Structural elucidation of hexavalent Cr adsorbed on surfaces and bulks of Fe3O4 and α-FeOOH

Magnetite (Fe3O4) and goethite (α-FeOOH) were synthesized via a hydrothermal approach and utilized as adsorbents for Cr6+ removal in an aqueous medium. The typical crystal structures of the synthesized Fe3O4 and α-FeOOH were confirmed by XRD and TEM. Fe3O4 in a spherical shape with a surface area of 32 m2 g-1 was established. While α-FeOOH had a rod-like form with a larger surface area of 84 m2 g-1. Cr6+ removal in an aqueous solution was studied in various conditions to evaluate thermodynamic and kinetic parameters. The adsorption isotherms on both adsorbents fit the Langmuir model indicating monolayer adsorption. Fe3O4 showed a better adsorption ability than α-FeOOH even though it had a lower surface area. XAS and XPS analysis strongly evidenced the production of stable Cr3+ species of Fe(1-x)Cr x OOH and Fe(3-x)Cr x O4 by Cr6+ reduction and migration processes into the bulk structure. Thus, the existence of stable Cr-species in Fe3O4 structure strongly affected Cr-adsorption ability rather than the surface area of the adsorbent. However, the precipitated Cr2O3 and HCrO4 - molecules electrostatically adsorbed on the outer surface of α-FeOOH without bulk transformation. The presence of physisorbed FeO-HCrO4 species on α-FeOOH led to low reducibility and adsorption capability of Cr6+.

Fabrication of aluminum beads derived from selectively recovered Al-rich precipitates and their application into defluoridation

This work introduced a new way of fabricating a granular material with the supply of Al-rich precipitates selectively obtained from acid mine drainage (AMD), and its potential as a promising adsorbent for fluoride (F) was evaluated. Through the selective sequential precipitation (SP) process in the field, Al-rich precipitates with high purity (>81%) were collected at the high recovery rate (>99.8%) as a raw material for adsorbent fabrication. The granular adsorbent (ALB) was synthesized through encapsulation of precipitate powders by chemically inducing polymeric bead formation. The characterization results revealed that ALB possessed a highly porous structure and embedded a large number of nanoparticles of amorphous Al hydroxides inside its framework. Less adsorption of F occurred at an alkaline pH condition due to the competitive effect of hydroxyl ions. The adsorption process can be divided into fast adsorption by the outer surface and slow diffusion in the inner phase. The maximum adsorption capacity of ALB for F was calculated to be 17.7 mg g^-1 in the Langmuir isotherm model fitting results. By the repetitive adsorption/desorption and XPS results, it turned out that both chemisorption and physisorption gave a contribution in the removal of F, and the regeneration of adsorbent using NaOH was effective to restore the adsorption capability but accompanied the loss of adsorption sites. As a result, it can be concluded that a granule-type material fabricated using Al-rich precipitates selectively recovered from AMD neutralization can be considered as a promising adsorbent for F removal in aqueous solution.

Enhanced sequestration of Cr(VI) onto plant extract anchored on carbon-coated aluminium oxide composite

Aluminium oxide (ALU) and carbon-coated aluminium oxide modified with Kigelia africana leaf extract (KECA) were employed for the removal of toxic hexavalent chromium (Cr(VI)) from the aqueous phase. The adsorbents (ALU and KECA) were characterized by TGA, BET, FESEM, FTIR, Raman and XRD spectroscopic techniques. The potential of KECA and ALU to remove Cr(VI) from simulated wastewater was optimum at pH 2, sorbent dose of 0.025 g and a contact time of 200 min. Meanwhile, the uptake capacity of KECA and ALU was enhanced with an increase in sorbent dose, contact time and initial Cr(VI) concentration. The uptake of Cr(VI) onto the adsorbents ALU and KECA was kinetically best described by the pseudo-second-order and Elovich models, respectively. Besides, the equilibrium data acquired for ALU and KECA obeyed Freundlich and Langmuir isotherm models, respectively. ALU and KECA were observed to have optimum adsorption capacity of 56.45 mg g^-1 and 258.2 mg g^-1, respectively. The adsorption of Cr(VI) onto the adsorbents was thermodynamically feasible, endothermic in nature and entropy-driven. A decrease in efficiency was observed on regeneration of the absorbents, thus limiting their reusability. However, the presence of functional groups with reducing property in the extract of Kigelia africana leaves was noticed to enhance the capacity of the adsorbent to abstract Cr(VI) from the solution. Hence, this study demonstrates the potential of KECA to sequestrate Cr(VI) from an aqueous solution and provides a reference for its application to the treatment of Cr(VI)-laden industrial wastewater.

Efficient removal of chromate ions from aqueous solution using a highly cost-effective ferric coordinated [3-(2-aminoethylamino)propyl]trimethoxysilane-MCM-41 adsorbent

The present investigation involves synthesis and characterization of MCM-41-AEAPTMS-Fe(iii)Cl using coordinated Fe(iii) on MCM-41-AEAPTMS for efficient removal of hazardous Cr(vi) ions from aqueous solution. The adsorbent MCM-41-AEAPTMS-Fe(iii)Cl was characterized using small-angle X-ray diffraction (SAX), transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy dispersive X-ray (EDX), Fourier-transform infrared (FT-IR) and Brunauer-Emmett-Teller (BET) surface analyzer techniques. The BET surface area was found to be 87.598 m2 g-1. The MCM-41-AEAPTMS-Fe(iii)Cl effectively adsorbs Cr(vi) with an adsorption capacity acquiring the maximum value of 84.9 mg g-1 at pH 3 at 298 K. The data followed pseudo-second-order kinetics and obeyed the Langmuir isotherm model. The thermodynamic data proved the exothermic and spontaneous nature of Cr(vi) ion adsorption on MCM-41-AEAPTMS-Fe(iii). Further, the higher value of ΔH° (-64.339 kJ mol-1) indicated that the adsorption was chemisorption in nature.

Synthesis of mesoporous rebar MWCNT/alumina composite (RMAC) nodules for the effective removal of methylene blue and Cr (VI) from an aqueous medium

The distinctive and tuneable physical, chemical and configurational properties of carbon nanotubes (CNTs), has prompted their combination with metal oxides to contrive carbon composites showing entrancing adsorption property with incredible potential in water treatment. MWCNT/Alumina (RMAC) nodules with effective adsorption capacity were synthesized following aqueous sol-gel route. Batch sorption experiments examined the efficiency of removal of dyes and heavy metal ions from an aqueous solution on RMAC nodules. The factors affecting adsorption were studied for adsorption of methylene blue dye (MB) and hexavalent chromium by altering the MWCNT concentration from 1 wt.% to 5 wt.%. The adsorption experiment demonstrated an adsorption capacity of 187.5 and 597 mg g^-1 at 25 °C for MB and Cr (VI) respectively. Various characterization techniques such as XRD, BET, TEM, Raman, FTIR, TPD and CHN were employed to study the initial development of the material. Multiple adsorption interaction mechanisms (electrostatic interactions, hydrogen bonding, π-π electron-donor-acceptor interactions) may be credited for the remarkable adsorption capacity of these nodules. Results of this work are of great significance for environmental applications of Alumina/MWCNT composite as a promising adsorbent nanomaterial for organic pollutants from aqueous solutions. Apart from high sorption ability, these nodules offer ease of separation with splendid regeneration ability.

Eucalyptus sawdust derived biochar generated by combining the hydrothermal carbonization and low concentration KOH modification for hexavalent chromium removal

In this study, Eucalyptus sawdust was hydrothermally carbonized, and the resulting biochar was modified by a low concentration potassium hydroxide. The morphology and surface property was characterized by SEM-EDS, BET, FTIR and XPS techniques. A series of batch adsorption experiments were conducted to screen out the optimum conditions, and to investigate the isotherm, kinetics and thermodynamic behaviors. The results indicated that a high adsorption capacity of hexavalent chromium (q_e 45.88 mg/g) was achieved by the combining of hydrothermal carbonization at 220 °C and 0.05 N potassium hydroxide modification, and a high biochar yield (47.61%) was obtained. The isotherm, kinetics and thermodynamic studies suggested that the spontaneously and endothermically chemical adsorption was the main mechanism, which was partially supported by BET, FTIR and XPS results. This finding suggested that the combination of hydrothermal carbonization and a subsequent low alkali modification was an effective method to prepare a high-performance adsorbent for hexavalent chromium removal.

The effective removal of Cr(vi) ions by carbon dot–silica hybrids driven by visible light

A carbon dot–silica hybrid with a high dosage of CDs demonstrated significant visible light absorption, a large Cr(vi) adsorption capacity and good photocatalytic activity for the reduction of Cr(vi) in visible light.

Sodium alginate-based magnetic carbonaceous biosorbents for highly efficient Cr(vi) removal from water

A novel magnetic alginate-based biosorbent, aiming for efficient removal of Cr(vi) from aqueous system, was successfully synthesized.