Formatted Title
Using Zero-Valent Bimetals for the Degradation of Chlorinated Solvents Vapors in the Unsaturated Zone
Background/Objectives
Chlorinated solvents, such as trichloroethylene (TCE), have been involved in various civil and industrial applications in the past decades due to their chemical and physical properties. These compounds are characterized by high mobility and low biodegradability at the same time, with consequent persistence in the environment. As a result, many groundwater bodies are currently characterized by diffuse contamination by chlorinated compounds, which can cause potential long-term risks to human health. In particular, the presence of these contaminants in groundwater is challenging for the management of the migration pathway of chlorinated solvents vapors to ambient air or into buildings (i.e., vapor intrusion). In sites characterized by diffuse contamination of chlorinated solvents, traditional remediation techniques are not technically and economically sustainable as they typically require high amounts of reagents or energy. In this scenario, it is more suitable to adopt risk management strategies to mitigate the volatilization pathway of chlorinated solvents vapors. Recently, it was proposed to use horizontal permeable reactive barriers (HPRBs) placed in the unsaturated zone aimed at treating upward volatile organic compounds (VOCs), in imitation of vertical PRBs for the treatment of groundwater contaminated by chlorinated solvents. Zero-valent iron (ZVI) was proposed as reactive material for HPRBs and tested for TCE degradation in the gas phase through reductive dehalogenation. In the last years, zero-valent bimetals based on iron and a secondary transition metal have also been widely investigated for the enhancement of chlorinated compounds degradation in the aqueous phase. In particular, the addition of a secondary transition metal (e.g., Cu or Ni) to iron increases the rate of reduction of chlorinated contaminants, as these metals are catalysts of iron corrosion reaction and of the dissociation of molecular hydrogen on the surface of the material. However, such bimetals have been so far poorly investigated for the treatment of chlorinated solvents in the vapor phase. In this study, we investigate the use of zero-valent Fe-Cu ad Fe-Ni bimetals for the degradation of TCE in the vapor phase at partially saturated conditions in view of using them as filling materials for HPRBs.
Approach/Activities
Different bimetals were synthesized mixing Fe and Ni or Cu powders at different weight percentages (i.e. 1%, 5%, 20%) using disc milling. Once produced, the bimetals were characterized using SEM-EDS, XRD and BET. Then, the produced bimetals were tested in laboratory batch degradation tests at different time intervals for the treatment of TCE vapors at partially saturated conditions to evaluate their reactivity towards dechlorination and their effectiveness as constituent materials for HPRBs.
Results/Lessons Learned
From the characterization analyses, the disc-milled bimetals produced presented particles with micrometric size and homogenous distribution of Cu or Ni in the iron phase. In all the tests, up to 99.9% degradation of TCE vapors was achieved in maximum 4 days with zero-order degradation kinetics. Fe-Ni bimetals have shown better performances in terms of TCE removal compared to Fe-Cu ones leading to up to 99.9% TCE degradation in the vapor phase after 2 days of reaction. These results showed a significant enhancement in TCE removal compared to using ZVI alone, which in previous studies ensured analogue results on TCE vapors degradation after minimum 2 weeks of reaction. The only detectable reaction byproducts in the tested conditions were C3–C6 hydrocarbons, with no presence of vinyl chloride (VC) or dichloroethylene (DCE). In view of using the tested bimetals as constituents for HPRBs to treat chlorinated solvent vapors in the subsoil, the experimental results achieved were integrated into an analytical model to simulate the reactive transport of contaminated vapors through the barrier. From this model, it was found that an HPRB of approximately 20 cm could ensure a complete degradation of TCE in the vapor phase.