Synthesis of an attapulgite clay@carbon nanocomposite adsorbent by a hydrothermal carbonization process and their application in the removal of toxic metal ions from water

Redox-responsive copper(I) metallogel: a metal-organic hybrid sorbent for reductive removal of chromium(VI) from aqueous solution

Herein, we report a new strategy to remove toxic Cr(VI) ion from aqueous solution using metal-organic hybrid gel as sorbent. The gel could be easily synthesized from the commercially available organic ligand 2-mercaptobenzimidazole (2-MBIm) and copper(II) chloride in alcoholic medium. The synthesis involves one-electron reduction of Cu(II) to Cu(I) by 2-MBIm, and then gel formation is triggered through Cu(I)-ligand coordination and extensive hydrogen-bonding interactions involving the "-NH" protons (of 2-MBIm ligand), solvent molecules, and chloride ions. The gel shows entangled network morphology. Different microanalytical techniques (FTIR, powder XRD, FESEM, TEM, rheology etc.) have been employed for complete characterizations of the gel sample. Both Cu(I) (in situ formed) and Cl(-) ions trigger the gel formation as demonstrated from systematic chemical analyses. The gel also exhibits its stimuli-responsive behavior toward different interfering chemical parameters (pH, selective metal ions and anions, selective complexing agents, etc.). Finally the gel shows its redox-responsive nature owing to the distinguished presence of Cu(I) metal centers throughout its structural backbone. And this indeed helps in the effective removal of Cr(VI) ions from aqueous solution. Reduction of Cr(VI) to Cr(III) ions and its subsequent sorption take place in the gel matrix. The reductive removal of Cr(VI) has been quantitatively interpreted through a set of different kinetic measurements/models, and the removal capacity of the gel matrix has been observed to be ∼331 mg g(-1) at pH ∼ 2.7, which is admirably higher than the commonly used adsorbents. However, the capacity decreases with the increase in pH of the solution. The overall removal mechanism has been clearly demonstrated. Again, the gel could also be recycled. Thus, the low-cost and large-scale fabrication of the redox-active metallogel makes it an efficient matrix for the toxic ion removal and hence indicates the high promise of this new generation hybrid material for environmental pollution abatement.

Furfural-induced hydrothermal synthesis of ZnO@C gemel hexagonal microrods with enhanced photocatalytic activity and stability

Here we report the synthesis of ZnO@C coaxial gemel hexagonal microrods with a thin hydrothermal carbon (HTC) layer on their surface by a facile one-step hydrothermal method with furfural as the carbon precursor. The furfural has a unique dual role, which not only induces the nucleation of ZnO in the initial stage of hydrothermal process, but also forms a thin HTC layer deposited on the ZnO surface. The thickness of the surface HTC layer increases with the hydrothermal time until 16 h under the conditions adopted in the present study. It has been found that the HTC layer has resulted in a significant improvement in the photocatalytic activities and photostabilities of the ZnO@C microrods for the UV-irradiated photodegradation of methylene blue solution. The mechanism involved in the process is proposed and discussed in terms of the photodegradation scheme and the properties of the ZnO@C microrods.

Poly(itaconic acid)-grafted chitosan adsorbents with different cross-linking for Pb(II) and Cd(II) uptake

Two novel chitosan (CS) adsorbents were prepared in powder form, after modification with the grafting of itaconic acid (CS-g-IA) and cross-linking with either glutaraldehyde (CS-g-IA(G)) or epichlorohydrin (CS-g-IA(E)). Their adsorption properties were evaluated in batch experiments for Cd(II) or Pb(II) uptake. Characterization techniques were applied to the prepared adsorbents as swelling experiments, TGA, SEM, XRD, and FTIR. Adsorption mechanisms were suggested for different pH conditions. Various adsorption parameters were determined as the effect of pH, contact time, and temperature. The maximum adsorption capacities for Cd(II) uptake were 405 and 331 mg/g for CS-g-IA(G) and CS-g-IA(E), respectively, revealing the capacity enhancement after grafting (124 and 92 mg/g were the respective values before grafting, respectively). A similar grafting effect was observed for Pb(II) uptake, proving its adsorption effectiveness on the CS backbone. The reuse of adsorbents was tested with 20 adsorption-desorption cycles.

Efficient visible light driven photocatalytic hydrogen production from water using attapulgite clay sensitized by CdS nanoparticles

Hydrogen production through water splitting using photocatalysts with solar energy can produce clean fuel from renewable resources. In this study, CdS nanoparticle sensitized attapulgite (ATP) nanocomposites were successfully prepared by a facile approach. Under visible-light irradiation, the as-prepared photocatalysts were used for photocatalytic water splitting for hydrogen evolution from aqueous solutions containing Na2SO3 and Na2S as sacrificial reagents even without the noble metals. Photocatalytic hydrogen production activity is ascribed to the presence of CdS nanocrystals that alter the energy levels of the conduction band and valence band in the coupled semiconductor system. Furthermore, the theoretical calculations show that the natural Fe doping (two ATP cells sharing one Fe atom) can promote the photocatalytic process.

Hydrothermal reactions of agricultural and food processing wastes in sub- and supercritical water: a review of fundamentals, mechanisms, and state of research

Hydrothermal (HT) reactions of agricultural and food-processing waste have been proposed as an alternative to conventional waste treatment technologies due to allowing several improvements in terms of process performance and energy and economical advantages, especially due to their great ability to process high moisture content biomass waste without prior dewatering. Complex structures of wastes and unique properties of water at higher temperatures and pressures enable a variety of physical-chemical reactions and a wide spectra of products. This paper's aim is to give extensive information about the fundamentals and mechanisms of HT reactions and provide state of the research of agri-food waste HT conversion.

Superhydrophobic activated carbon-coated sponges for separation and absorption

Highly porous activated carbon with a large surface area and pore volume was synthesized by KOH activation using commercially available activated carbon as a precursor. By modification with polydimethylsiloxane (PDMS), highly porous activated carbon showed superhydrophobicity with a water contact angle of 163.6°. The changes in wettability of PDMS- treated highly porous activated carbon were attributed to the deposition of a low-surface-energy silicon coating onto activated carbon (confirmed by X-ray photoelectron spectroscopy), which had microporous characteristics (confirmed by XRD, SEM, and TEM analyses). Using an easy dip-coating method, superhydrophobic activated carbon-coated sponges were also fabricated; those exhibited excellent absorption selectivity for the removal of a wide range of organics and oils from water, and also recyclability, thus showing great potential as efficient absorbents for the large-scale removal of organic contaminants or oil spills from water.

Superb adsorption capacity and mechanism of flowerlike magnesium oxide nanostructures for lead and cadmium ions

A facile method based on microwave-assisted solvothermal process has been developed to synthesize flowerlike MgO precursors, which were then transformed to MgO by simple calcinations. All the chemicals used (magnesium nitrate, urea, and ethanol) were low cost and environmentally benign. The products were characterized by X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, high-resolution TEM, and N(2) adsorption-desorption methods. These flowerlike MgO nanostructures had high surface area and showed superb adsorption properties for Pb(II) and Cd(II), with maximum capacities of 1980 mg/g and 1500 mg/g, respectively. All these values are significantly higher than those reported on other nanomaterials. A new adsorption mechanism involving solid-liquid interfacial cation exchange between magnesium and lead or cadmium cations was proposed and confirmed.

High-strength hydrogel as a reusable adsorbent of copper ions

A mechanically strong hydrogel was prepared by photoinitiated polymerization of oligo(ethylene glycol) methacrylate (OEGMA), 2-vinyl-4,6-diamino-1,3,5-triazine (VDT) and cross-linker N,N'-methylenebisacrylamide (MBAA). Introduction of the monomer VDT considerably strengthened the mechanical properties of the hydrogel by self-hydrogen bonding of diaminotriazine, and enhanced the adsorption of copper ion onto the hydrogel by chelation between amino groups and metal ion. Adsorption studies were carried out by varying the OEGMA/VDT ratio, contact time, pH value, counterion and initial concentration of Cu(2+) ions. The evaluation of adsorption properties showed that the hydrogel exhibited better correlation with Langmuir isotherm model. This adsorbent could be used repeatedly with little loss in adsorption capacity.