Chapter Three - Biological treatments of mercury and nitrogen oxides in flue gas: biochemical foundations, technological potentials, and recent advances

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Abstract

Nitrogen oxides (NOx) and mercury (Hg) are commonly found coexistent pollutants in combustion flue gas. Ever-increasing emission of atmospheric Hg and NOx has caused considerable environmental risks. Traditional flue gas demercuration and denitration techniques have many socioeconomic, technological and environmental drawbacks. Biotechnologies can be a promising and prospective alternative strategy. This article discusses theoretical foundation (biochemistry and genomic basis) and technical potentials (Hg0 bio-oxidation coupled to denitrification) of bioremoval of Hg and NOx in flue gas and summarized recent experimental and technological advances. Finally, several specific technical perspectives have been put forward to better guide future researches.

Introduction

Fossil fuel combustion, waste incineration and industrial metal and cement production accounted for 95% of anthropogenic atmospheric mercury (Hg) emission (Pacyna, Pacyna, Steenhuisen, & Wilson, 2006; Stolle, Koeser, & Gutberlet, 2014). Moreover, combustion of coal contributes the largest proportion to atmospheric Hg emission (Yang, Yang, Shao, Niu, & Wang, 2011). In coaled-fired flue gas, atmospheric Hg pollutants typically exists in the chemical forms of elemental mercury (Hg0), oxidized mercury (Hg2 +) and particulate mercury (Hgp) (Cheng, Bai, Zhang, & Cai, 2014; Chi, Yan, Qu, Qiao, & Jia, 2009). Herein, Hg0 accounts for over 90% of total Hg species in coal-fired flue gas and plays a predominant role in global atmospheric Hg pollution (Liu, Zhang, & Pan, 2014; Presto & Granite, 2006; Wang et al., 2014). Furthermore, Hg0 is comparatively stable, chemically non-reactive, but highly volatile and extremely mobile (Cheng, Zhang, & Bai, 2014). The stability and mobility of Hg0 enables its long-time residence in atmosphere (from several months to 1 year), wide-range transport across global atmosphere (hemispherical scale), and causes undesired technical difficulties in its removal (Hsu-Kim, Kucharzyk, Zhang, & Deshusses, 2013; O'Connor et al., 2019). On the other hand, fossil fuel combustion contributed to almost all of anthropogenic emission of nitrogen oxides (NOx) (Amoatey, Omidvarborna, Baawain, & Al-Mamun, 2019; Jin, Veiga, & Kennes, 2005). In chemistry, atmospheric NOx generally refers to an extensive variety of nitrogen-oxygen compounds, including nitric oxide (NO), nitrogen dioxide (NO2), nitrous oxide (N2O), dinitrogen trioxide (N2O3), dinitrogen tetroxide (N2O4), dinitrogen pentoxide (N2O5) and so on (Gholami, Tomas, Gholami, & Vakili, 2020). Dependent on the formation mechanisms, NOx produced in fuel combustion can be categorized into thermal NOx (formed by reaction between atmospheric N2 and O2 at high temperature), fuel NOx (formed by oxidation of nitrogen in fuel during combustion), and prompt NOx (formed by reaction between atmospheric N2 and hydrocarbon free radicals during fuel combustion at low temperature) (Gholami et al., 2020; Raghunath & Mondal, 2017; Rao, Mehra, Duan, & Ma, 2017). Herein, NO typically accounts for over 95% of NOx in coal-fired flue gas and acts as a significant role in global atmospheric NOx pollution (Wang, Yuan, Hao, Zhao, & Wang, 2019). NO is fairly stable, hardly water-soluble, less chemically reactive and highly mobile, which also incurs a great many technological challenges in its decontamination (Lee & Sublette, 1991; Wen et al., 2019). Both Hg0 and NO are commonly coexistent atmospheric pollutants in coaled-fired flue gas and pose extremely complicated technological difficulties in their removal (Wang et al., 2019; Yang et al., 2018; Zhao, Hao, Xue, & Feng, 2017). The ubiquitous co-existence and unfavorable physicochemical properties of Hg0 and NO in coal-fired flue gas called for development of integrated techniques for simultaneous removal of both (Huang, Tan, et al., 2020; Zhao et al., 2017, Zhao et al., 2019). Based on the current situations, a novel technique for simultaneous removal of Hg0 and NO is bound to be of strategic importance, technological necessity and environmental significance.

Almost all chemical forms of mercury species tend to more or less induce acute or chronic toxic effects on human and mammals (Osborn, Bruce, Strike, & Ritchie, 1997; Ullrich, Tanton, & Abdrashitova, 2001). Different toxicological mechanisms of various forms of Hg species in terms of transport, accumulation and physiochemical fate have been detailed in previous publication (Biester, Müller, & Schöler, 2002; Kaschak, Knopf, Petersen, Bings, & König, 2014; Mukkata, Kantachote, Wittayaweerasak, Megharaj, & Naidu, 2019). Herein, the most abominable and problematic are the organic forms of Hg species such as Methylmercury (MeHg) and Dimethylmercury (Me2Hg), which are the deadly neurotoxins and the main culprit responsible for “Minamata disease,” outbreak of which resulted from poisoning by MeHg through dietary intake of contaminated fish (Grandjean et al., 1997; Simpson & Spadaro, 2016; Yang et al., 2020). Approximately 95% of the total atmospheric Hg exists in the form of Hg0, which can be inhaled by human respiratory systems, retained in blood, and transported to the vital organs through blood circulation (Liu & Wang, 2014; Xu, Pan, Fan, & Liu, 2019; Zeng, Jin, & Guo, 2004). Meanwhile, the absorbed Hg0 in vivo can be swiftly oxidized to inorganic Hg2 + and further methylated to produce MeHg (Ha et al., 2017). Bacterial methylation is another important pathway of Hg bioconversion, Inorganic Hg2 + is methylated to the MeHg most typically by iron-reducing bacteria (IRB) and sulfate-reducing bacteria (SRB) (Benoit, Gilmour, & Mason, 2001; Bridou, Monperrus, Gonzalez, Guyoneaud, & Amouroux, 2011; Moreau et al., 2015). Phenylmercury acetate, a widely used agricultural pesticide, can easily degrade to inorganic Hg2 + and further transform into MeHg in soil (Yang et al., 2020). Approximately 70–80% of total Hg takes the eventual predominant form of MeHg in the human body (Mortensen, Caudill, Caldwell, Ward, & Jones, 2014). The accumulation of MeHg in vivo consequently resulted in retarded intelligence, sensory disorder, blurred vision, impaired hearing, tremors, ataxia, dysarthria and etc. (Bailey et al., 2017; Biswas et al., 2011). Besides its neurotoxicity, long-term exposure to any form of Hg species mercury can also lead to renal disorder, myocardial infarction, immune malfunction and abnormal blood pressure (Ha et al., 2017; Wells et al., 2017). Currently, environmental risks raised by atmospheric Hg contamination lies in chronic Hg accumulation and poisoning via bio-amplification on food chain(Bridou et al., 2018; Colombo, Ha, Reinfelder, Barkay, & Yee, 2013). The chemically inert Hg0 though as the predominant atmospheric mercury species can be eventually converted into the deadly neurotoxic MeHg in human body through a diverse variety of long-term biochemical processes (Merritt & Amirbahman, 2009). On the other hand, NOx are notorious as the major culprit responsible for environmental issues such as acid precipitation, photochemical smog, tropospheric ozone depletion, urban haze and etc. (Jin et al., 2005; Qie, Zhu, Rong, & Zong, 2019). NO and NO2 as the principal components of NOx in coal-fired flue gas severely threaten human health (Thiemann, Scheibler, & Wiegand, 2005). NO in vivo can react with hemoglobin to form methemoglobin and consequently induce cyanosis (Strak et al., 2017). Inhalation of water and lipid-soluble NO2 can induce pulmonary edema, capillary walls damage, exudative inflammation and respiratory obstruction (Kampa & Castanas, 2008). Long-term exposure to low doses of NOx also results in chronic effects such as coughing, headache, appetite loss and gastrointestinal disorders (Strak et al., 2017). Both Hg0 and NO are the leading causative factors of numerous potential environmental risks and human health hazard. The globally ever-increasing emission of coal-fired flue gas, in which both Hg0 and NOx are frequently coexistent, further aggravates the situation, therefore simultaneous removal of Hg0 and NO in the coal-fired flue gas becomes a very critical strategy to alleviate the Hg and NOx-induced environmental risks (Zhao et al., 2017).

Control of NOx emission can be actualized before, during or after combustion process, from which pre-combustion techniques, combustion techniques, and post-combustion techniques are derived, respectively (Skalska, Miller, & Ledakowicz, 2010). Pre-combustion techniques aim at reduction of total nitrogen content in fuels by means of physicochemical techniques to limit NOx emission at its source (Wojciechowska & Lomnicki, 1999). Combustion techniques aim to optimize operational conditions during combustion such as temperature distribution, residence time, fuel-oxygen ratio and etc. to limit NOx formation based on improved and upgraded designs of furnaces and incinerators (Gholami et al., 2020). NOx emission abatement generally reaches up to 50% by means of pre-combustion and combustion techniques (Skalska et al., 2010). Traditional post-combustion techniques deal specifically with NOx removal in exhaust flue gas by utilization of multifarious forms of physicochemical techniques such as selective catalytic reduction (SCR), selective non-catalytic reduction (SNCR), wet absorption, adsorption, electron beam, non-thermal plasma, which are extensively applied in engineering field and generally attain considerably high removal efficiency over 80% (Gholami et al., 2020; Skalska et al., 2010; Wojciechowska & Lomnicki, 1999). These denitration techniques vary substantially from reactants (SCR catalysts: Cu-zeolite, V2O5-WO3/TiO2, CeOx-based mesoporous silica, WO3(x)-CeO2, Mg-Mn/TiO2, Cu-Mn/TiO2, Mn-Sb/TiO2, Mn-Fe/TiO2, Ni-MnOx, MnOx/CePO4, and etc.; adsorbents: activated carbon, zeolites, solid amine, metal oxides, metal-organic frameworks, silica gel and etc.; absorbent: solutions of Na2SO3, NaHSO3, and Na2S, Fe(II)-EDTA, H2O2, KMnO4, urea and etc.; SNCR reducing agent: NH3, cyanuric acid, urea and etc.), reactor configurations (tubular reactor, flow reactor, bubbling reactor, spraying reactor, corona discharge reactor, plasma reactor and etc.), operational conditions (temperature, residence time, flue gas components and etc.) and the overall performance(Andreoli, Deorsola, Galletti, & Pirone, 2015; Cao et al., 2015; Ganduglia-Pirovano, Hofmann, & Sauer, 2007; Gholami et al., 2020; Putluru et al., 2015; Shan, Liu, He, Shi, & Zhang, 2012; Shen, Wang, Wang, & Liu, 2014; Skalska et al., 2010; Tang et al., 2018; Zhan et al., 2017). Herein, SCR and SNCR are the most extensively applied techniques dealing with post-combustion flue gas denitration (Mahmoudi, Baeyens, & Seville, 2010; Praveena & Martin, 2018). Control of Hg emission lies particularly in Hg0 removal in exhaust flue gas, since both Hg2 + and Hgp can be effectively removed by means of typically used air pollution control devices such as flue gas desulfurization (WFGD) and electrostatic precipitators (ESPs) (Rodríguez-Pérez, López-Antón, Díaz-Somoano, García, & Martínez-Tarazona, 2013; Wang et al., 2014). Traditional post-combustion techniques accomplish Hg0 removal in flue gas by application of four major forms of physicochemical approaches, namely, adsorption, chemical oxidation, catalytic oxidation and advanced oxidation (Dranga, Lazar, & Koeser, 2012; Granite, Pennline, & Hargis, 2000; Liu & Adewuyi, 2016). The oxidation processes aim ultimately to oxidize Hg0 to water-soluble Hg2 + chemically and subsequently remove Hg2 + by scrubbing techniques. These demercuration techniques differ substantially from catalysts (noble metals: Au/Al2O3, Pd/Al2O3, Ir/Al2O3, Ag/TiO2, Au/TiO2 and etc.; metal oxides: MnO2, nano-CuO, nano-Fe2O3 and etc.; composite metal oxides and their modified or supported products: CuO/TiO2, V2O5-MoO3/TiO2, CeO2-TiO2, Co-MnO2, MnOx/Al2O3, CuCoO4/γ-Al2O3, MnO2-TiO2 and etc.), adsorbents (Ca-based adsorbents: hydrated lime, advacate, the modified advacate and etc.; C-based adsorbents: activated carbon, bamboo charcoal, rice husk ash, biochar, mulberry twig chars and etc.; metal oxides and sulfides: MnO2, Cr2O3, MoS2, Fe2O3, FeS, FeS2, MoO3/MgSiO3 and etc.; zeolites; silicone; natural minerals), oxidizing agents (H2O2, KMnO4, O2, O3, NaClO2, halogens, ClO2, KClO, FeO42 ), advance oxidizing agents (UV/K2S2O8/H2O2, plasma-fenton, UV/O3/H2O2, UV/photocatalysis, microwave/H2O2, UV/Fenton, supercritical oxidation, ultrasonic/O3/H2O2), reactor configurations (wet scrubber, photochemical reactor, bubble column reactor, vaporizer, spray reactor, membrane reactor and etc.) and the operational conditions (temperature, density of catalysts or adsorbents, power supply, inlet concentration, residence time, flue gas components and etc.) (Barnea, Sachs, Chidambaram, & Sasson, 2013; Chen, Hsi, Nian, & Chiu, 2014; Chen, Ma, et al., 2014; Hou, Zhou, Qi, Gao, & Luo, 2014; Klasson, Boihem, Uchimiya, & Lima, 2014; Liu & Adewuyi, 2016; Rodríguez-Pérez et al., 2013; Wang et al., 2010, Wang et al., 2014; Wiatros-Motyka, Sun, Stevens, & Snape, 2013; Zhao et al., 2014). However, these denitration and demercuration techniques have a sizable number of socioeconomic disadvantages, including but not limited to catalyst poisoning, limited catalyst life span, ammonia slip, high operational cost, high initial investment, large equipment size, secondary waste production, equipment corrosion, flying ash, high energy consumption and etc. (Guan, Zhan, Lin, & Huang, 2014; Mladenović, Paprika, & Marinković, 2018; Skalska et al., 2010). This overwhelmingly lot of socioeconomic, technological and environmental drawbacks severely limit the applicability of these traditional techniques and call urgently for the development of a novel, efficient, cost-effective, environment-friendly technique for flue gas denitration and demercuration(Guan et al., 2014; Mladenović et al., 2018).

Biotechnology has extraordinarily stood out as a technically feasible, cost-effective, energy-saving, environment-friendly and highly prospective alternative strategy for atmospheric pollutants decontamination in recent decades. Dependent upon the physiochemical and biochemical metabolic processes, biological flue gas denitration techniques can be categorized into four major types in principle: (1) nitrification; (2) denitrification; (3) microalgal assimilation; and (4) BioDeNOx (Jin et al., 2005; Wang et al., 2016; Wang, Xu, Zou, Yang, & Zhang, 2018; Xu, Dai, & Chai, 2018; Xu et al., 2017). Microbial nitrification aims to convert NOx in flue gas into water soluble nitrate (NO3) or nitrite (NO2), which can be subsequently removed by scrubbing device, via chemoautotrophism of ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB) (Han, Shaobin, Zhendong, Pengfei, & Yongqing, 2016; Peng et al., 2018). Microbial denitrification converts NOx in flue gas to N2 thoroughly, an environmentally benign product, by virtues of a sequential series of enzymatic dissimilatory reductive bio-processes enabled by denitrifying bacteria (DNB) (Di Capua, Lakaniemi, Puhakka, Lens, & Esposito, 2017). As for microalgal assimilation, NOx in flue gas can be utilized as nitrogen source by microalgae to biosynthesize a large variety of essential nitrogen compounds (glutamate, glutamine, and etc.) and thereby NOx in flue gas takes its final form of the vital components in microalgal proteins, which increasingly become an emerging source of dietary supplements for human (Jin et al., 2005; Qie et al., 2019). BioDeNOx is the integration of physicochemical absorption and microbial denitrification, in which NOx in flue gas is first physico-chemically absorbed by the chelating agent Fe (II)-EDTA and subsequently biochemically converted into the environmentally benign N2 by the denitrifying bacterial community (Jin et al., 2005). After microbial denitrification, NOx-Fe (II)-EDTA is dechelated and the regenerated Fe (II)-EDTA is reusable for secondary absorption. Technologically, biological flue gas denitration is actualized via four major forms of bioreactors: (1) bioscrubber; (2) biofilters; (3) biotrickling filters; and (4) membrane biofilm reactors (Can, Syron, & Ergenekon, 2021; Chen, Gu, Zheng, & Chen, 2016; Jiang, Huang, Chow, & Yang, 2009; Wei et al., 2017). Nevertheless, biological flue gas demercuration technologies did not reach as much substantial progress as that of denitration in the fields of both experimental researches and engineering applications. Biosorption of Hg2 + in waste water by a wide variety of biosorbents (bacteria, fungi and algae) and phytoremediation techniques (phytofiltration, phytostabilization, phytovolatilization and rhizodegradation) are currently the two major forms of biotechnologies dealing with bioremediation of Hg contamination (Chang & Hong, 1994; Velásquez & Dussan, 2009). Pertinent researches into biological Hg0 removal in flue gas are not widely and commonly found. A previous study indicated that a sulfur-oxidizing biotrickling filter can potentially bio-convert Hg0 in synthetic flue gas into immobile Hg-S complexes to achieve stabilization (Philip & Deshusses, 2008). Our previous works confirmed that synergistic integration of Hg0 bio-oxidation with sulfate bio-reduction, branched sulfur oxidation, nitrification and denitrification in the membrane biofilm reactor (MBfR) bio-convert Hg0 in synthetic flue gas into mercury sulfide (HgS) and humic acids-bound mercury (HA-Hg), both of which are the less toxic, less reactive, and less bioavailable forms of Hg species (Huang, Wei, et al., 2020; Huang, Tan, et al., 2020; Huang et al., 2019a, Huang et al., 2019b, Huang et al., 2019c). On the basis of these evidences, biological flue gas demercuration is of theoretical foundation and technological potential, and can be synergistically coupled with the mature biological denitration techniques.

Section snippets

Biotransformation of mercury

Understanding the bacterial biotransformation of Hg from the perspective of genomic foundation is the very key to substantiate technological potential of biological flue gas demercuration. Bacterial biotransformation of Hg can be categorized into four major pathways: (1) Hg0 bio-oxidation; (2) Hg2 + bio-reduction; (3) Hg2 + methylation; and (4) CH3Hg+ demethylation. Biological flue gas demercuration aims at bioconversion of Hg0 into chemically stable, immobile, less toxic and less bioavailable Hg

Technological advances in bioremoval of mercury and nitrogen oxides

Simultaneous biological flue gas demercuration and denitration is theoretically viable, technologically feasible and applicable in engineering field. Currently, biological flue gas denitration has been extensively studied in terms of bioreactor forms, parametrical optimization, long-term performance, functional bacterial community, metagenomic insights and etc. However, researches into biological flue gas demercuration are less widely found, let alone that of simultaneous biological

Prospects and outlooks in future researches

Currently found researches into bioremoval of Hg0 and NOx in flue gas have reached substantial progresses in performance enhancement (removal efficiency of Hg0 and NO over 90%), operation optimization (optimal GRT, temperature, loadings, carbon source, pH, inhibitive substrates and etc.), bacterial taxonomic analyses (DNB, AOB, NOB, MOB and multi-functional bacteria), metagenomic investigations of nitrogen metabolism and Hg0 bio-oxidation (nar, nir, nor, nos and kat), mechanisms of NOx

Acknowledgments

We gratefully acknowledge the funding by the Guangdong Basic and Applied Basic Research Foundation (2019B1515120021) and National Natural Science Foundation of China (21677178).

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