Converting Spent Cu/Fe Layered Double Hydroxide into Cr(VI) Reductant and Porous Carbon Material

Recycling solid waste as functional materials is important for both environmental remediation and resource recycling. This study attempts to recycle spent Cu/Fe layered double hydroxide (Cu/Fe-LDH) which is generated from the adsorption of dyes by converting to Cr(VI) reductant and porous carbon material. Results showed that the obtained reductant was mainly composed of Fe-0 and Cu-0, and exhibited good reductive activity toward Cr(VI). The species of Fe-0, Fe2+, Cu-0, and Cu+ all favored the reduction of Cr(VI) according to X-ray photoelectron spectroscopy analysis. During Cr(VI) removal, solution pH could increase to neutral which caused the metal ions to precipitate near completion. On the other hand, the spent Cu/Fe-LDH could be employed to produce porous carbon materials, and the generated waste metals solution herein could be reused for LDH synthesis. Specific surface areas of the obtained carbon materials varied from 141.3-744.2 m(2)/g with changes in adsorbed amount of dyes on the LDH. This study illustrates that all the components of wastes can be useful resources, offering a simple recycling approach for similar organic-inorganic solid wastes. This work also enlightens us that designing a proper initial product is crucial to make waste recycling simpler

Calcined Mg/Al layered double hydroxides as efficient adsorbents for polyhydroxy fullerenes

The environmental behaviors and pollution control of engineered nanomaterials are drawing increasing interests nowadays. This work showed that the calcined layered double hydroxides (LDH), i.e., layered double oxides (LDO), could effectively adsorb polyhydroxy fullerenes (PHF) from aqueous solution. The maximum adsorption capacity of LDO reached similar to 476 mg/g, much higher than that on LDH (similar to 47 mg/g) and activated carbon (similar to 28 mg/g). All of the three equilibrium adsorption isotherms could be well fitted with the Langmuir equation. The high adsorption capacity of PHF on LDO can be attributed to the enhanced accessibility to the adsorption sites for PHF during structural reconstruction of LDO. In addition, the rehydrated LDH, with a net positive surface charge, has high affinity for negatively charged PHF through an electrostatic interaction. Cl-, CO32-, and SO42- could slightly enhance the adsorption of the PHF on LDO, while HPO42- showed an evident inhibiting effect in the whole concentration range of PHF. The adsorbents before and after the adsorption of PHF were characterized by XRD, FT-IR, and TG. The obtained results indicated that the adsorbed PHF could not intercalate into the interlayer spaces of the reconstructed LDH, but could effectively compete with CO32- during the adsorption process

From spent Mg/Al layered double hydroxide to porous carbon materials

Adsorption has been considered as an efficient method for the treatment of dye effluents, but properdisposal of the spent adsorbents is still a challenge. This work attempts to provide a facile methodto reutilize the spent Mg/Al layered double hydroxide (Mg/Al-LDH) after the adsorption of orange II(OII). Herein, the spent hybrid was carbonized under the protection of nitrogen, and then washedwith acid to obtain porous carbon materials. Thermogravimetric analysis results suggested that thecarbonization could be well achieved above 600◦C, as mass loss of the spent hybrid gradually stabilized. Therefore, the carbonization process was carried out at 600, 800, and 1000 ◦C, respectively. Scanning electron microscope showed that the obtained carbon materials possessed a crooked flaky morphology. Nitrogen adsorption–desorption results showed that the carbon materials had large BET surface area and pore volume, e.g., 1426 m2/g and 1.67 cm3/g for the sample carbonized at 800 ◦C. Moreover, the pore structure and surface chemistry compositions were tunable, as they were sensitive to the temperature. Toluene adsorption results demonstrated that the carbon materials had high efficiency in toluene removal. This work provided a facile approach for synthesizing porous carbon materials using spent Mg/Al-LDH

Calcined Mg/Al-LDH for acidic wastewater treatment: Simultaneous neutralization and contaminant removal

Acid drainage (AD) poses a significant concern for water pollution due to its strong acidity and the toxicity of its various contaminants (e.g., heavy metal ions). In order to minimize the harmful effects of AD, the acidity must be neutralized and the contaminants be removed. The capacity of calcined Mg/Al layered double hydroxide (Mg/ Al-CLDH) for simultaneously neutralizing the pH of AD and removing various heavy metal cations and oxyanions (Cr(VI) and phosphate) was studied herein. The interactions throughout co-removal between metal cations (in short M) and oxyanions were particularly investigated. In the solution with only M, Mg/Al-CLDH was capable of neutralizing solution pH and removing M. The reconstruction of LDH from Mg/Al-CLDH produced OH-to neutralize pH and partially remove M through precipitation. FT-IR results suggested that forming H-bonds with the reconstructed LDH (R-LDH) might also contribute to M removal. In the solution containing both M and oxyanions, M and oxyanions could mutually affect their removal efficiency by Mg/Al-CLDH. M weakened the removal capacities of Cr(VI) and phosphate, because it could compete for adsorption sites on R-LDH. Cr(VI) and phosphate showed complex effects on the removal of M: Low concentrations of Cr(VI) promoted the removal of M by providing extra adsorption sites; high concentrations of Cr(VI), however, had the opposite effect, as a high concentration of Cr(VI) might largely occupy the adsorption sites on R-LDH. By contrast, phosphate inhibited the removal of M considerably, which might be ascribed to its strong buffering ability that maintained a relatively strong acidic nature of the solution. Our results, for the first time, showed that Mg/Al-CLDH is particularly suitable for the treatment of AD containing various contaminants

Structural effects on dissolution of silica polymorphs in various solutions

The dissolution and precipitation of silica minerals in rocks, soils, and sediments are essential processes of material transformations near the Earth's surface. In this study, the dissolution of alpha-quartz and alpha-cristobalite are investigated at 25 degrees C in HNO3, NaOH, KCl, and MgCl2 solutions. The amounts of silicon release from alpha-quartz in HNO3 and electrolyte solutions are larger than those from alpha-cristobalite, and the circumstance is in consistence with the density of surface silanols. In NaOH solutions, the amounts of silicon release increase significantly and the maximum amount is about 30 times higher than that in acid and electrolyte solutions. Moreover, the maximum silicon release is inversely proportional to the density of surface silanols, resulting in a lower silicon release from alpha-quartz in comparison to that from alpha-cristobalite. It could be deduced that there is a possible correlation with structural defects, for instance, oxygen vacancies, demonstrated by the electron paramagnetic resonance (EPR) spectrum. By analyzing the surface composition of samples after dissolution, the result of X-ray photoelectron spectroscopy (XPS) confirms that no stable amorphous silica layer form on the surface of alpha-quartz and alpha-cristobalite under the current experimental conditions, along with the verification of high-resolution transmission electron microscope (TEM). Furthermore, the surface species of alpha-quartz and alpha-cristobalite after reacting with different solutions are also qualitatively determined by XPS. (C) 2017 Elsevier B.V. All rights reserved

Enhanced photocatalytic activity of Zn/Ti-LDH via hybridizing with C60

A novel and efficient photocatalyst was synthesized by surface hybridizing Zn/Ti-LDH (LDH) with C60 molecules. The structural characteristics of the resulting products (C60/LDH) were studied using a variety of characterization methods, and the photocatalytic activities of C60/LDH were tested using Orange II (OII) as a model contaminant under simulated solar irradiation. The FT-IR spectra showed that the characteristic mode of C60 at 1182 cm-1 split into two peaks at approximately 1240 and 1156 cm-1; in addition, the Raman spectra showed the band at 1422 cm-1 for C60 was upshifted to 1431 cm-1 with the increased doping amount of C60. In addition, the XPS spectra revealed that the peaks of O1s, Zn 2p, and Ti 2p were downshifted slightly. These results demonstrated that chemical interaction existed between C60 and LDH on C60/LDH. The UV-vis diffuse reflectance spectra confirmed that the absorbance of C60/LDH in visible-light region enhanced markedly, as compared with that of pristine LDH. The photoluminescence (PL) spectroscopic measurement revealed that the C60/LDH composites exhibited much weaker PL intensity than that of LDH sample, and the transient photocurrent (I-V) analysis showed that C60/LDH had higher photocurrent density than pristine LDH. The results of PL spectra and I-V analysis suggested that C60 molecule could effectively transfer the photoelectrons from the conduction band of LDH, leading to a lower recombination rate of the photo-induced electrons and holes. The photocatalytic experiments showed that C60/LDH had much higher photocatalytic activity in the decolorization of OII than that of pristine LDH, and 2%C60/LDH composite exhibited the highest photocatalytic activity. Furthermore, the stability test indicated that the decolorization efficiency of OII by 2%C60/LDH was still quite efficient after being used for 5 cycles

Layered intercalation compounds: Mechanisms, new methodologies, and advanced applications

The structural characteristics of two-dimensional (2-D) materials result in unique physical, electronic, chemical, and optical properties, making them potentially useful in a wide range of applications. These unique properties can be fine-tuned and enhanced via intercalation, expanding the applications of various 2-D intercalation compounds to a much wider scope. This article aims to provide an overview of innovations in the field of intercalation chemistry of 2-D intercalation materials, as well as to highlight their leading applications. A brief perspective on the intercalation of 2-D layered compounds is provided, focusing on mechanisms, approaches, and influential factors involving intercalation. Insights into the potential applications, challenges, and future opportunities of 2-D intercalated materials are discussed

Interlayer Anions of Layered Double Hydroxides as Mobile Active Sites To Improve the Adsorptive Performance toward Cd²⁺

Layered double hydroxides (LDHs) have been considered important sinks for ionic contaminants in nature and effectively engineered adsorbents for environmental remediation. The availability of interlayer active sites of LDHs is critical for their adsorptive ability. However, inorganic LDHs generally have a nano-confined interlayer space of ca. 0.3–0.5 nm, and it is unclear how LDHs can utilize their interlayer active sites during the adsorption process. Thus, LDHs intercalated with SO42–, PO43–, NO3–, Cl–, or CO32– were taken as examples to reveal this unsolved problem during Cd2+ adsorption. New adsorption behaviors and pronounced differences in adsorption performance were observed. Specifically, SO42–/PO43– intercalated LDHs showed a maximum Cd2+ adsorption capacity of 19.2/9.8 times higher than other LDHs. The ligand exchange of H+ (on the surface −OH) by Cd2+ and formation of Cd-SO42–/PO43– complexes led to the efficient removal of Cd2+. Interestingly, interlayer SO42– was demonstrated to be able to move to the edges/outer surfaces of LDHs, providing abundant movable adsorption sites for Cd2+. This novel phenomenon made the SO42– intercalated LDH a superior adsorbent for Cd2+ among the tested LDHs, which also suggests that LDHs with a nano-confined interlayer space can also highly utilize their interlayer active sites based on the mobility of interlayer anions, offering a new method for constructing superior LDH adsorbents