Reductive dechlorination in water: Interplay of sorption and reactivity

Abstract

Trichloroethene (TCE) dechlorination by nanoscale zero-valent iron (nZVI) in aqueous suspension was investigated in absence and presence of microscale activated carbon (AC). The TCE adsorbed within the AC micropores is available to dechlorination, although a direct contact between sorbed TCE and nZVI is ruled out. The AC functions as a mediator of redox equivalents between the external reductant and the internal sorbate. Therefore, AC plays the role of an adsorbent as well as a dechlorination catalyst driven by the reductive power of nZVI. The dechlorination product patterns in the AC–nZVI mixture and the AC-free slurry are similar, yielding ethane, ethene and ethyne as primary products. One can interpret this as spillover-like transport of mobile reactive species from the iron surface via the carbon matrix to the sorbate. In order to distinguish between transported electrons or hydrogen species, electrochemical experiments with and without the generation of nascent hydrogen (H*) were conducted. They prove the significant role of reactive hydrogen in the nZVI–AC system and favour the spillover mechanism. Ethyne and cis-DCE are the dominant products from TCE in the absence of H* species, whereas total dechlorination and hydrogenation occurred in the presence of H*, yielding ethane and ethene as the dominant primary products. The observed interplay between sorptive enrichment and chemical degradation of pollutants inside the AC pores may offer options for improved iron-carbon composite materials and reactor designs.