Abstract
Hexavalent chromium (CrVI) compounds are used in a variety of industrial applications and, as a result, large quantities of CrVI have been released into the environment due to inadequate precautionary measures or accidental releases. CrVI is highly toxic to most living organisms and a known human carcinogen by inhalation route of exposure. Another major issue of concern about CrVI 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 (Fe0) for the abatement of CrVI polluted waters. Despite a great deal of research, the mechanisms behind the efficient aqueous CrVI removal in the presence of Fe0 (Fe0/H2O systems) remain deeply controversial. The introduction of the Fe0-based filtration technology, at the beginning of 1990s, was coupled with the broad consensus that direct reduction of CrVI by Fe0 was followed by co-precipitation of resulted cations (CrIII, FeIII). This view is still the dominant removal mechanism (reductive-precipitation mechanism) within the Fe0 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 Fe0/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 Fe0/CrVI/H2O and CrVI/H2O systems, and clearly demonstrates that direct reduction with Fe0 followed by precipitation is not acceptable, under environmental relevant conditions, as the sole/main mechanism of CrVI removal in the presence of Fe0.
Highlights
Heavy metal pollution has become a major area of concern because of high concentrations released into the environment
It is important to underline several details: (1) the direct reduction mechanism (Equation (1)) was considered to have the main contribution to CrVI removal; (2) the iron species resulted from the direct reduction of CrVI was FeIII ; and (3) the source of FeII acting as reducing agent in the indirect reduction of CrVI (Equation (3)) was considered to be both Fe0 corrosion (Equation (2))
The first concept proposed in the early nineties for the removal of CrVI with Fe0, and widely accepted since was the reductive-precipitation mechanism
Summary
Heavy metal pollution has become a major area of concern because of high concentrations released into the environment. There are no typical iron fillings with a characteristic range of reactivity This evidence suggests that the primary reason for controversial reports in the Fe0 literature relies on the ill-defined nature of tested and used materials. Water 2018, 10, 651 critically review information concerning the mechanism of CrVI removal from contaminated waters in the Fe0 /H2 O system. Reductive immobilization of chromium likely involves multiple pathways including reaction with dissolved FeII and reaction with ferrous minerals, including mackinawite.”. This statement is representative of the majority of reports on CrVI removal in Fe0 /H2 O systems.
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