Electrochemical reduction of Cr(VI) in water: lessons learned from fundamental studies and applications

Development of a continuous electrochemical reactor incorporated with waste-derived activated carbon electrode for the effective removal of hexavalent chromium from industrial effluent

Being a recognized carcinogen, hexavalent chromium is hazardous to both human and environmental health. Thus, it is imperative to regulate and oversee their levels in a variety of industries, including textiles, dyes, pigments, and metal finishing. This study strives to reduce Cr(VI) in wastewater by using capacitive deionization in conjunction with an activated carbon-based electrode and a continuous electrochemical reactor (CER). Activated carbon derived from rubberwood sawdust demonstrated excellent properties, including a high surface area of 1157 m2 g-1. The electrical conductivity and mechanical stability of the electrode were enhanced by the incorporation of synthesized expanded graphite (EG) into the AC. Key parameters were optimized via systematic batch electroreduction experiments with an optimal response surface design. The efficacy of the fabricated CER was proved when it successfully reduced Cr(VI) in a 5 mg L-1 solution within 15 min under optimized conditions, in contrast to the considerably longer durations anticipated by conventional methods. Validation of these findings was done by treating industrial wastewater of 30 mg L-1 in the CER. The electroreduction of Cr(VI) followed the Langmuir isotherm with a maximum capacity of 13.491 mg g-1 and pseudo-second-order kinetics. These results indicate that the combined use of the modified AC electrode and CER holds potential as a sustainable and economical approach to effectively eliminate Cr(VI) from wastewater.

Ultrasonic controllable synthesis of sulfur-functionalized metal-organic frameworks (S-MOFs) and their application in piezo-photocatalytic rapid reduction of hexavalent chromium (Cr)

The United Nations' Sustainable Development Goals (SDGs) are significant in guiding modern scientific research. In recent years, scholars have paid much attention to MOFs materials as green materials. However, piezo catalysis of MOFs materials has not been widely studied. Piezoelectric materials can convert mechanical energy into electrical energy, while MOFs are effective photocatalysts for removing pollutants. Therefore, it is crucial to design MOFs with piezoelectric properties and photosensitivity. In this study, sulfur-functionalized metal-organic frameworks (S-MOFs) were prepared using organic sulfur-functionalized ligand (H2TDC) ultrasonic synthesis to enhance their piezoelectric properties and visible light absorption. The study demonstrated that the S-MOFs significantly enhanced the reduction of a 10 mg/L solution of hexavalent chromium to 99.4 % within 10 min, using only 15 mg of catalyst. The orbital energy level differences of the elements were analyzed using piezo response force microscopy (PFM) and X-ray photoelectron spectroscopy (XPS). The results showed that MOFs functionalized with sulfur atom ligands have a built-in electric field that facilitates charge separation and migration. This study presents a new approach to enhance the piezoelectric properties of MOFs, which broadens their potential applications in piezo catalysis and piezo-photocatalysis. Additionally, it provides a sustainable method for reducing hexavalent chromium, contributing to the achievement of sustainable development goals, specifically SDG-6, SDG-7, SDG-9, and SDG-12.

Enhanced Chromium (VI) Removal by Micron-Scale Zero-Valent Iron Pretreated with Aluminum Chloride under Aerobic Conditions

Micron-scale zero-valent iron (ZVI)-based material has been applied for hexavalent chromium (Cr(VI)) decontamination in wastewater treatment and groundwater remediation, but the passivation problem has limited its field application. In this study, we combined aluminum chloride solution with ZVI (pcZVI-AlCl3) to enhance Cr(VI) removal behavior under aerobic conditions. The optimal pre-corrosion conditions were found to be 2.5 g/L ZVI, 0.5 mM AlCl3, and a 4 h preconditioning period. Different kinds of techniques were applied to detect the properties of preconditioned ZVI and corrosion products. The 57Fe Mössbauer spectra showed that proportions of ZVI, Fe3O4, and FeOOH in pcZVI-AlCl3 were 49.22%, 34.03%, and 16.76%, respectively. The formation of Al(OH)3 in the corrosion products improved its pHpzc (point of zero charge) for Cr(VI) adsorption. Continuous-flow experiments showed its great potential for Cr(VI) removal in field applications. The ZVI and corrosion products showed a synergistic effect in enhancing electron transfer for Cr(VI) removal. The mechanisms underlying Cr(VI) removal by pcZVI-AlCl3 included adsorption, reduction, and precipitation, and the contribution of adsorption was less. This work provides a new strategy for ZVI pre-corrosion to improve its longevity and enhance Cr(VI) removal.

Scaling up of dual-chamber microbial electrochemical systems - An appraisal using systems design approach

Impetus to minimise the energy and carbon footprints of evolving wastewater resource recovery facilities has promoted the development of microbial electrochemical systems (MES) as an emerging energy-neutral and sustainable platform technology. Using separators in dual-chamber MES to isolate anodic and cathodic environments creates endless opportunities for its myriad applications. Nevertheless, the high internal resistance and the complex interdependencies among various system factors have challenged its scale-up. This critical review employed a systems approach to examine the complex interdependencies and practical issues surrounding the implementation and scalability of dual-chamber MES, where the anodic and cathodic reactions are mutually appraised to improve the overall system efficiency. The robustness and stability of anodic biofilms in large-volume MES is dependent on its inoculum source, antecedent history and enrichment strategies. The composition and anode-respiring activity of these biofilms are modulated by the anolyte composition, while their performance demands a delicate balance between the electrode size, macrostructure and the availability of substrates, buffers and nutrients when using real wastewater as anolyte. Additionally, the catholyte governed the reduction environment and associated energy consumption of MES with scalable electrocatalysts needed to enhance the sluggish reaction kinetics for energy-efficient resource recovery. A comprehensive assessment of the dual-chamber reactor configuration revealed that the tubular, spiral-wound, or plug-in modular MES configurations are suitable for pilot-scale, where it could be designed more effectively using efficient electrode macrostructure, suitable membranes and bespoke strategies for continuous operation to maximise their performance. It is anticipated that the critical and analytical understanding gained through this review will support the continuous development and scaling-up of dual-chamber MES for prospective energy-neutral treatment of wastewater and simultaneous circular management of highly relevant environmental resources.

Simultaneous Cr(VI) reduction and Cr(III) sequestration in a wide pH range by using magnetic chitosan-based biopolymer

The simultaneous reduction of Cr(VI) and sequestration of the resulting Cr(III) in one process is highly desirable as a cost-effective and environmental-friendly approach for the decontamination of Cr(VI)-polluted wastewater. However, most of the existing adsorptive materials are only effective in low pH environments (pH = 1-3), severely restricting the adsorption efficiency and cost effectiveness. Herein, we proposed a chitosan-based magnetic porous microsphere (PPy@PMCS) for simultaneous Cr(VI) reduction and Cr(III) sequestration in a wide pH range. Benefiting from its abundant interaction sites, Cr(VI) was effectively adsorbed on the surface and then immediately reduced to Cr(III) with much lower toxicity. Most importantly, the resulting Cr(III) was in-situ sequestrated by the complexation of chitosan matrix. As a result, PPy@PMCS exhibited a maximum Cr(VI) adsorption capacity of 330.42 mg/g at pH 2.0 and an adsorption capacity of 167.82 mg/g even at near neutral pH (6.0), which is superior to most reported adsorbents. Furthermore, the exhausted PPy@PMCS can be rapidly separated from solutions under an external magnetic field and facilely regenerated. The proposed novel biopolymer-based material shows great application potentials in wastewater treatment.

A water-soluble conjugated polyelectrolyte for selective and sensitive detection of carcinogenic chromium(VI)

Environmental water pollution caused by hexavalent chromium (Cr(VI)) is a threat to living beings due to its carcinogenic nature. Herein, we report the synthesis of a highly fluorescent water-soluble conjugated polyelectrolyte PPMI and its application as a fluorescence sensor to monitor traces of carcinogenic Cr(VI) ions in water. PPMI was synthesized via the oxidative polymerization method followed by post-polymer functionalization. Fluorescent PPMI exhibited a photoluminescence quantum yield of 23.87 and displayed a rapid, very selective, and sensitive turn-off fluorescence signal in response to Cr(VI), with a significantly high quenching constant of 1.32 × 106 M-1. The mechanism of sensing was found to be static quenching. The limit of detection of this highly accessible analytical method was found to be in nanomolar ranges, i.e. 0.85 nM. Additionally, sensing on solid platforms such as economical paper strips was successfully achieved, which is very challenging and highly recommended for any reliable, portable, and economical analytical method.

High-Efficiency Adsorption of Cr(VI) and Mn(VII) from Wastewater by a Two-Dimensional Copper-Based Metal-Organic Framework

Cr(VI) and Mn(VII) in industrial wastewater have certain toxicity, and they pose a threat to the environment and human health and safety. Metal-organic frameworks (MOFs) usually have rich adsorption sites and a large specific surface area. They can effectively adsorb Cr(VI) and Mn(VII) from wastewater. In this paper, a two-dimensional copper-based metal-organic framework, {[Cu·(4,4'-bpy)2·(H2O)]·2(NO3)·6(H2O)·(CH3OH)}n (1), is synthesized by hydrothermal synthesis. The structure of 1 is characterized by Fourier transform infrared (IR) spectroscopy, single-crystal X-ray diffraction, element analysis, and X-ray photoelectron spectroscopy (XPS). The results showed that 1 had a two-dimensional network structure, and the specific surface area of the nanostructure was 67.63 m2/g. The nanostructure of 1 could efficiently adsorb Cr(VI) and Mn(VII) from wastewater. The adsorption properties of Cr(VI) and Mn (VII) of 1 showed that the optimal concentration of both adsorbents was 0.2 g/L. It has good adsorption performance in the pH range 4-8. The adsorption performance is the best when pH is 7, which can reach 145 and 83 mg/g, respectively.

Electrochemical reduction of wastewater by non-noble metal cathodes: From terminal purification to upcycling recovery

A shift beyond conventional environmental remediation to a sustainable pollutant upgrading conversion is extremely desirable due to the rising demand for resources and widespread chemical contamination. Electrochemical reduction processes (ERPs) have drawn considerable attention in recent years in the fields of oxyanion reduction, metal recovery, detoxification and high-value conversion of halogenated organics and benzenes. ERPs also have the potential to address the inherent limitations of conventional chemical reduction technologies in terms of hydrogen and noble metal requirements. Fundamentally, mechanisms of ERPs can be categorized into three main pathways: direct electron transfer, atomic hydrogen mediation, and electrode redox pairs. Furthermore, this review consolidates state-of-the-art non-noble metal cathodes and their performance comparable to noble metals (e.g., Pd, Pt) in electrochemical reduction of inorganic/organic pollutants. To overview the research trends of ERPs, we innovatively sort out the relationship between the electrochemical reduction rate, the charge of the pollutant, and the number of electron transfers based on the statistical analysis. And we propose potential countermeasures of pulsed electrocatalysis and flow mode enhancement for the bottlenecks in electron injection and mass transfer for electronegative pollutant reduction. We conclude by discussing the gaps in the scientific and engineering level of ERPs, and envisage that ERPs can be a low-carbon pathway for industrial wastewater detoxification and valorization.

Construction of Core-shell Sb2 s3 @Cds Nanorod with Enhanced Heterointerface Interaction for Chromium-Containing Wastewater Treatment

How to collaboratively reduce Cr(VI) and break Cr(III) complexes is a technical challenge to solve chromium-containing wastewater (CCW) pollution. Solar photovoltaic (SPV) technology based on semiconductor materials is a potential strategy to solve this issue. Sb2 S3 is a typical semiconductor material with total visible-light harvesting capacity, but its large-sized structure highly aggravates disordered photoexciton migration, accelerating the recombination kinetics and resulting low-efficient photon utilization. Herein, the uniform mesoporous CdS shell is in situ formed on the surface of Sb2 S3 nanorods (NRs) to construct the core-shell Sb2 S3 @CdS heterojunction with high BET surface area and excellent near-infrared light harvesting capacity via a surface cationic displacement strategy, and density functional theory thermodynamically explains the breaking of SbS bonds and formation of CdS bonds according to the bond energy calculation. The SbSCd bonding interaction and van der Waals force significantly enhance the stability and synergy of Sb2 S3 /CdS heterointerface throughout the entire surface of Sb2 S3 NRs, promoting the Sb2 S3 -to-CdS electron transfer due to the formation of built-in electric field. Therefore, the optimized Sb2 S3 @CdS catalyst achieves highly enhanced simulated sunlight-driven Cr(VI) reduction (0.154 min-1 ) and decomplexation of complexed Cr(III) in weakly acidic condition, resulting effective CCW treatment under co-action of photoexcited electrons and active radicals. This study provides a high-performance heterostructured catalyst for effective CCW treatment by SPV technology.

Chromium removal at neutral pHs via electrochemical Cr(VI) reduction and subsequent Cr(III) adsorption with MoS2 nanoflowers-modified graphite felt

The operation performance and stability of electrochemical Cr(VI) reduction are strongly restricted at neutral pHs (e.g., drinking water and groundwater) by the high Cr(VI) oxidation potentials and cathode passivation of Cr(OH)₃ precipitates. Herein, we fabricated MoS₂ nanoflowers-modified graphite felt (GF-MoS₂) to construct the electrochemical apparatus (EA) and adsorption column (AC), attempting to stable and effective Cr(VI) removal at neutral pHs via electrochemical Cr(VI) reduction and subsequent Cr(III) adsorption. In EA with a sequential oxidation-reduction process, Cr(VI)-contaminated influent (5 mg/L) at neutral pHs (6.0-8.0) was oxidized first by anode to generate large amounts of H⁺ ions via H₂O oxidation, decreasing the pH of anode-oxidized influent to ∼2.5 at 2.6 V and 1000 L/m²/h. Subsequently, the acidic anode-oxidized influent was further reduced by GF-MoS₂ cathode, promoting significantly Cr(VI) reduction via decreasing Cr(VI) oxidation potentials and alleviating Cr(III) precipitation on cathode. These results enabled the stable and effective operation of GF-MoS₂-based EA with almost Cr(VI) reduction to Cr(III). With further assembling GF-MoS₂-based AC, Cr(III) species in EA effluent were easily adsorbed or intercepted by GF-MoS₂, achieving undetectable Cr species in AC effluent. Combination techniques of GF-MoS₂-based electrochemical reduction and adsorption can be an effective approach for remediating Cr(VI)-contaminated water at neutral pHs.

One-step removal of hexavalent chromium in wide pH range using thiourea dioxide: the role of reactive species

One-step removal of hexavalent chromium in a wide pH range is of great significance. In this paper, a single thiourea dioxide (TD) and two-components thiourea dioxide/ethanolamine (MEA) were used as a green reducing agent for the efficient removal of Cr(vi), respectively. The reduction of Cr(vi) and the precipitation of Cr(iii) were carried out simultaneously under this reaction system. The experimental results proved that TD was activated by amine exchange reaction with MEA. In other words, MEA promoted the generation of an active isomeride of TD by changing the equilibrium position of the reversible reaction. After adding MEA, the removal rate of Cr(vi) and total Cr could reach industrial water discharge standards in a wide pH range of 8-12. The change of pH, reduction potential and the decomposition rate of TD were investigated in the reaction processes. Meanwhile, reductive and oxidative reactive species were produced simultaneously during this reaction process. Further, oxidative reactive species (O₂˙^- and ¹O₂) were beneficial for the decomplexation of Cr(iii) complexes and the formation of Cr(iii) precipitation. The experimental results also demonstrated that TD/MEA was effective in practical industrial wastewater. Hence this reaction system has a significant industrial application prospect.

A metal-organic framework surrounded with conjugate acid-base pairs for the efficient capture of Cr(VI) via hydrogen bonding over a wide pH range

Given the large amount of toxic Cr(VI) wastewater from various industries, it is urgent to take effective treatment measures. Adsorption has been regarded as highly desirable for Cr(VI) removal, but the effectiveness of most adsorbents is significantly dependent on pH value, in which precipitous performance drop and even structural collapse generally occur in strong acidic/alkaline aqueous. Thus, maintaining high adsorption performance and structural integrity over a wide pH range is challenging. To efficiently remove Cr(VI), we designed and prepared of an acid-base resistant metal-organic framework (MOF) Zr-BDPO, by introducing weak acid-base groups (-NH-, -N= and -OH) onto the ligand. Zr-BDPO achieved a maximum adsorption capacity of 555.6 mg·g^-1 and retained skeletal structure at pH= 1-11. Interestingly, all these groups can generate conjugate acid-base pairs by means of H⁺ and OH^- in the external solution and then form buffer layer. The removal of Cr(VI) at a broad range of pH values primarily via hydrogen bonds between -NH- and -OH, and the oxoanion species of Cr(VI) is unusual. This strategy that insulating high concentrations of acids and bases and relying on hydrogen bonds to capture Cr(VI) oxoanions provides a new perspective for actual Cr(VI) wastewater treatment.

Enhancing the spatial separation of photogenerated charges on Fe-based MOFs via structural regulation for highly-efficient photocatalytic Cr(VI) reduction

Although iron-based metal-organic frameworks (Fe-MOFs) have displayed the photocatalytic activity, there is still abundant room for improving their photocatalytic performance through tuning the structures. In this work, four novel iron-based metal-organic frameworks (Fe-MOFs) were successfully synthesized via ligand modulation for better photocatalytic Cr(VI) reduction, in which MTBDC-TPT-Fe had the highest catalytic activity (MTBDC = 2,5-bis(methylthio)terephthalic acid, TPT = 2,4,6-tri(4-pyridyl)- 1,3,5-triazine). The boosted photocatalytic reduction may be mainly ascribed to the enhanced electron push-pull effect between iron-oxygen clusters and organic ligands. The introduction of -SCH₃ groups can enhance the light absorption and donate electrons to iron center under visible-light irradiation, meanwhile the separation and transfer of photogenerated charge carriers can be enhanced resulting from the electron-pulling effect when introducing TPT. Moreover, enhanced specific surface areas and positive skeleton charge due to the introduction of TPT may improve active sites exposure and Cr(VI) adsorption, thereby enhancing photocatalytic Cr(VI) reduction activity without the presence of any assisted scavengers. In addition, the photocatalytic mechanism (i.e. active species) were also studied and presented. This work confirmed an effective structure-performance regulation strategy on Fe-MOFs for photocatalytic Cr(VI) reduction.

Selective Chromium(VI) Trapping by an Acetate-Releasing Coordination Polymer

We report the high-capacity and selective uptake of Cr(VI) from water using the coordination polymer silver bipyridine acetate (SBA, [Ag(4,4'-bipy)][CH₃CO₂]·3H₂O). Cr capture involves the release of acetate, and we have structurally characterized two of the product phases that form: silver bipyridine chromate (SBC, SLUG-56, [Ag(4,4'-bipy)][CrO₄]_0.5·3.5H₂O) and silver bipyridine dichromate (SBDC, SLUG-57, [Ag(4,4'-bipy)][Cr₂O₇]_0.5·H₂O). SBA maintains a high Cr uptake capacity over a wide range of pH values (2-10), reaching a maximum of 143 mg Cr/g at pH 4. This Cr uptake capacity is one of the highest among coordination polymers. SBA offers the additional benefits of a one-step, room temperature, aqueous synthesis and its release of a non-toxic anion following Cr(VI) capture, acetate. Furthermore, SBA capture of Cr(VI) remains >97% in the presence of a 50-fold molar excess of sulfate, nitrate, or carbonate. We also investigated the Cr(VI) sequestration abilities of silver 1,2-bis(4-pyridyl)ethane nitrate (SEN, [Ag(4,4'-bpe)][NO₃]) and structurally characterized the silver 1,2-bis(4-pyridyl)ethane chromate (SEC, SLUG-58, [Ag(4,4'-bpe)][CrO₄]_0.5) product. SEN was, however, a less effective Cr(VI) sequestering material than SBA.

Hierarchical S-Scheme Heterostructure of CdIn2S4@UiO-66-NH2 toward Synchronously Boosting Photocatalytic Removal of Cr(VI) and Tetracycline

Developing highly efficient photocatalysts toward synchronously removing heavy metals and organic pollutants is still a serious challenge. Herein, we depict hierarchical S-scheme heterostructured photocatalysts prepared via in situ anchoring UiO-66-NH₂ nanoparticles onto the CdIn₂S₄ porous microsphere structures assembled with numerous nanosheets. In the mixed system of Cr(VI) and tetracycline (TC), the optimal photocatalyst (CIS@U66N-30) shows remarkable photocatalytic activities toward the synchronous removal of Cr(VI) (97.26%) and TC (close to 100% of) under visible-light irradiation for 60 min, being the best removal rates among those of the reported photocatalysts, and sustains the outstanding stability and reusability. Its reaction rate constants of Cr(VI) reduction and TC degradation are about 2.06 and 1.58 folds that in the single Cr(VI) and TC systems, respectively. The enhanced photocatalytic activities of CIS@U66N-30 mainly result from the following synergism: (1) its hierarchical structure offers abundant active sites, and the S-scheme migration mechanism of charge carriers in the heterostructure accelerates the separation and migration of the useful photoinduced electrons and holes with the high redox capability; (2) Cr(VI) and TC can serve as the electron scavenger for TC oxidation degradation and the hole and ^•OH scavenger for Cr(VI) reduction, respectively, further enhancing the separation and utilization efficiency of photoinduced electrons and holes. Besides, the possible TC degradation pathway and plausible S-scheme photocatalytic mechanism over CIS@U66N-30 for the concurrent elimination of Cr(VI) and TC are proposed.

Electrochemical reduction of Cr (VI) using a palladium/graphene modified stainless steel electrode

In this study, a palladium/graphene modified stainless steel electrode was successfully prepared and applied in an electrochemical reduction device to remove Cr (VI) from the wastewater. Pd was modified onto the electrode mainly via interacting with the carboxyl group of graphene. The Cr (VI) removal efficiency was up to 99.70 ± 0.00% under the optimal condition (Pd content proportion of 3%, electrode potential of -0.9 V, pH = 2 and electrolyte concentration of 6 g/L). It was found that Cr (VI) was removed via the following processes: (1) direct electrochemical reduction by accepting electrons, (2) indirect electrochemical reduction by H₂O₂ that was generated from H₂ in the presence of Pd, (3) adsorption through hydrogen bond, and (4) chemical reduction through alkoxy groups donating electrons. The indirect electrochemical reduction considerably promoted the Cr (VI) removal while a small amount of Cr (VI) was removed via adsorption and chemical reduction. The method could not only be used as a pretreatment technology to solve the problem of excessive Cr (VI) concentration of industrial wastewater, but also could provide reference for the electrochemical reduction of similar metal ions.

Efficient removal of Cr(VI) at alkaline pHs by sulfite/iodide/UV: Mechanism and modeling

Efficient removal of toxic hexavalent chromium (Cr(VI)) under alkaline conditions is still a challenge due to the relatively low reactivity of CrO₄^2-. This study proposed a new sulfite/iodide/UV process to remove Cr(VI). The removal of Cr(VI) followed pseudo-zero-order kinetics at alkaline pHs, and was enhanced by sulfite and iodide with synergy. Compared with sulfite/UV, iodide in sulfite/iodide/UV showed about 40 times higher concentration-normalized enhancement for Cr(VI) removal, and reduced the requirement of sulfite ([S(IV)]₀/[Cr(VI)]₀ of about 2.1:1) by more than 90%. The Cr(VI) removal was accelerated by decreasing pH and by increasing temperature, and was slightly influenced by dissolved oxygen, carbonate, and humic acid. The process was still effective in real surface water and industrial wastewater. Mechanism and pathways of Cr(VI) removal were revealed by quenching experiments, competition kinetic analysis, product identification and quantification, and mass and electron balance. Both e_aq^- and SO₃^•- were responsible for Cr(VI) removal, making contributions of about 75% and 25%, respectively. When e_aq^- mainly reacted with Cr(VI), SO₃^•- participated in reduction of Cr(V) and Cr(IV) intermediates, with Cr(III), S₂O₆^2-, and SO₄^2- as the final products. A model was developed to predict removal kinetics of Cr(VI), and well interpreted the roles of S(IV) and iodide in the process. This study sheds light on mechanism of Cr(VI) removal at alkaline pHs by kinetic modeling, and thus advances the applicability of this promising process for water decontamination.

Cellulose-based aerogel beads for efficient adsorption-reduction-sequestration of Cr(VI)

The reduction and sequestration of toxic Cr(VI) via a one-step process in an aqueous solution is critical to eliminate its environmental risk. In this study, amine functionalized cellulose-based aerogel beads (CGP) was developed for simultaneous and efficient adsorption- reduction- sequestration of Cr(VI). CGP showed a maximum Cr(VI) adsorption capacity of 386.40 mg/g at 25 °C due to its strong electrostatic attraction towards Cr(VI). The simultaneous Cr(VI) adsorption- reduction- sequestration performance of CGP over a wide Cr(VI) concentration range was examined. The mechanism was investigated in-depth via the analysis of adsorption kinetics, XPS spectra, and FTIR spectra. Moreover, the Cr immobilization stability of CGP after adsorption was evaluated in simulated neutral, acidic, and alkaline conditions. The effect of pH, temperature, ionic strength and the presence of interfering ions on CGP adsorption performance were investigated by batch adsorption experiments. Fixed-bed column adsorption study was performed to explore the application potential of CGP beads in a wastewater treatment process.

Photo–Redox Properties of –SO3H Functionalized Metal-Free g-C3N4 and Its Application in the Photooxidation of Sunset Yellow FCF and Photoreduction of Cr (VI)

In this work, we synthesized a metal-free sulfonic functionalized graphitic carbon nitride using sulfuric acid through the wet impregnation technique. The functionalization of sulfonic groups (–SO3H) on g-C3N4 will promote a high surface charge density and charge separation owing to its high electronegativity. The g-C3N4–SO3H shows excellent optical/electronic and surface properties towards enhanced photo–redox reactions. The sulfonic groups also facilitate the availability of more separated charge carriers for photocatalytic oxidation and reduction reactions. The as-synthesized material has been characterized by different spectroscopic tools to confirm the presence of functionalized –SO3H groups and optoelectronic possessions. The photocatalytic responses of g-C3N4–SO3H result in 99.56% photoreduction of Cr (VI) and 99.61% photooxidation of Sunset Yellow FCF within 16 min and 20 min, respectively, of visible light irradiation. The g-C3N4–SO3H catalyst exhibits a high apparent rate constant (Kapp) towards the degradation of Cr (VI), and SSY, i.e., 0.783 min−1 and 0.706 min−1, respectively. The intense optical–electrochemical properties and potentially involved active species have been analyzed through transient photocurrent, electrochemical impedance, and scavenging studies. Consequently, the photocatalytic performances are studied under different reaction parameters, and the plausible photocatalytic mechanism is discussed based on the results.

Fabrication of Ru-CoFe2O4/RGO hierarchical nanostructures for high-performance photoelectrodes to reduce hazards Cr(VI) into Cr(III) coupled with anodic oxidation of phenols

Dual-functional photo (electro)catalysis (PEC) is a key strategy for removing coexisting heavy metals and phenolic compounds from wastewater treatment systems. To design a PEC cell, it is crucial to use chemically stable and cost-effective bifunctional photocatalysts. The present study shows that ruthenium metallic nanoparticles decorated with CoFe₂O₄/RGO (Ru-CoFe₂O₄/RGO) are effective bifunctional photoelectrodes for the reduction of Cr(VI) ions. Ru-CoFe₂O₄/RGO achieves a maximum Cr(VI) reduction rate of 99% at 30 min under visible light irradiation, which is much higher than previously reported catalysts. Moreover, PEC Cr(VI) reduction rate is also tuned by adding varying concentration of phenol. A mechanism for the concurrent removal of Cr(VI) and phenol has been revealed over a bifunctional Ru-CoFe₂O₄/RGO catalyst. A number of key conclusions emerged from this study, demonstrating the dual role of phenol during Cr(VI) reduction by PEC. Anodic oxidation of phenol produces the enormous H⁺ ion, which appears to be a key component of Cr(VI) reduction. Additionally, phenolic molecules serve as hole (h⁺) scavengers that reduce e^-/h⁺ recombination, thus enhancing the reduction rate of Cr(VI). Therefore, the Ru-CoFe₂O₄/RGO photoelectrode exhibits a promising capability of reducing both heavy metals and phenolic compounds simultaneously in wastewater.

MnIn2S4 nanosheets growing on rods-like β-MnO2 via covalent bonds as high-performance photocatalyst for boosting Cr(VI) photocatalytic reduction under visible light irradiation: Behavior and mechanism study

It is an urgent and onerous task to develop catalysts for photocatalytic reduction of Cr(VI) in wastewater under wide pH range. In this work, a novel hierarchical Z-scheme MnO₂/MnIn₂S₄ (MISO) heterojunction photocatalyst with MnIn₂S₄ nanosheets growing on the surface of β-MnO₂ nanorods is constructed for efficient photocatalytic reduction of Cr(VI). The optimized 2.0-MISO photocatalyst exhibits the almost 100% reduction efficiency in the pH range of 2.1-5.6 under visible light irradiation, and the apparent rate constant is 0.05814 min^-1, which is 29.96 and 3.27 times higher than the pure β-MnO₂ and MnIn₂S₄, respectively. A efficient photocatalytic reduction of Cr(VI) to Cr(III) species on 2.0-MISO photocatalyst in actual industry wastewater (286.7 mg/L) up to 99.8% is achieved. Under natural light, the 2.0-MISO photocatalyst also shows rapid reduction of Cr(VI) species. The photocorrosion of MnIn₂S₄ was significantly hindered by the construction of heterojunction. And the O₂^- and e^- species are the main active species during the Cr(VI) photoreduction process. The connection mode between MnIn₂S₄ and β-MnO₂ is verified by DFT calculations and a possible photocatalytic mechanism is also proposed.

Vacancy-mediated transition metals as efficient electrocatalysts for water splitting

Water splitting using renewable electricity provides a promising way for large-scale hydrogen production due to its zero-carbon emission properties. However, the development of highly efficient, low-cost and durable electrocatalysts remains an ongoing challenge in industrial applications. Herein, a strategy integrating vacancy engineering and metal doping was proposed to design and screen M@CuS catalysts with excellent catalytic activity via density functional theory (DFT) calculations. TM single atoms anchored by the vacancy of the CuS surface show high stability, and serve as the active centers for water splitting. Ti@CuS and Co@CuS exhibit exceptional performance towards the hydrogen evolution reaction (HER). Ti@CuS and Co@CuS can achieve hydrogen adsorption free energies (ΔG_H*) of 0.01 eV and -0.03 eV, respectively. The HER process of Ti@CuS is controlled by the Heyrovsky mechanism. Co@CuS also shows superior catalytic activity towards the oxygen evolution reaction (OER), and presents a relatively lower OER overpotential of 0.41 V. Co@CuS serves as a promising candidate of bifunctional HER/OER electrocatalysts. This work not only provides highly efficient electrocatalysts for water splitting, but also inspires a novel concept to guide the extending design of catalysts in other catalysis fields.

Insight into the Key Role of Cr Intermediates in the Efficient and Simultaneous Degradation of Organic Contaminants and Cr(VI) Reduction via g-C3N4-Assisted Photocatalysis

Photocatalysis provides an impetus for the synergetic removal of Cr(VI) and organic contaminants, but the generation of Cr intermediates and their potential oxidizability may be overlooked in pollutant conversion. Herein, the Cr intermediates in the Cr(VI) reduction process were emphasized in Cr(VI)/bisphenol A (BPA) by using graphitic carbon nitride as a photocatalyst. The active species for BPA photodegradation in the BPA system and Cr(VI)/BPA system suggested that the Cr(VI) reduction process indeed promotes BPA photodegradation. Electron paramagnetic resonance (EPR) of Cr complexes and in situ variable-temperature EPR analysis demonstrated Cr(V) intermediate (g = 1.978) generation in Cr(VI) reduction and its oxidization for BPA degradation in photocatalysis. By adding the electron donor Na₂SO₃, BPA degradation was induced in Cr(VI)/BPA solution, further confirming the positive effect of Cr(V). Moreover, the difference in BPA degradation products in the BPA/air, Cr(VI)/BPA/air, and Cr(VI)/BPA/Ar systems indirectly explained why the Cr(V) intermediate was involved in BPA degradation. Density functional theory calculations revealed that photogenerated electrons can reduce the free energy (0.98 eV) of converting Cr(VI) into Cr(V), which can facilitate the subsequent Cr(V) oxidation step for BPA degradation. This work contributes to the exploration of the Cr(VI) reduction process and the synergistic removal of organic pollutants in Cr(VI)/organics systems.

Highly efficient removal of Cr(VI) by hexapod-like pyrite nanosheet clusters

Pyrite nanomaterials show an excellent performance in remediating Cr(VI) contaminated wastewater. However, the high surface reactivity makes them easy to agglomerate to reduce their removal efficiency for Cr(VI). In this study, a novel hexapod-like pyrite nanosheet clusters material was successfully synthesized via a facile hydrothermal method with the assistance of fluorides. The products were pyrite microspherulites without fluoride ion. The hexapod-like pyrite nanosheet clusters had dramatically higher Cr(VI) removal efficiencies than microspherulites due to more dissolved Fe(II) and S(-II) into the suspension released for nanosheet clusters should be responsible for the enhanced removal rate of Cr(VI). The XPS analysis revealed that the rapid adsorption on the surface of pyrite nanosheet clusters followed by reduction of Cr(VI) to Cr(III) by FeS₂ and subsequent precipitation of Cr(III) hydroxides/oxyhydroxides are responsible for the high removal capacity of Cr(VI). The hexapod-like pyrite nanosheet clusters material had high stability and longevity, and did not aggregate during the Cr(VI) removal process. The removal efficiency of Cr(VI) was still 100% after 5 cycles. Our study shows that the hexapod-like pyrite nanosheet clusters material could be acted as a recyclable and promising mineral material with high activity, stability, feasibility for remediating Cr(VI) contaminated environment.

Impact of the choice of buffer on the electrochemical reduction of Cr(vi) in water on carbon electrodes

The nature of the buffer influences the PCET step gating Cr(vi) reduction in water at pH 4.75, as well as the extent of deposition on carbon electrodes. Electrode activity is recovered without polishing, through a simple acid wash step.

A Review on Promising Membrane Technology Approaches for Heavy Metal Removal from Water and Wastewater to Solve Water Crisis

Due to the impacts of water scarcity, the world is looking at all possible solutions for decreasing the over-exploitation of finite freshwater resources. Wastewater is one of the most reliable and accessible water supplies. As the population expands, so do industrial, agricultural, and household operations in order to meet man’s enormous demands. These operations generate huge amounts of wastewater, which may be recovered and used for a variety of reasons. Conventional wastewater treatment techniques have had some success in treating effluents for discharge throughout the years. However, advances in wastewater treatment techniques are required to make treated wastewater suitable for industrial, agricultural, and household use. Diverse techniques for removing heavy metal ions from various water and wastewater sources have been described. These treatments can be categorized as adsorption, membrane, chemical, or electric. Membrane technology has been developed as a popular alternative for recovering and reusing water from various water and wastewater sources. This study integrates useful membrane technology techniques for water and wastewater treatment containing heavy metals, with the objective of establishing a low-cost, high-efficiency method as well as ideal production conditions: low-cost, high-efficiency selective membranes, and maximum flexibility and selectivity. Future studies should concentrate on eco-friendly, cost-effective, and long-term materials and procedures.

Bifunctional Ionic Covalent Organic Networks for Enhanced Simultaneous Removal of Chromium(VI) and Arsenic(V) Oxoanions via Synergetic Ion Exchange and Redox Process

Chromium (VI) and arsenic (V) oxoanions are major toxic heavy metal pollutants in water threatening both human health and environmental safety. Herein, the development is reported of a bifunctional ionic covalent organic network (iCON) with integrated guanidinium and phenol units to simultaneously sequester chromate and arsenate in water via a synergistic ion-exchange-redox process. The guanidinium groups facilitate the ion-exchange-based adsorption of chromate and arsenate at neutral pH with fast kinetics and high uptake capacity, whereas the integrated phenol motifs mediate the Cr(VI)/Cr(III) redox process that immobilizes chromate and promotes the adsorption of arsenate via the formation of Cr(III)-As(V) cluster/complex. The synergistic ion-exchange-redox approach not only pushes high adsorption efficiency for both chromate and arsenate but also upholds a balanced Cr/As uptake ratio regardless of the change in concentration and the presence of interfering oxoanions.

Innovative Magnetite Based Polymeric Nanocomposite for Simultaneous Removal of Methyl Orange and Hexavalent Chromium from Water

One of the most important directions for environmental remediation is the effective removal of dyes and toxic heavy metals from water using newly fabricated nanoadsorbents. Here, magnetic Fe3O4 nanoparticles were combined with nitrogen-containing functional group polymers chitosan (CS) and polypyrrole (ppy) to synthesize a nanocomposite (polypyrrole@magnetic chitosan) useful for removing methyl orange (MO) and hexavalent chromium (Cr (VI)) from water. The physicochemical properties of the nanocomposite were determined using SEM, TEM, XRD, FT–IR, and TGA techniques. The effect of different factors on the adsorption system was studied including the contact time, pH, and the effect of co-existed ions. The kinetic study illustrated that the adsorption fit well with Langmuir isotherm. The maximum adsorption capacity of MO and Cr (VI) was found to be 95 and 105 mg/g, respectively. The reusability of the nanocomposite was studied for up to five cycles using 0.1 M NaOH as eluent with a slight decrease of adsorbent efficiency. Furthermore, the removal mechanism studied suggested the removal of MO via adsorption and Cr (VI) via chemical reduction and adsorption. This study suggests that a ppy@magnetic chitosan nanocomposite is a promising nanoadsorbent for removing MO and Cr (VI) from water.

Simple fabrication of Z-scheme MgIn2S4/Bi2WO6 hierarchical heterostructures for enhancing photocatalytic reduction of Cr(vi)

Direct Z-scheme MgIn2S4/Bi2WO6 hierarchical heterostructures were simply fabricated, exhibiting distinctly enhanced photocatalytic activity for Cr(vi) reduction under simulated sunlight irradiation.

Direct Z-scheme hierarchical heterostructures of oxygen-doped g-C3N4/In2S3 with efficient photocatalytic Cr(vi) reduction activity

Direct Z-scheme hierarchical heterostructures of O-doped g-C3N4/In2S3 were simply prepared, exhibiting efficient photocatalytic Cr(vi) reduction under visible light illumination.