For the artificial ground freezing (AGF) projects in highly permeable formations, the effect of groundwater flow cannot be neglected. Based on the heat transfer and seepage theory in porous media with the finite element method, a fully coupled numerical model was established to simulate the changes of temperature field and groundwater flow field. Firstly, based on the classic analytical solution for the frozen temperature field, the model’s ability to solve phase change problems has been validated. In order to analyze the influences of different parameters on the closure time of the freezing wall, we performed the sensitivity analysis for three parameters of this numerical model. The analysis showed that, besides the head difference, the thermal conductivity of soil grain and pipe spacing are also the key factors that control the closure time of the frozen wall. Finally, a strengthening project of a metro tunnel with AGF method in South China was chosen as a field example. With the finite element software COMSOL Multiphysics^® (Stockholm, Sweden), a three-dimensional (3D) numerical model was set up to simulate the change of frozen temperature field and groundwater flow field in the project area as well as the freezing process within 50 days. The simulation results show that the freezing wall appears in an asymmetrical shape with horizontal groundwater flow normal to the axial of the tunnel. Along the groundwater flow direction, freezing wall forms slowly and on the upstream side the thickness of the frozen wall is thinner than that on the downstream side. The actual pipe spacing has an important influence on the temperature field and closure time of the frozen wall. The larger the actual pipe spacing is, the slower the closing process will be. Besides this, the calculation for the average temperature of freezing body (not yet in the form of a wall) shows that the average temperature change of the freezing body coincides with that of the main frozen pipes with the same trend.

Metallic iron (Fe⁰) materials have been industrially used for water treatment since the 1850s. There are still many fundamental challenges in affordably and reliably characterizing the Fe⁰ intrinsic reactivity. From the available methods, the one using Fe⁰ dissolution in ethylenediaminetetraacetic acid (EDTA—2 mM) was demonstrated the most applicable as it uses only four affordable chemicals: Ascorbic acid, an ascorbate salt, EDTA and 1,10-Phenanthroline (Phen). A careful look at these chemicals reveals that EDTA and Phen are complexing agents for dissolved iron species. Fe³-EDTA is very stable and difficult to destabilize; ascorbic acid is one of the few appropriate reducing agents, therefore. On the other hand, the Fe²-Phen complex is so stable that oxidation by dissolved O₂ is not possible. This article positively tests Fe⁰ (0.1 g) dissolution in 2 mM Phen (50 mL) as a characterization tool for the intrinsic reactivity, using 9 commercial steel wool (Fe⁰ SW) specimens as probe materials. The results are compared with those obtained by the EDTA method. The apparent iron dissolution rate in EDTA (k_EDTA) and in Phen (k_Phen) were such that 0.53 ≤ k_EDTA (μg h⁻¹) ≤ 4.81 and 0.07 ≤ k_Phen (μg h⁻¹) ≤ 1.30. Higher k_EDTA values, relative to k_Phen, are a reflection of disturbing Fe³ species originating from Fe² oxidation by dissolved O₂ and dissolution of iron corrosion products. It appears that the Phen method considers only the forward dissolution of Fe⁰. The Phen method is reliable and represents the most affordable approach for characterizing the suitability of Fe⁰ for water treatment.

The ambitious United Nations Sustainable Development Goal for 2030 to “leave no one behind” concerning safe drinking water calls for the development of universally applicable and affordable decentralized treatment systems to provide safe drinking water. Published results suggest that well-designed biological sand filters (BSFs) amended with metallic iron (Fe⁰-BSFs) have the potential to achieve this goal. Fe⁰-BSFs quantitatively remove pathogens and a myriad of chemical pollutants. The available data were achieved under various operating conditions. A comparison of independent research results is almost impossible, especially because the used Fe⁰ materials are not characterized for their intrinsic reactivity. This communication summarizes the state-of-the-art knowledge on designing Fe⁰-BSFs for households and small communities. The results show that significant research progress has been made on Fe⁰-BSFs. However, well-designed laboratory and field experiments are required to improve the available knowledge in order to develop the next generation of adaptable and scalable designs of Fe⁰-BSFs in only two years. Tools to alleviate the permeability loss, the preferential flow, and the use of exhausted filters are presented.

Researchers and engineers using metallic iron (Fe⁰) for water treatment need a tutorial review on the operating mode of the Fe⁰/H₂O system. There are few review articles attempting to present systematic information to guide proper material selection and application conditions. However, they are full of conflicting reports. This review seeks to: (i) Summarize the state-of-the-art knowledge on the remediation Fe⁰/H₂O system, (ii) discuss relevant contaminant removal mechanisms, and (iii) provide solutions for practical engineering application of Fe⁰-based systems for water treatment. Specifically, the following aspects are summarized and discussed in detail: (i) Fe⁰ intrinsic reactivity and material selection, (ii) main abiotic contaminant removal mechanisms, and (iii) relevance of biological and bio-chemical processes in the Fe⁰/H₂O system. In addition, challenges for the design of the next generation Fe⁰/H₂O systems are discussed. This paper serves as a handout to enable better practical engineering applications for environmental remediation using Fe⁰.

A critical survey of the abundant literature on environmental remediation and water treatment using metallic iron (Fe⁰) as reactive agent raises two major concerns: (i) the peculiar properties of the used materials are not properly considered and characterized, and, (ii) the literature review in individual publications is very selective, thereby excluding some fundamental principles. Fe⁰ specimens for water treatment are typically small in size. Before the advent of this technology and its application for environmental remediation, such small Fe⁰ particles have never been allowed to freely corrode for the long-term spanning several years. As concerning the selective literature review, the root cause is that Fe⁰ was considered as a (strong) reducing agent under environmental conditions. Subsequent interpretation of research results was mainly directed at supporting this mistaken view. The net result is that, within three decades, the Fe⁰ research community has developed itself to a sort of modern knowledge system. This communication is a further attempt to bring Fe⁰ research back to the highway of mainstream corrosion science, where the fundamentals of Fe⁰ technology are rooted. The inherent errors of selected approaches, currently considered as countermeasures to address the inherent limitations of the Fe⁰ technology are demonstrated. The misuse of the terms “reactivity”, and “efficiency”, and adsorption kinetics and isotherm models for Fe⁰ systems is also elucidated. The immense importance of Fe⁰/H₂O systems in solving the long-lasting issue of universal safe drinking water provision and wastewater treatment calls for a science-based system design.