A comparative evaluation of hexavalent chromium treatment in contaminated soil by calcium polysulfide and green-tea nanoscale zero-valent iron

Harnessing plant extracts for eco-friendly synthesis of iron nanoparticle (Fe-NPs): Characterization and their potential applications for ameliorating environmental pollutants

Iron-nanoparticles (Fe-NPs) are increasingly been utilized in environmental applications due to their efficacy and strong catalytic activities. The novelty of nanoparticle science had attracted many researchers and especially for their green synthesis, which can effectively reuse biological resources during the polymerization reactions. Thus, the synthesis of Fe-NPs utilizing plant extracts could be considered as the eco-friendly, simple, rapid, energy-efficient, sustainable, and cost-effective. The green synthesis route can be recognized as a practical, valuable, and economically effective alternative for large-scale production. During the production process, some biomolecules present in the extracts undergo metal salts reduction, which can serve as both a capping and reducing mechanism, enhancing the reactivity and stability of green-synthesized Fe-NPs. The diversity of species provided a wide range of potential sources for green synthesis of Fe-NPs. With improved understanding of the specific biomolecules involved in the bioreduction and stabilization processes, it will become easier to identify and utilize new, potential plant materials for Fe-NPs synthesis. Newly synthesized Fe-NPs require different characterization techniques such as transmission electron microscope, ultraviolet-visible spectrophotometry, and X-ray absorption fine structure, etc, for the determination of size, composition, and structure. This review described and assessed the recent advancements in understanding green-synthesized Fe-NPs derived from plant-based material. Detailed information on various plant materials suitable of yielding valuable biomolecules with potential diverse applications in environmental safety. Additionally, this review examined the characterization techniques employed to analyze Fe-NPs, their stability, accumulation, mobility, and fate in the environment. Holistically, the review assessed the applications of Fe-NPs in remediating wastewaters, organic residues, and inorganic contaminants. The toxicity of Fe-NPs was also addressed; emphasizing the need to refine the synthesis of green Fe-NPs to ensure safety and environmental friendliness. Moving forward, the future challenges and opportunities associated with the green synthesis of Fe-NPs would motivate novel research about nanoparticles in new directions.

Reduction of organic contaminants from industrial effluent using the advanced oxidation process, chemical coagulation, and green nanotechnology

Municipal wastewater treatment systems use the chemical oxygen demand test (COD) to identify organic contaminants in industrial effluents that impede treatment due to their high concentration. This study reduced the COD levels in tannery wastewater using a multistage treatment process that included Fenton oxidation, chemical coagulation, and nanotechnology based on a synthetic soluble COD standard solution. At an acidic pH of 5, Fenton oxidation reduces the COD concentration by approximately 79%. It achieves this by combining 10 mL/L of H2O2 and 0.1 g/L of FeCl2. Furthermore, the author selected the FeCl3 coagulant for the coagulation process based on the best results of comparisons between different coagulants. At pH 8.5, the coagulation dose of 0.15 g/L achieved the maximum COD removal efficiency of approximately 56.7%. Finally, nano bimetallic Fe/Cu was used to complete the degradation and adsorption of the remaining organic pollutants. The XRD, SEM, and EDX analyses proved the formation of Fe/Cu nanoparticles. A dose of 0.09 g/L Fe/Cu NPs, 30 min of contact time, and a stirring rate of 200 rpm achieve a maximum removal efficiency of about 93% of COD at pH 7.5. The kinetics studies were analyzed using pseudo-first-order P.F.O., pseudo-second-order P.S.O., and intraparticle diffusion models. The P.S.O. showed the best fit among the kinetic models, with an R2 of 0.998. Finally, the authors recommended that technique for highly contaminated industrial effluents treatment for agriculture or industrial purposes.

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

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

Combined effects of biochar and biodegradable mulch film on chromium bioavailability and the agronomic characteristics of tobacco

Biochar (BC) and biodegradable mulch film (BMF) are both commonly used means of production in agriculture. In recent years, most studies have focused on the effects of BC or BMF on soil heavy metal pollution, while they have neglected the combined effects. In this study, a pot experiment was conducted to examine the impacts of BMF, BC, and combined BMF and BC (CMB) on the mobility of chromium (Cr) and the agronomic characteristics of flue-cured tobacco. Compared with the control, BMF, BC, and CMB significantly reduced the concentrations of diethylenetriamine pentaacetic acid (DTPA) extractable Cr in soils by 29.07-29.75%, 45.35-48.54%, and 34.21-37.92%, respectively. In comparison to the application of BMF and BC alone, co-application reduced the availability of Cr in soil via increasing the adsorption of soil Cr and soil enzyme activity, which resulted in the decrease of Cr content and bioconcentration factor and in plants. Moreover, the combined application increased the plant height, stem diameter, leaf area, total root area, root tip number, and root activity of tobacco, which leaded to increase in leaf and root biomass by 11.40-67.01% and 23.91-50.74%, respectively. Therefore, the application of CMB can reduce the heavy metal residues in tobacco leaves and improve tobacco yield and quality.

Long-term stability and toxicity effects of three-dimensional electrokinetic remediation on chromium-contaminated soils

The three-dimensional electrokinetic remediation (3D EKR) achieved efficient removal of chromium (Cr) from the soil through mechanisms including electromigration, electroosmosis, and redox reactions. In this study, the long-term stability, leaching toxicity, bioavailability, and phytotoxicity of Cr in remediated soils were systematically analyzed to comprehensively evaluate the effectiveness of the 3D EKR method. The results showed that the concentration of hexavalent chromium (Cr (VI)) in the leachate of the 3D EKR system with sulfidated nano-scale zerovalent iron (S-nZVI) was more than 40% lower than those of the other 3D electrode groups, and the time required to reach the level III standard of groundwater quality criterion in China (0.05 mg/L, GB/T 14848-2017) was significantly shortened. The stabilization of Cr(VI) in contaminated soil after 3D EKR was maintained for 300 pore volumes (PVs), indicating that the treated Cr(VI) had good long-term stability. The leaching toxicity and bioaccessibility of Cr were assessed by the synthetic precipitation leaching procedure (SPLP), the toxicity characteristic leaching procedure (TCLP), and the physiologically based extraction test (PBET). The concentration of Cr(VI) in the SPLP, TCLP, and PBET leachates of the S-nZVI group decreased by more than 25% compared to the other 3D electrode groups, corresponding to the decrease in leaching toxicity and bioavailability of the treated Cr during the 15-day remediation period. In addition, the germination rate of wheat seeds and the average biomass of wheat seedlings in the S-nZVI group under alkaline conditions (EE) were higher than those in the non-polluting group (Blank-OH), indicating that the remediated soil had no obvious toxicity to wheat. In summary, 3D EKR achieved a satisfactory and stable remediation effect on Cr-contaminated soil, especially when using S-nZVI as the 3D electrode.

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.

Effective Cr(VI) reduction and immobilization in chromite ore processing residue (COPR) contaminated soils by ferrous sulfate and digestate: A comparative investigation with typical reducing agents

Chemical reduction combined with microbial stabilization is a green and efficient method for the remediation of hexavalent chromium (Cr(VI)) contaminated soil. In this study, the combination of ferrous sulfate with kitchen waste digestate was applied to reduce and immobilize Cr(VI) in chromite ore processing residue (COPR) contaminated soils, and systematically evaluated the remediation performance of Cr(VI) compared with several typical reducing agents (i.e., ferrous sulfate, zero valent iron, sodium thiosulfate, ferrous sulfide, and calcium polysulfide). The results showed that the combination of ferrous sulfate and digestate had superior advantages of a lower dosage of reducing agent and a long-term remediation effect compared to other single chemical reductants. Under an Fe(II):Cr(VI) molar ratio of 3:1% and 4% digestate (wt), the content of Cr(VI) in the soil decreased to 5.07 mg/kg after 60 days of remediation. Meanwhile, the leaching concentrations of Cr(VI) were below detection limit, which can meet the hazardous waste toxicity leaching standard. The risk level of Cr pollution was decreased from very high risk to low risk. The X-ray photoelectron spectroscopy (XPS) results further demonstrated that the combined treatments were beneficial to Cr(VI) reduction and stabilization. The abundance of bacteria with Cr(VI) reducing ability was higher than other treatments. Moreover, the high abundance of carbon and nitrogen metabolism in the combined treatments demonstrated that the addition of digestate was beneficial to the recovery and flourishing of Cr(VI)-reducing related microorganisms in COPR contaminated soils. This work provided an alternative way on Cr(VI) remediation in COPR contaminated soils.

Functional materials contributing to the removal of chlorinated hydrocarbons from soil and groundwater: Classification and intrinsic chemical-biological removal mechanisms

Chlorinated hydrocarbons (CHs) are the main contaminants in soil and groundwater and have posed great challenge on the remediation of soil and ground water. Different remediation materials have been developed to deal with the environmental problems caused by CHs. Remediation materials can be classified into three main categories according to the corresponding technologies: adsorption materials, chemical reduction materials and bioaugmentation materials. In this paper, the classification and preparation of the three materials are briefly described in terms of synthesis and properties according to the different types. Then, a detailed review of the remediation mechanisms and applications of the different materials in soil and groundwater remediation is presented in relation to the various properties of the materials and the different challenges encountered in laboratory research or in the environmental application. The removal trends in different environments were found to be largely similar, which means that composite materials tend to be more effective in removing CHs in actual remediation. For instance, adsorbents were found to be effective when combined with other materials, due to the ability to take advantage of the respective strengths of both materials. The rapid removal of CHs while minimizing the impact of CHs on another material and the material itself on the environment. Finally, suggestions for the next research directions are given in conjunction with this paper.

Understanding and eliminating the reductant interference on Chromium VI measurement with USEPA method 3060A

Excessive reductants are used in engineering to ensure a reliable remediation effect of chromite ore processing residue (COPR), however, re-yellowing phenomenon of remediated COPR occurs after some time though the Cr(VI) content meets regulatory requirements after curing period. This problem is due to a negative bias on Cr(VI) determination using USEPA method 3060A. To address this issue, this study tried to reveal the interference mechanisms and proposed two methods to amend the bias. Results of ion concentrations, UV-Vis spectrum, XRD, and XPS together showed that Cr(VI) was reduced by ions (Fe²⁺, S₅^2-) in the digestion stage of USEPA method 3060A, and as a result, method 7196A would not reflect the true Cr(VI) concentration. The interference on Cr(VI) determination generated by excess reductants mainly occurs during the curing period of remediated COPR, but it decreases over time as reductants being oxidized gradually by the air. Compared with the thermal oxidation, the chemical oxidation with K₂S₂O₈ prior to alkaline digestion performs better to eliminate the masking effect brought by excess reductants. This study provides an approach on how to accurately determine the Cr(VI) concentration in the remediated COPR. It might be helpful to reduce the occurrence possibility of re-yellowing phenomenon.

A Comprehensive Review of the Current Progress of Chromium Removal Methods from Aqueous Solution

Chromium (Cr) exists in aqueous solution as trivalent (Cr³⁺) and hexavalent (Cr⁶⁺) forms. Cr³⁺ is an essential trace element while Cr⁶⁺ is a dangerous and carcinogenic element, which is of great concern globally due to its extensive applications in various industrial processes such as textiles, manufacturing of inks, dyes, paints, and pigments, electroplating, stainless steel, leather, tanning, and wood preservation, among others. Cr³⁺ in wastewater can be transformed into Cr⁶⁺ when it enters the environment. Therefore, research on Cr remediation from water has attracted much attention recently. A number of methods such as adsorption, electrochemical treatment, physico-chemical methods, biological removal, and membrane filtration have been devised for efficient Cr removal from water. This review comprehensively demonstrated the Cr removal technologies in the literature to date. The advantages and disadvantages of Cr removal methods were also described. Future research directions are suggested and provide the application of adsorbents for Cr removal from waters.

Ferrochrome slag: A critical review of its properties, environmental issues and sustainable utilization

Ferrochrome slag (FCS) is a by-product of ferrochrome industries and is produced during the extraction of ferrochrome from chromite ore. The chemical composition of FCS comprises of 27-33% SiO₂, 15-25% Al₂O₃, 20-35% MgO, and 10-15% iron-chromium compounds. The high chromium content of FCS and the possibility of its leaching into the environment categorize FCS as hazardous waste material. For each ton of ferrochrome production, nearly 1.2-1.5 tons of FCS is generated, which becomes a significant challenge for the ferrochrome producers while managing this hazardous waste. Therefore, several research attempts have been made to observe the leaching characteristics of chromium (VI) in FCS, its stabilization, and subsequent potential utilization. The high mechanical properties of FCS have led many researchers worldwide to utilize it as a construction material. This review work has undertaken FCS's physical, chemical, and microstructural characteristics and its following utilization as a fine and coarse aggregate in producing green and sustainable concrete. Different methods of stabilizing chromium (VI), including the physical, chemical, and biological methods, are extensively discussed in this review. This article also accommodated FCS as a precursor material in geopolymer and alkali-activated binders. However, the compressive strength achieved with FCS as a binder in geopolymer is very low, and thus more studies are needed to establish the possibility of strength enhancement. The leaching aspects of geopolymers with FCS also need to be studied extensively for their successive application. Lastly, the conclusions and discussion of this study have keenly addressed the significant challenges to the safe utilization of FCS in construction applications. Also, it deliberates on how the emerging research on FCS, such as refractory, composites, and coating material, can be new avenues for its utilization without any potential threat to the environment.

Rapid Cr(VI) reduction structure in chromium contaminated soil: The UV-assisted electrokinetic circulation of background iron

Substantially decreasing the severe hazards connected with the toxic Cr(VI), developing effective reduction remediation strategies may be crucial under favorable economic conditions for the contaminated soil containing Cr(VI) to protect human health. Several typical enhancers (phosphate, fulvic acid, citric acid) were used to test electrokinetic remediation (EKR) coupled with UV radiation-induced photochemical reduction for contaminated soil containing Cr(VI). The added citrate, while improving the Cr(VI) electromigration, worked as the ultimate sacrificial electron donors, with the dissolved soil background Fe(III) as electron shuttle, to Cr(VI) rapid reduction. The dissolved soil background Fe(III) convert into Fe(II) ions through the UV radiation-induced ligand-metal charge transfers reaction, which constituted a novel electrokinetic circulation reduction pathway for the elimination of surface-bound/dissolved Cr(VI) (difficult to electromigration) in the near-anodic soil layers. More than 80% dissolved and surface-bound Cr(VI) was eliminated from the soil. In particular, the dissolved and surface-bound Cr(VI) was enhanced by more than 62.37% removal in near-anodic soil layers compared to conventional citric acid-enhanced EKR and provided no extra cost other than UV radiation. This configuration may be a cost-effective and feasible remediation design in the future for the in-situ Cr(VI) reduction of contaminated sites.

Key Cr species controlling Cr stability in contaminated soils before and chemical stabilization at a remediation engineering site

Linking chromium (Cr) speciation with its stability in soils is vital because insoluble Cr(VI) and chemically adsorbed Cr(VI) could hinder the remediation efficiency and release Cr(VI) for a prolonged period of time. In this study, we investigated key Cr species to probe the mechanisms controlling the release of insoluble Cr(VI) at Cr-contaminated sites using synchrotron-based X-ray absorption near-edge structure (XANES) for the first time. Chromite, stichtite and Cr-silicate were predominant forms of Cr(III). Insoluble Cr(VI) was hosted by layered double hydroxides (LDHs) such as brownmilerite and hydrotalcite. Anion competition tests documented a substitution of absorbed Cr(VI) by SO₄^2- and NO₃^-. Acid extraction released 6.7-25.7% more Cr(VI) than anion extraction, possibly attributing to the erosion of LDH and CaCrO₄ in calcite rather than Cr-bearing minerals. Brown and red soils released maximally 62% and 44% of total Cr(VI) by 10 mol/(kg soil) and 2 mol/(kg soil) of H⁺, respectively. SO₄^2-, H₂O and H⁺ contributed to more release of total Cr(VI) in brown soils (22%, 33% and 7%) than red soils (25%, 17% and 2%). More crystalline Cr structures were found after chemical stabilization, indicating a higher Cr stability in chemically stabilized soils. Cr and Mn exhibited an overlapped distribution pattern in both contaminated and chemically stabilized soils, hinting at the re-oxidation of Cr(III). Insoluble Cr(VI) could be released by acidic rainfalls and soil organic matters, posing potential threats to Cr long-term stability in field-scale remediation.

Adsorption and Reduction of Aqueous Cr by FeS-Modified Fe-Al Layered Double Hydroxide

To remedy the widespread chromium (Cr) pollution in the environment, this study mainly used the ultrasonic-assisted co-precipitation and precipitation methods to prepare FeS-modified Fe-Al-layered double hydroxide (FeS/LDH) composite material. The experimental results showed that FeS/LDH has higher removal efficiency of Cr in aqueous solution and stronger anti-interference ability than unmodified LDH. Under the same reaction conditions, the removal efficiency of total Cr(Cr(T)) using LDH was 34.85%, and the removal efficiency of Cr(VI) was 46.76%. For FeS/LDH, the removal efficiency of Cr(T) and Cr(VI) reached 99.57% and 100%, respectively. The restoration of Cr(T) and Cr(VI) by FeS/LDH satisfied the Langmuir adsorption isotherm. The maximum adsorption capacity of Cr(T) and Cr(VI) achieved 102.9 mg/g and 147.7 mg/g. The efficient removal of Cr by FeS/LDH was mainly based on the triple synergistic effect of anion exchange between Cr(VI) and interlayer anions, redox of Cr(VI) with Fe2+ and S2−, and co-precipitation of Fe3+ and Cr3+.

Evaluation of remediation of Cr(VI)-contaminated soils by calcium polysulfide: Long-term stabilization and mechanism studies

In the remediation of Cr(VI)-contaminated soils, the effectiveness and long-term stability are critical qualities for the selection of a reductant. In current engineering practices, iron-based materials and sulfides are the most prevalent reductants, and calcium polysulfide (CaS₄) is considered as the one with the highest effectiveness and strongest long-term stabilization ability. But this opinion is questioned by the high interference ability of CaS₄ to soil Cr(VI) analysis. This study provides a pretreatment method to eliminate the interference of residual ferrous and sulfides to soil Cr(VI) analysis. By this pretreatment method and comparing with FeSO₄ and Na₂S, the mechanisms of the false high effectiveness and strong long-term stabilization ability of CaS₄ is revealed. In the remediation process, CaS₄ produces much elemental sulfur (S⁰) which remains in the soils. During the alkaline digestion, the S⁰ generates polysulfide which reduces the extracted Cr(VI), inducing serious negative analysis bias. When this negative bias is eliminated by pretreatment method, analysis results show that CaS₄ exhibits lowest effectiveness. The S⁰ cannot be leached away from soils and oxidized by oxygen under natural conditions, this makes CaS₄ exhibit a persistent interference ability, which is mistaken for a strong long-term stabilization ability.

Synthesis of montmorillonite-supported nano-zero-valent iron via green tea extract: Enhanced transport and application for hexavalent chromium removal from water and soil

The nano-zero-valent iron composite (nZVI@TP-Mont) was successfully prepared using a low-cost and environmental-friendly green synthesis via tea leaves extract (tea polyphenols, TPs) and the montmorillonite (Mont). The batch and column experiments and characterization were conducted to investigate the transport behavior and Cr(VI) remediation by nZVI@TP-Mont in water/soil. Due to its particular surface characteristics and morphology (i.e., the Fe⁰ core wrapped by TPs, the doped sulfur, and interlayer structure), the nZVI@TP-Mont composite showed a great removal capacity of Cr(VI) and sufficient mobility under different soil conditions. We opine the increase in the Cr(VI) reduction of nZVI@TP-Mont was attributed to the tethering of Fe₂O₃ on the surface of Fe⁰ core by the support of Mont interlayer, especially the TP-coverage around nZVI@TP-Mont surface unwrapped, thereby increasing the regenerated reactive Fe²⁺ and the exposed reaction sites of Fe⁰ cores to Cr(VI). The increased transportability of nZVI@TP-Mont slightly depends on the heterogeneous soil properties (i.e., ionic strength, sand/soil ratio, and pH). The two-site kinetic attachment model fitting results suggest Cr(VI)/Cr(III) speciation associated with the agglomerated nZVI@TP-Mont were efficiently immobilized in soil. Therefore, this study would benefit the efficient application of the green-synthesized nZVI@TP-Mont in in-situ remediation of soils contaminated by Cr(VI).

Microwave-enhanced reductive immobilization of high concentrations of chromium in a field soil using iron polysulfide

High concentrations of Cr(VI) are often detected in contaminated soil. Yet, cost-effective remediation technologies have been lacking. In this study, we prepared a type of FeS_x based on commercial FeSO₄.7H₂O and CaS_x and tested a microwave-assisted technology based on FeS_x for reductive immobilization of high concentrations of Cr(VI) in a field contaminated soil. The as-prepared FeS_x particles appeared as a honeycomb-like and highly porous structure. The microwave-assisted FeS_x reduction process was able to rapidly reduce the TCLP-based reachability of Cr(VI) from 391.8 to 2.6 mg·L^-1. The dosage of FeS_x, S/Fe molar ratio, initial moisture content, microwave power, and irradiation time can all affect the treatment effectiveness. After 500 days curing under atmospheric conditions, the TCLP-leached concentration of Cr remained below the regulatory limit of 5 mg·L^-1, while other treatments failed to meet the goal. S_x^2- or S^2- served as the primary electron donors, whereas Fe facilitated the microwave absorption and the formation of the stable final product of FeCr₂O₄. S and Fe are mostly precipitated in soil. The microwave-assisted FeS_x reduction was shown to be an effective approach to rapidly reduce the leachability of Cr(VI) in contaminated soil, especially in heavily contaminated soil.

A Review on Montmorillonite-Supported Nanoscale Zerovalent Iron for Contaminant Removal from Water and Soil

Nanoscale zerovalent iron (nZVI) has shown great promise for water treatment and soil remediation. However, the rapid aggregation of nZVIs significantly affects their mobility and reactivity, which considerably limits the practical applications. Montmorillonite- (Mt-) supported nZVI (Mt-nZVI) has received increasing attention for the past decade because it can prevent the aggregation of nZVI and incorporate the advantages of both nZVI and Mt in soil and water treatment. This work thus had a comprehensive review on the use of Mt-nZVI for soil and water treatment. We first summarized existing methods used to prepare Mt-nZVI, indicating the advantages of using Mt to support nZVI (e.g., increase of the dispersion and mobility of nZVI, reduction of the size and oxidation tendency of nZVI). We then presented the reaction mechanisms of Mt-nZVI for contaminant removal and evaluated the critical factors that influence the removal (e.g., pH, temperature, and dosage of the adsorbent). We further presented examples of applications of Mt-nZVI for the removal of typical contaminants such as heavy metals and organic compounds in soil and water. We finally discussed the limitations of the use of Mt-nZVI for water treatment and soil remediation and presented future directions for the application of nZVI technology for soil and water treatment.

Reductive materials for remediation of hexavalent chromium contaminated soil - A review

Chemical reduction of Cr(VI) to Cr(III) by reductive materials is the most widely used technology for the remediation of Cr(VI)-contaminated soil due to its high efficiency, adaptability and low cost. This paper reviews chromium chemistry and the materials that can effectively reduce Cr(VI) to Cr(III) for the remediation of Cr(VI)-contaminated soil, namely iron-bearing reductants, sulfur-based compounds and organic amendments. Moreover, we discuss the corresponding mechanisms involved in the process of immobilization of Cr(VI) in polluted soil, and emphasize the relationship between the materials remediation performance and soil environmental conditions. Besides, perspectives on the potential future researches of novel materials design and technological development in the remediation of Cr(VI) contaminated soil are also put forward.

Enhanced remediation efficiency of Cr(VI)-contaminated heterogeneous aquifers: Improved sweeping efficiency using shear-thinning fluids

Low permeability zones (LPZs) are typically bypassed when remedial reagents are injected into heterogeneous aquifers, which hinders the in situ remediation. Although shear-thinning polymers have emerged as promising tools to meet this challenge, their applicability in complex remedial systems remains unconfirmed. We investigated the sweeping efficiencies of calcium polysulfide (CPS) into Cr(VI)-contaminated LPZs using xanthan gum (XG) as the model shear-thinning polymer. Firstly, the compatibility of XG-CPS fluids and their reduction capacities toward Cr(VI) were demonstrated based on batch experiments. The removal rates of Cr(VI) exceeded 85% in the presence of 250-2000 mg/L of XG. Besides, XG-CPS fluids exhibited a greater impact on the permeability decrease of transmissive zones than that of LPZs as confirmed by sand column experiments. Furthermore, the sweeping efficiencies in LPZs during XG-CPS flooding were investigated by multiple sand tank experiments. The sweeping rate in LPZs (r_s) in Cr(VI)-contaminated aquifer (1.68 × 10^-3/min) was found to be approximately 11% higher than that of uncontaminated system, and two possible reasons behind this phenomenon were proposed. The spatial distribution profiles of Cr under different XG-CPS flooding conditions were depicted based on 20 representative samples. The results indicated that all Cr(VI) in LPZs can be effectively removed either by displacement or immobilization as Cr(III). The percentages of displaced Cr(VI) and immobilized Cr(III) were calculated to be 65%-75% and 25-35%, respectively. This work demonstrates the applicability of XG-CPS fluids as remedial materials for Cr(VI)-contaminated heterogeneous aquifers and provides novel insights into the role of Cr(VI) in in situ remediation using shear-thinning polymers.

Hexavalent chromium reduction by gallic acid

Cr(VI) is widely used in industry and often large quantities have been accidently or due to lack of precautions released at the environment. The presence of high concentrations of Cr(VI) in waste streams causes concern due to its toxicity and solubility. However the Cr(III) is a useful micronutrient in human diet. The reduction of Cr(VI) to Cr(III) is a promising technology for a clean environment. Polyphenols are commonly found in plants and they are considered as strong natural antioxidants. In this study the reduction of hexavalent to trivalent chromium using gallic acid (GA), a polyhydroxyphenolic compound, was evaluated. The effect of pH, temperature, and Cr(VI) concentration on the kinetics of Cr(VI) reduction by GA were examined by conducting batch tests. It was found that reduction of Cr(VI) by GA is a rapid reaction under mild acidic conditions. Reduction was found to follow a 2nd order kinetics with respect to Cr(VI) concentration in the pH range 3-5. The results of the present study indicated that the treatment of contaminated waters with initial concentration of Cr(VI) 100 μΜ by gallic acid, below the environmental limit of 0.96 μM, required 20 min at pH 3 and 40 min at pH 4. At higher pH a different reduction mechanism seems to prevail and the required time for Cr(VI) reduction was approximately 10 h at pH 5 and 42 h at pH 6. The increase of temperature25 °C to 35 °C, at pH 6, was slightly accelerated the reaction.

Carbon dots derived from kanamycin sulfate with antibacterial activity and selectivity for Cr6+ detection

Among antibacterial nanomaterials, carbon dots (CDs) have attracted much attention because of their unique physical and chemical properties and good biosafety. In this study, kanamycin sulfate (Kan), a broad-spectrum antibiotic, was used to synthesize novel carbon dots (CDs-Kan) by a one-step hydrothermal method. CDs-Kan showed good inhibitory effects on Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus. Further, scanning electron microscopy revealed that treatment with CDs-Kan and Kan resulted in the same phenomena. In particular, the morphologies of S. aureus cells treated with CDs-Kan and Kan became smaller and irregular, whereas the surfaces of E. coli cells protruded and formed vesicles. These results indicated that CDs-Kan was shown to retain the good antibacterial activity of Kan as well as its main bactericidal functional groups, namely, the amino sugar and amino cyclic alcohol, We refer to this phenomenon as the "preservation property". We also found that CDs-Kan has good biocompatibility and nontoxic properties. Moreover, CDs-Kan was successfully applied to the biological imaging of fungi and plant cells. In addition, CDs-Kan could be used as a fluorescent probe for the quick, sensitive, and selective detection of Cr⁶⁺. Therefore, CDs-Kan not only retained the good bacteriostatic properties of Kan but also expanded its application in bioimaging and biosensors.

Continuous Flow Process for Cr(VI) Removal from Aqueous Solutions Using Resin Supported Zero-Valent Iron

The objective of the present study was to evaluate the performance of a nanocomposite material consisting of nano zero valent iron and a cation exchange resin, for the reduction of chromate, by conducting column tests. A cationic resin, Amberlyst 15, was selected as porous host material. The synthesis of the nanocomposite material (R-nFe) was carried out using Green Tea extract to obtain the reduction of adsorbed Fe(III) to the elemental state Fe(0). Three column tests were implemented with different dimensions, corresponding to variable contact times between the aqueous solution and the resin beads loaded with Fe(0), namely 168, 744 and 1260 s respectively for columns I, II and III. The results indicated that the removal of Cr(VI) follows a first order kinetic law with a chemical constant equal to 0.0526 min^-1 (8.8 × 10^-4 s^-1).

Interference of sulfide with iron ions to the analysis of Cr(Ⅵ) by Method 3060a & Method 7196a

When Cr(Ⅵ)-contaminated soil was remediated with sulfide, due to incomplete reaction and overdose, excess sulfide could remain in the soil inducing unacceptable underestimation of residual Cr(Ⅵ) analysis, which would get worse when iron ions were introduced in. Unfortunately, the quality control policy of Method 3060a cannot distinguish the difference between real zero and false zero residual Cr(Ⅵ) when their spike recoveries are zero. This paper systematically investigated the sulfide-induced Cr(Ⅵ) reduction in Cr(Ⅵ) analysis by Method 3060a & Method 7196a. Experimental results indicate that the sulfide-Cr(Ⅵ) reaction mainly occurs during alkaline digestion and pH adjustment processes, and iron ions as an electron transfer carrier between sulfide and Cr(Ⅵ) can catalyze the redox reaction during alkaline digestion. Besides, the high temperature in alkaline digestion significantly accelerates the redox reaction which usually is negligible at room temperature. Sulfur of high concentrations in remediated soils can also cause unacceptable underestimation of Cr(Ⅵ) due to the production of sulfide ions by disproportionation reaction in alkaline digestion. This paper also provides a method to eliminate sulfide ions from alkaline digestion solution before pH adjustment and suggests some possible solutions to the ferrous or sulfide-induced Cr(Ⅵ) analysis bias.

Reduction and stabilization of Cr(VI) in soil by using calcium polysulfide: Catalysis of natural iron oxides

Cr(VI)-contaminated soils could be remediated by using calcium polysulfide (CPS), while natural iron oxides as a main composition of soil would influence the pathways of the remediation. Through kinetic batch tests, the kinetics of Cr(VI) removal from soil, the effects of the contents of natural iron oxides, soil environmental conditions and mechanisms of Cr(VI) removal by using CPS with the presence of natural iron oxides were investigated. The results show that the removal of Cr(VI) by using CPS in soil fitted the pseudo-second-order model best, and the appearance of goethite increased the apparent rate constant from 0.0002 kg mg^-1 h^-1 to 0.0005 kg mg^-1 h^-1. The presence of iron oxides enhanced the removal of Cr(VI) by using CPS, and an extended reductive atmosphere of soil was created. The enhancement of Cr(VI) removal increased with the contents of iron oxides from 0 to 9 g kg^-1, and declined from 9 to 12 g kg^-1. Acidic environment favored the removal of Cr(VI) from soil by using CPS with or without the iron oxides compared to neutral soil and increased it from 87% to 100% because of proton-consuming reactions and electrostatic attraction. Twenty-nine percent of exchangeable and bound-to-carbonates species of chromium declined after the remediation, while 24% bound-to-iron-and-manganese-oxide species increased simultaneously. The findings of the study indicate that natural iron oxides in soils catalyze the reduction of Cr(VI) in soil and facilitate significantly the remediation of Cr(VI)-contaminated soil by using CPS.

Nanotechnology in soil remediation - applications vs. implications

Engineered nanomaterials (ENMs) and nanotechnology have shown great potential in addressing complex problems and creating innovative approaches in soil remediation due to their unique features of high reactivity, selectivity and versatility. Meanwhile, valid concerns exist with regard to their implications towards the terrestrial environment and the ecosystem. This review summarizes: (i) the applications and the corresponding mechanisms of various types of ENMs for soil remediation; (ii) the environmental behavior of ENMs in soils and their interactions with the soil content; (iii) the environmental implications of ENMs during remedial applications. The overall objective is to promote responsible innovations so as to take optimal advantage of ENMs and nanotechnology while minimizing their adverse effects to the ecological system. It is critical to establish sustainable remediation methods that ensure a healthy and safe environment without bringing additional risk.

Remediation of hexavalent chromium in contaminated soil using amorphous iron pyrite: Effect on leachability, bioaccessibility, phytotoxicity and long-term stability

A large amounts of arable land is facing a high risk of hexavalent chromium (Cr(VI)) pollution, which requires remediation using a low toxic agent. In this study, the remediation effect of amorphous iron pyrite (FeS_2(am)) on Cr(VI) in Cr(VI)-contaminated soil was evaluated by systematically analyzing the variation of the leachability, bioaccessibility, phytotoxicity, and long-term stability of the remediated soil. The effectiveness of FeS_2(am) on the leachability was assessed by alkaline digestion and the toxicity characteristic leaching procedure (TCLP); the effect on the bioaccessibility was evaluated via the physiologically based extraction test (PBET) and the Tessier sequential extraction; the effect on the phytotoxicity was assessed via phytotoxicity bioassay (seed germination experiments) based on rape (Brassica napus L.) and cucumber (Cucumis Sativus L.), and the long-term stability of the Cr(VI)-remediated soil was appraised using column tests with groundwater and acid rain as the influents. The results show that FeS_2(am), with a stoichiometry of 4× exhibited a high efficiency in the remediation of Cr(VI) and decreased its leachability and bioaccessibility during the 30-day remediation period. In addition, seed germination rate, accumulation and translocation of Cr, and root and shoot elongation of rape and cucumber of remediated soil are not significantly different from those of clean soil, illustrating that FeS_2(am) is suitable for remediating Cr(VI) contaminated arable soil. The stabilization of Cr(VI) in contaminated soil using FeS_2(am) was maintained for 1575 days. The long-term effectiveness was further confirmed by the increasing amount of free Fe and Mn in the effluent and the decreasing redox potential. In summary, FeS_2(am) has an excellent efficiency for the remediation of Cr(VI), demonstrating it is a very promising alternative for use in the contaminated arable soil.

Remediation of chromium-contaminated soil using delaminated layered double hydroxides with different divalent metals

Soil is commonly polluted by chromium, and layered double hydroxides (LDHs) are widely used for chromium removal due to their strong adsorption capacity and the unique properties of their delaminated products. In this study, delaminated LDHs (S-Mg-LDH and S-Ca-LDH) and their original LDHs were used to remediate Cr(VI)-contaminated soil. A series of characterizations confirmed the successful synthesis of delaminated LDHs whose sheet structure was thinner with a greater surface energy than the original LDHs. The remediation results indicated that delaminated LDHs could more efficiently immobilize Cr(VI) in soil. The immobilization rate of S-Mg-LDH was 64.32%, while Mg-LDH was only 8.09%. However, at low dosages, the efficiency of S-Ca-LDH was 28.1% while Ca-LDH was 5.16%, but they had similar effects at high doses. Moreover, soil pH had little effect on their removal efficiencies. The toxicity characteristic leaching procedure (TCLP) results showed that the leaching of Cr(VI) in soil after treatment with S-Mg-LDH, S-Ca-LDH, Mg-LDH, and Ca-LDH was reduced by 75.43%, 72.43%, 86.55%, and 75.90%, respectively. The phytotoxicity tests of soil treated by S-Mg-LDH and S-Ca-LDH revealed that they effectively reduced the toxicity of chromium and lowered its bioaccumulation. Overall, this study confirms the feasibility of delaminated LDHs for Cr(VI) immobilization in soils.

Influence of redox potential on leaching behavior of a solidified chromium contaminated soil

Cement-based solidification/stabilization (S/S) technology is often used to remediate chromium (Cr) contaminated soils. The valence state and mobility of Cr in soils are closely related with redox potential (E_H). However, Cr mobilization from the solidified soils influenced by E_H has received little attention. In this study, semi-dynamic leaching tests and the toxicity characteristic leaching procedure (TCLP) were performed on a S/S treated Cr contaminated soil under various E_H conditions. The effective diffusion coefficient and leachability index were obtained from the leaching data to investigate the leaching behavior of Cr from the S/S treated soil. Speciation of Cr remained in the sample after the leaching process was obtained through the sequential extraction procedures. The results show that an increase in E_H increases the effective diffusion coefficient of Cr and, therefore, the amount of Cr leached. This result is attributed to immobile Cr(III) being oxidized to highly mobile Cr(VI). The leachability index results indicate that the cement solidification of Cr contaminated soil may not be appropriate under oxidizing conditions. For the TCLP and sequential extraction procedures, the leached amount of Cr exhibits a strong dependence on E_H. As E_H increases, the content of Cr remaining in the soil in unstable phases reduced, and more Cr was released to leachant.

Sustainable Synthesis of Nanoscale Zerovalent Iron Particles for Environmental Remediation

Nanoscale zerovalent iron (nZVI) particles represent an important material for diverse environmental applications because of their exceptional electron-donating properties, which can be exploited for applications such as reduction, catalysis, adsorption, and degradation of a broad range of pollutants. The synthesis and assembly of nZVI by using biological and natural sustainable resources is an attractive option for alleviating environmental contamination worldwide. In this Review, various green synthesis pathways for generating nZVI particles are summarized and compared with conventional chemical and physical methods. In addition to describing the latest environmentally benign methods for the synthesis of nZVI, their properties and interactions with diverse biomolecules are discussed, especially in the context of environmental remediation and catalysis. Future prospects in the field are also considered.

Microstructural analyses of Cr(VI) speciation in chromite ore processing residue from the soda ash process

The microstructure of Chromite Ore Processing Residue (COPR) derived from the soda ash roasting process was investigated prior to and after removal of water exchangeable chromate using a host of microscopy and spectroscopy techniques. Soda ash COPR consists mostly of a magnesioferrite (MgFe₂O₄) matrix that has substantial substitution of trivalent chromium (Cr) for iron. The chromite particles are generally larger than the overall particle size distribution of COPR, containing most of the Cr mass in areas that are greater than 20 μm in diameter; chromite particles are also associated with most of the non-exchangeable hexavalent Cr (Cr(VI)), even though the binding mechanism is not well understood. The remaining non-exchangeable Cr(VI) was found in association with the surrounding Si- and Al-matrix, with spectroscopic evidence of the presence of Cr(VI)-hydrotalcite.

The Immobilization Effect of Natural Mineral Materials on Cr(VI) Remediation in Water and Soil

The effects of sepiolite, montmorillonite, and attapulgite on the removal and immobilization of Cr(VI) in water and soil were studied. X-ray diffraction (XRD) characterizations showed that the purities of these three mineral materials decreased in the following order: montmorillonite > attapulgite > sepiolite, and that their surface molecular bond types were similar. The adsorption potential of Cr(VI) in aqueous solutions of the three mineral materials was in the following order: sepiolite > attapulgite > montmorillonite. The adsorption mechanism for attapulgite was consistent with the Freundlich isotherm adsorption model, whereas that for montmorillonite was more consistent with the Langmuir model. Sepiolite had a good fitting effect for both isothermal adsorption models. For montmorillonite and attapulgite, a lower pH corresponded to a higher removal of Cr(VI). For sepiolite, however, the removal efficiency of Cr(VI) from an aqueous solution was the lowest at a pH of approximately 5.0. The results of the soil toxicity characteristic leaching procedure showed that, following the addition of 15% sepiolite, attapulgite, or montmorillonite to the contaminated soil, Cr(VI) concentrations in the leachates decreased by 16.8%, 18.9%, and 15.9%, respectively, and the total Cr concentrations in the leachates were reduced by 21.2%, 29.2%, and 17.6%. Of the three mineral materials, attapulgite demonstrated the highest Cr(VI) immobilization efficiency in soil. This study emphasizes the effect of attapulgite on the immobilization of Cr(VI) in soil and aqueous solutions, thus providing a theoretical basis for the potential application of natural mineral material remediation of Cr(VI)-contaminated aqueous solutions and soils.

Enhanced Cr(VI) stabilization in soil by carboxymethyl cellulose-stabilized nanosized Fe0 (CMC-nFe0) and mixed anaerobic microorganisms

A collaborative system of carboxymethyl cellulose stabilized nanosized zero-valent iron (CMC-nFe⁰) and microorganisms was set up to enhance the stabilization of Cr(VI) in soil. In comparison with an aqueous-bound Cr(VI) removal of 18.9% in the nFe⁰ system, a higher Cr(VI) removal of 68.9% was achieved in the nFe⁰ and microorganisms system after 14 d remediation because the microorganisms on the nFe⁰ surface promoted nFe⁰ corrosion and enhanced abiotic and biotic Cr(VI) stabilization by generating highly active minerals such as magnetite, lepidocrocite and green rust on the nFe⁰ surface. As a stabilizing agent for nFe⁰ and an organic substrate for microorganisms, CMC on the nFe⁰ surface not only enhanced the dispersion of nFe⁰, but also boosted the activity of microorganisms, resulting in a promotion of 0.9 and 0.5 times higher aqueous-bound Cr(VI) removal via the improvement of nFe⁰ and microorganisms respectively, thus a total 4 times higher aqueous-bound Cr(VI) removal of 95.3% was achieved in the CMC-nFe⁰ and microorganisms system as compared to the nFe⁰ system. After 14 d remediation, easily available species of Cr(VI) and Cr_total, such as water soluble (WS), exchangeable (EX) and bounded to carbonates (CB), were mainly transformed to less available Fe-Mn oxides-bounded (OX) and residual (RS) species because of the production of ferrochrome precipitates (Cr_xFe_1-xOOH or Cr_xFe_1-x(OH)₃). Besides, the stabilization of Cr(VI) in the CMC-nFe⁰ and microorganisms system was pH-dependent and it increased with CMC-nFe⁰ dosage. Due to excellent Cr(VI) stabilization and Cr immobilization, coupled CMC-nFe⁰ and anaerobic microorganisms process is of great potential in remediating Cr(VI)-containing soil.

Neurotoxicity of green- synthesized magnetic iron oxide nanoparticles in different brain areas of wistar rats

AIMS

The aim of the present study was to evaluate the toxicity of magnetic iron oxide nanoparticles (MIONs) which were synthesized using carob leaf extract on various brain areas of Wistar rats.

MAIN METHODS

Carob leaf synthesized-MIONs were characterized using different techniques: Dynamic Light Scattering (DLS), Transmission Electron Microscope (TEM), UV-vis spectrophotometer, Fourier Transform infrared (FTIR), X-Ray Diffraction (XRD) and Atomic Force Microscope (AFM). The toxicity of MIONs in vivo was evaluated by: monitoring rat's body weight, measuring iron content in different brain areas, evaluating some oxidative stress parameters, estimating acetylcholinesterase (AChE) in addition to histopathological investigations.

KEY FINDINGS

The present study demonstrated no body weight changes of MIONs- treated rats. According to the conditions of the present study, the hippocampus and striatum were the most affected areas and demonstrated neuronal degeneration due to MIONs exposure. MIONs treatment of Wistar rats, also affected the iron homeostasis in both striatum and midbrain by decreasing iron content in these areas. The least affected areas were thalamus and cerebellum. The histopathological examination of brain areas demonstrated moderate neuronal degeneration in hippocampus and striatum, mild neuronal degeneration in cortex and slight degeneration in hypothalamus and pons-medulla areas were detected.

SIGNIFICANCE

The results suggested that MIONs have a toxic impact on different brain areas and the effect varies according to the brain area.

An interpretation to Cr(Ⅵ) leaching concentration rebound phenomenon with time in ferrous-reduced Cr(Ⅵ)-bearing solid matrices

Toxicity characteristic leaching procedure (TCLP) is a prevalent way to evaluate the treatment effectiveness for Cr(Ⅵ)-bearing solid matrices (CBSM). But when a certain amount of residual reductants are present in the treated CBSM, Cr(Ⅵ) leaching concentration rebound phenomenon (CLCRP) occurs, which invalidates the TCLP. This study explores the microstructure of ferrous-reduced CBSM and proves that the residual Cr(Ⅵ), Fe_xCr_1-x(OH)₃ precipitate and residual ferrous are separately distributed in a three-layer structure. In natural scenarios, the residual ferrous in the out-layer is firstly flushed away by rainfall and groundwater or oxidized by dissolved oxygen, resulting in the decrease of ferrous with time. Residual Cr(Ⅵ), due to being blocked by precipitate layer, is less flushed away. While in TCLP, all of released residual ferrous and Cr(Ⅵ) are in the leachate and react till one of them is almost exhausted, resulting in the underestimation of Cr(Ⅵ) leaching concentrations. The longer the samples experience the natural scenarios, the less of the residual ferrous, resulting in the decline of underestimation of Cr(Ⅵ) leaching concentrations with time. This study also provides a pretreatment which can effectively reduce the residual ferrous, achieving more accurate Cr(Ⅵ) leaching concentrations and eliminating CLCRP.

Removal of hexavalent chromium from groundwater using sodium alginate dispersed nano zero-valent iron

Hexavalent chromium (Cr), one of the most common heavy metals, is widely found in contaminated soil and groundwater. Nano zero-valent iron (nZVI) is used to treat Cr(VI) in polluted groundwater. However, due to agglomeration, rapid sedimentation, and limited mobility of nanoparticles in the aquatic environment, nZVI is not widely used in groundwater treatment. In this study, we used sodium alginate (SA) to modify nZVI to generate dispersed SA-nZVI. SA-nZVI particles were found to embed in the polymer material and exist as an amorphous state with a diameter less than 100 nm. Compared with traditional nZVI and carboxymethyl cellulose (CMC)-nZVI, SA-nZVI had better stability and higher absolute zeta potential. The presence of SA enhanced mobility of nZVI and effectively prevented sedimentation and aggregation. Furthermore, SA-nZVI had a higher Cr(VI) removal rate than (CMC)-nZVI under both acidic and alkaline conditions. XPS analysis showed that Cr(VI) was reduced to Cr(III) and formed Cr(OH)₃ as precipitates after treatment with SA-nZVI. In addition, NO₃^- had no effect on the final removal rate of Cr(VI) by SA-nZVI. These results suggest that SA-nZVI has high penetration and a high removal rate in Cr(VI) removal and can be used to stabilize nZVI to remediate Cr(VI)-contaminated groundwater in the future.

Iron-based nanoparticles prepared from yerba mate extract. Synthesis, characterization and use on chromium removal

Iron-based nanoparticles were synthesized by a rapid method at room temperature using yerba mate (YM) extracts with FeCl₃ in different proportions. Materials prepared from green tea (GT) extracts were also synthesized for comparison. These materials were thoroughly characterized by chemical analyses, XRD, magnetization, SEM-EDS, TEM-SAED, FTIR, UV-Vis, Raman, Mössbauer and XANES spectroscopies, and BET area analysis. It was concluded that the products are nonmagnetic iron complexes of the components of the extracts. The applicability of the materials for Cr(VI) (300 μM) removal from aqueous solutions at pH 3 using two Cr(VI):Fe molar ratios (MR), 1:3 and 1:0.5, has been tested. At Cr(VI):Fe MR = 1:3, the best YM materials gave complete Cr(VI) removal after two minutes of contact, similar to that obtained with commercial nanoscale zerovalent iron (N25), with dissolved Fe(II), and with a likewise prepared GT material. At a lower Cr(VI):Fe MR (1:0.5), although Cr(VI) removal was not complete after 20 min of reaction, the YM nanoparticles were more efficient than N25, GT nanoparticles and Fe(II) in solution. The results suggest that an optimal Cr(VI):Fe MR ratio could be reached when using the new YM nanoparticles, able to achieve a complete Cr(VI) reduction, and leaving very low Cr and Fe concentrations in the treated solutions. The rapid preparation of the nanoparticles would allow their use in removal of pollutants in soils and groundwater by direct injection of the mixture of precursors.

Reduction and immobilization of hexavalent chromium in chromite ore processing residue using amorphous FeS2

In this work, a series of long-term treatment trials were conducted to evaluate the remediation performance of amorphous iron pyrite (FeS₂₍am₎) toward hexavalent chromium (Cr(VI)) in chromite ore processing residue (COPR). The effectiveness of FeS₂₍am₎ was assessed using alkaline digestion, the synthetic precipitation leaching procedure (SPLP) and the physiologically based extraction test (PBET). Reaction mechanisms were explored by monitoring the changes in the solid pH, redox potential (Eh), the chemical states of relevant elements as well as the crystal forms present in COPR. The results showed that, using a proper dosage, the total content of Cr and Cr(VI) in the leachate from treated COPR met the Chinese standard regulatory limits for the extraction toxicity of hazardous wastes (GB 5085.3-2007). In addition, the in vitro bioaccessibility of Cr(VI) in COPR was also significantly reduced. Moreover, the remediation effect was maintained for the subsequent six months. This long-term effect was attributed to the presence of reductive sulfur (S) species and Fe(II) that remained even after 180 days of treatment, which were identified using high-resolution X-ray photoelectron spectroscopy (HR-XPS) and X-ray diffraction (XRD). To maximize the effect of FeS₂₍am₎, 5% lime was employed as an additive to adjust the pH and thus contributed to Cr(VI) reduction and immobilization. To remediate COPR within 30 days, the use of 5% lime and a specific FeS₂₍am₎ dosage (FeS₂₍am₎:Cr(VI) = 1.25:1) is recommended based on the results. For rapid remediation (< 1 day), the use of 5% lime and a 2.5:1 FeS₂₍am₎:Cr(VI) ratio is recommended. In comparison with widely studied reductants, FeS₂₍am₎ showed excellent efficiency for the remediation of COPR over both short- and long-term treatment trials, demonstrating it is a very promising alternative treatment method.

Chromium Removal with Environmentally Friendly Iron Nanoparticles in a Pilot Scale Study

Nano zero valent iron (nZVI) is highly reactive, targets a wide range of contaminants and is considered as a promising material for the in situ remediation of contaminated aquifers. Injection of nZVI suspension is a remedial option for the in-situ reduction and immobilization of pollutants in aquifers. The goal of the present study was to evaluate the efficiency of iron nanoparticles (GT-nZVI), synthesised by a green synthesis route, for Cr(VI) removal in a pilot scale application. A tank with dimensions 2.5 m (width) × 3.75 m (length) × 1.5 m (height) was constructed and filled with 24 tons of soil. Firstly 3 m³ GT-nZVI suspension was injected and then the supply of contaminated water containing 5 mg/L Cr(VI) was initiated. Samples were analysed for pH, redox potential and electric conductivity, as well as Fe_tot and Cr(VI). The reactive zone operated efficiently for the treatment of contaminated water for 1 year.

Leachability and Stability of Hexavalent-Chromium-Contaminated Soil Stabilized by Ferrous Sulfate and Calcium Polysulfide

Ferrous sulfate (FeSO4) and calcium polysulfide (CaS5) stabilization are practical approaches to stabilizing hexavalent chromium (Cr(VI))-contaminated soil. The leachability and stability of Cr(VI) and Cr are important factors affecting the effectiveness of stabilized Cr(VI)-contaminated soil. This study compared the leachability and stability of Cr(VI) and Cr in Cr(VI)-contaminated soil stabilized by using FeSO4 and CaS5. The contaminated soil was characterized before and after stabilization, and the effectiveness of FeSO4 and CaS5 stabilization was assessed using leaching, bioaccessibility, alkaline digestion, sequential extraction, and X-ray diffraction tests. Results showed that FeSO4 and CaS5 significantly reduced the leachability and Cr(VI) content in the contaminated soil. The acid-buffering capacity and stability (leachability, bioaccessibility, speciation distribution, and mineral composition) of the Cr(VI)/Cr and Cr(VI) content of CaS5 were better than those of FeSO4. This study demonstrated that CaS5 had a better effect than FeSO4 on the stabilization of Cr(VI) in Cr(VI)-contaminated soil. The CaS5 significantly enhanced the stabilization and immobilization of Cr(VI) and reduced its leachability and toxicity.

Iron and Iron Oxide Nanoparticles Synthesized Using Green Tea Extract: Improved Ecotoxicological Profile and Ability to Degrade Malachite Green

In recent years, iron-based nanoparticles (FeNPs) have been successfully used in environmental remediation and water treatment. This study examined ecotoxicity of two FeNPs produced by green tea extract (smGT, GTFe) and their ability to degrade malachite green (MG). Their physicochemical properties were assessed using transmission electron microscopy, X-ray powder diffraction, dynamic light scattering, and transmission Mössbauer spectroscopy. Using a battery of ecotoxicological bioassays, we determined toxicity for nine different organisms, including bacteria, cyanobacterium, algae, plants, and crustaceans. GTFe, amorphous complex of Fe(II, III) ions and polyphenols from green tea extract, proved low capacity to degrade MG and was toxic to all tested organisms. Superparamagnetic iron oxide NPs (smGT) derived from GTFe, showed no toxic effect on most of the tested organisms up to a concentration of 1g/L, except for algae and cyanobacterium and removed 93 % MG at concentration 125 mg Fe/L after 60 minutes. The procedure described in this paper generates new superparamagnetic iron oxide NPs from existing and toxic GTFe, which are nontoxic and has degradative potential for organic compounds. These findings suggest low ecotoxicological risks and suitability of this green-synthesized FeNPs for environmental remediation purposes.

Reductive decolorization of azo dyes via in situ generation of green tea extract-iron chelate

In this study, rapid decolorization of azo dyes was achieved by in situ-generated green tea extract-iron (GTE-Fe) chelate for the first time. When changing reaction conditions from the aerobic condition to the anaerobic condition, the decolorization efficiencies of two azo dyes, i.e., acid orange 7 (AO7) and acid black 1 (AB1), increased from 46.38 and 83.17 to 90.13 and 95.37%, respectively. The recalcitrant AO7 was then selected as the targeting pollutant in subsequent optimization and mechanism studies. Experimental evidences showed that the initial concentrations of AO7, Fe(III), and GTE are the key factors to optimize the decolorization efficiency. Further characterization studies by spectroscopic analysis, including FESEM, FTIR, and XPS, suggested that the major mechanism of AO7 decolorization is the nucleophilic attack of the oxygen in green tea polyphenols (GTP), and this attack could be facilitated by the organometal chelation. This study provided an efficient and environmental friendly strategy to decolorize azo dyes via in situ generation of the GTE-Fe chelate, as well as its mechanistic insights, shedding lights on in situ remediation of azo dye pollution. Graphical abstract ᅟ.

Evaluation of mercury stabilization mechanisms by sulfurized biochars determined using X-ray absorption spectroscopy

The application of biochar to treat mercury (Hg) in the environment is being proposed on an increasing basis due to its widespread availability and cost effectiveness. However, the efficiency of Hg removal by biochars is variable due to differences in source material composition. In this study, a series of batch tests were conducted to evaluate the effectiveness of sulfurized biochars (calcium polysulfide and a dimercapto-related compound, respectively) for Hg removal; Hg-loaded biochars were then characterized using synchrotron-based techniques. Concentrations of Hg decreased by >99.5% in solutions containing the sulfurized biochars. Sulfur X-ray absorption near-edge structure (XANES) analyses indicate a polysulfur-like structure in polysulfide-sulfurized biochar and a thiol-like structure (shifted compared to dimercapto) in the dimercapto-sulfurized biochar. Micro-X-ray fluorescence (μ-XRF) mapping and confocal X-ray micro-fluorescence imaging (CXMFI) analyses indicate Hg is distributed primarily on the edges of sulfurized biochar and throughout unmodified biochar particles. Hg extended X-ray absorption fine structure (EXAFS) analyses show Hg in enriched areas is bound to chlorine (Cl) in the unmodified biochar and to S in sulfurized biochars. These results indicate that Hg removal efficiency is enhanced after sulfurization through the formation of strong bonds (Hg-S) with S-functional groups in the sulfurized biochars.