Screening for the action mechanisms of Fe and Ni in the reduction of Cr(VI) by Fe/Ni nanoparticles

Enhanced Cr(VI) removal via CPBr-modified MIL-88A@amine-functionalized GO: synthesis, performance, and mechanism

Water contamination by heavy metals, especially chromium (VI), poses a critical environmental issue due to its carcinogenic nature and persistence in the environment. Addressing this, the current study develops an efficient adsorbent, CPBr-MIL-88A@AmGO, which utilizes the synergistic capabilities of Cetylpyridinium bromide-modified MIL-88A and amine-functionalized graphene oxide to enhance Cr(VI) removal from aqueous solutions. The obtained results indicate that CPBr-MIL-88A@AmGO achieves its highest removal efficacy at pH 2, where the interaction of CPBr and AmGO's positively charged centers significantly contributes to the adsorption processes. According to the Langmuir isotherm model, the composite's adsorption capacity reached a maximum of 306.75 mg/g. The adsorption kinetics adhered to a pseudo-second-order model along with the endothermic nature of the process. Although the presence of SO42- ions significantly reduces adsorption capacity, other interfering ions including Na+, K+, Ca2+, Cl-, and NO3- only slightly affect it. Remarkably, the composite maintains high removal efficiency, over 82%, even after 7 recycling tests, underscoring its potential for practical applications in water treatment systems. The proposed mechanism involves the contribution of electrostatic attractions, ion exchange, complexation, and the reduction of Cr(VI) to Cr(III) in the removal process. This study not only offers a potent solution for Cr(VI) remediation but also contributes to sustainable water resource management.

Optimized preparation of Ni-Febm bimetallic particles by ball milling NiSO4 and iron powder for efficient removal of triclosan

The excessive usage and emissions of triclosan (TCS) pose a serious threat to aquatic environments. Iron-based bimetallic particles (Pd/Fe, Ni/Fe, and Cu/Fe, etc.) were widely used for the degradation of chlorophenol pollutants. This study proposed a novel synthesis method for the preparation of Ni/Fe bimetallic particles (Ni-Febm) by ball milling microscale zero valent iron ZVI (mZVI) and NiSO4. Ball-milling conditions such as ball-milling time, ball-milling speed and ball-to-powder ratio were optimized to prepare high activity Ni-Febm bimetallic particles. During the ball-milling process, Ni2+ was reduced to Ni0 and formed a coupled structure with ZVI. The amount of Ni0 on ZVI significantly affected the activity of Ni-Febm bimetallic particles. The highest activity Ni-Febm bimetallic particles with Ni/Fe ratio of 0.03 were synthesized under optimized conditions, which could remove 86.56% of TCS (10 μM) in aerobic aqueous solution within 60 min. In addition, higher particle dosage, lower pH condition and higher reaction temperature were more conducive for TCS degradation. The higher corrosion current and lower electron transfer impedance of Ni-Febm bimetallic particles were the main reasons for its high activity. The hydrogen atom (•H) on the surface of Ni-Febm bimetallic particles was mainly contributed to the removal of TCS, as reductive transformation products of TCS were detected by LC-TOF-MS. Notably, a small amount of oxidation products were discovered. The total dechlorination rate of TCS was calculated to be 39.67%. After eight reaction cycles, the residual Ni-Febm bimetallic particles could still degrade 28.34% of TCS within 6 h. Low Ni2+ leaching during reaction indicated that Ni-Febm bimetallic particles did not pose potential environmental risks. The prepared environmental-friendly Ni-Febm bimetallic particles with high activity have great potential in the degradation of other chlorinated organic compounds in wastewater.

Fe/Au galvanic nanocells to generate self-sustained Fenton reactions without additives at neutral pH

The generation of reactive oxygen species (ROS) via the Fenton reaction has received significant attention for widespread applications. This reaction can be triggered by zero-valent metal nanoparticles by converting externally added H2O2 into hydroxyl radicals (˙OH) in acidic media. To avoid the addition of external additives or energy supply, developing self-sustained catalytic systems enabling onsite production of H2O2 at a neutral pH is crucial. Here, we present novel galvanic nanocells (GNCs) based on metallic Fe/Au bilayers on arrays of nanoporous silica nanostructures for the generation of self-sustained Fenton reactions. These GNCs exploit the large electrochemical potential difference between the Fe and Au layers to enable direct H2O2 production and efficient release of Fe2+ in water at neutral pH, thereby triggering the Fenton reaction. Additionally, the GNCs promote Fe2+/Fe3+ circulation and minimize side reactions that passivate the iron surface to enhance their reactivity. The capability to directly trigger the Fenton reaction in water at pH 7 is demonstrated by the fast degradation and mineralization of organic pollutants, by using tiny amounts of catalyst. The self-generated H2O2 and its transformation into ˙OH in a neutral environment provide a promising route not only in environmental remediation but also to produce therapeutic ROS and address the limitations of Fenton catalytic nanostructures.

Promoted iron corrosion and subsequent hexavalent chromium removal in zero-valent iron systems by oxidant activation

Zero-valent iron (ZVI), as an effective medium, is widely used to eliminate heavy metal ions in filter tanks. However, it will react with Cr(VI) to generate Fe-Cr precipitates with low conductivity on its surface, resulting in slow iron corrosion and low Cr(VI) removal efficiency. In this study, three oxidants (KMnO4, NaClO, and Na2S2O8) were employed to promote iron corrosion in ZVI systems for enhanced Cr(VI) removal at a concentration of 5 mg/L through batch tests and column experiments. The ZVI/KMnO4, ZVI/NaClO, and ZVI/Na2S2O8 systems achieved significantly higher Cr(VI) removal rates of 31.5%, 52.8%, and 65.9% than the ZVI system (9.8%). Solid phase characterization confirmed that these improvements were attributed to promoted iron corrosion and secondary mineral formation (e.g., lepidocrocite, ferrihydrite, and magnetite) by oxidants. Those minerals offered more reaction sites for Cr(VI) reduction, adsorption, and sequestration. Cycle experiments indicated that ZVI/oxidant systems could stably remove Cr(VI). In long-term column experiment, the ZVI/NaClO column showed a much longer life-span and exhibited a 34.8 times higher Cr(VI) removal capacity than that of the ZVI column. These findings demonstrated that ZVI in combination with a reasonable amount of oxidants was a promising method for removing Cr(VI) in practical filter tanks and provided a new insight to enhance Cr(VI) removal.

Recent advances in bimetallic nanoscale zero-valent iron composite for water decontamination: Synthesis, modification and mechanisms

The environmental pollution of water is one of the problems that have plagued human society. The bimetallic nanoscale zero-valent iron (BnZVI) technology has increased wide attention owing to its high performance for water treatment and soil remediation. In recent years, the BnZVI technology based on the development of nZVI has been further developed. The material chemistry, synthesis methods, and immobilization or surface stabilization of bimetals are discussed. Further, the data of BnZVI (Fe/Ni, Fe/Cu, Fe/Pd) articles that have been studied more frequently in the last decade are summarized in terms of the types of contaminants and the number of research literatures on the same contaminants. Five contaminants including trichloroethylene (TCE), Decabromodi-phenyl Ether (BDE209), chromium (Cr(VI)), nitrate and 2,4-dichlorophenol (2,4-DCP) were selected for in-depth discussion on their influencing factors and removal or degradation mechanisms. Herein, comprehensive views towards mechanisms of BnZVI applications including adsorption, hydrodehalogenation and reduction are provided. Particularly, some ambiguous concepts about formation of micro progenitor cell, production of hydrogen radicals (H·) and H2 and the electron transfer are highlighted. Besides, in-depth discussion of selectivity for N2 from nitrates and co-precipitation of chromium are emphasized. The difference of BnZVI is also discussed.

Dechlorination of 2,4-dichlorophenol by Fe/Ni nanoparticles: the pathway and the effect of pH and the Ni mass ratio

ABSTRACTThe dechlorination of 2,4-dichlorophenol (2,4-DCP) by a nanoscale Fe/Ni material was investigated at room temperature. 2,4-DCP can be removed more quickly by an Fe/Ni material with 2% Ni. Fe/Ni exhibited excellent adsorption and reduction efficiency toward 2,4-DCP in an aqueous solution over a wide range of pH values. The removal rate of 2,4-DCP exceeded 95% in 60 min in the pH range of 3.0-9.0, and more than 75% was dechlorinated to phenol (CA). The degradation pathway of 2,4-DCP was confirmed based on analysis of the intermediate and end products. A portion of 2,4-DCP was first dechlorinated with a chlorine atom to produce 2-chlorophenol and 4-chlorophenol, and then dechlorination was performed sequentially to form CA. The other portion of 2,4-DCP was dechlorinated to remove two chlorine atoms simultaneously to generate CA. The investigations are essential to the application of iron-based remediation technology.

Fabrication of Chitin microspheres supported sulfidated nano zerovalent iron and their performance in Cr (VI) removal

Sulfidated nanoscale zerovalent iron (S-nZVI) has been extensively studied for the reductive removal of Cr(VI), but its applicability is limited by agglomeration and unexpected efficiency reduction. In this study, chitin microsphere supported sulfidated nanoscale zero-valent iron (S-nZVI@Chi-M) was prepared by in-situ one-step reduction method and used to remove Cr(VI) from water. Compared to chitin and chitosan powder, Chi-M with nanofibrous structure and large surface area performed best in stabilizing S-nZVI with a Fe0 loading content of 3.01 wt%. The S-nZVI particles were homogeneously distributed on the surface of Chi-M, effectively avoiding agglomeration. Compared with bare nanoparticles and supported nZVI, S-nZVI@Chi-M showed significantly enhanced Cr(VI) removal capacity (924.5 mg Cr(VI) for per gram of effective Fe0). The influences of sulfidation degree, dosages, initial Cr(VI) concentration, pH, DO, humic acid and typical ions on Cr(VI) removal kinetics were further studied. S-nZVI@Chi-M could be recycled for at least 4 times with acceptable reactivity. The mechanism investigation results indicated that the Cr(VI) removal was a complex process of reduction, adsorption and co-precipitation under the synergistic effect of Chi-M and S-nZVI. This work provides new ideas for the continuous fabrication of highly reactive nanoparticles, hopefully expanding the application scope of biomass resources in pollution remediation.

Multifunctional interfaces for multiple uses: Tin(II)-hydroxyapatite for reductive adsorption of Cr(VI) and its upcycling into catalyst for air protection reactions

Evidence collected to date by our group has demonstrated that tin(II)-functionalized hydroxyapatites (Sn/HAP) are a newly discovered class of ecofriendly reductive adsorbents for Cr(VI) removal from wastewaters. In this work an upgraded series of Sn/HAP materials assured a maximum removal capacity of ≈ 20 mgCr/g, doubling the previously reported value for Sn/HAP materials, thanks to higher Sn-dispersion as proved by X-ray photoelectron spectroscopy and electron microscopy. Insights on kinetics and thermodynamics of the reductive adsorption process are provided and the influence of pH, dosage, and nature of Cr(VI) precursors on chromium removal performances have been investigated. Pseudo-second-order kinetics described the interfacial reductive adsorption process on Sn/HAP, characterized by low activation energy (21 kJ mol-1), when measured in the 278-318 K range. Tests performed in the 2-6 pH interval showed similar efficiency in terms of Cr(VI) removal. Conventional procedures of recycling and regeneration resulted ineffective in restoring the pristine performances of the samples due to surface presence of both Sn(IV) and Cr(III). To overcome these weaknesses, the used samples (Sn + Cr/HAP) were upcycled into catalysts in a circular economy perspective. Used samples were tested as catalysts in gas-phase catalytic processes for air pollution remediation: selective catalytic reduction of NOx (NH3-SCR), NH3 selective catalytic Oxidation (NH3-SCO), and selective catalytic oxidation of methane to CO2. Catalytic tests enlightened the interesting activity of the upcycled Sn + Cr/HAP samples in catalytic oxidation processes, being able to selectively oxidize methane to CO2 at relatively low temperature.

Polyacrylate stabilized ZVI/Cu bimetallic nanoparticles for removal of hexavalent chromium from wastewater

In this work, a magnetic core-shell composite zero-valent iron/copper-polyacrylate (ZVI/Cu-PAA) was synthesized by a simple liquid-phase reduction process and used for hexavalent chromium Cr(VI) removal from wastewater. The optimization experiments show that the optimal dosages of polyacrylate and Cu are 7.00 wt% and 8.25 wt%, respectively. The maximum adsorption capacity and removal rate of Cr(VI) by ZVI/Cu-PAA reached 106.12 mg g^-1 and 99.05% at pH 5.5, respectively. Furthermore, the presence of coexisting ions such as Ca²⁺, Mg²⁺, Na⁺, and NO₃^- had no significant effect on its Cr(VI) removal performance. The excellent performance of ZVI/Cu-PAA is attributed to that the modification of polyacrylate can not only give more active sites but also inhibit agglomeration of nano-metallic particles, while Cu doping promotes the electron generation and transformation of Fe(III)/Fe(II) and Cu(II)/Cu(I) redox cycles. This makes ZVI/Cu-PAA has rich active sites and excellent stability, and has broad application prospects in the remediation of Cr (VI) polluted wastewater. The magnetic core-shell composite ZVI/Cu-PAA has excellent Cr (VI) removal performance because of its rich active sites and high electron transformation efficiency.

Constructing Ni4/Fe@Fe3O4-g-C3N4 nanocomposites for highly efficient degradation of carbon tetrachloride from aqueous solution

Catalytic hydrodechlorination is one of the most potential remediation methods for chlorinated organic pollutants. In this study, Ni₄/Fe@Fe₃O₄-g-C₃N₄ (NFFOCN) nanocomposites were synthesized for carbon tetrachloride (CT) removal and characterized by SEM, XPS and FTIR. The characterization results demonstrated that the special functional groups of g-C₃N₄, especially NH groups, effectively alleviated the aggregation of nanoparticles. In addition, the C and N groups of g-C₃N₄ enhanced the catalytic dechlorination of CT by providing binding sites. The experimental results showed that NFFOCN could effectively remove CT over a wide initial pH range of 3-9, and the CT removal efficiency reached 94.7% after 35 min with only 0.15 g/L of NFFOCN at pH 5.5. The Cl^-, SO₄^2-, and HCO₃^- promoted the removal of CT, while HA and NO₃^- had the opposite effect. Furthermore, good sequential CT removal by NFFOCN nanocomposites was observed, and the CT removal efficiency reached 77.3% after four cycles. Based on the identification of products, a possible degradation pathway of CT was proposed. Moreover, the main mechanisms regarding CT removal included the direct reduction of nZVI (about 40.51%), adsorption (around 34.79%), and hydrodechlorination of CT by Ni⁰ using H₂ (about 19.40%).

Fate and mechanistic insights into nanoscale zerovalent iron (nZVI) activation of sludge derived biochar reacted with Cr(VI)

While nanoscale zero-valent iron modified biochar (nZVI-BC) have been widely investigated for the removal of heavy metals, the corrosion products of nZVI and their interaction with heavy metals have not been revealed yet. In this paper, nZVI-BC was synthesized and applied for the removal of Cr(VI). Batch experiments indicated that the adsorption of Cr(VI) fit Langmuir isotherm, with the maximum removal capacity at 172.4 mg/g at pH 2.0. SEM-EDS, BET, XRD, FT-IR, Raman and XPS investigation suggested that reduction of Cr(VI) to Cr(III) was the major removal mechanism. pH played an important role on the corrosion of nZVI-BC, at pH 4.5 and 2.0, FeOOH and Fe₃O₄ were detected as the major iron oxide, respectively. Therefore, FeOOH-BC and Fe₃O₄-BC were further prepared and their interaction with Cr were studied. Combining with DFT calculations, it revealed that Fe₃O₄ has higher adsorption capacity and was responsible for the effective removal of Cr(VI) through electrostatic attraction and reduction under acidic conditions. However, Fe₃O₄ will continue to convert to the more stable FeOOH, which is the key to for the subsequent stabilization of the reduced Cr(III). The results showed that the oxide corrosion products of nZVI-BC were subjected to the environment, which will eventually affect the fate and transport of the adsorbed heavy metal.

New insights into iron/nickel-carbon ternary micro-electrolysis toward 4-nitrochlorobenzene removal: Enhancing reduction and unveiling removal mechanisms

The ternary micro-electrolysis material iron/nickel-carbon (Fe/Ni-AC) with enhanced reducibility was constructed by introducing the trace transition metal Ni based on the iron/carbon (Fe/AC) system and used for the removal of 4-nitrochlorobenzene (4-NCB) in solution. The composition and structures of the Fe/Ni-AC were analyzed by various characterizations to estimate its feasibility as reductants for pollutants. The removal efficiency of 4-NCB by Fe/Ni-AC was considerably greater than that of Fe/AC and iron/nickel (Fe/Ni) binary systems. This was mainly due to the enhanced reducibility of 4-NCB by the synergism between anode and double-cathode in the ternary micro-electrolysis system (MES). In the Fe/Ni-AC ternary MES, zero-iron (Fe⁰) served as anode involved in the formation of galvanic couples with activated carbon (AC) and zero-nickel (Ni⁰), respectively, where AC and Ni⁰ functioned as double-cathode, thereby promoting the electron transfer and the corrosion of Fe⁰. The cathodic and catalytic effects of Ni⁰ that existed simultaneously could not only facilitate the corrosion of Fe⁰ but also catalyze H₂ to form active hydrogen (H*), which was responsible for 4-NCB transformation. Besides, AC acted as a supporter which could offer the reaction interface for in-situ reduction, and at the same time provide interconnection space for electrons and H₂ to transfer from Fe⁰ to the surface of Ni⁰. The results suggest that a double-cathode of Ni⁰ and AC could drive much more electrons, Fe²⁺ and H*, thus serving as effective reductants for 4-NCB reduction.

Rapid and efficient reduction of chromate by novel Pd/Fe@biomass derived from Enterococcus faecalis

Efficient reduction of chromate is highly desirable for its detoxification and remediation of the contaminated environment. This study described a fusion of the concepts of precious metal biorecovery and fabrication of Pd/Fe@biomass derived from simulated wastewater. The effectiveness of Pd/Fe@biomass during reduction process of Cr(VI) was evaluated by comparing with pure nZVI, E. faecalis and Pd@biomass. Results showed that Pd(II) could be recovered by E. faecalis with Fe(II) as the electron donor, and precipitation could yield nZVI anchored onto Pd-loaded E. faecalis. The nano particles (NPs) on Pd/Fe@biomass were well-dispersed, which provided 2.70 folds specific surface area comparing with nZVI. Efficient Cr(VI) reduction could be achieved at a higher catalyst dosage, the most appropriated Pd/Fe molar ratio of 2% and a wide pH range. Typically, 0.5 mM Cr(VI) could be completely reduced in 5 min driven by Pd/Fe@biomass under the conditions of dosage of 1.0 g/L and pH 3. Moreover, the mechanisms of Cr(VI) reduction by Pd/Fe@biomass were proposed, which intimately related to nZVI electron donating capacities, Pd catalysis for hydrogenation and galvanic cell effects between Fe and Pd. Therefore, Pd/Fe@biomass could be an alternative for rapid and complete reduction of Cr(VI).

Experimental Identification of the Roles of Fe, Ni and Attapulgite in Nitroreduction and Dechlorination of p-Chloronitrobenzene by Attapulgite-Supported Fe/Ni Nanoparticles

The porous-material loading and noble-metal doping of nanoscale zero-valent iron (nFe) have been widely used as countermeasures to overcome its limitations. However, few studies focused on the experimental identification of the roles of Fe, the carrier and the doped metal in the application of nFe. In this study, the nitroreduction and dechlorination of p-chloronitrobenzene (p-CNB) by attapulgite-supported Fe/Ni nanoparticles (ATP-nFe/Ni) were investigated and the roles of Fe, Ni and attapulgite were examined. The contributions of Ni are alleviating the oxidization of Fe, acting as a catalyst to trigger the conversion of H₂ to H*(active hydrogen atom) and promoting electron transfer of Fe. The action mechanisms of Fe in reduction of -NO₂ to -NH₂ were confirmed to be electron transfer and to produce H₂ via corrosion. When H₂ is catalyzed to H* by Ni, the production H* leads to the nitroreduction. In additon, H* is also responsible for the dechlorination of p-CNB and its nitro-reduced product, p-chloroaniline. Another corrosion product of Fe, Fe²⁺, is incapable of acting in the nitroreduction and dechlorination of p-CNB. The roles of attapulgite includes providing an anoxic environment for nFe, decreasing nFe agglomeration and increasing reaction sites. The results indicate that the roles of Fe, Ni and attapulgite in nitroreduction and dechlorination of p-CNB by ATP-nFe/Ni are crucial to the application of iron-based technology.

Bimetallic FeNi nanoparticles immobilized by biomass-derived hierarchically porous carbon for efficient removal of Cr(VI) from aqueous solution

Nano zero-valent iron (nZVI) is an effective material for Cr(VI) treatment, however excessive agglomeration and surface oxidation limit its application. Herein, straw derived hierarchically porous carbon supported FeNi bimetallic nanoparticles (FeNi@HPC) was prepared for effective removal of Cr(VI) from water. FeNi nanoparticles were successfully loaded onto HPC with good dispersibility, and HPC caused an increase in specific surface area of FeNi nanoparticles. FeNi@HPC exhibited a significantly enhanced removal efficiency for Cr(VI) in comparison to Fe@HPC and FeNi NPs. The Ni doping content was further optimized, and the best Ni content in bimetallic NPs was estimated as 10 wt%. The conditions optimal for the activity of FeNi@HPC were assessed, and the highest removal efficiency equivalent to 30 mg L^-1 of Cr(VI) was achieved at pH= 4.0 in 360 min with a dosage of 0.5 g L^-1. Higher temperatures favored the removal of Cr(VI) and FeNi@HPC manifested the lowest activation energy as compared to Fe@HPC and FeNi NPs. The action mechanisms of FeNi@HPC presumably involved electron transfer from Fe⁰, Fe²⁺and atomic hydrogen. This work not only provide a cost-effective and available HPC material to stabilize nZVI but also revealed that using FeNi@HPC is a promising approach for the remediation of water pollution.

Enhanced removal of Cr(VI) from aqueous solution by stabilized nanoscale zero valent iron and copper bimetal intercalated montmorillonite

Batch experiments were conducted to study the Cr(VI) removal by nanoscale zero valent iron and copper intercalated montmorillonite (MMT-nFe⁰/Cu⁰) nanocomposite. MMT-nFe⁰/Cu⁰ was characterized using SEM, TEM, XRD, FTIR, N₂ adsorption-desorption isotherms and XPS. The results demonstrated that highly dispersed nanoscale Fe⁰/Cu⁰ (nFe⁰/Cu⁰) were successfully introduced into the montmorillonite (MMT) layers. In the reaction process, the combination of Cu⁰ and Fe⁰ acted as a galvanic cell, and electrocorrosion not only speeded up the reaction rate, but also increased reduction activity of nFe⁰. MMT-nFe⁰/Cu⁰ as an excellent carrier had good functions in dispersing nFe⁰ and Cu⁰ particles, pH buffering and could keep nFe⁰ and Cu⁰ particles from being released. Besides, no iron ions and very low concentrations of copper ions released in the reaction system, which greatly avoided the influence of secondary environmental pollution.

Bi/mZVI Combined with Citric Acid and Sodium Citrate to Mineralize Multiple Sulfa Antibiotics: Performance and Mechanism

The oxidative mineralization of sulfanilamide drugs (SAs) using micro-size zero-valent iron (mZVI) cooperated with a citric acid buffer solution was evaluated. In this study SM2, SMX, and SD could be removed at 66%, 89%, and 83%, respectively, in a 0.5% Bi/mZVI+CA+NaCA system within 2 h. Based on our analysis, the produced ·OH could be ascribed from the complexation between citrate iron (Fe(II)[Cit]⁻) and the generated H₂O₂ resulting from the activation of O₂ on the mZVI surface in the Bi/mZVI+CA+NaCA system, further inducing the mineralization of antibiotics. The related possible degradation pathways were proposed. Two similar degradation pathways of SM2, SMX, and SD in the mixed liquid, including hydroxylation and SO₂ extrusion, were solved. Meanwhile, there was an additional proposed degradation pathway for SMX to be degraded more effectively, as reflected in the opening of the N-O bond on the benzene ring. Therefore, this work provides an experimental basis and theoretical support for the efficient treatment of antibiotic wastewater in real industry by using an iron-based method.

Novel core-shell sulfidated nano-Fe(0) particles for chromate sequestration: Promoted electron transfer and Fe(II) production

Sulfidated nanoscale valent iron in form of FeS/Fe (0) shell-core nanoparticle has the aptitude to be a promising remediation material toward reductive removal of metal oxyanions. However, disrupted contact between Fe (0) core and FeS shell by thick iron oxides limited its reactivity improvement, and its mechanism of electron transfer remains unveiled. In this study, a novel sulfidated nZVI core-shell particles (FeS/Fe (0)) was fabricated via a modified post sulfidation approach to achieve a more uniform coverage of FeS for aqueous Cr(VI) sequestration. SEM and STEM tests confirmed the formation of the core-shell FeS/Fe (0) structure with a more solid interaction between FeS layer and Fe (0) core. The highest Cr(VI) removal rate was offered at optimal S/Fe molar ratio of 1/25 that the most chelated Fe²⁺ was also observed. The improved performance was due to that FeS shell with greater electronegativity could significantly accelerate the corrosion of Fe (0), facilitate the electron transfer form Fe (0) core to FeS shell according to the electrochemical tests. Moreover, FeS shell provided a protective layer for Fe (0) core so as to alleviate its anoxic passivation in water that FeS/Fe (0) had a better longevity for Cr(VI) removal than nFe (0). Characterizations of STEM and XPS revealed that Cr(VI) was reduced to Cr(III) and evenly coprecipitated with surface Fe(II)/Fe(III).

"In-situ synthesized" iron-based bimetal promotes efficient removal of Cr(VI) in by zero-valent iron-loaded hydroxyapatite

Anionic Cr(VI) and cationic heavy metals generally co-exist in industrial effluents and threaten the public health. Zero-valent iron (ZVI) particles tent to passivate rapidly, which results in a gradual drop in its reactivity. In this work, a strategy of "in-situ synthesized" iron-based bimetal was first developed to stimulate the self-activation of passivated ZVI. During this process, ZVI-loaded hydroxyapatite (ZVI/HAP) was prepared to enhance the affinity for co-existing Cu²⁺, which promoted the in-situ Cu⁰ deposition on ZVI/HAP to form a Fe-Cu bimetal. The deposited Cu⁰ significantly decreased the activation energy (E_a) of Cr(VI) reduction by 24.9%, and its corresponding Cr(VI) removal (96.53%) was much higher that of single Cr(VI) system (68.67%) within 9 h. More importantly, the removal of Cr(VI) and Cu²⁺ were synchronously achieved. Systematical electrochemical characterizations were first introduced to explore the galvanic behaviors of iron-based bimetal. The charge transfer resistance and the negative open circuit potential of ZVI/HAP significantly decreased with the Cu⁰ deposition, thereby accelerating the electron transfer from Fe⁰ to Cu²⁺. The enhanced electron transfer further facilitated the Fe(II) release to promote Cr(VI) reduction. This "in-situ synthesized" iron-based bimetal strategy provides a novel pattern for ZVI activation and exhibits practical application in remediation of combined contaminant.

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.

Carbothermal Synthesis of Ni/Fe Bimetallic Nanoparticles Embedded into Graphitized Carbon for Efficient Removal of Chlorophenol

The reactivity of nanoscale zero-valent iron is limited by surface passivation and particle agglomeration. Here, Ni/Fe bimetallic nanoparticles embedded into graphitized carbon (NiFe@GC) were prepared from Ni/Fe bimetallic complex through a carbothermal reduction treatment. The Ni/Fe nanoparticles were uniformly distributed in the GC matrix with controllable particle sizes, and NiFe@GC exhibited a larger specific surface area than unsupported nanoscale zero-valent iron/nickel (FeNi NPs). The XRD results revealed that Ni/Fe bimetallic nanoparticles embedded into graphitized carbon were protected from oxidization. The NiFe@GC performed excellently in 2,4,6-trichlorophenol (TCP) removal from an aqueous solution. The removal efficiency of TCP for NiFe@GC-50 was more than twice that of FeNi nanoparticles, and the removal efficiency of TCP increased from 78.5% to 94.1% when the Ni/Fe molar ratio increased from 0 to 50%. The removal efficiency of TCP by NiFe@GC-50 can maintain 76.8% after 10 days of aging, much higher than that of FeNi NPs (29.6%). The higher performance of NiFe@GC should be ascribed to the significant synergistic effect of the combination of NiFe bimetallic nanoparticles and GC. In the presence of Ni, atomic H* generated by zero-valent iron corrosion can accelerate TCP removal. The GC coated on the surface of Ni/Fe bimetallic nanoparticles can protect them from oxidation and deactivation.

Inorganic nanoparticles for reduction of hexavalent chromium: Physicochemical aspects

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