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

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.

Bio-oxidation of Elemental Mercury into Mercury Sulfide and Humic Acid-Bound Mercury by Sulfate Reduction for Hg0 Removal in Flue Gas.

Bioconversion of elemental mercury (Hg0) into immobile, nontoxic, and less bioavailable species is of vital environmental significance. Here, we investigated bioconversion of Hg0 in a sulfate-reducing membrane biofilm reactor (MBfR). The MBfR achieved effective Hg0 removal by sulfate bioreduction. 16 S rDNA sequencing and metagenomic sequencing revealed that diverse groups of mercury-oxidizing/sulfate-reducing bacteria (Desulfobulbus, Desulfuromonas, Desulfomicrobium, etc.) utilized Hg0 as the initial electron donor and sulfate as the terminal electron acceptor to form the overall redox. These microorganisms coupled Hg0 bio-oxidation to sulfate bioreduction. Analysis on mercury speciation in biofilm by sequential extraction processes (SEPs) and inductively coupled mass spectrometry (ICP-MS) and by mercury temperature programmed desorption (Hg-TPD) showed that mercury sulfide (HgS) and humic acid-bound mercury (HA-Hg) were two major products of Hg0 bio-oxidation. With HgS and HA-Hg comprehensively characterized by X-ray diffraction (XRD), excitation-emission matrix spectra (EEM), scanning electron microscopy-energy disperse spectroscopy (SEM-EDS), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR), it was proposed that biologically oxidized mercury (Hg2+) further reacted with biogenic sulfides to form cubically crystallized metacinnabar (ß-HgS) extracellular particles. Hg2+ was also complexed with functional groups -SH, -OH, -NH-, and -COO- in humic acids from extracellular polymeric substances (EPS) to form HA-Hg. HA-Hg may further react with biogenic sulfides to form HgS. Bioconversion of Hg0 into HgS was therefore achieved and can be a feasible biotechnique for flue gas demercuration.

Simultaneous removal of elemental mercury and NO by mercury induced thermophilic community in membrane biofilm reactor.

Thermophilic membrane biofilm reactor (TMBR) for elemental mercury (Hg0) and NO removal in simulated flue gas was investigated at oxygen content of 6% and 60 °C. The performance, the microbial community structures, gene function and the mechanism for Hg0 and NO removal in the TMBR were evaluated. TMBR achieved effective simultaneous Hg0 and NO removal in 210 days of operation, Hg0 and NO removal efficiency were up to 88.9% and 85.3%, respectively. Mercury induced thermophilic community had been formed significantly. Comamonas, Pseudomonas, Desulfomicrobium, Burkholderia and Halomonas were thermophilic mercury resistant bacteria. Brucella, Paracoccus, Tepidiphilus, Proteobacteria, Pseudomonas and Symbiobacterium were nitrifying/denitrifying genera, and had functional genes of mercury and nitrogen metabolism, as shown by16S rDNA and metagenomic sequencing. The biofilm in TMBR was characterized by XPS, HPLC. XPS and HPLC spectra indicate the formation of a mercuric species (Hg2+) from mercury oxidation. TMBR used oxygen as electron acceptor, NO and Hg0 as electron donor in nitrification; O2, NO and NO3- could be used as electron acceptor and Hg0 as electron donor in denitrification.

Nitrification/denitrification shaped the mercury-oxidizing microbial community for simultaneous Hg0 and NO removal.

A denitrifying/nitrifying membrane biofilm reactor for simultaneous removal of Hg0 and NO was investigated. Hg0 and NO removal efficiency attained 94.5% and 86%, respectively. The mercury-oxidizing microbial community was significantly shaped by nitrification/denitrification after the supply of gaseous Hg0and NO continuously. Dominant genera Rhodanobacter and Nitrosomonas participated in Hg0 oxidation, nitrification and denitrification simultaneously. Hg0 oxidizing bacteria (Gallionella, Rhodanobacter, Ottowia, Nitrosomonas and etc.), nitrifying bacteria (Nitrosomonas, Rhodanobacter, Diaphorobacte and etc.) and denitrifying bacteria (Nitrosomonas, Rhodanobacter, Castellaniella and etc.) co-existed in the MBfR, as shown by metagenomic sequencing. X-ray photoelectron spectroscopy (XPS) and high performance liquid chromatography-inductively coupled plasma mass spectrometry (HPLC-ICP-MS) confirmed the formation of a mercuric species (Hg2+) from mercury bio-oxidation. Mechanism of mercury oxidation can be described as the bacterial oxidation of Hg0 in which Hg0 serves as electron donor, NO serves as electron donor in nitrification and electron acceptor in denitrification, oxygen serves as electron acceptor.