Abstract

In this study, feasibility of nanoscale zero-valent iron (nZVI) application to enhance microbial reductive dechlorination of trichloroethylene (TCE) was evaluated. Batch experiments were conducted to assess the potential of hydrogen production by nZVI and the biodegradability of TCE at a TCE-contaminated site. Results of hydrogen production experiments show that the rates and efficiency of hydrogen production by iron nanoparticles mainly ranged from 8.0 x 10-2 to 15 x 10-2 mg H2/g nFe/hr and from 30 to 76%, respectively. This indicates that iron nanoparticles have high potential to be used as hydrogen producers. Results of microcosm study show that indigenous microorganisms were capable of degrading TCE under anaerobic reductive dechlorinating conditions. Compared to the live control, autoclaved control, and cane molasses microcosms, microcosms with hydrogen addition significantly enhanced the TCE removal rates. Although cane molasses was also used as the electron donor in cane molasses microcosms, lower TCE removal rate was observed. This reveals that hydrogen was more bioavailable than cane molasses during the reductive dechlorinatng process. Although TCE in nZVI microcosms was totally consumed by iron nanoparticles, results of microcosms with hydrogen addition demonstrated that hydrogen was able to improve the efficiency of anaerobic intrinsic biodegradation of TCE. Results imply that nZVI can be applied as the source of hydrogen to bioremediate TCE-contaminated groundwater under anaerobic conditions. Advantages of using iron nanoparticles as the source of hydrogen for the enhancement of anaerobic bioremediation include: (1) rapid removal of significant contaminant concentrations in early stage, (2) creation of a more reducing environment; (3) safer than liquid hydrogen, which is stored in steel containers; (4) direct hydrogen supply without transfer of biological mechanisms; and (5) ease of hydrogen delivery due to the high mobility of nZVI in the subsurface. Except for the biotic mechanism, the supplied nZVI can also cause the TCE degradation via abiotic mechanism through oxidation- reduction process. In the future field application, nZVI contained barrier system can be installed in the mid or downgradient areas of the TCE-spill site for plume control through the hybrid reactions (chemical and biological reactions) to enhance the removal of the contaminants and their byproducts. Knowledge and comprehension obtained in this study will be helpful in designing an enhanced in situ anaerobic bioremediation system for a TCE-contaminated site. The nZVI treatment scheme would be expected to provide a more costeffective alternative to remediate chlorinated-solvent contaminated aquifers.

Keywords: Enhanced bioremediation, electron donor, hydrogen, nanoscale zero-valent iron (nZVI), trichloroethylene (TCE), anaerobic reductive dechlorinating conditions, indigenous microorganisms, biotic mechanism