Previous column experiments have showed the effectiveness of nZVI for the in situ immobilization of heavy metals, which reduces their potential leachability, as a strategy to prevent their transport into deeper soil layers, rivers, and groundwater [ 21 ]. Iron nanoparticles are particularly attractive for environmental remediation because these are much more reactive than iron powders and they can be suspended in slurry and moved to the polluted site [ 22 , 23 ]. Recently, the synthesis and utilization of iron-based nanomaterials with novel properties and functions have been widely studied, both for their nanosize and for their magnetic characteristics [ 14 ].
For this reason, these nanoparticles are being used for in situ experiments [ 15 , 16 ]. For example, during the remediation of contaminated soils, nZVI has become a widespread amendment for in situ applications, since it can form a permeable barrier in the soil in order to prevent the dissemination of contaminants by the soil pore water, thus achieving their immobilization [ 18 , 24 ]. Efficient nZVI remediation of groundwater contaminants has been shown in multiple studies; however, regarding nZVI-induced soil toxicity, limited data have been reported, providing preliminary results about the effects of nZVI on soil biota and some plant species [ 19 ].
Most of the reported studies have been conducted either under no real conditions or only considering short-term exposure. Therefore, the impact of nZVI treatment on soil properties and functionality remains unclear. On the other hand, very few investigations of nZVI materials present a detailed study of the products formed in the remediation process for reusability of these nZVI after treatment [ 25 , 26 , 27 ]. In the nZVI reaction, metallic iron is oxidized in the presence of water, which can remove other metal ions from aqueous media by chemical absorption.
Hematite has been studied for catalytic applications because of the presence of active photochemical properties [ 28 ]. Even so, the application of hematite in PSC is a challenge for the scientist community because this species is highly active in UV range but not absorbed in the visible range. Of all the metallic contaminants, cadmium draws special attention because of its high affinity and water solubility [ 32 ].
Cadmium species have been detected in aquatic ecosystems and found to bioaccumulate in organisms in nanomolar to micromolar concentrations [ 33 ]. Efficient nZVI remediations of groundwater contaminants have been shown in multiple studies [ 33 , 34 , 35 , 36 , 37 ]. However, in the literature, there is a lack of comparable studies for different nZVI materials and deployment strategies [ 38 ].
Results from our study provide important information of the products formed during the remediation process of cadmium. Studies of redox and adsorption processes after treatment of nZVI have been evaluated [ 39 ].
However, it is necessary to understand in detail what occurs in the cadmium adsorption process at the nanoparticle surface. These results show that nZVI is an alternative to decrease high Cd concentration in contaminated sites. However, there are no sufficient data about the possible formation of toxic product after treatment with nZVI. For this reason, a structural analysis of used nZVI was deemed imperative to gain an understanding of the interactions between the nZVI and cadmium.
This new knowledge may serve to optimize the remediation process and to provide alternative uses for the remediation product. The formations of unexpected nanofibers and cadmium ferrite structures have been reported. This remediation product or environmental waste has been suggested as a photocatalyst material that has great potential application for light harvesting [ 40 ]. These results could be useful because we can prevent the waste formation after chemical process, and reuse the products of remediation processes for other energy applications.
This will decrease the amount of new hazardous substances produced after water decontamination processes. This conceptual model of Fe 0 nanofibers was presented considering the results of X-ray diffraction patterns, X-photoelectron spectroscopy results, X-ray absorption spectroscopy, and high-resolution transmission electron microscopy HRTEM images.
Conceptual model of cadmium adsorption process on nZVI nanostructures with cadmium-iron oxides on the surface . The oxyhydroxide iron FeOOH has a crystal structure containing tunnel-shaped cavities that run parallel to the c-axis. These sites are bound by double rows of fused octahedral, in which cadmium ions probably reside [ 41 ].
These clusters of nanoparticles are caused by magnetic dipole-dipole interactions of the individual particles [ 42 ]. The unintended formation of CdFe 2 O 4 nanofibers as a remediation product presents an opportunity to reuse the remediation products for applications pertaining to light harvesting. Photoelectrochemical solar cells use light to carry out a chemical reaction, converting light to chemical energy or power [ 44 , 45 , 46 ]. A photoelectrochemical cell is a photocurrent-generating device that has a semiconductor in contact with an electrolyte.
It consists of a photoactive semiconductor working electrode either n-type or p-type and counter electrode made of either metal e. These electrodes are immersed in the electrolyte containing redox species with its standard potential being within the semiconductor bandgap potential region. In a metal-electrolyte junction, the potential drop occurs entirely on the solution site, whereas in a semiconductor-electrolyte junction, the potential drop occurs on the semiconductor site as well as the solution site [ 47 ]. The charge on the semiconductor side is distributed in the interior of the semiconductor, creating a space charge region.
If the junction of the semiconductor-electrolyte is illuminated with a light having energy greater than the semiconductor bandgap, photogenerated electron-hole pairs are separated in the space charge region [ 48 , 49 ].
The photogenerated minority carriers arrive at the interface of the semiconductor-electrolyte where a redox reaction will occur. Photoelectrochemical devices require exhaustive optimization of their quantum conversion efficiency, which is affected by the electron transfer processes.
Arsenic sorption on zero-valent iron-biochar complexes. Open Access. Effects of biochar from slow pyrolysis of papermill waste on agronomic performance and soil fertility. Nanoscience and nanotechnologies: opportunities and uncertainties. A photoelectrochemical cell is a photocurrent-generating device that has a semiconductor in contact with an electrolyte. These studies indicate a potential for adverse health effects from exposure and uptake of Fe oxide nanoparticles into mam-malian cells. Thus, whereas the nanoparticles themselves may not possess toxic properties, the pollutants they could carry with them may.
Methods such as doping with other metals and changing the structural arrangement of the system have been employed to overcome challenges regarding electron transfer processes [ 28 ]. The incorporation of Cd ions on the surface of the oxidized nZVI may produce surface structure changes. It has been found that oxide structures such as Fe x O y and the formation of CdFe 2 O 4 may be present at the surface of the nanoparticles [ 43 ].
As described in recent reports, this surface process may occur without using high-temperature processes, a common surface reaction described in the literature [ 29 , 30 ]. Moreover, photovoltaic and photoelectrochemical processes have been studied with CdFe 2 O 4. However, few reports have shown the use of heavy metal doped ferrite particles as semiconductors in photovoltaic and photoelectrochemical devices [ 30 , 50 , 51 , 52 ]. Photoelectrochemical devices are challenging due to the optimization of their quantum conversion efficiency, which is affected by the electron transfer processes in the system.
The novelty of these results was to analyze the material produced after the Cd decontamination processes in water using nZVI as a photoactive substance.
The product formed exhibited capable photoactive behavior for photoelectrochemical solar cell applications. The samples prepared using the nZVI do not display significant signals lowest curve. Such a high photovoltage can be explained by an improvement in the electron transfer dynamics of the material in the PSC at higher cadmium concentrations due to structural changes as previously suggested in the literature [ 30 ]. As one of the principle of Green Chemistry, these results provide a new alternative to reuse nanomaterials used in decontamination processes and generate modified iron oxide photocatalyst without using high temperature.
This report provides up-to-date technical information and state-of-the-art research findings on the use of zero-valent ironreactive materials to remove contaminants frequently found in groundwater. Covering both the theoretical and the practical, this book covers the comprehensive principles, latest research findings, and innovative developments in hazardous waste and inorganics removal. To establish a basic understanding of the reactive materials, the scientific principles of chemical reduction reactions, reductive precipitation, and adsorption processes are explained.
Then the book describes the design methodology for the full-scale application ofzero-valent ironin contaminated sites, as well as approaches to performance evaluation of the reactive materials. The book includes case studies and covers the design, installation, and performance evaluation of permeable reactive barriers based on zero-valent iron. This book will be beneficial to engineers, scientists, and decisionmakers applying zero-valent iron for hazardous waste and inorganics removal. See All Customer Reviews. Shop Textbooks.