Progress in Understanding the Mechanism of CrVI Removal in Fe0-Based Filtration Systems

Hexavalent chromium (Cr-VI) compounds are used in a variety of industrial applications and, as a result, large quantities of Cr-VI have been released into the environment due to inadequate precautionary measures or accidental releases. Cr-VI is highly toxic to most living organisms and a known human carcinogen by inhalation route of exposure. Another major issue of concern about Cr-VI compounds is their high mobility, which easily leads to contamination of surface waters, soil, and ground waters. In recent years, attention has been focused on the use of metallic iron (Fe-0) for the abatement of Cr-VI polluted waters. Despite a great deal of research, the mechanisms behind the efficient aqueous Cr-VI removal in the presence of Fe-0 (Fe-0/H2O systems) remain deeply controversial. The introduction of the Fe-0-based filtration technology, at the beginning of 1990s, was coupled with the broad consensus that direct reduction of Cr-VI by Fe-0 was followed by co-precipitation of resulted cations (Cr-III, Fe-III). This view is still the dominant removal mechanism (reductive-precipitation mechanism) within the Fe-0 remediation industry. An overview on the literature on the Cr geochemistry suggests that the reductive-precipitation theory should never have been adopted. Moreover, recent investigations recalling that a Fe-0/H2O system is an ion-selective one in which electrostatic interactions are of primordial importance is generally overlooked. The present work critically reviews existing knowledge on the Fe-0/Cr-VI/H2O and Cr-VI/H2O systems, and clearly demonstrates that direct reduction with Fe-0 followed by precipitation is not acceptable, under environmental relevant conditions, as the sole/main mechanism of Cr-VI removal in the presence of Fe-0.