ-Al2O3 catalyst at low temperatures
Chemosphere
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