dc.description.abstract | The purposes of this study were to investigate the characteristics of the zero-valent iron (ZVI) and various groundwaters quality that may affect the reductive dechlorination of trichloroethylene (TCE). Also, techniques, enhanced TCE removal by ZVI reactive barrier, and the combination of biobarrier would be examined to establish the treatment model of remediation of TCE-contaminated groundwater by ZVI reactive barrier at in-situ field.
Experimental results indicated that the binding energy of Fe 2p on the iron surface was close to the condition of zero valent after acid-washing by XPS analysis, meaning electron released easily from the iron surface. Also, the impurities on the iron surface were reduced and conformation of iron hydroxides was changed to more hydrated α-FeOOH form and reduced the passivation ofγ-Fe2O3 simultaneously, therefore, enhancing the reductive dechlorination of TCE. The efficiency of TCE degradation decreased with the increase in ferrous concentration due to the production of ferrous hydroxide on the surface of acid-washed ZVI. However, ferrous were adsorbed onto the surface of unacid-washed ZVI and transformed to magnetite (Fe3O4). Thus, these permitted electrons to pass through precipitates from iron surface and allowed reductive dechlorination of TCE to occur in bulk solution. The effect of sulfate was similar to ferrous function. But, the sulfate could also react with the ferrous hydroxides and remove the precipitates coated on the iron surface, enhancing the TCE degradation. Nitrate would be preferentially reduced by ZVI so that the competition effect with TCE would appear until nitrate was reduced to ammonium. Effect of humic acid on the TCE degradation was apparent because the humic acid continuously accumulated on the iron surface, therefore the reaction sites of iron surface were decreased obviously.
The direct current was externally supplied to ZVI reactive barrier was feasible to enhance TCE degradation because the surface of the iron particles was acid-washed by H+, owing to water oxidation around the anode and promoted the reactivity of the surface iron. An anode electrode should be placed in contact with the iron filling near the inlet of the column, and then a cathode was located on the top of the column in that the TCE removal efficiency reached 100% and maintained this high level after 25 days with 1.0V/cm of potential gradient applied, displaying excellent potential for development in long-term operations.
Results of batch tests indicated that aerobic co-metabolism with toluene as growth substrate could effectively biodegrade TCE by indigenous soil cells, which were sampled from a doubtful TCE-contaminated site. These indigenous microorganisms could biodegrade vinyl chloride (VC) directly under an aerobic condition. In addition, results of soil microcosm column tests demonstrated that the concentrations of TCE and VC in the column effluent decreased with the increase of operation time. The removal efficiencies of TCE and VC were greater than 99% after 30 days of operation time and no other chlorinated byproducts were detected while TCE and VC were biodegraded. The predominant species were identified as both Burkholderia cepacia ATCC and Burkholderia sp. that were ability to biodegraded TCE by aerobic co-metabolism. The process of iron wall associated with biobarrier could degrade TCE and their chlorinated intermediates completely, thus obtaining the stable groundwater quality. | en_US |