Fe-based nanomaterial transformation to amorphous Fe: Enhanced alfalfa rhizoremediation of PCBs-contaminated soil

Ferric Oxide Nanomaterials and Plant-Rhizobacteria Symbionts Cogenerate Iron Plaque for Removing Highly Chlorinated Contaminants in Dryland Soils

Rhizosphere iron plaques derived from Fe-based nanomaterials (NMs) are a promising tool for sustainable agriculture. However, the requirement for flooded conditions to generate iron plaque limits the scope of the NM application. In this study, we achieved in situ Fenton oxidation of a highly chlorinated persistent organic pollutant (2,2',4,5,5'-pentachlorobiphenyl, PCB101) through iron plaque mediated by the interaction between α-Fe2O3 NMs and plant-rhizobacteria symbionts under dryland conditions. Mechanistically, the coexistence of α-Fe2O3 NMs and Pseudomonas chlororaphis JD37 stimulated alfalfa roots to secrete acidic and reductive agents as well as H2O2, which together mediated the rhizosphere Fenton reaction and converted α-Fe2O3 NMs into iron plaque rich in Fe(II)-silicate. Further verifications reproduced the Fenton reaction in vitro using α-Fe2O3 NMs and rhizosphere compounds, confirming the critical role of •OH in the oxidative degradation of PCB101. Significant reductions in PCB101 content by 18.6%, 42.9%, and 23.2% were respectively found in stem, leaf, and soil after a 120-d treatment, proving the effectiveness of this NMs-plant-rhizobacteria technique for simultaneously safe crop production and soil remediation. These findings can help expand the potential applications of nanobio interaction and its mediated iron plaque generation for both agricultural practice and soil remediation.

The effects of iron-based nanomaterials (Fe NMs) on plants under stressful environments: Machine learning-assisted meta-analysis

Mitigating the adverse effects of stressful environments on crops and promoting plant recovery in contaminated sites are critical to agricultural development and environmental remediation. Iron-based nanomaterials (Fe NMs) can be used as environmentally friendly nano-fertilizer and as a means of ecological remediation. A meta-analysis was conducted on 58 independent studies from around the world to evaluate the effects of Fe NMs on plant development and antioxidant defense systems in stressful environments. The application of Fe NMs significantly enhanced plant biomass (mean = 25%, CI = 20%-30%), while promoting antioxidant enzyme activity (mean = 14%, CI = 10%-18%) and increasing antioxidant metabolite content (mean = 10%, CI = 6%-14%), reducing plant oxidative stress (mean = -15%, CI = -20%∼-10%), and alleviating the toxic effects of stressful environments. The observed response was dependent on a number of factors, which were ranked in terms of a Random Forest Importance Analysis. Plant species was the most significant factor, followed by Fe NM particle size, duration of application, dose level, and Fe NM type. The meta-analysis has demonstrated the potential of Fe NMs in achieving sustainable agriculture and the future development of phytoremediation.

Groundwater-level fluctuation effects on petroleum hydrocarbons in vadose zones and their potential risks: Laboratory studies

Despite the role of the vadose zone protecting groundwater from contamination, the non-stationarity in this zone makes it difficult to predict the behavior of petroleum hydrocarbons (PH) therein. In laboratory soil columns with sandy and sandy loam soils, we simulated a vadose zone subjected to repeated groundwater-level fluctuation (GLF) to evaluate the behavior of PH under hydrodynamic conditions. The GLF vertically redistributed the PH, the extent of which was pronounced in the sandy soil with a high initial concentration due to the enhanced transport of the immiscible PH through the larger pores. The frequency of GLF did not show a substantial effect on the extent of PH redistribution but largely affected their attenuation. The greater GLF hindered PH volatilization by maintaining a high degree of water saturation, while the subsequent development of a local anaerobic regime inhibited biodegradation, which was more apparent in the sandy loam. Finally, a specific potential risk index was introduced to quantitatively compare the potential risk of PH contamination in different vadose zones exposed to GLF. Overall, the sandy soil contaminated with the higher total PH (TPH) concentration showed markedly higher potential risk indices (i.e., 18.4-29.0%), while the ones comprised of the sandy loam showed 0.6-4.9%, which increased under the greater number of GLF cycles.

Occurrences of typical PPCPs during wastewater treatment and the composting of sewage sludge with micron-sized and nano-sized Fe3O4

New pollutants, pharmaceuticals and personal care products (PPCPs), accumulate in sewage sludge (SS) in wastewater treatment plants (WWTPs), posing risks to the environment and to human health. In the present study, the fates of typical PPCPs, carbamazepine (CBZ), triclosan (TCS), ibuprofen (IBU) and galaxolide (HHCB), were examined during WW treatment. Additionally, SS collected from a WWTP was used for aerobic composting to investigate the influences of micron-sized Fe3O4 (M-Fe) and nano-sized Fe3O4 (N-Fe) on the degradation of these PPCPs and the succession of microbial communities during the composting process. The results showed that the mean concentrations of CBZ, TCS, IBU and HHCB in the influent of the WWTP were 926.5, 174.4, 8869, and 967.3 ng/g, respectively, and in the effluent were 107.6, 47.0, 283.4, and 88.4 ng/g, respectively. The removal rate averaged ∼80%, while the enrichment rates of the PPCPs in SS ranged from 37.2% to 60.5%. M-Fe and N-Fe reduced NH3 emissions by 32.9% and 54.1% and N2O emissions by 26.2% and 50.8%, respectively. Moreover, the addition of M-Fe and N-Fe effectively increased PPCP degradation rates 1.12-1.66-fold. During the whole process, the additions of M-Fe and N-Fe significantly shifted microbial community structure, and the abundances of Proteobacteria, Chloroflexi, and Actinobacteria were increased during the thermophilic stage, marking them as key PPCP-degrading phyla. Taken together, our results indicated that the addition of M-Fe and N-Fe is an effective method for improving the quality of end compost and accelerating the degradation of PPCPs.

Unraveling the roles of modified nanomaterials in nano enabled agriculture

Nanotechnology has emerged as a key empowering technology for agriculture production due to its higher efficiency and accurate target delivery. However, the sustainable and effective application of nanotechnology requires nanomaterials (NMs) to have higher stability and less aggregation/coagulation at the reaction sites. This can ideally be achieved by modifying NMs with some surfactants or capping agents to ensure higher efficiency. These modified nanomaterials (MNMs) stabilize the interface where NMs interact with their medium of preparation and showed a significant improvement in mobility, reactivity, and controlled release of active ingredients for nano-enabled agriculture. Several environmental factors (e.g., pH, organic matter and the oxidation-reduction potential) could alter the interaction of MNMs with agricultural plants. Firstly, this novel review article introduces production technologies and a few frequently used modification agents in synthesizing MNMs. Next, we critically elaborate the leveraging progress in the modified nano-enabled agronomy and unveil their phytoremediation potential. Lastly, we propose a framework to overcome current challenges and develop a strategy for safe, effective and acceptable applications of MNMs in nano-enabled agriculture. However, the long-term effectiveness and reactivity of MNMs should be investigated to assess their technology effectiveness and optimize the process design to draw definite conclusions.

Fe-Based Nanomaterials and Plant Growth Promoting Rhizobacteria Synergistically Degrade Polychlorinated Biphenyls by Producing Extracellular Reactive Oxygen Species

Plant growth promoting rhizobacteria (PGPR) produce extracellular reactive oxygen species (ROS) to protect plants from external stresses. Fe-based nanomaterials can potentially interact with PGPR and synergistically degrade organic pollutants, yet they have received no study. Here, we studied how the interaction between a typical PGPR (Pseudomonas chlororaphis, JD37) and Fe-based nanomaterials facilitated the degradation of 2,4,4'-trichlorobiphenyl (PCB28), by comparing the zerovalent iron of 20 nm (nZVI20), 100 nm (nZVI100), and 5 μm; iron oxide nanomaterials (α-Fe2O3, γ-Fe2O3, and Fe3O4) of ca. 20 nm; and ferrous and ferric salts. Although all Fe materials (0.1 g L-1) alone could not degrade aqueous PCB28 (0.1 mg L-1) under dark or aerobic conditions, nZVI20, nZVI100, α-Fe2O3, and Fe2+ promoted PCB28 degradation by JD37, with the half-life of PCB28 shortened from 16.5 h by JD37 alone to 8.1 h with nZVI100 cotreatment. Mechanistically, the nanomaterials stimulated JD37 to secrete phenazine-1-carboxylic acid and accelerated the NADH/NAD+ conversion, promoting O2*- generation; JD37 increased Fe(II) dissolution from the nanomaterials, facilitating *OH generation; and the ROS gradually degraded PCB28 into benzoic acid through dihydroxy substitution, oxidation to quinone, and Michael addition. These findings provide a new strategy of nanoenabled biodegradation of organic pollutants by applying Fe-based nanomaterials and PGPR.

Adsorption and desorption of polychlorinated biphenyls on biochar colloids with different pyrolysis temperatures: the effect of solution chemistry

Biochar releases colloidal particles into the environment during applications and aging which can become carriers of pollutants and influence on the environmental risk of pollutants due to the excellent adsorption and migration properties of biochar colloids (BCCs). The adsorption and desorption behaviors of BCCs can be different from their bulk ones due to the colloidal size, which merits specific studies. Herein, the adsorption and desorption of 2,4,4'-trichlorobiphenyl (PCB28) as a representative on BCCs released from bulk biochars prepared from bamboo chips at 300, 500, and 700 C and the effects of solution properties were specifically investigated. Results show that the adsorption was dominated by pore filling and π-π interaction, and thus, BCCs prepared at higher temperature with greater pore volume and aromaticity had higher adsorption of PCB28. Results show that the adsorption was dominated by pore filling and π-π interaction, and thus, BCCs prepared at higher temperature with greater pore volume and aromaticity had higher adsorption of PCB28. The saturation adsorption amounts of PCB28 on BCC300, BCC500, and BCC700 were 21.9, 40.3, and 62.4 mg/g, respectively. It is noteworthy that PCB28 possessed a significant desorption hysteresis from BCCs, with the hysteresis index (Ce = 80 μg/L) increased from 0.380 to 0.661 as the preparation temperature of BCCs rising from 300 to 700 ℃. High concentration of NaCl (100 mmol/L) was unfavorable for the adsorption and desorption. The presence of humic acid or fulvic acid (FA), especially the smaller FA, could inhibit the adsorption and desorption of PCB28 on BCCs due to micropore blocking. In seawater, groundwater, surface water, and soil solution samples, the PCB28 adsorption of BCCs was inhibited to varying degrees in comparison with that in deionized water, and the desorption was noticeably inhibited in the groundwater sample. These findings provide valuable information for the understanding of interactions between BCCs and organic contaminants in natural waters and for the environmental application of biochars as well.

Chemodiversity of soil organic matters determines biodegradation of polychlorinated biphenyls by a graphene oxide-assisted bacterial agent

A promising strategy for degrading persistent organic pollutants (POPs) in soil is amendment with nanomaterial-assisted functional bacteria. However, the influence of soil organic matter chemodiversity on the performance of nanomaterial-assisted bacterial agents remains unclear. Herein, different types of soil (Mollisol soil, MS; Ultisol soil, US; and Inceptisol soil, IS) were inoculated with a graphene oxide (GO)-assisted bacterial agent (Bradyrhizobium diazoefficiens USDA 110, B. diazoefficiens USDA 110) to investigate the association between soil organic matter chemodiversity and stimulation of polychlorinated biphenyl (PCB) degradation. Results indicated that the high-aromatic solid organic matter (SOM) inhibited PCB bioavailability, and lignin-dominant dissolved organic matter (DOM) with high biotransformation potential was a favored substrate for all PCB degraders, which led to no stimulation of PCB degradation in MS. Differently, high-aliphatic SOM in US and IS promoted PCB bioavailability. The high/low biotransformation potential of multiple DOM components (e.g., lignin, condensed hydrocarbon, unsaturated hydrocarbon, etc.) in US/IS further resulted to the enhanced PCB degradation by B. diazoefficiens USDA 110 (up to 30.34%) /all PCB degraders (up to 17.65%), respectively. Overall, the category and biotransformation potential of DOM components and the aromaticity of SOM collaboratively determine the stimulation of GO-assisted bacterial agent on PCB degradation.

Adaptation of bacterial community in maize rhizosphere for enhancing dissipation of phthalic acid esters in agricultural soil

Rhizospheric degradation is a green and in situ strategy to accelerate dissipation of organic pollutants in soils. However, the mechanism on microbial degradation of phthalic acid esters (PAEs) in rhizosphere is still unclear. Here, the bacterial community and function genes in bulk and rhizospheric soils of maize (Zea mays L.) exposed to gradient concentrations of di-(2-ethylhexyl) phthalate (DEHP) were analyzed with 16 S rRNA, metagenomic sequencing and quantitative PCR (qPCR). Maize rhizosphere significantly increased the dissipation of DEHP by 4.02-11.5% in comparison with bulk soils. Bacterial community in rhizosphere exhibited more intensive response and shaped its beneficial structure and functions to DEHP stress than that in bulk soils. Both rhizospheric and pollution effects enriched more PAE-degrading bacteria (e.g., Bacillus and Rhizobium) and function genes in rhizosphere than in bulk soil, which played important roles in degradation of PAEs in rhizosphere. The PAE-degrading bacteria (including genera Sphingomonas, Sphingopyxis and Lysobacter) identified as keystone species participated in DEHP biodegradation. Identification of PAE intermediates and metagenomic reconstruction of PAE degradation pathways demonstrated that PAE-degrading bacteria degraded PAEs through cooperation with PAE-degrading and non-PAE-degrading bacteria. This study provides a comprehensive knowledge for the microbial mechanism on the superior dissipation of PAEs in rhizosphere.

nZVI-induced iron poisoning aggravated the toxicity of TCEP to earthworm in soil

Tris (2-chloroethyl) phosphate (TCEP) is a newly developed organophosphorus flame retardant that has been increasingly detected in soil as a contaminant. Nanoremediation is a potential solution for the control of TCEP, while the effectiveness and ecological risks are poorly understood. Here, we investigated the physicochemical interactions and joint toxicity of nano zero-valent iron (nZVI) (50-5000 mg/kg) and TCEP (50-5000 μg/kg) at environmental relevant concentrations to earthworms (Eisenia fetida) in soil. During a 28-d exposure, TCEP in soil was neither self-degraded nor removed by nZVI, and the individual toxicity of TCEP on the physiology of earthworms was significantly higher than that of nZVI. Notably, nZVI was found to synergize the toxicity of TCEP to earthworms without showing the classical "Trojan horse effect". Mechanically, TCEP mainly induced a typical neurotoxicity, and indirectly inhibited the food ingestion and growth performance of earthworms; nZVI induced iron poisoning aggravated the intestinal damage and directly inhibited the energy metabolism, therefore exacerbated the TCEP-induced malnutrition. Our findings provide new insights into the toxic mechanisms of nZVI-TCEP co-exposure to soil organisms, and emphasize the necessity of risk assessment and cautious usage of nanoremediation in newly emerged contaminations.

Enhanced rhizoremediation of polychlorinated biphenyls by resuscitation-promoting factor stimulation linked to plant growth promotion and response of functional microbial populations

Rhizoremediation is acknowledged as a green technology for removing polychlorinated biphenyls (PCBs) in soil. However, rhizoremediation is limited because most soil microorganisms enter into a viable but non-culturable (VBNC) state under PCBs stress. This work was to study the effect of resuscitation-promoting factor (Rpf) on rhizoremediation efficiency of PCBs in alfalfa and rhizosphere microbiological communities. Results suggested that Rpf promoted alfalfa growth in PCB-contaminated soil by improving antioxidant enzymes and detoxification metabolites in alfalfa. After 40 d Rpf treatment, removal rate for five selected PCBs significantly increased by 0.5-2.2 times. Rpf enhanced relative abundances of bphA and bphC responsible for degrading PCBs, and enzymatic activities of metabolizing exogenous compounds in rhizosphere soil. High-throughput sequencing showed that Rpf did not change the dominant microbial population at phyla and genera levels, but caused variation of the bacterial community structures. The promoting function of Rpf was linked to the shift of various key populations having different functions depending on Rpf concentrations. Pseudomonas and Rhizobium spp. enrichment might stimulate PCB degradation and Streptomyces and Bacillus spp. primarily contributed to alfalfa growth. Predicted functions in rhizosphere soil bacterial community indicated Rpf facilitated soil nutrient cycling and environmental adaptation. This study indicated that Rpf was an active additive for strengthening rhizoremediation efficiency of PCB-contaminated soil and enhancing their in-situ remediation.

Application of α-Fe2O3 nanoparticles in controlling antibiotic resistance gene transport and interception in porous media

Metal oxide nanoparticles (MONPs) with a large specific surface area are expected to bind with antibiotic resistance genes (ARGs), thereby controlling ARGs' contamination by reducing their concentration and mobilization. Here, adsorption experiments were carried out and it was found that α-Fe₂O₃ NPs could chemically bind with ARGs (tetM-carrying plasmids) in water with an adsorption rate of 0.04 min^-1 and an adsorption capacity of 7.88 g/kg. Mixing α-Fe₂O₃ NPs into quartz sand column markedly increased the interceptive removal of ARGs from inflow water. The interception rate of 1.0 μg/mL ARGs in ultrapure water (25 mL, 5 pore volumes) through the sand column (plexiglass, length 8 cm, internal diameter 1.4 cm) with 1 g/kg α-Fe₂O₃ NPs was 1.73 times of that through the pure sand column; the interception rate overall increased with increasing addition of α-Fe₂O₃ NPs, reaching 68.8% with 20 g/kg α-Fe₂O₃ NPs. Coexisting Na⁺ (20 mM), Ca²⁺ (20 mM), and acidic condition (pH 4.0) could further increase the interception rate of ARGs by 1 g/kg α-Fe₂O₃ NPs from 21.1% to 86.2%, 90.7%, and 96.2%, respectively. The presence of PO₄^3- and humic acid at environmentally relevant concentrations would not significantly affect the interception of ARGs. In the treatment groups with PO₄^3- and humic acid, the removal rate decreased by only 1.8% and 0.1%, respectively. In addition, the interceptive removal of ARGs by α-Fe₂O₃ NPs-incorporated sand column was even better in actual surface water samples (87.2%) than that in the ultrapure water (21.1%). The findings provide a promising approach to treat ARGs-polluted water.

Emerging Trends in the Remediation of Persistent Organic Pollutants Using Nanomaterials and Related Processes: A Review

Persistent organic pollutants (POPs) have become a major global concern due to their large amount of utilization every year and their calcitrant nature. Due to their continuous utilization and calcitrant nature, it has led to several environmental hazards. The conventional approaches are expensive, less efficient, laborious, time-consuming, and expensive. Therefore, here in this review the authors suggest the shortcomings of conventional techniques by using nanoparticles and nanotechnology. Nanotechnology has shown immense potential for the remediation of such POPs within a short period of time with high efficiency. The present review highlights the use of nanoremediation technologies for the removal of POPs with a special focus on nanocatalysis, nanofiltration, and nanoadsorption processes. Nanoparticles such as clays, zinc oxide, iron oxide, aluminum oxide, and their composites have been used widely for the efficient remediation of POPs. Moreover, filtrations such as nanofiltration and ultrafiltration have also shown interest in the remediation of POPs from wastewater. From several pieces of literature, it has been found that nano-based techniques have shown complete removal of POPs from wastewater in comparison to conventional methods, but the cost is one of the major issues when it comes to nano- and ultrafiltration. Future research in nano-based techniques for POP remediation will solve the cost issue and will make it one of the most widely accepted and available techniques. Nano-based processes provide a sustainable solution to the problem of POPs.

Photocatalytic Degradation of Dielectric Mineral Oil with PCBs Content Coupled with Algae Treatment

Insulating oil contaminated with polychlorinated biphenyls (PCBs) is an environmentally important pollutant. This research focused on the establishment of the optimum conditions under which photocatalytic oxidation can be used together with biotreatment using the Nostoc sp. microorganism to degrade PCBs present in used dielectric oils. Among the optimal conditions studied were PCB concentration, initial pH, and titanium dioxide (TiO₂) concentration for the photocatalytic step, and PCB concentration and photoperiod for the biotreatment step. The results indicate that the optimal conditions necessary for photocatalytic degradation were a pH of 6.10, 113 mg/L TiO₂, and 765 mg/L PCBs, achieving close to 90% removal. For the biotreatment step, the results showed that PCBs progressively inhibited the microbiological growth, with the lowest cellular growth observed in the medium with the highest PCB concentration.