Application of nanoscale zero valent iron (NZVI) for groundwater remediation in Europe

Enhanced breakage of the aggregates of nanoscale zero-valent iron via ball milling

Aggregates of nanoscale zero-valent iron (nZVI) are commonly encountered for nZVI in aqueous solution, particularly during large-scale nZVI applications where nZVI is often in a highly concentrated slurry, and such aggregates lower nZVI mobility during its in-situ remediation applications. Herein, we report that the ball milling is an effective tool to break the nZVI aggregates and thereby improve the nZVI mobility. Results show that the milling (in just five minutes) can break the aggregates of a few tens of microns to less than one micron, which is one-tenth of the size that is acquired via the breakage using the mechanical mixing and ultrasonication. The milling breakage can also improve the efficacy of the chemical conditioning method that is commonly used for the nanoparticle stabilization and dispersion. The milling breakage is further optimized via a study of the milling operational factors including milling time, bead velocity, bead diameter, and chamber porosity, and an empirical equation is proposed combining the bead collision number during the milling. Mechanistic study shows that the high efficacy of the milling to break the aggregates can be explained by the small eddy created by the high shear rate produced by the close contact of the milling beads and may also relate to the direct mechanical pulverization effect. This study provides a high efficacy physical method to break the nanoparticle aggregates. The method can be used to improve the nZVI mobility performance by milling the nZVI slurry before its injection for in-situ remediation, and the milling may also replace the mechanical mixing during the nZVI stabilization via surface modification.

A review of passive acid mine drainage treatment by PRB and LPB: From design, testing, to construction

An extensive volume of acid mine drainage (AMD) generated throughout the mining process has been widely regarded as one of the most catastrophic environmental problems. Surface water and groundwater impacted by pollution exhibit extreme low pH values and elevated sulfate and metal/metalloid concentrations, posing a serious threat to the production efficiency of enterprises, domestic water safety, and the ecological health of the basin. Over the recent years, a plethora of techniques has been developed to address the issue of AMD, encompassing nanofiltration membranes, lime neutralization, and carrier-microencapsulation. Nonetheless, these approaches often come with substantial financial implications and exhibit restricted long-term sustainability. Among the array of choices, the permeable reactive barrier (PRB) system emerges as a noteworthy passive remediation method for AMD. Distinguished by its modest construction expenses and enduring stability, this approach proves particularly well-suited for addressing the environmental challenges posed by abandoned mines. This study undertook a comprehensive evaluation of the PRB systems utilized in the remediation of AMD. Furthermore, it introduced the concept of low permeability barrier, derived from the realm of site-contaminated groundwater management. The strategies pertaining to the selection of materials, the physicochemical aspects influencing long-term efficacy, the intricacies of design and construction, as well as the challenges and prospects inherent in barrier technology, are elaborated upon in this discourse.

Effect of Stabilized nZVI Nanoparticles on the Reduction and Immobilization of Cr in Contaminated Soil: Column Experiment and Transport Modeling

Batch and transport experiments were used to investigate the remediation of loamy sand soil contaminated with Cr(VI) using zero-valent iron nanoparticles (nZVI) stabilized by carboxymethylcellulose (CMC-nZVI). The effect of pH, ionic strength (IS), and flow rate on the removal efficiency of Cr(VI) were investigated under equilibrium (uniform transport) and non-equilibrium (two-site sorption) transport using the Hydrus-1D model. The overall removal efficiency ranged from 70 to over 90% based on the chemical characteristics of the CMC-nZVI suspension and the transport conditions. The concentration and pH of the CMC-nZVI suspension had the most significant effect on the removal efficiency and transport of Cr(VI) in the soil. The average removal efficiency of Cr(VI) was increased from 24.1 to 75.5% when the concentration of CMC-nZVI nanoparticles was increased from 10 to 250 mg L-1, mainly because of the increased total surface area at a larger particle concentration. Batch experiments showed that the removal efficiency of Cr(VI) was much larger under acidic conditions. The average removal efficiency of Cr(VI) reached 90.1 and 60.5% at pH 5 and 7, respectively. The two-site sorption model described (r2 = 0.96-0.98) the transport of Cr(VI) in soil quite well as compared to the uniform transport model (r2 = 0.81-0.98). The average retardation of Cr(VI) was 3.51 and 1.61 at pH 5 and 7, respectively, indicating earlier arrival for the breakthrough curves and a shorter time to reach maximum relative concentration at lower pH. The methodology presented in this study, combining column experiment and modeling transport using the Hydrus-1D model, successfully assessed the removal of Cr(VI) from polluted soils, offering innovative, cost-effective, and environmentally friendly remediation methodologies.

Affecting factors and mechanism of removing antibiotics and antibiotic resistance genes by nano zero-valent iron (nZVI) and modified nZVI: A critical review

Antibiotics and antibiotic resistance genetic pollution have become a global environmental and health concern recently, with frequent detection in various environmental media. Therefore, finding ways to control antibiotics and antibiotic resistance genes (ARGs) is urgently needed. Nano zero-valent iron (nZVI) has shown a positive effect on antibiotics degradation and restraining ARGs, making it a promising solution for controlling antibiotics and ARGs. However, given the current increasingly fragmented research focus and results, a comprehensive review is still lacking. In this work, we first introduce the origin and transmission of antibiotics and ARGs in various environmental media, and then discuss the affecting factors during the degradation of antibiotics and the control of ARGs by nZVI and modified nZVI, including pH, nZVI dose, and oxidant concentration, etc. Then, the mechanisms of antibiotic and ARGs removal promoted by nZVI are also summarized. In general, the mechanism of antibiotic degradation by nZVI mainly includes adsorption and reduction, while promoting the biodegradation of antibiotics by affecting the microbial community. nZVI can also be combined with persulfates to degrade antibiotics through advanced oxidation processes. For the control of ARGs, nZVI not only changes the microbial community structure, but also affects the proliferation of ARGs through affecting the fate of mobile genetic elements (MGEs). Finally, some new ideas on the application of nZVI in the treatment of antibiotic resistance are proposed. This paper provides a reference for research and application in this field.

Performance of field demonstration nanoscale zero-valent iron in groundwater remediation: A review

Nanoscale zero-valent iron (nZVI) has gained widespread usage in groundwater remediation due to its exceptional reactivity. Since its initial deployment in field demonstrations in 2001, nZVI has proven to be an effective nanomaterial for addressing groundwater contaminants. Subsequent research has highlighted the versatility of nZVI, showcasing its potential to overcome critical limitations associated with conventional remediation technologies. The effectiveness of nZVI in remediation varies, contingent on factors such as the type of nZVI, contaminant nature, site conditions, and injection methodologies employed. This review aims to present a comprehensive progress report on the field application of nZVI spanning 22 years across eight countries. Drawing from a database encompassing 32 pilot or full-scale remediation sites, the study delineates the various types of nZVI, modification methods, demonstration sites, and primary contaminants targeted in field tests. Specific attention is given to the application effects and mechanisms of unmodified nZVI, Pd, surfactants, and carbon-modified nZVI in diverse field demonstrations. An analysis of the key factors influencing their performance is provided, and potential future applications of nZVI in groundwater remediation are discussed.

From lab to ecosystem: Understanding the ecological footprints of engineered nanoparticles

Nanotechnology has attained significant attention from researchers in past decades due to its numerous advantages, such as biocompatibility, biodegradability, and improved stability over conventional drug delivery systems. The fabrication of engineered nanoparticles (ENPs), including carbon nanotubes (CNTs), fullerenes, metallic and metal oxide-based NPs, has been steadily increasing day due to their wide range of applications from household to industrial applications. Fabricated ENPs can release different materials into the environment during their fabrication process. The effect of such materials on the environment is the primary concern with due diligence on the safety and efficacy of prepared NPs. In addition, an understanding of chemistry, reactivity, fabrication process, and viable mechanism of NPs involved in the interaction with the environment is very important. To date, only a limited number of techniques are available to assess ENPs in the natural environment which makes it difficult to ascertain the impact of ENPs in natural settings. This review extensively examines the environmental effects of ENPs and briefly discusses useful tools for determining NP size, surface charge, surface area, and external appearance. In conclusion, the review highlights the potential risks associated with ENPs and suggests possible solutions.

Research progress on remediation of organochlorine pesticide contamination in soil

Deteriorated soil pollution has grown into a worldwide environmental concern over the years. Organochlorine pesticide (OCP) residues, featured with ubiquity, persistence and refractoriness, are one of the main pollution sources, causing soil degradation, fertility decline and nutritional imbalance, and severely impacting soil ecology. Furthermore, residual OCPs in soil may enter the human body along with food chain accumulation and pose a serious health threat. To date, many remediation technologies including physicochemical and biological ways for organochlorine pollution have been developed at home and abroad, but none of them is a panacea suitable for all occasions. Rational selection and scientific decision-making are grounded in in-depth knowledge of various restoration techniques. However, soil pollution treatment often encounters the interference of multiple factors (climate, soil properties, cost, restoration efficiency, etc.) in complex environments, and there is still a lack of systematic summary and comparative analysis of different soil OCP removal methods. Thus, to better guide the remediation of contaminated soil, this review summarized the most commonly used strategies for OCP removal, evaluated their merits and limitations and discussed the application scenarios of different methods. It will facilitate the development of efficient, inexpensive and environmentally friendly soil remediation strategies for sustainable agricultural and ecological development.

Soil properties determine the impact of nZVI on Lactuca sativa L and its rhizosphere

Nanoscale zero-valent iron (nZVI) is a promising material tool for the remediation of metal(loid)-contaminated soils since it reduces metal(loid) availability and plant uptake, thereby enhancing the development of the plants. However, the effects of nZVI as nanoparticles on soil properties, plants, and the microbial rhizosphere in unpolluted soils are poorly understood. Here we tested the impact of nZVI at different doses (0.5 and 5% of commercial suspension) on soil properties, lettuce plants, and their microbial rhizosphere in two non-contaminated soils with distinct physico-chemical properties (alkaline versus acidic soil). To this end, a pot experiment was performed with lettuce plants in a growth chamber for a month. Both soils showed an increase in of pH and available Fe after nZVI application. However, these effects were more marked in the acidic soil. In this regard, the plants in this soil showed increased biomass and Fe content. TEM analysis revealed that although the roots and leaves of plants grown in the alkaline soil showed better cell integrity than those in acidic soil-an observation that was consistent with the visual appearance of the plants-the former were more affected by the nZVI treatment. Regarding the microbial rhizosphere, in general, nZVI enhanced enzyme activity regardless of the soil type. Microbial functional diversity showed a significant decline in response to nZVI in alkaline soil. In contrast, the 0.5% nZVI treatment had a positive effect on this parameter in acidic soil. Bacterial genetic diversity was less affected by the presence of nZVI than fungal diversity, which was higher in nZVI-treated acidic soils. In addition, alterations of bacterial and fungal communities were associated with available Fe in acidic soil. In conclusion, soil properties play a key role in determining the effects of nZVI on lettuce plants and their rhizosphere.

Corrosion behaviors and kinetics of nanoscale zero-valent iron in water: A review

Knowledge on corrosion behaviors and kinetics of nanoscale zero-valent iron (nZVI) in aquatic environment is particularly significant for understanding the reactivity, longevity and stability of nZVI, as well as providing theoretical guidance for developing a cost-effective nZVI-based technology and designing large-scale applications. Herein, this review gives a holistic overview on the corrosion behaviors and kinetics of nZVI in water. Firstly, Eh-pH diagram is introduced to predict the thermodynamics trend of iron corrosion. The morphological, structural, and compositional evolution of (modified-) nZVI under different environmental conditions, assisted with microscopic and spectroscopic evidence, is then summarized. Afterwards, common analytical methods and characterization technologies are categorized to establish time-resolved corrosion kinetics of nZVI in water. Specifically, stable models for calculating the corrosion rate constant of nZVI as well as electrochemical methods for monitoring the redox reaction are discussed, emphasizing their capabilities in studying the dynamic iron corrosion processes. Finally, in the future, more efforts are encouraged to study the corrosion behaviors of nZVI in long-term practical application and further build nanoparticles with precisely tailored properties. We expect that our work can deepen the understanding of the nZVI chemistry in aquatic environment.

Controlled-Release Materials for Remediation of Trichloroethylene Contamination in Groundwater

Groundwater contamination by trichloroethylene (TCE) presents a pressing environmental challenge with far-reaching consequences. Traditional remediation methods have shown limitations in effectively addressing TCE contamination. This study reviews the limitations of conventional remediation techniques and investigates the application of oxidant-based controlled-release materials, including encapsulated, loaded, and gel-based potassium permanganate since the year 2000. Additionally, it examines reductant controlled-release materials and electron donor-release materials such as tetrabutyl orthosilicate (TBOS) and polyhydroxybutyrate (PHB). The findings suggest that controlled-release materials offer a promising avenue for enhancing TCE degradation and promoting groundwater restoration. This study concludes by highlighting the future research directions and the potential of controlled-release materials in addressing TCE contamination challenges.

Decoupling Fe0 Application and Bioaugmentation in Space and Time Enables Microbial Reductive Dechlorination of Trichloroethene to Ethene: Evidence from Soil Columns

Fe⁰ is a powerful chemical reductant with applications for remediation of chlorinated solvents, including tetrachloroethene and trichloroethene. Its utilization efficiency at contaminated sites is limited because most of the electrons from Fe⁰ are channeled to the reduction of water to H₂ rather than to the reduction of the contaminants. Coupling Fe⁰ with H₂-utilizing organohalide-respiring bacteria (i.e., Dehalococcoides mccartyi) could enhance trichloroethene conversion to ethene while maximizing Fe⁰ utilization efficiency. Columns packed with aquifer materials have been used to assess the efficacy of a treatment combining in space and time Fe⁰ and aD. mccartyi-containing culture (bioaugmentation). To date, most column studies documented only partial conversion of the solvents to chlorinated byproducts, calling into question the feasibility of Fe⁰ to promote complete microbial reductive dechlorination. In this study, we decoupled the application of Fe⁰ in space and time from the addition of organic substrates andD. mccartyi-containing cultures. We used a column containing soil and Fe⁰ (at 15 g L^-1 in porewater) and fed it with groundwater as a proxy for an upstream Fe⁰ injection zone dominated by abiotic reactions and biostimulated/bioaugmented soil columns (Bio-columns) as proxies for downstream microbiological zones. Results showed that Bio-columns receiving reduced groundwater from the Fe⁰-column supported microbial reductive dechlorination, yielding up to 98% trichloroethene conversion to ethene. The microbial community in the Bio-columns established with Fe⁰-reduced groundwater also sustained trichloroethene reduction to ethene (up to 100%) when challenged with aerobic groundwater. This study supports a conceptual model where decoupling the application of Fe⁰ and biostimulation/bioaugmentation in space and/or time could augment microbial trichloroethene reductive dechlorination, particularly under oxic conditions.

Aging and reactivity assessment of nanoscale zerovalent iron in groundwater systems

In this study, changes in the reactivity of nanoscale zerovalent iron (NZVI) in five different groundwater (GW) systems under anoxic and oxic conditions were examined over a wide range of aging time (0 - 60 d). p-nitrophenol (p-NP) was used as a redox-sensitive probe, whereas nalidixic acid (NA), a typical antibiotic found in the natural environment, was used as a sorbing compound. Investigation of the p-NP reduction in pure water systems showed that NZVI lost 41% and 98% of its reductive activity under anoxic and oxic conditions after 60 d, while enhancement of its reactivity was observed after short-term aging in GW (1 - 5 d), followed by a further decline. This behavior has been ascribed to the formation of secondary Fe(II)-bearing phases, including magnetite and green rust, resulting from NZVI aging in GW. Adsorption experiments revealed that GW-anoxic-aged NZVI samples exhibited a good affinity toward NA, and a greater NA adsorption (∼27 µmol g ^- 1) than that of pristine NZVI (∼2 µmol g ^- 1) at alkaline pH values. Surface complexation modeling showed that the enhanced adsorption of NA onto secondary minerals can be attributed to the Fe(II)-NA surface complexation. This considerable change in the reductive ability and the adsorption capacity of NZVI arising from groundwater corrosion calls for greater attention to be paid in assessment studies, where NZVI is injected for long-term remediation in groundwater.

The combined enhancement of RL, nZVI and AQDS on the microbial anaerobic-aerobic degradation of PAHs in soil

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous persistent organic pollutants in soil, which have carcinogenic, teratogenic and mutagenic hazards. The effects of rhamnolipid (RL), nano zero-valent iron (nZVI), and anthraquinone-2,6-disulfonic acid (AQDS) on the degradation of PAHs in soil were studied. It was found that the treatment of 5 mg·kg^-1RL + 1% nZVI +0.2 mmol·kg^-1AQDS had the highest degradation rate. The degradation rate of total PAHs and HMW-PAHs was 72.81% and 79.47% respectively after 90 days. High-throughput sequencing showed that in RL + nZVI + AQDS enhanced soil, Clostridium, Geobacter, Anaeromyxobacter and Sphingomonas were the dominant species for anaerobic degradation of PAHs. Rhodococcus, Nocardioides, and Microvirga are the dominant species for aerobic degradation of PAHs. The activities of methyltransferase, dehydrogenase and catechol 1,2-dioxygenase in the anaerobic-aerobic degradation process of PAHs were consistent with the degradation process of PAHs, indicating the role of these enzymes in the degradation of PAHs. RL, nZVI, and AQDS combined enhanced microbial anaerobic-aerobic degradation has great application potential in remediation of PAHs-contaminated soil.

The Widespread Use of Nanomaterials: The Effects on the Function and Diversity of Environmental Microbial Communities

In recent years, as an emerging material, nanomaterials have rapidly expanded from laboratories to large-scale industrial productions. Along with people's productive activities, these nanomaterials can enter the natural environment of soil, water and atmosphere through various ways. At present, a large number of reports have proved that nanomaterials have certain toxic effects on bacteria, algae, plants, invertebrates, mammalian cell lines and mammals in these environments, but people still know little about the ecotoxicology of nanomaterials. Most relevant studies focus on the responses of model strains to nanomaterials in pure culture conditions, but these results do not fully represent the response of microbial communities to nanomaterials in natural environments. Over the years, the effect of nanomaterials infiltrated into the natural environment on the microbial communities has become a popular topic in the field of nano-ecological environment research. It was found that under different environmental conditions, nanomaterials have various effects on the microbial communities. The medium; the coexisting pollutants in the environment and the structure, particle size and surface modification of nanomaterials may cause changes in the structure and function of microbial communities. This paper systematically summarizes the impacts of different nanomaterials on microbial communities in various environments, which can provide a reference for us to evaluate the impacts of nanomaterials released into the environment on the microecology and has certain guiding significance for strengthening the emission control of nanomaterials pollutants.

Dissolved iron released from nanoscale zero-valent iron (nZVI) activates the defense system in bacterium Pseudomonas putida, leading to high tolerance to oxidative stress

Nanoscale zero-valent iron (nZVI) has increasingly been applied to remediate aquifers polluted by organochlorines or heavy metals. As a result, bacteria in the vicinity of remediate action can be stressed by surplus iron released from nZVI. However, the understanding of the iron stress defense pathways during this process is currently incomplete. Therefore, we aimed to elucidate the physiological and transcriptomic response of the bacterium, Pseudomonas putida NCTC 10936, to 100 mg/L of nZVI and 44.5 µg/L of dissolved iron obtained from nZVI suspension. Cell viability was neither affected by nZVI nor dissolved iron, although the dissolved iron caused stress that altered the cell physiology and caused the generation of smaller cells, whereas cells were elongated in the presence of nZVI. Transcriptomic analysis confirmed the observed stronger physiological effect caused by dissolved iron (in total 3839 differentially expressed genes [DEGs]) than by nZVI (945 DEGs). Dissolved iron (but not nZVI) activated genes involved in oxidative stress-related pathways, antioxidant activity, carbohydrate and energy metabolism, but downregulated genes associated with flagellar assembly proteins and two-component systems involved in sensing external stimuli. As a result, bacteria very effectively faced oxidative insults and cell viability was not affected.

Remediation of groundwater contaminated with dye using carbon dots technology: Ecotoxicological and microbial community responses

Groundwater pollution poses a serious threat to the main source of clean water globally. Nanoparticles have the potential for remediation of polluted aquifers; however, environmental safety concerns associated with in situ deployments of such technology include potential detrimental effects on microorganisms in terms of toxicity and functional disruptions. In this work, we evaluated a new and ecofriendly approach using carbon dots (CDs) as Fenton-like catalysts to catalyse the degradation of dye-containing groundwater samples. This investigation aimed at evaluating the efficacy of a novel remediation technology in terms of dye degradation and toxicity reduction while assessing its impacts on aquatic microorganisms. Uncontaminated Australian groundwater samples were spiked with methylene blue and incubated in the dark, at 18 °C, under slow agitation, using CDs at 0.5 mg mL^-1 and H₂O₂ at 73.5 mM for 25 h. The dye degradation rate was determined as well as the toxicity of the treated solutions using the Microtox® bioassay. Further, to determine the changes in the groundwater microbial community, 16 S rRNA sequencing was used and evenness and diversity indices were analysed using Pielou's evenness and Simpson index, respectively. This study revealed that dye-containing groundwater were effectively treated by CDs showing a degradation rate of 78-82% and a significant 4-fold reduction in the toxicity. Characterisation of the groundwater microbiota revealed a predominance of at least 60% Proteobacteria phylum in all samples where diversity and evenness were maintained throughout the remediation process. The results showed that CDs could be an efficient approach to treat polluted groundwater and potentially have minimum impact on the environmental microbiome.

Real-time monitoring of aqueous Hg2+ reduction dynamics by magnetite/iron metal composite powders synthesized hydrothermally

An iron-based powder material composed of zerovalent iron (8 wt.%) and magnetite (92 wt.%), has been synthesized hydrothermally at 200 °C from zero-valent iron. Its effect on the reduction of aqueous Hg²⁺ into gaseous Hg⁰ has been investigated at ambient conditions for pH comprised between 4 and 8.5. The production of Hg⁰ was monitored with an online mercury vapor analyzer at the picogram level for concentrations of iron-based composite of a few tenths of mg L^-1. Starting from a solution having an Hg²⁺ concentration of 25 ng L^-1 at pH = 4, a succession of two Hg⁰ production events was recorded. The first event is related to the Hg²⁺ reduction by ZVI which fully dissolved within the first hours. Upon ZVI consumption, pH drifted towards the pH window where magnetite can efficiently reduce Hg²⁺ at the hour timescale, resulting in a second Hg⁰ production peak. The combined use of ZVI and magnetite to remove aqueous Hg²⁺ by formation of Hg⁰ (volatile) under mild acidic pH allows (1) to maximize the Hg²⁺ reduction rate and (2) to take benefit of the longer lifetime of magnetite compared to ZVI.

Evaluating the Effectiveness of Nanotechnology in Environmental Remediation of a Highly Metal-Contaminated Area—Minas Gerais, Brazil

A column experiment at a laboratory level was carried out to assess the effect of the application of nanotechnology in the decontamination of soils and alluvial deposits with high levels of potentially toxic elements (PTEs). A suspension of zero-valent iron nanoparticles (nZVI) was injected at three different concentrations in selected samples (two sediments, one soil). For most of the elements, the retention by nZVI was proportional to the concentration of the suspension and the trend was similar. Metals were immobilized by adsorption on the surface layer of the nanoparticles and/or by complexation, co-precipitation, and chemical reduction. By day 60 following injection, the nZVI lost reactivity and the retained species were desorbed and back into the soluble phase. The definition of spatial patterns for PTEs’ distribution allowed for the construction of contamination risk maps using a geostatistical simulation approach. The analysis obtained from the extractable contents of five target elements (Zn, Cu, Cd, Pb, As) was cross-checked with the estimated map network to assess their retention efficiency. Data from the analysis of these elements, in the extractable phase and in the porewater of the sediments/soils, indicate the nZVI injection as a suitable technique for reducing the risk level of PTEs in contaminated Fe-rich tropical environments.

Characteristic and mechanism analysis of sludge polymer by adding carboxymethyl cellulose-g-acrylic acid for sludge utilization

Herein, a nutrient water retention agent is prepared by fully mixing sludge with carboxymethyl cellulose-g-acrylic acid (CMC-g-AA) gel and nanoscale zero-valent iron (nZVI) using polymer modifying curing technology. Experimental results show that when CMC:AA = 1:12 and CMC-g-AA gel content is 50%, sludge polymer has better water absorption and retention performance and the water retention time is extended for ∼14 days. At the same time, sludge polymer can preserve the characteristics of nutrient-rich elements and organic matter and promote plant growth. The addition of nZVI has a significant impact on reducing the risk of heavy metal toxic leaching in sludge. Moreover, analysis of variance and multiple comparisons shows that sludge polymer's particle size and water absorption times have significant effects on the water absorption and retention properties of sludge polymer. Scanning electron microscopy, X-ray diffraction, Fourier-transform infrared spectroscopy and ¹³C-nuclear magnetic resonance analyses show that the addition of an appropriate amount of gel could increase the number of hydrophilic groups and hydrophilic mineral components in sludge polymer, increase its overall porosity and improve its water absorption and retention properties.

Enhanced nitrobenzene removal in soil by biochar supported sulfidated nano zerovalent iron: Solubilization effect and mechanism

Sulfidated nano zerovalent iron (S-nZVI) is reported to be effective in removal of aqueous organic contaminants. However, little is known about its potential use in reductive degradation of soil-sorbed contaminants. In this study, biochar (BC) supported S-nZVI (S-nZVI@BC) was successfully synthesized through sulfidation and carbon loading modification, which effectively combined the solubilization characteristics of BC and high reduction characteristics of S-nZVI. Transmission electron microscopy (TEM) with an energy-dispersive X-ray spectroscopy (EDS) analysis suggested that sulfur and iron were evenly distributed throughout BC matrix. The degradation of nitrobenzene (NB) in soil was achieved more efficiently with the as-synthesized S-nZVI@BC composites. Results indicated that S-nZVI@BC with S-nZVI/BC mass ratio of 3:1, dosage of 10 mg/g exhibited superior NB removal (98%) and aniline (AN) formation (90%) efficiency within 24 h without formation of other intermediates, higher than those of S-nZVI. Meanwhile, the surface FeS_X layer enhanced the antioxidant capacity of S-nZVI@BC and participated in the reduction of NB. The soil-sorbed NB decreased from 14% to 1.4%, indicating that the addition of BC played an important role in solubilization of NB from soil. Solubilization-reduction was the dominant mechanism for NB removal. This research indicated that S-nZVI@BC held the potential to enhance in-situ remediation of NB-contaminated soil.

Sulfidation extent of nanoscale zerovalent iron controls selectivity and reactivity with mixed chlorinated hydrocarbons in natural groundwater

Sulfidated nanoscale zerovalent iron (S-nZVI) exhibits low anoxic oxidation and high reactivity towards many chlorinated hydrocarbons (CHCs). However, nothing is known about S-nZVI reactivity once exposed to complex CHC mixtures, a common feature of CHC plumes in the environment. Here, three S-nZVI materials with varying iron sulfide (mackinawite, FeS_m) shell thickness and crystallinity were exposed to groundwater containing a complex mixture of chlorinated ethenes, ethanes, and methanes. CHC removal trends yielded pseudo-first order rate constants (k_obs) that decreased in the order: trichloroethene > trans-dicloroethene > 1,1-dichlorethene > trichloromethane > tetrachloroethene > cis-dichloroethene > 1,1,2-trichloroethane, for all S-nZVI materials. These k_obs trends showed no correlation with CHC reduction potentials based on their lowest unoccupied molecular orbital energies (E_LUMO) but absolute values were affected by the FeS_m shell thickness and crystallinity. In comparison, nZVI reacted with the same CHCs groundwater, yielded k_obs that linearly correlated with CHC E_LUMO values (R² = 0.94) and that were lower than S-nZVI k_obs. The CHC selectivity induced by sulfidation treatment is explained by FeS_m surface sites having specific binding affinities towards some CHCs, while others require access to the metallic iron core. These new insights help advance S-nZVI synthesis strategies to fit specific CHC treatment scenarios.

Iron Nitride Nanoparticles for Enhanced Reductive Dechlorination of Trichloroethylene

Nitriding has been used for decades to improve the corrosion resistance of iron and steel materials. Moreover, iron nitrides (Fe_xN) have been shown to give an outstanding catalytic performance in a wide range of applications. We demonstrate that nitriding also substantially enhances the reactivity of zerovalent iron nanoparticles (nZVI) used for groundwater remediation, alongside reducing particle corrosion. Two different types of Fe_xN nanoparticles were synthesized by passing gaseous NH₃/N₂ mixtures over pristine nZVI at elevated temperatures. The resulting particles were composed mostly of face-centered cubic (γ'-Fe₄N) and hexagonal close-packed (ε-Fe_2-3N) arrangements. Nitriding was found to increase the particles' water contact angle and surface availability of iron in reduced forms. The two types of Fe_xN nanoparticles showed a 20- and 5-fold increase in the trichloroethylene (TCE) dechlorination rate, compared to pristine nZVI, and about a 3-fold reduction in the hydrogen evolution rate. This was related to a low energy barrier of 27.0 kJ mol^-1 for the first dechlorination step of TCE on the γ'-Fe₄N(001) surface, as revealed by density functional theory calculations with an implicit solvation model. TCE dechlorination experiments with aged particles showed that the γ'-Fe₄N nanoparticles retained high reactivity even after three months of aging. This combined theoretical-experimental study shows that Fe_xN nanoparticles represent a new and potentially important tool for TCE dechlorination.

Green synthesis of Fe0 nanoparticles using Eucalyptus grandis leaf extract: Characterization and application for dye degradation by a (Photo)Fenton-like process

Zero-valent iron nanoparticles (EGnZVI) were synthesized using Eucalyptus grandis (EG) leaf extract as a reducing/stabilizing agent. The studied materials (EG leaves, extract and EGnZVI) were characterized using the XRD, FTIR, Raman spectroscopy, SEM, TEM/EDS techniques. The results indicate that several organic compounds, including phenolics, present in the EG leaves were successfully extracted and incorporated into the structure of the material, possibly promoting the capping and stabilization of the formed zero-valent iron particles. The EGnZVI presented low crystallinity, varied size (50-500 nm), approximately spherical shape, and formed aggregates. The EGnZVI were utilized in the removal of the Direct Red 80 (DR80), an azo dye. The effects of the temperature (15-35 °C), initial DR80 concentration (10-250 mg L-1), initial pH (2.5-8.5), the doses of H2O2 (0.5-5 mmol L-1) and EGnZVI (0.2-10 mg L-1), and the incidence of UV-light were evaluated. The EGnZVI did not present reactivity towards the DR80 in the absence of H2O2. However, in the presence of H2O2, the EGnZVI was highly efficient at removing the DR80 at slightly acidic pH0 values (4 and 5.5). Under these pH0 conditions, the EGnZVI/Fenton process proved to be more effective than the classic homogenous Fenton. Finally, in the presence of the UV-light, the process was highly efficient throughout the studied pH0 interval, with increased removal rates. Therefore, the nZVI/Fenton process, using the synthesized material, presents itself as a promising alternative for the degradation of organic pollutants, and the incidence of UV light can considerably improve its efficiency.

Coupling Removal of P-Chloronitrobenzene and Its Reduction Products by Nano Iron Doped with Ni and FeOOH (nFe/Ni-FeOOH)

The removal of chlorinated pollutants from water by nanoparticles is a hot topic in the field of environmental engineering. In this work, a novel technique that includes the coupling effect of n-Fe/Ni and its transformation products (FeOOH) on the removal of p-chloronitrobenzene (p-CNB) and its reduction products, p-chloroaniline (p-CAN) and aniline (AN), were investigated. X-ray diffraction (XRD) and transmission electron microscopy (TEM) were employed to characterize the nano-iron before and after the reaction. The results show that Fe⁰ is mainly oxidized into lath-like lepidocrocite (γ-FeOOH) and needle-like goethite (α-FeOOH) after 8 h of reaction. The coupling removal process and the mechanism are as follows: Fe⁰ provides electrons to reduce p-CNB to p-CAN and then dechlorinates p-CAN to AN under the catalysis of Ni. Meanwhile, Fe⁰ is oxidized to FeOOH by the dissolved oxygen and H₂O. AN is then adsorbed by FeOOH. Finally, p-CNB, p-CAN, and AN were completely removed from the water. In the pH range between 3 and 7, p-CAN can be completely dechlorinated by n-Fe/Ni within 20 min, while AN can be nearly 100% adsorbed by FeOOH within 36 h. When the temperature ranges from 15 °C to 35 °C, the dechlorination rate of p-CAN and the removal rate of AN are less affected by temperature. This study provides guidance on the thorough remediation of water bodies polluted by chlorinated organics.

Iron nanoparticles to recover a co-contaminated soil with Cr and PCBs

Little attention has been given to the development of remediation strategies for soils polluted with mixture of pollution (metal(loid)s and organic compounds). The present study evaluates the effectiveness of different types of commercial iron nanoparticles (nanoscale zero valent iron (nZVI), bimetallic nZVI-Pd, and nano-magnetite (nFe₃O₄)), for the remediation of an industrial soil co-contaminated with Cr and PCBs. Soil samples were mixed with nZVI, nZVI-Pd, or nFe₃O₄ at doses selected according to their reactivity with PCBs, homogenized, saturated with water and incubated at controlled conditions for 15, 45 and 70 days. For each sampling time, PCBs and chromium were analyzed in aqueous and soil fractions. Cr(VI) and Cr leachability (TCLP test) were determined in the soil samples. The treatment with the three types of iron nanoparticles showed significant reduction in Cr concentration in aqueous extracts at the three sampling times (> 98%), compared to the control samples. The leachability of Cr in treated soil samples also decreased and was stable throughout the experiment. Results suggested that nZVI and nZVI-Pd immobilized Cr through adsorption of Cr(VI) on the shell and reduction to Cr(III). The mechanism of interaction of nFe₃O₄ and Cr(VI) included adsorption and reduction although its reducing character was lower than those of ZVI nanoparticles. PCBs significantly decreased in soil samples (up to 68%), after 15 days of treatment with the three types of nanoparticles. However, nFe₃O₄ evidenced reversible adsorption of PCBs after 45 days. In general, nZVI-Pd reduced PCB concentration in soil faster than nZVI. Control soils showed a similar reduction in PCBs concentration as those obtained with nZVI and nZVI-Pd after a longer time (45 days). This is likely due to natural bioremediation, although it was not effective for Cr remediation. Results suggest that the addition of nZVI or nZVI-Pd and pseudo-anaerobic conditions could be used for the recovery of soil co-contaminated with Cr and PCBs.

Nanowaste: Another Future Waste, Its Sources, Release Mechanism, and Removal Strategies in the Environment

Nanowaste is defined as waste derived from materials with at least one dimension in the 1–100 nm range. The nanomaterials containing products are considered as “nanoproducts” and they can lead to the development of nanomaterial-containing waste, also termed as “nanowaste”. The increased production and consumption of these engineered nanomaterials (ENMs) and nanoproducts that generate enormous amounts of nanowaste have raised serious concerns about their fate, behavior, and ultimate disposal in the environment. It is of the utmost importance that nanowaste is disposed of in an appropriate manner to avoid an adverse impact on human health and the environment. The unique properties of ENMs, combined with an inadequate understanding of appropriate treatment techniques for many forms of nanowaste, makes nanowaste disposal a complex task. Presently, there is a lack of available information on the optimized standards for identifying, monitoring, and managing nanowaste. Therefore, this review highlights concerns about nanowaste as future waste that need to be addressed. The review focuses on ENMs waste (in the form of NP, nanotubes, nanowires, and quantum dots) generated from the manufacture of a wide variety of nanoproducts that end up as nanowaste and adversely affect the environment. Furthermore, the review considers different types of ENMs in waste streams and environmental compartments (i.e., soil, water, and air). Detailed studies are still required to identify data gaps and implement strategies to remove and control this future waste.

Long-Term Exposure and Effects of rGO-nZVI Nanohybrids and Their Parent Nanomaterials on Wastewater-Nitrifying Microbial Communities

Single nanomaterials and nanohybrids (NHs) can inhibit microbial processes in wastewater treatment, especially nitrification. While existing studies focus on short-term and acute exposures of single nanomaterials on wastewater microbial community growth and function, long-term, low-exposure, and emerging NHs need to be examined. These NHs have distinctly different physicochemical properties than their parent nanomaterials and, therefore, may exert previously unknown effects onto wastewater microbial communities. This study systematically investigated long-term [∼6 solid residence time [(SRT)] exposure effects of a widely used carbon-metal NH (rGO-nZVI = 1:2 and 1:0.2, mass ratio) and compared these effects to their single-parent nanomaterials (i.e., rGO and nZVI) in nitrifying sequencing batch reactors. nZVI and NH-dosed reactors showed relatively unaffected microbial communities compared to control, whereas rGO showed a significantly different (p = 0.022) and less diverse community. nZVI promoted a diverse community and significantly higher (p < 0.05) biomass growth under steady-state conditions. While long-term chronic exposure (10 mg·L^-1) of single nanomaterials and NHs had limited impact on long-term nutrient recovery, functionally, the reactors dosed with higher iron content, that is, nZVI and rGO-nZVI (1:2), promoted faster NH₄⁺-N removal due to higher biomass growth and upregulation of amoA genes at the transcript level, respectively. The transmission electron microscopy images and scanning electron microscopy─energy-dispersive X-ray spectroscopy analysis revealed high incorporation of iron in nZVI-dosed biomass, which promoted higher cellular growth and a diverse community. Overall, this study shows that NHs have unique effects on microbial community growth and function that cannot be predicted from parent materials alone.

Application of porous styrene resin loaded carboxymethyl cellulose-stabilized nano-zero-valent iron for highly efficient hexavalent chromium removal

The results of this study provide a new idea for the design of efficient Cr(vi) removal materials based on nZVI.

Magnetic Solid-Phase Extraction Based on Amino-functionalized Magnetic Starch for Analysis of Organochlorine Pesticides

A magnetic starch modified with 3,5-diaminobenzoic acid and 3-aminopropyltriethoxysilane (Fe-starch@DABA-APTES) was synthesized and applied as adsorbent for the extraction of organochlorine pesticides (OCPs). Magnetic solid-phase extraction was developed using 75 mg of the sorbent and 15 mL of a sample solution. Extraction was conducted on a vortex mixer for 40 s. The adsorbent was collected using an external magnet before eluting the analytes using 0.5 mL of methanol. Quantification of the analytes was performed using gas chromatography with a micro-electron capture detector. The linearity was obtained for the studied OCPs in the range between 0.01 - 2.00 μg L^-1. The detection and quantification limits were obtained in ranges of 0.5 - 4.0 and 1.5 - 15.0 ng L^-1, respectively, with enrichment factors of up to 39. The precision in terms of the intra- and inter-day relative standard deviations (RSDs) were below 4.75, and 9.25%, respectively. The developed method has been applied in natural water and agricultural product samples. The recoveries ranged between 59.83 - 132.67% (RSDs < 10.73%).

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.

Mechanisms of metabolic performance enhancement and ARGs attenuation during nZVI-assisted anaerobic chloramphenicol wastewater treatment

Anaerobic wastewater treatment is a promising technology for refractory pollutant treatment. The nano zero-valent iron (nZVI) assisted anaerobic system could enhance contaminant removal. In this work, we added nZVI into an anaerobic system to investigate the effects on system performances and metabolic mechanism for chloramphenicol (CAP) wastewater treatment. As nZVI concentrations increased from 0 to 1 g/L, the CAP removal efficiency was appreciably improved from 46.5% to 99.2%, while the CH₄ production enhanced more than 20 times. The enhanced CAP removal resulted from the enrichments of dechlorination-related bacteria (Hyphomicrobium) and other functional bacteria (e.g., Zoogloea, Syntrophorhabdus) associated with refractory contaminants degradation. The improved CH₄ production was ascribed to the increases in fermentative-related bacteria (Smithella and Acetobacteroides), homoacetogen (Treponema), and methanogens. The increased abundances of anaerobic functional genes further verified the mechanism of CH₄ production. Furthermore, the abundances of potential hosts of antibiotic resistance genes (ARGs) were reduced under high nZVI concentration (1 g/L), contributing to ARGs attenuation. This study provides a comprehensive analysis of the mechanism in metabolic performance enhancement and ARGs attenuation during nZVI-assisted anaerobic CAP wastewater treatment.

Detection and remediation of pollutants to maintain ecosustainability employing nanotechnology: A review

Environmental deterioration due to anthropogenic activities is a threat to sustainable, clean and green environment. Accumulation of hazardous chemicals pollutes soil, water and air and thus significantly affects all the ecosystems. This article highlight the challenges associated with various conventional techniques such as filtration, absorption, flocculation, coagulation, chromatographic and mass spectroscopic techniques. Environmental nanotechnology has provided an innovative frontier to combat the aforesaid issues of sustainable environment by reducing the non-requisite use of raw materials, electricity, excessive use of agrochemicals and release of industrial effluents into water bodies. Various nanotechnology based approaches including surface enhance scattering, surface plasmon resonance; and distinct types of nanoparticles like silver, silicon oxide and zinc oxide have contributed significantly in detection of environmental pollutants. Biosensing technology has also gained significant attention for detection and remediation of pollutants. Furthermore, nanoparticles of gold, ferric oxide and manganese oxide have been used for the on-site remediation of antibiotics, organic dyes, pesticides, and heavy metals. Recently, green nanomaterials have been given more attention to address toxicity issues of chemically synthesized nanomaterials. Hence, nanotechnology has provided a platform with tremendous applications to have sustainable environment for present as well as future generations. This review article will help to understand the fundamentals for achieving the goals of sustainable development, and healthy environment.

Removal of type-A, type-B, and borderline metals from contaminated soils using zero valent iron and magnetic separation technology: A predictive approach for metal resources recovery

Despite the limitations reported on the efficiency of metals used as sorbents, recent advances in chemical and material sciences make it possible to use remediation technologies based on zero valent iron (ZVI) to restore the ecosystem services of metal-contaminated soils. In addition, recent studies showed that remediation by in situ immobilization could be avoided by taking advantage of the strong magnetic characteristics of ZVI. We combined these well-established concepts and conducted laboratory experiments to predict the removal efficiency of metals from contaminated soils based on their chemical classification into type-A, type-B and borderline metals. The Nieboer-Richardson separation of metal ions based on covalent and ionic indexes was used, and beryllium (Be²⁺), mercury (Hg²⁺) and lead (Pb²⁺) were selected as representative of type-A, type-B and borderline, respectively. The results showed a significant decrease in total metal concentrations of treated soils, with a removal efficiency of about 80% for Be, 90% for Pb and 97% for Hg. This ranking followed the increasing order of the covalent indexes, which are 1.11, 3.36, and 3.92 for Be, Pb and Hg, respectively. Therefore, the ability to form strong covalent bonds with oxygen atoms in maghemite (Fe₂O₃, γ-Fe₂O₃) identified on ZVI surfaces seems to drive metal recovery. Validation studies conducted on soil samples collected from sites contaminated with either Pb or Hg, confirmed the above trend. Overall, the results suggest that borderline and type-B metals can be successfully recovered from contaminated soils with rates ≥90%, while the performance would be much lower for type-A metals.

Compositional evolution of nanoscale zero valent iron and 2,4-dichlorophenol during dechlorination by attapulgite supported Fe/Ni nanoparticles

Transformation of chloro-organic compounds by nFe(0) has been studied extensively, but limited study exists on the transformation and fate of nFe(0) during the dechlorination of chloro-organics even though such knowledge is important in predicting its surface chemistry, particularly, toxicity in the environment. In this study, the nFe(0) core became hollowed, collapsed and gradually corroded into poorly crystallized ferrihydrite (Fe₅O₃(OH)₉) at the pristine reaction time, which later gave rise to lath-like lepidocrocite (γ-FeOOH), acicular goethite (α-FeOOH) and cubic magnetite (Fe₃O₄) by the end of the reaction time (120 min). Also, dechlorination of 2,4-DCP into 2-CP, 4-CP and phenol was achieved within 120 min. The rapid dechlorination of 2,4-DCP and transformation of nFe(0) could not be achieved significantly without doping Ni on nFe(0) and supporting on attapulgite. The schematic representation of the transformation and compositional evolution of nFe(0) in A-nFe/Ni was proposed. These findings are critical in understanding the compositional evolution and the fate of nFe(0) upon reaction with chloro-organics and can provide guidance for more efficient uses of the nFe(0) reactivity towards the target contaminants in groundwater remediation.

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.

The application status, development and future trend of nano-iron materials in anaerobic digestion system

Growing environment problem and emphasis of environmental protection motivate intense research efforts in exploring technology to improve treatment efficiency on refractory organic pollutants. Hence, finding a method to make up for the deficiency of anaerobic digestion (AD) is very attractive and challenging tasks. The recent spark in the interest for the usage of some nanomaterials as an additive to strengthen AD system. The adoption of iron compounds can influence the performance and stability in AD system. However, different iron species and compounds can influence AD system in significantly different ways, both positive and negative. Therefore, strengthening mechanism, treatment efficiency, microbial community changes in Nanoscale Zero Valent Iron (nZVI) and Fe₃O₄ nanoparticles (Fe₃O₄ NPs) added AD systems were summarized by this review. The strengthening effects of nZVI and Fe₃O₄ NPs in different pollutants treatment system were analyzed. Previous study on the effects of nZVI and Fe₃O₄ NPs addition on AD have reported the concentration of nZVI and Fe₃O₄ NPs, and the types and biodegradability of pollutants might be the key factors that determine the direction and extent of effect in AD system. This review provides a summary on the nZVI and Fe₃O₄ NPs added AD system to establish experiment systems and conduct follow-up experiments in future study.

Effects of in situ Remediation With Nanoscale Zero Valence Iron on the Physicochemical Conditions and Bacterial Communities of Groundwater Contaminated With Arsenic

Nanoscale Zero-Valent Iron (nZVI) is a cost-effective nanomaterial that is widely used to remove a broad range of metal(loid)s and organic contaminants from soil and groundwater. In some cases, this material alters the taxonomic and functional composition of the bacterial communities present in these matrices; however, there is no conclusive data that can be generalized to all scenarios. Here we studied the effect of nZVI application in situ on groundwater from the site of an abandoned fertilizer factory in Asturias, Spain, mainly polluted with arsenic (As). The geochemical characteristics of the water correspond to a microaerophilic and oligotrophic environment. Physico-chemical and microbiological (cultured and total bacterial diversity) parameters were monitored before and after nZVI application over six months. nZVI treatment led to a marked increase in Fe(II) concentration and a notable fall in the oxidation-reduction potential during the first month of treatment. A substantial decrease in the concentration of As during the first days of treatment was observed, although strong fluctuations were subsequently detected in most of the wells throughout the six-month experiment. The possible toxic effects of nZVI on groundwater bacteria could not be clearly determined from direct observation of those bacteria after staining with viability dyes. The number of cultured bacteria increased during the first two weeks of the treatment, although this was followed by a continuous decrease for the following two weeks, reaching levels moderately below the initial number at the end of sampling, and by changes in their taxonomic composition. Most bacteria were tolerant to high As(V) concentrations and showed the presence of diverse As resistance genes. A more complete study of the structure and diversity of the bacterial community in the groundwater using automated ribosomal intergenic spacer analysis (ARISA) and sequencing of the 16S rRNA amplicons by Illumina confirmed significant alterations in its composition, with a reduction in richness and diversity (the latter evidenced by Illumina data) after treatment with nZVI. The anaerobic conditions stimulated by treatment favored the development of sulfate-reducing bacteria, thereby opening up the possibility to achieve more efficient removal of As.

The Era of Nanomaterials: A Safe Solution or a Risk for Marine Environmental Pollution?

In recent years, the application of engineered nanomaterials (ENMs) in environmental remediation gained increasing attention. Due to their large surface area and high reactivity, ENMs offer the potential for the efficient removal of pollutants from environmental matrices with better performances compared to conventional techniques. However, their fate and safety upon environmental application, which can be associated with their release into the environment, are largely unknown. It is essential to develop systems that can predict ENM interactions with biological systems, their overall environmental and human health impact. Until now, Life-Cycle Assessment (LCA) tools have been employed to investigate ENMs potential environmental impact, from raw material production, design and to their final disposal. However, LCA studies focused on the environmental impact of the production phase lacking information on their environmental impact deriving from in situ employment. A recently developed eco-design framework aimed to fill this knowledge gap by using ecotoxicological tools that allow the assessment of potential hazards posed by ENMs to natural ecosystems and wildlife. In the present review, we illustrate the development of the eco-design framework and review the application of ecotoxicology as a valuable strategy to develop ecosafe ENMs for environmental remediation. Furthermore, we critically describe the currently available ENMs for marine environment remediation and discuss their pros and cons in safe environmental applications together with the need to balance benefits and risks promoting an environmentally safe nanoremediation (ecosafe) for the future.

Sulfidated nano-scale zerovalent iron is able to effectively reduce in situ hexavalent chromium in a contaminated aquifer

In a number of laboratory studies, sulfidated nanoscale zero-valent iron (S-nZVI) particles showed increased reactivity, reducing capacity, and electron selectivity for Cr(VI) removal from contaminated waters. In our study, core-shell S-nZVI particles were successfully injected into an aquifer contaminated with Cr(VI) at a former chrome plating facility. S-nZVI migrated towards monitoring wells, resulting in a rapid decrease in Cr(VI) and Cr_tot concentrations and a long-term decrease in groundwater redox potential observed even 35 m downstream the nearest injection well. Characterization of materials recovered from the injection and monitoring wells confirmed the presence of nZVI particles, together with iron corrosion products. Chromium was identified on the surface of the recovered iron particles as Cr(III), and its occurrence was linked to the formation of insoluble chromium-iron (oxyhydr)oxides such as Cr_xFe_(1-x)(OH)_3(s). Injected S-nZVI particles formed aggregates, which were slowly transformed into iron (oxyhydr)oxides and carbonate green rust. Elevated contents of Fe⁰ were detected even several months after injection, indicating good S-nZVI longevity. The sulfide shell was gradually disintegrated and/or dissolved. Geochemical modelling confirmed the overall stability of the resulting Cr(III) phase at field conditions. This study demonstrates the applicability of S-nZVI for the remediation of a Cr(VI)-contaminated aquifer.

Porous media transport of iron nanoparticles for site remediation application: A review of lab scale column study, transport modelling and field-scale application

Injection of surface modified zero valent iron nanoparticles for in situ remediation of soil, contaminated with an array of pollutants has attracted great attention due to the high reactivity of zero valent iron towards a broad range of contaminants, its cost effectiveness, minimal physical disruption and low toxicity. The effectiveness of this technology relies on the stability and mobility of injected iron nanoparticles. Hence the development of a modelling tool capable of predicting nZVI transport is indispensable. This review provides state of the art knowledge on the mobility of iron nanoparticles in porous media, mechanisms involved in subsurface retention of nZVI based on continuum models and field scale application. Special attention is given to the identification of the influential parameters controlling the transport potential of iron nanoparticles and the available numerical models for the simulation of laboratory scale transport data.

Reduction of chlorinated hydrocarbons using nano zero-valent iron supported with an electric field. Characterization of electrochemical processes and thermodynamic stability

Electric field assisted remediation using nano iron has shown outstanding results as well as economic benefits during pilot applications (Černíková et al., 2020). This method is based on donating electrons to the zero-valent iron that possess an inherently strong reductive capacity. The reduction of chlorinated hydrocarbons may be characterized by a decrease in contaminants or better still by the evolution of ethene and ethane originating from the reduction of chlorinated ethenes. The evolution of ethene and ethane was observed predominantly in the vicinity of the anode despite reduction processes being expected near the cathode - the electron donor. The reduction near the anode occurred due to dissolved Fe²⁺ ions, whose presence was suggested by a Pourbaix diagram that combines Eh/pH values to characterize electrochemical stabilities between different species. No products of dechlorination were observed in the area of the cathode due to presence of oxidized Fe in the form of Fe³⁺ or Fe(OH)₄^-. The experimental work described in this research provides a deeper view of the processes of electrochemical reductive dechlorination using zero-valent iron and DC. It also showed an increase in the efficiency compared to the method using zero-valent iron only.

Impact of Zero-Valent Iron on Freshwater Bacterioplankton Metabolism as Predicted from 16S rRNA Gene Sequence Libraries

The application of zero-valent iron particles (ZVI) for the treatment of heavily polluted environment and its biological effects have been studied for at least two decades. Still, information on the impact on bacterial metabolic pathways is lacking. This study describes the effect of microscale and nanoscale ZVI (mZVI and nZVI) on the abundance of different metabolic pathways in freshwater bacterial communities. The metabolic pathways were inferred from metabolism modelling based on 16S rRNA gene sequence data using paprica pipeline. The nZVI changed the abundance of numerous metabolic pathways compared to a less influencing mZVI. We identified the 50 most affected pathways, where 31 were related to degradation, 17 to biosynthesis, and 2 to detoxification. The linkage between pathways was two times higher in nZVI samples compared to mZVI, and was specifically higher considering the arsenate detoxification II pathway. Limnohabitans and Roseiflexus were linked to the same pathways in both nZVI and mZVI. The prediction of metabolic pathways increases our knowledge of the impacts of nZVI and mZVI on freshwater bacterioplankton.

The enhanced reduction of bromate by highly reactive and dispersive green nano-zerovalent iron (G-NZVI) synthesized with onion peel extract

In this study, novel green nano-zerovalent iron (G-NZVI) is synthesized for the first time using onion peel extract for the prevention of rapid surface oxidation and the enhancement of particle dispersibility with a high reductive capacity. The results from various surface analyses revealed that the spherical shape of G-NZVI was fully covered by the onion peel extract composed of polyphenolic compounds with C[double bond, length as m-dash]C-C[double bond, length as m-dash]C unsaturated carbon, C[double bond, length as m-dash]C, C-O, and O-H bonds, resulting in high mobility during column chromatography. Furthermore, the obtained G-NZVI showed the complete removal of 50 mg L-1 of bromate (BrO3 -) in 2 min under both aerobic (k = 4.42 min-1) and anaerobic conditions (k = 4.50 min-1), showing that G-NZVI had outstanding oxidation resistance compared to that of bare NZVI. Moreover, the observed performance of G-NZVI showed that it was much more reactive than other well-known reductants (e.g., Fe and Co metal organic frameworks), regardless of whether aerobic or anaerobic conditions were used. The effects of G-NZVI loading, the BrO3 - concentration, and pH on the BrO3 - removal kinetics using G-NZVI were also investigated in this study. The results provide the novel insight that organic onion peel waste can be reused to synthesize highly reactive anti-oxidative nanoparticles for the treatment of inorganic chemical species and heavy metals in water and wastewater.

Applications of Nanomaterials for Heavy Metal Removal from Water and Soil: A Review

Heavy metals are toxic and non-biodegradable environmental contaminants that seriously threaten human health. The remediation of heavy metal-contaminated water and soil is an urgent issue from both environmental and biological points of view. Recently, nanomaterials with excellent adsorption capacities, great chemical reactivity, active atomicity, and environmentally friendly performance have attracted widespread interest as potential adsorbents for heavy metal removal. This review first introduces the application of nanomaterials for removing heavy metal ions from the environment. Then, the environmental factors affecting the adsorption of nanomaterials, their toxicity, and environmental risks are discussed. Finally, the challenges and opportunities of applying nanomaterials in environmental remediation are discussed, which can provide perspectives for future in-depth studies and applications.