Remediation of hexavalent chromium contaminated soil by nano-FeS coated humic acid complex in combination with Cr-resistant microflora

Remediation of Cr(VI)-contaminated soil by CS/PPy coupling with Microbacterium sp. YL3

In this research, a new material, chitosan/polypyrrole (CS/PPy), was synthesized and linked with the Cr(VI)-reducing bacterial strain YL3 to treat Cr(VI)-polluted soil. The findings demonstrated that the synergistic application of strain YL3 and CS/PPy achieved the greatest reduction (99.6 %). During the remediation process, CS/PPy served as a mass-storage and sustained release agent in the soil, which initially decreased the toxic effects of high concentrations of Cr(VI) on strain YL3, thereby enhancing the Cr(VI) reduction efficiency of strain YL3. These combined effects significantly mitigated Cr(VI) stress in the soil and restored enzyme activities. Furthermore, wheat growth in the treated soil also significantly improved. High-throughput sequencing of the microorganisms in the treated soil revealed that CS/PPy was not only effective at removing Cr(VI) but also at preserving the original microbial diversity of the soil. This suggests that the combined treatment using strain YL3 and CS/PPy could rehabilitate Cr(VI)-contaminated soil, positioning CS/PPy as a promising composite material for future bioremediation efforts in Cr(VI)-contaminated soils.

A META analysis on the efficacy of functional materials for soil chromium remediation

Metallic chromium pollution in soil is widespread, which aroused intensive research in recent decades. In mainstream research, most studies use materials with a reducing ability to adsorb and reduce hexavalent chromium. However, comprehensive analyses and systematic verifications of these different materials are scarce. Therefore, this study conducted a meta-analysis of relevant papers published from 2013 to October 2024 to compare and analyze the performance and usage conditions of some common materials, such as iron-based materials, mineral inorganic materials, organic materials, and layered double hydroxide materials. We synthesized 31 papers for 186 pairwise comparisons and selected the Standardized Mean Difference (SMD) as the appropriate effect size for mean-to-mean comparisons. Fe-based materials had the most stable performance based on its numerous data support, while organic materials had the worst performance. The difference in performance between inorganic mineral materials was the greatest, which was closely related to the selection of components. The difference in the effectiveness of inorganic materials was the greatest, which was closely related to the selection of components and there was room for further improvement. Through further analysis of the impact of environmental factors on material performance, it can be concluded that the effect of the material was better under alkaline, non-sandy, low organic matter, and high CEC soil conditions.

Remediation materials for the immobilization of hexavalent chromium in contaminated soil: Preparation, applications, and mechanisms

Hexavalent chromium is a toxic metal that can induce severe chromium contamination of soil, posing a potential risk to human health and ecosystems. In recent years, the immobilization of Cr(VI) using remediation materials including inorganic materials, organic materials, microbial agents, and composites has exhibited great potential in remediating Cr(VI)-contaminated soil owing to the environmental-friendliness, short period, simple operation, low cost, applicability on an industrial scale, and high efficiency of these materials. Therefore, a systematical summary of the current progress on various remediation materials is essential. This work introduces the production (sources) of remediation materials and examines their characteristics in detail. Additionally, a critical summary of recent research on the utilization of remediation materials for the stabilization of Cr(VI) in the soil is provided, together with an evaluation of their remediation efficiencies toward Cr(VI). The influences of remediation material applications on soil physicochemical properties, microbial community structure, and plant growth are summarized. The immobilization mechanisms of remediation materials toward Cr(VI) in the soil are illuminated. Importantly, this study evaluates the feasibility of each remediation material application for Cr(VI) remediation. The latest knowledge on the development of remediation materials for the immobilization of Cr(VI) in the soil is also presented. Overall, this review will provide a reference for the development of remediation materials and their application in remediating Cr(VI)-contaminated soil.

Remediation of Cr(VI) contaminated soil by chitosan stabilized FeS composite and the changes in microorganism community

In-suit immobilization is one of the major strategies to remediate heavy metals contaminated soil with the effectiveness largely depends on the characteristics of the added chemical reagents/materials. In this study, chitosan stabilized FeS composite (CS-FeS) was prepared to evaluate the performance of remediating the high and toxic hexavalent chromium contaminated soil from the effectiveness and microbial response aspects. The characterization analysis confirmed the successful preparation of composite, and the introduction of chitosan successfully stabilized FeS to protect it from rapid oxidation as compared to bare FeS particles. With the addition dosage at 0.1%, about 85.6% and 81.3% of Cr(VI) was reduced in 3 d based on toxicity characteristic leaching procedure (TCLP) and CaCl₂ extraction, and the reduction efficiency increased to 96.6% and 94.8% in 7 d, respectively. The Cr(VI) was non-detected in the TCLP leachates with increase the CS-FeS composites to 0.5%. The percentages of HOAc-extractable Cr decreased from 25.17% to 6.12% accompanied with the increase in the residual Cr from 4.26% to 13.77% and improvement of soil enzyme activity under CS-FeS composites addition. Cr(VI) contamination reduced the diversity of microbial community in soil. Three dominate prokaryotic microorganisms, namely Proteobacteria, Actinobacteria and Firmicutes, were observed in Cr-contaminated soil. The addition of CS-FeS composites increased the microbial diversity especially for that in relative lower abundance. The relative abundance of Proteobacteria and Firmicute related to Cr-tolerance and reduction increased in CS-FeS composites added soils. Taking together, these results demonstrated the potential and promising of using the CS-FeS composites for Cr(VI) polluted soil remediation.

PBAT/PLA humic acid biodegradable film applied on solar greenhouse tomato plants increased lycopene and decreased total acid contents

This work aims to resolve residual film pollution in farmlands and improve tomato quality. The mechanical properties and degradation of PBAT/PLA lignin (MZS) and PBAT/PLA humic acid (FZS) composite biodegradable film were analyzed, and its effect on soil temperature and humidity, soil microorganisms, soil physical and chemical properties, tomato yield, and quality was studied. Polyethylene film (PE) was used as a control. The results demonstrate a higher degradation degree of FZS film than of MZS film. The degradation degree of FZS and MZS films reached level 2 and level 1, respectively, after 131 days of film covering. The weight loss rate of FZS and MZS films reached 52.74 % and 57.82 %, respectively, when buried for 160 days. Compared to the coverings of PE and MZS films, FZS film could significantly increase the soil's electric conductivity and organic matter content (p < 0.05). The relative abundance of soil fungi Chaetomium also increased. The yield, soluble solids, vitamin C (Vc), soluble sugar, and lycopene of tomato plants covered with FZS film significantly increased by 6.74 %, 8.75 %, 15.41 %, 8.30 %, and 27.27 % compared to plants covered with PE film, and the total acid and hardness significantly decreased by 24.95 % and 8.46 %, respectively (p < 0.05). Using 10 μm PBAT/PLA humic acid biodegradable film for tomato cultivation in autumn and winter increased the lycopene and decreased the total acid content by changing the soil's physical and chemical characteristics and increasing the content of Chaetomium soil.

Role of humic acid in the transformation of hexavalent chromium in a sulfidated ferrihydrite system

Ferrihydrite (Fh) often coexists with organic matter (i.e., humic acid (HA)) in the environment; however, its impacts on the transformation of hexavalent chromium (Cr(VI)) is poorly understood during the sulfidation of Fh. Upon exposed to 2 mM sulfide for 12 h, the total amount of Fe(II) (Fe(II)_tot, 0.78 mM) in treatments with HA (300 mg HA/L) was higher than that (0.67 mM) in treatments without HA, since HA could enhance the solubility of Fe(II). After then, the Cr(VI) was reduced by sulfidated Fh. Aqueous Cr(VI) concentration (Cr(VI)_aq) declined from 0.67 to 0.43 mM with the increase of HA concentration from 0 to 400 mg/L, which was partly ascribed to the inhibition of surface passivation by HA. Moreover, the increase in Fe(II) during the sulfidation of Fh also exerted a strong impact on the transformation of Cr(VI) subsequently. In addition of HA, batch experiments suggested that EDTA could also promote the formation of Fe(II) (Fe(II)_tot, 0.80 mM), rendering a lower Cr(VI)_aq (0.59 mM) in EDTA-300 treatments. This study further demonstrated that HA played an important role in the transformation of Cr(VI), hence providing a theoretical basis for in-situ remediation of Cr(VI) in the future.

Calcium ion removal at different sodium chloride concentrations by free and immobilized halophilic bacteria

Much attention has been paid to Ca²⁺ ion removal by biomineralization due to the dangers of Ca²⁺ on industrial processes and human health. However, Ca²⁺ removal from hypersaline water by biomineralization is quite difficult due to there being few halophilic bacteria tolerating higher salinities. In this study, free and immobilized Virgibacillus massiliensis C halophilic bacteria exhibiting carbonic anhydrase activity were used to remove Ca²⁺ ions from water at different NaCl concentrations. With increasing NaCl concentrations (10, 50, 100, 150 and 200 g/L), Ca²⁺ ion concentrations in the presence of free bacteria and in two groups of immobilized bacteria for a period of 6 days sharply decreased from 1200 mg/L to 219-562 mg/L, 71-214 mg/L and 21-159 mg/L, respectively; Ca²⁺ precipitation ratios were 55%-81%, 82%-94% and 87%-98%, respectively. The humic acid-like substances, protein, DNA and polysaccharide, released by the bacteria, promoted the Ca²⁺ ion removal. The immobilized bacteria were able to be recycled and precultured, which would save industry costs and increase Ca²⁺ ion removal efficiency. Biological processes for Ca²⁺ ion removal include cell surface, intracellular and extracellular biomineralization. The biogenesis of calcium carbonate was proved by SEM-EDS, FTIR, XPS and stable carbon isotope values. This study provides insights into the effective removal of Ca²⁺ ions by biomineralization in hypersaline water.

Citric acid secretion from rice roots contributes to reduction and immobilization of Cr(VI) by driving microbial sulfur and iron cycle in paddy soil

Root exudates released by plants can promote microbial growth and activity, thereby affecting the transformation and availability of soil pollutants. However, the effects of the root exudates of rice plants on chromium (Cr) transformation in paddy soils and the underlying mechanisms are yet to be elucidated properly. The present study investigated how rice root exudates interact with rhizosphere microorganisms to influence the transformation of Cr and explored the key components in root exudates that affect Cr(VI) reduction. The results showed that the addition of root exudate and citric acid markedly decreased soil pH and increased dissolved organic carbon content that created favorable conditions and provided sufficient electron donors for Cr(VI) reduction, thereby greatly facilitating the reduction of Cr(VI) and the transformation of HOAc-extractable Cr into more stable oxidizable and residual Cr. Additionally, Desulfovibrio-related sulfate-reducing bacteria, Thiobacillus-related sulfide-oxidizing bacteria, and Geobacter-related Fe(III)-reducing bacteria were enriched with the addition of root exudate and citric acid. Among them, sulfate would be reduced by Desulfovibrio to sulfide, which would be further utilized by Thiobacillus to reduce Cr(VI), thereby enabling the continuous reduction of Cr(VI); simultaneously, Geobacter would sustain the reduction of Cr(VI) by reducing Fe(III) to Fe(II). Furthermore, based on the high-level secretion of citric acid in response to Cr(VI) exposure and the similar effects of root exudates and citric acid on Cr(VI) reduction, it is proposed that citric acid is the key component in rice root exudates that affects Cr(VI) reduction. These results suggest that root exudates (citric acid as the key component) contribute to the reduction and immobilization of Cr(VI) by driving microbial S and Fe cycles, with Desulfovibrio, Thiobacillus, and Geobacter being the keystone genera. The study provides a novel insight into the Fe/S/Cr co-transformation processes with microbial involvement, and the artificial root exudate mixtures designed to reduce Cr(VI).

Efficacy of metallic nanoparticles in attenuating the accumulation and toxicity of chromium in plants: Current knowledge and future perspectives

Nanoparticles have emerged as cutting-edge technology for the improvement of crops yield and safe cultivation of agricultural plants, especially in peripheral areas impaired with toxic heavy metals including chromium (Cr). The uncontrolled release of Cr mainly from anthropogenic factors is substantially polluting the surrounding environment, thereby extensively accumulated in soil-plant system. The excessive Cr-accretion in plant tissues disturbed the morph-physiological, biochemical, cellular, metabolic and molecular processes, and impaired the plants functionality. Therefore, it is obligatory to restrict the accumulation and toxic effects of Cr in plant organs. Recent studies on metallic nanoparticles (MNPs) such as iron oxide, silicon dioxide, copper oxide and zinc oxide have approved their efficacy as potent pool to curb the Cr-induced phytotoxicities and improved the plant tolerance. MNPs attenuated the bioaccumulation and phytotoxicity of Cr by utilizing key mechanisms such as improved photosynthetic machinery, regulation of cellular metabolites, greater chelation capacity to bind with Cr, release of corresponding metallic ions, upsurge in the uptake of essential nutrients, activation of antioxidants (enzymatic and non-enzymatic), reduction in oxidative stress, and cellular injuries, thus improvement in plant growth performances. We have briefly discussed the current knowledge and research gaps in existing literature along with possible recommendations for future research. Overall, Cr-detoxification by MNPs may depends upon the target plant species, Cr speciation, plant growth stages (seedling, vegetative and ripening etc.), treatment methods (foliar spray, seed priming and nutrient solution etc.), type, size, dose and coating of applied MNPs, and conditions (hydroponic and soil environment etc.). This review would help plant scientists to develop MNPs based strategies such as nano-fertilizers to alleviate the Cr-accumulation and its toxic impacts. This may leads to safe and healthy food production. The review outcomes can provide new horizons for research in the applications of MNPs for the sustainable agriculture.

Responsive change of crop-specific soil bacterial community to cadmium in farmlands surrounding mine area of Southeast China

In arable soils co-influenced by mining and farming, soil bacteria significantly affect metal (Cadmium, Cd) bioavailability and accumulation. To reveal the soil microecology response under this co-influence, three intersection areas (cornfield, vegetable field, and paddy field) were investigated. With a similar nutrient condition, the soils showed varied Cd levels (0.31-7.70 mg/kg), which was negatively related to the distance from mining water flow. Different soils showed varied microbial community structures, which were dominated by Chloroflexi (19.64-24.82%), Actinobacteria (15.49-31.96%), Acidobacteriota (9.46-20.31%), and Proteobacteria (11.88-14.57%) phyla. A strong correlation was observed between functional microbial taxon (e. g. Acidobacteriota), soil physicochemical properties, and Cd contents. The relative abundance of tolerant bacteria including Vicinamibacteraceae, Knoellia, Ardenticatenales, Lysobacter, etc. elevated with the increase of Cd, which contributed to the enrichment of heavy metal resistance genes (HRGs) and integration genes (intlI), thus enhancing the resistance to heavy metal pollution. Cd content rather than crop species was identified as the dominant factor that influenced the bacterial community. Nevertheless, the peculiar agrotype of the paddy field contributed to its higher HRGs and intlI abundance. These results provided fundamental information about the crop-specific physiochemical-bacterial interaction, which was helpful to evaluate agricultural environmental risk around the intersection of farmland and pollution sources.

Remediation of Cr(VI)-Contaminated Soil by Biochar-Supported Nanoscale Zero-Valent Iron and the Consequences for Indigenous Microbial Communities

Biochar/nano-zero-valent iron (BC-nZVI) composites are currently of great interest as an efficient remediation material for contaminated soil, but their potential to remediate Cr-contaminated soils and effect on soil microecology is unclear. The purpose of this study was to investigate the effect of BC-nZVI composites on the removal of Cr(VI) from soil, and indigenous microbial diversity and community composition. The results showed that after 15 days of remediation with 10 g/kg of BC-nZVI, 86.55% of Cr(VI) was removed from the soil. The remediation of the Cr-contaminated soil with BC-nZVI resulted in a significant increase in OTUs and α-diversity index, and even a significant increase in the abundance and diversity of indigenous bacteria and unique bacterial species in the community by reducing the toxic concentration of Cr, changing soil properties, and providing habitat for survival. These results confirm that BC-nZVI is effective in removing Cr(VI) and stabilizing Cr in soil with no significant adverse effects on soil quality or soil microorganisms.

Strategy for enhancing Cr(VI)-contaminated soil remediation and safe utilization by microbial-humic acid-vermiculite-alginate immobilized biocomposite

Bioreduction is an efficient approach to in-situ remediate Cr(VI)-contaminated soil, but further strengthening methods are still urgently needed. Herein, a novel immobilized biocomposite (B-HA-VE-SA) was successfully synthesized by embedding a efficient strain Bacillus sp. CRB-7 with humic acid (HA) combined vermiculite (VE) and sodium alginate (SA). The performance and enhancement mechanism of the immobilized biocomposite on remediating Cr(VI)-contaminated soil were also investigated by analyzing the whole-genome of CRB-7, Cr(VI) detoxification, soil microecological regulation, and subsequent crop growth response. Genomic annotation demonstrated that CRB-7 contains multiple genes contributed to Cr(VI) tolerance, Cr(VI) reduction and other metals resistance. Results showed that embedded CRB-7 biocomposites exhibited more effective reduction of Cr(VI) in soil compared with control and free CRB-7 treatment, especially B-HA-VE-SA achieved the highest Cr(VI) removal efficiency (96.18%) and the residual Cr proportion (49.04%) via multiple mechanisms including carrier effects, nutrient sustained-release, and electron-shuttle effect enhanced the bioremediation process. Furthermore, the synergies of CRB-7 and immobilizers (HA, VE and SA) significantly improved soil microecology (soil enzyme activities, microbial quantity and diversity), and engendered the evolution of microbial community composition and functional pathways. Consequently, pot experiments (Brassica napus L.) verified the plant-growth-promoting (12.00-18.00% and 43.82-69.00% higher in emergence rate and biomass) and Cr-accumulation-reducing effects (19.47-91.09% and 29.11-89.80% lower in root and aerial parts) of free and immobilized CRB-7. Taken together, these findings highlighted the superiority of B-HA-VE-SA in simultaneous remediation, microecological improvement and safe utilization of Cr(VI)-contaminated soil.

Calculation and Analysis of Nonlinear Algorithm for Stability of Nanosilica Powder Soft Soil Pile Foundation

In order to reasonably evaluate the stability of the embankment supported by the rigid pile composite foundation, a nonlinear algorithm calculation and analysis method for the stability of the nanosilica powder soft soil pile foundation is proposed. First, the soft soil foundation reinforced by the composite structure of "piles (prestressed pipe piles, (cement fly-ash gravel, CFG) piles)-pile caps-geotextile pads" is analyzed through the soft soil embankment test section of the Wenfu high-speed railway. Second, the compressive modulus under one-dimensional confining compression is calculated using the layered summation method. Finally, the deep lateral deformation of the pile foundation soil is measured by an inclinometer to evaluate the actual displacement of the pile foundation soil. In the standard method, the empirical coefficient 1.1-1.7 is multiplied on the basis of the calculation result of this method to correct the calculation error. Combined with the test results, it is shown that nanosilica powder can give full play to its excellent characteristics: promoting hydration speed and hydration degree through the reaction of pozzolan refining and consuming Ca(OH)2 crystals produced by cement hydration. Soft soils and modified soft soils: a certain amount of nanosilica powder can significantly improve the strength of soft soil at different ages.

Reduction, mineralization, and magnetic removal of chromium from soil by using a natural mineral composite

Reductive immobilization has been a commonly used technique to detoxify Cr(VI) from soil; however, it's challenging to remove the reduced Cr from soil to prevent its re-oxidation. This work explored a natural magnetic composite for the remediation, mineralization, and magnetic removal of Cr(VI) from the soil. It consists of 77% magnetite and 23% pyrrhotite with strong magnetic properties. A series of characterization tests show that composites of magnetite and pyrrhotite are interlaced and closely bonded, and contain no other heavy metals. The Cr(VI) removal rate increases with the decrease in composite particle size. A kinetics study shows that removing Cr(VI) by the composite is likely through both adsorption and reduction. Acidic conditions are more favorable for the immobilization of Cr(VI), at 45.8 mg Cr(VI) removal per g of composite at pH 2. After 100 days of in-situ treatment by the composite, the leaching concentration (TCLP) of Cr(VI)-contaminated soil was 1.95 mg L^-1, which was below the EPA limit (5 mg L^-1) for hazardous waste. After reduction, the composite was separated from soil by magnetic characteristics, and 58.2% of Cr was found mineralized. The post-treatment Cr-containing composite was analyzed by SEM-EDS, Raman spectra, and XPS. It was found that Cr was mineralized on the surface of the composite in the form of Cr(OH)₃, Cr₂O₃, and FeCr₂O_4. This indicates that reduction and mineralization of Cr(VI) in the soil can be accomplished through natural magnetic mineral composites and easily separated and removed from the soil, achieving a complete soil cleanup.

Unveiling the Potential of Novel Struvite–Humic Acid Composite Extracted from Anaerobic Digestate for Adsorption and Reduction of Chromium

A novel struvite–humic acid composite (S–HA) was derived from an anaerobic digestate and evaluated for the adsorption and reduction of chromium [Cr (VI)] in this study. The results indicated that the struvite–humic acid composite (S–HA) contains higher contents of oxygen-containing and aromatic functional groups (47.05% and 34.13%, respectively) and a higher specific surface area (19.3 m2 g−1). These special characteristics of S–HA contributed to its higher adsorption capacity (207.69 mg g−1 and 254.47 mg g−1 for pseudo-first and second-order kinetic models, respectively) for chromium. Furthermore, XPS analysis showed that a portion of the bonded Cr (VI) was reduced to Cr (III) by carboxyl and hydroxyl functional groups, which oxidized and changed into ketone and phenol functional groups. Based on the findings, it was concluded that the phosphate–humic acid composite has an outstanding chromium adsorptive and reduction capacity. However, more research is needed to fully understand the potential of the struvite–humic acid composite for chromium adsorption and reduction.

Recent advances in nanoremediation: Carving sustainable solution to clean-up polluted agriculture soils

Agriculture is one of the foremost significant human activities, which symbolizes the key source for food, fuel and fibers. This activity results in a lot of ecological harms particularly with the excessive usage of chemical fertilizers and pesticides. Different agricultural practices have remained industrialized to advance food production, due to the growth in the world population and to meet the food demand through the routine use of more effective fertilizers and pesticides. Soil is intensely embellished by environmental contamination and it can be stated as "universal incline." Soil pollution usually occurs from sewage wastes, accidental discharges or as byproducts of chemical residues of unrestrained production of numerous materials. Soil pollution with hazardous materials alters the physical, chemical, and biological properties, causing undesirable changes in soil fertility and ecosystem. Engineered nanomaterials offer various solutions for remediation of contaminated soils. Engineered nanomaterial-enable technologies are able to prevent the uncontrolled release of harmful materials into the environment along with capabilities to combat soil and groundwater borne pollutants. Currently, nanobiotechnology signifies a hopeful attitude to advance agronomic production and remediate polluted soils. Studies have outlined the way of nanomaterial applications to restore the eminence of the environment and assist the detection of polluted sites, along with potential remedies. This review focuses on the latest developments in agricultural nanobiotechnology and the tools developed to combat soil or land and or terrestrial pollution, as well as the benefits of using these tools to increase soil fertility and reduce potential toxicity.

Carboxymethyl cellulose stabilized ferrous sulfide@extracellular polymeric substance for Cr(VI) removal: Characterization, performance, and mechanism

Iron-based materials, especially ferrous sulfide (FeS), effectively remediate chromium pollution. However, the agglomeration of FeS reduces its reactivity to chromium. Herein, carboxymethyl cellulose stabilized ferrous sulfide@extracellular polymeric substance (CMC-FeS@EPS) was developed to remove hexavalent chromium (Cr(VI)) from water. CMC-FeS@EPS (98.00%) exhibited excellent removal efficiency of 40 mg/L Cr(VI) than those of FeS (57.35%) and CMC-FeS (68.60%). CMC-FeS@EPS showed good removal efficiency of Cr(VI) in wide pH range (from 4 to 9) and the co-existence of ions. FTIR and XPS results demonstrated that EPS functional group accelerated the process of adsorption and precipitation. Electrochemical results showed that CMC-FeS@EPS transferred electrons to Cr(VI) faster than CMC-FeS. In total, this study started from a new idea of using EPS to improve the performance of CMC-FeS, and provided a simple and effective way to remediate chromium pollution without secondary pollution.

Nano-biotechnology, an applicable approach for sustainable future

Nanotechnology is one of the most emerging fields of research within recent decades and is based upon the exploitation of nano-sized materials (e.g., nanoparticles, nanotubes, nanomembranes, nanowires, nanofibers and so on) in various operational fields. Nanomaterials have multiple advantages, including high stability, target selectivity, and plasticity. Diverse biotic (e.g., Capsid of viruses and algae) and abiotic (e.g., Carbon, silver, gold and etc.) materials can be utilized in the synthesis process of nanomaterials. "Nanobiotechnology" is the combination of nanotechnology and biotechnology disciplines. Nano-based approaches are developed to improve the traditional biotechnological methods and overcome their limitations, such as the side effects caused by conventional therapies. Several studies have reported that nanobiotechnology has remarkably enhanced the efficiency of various techniques, including drug delivery, water and soil remediation, and enzymatic processes. In this review, techniques that benefit the most from nano-biotechnological approaches, are categorized into four major fields: medical, industrial, agricultural, and environmental.

Contribution of Piriformospora indica on improving the nutritional quality of greenhouse tomato and its resistance against cu toxicity after humic acid addition to soil

Protected cultivation has a significant contribution in vegetable production. We investigated whether humic acid addition to soil and Piriformospora indica can improve the nutritional quality of greenhouse tomato. We conducted a pot experiment, in which the effects of P. indica inoculation, humic acid addition, and Cu spiking to soil (0, 120, 240, and 500 ppm Cu) were tested. Humic acid addition to soil spiked with 500 ppm Cu decreased the Cu concentration in the fruits of plants inoculated with P. indica from 0.65 to 0.40 mg 100 g Fw^-1, which is still above the maximum allowed limits of Cu in tomato by World Health Organization (WHO). The lycopene and ascorbic acid content of tomato fruits were consistently improved by humic acid addition and P. indica inoculation. The antioxidant enzymes' activity changed in response to humic acid addition, Cu spiking to soil, and P. indica inoculation. With increasing Cu level up to 240 ppm, the activity of superoxide dismutase (SOD) and peroxidase (POD) increased significantly. However, with spiking more Cu to soil, the activity of antioxidant enzymes reduced and the MDA content increased significantly. Addition of humic acid to soil and/or presence of P. indica increased the activity of antioxidant enzymes when the soil spiked with 500 ppm Cu. This study indicated that addition of P. indica and humic acid to the soil can enhance the nutritional quality of greenhouse tomato by reduction of Cu toxicity as a common pollutant in the greenhouse media and increasing the antioxidant content of fruits.

A Bacillus and Lysinibacillus sp. bio-augmented Festuca arundinacea phytoremediation system for the rapid decontamination of chromium influenced soil

Phytoremediation as an efficient and eco-friendly soil detoxification method has received widespread attention. In this study, two newly screened Chromium (Cr) reducing strains (Bacillus sp. AK-1 and Lysinibacillus sp. AK-5) were used to remediate Cr contaminated soil in conjunction with the application of hyperaccumulator tall fescue (Festuca arundinacea), thus establishing a soil Cr decontamination system. In this system, soil urease and dehydrogenase activities were increased, the malondialdehyde (MDA) contents in leaves of tall fescue were significantly decreased, while glutathione (GSH) contents increased. In terms of Cr fractions, the proportion of acetic acid extractable Cr decreased by 12.82-20.00% in treatment groups, respectively, compared with CK, while residual Cr increased by 9.41-22.37%. Moreover, biomass, root length and shoot length of tall fescue in treatment groups increased by 80.77-139.74%, 60.85-68.04%, 7.06-27.10%, respectively. In addition, the root system of tall fescue accumulated 303.887-372.167 mg kg^-1 of Cr, and the aboveground part accumulated 16.289-19.289 mg kg^-1 of Cr. Therefore, the application of strains AK-1 and AK-5 reduced the toxicity of Cr to plants and greatly increased plant accumulation potential, which indicated that AK-1 and AK-5 could improve removal efficiency of phytoremediation in Cr contaminated soil by reducing its bio-toxicity and promoting growth of tall fescue growth.

Field-scale studies on the change of soil microbial community structure and functions after stabilization at a chromium-contaminated site

Various remediation strategies have been developed to eliminate soil chromium (Cr) contamination which challenges the ecosystem and human health, and chemical stabilization is the most popular one. Limited work focuses on the change of soil microbial community and functions after chemical stabilization. The present study examined the diversity and structure of bacterial, fungal and archaeal communities in 20 soils from a Cr-contaminated site in China after chemical stabilization and ageing. Cr contamination significantly reduced microbial diversity and shaped microbial community structure. After chemical stabilization, bacterial and fungal communities had higher richness and evenness, whereas archaea behaved oppositely. Microbial community structure after stabilization were more similar to uncontaminated soils. Among all environmental variables, pH and Al explained 25.2% and 9.4% of the total variance of bacterial diversity, whereas the major variable affecting fungal community was pH (29.3%). Cr, organic matters, extractable-Al and moisture explained 25.8%, 22.4%, 9.9% and 9.9% of the total variance in archaeal community, respectively. This work for the first time unraveled the change of the whole soil microbial community structures and functions at Cr-contaminated sites after chemical stabilization on field scale and proved chemical stabilization as an effective approach to detoxicate Cr(VI) and recover microbial communities in soils.

Transformation and implication of nanoparticulate zero valent iron in soils

Knowledge of nanoparticulate zero-valent iron (nZVI) transformation in soils and its relationship with the potential impacts on soil properties are crucial to evaluate the environmental implication and application of nZVI. This study investigated nZVI transformation and the effects on soil properties in eight soils with various ageing time and soil moisture content (SMC). Spherical nZVI was gradually oxidized, collapsed, and adhered to clay minerals, and crystalline maghemite and magnetite were the primary oxidation products. Compared with the flooded condition, nZVI oxidation was accelerated under 70% SMC but was limited under 30% SMC. Acidic soil with lower content of dissolved aromatic carbon was advantage to nZVI oxidation under the flooded condition, while carboxymethylcellulose coating and iron oxides on nZVI surface limited nZVI oxidation. The aged nZVI existed mainly in the form of association with soil mineral or organic matter rather than in ion-exchangeable or carbonate form. nZVI treatment promoted soil aromatic carbon sequestration and decreased soil redox potential, and the impacts of nZVI on soil pH, electrical conductivity, ζ-potential, dissolved organic carbon, and catalase and urease activities were dependent on soil type and SMC. The findings are of significance for the evaluation of the environmental risk and proper application of nZVI.

Remediation of Chromium-Contaminated Soil Based on Bacillus cereus WHX-1 Immobilized on Biochar: Cr(VI) Transformation and Functional Microbial Enrichment

Microorganisms are applied to remediate chromium (Cr)-contaminated soil extensively. Nevertheless, the microbial loss and growth inhibition in the soil environment restrain the application of this technology. In this study, a Cr(VI)-reducing strain named Bacillus cereus WHX-1 was screened, and the microbial aggregates system was established via immobilizing the strain on Enteromorpha prolifera biochar to enhance the Cr(VI)-reducing activity of this strain. The mechanism of the system on Cr(VI) transformation in Cr-contaminated soil was illuminated. Pot experiments indicated that the microbial aggregates system improved the physicochemical characteristics of Cr-contaminated soil obviously by increasing organic carbon content and cation exchange capacity, as well as decreasing redox potential and bulk density of soil. Moreover, 94.22% of Cr(VI) was transformed into Cr(III) in the pot, and the content of residue fraction Cr increased by 63.38% compared with control check (CK). Correspondingly, the physiological property of Ryegrass planted on the Cr-contaminated soil was improved markedly and the main Cr(VI)-reducing microbes, Bacillus spp., were enriched in the soil with a relative abundance of 28.43% in the microbial aggregates system. Considering more active sites of biochar for microbial aggregation, it was inferred that B. cereus WHX-1 could be immobilized by E. prolifera biochar, and more Cr(VI) was transformed into residue fraction. Cr stress was decreased and the growth of plants was enhanced. This study would provide a new perspective for Cr-contaminated soil remediation.

Inorganic nanoparticles for reduction of hexavalent chromium: Physicochemical aspects

Hexavalent Chromium [Cr(VI)] is a highly carcinogenic and toxic material. It is one of the major environmental contaminants in aquatic system. Its removal from aqueous medium is a subject of current research. Various technologies like adsorption, membrane filtration, solvent extraction, coagulation, biological treatment, ion exchange and chemical reduction for removal of Cr(VI) from waste water have been developed. But chemical reduction of Cr(VI) to Cr(III) has attracted a lot of interest in the past few years because, the reduction product [Cr(III)] is one of the essential nutrients for organisms. Various nanoparticles based systems have been designed for conversion of Cr(VI) into Cr(III) which have not been critically reviewed in literature. This review present recent research progress of classification, designing and characterization of various inorganic nanoparticles reported as catalysts/reductants for rapid conversion of Cr(VI) into Cr(III) in aqueous medium. Kinetics and mechanism of nanoparticles enhanced/catalyzed reduction of Cr(VI) and factors affecting the reduction process have been discussed critically. Personal future insights have been also predicted for further development in this area.

Microbial Cd(II) and Cr(VI) resistance mechanisms and application in bioremediation

The heavy metals cadmium (Cd) and chromium (Cr) are extensively used in industry and result in water and soil contamination. The highly toxic Cd(II) and Cr(VI) are the most common soluble forms of Cd and Cr, respectively. They enter the human body through the food chain and drinking water and then cause serious illnesses. Microorganisms can adsorb metals or transform Cd(II) and Cr(VI) into insoluble or less bioavailable forms, and such strategies are applicable in Cd and Cr bioremediation. This review focuses on the highlighting of novel achievements on microbial Cd(II) and Cr(VI) resistance mechanisms and their bioremediation applications. In addition, the knowledge gaps and research perspectives are also discussed in order to build a bridge between the theoretical breakthrough and the resolution of Cd(II) and Cr(VI) contamination problems.

Successful Outcome of Phytostabilization in Cr(VI) Contaminated Soils Amended with Alkalizing Additives

This study analysed the effect of three alkalizing soil amendments (limestone, dolomite chalcedonite) on aided phytostabilization with Festuca rubra L. depending on the hexavalent chromium (Cr(VI)) level in contaminated soil. Four different levels of Cr(VI) were added to the soil (0, 50, 100 and 150 mg/kg). The Cr contents in the plant roots and above-ground parts and the soil (total and extracted Cr by 0.01 M CaCl2) were determined with flame atomic absorption spectrometry. The phytotoxicity of the soil was also determined. Soil amended with chalcedonite significantly increased F. rubra biomass. Chalcedonite and limestone favored a considerable accumulation of Cr in the roots. The application of dolomite and limestone to soil contaminated with Cr(VI) contributed to a significant increase in pH values and was found to be the most effective in reducing total Cr and CaCl2-extracted Cr contents from the soil. F. rubra in combination with a chalcedonite amendment appears to be a promising solution for phytostabilization of Cr(VI)-contaminated areas. The use of this model can contribute to reducing human exposure to Cr(VI) and its associated health risks.

Electro-enhanced leaching method for the mobilization of Cr(VI) in contaminated groundwater aquifer

Removal of hexavalent chromium [Cr(VI)] from soils and water has been widely studied for its high toxicity. Although leaching method is viewed as an effective approach to eliminate Cr(VI) and some studies attempted to enhance leaching performance via the external electric field, there is little knowledge about the influential factor in electro-leaching system on Cr(VI) removal performance. In this study, an electro-leaching technology was developed for removing Cr(VI) from groundwater aquifer to comprehensively discuss the correlation between the operational parameters and Cr(VI) removal efficiency. When the applied voltage was 20 V and the initial Cr(VI) concentration was 40 mg/kg, Cr(VI) removal efficiency achieved 99.9% in 120 min in the electro-leaching system, 15% higher than the system without the electric field. Cr(VI) removal efficiencies increased with the voltage demonstrating the significant enhancement of the electro-leaching method in removing Cr(VI). When Cr(VI) concentration climbed to 120 mg/kg, Cr(VI) removal efficiency remained above 85%. The effects of different voltages, Cr(VI) concentrations, pollutant distribution and salt content of leaching solution on the leaching effect were also investigated. Meanwhile, the relationship between the current intensity change and the amount of removed Cr(VI) during the electro-leaching process was first investigated, and the relevant model was fitted. There is a quadratic linear correlation between the amount of current change and the amount of removed Cr(VI). This novel electro-enhanced leaching method can effectively remove Cr(VI) from contaminated groundwater aquifer by enhancing the migration of charged contaminant ions during the leaching process, and it is worthy of further study of heavy metal remediation.