Partial Reduction Of Cr Research Articles

Incorporation of Zr-based metal-organic polyhedron into the PDA/PVDF adsorptive membrane for rapid removal of Cr(VI)

Zirconium-based metal-organic polyhedra (ZrMOPs) have gained significant attention owing to their well-defined cavities, stability and potential applications. Herein, a novel composite membrane (PDA/ZrT-1-NH2/PVDF) was developed by incorporating amino-functionalized ZrMOP (ZrT-1-NH2) into polydopamine (PDA) and trimesoyl chloride (TMC) cross-linking system. Notably, the cationic ZrT-1-NH2 cages were immobilized onto the membrane surface by a simple vacuum filtration method, which thus endowed the surface positive charge for the membrane. A pre-prepared composite membrane was used to remove Cr(VI) from the wastewater. The effects of pH, contact time, initial Cr(VI) concentration, temperature, coexisting ions competition and reusability were investigated in batch experiments. The results revealed the as-prepared composite membrane exhibited high removal efficiency of 96.6 % for Cr(VI) ions within 5 min. In addition, the adsorption rate was very fast, reaching adsorption equilibrium in less than 30 min. Finally, a possible adsorption mechanism was proposed. The adsorption process was dominated by physical adsorption accompanied by a partial reduction of Cr(VI) to Cr(III).

Enhanced photocatalytic Cr(Ⅵ) reduction over the Z-scheme ZnO/ZnIn2S4 composite under visible light

In the present study, a series of ZnO/ZnIn2S4 photocatalysts with different mass ratios were synthesized for Cr(VI) removal. The surface morphology and chemical composition analysis results showed a well-distributed presence of ZnO nanoparticles on the surface of ZnIn2S4 with self-assembled microporous or mesoporous flower-like structure. The Mott−Schottky diagram indicated compatibility between the band structures of ZnO and ZnIn2S4. Moreover, the maximum Cr(VI) removal efficiency was achieved with 5% ZnO/ZnIn2S4 at pH 4.0, 1.6-fold and 14.5-fold higher than pristine ZnIn2S4 and ZnO, respectively. The pseudo-first-order kinetic model effectively simulated the Cr(VI) removal by ZnO/ZnIn2S4. The enhanced performance of ZnO/ZnIn2S4 was mainly attributed to the interaction between ZnO and ZnIn2S4, resulting in a Z-scheme heterojunction that improved visible-light absorption and charges separation efficiency. Fourier transform infrared and X-ray photoelectron spectroscopy indicated a partial reduction of Cr(VI) to Cr(III). At the same time, TR-PL tests demonstrated longer showed that the photogenerated carrier lifetimes in the compared heterojunction materials were higher than those of ZnO and ZnIn2S4. Free radical capture and electron paramagnetic resonance (EPR) experiments further confirmed that photogenerated electrons (e−) and superoxide radicals (·O2−) were the primary active species in the photoreduction of Cr(VI). These results highlight the potential of ZnO/ZnIn2S4 Z-scheme heterojunction as an efficient photocatalyst for wastewater treatment under visible light irradiation.

Coupling few-layer MXene nanosheets with NiFe layered double hydroxide as 3D composites for the efficient removal of Cr(VI) and 1-naphthol

The application of few-layer MXene combined with layered double hydroxides (LDH) as adsorbent for the removal of pollutant is scarce. Herein, a series of novel flower-like MXene modified NiFe-LDH composites (MNFL) were fabricated via in situ growth of LDH on the MXene substrate. The results demonstrated that the added amount of MXene played a significant role in the physicochemical properties of MNFL. Among them, the formed MNFL-60 possessed wonderful dispersion, appreciable surface area (64.04 m2/g) and saturation magnetization (∼10 emu/g). The maximum removal capacities of MNFL-60 were highest, with the value of 372.9 mg/g for Cr(VI) and 201.8 mg/g for 1-naphthol at 288 K. Furthermore, the removal capacities of MNFL-60 for Cr(VI)/1-naphthol increased with the rise of temperature, indicating that the adsorption processes were more favorable under a higher temperature. The addition of LDH decreased the bio-toxicity of MXene, with mortality rates of more than 76 % for E. coli and 48 % for S. aureus at 100 mg/L MNFL-60 and 80 % for E. coli and 57 % for S. aureus at 200 mg/L MNFL-60. Based on the results of XPS, the removal of 1-naphthol was ascribed to the inter-reaction with N/O–containing groups, whereas the eliminate of Cr(VI) was attributed to the electrostatic attraction, complexation and partial reduction of Cr(VI) to Cr(III). Additionally, the outstanding reusability and easy recyclability could make MNFL-60 can be a promising material for remediating organic and inorganic pollutants from wastewaters.

Malic acid cross-linked chitosan based hydrogel for highly effective removal of chromium (VI) ions from aqueous environment

The acrylamide grafted chitosan hydrogel was synthesized through cross-linking of malic acid using a simple one pot synthesis method involving free radical polymerization. The hydrogel with porous infrastructure and diverse functionalities offered large surface area for adsorption. The synthesized hydrogel exhibited high adsorption capacity (427 mg/g) for Cr(VI) ions in an aqueous medium under optimized conditions and it was quite high in comparison to the previously reported literature. The process of adsorption of Cr(VI) as oxyanion at pH 2 was endothermic and driven by entropy. It demonstrated the suitability of Langmuir adsorption isotherm, specifying monolayer adsorption of the metal ions over the hydrogel. The kinetics of adsorption was well suited with pseudo-second-order kinetic model indicating chemisorption and dependence of adsorption rate upon the adsorption capacity of hydrogel. The adsorption mechanism studied using FTIR spectroscopy and XPS indicated the strong intermolecular interactions of the negatively charged Cr(VI) species (HCrO4− & CrO42− at pH 2) to the various functional groups (hydroxyl, protonated amine and amide groups) of the hydrogel and further partial reduction of Cr(VI) to Cr(III) by the functionalities present on the hydrogel. The synthesized hydrogel with good mechanical and thermal characteristics and recycling potential can act as an efficient adsorbent for adsorption and removal of Cr(VI) ions in an aqueous environment.

Construction of Ni-based N-doped mesoporous carbon sphere for efficiently catalytic dichromate reduction with HCOOH at room temperature

The pollution of poisonous Cr(VI) poses a severe threat to our health, and the reduction of Cr(VI) to Cr(III) has become a high-efficiency way for wastewater remediation. Herein, the catalyst with metallic Ni confined in an N-doped mesoporous carbon sphere was prepared by the easy pyrolysis method of carbon sphere with Ni salts and showed remarkable activity, selectivity, and stability for reducing Cr(VI) with HCOOH. Notably, the Ni-N-C600 can rapid and 100 % reduction of the high concentrations of Cr(VI) (100 mg/L, 100 ml) using HCOOH within 7 min at room temperature over the reported Ni-based catalysts. Even in the presence of coexisting ions (0.1 M of Na+, K+, Ca2+, Zn2+, NO3−, Cl−, CO32−, and SO42−), its catalytic efficiency is not affected at all. The catalytic activity of the Ni-N-C600 declined little even after five cycles because the confinement effect of the mesoporous carbon sphere physical barrier prevented the leaching and aggregation of Ni NPs. The magnetism of metallic Ni greatly facilitates the separation of the catalyst. The reduction mechanism is mainly achieved by active hydrogen produced through the dehydrogenation of HCOOH. In addition, the electronic donation of C-containing functional groups also caused a partial reduction of Cr(VI). This work provides a rational design and development of high-efficiency non-noble metal catalysts to reduce Cr(VI).

Removal of chromium from water using manganese (II, III) oxides coated sand: adsorption and transformation of Cr(VI) and Cr(III)

ABSTRACT A manganese coated sand (MCS) sorbent containing manganese (II,III) oxides was developed for adsorption and transformation of chromium [Cr(VI) and Cr(III)] with potential application in flow-through permeable media adsorption filters. Characterization of the MCS sorbent using XRD and XPS showed that the oxides of manganese (II) and manganese (III) were present on the MCS sorbent surface. Adsorption of both Cr(VI) and Cr(III) onto the MCS sorbent occurred over a broad pH range from 3 to 10. Surface charge analysis of the MCS sorbent determined a pHPZC of 7.8, which may facilitate the uptake of both oxy-anionic Cr(VI) species and cationic Cr(III) species. Favorable adsorption of Cr(VI) and Cr(III) onto the MCS sorbent occurred according to the Langmuir and the Freundlich adsorption equations, with a higher adsorption capacity for Cr(III) than Cr(VI). Adsorption parameters from the Langmuir, the Freundlich and the Temkin adsorption equations showed a stronger binding of Cr(VI) than Cr(III). Adsorption of Cr(III) decreased with increasing calcium concentration while adsorption of Cr(VI) decreased with increasing concentration of common anions found in natural water in the following order: phosphate > sulfate> bicarbonate. Transformation of chromium occurred on the surface of the MCS sorbent due to the partial reduction of Cr(VI) and the partial oxidation of Cr(III), which may be attributed to the role of surface manganese (II,III) oxides as either reducing or oxidizing agents. The MCS sorbent is a recyclable and sustainable adsorbent for removal of chromium from water with an environmental impact comparable to ion-exchange technology.

Performance of Halloysite-Mg/Al LDH Materials for Aqueous As(V) and Cr(VI) Removal.

This research focused on the investigation of layered double hydroxide (LDH)/halloysite materials’ adsorption efficiency and mechanisms in reactions with aqueous As(V) and Cr(VI) in a broad pH range. The materials consisting of Mg/Al LDH and halloysite were synthesized using both direct precipitation and physical mixing methods. The XRD, FTIR, DTA, SEM and XPS methods were used to evaluate the quality of the obtained materials and get insight into removal mechanisms. The XRD, FTIR and DTA confirmed LDH formation and showed the dominating presence of intercalated carbonates in the LDH structure. The SEM of the materials revealed characteristic agglomerates of layered LDH particles deposited on halloysite tubular forms. The raw LDH phases showed high removal efficiency of both As(V) and Cr (VI) for initial pH in the range of 3–7. In the studied concentration range the materials containing 25 wt % of LDH exhibited a removal efficiency very similar to the raw LDH. In particular, the halloysite presence in the materials’ mass had a positive effect in the reactions with As(V), which was removed by chemisorption. At a low pH the LDH component underwent partial dissolution, which lowered the adsorption efficiency. Apart from the anion exchange mechanism at a low pH the Cr(VI) was removed via formation of MgCrO4 with Mg (II) being released from the LDH structure. The XPS spectra for As(V) did not show changes in oxidation state in the reactions. In turn, a partial reduction of Cr(VI) to Cr(III) was observed, especially at a high pH. The use of materials composed of two different minerals is promising due to reduction of costs as well as prevention of adsorbent swelling. This opens the possibility of its use in dynamic adsorption flow through systems.

Efficient Removal of Cr(VI) from Water by Biochar and Activated Carbon Prepared through Hydrothermal Carbonization and Pyrolysis: Adsorption-Coupled Reduction Mechanism

Three carbonaceous porous materials (biochar and activated carbon) were developed from the Tectona grandis tree sawdust. The applied process of two-stage preparation included pre-treatment through hydrothermal carbonization at 190 °C and subsequent pyrolysis at 800 °C. Two chemical activating agents (K2CO3 and ZnCl2) were used to prepared activated carbons (K2CO3-AC and ZnCl2-AC), respectively. They were characterized by textural property, morphology, and surface element components and applied to remove Cr(VI) from solution at various solution pH values and initial Cr(VI) concentrations. Results showed that the textural parameters (SBET and VTotal) of the prepared material were 1757 m2/g and 1.027 cm3/g for Zn-Cl2-AC, 1013 m2/g and 0.418 cm3/g for K2CO3-AC, and 792 m2/g and 0.345 cm3/g for biochar. The adsorption process reached the highest efficiency at pH 3.0. The Langmuir maximum adsorption capacity indicated the decreasing order: ZnCl2-AC (127 mg/g) > K2CO3-AC (103 mg/g) > biochar (83.5 mg/g). The removal mechanism of Cr(V) from solution was regarded as an adsorption-coupled reduction, namely (1) partial reduction of Cr(VI) into Cr(III) during the adsorption process and (2) adsorption of the Cr(VI) anions through electrostatic attraction and pore filling and the reduced Cr(III) cations through complexation, Cπ–cation interaction, cation exchange, and pore filing. Therefore, the prepared biochar and activated carbon can server as promising adsorbents to efficiently remove both Cr(VI) and Cr(III) from water.

One-Pot Hydrothermal Synthesis of Polyethylenimine Functionalized Magnetic Clay for Efficient Removal of Noxious Cr(VI) from Aqueous Solutions

In this work, the polyethylenimine functionalized magnetic montmorillonite clay (MMT-Fe3O4-PEI) was synthesized via a facile hydrothermal method. The formed nanocomposite was applied as a magnetically separable adsorbent for hexavalent chromium (Cr(VI)) removal from aqueous solution. The impact of several influential parameters including initial adsorbate concentration, pH, adsorbent mass and temperature were investigated to optimize the conditions for maximum Cr(VI) adsorption. The Langmuir, Freundlich, and Temkin models were employed to describe isotherm constants. The adsorption process was well interpreted with the Langmuir adsorption isotherm and the maximum adsorption capacity of Cr(VI) was achieved as 62.89 mg g−1. The pseudo-second-order kinetic model was fitted well suggested that the rate-controlling step of the adsorption could be controlled by chemical reactions. The main Cr(VI) uptake mechanism was confirmed to be electrostatic attractions between protonated amino groups with Cr(VI) anions, and followed by partial reduction of Cr(VI) to Cr(III). Moreover, reusability studies showed that the prepared magnetic adsorbent can be reused after successive Cr(VI) removal, which will be beneficial in water treatment.

Behaviour of chromium isotopes in the eastern sub-tropical Atlantic Oxygen Minimum Zone

Constraints on the variability of chromium (Cr) isotopic compositions in the modern ocean are required to validate the use of Cr isotopic signatures in ancient authigenic marine sediments for reconstructing past levels of atmospheric and ocean oxygenation. This study presents dissolved Cr concentrations (CrT, where CrT = Cr(VI) + Cr(III)) and Cr isotope data (δ53Cr) for shelf, slope and open ocean waters within the oxygen minimum zone (OMZ) of the eastern sub-tropical Atlantic Ocean. Although dissolved oxygen concentrations were as low as 44–90 μmol kg−1 in the core of the OMZ, there was no evidence for removal of Cr(VI). Nonetheless, there was significant variability in seawater δ53Cr, with values ranging from 1.08 to 1.72‰. Shelf CrT concentrations were slightly lower (2.21 ± 0.07 nmol kg−1) than in open ocean waters at the same water depth (between 0 and 160 m, 2.48 ± 0.07 nmol kg−1). The shelf waters also had higher δ53Cr values (1.41 ± 0.14‰ compared to 1.18 ± 0.05‰ for open ocean waters shallower than 160 m). This is consistent with partial reduction of Cr(VI) to Cr(III), with subsequent removal of isotopically light Cr(III) onto biogenic particles. We also provide evidence for input of relatively isotopically heavy Cr from sediments on the shelf. Intermediate and deep water masses (AAIW and NADW) show a rather limited range of δ53Cr values (1.19 ± 0.09‰) and inputs of Cr from remineralisation of organic material or re-oxidation of Cr(III) appear to be minimal. Authigenic marine precipitates deposited in deep water in the open ocean therefore have the potential to faithfully record seawater δ53Cr, whereas archives of seawater δ53Cr derived from shelf sediments must be interpreted with caution.

Efficient removal of Cr(VI) from water by quaternized chitin/branched polyethylenimine biosorbent with hierarchical pore structure

A novel chitin-based biosorbent (QCP) was synthesized by cross-linking quaternized chitin and branched polyethylenimine with the aid of epichlorohydrin for efficient removal of Cr(VI) from water. Because it possessed both quaternary ammonium groups and amino groups as well as the hierarchical pore structure, QCP presented a maximum adsorption capacity of 387.7 mg/g according to the Langmuir isotherm at 25 °C. The biosorption of QCP achieved the equilibrium within 40 min and followed the pseudo-second-order kinetic model. QCP worked well even in the solution with high pH and high content of competing anions and, it exhibited an excellent reusability. The main Cr(VI) uptake mechanism was confirmed to be electrostatic attractions between Cr(VI) anions and quaternary ammonium groups as well as the protonated amino groups, and followed by partial reduction of Cr(VI) to Cr(III) by amines and hydroxyls. This work may provide a potential for Cr(VI) removal by chitin-based biosorbents.

Remediation of Cr (VI) by inorganic-organic clay

Application of inorganic–organic clays (IOCs) for the remediation of hexavalent chromium (Cr (VI)) and the effect of bisphenol-A (BPA) were investigated. IOCs were better adsorbents for the removal of Cr (VI) than organoclay. Solution pH significantly affected the surface charge of IOCs and speciation of Cr in the solution; hence, the adsorption of Cr onto IOCs. Pseudo-second order model and Langmuir model were the best fit for the kinetics and equilibrium data, indicating chemisorption and formation of monolayer on homogenous adsorption sites, respectively. Cr (VI) adsorption by IOCs was driven by electrostatic interactions between positively charged IOC surfaces and HCrO4- anions. The IOC adsorption capacity for Cr (VI) correlates with the amount of loosely held ODTMA on the external surface of the clay. Involvement of an anion-exchange reaction was suggested in which the counter halide ions of the cationic surfactants were replaced by HCrO4- anions. Cr (VI) adsorption was not affected by the presence of BPA in the medium and two mechanisms for the adsorption of BPA and Cr (VI) were suggested. Partial reduction of Cr (VI) to Cr (III) was observed on the surface of IOC and the reducing agent responsible was suggested to be the Fe (II) in montmorillonite.

Efficient Removal of Cr(VI) from Aqueous Solution with Fe@Fe2O3 Core−Shell Nanowires

The batch removal of Cr(VI) from simulated wastewater with Fe@Fe2O3 core-shell nanowires (FCSNs) was investigated in this study. Itwas found that each gram of the FCSNs could remove 7.78 mg of Cr(VI) from simulated wastewater containing 8.0 mg L(-1) of Cr(VI) with an initial pH of 6.5 at room temperature. The Freundlich adsorption isotherm was applicable to describe the removal processes. Kinetics of the Cr(VI) removal was found to follow pseudo-second-order rate equation. Furthermore, the as-prepared and Cr(VI)-adsorbed FCSNs were carefully examined by scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), Fourier transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopic analysis (XPS). The characterization results suggested that the adsorbed Cr(VI) was partially reduced to Cr(lll) in Cr2O3/Cr(OH)3 on the FCSNs. The possible mechanism of removal of Cr(VI) on FCSNs was proposed, which involved the dominant Cr(VI) adsorption, followed by the partial reduction of Cr(VI) to Cr(III) (chromium(III) oxyhydroides) on the surface of FCSNs. These Fe@Fe2O3 core-shell nanowires with high specific surface area and strong magnetic property are very attractive for the removal of Cr(VI) from wastewater.