Novel carbon-based sorbents for elemental mercury removal from gas streams: A review

Journal Pre-proofs Review Novel Carbon-based Sorbents for Elemental Mercury Removal from Gas Streams: A Review Dongjing Liu, Chaoen Li, Jiang Wu, Yangxian Liu PII: DOI: Reference:

S1385-8947(19)32929-8 https://doi.org/10.1016/j.cej.2019.123514 CEJ 123514

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Chemical Engineering Journal

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Please cite this article as: D. Liu, C. Li, J. Wu, Y. Liu, Novel Carbon-based Sorbents for Elemental Mercury Removal from Gas Streams: A Review, Chemical Engineering Journal (2019), doi: https://doi.org/10.1016/j.cej.2019.123514

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Novel Carbon-based Sorbents for Elemental Mercury Removal from

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Gas Streams: A Review

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Dongjing Liu 1, Chaoen Li 2, Jiang Wu 3, Yangxian Liu 1*

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1 School

of Energy and Power Engineering, Jiangsu University, Zhenjiang 212013, China

2 College 3 College

of Mechanical Engineering, Tongji University, Shanghai 200092, China

of Energy and Mechanical Engineering, Shanghai University of Electric Power, Shanghai 200090, China

8 9 10

*Contact information: Tel.: +86-0511-88780211; Fax: +86-0511-88780211;

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E-mail: [email protected] (X.Y. Liu)

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Abstract: Mercury emission, derived from natural gas exploitation, metal sintering, coal

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gasification and combustion, gold mining, etc., has posed great threaten to the environment and

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human beings due to its volatility, mobility, toxicity, and bioaccumulation in ecosystem and food

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chains. Adsorption using carbon materials is regarded as one of the most promising techniques for

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mercury emission control due to its simple equipment, convenient operation, high removing

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efficacy, less secondary pollution, etc. This review comments the new research progress of

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elemental mercury capture by novel carbon-based sorbents in recent five years, particularly

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emerging carbon materials, such as bio-chars, graphene and graphene oxide, carbon nanotubes and

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nanofibers, carbon spheres, carbon aerogels, metal-organic frameworks, and graphitic carbon

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nitrides. The mercury removal performances and reaction mechanisms of diverse carbon-based

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sorbents along with their merits and drawbacks are fully discussed, which aims to strengthen the

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understanding of this emerging research topic and outline future research directions for the

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development of sustainable, recyclable, and cost-effective carbon-based mercury sorbents.

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Keywords: Elemental mercury; graphitic carbon nitride; metal-organic framework; graphene;

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carbon sphere; bio-char

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1. Introduction

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Mercury, as a trace heavy metal element, is one of the primary lethal pollutants due to its

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toxicity, mobility and bioaccumulation in ecosystem and food chains [1-3]. Inorganic mercury can

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be methylated biotically to its most toxic form, dimethyl mercury which can cause nervous system

32

disorder, kidney and liver damage, and impair childhood development [4-5]. Mercury is naturally

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abundant in coal and heavy metal-rich geologic deposits; however, it has been largely liberated into

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the atmosphere by human productive activities, such as natural gas exploitation, coal gasification

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and combustion, metal sintering, gold mining and so forth [6-8]. Hence, abatement of

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anthropogenic mercury emissions from gas streams, such as natural gas, coal-fired flue gas, syngas,

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sintering gas, etc., has gained increasingly environmental and regulatory concern. The released

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mercury primarily existed in three forms, i.e. elemental mercury (Hg0), oxidized mercury (Hg2+)

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and particulate bond mercury (Hgp) [9-11]. Hg2+ species is easily absorbed by amine solution. Hgp

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can be readily captured by baghouse, electrostatic precipitator or fabric filter. However, Hg0 is

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hardly removed because of its high volatility and insolubility [12-14]. Therefore, efficient mercury

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control technologies are urgently needed. Currently, there are a number of technologies available to

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remove Hg0 from gas streams, for instance, wet advanced oxidation [15-17], catalytic oxidation

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[18-20], photochemical oxidation [21-23], and adsorption method [24-26]. Adsorption technology,

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characterized by simple equipment, convenient operation, good removing efficacy, and less

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secondary pollution, appears to be one of the most effective pathway to remove Hg0 from gas

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streams at low temperature. Because of its large surface area, flexible surface chemistry, and variety

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diversity, carbon material is regarded as the most promising sorbent for mercury abatement [27-29].

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The recent advances in Hg0 removal techniques have been summarized by some researchers.

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Liu et al. [30] first comprehensively commented the progress of four types of advanced oxidation

51

technologies for Hg0 removal. The reaction mechanism, kinetics, reactor types and process system

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as well as their effects on Hg0 abatement were evaluated, with insights into the challenges of

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large-scale application. Sowlat et al. [31] systematically discussed the effects of different

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calcinating temperatures, reaction temperatures, and flue gas components on the Hg0 removal

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performances of cerium-modified activated carbons. Xu et al. [32] reviewed the development of

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sorbent modification methods for Hg0 removal. The merits and shortcomings of diverse

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modification routes, such as halide and sulfur modification, acid and alkaline impregnation, noble

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metal and metal oxide decoration, microwave and plasma treatment, and miscellaneous

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modification were fully discussed. Yang et al. [33] highlighted the progress in Hg0 removal by

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gas-solid heterogeneous oxidation method from the perspective of key process parameters, reaction

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mechanisms, and properties of catalysts or traditional sorbents. Chalkidis et al. [34] critically

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commented the conventional and alternative sorbents as well as the processes adopted for mercury

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abatement and remediation during natural gas processing. Liu et al. [35] discussed the advances in

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Hg0 removal by mineral sorbents. The physiochemical features and common modification

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approaches for various mineral materials were briefly summarized. The influencing factors and

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possible reaction mechanisms with respect to Hg0 capture by mineral sorbents were highlighted.

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However, most of these reviews mainly paid attention on conventional activated carbons,

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metallic oxides, and mineral sorbents in perspective of processes and kinetics. Due to the rapid

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development of material synthesis method and nanotechnology, many novel carbon-based polymers

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or composites have been applied for Hg0 removal due to their unique physicochemical properties. In

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this work, the research progress of Hg0 capture by novel carbon-based materials in recent five years,

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especially emerging carbon materials, such as bio-chars, graphene and graphene oxide, carbon

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nanotubes and nanofibers, carbon spheres, metal-organic frameworks, graphitic carbon nitrides, and

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carbon aerogels, are reviewed. The mercury removal performances and reaction mechanisms of

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diverse carbon-based sorbents associated with their merits and drawbacks are fully commented as

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well.

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2. Activated carbon

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Activated carbon is the most widely used mercury sorbent due to the merits of high surface

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area, favorable pore structures, and sufficient surface oxygen functional groups [36]. Raw activated

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carbon usually shows limited Hg0 removal performance, which can be greatly enhanced by halide,

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metal oxide, and sulfide modification [37-39].

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Halides have been extensively employed as modifiers for activated carbon sorbents. The

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efficiency of halogen-modified activated carbon for Hg0 removal increases in the order Cl < Br < I

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[40]. As for metal chlorides, FeCl3 is evidenced as the most effective modifier for activated carbon

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as compared to CuCl2, CeCl3, and MnCl2 [41]. The reaction mechanism of metal chlorides and

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elemental mercury has been widely studied. Chen et al. [42] claimed that the cupric and chloride in

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CuCl2-impregnated activated carbon had a cooperative effect, which significantly enhanced the Hg0

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removal performance. Tong et al. [43] proposed that I2 was the major active sites of

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KI-impregnated activated carbon for mercury removal. A high efficient I-modified activated carbon

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was prepared via iodine vapor deposition [44]. The Hg0 capture ability of I-modified activated

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carbon was superior to the one modified by bromide deposition. Besides, ZnCl2 in-situ activation

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could also greatly improve the Hg0 removal efficiency of the activated carbon owing to the

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production of ZnO [45].

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However, halide-modified activated carbon often suffers from low thermal stability and

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corrosion effect to the duct because of the release of halides at elevated temperatures. Therefore,

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precious metals and transition-metal oxides had been regarded as alternative modifying reagents for

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activated carbons due to their high thermal stability and excellent redox property. Three types of Pd

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species were identified in Pd-loaded activated carbon, i.e. Pd0, Pd2+ and Pd2H, among which Pd0

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was recognized as the dominant active species for Hg0 removal [46]. Presence of H2 promoted Hg0

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adsorption attributed to PdO reduction. However, existence of H2 and O2 disfavored mercury

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removal probably owing to the formation of inactive Pd(OH)2 and PdO, respectively. The presence

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of PdO would decrease Pd0 content; in addition, PdO was easily sulfided by H2S. Han et al. [47]

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pointed out that Fe addition could enhance the Hg0 removal performance of Pd-loaded activated

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carbon. Fe2O3 could compete with PdO to react with H2S, thus protecting sorbent from H2S

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poisoning. Au-loaded activated carbon also displayed good ability towards Hg0 adsorption due to

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the formation of Hg-Au amalgam [48].

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Though precious metal-modified activated carbon had good Hg0 removal performance, the

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high capital costs and scarce sources of noble metals still restrained their large-scale application.

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Thus, inexpensive transition-metal oxides have attracted increasing research concern recently. A

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sequence of CeO2-MnOx modified activated carbons were used to remove Hg0 at low temperature

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[49]. The existence of the Ce4+/Ce3+ redox couple and multiple valence states of Mn ions enhanced

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the Hg0 oxidation ability of the activated carbon. CeO2-V2O5 modified activated carbons were also

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applied for Hg0 capture at 100-200 °C [50]. V2O5 and CeO2 displayed the optimal cooperative

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effect towards Hg0 oxidation with 1 wt% V2O5 and 8 wt% CeO2 in the sorbent. However, most

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transition-metal oxides showed lower SO2 resistance, especially manganese oxides [51].

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Recently, metal sulfides are considered as effective modifying reagents for activated carbons

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due to their higher thermal stability and stronger affinities towards elemental mercury compared to

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halides, precious metals, and transition-metal oxides. Yang et al. [52] found that copper polysulfide

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modified activated carbon displayed better Hg0 removal performance and higher SO2 resistance

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than those of copper or sulfur modified ones. The process of Hg0 adsorption on copper polysulfide

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modified activated carbon is presented in Fig. 1. First, Hg0 was physically adsorbed on sorbent

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surface. Then, the adsorbed Hg0 bonded with the active sulfur species in copper polysulfide to

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generate stable HgS.

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Fig. 1 The process of Hg0 capture from flue gas by copper polysulfide modified activated carbon

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(AC).

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A ZnS-modified activated carbon was attained by in-situ calcination of high sulfur-containing

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petroleum coke and zinc nitrate under nitrogen atmosphere [53]. It sustained Hg0 removal efficiency 6

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>84% within 360 min at or below 120 °C. But Hg0 removal was inhibited by rising reaction

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temperature. H2S greatly enhanced Hg0 capture performance of the ZnS-modified activated carbon,

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while H2S content had nearly no influence on Hg0 removal efficiency. Shi et al. [54] employed

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non-thermal plasma to modify activated carbons in the presence of H2S to enhance Hg0 removal

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performance. Sulfur content and Hg0 removal efficiency augmented with incremental H2S content.

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The enhancement of Hg0 capture performance was due to the chemisorption rather than

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physisorption. Hg0 was removed by binding with the sulfur species on the surface of activated

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carbon, which yielded HgS.

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3. Activated coke

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Activated coke is a kind of porous carbon-based sorbent, which is not fully activated or

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retorted as compared to activated carbon [55]. Activated coke owns higher mechanical strength,

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better regenerability, and lower capital cost than activated carbons, making it a promising candidate

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for mercury removal. It can also resist abrasion and crushing during circulation and handling

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processes because of its high mechanical strength [56].

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MnOx-CeO2 modified activated coke showed good Hg0 oxidation ability, which was plausibly

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ascribed to the lattice oxygen and chemisorbed oxygen or hydroxyl groups on sorbent surface [57].

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The CoOx-CeO2 modified activated coke also performed well towards Hg0 removal, which was

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attributed to the synergetic effect between CoOx and CeO2 [58]. Zhao et al. [59] employed

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copper-modified activated coke for Hg0 removal. Copper nitrates would decompose into CuO at

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lower calcinating temperatures, and partially into Cu2O at higher temperatures. Furthermore, copper

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decoration broadened the pore distribution of the activated coke, benefiting Hg0 removal. The

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adsorbed mercury species on sorbent surface were identified as Hg0 and HgO, testifying that the

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mechanism of Hg0 removal by Cu-modified activated coke was a combination of adsorption and

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oxidation.

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Chen et al. [60] used CoOx-CeO2 modified activated coke to simultaneously remove Hg0 and

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HCHO. The good texture property, high dispersion of CoOx and strong redox ability contributed to

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its good removal performances of Hg0 and HCHO. Furthermore, the interaction between CoOx and

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CeO2 facilitated the mobility of the active oxygen species, significantly promoting the removal

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performances of Hg0 and HCHO. Liu et al. [61] reported that MoO3-CeO2 loaded cylindrical

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activated coke with Mo/Ce molar ratio of 1:2 performed the best towards Hg0 removal with good

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stability and remarkable resistance to SO2 and H2O, which was attributed to the synergy effect of

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Mo6+ and Ce3+ ions. The possible reason for its high SO2 tolerance was as follows: (i) the

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preferential interaction between sulfate and MoO3 prevented ceria from SO2 poisoning; (ii) the

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presence of SO2 resulted in the generation of weakly-HgO and HgSO4, appreciably promoting Hg0

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removal. Mn-Ni co-loaded activated coke also displayed good Hg0 removal performance with

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Mn/Ni ratio of 6:0.5. [62].

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Semi-coke, as potentially inexpensive mercury sorbent, owns high mechanical strength,

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well-developed pore structures, and abundant surface functional groups [63]. Fe-Ce modified lignite

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semi-coke showed good Hg0 oxidation ability with the optimal Fe/Ce molar ratio of 0.4:0.2. H2S

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reacting with Fe-Ce oxides yielded active sulfur, benefiting Hg0 removal via generating HgS [64].

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However, the presence of exorbitant H2S restrained Hg0 removal, because excessive reaction of H2S

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with Fe-Ce oxides would consume active metal oxides.

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Zhang et al [65, 66] studied the effects of water vapor and fly ash on the Hg0 removal

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performance of CeO2-modified semi-coke via experimental and theoretical calculation method. Hg0

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removal efficiency dropped by 30% in the presence of 10% water vapor, which was probably owing

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to the formation of Ce-OH groups and the depletion of lattice oxygen. The addition of 0.5 g fly ash

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showed little influence on mercury removal performance. However, the Hg0 removal efficiency

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reduced by only 15% in coexistence of water vapor and fly ash. This indicated that the production

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of Fe-OH groups stemmed from the interaction of H2O and Fe2O3 in the fly ash reduced the

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inhibitive effect of water steam. The effects of water vapor and fly ash on Hg0 removal over

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CeO2-modified semi-coke is presented in Fig. 2. Fly ash slowed down the suppressive impact of

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H2O on Hg0 removal over CeO2-modified semi-coke. Because some lattice oxygen on Fe2O3

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surfaces was converted to highly active Fe-OH groups. The content of Fe-OH group on γ-Fe2O3

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surface augmented faster than that of α-Fe2O3 after steam treatment, contributing to the higher Hg0

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removal performance of γ-Fe2O3 than α-Fe2O3.

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Fig. 2 The promotive mechanism of H2O on Hg0 removal over Fe2O3.

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Petroleum coke is a waste by-product of petroleum refining process with large amounts readily

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available worldwide. Due to its high intrinsic sulfur content, petroleum coke is regarded as an

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attractive raw material for developing carbon-based mercury sorbent. Brominated petroleum cokes

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had been recently employed for Hg0 removal [67, 68]. The bromine loaded on petroleum coke was

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stable up to 200 °C. The Br-modified petroleum coke displayed excellent Hg0 removal efficiency at 9

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100 and 200 °C. The intrinsic sulfur, primarily in the form of thiophene and organic sulfide, could

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raise bromine depositing and increased mercury capture capacity.

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4. Bio-char

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Biomass, as a renewable resource, has been extensively applied in preparation of

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carbon-based sorbents for catalytic and adsorption reactions due to its ample source, sustainability,

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economy, and environmental benign. Bio-char, as a new kind of mercury sorbent, can be easily

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attained via thermal pyrolysis of diverse terrestrial or marine biomass and biomass wastes, such as

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woody and fibrous plants [69, 70], agricultural waste [71], nut waste [72], seaweed [73] and so

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forth. After devolatilization and reconstruction of carbohydrate polymers, the resultant bio-chars

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with loosely developed pore structures and highly active surface oxygen functional groups possess

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excellent prospect as mercury sorbents [74].

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4.1 Agricultural waste-derived bio-char

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Lots of agricultural waste, such as rice straw, wheat straw, maize straw etc., can be used for

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bio-char preparation. Cu-Ce modified bio-chars derived from rice straw demonstrated good Hg0

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removal performance [76]. The optimal Cu/Ce molar ratio, calcination temperature, and reaction

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temperature were 1:5, 260 °C, and 150 °C, correspondingly. Yang et al. [78] synthesized a

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magnetic Co-Fe impregnated porous carbon stemmed from rice straw by microwave and steam

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activation [78]. Ultrasound-assisted impregnation improved the dispersion of Co3O4 and Fe3O4 on

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sorbent surface. While the activation of microwave and steam greatly improved the pore structures.

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The surface chemisorbed oxygen and the lattice oxygen stemmed from Co3O4 and Fe3O4 involved

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in Hg0 removal process. Besides, Co-Fe modified porous carbon showed good regeneration

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performance. The plausible mechanism of Hg0 capture on Co-Fe loaded porous carbon is illustrated

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in Fig. 3. The adsorbed Hg0 was oxidized into Hg2+ by high-valence Co3+ and Fe3+ ions. The

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low-valence Co2+ and Fe2+ ions could be replenished by the oxygen from gas stream.

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Fig. 3 The mechanism of Hg0 capture by Co-Fe doped porous carbon.

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K2FeO4-modified bio-chars derived from wheat straw were developed for Hg0 removal [79].

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K2FeO4 modification improved the pore structures and introduced new active sites, greatly

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promoting Hg0 removal performance. Hg0 removal efficiency reduced with rising Hg0 inlet content.

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NO facilitated Hg0 removal, because it could oxidize Hg0 into Hg(NO3)2. SO2 greatly decreased Hg0

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removal efficiency because of the competitive adsorption between SO2 and Hg0. Yang et al. [80]

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used a wheat straw derived bio-char modified with Mn-Ce oxides to remove Hg0 at low

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temperature. NO and O2 improved Hg0 removal performance. Low content of steam and SO2

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enhanced Hg0 removal, but high content of steam and SO2 restrained Hg0 removal. Magnetic Fe-Cu

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doped bio-chars stemmed from wheat straw were employed for capturing Hg0 from flue gas [81].

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The Fe-Cu active species, chemisorbed oxygen, and lattice oxygen on sorbent surface were

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profitable for Hg0 removal.

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Corn stalk derived bio-chars were treated with dielectric barrier discharge plasma to promote

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its Hg0 adsorption capacity [82]. After plasma treatment, the surface area and pore volume reduced

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subtly. While the oxygen-containing functional groups, especially C=O groups, augmented on

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sorbent surface. The bio-chars with plasma treatment showed higher Hg0 removal efficiency than

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that of untreated ones. The Hg0 adsorption process was probably dominated by chemisorption,

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attributed to the highly active carbonyl and ester functional groups. Gao et al. [83] developed a

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maize straw derived activated carbon modified with Co-Ce to simultaneously remove NO and Hg0.

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There was a cooperative effect between Co3O4 and CeO2, leading to production of more Co3+ and

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Ce3+ cations to induce more anionic defects and form more active oxygen and oxygen vacancies.

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NO reduction was primarily governed by Langmuir-Hinshlwood mechanism, while Hg0 oxidation

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related to Mars-Masson mechanism. Shan et al. [84] developed a magnetic Ce-Fe modified

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bio-char stemmed from maize straw for elemental mercury abatement. During Hg0 removal

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processes, Ce4+ and Fe3+ ions were reduced to Ce3+ and Fe2+ ions, respectively. The chemisorbed

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oxygen species were significantly depleted as well.

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Ammonium halide-modified cotton straw derived bio-chars were adopted for Hg0 removal

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[85]. Increasing temperature could increase the mercury adsorption capacity of NH4Br and NH4I

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modified bio-chars, whereas, high temperature would decrease the mercury adsorption capacity of

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NH4Cl modified bio-chars. The C-X groups were identified as the dominant adsorption sites for

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Hg0. Magnetic Mn-Fe modified cotton straw derived bio-chars performed well with respect to

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elemental mercury abatement [86]. Microwave activation was profitable for the development of

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pore structures. And ultrasound treatment facilitated the dispersion of Mn and Fe active

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components.

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To further improve the Hg0 removal performance of bio-chars. Non-thermal plasma method

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had been adopted to introduce chlorine active sites on bio-char surface. HCl plasma activation

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increased the chlorine active sites on the surface of sorghum straw derived bio-chars, contributing

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to the significant rise in Hg0 removal efficiency [87]. X-ray absorption near edge structure

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(XANES) analysis disclosed that the adsorbed mercury on HCl-modified bio-chars was primarily

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presented in the form of Hg+. Hg0 vapor reacting with chlorinated active sites via electron-transfer

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yielded Hg2Cl2. A lot of biomasses, i.e., rice straw, tobacco straw, corn straw, wheat straw, millet

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straw, and black bean straw had been used for preparing bio-chars which were then activated with

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Cl2 non-plasma approach [88].The number of C-Cl groups was increased after Cl2 plasma

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treatment. The produced C-Cl groups served as active sites which enhanced Hg0 removal process.

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The six biomasses derived bio-chars were also activated by H2S plasma, leading to the

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remarkable increase of sulfur-containing and carboxyl groups on bio-char surface [89]. The

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bio-chars after H2S plasma modification presented better Hg0 removal ability than unmodified ones.

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C-S and carboxyl were recognized as the dominant functional groups producing HgS and HgO,

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respectively. The process of Hg0 capture by H2S plasma treated bio-chars is illustrated in Fig. 4.

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Active C-S bonds could be formed on bio-char surface after H2S plasma treatment. Active sulfur

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species was released from the C-S bonds at high temperature and further oxidized Hg0 into HgS.

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Besides, Hg0 could also be oxidized into HgO by the oxygen in carboxyl groups.

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Fig. 4 Hg0 adsorption on H2S plasma activated bio-chars. A sequence of Co-doped MnOx/biomass activated carbons were developed for simultaneously

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removing NO and Hg0 from flue gas [90]. Co addition increased the surface active oxygen

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components and Mn4+ content of MnOx/biomass activated carbons. The enhancement of redox

273

ability and the strength of surface acidic sites restrained the crystallization of MnOx, possibly

274

contributing to the improvement of mercury removal performance and resistance to SO2 and H2O.

275

Xu et al. [91] prepared magnetic chlorinated bio-chars by one-step pyrolysis of Fe(NO3)3-laden

276

wood and polyvinyl chloride mixture. The Fe-Cl co-modified bio-chars displayed far better Hg0

277

removal performance than Cl-modified bio-chars. The produced Fe3O4, C-Cl bonds, and C=O

278

bonds were identified as the major active sites for Hg0 removal. The synthesis route and Hg0

279

capture process of magnetic Fe-Cl co-modified bio-chars are depicted in Fig. 5. Magnetic Fe-Cl

280

co-modified bio-chars were prepared by prolysis of Fe-laden wood and polyvinyl chloride (PVC)

281

mixture. Hg0 could be oxidized into Hg2+ by Fe3O4, C-Cl, and C=O groups.

282 283 284

Fig. 5 Synthesis and performance of magnetic Fe-Cl co-modified bio-chars. 4.2 Seaweed-derived biochar

285

The bio-chars stemmed from agricultural wastes cannot be produced on a large scale owing to

286

the geographical restriction of crop planting and limited arable lands [92]. However, ocean owns

287

huge amounts of seaweed biomass resources. The development and utilization of the seaweed

288

biomass resources, such as Sargassum, Enteromorpha, etc., could be a feasible pathway to tackle

14

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these issues [93, 94].

290

NH4Cl- and NH4Br-modified Sargassum chars were employed for elemental mercury

291

abatement [95, 96]. Low contents of SO2 and H2O favored Hg0 removal, whereas, high contents of

292

SO2 and H2O restrained Hg0 adsorption. C-Br, C-Cl, and C=O covalent functional groups were the

293

predominant active sites. The Hg0 capture on NH4Cl- and NH4Br-modified Sargassum chars both

294

subjected to pseudo-second-order kinetic model, suggesting that Hg0 removal was controlled by

295

chemisorption process. NH4Cl- and NH4Br-modified Enteromorpha chars were also used to remove

296

Hg0 from flue gas [97]. Chlorine and bromine impregnation both promoted Hg0 adsorption. The

297

Br-modified Enteromorpha chars performed better than Cl-modified ones. The Hg0 adsorption on

298

Cl- and Br-modified Enteromorpha chars was endothermic process.

299

To avoid the release of harmful ammonia during bio-char modification process, potassium

300

halide was adopted to modify Enteromorpha chars and Sargassum chars for elemental mercury

301

abatement [98]. The Hg0 removal performances of KI-modified bio-chars were superior to KCl- and

302

KBr-modified ones. The C-I covalent groups and chemisorbed oxygen and/or weakly bonded

303

oxygen species were the dominant chemisorption sites. The application scenarios of seaweed

304

biomass are presented in Fig. 6. Bio-chars and bio-oil could be simultaneously attained by

305

high-temperature pyrolysis of seaweeds. The potassium halide-modified bio-chars were then used to

306

remove Hg0.

15

307 308

Fig. 6 The utilization of seaweed biomass for producing bio-oil and bio-chars.

309

Activated carbon was attained by pyrolysis of Sargassum and Enteromorpha associated with

310

ZnCl2 activation [99]. The Hg0 removal performances of the seaweed-derived activated carbons

311

were much better than those of the seaweed-derived chars. The optimal activation temperature was

312

800 °C. The Hg0 removal process was governed by exterior mass transfer at 80 °C and controlled

313

by chemisorption at 120 and 160 °C. The Hg0 removal process of the seaweed-derived activated

314

carbon was a combination of physisorption and chemisorption, attributed to the large specific

315

surface area and the C-Cl functional groups and active oxygen species, respectively. Though

316

seaweed-derived bio-chars have been considered as good candidates for mercury removal, the

317

harvesting cost of seaweed and large consumption of thermal energy for wet seaweed drying are the

318

major defects.

319

5. Low-dimensional carbon

320

Low dimensional carbons, as emerging carbon materials, are those own at least one physical

321

border small enough to confine electrons. This quantum confinement effect reduces the

322

dimensionality of the materials, imparting them distinct and novel properties that are not presented

323

in their bulk forms [100]. The low-dimensional carbons have also been applied for Hg0 removal in

324

recent years.

325

5.1 Graphene and graphene oxide 16

326

Graphene, known as a new member of two-dimensional carbon-based nanomaterials with a

327

single layer of sp2-hybrid carbon atoms packed into a benzenering structure, has recently aroused

328

widespread attention due to its exceptional electrical, mechanical, and thermal property [101, 102].

329

Graphene oxide possesses ample oxygen-containing functional groups, such as hydroxyl, epoxy and

330

carboxyl groups [103], which are favorable for adsorption and oxidation reactions. Therefore,

331

graphene and its derivates have been applied in many fields due to their large specific surface area,

332

fast electron transferring speed, and good mechanical strength [104].

333

Xu et al. [105, 106] prepared MnOx/graphene and Ag/graphene composites for Hg0 capture

334

at low temperature. Graphene seemed to be a perfect carrier for MnOx particles. It could be readily

335

deposited on graphene surface via hydrothermal process. Graphene also acted as electron transfer

336

channels in Hg0 oxidation reaction. Graphene facilitated the electrical conductivity of MnOx, which

337

was profitable for Hg0 oxidation reaction. Besides, MnOx/graphene and Ag/graphene both showed

338

splendid regenerative ability. Mercury as a resource could be recycled by using Ag/grapheme

339

sorbent via a facile thermal regeneration route. Though silver is a precious metal, its dosage is much

340

less than that of transition-metal oxide. Beside, the Ag-deposited graphene can be recycled after

341

heating treatment, making silver a promising modifying reagent for graphene. The synthesis route

342

of Ag/graphene and its application in Hg0 capture from flue gas is presented in Fig. 7. Graphene

343

was first synthesized using Hummers method. AgNO3 and N2H4 were then mixed together with the

344

graphene and subsequently dried and calcinated under nitrogen stream to yield Ag/graphene. The

345

Hg0 capture performance was evaluated in a horizontal a fix-bed reactor.

17

346 347

Fig. 7 The synthesis process of Ag/graphene and its application for Hg0 removal.

348

Graphene was employed to enhance the Hg0 removal performance of Mn-Ce binary metal

349

oxides [107]. The high valances of Mn4+ and Mn3+ favored Hg0 oxidation. Ceria offered abundant

350

oxygen for mercury oxidation. Graphene promoted Hg0 oxidation and adsorption process via

351

providing more reaction space and facilitating electron transfer. The field emission scanning

352

electron microscopy images of graphene oxide, MnOx@graphene oxide, and Mn-Ce@graphene

353

oxide are shown in Fig. 8. The pristine graphene oxide exhibited typical nanosheet structures. For

354

MnOx@graphene oxide, MnOx nanoparticles were evenly dispersed on the surface of graphene

355

oxide. Mn-Ce@graphene oxide presented a three-dimensional morphology with some graphene

356

oxide being covered by MnOx and CeOx grains.

357 18

358

Fig. 8 Field emission scanning electron microscopy pictures of (a) and (b) graphene oxide, (c)

359

MnOx@graphene oxide, and (d) Mn-Ce@graphene oxide.

360

Ag nanoparticles were deposited on graphene oxide for Hg0 abatement [108]. The

361

Ag/graphene oxide composite could efficiently capture Hg0 from flue gas up to 150-200 °C,

362

plausibly owing to the amalgamation of Hg0 on Ag nanoparticles. An et al. [109] reported that

363

loading Mn-Fe-Ce mixed oxides on graphene oxide could effectively remove Hg0 from flue gas at

364

170-250 °C. The Mn-Fe-Ce active components were readily dispersed on graphene surface due to

365

its large specific surface area and good mechanical property. A magnetic Ag-Fe3O4@graphene

366

oxide ternary composite was synthesized for gaseous Hg0 capture from coal-fired flue gas [110].

367

The ternary composite performed better than pristine graphene oxide and Fe3O4@graphene oxide.

368

The Hg0 was captured via Ag-Hg amalgam mechanism. The magnetic ternary composite was

369

reusable and could be possibly separated from fly ashes for multicycle utilization.

370

Apart from experimental studies, theoretical calculation method has also been applied to

371

explore the potential of graphene towards Hg0 capture and the mechanism of Hg0 adsorption on

372

graphene. Meeprasert et al. [111] studied the reactivity of single-vacancy defective graphene and

373

defective graphene-supported Pdn and Agn (n = 1, 13) towards Hg0 adsorption by using density

374

functional theory calculation method. Pdn showed higher binding ability with the defective site of

375

graphene than Agn. Metal nanocluster bonded stronger with defective graphene than single metal

376

atom. Jungsuttiwong et al. [112] studied the mechanism of Hg0 adsorption on transition-metal

377

decorated boron-doped graphene with periodic density functional theory. Pd4 type A structure

378

exhibited the highest Hg0 adsorption performance. The interaction of Hg-5d orbital and Pd-4d,

379

Pt-5d orbital contributed to its high Hg0 adsorption ability. The top view of the optimized structure

19

380

of B-doped graphene surface is presented in Fig. 9. The optimized B-doped graphene sheet

381

encompassed 12 B atoms (in red color) and 60 C atoms (in gray color). The optimal B-doped

382

graphene surface was then prepared for transition-metal atom and cluster deposition.

383 384

Fig. 9 Top view of the optimized structure of B-doped graphene in a 6 × 6 unit cells.

385

Density functional theory calculations were performed to investigate the adsorption processes

386

of mercury and arsenic on Pdn (n=1-6) decorated pyridine-like N-doped graphene [113]. The

387

vacancy site was confirmed as the most favorable adsorption site for Pd atom. The Pd atoms or

388

clusters could promote the reactivity of N-doped graphene with respect to Hg and AsH3 adsorption.

389

The capture ability of Hg on Pdn/N-doped graphene was linked to the d-band center of Pdn. The

390

closer the d-band center of Pdn to the Fermi level was, the higher adsorption strength of Hg on

391

Pdn/N-doped graphene was.

392

The adsorption processes of Hg0 on perfect, defective, and Al-doped graphene were studied

393

based on density functional theory [114]. The adsorption energies of Hg0 on perfect and defective

394

graphene were -0.220 eV and -0.342 eV, respectively, which were physisorption processes.

395

Whereas, the adsorption energy of Hg0 on Al-doped graphene was -0.570 eV, which was a

396

chemisorption process. The adsorption energy increased with increasing Al-doped vacancy sites.

397

However, the incremental Al atoms doped on a single vacancy site would reduce the adsorption

20

398

energy, which was unfavorable for Hg0 adsorption.

399

5.2 Carbon nanotube and nanofiber

400

Carbon nanotubes and nanofibers are new types of one-dimensional nanomaterials. The

401

carbon nanotubes possess perfect hexagonal connected structures, distinct thermal, electrical and

402

mechanical characteristics [115]. The carbon nanofiber features low density, large length-diameter

403

ratio, high strength, good electrical property and corrosion resistance. Furthermore, the oxygen

404

functional groups can be easily produced on the surface of carbon nanotubes and nanofibers via

405

chemical modification [116], which is conducive to Hg0 removal.

406

Mn-Mo oxides were loaded on carbon nanotubes for oxidation removal of Hg0 at low

407

temperature [117]. Mn-modified carbon nanotube displayed high Hg0 oxidation ability at 150-250

408

℃. However, it nearly lost Hg0 oxidation ability in existence of 500 ppm SO2 due to the generation

409

of MnSO4. In contrast, SO2 promoted the Hg0 oxidation ability of Mn-Mo doped carbon nanotube.

410

The probable reason was that Mo addition could facilitate the conversion of SO2 to SO3, which was

411

profitable for Hg0 oxidation. Mo species could also protect MnO2 from SO2 poisoning.

412

Fe-Ce decorated multi-walled carbon nanotube was also used for simultaneous removal of

413

Hg0 and NO from flue gas [118]. The iron-cerium hybrid oxides loaded both inside and outside of

414

the multi-walled carbon nanotube in evenly dispersion form with sizes of 3-5 nm. Some cerium

415

presented in the form of fluorite-like crystal ceria, others entered into the spinel structure of

416

γ-Fe2O3, resulting in the distortion of the γ-Fe2O3 lattice. The amounts of chemisorbed oxygen

417

species and the oxidation activities of catalysts increased remarkably after cerium addition,

418

contributing to the outstanding Hg0 and NO removal performance.

419

Dimethyl sulfoxide was adopted to modify carbon nanofibers for gas-phase Hg0 capture

21

420

[119]. Though sulfur modification reduced the surface areas and pore volumes of carbon

421

nanofibers, mercury uptake capacities augmented as compared to unmodified ones. The sulfur

422

atoms were incorporated into carbon matrix in the form of sulfides and sulfates. The sulfide groups

423

performed more effectively than sulfate groups towards Hg0 removal. Because the lone pair

424

electrons of sulfide groups acted as active sites for sulfur interacting with mercury, or at least as a

425

point of initial attachment. The micro-morphologies of pristine and sulfur-treated carbon nanofibers

426

are shown in Fig. 10. The carbonaceous species were coated on nonwoven fiber glass with diameter

427

of 5-7 μm. No sulfur clusters were observed on the surface of sulfur-treated carbon nanofibers,

428

confirming that sulfur atoms might be built-in with carbon matrix.

429 430

Fig. 10 The scanning electron microscope images of (A) pristine and (B) sulfur-treated carbon

431

nanofibers.

432

The probable mechanism of Hg0 capture by sulfur-treated carbon nanofibers is shown in Fig. 11.

433

First, Hg0 was oxidized into Hg(II) and formed a double bond with S atom via donating two pairs of

434

shared electrons. Then, one electron transferred from Hg=S double bond to C-S single bond,

435

producing active Hg and C species with one electron. Finally, the reactive electrons of Hg and C

436

species generated a single bond. The original C-S bond broke and yielded a lone pair of electrons.

22

437 438

Fig. 11 The mechanism of Hg0 adsorption on sulfur-treated carbon nanofibers.

439

Magnetic Fe-Mn oxide loaded carbon nanofibers were synthesized via one-step

440

solvothermal approach for Hg0 removal [120]. The Hg0 removal performance greatly augmented

441

with incremental Mn doping amount into Fe3O4 spinel structures. Carbon nanofiber addition

442

improved the dispersion of Fe-Mn oxide grains and promoted the electron transfer mobility, which

443

favored Hg0 removal. Mn cations and lattice or chemisorbed oxygen were regarded as active sites

444

for Hg0 oxidation. Mn-Ce oxides were supported on carbon nanofibers for Hg0 abatement [121].

445

Mn-Ce oxides were highly dispersed on carbon nanofibers and offered sufficient active components

446

for Hg0 oxidation, such as active oxygen and Mn4+ ions. The carbon nanofiber framework provided

447

channels for electron transfer during Hg0 oxidation process.

448

5.3 Carbon sphere

449

Carbon sphere, as a zero-dimensional carbon material, displays numerous advantages over

450

powder or granular activated carbon for catalyst support and adsorption process, probably owing to

451

its high surface area, excellent mobility, high compressive strength, low pressure drop, and potential

23

452

application in large-scale fluidized bed [122, 123].

453

Hierarchical α-MnO2/carbon sphere composite was fabricated via hydrothermal route for

454

Hg0 oxidation and adsorption [124]. Carbon spheres served as the core on which α-MnO2 nanorods

455

grew. The α-MnO2/carbon sphere composite performed stably with Hg0 removal efficiency of

456

>99% over 600 min on stream. Besides, it had better SO2 resistance than unsupported α-MnO2. This

457

three-dimensional α-MnO2/carbon sphere composite composed of zero-dimensional carbon sphere

458

and one-dimensional α-MnO2 was appealing for gas purification.

459

Resin-based carbon spheres were synthesized by suspension polymerization of alkyl phenol

460

and formaldehyde and steam activation in combination with surface modification by heat treatment

461

[125]. The pure carbon spheres attained via oxidation modification at 300 °C performed well,

462

probably attributed to the sufficient oxygen functional groups of C=O and C(O)-O-C. These oxygen

463

functional groups acted as strong oxidizer and promoted electron transfer for converting Hg0 to

464

Hg2+ during Hg0 chemisorption process.

465

Zhang et al. [126, 127] further modified the resin-based carbon spheres with CuO or CeO2

466

to improve its Hg0 removal ability. CuO enhanced the redox ability of the carbon sphere and was

467

profitable to activate its active species, which appreciably improved Hg0 removal performance.

468

CeO2 greatly enhanced the Hg0 removal ability of the carbon sphere through forming active species,

469

such as C=O or C-O, and lattice oxygen via Ce4+/Ce3+ redox couple for oxidizing Hg0 into HgO.

470

The micro-morphology of resin-based carbon sphere is displayed in Fig. 12. The carbon sphere

471

exhibited a relatively smooth surface with a large specific surface area of >1000 m2/g. Few pores

472

and pitted morphology were formed on the surface of the carbon sphere after steam activation.

24

473 474

Fig. 12 The scanning electron microscope image of resin-based carbon sphere.

475

The proposed mechanism for Hg0 capture on CuO-modified carbon sphere is displayed in Fig. 13.

476

Hg0 vapor was first adhered on the surface or in the pore channels of the carbon sphere. Then, it

477

was oxidized by the active components of the carbon sphere, such as C=O or C-O. Furthermore,

478

CuO would promote the activation of the active components and could also oxidize the adsorbed

479

Hg0 by itself.

480

481 482 483

Fig. 13 The mechanism of Hg0 adsorption on CuO-modified carbon sphere. 6. Carbon composite and polymer

25

484

Incorporating heteroatoms into carbon matrix can alter the electronic structures of

485

carbonaceous materials [128], which may lead to improvement in catalytic and adsorption ability.

486

Carbon bonded with metals could form metal-organic framework, which shows great potential in

487

gas separation. Carbon bonded nitrogen could produce graphitic carbon nitride, which is an

488

emerging carbon-based polymers. The metal-organic framework and graphitic carbon nitride have

489

recently been employed for Hg0 removal as well, which has become a research hotspot.

490

6.1 Metal-organic framework

491

Metal-organic frameworks, as a new class of porous materials or porous coordination

492

polymers assembled by metal ions and polyfunctional organic ligands, exhibit low density, high

493

porosity and apparent surface area, controllable pore size, well-defined structure, and chemical

494

tunability [129-132]. Since Yaghi and coworkers [133, 134] first proposed crystalline metal

495

organic frameworks, they have attracted great attention and have been applied in many fields, such

496

as hydrogen storage [135, 136], gas adsorption and separation [137, 138], catalytic reactions [139,

497

140], and environmental remediation [141, 142]. Recently, elemental mercury capture by using

498

metal-organic frameworks has gained increasing research interest due to the maturity of

499

metal-organic framework synthesis methods and the increasingly stringent regulations for mercury

500

emissions.

501

Liu et al. [143] first studied the adsorption potential of metal-organic frameworks towards

502

mercury species, i.e., Hg0, HgCl2, HgO, and HgS, by theoretical calculations based on density

503

functional theory method. The Hg0 was stably physi-sorbed on the unsaturated metal centers of

504

Mg-based metal-organic framework. The Cl endings of HgCl2 multi-interacted with two neighbored

505

Mg ions simultaneously, leading to strong adsorption strength. HgO and HgS were chemisorbed on

26

506

Mg-based metal-organic framework. Therefore, metal-organic frameworks were theoretically

507

predicted to be promising for effective removing mercury species from coal-fired flue gas.

508

Later, phenyl bromine-appended metal-organic frameworks were employed for Hg0 removal

509

[144]. Phenyl bromine was the primary chemisorption site for Hg0 capture. The optimal Br-based

510

metal-organic framework performed much better than un-functionalized metal-organic framework

511

and Br-impregnated activated carbon. The Br-based metal-organic framework showed enhanced

512

Hg0 removal efficiency in the presence of SO2, whereas steam suppressed its Hg0 removal

513

performance. The Br-based metal-organic framework showed high bromine stability during Hg0

514

adsorption, which could avoid possible bromine pollution issue often along with Br-modified

515

sorbents.

516

Mn-Ce modified metal-organic framework was also used for Hg0 and NO removal [145]. It

517

performed better than conventional Mn-Ce loaded ZrO2, especially at low temperature. The

518

metal-organic framework promoted the dispersion of active components, improved Hg0 adsorption

519

and enhanced oxygen utilization, resulting in a desirable catalytic activity. Zhao et al. [146]

520

investigated the combined effect of Ag and Zr-based metal-organic framework with respect to Hg0

521

removal. Ag addition promoted the redox activity of Zr-based metal-organic framework. Ag and

522

Zr-based metal-organic framework presented a significant cooperative effect towards Hg0 removal.

523

The plausible reaction pathways for Hg0 removal by Ag-modified Zr-based metal-organic

524

framework is illustrated in Fig. 14. At low temperature, Hg0 vapor was predominantly captured via

525

generating an Ag-Hg amalgam and channel adsorption. At high temperature, Ag transferred an

526

electron to the oxygen of C=O group to form active oxygen which then attracted an electron from

527

Hg0 through Ag. Besides, the formed active Zr combined with Hg0, which could be captured via

27

528

channel adsorption as well.

529 530

Fig. 14 The reaction pathways of Hg0 removal by Ag-modified Zr-based metal-organic framework.

531

A Cu-based metal-organic framework was adopted for Hg0 removal from sintering gas [147].

532

The Cu-based metal-organic framework showed limited Hg0 removal ability without HCl. Presence

533

of 15 ppm HCl could greatly enhanced its Hg0 removal efficiency. SO2 inhibited Hg0 removal,

534

whereas NO enhanced Hg0 adsorption. Furthermore, Cu-based metal-organic framework exhibited

535

good water-resistance. The mechanism Hg0 removal by Cu-based metal-organic framework is

536

depicted in Fig. 15. Without HCl in sintering gas, Hg0 was removed primarily by active oxygen via

537

forming HgO. With HCl in sintering gas, HCl was adsorbed on Cu-based metal-organic framework

538

to produce intermediates of CuCl or CuCl2. Hg0 was first oxidized into Hg2+ by chemisorbed

539

oxygen species. Then, the produced Hg2+ reacted with CuCl or CuCl2 to generate HgCl2.

540

Furthermore, Cl- ions may be activated by oxygen to produce active chorine species which could

541

oxidize Hg0 into HgCl2.

28

542 543

Fig. 15 The mechanism of Hg0 removal by Cu-based metal-organic framework.

544

It was reported that Cr-based metal-organic framework performed better than Cu-based and

545

Zr-based metal-organic framework [148]. Hg0 removal efficiency augmented with the increasing

546

reaction temperature and oxygen content. The open metal site of Cr-based metal-organic framework

547

was the key factor for Hg0 removal. Hg0 was first adsorbed on Cr-based metal-organic framework

548

and then oxidized into Hg2+ by the open metal site Cr3+. The yielding Hg2+ subsequently reacted

549

with chemisorbed oxygen to produce HgO. While the reduced open metal site Cr2+ was reoxidized

550

into Cr3+ by surface active oxygen. However, high-valence chromium is harmful to human health,

551

which might limit its further application.

552

Zhou et al. [149] loaded α-MnO2 onto Al-based metal-organic framework for Hg0 removal.

553

Al-based metal-organic framework could expose more active sites of α-MnO2, contributing to the

554

enhancement in the Hg0 oxidation ability of α-MnO2. A Se-functionalized metal-organic framework

555

was synthesized for Hg0 removal [150]. The Se-based metal-organic framework displayed a

556

distinguished mercury adsorption capacity of 148.19 mg/g which was about 154 to 705 times bigger

557

than that of commercial activated carbons. Besides, its prominent mercury adsorption stability could

558

still retain under simulated flue gas atmosphere. The gas-phase Hg0 was oxidized into stable and

559

water-insoluble HgSe, ensuring the minimal re-emission or even sequestration of mercury. 29

560

However, the scarcity and toxicity of selenium might be an obstruction for its practical application.

561

6.3 Graphitic carbon nitride

562

Graphitic carbon nitride (g-C3N4), the most stable allotrope of polymeric carbon nitride, has

563

attracted increasing attention due to the facile synthesis route, abundant sources, high thermal and

564

chemical stability, environmentally benign and good biocompatibility, and distinctive electronic

565

structure [151-154]. These unique properties allow for its diverse applications, such as

566

photocatalytic hydrogen evolution [155], wastewater decontamination [156], nitric oxide removal

567

[157], and carbon dioxide reduction [158]. Recently, Liu et al. [159] first explored the potential of

568

g-C3N4 for Hg0 adsorption. They found that g-C3N4 exhibited strong affinity with Hg0. Precursors

569

had a big impact on Hg0 removal performance. The g-C3N4 stemmed from urea high-temperature

570

polymerization displayed a striking Hg0 capture ability with Hg0 removal efficiency of ~84%,

571

which was significantly higher than those of dicyandiamide- and melamine-derived g-C3N4. The

572

probable reason was that the specific surface area of the urea-derived g-C3N4 was much bigger than

573

those of the dicyandiamide- and melamine-derived g-C3N4.

574

CuO/g-C3N4 nanocomposites were prepared via incipient-wetness impregnation method for

575

elemental mercury abatement [160]. Pristine g-C3N4 was attained via a two-step calcinating

576

approach. g-C3N4 displayed good Hg0 adsorption performance at 40-200 °C. CuO decoration

577

greatly enhanced the Hg0 removal ability of g-C3N4 owing to the close interaction of CuO and

578

g-C3N4. Liu et al. [161] fabricated a porous g-C3N4 via a facile two-step thermal etching oxidation

579

method for Hg0 removal. The porous g-C3N4 also displayed good reactivity towards Hg0 adsorption

580

at 40-200 °C. CuO decoration efficiently activated g-C3N4, imparting it enhanced Hg0 oxidation

581

ability due to the Mott-Schottky effect at the interface of CuO and g-C3N4.

30

582

It is well established that g-C3N4 nanosheet owns big surface area for exposing plentiful

583

active sites and short bulk diffusion length for promoting electron transfer, which are conducive to

584

adsorption and catalytic reactions [162, 163]. A two-dimensional g-C3N4 nanosheet was synthesized

585

via thermal exfoliation of bulk g-C3N4 [164, 165]. The g-C3N4 nanosheet performed well with

586

respect to Hg0 removal at 60-240 °C. The maximal Hg0 removal efficiency of ~65.8% was reached

587

at 150 °C due to the two-dimensional planar structure and big specific surface area of g-C3N4

588

nanosheet. The transmission electron microscopy photo of g-C3N4 nanosheet is shown in Fig. 16.

589

The pristine g-C3N4 nanosheet displayed ultrathin sheet-like morphology with thickness of ~1-10

590

nm and lateral size of ~1-3 µm. The specific surface area of the g-C3N4 nanosheet was ~109 m2/g.

591 592

Fig. 16 The transmission electron microscopy image of g-C3N4 nanosheet.

593

MnO2 and Co3O4 decoration significantly enhanced the Hg0 removal performance of the

594

g-C3N4 nanosheet. The plausible reason was attributed to the excellent redox abilities of MnO2 and

595

Co3O4 and the enhanced electron mobility at the interface of MnO2 or Co3O4 and g-C3N4.

596

MnO2-decorated g-C3N4 nanosheet displayed Hg0 removal efficiency all above 91% within 90-240

597

°C. Co3O4-modified g-C3N4 nanosheet presented excellent Hg0 capture ability with Hg0 removal

598

efficiency of ~100% within 120-240 °C. However, NO and SO2 significantly inhibited the Hg0

599

removal performance of the Co3O4-modified g-C3N4 nanosheet, probably due to the competitive 31

600

adsorption and side reactions. The mechanism of Hg0 removal over MnO2-modified g-C3N4

601

nanosheet is depicted in Fig. 17. The gas-phase Hg0 was oxidized into HgO by surface active

602

oxygen (Oad) and lattice oxygen ([O]) of MnO2 which was reduced to Mn2O3. Gaseous O2

603

replenished the depleted chemisorbed oxygen and lattice oxygen and simultaneously oxidized

604

Mn2O3 into MnO2 to refresh the active component for Hg0 oxidation.

605 606

Fig. 17 The process of Hg0 oxidation by MnO2-loaded g-C3N4 nanosheet.

607

In addition, Liu et al. [166] first employed CuCl2-loaded carbon aerogels for elemental

608

mercury removal at low temperature. It displayed very stable Hg0 removal performance with

609

~100% Hg0 removal efficiency over 600 min on stream. To further lower down the capital cost of

610

carbon aerogel, a seaweed-templated pathway was developed for controllable synthesis of

611

SnO2-loaded carbon aerogels to simultaneously remove Hg0 and H2S [167]. The carbon aerogels

612

possess distinct three-dimensional network structures. The nanoconfinement of carbon nanoshells

613

could prevent the growth and agglomeration of SnO2 nanoparticles during regeneration process,

614

which favored removal of Hg0 and H2S [168-170]. The three-dimensional interconnected macro-

615

and meso-pores of carbon aerogels was conducive to Hg0 and H2S removal at high space velocities.

32

616

Carbon aerogel might also be a potential carbon-based sorbent for Hg0 removal provided the capital

617

cost is reduced and synthesis process is simplified. In addition, some practical industrial gas streams

618

are extremely complex, and often simultaneously contain multiple gas components such as H2O,

619

NOx, HCl, H2S, SO2, CO, SO3, VOCs, etc [171-180]. Thus much more work should be conducted

620

to further improve the stability of these carbon-based adsorbents in the presence of H2O, NOx, HCl,

621

H2S, SO2, CO, SO3, VOCs, etc., and reveal the effects of these gas components on Hg0 removal.

622

7. Conclusions and prospects

623

Atmospheric mercury pollution has become a global concern mainly due to the large

624

consumption of fossil fuels and gold mining. The primary challenge for reducing elemental mercury

625

from gas streams is to search for environmentally benign, inexpensive, regenerable, and efficient

626

mercury sorbents with good resistance to NO, SO2, and H2O. Carbon-based materials are deemed to

627

be appealing candidates for removing Hg0 from gas streams at low temperature. This review

628

provided the state-of-the-art of carbon-based sorbents for elemental mercury abatement in the view

629

point of material and chemistry. To date, massive carbon materials have been employed for Hg0

630

capture. The advantages and disadvantages of versatile carbon-based materials towards elemental

631

mercury removal are summarized as follows.

632

Activated carbon is the only commercialized mercury sorbent due to its wealthy pore

633

structure and sufficient surface oxygen functional groups. Activated carbon injection technology

634

has been widely applied in coal-fired power plants. However, the high operation cost and

635

detrimental effect on fly ash utilization hampered its industrial application. Activated coke,

636

including semi-coke and petroleum coke, could be alternatives to the activated carbon because of its

637

higher mechanical strength, better regenerability, and lower capital cost. Ongoing and future efforts

33

638

should pay more attention on how to improve its reusability and efficiency. Combing magnetic

639

metal oxides with activated coke is a feasible pathway to tackle this issue.

640

Bio-chars, obtained from direct pyrolysis of diverse terrestrial or marine biomass and biomass

641

wastes at high temperature, are regarded as sustainable and inexpensive mercury sorbents. So far,

642

bio-chars have been extensively investigated for elemental mercury removal owing to their

643

sustainability and sufficient sources. However, virgin bio-chars display fairly low surface area and

644

poor activity towards Hg0 adsorption. In situ activation can greatly improve the porosity and surface

645

area of the bio-chars, and thereby strengthening Hg0 removal performance. Besides, appropriate

646

modification methods, e.g., green, efficient, and inexpensive, associating with high removing

647

efficiency and less contamination to the environment, plays a pivotal role in the large-scale

648

application of bio-chars.

649

Low-dimensional carbons own good electronic conductivity and excellent electron transfer

650

mobility, making them good mercury sorbents after loading with active components. However, the

651

synthesis routes for most low-dimensional carbons, particularly graphene, are time consuming and

652

energy consumption, leading to high capital costs of the low-dimensional carbons. Furthermore, the

653

low activities of pristine low-dimensional carbons restrained their practical utilizations.

654

Metal-organic frameworks are characterized by tunable chemical properties and porous structures

655

through altering their carbonaceous ligands. Some research works showed that metal and halogen

656

modified metal-organic frameworks displayed good Hg0 removal performance. Nevertheless, the

657

high cost and sensitivity towards steam and acidic and basic gases restricted their large-scale

658

application. Thus, the development of low-dimensional carbon and metal-organic framework

659

sorbents highly depend on the progress in material science and engineering. The development of

34

660

new synthetic processes with low cost and simple operation and recyclable characteristics of

661

sorbents are the key factors in the further development of low-dimensional carbon and

662

metal-organic framework mercury sorbents.

663

Graphitic carbon nitride is a new kind of carbon-based sorbent for elemental mercury

664

abatement from coal-derived flue gas. Recent experimental studies found that g-C3N4 performed

665

effectively towards Hg0 adsorption at low temperature. The strong affinity of g-C3N4 with Hg0 is

666

plausibly attributed to its unique electronic characteristics and layered structure. Besides, g-C3N4

667

features high thermal and chemical stability and can be facilely attained from diverse sources. Thus,

668

g-C3N4 shows appealing prospect as novel carbon-based sorbents for Hg0 removal. However, the

669

performance of g-C3N4 is significantly affected by gas components. Some practical industrial gas

670

streams are extremely complex, and often simultaneously contain multiple gas components such as

671

H2O, NOx, HCl, H2S, SO2, CO, SO3, VOCs, etc. Thus much more work should be conducted to

672

further improve the stability of these carbon-based adsorbents in the presence of H2O, NOx, HCl,

673

H2S, SO2, CO, SO3, VOCs, etc., and reveal the effects of these gas components on Hg0 removal.

674

Furthermore, the disposal of spent sorbents after mercury capture also plays an important role in

675

mercury emission control by adsorption technique. Generally, the spent sorbents can be regenerated

676

and reused after heating treatment at middle temperature under nitrogen stream.

677

Acknowledgements

678

This work is supported by the National Natural Science Foundation of China (51576094;

679

U1710108), and the Senior Talent Foundation of Jiangsu University (18JDG017).

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Graphical Abstract

1078 1079 1080 1081 1082

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1. The prospect and limitation of low-dimensional carbon for Hg0 removal are commented.

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2. Bio-char, carbon nitride, and metal-organic framework are promising mercury sorbents.

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3. The performances and mechanisms of diverse carbon-based sorbents are fully discussed.

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Declaration of Interest Statement

1090 1091 1092

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(1) All authors declare that we have no financial and personal relationships with other people,

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organizations or company that can inappropriately influence our work. The authors also state

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that the processing of this manuscript will not receive any organizational or personal

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interference.

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(2) All authors declare that the copyright of this manuscript is entirely determined by the

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submitting publisher and/or the journal. The authors have no interest disputes about this

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publishing rights or future commercial interests.

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(3) All authors agree to the corresponding author's submission decision and ensure that there are no disputes of interest.

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