Reduction Efficiency Of Cr Research Articles

In-situ and ex-situ preparation of Bi2S3 on the BiVO4/TiO2 to construct Bi2S3/BiVO4/TiO2 heterojunction for efficient Cr(VI) reduction

The environmentally friendly and rapid removal of hexavalent chromium (Cr(VI)) from wastewater has garnered significant attention, driven by the severe acute toxicity and carcinogenic properties associated with Cr(VI). The design and synthesis of heterojunctions are recognized as effective solutions for enhancing the photocatalytic performance of TiO2 based materials. BiVO4/TiO2 and Bi2S3/BiVO4/TiO2 composite films (BVT) were prepared via a simple hydrothermal method. The influence of Bi2S3 preparation methods (In-situ and Ex-situ) on the morphology, structure and properties of BVT was studied. The relative crystallinity of TiO2, BiVO4/TiO2, BVT(In-situ) and BVT(Ex-situ) were 50 %, 62 %, 72 % and 92 %, respectively. SEM shows that the morphology of Bi2S3 in BVT(In-situ) was nanoribbon and that of Bi2S3 in BVT(Ex-situ) was radioactive sphere. Compared with pure TiO2 and BiVO4/TiO2, the light absorption range of BVT composite material has been expanded from the ultraviolet light region to the visible region, the bandgap has been significantly narrowed, and the photocurrent density has been increased. The BVT(In-situ) shows stronger photoelectrical performance and reduction efficiency of Cr (VI) compared to BVT(Ex-situ), reaching 93.9 %. The photocatalytic reaction rate of BVT(In-situ) and BVT(Ex-situ) are 0.0291 min−1 and 0.0266 min−1, respectively. Density functional theory (DFT) calculations indicate that the work function of BiVO4 is higher than that of Bi2S3, and electrons will transfer from Bi2S3 to BiVO4 at the heterojunction interface. The BVT(In-situ) heterojunction constructed in this experiment greatly improves the separation and transport efficiency of photo-generated carriers, laying the foundation for the wide application of BVT heterojunction in the field of optoelectronics and providing a strategy for photocatalytic reduction of Cr(VI). The BVT(In-situ) with FTO conductive glass, it also can be applied to photovoltaic fields such as solar cells, photocatalysis and photodetectors.

Electron beam synergetic removal of microplastics and hexavalent chromium: Synergetic removal process and mechanism

Microplastics are ubiquitous in the environment and aged microplastics are highly susceptible to absorbing pollutants from the environment. In this study, electron beam was innovatively used to treat PVC composite Cr(VI) pollutants (Composite contaminant formed by polyvinyl chloride microplastics with the heavy metal hexavalent chromium). Experiments showed that electron beam was able to achieve synergistic removal of PVC composite Cr(VI) pollutants compared to degrading the pollutants alone. During the electron beam removal of PVC composite Cr(VI) pollutants, the reduction efficiency of Cr(VI) increased from 57% to 92%, Cl− concentration increased from 3.52 to 12.41 mg L−1, and TOC concentration increased from 16.72 to 26.60 mg L−1. The research confirmed that electron beam can effectively promote the aging degradation of PVC, alter the physicochemical properties of microplastics, and generate oxygen-containing functional groups on the surface of microplastics. Aged microplastics enhanced the adsorption capacity for Cr(VI) through electrostatic and chelation interactions, and the adsorption process followed second-order kinetics and the Freundlich model. Theoretical calculations and experiments demonstrated that PVC consumed oxidizing free radical through dechlorination and decarboxylation processes, generating inorganic ions and small organic molecules. These inorganic ions and small organic molecules further reacted with oxidizing free radical to produce reducing free radicals, facilitating the reduction of Cr(VI). Cr(VI) continuously consumed the educing free radicals to transform into Cr (Ⅲ), enhancing the system oxidative atmosphere and promoting the oxidation degradation of PVC. This study investigated the formation and synergistic removal processes of PVC composite pollutants, offering new insights for controlling microplastics composite pollution.

New insight into the effects of p-benzoquinone on photocatalytic reduction of Cr(VI) over Fe-doped g-C3N4

It is of great significance to establish an effective method for removing Cr(VI) from wastewater. Herein, Fe-doped g-C3N4 (namely Fe-g–C3N4–2) was synthesized and then employed as photocatalyst to conduct the test of Cr(VI) reduction. Notably, the embedding of Fe ion in g-C3N4 can offer the Fe2+/Fe3+ redox couples, so reducing the interfacial resistance of charge transfer and suppressing the recombination of photogenerated electrons and holes. The impurity energy levels will form in g-C3N4 after the introduction of Fe ion, thereby boosting the light absorption capacity of catalyst. Thus, Fe-g–C3N4–2 showed good performance in photocatalytic Cr(VI) reduction, and the reduction efficiency of Cr(VI) can reach 39.9% within 40 min. Different with many previous studies, current work unexpectedly found that the addition of p-benzoquinone (BQ) can promote the Cr(VI) reduction, and the reduction efficiency of Cr(VI) over Fe-g–C3N4–2 was as high as 93.2% in the presence of BQ (1.5 mM). Further analyses showed that BQ can be reduced to hydroquinone (HQ) by photogenerated electrons, and UV light can also directly induce BQ to generate HQ by using H2O as the hydrogen donor. The HQ with reducing ability can accelerate the Cr(VI) reduction. In short, current work shared some novel insights into photocatalytic Cr(VI) reduction in the presence of BQ. Future research should consider possible reactions between photogenerated electrons and BQ. For the UV-induced photocatalysis, the suitability of BQ as the scavenger of O2•‒ must be given carefully consideration.

S-scheme MOF-on-MOF heterojunctions for enhanced photo-Fenton Cr(VI) reduction and antibacterial effects

The photocatalytic efficiency is commonly restrained by inferior charge separation rate. Herein, the S-scheme MIL-100(Fe)/NH2-MIL-125(Ti) (MN) photo-Fenton catalyst with the built-in electric field (BEF) was successfully constructed by a simple ball-milling technique. As a result, the MN-3 (the mass ratio of MIL-100(Fe) to NH2-MIL-125(Ti) was 3) composite presented the best visible-light-induced photocatalytic ability, in contrast to pure MIL-100(Fe) and NH2-MIL-125(Ti). The reduction efficiency of Cr(VI) almost reached 100% within 35 min of illumination. Moreover, the MN-3 heterojunction also exhibited the highest antibacterial activity, and about 100% E. coli and more than 90% S. aureus were killed within 60 min of illumination. In photo-Fenton system, In the photo-Fenton system, e−, O2•- and Fe2+ played vital roles for Cr(VI) reduction, and •OH, h+ and O2•- and 1O2 were responsible for sterilization. Additionally, 5 cyclic tests and relevant characterizations confirmed the excellent repeatability and stability of the composite. Also, the S-scheme charge transfer process was put forward. This work offers a novel idea for establishing the MOF-on-MOF photo-Fenton catalyst for high-efficiency environmental mitigation.

Preparation of chromium oxide by hydrothermal reduction of sodium chromate aqueous solution with hydrogen and application to chromite ore processing

Chromium oxide (Cr2O3) has wide application in various industries. Its traditional production process, uses sodium carbonate (Na2CO3), which is converted into cheaper byproducts, including sodium sulfate (Na2SO4) and sodium bisulfate (NaHSO4). These byproducts contain hexavalent chromium (Cr(VI)), which can lead to serious environmental pollution. In this work, a cleaner production process for Cr2O3 through the hydrothermal reduction of a sodium chromate (Na2CrO4) aqueous solution with hydrogen (H2) was studied. Nearly single-phase chromium oxide hydroxide (CrOOH) formed as a solid product after the hydrothermal reduction. Then, Cr2O3 was obtained by the thermal decomposition of CrOOH. The advantages of the new process are remarkable. First, the raw material, the Na2CrO4 aqueous solution, was the direct intermediate product in the pressure oxidative leaching of chromite ore with sodium hydroxide (NaOH). Second, the reduction efficiency of Cr(VI) increased up to 97.6%, and the total yield of chromium from Na2CrO4 to Cr2O3 was as high as 94.2%. Finally, 96.6% of sodium could be converted into NaOH, so the aqueous solution in the hydrothermal hydrogen reduction process could be recycled as the leaching agent for the pressure oxidative leaching process of chromite ore. Consequently, a complete cleaner and shorter preparation route from chromite ore to Cr2O3 was achieved by integrating three processes: the pressure oxidative leaching of chromite ore, the hydrothermal reduction of the Na2CrO4 aqueous solution, and the thermal decomposition of CrOOH. The new method achieves the high-efficiency use of chromium resources, cyclic utilization of material, and extremely low environmental pollution of Cr(VI), and well exemplifies the principle of the 3Rs (Reduce, Recycle, Reuse).

Detoxification and resource utilization of soda ash chromite ore processing residue by copper slag

Chromite ore processing residue (COPR) and copper slag (CS) are industrial waste slags that cause environmental pollution and require effective treatment. This study proposes a new strategy for the co-treatment and resource utilization of COPR and CS. Under the optimal conditions (liquid-solid ratio of 9.99; H2SO4 concentration of 0.85 M; the CS/COPR mass ratio of 9.98%; temperature of 88.20 ℃; and duration of 60 min), the reduction efficiency of Cr(VI) reached 99.48%, and the total Cr leaching concentration of residue reached 1.35 mg/L, which was below the USEPA regulatory limit of 5 mg/L. In addition, the detoxicated slag containing 61.44 wt% Fe2O3 can be used as a raw material in the steel industry. Through kinetic, characterization, and DFT analyses, the effective remediation of Cr(VI) in COPR by CS is a combination of adsorption and reduction. The unreacted shrinkage nuclear reaction model under the control of the surface chemical reaction is the most suitable model to describe the process, and when the apparent activation energy is 63.90 kJ/mol, the apparent rate equation is:1-(1-x)1/3= 1·371×107[CS/COPR]2·35004 [H2SO4]2·3945 [L/S]1·95338 exp(–63·90/RT)This is an important study for waste-waste co-treatment and resource utilization of industrial waste.

Construction of S-scheme CdS/g-C3N4 heterojunctions with enhanced photocatalytic H2 evolution and Cr(VI) reduction performance

Construction of heterojunctions is one of the most attractive approaches to synthesize highly active photocatalysts. In this work, we prepared CdS/g-C3N4 (CSCN) heterojunctions with different molar ratio of CdS/g-C3N4 via ball milling for photocatalytic hydrogen production and removal of Cr(VI). The successful construction of CSCN heterojunctions was proved by the various characterization methods. Formation of CSCN heterojunctions not only generates more vacancies to act as active site, but also accelerates the separation of photoinduced charge carriers, endowing with more electrons to participate in the photocatalytic reduction reaction. 2%CSCN sample was confirmed to hold the highest separation and transfer performance of photoinduced e−/h+ pairs by photoelectric tests. 2%CSCN samples exhibit the highest photocatalytic H2 evolution rate (680.04 μmol/g/h), which is 3.63-fold higher than that of the bare g-C3N4 (146.83 μmol/g/h). The activity of photocatalyst was further examined by removal of Cr(VI) experiment. The results show that the reduction efficiency of Cr(VI) on 2%CSCN is 2.46-fold of that of on the single g-C3N4, and the elimination rate of Cr(VI) on 2%CSCN can reach 64% after 1 h under simulated sunlight illumination. The improved photocatalytic performance can be ascribed to the creation of appropriate C vacancies and the widened light absorption range induced by CdS. A rational mechanism for enhanced photocatalytic activity was proposed to further understand the separation and transfer of photogenerated carriers produced by heterojunctions.

Asymmetric Alternative Current Electrochemical Method Coupled with Amidoxime-Functionalized Carbon Felt Electrode for Fast and Efficient Removal of Hexavalent Chromium from Wastewater

A large amount of Cr (VI)-polluted wastewater produced in electroplating, dyeing and tanning industries seriously threatens water ecological security and human health. Due to the lack of high-performance electrodes and the coulomb repulsion between hexavalent chromium anion and cathode, the traditional DC-mediated electrochemical remediation technology possesses low Cr (VI) removal efficiency. Herein, by modifying commercial carbon felt (O-CF) with amidoxime groups, amidoxime-functionalized carbon felt electrodes (Ami-CF) with high adsorption affinity for Cr (VI) were prepared. Based on Ami-CF, an electrochemical flow-through system powered by asymmetric AC was constructed. The mechanism and influencing factors of efficient removal of Cr (VI) contaminated wastewater by an asymmetric AC electrochemical method coupling Ami-CF were studied. Scanning Electron Microscopy (SEM), Fourier Transform Infrared (FTIR), and X-ray photoelectron spectroscopy (XPS) characterization results showed that Ami-CF was successfully and uniformly loaded with amidoxime functional groups, and the adsorption capacity of Cr (VI) was more than 100 times higher than that of O-CF. In particular, the Coulomb repulsion effect and the side reaction of electrolytic water splitting were inhibited by the high-frequency anode and cathode switching (asymmetric AC), the mass transfer rate of Cr (VI) from electrode solution was increased, the reduction efficiency of Cr (VI) to Cr (III) was significantly promoted and a highly efficient removal of Cr (VI) was achieved. Under optimal operating conditions (positive bias 1 V, negative bias 2.5 V, duty ratio 20%, frequency 400 Hz, solution pH = 2), the asymmetric AC electrochemistry based on Ami-CF can achieve fast (30 s) and efficient removal (>99.11%) for 0.5-100 mg·L-1 Cr (VI) with a high flux of 300 L h-1 m-2. At the same time, the durability test verified the sustainability of the AC electrochemical method. For Cr (VI)-polluted wastewater with an initial concentration of 50 mg·L-1, the effluent concentration could still reach drinking water grade (<0.05 mg·L-1) after 10 cycling experiments. This study provides an innovative approach for the rapid, green and efficient removal of Cr (VI) containing wastewater at low and medium concentrations.

Construction of novel type II heterojunction WO3/Bi2WO6 and Z-scheme heterojunction CdS/Bi2WO6 photocatalysts with significantly enhanced photocatalytic activity for the degradation of rhodamine B and reduction of Cr(VI)

As a novel photocatalyst with visible light responsiveness, Bi2WO6 has excellent activity in treating wastewater containing organic pollutants and noble metals. However, its photocatalytic action is limited due to the rapid recombination of photogenerated electrons and holes. Based on the study of Bi2WO6, a type Ⅱ heterojunction photocatalyst (WO3/Bi2WO6) and a Z-scheme heterojunction photocatalyst (CdS/Bi2WO6) were constructed. Composites exhibited higher catalytic activity than pure Bi2WO6. The degradation efficiency of RhB(50 mg/L) and reduction efficiency of Cr(VI) (50 mg/L) by 5%WO3/Bi2WO6 catalyst within 60 min of photocatalytic reaction was 98.82% and 53.82%, respectively. The degradation efficiency of RhB and reduction efficiency of Cr(VI) by 15%CdS/Bi2WO6 catalyst was 99.52% and 69.52%, respectively. By studying the structural composition, microscopic morphology, photoelectrochemical performance, and light absorption performance of the catalysts of the two different systems. These mechanisms may help to explain the reasons for the enhanced activity of the composite photocatalysts. In addition, mixed wastewater containing organic pollutants (RhB) and heavy metal ions (Cr(VI)) was used to simulate real and complex wastewater, which provided more ideas for wastewater treatment problems.

Removal mechanism of tetracycline-Cr(Ⅵ) combined pollutants by different S-doped sludge biochars: Role of environmentally persistent free radicals

In this work, by using sodium thiosulfate as the S source, S-doped biochars were prepared to remove tetracycline/hexavalent chromium (TC-Cr (Ⅵ)) combined pollutants in aqueous solutions. The concentration of environmentally persistent free radicals (EPFRs) was used to directly determine the degradation of TC and the reduction of Cr (Ⅵ). The concentration of EPFRs in S-doped hydrothermal + pyrocarbon (S-HPBC) (3.64 × 1019 spins/g) was greater than that of S-doped hydrochar (S-HBC) (5.64 × 1018 spins/g) and S-doped pyrocarbon (S-PBC) (6.53 × 1018 spins/g). The increase in EPFRs concentration after S doping was positively correlated with the number of defect structures. In the TC-Cr (Ⅵ) system, the reduction efficiency of Cr (Ⅵ) decreased due to competition for electrons, while the TC degradation efficiency remained high. This was likely because Cr (Ⅵ) reduction promoted the degradation of TC. In addition, de-complexation was the primary factor for the removal of TC-Cr (Ⅵ), and some ROS were consumed during this process. The thiophene groups (-C-S-C-) that formed after S-doping of biochar were the main functional groups involved in the catalytic degradation of TC. In the radical pathway, SO4•- and •OH provided the greatest contribution to the degradation of TC, while 1O2 contributed the most to TC degradation via a non-radical pathway. The regeneration experiment confirmed that S-doped biochar could be reused and maintained a high pollutant removal efficiency. S-HPBC is a promising modified biochar material for removing mixed pollutants.

Synergistic Cr(VI) reduction and adsorption of Cu(II), Co(II) and Ni(II) by zerovalent iron-loaded hydroxyapatite

Multi-metal contaminated soil, such as Cr(VI), Cu(II), and Co(II), still challenge the environmental remediation. In this work, zerovalent iron-loaded hydroxyapatite (ZVI/HAP) was first applied to simultaneously adsorb multi-metal in contaminated soil. During the remediation, the co-existing Cu(II), Ni(II), and Co(II) were adsorbed and precipitated onto ZVI/HAP. This “spontaneous deposition” simultaneously achieved the adsorption of the cationic metals and improved the isoelectric point of ZVI/HAP to 4.83 from 1.59, thus significantly alleviating the electronegativity to enhance the capture and reduction efficiency of Cr(VI). The application of ZVI/HAP resulted in the reduction of more than 99% of total Cr(VI) in contaminated soil, and the almost complete adsorption of water-soluble and DTPA-extractable Cu, Ni and Co within 20 d. Based on the sequential extraction and risk reduction assessment, soil Cr, Cu, Ni, and Co speciation was transformed from an unstable state (exchangeable and carbonate-bound fractions) to a relatively stable state, reducing the risk of heavy metals in contaminated soil significantly. This study developed an efficient strategy for the remediation of multi-metal contaminated soil.

Novel 3D Cu2O/N-CQD/ZIF-8 composite photocatalyst with Z-scheme heterojunction for the efficient photocatalytic reduction of Cr(Ⅵ)

ZIF-8 exhibits poor visible light response and rapid electron-hole recombination. To address these problems, we constructed an N-CQD/ZIF-8 framework by in-situ synthesis method, then prepared a novel Z-scheme heterostructure photocatalyst (Cu2O/N-CQD/ZIF-8) by reduction precipitation method for the efficient photocatalytic reduction of Cr(Ⅵ). Cu2O/N-CQD/ZIF-8 (with a 10% Cu2O loading) had a unique heterostructure and exhibited significantly improved performance for Cr(Ⅵ) photoreduction. This photocatalyst successfully reduced 98.99% of the Cr(Ⅵ) in a pH = 3 solution system, which was 6 times higher than that of ZIF-8. Meantime, due to its stable interfacial adhesion, the reduction efficiency of Cr(Ⅵ) by Cu2O/N-CQD/ZIF-8 still reached 97.13% after five cycles. Therefore, Cu2O/N-CQD/ZIF-8 exhibited superior photocatalytic activity and recyclability compared with unmodified ZIF-8. The charge transfer path of this Z-scheme heterojunction was verified by free radical trapping experiments and X-ray photoelectron spectroscopy. Moreover, a possible charge transfer mechanism was proposed. N-CQD provided a potential driving force for the construction of the internal Z-scheme charge transfer system. Moreover, the synergistic effect between Cu2O and ZIF-8 inhibited the rapid recombination of photogenerated electron-hole pairs. This work provides potential insights for the innovative design of high-performance photocatalysts based on metal-organic frameworks for the restoration of Cr(Ⅵ)-containing wastewater.

N, S co-doped carbon quantum dots/TiO2 composite for visible-light-driven photocatalytic reduction of Cr (VI)

A novel carbon quantum dots (CQDs) sensitized TiO2 (N, S-CQDs/TiO2) composite was prepared and used as a visible-light-active photocatalyst for the reduction of Cr (VI), one of the most toxic heavy metal ions. Experimental results from the batch reaction process indicated that the composite was able to completely reduce Cr (VI) to Cr (III) at pH 2.0 after 3 h′ exposure to visible light with the initial Cr (VI) concentration being 20.0 mg/L and a catalyst dosage of 1.0 g/L. It was also found that lower pH and lower initial concentration of Cr (VI) led to high reduction efficiency. Compare with pure TiO2, a four-fold increase in reduction efficiency of Cr (VI) was achieved by the N, S-CQDs/TiO2 composite under the same reaction conditions. The prepared composite also demonstrated good reusability and stable photocatalytic performance in real water samples. Results from this study confirm that sensitization of TiO2 with CQDs is efficient in broadening the light absorption range of TiO2, expanding its application in visible-light-driven photocatalysis.

Fabrication of 3D self-supported MoS2-Co-P/nickel foam electrode for adsorption-electrochemical removal of Cr(Ⅵ).

Electrochemistry is considered to be one of the most efficient and environment-friendly methods for removing highly toxic Cr (Ⅵ). In this study, a 3D self-supported MoS2-Co-P/nickel foam (NF) electrode was prepared via a calcination-hydrothermal process to remove the Cr (Ⅵ) in aqueous medium. Scanning electron microscope (SEM) analysis indicated that the pine-needle-like Co2P nanoneedle and flower-like MoS2 nanosheets were successfully loaded on the three-dimensional (3D) framework of NF, which provided abundant active sites. The electrode modified by Co, P and MoS2 exhibited high removal efficiency of Cr (Ⅵ) (96.9%) at pH 3.0, current of 0.128mA and voltage of 2.5V. Co, P and MoS2 have the synergistic promotion on the catalytic performance of electrodes, and the reduction efficiency of Cr (Ⅵ) was greatly improved by 127.5 times relative to pure NF. The enhanced removal of Cr (Ⅵ) was related to the coupling effect of adsorption and electrocatalytic reduction. The mechanism study indicated that electron (e-) is the active species of Cr (Ⅵ) reduction. The Cr (Ⅵ) removal rate was maintained at 90±1% after five successive cycle experiments, demonstrating good stability and potential industrial applications of MoS2-Co-P/NF.

Novel ZnFe2O4/Bi2S3 high-low junctions for boosting tetracycline degradation and Cr(VI) reduction

The fabrication of photocatalysts with high reaction activity for decomposition of pharmaceutical contaminants and detoxication of heavy metal ions in wastewater is challenging. In this study, novel ZnFe2O4/Bi2S3 high-low junctions with different work functions and tight interface were constructed by the growth of Bi2S3 over ZnFe2O4 under hydrothermal conditions, and the content of ZnFe2O4 was optimized. The optimal 12 % ZnFe2O4/Bi2S3 sample exhibited the best visible-light photocatalytic performance of 91.6 % tetracycline removal at solution pH of 4.72 and 96.7 % reduction efficiency of Cr(VI) at solution pH of 5.51 within 2 h. Holes (h+), hydroxyl radicals (OH) and superoxide radicals (O2−) jointly attacked tetracycline, leading to efficacious decomposition of tetracycline and sharp toxicity reduction. The toxicity prediction of degradation intermediates by ECOSAR software and E. coli growth further confirmed the significant role of ZnFe2O4/Bi2S3 high-low junction in toxicity reduction of tetracycline. The photogenerated electrons were involved in Cr(VI) reduction. The relationship between structure and photocatalytic activity was set forth from the view of light absorption, band structure, internal electric field at interface, charge separation and migration behaviors. Importantly, the photocatalytic mechanism of ZnFe2O4/Bi2S3 high-low junctions with different work functions and intimate interface was first provided based on edge energy offset. This work will provide new insights for the preparation and application of high-low junction photocatalytic materials based on CB/VB edge energy shift and unique photogenerated charge transfer mechanism.

Treatment of soda ash chromite ore processing residue by Waste-Molasses-Based Ball Milling: A new strategy for disposal of waste with waste

The current technology of soda ash Chromium Ore Processing Residue (COPR) detoxification consumes a large amount of acids (as in the wet reduction process) and requires high temperature, along with the generation of carbon dioxide (as in the dry reduction process), and thus, is not cost-effective and environmentally-friendly. Waste molasses (WM) is waste biomass consisting of various reducing sugars, a cost-effective reducing agent. According to the idea of treating waste with waste, this study put forward a ball milling + WM method to detoxify leached-soda ash COPR (e.g., dried COPR containing 1622 ± 28 Cr(VI)/kg). Through response surface experiments, it was determined that under the best conditions (i.e., the milling speed = 600 rpm, the milling time = 2 h, WM dosage = 7.5% of the dry mass of COPR, and the ball-to-powder weight ratio, BPR = 12), the reduction efficiency of Cr(VI) reached 98.35%. The Toxicity Characteristic Leaching Procedure (TCLP) total Cr concentration reached 1.82 mg/L, meeting the standard (total Cr < 5 mg/L) of the US Environmental Protection Agency (USEPA). The associated reaction mechanism was determined as: under the action of mechanical energy, the structure of Cr(VI)-embedded matrixes was destroyed, and the internal, otherwise non-exchangeable, Cr(VI) was exposed and underwent redox reactions with WM under alkaline conditions. In brief, WM-based ball milling to detoxify COPR is cost-effective and provides a new reference for the disposal of waste with waste.

Photocatalytic Reduction of Cr(VI) and Degradation of Organic Pollutants by Z-Scheme g-C3N4/Bi2S3 Heterojunction

Photocatalytic reduction of hexavalent Cr(VI) couping oxidative degradation of organic contamination is an emerging and practical approach for water treatment. In this study, Z-scheme g-C3N4/Bi2S3 heterojunctions with intimate interface were successfully synthesized by direct growth of Bi2S3 on g-C3N4 surface. Notably, the photocatalytic performance of Z-scheme g-C3N4/Bi2S3 was influenced by g-C3N4 content. The optimized 2% g-C3N4/Bi2S3 heterojunction shows the highest photocatalytic reduction performance with 93.4% reduction efficiency of Cr(VI) under UV-visible light due to efficient separation and transfer of charge carriers and proper band structure. Furthermore, 2% g-C3N4/Bi2S3 can degrade tetracycline and Rhodamine B. Free radical capturing and quantitative tests indicate that holes and superoxide radicals are primary active species for the degradation of organic pollutants, while Cr(VI) was reduced to Cr(III) by the photogenerated electrons. Overall, this study provides new insight into the synthesis of high-performance Z-scheme heterojunctions for the future advancement of photocatalysis technology.

Enhancement of chromium contaminated soil remediation by ferrous sulfate with the addition of biogas residue

AbstractIn this study, a cost‐effective and eco‐friendly method was developed to remediate soil contaminated by hexavalent chromium (Cr[VI]) using ferrous sulfate and biogas residue (BR). The results show that the addition of BR at mole ratio of Fe(II)/Cr(VI) = 3:1 and mass ratio of BR (dry mass)/Cr(VI) = 70:1 significantly improved the reduction of Cr(VI) and reduced its mobility. Moreover, the reduction efficiency of Cr(VI) was higher than 85% at day 1 and reached a maximum of 96.7% at day 7. The X‐ray photoelectron spectroscopy, scanning electron microscopy, and microbial diversity analyses indicated that the microbial mechanism created by the BR addition was crucial for the long‐term and stable reduction of Cr(VI). The enriched Cr(VI)‐reducing bacteria genera in the BR directly reduced Cr(VI), whereas the reducing bacteria in the BR indirectly reduced Cr(VI) to Cr(III) through the electron transport system in the reaction system. The addition of BR can lead to a high reduction efficiency and long‐term stabilization of Cr(VI)‐contaminated soils. This study provides reference values for the combined remediation technology for Cr(VI)‐contaminated soils.

Electrochemical synthesis of PANI-ERGO composite electrode and its application in the reduction of hexavalent chromium

Electroreduction is a promising approach for toxic Cr (VI) remediation due to its efficiency and excellent environmental compatibility. This study researches the facile electrode preparation and high mass transfer efficiency in Cr (VI) reduction application. Firstly, polyaniline (PANI)/electrochemically reduced graphene oxide (ERGO)/Ti electrode was synthesized in situ on a titanium mesh substrate via a three-step electrochemical method. The pretreated electrodes were characterized by SEM, FTIR, XPS. The EIS and CV were also adopted to prove further at the performance of electron transfer and electrocatalytic activity of Cr (VI) on the interface of the composite film. Subsequently, the reduction efficiency of Cr (VI) was invested in the electroreduction process by the PANI/ERGO/Ti electrode, factors such as potential condition, pH, and initial concentration of Cr (VI) were also considered. It showed higher degradation percentage and current efficiencies of the PANI/ERGO/Ti compared to the PANI/Ti and the ERGO/Ti. And revealed that lower pH is more favorable, and higher initial concentration is more accessible to reduce. The proposed test result reached 99.29% Cr (VI) reduction efficiency with the potential of − 0.8 V, pH of 2, and concentration of Cr (VI) is 10 mg/L within 2 h. Based on the above analysis, reasons for these results may attribute to a combination mechanism of adsorption and electro-reduction. Furthermore, PANI/ERGO/Ti presented good durability and achieved the expected application in actual wastewater in the system. This research could offer an effective strategy for graphene-supported polymer film electrodes which can be utilized for effective electrochemical reduction of Cr (VI)-containing wastewater.

Mechanism of reduction and immobilization of Cr(VI) by application modified nano-zerovalent iron

ABSTRACT Hexavalent chromium [Cr(VI)] is a highly toxic agent that seriously threatens the environment and human health. In-situ reduction and immobilization are potential strategies to treat Cr(VI)-contaminated soil. In this study, sodium alginate (SA) and carboxymethylcellulose (CMC) were used to modify nano-zero-valent iron (NZVI) to prepare SA-NZVI and CMC-NZVI to study its effect on the reduction efficiency of Cr(VI) in the soil. Scanning electron microscope (SEM) images found that compared with the agglomeration of NZVI and CMC-NZVI, SA-NZVI particles have a significant improvement, better dispersion, chain-like distribution, and a particle size of 50–100 nm. And the X-ray diffraction (XRD) pattern further showed that SA-NZVI has only Fe0 peaks and no Fe3O4 peaks, which further proved that it is pure nano-zero-valent iron. The batch tests showed that SA-NZVI could quickly and effectively reduce the Cr(VI) in the soil, and SA-NZVI could reduce the Cr(VI) by 96.68%. For Cr(VI)-contaminated soil, the Cr(VI) reduction rate of SA-NZVI was higher than NZVI and CMC-NZVI. After 28 d remediation, the fractions of Cr forms in soil, such as exchangeable (EX), carbonate-bound (CB), Fe-Mn oxides-bound (OX), and organic matter-bound (OM), were mainly transformed to residual fraction (RS) species because of the production of ferrochrome precipitates. X-ray photoelectron spectroscopy (XPS) showed that after SA-NZVI treatment, Cr(VI) was reduced to trivalent chromium [Cr(III)] and the formation of Cr(OH)3 precipitation. The research results provide a theoretical basis for applying SA-NZVI in Cr(VI) contaminated soil remediation.