Highly active nanoscale zero-valent iron (nZVI)-Fe3O4 nanocomposites for the removal of chromium(VI) from aqueous solutions

Controllable chemical redox reactions to couple microbial degradation for organic contaminated sites remediation: A review

Global environmental concern over organic contaminated sites has been progressively conspicuous during the process of urbanization and industrial restructuring. While traditional physical or chemical remediation technologies may significantly destroy the soil structure and function, coupling moderate chemical degradation with microbial remediation becomes a potential way for the green, economic, and efficient remediation of contaminated sites. Hence, this work systematically elucidates why and how to couple chemical technology with microbial remediation, mainly focused on the controllable redox reactions of organic contaminants. The rational design of materials structure, selective generation of reactive oxygen species, and estimation of degradation pathway are described for chemical oxidation. Meanwhile, current progress on efficient and selective reductions of organic contaminants (i.e., dechlorination, defluorination, -NO2 reduction) is introduced. Combined with the microbial remediation of contaminated sites, several consideration factors of how to couple chemical and microbial remediation are proposed based on both fundamental and practical points of view. This review will advance the understanding and development of chemical-microbial coupled remediation for organic contaminated sites.

Reaction Thermodynamics of Zerovalent Iron and Hexavalent Chromium at the Solid-Liquid Interface and in Solution

Zerovalent iron (Fe0) is a promising candidate for remediating hexavalent chromium (Cr(VI)) via adsorption and (or) reduction. Herein, the reaction between Fe0 and Cr(VI) at the solid-liquid interface and in solution under varying pHs was inspected using the methodology of equilibrium thermodynamics. First, species distribution functions of aqueous Cr(VI), Cr(III), Fe(III), and Fe(II) are deduced to illuminate the quantitative distribution of aqueous metal species. Second, the plausible reaction at pH = 0-14 either at the solid-liquid interface or in solution is determined according to the species distribution function. Third, the spontaneity of each reaction is evaluated via a thermodynamic calculation based on the van't Hoff equation. The results present the following. (1) At the solid-liquid interface, the redox reaction 2Cr(VI) + 3Fe0 → 2Cr(III) + 3Fe(II) is spontaneous, inducing complete Cr(VI) → Cr(III) reduction at pH = 0-14. Especially, the high spontaneity of the redox reaction is mainly ascribed to Fe0 oxidation, which serves as a highly spontaneous subreaction. (2) In solution, the redox reaction Cr(VI) + 3Fe(II) → Cr(III) + 3Fe(III) is nonspontaneous at pH = 6 and 7, whereas it is spontaneous at pH = 6-7, 0-5, and 8-14. Accordingly, no Cr(VI) → Cr(III) reduction at pH = 6-7 and complete Cr(VI) → Cr(III) reduction at pH = 0-5 and 8-14 are expected. Particularly, the nonspontaneity of the Cr(VI) reduction at pH = 6-7 is majorly attributed to water ionization, which is involved as a highly nonspontaneous subreaction. On the contrary, the spontaneity of the Cr(VI) reduction at pH = 0-5 and 8-14 is mainly owing to acid-base neutralization, which is involved as a highly spontaneous subreaction. This work may deepen our knowledge about the chemistry involved in hexavalent chromium remediation by the zerovalent iron.

Synergistic effect of novel pyrite/N-doped reduced graphene oxide composite with heterojunction structure for enhanced photo-assisted reduction of Cr(VI) in oxic water: Specific role of molecular oxygen

To avoid severe aggregation and synergistically utilize the intrinsic and photocatalytic reducibility, pyrite (FeS2) was loaded onto N-doped reduced graphene oxides (N-rGO) to fabricate a novel FeS2/N-rGO heterojunction catalyst for enhanced chromium (Cr(VI)) reduction in oxic condition to simultaneously investigate the specific effect and role of dissolved oxygen (DO). Characterization results showed that strong interaction and combination of FeS2 and N-rGO not only achieved the uniform distribution of FeS2, but also increased the defects, and exposed more functional groups. Meanwhile, the Type II heterojunction was formed in FeS2/N-rGO, which facilitated the separation efficiency of photo-generated carriers and electrons, endowing FeS2/N-rGO a superior photocatalytic activity. Cr(VI) was almost completely reduced via FeS2/N-rGO within 60 min under irradiation (Cr(VI) = 10 mg/L, dosage = 0.2 g/L), 3 times that of pristine FeS2 (18.7 %). Trapping and Electron Spin Resonance (ESR) experiments indicated that photo-generated e- and derived O2- species from photoactivation of dioxygen (DO) were the key reactive species for the enhancement of photo-assisted Cr(VI) reduction, rather than reductive Fe2+ and S22- species. Although the photocatalysis of FeS2/N-rGO cannot directly generate hydroxyl radicals (OH), the oxidative OH ascribed to superoxide radicals (O2-), photo-induced holes and free DO preferentially consumed by Fe2+ and S22- with stronger reducibility. Hence, as compared to the anoxic condition, the reduction rate of Cr(VI) was slightly decreased, but still could be totally removed within 60 min in the oxic conditions. Due to the excessive amount of FeS2/N-rGO, Cr(III) after reduction would not be influenced by oxidative species and maintain stability under oxic condition. This study provided a facile modification strategy for FeS2 based composites and uncovered its working mechanism for Cr(VI) decontamination.

Cost-effective core@shell structured zero-valent iron nanoparticles @ magnetic (nZVI@Fe3O4) for Cr(vi) removal from aqueous solutions: preparation by disproportionation of Fe(ii)

Nanoscale zero-valent iron (nZVI) and its composites are known for their excellent ability to remove Cr(vi), but their preparation can be expensive due to the reduction processes. This study presents a cost-effective method to prepare core@shell structured nZVI@Fe3O4 nanocomposites using a novel Fe(ii) disproportionation reaction. The nZVI@Fe3O4 was thoroughly characterized using various techniques, including FESEM, HRTEM, EDS, XPS, XRD, FTIR, and VSM. Batch experiments were performed to evaluate the removal efficiency of nZVI@Fe3O4 in eliminating Cr(vi) ions from aqueous solutions, while classical models were employed to investigate the influencing factors associated with the removal process. The results showed that a 0.7 mg per ml NaOH solution reacted with Fe(ii) at 150 °C for 0.5 h could be used to prepare nZVI@Fe3O4 composites efficiently and inexpensively. nZVI@Fe3O4 was able to remove more than 99% of Cr(vi) from both simulated Cr(vi) solutions and real electroplating wastewater, and the recovery and preparation could be easily performed using external magnets to separate it from the solution. At pH 6.0, the maximum adsorption capacity (qmax) for Cr(vi) reached 58.67 mg g-1. The reaction mechanism was discussed from the perspective of electron transfer. Overall, the results suggest that nZVI@Fe3O4, an efficient adsorbent prepared using an environmentally friendly and inexpensive Fe(ii) disproportionation reaction, is a promising option for the treatment of Cr(vi) from industrial wastewater and other contaminated water sources.

Tin Oxide Nanoparticles via Solar Vapor Deposition for Hexavalent Chromium Remediation

Tin oxide nanoparticles optimized to capture low concentrations of hexavalent chromium from water were developed through a facile, scalable, and low-cost one-step solar vapor deposition methodology. Considering the preservation of high electron donation capacity as the key to support the reduction of mobile Cr(VI) into insoluble forms, the growth of SnO nanoparticles was favored by the co-evaporation of SnO2 with Fe powders at various mass ratios. Characterization techniques indicated that the percentage and the stability of SnO is proportional to the Fe content in the target with a requirement of at least 50% wt to inhibit the formation of a passive SnO2 surface layer. The produced particles were evaluated regarding their efficiency to capture Cr(VI) under conditions similar to water treatment for drinking purposes (pH 7). It was revealed that passivation-free SnO nanoparticles deliver significant improvement in the adsorption capacity corresponding to the residual concentration of 25 μg/L, reaching a value of 1.74 mg/g for the sample prepared with 50% wt Fe in the target. The increase of water acidity was found responsible for the activation of more reduction sites on the particle surface, as reflected through the elevation of efficiency by more than 20% at pH 6.

Lipid production and cellular changes in Fremyella diplosiphon exposed to nanoscale zerovalent iron nanoparticles and ampicillin

With the dramatic decrease in fossil fuel stocks and their detrimental effects on the environment, renewable energy sources have gained imminent importance in the mitigation of emissions. As lipid-enriched energy stocks, cyanobacteria are the leading group of microorganisms contributing to the advent of a new energy era. In the present study, the impact of Nanofer 25 s nanoscale zero-valent iron nanoparticles (nZVIs) and ampicillin on lipid production and cellular structural changes in Fremyella diplosiphon strain B481-SD were investigated. Total lipid abundance, fatty acid methyl ester (FAME) compositions, and alkene production as detected by high-resolution two-dimensional gas chromatography with time-of-flight mass spectrometry (GC × GC/TOF-MS) was significantly higher (p < 0.05) in the individual application of 0.8 mg/L ampicillin, 3.2 mg/L nZVIs, and a combined regimen of 0.8 mg/L ampicillin and 3.2 mg/L nZVIs compared to the untreated control. In addition, we identified significant increases (p < 0.05) in monounsaturated fatty acids (MUFAs) in F. diplosiphon treated with the combination regimen compared to the untreated control, 0.8 mg/L of ampicillin, and 3.2 mg/L of nZVIs. Furthermore, individual treatment with 0.8 mg/L ampicillin and the combination regimen (0.8 mg/L ampicillin + 3.2 mg/L nZVIs) significantly increased (p < 0.05) Nile red fluorescence compared to the untreated control, indicating neutral membrane lipids to be the main target of ampicillin added treatments. Transmission electron microscopy studies revealed the presence of single-layered thylakoid membranes in the untreated control, while complex stacked membranes of 5-8 layers were visualized in ampicillin and nZVI-treated F. diplosiphon. Our results indicate that nZVIs in combination with ampicillin significantly enhanced total lipids, essential FAMEs, and alkenes in F. diplosiphon. These findings offer a promising approach to augment the potential of using the strain as a large-scale biofuel agent.

A review of novel green adsorbents as a sustainable alternative for the remediation of chromium (VI) from water environments

The presence of heavy metal, chromium (VI), in water environments leads to various diseases in humans, such as cancer, lung tumors, and allergies. This review comparatively examines the use of several adsorbents, such as biosorbents, activated carbon, nanocomposites, and polyaniline (PANI), in terms of the operational parameters (initial chromium (VI) concentration (C_o), temperature (T), pH, contact time (t), and adsorbent dosage) to achieve the Langmuir's maximum adsorption capacity (q_m) for chromium (VI) adsorption. The study finds that the use of biosorbents (fruit bio-composite, fungus, leave, and oak bark char), activated carbons (HCl-treated dry fruit waste, polyethyleneimine (PEI) and potassium hydroxide (KOH) PEI-KOH alkali-treated rice waste-derived biochar, and KOH/hydrochloric acid (HCl) acid/base-treated commercial), iron-based nanocomposites, magnetic manganese-multiwalled carbon nanotubes nanocomposites, copper-based nanocomposites, graphene oxide functionalized amino acid, and PANI functionalized transition metal are effective in achieving high Langmuir's maximum adsorption capacity (q_m) for chromium (VI) adsorption, and that operational parameters such as initial concentration, temperature, pH, contact time, and adsorbent dosage significantly affect the Langmuir's maximum adsorption capacity (q_m). Magnetic graphene oxide functionalized amino acid showed the highest experimental and pseudo-second-order kinetic model equilibrium adsorption capacities. The iron oxide functionalized calcium carbonate (IO@CaCO₃) nanocomposites showed the highest heterogeneous adsorption capacity. Additionally, Syzygium cumini bark biosorbent is highly effective in treating tannery industrial wastewater with high levels of chromium (VI).

Highly efficient removal of hexavalent chromium by magnetic Fe-C composite from reed straw and electric furnace dust waste

Reed straw and electric furnace dust (EFD) waste were used to prepare magnetic Fe-C composite (EFD&C) by co-precipitation and high-temperature activation method to remove Cr(VI) from water. The magnetic EFD&C owned a large specific surface (536.61 m²/g) and a porous structure (micropores and mesopores), and had an efficient removal capacity for Cr(VI). Under conditions of pH (2), the addition amount of EFD&C (1 g/L), the adsorption time (760 min), and the temperature (45 °C), the maximum adsorption capacity reached 111.94 mg/g. The adsorption mechanism mainly attributed to chemical adsorption (redox), Cr(VI) reduced to Cr(III) by Fe(II) and Fe(0) (from Fe₃O₄ and Fe components in EFD) and surface functional groups of -OH, C = C, C-C and O-C = O (from biochar), and secondary attributed to physical adsorption, Cr(VI) and Cr(III) (from reduced Cr(VI)) adsorbed into the porous structure of EFD&C. This study provided a feasible solution for the preparation of adsorbents for adsorbing heavy metals from iron-containing metallurgical solid waste and biomass waste, which contributed to reducing the environmental pollution and lowering the cost of adsorbent preparation.

Facile one-pot synthesis of polyethyleneimine functionalized α-FeOOH nanoraft consisted of single-layer parallel-aligned ultrathin nanowires for efficient removal of Cr (VI): Synergy of reduction and adsorption

Two-dimensional (2D) iron oxide-hydroxide (FeOOH) nanomaterials as low-cost and environmental-friendly composites are promising materials for application in heavy metal elimination. However, developing 2D FeOOH adsorbents with high adsorption capacity and excellent durability toward Cr (VI) removal is still a challenge due to the intrinsically non-layered structure. Here, a novel polyethyleneimine (PEI) functionalized 2D single-layer nano-raft-like α-FeOOH (α-FeOOH NF) consisted of parallel-aligned ultrathin nanowires was obtained via a facile one-pot hydrothermal approach. It was found that the 2D α-FeOOH NF nanostructure was formed by an in-plane iterative self-assembly mechanism, where α-FeOOH nanoparticles acted as intermediates and iterative seeds with anisotropic growth. The as-prepared 2D α-FeOOH NF possessed porous structure and high surface area, which provided a strong ability to capture the Cr (VI) ions in water. Benefiting from the unique structure and PEI modification, it exhibited fast adsorption kinetic rate, high reusability, and high adsorption capacity toward Cr(VI) removal. The removal mechanism involved adsorption and reduction process. Besides, the molecular dynamic simulations disclosed a facet-dependent Cr(VI) adsorption behavior of α-FeOOH. The maximum adsorption capacity was 67.1 mg/g and the removal efficiency still maintained 83.9 % in the fifth cycle. This work demonstrated that 2D α-FeOOH NF could be a promising adsorbent for Cr(VI) removal.

Insights into enhanced removal of Cd2+ from aqueous solutions by attapulgite supported sulfide-modified nanoscale zero-valent iron

The sulfidation of nanoscale zerovalent iron (nZVI) has received increasing attention for reducing the oxidizability of nZVI and improving its reactivity toward heavy metal ions. Here, a sulfide (S)-modified attapulgite (ATP)-supported nanoscale nZVI composite (S-nZVI@ATP) was rapidly synthesized under acidic conditions and used to alleviate Cd²⁺ toxicity from an aqueous solution. The degree of oxidation of S-nZVI@ATP was less than that of nZVI@ATP, indicating that the sulfide modification significantly reduced the oxidation of nZVI. The optimal loading ratio was at an S-to-Fe molar ratio of 0.75, and the adsorption performance of S-nZVI@ATP for Cd²⁺ was significantly improved compared with that of nZVI@ATP. The removal of Cd²⁺ by S-nZVI@ATP was 100% when the adsorbent addition was 1 g/L, the solution was 30 mL, and the adsorption was performed at 25 °C for 24 h with an initial Cd²⁺ concentration of 100 mg/L. Kinetics studies showed that the adsorption process of Cd followed the pseudo-second-order model, indicating that chemisorption was the dominant adsorption mechanism. The adsorption of Cd²⁺ by S-nZVI @ATP is dominated by the complexation between the iron oxide or iron hydroxide shell of S-nZVI and Cd²⁺ and the formation of Cd(OH)₂ and CdS precipitates.

Chromium toxicity and its remediation by using endophytic bacteria and nanomaterials: A review

Chromium (Cr) is a crucial element for all life forms. Various anthropogenic activities have been responsible for environmental contamination with Cr (VI) in recent years. For this review, articles were collected using electronic databases such as Web of Science, Pubmed, ProQuest, and Google Scholar as per the guidelines of PRISMA-2015, applying the Boolean search methods. Chromium can cause severe health complications in humans and animals and threatens the surrounding environment, with negative impacts on crop yield, development, and quality. Hence, monitoring Cr contamination is essential, and various remediation technologies have emerged in the past 50 years to reduce the amount of Cr in the environment. This review focuses on chromium exposure and the associated environmental health risks. We also reviewed sustainable remediation processes, with emphasis on nanoparticle and endophytic remediation processes.

Structural elucidation of hexavalent Cr adsorbed on surfaces and bulks of Fe3O4 and α-FeOOH

Magnetite (Fe3O4) and goethite (α-FeOOH) were synthesized via a hydrothermal approach and utilized as adsorbents for Cr6+ removal in an aqueous medium. The typical crystal structures of the synthesized Fe3O4 and α-FeOOH were confirmed by XRD and TEM. Fe3O4 in a spherical shape with a surface area of 32 m2 g-1 was established. While α-FeOOH had a rod-like form with a larger surface area of 84 m2 g-1. Cr6+ removal in an aqueous solution was studied in various conditions to evaluate thermodynamic and kinetic parameters. The adsorption isotherms on both adsorbents fit the Langmuir model indicating monolayer adsorption. Fe3O4 showed a better adsorption ability than α-FeOOH even though it had a lower surface area. XAS and XPS analysis strongly evidenced the production of stable Cr3+ species of Fe(1-x)Cr x OOH and Fe(3-x)Cr x O4 by Cr6+ reduction and migration processes into the bulk structure. Thus, the existence of stable Cr-species in Fe3O4 structure strongly affected Cr-adsorption ability rather than the surface area of the adsorbent. However, the precipitated Cr2O3 and HCrO4 - molecules electrostatically adsorbed on the outer surface of α-FeOOH without bulk transformation. The presence of physisorbed FeO-HCrO4 species on α-FeOOH led to low reducibility and adsorption capability of Cr6+.

Bacterial capture and inactivation in sand filtration systems with addition of zero-valent iron as permeable layer under both slow and fast filtration conditions

To improve bacterial capture performance and inactivate bacteria, zero-valent iron (ZVI) were added into sand columns as permeable filtration media. Both Gram-negative Escherichia coli and Gram-positive Bacillus subtilis (1.25 ×10⁷ cells/mL) could be completely retained in 10 wt% ZVI amended sand columns in different ionic strength solutions (1-100 mM NaCl) at both slow (4 m/day) and fast (90 m/day) flow velocities. The strong adsorption property of ZVI contributed to the improved bacterial capture performance of sand columns. Moreover, ZVI could inactivate nearly all captured bacteria. Clearly, ZVI added as permeable layer not only could significantly enhance bacterial capture but also would inactivate the captured bacteria. ZVI could destroy the structure of extracellular polymeric substance and cell membrane. Intracellular oxidative stress was then increased and ATP content was decreased, causing bacterial death. Furthermore, high bacterial capture efficiencies were achieved with the coexisting of humic acid (0.2-5 mg/L), in actual river water samples, and longtime filtration processes. ZVI could be regenerated and reused as permeable layer to efficiently capture bacteria. Furthermore, sand columns with 10 wt% ZVI amendment could completely capture and inactivate 4.0 × 10⁶ cells/mL algae. Clearly, ZVI amended sand filtration systems have potentials to purify water contaminated by pathogenic bacteria and algae.

Integrated green complexing agent and biochar modified nano zero-valent iron for hexavalent chromium removal: A characterisation and performance study

In this study, nano zero-valent iron (nZVI) was loaded on biochar (BC) prepared from recycled waste peanut shells. The loaded BC in the nZVI@BC composite was assumed to weaken the agglomeration of nZVI and the environmentally-friendly complexing agents sodium citrate (Cit) and sodium carboxymethyl cellulose (CMC) were used to establish Cit-nZVI@BC and CMC-nZVI@BC for the effective removal of Cr(VI) from aqueous environments. The characterisation results suggested that Cit and CMC not only inhibited the oxidation of nZVI, but also effectively improved its reactivity. The experimental results demonstrated that the Cr(VI) removal efficiency by nZVI was less than 20%, while CMC-nZVI@BC enhanced the Cr(VI) removal efficiency to 80.73%, because CMC was coated on the nZVI surface for anti-passivation and improved the surface activity of nanoparticles. In addition, the Cr(VI) removal efficiency reached almost 100% with Cit-nZVI@BC, and the citrate dissociated the passivation layer on the surface of the zero-valent iron particles to ensure the reactivity of the zero-valent iron. The reaction mechanism of Cit-nZVI@BC includes adsorption, reduction, and co-precipitation, whereas CMC-nZVI@BC also involves surface complexation reactions. The kinetic studies revealed that the removal of Cr(VI) by Cit-nZVI@BC and CMC-nZVI@BC followed the second-order reaction kinetic model, and the reaction rates of Cit-nZVI@BC and CMC-nZVI@BC were both higher than that of nZVI. The results indicate that the prepared systems are promising for Cr(VI) remediation in contaminated environments.

Tuning the Fe(II)/hydroxide Ratio during Synthesis of Magnetite Nanoparticles to Maximize Cr(VI) Uptake Capacity

The impact of hydroxyl excess as defined by the Fe(II)/hydroxide ratio during the synthesis of Fe3O4 nanoparticles by oxidative precipitation of FeSO4 was examined as a critical parameter determining the potential for Cr(VI) uptake from polluted water. Various samples were prepared by varying the OH− excess in the range of −0.10 up to +0.03 M and characterized according to their composition, morphology, and surface configuration. Their efficiency for Cr(VI) removal was evaluated by batch adsorption tests, carried out under similar conditions with drinking water purification in the concentration range below 10 mg/L. Results indicate that near the zero-excess point for hydroxyl balance, the uptake capacity for residual Cr(VI) concentration equal to 25 μg/L remains at very low levels (<0.5 mg/g). However, a small increase above +0.02 M features synthesized nanoparticles with an uptake capacity of 2.5 mg/g owed to the decrease in particles size (28 nm) and enhancement of the reducing potential (Fe2+/Fe3+ = 0.42). In addition, utilizing negative excess values below −0.05 M triggers a similar efficiency rise, although the morphology of the obtained aggregates is rather different. Such finding is attributed to a possible exchange mechanism between adsorbed sulfates and chromate anions that assist approach of Cr(VI) to the material’s surface. Overall, proper tuning of hydroxyl excess offers multiple options for the implementation of monodisperse magnetically responsive nanoparticles or larger aggregates with optimized purification efficiency in water technology.

Use of a Hybrid Porous Carbon Material Derived from Expired Polysaccharides Snack/Iron Salt Exhibiting Magnetic Properties, for Hexavalent Chromium Removal

Nowadays, the scientific interest is focused more and more on the development of new strategies in recycling of waste products as well as on the development of clean technologies due to the increased environmental pollution. In this work we studied the valorization of an expired cheese-tomato flavor corn snack, which is polysaccharide food product, by producing advanced hybrid magnetic materials for environmental remediation purposes. The carbonization-chemical activation of this snack using potassium hydroxide leads to a microporous activated carbon with high surface area (SgBET ~800 m2/g). The magnetic hybrid material was synthesized via an in-situ technique using iron acetate complex as the precursor to produce iron based magnetic nanoparticles. The resulting material retains a fraction of the microporous structure with surface area SgBET ~500 m2/g. Such material consists, of homogenously dispersed magnetic isolated zero valent iron nanoparticles and of iron carbides (Fe3C), into the carbon matrix. The magnetic carbon exhibited high adsorption capacity in Cr(VI) removal applications following a pseudosecond order kinetic model. The maximum adsorption capacity was 88.382 mgCr(VI)/gAC at pH = 3. Finally, oxidation experiments, in combination with FT-IR, Mössbauer, and VSM measurements indicated that the possible Cr6+ removal mechanism involves oxidation of iron phases and reduction of Cr6+ to Cr3+.

Sustainable and efficient technologies for removal and recovery of toxic and valuable metals from wastewater: Recent progress, challenges, and future perspectives

Due to their numerous effects on human health and the natural environment, water contamination with heavy metals and metalloids, caused by their extensive use in various technologies and industrial applications, continues to be a huge ecological issue that needs to be urgently tackled. Additionally, within the circular economy management framework, the recovery and recycling of metals-based waste as high value-added products (VAPs) is of great interest, owing to their high cost and the continuous depletion of their reserves and natural sources. This paper reviews the state-of-the-art technologies developed for the removal and recovery of metal pollutants from wastewater by providing an in-depth understanding of their remediation mechanisms, while analyzing and critically discussing the recent key advances regarding these treatment methods, their practical implementation and integration, as well as evaluating their advantages and remaining limitations. Herein, various treatment techniques are covered, including adsorption, reduction/oxidation, ion exchange, membrane separation technologies, solvents extraction, chemical precipitation/co-precipitation, coagulation-flocculation, flotation, and bioremediation. A particular emphasis is placed on full recovery of the captured metal pollutants in various reusable forms as metal-based VAPs, mainly as solid precipitates, which is a powerful tool that offers substantial enhancement of the remediation processes' sustainability and cost-effectiveness. At the end, we have identified some prospective research directions for future work on this topic, while presenting some recommendations that can promote sustainability and economic feasibility of the existing treatment technologies.

Recent developments in alginate-based adsorbents for removing phosphate ions from wastewater: a review

The huge development of the industrial sector has resulted in the release of large quantities of phosphate anions which adversely affect the environment, human health, and aquatic ecosystems. Naturally occurring biopolymers have attracted considerable attention as efficient adsorbents for phosphate anions due to their biocompatibility, biodegradability, environmentally-friendly nature, low-cost production, availability in nature, and ease of modification. Amongst them, alginate-based adsorbents are considered one of the most effective adsorbents for removing various types of pollutants from industrial wastewater. The presence of active COOH and OH- groups along the alginate backbone facilitate its physical and chemical modifications and participate in various possible adsorption mechanisms of phosphate anions. Herein, we focus our attention on presenting a comprehensive overview of recent advances in phosphate removal by alginate-based adsorbents. Modification of alginate by various materials, including clays, magnetic materials, layered double hydroxides, carbon materials, and multivalent metals, is addressed. The adsorption potentials of these modified forms for removing phosphate anions, in addition to their adsorption mechanisms are clearly discussed. It is concluded that ion exchange, complexation, precipitation, Lewis acid-base interaction and electrostatic interaction are the most common adsorption mechanisms of phosphate removal by alginate-based adsorbents. Pseudo-2nd order and Freundlich isotherms were figured out to be the major kinetic and isotherm models for the removal process of phosphate. The research findings revealed that some issues, including the high cost of production, leaching, and low efficiency of recyclability of alginate-based adsorbents still need to be resolved. Future trends that could inspire further studies to find the best solutions for removing phosphate anions from aquatic systems are also elaborated, such as the synthesis of magnetic-based alginate and various-shaped alginate nanocomposites that are capable of preventing the leaching of the active materials.

Ni-Fe/Reduced Graphene Oxide Nanocomposites for Hexavalent Chromium Reduction in an Aqueous Environment

We designed and synthesized a novel high efficiency Cr(VI) removal material using reduced graphene oxide (RGO) as a support with high specific surface area and a mixture of Fe and Ni nanoparticles (NPs) as a catalytic reducing agent. Such a design enables the composite particle to be integrated with three functions of adsorption, catalysis, and reduction, where RGO could enhance Cr(VI) adsorption, while Fe/Ni NPs increase the catalytic reducing efficiency. The application of a microchip mixer guaranteed a better mixing of GO and subsequent decoration of Fe and Ni NPs on RGO. Cr(VI) removal experiments with various materials are performed, and the results demonstrated that the Ni-Fe/RGO achieved an adsorption capacity of 150.45 mg/g at pH = 7 and 197.43 mg/g at pH = 5 for Cr(VI), which is higher than those of other reported materials at a pH of ∼7. To the best of our knowledge, this is the first example of Ni-Fe/RGO for efficient Cr(VI) removal by using the synergistic effects of increased adsorption, catalysis-assisted reduction, and enhanced mixing effect of a microchip mixer. This work also provides us with a simple and low-cost method for the fabrication of an effective Cr(VI) removal material.

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.

Interfacial engineering of CuFeS2 quantum dots via platinum decoration with enhanced Cr(VI) reduction dynamics under UV-Vis-NIR radiation

Configuring reactive and stable catalytic interfaces is crucial to design efficient photocatalysts for Cr(VI) reduction. Herein, via the platinum decoration approach based on interfacial engineering, we developed an effective catalytic interface within novel semiconducting chalcopyrite quantum dots (Pt/CuFeS₂ QDs). Benefiting from the catalytic merits of the Pt modulated interfacial structure and electronic structure, Pt/CuFeS₂ QDs show a broader light absorption capability extending to near-infrared radiation (NIR) range with superior carriers separation performance and faster charge transfer efficiency, which delivers a three-folder faster photocatalytic Cr(VI) reduction efficiency comparing to the original CuFeS₂ QDs. Density functional theory (DFT) calculations unravel that Pt atoms prefer to be anchored with the surface S atoms to form a stable interfacial structure with faster electron transfer and Cr(VI) reduction dynamics. This work demonstrates that platinum decoration based on interfacial engineering is an effective strategy to simultaneously modulate the band structure and accelerate the interfacial reaction dynamics for semiconductor photocatalysts, which paves the way for designing highly efficient photocatalysts for light-driven environmental and energy engineering applications.

One-step pyrolysis for preparation of sulfur-doped biochar loaded with iron nanoparticles as an effective peroxymonosulfate activator for RhB degradation

In this study, sulfur-doped biochar loaded with iron nanoparticles (Fe/S-BC) was easily prepared by a one-pot pyrolysis method using anhydrous FeCl3, Na2S2O3·5H2O, and cherry stone powder as raw materials and...

Nanoscale zero-valent iron particles supported on sludge-based biochar for the removal of chromium (VI) from aqueous system

Biochar (BC) obtained by the co-pyrolysis of municipal sewage sludge (MSS) and sunflower seed shells (SSS) was utilized to support nanoscale zero-valent iron particles (nZVI) for the synthesis of a composite material (nZVI-BC) for Cr(VI) removal from aqueous systems. A series of characterization methods confirmed successful immobilization of nZVI on the surface of biochar with no aggregation. Batch experiments showed that the initial pH, initial Cr(VI) concentration, and nZVI-BC dose all significantly affected the Cr(VI) removal using nZVI-BC. The kinetics for Cr(VI) removal via nZVI-BC could be better explained by the pseudo-second-order (PSO) adsorption model. Adsorption isotherms analysis demonstrated the superior Cr(VI) removal capability of nZVI-BC in comparison to bare nZVI and BC. nZVI-BC can be reused after the regeneration process by applying 0.1 M H2SO4 and 0.1 M NaBH4 solutions. The reaction mechanism for Cr(VI) removal might involve its chemical reduction on the nZVI-BC surface. Overall, environmentally friendly nZVI-BC was highly efficient in Cr(VI) removal from aqueous systems.

Confining polyacrylic acid on the surface of nanoscale zero-valent iron by aluminum hydroxide for in-situ anti-passivation

Aggregation and surface passivation of nanoscale zero-valent iron (NZVI) particles have limited their reactivity and application for environmental remediation. In this study, an aluminum hydroxide/polyacrylic acid (Al(OH)₃/PAA) hybrid shell was homogeneously coated on the NZVI surface to overcome the limitations. PAA molecules were confined onto the NZVI surface by hydration of Al(III) cations. The Al(OH)₃/PAA coating shell provided more electrostatic repulsion forces between NZVI particles to hinder the particle aggregation and preserve the NZVI reactivity. On the other hand, the surface-anchored PAA provided a thickened reactive layer for Cr(VI) reduction. Besides, XPS and TEM results showed that the surface carboxylic groups bound produced Cr(III) and Fe(III) cations and inhibited the precipitation of hydroxides on the NZVI surface. The reduced passivation layer increased the longevity of NZVI for surface reactions. As a result, the 24-h Cr(VI) reduction capacity of NZVI particles was improved from 49.4 to 92.6 mg/g with a 2 wt% (Al/Fe) Al(OH)₃/PAA coating shell. Overall, this study presented a promising strategy to effectively tune the surface properties of nanoparticles and improve the feasibility of NZVI for environmental remediation.

Development of copper-iron bimetallic nanoparticle impregnated activated carbon derived from coconut husk and its efficacy as a novel adsorbent toward the removal of chromium (VI) from aqueous solution

The present work demonstrates the preparation of biodegradable coconut husk derived activated carbon (CAC) impregnated with bimetallic nanoparticles consisting of zerovalent iron and copper to produce a new adsorbent (Fe⁰ /Cu-CAC). The new adsorbent was further employed to determine the removal efficiency of Chromium (VI) from aqueous solutions. Chromium (VI) adsorption process by Fe⁰ /Cu-CAC was found to be favorable at pH ~ 3, attaining 95.28% removal efficiency while its counterpart CAC attained only ~41%. Negative ΔG⁰ value suggests that the process was thermodynamically spontaneous and ΔH⁰ was observed to be 8.496 kJ/mol, further corroborating the endothermic nature of the process. Pseudo-second order model was best suited to explain the kinetics of the process with R² value of 0.99683 and an error of 6.73%. Equilibrium parameter (R_L ) derived from Langmuir isotherm was calculated to be 0.1103, indicating favorable adsorption and thus Langmuir isotherm can be used to describe the process of adsorption of Cr (VI) by Fe⁰ /Cu-CAC. Finally, a maximum monolayer adsorption capacity of 173.9 mg/g indicated the suitability of the prepared adsorbent in treating chromium contaminated wastewater streams. PRACTITIONER POINTS: Facile development of novel adsorbent from biodegradable coconut husk via cost effective route. Utilization of the adsorbent towards removal of toxic pollutant like Cr (VI) from its aqueous solution. Validation of the effective adsorption mechanism by Kinetic and Thermodynamic study.

Waste foundry dust (WFD) as a reactive material for removing As(III) and Cr(VI) from aqueous solutions

This study evaluates the use of waste foundry dust (WFD) as a reactive material for mitigating water pollution using As(III) and Cr(VI) as model contaminants. A detailed structural characterization of WFD was performed using X-ray diffraction (XRD), X-ray fluorescence (XRF), scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX), Fourier-transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS). Batch removal experiments and kinetic studies for removal of both As(III) and Cr(VI) were conducted at various initial pH values (2-10), concentrations (1-100 mg/L), and solid-to-liquid ratios (2.5-125 g/L). The results show that WFD consisted of small particles (< 30 µm) with magnetic properties, mainly composed of quartz (SiO2) and magnetite (Fe3O4). The maximum removal capacity of WFD was 12.6 mg/g for As(III) at pH 3.0 and 6.1 mg/g for Cr(VI) at pH 5.0. WFD was effective in a wide pH range, from 3.0 to 8.0, and in high concentrations, up to 100 mg/L. WFD removed As(III) and Cr(VI) from aqueous solutions through complex processes including adsorption, precipitation, and redox reactions by oxidation of Fe(II). The results of this study suggest that WFD can be used as a reactive material for removal of As(III) and Cr(VI) from aqueous solutions.

Advanced Simulation of Removing Chromium from a Synthetic Wastewater by Rhamnolipidic Bioflotation Using Hybrid Neural Networks with Metaheuristic Algorithms

This work aims at presenting an advanced simulation approach for a novel rhamnolipidic-based bioflotation process to remove chromium from wastewater. For this purpose, the significance of key influential operating variables including initial solution pH (2, 4, 6, 8, 10 and 12), rhamnolipid to chromium ratio (RL:Cr = 0.010, 0.025, 0.050, 0.075 and 0.100), reductant (Fe) to chromium ratio (Fe:Cr of 0.5, 1.0, 1.5, 2.0, 2.5, 3.0), and air flowrate (50, 100, 150, 200 and 250 mL/min) were investigated and evaluated using Analysis of Variance (ANOVA) method. The RL as both collector and frother was produced using a pure strain of Pseudomonas aeruginosa MA01 under specific conditions. The bioflotation tests were carried out within a bubbly regimed column cell with the dimensions of 60 × 5.70 × 0.1 cm. Four optimization techniques based on Artificial Neural Network (ANN) including Cuckoo, genetic, firefly and biogeography-based optimization algorithms were applied to 113 experiments to identify the optimum values of studied factors. The ANOVA results revealed that all four variables influence the bioflotation performance through a non-linear trend. Their influences, except for aeration rate, were found statistically significant (p-value < 0.05), and all parameters followed the normal distribution according to Anderson-Darlin (AD) criterion. Maximum chromium removal of about 98% was achieved at pH of 6, rhamnolipid to chromium ratio of 0.05, air flowrate of 150 mL/min, and Fe to Cr ratio of 1.0. Flotation kinetics study indicated that chromium bioflotation follows the first-order kinetic model with a rate of 0.023 sec-1. According to the statistical assessment of the model accuracy, the firefly algorithm (FFA) with a structure of 4-9-1 yielded the highest level of reliability with the mean squared, root mean squared, percentage errors and correlation coefficient values of test-data of 0.0038, 0.0617, 3.08% and 96.92%, respectively. These values were evidences of the consistency of the well-structured ANN method to simulate the process.

One-step synthesis of mixed valence FeOX nanoparticles supported on biomass activated carbon for degradation of bisphenol A by activating peroxydisulfate

A novel FeO_X nanoparticles supported biomass activated carbon (BAC/FeO_X) composite was prepared through one-pot calcination method with FeCl₃ and cherry stone powder as precursors. The carbonization of biomass, reduction of Fe³⁺, and FeO_X anchored on carbon substrate could be achieved at the same time. Characterization with transmission electron microscope (TEM) and scanning electron microscope indicated that nanoscale FeO_X distributed uniformly on carbon substrate, and X-ray photoelectron spectroscopy, X-ray diffraction, and high resolution TEM characterization proved that the loaded FeO_X was high crystallinity of Fe₃O₄ and α-Fe⁰. Bisphenol A (BPA) was used to investigate the degradation performance of BAC/FeO_X activating peroxydisulfate (PDS). The ratio of raw materials affected degradation efficiency of BPA intensively through the content, valence state, and dispersibility of FeO_X nanoparticles, and the optimal material could degrade 20 mg/L BPA completely in 5 min at 0.1 g/L in the presence of 1 g/L PDS. Free radical determination and quenching experiments indicated that both SO₄^•- and •OH were involved in BPA degradation. The degradation pathway was proposed based on the identification of degradation intermediates. The facile synthesis method, high activation efficiency, and low-cost and environmental friendly raw materials made the BAC/FeO_X-50 an alternative catalyst for organic pollution water treatment.

Chromium contamination and effect on environmental health and its remediation: A sustainable approaches

Chromium (Cr) is a trace element critical to human health and well-being. In the last few decades, its contamination, especially hexavalent chromium [Cr(VI)] form in both terrestrial and aquatic ecosystems, has amplified as a result of various anthropogenic activities. Chromium pollution is a significant environmental threat, severely impacting our environment and natural resources, especially water and soil. Excessive exposure could lead to higher levels of accumulation in human and animal tissues, leading to toxic and detrimental health effects. Several studies have shown that chromium is a toxic element that negatively affects plant metabolic activities, hampering crop growth and yield and reducing vegetable and grain quality. Thus, it must be monitored in water, soil, and crop production system. Various useful and practical remediation technologies have been emerging in regulating chromium in water, soil, and other resources. A sustainable remediation approach must be adopted to balance the environment and nature.

Highly efficient adsorption and immobilization of U(VI) from aqueous solution by alkalized MXene-supported nanoscale zero-valent iron

A novel composite of zero-valent iron nanoparticles supported on alkalized Ti₃C₂T_x nanoflakes (nZVI/Alk-Ti₃C₂T_x) was constructed by an in-situ growth method for simultaneous adsorption and reduction U(VI) from aqueous solution in anoxic conditions. The effect of various factors such as adsorbent dose, pH, ionic strength, contact time, initial U(VI) concentration and environmental media were comprehensively investigated by batch experiments. Benefiting from the good dispersion uniformity of nZVI on MXene substrates, nZVI/Alk-Ti₃C₂T_x exhibited rapid removal kinetics, excellent selectivity, 100% removal efficiency and up to 1315 mg g^-1 uptake capacity for U(VI) capture. In the presence of mimic groundwater, 1.0 mM NaHCO₃ and 10 mg L^-1 humic acid, the removal percentages of U(VI) by the composites could reach 95.1%, 88.9% and 69.5%, respectively. The reaction mechanism between U(VI) and nZVI/Alk-Ti₃C₂T_x has been clarified based on FTIR, XANES, XPS and XRD analysis. Depending on the consumption of reactive nZVI in the composites and the solution pH, the elimination of U(VI) could be realized by different pathways including reductive immobilization in the form of UO₂, inner-sphere surface complexation and hydrolysis precipitation. The present study illustrates that the nZVI/Alk-Ti₃C₂T_x composite may be an efficient scavenger for radioactive wastewater purification in environmental remediation.

Ultrasound-assisted catalytic reduction of Cr(VI) by an acid mine drainage based nZVI coupling with FeS2 system from aqueous solutions: Performance and mechanism

Nowadays, nanoscale zero valent iron (nZVI) has been extensively applied for the decontamination of various pollutants, but passivation of nZVI severely affects its reactivity in use. In this study, ultrasound (US)-assisted catalytic reduction of Cr(VI) by an acid mine drainage based nZVI (AMD-nZVI) coupling with FeS₂ system was systematically examined. Results show that the presence of FeS₂ and US induced a synergistic enhancement of Cr(VI) removal by AMD-nZVI. Nearly 98% of Cr(VI) removal was achieved by AMD-nZVI/FeS₂/US process within 60 min under optimal reaction conditions. Several coexisting substances with lower concentration including Pb(II), Ni(II), bisphenol A (BPA) and 2,4-diclorophenol (2,4-DCP) could be effectively removed in simultaneous manner with Cr(VI) removal. The inhibitory order of water matrix species on Cr(VI) removal was NO₃^- > PO₄^3- > HCO₃^- > Ca²⁺ > Mg²⁺ > Cl^-, and a serious suppression effect was induced by humic acid (HA). Addition of ethylene diamine tetra-acetic acid (EDTA) and citric acid (CA) could enhance Cr(VI) removal rate. An enhanced reaction mechanism was proposed, which involved the regeneration of more Fe²⁺ and H⁺ by AMD-nZVI/FeS₂/US process, leading to the reduction of Cr(VI) by AMD-nZVI and FeS₂ into Cr(III) species inculding Cr₂O₃ and Cr(OH)₃. This study well demonstrates that AMD-nZVI/FeS₂/US process is considered as a potential candidate for the remediation of Cr(VI) in real wasterwater.

Applications of Nanomaterials for Heavy Metal Removal from Water and Soil: A Review

Heavy metals are toxic and non-biodegradable environmental contaminants that seriously threaten human health. The remediation of heavy metal-contaminated water and soil is an urgent issue from both environmental and biological points of view. Recently, nanomaterials with excellent adsorption capacities, great chemical reactivity, active atomicity, and environmentally friendly performance have attracted widespread interest as potential adsorbents for heavy metal removal. This review first introduces the application of nanomaterials for removing heavy metal ions from the environment. Then, the environmental factors affecting the adsorption of nanomaterials, their toxicity, and environmental risks are discussed. Finally, the challenges and opportunities of applying nanomaterials in environmental remediation are discussed, which can provide perspectives for future in-depth studies and applications.

Effects of humic acid on enhanced removal of lead ions by polystyrene-supported nano-Fe (0) nanocomposite

Polymer-supported nanozero-valent iron composites (D001-nZVI) were fabricated for the removal of lead ions from aqueous solutions by embedding nZVI into the porous polystyrene anion exchanger D001. Humic acid (HA) was selected as a model species because of its ubiquitous existence to gain insight into the influencing factors in the actual application process. The iron contents of the composites were approximately 11.2%, and the smallest ZVI particle size was ~ 5 nm. The experimental results showed that the effect of HA on the reduction of lead ions by D001-nZVI was a concentration-dependent process. At low HA concentrations, the surface-competitive adsorption of HA and Pb²⁺ dominated; therefore, the removal efficiency of Pb²⁺ by D001-nZVI decreased from 97.5 to 90.2% with an increasing HA concentration. When the HA concentration increased to 30 mg/L or more, the lead ions removal remained constant with the following possible cooperation mechanism: the competitive adsorption of HA and Pb²⁺ on the nZVI surface and the well-dispersed particles were caused by electrostatic interactions between the HA coating and the nZVI surface. In addition, the adsorption complexation between HA and Pb²⁺ also had a positive effect on the removal of Pb²⁺ at higher concentrations of HA.

Insight into the Mechanism of Arsenic(III/V) Uptake on Mesoporous Zerovalent Iron-Magnetite Nanocomposites: Adsorption and Microscopic Studies

Mesoporous zerovalent iron-magnetite nanocomposites (ZVI-MNCs) were developed to circumvent the limitations of magnetite, such as its susceptibility to phase transition in air-water interfaces. High-resolution transmission electron microscopy images revealed the presence of Fe⁰ and Fe₃O₄ in the as-prepared adsorbent. High-resolution X-ray photoelectron spectroscopy (HR-XPS) Fe 2p deconvoluted spectra showed that electron transfer between Fe⁰ and Fe₃O₄ controlled the magnetite transformation. The isotherm equilibrium data for As(III) and As(V) are described by the Sips model, which suggests single- and multilayer formation onto a heterogeneous surface with different binding sites, whereas adsorption is controlled by a pseudo-second-order kinetic model, which indicates chemisorption. The maximum sorption capacities (q_m) for As(III) and As(V) are 632.6 and 1000 μmol g^-1, respectively, which are larger than the q_m of similar adsorbents. The greater q_m for As(V) is attributed to a higher multilayer formation and a stronger bonding force compared with As(III). The arsenic uptake capacity showed that the as-prepared adsorbent was effective over a wide pH range, and an optimal uptake capacity was recorded between pH 5.0 and 9.0 for As(III) and 3.0 and 7.0 for As(V). The adsorbent exhibited a remarkable regeneration performance for As(III) and As(V) uptake. Several microscopic analytical tools, including Fourier transform infrared spectroscopy, HR-XPS, and X-ray absorption near-edge structure together with zeta potential, confirmed that the binding mode of As(III) and As(V) on ZVI-MNCs was predominantly inner-sphere coordination. Partial redox transformation occurred for As(III) and As(V) on nearly 10 nm of the adsorbent, which indicates that a surface redox mechanism contributed partially to arsenic uptake on the near surface of the ZVI-MNCs. Extended X-ray absorption fine structure spectral analysis proposed that a corner-sharing monodentate mononuclear (¹V) complex occurred for As(III) with a small portion of a corner-sharing bidentate binuclear (²C) complex, whereas As(V) formed a corner-sharing bidentate binuclear (²C) complex with octahedral Fe bonding.