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 (Hg-0) into immobile, nontoxic, and less bioavailable species is of vital environmental significance. Here, we investigated bioconversion of Hg-0 in a sulfate-reducing membrane biofilm reactor (MBfR). The MBfR achieved effective Hg-0 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 Hg-0 as the initial electron donor and sulfate as the terminal electron acceptor to form the overall redox. These microorganisms coupled Hg-0 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 Hg-0 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 (beta-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 Hg-0 into HgS was therefore achieved and can be a feasible biotechnique for flue gas demercuration.