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Open AccessFeature PaperArticle

Developing the Ascorbic Acid Test: A Candidate Standard Tool for Characterizing the Intrinsic Reactivity of Metallic Iron for Water Remediation

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Water 2023, 15(10), 1930; https://doi.org/10.3390/w15101930 - 19 May 2023

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Abstract

Granular metallic iron (gFe⁰) materials have been widely used for eliminating a wide range of pollutants from aqueous solutions over the past three decades. However, the intrinsic reactivity of gFe⁰ is rarely evaluated and existing methods for such evaluations have [...] Read more.

Granular metallic iron (gFe⁰) materials have been widely used for eliminating a wide range of pollutants from aqueous solutions over the past three decades. However, the intrinsic reactivity of gFe⁰ is rarely evaluated and existing methods for such evaluations have not been standardized. The aim of the present study was to develop a simple spectrophotometric method to characterize the intrinsic reactivity of gFe⁰ based on the extent of iron dissolution in an ascorbic acid (AA—0.002 M or 2 mM) solution. A modification of the ethylenediaminetetraacetic acid method (EDTA method) is suggested for this purpose. Being an excellent chelating agent for Fe^II and a reducing agent for Fe^III, AA sustains the oxidative dissolution of Fe⁰ and the reductive dissolution of Fe^III oxides from gFe⁰ specimens. In other words, Fe⁰ dissolution to Fe^II ions is promoted while the further oxidation to Fe^III ions is blocked. Thus, unlike the EDTA method that promotes Fe⁰ oxidation to Fe^III ions, the AA method promotes only the formation of Fe^II species, despite the presence of dissolved O₂. The AA test is more accurate than the EDTA test and is considerably less expensive. Eight selected gFe⁰ specimens (ZVI1 through ZVI8) with established diversity in intrinsic reactivity were tested in parallel batch experiments (for 6 days) and three of these specimens (ZVI1, ZVI3, ZVI5) were further tested for iron leaching in column experiments (for 150 days). Results confirmed the better suitability (e.g., accuracy in assessing Fe⁰ dissolution) of the AA test relative to the EDTA test as a powerful screening tool to select materials for various field applications. Thus, the AA test should be routinely used to characterize and rationalize the selection of gFe⁰ in individual studies. Full article

(This article belongs to the Special Issue Sustainable Remediation Using Metallic Iron: Quo Vadis?)

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Open AccessArticle

Comparison between Different Technologies (Zerovalent Iron, Coagulation-Flocculation, Adsorption) for Arsenic Treatment at High Concentrations

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Water 2023, 15(8), 1481; https://doi.org/10.3390/w15081481 - 11 Apr 2023

Cited by 2 | Viewed by 1003

Abstract

The presence of arsenic in water for human consumption is of concern, especially in developing countries, and the design of simple and economic treatments for arsenic removal is imperative. In this paper, three low-cost technologies were evaluated for As(V) or As(III) (5 mg [...] Read more.

The presence of arsenic in water for human consumption is of concern, especially in developing countries, and the design of simple and economic treatments for arsenic removal is imperative. In this paper, three low-cost technologies were evaluated for As(V) or As(III) (5 mg L⁻¹) removal: (1) zerovalent iron (Fe(0)), as powdered (μFe(0)) and iron wool (wFe(0)); (2) coagulation-flocculation with Al₂(SO₄)₃ or FeCl₃; and (3) adsorption on a natural clay. μFe(0) was more efficient than wFe(0), requiring a minimal dose of 0.25 g L⁻¹ to achieve [As(V)] < 0.01 mg L⁻¹ after 288 h; the reaction time was reduced to 168 h under stirring. When starting from As(III), partial oxidation to As(V) was observed, and removal was not complete even after 648 h with 1 g L⁻¹ μFe(0). As(V) removal using FeCl₃ and Al₂(SO₄)₃ was very fast and completed in 15 min with 0.25 g L⁻¹ of both reagents. However, Al₂(SO₄)₃ was not efficient to remove As(III). With the clay, doses higher than 50 g L⁻¹ and times longer than 648 h were needed to remove both As species. Arsenic leached from μFe(0) used to treat As(III) was almost negligible. Thus, Fe(0) may be the best alternative for low-cost, small-scale applications. Full article

(This article belongs to the Special Issue Sustainable Remediation Using Metallic Iron: Quo Vadis?)

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Open AccessFeature PaperEditor’s ChoiceArticle

Effect of Sand Co-Presence on Cr^VI Removal in Fe⁰-H₂O System

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Marius Gheju

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Ionel Balcu

Water 2023, 15(4), 777; https://doi.org/10.3390/w15040777 - 16 Feb 2023

Cited by 3 | Viewed by 946

Abstract

The aim of the present study was to provide new knowledge regarding the effect of non-expansive inert material addition on anionic pollutant removal efficiency in Fe⁰-H₂O system. Non-disturbed batch experiments and continuous-flow-through column tests were conducted using Cr^VI [...] Read more.

The aim of the present study was to provide new knowledge regarding the effect of non-expansive inert material addition on anionic pollutant removal efficiency in Fe⁰-H₂O system. Non-disturbed batch experiments and continuous-flow-through column tests were conducted using Cr^VI as a redox–active contaminant in three different systems: “Fe⁰ + sand”, “Fe⁰ only” and ”sand only”. Both experimental procedures have the advantage that formation of (hydr)oxide layers on Fe⁰ is not altered, which makes them appropriate proxies for real Fe⁰-based filter technologies. Batch experiments carried out at pH 6.5 showed a slight improvement of Cr^VI removal in a 20% Fe⁰ system, compared to 50, 80 and 100% Fe⁰ systems. Column tests conducted at pH 6.5 supported results of batch experiments, revealing highest Cr^VI removal efficiencies for “Fe⁰ + sand” systems with lowest Fe⁰ ratio. However, the positive effect of sand co-presence decreases with increasing pH from 6.5 to 7.1. Scanning electron microscopy—energy dispersive angle X-ray spectrometry and X-ray diffraction spectroscopy employed for the characterization of Fe⁰ before and after experiments indicated that the higher the volumetric ratio of sand in “Fe⁰ + sand” system, the more intense the corrosion processes affecting the Fe⁰ grains. Results presented herein indicate the capacity of sand at sustaining the efficiency of Cr^VI removal in Fe⁰-H₂O system. The outcomes of the present study suggest that a volumetric ratio Fe⁰:sand = 1:3 could assure not only the long-term permeability of Fe⁰-based filters, but also enhanced removal efficiency of Cr^VI from contaminated water. Full article

(This article belongs to the Special Issue Sustainable Remediation Using Metallic Iron: Quo Vadis?)

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Open AccessArticle

Hydrodynamic Decontamination of Groundwater and Soils Using ZVI

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David D. J. Antia

Water 2023, 15(3), 540; https://doi.org/10.3390/w15030540 - 29 Jan 2023

Cited by 1 | Viewed by 1906

Abstract

Polluted aquifers can be decontaminated using either ZVI (zero valent iron) permeable reactive barriers (PRB) or injected ZVI. The placement of ZVI within the aquifer may take several decades to remediate the contaminant plume. Remediation is further complicated by ZVI acting as an [...] Read more.

Polluted aquifers can be decontaminated using either ZVI (zero valent iron) permeable reactive barriers (PRB) or injected ZVI. The placement of ZVI within the aquifer may take several decades to remediate the contaminant plume. Remediation is further complicated by ZVI acting as an adsorbent to remove some pollutants, while for other pollutants, it acts as a remediation catalyst. This study investigates an alternative aquifer decontamination approach to PRB construction or n-Fe⁰ injection. The alternative approach reconstructs the potentiometric surface of the aquifer containing the contaminant. This reconstruction confines the contaminant plume to a stationary, doughnut shaped hydrodynamic mound. Contaminated water from the mound is abstracted, decontaminated, and then reinjected, until all the water confined within the mound is decontaminated. At this point, the decontaminated mound is allowed to dissipate into the surrounding aquifer. This approach is evaluated for potential use in treating the following: (i) immiscible liquid plumes; (ii) miscible contaminant and ionic solute plumes; (iii) naturally contaminated aquifers and soils; and (iv) contaminated or salinized soils. The results indicate that this approach, when compared with the PRB or injection approach, may accelerate the decontamination, while reducing the overall amount of ZVI required. Full article

(This article belongs to the Special Issue Sustainable Remediation Using Metallic Iron: Quo Vadis?)

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Open AccessFeature PaperArticle

Remediation of Saline Wastewater Producing a Fuel Gas Containing Alkanes and Hydrogen Using Zero Valent Iron (Fe⁰)

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David Dorab Jamshed Antia

Water 2022, 14(12), 1926; https://doi.org/10.3390/w14121926 - 15 Jun 2022

Cited by 9 | Viewed by 1469

Abstract

Zero valent iron (Fe⁰) water remediation studies, over the last 40 years, have periodically reported the discovery of C_nH_2n+2 in the product water or product gas, where n = 1 to 20. Various theories have been proposed for [...] Read more.

Zero valent iron (Fe⁰) water remediation studies, over the last 40 years, have periodically reported the discovery of C_nH_2n+2 in the product water or product gas, where n = 1 to 20. Various theories have been proposed for the presence of these hydrocarbons. These include: (i) reductive transformation of a more complex organic chemical; (ii) hydrogenation of an organic chemical, as part of a degradation process; (iii) catalytic hydrogenation and polymerisation of carbonic acid; and (iv) redox transformation. This study uses wastewater (pyroligneous acid, (pH = 0.5 to 4.5)) from a carbonization reactor processing municipal waste to define the controls for the formation of C_nH_2n+2 (where n = 3 to 9), C₃H₄, and C₃H₆. A sealed, static diffusion, batch flow reactor, containing zero-valent metals [181 g m-Fe⁰ + 29 g m-Al⁰ + 27 g m-Cu⁰ + 40 g NaCl] L⁻¹, was operated at two temperatures, 273–298 K and 348 K, respectively. The reactions, reactant quotients, and rate constants for the catalytic formation of H_2(g), CO_2(g), C₃H_4(g), C₃H_6(g), C₃H_8(g), C₄H_10(g), C₅H_12(g), C₆H_14(g,l), and C₇H_16(g,l), are defined as function of zero valent metal concentration (g L⁻¹), reactor pressure (MPa), and reactor temperature (K). The produced fuel gas (422–1050 kJ mole⁻¹) contained hydrogen + C_nH_y(gas), where n = 3 to 7. The gas production rate was: [1058 moles C_nH_y + 132 moles H₂] m⁻³ liquid d⁻¹ (operating pressure = 0.1 MPa; temperature = 348 K). Increasing the operating pressure to 1 MPa increased the fuel gas production rate to [2208 moles C_nH_y + 1071 moles H₂] m⁻³ liquid d⁻¹. In order to achieve these results, the Fe⁰, operated as a “Smart Material”, simultaneously multi-tasking to create self-assembly, auto-activated catalysts for hydrogen production, hydrocarbon formation, and organic chemical degradation (degrading carboxylic acids and phenolic species to CO₂ and CO). Full article

(This article belongs to the Special Issue Sustainable Remediation Using Metallic Iron: Quo Vadis?)

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Open AccessArticle

Fe⁰-Supported Anaerobic Digestion for Organics and Nutrients Removal from Domestic Sewage

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Omari Bakari

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Karoli N. Njau

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Chicgoua Noubactep

Water 2022, 14(10), 1623; https://doi.org/10.3390/w14101623 - 18 May 2022

Cited by 5 | Viewed by 1071 | Correction

Abstract

Results from different research suggest that metallic iron (Fe⁰) materials enhance anaerobic digestion (AD) systems to remove organics (chemical oxygen demand (COD)), phosphorus and nitrogen from polluted water. However, the available results are difficult to compare because they are derived from [...] Read more.

Results from different research suggest that metallic iron (Fe⁰) materials enhance anaerobic digestion (AD) systems to remove organics (chemical oxygen demand (COD)), phosphorus and nitrogen from polluted water. However, the available results are difficult to compare because they are derived from different experimental conditions. This research characterises the effects of Fe⁰ type and dosage in AD systems to simultaneously remove COD and nutrients (orthophosphate (PO₄³⁻), ammonium (NH₄⁺), and nitrate (NO₃⁻)) Lab-scale reactors containing domestic sewage (DS) were fed with various Fe⁰ dosages (0 to 30 g/L). Batch AD experiments were operated at 37 ± 0.5 °C for 76 days; the initial pH value was 7.5. Scrap iron (SI) and steel wool (SW) were used as Fe⁰ sources. Results show that: (i) SW performed better than SI on COD and PO₄³⁻ removal (ii) optimum dosage for the organics and nutrients removal was 10 g/L SI (iii) (NO₃⁻ + NH₄⁺) was the least removed pollutant (iv) maximum observed COD, PO₄³⁻ and NO₃⁻ + NH₄⁺ removal efficiencies were 88.0%, 98.0% and 40.0% for 10 g/L SI, 88.2%, 99.9%, 25.1% for 10 g/L SW, and 68.9%, 7.3% and 0.7% for the reference system. Fe⁰-supported AD significantly removed the organics and nutrients from DS. Full article

(This article belongs to the Special Issue Sustainable Remediation Using Metallic Iron: Quo Vadis?)

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Review

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Open AccessFeature PaperReview

Kanchan Arsenic Filters for Household Water Treatment: Unsuitable or Unsustainable?

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Arnaud Igor Ndé-Tchoupé

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Bernard Konadu-Amoah

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Nadège Gatcha-Bandjun

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Rui Hu

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Willis Gwenzi

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Chicgoua Noubactep

Water 2022, 14(15), 2318; https://doi.org/10.3390/w14152318 - 26 Jul 2022

Cited by 1 | Viewed by 1055

Abstract

This article critically evaluates the conventional Kanchan Arsenic Filter (KAF) in order to determine the main reasons for its reported poor performance. The KAF was introduced in 2004 in Nepal and makes use of non-galvanized nails as a Fe⁰ source for As [...] Read more.

This article critically evaluates the conventional Kanchan Arsenic Filter (KAF) in order to determine the main reasons for its reported poor performance. The KAF was introduced in 2004 in Nepal and makes use of non-galvanized nails as a Fe⁰ source for As removal. As early as 2009, the KAF was demonstrated to be ineffective for As removal in many cases. This was unambiguously attributed to the Fe⁰ layer which is placed on top of a sand filter instead of being incorporated into a sand matrix. Despite this conceptual mistake, the conventional KAF has been largely distributed in Asia, and recent articles have assessed its sustainability. This study reiterates that the suitability of the technology, rather than its sustainability, should be addressed. Evidence shows that the KAF has the following design limitations: (i) uses iron nails of unknown reactivity, and (ii) operates on the principle of a wet/dry cycle. The latter causes a decrease in the corrosion rate of the used nails, thereby limiting the availability of the iron corrosion products which act as contaminant scavengers. Taken together, these results confirm the unsuitability of the conventional KAF. Besides correcting the design mistakes, more attention should be paid to the intrinsic reactivity of the used iron nails, including using alternative Fe⁰ materials (e.g., iron filings, steel wool) for filters lasting for just 6 or 18 months. Specific design considerations to be addressed in the future are highlighted. Full article

(This article belongs to the Special Issue Sustainable Remediation Using Metallic Iron: Quo Vadis?)

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Other

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Open AccessOpinion

Conceptualizing the Fe⁰/H₂O System: A Call for Collaboration to Mark the 30th Anniversary of the Fe⁰-Based Permeable Reactive Barrier Technology

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Viet Cao

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Omari Bakari

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Joseline Flore Kenmogne-Tchidjo

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Nadège Gatcha-Bandjun

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Arnaud Igor Ndé-Tchoupé

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Willis Gwenzi

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Karoli N. Njau

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Chicgoua Noubactep

Water 2022, 14(19), 3120; https://doi.org/10.3390/w14193120 - 03 Oct 2022

Cited by 3 | Viewed by 1031

Abstract

Science denial relates to rejecting well-established views that are no longer questioned by scientists within a given community. This expression is frequently connected with climate change and evolution. In such cases, prevailing views are built on historical facts and consensus. For water remediation [...] Read more.

Science denial relates to rejecting well-established views that are no longer questioned by scientists within a given community. This expression is frequently connected with climate change and evolution. In such cases, prevailing views are built on historical facts and consensus. For water remediation using metallic iron (Fe⁰), also known as the remediation Fe⁰/H₂O system, a consensus on electro-chemical contaminant reduction was established during the 1990s and still prevails. Arguments against the reductive transformation concept have been regarded for more than a decade as ‘science denial’. However, is it the prevailing concept that denies the science of aqueous iron corrosion? This article retraces the path taken by our research group to question the reductive transformation concept. It is shown that the validity of the following has been questioned: (i) analytical applications of the arsenazo III method for the determination of uranium, (ii) molecular diffusion as sole relevant mass-transport process in the vicinity of the Fe⁰ surface in filtration systems, and (iii) the volumetric expansive nature of iron corrosion at pH > 4.5. Item (i) questions the capability of Fe⁰ to serve as an electron donor for U^VI reduction under environmental conditions. Items (ii) and (iii) are inter-related, as the Fe⁰ surface is permanently shielded by a non-conductive oxide scale acting as a diffusion barrier to dissolved species and a barrier to electrons from Fe⁰. The net result is that no electron transfer from Fe⁰ to contaminants is possible under environmental conditions. This conclusion refutes the validity of the reductive transformation concept and calls for alternative theories. Full article

(This article belongs to the Special Issue Sustainable Remediation Using Metallic Iron: Quo Vadis?)