Cr Removal Efficiency Research Articles

Enhanced bioelectroremediation of heavy metal contaminated groundwater through advancing a self-standing cathode

Hexavalent chromium (Cr(VI)) contamination in groundwater poses a substantial global challenge due to its high toxicity and extensive industrial applications. While the bioelectroremediation of Cr(VI) has attracted huge attention for its eco-friendly attributes, its practical application remains constrained by the hydrogeochemical conditions of groundwater (mainly pH), low electron transfer efficiency, limitations in electrocatalyst synthesis and electrode fabrication. In this study, we developed and investigated the use of N, S co-doped carbon nanofibers (CNFs) integrated on a graphite felt (GF) as a self-standing cathode (NS/CNF-GF) for the comprehensive reduction of Cr(VI) from real contaminated groundwater. The binder free cathode, prepared through electro-polymerization, was employed in a dual-chamber microbial fuel cell (MFC) for the treatment of Cr (VI)-laden real groundwater (40 mg/L) with a pH of 7.4. The electrochemical characterization of the prepared cathode revealed a distinct electroactive surface area, more wettability, facilitating enhanced adsorption and rapid electron transfer, resulting in a commendable Cr(VI) reduction rate of 0.83 mg/L/h. The MFC equipped with NS/CNF-GF demonstrated the lowest charge transfer resistance (Rct) and generated the highest power density (155 ± 0.3 mW/m2) compared to control systems. The favorable electrokinetics for modified cathode led to swift substrate consumption in the anode, releasing more electrons and protons, thereby accelerating Cr(VI) reduction to achieve the highest cathodic coulombic efficiency (C.Eca)of80 ± 1.3 %. A similar temporal trend observed between Cr(VI) removal efficiency, COD removal efficiency, and C.Eca, underscores the effective performance of the modified electrode. The reusability of the binder free cathode, exemption from catholyte preparation and the absence of pH regulation requirements highlighted the potential scalability and applicability of our findings on a larger scale.

Efficient removal of Cr(VI) from wastewater by composite adsorptive membrane modified with polyethyleneimine (PEI)

The development of an efficient and economical method to remove the toxic Cr(VI) from water is a global challenge. This study developed a new composite adsorptive membrane by combining polyacrylonitrile (PAN) as a substrate with crosslinked polyethyleneimine (PEI) to efficiently remove Cr(VI) from wastewater. After optimizing the preparation conditions of the material, the Cr(VI) removal by PEI/GMA@PAN was investigated using batch adsorption and longer-term filtration experiments. The composite adsorptive membrane exhibited a high adsorption capacity, reaching a maximum of 89.16 mg/g, along with fast adsorption kinetics that achieved equilibrium within 100 min for Cr(VI) removal. Even in the presence of coexisting ions, PEI/GMA@PAN membrane maintained a high efficiency in removing Cr(VI). The removal mechanism mainly relied on electrostatic interactions and the reduction of Cr(VI) to Cr(III). Notably, long-term filtration experiments substantiated the membrane's effectiveness in removing Cr(VI) from synthetic wastewater, with an impressive treatment volume of 342 L/m2. The optimized PEI/GMA@PAN membrane also demonstrated low PEI leaching across a wide pH range and the potential for re-utilization through four-round cycles, underscoring its promising applicability in wastewater treatment. Therefore, the newly synthesized composite membranes show potential for the removal of Cr(VI) from wastewater.

Efficient removal of Cr(VI) by sulfide nanoscale zero-valent iron: Characterization, aging and mechanism

In the research, sulfide nanoscale zero-valent iron (S-nZVI) was synthesized, characterized and further applied to remove hexavalent chromium Cr(VI) from water. The results showed that a maximum adsorption (81 mg/g) and repeatability was reached at a sulfur/iron molar ratio of 0.1. Sulfide modification could form amorphous FeS on the surface of nZVI, which enhanced the antioxidant activity and hydrophobicity of S-nZVI0.1. The removal of Cr(VI) by S-nZVI0.1 was in accordance with the pseudo-second-order and Langmuir model with the theoretical removal capacity of 83.51 mg/g. After aging 45 d, 60 mg/g of Cr(VI) still was adsorbed by S-nZVI0.1, suggesting that S-nZVI0.1 exhibited a better stability. 84.1 % of Cr(VI) was reduced to Cr(III) on the surface of S-nZVI0.1, Fe0, FeS, Fe2+ and S2− were involved in the reduction reaction. This study provided guidance for synthesizing well stability and high reaction of S-nZVI to remove Cr(VI).

Simultaneous removal of methylene blue and Cr(VI) in a dual-chamber photocatalytic microbial fuel cell with WO3/MoS2/FTO photocathode

Carbon felt was used as the anode and WO3/MoS2/FTO (fluorine-doped tin oxide) was used as the photocathode in a photocatalytic microbial fuel cell (PMFC). The photoelectric performance of the WO3/MoS2/FTO photocathode and the removal efficiency of methylene blue (MB) and Cr(VI) mixed pollutants were systematically investigated in the cathode chamber. The results showed that after 12 h of light irradiation in the PMFC with WO3/MoS2/FTO as the photocathode, the removal rates of MB and Cr(VI) were 84.56 and 68.11 %, respectively, which were much higher than those using WO3/FTO as a photocathode (55.57 % and 45.26 %, respectively). The corresponding maximum output power was 33.14 mW/m2, which was 1.85 times that of the WO3/FTO photocathode PMFC. These results can be attributed to the fact that WO3 is an n-type semiconductor and MoS2 is a p-type semiconductor. Analysis of trapping experiments showed that the composite of WO3 and MoS2 formed a Z-scheme heterojunction, which improved the separation efficiency of the photoelectric carriers and enhanced the pollutant removal efficiency of the photocathode. PMFCs are a new and environment-friendly technology for removing pollutants thereby providing an experimental basis for future engineering applications.

Actual Adsorptive Efficiency of Aqueous Cr(VI) Using Modified Banana Pseudostems

A wide application of chromium (Cr) in industries has discharged large quantity of Cr(VI) into the environment. Cr(VI) is a strong oxidizing agent that can react rapidly to produce toxic compounds to humans. Various agricultural products and byproducts have been studied for the sorptive removal of Cr(VI). However, less attention has been given to the presence of Cr(III) in solution after the adsorption with significant effect to actual adsorption capacity of the material. Here the Cr(VI) removal efficiency was investigated by using modified banana pseudostems (m-BP). Banana pseudostems were treated with 0.08 M HCl for 4 h and washed thoroughly. The maximum adsorption capacity of Cr(VI) was found at pH 2, adsorbent dose 34 g/L, contact time of 180 min and 100 mg/L initial Cr(VI) concentration. The adsorption efficiency of Cr(VI) onto m-BP was recorded to be 73.6% while the total conversion of Cr(VI) was 99.6%. The adsorption efficiency was lower than the total conversion of Cr(VI) due to the reduction of Cr(VI) to Cr(III) by the presence of organic groups in banana pseudostems. Owing to relatively high efficiency, m-BP can be used as effective and very low cost adsorbent for Cr(VI) removal from wastewater.

Immature persimmon residue as a novel biosorbent for efficient removal of Pb(II) and Cr(VI) from wastewater: Performance and mechanisms

Owing to the powerful affinity of tannin toward heavy metal ions, it is frequently immobilized on adsorbents to enhance their adsorption properties. However, natural adsorbents containing tannin have been overlooked owing to its water solubility. Herein, a novel natural adsorbent based on the immature persimmon residue (IPR) with soluble tannin removed was fabricated to eliminate Pb(II) and Cr(VI) in aquatic environments. The insoluble tannin in IPR endowed it with prosperous properties for eliminating Pb(II) and Cr(VI), and the IPR achieved maximum Pb(II) and Cr(VI) adsorption quantities of 68.79 mg/g and 139.40 mg/g, respectively. Kinetics and isothermal adsorption analysis demonstrated that the removal behavior was controlled by monolayer chemical adsorption. Moreover, the IPR exhibited satisfactory Pb(II) and Cr(VI) removal efficiencies even in the presence of multiple coexisting ions and showed promising regeneration potential after undergoing five consecutive cycles. Additionally, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) analysis unveiled that the elimination mechanisms were primarily electrostatic attraction, chelation and reduction. Overall, the IPR, as a tannin-containing biosorbent, was verified to possess substantial potential for heavy metal removal, which can provide new insights into the development of novel natural adsorbents from the perspective of waste resource utilization.

Catalytic role of biogenic gold nanoparticles in improving Cr(VI) removal efficiency of biocathode microbial fuel cells

AbstractBACKGROUNDBiogenic metal nanoparticle‐modified electrodes have a promising prospect for improving the efficiency of microbial fuel cells (MFCs) for hexavalent chromium (Cr(VI))‐containing wastewater treatment. In this study, a graphene (GO) electrode was modified with chemical gold nanoparticles (ChemAu) and biogenic gold nanoparticles (BioAu), respectively, and the two modified electrodes were then used as MFC biocathode electrodes to treat Cr(VI)‐containing wastewater.RESULTSThe results demonstrated that the BioAu/GO biocathode‐based MFC obtained the highest power density (95.78 ± 1.11 mW m−2) and Cr(VI) removal rate (2.17 ± 0.51 mg L−1 h), which were 13.19 and 1.03 times higher than those of the graphite paper biocathode‐based MFC, respectively. The Cr(VI) removal efficiency of the BioAu/GO biocathode‐based MFC under close‐circuit condition reached 87.61 ± 0.19%, which was 3.74 times higher than that recorded under open‐circuit conditions, indicating the critical role of the bioelectrochemical reduction reaction mediated by the BioAu/GO biocathode on Cr(VI) removal.CONCLUSIONThe BioAu/GO electrode first confirmed its superior performance to the ChemAu/GO electrode in Cr(VI)‐reducing MFCs due to its excellent material properties. This study provides a technical reference for the exploration of efficient bioelectrode materials based on biogenic metal nanoparticles for MFCs to treat recalcitrant pollutant‐containing wastewater. © 2024 Society of Chemical Industry (SCI).

Removal of hexavalent chromium in water by chitosan-modified Enteromorpha prolifera biochar loaded with iron-manganese oxides: Application performances and reaction mechanisms

This study successfully synthesized magnetic iron-manganese biochar (F1M3BC) and iron-manganese chitosan biochar (Chi-F1M3BC) using Enteromorpha prolifera (EP) as the raw material, aiming to efficiently remove hexavalent chromium (Cr(VI)) from water. Compared to F1M3BC, Chi-F1M3BC, after chitosan cross-linking encapsulation, exhibited significant advantages in terms of specific surface area, pH adaptability, and adsorption performance. The specific surface area of Chi-F1M3BC reached 101.47 m³/g, approximately double that of F1M3BC (52.89 m³/g), making it rich in reactive groups for Cr(VI) removal. The addition of chitosan elevated the pHpzc value of Chi-F1M3BC from 5.5 to 9, and the abundant active groups enabled it to efficiently remove Cr(VI) over a wider pH range. Under 30 °C conditions, 0.5 g/L of Chi-F1M3BC could completely remove 30 mg/L of Cr(VI) solution within 1 h. Adsorption isotherm and kinetic studies indicated that Chi-F1M3BC complied with the Langmuir model and pseudo-second-order kinetics. At 30 °C, the adsorption capacity of Chi-F1M3BC reached 104.5 mg/g, approximately twice that of F1M3BC (58.93 mg/g). Moreover, after five consecutive usage cycles, the removal efficiency of Cr(VI) by Chi-F1M3BC only slightly decreased by 3%, demonstrating its superior stable removal performance. Subsequent characterization and experimental investigations elucidated the elimination of Cr(VI) by Chi-F1M3BC, achieved through the synergistic interplay of ion exchange, electrostatic attraction involving surface hydroxyl and amino functional groups, chelation, and the reduction of iron and manganese oxides. In conclusion, the synthesized Chi-F1M3BC has demonstrated its efficiency and stable adaptability in removing Cr(VI), coupled with the additional benefit of utilizing waste Enteromorpha prolifera.

Magnetic triiron tetraoxide/biochar–loaded nanoscale zero-valent iron for chromium(VI) removal from aqueous solution

BackgroundTo inhibit the agglomeration of nanoscale zero-valent iron (nZVI or Fe0), increase its stability and facilitate electron transfer, magnetic triiron tetraoxide/biochar (MBC) was used as the carrier to load nZVI. Consequently, a new pathway was created for electron transfer from nZVI to triiron tetraoxide (Fe3O4) and then to contaminants, thereby improving the chemical reactivity of nZVI. MethodsMagnetic triiron tetraoxide/biochar–loaded nanoscale zero-valent iron (nZVI@MBC) composites were prepared for chromium(VI) adsorption. Different influencing aspects were studied, including the loading percentage of nZVI, solution pH, co-existing anions(PO43−, SO42−, Cl−, NO3−), temperature and reusability. Furthermore, the adsorption isotherms, kinetics and thermodynamics for Cr(VI) removal were performed. Significant findingsnZVI@MBC exhibited an adsorption amount of 117.25 mg g−1 at the optimal conditions (dosage 0.4 g L−1, initial concentration 50 mg L−1, temperature 30 °C, nZVI loading percentage onto MBC 5.0 wt% and pH 2). The adsorption data obeyed the pseudo-second-order kinetics model and followed the Langmuir isotherm, thermodynamic characteristics revealed that Cr(VI) adsorption onto nZVI@MBC was a spontaneous endothermic process. After four recycles, the Cr(VI) removal efficiency remained higher than 70 %, exhibiting a good reusable ability. The mechanisms were confirmed to be physical adsorption, chemical reduction and co-precipitation processes.

Synergistic adsorption removal of heavy metal oxoanions and cations from aqueous solution by a polyacrylate-coated ferrous sulfide and layered double hydroxide nanocomposite

Heavy metal oxoanions and cations often co-exist in the environment and it is a critical challenge to achieve their simultaneous and efficient removal due to their presence of opposite charges. Here, a polyacrylate-coated ferrous sulfide and layered double hydroxide (PAA/FeS@LDH) nanocomposite was developed. Results illustrated that PAA/FeS@LDH exhibited excellent reductivity and stability due to the dispersion of PAA and the support by LDH. The removal efficiency of single Cr(VI) (9.73 mg/L) using PAA/FeS@LDH reached 100 % within 120 min, which was significantly higher than that of LDH (35.39 %), FeS (73.13 %), and FeS/LDH (96.47 %). Attractively, an enhanced removal of Cr(VI) (19.46 mg/L) by PAA/FeS@LDH in the presence of Cu(II), Ni(II), or Cd(II) was observed, with Cr(VI) removal efficiency increasing from 49.63 % (single Cr(VI)) to 99.51 %, 82.46 %, and 72.18 % within 30 min, respectively. The adsorption capacities of Cr(VI) significantly enlarged from 140 mg Cr/g (single Cr(VI)) to 221, 175, and 171 mg Cr/g when co-existing with Cu(II), Ni(II), or Cd(II), respectively. Another interesting outcome was the significant enhancement of reusability of PAA/FeS@LDH by the co-existence of Cr(VI) and Cu(II). After 5 cycles, PAA/FeS@LDH could still remove 96.97 % of Cr(VI) together with >70 % of Cu(II) in the mixed solution, by contrast, the removal efficiency of Cr(VI) was only 38.04 % in a single solution. The characterization analysis and density functional theory calculations demonstrated that besides surface reduction, complexation and precipitation, the cation-anion bridge interaction was an essential mechanism of achieving the synergistic removal of heavy metal oxoanions and cations.

The structural and magnetic properties of Co0.9Ni0.1Fe2O4 as a heavy metal absorbing material

This research uses co-precipitation to synthesize cobalt nickel ferrite as a heavy metal absorbing material from natural iron sand. The co-precipitation method was applied with temperature calcination of 700 °C for 2 h. Characterization of crystal structure using X-ray diffraction (XRD), characterization of magnetic properties using Vibrating Sample Magnetometer (VSM), and characterization of particle morphology using Field Emission Scanning Electron Microscopes (FE-SEM). Characterization of adsorption capacity of samples using Atomic Absorption Spectrophotometer (AAS). From the characterization results, it is known that cobalt nickel ferrite is a hard magnet and has a cubic crystal structure. The magnetic hysteresis curve shows a coercivity of 715.68 Oe and a remanent magnetization of 18.38 emu/g. The absorption capacity obtained from Cu and Cr removal efficiency was 96.26%, with an adsorption capacity of 10.83 mg/L.

Biomass polyamine-functionalized nanocellulose-loaded covalent organic framework to construct composite aerogels for highly efficient removal of Cr (VI) and methyl orange

Heavy metal ions and organic dyes cause ecological pollution, which is a problem that needs to be solved urgently. In this study, a novel biomass polyamine-functionalized nanocellulose loaded-covalent organic framework (COF) successfully constructed a hybrid aerogel (TDB). Due to the synergistic effect of biomass polyamines and COF, a vast number of active sites are present on the surface of TDB aerogel. This made the TDB aerogel have the largest adsorption capacity (470.67 mg/g, 488.22 mg/g) and the highest removal efficiency (98.49 %, 99.51 %) for Cr (VI) or MO (methyl orange). In the binary pollutant system, TDB aerogel also showed excellent removal efficiency for Cr (VI) and MO (91.29 %, 82.84 %). The adsorption process and adsorption mechanism of TDB aerogel were analyzed by adsorption kinetics, adsorption isotherms, thermodynamics, FTIR, and XPS. Its crystalline structure also didn't change. Fixed-bed column adsorption experiments showed that TDB aerogel could effectively purify 4510 mL of wastewater that contained Cr (VI) or 4681 mL of wastewater that contained MO. Therefore, TDB aerogel can be considered a promising adsorbent for the treatment of heavy metal ions and organic dyes in wastewater.

Development of Fe0 biochar composites synergistically activated with MgFe2O4 and Fe2+ through one-step pyrolyzing Fe sludge and MgCl2 for enhanced aqueous Cr(VI) removal

The surface passivation layer on Fe0 severely restricts its reactivity during environmental decontamination. In this study, a new composite—an Fe0 biochar composite (Fe0/BC) coupled with MgFe2O4 and Fe2+—was first developed for enhanced Cr(VI) removal by simple co-pyrolysis of Fe sludge (FS) with MgCl2. The characterization results demonstrated that MgFe2O4 and Fe0 could be formed simultaneously at 700 ℃. However, MgFe2O4 disappeared at temperatures of 800 and 900 ℃. By varying the flow rate of N2 and holding time, Fe0/BC was further optimized based on the determined MgCl2 to FS mass ratio (1.9:10.0) and temperature (700 °C). The optimal Fe0/BC (MBC700120-400–30) achieved a Cr(VI) removal efficiency of 97.67%, which was 2.43 times higher than that of the unmodified Fe0/BC obtained at 700 ℃. This enhanced reactivity was directly related to the electron transfer of Fe0 stimulated by MgFe2O4 as well as the conversion of passive corrosion products into Fe3O4 facilitated by soluble Fe2+ components. The predicted maximum adsorption capacity of MBC700120-400–30 was 85.43 mg/g (pH = 5) and the removal of Cr(VI) was dominated by chemisorption. The reduction of Cr(VI) to Cr(Ⅲ) was identified as the principal removal mechanism. Overall, this study demonstrates a novel strategy for developing functional Fe0-based materials for water contamination treatment.

Heavy Metal Ions Removal from Aqueous Solution using NiZnFe2O4/TiO2 Nanocomposites Adsorbent

This research explores the adsorption effectiveness of NiZnFe2O4/TiO2 nanocomposites regarding Cr(VI). The nanocomposites were effectively synthesized utilizing coprecipitation and Stöber methods, incorporating diverse molar ratios of TiO2. The samples were subjected to characterization using methods such as X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy-energy dispersive X-ray (SEM-EDX), Fourier-transform infrared (FTIR), vibrating sample magnetometer (VSM), and ultraviolet-visible spectroscopy (UV-Vis). These analyses were conducted to evaluate the crystal structure, morphology, chemical bond formation, optical properties, magnetic properties, and removal efficiency of the specimens. XRD results showed that NiZnFe2O4 and TiO2 have a cubic and tetragonal structure. The crystallite size decreased as the TiO2 concentration increased. TEM image of NiZnFe2O4 reveals the formation of clusters, indicating uneven dispersion under agglomerated conditions. The average particle size is measured at (10.6 ± 0.8) nm. The Fourier-transform infrared (FTIR) analysis demonstrated the presence of functional groups O-H, C-H, and H-O-H, indicating the successful synthesis of the material. Moreover, the identification of MO-octahedral, MO-tetrahedral, and Ti-O functional groups suggested the formation of NiZnFe2O4/TiO2 nanocomposites. The incorporation of TiO2 had an impact on both the saturation magnetization and coercivity values, which fell within the ranges of 12.4 to 22.9 emu/g and 47 to 55 Oe, respectively. This finding indicates the presence of advantageous magnetic properties. The absorbance spectrum of these nanocomposites displayed a shift to the right (redshift), allowing them to absorb ultraviolet rays. The band gap of these nanocomposites ranges from (2.85 ± 0.02) to (3.29 ± 0.02) eV. Notably, NiZnFe2O4/TiO2 nanocomposites with a concentration ratio of 1:5 exhibit effective Cr(VI) removal efficiency, achieving a degradation value of 65.6%. The pseudo-kinetic model was first investigated to describe kinetic data and Cr(VI) removal determination. The SEM-EDX adsorbent results after adsorption showed the presence of Cr(VI) in the nanocomposites. Therefore, these results can promote NiZnFe2O4/TiO2 nanocomposites as a promising candidate in the removal of heavy metal waste.

Insights into the Adsorption of Cr(VI) on Activated Carbon Prepared from Walnut Shells: Combining Response Surface Methodology with Computational Calculation

Walnut shells were used to produce highly microporous activated carbon. The prepared activated walnut shells were found to be an efficient adsorbent for removing Cr(VI). The study used the response surface methodology to investigate four independent variables effect: Cr(VI) concentration, pH, AC-Ws dose, and temperature on the Cr(VI) removal efficiency, which was studied in the concentration range of 0.1 to 0.3 g/L, 4 to 10, 15 to 35 °C and 1 to 5 mg/L, respectively. Through experiments designed, the optimum conditions were determined to be 4, 0.23 g/L, 298 k, and 2 g/L, respectively. At these conditions, the efficiency of removal was found to be 93%. The thermodynamic study of the adsorption process showed a spontaneous and exothermic nature. The kinetic model that explains the experimental data is the pseudo-second-order model. Furthermore, the Langmuir isotherm model was estimated to be an excellent representation of the equilibrium data. Quantum calculations and NCI analyses were also performed to get more light on the adsorption mechanism of the Cr(VI) atom and its complex form on the prepared AC-Ws surface.

Preparation of PVA/SA-FMB Microspheres and Their Adsorption of Cr(VI) in Aqueous Solution

Biochar, a carbon-dense material known for its substantial specific surface area, remarkable porosity, diversity of functional groups, and cost-effective production, has garnered widespread acclaim as a premier adsorbent for the elimination of heavy metal ions and organic contaminants. Nevertheless, the application of powdered biochar is hindered by the challenges associated with its separation from aqueous solutions, and without appropriate management, it risks becoming hazardous waste. To facilitate its use as an immobilization medium, biochar necessitates modification. In this investigation, sodium alginate, celebrated for its superior gelation capabilities, was amalgamated with polyvinyl alcohol to bolster mechanical robustness, thereby embedding biochar to formulate sodium alginate biochar microspheres (PVA/SA-FMB). A meticulously designed response surface methodology experiment was employed to ascertain the optimal synthesis conditions for PVA/SA-FMB. Characterization outcomes unveiled a highly developed surface abundant in functional groups and confirmed the successful incorporation of iron ions. Adsorption trials revealed that at a temperature of 25 °C and a pH of 2, the adsorption capacity of PVA/SA-FMB for Cr(VI) was 13.7 mg/g within the initial 30 min, reaching an equilibrium capacity of 26.03 mg/g after 1440 min. Notably, the material sustained a Cr(VI) removal efficiency exceeding 90% across five cycles, underscoring its rapid and effective Cr(VI) eradication performance. Kinetic and isothermal adsorption analyses suggested that the adsorption of Cr(VI) adheres to a pseudo-second-order kinetic model and the Freundlich isotherm, indicative of monolayer adsorption dominated by reaction mechanisms. X-ray photoelectron spectroscopy (XPS) analysis inferred that the adsorption mechanism predominantly encompasses electrostatic attraction, redox processes, and complex formation.

Application of coconut shell activated carbon filter in vertical subsurface flow constructed wetland for enhanced multi-metal bioremediation and antioxidant response of Salvinia cucullate

Coconut shell activated carbon (CNSAC) was applied as a filter layer in hybrid vertical subsurface flow constructed wetland (H-VSSF-CW), in order to enhance the multi-metal removal efficiency of the constructed wetland (CW) and to reduce heavy metal accumulation on Salvinia cucullata. Treatment P + AC, (having CNSAC filter layer), showed 32, 21 and 34% more Cd, Cr, and Pb removal efficiency than treatment P (without CNSAC layer). CNSAC activated carbon adsorbed Cd and Pb and Cr by functional groups –NH, -NO2, -C-O, –OH and –CO, and significantly reduced Cd and Pb exposure to S. cucullate. Chromium adsorption by CNSAC filter layer was half (just 25% of total input) of the Cd and Pb. In treatment P, due to high Cd, Pb and Cr accumulation in S. cucullate, the antioxidant defense mechanism of the plant was collapsed and cell death was observed, which in turn has resulted reduced biomass gain (5% reduction). On the other hand, in treatment P + AC, an antioxidant defense mechanism was active in the form significantly (p ≤ 0.05) increased of SOD, CAT and proline content while reduced MDA, EL, %EB and soluble sugar. So, the application of CNSAC increased the heavy metal removal efficiency of H-VSSF-CW by adsorption of a considerable share of heavy metal and hence, reduced the heavy metal load/exposure to S. cucullate.

Integration of galvanic reactions and engineered nanoconfinement in iron-copper nanocomposites for highly-efficient remediation of Cr(VI)-contaminated acidic/alkaline water

To eliminate trace toxic hexavalent chromium (Cr(VI)) from water with different pHs, researchers focused on adsorption, reduction of Cr(VI) to trivalent chromium with followed precipitation while the complex and costly reactivity regulations via adjusting pHs was inevitable. Here we exploit Fe-Cu nanocomposites (CnZVI/Cu) with galvanic reactions and engineered nanoconfinement effect for removing Cr(VI) from acidic/alkaline water directly, consisting of phosphorylated cracked nanoscale zero-valent iron (CnZVI, crack width of 1–2 nm) anode and Cu aggregates cathode. The formed primary battery exhibits strong electrostatic interactions towards ions and outstanding reactivity of reduction and precipitation at acidic/alkaline conditions. The pH inside the cracks at neutral or alkaline conditions is calculated to turn lower for enhancing the dynamics relevant to remediation. Benefiting from those two effects, the removal efficiencies of Cr(VI) through CnZVI/Cu attain more than 99.5 % at pH ≤ 8 and even 96 % at pH of 12 in only 30 min, outperforming the previous works. Its protection on beings (water spinach, zebrafish, and cell) from suffering poisonous Cr(VI), control of adsorbed Cr from migration, and magnetic collectability demonstrate the admirable removal performance, biosafety, and stability, respectively. This work proposes highly-efficient, eco-friendly, convenient, and low-cost technology to scavenge Cr(VI) from acidic/alkaline water.

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.

One-step synthesis of ferrous disulfide and iron nitride modified hydrochar for enhanced adsorption and reduction of hexavalent chromium in Bacillus LD513 by promoting electron transfer and microbial metabolism

Microbial immobilization technology is effective in improving bioremediation efficiency and heavy metal pollution. Herein, Bacillus LD513 with hexavalent chromium (Cr(VI)) tolerance was isolated and immobilized on a novel ferrous disulfide (FeS2)/iron nitride (FeN) modified hydrochar (Fe3-SNHC) prepared from waste straws. The prepared Fe3-SNHC-based LD513 (FeLD) significantly improves Cr(VI) adsorption and reduction by 31.4 % and 15.7 %, respectively, compared to LD513 alone. Furthermore, the FeLD composite system demonstrates efficient Cr(VI) removal efficiency and good environmental adaptability under different culture conditions. Microbial metabolism and electrochemical analysis indicate that Fe3-SNHC is an ideal carrier for protecting LD513 activity, promoting extracellular polymer secretion, and reducing oxidative stress. Additionally, the carrier serves as an electron shuttle that accelerates electron transfer and promotes Cr(VI) reduction. Overall, FeLD is an environmentally friendly biocomposite that shows good promise for reducing Cr(VI) contamination in wastewater treatment.