Role of acid gases in Hg0 removal from flue gas over a novel cobalt-containing biochar prepared from harvested cobalt-enriched phytoremediation plant

doi:10.1016/j.fuproc.2020.106478

Fuel Processing Technology

Volume 207, October 2020, 106478

https://doi.org/10.1016/j.fuproc.2020.106478 Get rights and content

Highlights

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The biochar prepared from cobalt-enriched phytoremediation plant was used for Hg⁰ oxidation removal.
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The Co³⁺/Co²⁺ redox cycle from highly dispersed cobalt oxides in the biochar facilitated Hg⁰ removal.
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SO₂ reacted with active Co³⁺ to generate sulfate, which inhibited Hg⁰ removal.
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NO (HCl) contributed to Hg⁰ removal mainly through the Eley-Rideal mechanism.
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Formation of active nitrogenous (chlorine) species and its reaction with Hg⁰ was hampered by SO₂.

Abstract

A novel biochar containing highly-dispersed cobalt oxides was prepared by one-step pyrolysis using Co-enriched Siberian Iris as the raw material, and it was used for elemental mercury removal from flue gas. The presence of NO or HCl could approximately keep Hg⁰ removal efficiency over 85%, while the presence of SO₂ suppressed Hg⁰ removal. The role of aforementioned acid gases in Hg⁰ removal over such biochar was systematically studied. The surface active oxygen species derived from the reversible conversion between Co³⁺ and Co²⁺ could activate the adsorbed NO (HCl) to produce active nitrogenous (chlorine) species, which functioned as active site for Hg⁰ oxidation removal. The reaction between NO (HCl) and Hg⁰ occurred mainly through the Eley-Rideal mechanism. SO₂ consumed active oxygen and reacted with active Co³⁺ to generate Co²⁺ bonded with SO₄²⁻, resulting in irreversible damage on the biochar. The formation of active nitrogenous (chlorine) species as well as its reaction with Hg⁰ would be hampered by the generation of sulfate. This study can give some basic insights into the application of such heavy metal enriched phytoremediation plants for elemental mercury removal from flue gas.

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 (Hg^p). Hg²⁺ and Hg^p 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.

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Role of acid gases in Hg0 removal from flue gas over a novel cobalt-containing biochar prepared from harvested cobalt-enriched phytoremediation plant

Highlights

Abstract

Introduction

Section snippets

Sample preparation

Physicochemical properties of the biochar

Conclusions

Declaration of competing interest

Acknowledgements

Credit author statement

Fuel

Fuel Process. Technol.

Int. J. Coal Geol.

J. Hazard. Mater.

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Chem. Eng. J.

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Chemosphere

Role of acid gases in Hg⁰ removal from flue gas over a novel cobalt-containing biochar prepared from harvested cobalt-enriched phytoremediation plant