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Sustainable Remediation Using Metallic Iron: Quo Vadis?

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A special issue of Water (ISSN 2073-4441). This special issue belongs to the section "Wastewater Treatment and Reuse".

Deadline for manuscript submissions: closed (10 November 2023) | Viewed by 17223

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Dr. Chicgoua Noubactep

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Guest Editor

Angewandte Geologie, Universität Göttingen, Goldschmidtstraße 3, D-37077 Göttingen, Germany
Interests: adsorption; decentralized systems; filtration; rainwater harvesting; water treatment; zerovalent iron
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Dr. Marius Gheju

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Guest Editor

Faculty of Industrial Chemistry and Environmental Engineering, Politehnica University Timisoara, Bd. V. Parvan Nr. 6, 300223 Timisoara, Romania
Interests: water treatment; adsorption processes; zerovalent iron; biosorbents; heavy metals

Special Issue Information

Dear Colleagues,

During the past three decades, groundwater remediation using permeable reactive barriers (PRBs) containing metallic iron (Fe⁰) has become a well-established technology. However, many uncertainties exist regarding their design, suggesting that Fe⁰ PRBs is still an innovative technology.

Research on Fe⁰ PRBs started in the early 1990s and has boomed in the past three decades. Sufficient data and observations have been accumulated to establish the science of the Fe⁰/H₂O system. To explain the initial observation that there were losses of chlorinated organic contaminants from aqueous solutions in contact with a variety of metals (including Fe⁰), it was proposed that reductive dechlorination was the main cause, with electrons coming from the metal body. In the meantime, Fe⁰ is described in the literature as “reservoir of electrons” for contaminant transformation. However, considering Fe⁰ as source of electrons for dissolved species, including O₂, contradicts the century-old knowledge that, under environmental conditions, the Fe⁰ surface is covered by a non-conductive oxide scale, hindering any electron transfer from the Fe⁰ body. This state of affairs implies that abiotic reductive transformations in Fe⁰/H₂O systems are mediated by secondary reducing agents such as Fe^II species, Fe^II/Fe^III species, and H₂. Biotic transformations are also reported, using Fe^II or H₂ as electron sources. In other words, the still widely accepted operating mode of Fe⁰/H₂O systems seems not to have a scientific basis. The other evidence against this operating mode is that its establishment was not based on any holistic approach since the essence of aqueous iron corrosion is by water and is found in trace amounts (humidity). Clearly, the conversion of data and observations into knowledge constitutes a great challenge for the Fe⁰ research community.

This Special Issue welcomes papers highlighting: (i) established or innovative Fe⁰ materials for water treatment; (ii) models and research tools applied to advancing the understanding of the Fe⁰/H₂O system; and (iii) case studies, conceptual frameworks, viewpoints, and field applications. Special attention is given (but is not limited) to the assessment of operation modes of innovative materials (e.g., composites) and their sustainability under field conditions. The objective is to give state-of-the-art knowledge of the development of Fe⁰ PRBs-based recent advances in understanding the operating mode of the Fe⁰/H₂O system, and in particular, the view that Fe⁰ is not a stand-alone reducing agent under environmental conditions.

Dr. Chicgoua Noubactep
Dr. Marius Gheju
Guest Editors

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Keywords

corrosion kinetics
groundwater remediation
iron corrosion, permeable reactive barrier (PRB)
permeability loss, reactivity loss, scrap iron filing
zero-valent iron

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20 pages, 3228 KiB

Open AccessArticle

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

by Luis E. Lan, Fernando D. Reina, Graciela E. De Seta, Jorge M. Meichtry and Marta I. Litter

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

Cited by 19 | Viewed by 3112

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 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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19 pages, 3717 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

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

by Marius Gheju and Ionel Balcu

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

Cited by 8 | Viewed by 2487

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 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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76 pages, 15705 KiB

Open AccessArticle

Hydrodynamic Decontamination of Groundwater and Soils Using ZVI

by David D. J. Antia

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

Cited by 4 | Viewed by 4313

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 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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37 pages, 8450 KiB

Open AccessFeature PaperArticle

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

by David Dorab Jamshed Antia

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

Cited by 12 | Viewed by 3037

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 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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19 pages, 394 KiB

Open AccessOpinion

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

by Viet Cao, Omari Bakari, Joseline Flore Kenmogne-Tchidjo, Nadège Gatcha-Bandjun, Arnaud Igor Ndé-Tchoupé, Willis Gwenzi, Karoli N. Njau and Chicgoua Noubactep

Water 2022, 14(19), 3120; https://doi.org/10.3390/w14193120 - 3 Oct 2022

Cited by 6 | Viewed by 2206

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 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?)

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