Construction of novel graphene-based materials GO@SiO2@C@Ni for Cr(VI) removal from aqueous solution

Synthesis of graphene oxide-modified porous chitosan cross-linked polyaniline composite for static and dynamic removal of Cr(VI)

A novel ternary composite was synthesized comprising graphene oxide-modified porous chitosan cross-linked polyaniline (GO@CS-PANI) by improved Hummers method, followed by cross-linking and grafting. The morphological, structural, and electrical properties of the composite were characterized by FESEM, BET, XRD, RAMAN, FTIR spectra, and zeta potential. It was found that the composite shows excellent Cr(VI) removal performance both in static and dynamic adsorption. The optimal adsorption parameters were solution at pH of 2.0, adsorbent dosage of 0.4 g/L, time of 45 min, and temperature of 35 °C. The Langmuir isotherm model was the best-fitted model, indicating homogeneous adsorption with maximum uptake of 539.83 mg/g. Pseudo-second-order was the best-fitted kinetic model, and the rate was controlled by film diffusion. Thermodynamic data demonstrated that the process was spontaneous, endothermic, and feasible. From the dynamic study, it was witnessed that a lower flow rate and a higher bed height were suitable for maximum adsorption performance. The Thomas model was the best-fitted model for data obtained from the dynamic study. Competition from interfering ions showed that anions have little effect on Cr(VI) removal, whereas cations have no such effect. The adsorption mechanism involved electrostatic attraction, π-π interaction, ion exchange, and metal ion complexion. After five cycles of adsorption-desorption study, the composite still removed 76% Cr(VI). These findings of the present study and the reusable nature of GO@CS-PANI composite signify the innovative and excellent adsorbent for wastewater treatment.

Removal of Chromium Species by Adsorption: Fundamental Principles, Newly Developed Adsorbents and Future Perspectives

Emerging chromium (Cr) species have attracted increasing concern. A majority of Cr species, especially hexavalent chromium (Cr(VI)), could lead to lethal effects on human beings, animals, and aquatic lives even at low concentrations. One of the conventional water-treatment methodologies, adsorption, could remove these toxic Cr species efficiently. Additionally, adsorption possesses many advantages, such as being cost-saving, easy to implement, highly efficient and facile to design. Previous research has shown that the application of different adsorbents, such as carbon nanotubes (carbon nanotubes (CNTs) and graphene oxide (GO) and its derivatives), activated carbons (ACs), biochars (BCs), metal-based composites, polymers and others, is being used for Cr species removal from contaminated water and wastewater. The research progress and application of adsorption for Cr removal in recent years are reviewed, the mechanisms of adsorption are also discussed and the development trend of Cr treatment by adsorption is proposed.

Nanoscale zero-valent iron immobilized by ZIF-8 metal-organic frameworks for enhanced removal of hexavalent chromium

nZVI is considered to be a promising material for environmental remediation. However, the drawbacks of easy agglomeration and low activity severely limit its application. In this work, nZVI/ZIF-8 was obtained by in-situ reduction of nZVI in the presence of performed ZIF-8. The reactivity of the as-obtained nZVI/ZIF-8 nanocomposites was investigated by removing hexavalent chromium (Cr(VI)) from wastewater. The as-obtained nZVI/ZIF-8 nanocomposites showed a superior activity for Cr(VI) removal, with an optimum activity (91.27%) achieved over 0.25 nZVI/ZIF-8 (i e., the mass ratio of ZIF-8 to nZVI was 0.25), higher than that of nZVI (64.55%), and this could be owned to the excellent dispersion of nZVI in nZVI/ZIF-8 and the high specific surface area as compared with the bare nZVI. The results of XPS characterization, quenching experiment analysis and kinetics fitting indicated that the Cr(VI) elimination was a surface-dominated chemical reduction process. Besides, more than 99.00% Cd(II), Cu(II), Cr(VI) and Pb(II) was removed from wastewater over nZVI/ZIF-8 nanocomposites, and negligible zinc ion was detected in the aqueous solutions. The results of our finding demonstrate that the introduction of MOFs is an effective strategy in developing a highly efficient nZVI-based nanocomposites system, and also highlight the promising role of using nZVI/MOFs in heavy metal treatment for practical wastewater.

Metallic nanoparticles for catalytic reduction of toxic hexavalent chromium from aqueous medium: A state-of-the-art review

The ever increasing concentration of toxic and carcinogenic hexavalent chromium (Cr (VI)) in various environmental mediums including water-bodies due to anthropogenic activities with rapid civilization and industrialization have become the major issue throughout the globe during last few decades. Therefore, developing new strategies for the treatment of Cr(VI) contaminated wastewaters are in great demand and have become a topical issue in academia and industry. To date, various techniques have been used for the remediation of Cr(VI) contaminated wastewaters including solvent extraction, adsorption, catalytic reduction, membrane filtration, biological treatment, coagulation, ion exchange and photo-catalytic reduction. Among these methods, the transformation of highly toxic Cr(VI) to benign Cr(III) catalyzed by metallic nanoparticles (M-NPs) with reductant has gained increasing attention in the past few years, and is considered to be an effective approach due to the superior catalytic performance of M-NPs. Thus, it is a timely topic to review this emerging technique for Cr(VI) reduction. Herein, recent development in synthesis of M-NPs based non-supported, supported, mono-, bi- and ternary M-NPs catalysts, their characterization and performance for the reduction of Cr(VI) to Cr(III) are reviewed. The role of supporting host to stabilize the M-NPs and leading to enhance the reduction of Cr(VI) are discussed. The Cr(VI) reduction mechanism, kinetics, and factors affecting the kinetics are overviewed to collect the wealthy kinetics data. Finally, the challenges and perspective in Cr(VI) reduction catalyzed by M-NPs are proposed. We believe that this review will assist the researchers who are working to develop novel M-NPs catalysts for the reduction of Cr(VI).

Sulfidized Nanoscale Zerovalent Iron Supported by Oyster Powder for Efficient Removal of Cr (VI): Characterization, Performance, and Mechanisms

In this study, sulfidized nanoscale zerovalent iron (S-nZVI) supported by oyster shell (OS) powder (S-nZVI@OS) was synthesized by controlling the initial S/Fe ratios (0.1–0.5) to explore the potential synergistic effects during the adsorption and reduction of Cr (VI). X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) analyses showed that Fe (0) and FeS were well dispersed on the OS surface. Furthermore, the stability of S-nZVI@OS composite was higher than that of nZVI, which was proved by the material ageing experiment. The effects of different S/Fe molar ratios, time, temperature, the initial concentration of Cr (VI), and initial pH on the removal efficiency were also studied. The results indicated that with the increase of the S/Fe molar ratio, the removal capacity of Cr (VI) first increased rapidly and then decreased slowly. Batch experiments showed that an optimal S/Fe molar ratio of 0.2 offered a Cr (VI) removal capacity of about 164.7 mg/g at pH 3.5. The introduction of S can not only promote Cr (VI) reduction but also combine with Cr (III) by forming precipitate on S-nZVI@OS mainly as Cr_xFe_(1−x) OOH and Cr₂S₃. The adsorption thermodynamics and kinetics demonstrated that the Langmuir model and pseudo-second-order kinetics model can describe the adsorption isotherms and kinetics. These results suggest that S-nZVI@OS is an effective and safe material for removing Cr (VI) from aqueous solutions.

Synthesis of ferroferric oxide@silicon dioxide/cobalt-based zeolitic imidazole frameworks for the removal of doxorubicin hydrochloride from wastewater

Due to its low-cost, eco-friendliness and easy mode of separation biosynthesized magnetic ferroferric oxide (Fe₃O₄) can be successfully used for the removal of organic contaminants from wastewater. However, there are some challenges that to date have limited this compound's practical removal efficiency. Thus, in this study, a cobalt-based zeolitic imidazole frameworks (ZIF-67) coated biosynthesized ferroferric oxide@silicon dioxide (Fe₃O₄@SiO₂) magnetic composite (Fe₃O₄@SiO₂/ZIF-67) was prepared to address these issues and subsequently used to remove doxorubicin hydrochloride (DOX). Characterization results showed that the fabricated composite exhibited significant magnetic properties (16.1 emu·g^-1) with a size ranging between 50 and 250 nm. The amount of DOX adsorbed by the composite (90.7 mg·g^-1) was much higher than either of the component parts, which were only 35.7 and 82.5 mg·g^-1 for Fe₃O₄@SiO₂ and ZIF-67 respectively. This indicated enhanced DOX adsorption by Fe₃O₄@SiO₂/ZIF-67. The DOX adsorption best fit a pseudo-second order kinetic and Langmuir adsorption model. These studies suggested that the DOX adsorption mechanism involved a combination of electrostatic interactions, π-π stacking, hydrogen bonding and pore filling. Regeneration and application studies, exposing Fe₃O₄@SiO₂/ZIF-67 to real water samples, practically demonstrated that Fe₃O₄@SiO₂/ZIF-67 with propensity for magnetic separation and recycle is a promising nanomaterial for DOX removal.

Two Birds with One Stone: Preparation of 4, 4-Diaminodiphenylmethane Functionalized GO@SiO2 with Mechanical Reinforcement and UV Shielding Properties and Its Application in Thermoplastic Polyurethane

This study prepared 4,4-diaminodiphenylmethane (DDM)-functionalized graphene oxide (GO)@silica dioxide (SiO2) nano-composites through amidation reaction and low-temperature precipitation. The resulting modified GO, that was DDM-GO@SiO2. The study found that DDM-GO@SiO2 showed good dispersion and compatibility with thermoplastic polyurethane (TPU) substrates. Compared with pure TPU, the tensile strength of the TPU composites increased by 41% to 94.6 MPa at only 0.5 wt% DDM-GO@SiO2. In addition, even when a small amount of DDM-GO@SiO2 was added, the UV absorption of TPU composites increased significantly, TPU composites can achieve a UV shielding efficiency of 95.21% in the UV-A region. These results show that this type of material holds great promise for the preparation of functional coatings and film materials with high strength and weather resistance.

Investigating the selectivity and interference behavior for detoxification of Cr(VI) using lanthanum phosphate polyaniline nanocomposite via adsorption-reduction mechanism

A novel Lanthanum phosphate polyaniline (LaPO₄-PANI) nanocomposite was synthesized by the simple sol-gel technique. The nanocomposite prepared at 1:1 ratio provided the highest ion exchange capacity and selective adsorption of Cr(VI). The phase composition and particle morphology of the as-prepared material was evaluated by XRD, FESEM and TEM analyses. The FTIR, Raman, and TGA data inferred the definite chemical interaction between the organic and inorganic counterparts in the formation of LaPO₄-PANI. The selective adsorption of Cr(VI) was estimated by evaluating the distribution coefficient, electrical double layer theory as well as valency and Pauling's ionic radii of interfering ions (phosphate, iodide, sulfate, chloride, sulfide). The high tolerance capability of LaPO₄-PANI against the interfering ions made it appropriate for selective and efficient removal of Cr(VI) ions from solutions. The nanocomposite showed the highest removal percentage of 98.6% towards Cr(VI) in a wide pH range of 2-6 at room temperature, as compared to sole lanthanum phosphate (56%) and polyaniline (75%). The XPS analysis revealed the adsorption mechanism due to the combined effect of both adsorption and reduction. Cr(VI) is adsorbed through electrostatic interactions while the = N-/-NH- group facilitated the in situ chemical reduction. The procured results make the LaPO₄-PANI nanocomposite a promising adsorbent for the removal of Cr(VI).

Interconnected hierarchical nickel-carbon hybrids assembled by porous nanosheets for Cr(VI) reduction with formic acid

Magnetic carbon materials promise distinct advantages in the decontamination of heavy metal ions. In this work, a novel interconnected hierarchical nickel-carbon (Ni@IHC) hybrid was synthesized by combining the solvothermal method with a one-step pyrolysis under argon atmosphere. Benefitting from 3D flower-like morphology, interconnected porous nanosheets, large surface area, and abundant Ni nanoparticles, Ni@IHC hybrids can remove Cr(VI) within 25 min by using formic acid (FA) as a reductant at 25 ℃. Furthermore, the experimental parameters that can affect the material catalytic performance such as initial Cr(VI) concentration, catalyst dosage, FA concentration, and temperature were also investigated in detail. It was found that highly dispersed Ni nanoparticles contributed significantly to the reduction process. More importantly, the embedded Ni nanoparticles favor fast separation by a magnet and were helpful for the recycles use. This Ni@IHC hybrid was obtained by a facile and easy scale-up method, resulting in the fast transformation of Cr(VI) into Cr(III).

Silica-graphene porous nanocomposites for environmental remediation: A critical review

With the increase of industrialization, there is an urgent need for developing technologies to detect and remove toxic pollutants from water bodies. The pollutants are often released to the environment due to the consumption of raw materials that are necessary for the production of technological goods (such as chemical and pharmaceutical compounds, metals, and alloys or foods). Amongst all the remediation techniques, adsorption is considered as one of the preferred techniques, due to its fast and efficient removal of contaminants. Novel materials, which are engineered for selective and responsive water remediation, have also recently revealed a strong potential in the detection of pollutants. Here, current trends of silica-graphene (SG) porous composites for the removal of oils, organic solvents, heavy metals, and dyes are reviewed in detail. Insights on the modifications of composites to enhance their sorption performance have been highlighted. In addition, the detection of pollutants using porous SG nanocomposites is also critically reviewed. Overall, SG composites reveal a strong potential as nanostructure materials with improved efficiency for environmental-based applications.

An overview of palladium supported on carbon-based materials: Synthesis, characterization, and its catalytic activity for reduction of hexavalent chromium

Palladium plays a pivotal role in most of the industrial heterogeneous catalysts, because of its unique properties such as well-defined structure, great intrinsic carrier, outstanding electronic, mechanical and thermal stability. The combination of palladium and various porous carbons (PCs) can widen the use of heterogeneous catalysts. This review highlights the advantages and limitations of carbon supported palladium-based heterogeneous catalyst in reduction of toxic hexavalent chromium (Cr^(VI)). In addition, we address recent progress on synthesis routes for mono and bimetallic palladium nanoparticles supported by various carbon composites including graphene-based materials, carbon nanotubes, mesoporous carbons, and activated carbons. The related reaction mechanisms for the Cr^(VI) reduction are also suggested. Finally, the challenge and perspective are proposed.

Three dimensional flower-like magnetic polyethyleneimine@MoS2 composites for highly efficient removal of Cr(VI) and Pb(II) ions

Magnetic Fe₃O₄ nanoparticles were coated by polyethyleneimine (PEI), and then Fe₃O₄@PEI was further modified with MoS₂ by the hydrothermal method to fabricate 3D flower-like structured magnetic polyethyleneimine@MoS₂ (MP@MoS₂) composites, and the composites were served as efficient adsorbents to capture Cr(VI) and Pb(II) from aqueous solution. The effects of temperature, pH, shaking time and environmental conditions on adsorption performance of MP@MoS₂ towards Cr(VI) and Pb(II) have been conducted by batch adsorption experiments. The prepared MP@MoS₂ exhibited high adsorption capacities (192.30 mg/g for Cr(VI) at pH 3.0 and 256.41 mg/g for Pb(II) at pH 6.0) and the adsorption equilibrium could be achieved in a short time. Moreover, MP@MoS₂ composites with high saturation magnetization could be simply separated under an external magnet. Combined experiments and spectral analysis, the underlying adsorption mechanism for Cr(VI) on MP@MoS₂ was mainly attributed to the reduction of Cr(VI) to Cr(III), and the removal of Pb(II) was due to the complexation with sulfur groups and amino-groups. Consequently, the prepared 3D flower-like structured MP@MoS₂ has a great potential for the practical application in removing Cr(VI) and Pb(II) from the aquatic environment.

Redox mechanisms of conversion of Cr(VI) to Cr(III) by graphene oxide-polymer composite

Alternative methods of aqueous chromium removal have been of great research interest in recent years as Cr (VI) is a highly toxic compound causing severe human health effects. To achieve better removal of Cr (VI), it is essential to understand the chemical reactions that lead to the successful removal of Cr species from the solution. Recent studies have demonstrated that graphene oxide (GO) based polymer beads cannot only adsorb Cr (VI) via electrostatic attractions but also reduce it to Cr (III), which is a much less toxic form of chromium. This conversion and the functional groups involved in this conversion, until now, were not elucidated. In the present study, we employed X-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy to investigate the conversion pathway of Cr (VI) to Cr (III) in graphene-based polymer beads. The results showed that alcoholic groups are converted to carboxylic groups while reducing Cr (VI) to Cr (III). The inclusion of GO in the polymer beads dramatically increased the potential of Cr (VI) uptake and conversion to Cr (III), indicating polymers and nanomaterials containing alcohol groups can remove and convert chromium in water. Other functional groups present in the polymer bead play an important role in adsorption but are not involved in the conversion of Cr (VI) to Cr (III).

Magnetic metal organic frameworks/graphene oxide adsorbent for the removal of U(VI) from aqueous solution

A well-defined magnetic metal organic frameworks (MOFs)/graphene oxide (Fe₃O₄@HKUST-1/GO) consisting of magnetic Fe₃O₄ nanoparticles, HKUST-1 nanocrystal and GO was synthesized through a simple and environmentally friendly approach. Characterizations of Fe₃O₄@HKUST-1/GO adsorbing U(VI) with high-resolution transmission electron microscopy suggested that the Fe₃O₄@HKUST-1/GO possessed good stability. The introduction of GO enhanced the ability of particles to uptake U(VI) from aqueous solution. The effects of solution pH, contact time and temperature on U(VI) adsorption were systematically tested by intermittent experiments. The adsorption process can be better described by the Langmuir model and the pseudo-second-order kinetic model. The results showed that the Fe₃O₄@HKUST-1/GO exhibited good adsorption capacity towards U(VI) at the initial solution pH value of 4.0 and T = 318 K. The X-ray photoelectron spectroscopy was used to analyze the U(VI) removal mechanism. This work represents the application of Fe₃O₄@HKUST-1/GO as a novel adsorbent to extract U(VI) from contaminated water.

Highly efficient removal of Cr(VI) from water based on graphene oxide incorporated flower-like MoS2 nanocomposite prepared in situ hydrothermal synthesis

An efficient adsorbent for the treatment of Cr(VI) was simply fabricated by combining graphene oxide with MoS₂ nanosheets via in situ hydrothermal process with CTAB as the surfactant. The experimental results indicated that the agglomeration of the MoS₂ nanosheets are reduced and uniformly grown on the graphene sheet during the in situ hydrothermal process, and the introduction of graphene oxide provided higher specific surface area and abundant oxygenic groups. Based on this, the removal efficiency of Cr(VI) onto MoS₂/rGO was 75.9% at pH 2.0, which was higher than that of bulk MoS₂ (61.0%). On account of Sips adsorption isotherm model, the highest uptake capacity of MoS₂/rGO toward Cr(VI) reached 80.8 mg g^-1. The adsorption kinetic consequences showed that the chemisorption process was the control step, and the removal mechanism for Cr(VI) is redox and adsorption; in this way, the adsorbed Cr(VI) was partially reduced to Cr(III). Furthermore, this as-prepared adsorbent also presented satisfying reusability for removal of Cr(VI) and can be used for the selective removal of Cr(VI) in the presence of NO₃^-. In short, it may provide a potential route to enhance the adsorption property of MoS₂ toward heavy metals through incorporating with GO, which would expand the applications of MoS₂ in the field of treatment of the heavy metal wastewater.

Hyperbranched thiourea-grafted electrospun polyacrylonitrile fibers for efficient and selective gold recovery

Research on the recovery of precious metals (including gold, Au), attracts attention of scientists worldwide. This paper reports synthesis of a novel fiber adsorbent consisting of hyperbranched thiourea-grafted electrospun polyacrylonitrile (HS-PAN) for Au(III) ion recovery. High-density functional groups of the hyperbranched structure allowed HS-PAN fibers to exhibit much higher affinity and selectivity towards Au(III) than towards Pt(IV), Cr(VI), Pb(II), Ni(II), Co(II), Fe(III), Mg(II), Cu(II) and Na(I). Au(III) adsorption proceeded according to the pseudo-second-order kinetic model and could be fitted very well using Langmuir isotherm. The maximum adsorption capacity of these fibers relative to Au(III) was 3257.3 mg/g, which is higher than the values reported in the literature for other Au(III) adsorbents. Our novel HS-PAN fibers can extract Au from real electronic waste with 99% recovery yield in just 1 h. Thus, this study demonstrates a very efficient and low-cost way to recover gold.