Reaction Thermodynamics of Zerovalent Iron and Hexavalent Chromium at the Solid-Liquid Interface and in Solution.

Zerovalent iron (Fe0) is a promising candidate for remediating hexavalent chromium (Cr(VI)) via adsorption and (or) reduction. Herein, the reaction between Fe0 and Cr(VI) at the solid-liquid interface and in solution under varying pHs was inspected using the methodology of equilibrium thermodynamics. First, species distribution functions of aqueous Cr(VI), Cr(III), Fe(III), and Fe(II) are deduced to illuminate the quantitative distribution of aqueous metal species. Second, the plausible reaction at pH = 0-14 either at the solid-liquid interface or in solution is determined according to the species distribution function. Third, the spontaneity of each reaction is evaluated via a thermodynamic calculation based on the van't Hoff equation. The results present the following. (1) At the solid-liquid interface, the redox reaction 2Cr(VI) + 3Fe0 → 2Cr(III) + 3Fe(II) is spontaneous, inducing complete Cr(VI) → Cr(III) reduction at pH = 0-14. Especially, the high spontaneity of the redox reaction is mainly ascribed to Fe0 oxidation, which serves as a highly spontaneous subreaction. (2) In solution, the redox reaction Cr(VI) + 3Fe(II) → Cr(III) + 3Fe(III) is nonspontaneous at pH = 6 and 7, whereas it is spontaneous at pH = 6-7, 0-5, and 8-14. Accordingly, no Cr(VI) → Cr(III) reduction at pH = 6-7 and complete Cr(VI) → Cr(III) reduction at pH = 0-5 and 8-14 are expected. Particularly, the nonspontaneity of the Cr(VI) reduction at pH = 6-7 is majorly attributed to water ionization, which is involved as a highly nonspontaneous subreaction. On the contrary, the spontaneity of the Cr(VI) reduction at pH = 0-5 and 8-14 is mainly owing to acid-base neutralization, which is involved as a highly spontaneous subreaction. This work may deepen our knowledge about the chemistry involved in hexavalent chromium remediation by the zerovalent iron.

Adsorption mechanism and removal efficiency of magnetic graphene oxide-chitosan hybrid on aqueous Zn(II).

Magnetic architecture incorporating graphene-chitosan has demonstrated encouraging application in wastewater purification. Herein, a ternary hybrid based on Fe3O4-graphene oxide-chitosan (MGOCS) was fabricated and employed as adsorbent to remove aqueous Zn(II). The adsorption mechanism was intensively inspected based on the hard and soft acid base (HSAB) theory. Results present, MGOCS removes 96.73 % of Zn(II) in 38 min, with adsorption quantity 386.92 mg·g-1. Electron transfer and energy lowering determined by the HSAB theory illuminate the plausible adsorption sites in each component of MGOCS: O2- in Fe3O4, -C(=O)NH-, -NH2 in chitosan and -OH in graphene oxide. The exploration was upheld by spectroscopic analyses. Thereby, following adsorption mechanism was proposed. (1) ZnO bond was formed featured by electron donation. (2) The -C(=O)NH- group formed via amidation between graphene oxide and chitosan contributes to Zn(Π) uptake. This work may inspire the development of efficient adsorbent based on magnetic graphene-chitosan for wastewater remediation.

Combination mechanism of the ternary composite based on Fe3O4-chitosan-graphene oxide prepared by solvothermal method.

Magnetic nanohybrid combining chitosan and graphene have demonstrated promising application in environmental remediation. Herein, ternary composite MCG based on Fe3O4, chitosan (CS) and graphene oxide (GO) was facilely prepared via solvothermal method. The as prepared composite was characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), Raman, Brunauer/Emmett/Teller-Barret/Joyner/Halenda (BET-BJH) and thermo gravimetric-differential thermal analysis (TG-DTA). The combination mechanism of MCG was unveiled via employing the hard-soft acid-base (HSAB) theory and spectroscopic investigations including X-ray photoelectron spectroscopy (XPS), Ultraviolet-visible (UV-Vis) and fluorescent emission spectra. Particularly, combination mechanism of MCG was elucidated by the probable site to site interaction of the couplet components in MCG, as follows. (1) CS-Fe3O4. The primary interaction is N(NH2)-Fe(III), electron donates from N to Fe, transforming one half of the amino groups of chitosan into positive N+. (2) GO-CS. Amidation reaction is the primary interaction form, converting the other half of the amino groups of chitosan into -C(O)NH-. (3) GO-Fe3O4. Dominant interactions are those of epoxy, hydroxyl and aromatic ring with Fe(III). Moreover, MCG exhibits fair adsorption performance on divalent heavy metals in six consecutive cycles. These explorations may shed light on the design of efficient adsorbent based on Fe3O4-chitosan-graphene architecture.

Mechanism of Cd(II) and Cu(II) Adsorption onto Few-Layered Magnetic Graphene Oxide as an Efficient Adsorbent.

Heavy metal contamination caused by industrial discharge is a challenging environmental issue. Herein, an efficient adsorbent based on few-layered magnetic graphene oxide (FLMGO) was fabricated, characterized, and utilized to remove aqueous Cd(II) and Cu(II). Results present that the two components graphene oxide (GO) and Fe3O4 of FLMGO promote mutually, enabling FLMGO to outperform either GO or Fe3O4. Specifically, FLMGO adsorbs Cd(II) and Cu(II) with adsorption quantities of 401.14 and 1114.22 mg·g-1 in 5 and 7 min, respectively. Moreover, FLMGO can be readily recovered via magnetic separation using a hand-held magnet. Adsorptions are spontaneous, endothermic, and entropy increasing, which are the best described by the Freundlich and pseudo-second-order model. The interaction mechanism is as follows: lone pair electrons in C=O- and C-O-related groups were coordinated toward Cd(II) and Cu(II) to induce chemical interaction. The high adsorption efficiency endows FLMGO with encouraging application potential in heavy metal remediation.

Synthesis of 1,3-dicarbonyl-functionalized reduced graphene oxide/MnO2 composites and their electrochemical properties as supercapacitors.

A novel 1,3-dicarbonyl-functionalized reduced graphene oxide (rDGO) was prepared by N-(4-aminophenyl)-3-oxobutanamide interacting with the epoxy and carboxyl groups of graphene oxide. The high-performance composite supercapacitor electrode material based on MnO2 nanoparticles deposited onto the rDGO sheet (DGM) was fabricated by a hydrothermal method. The morphology and microstructure of the composites were characterized by field-emission scanning electron microscopy, transmission electron microscopy, Raman microscopy and X-ray photoelectron spectroscopy. The obtained results indicated that MnO2 was successfully deposited on rDGO surfaces. The formed composite electrode materials exhibit excellent electrochemical properties. A specific capacitance of 267.4 F g-1 was obtained at a current density of 0.5 A g-1 in 1 mol L-1 H2SO4, while maintaining high cycling stability with 97.7% of its initial capacitance after 1000 cycles at a current density of 3 A g-1. These encouraging results are useful for potential energy storage device applications in high-performance supercapacitors.

High-Efficient Liquid Exfoliation of Boron Nitride Nanosheets Using Aqueous Solution of Alkanolamine.

As one of the simple and efficient routes to access two-dimensional materials, liquid exfoliation has received considerable interest in recent years. Here, we reported on high-efficient liquid exfoliation of hexagonal boron nitride nanosheets (BNNSs) using monoethanolamine (MEA) aqueous solution. The resulting BNNSs were evaluated in terms of the yield and structure characterizations. The results show that the MEA solution can exfoliate BNNSs more efficiently than the currently known solvents and a high yield up to 42% is obtained by ultrasonic exfoliation in MEA-30 wt% H2O solution. Finally, the BNNS-filled epoxy resin with enhanced performance was demonstrated.

High-efficient Synthesis of Graphene Oxide Based on Improved Hummers Method.

As an important precursor and derivate of graphene, graphene oxide (GO) has received wide attention in recent years. However, the synthesis of GO in an economical and efficient way remains a great challenge. Here we reported an improved NaNO3-free Hummers method by partly replacing KMnO4 with K2FeO4 and controlling the amount of concentrated sulfuric acid. As compared to the existing NaNO3-free Hummers methods, this improved routine greatly reduces the reactant consumption while keeps a high yield. The obtained GO was characterized by various techniques, and its derived graphene aerogel was demonstrated as high-performance supercapacitor electrodes. This improved synthesis shows good prospects for scalable production and applications of GO and its derivatives.