Aqueous Cr Research Articles

Recognizing adsorption of aqueous Cr(VI), As(III), Cd(II), and Pb(II) ions by amino groups hollow polymer adsorbent

The mechanism from the molecular level and the selectivity for the removal of metal ions from aqueous has become a hotspot in current water treatment. In this study, a sulphydryl-modified hollow polymer adsorbent was designed with intrinsic amine and grafted sulphydryl groups for efficient uptake of Cr(VI), As(III), Cd(II), and Pb(II) by taking advantage of their properties of abundant functional groups, discrete hollow chambers, and high porosity. Interestingly, the benzenoid amine/quinoid imine functional groups have a stronger affinity for Cr(VI) than Cd(II) or As(III) and Pb(II). The formation of approximately 65.7% Cr(III) indicates the redox reaction from Cr(VI) to Cr(III). The electron-donating comes from benzenoid amine and oxidized to quinoid imine or protonated oxidized nitrogen units (-N = +) with Cr(VI) reduced to Cr(III) for the increase of more than double of quinoid imine and a new functional group of -N = +. Ion exchange between –COO- groups and As(III)/Cd(II), the chelation of –SH groups for Pb(II)/As(III) are plausible mechanisms.

1D/2D Rod-sheet Shape Bi2S3 Photocatalyst for Photocatalytic Reduction Cr(VI) under Visible Light

The crystal structure and morphology of photocatalysts play a crucial role in determining their photocatalytic performance. In this study, we synthesized and investigated 1D/2D Bi2S3 as a potential visible-light-activated photocatalyst for the reduction of aqueous Cr(VI). The 1D/2D Bi2S3 was synthesized using hydrothermal synthesis technique by heating Bi2(H2O)2(SO4)2(OH)2 precursor and sodium sulfide at 190 ℃ for 24 h, where the molar ratio of Bi to S elements in the reaction reagents was changed from 1:6 to 2:3. The structure, composition, and optoelectronic properties of the prepared Bi2S3 were characterized using X-ray diffraction, UV-vis diffuse reflectance spectra, field emission scanning electron microscopy, electrochemical impedance spectra, and transient photocurrent. It is shown that the prepared orthorhombic Bi2S3 has full-spectrum photoresponsivity. Bi2S3-B with 1D/2D heterogeneous structure exhibits the lowest charge carrier transport resistance, and its photocurrent intensity is nearly twice that of Bi2S3-C. It demonstrates the highest photocatalytic activity in visible-light photocatalytic reduction of aqueous Cr(VI), with a reduction rate of 54.5% after 140 minutes of light exposure. According to the bandgap of Bi2S3 and radical scavenger experiments, a reaction mechanism for the photocatalytic reduction of Cr(VI) by Bi2S3 was proposed. Furthermore, the results highlight the economic and environmentally friendly nature of the hydrothermal synthesis method using homemade precursors, which allows for the regulation of Bi2S3 morphology and the enhancement of its visible photocatalytic activity. Copyright © 2023 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0).

Using the inherent elements in yeast biomass to produce Ni2P/N-doped biocarbon composites for efficient hexavalent chromium reduction.

The heterogeneous catalytic reduction of Cr(VI) to Cr(III) is an effective strategy for aqueous Cr(VI) contamination abatement, which requires the development of highly efficient, low-cost, and recyclable catalysts. Herein, Ni2P/N-doped biocarbon composites (Ni2P/N-BC) were fabricated through an anoxic pyrolysis process using NaCl and KCl as activators. A precursor of yeast biomass provided the essential C, N, and P elements for Ni2P/N-BC formation. When adopted for Cr(VI) reduction in the presence of oxalic acid as a reductant, the fabricated Ni2P/N-BC performed superior catalytic activity with a 100% Cr(VI) reduction efficiency within 10 min (Ni2P/N-BC-5 = 0.2 g L-1, oxalic acid = 0.4 g L-1, Cr(VI) = 20 mg L-1). Typical affecting parameters, e.g., catalyst dosage, oxalic acid loading, reaction temperature, initial solution pH, and water matrix, were investigated. Ni2P/N-BC exhibited good applicability in a broad pH range from 3.0 to 9.0 and in actual aquatic systems. Cr(VI) reduction efficiency remained 92.7% after five recycle runs. Such promising catalytic activity may originate from the well-crystallized Ni2P, N-doped biocarbon framework and high specific surface area of the materials. Preliminary reaction mechanism analysis indicated that the favorable charge state of Ni2P, fast hydrogen transfer, affinity of oxalic acid to Cr(VI), and inherent electron transfer in the biocarbon matrix contributed to effective Cr(VI) reduction. This work not only provides a facile and low-cost strategy to construct Ni2P/N-doped biocarbon nanosheet composite using environmentally benign biomass but also brings new insights for the remediation of Cr(VI) contamination.

Efficient removal of Cr (VI) from aqueous solution using memory effect property of layered double hydroxide material

Treating wastewater containing pollutants with layered double hydroxide (LDH) material attracts excellent interest. LDH materials are known by the memory effect property, which leads to the reconstruction of the LDH structure after its calcination and rehydration. In this study, LDH material was prepared, calcined, and then rehydrated in an aqueous Cr(VI) solution. XRD, FTIR, and SEM-EDS analysis confirm the successful reconstruction of LDH-loading chromium on its surface and layered space. Response surface methodology (RSM) results showed that LDH mass, contact time, and chromium concentration are the main factors controlling the removal of Cr(VI). The heterogeneous sorption of chromium was described by fitting the equilibrium data to the Freundlich model. Analytical techniques, thermodynamic data, activation, and adsorption energies confirm that the removal process of Cr(VI) is endothermic, spontaneous, and physical nature. LDH exhibits good reusability performance with only a 7% reduction of initial adsorption capacity after five cycles of the calcination-rehydration process. These results show that the memory effect of LDH is helpful for the intercalation and the removal of emergent pollutants, especially for wastewater treatment.

Modified silver phosphate nanocomposite as an effective visible-light-driven photocatalyst in the reduction of aqueous Cr(VI)

A simple precipitation technique was used to synthesize novel modified Ag3PO4 (AP) with Ag2S (AS) and reduced graphene oxide (G). XRD, FT-IR, Raman spectra, HR-TEM, XPS, UV-Vis, and PL, were used to investigate the physicochemical and optical properties of the as-prepared materials. The produced AS/G/AP nanocomposites exhibited a more visible-light response, diminished bandgap energy, and enhanced the charge separation rate. For the first time, the photocatalytic efficiency of modified Ag3PO4 nanocomposites was tested in aqueous Cr(VI) reduction under visible light illumination. After 45 min of irradiation and adapting the dose to 0.05 g/L, the optimized composite with 20 wt%AS (20-AS/G/AP) attained the maximum photo-reduction efficiency of 98.7% for Cr(VI) with a rate constant of 0.0734 min−1, which is almost 49 times higher than pure AP. The XPS analysis of the spent catalyst confirmed the distinctive Cr(III) peaks. Moreover, the photocatalyst showed notable reusability for four cycles without substantial activity loss. Finally, the photo-reduction mechanism of the catalyst was suggested.

Pseudocapacitive deionization of high concentrations of hexavalent chromium using NiFe-layered double hydroxide/polypyrrole asymmetric electrode

The wastewater polluted by high concentrations of aqueous hexavalent chromium (Cr(VI)) has serious adverse effects on ecological environment and human health. As a burgeoning, environmental-friendly, and promising wastewater treatment technology, capacitive deionization (CDI) process for efficient removal of aqueous Cr(VI) needs to be investigated. In this work, pseudocapacitive NiFe-layered double hydroxide/ polypyrrole (NiFe-LDH/PPy, denoted as NiFe/PPy) electrode was synthesized by the in-situ polymerization of PPy on the NiFe-LDH surface. Results of characterizations manifested that NiFe/PPy had the chain spherical structure of PPy. In particular, excellent PPy provides effective reversible capacitance and ion diffusion and improves the conductivity and pseudocapacitance performance of NiFe-LDH. Then, the NiFe/PPy anode and activated carbon (AC) cathode were assembled into the CDI cell for capacitive removal of high concentrations of aqueous Cr(VI). In 20 mL/min of flow velocity, 1.2 V, and 100 mL 100 mg/L of Cr(VI) solution, the Cr(VI) electrosorption capacity and removal ratio attained to 47.95 mg/g and 95.89%, respectively. The electrosorption of Cr(VI) slightly decreased under the common competing anions of Cl−, NO3−, SO42−, HCO3−, and CO32−. Based on Langmuir isotherm model, the maximum theoretical removal capacity of NiFe/PPy for Cr(VI) was 111 mg/g. The electrostatic attraction, reduction, and surface complexation played primary roles in the CDI system of NiFe/PPy//AC asymmetric electrodes. The outstanding electrosorption performance makes this CDI system have a favorable application prospect in the actual wastewater treatment containing high concentrations of Cr(VI).

Removal performance and mechanisms of aqueous Cr (VI) by biochar derived from waste hazelnut shell.

Cr (VI) is still of great concern due to its high toxicity, solubility, and mobility. The transformation of waste biomass to biochar is favorable for sustainable development. Hazelnut shell, an agriculture waste, was utilized as precursor to prepare biochar at 700 °C and firstly conducted for Cr (VI) removal. Nearly all 50 mg L-1 of Cr (VI) was removed from aqueous media in 180 min under the optimal conditions. The best compliance with pseudo-second-order kinetic model (R2 = 0.999)and Langmuir isotherm model (R2 = 0.999) indicated Cr (VI) removal was a monolayer chemisorption process. The hazelnut shell biochar exhibited superior performance on Cr (VI) removal at low pH (2.0) and Cr (VI) concentrations (≤ 50 mg L-1). Various techniques illustrated that the predominant mechanism of Cr (VI) removal by hazelnut shell biochar involved electrostatic attraction, reduction, and complexation. This study provides a promising low-cost alternative for Cr (VI) elimination from acidic wastewater and groundwater after extraction following by pH adjustment to 2.0.

Influencing factors and mechanism of Cr(VI) reduction by facultative anaerobic Exiguobacterium sp. PY14.

Microbial reduction is an effective way to deal with hexavalent chromium [Cr(VI)] contamination in the environment, which can significantly mitigate the biotoxicity and migration of this pollutant. The present study investigated the influence of environmental factors on aqueous Cr(VI) removal by a newly isolated facultative anaerobic bacterium, Exiguobacterium sp. PY14, and revealed the reduction mechanism. This strain with a minimum inhibitory concentration of 400 mg/L showed the strongest Cr(VI) removal capacity at pH 8.0 because of its basophilic nature, which was obviously depressed by increasing the Cr(VI) initial concentration under both aerobic and anaerobic conditions. In contrast, the removal rate constant for 50 mg/L of Cr(VI) under anaerobic conditions (1.82 × 10-2 h-1) was 3.3 times that under aerobic conditions. The co-existence of Fe(III) and Cu(II) significantly promoted the removal of Cr(VI), while Ag(I), Pb(II), Zn(II), and Cd(II) inhibited it. Electron-shuttling organics such as riboflavin, humic acid, and anthraquinone-2,6-disulfonate promoted the Cr(VI) removal to varying degrees, and the enhancement was more significant under anaerobic conditions. The removal of aqueous Cr(VI) by strain PY14 was demonstrated to be due to cytoplasmic rather than extracellular reduction by analyzing the contributions of different cell components, and the end products existed in the aqueous solution in the form of organo-Cr(III) complexes. Several possible genes involved in Cr(VI) metabolism, including chrR and chrA that encode well-known Chr family proteins responsible for chromate reduction and transport, respectively, were identified in the genome of PY14, which further clarified the Cr(VI) reduction pathway of this strain. The research progress in the influence of crucial environmental factors and biological reduction mechanisms will help promote the potential application of Exiguobacterium sp. PY14 with high adaptability to environmental stress in Cr(VI) removal in the actual environment.

Adsorption characteristics of aqueous Cr(VI) by using magnetic γFe2O3/Fe3O4 lotus stem biochar by NaOH modification

AbstractThe article aims to investigate the adsorption characteristics of aqueous Cr(VI) biochar by magnetic γFe2O3/Fe3O4/NaOH lotus stem biochar (SMBC) which was synthesized with modification of γFe2O3, Fe3O4, and sodium hydroxide. Biochar samples before and after adsorption were characterized with Brunauer Emmett Teller, X‐ray diffractometer, Fourier transform infrared spectroscopy, X‐ray photoelectron spectrometer, magnetism. The results showed that SMBC was successfully prepared with the saturation magnetization of 66.608 emu/g and the BET specific surface area of 239.56 m2/g. The adsorption isotherm study showed that the theoretical maximum adsorption capacity of SMBC for Cr(VI) was 196.270 mg/g. The process of its adsorption of Cr(VI) fit the Langmuir isotherm adsorption model and the pseudo‐second‐order kinetic model, indicating that this process was chemical adsorption. The adsorption mechanism of SMBC to Cr(VI) mainly included electrostatic interaction, redox, and complexation. Desorption and regeneration experiment results showed that SMBC after adsorption could be easily recycled from the solution under the force of an external magnetic field. Furthermore, the adsorption capacity of SMBC reached about 80% of the original even after six desorption/regeneration cycles. This study proves that SMBC showed great potential as a separable and reproducible adsorbent to remove Cr(VI) in wastewater with excellent adsorption performance, strong adaptability, environmental friendliness, and good recycle.

Rapid reduction of aqueous Cr(VI) by oxalic acid on N-doped lignin charcoal: A significant contribution of structural defects and electronic shuttle effect

Rapid reduction of aqueous Cr(VI) to Cr(III) with waste biomass N-doped lignin charcoals (PLCNx) as a catalyst via mediate oxalic acid (OA) for hexavalent chromium wastewater. The experiment results showed that the PLCN20 composite had the greatest capacity for removing Cr(VI), and the removal rate of Cr(VI) kept higher than 98% when the addition of 5 mM OA in the system. The bubble-shaped structures of PLCNx could act as an electron shuttle mediating the electron transfer and participate in the reduction of Cr(VI), and the N vacancies defects and graphitic N on its surface were a premise for initiating the Cr(VI) reduction process. More importantly, the formation of the Cr(VI)-OA complex facilitated electron transfer as well, which significantly enhanced Cr(VI) reduction. The combined effects of the electron-donating ability of OA and the catalysis of Cr(VI)-OA complex, lead to the enhancement of Cr(VI) reduction on the PLCNx. This work not only offers novel insights into the synthesis of a superior high-efficiency heterostructure for catalyst but also proposes one new mechanism of PLCNx activate OA enhanced catalysis capacity, which helps design and optimize the catalyst process.

Distinct chromium removal mechanisms by iron-modified biochar under varying pH: Role of iron and chromium speciation

Iron-modified biochar (Fe-biochar) has been widely developed to attenuate Cr(VI) pollution in both acid and alkaline environments. However, there are few comprehensive studies on how the iron speciation in Fe-biochar and chromium speciation in solution influencing the removal of Cr(VI) and Cr(III) under varying pH. Here, multiple Fe-biochar containing Fe3O4 or Fe(0) were prepared and applied to remove aqueous Cr(VI). Kinetics and isotherms suggested that all Fe-biochar could efficiently remove Cr(VI) and Cr(III) via adsorption-reduction-adsorption. The Fe3O4-biochar immobilized Cr(III) by forming FeCr2O4, while amorphous Fe–Cr coprecipitate and Cr(OH)3 was formed when using Fe(0)-biochar. Density functional theory (DFT) analysis further indicated that pH increase caused more negative adsorption energies between Fe(0)-biochar and the pH-dependent Cr(VI)/Cr(III) species. Consequently, the adsorption and immobilization of Cr(VI) and Cr(III) species by Fe(0)-biochar was more favored at higher pH. In comparison, Fe3O4-biochar exhibited weaker adsorption abilities for Cr(VI) and Cr(III), which were in consistent with their less negative adsorption energies. Nonetheless, Fe(0)-biochar merely reduced ∼70% of adsorbed Cr(VI), while ∼90% of adsorbed Cr(VI) was reduced by Fe3O4-biochar. These results unveiled the importance of iron and chromium speciation for chromium removal under varying pH, and might guide the application-oriented design of multifunctional Fe-biochar for broad environmental remediation.

The effect of FeS on the fate of Cr(VI) in the presence of organic matters under dynamic anoxic/oxic conditions.

The reduction of Cr(VI) by FeS under anoxic conditions has been studied extensively. However, when the redox environments alternate from anoxic to oxic, the effect of FeS on the fate of Cr(VI) in the presence of organic matters still remains unknown. Therefore, this study investigated the effect of FeS coupled with humic acids (HA) and alga on the transformation of Cr(VI) under dynamic anoxic/oxic conditions. HA could promote the reduction of Cr(VI) from 86.6% to 100% under anoxic conditions due to the enhancement of the dissolution and dispersibility of FeS particles by HA. However, the strong complexing and oxidizing properties of alga inhibited the reduction of FeS. Reactive oxygen species (ROS) generated from FeS oxidation under oxic conditions could oxidize 3.80μM of Cr(III) to aqueous Cr(VI) at pH 5.0, while aqueous Cr(VI) reached to 4.83μM in the presence of HA, which was ascribed to the increasing amount of free radicals. In addition, acidic conditions and excess FeS would increase strong reducing Fe(II) and S(-II) species, and improve the efficiency of Fenton reaction. The findings provided new insights into the fate of Cr(VI) in aquatic systems containing FeS and organic matters under dynamic anoxic/oxic conditions.

Hydrochar effectively removes aqueous Cr(VI) through synergistic adsorption and photoreduction

Photoreduction is recognized as a desirable treatment method for hexavalent chromium Cr(VI) because of the advantages of high efficiency, energy-saving, and environmentally friendly. Herein, three kinds of hydrochars were synthesized by one-step hydrothermal carbonization of agricultural and forestry residuals for synergistic adsorption and photoreduction of Cr(VI). All of them showed relatively strong adsorption capability to Cr(VI). Without illumination, the maximum adsorption capacities of bamboo hydrochar (BHC), cornstalk hydrochar (CHC), and pine wood hydrochar (PHC) were 177.0, 164.2, and 156.5 mg/g, respectively. Moreover, the hydrochars also had excellent photocatalytic properties as indicated by various characterization techniques. Under light irradiation, they created synergistic effects between adsorption and photoreduction to effectively remove Cr(VI) under various conditions. The synergistic removal of Cr(VI) by adsorption and photoreduction in ultrapure water, tap water, and lake water on BHC all reached almost 100% in 40 min. Besides, the photoreduction of Cr(VI) was almost unaffected with various coexisting ions (Cl-, SO42-, PO43-), and the removal rate of Cr(VI) remained above 90% within 60 min after 5 runs of synergistic adsorption-photoreduction. The photocatalytic mechanism suggests that the BHC had suitable band energies which can reduce Cr(VI) to Cr(III) by photo-induced electron. Findings of this study demonstrate that hydrochar is a promising dual-functional material for the treatment of Cr(VI) contaminated water.

Optimized preparation and dyeing of Pterocarpus santalinus waste extract for enhancing healthy and environmental care performance of wool fabric

P. santalinus waste powders were utilized to prepare the aqueous extracts for the dyeing of wool fabric. The extraction and dyeing processes of P. santalinus were optimized for obtaining the dyed fabric with high color strength and flavonoids content. The antibacterial, antioxidant and aqueous Cr(VI) removal capacity of the dyed fabric was then examined and compared with other two plant (P. tinctorius and Lycium ruthenicum Murray) material extract dyed wool fabrics. The results indicated that the addition of ethanol in aqueous solvent could increase the content of colorants and flavonoid compounds in P. santalinus extract when soxhlet apparatus was used. The highest extraction effectiveness of P. santalinus was achieved at ethanol concentration of 80%. High dyeing temperature or low pH level caused an almost linear increment in color strength and flavonoid content of the dyed fabric. Optimum dyeing of wool fabric could be conducted when 35 mL of P. santalinus extract was used to prepare 100 mL of dyeing bath. The dyed fabric not only showed unique color, but also presented better antibacterial, antioxidant and Cr(VI) removal capacity, which were higher than those of the other two dyed fabrics. Increasing color strength or flavonoid content could enhance their functionalities.

Construction of bubbles enrichment sites and gas-diffusion-microchannel to assist Cr(VI) electrochemical reduction in a flow-through electrochemical system

Due to characteristics of the forced convection and enhanced mass transfer, graphite felt cathode based flow-through electrochemical system presented high electroreduction efficiency for aqueous Cr(VI). However, disadvantage is that bubbles generated by water electrolysis cannot pass through the liquid filled microchannel in time, which is easy to cause gas accumulation and an unstable electrochemical operation. Herein, we demonstrated graphite felt flow-through cathode designed with hydrophobic polytetrafluoroethylene (PTFE) as bubbles enrichment sites and construction of gas-diffusion-microchannel to assist in Cr(VI) electrochemical reduction. Cr(VI) reduction kinetics constant by graphite felt flow-through cathode increased 4.6 times after loading PTFE and was 2.6 ∼ 5.7 times higher than parallel-plate electrodes system. Typical electrochemical analysis suggested that PTFE can enrich and accelerate the dissolved oxygen mass transfer rate at PTFE-electrode-solution micro-interface, realizing oxygen digestion via two-electron reduction reaction. The unconsumed bubbles were transported out of electrochemical cell along gas-diffusion-microchannel to avoid gas accumulation and maintained a stable operation. Batch experiments indicated the ∼ 99.9% Cr(VI) reduction efficiency when the initial pH 2 and 100 mg/L Cr(VI) feed was treated under a single-pass pattern with flow rate of 1.5 mL/min and current density of 12.74 mA/cm2. Mechanism study revealed that faraday reaction was the main driving force for Cr(VI) reduction, while Cr(VI)-induced activation of in-situ electro-generated H2O2 contributed to formations of •OH and intermediate chromium species. This work also shows the feasibility of remediation for Cr(VI)-organic coexistence environment by a catalyst-free flow-through electrochemical system.

Removal of Aqueous Cr(VI) by Tea Stalk Biochar Supported Nanoscale Zero-Valent Iron: Performance and Mechanism

Removal of Aqueous Cr(VI) by Tea Stalk Biochar Supported Nanoscale Zero-Valent Iron: Performance and Mechanism

Dual-Functional CuO-Decorated Bi3.84W0.16O6.24/Bi2WO6 Nanohybrids for Enhanced Electrochemical Hydrogen Evolution Reaction and Photocatalytic Cr(VI) Reduction Performance

The development of dual-functional nanohybrid-based electrocatalysts/photocatalysts for producing renewable energy and removing heavy metal pollutants describes an environmentally sustainable technique to address the challenges in energy and environment applications. Herein, we adopted microwave-assisted synthesis to develop CuO-decorated Bi3.84W0.16O6.24/Bi2WO6 (CuO/biphase-BW) dual-functional nanohybrids for electrochemical hydrogen evolution reaction (HER) and photocatalytic Cr(VI) reduction applications. CuO nanoparticles with ∼50 nm were uniformly distributed on an octahedron-shaped bismuth tungstate analyzed via a high-resolution transmission electron microscope technique. Biphase bismuth tungstate optimization was meticulously controlled via the amount of copper precursor added to the reaction. Among synthesized materials, CuO1.0/biphase-BW catalysts exhibit excellent HER activity in neutral media (0.5 M Na2SO4) with a relatively low overpotential of 111 mV at a current density of 10 mA/cm2, and also it exhibits a lower Tafel slope of 237 mV/dec. Moreover, CuO1.0/biphase-BW catalyst-coated carbon cloth exhibits good conductivity and excellent electrochemical stability for 24 h. Maximum photocatalytic reduction of aqueous Cr(VI) was achieved on the CuO1.0/biphase-BW catalyst. It is worth mentioning that the tremendous activity of the synthesized electrocatalyst/photocatalyst can be ascribed to the formation and electronic interaction of CuO with biphase Bi3.84W0.16O6.24/Bi2WO6, thus increasing its surface-active sites and charge transfer. Our work provides a facile and effective way to engineer dual-functional nanohybrids for efficient water splitting in a neutral medium and reducing toxic (Cr(VI)) metal.

Fabrication of sandwich structure membrane and its performance for photocatalytic reduction of Cr(VI)

The regenerated cellulose membrane (RC) was synthesized by dissolving cotton cellulose in NaOH/CO(NH2)2 system. The Polydopamine/Bismuth tungstate/RC composite membrane (RCPB) with photocatalytic activity was synthesized by loading polydopamine-modified bismuth tungstate (PDA/BWO) composite onto the RC by blending method. The RCPB/PAN/RCPB sandwich structure membrane was synthesized by combining the polyacrylonitrile (PAN) nanofiber membrane and RCPB by scraping method, which can reduce aqueous Cr(VI) under visible light effectively. Characterization shows that the tensile strength, elongation at break, roughness and initial water contact angle of RCPB/PAN/RCPB are 32.1 MPa, 5.34%, 0.658 μm and 69.0°, respectively. The photoreduction percent of Cr(VI) by RCPB/PAN/RCPB can reach 99.7% within 120 min with a rate constant of 0.0869 min−1, and the photoreduction percent remains above 84.6% after four cycles. The introduction of PAN nanofiber membrane further improves the mechanical properties and recycling ability of RCPB. Meanwhile, the capture experiments reveal that the main active substance for photocatalytic reduction of Cr(VI) by RCPB/PAN/RCPB is photogenerated e−. This work provides a new idea for the treatment of Cr(VI)-containing wastewater.

A novel insight into the microwave induced catalytic reduction mechanism in aqueous Cr(VI) removal over ZnFe2O4 catalyst.

Aqueous Cr(VI) pollution is an emerging environmental issue. Herein, a sphere-like ZnFe2O4 catalyst with a size of ∼430nm was prepared by a solvothermal method, by which the aqueous Cr(VI) in a 50mL solution with concentration of 50mg/L was completely removed after 10min-microwave (MW) irradiation. "Surface temperature visualization" tests and COMSOL simulations showed that the surface temperature of the as-prepared ZnFe2O4 catalysts could be as high as >1000°C only after 300sMW irradiation, and the work function calculations and scavenging experiments demonstrated that the excited electrons derived by the "hot spots" effect of the ZnFe2O4 catalysts reduced the Cr(VI) to Cr(III). Kinetic reaction process of the reduction of *Cr2O72- to *CrO3H3 over the ZnFe2O4 catalysts was clarified by using DFT calculation, and the results indicated that *Cr2O72- adsorbed on the Fe atoms was more easily to be reduced, and that Fe atoms played more significant roles than the Zn and O atoms in ZnFe2O4 catalysts. The present study not only proves that the MW induced ZnFe2O4 catalytic reduction was promising for ultrafast remediation of toxic Cr(VI), but also provides a new insight into the corresponding mechanism.

Aqueous Cr (VI) removal performance of an invasive plant-derived biochar modified by Mg/Al-layered double hydroxides

In this study, MgAl layered double hydroxides (Mg-Al/LDH)-decorated invasive plant (Praxelis clematidea)-derived biochar (MgAl/LDH-PBC) were prepared by the co-precipitation method. The characterization results revealed that the modified biochar surface offered more active adsorption sites, facilitating the Cr (VI) adsorption. MgAl/LDH-PBC showed an excellent adsorption capacity of 177.88 mg/g in a monolayer, with a rapid uptake equilibrium within 180 min. In addition, even after five cycles, the desorption rates remained at about 85%, and the Cr (VI) adsorption was quite stable despite the presence of a number of other ions. Moreover, MgAl/LDH-PBC bonded efficiently with Cr (VI) via electrostatic interaction, oxidation-reduction reaction, ion-exchange reaction, complexation, and co-precipitation. Based on these findings, MgAl/LDH-PBC may be used as an adsorbent for the removal of Cr (VI) from sewage and the management of invasive plant species.