Visible Light Accelerates Cr(III) Release and Oxidation in Cr-Fe Chromite Residues: An Overlooked Risk of Cr(VI) Reoccurrence

The reduced chromite ore processing residue (rCOPR) deposited in environments is susceptible to surrounding factors and causes reoccurrence of Cr(VI). However, the impact of natural sunlight on the stability of rCOPR is still unexplored. Herein, we investigated the dissolution and transformation behaviors of Cr(III)-Fe(III) hydroxide, a typical Cr(III)-containing component in rCOPR, under visible light. At acidic conditions, the release rate of Cr(III) under illumination markedly increased, up to 7 times higher than that in the dark, yet no Cr(VI) was produced. While at basic conditions, only Cr(VI) was obtained by photo-oxidation, with an oxidation rate of ∼7 times higher than that by δ-MnO₂ under dark conditions at pH 10, but no reactive oxygen species was generated. X-ray absorption near-edge structure and density functional theory analyses reveal that coexisting Fe in the solid plays a critical role in the pH-dependent release and transformation of Cr(III), where photogenerated Fe(II) accelerates Cr(III) produced at acidic conditions. Meanwhile, at basic conditions, the production of intermediate Cr(III)-Fe(III) clusters by light leads to the oxidation of Cr(III) into Cr(VI) through the nonradical "metal-to-metal charge transfer" mechanism. Our study provides a new insight into Cr(VI) reoccurrence in rCOPR and helps in predicting its environmental risk in nature.

Compact Carbon-Based Membrane Reactors for the Intensified Anaerobic Decolorization of Dye Effluents

Carbon-based membranes integrated with anaerobic biodegradation are presented as a unique wastewater treatment approach to deal with dye effluents. This study explores the scope of ceramic-supported carbon membrane bioreactors (B-CSCM) and ceramic-supported graphene oxide membrane bioreactors (B-CSGOM) to decolorize azo dye mixtures (ADM) and other dyes. The mixture was prepared using an equimolar composition of monoazo Acid Orange 7, diazo Reactive Black 5, and triazo Direct Blue 71 dye aqueous solution. Afterwards, as in the ADM experiment, both compact units were investigated for their ability in the biodecolorization of Methylene Blue (MB) and Rhodamine B (RhB) dye solutions, which do not belong to the azo family. The obtained outcomes revealed that the conductive surface of the graphene oxide (GO) membrane resulted in a more efficient and higher color removal of all dye solutions than B-CSCM under a wide feed concentration and permeate flux ranges. The maximum color removal at low feed concentration (50 mg·L⁻¹) and permeate flux (0.05 L·m⁻²·h⁻¹) was 96% for ADM, 98% for MB and 94% for RhB, whereas it was 89%, 94% and 66%, respectively, for B-CSCM. This suggests that the robust, cost-effective, efficient nanostructures of B-CSGOM can successfully remove diverse azo dye solutions from wastewater better than the B-CSCM does.

Ultra-efficient catalytic degradation of malachite green dye wastewater by KMnO4-modified biochar (Mn/SRBC)

In this work, KMnO₄-modified biochar was prepared from spirulina residue as the research object. Herein, we report the synthesis, characterization, and catalytic degradation performance of KMnO₄-modified biochar, given that heterogeneous catalytic oxidation is an effective way to treat dye wastewater rapidly. The Mn/SRBC catalyst prepared by KMnO₄ modification was characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffractometry, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, nitrogen adsorption–desorption and laser Raman spectroscopy. In addition, we compared the results with that of the unmodified SRBC. The results showed that the Mn/SRBC catalyst prepared by KMnO₄ modification had a rich pore structure, which provided sufficient contact area for the catalytic reaction. In the presence of H₂O₂, the catalyst could be used to catalyze the oxidative degradation of malachite green in aqueous solution with ultra-high efficiency. In the experiment, the initial pH values of the reaction system had a significant influence on the reaction rate. The removal effect of biochar on the malachite green was poor in an alkaline environment. Within a specific range, the removal rate of malachite green was proportional to the concentration of H₂O₂ in the reaction system. The degradation rate of malachite green dye at 8000 mg L⁻¹ was about 99% in the presence of the catalyst over 5 mmol L⁻¹ hydrogen peroxide for 30 min. These results show the potential application of algae residue biochar and carbon-based composite catalysts for degrading and removing dye wastewater.

In this work, KMnO₄-modified biochar was prepared from spirulina residue as the research object.

Polyacrylonitrile homogeneous blend hollow fiber membrane with stable structure as a substrate to support Fe/Mn oxide and its enhanced capability to purify dye wastewater

Blending different molecular weight polyacrylonitrile (PAN) was adopted to solve the shrinkage problem of high molecular weight PAN hollow fiber membrane, to enhance the application performance of low molecular weight PAN membrane, and to adjust the porosity, pore size distribution, and hydrophilicity of the end product. The structurally-optimized membrane was chosen as a substrate to support Fe/Mn oxides and then used as a reactor to remove dyes from their solutions in the presence of H2O2. The results showed that the flux of methylene blue (MB) aqueous solution was 83.7 L/m2 h for the PAN homogeneous blend membrane, much higher than 29.1 L/m2 h of high molecular weight PAN membrane; MB removal efficiency was 97.3%, higher than 62.3% of low molecular weight PAN membrane, and it could be reused 25 times to remove dyes from their solutions without any loss in removal efficiency. The membrane was also found to have the application advantages of decreasing H2O2 dosage, reducing operation pressure, and raising MB removal efficiency compared with other membranes reported in the pieces of literature. Therefore, we were confident that the hollow fiber membrane fabricated by us would exhibit great application potential in the field of decontaminating dye wastewater.