Bioconversion of Hg0 into HA-Hg for simultaneous removal of Hg0 and NO in a denitrifying membrane biofilm reactor.

Abstract

Bacterial mercury oxidation coupled to denitrification offers great potential for simultaneous removal of elemental mercury (Hg0) 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 Hg0/NO removal in MBfR. Herein, Hg0 bio-oxidation, oxidative Hg0 biosorption and denitrification played leading roles in simultaneous removal of Hg0 and NO. Living microbes performed simultaneous Hg0 bio-oxidation and denitrification, in which Hg0 as electron donor was biologically oxidized to oxidized mercury (Hg2+), while NO as the terminal electron acceptor was denitrified to N2. The Hg2+ 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 Hg0 biosorption, in which Hg0 may be physically adsorbed by cellular matrix, then non-metabolically oxidized to Hg2+ 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 Hg0 to HA-Hg by Hg0 bio-oxidation and oxidative Hg0 biosorption coupled with NO denitrification to N2 dynamically cooperated to accomplish simultaneous removal of Hg0 and NO in MBfR.