Alkaline electrochemical advanced oxidation process for chromium oxidation at graphitized multi-walled carbon nanotubes

A Critical Review of Groundwater Table Fluctuation: Formation, Effects on Multifields, and Contaminant Behaviors in a Soil and Aquifer System

The groundwater table fluctuation (GTF) zone is an important medium for the hydrologic cycle between unsaturated soil and saturated aquifers, which accelerates the migration, transformation, and redistribution of contaminants and further poses a potential environmental risk to humans. In this review, we clarify the key processes in the generation of the GTF zone and examine its links with the variation of the hydrodynamic and hydrochemistry field, colloid mobilization, and contaminant migration and transformation. Driven by groundwater recharge and discharge, GTF regulates water flow and the movement of the capillary fringe, which further control the advection and dispersion of contaminants in soil and groundwater. In addition, the formation and variation of the reactive oxygen species (ROS) waterfall are impacted by GTF. The changing ROS components partially determine the characteristic transformation of solutes and the dynamic redistribution of the microbial population. GTF facilitates the migration and transformation of contaminants (such as nitrogen, heavy metals, non-aqueous phase liquids, and volatile organic compounds) through colloid mobilization, the co-migration effect, and variation of the hydrodynamic and hydrochemistry fields. In conclusion, this review illustrates the limitations of the current literature on GTF, and the significance of GTF zones in the underground environment is underscored by expounding on the future directions and prospects.

Room-temperature ultrasonic-assisted self-assembled synthesis of silkworm cocoon-like COFs@GCNTs composite for sensitive detection of diuron in food samples

Diuron (DU) exhibits good weed control effect but possesses strong hazard to human health, thereby designing a fast and sensitive method to detect DU is highly urgent. Herein, we report the ultrasonic-assisted self-assembly synthesis of porous covalent organic frameworks (COFs) spheres@graphitized multi-walled carbon nanotubes (GCNTs) composite based on π-π conjugation effect at room temperature, which was employed for DU determination. For the COFs@GCNTs composite, COFs with ultrahigh specific surface area shows strong adsorption ability towards DU, whereas GCNTs with favorable conductivity help to form the 3D interconnected conductive network around COFs spheres, thereby effectively compensating for the poor conductivity of COFs. Because of the synergistic effect between COFs and GCNTs, the developed sensor presented a low detection limit of 0.08 µM in the concentration range of 0.30-18.00 µM. Moreover, the actual sample analysis in the tomato and cucumber yielded satisfactory recoveries (96.40%-103.20%), proving reliable practicability of the developed sensor.

Oxidative leaching of V-Cr-bearing reducing slag via a Cr(III) induced Fenton-like reaction in concentrated alkaline solutions

V-Cr-bearing reducing slag (VCRS) is considered a hazardous waste that can create ecosystem disasters if handled improperly. It consists of a considerable amount of heavy metals, such as vanadium (V) and chromium (Cr). In this study, we propose a novel process featuring a VCRS self-induced Cr(III)-Fenton-like reaction to efficiently recover V and Cr from hazardous VCRS. The generation of hydroxyl radicals (·OH) and determination of their effect on V and Cr oxidation were examined via electron spin resonance detection, free radical quenching, and terephthalic acid fluorescence probe methods. The V and Cr oxidative leaching processes were directly controlled by the amount of added H₂O₂ and generated·OH from the Cr(III)-Fenton-like reaction, which in turn was dependent on the amount of dissolved Cr(OH)₄^-. In a single oxidative leaching process, the leaching efficiencies of V and Cr reached 97.5 ± 0.6 % and 85.2 ± 0.8 %, respectively, and reached 99.4 ± 0.5 % and 94.6 ± 0.9 %, respectively, from circular leaching owing to a continuous supply of dissolved Cr(OH)₄^- from fresh VCRS. This study identifies a novel approach to discovering deep oxidation of the VCRS while minimizing environmental contamination via a waste control strategy and can be considered an attractive alternative approach for the green treatment of VCRS.

FeS-mediated mobilization and immobilization of Cr(III) in oxic aquatic systems

Reduction of soluble Cr(VI) into insoluble Cr(III) by iron sulfide (FeS) minerals under anoxic conditions has been widely observed in natural and engineered systems. Yet, information has been lacking on the FeS-mediated oxidation and remobilization potential of Cr(III) under varying environmental conditions. The objective of this study was to investigate FeS-mediated redox transformation of Cr(III) to Cr(VI) and the associated mobilization and immobilization when Cr(III)-FeS systems are exposed to atmospheric conditions. The results showed that FeS nanoparticles facilitated rapid and strong Fenton-like reactions during the early-stage oxygenation of FeS, resulting in rapid production of hydroxyl radicals (•OH). Consequently, Cr(III) was rapidly oxidized into Cr(VI). Yet, as the reactions proceeded, the oxidative potential was counteracted by competitive scavenging of •OH by Fe(II) and S(-II) from FeS and the reduction reactions by these electron donors. At equilibrium, all Cr(VI) was reduced back to Cr(III) at an FeS-Cr(III) molar ratio of 10:1, while a small fraction of Cr(VI) persisted in solid products of Cr(OH)₃(s) at an FeS-Cr(III) molar ratio of 1:1. Acidic conditions favored the generation of Cr(VI) and the equilibrium concentration of Cr(VI) in oxic FeS NPs systems at pH 5.0 was 1.7 times higher than at pH 9.0. Overall, the FeS-induced Fenton-like reactions and the oxidation of Cr(III) were favored in the early stage, but quenched in the later stage and outcompeted by the reduction of Cr(VI) if sufficient FeS was available. The findings provide new insights into the hydrochemical processes that can affect the speciation, toxicity, and mobility of Cr in aquatic systems containing FeS and Cr.

Pulmonary inflammatory and fibrogenic response induced by graphitized multi-walled carbon nanotube involved in cGAS-STING signaling pathway

Graphitized multi-walled carbon nanotubes (GMWCNTs) are a new type of nanomaterial. Recently, their production and application in biological medicine have grown rapidly. However, GMWCNTs may cause adverse health effects, including the common occupational disease of pulmonary fibrosis. Pulmonary fibrosis is a serious progressive disease that often leads to lung failure, high mortality, and disability, and there is no effective therapy currently available. Therefore, identifying new biomarkers of the disease is important to better understand the disease mechanisms and explore new therapeutic strategies. In this study, 40 μg of GMWCNTs was used to treat mice in vivo by pharyngeal aspiration, and different genes were screened by transcriptome sequencing. Activation of the cyclic GMP-AMP synthase (cGAS)-stimulator of interferon gene (STING) signal pathway had an important effect on the development of pulmonary inflammation and fibrosis. GMWCNTs were then administered to the mice with a STING inhibitor (C-176). Inhibition of STING effectively decreased pulmonary inflammation and fibrosis in mice induced by GMWCNTs. Collectively, activation of the cGAS-STING signaling pathway is involved in GMWCNT-induced pulmonary inflammation and fibrosis in mice.

Functionalized Carbon Nanotubes (CNTs) for Water and Wastewater Treatment: Preparation to Application

As the world human population and industrialization keep growing, the water availability issue has forced scientists, engineers, and legislators of water supply industries to better manage water resources. Pollutant removals from wastewaters are crucial to ensure qualities of available water resources (including natural water bodies or reclaimed waters). Diverse techniques have been developed to deal with water quality concerns. Carbon based nanomaterials, especially carbon nanotubes (CNTs) with their high specific surface area and associated adsorption sites, have drawn a special focus in environmental applications, especially water and wastewater treatment. This critical review summarizes recent developments and adsorption behaviors of CNTs used to remove organics or heavy metal ions from contaminated waters via adsorption and inactivation of biological species associated with CNTs. Foci include CNTs synthesis, purification, and surface modifications or functionalization, followed by their characterization methods and the effect of water chemistry on adsorption capacities and removal mechanisms. Functionalized CNTs have been proven to be promising nanomaterials for the decontamination of waters due to their high adsorption capacity. However, most of the functional CNT applications are limited to lab-scale experiments only. Feasibility of their large-scale/industrial applications with cost-effective ways of synthesis and assessments of their toxicity with better simulating adsorption mechanisms still need to be studied.

Carbonaceous cathode materials for electro-Fenton technology: Mechanism, kinetics, recent advances, opportunities and challenges

Electro-Fenton (EF) technique has gained significant attention in recent years owing to its high efficiency and environmental compatibility for the degradation of organic pollutants and contaminants of emerging concern (CECs). The efficiency of an EF reaction relies primarily on the formation of hydrogen peroxide (H₂O₂) via 2e^─ oxygen reduction reaction (ORR) and the generation of hydroxyl radicals (^●OH). This could be achieved through an efficient cathode material which operates over a wide pH range (pH 3-9). Herein, the current progresses on the advancements of carbonaceous cathode materials for EF reactions are comprehensively reviewed. The insights of various materials such as, activated carbon fibres (ACFs), carbon/graphite felt (CF/GF), carbon nanotubes (CNTs), graphene, carbon aerogels (CAs), ordered mesoporous carbon (OMCs), etc. are discussed inclusively. Transition metals and hetero atoms were used as dopants to enhance the efficiency of homogeneous and heterogeneous EF reactions. Iron-functionalized cathodes widened the working pH window (pH 1-9) and limited the energy consumption. The mechanism, reactor configuration, and kinetic models, are explained. Techno economic analysis of the EF reaction revealed that the anode and the raw materials contributed significantly to the overall cost. It is concluded that most reactions follow pseudo-first order kinetics and rotating cathodes provide the best H₂O₂ production efficiency in lab scale. The challenges, future prospects and commercialization of EF reaction for wastewater treatment are also discussed.

A label-free electrochemical DNA biosensor for kanamycin detection based on diblock DNA with poly-cytosine as a high affinity anchor on graphene oxide

It is urgent to develop a more simple and sensitive method to detect antibiotic residues considering the harm of antibiotic residues in food to the human body. Herein we designed a label-free electrochemical DNA biosensor for the sensitive detection of kanamycin (KAN) based on diblock DNA with a 15-mer of poly-cytosine (poly-C). The diblock DNA can be immobilized on graphene oxide (GO) due to strong physical adsorption between the 15-mer of poly-C and GO. The aptamer of KAN acted as the other block for rapidly binding the target. It can specifically capture the target, which leads to the change of electrochemical signal. Consequently, the DNA biosensor exhibited high sensitivity and specificity towards KAN, the linear range was from 0.05 pM to 100 nM with a detection limit of 0.0476 pM. The developed DNA biosensor was constructed easily and showed promising applications for the detection of antibiotic residues for food safety.

Recovery and Separation of Vanadium and Chromium by Two-Step Alkaline Leaching Enhanced with an Electric Field and H2O2

This paper focused on the treatment of vanadium-chromium reducing residue with a two-step alkaline leaching process: electro-oxidation leaching of vanadium and H₂O₂ as well as oxidation leaching of chromium in an alkaline medium. The effects of experimental parameters on the leaching performance of vanadium and chromium were investigated. The experimental data showed that in the first alkaline leaching in stage I, the leaching efficiency of vanadium reached up to 95.32% under optimal conditions, while most of the chromium could not leach out (about 4% of chromium was leached out). Chromium was easily oxidized to high valence (CrO₄ ^2-) with H₂O₂ in the second alkaline leaching stage II. Under optimal conditions, 96.24% chromium was leached out.

In situ anodic induction of low-valence copper in electro-Fenton system for effective nitrobenzene degradation

To achieve superior advanced oxidation processes (AOPs), transitional state activators are of great significance for the production of active radicals by H₂O₂, while instability limits their activation efficiency. In this study, density functional theory calculation (DFT) results showed that Cu⁺ exhibits excellent H₂O₂ activation performance, with Gibbs free energy change (ΔG) of 33.66 kcal/mol, two times less than that of Cu²⁺ (77.83 kcal/mol). Meanwhile, an electro-Fenton system using Cu plate as an anode was proposed for in situ generation of Cu⁺. The released Cu with low-valence state can be well-confined on the surface of the exciting electrode, which was confirmed by X-ray photoelectron spectroscopy (XPS), Raman, and UV-vis spectroscopy. The hydroxyl radicals in this Cu-based electro-Fenton system were determined by the electron spin resonance (ESR). The nitrobenzene degradation ratio was greatly increased by 43.90% with the introduction of the proposed in situ electrochemical Cu⁺ generation process. Various characterization results indicated that the production of Cu⁺ was the key factor in the highly efficient Cu-based electro-Fenton reaction.

Effect of surface properties of activated carbon fiber cathode on mineralization of antibiotic cefalexin by electro-Fenton and photoelectro-Fenton treatments: Mineralization, kinetics and oxidation products

Solutions of 200 mg L^-1 cefalexin (CLX), an antibiotic with high usage frequency and biodegradation resistance, have been comparatively degraded by electro-Fenton (EF) and photoelectro-Fenton (PEF) processes using two kinds of activated carbon fiber (ACF) cathodes with different physical properties. These two ACFs shared similar pore volumes and pore diameters but varied BET surface areas, which were confirmed to be 0.5210 cm³ g^-1, 2.26 nm and 921 m² g^-1 for ACF1, while 0.6508 cm³ g^-1, 2.16 nm and 1206 m² g^-1 for ACF2, respectively. Their oxidation abilities were comparatively assessed in terms of degradation kinetics and mineralization rates, which increased in the order: ACF1-EF < ACF2-EF < ACF1-PEF < ACF2-PEF. These results confirmed the superiority of ACF with higher surface area, which was correlated to faster H₂O₂ and OH accumulation in more reaction sites provided. After 120 min electrolysis, ACF1 exhibited 1510 μM H₂O₂ and 37 μM OH accumulation, while ACF2 generated 1934 μM H₂O₂ and 85 μM OH. Moreover, ACF cathode with more developed pore structure also revealed faster formation of degradation by-products like inorganic ions (NH₄⁺ and NO₃^- ions) and short-chain carboxylic acids (acetic, formic, oxamic and oxalic acids), as well as enhanced removal for partial acids. In order to gain a deeper understanding of degradation mechanisms for ACF2-PEF system, evolutions of six aromatic by-products generated from sulfoxidation, hydroxylation and decarboxylation were confirmed by UPLC-QTOF-MS/MS determination. Based on the above identifications of the degradation intermediates, a plausible reaction pathway for CLX removal was proposed.

Highly Efficient Recovery of Vanadium and Chromium: Optimized by Response Surface Methodology

Response surface methodology was applied to optimize the processing parameters (dosage of NaOH, dosage of H₂O₂, reaction temperature, liquid-to-solid ratio, stirring rate, and reaction time) that affected the leaching process of vanadium and chromium. The results indicated that the leaching process of vanadium was significantly affected by the dosage of NaOH and dosage of H₂O₂ used in the experiments, whereas the processing parameters affected the leaching efficiency of chromium in the following order: dosage of H₂O₂ (F) > reaction temperature (C) > dosage of NaOH (A) > reaction time (B) > stirring rate (D) > liquid-to-solid ratio (E). Almost 98.60% of vanadium and 79.82% of chromium were leached out during the leaching process.

Highly efficient oxidation of chromium (III) with hydrogen peroxide in alkaline medium

Many technologies have been proposed to oxidize chromium, such as roasting-water leaching technology and hydrometallurgical methods such as pressure oxidative leaching coupled with oxygen, ozone, permanganate and ferrate, but the problems associated with the high temperature, low overall resource utilization efficiency, high energy consumption, and the environmental pollution, still remain unsolved. This paper focuses on the oxidation process of chromium (III) with hydrogen peroxide (H₂O₂) in an alkaline medium. The effect of parameters including dosage of H₂O₂, dosage of NaOH, reaction time, reaction temperature and stirring rate on the oxidation efficiency of chromium were investigated. The oxidation efficiency was significantly affected by the dosage of H₂O₂ and NaOH, reaction time and reaction temperature took second place; last was the stirring rate. Oxidation efficiency was nearly 100% under the optimal conditions: volume ratio of H₂O₂ to mass of Cr₂(SO₄)₃ of 2.4 mL/g, mass ratio of NaOH to Cr₂(SO₄)₃ 0.6 g/g, reaction time of 90 min, reaction temperature of 90 °C and stirring rate of 500 rpm.

Multiplexed aptasensor based on metal ions labels for simultaneous detection of multiple antibiotic residues in milk

A dual-target electrochemical aptasensor was developed for the simultaneous detection of multiple antibiotics based on metal ions as signal tracers and nanocomposites as signal amplification strategy. Metal ions such as Cd²⁺ and Pb²⁺ could generate distinct differential pulse voltammetry (DPV) peaks. When targets were present, kanamycin (KAN) and streptomycin (STR) as models, the KAN aptamer (KAP) and STR aptamer (STP) were released from their complementary strands, with more change of Cd²⁺ and Pb²⁺ corresponding to peak currents. At the same time, complementary strand of KAP (cKAP) and STP (cSTP) were linked with the poly (A) structure (cSTP-PolyA-cKAP) to increase their conformational freedom. Graphitized multi-walled carbon nanotubes (MWCNT_Gr) and carbon nanofibers-gold nanoparticles (CNFs-AuNPs) as a biosensor platform enhanced the surface area to capture a large amount of cSTP-PolyA-cKAP, thus amplifying the detection response. Under the optimal conditions, the aptasensor could detect KAN and STR as low as 74.50 pM and 36.45 pM respectively with the range from 0.1 to 100 nM and exhibited excellent selectively. Moreover, this aptasensor showed promising applications for the detection of other analytes by changing corresponding aptamers.