Highly efficient immobilization of NZVI onto bio-inspired reagents functionalized polyacrylonitrile membrane for Cr(VI) reduction

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.

Electrospun TiO2-GO/PAN-CA nanofiber mats: A novel material for remediation of organic contaminants and nitrophenol reduction

The outstanding properties of nanofiber composites have made them a popular choice for various structural applications. Recently, there has been a growing interest in using electrospun nanofibers as reinforcement agents, which possess exceptional properties that can enhance the performance of these composites. Herein, TiO2-graphene oxide (GO) nanocomposite incorporated into polyacrylonitrile (PAN)/cellulose acetate (CA) nanofibers were fabricated by an effortless electrospinning technique. The chemical and structural characteristics of the resulting electrospun TiO2-GO nanofibers were examined employing diverse techniques such as XRD, FTIR, XPS, TGA, mechanical properties, and FESEM. Remediation of organic contaminants and organic transformation reactions with electrospun TiO2-GO nanofibers were performed. The results indicated that the incorporation of TiO2-GO with various TiO2/GO ratios did not affect the molecular structure of PAN-CA. Still, they did significantly increase the mean fiber diameter (234-467 nm) and the mechanical properties of the nanofibers comprising UTS, elongation, Young's modulus, and toughness compared to PAN-CA. From various ratios of TiO2/GO (0.01TiO2/0.005GO and 0.005TiO2/0.01GO) in the electrospun NFs, the nanofiber containing a high content of TiO2 showed over 97% of the initial MB dyes were degraded after 120 min of visible light exposure and the same nanofibers also, achieved 96% nitrophenol conversion to aminophenol in just 10 min with activity factor kAF value of 47.7 g-1min-1. These findings illustrate the promise of TiO2-GO/PAN-CA nanofibers for use in various structural applications, particularly in the remediation of organic contaminants from water and organic transformation reactions.

Reactive membranes for groundwater remediation of chlorinated aliphatic hydrocarbons: competitive dechlorination and cost aspects

A nanocomposite membrane incorporating reactive Pd-Fe nanoparticles (NPs) was developed to remediate chlorinated aliphatic hydrocarbons (CAHs) from groundwater. Other than recapturing the produced Fen+ for in-situ regeneration, the functionalized polyanions prevented NPs agglomeration and resulting in a spherical Fe0 core (55 nm, O/Fe = 0.05) and an oxidized shell (4 nm, O/Fe = 1.38). The reactive membranes degraded 92% of target CAHs with a residence time of 1.7 seconds. After long-term treatment and regeneration, reusability was confirmed through recovered reactivity, recurrence of Fe0 in X-ray photoelectron spectroscopy, and >96% remaining of Fe and Pd. The total cost (adjusted present value for 20 years) was estimated to be 13.9% lower than the granular activated carbon system, following an EPA work breakdown structure-based cost model. However, non-target CAHs from groundwater can compete for active sites, leading to decreased surface-area normalized dechlorination rate ( k sa ) by 28.2-79.9%. A hybrid nanofiltration (NF)/reactive membrane was proposed to selectively intercept larger competitors, leading to 54% increased dechlorination efficiency and 1.3 to 1.9-fold enlarged k sa . Overall, the practical viability of the developed reactive membranes was demonstrated by the stability, reusability, and cost advantages, while the optional NF strategy could alleviate competitive degradation towards complex water chemistry.

Selective removal of Cr(VI) by tannic acid and polyethyleneimine modified zero-valent iron particles with air stability

In this study, a new composite adsorbent for Cr(VI) removal was developed by immobilizing polyethyleneimine (PEI) on the surface of zero-valent iron (ZVI) particles with tannic acid (TA) as a stabilizer. The adsorbent (denoted as Fe-TA-PEI-10) was easy to prepare and regenerate, requiring no conditions for storage. It was found to be particularly effective for Cr(VI) removal from wastewater via reduction and adsorption. Electrochemical analysis revealed that TA significantly reduced the electron transfer resistance of Fe-TA-PEI-10 and reduced the highly toxic Cr(VI)to the less toxic Cr(III). In addition, PEI endowed amino groups to Fe-TA-PEI-10, raising the zero charge point (pH_pzc) of Fe-TA-PEI-10 (pH_pzc= 7.80), allowing it to adsorb Cr(VI) from the solution rapidly under electrostatic forces and chelating effects. The adsorption process was consistent with the pseudo-first-order model (R² >0.99) and the Langmuir isotherm model (R² >0.99), and the maximum adsorption capacity could reach 161.6 mg/g. In particular, this study presented for the first time that TA-modified Fe(0) had excellent stability in the air, and the adsorbent showed no decrease in performance for Cr(VI) removal even after exposure to the air for 30 days. When tested with a simulated electroplating rinsing wastewater, the Fe-TA-PEI-10 showed very high selectivity for Cr(VI) removal. The mechanism of Cr(VI) removal with Fe-TA-PEI-10 was found to be based on adsorption and reduction. This work provided a new scheme for developing efficient and long-lasting reactive adsorbent for Cr(VI) removal.

Abiotic degradation behavior of polyacrylonitrile-based material filled with a composite of TiO2 and g-C3N4 under solar illumination

As some of the most promising alternatives to traditional non-degradable materials, photodegradable materials have advantages of environmental benignity and rapid degradation under simple conditions. In this work, nontoxic TiO₂ and cost-effective g-C₃N₄ have been compounded in a weight of 9:1 to form a photocatalytic additive with high activity. A 25 wt% loading of this photocatalytic additive has been incorporated into the polyacrylonitrile (PAN) by centrifugal electrospinning to prepare an abiotic degradable PAN material. Our results showed that the PAN chain could be almost fully degraded within 90 h in an aqueous medium under simulated sunlight in the absence of microorganisms. Product analysis implied that degradation of the PAN chain mainly involved the breaking of -CN and C-C bonds by radicals, followed by oxidation of terminal groups to carboxyl and gradual mineralization to CO₂ and H₂O. This design strategy may provide new insight for the production and degradation mechanism of photodegradable polymer.

Zerovalent Iron Nanoparticles-Alginate Nanocomposites for Cr(VI) Removal in Water—Influence of Temperature, pH, Dissolved Oxygen, Matrix, and nZVI Surface Composition

The immobilization of zerovalent iron nanoparticles (nZVI) is a way to facilitate their use in continuous flow systems for the treatment of aqueous pollutants. In this work, two types of nZVI (powdered, NSTAR; and slurry suspended, N25) were immobilized in millimetric alginate beads (AL) by coagulation, forming nanocomposites (NCs). These NCs, N25@AL and NSTAR@AL, were structurally studied and tested for Cr(VI) removal. For both NCs types, SEM analysis showed a uniform distribution of the nanoparticles in micron-scale agglomerates, and XRD analysis revealed the preservation of α-Fe as the main iron phase of the immobilized nanoparticles. Additionally, Raman spectroscopy results evidenced a partial oxidation of the initially present magnetite. For both nZVI types, the Cr(VI) removal efficiency increased with temperature, decreased with pH, and did not show any significant change in anoxic or oxic conditions. On the other hand, N25@AL resulted a faster removal agent than NSTAR@AL; however, both materials had the same maximum removal capacity: 133 mg of Cr(VI) per gram of nZVI at pH 3. Cr(III) formed during the removal of Cr(VI) was retained by the alginate matrix, constituting a clear advantage against the use of free nZVI in suspension at acidic pH.

Mussel-Inspired Fabrication of PDA@PAN Electrospun Nanofibrous Membrane for Oil-in-Water Emulsion Separation

Emulsified oily wastewater threatens human health seriously, and traditional technologies are unable to separate emulsion containing small sized oil droplets. Currently, oil–water emulsions are usually separated by special wettability membranes, and researchers are devoted to developing membranes with excellent antifouling performance and high permeability. Herein, a novel, simple and low-cost method has been proposed for the separation of emulsion containing surfactants. Polyacrylonitrile (PAN) nanofibers were prepared via electrospinning and then coated by polydopamine (PDA) by using self-polymerization reactions in aqueous solutions. The morphology, structure and oil-in-water emulsion separation properties of the as-prepared PDA@PAN nanofibrous membrane were tested. The results show that PDA@PAN nanofibrous membrane has superhydrophilicity and almost no adhesion to crude oil in water, which exhibits excellent oil–water separation ability. The permeability and separation efficiency of n-hexane/water emulsion are up to 1570 Lm⁻² h⁻¹ bar⁻¹ and 96.1%, respectively. Furthermore, after 10 cycles of separation, the permeability and separation efficiency values do not decrease significantly, indicating its good recycling performance. This research develops a new method for preparing oil–water separation membrane, which can be used for efficient oil-in-water emulsion separation.

Cu-Fe embedded cross-linked 3D hydrogel for enhanced reductive removal of Cr(VI): Characterization, performance, and mechanisms

Porous hydrogel, as a high-efficiency adsorbent for heavy metals, suffers the drawbacks of the use of expensive and toxic reagents during the process of preparation, further limiting its application ranges. Besides, the heavy metals couldn't be transformed into nontoxic species, which leads to the environmental pollution risk. Herein, a three-dimensionally (3D) structured Cu-Fe embedded cross-linked cellulose hydrogel (nFeCu-CH) was innovatively fabricated by a novel self-assembly and in-situ reduction method, which exhibited exceptionally enhanced adsorption-reduction property towards Cr(VI) wastewater. The results of degradation experiment exhibited that the removal reaction followed Langmuir-Hinshelwood first order kinetic model and the degradation rate constant decreased with solution pH and initial Cr(VI) concentration, while increased with nFeCu-CH dosage and temperature. Regeneration studies demonstrated that more than 88% of Cr(VI) was removed by nFeCu-CH even after five times of cycling. nFeCu-CH exhibited excellent reductive activity, which had a close connection with the superiority of 3D crosslinked architectures and bimetallic synergistic effect. And 97.1% of Cr(VI) could be removed when nFeCu-CH dosage was 9.5 g/L, pH was 5, initial concentration of Cr(VI) was 20 mg/L and temperature was 303 K. Combined with cellulose hydrogel not only could provide additional active sites, but also could restrain the crystallite growth and agglomeration of nano-metallic particles, leading to the promotion of Cr(VI) removal. In addition, coating with Cu facilitated the generation and transformation of electrons according to the continuous redox cycles of Fe(III)/Fe(II) and Cu(II)/Cu(I), leading to the further improvement of the reductivity of nFeCu-CH. Multiple interaction mechanisms including adsorption, reduction and co-precipitation between nFeCu-CH and Cr(VI) were realized. The current work suggested that nFeCu-CH with highly reactive sites, excellent stability and recyclability was considered as an potential material for remediation of Cr(VI) contaminated wastewater.

Immobilization of Ni (Ⅱ) at three levels of contaminated soil by rhamnolipids modified nano zero valent iron (RL@nZVI): Effects and mechanisms

A kind of biosurfactant rhamnolipid modified zero-valent iron nanoparticles have been synthesized and applied to evaluate the immobilization efficiency of Ni (Ⅱ) contaminated soil at three concentration levels (200Ni, 600Ni and 1800Ni). The results of SEM and XRD were clearly indicative of the well-attached phenomenon of rhamnolipid on the nZVI, featuring better stability and dispersity, and FTIR analysis proposed the interactions between rhamnolipid and nZVI through monodentate chelating between carboxylate groups and nZVI or hydrogen bonding with Fe-O groups on the surface. Sequential extraction procedures (SEP) analysis illustrated that the majority of labile fractions had been transformed into less accessible fractions (Fe-Mn oxide-bound fractions and residual fractions) after 28 days of incubation. And for low-concentrations polluted soil, soil self-remediation played a dominant role, while RL@nZVI exhibited a more significant stabilizing effect for medium and high-concentrations pollution. Furthermore, XPS and XRD analyses of Ni-adsorbed RL@nZVI identified the formation of NiO, Ni(OH)₂ and revealed the possible interaction mechanisms including reduction, adsorption and precipitation/co-precipitation. These results confirmed that RL@nZVI presented a promising prospect for the immobilization of Ni polluted soil.

Efficient Sequestration of Hexavalent Chromium by Graphene-Based Nanoscale Zero-Valent Iron Composite Coupled with Ultrasonic Pretreatment

Nanoscale zero-valent iron (nZVI) has attracted considerable attention for its potential to sequestrate and immobilize heavy metals such as Cr(VI) from an aqueous solution. However, nZVI can be easily oxidized and agglomerate, which strongly affects the removal efficiency. In this study, graphene-based nZVI (nZVI/rGO) composites coupled with ultrasonic (US) pretreatment were studied to solve the above problems and conduct the experiments of Cr(VI) removal from an aqueous solution. SEM-EDS, BET, XRD, and XPS were performed to analyze the morphology and structures of the composites. The findings showed that the removal efficiency of Cr(VI) in 30 min was increased from 45.84% on nZVI to 78.01% on nZVI/rGO and the removal process performed coupled with ultrasonic pretreatment could greatly shorten the reaction time to 15 min. Influencing factors such as the initial pH, temperature, initial Cr(VI) concentration, and co-existing anions were studied. The results showed that the initial pH was a principal factor. The presence of HPO₄²⁻, NO₃⁻, and Cl⁻ had a strong inhibitory effect on this process, while the presence of SO₄²⁻ promoted the reactivity of nZVI/rGO. Combined with the above results, the process of Cr(VI) removal in US-nZVI/rGO system consisted of two phases: (1) The initial stage is dominated by solution reaction. Cr(VI) was reduced in the solution by Fe²⁺ caused by ultrasonic cavitation. (2) In the following processes, adsorption, reduction, and coprecipitation coexisted. The addition of rGO enhanced electron transportability weakened the influence of passivation layers and improved the dispersion of nZVI particles. Ultrasonic cavitation caused pores and corrosion at the passivation layers and fresh Fe⁰ core was exposed, which improved the reactivity of the composites.

Green synthesis of hydrophilic activated carbon supported sulfide nZVI for enhanced Pb(II) scavenging from water: Characterization, kinetics, isotherms and mechanisms

For green synthesis of nZVI with low aggregation and high antioxidation, green tea extracts were explored as reductant during the synthesis with modification by hydrophilic porous activated carbon (HPAC) and sulfidation technology. Characterization results identified the effective preparation of porous activated carbon (PAC) with microporous and mesoporous characteristics, and the successful loading of S-nZVI nanoparticles on S-nZVI@HPAC. Moreover, HPAC was identified to have a higher degree of hydrophilicity surface compared to PAC, while the S-nZVI with an atomic ratio of S/Fe (0.16) further improved the hydrophilic performance of S-nZVI@HPAC. Batch adsorption revealed that the S-nZVI@HPAC possessed a pH-dependent adsorption performance with a fast kinetic equilibrium within 120 min and an outstanding Pb(II) binding of 295.30 mg/g at pH = 5.0 and 50 °C. Thermodynamic results exhibited positive ΔH° and ΔS°, clearly indicative of the endothermic property of Pb(II) uptake onto S-nZVI@HPAC with an increase in randomness, while the negative ΔG° uncovered a favorable and spontaneous process. Furthermore, the S-nZVI@HPAC was believed to enhance the Pb(II) uptake via the synergistic effects of electrostatic attraction, chemical precipitation, complexation and reduction. The results of this work highlighted the hydrophilic porous activated carbon supported sulfide nZVI for efficient remediation of Pb(II) contaminated water.

Simultaneous adsorption and reduction of hexavalent chromium on biochar-supported nanoscale zero-valent iron (nZVI) in aqueous solution

Flax straw biochar (FSBC)-supported nanoscale zero-valent iron (nZVI) composite (nZVI-FSBC) combining the advantages of nZVI and biochar was synthesized and tested for Cr(VI) removal efficiency from aqueous solution. Surface morphology and structure of FSBC and nZVI-FSBC were characterized by scanning electron microscopy, transmission electron microscopy, thermogravimetric analysis, X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and Brunauer-Emmett-Teller techniques, which help to clarify the mechanism of Cr(VI) removal from aqueous solution. The adsorption of Cr(VI) onto FSBC and nZVI-FSBC was best described by the pseudo-second-order and the Sips model. Compared with FSBC, nZVI-FSBC remarkably improved the performance in removing Cr(VI) under identical experimental conditions. Due to the collaborative effect of adsorption and reduction of nZVI-FSBC, the adsorption capacity of nZVI-FSBC for Cr(VI) is up to 186.99 mg/g. The results obtained by XPS, XRD, and FTIR confirmed that adsorption and reduction dominated the processes of Cr(VI) removal by nZVI-FSBC. As a supporter, FSBC not only improved the dispersion of nZVI, but also undertook the adsorption task of Cr(VI) removal. The surface oxygen-containing functional groups of nZVI-FSBC mainly participated in the adsorption part, and the nZVI promoted the Cr(VI) removal through the redox reactions. These observations indicated that the nZVI-FSBC can be considered as potential adsorbents to remove Cr(VI) for environment remediation.

Remediation of Cr(VI)-Contaminated Soil by Nano-Zero-Valent Iron in Combination with Biochar or Humic Acid and the Consequences for Plant Performance

Nano-scale zero-valent iron (nZVI) is among the most common nanoparticles widely used for the treatment of various environmental contaminants. However, little is known about the combined effects of nano-zero-valent iron (nZVI) and other soil amendments on soil remediation and plant performance. For the first time, we studied the remediation of Cr(VI)-contaminated soil using bare nZVI (B-nZVI) and starch-supported nZVI (S-nZVI) in combination with either biochar (BC) or humic acid (HA), and the consequent effects on plant growth and Cr accumulation. Both S-nZVI and B-nZVI decreased the contents of Cr(VI) and available Cr in soil, but increased available Fe content, with S-nZVI generally showing more pronounced effects at a higher dose (1000 mg/kg). B-nZVI exerted no inhibition and even stimulation on plant growth, but 1000 mg/kg S-nZVI produced significant phytotoxicity, resulting in decreased plant growth, low chlorophyll content in leaves, and excessive accumulation of Fe in roots. Each nZVI decreased shoot and root Cr concentrations. BC and HA produced synergistic effects with nZVI on Cr(VI) removal from soil, but HA decreased soil pH and increased the availability of Cr and Fe, implying a potential environmental risk. Addition of BC or HA did not alter the effects of either nZVI on plant growth. In conclusion, combined application of 100 mg/kg nZVI and BC could be an ideal strategy for the remediation of soil contaminated with Cr(VI), whereas high-dose S-nZVI and HA are not recommended in the remediation of agricultural soils for crop production or in the phytostabilization of Cr(VI).

Polyethylene glycol-stabilized nano zero-valent iron supported by biochar for highly efficient removal of Cr(VI)

In this study, polyethylene glycol (PEG)-stabilized nano zero-valent iron (nZVI) supported by biochar (BC) (PEG-nZVI@BC) was prepared to remedy Cr(VI) with high efficiency. The morphology, functional groups, and crystalline structure of PEG-nZVI@BC composites were characterized, revealing that when PEG was added, a large number of -OH functional groups were introduced, and nZVI was effectively dispersed on the BC surface with a smaller particle size. The results of Cr(VI) remediation experiments showed Cr(VI) removal rate by PEG-nZVI@BC (97.38%) was much greater than that by BC-loaded nZVI (nZVI@BC) (51.73%). The pseudo second-order and Sips isotherm models provide the best simulation for Cr(VI) removal experimental data, respectively. The main remediation mechanism of Cr(VI) was reduction and co-precipitation of Cr-containing metal deposits onto PEG-nZVI@BC. Ecotoxicity assessment revealed PEG-nZVI@BC (1.00 g/L) has little influence on rice germination and growth, but resisted the toxicity of Cr(VI) to rice. The modified Community Bureau of Reference (BCR) sequential extraction showed pyrolysis could increase the percentage of oxidizable and residual Cr and diminish the environmental risk of Cr release from post-removal composites.

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

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