Iron-based materials for the remediation of chlorinated ethenes



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Zero-valent iron (ZVI) is one of the most widely used engineered materials for the remediation of chlorinated ethenes in the subsurface environment. The material has been widely used in various in situ remediation technologies including Fe-permeable reactive barriers (Fe-PRB) and subsurface injection of nanoscale zero-valent iron (nZVI) for contaminant plume attenuation and source zone remediation. However, there are two serious drawbacks when ZVI is used for the remediation of chlorinated ethenes: (i) ZVI tends to undergo rapid passivation which undermines its longevity for remediation applications, and (ii) ZVI has low dechlorination reactivity in the absence of catalyst additives. Bimetallic nanoparticles (BNPs), prepared by doping a small amount of catalytic metals (e.g., Pd or Ni), can significantly enhance the particles’ dechlorination reactivity. However, BNPs suffer rapid deactivation when exposed to groundwater media, and the BNPs deactivation mechanisms are still poorly understood. The first part of this dissertation aims to investigate Ni-Fe BNPs and Pd-Fe BNPs deactivation mechanisms when exposing them to common groundwater solutes. Aging experiments were conducted by pre-immersing fresh prepared BNPs in solutions containing different groundwater solutes for 24 h prior to reacting the particles with trichloroethene (TCE) to assess their dechlorination reactivity. Analyses of reaction kinetics and product distribution and stable carbon isotope fractionation measurements suggest that Pd-Fe BNPs were sensitive to solute-induced deactivation, particularly in solutions containing chloride, bicarbonate, nitrate or sulfite ions. Although Ni-Fe BNPs possess higher electrochemical stability than Pd-Fe BNPs in the aqueous media, strong deactivation was observed in sulfate, nitrate, and phosphate solutions. Multiple modes of BNP deactivation were proposed for the two types of BNPs.
To overcome the intrinsic limitations of conventional and bimetallic ZVI materials, the second part of this dissertation aims to develop a new form of ZVI with higher dechlorination reactivity without the use of catalyst additives and a greater resistance to environmental passivation. A surface sulfidation treatment was designed and optimized for laboratory-made nZVI and commercial ZVI. The sulfided nZVI demonstrates remarkable improvements in dechlorination rates for chlorinated ethenes. Aging experiments indicated that sulfided nZVI possesses greater stability and maintains its dechlorination reactivity over long-term aging processes. Applying sulfidation treatment to commercial iron results in more efficient tetrachloroethene (PCE) and TCE degradation. Sulfidation treatment therefore represents a simple yet promising approach to increase the reactivity of ZVI using earth-abundant reagents in place of precious catalyst metals.

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Iron-Based Materials, Chlorinated Ethenes, Subsurface Remediation, Dechlorination