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

Abstract

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 /H 2 O 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 /H 2 O 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 /H 2 O and Cr VI /H 2 O 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 .

The Impact of Selected Pretreatment Procedures on Iron Dissolution from Metallic Iron Specimens Used in Water Treatment

“…Metallic iron (Fe 0 ) is a potential reducing agent for many reducible species, including water (H 2 O) and O 2 [15,26,56,57,114]. The redox potential of water is 0.00 V, making water a powerful oxidizing agent for Fe 0 (E 0 = −0.44 V).…”

“…Coagulation and flotation inducing coprecipitation and size-exclusion are less well understood [138,139] but are also well-established. However, it is clear from the published literature that there is a lack of a quantitative appreciation of the way in which these individual processes interact to provide efficient Fe 0 /H 2 O systems [9,26,28,33,47,[54][55][56][57][58][59][60]102,114,140] For the Fe 0 -based systems to become an accepted and dependable water treatment technology and play the expected wider role in decentralized water treatments, research is required that focuses neither on simply making a specific pollutant-centred application work (treatability studies) nor on any one of the named foundation processes but rather on the emphasis of explaining and quantifying the key interactions between electrochemistry, adsorption, coprecipitation and size-exclusion. The Fe 0 /MnO 2 /H 2 O system is a good candidate to start this systematic work.…”

Effect of the Surface Charge on the Adsorption Capacity of Chromium(VI) of Iron Oxide Magnetic Nanoparticles Prepared by Microwave-Assisted Synthesis

“…Common drinking water can be considered toxic when it contains more than 0.05 mg/L of Cr(VI), because this chromium state is found to be highly soluble and toxic. The chromates HCrO 4− and Cr 2 O 7 2− have been discharged over the years by many industrial activities in the fields of petroleum refining, electroplating, metal coating and batteries, among others [1,5,6]. In a common wastewater treatment process, the removal of these kind of compounds takes place through chemical and physical treatments using conventional methods such as coagulation and flocculation, membrane separation, oxidation, adsorption and ionic exchange [7,8].…”

Defeating Fluorosis in the East African Rift Valley: Transforming the Kilimanjaro into a Rainwater Harvesting Park

“…The simplest scenario would entail pumping water from an uncontaminated source to the local RWH tanks. A more reliable alternative is to treat available natural waters with available efficient (non-cost-effective) technologies as an interim measure [77][78][79][80].…”