Electrochemical reduction of hexavalent chromium on titania nanotubes with urea as an anolyte additive

Preparation of amino-modified carbon quantum dots-ZnO/cellulose nanofiber multifunctional hydrogel: Enhanced adsorption synergistic photoreduction and reversible fluorescence response visual recognition of Cr(VI)

This work innovatively used cellulose nanofibers as a photocatalyst carrier, which could recycle nano-photocatalysts and minimize nanoparticle aggregation. The morphology, structures, chemical composition, optical-electronic properties and photocatalytic performance of amino-modified carbon quantum dots-ZnO/cellulose nanofiber (N-CQDs-ZnO/CNF: ZCH-2) hydrogel were characterized by SEM, TEM, BET, EDS, XRD, FTIR, UV-vis, XPS, PL and other techniques. The mechanism of Cr(VI) adsorption synergistic photoreduction by ZCH-2 was discussed in detail. The results showed that the prepared ZCH-2 had excellent removal performance for Cr(VI). After 120 min of adsorption and 40 min of photoreduction, the removal efficiency of Cr(VI) was 98.9 %. Compared with ZnO/CNF hydrogel, the adsorption performance of ZCH-2 increased by 268 % and the photoreduction performance increased by 116 %. The adsorption of Cr(VI) by ZCH-2 was controlled by electrostatic attraction and chemical adsorption. The photoreduction kinetic constant of ZCH-2 was 0.106 min-1, which was 8.9 times that of ZnO/CNF hydrogel. The N-CQDs in ZCH-2 could form N-CQDs-metal complexes with Cr(VI), resulting in fluorescence quenching, so Cr(VI) could be visually identified by fluorescence changes. This study provides a new idea for the design and optimization of a new multifunctional hydrogel with efficient adsorption-photoreduction-fluorescence recognition.

Exploring the mechanism of enhanced Cr(VI) removal by Lysinibacillus cavernae microcapsules loaded with synthetic nano-hydroxyapatite

In this study, nano-scale hydroxyapatite (HAP) powder was successfully synthesized from waste eggshells and combined with Lysinibacillus cavernae CR-2 to form bio-microcapsules, which facilitated the enhanced removal of Cr(VI) from wastewater. The effects of various parameters, such as bio-microcapsule dosage, HAP dosage, and initial Cr(VI) concentration on Cr(VI) removal, were investigated. Under different treatment conditions, the Cr(VI) removal followed the order of LC@HAP (90.95%) > LC (78.15%) > Free-LC (75.61%) > HAP (6.56%) > NM (0.23%) at the Cr(VI) initial concentration of 50 mg L-1. Relative to other reaction systems, the LC@HAP treatment exhibited a considerable decrease in total Cr content in the solution, with removal rates surpassing 70%. Additionally, the bio-microcapsules maintained significant biological activity after reacting with Cr(VI). Further characterization using SEM, FTIR, XPS, and XRD revealed that the Cr(VI) removal mechanisms by bio-microcapsules primarily involved biological reduction and HAP adsorption. The adsorption of Cr(III) by HAP predominantly occurred through electrostatic interactions and surface complexation, accompanied by an ion exchange process between Cr(III) and Ca(II). Hence, bio-microcapsules, created by combining L. cavernae with HAP, represent a promising emerging material for the enhanced removal of Cr(VI) pollutants from wastewater.

Tubular Sediment-Water Electrolytic Fuel Cell for Dual-Phase Hexavalent Chromium Reduction

A novel tubular sediment-water electrolytic fuel cell (SWEFC) was fabricated for the reduction of Cr(VI) in a dual-phase system. The approach simulates a standing water body with Cr(VI)-contaminated overlying water (electrolyte) and bottom sediment phase with electrodes placed in both the phases, supplemented with urea as a potential electron donor. Cr(VI) reduction efficiency of 93.2 ± 1.3% from electrolyte (in 1.5 h) and 81.2 ± 1.3% from the sediment phase (in 8 h) with an initial Cr(VI) concentration of 1,000 mg/L was observed in a single-cell configuration. The effect of initial Cr(VI) concentration, variation in sediment salinity and pH, and different electron donors on the SWEFC performance were systematically investigated. SWEFC showed enhanced performance with 2.4-fold higher current (193.9 mA) at 400 mg/L Cr(VI) concentration when cow dung was used as a low-cost alternative to urea as an electron donor. Furthermore, reactor scalability studies were carried out with nine-anode and nine-cathode configuration (3 L electrolyte and 2 kg sediment), and reduction efficiencies of 98.9 ± 0.9% (in 1 h) and 97.6 ± 2.2% (in 8 h) were observed from the electrolyte and sediment phases, respectively. The proposed sediment-water electrolytic fuel cell can be an advanced and environmentally benign strategy for Cr(VI) remediation from contaminated sediment-water interfaces along with electricity generation.

A systematic review on adsorptive removal of hexavalent chromium from aqueous solutions: Recent advances

The contamination of natural resources by hexavalent chromium (Cr(VI)) originating from natural and anthropogenic activities is a serious environmental concern. Although many articles on chromium remediation have been published, a comprehensive understanding of the mechanisms involved in remediation with different sorbents is not yet available. In this systematic review, the performance and applicability of several adsorptive materials for Cr(VI) removal from aqueous media are discussed, along with a detailed analysis of the mechanisms involved. Statistical analysis is applied to compare the efficacies of different adsorbents, while a similar approach is used to determine the effects of sorbent properties and experimental conditions on the adsorption capacity. A detailed analysis of the factors involved in fixed-bed column studies is also presented. A suitable desorption approach to the regeneration of the spent adsorbent and its adsorption performance in reuse is also examined. Among the different sorbents, nanoparticles and mineral-doped biochar were found to be the most effective sorbents, while the adsorption was higher at low pH (~4.0) than that at intermediate pH (6-8). Contrary to our expectation, adsorption was high for sorbents with low specific surface areas, suggesting that the adsorption of Cr(VI) is largely influenced by the chemical properties of the sorbents. The optimum adsorption in fixed-bed column systems is obtained at a lower Cr(VI) ion concentration, a lower influent flow rate, and a higher bed height. Since most of the studies reviewed herein were merely experimental and utilized ideal conditions with the presence of a single contaminant, i.e. Cr(VI) in water, further studies on adsorption dynamics with the presence of other interfering ions are suggested. This review is promising for the further development of Cr(VI) removal strategies and closes the research gaps pertaining to their challenges.

Hexavalent chromium reduction by zero-valent magnesium particles in column systems

The discharge of hexavalent chromium in aquatic environments represents an issue of great concern. The chemical reduction of Cr⁶⁺ to Cr³⁺, which is less mobile and harmful, is a suitable approach for chromium removal. In this regard, in comparison to other reactive metals, the use of zero-valent magnesium (ZVM) has several advantages. Nevertheless, this element has been scarcely investigated in the decontamination of water and wastewater. In particular, no studies have been conducted by applying Mg⁰ particles fixed in column systems for Cr⁶⁺ reduction. In the present study, a wide experimental investigation was carried out to analyse the chromium abatement through zero valent magnesium particles in a packed batch column. The effects of pH, initial Cr⁶⁺ concentration and temperature were investigated. The experimental results proved that the process performances were mainly affected by pH values. High efficiencies were detected at pH 3, while unsatisfactory abatements were observed at pH 5 and under uncontrolled pH conditions. At pH 3, the process performance worsened with the rise in the initial chromium concentration. In particular, a complete abatement was detected in 180 min by treating solutions with initial Cr⁶⁺ concentrations up to 40 mg/L. The effect of temperature was negligible at pH 3 and under uncontrolled pH, while the increase from 20 to 30 and 40 °C produced a significant improvement in the removal yields at pH 5. By means of a kinetic analysis a theoretical law able to accurately describe the experimental removal trends was identified. Furthermore, a mathematical relation between the observed kinetic constants and the magnesium to initial chromium amount ratio was defined. Finally, the reaction pathways were proposed, and the reaction products identified.

Electrochemical recovery and high value-added reutilization of heavy metal ions from wastewater: Recent advances and future trends

Wastewater treatment for heavy metals is currently transitioning from pollution remediation towards resource recovery. As a controllable and environment-friendly method, electrochemical technologies have recently gained significant attention. However, there is a lack of systematic and goal oriented summarize of electrochemical metal recovery techniques, which has inhibited the optimized application of these methods. This review aims at recent advances in electrochemical metal recovery techniques, by comparing different electrochemical recovery methods, attempts to target recycling heavy metal resources with minimize energy consumption, boost recovery efficiency and realize the commercial application. In this review, different electrochemical recovery methods (including E-adsorption recovery, E-oxidation recovery, E-reduction recovery, and E-precipitation recovery) for recovering heavy metals are introduced, followed an analysis of their corresponding mechanisms, influencing factors, and recovery efficiencies. In addition, the mass transfer efficiency can be promoted further through optimizing electrodes and reactors, and multiple technologies (photo-electrochemical and sono-electrochemical) could to be used synergistically improve recovery efficiencies. Finally, the most promising directions for electrochemical recovery of heavy metals are discussed along with the challenges and future opportunities of electrochemical technology in recycling heavy metals from wastewater.

Selenium(Ⅳ) alleviates chromium(Ⅵ)-induced toxicity in the green alga Chlamydomonas reinhardtii

The wide range of industrial applications of chromium (Cr) has led to an increasing risk of water contamination by Cr(Ⅵ). However, efficient methods to remove or decrease the toxicity of Cr(Ⅵ) in situ are lacking. The main aim of this study was to investigate the mechanisms by which selenite alleviates chromium(Ⅵ)-induced toxicity in Chlamydomonas reinhardtii. Our results showed that K₂Cr₂O₇ had toxic effects on both the structure and physiology of C. reinhardtii in a dose-dependent manner. Adding selenite significantly alleviated chromium accumulation and toxicity in cells. RNA-seq data showed that the expression level of selenoproteins such as SELENOH was significantly increased. Both SELENOH-amiRNA knockdown mutants and selenoh insertional mutant produced more reactive oxygen species (ROS) and grew slower than the wild type, suggesting that SELENOH can reduce chromium toxicity by decreasing the levels of ROS produced by Cr(Ⅵ). We also demonstrated that selenite can reduce the absorption of Cr(Ⅵ) by cells but does not affect the process of Cr(Ⅵ) adsorption and efflux. This information on the molecular mechanism by which selenite alleviates Cr(Ⅵ) toxicity can be used to increase the bioremediation capacity of algae and reduce the human health risks associated with Cr(Ⅵ) toxicity.

Enhanced Cr(VI) removal from water using a green synthesized nanocrystalline chlorapatite: Physicochemical interpretations and fixed-bed column mathematical model study

Apatite-based minerals have attracted much attention in the remediation of heavy metal-contaminated environment. However, exploring apatite with efficient adsorption performance for inorganic oxyanions such as Cr(VI) remains a challenge. Herein, a novel nanocrystalline chlorapatite (ClAP) was promptly prepared by a green method using eggshell wastes as calcium source with the purpose to enhance Cr(VI) adsorption capability. The generated ClAP was characterized by XRD, SEM-EPMA, TEM, FTIR, and BET analyses. Batch and column experiments were subsequently carried out to explore the influencing factors, adsorption capacity and removal mechanism. Results showed that ClAP exhibited excellent stability and adsorption performance for Cr(VI) (63.47 mg g^-1), which was much greater than that of hydroxyapatite and most reported materials. The adsorption process was fitted well by the pseudo-second-order model and the Langmuir model. In fixed bed column experiments, a novel time-fractional derivative model exhibited much better suitability in interpreting the observed breakthrough curves of Cr(VI) than traditional models. Furthermore, the reusability of ClAP in column was evaluated. Results showed that the adsorption capacity maintained well after consecutively reused for five cycles. Studies of the effect of pH, as well as FTIR and XPS investigations indicated that Cr(VI) adsorption was mainly ascribed to electrostatic interactions and surface complexation, while the reduction of Cr(VI) to the low-toxicity Cr(III) also existed in the adsorption process. The ClAP adsorbent was also successfully used for Cr(VI) remediation from real wastewater. Hence, nanocrystalline ClAP can be a promising material for enhancing the elimination of oxyanion contaminants such as Cr(VI) from water.

Green rust functionalized geopolymer of composite cementitious materials and its application on treating chromate in a holistic system

Green rust functionalized geopolymer of composite cementitious materials (GR-CCM) was synthesized to improve the adsorption and subsequent stabilization/solidification of chromate in a holistic operating system. The initial pH in solution exhibited the most significant effect on the chromate removal by GR-CCM among three adsorption factors. The maximum monolayer adsorption capacity and theoretical saturation capacity of GR-CCM for Cr(VI) in the acidic condition were 55.01 mg/g and 41.70 mg/g, respectively. Amorphousness brought by loading GR weakened the crystallinity of composite cementitious materials (CCM), which enhanced the adsorption capacity of CCM and boosted the solidification process. The mixed-valent iron species in the GR-CCM not only directly engaged in the adsorption and reduction of chromate also positively strengthened the solidification of Cr species during the whole treatment. This study facilitates the application of GRs on the geopolymer materials and demonstrates the combination of adsorption and immobilization for the treatment of other potential heavy metal contamination.

Surface functionalization of recycled polyacrylonitrile fibers with ethylenediamine for highly effective adsorption of Hg(II) from contaminated waters

In this research, recycled polyacrylonitrile fibers (PANFs) acquired from the textile recycling process were amino-functionalized in one simple step by means of ethylenediamine (EDA). The amino-functionalized polyacrylonitrile fibers (AF-PANFs) were utilized for adsorption of Hg(II) ions from aquatic media. Temperature and contact time during the synthesis were optimized by the Central Composite Design (CCD) method. FE-SEM, EDS, BET, and FT-IR analysis, and pH_ZPC measurement were conducted to characterize the features of the AF-PANFs. The average diameter of raw fiber was 20 μm, which increased 20 percent after functionalizing. The impact of independent parameters on the adsorption process was investigated using the Box-Behnken Design (BBD) method during the batch experiments. The column tests were conducted in a semi-continuous system with the removal efficiency of over 99% for various initial concentrations after specific cycles. Freundlich, Langmuir, UT, Redlich-Peterson, and Temkin isotherm models were employed to analyze the relation between the final concentration of Hg(II) (C_o) and the equilibrium adsorption capacity (q_e) of the AF-PANFs. According to the isotherm models and experimental results, the maximum q_e of the AF-PANFs was 1116 mg g^-1 at initial Hg(II) concentration of 850 mg L^-1, contact time of 120 min, solution pH of 6, and at 40 °C. Kinetic and thermodynamic studies illustrated the approximate equilibrium time and endothermicity or exothermicity of the process. Regeneration of the AF-PANFs was accomplished for seven times without efficiency drop. The superb performance of the AF-PANFs in the presence of co-existing ions did not decline.

From children's toy to versatile sensor: One-step doping of Play-Doh with primary amino group for explosive detection both on surfaces and in solution

2,4,6-trinitrotoluene (TNT) sensing on surfaces and in solution is an important issue in sensor fabrication for homeland security and environmental protection. Herein, Play-Doh, a modeling material popular for kids, was proposed as a versatile sensor for on-site fluorescent (FL), visual FL (VFL), and colorimetric detection of TNT both on surfaces and in solution after being doped with -NH₂ through a one-step approach. Play-Doh exhibits FL emission due to the main ingredient of flour. After -NH₂ doping, amino-Play-Doh (APD) was utilized to construct a FL sensor based on FL resonance energy transfer and inner filter effect for TNT detection. The advantage of APD was that no additional fluorophore was needed compared with the traditional sensors for FL and VFL analysis. The orange complex visible to the naked eye was also recorded for smartphone-based colorimetric detection of TNT. In both cases, the APD demonstrated good analytical performance for TNT. Finally, APD was successfully utilized for TNT sensing on fingerprints, luggage, and in environmental water samples, respectively. Play-Doh might be a potential sensor for future on-site detection of TNT owing to the merits of being cost-effective and versatile.

Study of the properties and mechanism of deep reduction and efficient adsorption of Cr(VI) by low-cost Fe3O4-modified ceramsite

To efficiently treat low-concentration chromium wastewater at low cost, adsorbent iron-modified ceramsite (FCM) was successfully prepared. The adsorbent was characterized by SEM, XRD, FT-IR, XPS, ζ potential and VSM. From the experimental results, the optimum adsorption pH, adsorbent dosage and adsorption time were 4, 1g/L and 120min, respectively. The percentage removal of Cr(VI) was 93% and the reduction efficiency of Cr(VI) was 74.3% when the concentration was 2mg/L. After 5cycles, using 0.1mol/L NaOH as the regeneration agent, the removal rate of Cr(VI) did not decrease significantly. Furthermore, the mechanism of deep reduction and adsorption, the process of regeneration, the kinetics and adsorption isotherms were also explored. This paper can provide theoretical and technical support for reducing the toxicity and the advanced treatment of hexavalent chromium.

Hexavalent chromium reduction through redox electrolytic cell with urea and cow urine as anolyte

The present study demonstrates the potential utilization of urea/cow urine as anolyte for Cr(VI) reduction via a simple three-chambered electrolytic cell. The inherent chemical energy in the dual-waste stream (Cr(VI)-urea/urine) is employed for its self-oxidation-reduction without the need for any external energy supply. Ni foam as electroactive anode and catalyst-free carbon felt as cathode, along with the appropriate positioning of ion-selective separators, indirectly improved the cell performance by impeding electrolyte crossover. A fundamental study involving five different membrane configurations was conducted herein to improve Cr(VI) reduction efficiency. The Cr(VI) reduction efficiencies were 11.84 ± 0.27%, 10.55 ± 0.17%, 77.24 ± 0.38% at 24 h, 13.57 ± 0.25% at 72 h with glass frit, cation exchange membrane (CEM), sandwiched membrane, and anion exchange membrane (AEM) as separators in a dual-chambered H-cell, respectively, with an initial Cr(VI) concentration of 100 mg/L. The fifth configuration, consisting of a middle chamber between the anode and cathode with the CEM close to the anode and the AEM close to the cathode resulted in a reduction efficiency of 79.98 ± 2.24% within 45 min for an initial Cr(VI) concentration of 400 mg/L. The first order rate constants were determined to be 0.024, 0.018, and 0.013 min^-1 for Cr(VI) concentrations of 100, 200, and 400 mg/L, respectively. Moreover, when urea was replaced with cow urine as anolyte, a reduction efficiency of 98.94 ± 1.28% was achieved at pH 2 in 45 min with 400 mg/L as initial Cr(VI) concentration. Furthermore, the XPS spectra of reduced Cr corresponding to binding energies of 579.4 eV and 589.3 eV, respectively, confirmed the presence of low-toxic Cr(III). The effect of applied load, initial Cr(VI) and urea concentration, Cr(VI) reduction under different initial H₂SO₄ concentrations were succinctly investigated to evaluate the performance of the electrolytic cell. The redox electrolytic cell can thus be an alternative to the conventional chemical or energy intensive processes for the reduction of hexavalent chromium.