Residual impact of aged nZVI on heavy metal-polluted soils

Mineral transformation of poorly crystalline ferrihydrite to hematite and goethite facilitated by an acclimated microbial consortium in electrodes of soil microbial fuel cells

In this study, we investigated the biogenic mineral transformation of poorly crystalline ferrihydrite in the presence of an acclimated microbial consortium after confirming successful soil microbial fuel cell optimization. The acclimated microbial consortia in the electrodes distinctly transformed amorphous ferrihydrite into crystallized hematite (cathode) and goethite (anode) under ambient culture conditions (30 °C). Serial analysis, including transmission/scanning electron microscopy and X-ray/selected area electron diffraction, confirmed that the biogenically synthesized nanostructures were iron nanospheres (~100 nm) for hematite and nanostars (~300 nm) for goethite. Fe(II) ion production with acetate oxidation via anaerobic respiration was much higher in the anode electrode sample (3.2- to 17.8-fold) than for the cathode electrode or soil samples. Regarding the culturable bacteria from the acclimated microbial consortium, the microbial isolates were more abundant and diverse at the anode. These results provide new insights into the biogeochemistry of iron minerals and microbial fuel cells in a soil environment, along with physiological characters of microbes (i.e., iron-reducing bacteria), for in situ applications in sustainable energy research.

A fungal functionalized magnetized solid phase extractor for preconcentrations of Pb(II), Mn(II), and Co(II) from real samples

Due to increasing industrialization and overpopulation, the amount of toxic metals is increasing in the environment, including air, soil, water, and food. Solid phase extraction is an efficient and ideal technique to preconcentrate the toxic metals before their measurements by analytical instruments. Russula brevipes was immobilized on γ-Fe₂O₃ magnetic nanoparticles and employed as a SPE sorbent to preconcentrate the trace level of Pb(II), Mn(II), and Co(II). To investigate the extraction conditions, significant experimental parameters were examined in details. LODs were calculated as 0.022, 0.015, and 0.024 ng mL^-1 for Pb(II), Mn(II), and Co(II), respectively. The biosorption capacities of R. brevipes immobilized γ-Fe₂O₃ were calculated as 43.1 mg g^-1 for Pb(II), 54.9 mg g^-1 for Mn(II), and 49.7 mg g^-1 for Co(II). Pb(II), Mn(II), and Co(II) in food samples at trace levels were preconcentrated by applying the developed method.

Toxicity of Nanoscale Zero-Valent Iron to Soil Microorganisms and Related Defense Mechanisms: A Review

Soil pollution is a global environmental problem. Nanoscale zero-valent iron (nZVI) as a kind of emerging remedial material is used for contaminated soil, which can quickly and effectively degrade and remove pollutants such as organic halides, nitrates and heavy metals in soil, respectively. However, nZVI and its composites can enter the soil environment in the application process, affect the physical and chemical properties of the soil, be absorbed by microorganisms and affect the growth and metabolism of microorganisms, thus affecting the ecological environment of the entire soil. Because of the potential risks of nZVI to the environment and ecosystems, this paper summarizes the current application of nZVI in the remediation of contaminated soil environments, summarizes the various factors affecting the toxic effects of nZVI particles and comprehensively analyzes the toxic effects of nZVI on microorganisms, toxic mechanisms and cell defense behaviors to provide a theoretical reference for subsequent biosafety research on nZVI.

Impact on Some Soil Physical and Chemical Properties Caused by Metal and Metallic Oxide Engineered Nanoparticles: A Review

In recent years, the release of metal and metallic oxide engineered nanoparticles (ENPs) into the environment has generated an increase in their accumulation in agricultural soils, which is a serious risk to the ecosystem and soil health. Here, we show the impact of ENPs on the physical and chemical properties of soils. A literature search was performed in the Scopus database using the keywords ENPs, plus soil physical properties or soil chemical properties, and elements availability. In general, we found that the presence of metal and metallic oxide ENPs in soils can increase hydraulic conductivity and soil porosity and reduce the distance between soil particles, as well as causing a variation in pH, cation exchange capacity (CEC), electrical conductivity (EC), redox potential (Eh), and soil organic matter (SOM) content. Furthermore, ENPs or the metal cations released from them in soils can interact with nutrients like phosphorus (P) forming complexes or precipitates, decreasing their bioavailability in the soil solution. The results depend on the soil properties and the doses, exposure duration, concentrations, and type of ENPs. Therefore, we suggest that particular attention should be paid to every kind of metal and metallic oxide ENPs deposited into the soil.

Aging of colloidal contaminants and pathogens in the soil environment: Implications for nanoplastic and COVID-19 risk mitigation

Colloidal contaminants and pathogens are widely distributed in soil, whose tiny sizes and distinct surface properties render unique environmental behaviours. Because of aging, colloids can undergo dramatic changes in their physicochemical properties once in the soil environment, thus leading to diverse or even unpredictable environmental behaviour and fate. Herein, we provide a state-of-art review of colloid aging mechanisms and characteristics and implications for risk mitigation. First, we review aging-induced formation of colloidal contaminants and aging-associated changes. We place a special focus on emerging nanoplastic (NP) contaminants and associated physical, chemical, and biological aging processes in soil environments. Second, we assess aging and survival features of colloidal pathogens, especially viruses. Viruses in soils may survive from several days to months, or even several years in groundwater, depending on their rates of inactivation and the reversibility of attachment. Furthermore, we identify implications for risk mitigation based on aging mechanisms. Hotspots of (photo)chemical aging of NPs, including plastic gauzes at construction sites and randomly discarded plastic waste in rural areas, are identified as area requiring greater research attention. For COVID-19, we suggest taking greater care in regions where viruses are persist for long periods, such as cold climate regions. Soil amendment with quicklime (CaO) may act as an effective means for pathogen disinfection. Future risk mitigation of colloidal contaminants and pathogens relies on a better understanding of aging mechanisms and more sophisticated models accurately depicting processes in real soil environments.

Effect of Nanoscale Zero-Valent Iron on Arsenic Bioaccessibility and Bioavailability in Soil

Hand-to-mouth activity is considered to be the main way for children to come into contact with contaminated soil, and bioavailability is an important factor affecting their health risk. To reduce soil As risk to humans by oral exposure, nanoscale zero-valent iron (nZVI) has been extensively studied for immobilizing As-contaminated soil, but its efficiency has not been investigated using in vitro assay and its influence on As-RBA. In this study, two contaminated soil samples (A and B) were amended with 1% and 2% (w/w) nZVI for 56 days to study its effect on As fraction by sequence extraction, As bioaccessibility by SBRC assay, and As relative bioavailability (RBA) by the mouse liver and kidney model. Based on the sequence extraction, the As associated with the E1 (exchangeable fraction) and C2 (carbonate fraction) fractions were decreased from 3.00% to 1.68% for soil A and from 21.6% to 7.86% for soil B after being treated with 2% nZVI for 56 days. When assessing As bioaccessibility in all soils treated with nZVI by SBRC assay, it was found that As bioaccessibility was significantly higher in the gastric phase (GP) and lower in the intestinal phase (IP) (p < 0.05), and the bioaccessible Fe concentration decreased significantly from the gastric to intestinal phase at the same time. Based on the mouse liver–kidney model, the As-RBA in soil A increased from 21.6% to 22.3% and 39.9%, but in soil B decreased from 73.0% to 55.3% and 68.9%, respectively. In addition, there was a significant difference between As bioaccessibility based on GP or IP of SBRC assay and As-RBA in two soils after being treated with nZVI for 56 days. To more accurately assess the effects of nZVI human arsenic exposure, As-RBA should be considered in concert with secondary evidence provided through fraction and bioaccessibility assessments. In addition, it is necessary to develop a suitable in vitro assay to predict As-RBA in nZVI-amended soils.

Zero-valent iron-induced successive chemical transformation and biodegradation of lindane in historically contaminated soil: An isotope-informed metagenomic study

Zero-valent iron (ZVI) is widely used to mitigate environmental pollutants such as chlorinated pesticides through reductive reactions accompanied by extensive impacts on the soil microbial community. However, whether and how ZVI changes the biodegradation of target compounds remain poorly understood. Here, we monitor the fate of lindane using a ¹⁴C-labled tracer and evaluate the growth and functions of the bacterial community in ZVI-stressed conditions in a historically γ-hexachlorocyclohexane (lindane)-contaminated soil using a combination of isotopic (¹⁸O-H₂O) and metagenomic methods. ZVI promoted the biomineralization of lindane in a dose-dependent manner. Soil bacteria were inhibited by amendment with ZVI during the initial stages of incubation (first three days) but recovered during the subsequent six weeks. Metagenomic study indicates that the todC1/bedC1 genes involved in the oxidation of dechlorinated lindane intermediates were upregulated in the ¹⁸O-labeled bacterial community but the presence of the lin genes responsible for lindane dechlorination was not confirmed. In addition, the benzoate biodegradation pathway that links to downstream catabolism of lindane was enhanced. These findings indicate successive chemical and biological degradation mechanisms underlying ZVI-enhanced lindane mineralization and provide a scientific basis for the inclusion of an extended bioremediation stage in the environmental application of ZVI materials.

Copper and Chromium toxicity is mediated by oxidative stress in Caenorhabditis elegans: The use of nanoparticles as an immobilization strategy

Environmental contamination by heavy metals (HMs) has impelled searching for stabilization strategies, where the use of zero-valent iron nanoparticles (nZVI) is considered a promising option. We have evaluated the combined effect of Cu(II)-Cr(VI) on two Caenorhabditis elegans strains (N2 and RB1072 sod-2 mutant) in aqueous solutions and in a standard soil, prior and after treatment with nZVI (5% w/w). The results showed that HMs aqueous solutions had an intense toxic effect on both strains. Production of reactive oxygen species and enhanced expression of the heat shock protein Hsp-16.2 was observed, indicating increased HM-mediated oxidative stress. Toxic effects of HM-polluted soil on worms were higher for sod-2 mutant than for N2 strain. However, nZVI treatment significantly diminished all these effects. Our findings highlighted C. elegans as a sensitive indicator for HMs pollution and its usefulness to assess the efficiency of the nanoremediation strategy to decrease the toxicity of Cu(II)-Cr(VI) polluted environments.

In situ nanoremediation of soils and groundwaters from the nanoparticle's standpoint: A review

Anthropogenic pollution coming from industrial processes, agricultural practices and consumer products, results in the release of toxic substances into rural and urban environments. Once released, these chemicals migrate through the atmosphere and water, and find their way into matrices such as sediments and groundwaters, thus making large areas potentially uninhabitable. Common pollutants, including heavy metal(loid)s, radionuclides, aliphatic hydrocarbons and halogenated organics, are known to adversely affect physiological systems in animal species. Pollution can be cleaned up using techniques such as coagulation, reverse osmosis, oxidation and biological methods, among others. The use of nanoparticles (NPs) extends the range of available technologies and offers particular benefits, not only by degrading, transforming and immobilizing contaminants, but also by reaching inaccessible areas and promoting biotic degradation. The development of NPs is understandably heralded as an environmentally beneficial technology; however, it is only now that the ecological risks associated with their use are being evaluated. This review presents recent developments in the use of engineered NPs for the in situ remediation of two paramount environmental matrices: soils and groundwaters. Emphasis will be placed on (i) the successful applications of nano-objects for environmental cleanup, (ii) the potential safety implications caused by the challenging requirements of [high reactivity toward pollutants] vs. [none reactivity toward biota], with a thorough view on their transport and evolution in the matrix, and (iii) the perspectives on scientific and regulatory challenges. To this end, the most promising nanomaterials will be considered, including nanoscale zerovalent iron, nano-oxides and carbonaceous materials. The purpose of the present review is to give an overview of the development of nanoremediators since they appeared in the 2000s, from their chemical modifications, mechanism of action and environmental behavior to an understanding of the problematics (technical limitations, economic constraints and institutional precautionary approaches) that will drive their future full-scale applications.

Effects of nanoparticles on trace element uptake and toxicity in plants: A review

Agricultural soils are receiving higher inputs of trace elements (TEs) from anthropogenic activities. Application of nanoparticles (NPs) in agriculture as nano-pesticides and nano-fertilizers has gained rapid momentum worldwide. The NPs-based fertilizers can facilitate controlled-release of nutrients which may be absorbed by plants more efficiently than conventional fertilizers. Due to their large surface area with high sorption capacity, NPs can be used to reduce excess TEs uptake by plants. The present review summarizes the effects of NPs on plant growth, photosynthesis, mineral nutrients uptake and TEs concentrations. It also highlights the possible mechanisms underlying NPs-mediated reduction of TEs toxicity at the soil and plant interphase. Nanoparticles are effective in immobilization of TEs in soil through alteration of their speciation and improving soil physical, chemical, and biological properties. At the plant level, NPs reduce TEs translocation from roots to shoots by promoting structural alterations, modifying gene expression, and improving antioxidant defense systems. However, the mechanisms underlying NPs-mediated TEs uptake and toxicity reduction vary with NPs type, mode of application, time of NPs exposure, and plant conditions (e.g., species, cultivars, and growth rate). The review emphasizes that NPs may provide new perspectives to resolve the problem of TEs toxicity in crop plants which may also reduce the food security risks. However, the potential of NPs in metal-contaminated soils is only just starting to be realized, and additional studies are required to explore the mechanisms of NPs-mediated TEs immobilization in soil and uptake by plants. Such future knowledge gap has been highlighted and discussed.

Nanoremediation technologies for sustainable remediation of contaminated environments: Recent advances and challenges

A major and growing concern within society is the lack of innovative and effective solutions to mitigate the challenge of environmental pollution. Uncontrolled release of pollutants into the environment as a result of urbanisation and industrialisation is a staggering problem of global concern. Although, the eco-toxicity of nanotechnology is still an issue of debate, however, nanoremediation is a promising emerging technology to tackle environmental contamination, especially dealing with recalcitrant contaminants. Nanoremediation represents an innovative approach for safe and sustainable remediation of persistent organic compounds such as pesticides, chlorinated solvents, brominated or halogenated chemicals, perfluoroalkyl and polyfluoroalkyl substances (PFAS), and heavy metals. This comprehensive review article provides a critical outlook on the recent advances and future perspectives of nanoremediation technologies such as photocatalysis, nano-sensing etc., applied for environmental decontamination. Moreover, sustainability assessment of nanoremediation technologies was taken into consideration for tackling legacy contamination with special focus on health and environmental impacts. The review further outlines the ecological implications of nanotechnology and provides consensus recommendations on the use of nanotechnology for a better present and sustainable future.

Vitamin C mediates the activation of green tea extract to modify nanozero-valent iron composites: Enhanced transport in heterogeneous porous media and the removal of hexavalent chromium

Both green tea (GT) extract and vitamin C (VC) were used for the reduction of Fe³⁺ to Fe⁰ using a green synthesis method. Modified nanozero-valent iron (GT-nZVI@VC nanocomposites) was successfully obtained and characterized as α-Fe⁰-iron oxide/VC by multiple analytical methods. The GT-nZVI@VC nanocomposites showed better transportability than nZVI, in that transport behavior was slightly dependent on various ratios of sand/soil in water-saturated heterogeneous porous media. Breakthrough curves of GT-nZVI@VC nanocomposites in paddy soil exhibited "blocking effects" and were well described using a first-order straining coefficient (k₂) on site 2 obtained from a two-site kinetic attachment model. In particular, GT-nZVI@VC (VC/Fe = 0.6) showed higher Cr(VI) removal (especially reducibility) in both paddy soil and water compared to that of nZVI and VC. It is likely that the synergistic effects of VC (ascorbic acid) and tea polyphenols can increase the released free electrons into solution, favoring the high reduction of Cr(VI) into Cr(III) (i.e., FeOCr₂O₃, Cr(OH)₃ and Cr₂O₃), where Cr(III) is prone to be immobilized by the nanocomposites in soil. This research highlights that VC can mediate the activation of GT extract to successfully modify nZVI, which could be beneficial for efficient transport in subsurface and remediation of Cr(VI)-contaminated soil and underground water.

A highly porous animal bone-derived char with a superiority of promoting nZVI for Cr(VI) sequestration in agricultural soils

Paddy soil and irrigation water are commonly contaminated with hexavalent chromium [Cr(VI)] near urban industrial areas, thereby threatening the safety of agricultural products and human health. In this study, we develop a porous and high specific area bone char (BC) to support nanoscale zero-valent iron (nZVI) and apply it to remediate Cr(VI) pollution in water and paddy soil under anaerobic conditions. The batch experiments reveal that BC/nZVI exhibits a higher removal capacity of 516.7 mg/(g•nZVI) for Cr(VI) than nZVI when normalized to the actual nZVI content, which is 2.8 times that of nZVI; moreover, the highest nZVI utilization is the nZVI loading of 15% (BC/nZVI15). The Cr(VI) removal efficiency of BC/nZVI15 decreases with increasing pH (4 - 10). Coexisting ions (phosphate and carbonate) and humic acid can inhibit the removal of Cr(VI) with BC/nZVI15. Additionally, BC exhibits a strong advantage in promoting Cr(VI) removal by nZVI compared to the widely used biochar and activated carbon. Our results demonstrate that reduction and coprecipitation are the dominant Cr(VI) removal mechanisms. Furthermore, BC/nZVI15 shows a significantly higher reduction and removal efficiency as well as a strong anti-interference ability for Cr(VI) in paddy soil, as compared to nZVI. These findings provide a new effective material for remediating Cr(VI) pollution from water and soil.

Immobilisation of metal(loid)s in two contaminated soils using micro and nano zerovalent iron particles: Evaluating the long-term stability

The aim of this study was to compare the immobilisation of metal(loid)s in two differently contaminated soils using micro zerovalent iron (ZVI) and nano zerovalent iron (nZVI) particles. Chromated copper arsenate-contaminated soil contained high amounts of As, Cu, Cr, and Zn, whereas mining-contaminated soil contained high amounts of As, Cu, and Pb. Contaminated soils were amended using 2% ZVI and nZVI. As determined by the leaching procedures, nZVI was more efficient in immobilising all the studied metal(loid)s in the soils compared to ZVI. The greatest immobilisation was achieved for As in both soils. The long-term stability of immobilised metal(loid)s was studied in mining-contaminated soil by performing thermal oxidation (ageing). In the ZVI and nZVI-treated soils, high retention results were achieved for As and Cu, whereas in the ZVI and nZVI-treated soils, significant desorption of Pb was observed. The results also showed that retention of metal(loid)s over a long period of time could be more effective in soils treated with ZVI, as the crystallisation of Fe in ZVI-treated soil was to a lesser extent compared to the crystallisation of Fe in nZVI-treated soil.

Interference of goethite in the effects of glyphosate and Roundup® on ZFL cell line

Goethite (α-FeOOH) brings important perspectives in environmental remediation, as, due to its physicochemical properties, this iron oxide can adsorb a wide variety of compounds, including glyphosate. This study aimed to evaluate the effects of goethite nanoparticles (NPs), glyphosate (Gly), Roundup® (Rd), and co-exposures (Gly + NPs and Rd + NPs) on zebrafish liver cell line (ZFL). ZFL cells were exposed to NPs (1, 10, and 100 mg L-1), Gly (3.6 mg L-1), Rd (10 mg L-1), and co-exposures (Gly + NPs and Rd + NPs), or only to saline for 1, 6, and 12 h. Cell viability was assessed by Trypan blue, MTT, and neutral red assays. The generation of reactive oxygen species and total antioxidant capacity were also determined, while genotoxicity was quantified by the comet assay. Both NPs and Rd in isolation produced cytotoxic effects at 6 h and genotoxic effects at 1 and 6 h. Rd + NPs resulted in synergistic effects, intensifying the toxicity. Cells exposed to Gly did not present toxic effects and Gly + NPs resulted in the suppression of toxic effects observed for NPs. The presence of other components in Roundup® seems to favor its toxicity compared to the active ingredient. In conclusion, according to the in vitro model, the concentrations used were not safe for the ZFL lineage.

Aging of zero-valent iron-based nanoparticles in aqueous environment and the consequent effects on their reactivity and toxicity

A fundamental understanding of the long-term fate of nanoscale zero-valent iron (nZVI)-based particles in aqueous environment and the corresponding impacts on their reactivity and toxicity is essential for the responsible use and management of the nanoparticles in environmental applications. This paper comprehensively reviews the physicochemical transformations of nZVI-based particles and the consequent effects on the particle's reactivity and toxicity. The corrosions of nZVI in water under both anaerobic and aerobic conditions are summarized. The transformation of contaminant-bearing nZVI is also discussed. Besides, the factors influencing the transformation of nZVI (i.e., pH, typical anions and cations, natural organic matter, surface stabilizers, bimetal decoration, and sulfidation treatment) are summarized and discussed. In addition, the effects of particle aging on its reactivity and toxicity are discussed. Generally, the aging of nZVI-based particles would have negative impact on the removal of contaminants, especially for the degradation of organic pollutants. However, the aging process of nZVI-based particles would cause a significant reduction in their toxicity. It is suggested that the nZVI-based particles would finally transform to less toxic or benign materials (i.e., iron (oxyhydr)oxides) over time. Finally, future perspectives are proposed to better quantify and predict the transformation of nZVI-based particles in aqueous environment. PRACTITIONER POINTS: The corrosion rates and products of nZVI in water varied much under anaerobic and aerobic conditions. Typical anions and cations, natural organic matter, and iron types are critical factors influencing the physicochemical transformation of nZVI. The aging of nZVI would have negative impact its reactivity, especially for the degradation of organic pollutants. Although the fresh nZVI exhibits obvious toxicity, the aging process would cause a significant reduction in its toxicity.

Nanotechnological interventions for plant health improvement and sustainable agriculture

Agriculture is the source of food for both humans and animals. With the growing population demands, agricultural production needs to be scaled up where nanotechnology can play a significant role. The use of nanotechnology in agriculture can manage plant disease and growth for better and quality output. Therefore, this review focuses on the use of various nanoparticles for detection of nutrients and contaminants, nanosensors for monitoring the environmental stresses and crop conditions as well as the use of nanotechnology for plant pathogen detection and crop protection. In addition, the delivery of plant growth regulators and agrichemicals like nanopesticides and nanofertilizers to the plants along with the delivery of DNA for targeted genetic engineering and production of genetically modified (GM) crops are discussed briefly. Further, the future concerns regarding the use of nanoparticles and their possible toxicity, impact on the agriculture and ecosystem needs to be assessed along with the assessment of the nanoparticles and GM crops on the environment and human health.

Zero valent iron and goethite nanoparticles as new promising remediation techniques for As-polluted soils

The capacity of two iron-based nanomaterials, namely goethite nanospheres (nGoethite) and zero valent iron nanoparticles (nZVI), to immobilize As in a polluted soil was evaluated and compared. The composition and morphology of the products were studied by energy dispersive X-ray analysis and transmission electron microscopy, while zeta potential and average sizes were determined by dynamic light scattering. To assess As immobilization, soil subsamples were treated with nGoethite or nZVI at a range of Fe doses (0.5%, 2%, 5% and 10%) and then studied by the TCLP test and the Tessier sequential extraction procedure. The influence of both nanoparticles on As speciation was determined, as was impact on soil pH, electrical conductivity, Fe availability and phytotoxicity (watercress germination). For nZVI, notable results were achieved at a dose of 2% (89.5% decrease in As, TCLP test), and no negative effects on soil parameters were detected. Indeed, even soil phytotoxicity was reduced and only at the highest dose was a slight increase in As³⁺ detected. In contrast, excellent results were obtained for nGoethite at the lowest dose (0.2%) (82.5% decrease in As, TCLP test); however, soil phytotoxicity was increased at higher doses, probably due to a marked enhancement of electrical conductivity. For both types of nanoparticle, slight increases in Fe availability were observed. Thus, our results show that both nZVI and nGoethite have the capacity to effectively immobilize As in this brownfield. The use of lower doses of nGoethite emerges as a promising soil remediation strategy for soils affected by As pollution.

Bibliometric study of the toxicology of nanoescale zero valent iron used in soil remediation

The application of nanoscale zero-valent iron is one of the most widely used remediation technologies; however, the potential environmental risks of this technology are largely unknown. In order to broaden the knowledge on this subject, the present work consists of a bibliometric study of all of publications related to the toxicity of zero-valent iron nanoparticles used in soil remediation available from the Scopus (Elsevier) and Web of Science (Thompson Reuters) databases. This study presents a temporal distribution of the publications, the most cited articles, the authors who have made the greatest contribution to the theme, and the institutions, countries, and scientific journals that have published the most on this subject. The use of bibliometrics has allowed for the visualization of a panorama of the publications, providing an appropriate analysis to guide new research towards an effective contribution to science by filling the existing gaps. In particular, the lack of studies in several countries reveals a promising area for the development of further research on this topic.

The impact of nanoparticles zero-valent iron (nZVI) and rhizosphere microorganisms on the phytoremediation ability of white willow and its response

Soil contaminated with heavy metals (HMs) is a serious problem throughout the world that threatens all living organisms in the soil. Therefore, large-scale remediation is necessary. This study investigated a new combination of remediation techniques on heavy metal contaminated soil, phytoremediation, and soil amendment with nano-sized zero-valent iron (nZVI) and rhizosphere microorganisms. White willow (Salix alba L.) was grown for 160 days in pots containing Pb, Cu, and Cd and amended with 0, 150, and 300 (mg kg^-1) of nZVI and rhizosphere microorganisms, including the arbuscular mycorrhizal fungus (AMF), Rhizophagus irregularis, and the plant growth promoting rhizobacteria (PGPR), Pseudomonas fluorescens. The results showed that inoculation with PGPR and AMF, particularly dual inoculation, improved plant growth as well as the physiological and biochemical parameters of white willow, and increased the bioconcentration factor (BCF) of Pb, Cu, and Cd. The low dose of nZVI significantly increased the root length and the leaf area of the seedlings and increased the BCF of Cd. In contrast, the high dose of nZVI had negative effects on the seedlings growth and the BCF of Pb and Cu, about - 32% and - 63%, respectively. Our results demonstrate that nZVI at low doses can improve plant performance in a phytoremediation context and that the use of beneficial rhizosphere microorganisms can minimize nZVI stress in plants and make them less susceptible to stress even under high dose conditions.

Immobilization of cadmium in polluted soils by phytogenic iron oxide nanoparticles

While phytogenic nanomaterials have been successfully used to remove heavy metals in wastewater, the potential to successfully use such materials to immobilize heavy metals in soils is still unclear. In this study, phytogenic iron oxide nanoparticles (PION) were used to immobilize cadmium (Cd) in six soils. Amendment with PION effectively immobilized Cd, with a concomitant increase in the concentrations of iron oxides, soil pH and dissolved organic carbon (DOC) under both oxic and anoxic conditions. However, observed changes in soil properties and Cd fractions were different under oxic and anoxic conditions. After PION application, the exchangeable Cd fraction decreased by up to 91 and 69%, while the carbonate bound Cd fraction decreased by up to 61 and 75%, under oxic and anoxic conditions, respectively. Pearson correlation analysis revealed that under both oxic and anoxic conditions, Cd fractions were significantly and positively correlated with free iron oxide content and pH, where free iron oxide content was positively correlated with amorphous iron oxide, DOC and pH. The Cd immobilization mechanisms potentially involved either (1) formation of insoluble hydroxides at elevated pH; (2) participation of biomolecules released from PION in ligand complexation with Cd and (3) co-precipitated of Cd during the formation of iron oxides. This study provided new insights into the potential effects of PION applications for practical Cd immobilization in contaminated soils.

The impact of nanoscale zero-valent iron particles on soil microbial communities is soil dependent

The application of nanoscale zero-valent iron particles (nZVI) for the remediation of contaminated sites is very promising. However, information concerning the ecotoxicity of nZVI on soil microbial communities and, hence, soil quality, is still scarce. We carried out a three-month experiment to evaluate the impact of the application of different concentrations of nZVI (from 1 to 20 mg g DW soil^-1) on soil microbial properties in a clay-loam versus a sandy-loam soil. Data on microbial biomass (total bacteria and fungi by qPCR, microbial biomass carbon), activity (β-glucosidase, arylsulphatase and urease activities), and functional (Biolog Ecoplates^™) and structural (ARISA, 16S rRNA amplicon sequencing) diversity evidenced that the sandy-loam soil was more vulnerable to the presence of nZVI than the clay-loam soil. In the sandy-loam soil, arylsulphatase activity and bacterial abundance, richness and diversity were susceptible to the presence of nZVI. The high content of clay and organic matter present in the clay-loam soil may explain the observed negligible effects of nZVI on soil microbial properties. It was concluded that the impact of nZVI on soil microbial communities and, hence, soil quality, is soil dependent.

New insights into the impact of nZVI on soil microbial biodiversity and functionality

Nanoscale zero-valent iron (nZVI) is a strong reducing agent used for in situ remediation of soil. The impacts of nZVI (5-10% w/w) on the soil microbial biodiversity and functionality of two soils (Lufa 2.2 and 2.4) were assessed. Illumina MiSeq technology was used to evaluate the structure of soil microbiomes after 21 days of exposure. Proteobacteria, Verrucomicrobia, Firmicutes and Actinobacteria were the most abundant phyla in both soils. However, the dynamics of bacterial community composition following nZVI addition differed. nZVI exposure induced pronounced shifts in the microbial composition of soil 2.4, but not in soil 2.2; an increase in Verrucomicrobia abundance was the unique common taxonomic pattern observed in both soils. The PICRUSt approach was applied to predict the functional composition of each metagenome. Environmental information processing function (membrane transport) was decreased in both nZVI-spiked soils, although soil 2.4 samples were enriched in functions involved in cellular processes and metabolism. The effects of nZVI on autochthonous bacterial communities clearly varied with the soil type assessed; changes at the phylogenetic level appeared to be more abundant than those observed at the functional level, and thus, the overall effort of the soil ecosystem might involve the maintenance of functionality following nZVI exposure.

Aging effects on the mobility of Pb in soil: Influence on the energy requirements in electroremediation

This paper studies the possible differences in the behavior of lead as a contaminant in soil samples when it is present as "naturally-aged" for decades after the contamination, and when it has been spiked in the laboratory. This behavior differences are stablished mainly in two ways: as changes in the fractionation analysis obtained after a sequential extraction procedure (SEP) and as changes in the efficiency of the acid-enhanced electroremediation (EKR) technique. Additionally, aging effects have been studied for almost five years. In the case of the lead-spiked soil the influence of storage conditions on contaminant behavior have also been explored: 1) samples stored in capped containers at constant moisture conditions, and 2) samples in containers open to the atmosphere, with periods of water flooding and drying. Lab-spiked and the "naturally-aged" contaminants show very different behavior with respect not only to SEP analysis but also to EKR experiments. The soil spiked with a soluble lead salt presents a higher percent in the more mobile fractions. Regarding storage conditions, some changes were observed in the lead distribution along the vertical soil profile for samples stored in uncapped containers. The EKR results were also in agreement with those from fractionation analysis. Energy requirements for the remediation were estimated by a mathematical model with important differences obtained for the different soil samples. Results are indicating that it will be very unreliable to draw estimations for the "naturally-aged" soils from contaminant-spiked samples.

Performance and toxicity assessment of nanoscale zero valent iron particles in the remediation of contaminated soil: A review

Nanoscale zero valent iron (nZVI) particles have been studied in recent years as a promising technology for the remediation of contaminated soil. Although the potential benefits of nZVI are considerable, there is a distinct need to identify possible risks after environmental exposure to nZVI. This work firstly introduced the remediation of nZVI for heavy metals and chlorinated organic compounds in contaminated soil. And the corresponding stabilization mechanisms were discussed. We also highlighted the factors affecting nZVI reactivity, including nZVI surface area, nZVI stabilizers, soil pH, soil organic matter and soil types. In addition, this review shows a critical overview of the current understanding of toxicity of nZVI particles to soil bacteria and fungi. The toxicity mechanisms, cellular defenses behaviors and the factors affecting the toxicity of nZVI were summarized. Finally, the remaining barriers to be overcome in materials development for environment application are also discussed.

EDDS enhanced PCB degradation and heavy metals stabilization in co-contaminated soils by ZVI under aerobic condition

In the present study, biodegradable ligand EDDS was employed to assist ZVI on simultaneous remediation of PCB and heavy metals co-contaminated soils under aerobic condition. With addition of 4 mmol L^-1 EDDS and 5 g L^-1 ZVI, the total removal ratio of PCB reached 75.3%, and the stabilization ratio of Pb and Cu attained 97.1% and 91.9% respectively. EDDS played two key roles during the process. Firstly, the addition of EDDS could enhance hydroxyl radical generation by ZVI and oxygen for the oxidation of PCB including distribution in the soil phase and dissolved form in the aqueous phase. Secondly, free EDDS could accelerate the release of Cu and Pb from the soil phase to the aqueous phase. As the oxidation of EDDS and the increase of pH value during the process, the dissolved Cu and Pb could be efficiently stabilized by iron oxyhydroxide through coprecipitation. Compared with ZVI/Air, ZVI/EDDS/Air treatment could significantly enhance the stabilization of Pb and Cu. The reason was the dissolution of Cu and Pb by EDDS extraction could reduce the mass transfer limitations between heavy metals and iron oxyhydroxide. Therefore, our study suggests a promising alternative for remediation of organic compounds and heavy metals co-contaminated soil.

Combined chemical and microbiological degradation of tetrachloroethene during the application of Carbo-Iron at a contaminated field site

After the injection of Carbo-Iron® into an aquifer contaminated with tetrachloroethene (PCE), combined chemical and microbiological contaminant degradation processes were found in a long-term study of the field site in Lower Saxony (Germany). The applied composite material Carbo-Iron, which consists of colloidal activated carbon and embedded nanoscale zero-valent iron (ZVI) structures, functioned as intended: accumulating the pollutants and promoting their reductive dechlorination. Furthermore, the particles decreased the redox potential of the groundwater due to their reaction with oxygen and to the ZVI-corrosion-induced formation of molecular hydrogen up to 190 days after the injection, the latter promoting sulphate-reducing conditions. The emergence of cis-dichloroethene (cis-DCE), which was only found in trace quantities before the injection of Carbo-Iron, together with the presence of organisms related to Sulfospirillum multivorans, Desulfitobacterium spp. and Dehalococcoides mccartyi, indicate that Carbo-Iron is also able to support microbial degradation of PCE. However, cis-DCE did not accumulate in the present case study, although it is often observed at sites with active microbial dechlorination. The results of compound-specific isotope analysis in combination with pyrosequencing data suggested the oxidative degradation of cis-DCE by an organism related to Polaromonas sp. strain JS666. Consequently, the formation of the carcinogenic degradation intermediate vinyl chloride was circumvented. Overall, the moderate and slow change of environmental conditions mediated by Carbo-Iron not only supported organohalide-respiring bacteria, but also created the basis for a subsequent microbial oxidation step.

Effect of nano zero-valent iron application on As, Cd, Pb, and Zn availability in the rhizosphere of metal(loid) contaminated soils

Characterisation of geochemical transformations and processes in soils with special focus on the rhizosphere is crucial for assessing metal(loid) bioavailability to plants during in situ immobilisation and phytostabilisation. In this study, the effects of nano zero-valent iron (nZVI) were investigated in terms of the immobilisation of As, Zn, Pb and Cd in two soil types and their potential uptake by plants using rhizobox experiments. Such system allowed monitoring the behaviour of trace elements in rooted and bulk soil compartments separately. Sunflower (Helianthus annuus L.) and ryegrass (Lolium perenne L.) were tested for As-rich (15.9 g As kg^-1) and Zn-rich (4.1 g Zn kg^-1) soil samples, respectively. The application of nZVI effectively lowered the uptake of all target risk elements into plant tissues. Efficient immobilisation of As was determined in the As-soil without a significant difference between plant and bulk soil compartments. Similarly, a significant decrease was determined for CaCl₂-available fractions of Zn, Pb and Cd in nZVI-treated Zn-soil. The behaviour of As corresponded to changes in Eh, while Zn and Cd showed to be mainly pH-dependent. However, despite the observed stabilisation effect of nZVI, high amounts of As and Zn still remained available for plants. Furthermore, the accumulation of the target risk elements in roots and the overall effect of nZVI transformations in the rhizosphere were verified and visualised by SEM/EDS. The following immobilising mechanisms were suggested: (i) sorption onto both existing and newly formed Fe (hydr)oxides, (ii) formation of secondary Fe-As phases, and (iii) sorption onto Mn (hydr)oxides.

Nanoscale zero-valent iron-assisted soil washing for the removal of potentially toxic elements

The present study focuses on soil washing enhancement via soil pretreatment with nanoscale zero-valent iron (nZVI) for the remediation of potentially toxic elements. To this end, soil polluted with As, Cu, Hg, Pb and Sb was partitioned into various grain sizes (500-2000, 125-500 and <125 μm). The fractions were pretreated with nZVI and subsequently subjected, according to grain size, to Wet-High Intensity Magnetic Separation (WHIMS) or hydrocycloning. The results were compared with those obtained in the absence of nanoparticles. An exhaustive characterization of the magnetic signal of the nanoparticles was done. This provided valuable information regarding potentially toxic elements (PTEs) fate, and allowed a metallurgical accounting correction considering the dilution effects caused by nanoparticle addition. As a result, remarkable recovery yields were obtained for Cu, Pb and Sb, which concentrated with the nZVI in the magnetically separated fraction (WHIMS tests) and underflow (hydrocyclone tests). In contrast, Hg, concentrated in the non-magnetic fraction and overflow respectively, while the behavior of As was unaltered by the nZVI pretreatment. All things considered, the addition of nZVI enhanced the efficiency of soil washing, particularly for larger fractions (125-2000 μm). The proposed methodology lays the foundations for nanoparticle utilization in soil washing operations.

Remediation of contaminated soils by enhanced nanoscale zero valent iron

The use of nanoscale zero valent iron (nZVI) for in situ remediation of soil contamination caused by heavy metals and organic pollutants has drawn great concern, primarily owing to its potential for excellent activity, low cost and low toxicity. This reviews considers recent advances in our understanding of the role of nZVI and enhanced nZVI strategy in the remediation of heavy metals and persistent organic contaminants polluted soil. The performance, the migration and transformation of nZVI affected by the soil physical and chemical conditions are summarized. However, the addition of nZVI inevitably disturbs the soil ecosystem, thus the impacts of nZVI on soil organisms are discussed. In order to further investigate the remediation effect of nZVI, physical, chemical and biological method combination with nZVI was developed to enhance the performance of nZVI. From a high efficient and environmentally friendly perspective, biological method enhanced nZVI technology will be future research needs. Possible improvement of nZVI-based materials and potential areas for further applications in soil remediation are also proposed.

Environmental transformations and ecological effects of iron-based nanoparticles

The increasing application of iron-based nanoparticles (NPs), especially high concentrations of zero-valent iron nanoparticles (nZVI), has raised concerns regarding their environmental behavior and potential ecological effects. In the environment, iron-based NPs undergo physical, chemical, and/or biological transformations as influenced by environmental factors such as pH, ions, dissolved oxygen, natural organic matter (NOM), and biotas. This review presents recent research advances on environmental transformations of iron-based NPs, and articulates their relationships with the observed toxicities. The type and extent of physical, chemical, and biological transformations, including aggregation, oxidation, and bio-reduction, depend on the properties of NPs and the receiving environment. Toxicities of iron-based NPs to bacteria, algae, fish, and plants are increasingly observed, which are evaluated with a particular focus on the underlying mechanisms. The toxicity of iron-based NPs is a function of their properties, tolerance of test organisms, and environmental conditions. Oxidative stress induced by reactive oxygen species is considered as the primary toxic mechanism of iron-based NPs. Factors influencing the toxicity of iron-based NPs are addressed and environmental transformations play a significant role, for example, surface oxidation or coating by NOM generally lowers the toxicity of nZVI. Research gaps and future directions are suggested with an aim to boost concerted research efforts on environmental transformations and toxicity of iron-based NPs, e.g., toxicity studies of transformed NPs in field, expansion of toxicity endpoints, and roles of laden contaminants and surface coating. This review will enhance our understanding of potential risks of iron-based NPs and proper uses of environmentally benign NPs.

Metal(loid)s behaviour in soils amended with nano zero-valent iron as a function of pH and time

Nano zero-valent iron (nZVI) is currently investigated as a stabilising amendment for contaminated soils. The effect of pH (4-8) and time (48 and 192 h) on the behaviour of nZVI-treated Pb-Zn and As-contaminated soil samples was assessed. Additionally, soil leachates were subsequently used to study the direct interaction between soil solution components and nZVI particles in terms of mineralogical changes and contaminant retention. A typical U-shaped leaching trend as a function of pH was observed for Cd, Pb and Zn, while As was released predominantly under alkaline conditions. Oxidising conditions prevailed, so pH was the key controlling parameter rather than redox conditions. Generally, longer contact time resulted in increased soluble concentrations of metal(loid)s. However, the stabilisation effect of nZVI was only observed after the direct soil leachate-nZVI interactions, showing enhanced redox and sorption processes for the studied metals. A significant decrease of dissolved As concentrations was observed for both experimental soils, but with different efficiencies depending on neutralisation capacity, organic matter content or solid fractionation of As related to the origin of the soils. Scorodite (FeAsO₄·2H₂O) was predicted as a potential solubility-controlling mineral phase for As. Sorption of metal(loid)s onto secondary Fe- and Al-(oxyhydr)oxides (predicted to precipitate at pH > 5) represents an important scavenger mechanism. Moreover, transmission electron microscopy confirmed the retention of Zn and Pb under near-neutral and alkaline conditions by newly formed Fe oxides or aluminosilicates. This study shows that the efficiency of nZVI application strongly depends not only on soil pH-Eh conditions and contaminant type, but also on the presence of organic matter and other compounds such as Al/Fe/Mn oxyhydroxides and clay minerals.

The interactions between nanoscale zero-valent iron and microbes in the subsurface environment: A review

Nanoscale zero-valent iron (NZVI) particles, applied for in-situ subsurface remediation, are inevitable to interact with various microbes in the remediation sites directly or indirectly. This review summarizes their interactions, including the effects of NZVI on microbial activity and growth, the synergistic effect of NZVI and microbes on the contaminant removal, and the effects of microbes on the aging of NZVI. NZVI could exert either inhibitive or stimulative effects on the growth of microbes. The mechanisms of NZVI cytotoxicity (i.e., the inhibitive effect) include physical damage and biochemical destruction. The stimulative effects of NZVI on certain bacteria are associated with the creation of appropriate living environment, either through providing electron donor (e.g., H₂) or carbon sources (e.g., the engineered organic surface modifiers), or through eliminating the noxious substances that can cause bactericidal consequence. As a result of the positive interaction, the combination of NZVI and some microbes shows synergistic effect on contaminant removal. Additionally, the aged NZVI can be utilized by some iron-reducing bacteria, resulting in the transformation of Fe(III) to Fe(II), which can further contribute to the contaminant reduction. However, the Fe(III)-reduction process can probably induce environmental risks, such as environmental methylation and remobilization of the previously entrapped heavy metals.

Green Synthesis of Iron Nanoparticles and Their Environmental Applications and Implications

Recent advances in nanoscience and nanotechnology have also led to the development of novel nanomaterials, which ultimately increase potential health and environmental hazards. Interest in developing environmentally benign procedures for the synthesis of metallic nanoparticles has been increased. The purpose is to minimize the negative impacts of synthetic procedures, their accompanying chemicals and derivative compounds. The exploitation of different biomaterials for the synthesis of nanoparticles is considered a valuable approach in green nanotechnology. Biological resources such as bacteria, algae fungi and plants have been used for the production of low-cost, energy-efficient, and nontoxic environmental friendly metallic nanoparticles. This review provides an overview of various reports of green synthesised zero valent metallic iron (ZVMI) and iron oxide (Fe₂O₃/Fe₃O₄) nanoparticles (NPs) and highlights their substantial applications in environmental pollution control. This review also summarizes the ecotoxicological impacts of green synthesised iron nanoparticles opposed to non-green synthesised iron nanoparticles.

A review of the environmental implications of in situ remediation by nanoscale zero valent iron (nZVI): Behavior, transport and impacts on microbial communities

The increasing use of strategies incorporating nanoscale zero valent iron (nZVI) for soil and groundwater in situ remediation is raising some concerns regarding the potential adverse effects nZVI could have on indigenous microbial communities and ecosystem functioning. This review provides an overview of the current literature pertaining to the impacts of nZVI applications on microbial communities. Toxicity studies suggest that cell membrane disruption and oxidative stress through the generation of Fe(2+) and reactive oxygen species by nZVI are the main mechanisms contributing to nZVI cytotoxicity. In addition, nZVI has been shown to substantially alter the taxonomic and functional composition of indigenous microbial communities. However, because the physico-chemical conditions encountered in situ highly modulate nZVI toxicity, a better understanding of the environmental factors affecting nZVI toxicity and transport in the environment is of primary importance in evaluating the ecological consequences that could result from a more extensive use of nZVI.

The dual effects of carboxymethyl cellulose on the colloidal stability and toxicity of nanoscale zero-valent iron

Nanoscale zero-valent iron (NZVI) particles are usually modified with surface coating to mitigate the particle stability in water during the environmental application. However, the surface coating may not only influence the particle stabilization but also the particle cytotoxicity. In this study, we investigated the dual effects of carboxymethyl cellulose (CMC) on the colloidal stability and cytotoxicity of NZVI towards gram-negative Escherichia coli (E. coli) and discussed the interrelation between particle stability and cytotoxicity. The effect of CMC concentration, ionic strength (Ca(2+)) and aging treatment on the particle cytotoxicity were also examined. Specifically, the aqueous stability of NZVI suspensions with CMC ratio dose-dependently strengthened within 1 h. The inactivation of E. coli by bare NZVI was significant and concentration- and time-dependent. On the contrary, an increasing reduction in cytotoxicity of NZVI with CMC ratio increasing was observed, even though the particles became more dispersed. TEM analysis demonstrates the membrane disruption and the cellular internalization of nanoparticles after exposure of E. coli to NZVI. However, in the case of CMC-modified NZVI (CNZVI), the bacterial cell wall displays an outer shell of a layer of nanoparticles attached around the outer membrane, but the cell membrane was kept intact. The presence of Ca(2+) can either increase or decrease the cytotoxicity of NZVI and CNZVI, depending on the concentration. The aged NZVI and CNZVI particles did not seem to present obvious bactericidal effect due to the transformation of Fe(0) to the less toxic or non-toxic iron oxides, as indicated by the XRD analysis.