Bioconversion of Hg into HA-Hg for simultaneous removal of Hg and NO in a denitrifying membrane biofilm reactor.

Bacterial mercury oxidation coupled to denitrification offers great potential for simultaneous removal of elemental mercury (Hg) and nitric oxide (NO) in a denitrifying membrane biofilm reactor (MBfR). Four potentially contributory mechanisms tested separately, namely, membrane gas separation, medium absorption, biosorption and biotransformation, which contributed 4.9%/7.2%, 8.1%/8.9%, 38.8%/9.5% and 48.2%/84.9% of overall Hg/NO removal in MBfR. Herein, Hg bio-oxidation, oxidative Hg biosorption and denitrification played leading roles in simultaneous removal of Hg and NO. Living microbes performed simultaneous Hg bio-oxidation and denitrification, in which Hg as electron donor was biologically oxidized to oxidized mercury (Hg), while NO as the terminal electron acceptor was denitrified to N. The Hg further complexed with humic acids in extracellular polymeric substances via functional groups (-SH, -OH, -NH and -COO) and formed humic acids bound mercury (HA-Hg). Non-living microbial matrix performed oxidative Hg biosorption, in which Hg may be physically adsorbed by cellular matrix, then non-metabolically oxidized to Hg via oxidative complexation with -SH in humic acids and finally cleavage of S-H bond and surface charge transfer led to formation of HA-Hg. Therefore, bioconversion of Hg to HA-Hg by Hg bio-oxidation and oxidative Hg biosorption coupled with NO denitrification to N dynamically cooperated to accomplish simultaneous removal of Hg and NO in MBfR.