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

Metal nanoparticles and pesticides under global climate change: Assessing the combined effects of multiple abiotic stressors on soil microbial ecosystems

The soil is a vital resource that hosts many microorganisms crucial in biogeochemical cycles and ecosystem health. However, human activities such as the use of metal nanoparticles (MNPs), pesticides and the impacts of global climate change (GCCh) can significantly affect soil microbial communities (SMC). For many years, pesticides and, more recently, nanoparticles have contributed to sustainable agriculture to ensure continuous food production to sustain the significant growth of the world population and, therefore, the demand for food. Pesticides have a recognized pest control capacity. On the other hand, nanoparticles have demonstrated a high ability to improve water and nutrient retention, promote plant growth, and control pests. However, it has been reported that their accumulation in agricultural soils can also adversely affect the environment and soil microbial health. In addition, climate change, with its variations in temperature and extreme water conditions, can lead to drought and increased soil salinity, modifying both soil conditions and the composition and function of microbial communities. Abiotic stressors can interact and synergistically or additively affect soil microorganisms, significantly impacting soil functioning and the capacity to provide ecosystem services. Therefore, this work reviewed the current scientific literature to understand how multiple stressors interact and affect the SMC. In addition, the importance of molecular tools such as metagenomics, metatranscriptomics, proteomics, or metabolomics in the study of the responses of SMC to exposure to multiple abiotic stressors was examined. Future research directions were also proposed, focusing on exploring the complex interactions between stressors and their long-term effects and developing strategies for sustainable soil management. These efforts will contribute to the preservation of soil health and the promotion of sustainable agricultural practices.

Nano zero-valent iron and melatonin synergistically alters uptake and translocation of Cd and As in soil-rice system and mechanism in soil chemistry and microbiology

Nanoscale zero-valent iron (Fe) is a promising nanomaterial for remediating heavy metal-contaminated soils. Melatonin (MT) is essential to alleviate environmental stress in plants. However, the conjunction effects of Fe and MT (FeMT) on rice Cd, As accumulation and the mechanism of soil chemical and microbial factors interaction are unclear. Here, a pot experiment was conducted to evaluated the effects of the FeMT for rice Cd, As accumulation and underlying mechanisms. The findings showed that FeMT significantly reduced grains Cd by 92%-87% and As by over 90%, whereas improving grains Fe by over 213%. Soil available-Cd and iron plaques-Cd (extracted by dithionite-citrate-bicarbonate solution, DCB-Cd) significantly regulated roots Cd, thus affected Cd transport to grains. Soil pH significantly affected soil As and DCB-As, which further influenced roots As uptake and the transport to shoots and grains. The interactions between the soil bacterial community and soil Fe, available Fe, and DCB-Fe together affected root Fe absorption and transportation in rice. FeMT significantly influenced rhizosphere soil bacterial α- and β-diversity. Firmicutes as the dominant phylum exhibited a significant positive response to FeMT measure, and acted a key role in reducing soil Cd and As availability mainly by improving iron-manganese plaques. The increase of soil pH caused by FeMT was beneficial only for Actinobacteriota growth, which reduced Cd, As availability probably through complexation and adsorption. FeMT also showed greater potential in reducing human health and ecological risks by rice consumption and straw returning. These results showed the important role of both soil chemical and microbial factors in FeMT-mediated rice Cd, As reduction efficiency. This study opens a novel strategy for safe rice production and improvement of rice iron nutrition level in heavy-metals polluted soil, but also provides new insights into the intricate regulatory relationships among soil biochemistry, toxic elements, microorganism, and plants.

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.

Toxic effects and mechanisms of engineered nanoparticles and nanoplastics on lettuce (Lactuca sativa L.)

Engineered nanoparticles (ENPs) and nanoplastics (NPs) are typical nanoparticles in terrestrial environments. Till now, few studies have compared their toxicity and mechanism to plants. Here we investigated the effects of CuO, nZVI ENPs and polystyrene (PS) NPs on lettuce growth, metabolic functions, and microbial community structure. Results showed that low concentrations of nanoparticles decreased root biomass and promoted photosynthetic indicators, whereas increased reactive oxygen species (ROS) were detected in roots exposed to high concentrations of nanoparticles. High-dose CuO ENP exposure significantly raised the MDA content by 124.6 % compared to CK, causing the most severe membrane damage in the roots among the three types of nanoparticles. Although linoleic acid metabolism was down-regulated, the roots alleviated CuO stress by up-regulating galactose metabolism. Uptake of PS by roots similarly caused ROS production and activated the oxidative stress system by altering amino acid and vitamin metabolism. Faster microbial responses to nanoparticles were observed in the nZVI and PS networks. The root toxicity was indirectly mediated by ion release, NP uptake, or ROS generation, ultimately impacting root cell metabolism, rhizospheric microorganism and plant growth. These findings provide theoretical basis for assessing environmental impact of nanoparticles and their possible ecological risks.

Global analysis of the perturbation effects of metal-based nanoparticles on soil nitrogen cycling

Although studies have investigated the effects of metal-based nanoparticles (MNPs) on soil biogeochemical processes, the results obtained thus far are highly variable. Moreover, we do not yet understand how the impact of MNPs is affected by experimental design and environmental conditions. Herein, we conducted a global analysis to synthesize the effects of MNPs on 17 variables associated with soil nitrogen (N) cycling from 62 studies. Our results showed that MNPs generally exerted inhibitory effects on N-cycling process rates, N-related enzyme activities, and microbial variables. The response of soil N cycling varied with MNP type, and exposure dose was the most decisive factor for the variations in the responses of N-cycling process rates and enzyme activities. Notably, Ag/Ag2 S and CuO had dose-dependent inhibitory effects on ammonia oxidation rates, while CuO and Zn/ZnO showed hormetic effects on nitrification and denitrification rates, respectively. Other experimental design factors (e.g., MNP size and exposure duration) also regulated the effect of MNPs on soil N cycling, and specific MNPs, such as Ag/Ag2 S, exerted stronger effects during long-term (>28 days) exposure. Environmental conditions, including soil pH, organic carbon, texture, and presence/absence of plants, significantly influenced MNP toxicity. For instance, the effects of Ag/Ag2 S on the ammonia oxidation rate and the activity of leucine aminopeptidase were more potent in acid (pH <6), organic matter-limited (organic carbon content ≤10 g kg-1 ), and coarser soils. Overall, these results provide new insights into the general mechanisms by which MNPs alter soil N processes in different environments and underscore the urgent need to perform multivariate and long-term in situ trials in simulated natural environments.

Elevated CO2 altered the nano-ZnO-induced influence on bacterial and fungal composition in tomato (Solanum lycopersicum L.) rhizosphere soils

To investigate whether elevated CO₂ (eCO₂) changes the influence of nanoparticles (NPs) on soil microbial communities and the mechanisms, various nano-ZnO (0, 100, 300, and 500 mg·kg^-1) and CO₂ concentrations (400 and 800 µmol·mol^-1) were applied to tomato plants (Solanum lycopersicum L.) in growth chambers. Plant growth, soil biochemical properties, and rhizosphere soil microbial community composition were analyzed. In 500 mg·kg^-1 nano-ZnO-treated soils, root Zn content was 58% higher, while total dry weight (TDW) was 39.8% lower under eCO₂ than under atmospheric CO₂ (aCO₂). Compared with the control, the interaction of eCO₂ and 300 mg·kg^-1 nano-ZnO decreased and increased bacterial and fungal alpha diversities, respectively, which was caused by the direct effect of nano-ZnO (r =  - 1.47, p < 0.01). Specifically, the bacterial OTUs decreased from 2691 to 2494, while fungal OTUs increased from 266 to 307, when 800-300 was compared with 400-0 treatment. eCO₂ enhanced the influence of nano-ZnO on bacterial community structure, while only eCO₂ significantly shaped fungal composition. In detail, nano-ZnO explained 32.4% of the bacterial variations, while the interaction of CO₂ and nano-ZnO explained 47.9%. Betaproteobacteria, which are involved in C, N, and S cycling, and r-strategists, such as Alpha- and Gammaproteobacteria and Bacteroidetes, significantly decreased under 300 mg·kg^-1 nano-ZnO, confirming reduced root secretions. In contrast, Alpha- and Gammaproteobacteria, Bacteroidetes, Chloroflexi, and Acidobacteria were enriched in 300 mg·kg^-1 nano-ZnO under eCO₂, suggesting greater adaptation to both nano-ZnO and eCO₂. Phylogenetic Investigation of Communities by Reconstruction of Unobserved States 2 (PICRUSt2) analysis demonstrated that bacterial functionality was unchanged under short-term nano-ZnO and eCO₂ exposure. In conclusion, nano-ZnO significantly affected microbial diversities and the bacterial composition, and eCO₂ intensified the damage of nano-ZnO, while the bacterial functionality was not changed in this study.

Iron-modified phosphorus- and silicon-based biochars exhibited various influences on arsenic, cadmium, and lead accumulation in rice and enzyme activities in a paddy soil

Contamination of paddy soils with potentially toxic elements (PTEs) has become a severe environmental issue. Application of functionalized biochar for rice cultivation has been proposed as an effective means to reduce environmental risks of these PTEs in paddy soils. This work was undertaken to seek the positive effects of a rice husk-derived silicon (Si)-rich biochar (Si-BC) and a pig carcass-derived phosphorus (P)-rich biochar (P-BC), as well as their Fe-modified biochars (Fe-Si-BC and Fe-P-BC) on the enzyme activity and PTE availability in an As-Cd-Pb-contaminated soil. A rice cultivation pot trial was conducted using these functionalized biochars as soil amendments for the alleviation of PTE accumulation in rice plants. Results showed that Si-BC decreased the concentrations of As in rice grain and straw by 59.4 % and 61.4 %, respectively, while Fe-Si-BC significantly (P < 0.05) enhanced plant growth, increasing grain yield (by 38.6 %). Fe-Si-BC significantly (P < 0.05) elevated Cd and Pb accumulation in rice plants. P-BC enhanced the activities of dehydrogenase, catalase, and urease, and reduced grain-Pb and straw-Pb by 49.3 % and 43.2 %, respectively. However, Fe-P-BC reduced plant-As in rice grain and straw by 12.2 % and 51.2 %, respectively, but increased plant-Cd and plant-Pb. Thus, Fe-modified Si- and P-rich biochars could remediate paddy soils contaminated with As, and enhance the yield and quality of rice. Application of pristine P-rich biochar could also be a promising strategy to remediate the Pb-contaminated paddy soils and limit Pb accumulation in rice.

Key factors to consider in the use of environmental DNA metabarcoding to monitor terrestrial ecological restoration

Ecological restoration of terrestrial environments is a globally important process to combat the loss of biodiversity and ecosystem services. Holistic monitoring of restored biota and active management of restoration is necessary to improve restoration processes and outcomes, and provide evidence to stakeholders that targets are being achieved. Increasingly, environmental DNA (eDNA) metabarcoding is used as a restoration monitoring tool because it is able to generate biodiversity data rapidly, accurately, non-destructively, and reliably, on a wide breadth of organisms from soil microbes to mammals. The overall objective of this review is to discuss the key factors to consider in the use of environmental DNA for monitoring of restored terrestrial ecosystems, hopefully improving monitoring, and ultimately, restoration outcomes. We identified that the majority of eDNA based studies of ecosystem restoration are currently conducted in Europe, North America, and Australia, and that almost half of total studies were published in 2021-22. Soil was the most popular sample substrate, soil microbial communities the most targeted taxa, and forests the most studied ecosystem. We suggest there is no 'one size fits all' approach to restoration monitoring using eDNA, and discuss survey design. Factors to consider include substrate selection, sample collection and storage, assay selection, and data interpretation, all of which require careful planning to obtain reliable, and accurate information that can be used for restoration monitoring and decision making. We explore future directions for research and argue that eDNA metabarcoding can be a useful tool in the restoration monitoring 'toolkit', but requires informed application and greater accessibility to data by a wide spectrum of stakeholders.

Resistance and resilience of soil bacterial community to zero-valent iron disposal of lindane contamination

Zero-valent iron (ZVI, Fe⁰) enables chemical reduction of environmental pollutants coupled with reactivity loss due to surface oxidation. During ZVI treatment process, however, microbial community stability in terms of resistance and resilience remains largely unclear. Here, we monitored bacterial community succession over a 4 weeks period in soil microcosms with or without 2% (w/w) Fe⁰ amendment. To simulate soil pollution, 100 μg g^-1 chlorinated pesticide lindane (γ-hexachlorocyclohexane) was added to the microcosms as a model contaminant. In addition to microbial activity as measured by soil organic carbon mineralization, bacterial abundance, diversity and composition were determined using qPCR and high-throughput sequencing of 16 S rRNA genes. Co-occurrence analysis was performed to reveal the interaction patterns within the bacterial communities. The results indicated that ZVI caused near-complete transformation of lindane, while in the microcosms without Fe⁰ amendment the pesticide was recalcitrant. ZVI strongly inhibited CO₂-efflux at the early stage of incubation, but the bacterial community appeared to be less sensitive to Fe⁰ amendment. The ratios of negative to positive correlations between network nodes suggested that Fe⁰ had marginal influence on community stability compared to the lindane treatments, which destabilized the bacterial community. Community succession occurred in the presence of ZVI, as exemplified by a dominancy transition from anaerobic to aerobic taxa. Yet, ZVI alleviated the stress of lindane on soil bacteria by improving community structure and increasing network complexity. Taken together, these findings demonstrate the stability of soil bacterial community under Fe⁰ stress, which might be conducive to functional recovery of soil microorganisms following ZVI remediation.

Nanoscale zero-valent iron changes microbial co-occurrence pattern in pentachlorophenol-contaminated soil

Nanoscale zero-valent iron (nZVI) is a prominent nanomaterial for the remediation of organochlorine-contaminated soil and groundwater. However, a knowledge gap regarding the effects of the coexistence of nZVI and pollutants on soil microorganisms remains. Here, we studied the effects of nZVI on the microbial community structure, co-occurrence network, and keystone taxa in pentachlorophenol (PCP, a typical organochlorine pesticide) contaminated soils. The addition of nZVI (1000 mg/kg) had no obvious recovery effect on the microbial community structure of PCP-contaminated soil, but enhanced the connection and lowered the modularity of the microbial network. These changes were mainly present in the bacterial network rather than in the fungal or archaeal network. Moreover, the addition of nZVI increased the number of keystone taxa in the PCP-contaminated soil from 29 to 76. These keystone taxa are related to the degradation of organochlorine pollutants, carbon metabolism, and nitrogen metabolism and may thus be helpful in recovering soil ecological functions. These findings provide new insights into the interaction among nanomaterials, microorganisms, and pollutants.

Transformation of sulfidized nanoscale zero-valent iron particles and its effects on microbial communities in soil ecosystems

Sulfidized nanoscale zero-valent iron (S-nZVI) is a promising material for in situ soil remediation. However, its transformation (i.e., aging) and effects on the microbial community in soil ecosystems are largely unknown. In this study, S-nZVI having low (S-nZVI (L)) and high sulfur-doping (S-nZVI (H)) were incubated in soil microcosms and bare nZVI was used as a control. Their aged products were characterized using microspectroscopic analyses and the changes in the corresponding soil microbial community were determined using high-throughput sequencing analyses. The results indicate that severe corrosion of both bare and S-nZVI occurred over 56 days of aging with significant morphological and mineral changes. Magnetite, lepidocrocite, and goethite were detected as the main aged products. In addition, sulfate ions, pyrite, and iron polysulfide were formed in the aged products of S-nZVI. Cr(VI) removal test results indicated that S-nZVI(L) achieved the best results after aging, likely because of the optimal FeS arrangement on its nanoparticle surfaces. The presence of nZVI and S-nZVI increased the abundance of some magnetotactic microorganisms and altered bacterial and fungal community structures and compositions. Moreover, the addition of S-nZVI enriched some bacterial and fungal genera related to sulfur cycling because of the presence of sulfide-bearing material. The findings reveal the transformation of S-nZVI during aging and its effects on microbial communities in soil ecosystems, thereby helping to the evaluation of S-nZVI application in soil remediation.

Remediation of Pb-diesel fuel co-contaminated soil using nano/bio process: subsequent use of nanoscale zero-valent iron and bioremediation approaches

The effectiveness of the nano/bio process was investigated as a remediation option for co-contaminated soils. Nano/bio process is a hybrid treatment method that may be defined as the use of nanoscale zero-valent iron (nZVI) and bioremediation approaches subsequently/concurrently. Different bioremediation approaches (bioattenuation, biostimulation, and/or bioaugmentation) were performed together with nZVI application to remediate Pb- and diesel fuel-spiked soils. Nutrient (N and P) and activated sludge amendment were made to realize biostimulation and bioaugmentation, respectively. The nZVI application decreased the total percentage of the most mobile and bioavailable soil Pb fractions (exchangeable and carbonate-bound) from 68.3 to 31.7%. The biodegradation levels of nZVI-applied co-contaminated soils were significantly higher than the soils without nZVI indicating the positive effect of the reduced mobility, bioavailability, and toxicity of Pb content. The use of nano/biostimulation or nano/bioaugmentation treatments resulted in higher than 60% total n-alkane degradation, whereas 89.5% degradation was obtained by using nano/biostimulation + bioaugmentation. Hydrocarbon-degrader strains belonging to phyla Actinobacteria, Proteobacteria, or Firmicutes were identified from samples subjected to nano/bio process and the strains from biostimulation and bioaugmentation treatments were different. These results indicate that the stress on the microbial population caused by the co-contamination might be subsided and the biodegradation of alkanes might be improved by using the nano/bio process.

Cd and Pb immobilisation with iron oxide/lignin composite and the bacterial community response in soil

Iron oxide is a natural mineral that generally exists in the form of iron oxide-organic complexes (Fe-OM) in soil. Lignin is a naturally occurring polymer that is considered to be an important part of soil carbon cycling. In this study we prepared a composite material (MGE) with iron oxide and lignin based on the Fe-OM present in the soil. MGE was then applied to remediate Cd and Pb in contaminated soil. The results show that DTPA-Cd and DTPA-Pb levels were reduced by 58.87% and 78.09%, respectively. The bacterial community diversity index decreased in the iron oxide (GE) group, but a slight increase was observed in the MGE group. In terms of species composition in the MGE group, the abundance of Proteobacteria, Gemmatimonadota and Acidobacteriota increased, while the abundance of Bacteroidota, Actinobacteriota and Firmicutes decreased. The outcome in the GE group was the opposite. In the MGE group, HCl-Fe²⁺, HCl-Fe³⁺, and pH were significantly higher than in the other groups, indicating that MGE stimulated the growth of iron-reducing bacteria (FeRB) and promoted iron redox reactions. Iron oxide could be reduced to Fe²⁺ due to the activity of FeRB, and then Fe²⁺ would be oxidised and hydrolysed, which led to an increase in soil pH. Secondary minerals were formed during this process. With the oxidation of Fe²⁺ and the formation of secondary minerals, Cd and Pb could be stabilised in the oxides and were not easily released through a co-precipitation mechanism.

Recent Advances of Nanoremediation Technologies for Soil and Groundwater Remediation: A Review

Nanotechnology has been widely used in many fields including in soil and groundwater remediation. Nanoremediation has emerged as an effective, rapid, and efficient technology for soil and groundwater contaminated with petroleum pollutants and heavy metals. This review provides an overview of the application of nanomaterials for environmental cleanup, such as soil and groundwater remediation. Four types of nanomaterials, namely nanoscale zero-valent iron (nZVI), carbon nanotubes (CNTs), and metallic and magnetic nanoparticles (MNPs), are presented and discussed. In addition, the potential environmental risks of the nanomaterial application in soil remediation are highlighted. Moreover, this review provides insight into the combination of nanoremediation with other remediation technologies. The study demonstrates that nZVI had been widely studied for high-efficiency environmental remediation due to its high reactivity and excellent contaminant immobilization capability. CNTs have received more attention for remediation of organic and inorganic contaminants because of their unique adsorption characteristics. Environmental remediations using metal and MNPs are also favorable due to their facile magnetic separation and unique metal-ion adsorption. The modified nZVI showed less toxicity towards soil bacteria than bare nZVI; thus, modifying or coating nZVI could reduce its ecotoxicity. The combination of nanoremediation with other remediation technology is shown to be a valuable soil remediation technique as the synergetic effects may increase the sustainability of the applied process towards green technology for soil remediation.

The impacts of metal-based engineered nanomaterial mixtures on microbial systems: A review

The last decade has witnessed tremendous growth in the commercial use of metal-based engineered nanomaterials (ENMs) for a wide range of products and processes. Consequently, direct and indirect release into environmental systems may no longer be considered negligible or insignificant. Yet, there is an active debate as to whether there are real risks to human or ecological health with environmental exposure to ENMs. Previous research has focused primarily on the acute effects of individual ENMs using pure cultures under controlled laboratory environments, which may not accurately reveal the ecological impacts of ENMs under real environmental conditions. The goal of this review is to assess our current understanding of ENM effects as we move from exposure of single to multiple ENMs or microbial species. For instance, are ENMs' impacts on microbial communities predicted by their intrinsic physical or chemical characteristics or their effects on single microbial populations; how do chronic ENM interactions compare to acute toxicity; does behavior under simplified laboratory conditions reflect that in environmental media; finally, is biological stress modified by interactions in ENM mixtures relative to that of individual ENM? This review summarizes key findings and our evolving understanding of the ecological effects of ENMs under complex environmental conditions on microbial systems, identifies the gaps in our current knowledge, and indicates the direction of future research.

The influence of elevated CO2 on bacterial community structure and its co-occurrence network in soils polluted with Cr2O3 nanoparticles

Elevated CO₂ (eCO₂) and nanoparticles release are considered among the most noteworthy global concerns as they may impose negative effects on human health and ecosystem functioning. A mechanistic understanding of their combined impacts on soil microbiota is essential due to the profound eCO₂ effect on soil biogeochemical processes. In this study, the impacts of Cr₂O₃ nanoparticles (nano-Cr₂O₃) on the activity, structure and co-occurrence networks of bacterial communities under ambient and eCO₂ were compared between a clay loam and a sandy loam soil. We showed that eCO₂ substantially mitigated nano-Cr₂O₃ toxicity, with microbial biomass, enzyme activity and bacterial alpha-diversity in clay loam soil were much higher than those in sandy loam soil. Nano-Cr₂O₃ addition caused an increase in alpha-diversity except for clay loam soil samples under eCO₂. 16S rRNA gene profiling data found eCO₂ remarkably reduced community divergences induced by nano-Cr₂O₃ more efficiently in clay loam soil (P < 0.05). Network analyses revealed more complex co-occurrence network architectures in clay loam soil than in sandy loam soil, however, nano-Cr₂O₃ decreased but eCO₂ increased modularity and network complexity. Rising CO₂ favoured the growth of oligotrophic (Acidobacteriaceae, Bryobacteraceae) rather than the copiotrophic bacteria (Sphingomonadaceae, Caulobacteraceae, Bacteroidaceae), which may contribute to community recovery and increase available carbon utilization efficiency. Our results suggested that the degree to which eCO₂ mitigates nano-Cr₂O₃ toxicity is soil dependent, which could be related to the variation in clay and organic matter content, resilience of the resistant bacterial taxa, and microbial network complexity in distinct soils.

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.

Effects of novel bioorganic fertilizer application on soil enzymes and bacterial community in multi-site rice paddies in China

Application of a novel bioorganic fertilizer (BIO) has been effectively used to inhibit weeds in rice paddies. To identify changes in soil bacterial community and enzymes in response to BIO treatments, field experiments were carried out in five major rice-growing areas in China. The dominant phylogenetic groups recorded included Proteobacteria, Actinobacteria, Bacteroidetes, Firmicutes and Acidobacteria. Anaeromyxobacter, Bacteroides, Bifidobacterium, Escherichia- Shigella, Geobacter and Haliangium were significantly different between BIO-treatment and untreated control and aided in general function (R), amino acid transport, metabolism (E) and transcription (K) clusters. The soil chemical properties and enzyme activities were less affected by BIO at these study sites. RDA analysis showed that soil bacterial community had a significant positive correlations among northern latitude, eastern longitude, exchangeable K, total K, total P, soil pH, and total N, except for organic matter, hydrolytic N and extractable P. Overall, our work showed that application of BIO does not alter the main community structure and functional diversity of soil bacteria in rice paddies and should be encouraged for use as a sustainable weed management strategy.

Remediation of Cd-Contaminated Soil by Modified Nanoscale Zero-Valent Iron: Role of Plant Root Exudates and Inner Mechanisms

In this study, the role of exogenous root exudates and microorganisms was investigated in the application of modified nanoscale zero-valent iron (nZVI) for the remediation of cadmium (Cd)-contaminated soil. In this experiment, citric acid (CA) was used to simulate root exudates, which were then added to water and soil to simulate the pore water and rhizosphere environment. In detail, the experiment in water demonstrated that low concentration of CA facilitated Cd removal by nZVI, while the high concentration achieved the opposite. Among them, CA can promote the adsorption of Cd not only by direct complexation with heavy metal ions, but also by indirect effect to promote the production of iron hydroxyl oxides which has excellent heavy metal adsorption properties. Additionally, the H⁺ dissociated from CA posed a great influence on Cd removal. The situation in soil was similar to that in water, where low concentrations of CA contributed to the immobilization of Cd by nZVI, while high concentrations promoted the desorption of Cd and the generation of CA–Cd complexes which facilitated the uptake of Cd by plants. As the reaction progressed, the soil pH and cation exchange capacity (CEC) increased, while organic matter (OM) decreased. Meanwhile, the soil microbial community structure and diversity were investigated by high-throughput sequencing after incubation with CA and nZVI. It was found that a high concentration of CA was not conducive to the growth of microorganisms, while CMC had the effect of alleviating the biological toxicity of nZVI.

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.

Organic amendments exacerbate the effects of silver nanoparticles on microbial biomass and community composition of a semiarid soil

Increased utilization of silver nanoparticles (AgNPs) can result in an accumulation of these particles in the environment. The potential detrimental effects of AgNPs in soil may be associated with the low fertility of soils in semiarid regions that are usually subjected to restoration through the application of organic amendments. Microbial communities are responsible for fundamental processes related to soil fertility, yet the potential impacts of low and realistic AgNPs concentrations on soil microorganisms are still unknown. We studied the effects of realistic citrate-stabilized AgNPs concentrations (0.015 and 1.5 μg kg^-1) at two exposure times (7 and 30 days) on a sandy clay loam Mediterranean soil unamended (SU) and amended with compost (SA). We assessed soil microbial biomass (microbial fatty acids), soil enzyme activities (urease, β-glucosidase, and alkaline phosphatase), and composition of the microbial community (bacterial 16S rRNA gene and fungal ITS2 sequencing) in a microcosm experiment. In the SA, the two concentrations of AgNPs significantly decreased the bacterial biomass after 7 days of incubation. At 30 days of incubation, only a significant decrease in the Gram+ was observed at the highest AgNPs concentration. In contrast, in the SU, there was a significant increase in bacterial biomass after 30 days of incubation at the lowest AgNPs concentration. Overall, we found that fungal biomass was more resistant to AgNPs than bacterial biomass, in both SA and SU. Further, the AgNPs changed the composition of the soil bacterial community in SA, the relative abundance of some bacterial taxa in SA and SU, and fungal richness in SU at 30 days of incubation. However, AgNPs did not affect the activity of extracellular enzymes. This study demonstrates that the exposure time and organic amendments modulate the effects of realistic concentrations of AgNPs in the biomass and composition of the microbial community of a Mediterranean soil.

Effects of the application of an organic amendment and nanoscale zero-valent iron particles on soil Cr(VI) remediation

Chromium is considered an environmental pollutant of much concern whose toxicity depends, to a great extent, on its valence state, with Cr(VI) being more soluble, bioavailable, and toxic, compared to Cr(III). Nanoremediation is a promising strategy for the remediation of metal pollutants by changing their valence state. However, among other aspects, its effectiveness for soil remediation is seriously hampered by the interaction of nanoparticles with soil organic matter. In this study, soil was (i) amended with two doses of a municipal solid organic waste and (ii) artificially polluted with 300 mg Cr(VI) kg-1 DW soil. After a period of aging, a nanoremediation treatment with nanoscale zero-valent iron particles (1 g nZVI kg-1 DW soil) was applied. The efficiency of the remediation treatment was assessed in terms of Cr(VI) immobilization and recovery of soil health. The presence of the organic amendment caused (i) a decrease of redox potential, (ii) Cr(VI) immobilization via its reduction to Cr(III), (iii) a stimulation of soil microbial communities, and (iv) an improvement of soil health, compared to unamended soil. By contrast, nZVI did not have any impact on Cr(VI) immobilization nor on soil health. It was concluded that, unlike the presence of the organic amendment, nanoremediation with nZVI was not a valid option for soils polluted with Cr(VI) under our experimental conditions.

Assessment of iron-based and calcium-phosphate nanomaterials for immobilisation of potentially toxic elements in soils from a shooting range berm

Shooting range facilities in military areas have been indicated as a hotspot of land degradation with high contents of Potentially Toxic Elements (PTEs). Currently, based on the new nanomaterials with specific characteristics, nanoremediation technologies are used to immobilise and to reduce the availability of PTEs in field and laboratory conditions. In this study, the effects of nano-hydroxyapatite and/or hematite on PTEs immobilisation (As, Cd, Cu, Pb, Sb and Zn) in military shooting range soils were assessed through the measure of available and leachable forms with three single-extractions: calcium chloride (0.01M CaCl₂), low molecular weight organic acids (10 mM LMWOAs) and toxicity characteristic leaching procedure (TCLP). A sequential chemical extraction was used to determine the distribution of the PTEs in the different geochemical phases of the soils before and after the nanomaterial treatments. Results showed that the availability of PTEs decreased, especially for Pb (40-95%) and Zn (50-99%) after nanomaterial treatments. When both nanomaterial (hydroxyapatite + hematite) were combined, the immobilisation rate improved. However, when each nanomaterial was added individually to the soils, some elements, such as, Cu or Sb, showed a slight increment of their mobilisation. The sequential chemical extraction showed that the highest percentage of PTEs were mainly in the residual fraction before and after adding nanomaterials, being even higher in soils after the nanomaterial treatments. Likewise, the mobile fractions decreased after the treatment with nanomaterials. Our findings suggest that nanoremediation techniques improve the soil conditions, but they should be used carefully to avoid mobilisation of non-target PTEs or unexpected potentially impacts for soil biota.

Response of soil microbial communities to additions of straw biochar, iron oxide, and iron oxide-modified straw biochar in an arsenic-contaminated soil

Anthropogenic activities have caused extensive arsenic (As) contamination in soils. The role of biochar in the remediation of As-contaminated soils has been attracting attention lately. In this study, effects of straw biochar, iron oxide, and iron oxide-modified biochar on soil microbial community composition and soil chemical properties were tested in an As-contaminated soil. After 9 months of incubation, soil chemical properties and microbial communities were analyzed. Our results showed that biochar addition significantly increased soil pH value, soil organic carbon (SOC) concentration, and the ratio of soil carbon to nitrogen (soil C:N ratio) but decreased soil dissolved organic C. Adding iron oxide also increased soil pH value, while iron oxide-modified biochar decreased it. Interestingly, compared with the control, all treatments significantly decreased soil total microbial biomass and biomasses of soil bacteria, fungi, Actinomyces, and protozoa. In addition, significantly positive correlations were found between soil pH and soil total microbial biomass as well as bacterial, Actinomyces, and arbuscular mycorrhizal fungal biomass. There were negative relationships between SOC, soil C:N ratio, and all soil microbial biomass indicators in all treatments. These results indicated that biochar and iron oxide-modified biochar affected soil microbial community composition by altering the soil C:N ratio, but iron oxide affected it via adjusting soil pH. Furthermore, the iron oxide-modified biochar effects on soil microbial community and soil chemical properties are not the same as the additive effects of biochar and iron oxide alone, and its effect on soil microbial community is regulated by the soil C:N ratio. These findings will help guide the development of remediation practices for As-contaminated soil using biochar.

Impacts of a novel strain QY-1 allied with chromium immobilizing materials on chromium availability and soil biochemical properties

In-situ passivation of soil chromium (Cr) contamination based on chemical and biological passivators has been widely concerned, however, the cooperative effect of two types of passivators on Cr passivation and soil properties was little investigated. In this study, nano zero valent iron (nZVI) and humic acid (HA) as the chemical passivators were selected and were combined with a novel Cr resistant strain QY-1 to study these two points. Results demonstrated that the combination was more effective in Cr immobilization, among which, HA + QY-1 had the highest passivation rate (82.83%), followed by nZVI + QY-1. HA + QY-1 alleviated soil Cr stress most efficiently as its soil relevant fertility indicators, microbial quantity, respiration and seed gemination rate significantly increased. On the contrary, nZVI decreased soil respiration and microbial abundance, but the addition of QY-1 could relieve this phenomenon. The results highlighted the ability of HA + QY-1 to remediate Cr contaminated soil and improve soil stability.

The stabilizing mechanism of cadmium in contaminated soil using green synthesized iron oxide nanoparticles under long-term incubation

Despite numerous studies having been conducted on the stabilization of heavy metal contaminated soil, our understanding of the mechanisms involved remains limited. Here green synthesized iron oxide nanoparticles (GION) were applied to stabilize cadmium (Cd) in a contaminated soil. GION not only stabilized soil Cd, but also improved soil properties within one year of incubation. After GION application both the exchangeable and carbonate bound Cd fractions decreased by 14.2-83.5% and 18.3-85.8% respectively, and most of the Cd was translocated to the residual Cd fraction. The application of GION also strongly altered soil bacterial communities. In GION treatments, the abundance of Gemmatimonadetes, Proteobacteria, and Saccharibacteria increased which led to a shift in the dominant bacterial genera from Bacillus to Candidatus koribacter. The variation in bacteria confirmed the restoration of the contaminated soil. The most abundant bacterial genus and species found in GION treatments were related to (i) plant derived biomass decomposition; (ii) ammoxidation and denitrification; and (iii) Fe oxidation. GION application may enhance the formation of larger soil aggregates with anaerobic centers and coprecipitation coupled Fe (II) oxidization, ammoxidation and nitrite reduction followed by Fe mineral ripening may be involved in Cd stabilization. The predominant stabilization mechanism was thus coprecipitation-ripening-stabilization.