SEM and XPS Study of Cr6+ Removal from Wastewater via Reduction and Adsorption by Hierarchically Structured Carbon Composite in Neutral Media

Capacitive deionization of high concentrations of hexavalent chromium using nickel-ferric-layered double hydroxide/molybdenum disulfide asymmetric electrode

To decontaminate wastewater affected by high concentrations of aqueous hexavalent chromium (Cr(VI)) and improve the capability of layered double hydroxide (LDH) as an electrode in the capacitive deionization (CDI) process, nickel-ferric-LDH (NiFe-LDH) and NiFe-LDH/molybdenum disulfide (NiFe/MoS₂) were synthesized using a hydrothermal method. Characterization results indicated that the flower-like cluster framework of MoS₂ was decorated with the NiFe-LDH. Addition of MoS₂ improved the conductivity, capacitance reversibility, charge efficiency, coulombic efficiency, and stability of NiFe/MoS₂. The CDI performance of aqueous Cr(VI) was evaluated using NiFe/MoS₂ and activated carbon as the anode and cathode, respectively. The process reached equilibrium within 240 min. The deionization capacity and removal ratio for Cr(VI) (100 mg/L, 100 mL) were 49.71 mg/g and 99.42 %, respectively, at 1.2 V and 20 mL/min. The isothermal data were accurately described using the Langmuir model, and the theoretical maximum deionization capacity of NiFe/MoS₂ for Cr(VI) was 106.2 mg/g. The interaction mechanisms included electrostatic attraction, surface complexation, and reduction. These findings indicate that NiFe/MoS₂ has feasible applications in practical wastewater treatment for Cr(VI) removal.

Facile adsorption-electroflotation method for the removal of heavy metal ions from water using carbon nanomaterials

Due to the none-biodegradable and carcinogenic nature of toxic metal ions, a novel sorption-electroflotation method was developed using carbon nanomaterials. The metal ions removed were Ni(II), Co(II), Zn(II), Fe(II), and Cu(II) using carbon nanotubes (CNTs) and carbon nanoshells (CNSs). The porous structure, morphology, composition, and surface properties of carbon nanomaterials, viz. the presence and number of functional groups are characterized by methods of low-temperature nitrogen adsorption, scanning electron microscopy, Boem, X-ray photoelectron spectroscopy. The surface of the materials was rough with varied particle sizes. Regardless of the sorbed ion and the nature of the nanomaterial, the Langmuir, Temkin, Dubinin-Radushkevich, and Flory-Higgins models were applied to the data. The maximum sorption removal on CNTs were 15.0-69.0, 36.0-75.0, 33.0-72.0, 18.0-70.0, 29.0-69.0% for Fe(II), Zn(II), Co(II), Cu(II), and Ni(II) while these values on CNSs were 19.0-53, 23.0-58.0, 30.0-79.0, 12.0-46.0, and 41.0-86%. But after simultaneous sorption-electroflotation, the percentage removal was 99.0, 97.0, 95.0, 99.0, and 52% for these metal ions, indicating an excellent combination of sorption-electro flotation. The method is highly beneficial to work in varied pH ranges as sorption and electroflotation gave the best results in acidic and basic mediums. The method is very effective, efficient, and inexpensive and can be used for the removal of the reported metal ions in water.

Synthesis of calcium-aluminum-layered double hydroxide and a polypyrrole decorated product for efficient removal of high concentrations of aqueous hexavalent chromium

To efficiently remove high concentrations of hexavalent chromium (Cr(VI)), calcium-aluminum-layered double hydroxide (CaAl-LDH, denoted as CAL), and polypyrrole-modified CAL (CAL-PPy) were prepared by hydrothermal and in situ polymerization methods, respectively. The chemical structure, morphology, and elemental results indicated that the chain-like polypyrrole was decorated with hexagonal CAL. The specific surface area of CAL-PPy increased from 8.746 m²/g to 24.24 m²/g. The adsorption performances of CAL and CAL-PPy for aqueous Cr(VI) were investigated using batch equilibrium experiments. The decontamination process of aqueous Cr(VI) (100 mg/L) reached the equilibrium state within 50 min, and the kinetic data met the pseudo-second-order kinetic equation. The Langmuir model described the isothermal data properly, and the obtained theoretical adsorption capacity of CAL for Cr(VI) at 318 K was 34.06 mg/g, while that of CAL-PPy was 66.14 mg/g. The removal mechanisms involved electrostatic attraction, surface complexation, anion exchange, and reduction to low-toxicity Cr(III). Therefore, CAL and CAL-PPy have underlying applications in treating real wastewater containing Cr(VI).