Removal of Cr(VI) from aqueous solutions by Na2SO3/FeSO4 combined with peanut straw biochar

An ecological new approach for treating Cr(VI)-containing industrial wastewater: Photochemical reduction

An ecological new approach for photochemical reduction of Cr(VI) in aqueous solution by adding into water-soluble copolymer, polyethylene glycol (PEG), was investigated. Various influences including light intensity, initial solution pH value, PEG molecular weight and initial concentration ratio of PEG to Cr(VI) on photochemical reduction of Cr(VI) were discussed, and a possible reaction mechanism was proposed. Experimental results revealed that Cr(VI) could be reduced to Cr(III) by PEG under sunlight irradiation. The photo-reduction rate of Cr(VI) increased with the decrease of solution pH and PEG molecular weight, but increased with the light intensity. The reduction percentage of Cr(VI) increased with the initial concentration ratio of PEG to Cr(VI). When the initial solution pH value was below 3.0, almost all of Cr(VI) was completely reduced to Cr(III) within 20 min of 50 × 10(3) lux solar irradiation in the presence of PEG. After photo-reduction, PEG and Cr(III) in aqueous solutions could be recovered by adding into a high-concentrated Na2SO4 aqueous solution to induce the formation of a stable PEG-based aqueous biphasic system. By doing so, Na2SO4 in aqueous solution could also be removed. The present work highlights a promising new route for treating the industrial wastewater containing toxic Cr(VI) ions by adding into environmental-friendly PEG for photo-reduction of Cr(VI) to Cr(III), and then salting-out recovery of PEG and removal of Cr(III) in wastewater by adding into another high-salt wastewater, so that the high-salt wastewater could also be treated.

The roles of polycarboxylates in Cr(VI)/sulfite reaction system: Involvement of reactive oxygen species and intramolecular electron transfer

In this study, the effects of polycarboxylates on both Cr(VI) reduction and S(IV) consumption in Cr(VI)/S(IV) system was investigated in acidic solution. Under aerobic condition, the productions of reactive oxygen species (ROS), i.e., SO4(-) and OH, have been confirmed in S(IV) reducing Cr(VI) process by using electron spin resonance and fluorescence spectrum techniques, leading to the excess consumption of S(IV). However, when polycarboxylates (oxalic, citric, malic and tartaric acid) were present in Cr(VI)/S(IV) system, the affinity of polycarboxylates to CrSO6(2-) can greatly promote the reduction of Cr(VI) via expanding the coordination of Cr(VI) species from tetrahedron to hexahedron. Besides, as alternatives to S(IV), these polycarboxylates can also act as electron donors for Cr(VI) reduction via intramolecular electron transfer reaction, which is dependent on the energies of the highest occupied molecular orbital of these polycarboxylates. Notably, the variant electron donating capacity of these polycarboxylates resulted in different yield of ROS and therefore the oxidation efficiencies of other pollutants, e.g., rhodamine B and As(III). Generally, this study does not only shed light on the mechanism of S(IV) reducing Cr(VI) process mediated by polycarboxylates, but also provides an escalated, cost-effective and green strategy for the remediation of Cr(VI) using sulfite as a reductant.

Adsorption kinetics of magnetic biochar derived from peanut hull on removal of Cr (VI) from aqueous solution: Effects of production conditions and particle size

Magnetic biochar was made from peanut hull biomass using iron chloride in a simplified aqueous phase approach and pyrolysis at alternative peak temperatures (450-650 °C). Magnetic biochar showed an extreme capacity for adsorption of hexavalent chromium Cr (VI) from aqueous solution, which was 1-2 orders of magnitude higher compared to standard (non-magnetic) biochar from the same feedstock. Adsorption increased with pyrolysis temperature peaking at 77,542 mg kg(-1) in the sample pyrolysed at 650 °C. In contrast to magnetic biochar, the low adsorption capacity of standard biochar decreased with increasing pyrolysis temperature. The fine particle size of magnetic biochar and low aqueous pH were also important for adsorption. Surfaces of products from batch adsorption experiments were characterized by scanning electron microscopy, energy-dispersive X-ray analysis, X-ray diffraction, X-ray photoelectron spectroscopy and vibrating sample magnetometer. This revealed that γ-Fe2O3 was crucial to the properties (adsorbance and magnetism) of magnetic biochar. The removal mechanism was the Cr (VI) electrostatic attracted on protonated -OH on γ-Fe2O3 surface and it could be desorbed by alkaline solution. Findings suggest that pyrolysis has potential to create effective, magnetically recoverable adsorbents relevant to environmental application.

A novel graphene oxide coated biochar composite: synthesis, characterization and application for Cr(vi) removal

In the current work, a graphene oxide coated water hyacinth biochar composite (WHB-GO) was synthesized to remove Cr(vi) from aqueous solution.

Removal of phosphate and hexavalent chromium from aqueous solutions by engineered waste eggshell

In this work, a novel adsorbent derived from waste eggshell (ES) was used for phosphate and hexavalent chromium adsorption.

Cr(vi) removal by mesoporous FeOOH polymorphs: performance and mechanism

The mesoporous FeOOH polymorphs, i.e., goethite (α-FeOOH), akaganeite (β-FeOOH), lepidocrocite (γ-FeOOH), and feroxyhyte (δ-FeOOH) were synthesized and characterized before and after reaction with Cr(vi) using various analytical techniques.

Synergetic Transformations of Multiple Pollutants Driven by Cr(VI)-Sulfite Reactions

Reduction of Cr(VI) is often deemed necessary to detoxify chromium contaminants; however, few investigations utilized this reaction for the purpose of treating other industrial wastewaters. Here a widely used Cr(VI)-sulfite reaction system was upgraded to simultaneously transform multiple pollutants, namely, the reduction of Cr(VI) and oxidation of sulfite and other organic/inorganic pollutants in an acidic solution. As(III) was selected as a probe pollutant to examine the oxidation capacity of a Cr(VI)-sulfite system. Both (•)OH and SO4(•-) were considered as the primary oxidants for As(III) oxidation, based on the results of electron spin resonance, fluorescence spectroscopy, and specific radicals quenching. As(III)-scavenging, oxidative radicals greatly accelerated Cr(VI) reduction and simultaneously consumed less sulfite. In comparison with a Cr(VI)-H2O2 system with 50 μM Cr(VI), Cr(VI), the sulfite system had excellent performance for both As(III) oxidation and Cr(VI) reduction at pH 3.5. Moreover, in this escalated process, less sulfite was required to reduce Cr(VI) than the traditional Cr(VI) reduction by sulfite process. This effectively improves the environmental compatibility of this Cr(VI) detoxification process, alleviating the potential for SO2 release and sulfate ion production in water. Generally, this study provides an excellent example of a "waste control by waste" strategy for the detoxification of multiple industrial pollutants.

A preliminary investigation of the mechanism of hexavalent chromium removal by corn-bran residue and derived chars

Biochars were prepared from corn-bran residue (CBR) at 300–600 °C, which were utilized to removal Cr(vi) from aqueous solutions. The Cr(vi) removal capacity of CBR and CBR600 was attributed to the predominance of either reduction or adsorption.