Enhancement of methylene blue removal by anodic oxidation using BDD electrode combined with adsorption onto sawdust

Studies on the removal of methylene blue (MB) from aqueous solutions by anodic oxidation (AO) using a boron-doped diamond (BDD) electrode, adsorption onto sawdust, and combined treatment have been undertaken. The results proved that AO presents a high efficiency in removing both color and COD in a wide pH interval. The total mineralization of the dye solution was performed in 6 h, which corresponds to relatively high-energy consumption. On the other hand, high sawdust dosage (12 g·L⁻¹) was needed to ensure 98% of color and 81% of COD removal. Combining AO and adsorption onto sawdust constitute a very interesting technology. For instance, AO for 1 h followed by sorption permits a reduction in energy consumption by 80 W·h·g_COD⁻¹, a reduction of more than 24 times the adsorbent dose and an enhancement of color and COD removal, indicating that sawdust is efficient in removing not only the MB initial molecules but also the electrogenerated by-products.     

Enzyme-free impedimetric glucose sensor based on gold nanoparticles/polyaniline composite film

A non-enzymatic impedimetric glucose sensor was fabricated based on the adsorption of gold nanoparticles (GNPs) onto conductive polyaniline (PANI)-modified glassy carbon electrode (GCE). The modified electrode (GCE/PANI/GNPs) was characterized by using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and scanning electron microscopy (SEM). The determination of glucose concentration was based on the measurement of EIS with the mediation of electron transfer by ferricyanide ([Fe(CN)₆]^3?). The [Fe(CN)₆]^3? is reduced to ferrocyanide ([Fe(CN)₆]^4?), which in turn is oxidized at GCE/PANI/GNPs. An increase in the glucose concentration results in an increase in the diffusion current density of the [Fe(CN)₆]^4? oxidation, which corresponds to a decrease in the faradaic charge transfer resistance (R _ct). A wide linear concentration range from 0.3 to 10 mM with a lower detection limit of 0.1 mM for glucose was obtained. The proposed sensor shows high sensitivity, good reproducibility, and stability. In addition, the sensor exhibits no interference from common interfering substances such as ascorbic acid, acetaminophen, and uric acid.     

Ferricyanide Armed Anodes Enable Stable Water Oxidation in Saturated Saline Water at 2 A/cm2

The direct seawater electrolysis at high current density and low overpotential affords an effective strategy toward clean and renewable hydrogen fuel production. However, the severe corrosion of anode as a result of the saturation of Cl⁻ upon continuous seawater feeding seriously hamper the electrolytic process. Herein, cobalt ferricyanide / cobalt phosphide (CoFePBA/Co₂P) anodes with Cap/Pin structure are synthesized, which stably catalyze alkaline saturated saline water oxidation at 200–2000 mA cm⁻² over hundreds of hours without corrosion. Together with the experimental findings, the molecular dynamics simulations reveal that PO₄³⁻ and Fe(CN)₆³⁻ generated by the electrode play synergistic role in repelling Cl⁻ via electrostatic repulsion and dense coverage, which reduced Cl⁻ adsorption by nearly 5-fold. The novel anionic synergy endow superior corrosion protection for the electrode, and is expected to promote the practical application of saline water electrolysis.     

Detection of nicotine based on molecularly imprinted TiO2-modified electrodes

Amperometric detection of nicotine (NIC) was carried out on a titanium dioxide (TiO₂)/poly(3,4-ethylenedioxythiophene) (PEDOT)-modified electrode by a molecular imprinting technique. In order to improve the conductivity of the substrate, PEDOT was coated onto the sintered electrode by in situ electrochemical polymerization of the monomer. The sensing potential of the NIC-imprinted TiO₂ electrode (ITO/TiO₂[NIC]/PEDOT) in a phosphate-buffered saline (PBS) solution (pH 7.4) containing 0.1 M KCl was determined to be 0.88 V (vs. Ag/AgCl/saturated KCl). The linear detection range for NIC oxidation on the so-called ITO/TiO₂[NIC]/PEDOT electrode was 0-5 mM, with a sensitivity and limit of detection of 31.35 μA mM⁻¹ cm⁻² and 4.9 μM, respectively. When comparing with the performance of the non-imprinted one, the sensitivity ratio was about 1.24. The sensitivity enhancement was attributed to the increase in the electroactive area of the imprinted electrode. The at-rest stability of the ITO/TiO₂[NIC]/PEDOT electrode was tested over a period of 3 days. The current response remained about 85% of its initial value at the end of 2 days. The ITO/TiO₂[NIC]/PEDOT electrode showed reasonably good selectivity in distinguishing NIC from its major interferent, (−)-cotinine (COT). Moreover, scanning electrochemical microscopy (SECM) was employed to elucidate the surface morphology of the imprinted and non-imprinted electrodes using Fe(CN)₆³⁻/Fe(CN)₆⁴⁻ as a redox probe on a platinum tip. The imprinted electrode was further characterized by scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR).     

Removal of basic dye methylene blue from aqueous solutions using sawdust and sawdust coated with polypyrrole

This research deals with the application of wood sawdust obtained from walnut and its polypyrrole (PPy) coated form for the removal of methylene blue (MB) from aqueous solutions. MB, a typical cationic dye, was used as a test probe. Coating of sawdust was carried out chemically via direct addition of chemical oxidant (FeCl₃) on the sawdust which was previously soaked in monomer solution (0.2 M pyrrole). Adsorption experiments were carried out using both batch and fixed bed column systems. The effects of different system variables such as adsorbent dose, initial dye concentration, pH of test solution, contact time and breakthrough curves were studied. In order to find out the possibility of reuse, desorption study was also carried out in this investigation. It was found that both SD and PPy/SD are efficient and cost-effective adsorbents for the removal of MB dye from aqueous solutions. The collected data in this research indicate that compared with PPy/SD, adsorption of MB by SD is not a very pH dependent process, at least from pH 4 to 10. Higher sorption percentage of MB is obtained for PPy/SD at alkaline pHs.     

Preparation and Characterization of GCE Coatings Combining DDDMAB and PDADMAC with FAD and HCM for Electrocatalytic Detection of Oxygen and Sulfur Oxoanions

Electrochemically active hybrid coatings based on cationic films, didodecyldimethylammonium bromide (DDDMAB), and poly(diallyldimethylammonium chloride) (PDADMAC) are prepared on glassy carbon electrode surface by cycling the film‐covered electrode repetitively in a pH 7 solution containing flavin adenine dinucleotide (FAD), and anionic hexacyanometalate (HCM) complexes, Fe(CN)₆^3? and Ru(CN)₆^4?. Cyclic voltammetric features of hybrid coatings resemble that of electron transfer process of surface‐confined redox species. Electrochemical quartz crystal microbalance (EQCM) was used to monitor the deposition of FAD on DDDMAB film. Cyclic voltammetric peak potentials of modified electrode were found to be shifted to more negative region with increasing pH of contacting solution with a slope value of 63.3mV per pH unit. The electrocatalytic behavior of FAD‐modified DDDMAB‐coated GCE and hybrid film electrodes was tested towards reduction of oxygen, S₂O₈^2?, SO₅^2? and oxidation of SO₃^2?. The application of FAD‐modified DDDMAB‐coated GCE for S₂O₈^2? estimation was demonstrated in amperometric mode. The sensitivity and detection limit (S/N=3) were 267.6 μA mM^?1 and 2×10^?6 M, respectively.     

Carbon Nanotube Immobilized Electrode Using Amphiphilic Phospholipid Polymer with Anti‐fouling and Dispersion Property for Electrochemical Analysis

A carbon nanotube (CNT)‐modified electrode was fabricated by dropping a dispersion of multi‐walled CNTs in water‐soluble and amphiphilic phospholipid polymer with both dispersing ability and anti‐biofouling property onto a Au electrode. A poly(2‐methacryloyloxyethyl phosphorylcholine‐co‐n‐butyl methacrylate) (PMB) composed from 50 mol% of 2‐methacryloxylethyl phosphorylcholine and 50 mol% of n‐butyl methacrylate (PMB50) was used as dispersing reagent for CNTs. The dispersion of water‐insoluble material by PMB50 and its antifouling effects in electrochemical analysis were investigated. The CNT‐modified electrode showed an anodic peak potential that was shifted negatively and an increase in the current value for the electrolytic oxidation of nicotinamide adenine dinucleotide. In addition, the charge on PMB50 did not inhibit the electrochemical reaction of the redox compounds K₃[Fe(CN)₆], [Ru(NH₃)₆]Cl₃, and hydroxymethylferrocene. Cyclic voltammetry of K₃[Fe(CN)₆] in 4 % bovine serum albumin (BSA) using a bare Au electrode, the anodic peak current was reduced to 47 % of that without BSA. In contrast, the antifouling effect of the PMB50‐coated electrode meant that the current was only reduced to 70 % of that without BSA.     

The fabrication of a nanoscale polyoxometalate based magnetic adsorbent and its selective adsorption on cationic dyes

Amino group-functionalized Fe₃O₄ is loaded on a coordination complex-modified polyoxometalate nanoparticle. In this composite material, Fe₃O₄ and coordination complex-modified polyoxometalate are connected with intense hydrogen bonds as suggested by FTIR. This composite material exhibits excellent methylene blue (MB) adsorption, with adsorption capacity of 175.5 mg g^?1. It also possesses selective separation ability between cationic and anionic dye molecules. In binary solution of MB and methyl orange (MO), MB adsorption efficiency reaches 75%, but it exhibits almost no effect on the adsorption of methyl orange. The saturation magnetization value of this composite material is 18.89 emu g^?1, allowing magnetic separation, which facilitates the recycle and reuse of this composite adsorbent.     

Electrochemical behavior of the system ferricyanide-ferrocyanide at a graphite-epoxy composite electrode

Cyclic and direct voltammetry with linear potential sweep has been used for the investigation of the dependence of the reversibility and reduction current in the system Fe(CN)₆³⁻/Fe(CN)₆⁴⁻ on the concentrations of LiCl, NaCl, KCl, and CsCl solutions. The electrode was made of a graphite-epoxy composite and activated by mechanically cutting a surface layer directly in the solution and deactivated by the long-term storage in the air. The selected type of the graphite electrode and the method used to activate its surface provides the reversibility and diffusion control of the electrode process in the system Fe(CN)₆³⁻/Fe(CN)₆⁴⁻ regardless of the composition of the supporting solution. In the case of the deactivated electrode, the degree of irreversibility of this process depends on the form and concentration of metal chloride in the supporting electrolyte and the diffusion transfer is complicated by the adsorption of compounds formed between the ferricyanide and the cation of the supporting solution.     

A Carboxyl-Functionalized Covalent Organic Framework Synthesized in a Deep Eutectic Solvent for Dye Adsorption

Instead of using organic solvents, a deep eutectic solvent (DES) composed of tetrabutylammonium bromide and imidazole (Bu₄NBr/Im) was employed as a solvent for the first time to synthesize covalent organic frameworks (COFs). Due to the low vapor pressure of the Bu₄NBr/Im-based DES, a new carboxyl-functionalized COF (TpPa-COOH) was synthesized under environmental pressure. The as-synthesized TpPa-COOH has open channels, and the DES can be removed completely from the pores. The dye adsorption performance of TpPa-COOH was examined for three organic dyes with similar molecular sizes: one anionic dye (eosin B, EB) and two cationic dyes (methylene blue, MB and safranine T, ST). TpPa-COOH showed an excellent selective adsorption effect on MB and ST. The electronegative keto form in TpPa-COOH might help to form electrostatic and π–π interactions between the π-stacking frameworks of TpPa-COOH and the positive plane MB and ST molecules. The adsorption isotherms of MB and ST on TpPa-COOH were further investigated in detail, and the equilibrium adsorption was well modeled by using a Langmuir isotherm model. Together with hydrogen bonding, TpPa-COOH showed higher adsorption capacity for ST than for MB (1135 vs. 410 mg g⁻¹). These results could provide a guidance for the green synthesis of adsorbents in removing organic dyes from wastewater.     

Preparation, characterization, and electrocatalytic properties of hybrid coatings of hexacyanometalate-doped-cationic films

Electrochemically active hybrid coatings based on cationic films, didodecyldimethylammonium bromide (DDAB), and poly(diallyldimethylammonium chloride) (PDDAC) are prepared on electrode surface by cycling the film-covered electrode repetitively in a pH 6.5 solution containing Fe(CN)₆ ³⁻ and Ru(CN)₆ ⁴⁻ anions. Modified electrodes exhibited stable and reversible voltammetric responses corresponding to characteristics of Fe(CN)₆ ^3−/4− and Ru(CN)₆ ^4−/3− redox couples. The cyclic voltammetric features of hybrid coatings resemble that of electron transfer process of surface-confined redox couple. Electrochemical quartz crystal microbalance results show that more amounts of electroactive anionic complexes partitioned into DDAB coating than those doped into PDDAC coating from the same doping solution. Peak potentials of hybrid film-bound redox couples showed a negative shift compared to those at bare electrode and this shift was more pronounced in the case of DDAB. Finally, the advantages of hybrid coatings in electrocatalysis are demonstrated with sulfur oxoanions.     

Electrocatalytic evaluation of liquid phase deposited methylene blue/TiO2 hybrid films

Methylene blue (MB)/TiO₂ hybrid thin films were prepared on glassy carbon (GC) electrode surface by the liquid phase deposition (LPD) process. The electrochemical measurements indicated that MB retained its electrochemical activity in the hybrid films. The linear dependence of the reduction peak current for MB upon the scan rate and linear relationship between the middle point potential of MB and pH revealed the surface-confined two-proton two-electron electrochemical characteristics of MB entrapped in hybrid LPD films. Although the electron transfer of K₃[Fe(CN)₆] on GC surface was inhibited by TiO₂ film, the catalytic reduction of K₃[Fe(CN)₆] by MB was observed on the MB/TiO₂ hybrid films. The electrocatalytic activity of hybrid films was also demonstrated as an “artificial peroxidase” for the catalytic reduction of H₂O₂.     

Amperometric Sensor for Hydrazine Determination Based on Mechanically Immobilized Nickel Hexacyanoferrate Modified Electrode

An amperometric sensor constructed by mechanical immobilization of nickel hexacyanoferrate onto the paraffin impregnated graphite electrode is reported. The modified electrode exhibits a reversible redox peak, which corresponds to surface-confined Fe(CN)^4- ₆/Fe(CN)^3- ₆reaction. Electrocatalytic oxidation of hydrazine is effective on the modified electrode at a significantly reduced overpotential and in a broader pH range. Linear response for hydrazine is in the range 4.0 × 10^–4to 4.0 × 10^–3M with a correlation coefficient of 0.9963. The detection limit is 9.6 × 10^–5M (S/N = 3). The stability and reproducibility of the modified electrode for the determination of hydrazine is evaluated. The proposed method has been applied for the determination of hydrazine in drinking water.     

Preparation of an Anti-Aggregation Silica/Zinc/Graphene Oxide Nanocomposite with Enhanced Adsorption Capacity

Nanomaterials play a significant role in adsorption treatment of dye wastewater, but irreversible aggregation of nanoparticles poses a significant problem. In this work, nanomesoporous zinc-doped silicate (NMSZ) was prepared by an in situ method. To prevent agglomeration, NMSZ was covalently bonded to graphene oxide (GO) sheets to form a nano-silica/zinc/graphene oxide composite (GO-NMSZ), aimed at removal of cationic dye methylene blue (MB). For comparison, undoped mesoporous silica (MS) was also synthesized and modified to obtain a silica/graphene oxide composite (GO-MS). The materials were characterized by powder XRD, SEM, FTIR spectroscopy, TEM, nitrogen sorption, and X-ray photoelectron spectroscopy (XPS). Preservation of the oxygen-containing groups of GO in the composites led to higher adsorption capacities. The best GO-NMSZ composite exhibited an enhanced adsorption capacity of 100.4 mg g⁻¹ for MB compared to those of undoped GO-MS (80.1 mg g⁻¹) and nongrafted NMSZ (55.7 mg g⁻¹). The nonselective character of GO-NMSZ is demonstrated by effective adsorption of anionic dye Congo red (127.4 mg g⁻¹) and neutral dye isatin (289.0 mg g⁻¹). The adsorption kinetics, adsorption isotherms, and a thermodynamic study suggested that MB adsorption occurs by chemisorption and is endothermic in nature.     

Modified Calcium Alginate Beads with Sodium Dodecyl Sulfate and Clay as Adsorbent for Removal of Methylene Blue

In present study, we have investigated the effect of an anionic surfactant sodium dodecyl sulfate (SDS) and clay on calcium alginate beads was studied to remove methylene blue (MB) and to improve the adsorption capacity. The effects of various experimental parameters, such as shaking rate, initial dye concentration, temperature, and pH on the adsorption rate, have been studied. Equilibrium studies showed that the sorption of the dye was enhanced in presence of SDS. Scanning electron microscope (SEM) analysis showed that SDS entrapped beads have more pores and cavities which could be responsible for improved adsorption of MB. The kinetics of cationic dye adsorption nicely followed pseudo-second-order process. The evaluated thermodynamic parameters (ΔG ^o, ΔH ^o, ΔS ^o) suggest endothermic adsorption of MB. The results revealed that the surfactant entrapped alginate could be considered as potential adsorbents for MB removal from aqueous solutions.     

Layer-by-layer thin film-coated electrodes for electrocatalytic determination of ascorbic acid

Multilayer thin films composed of poly(allylamine hydrochloride) (PAH) and carboxymethyl cellulose (CMC) have been prepared on the surface of a gold (Au) disk electrode by a layer-by-layer deposition of PAH and CMC and ferricyanide ions ([Fe(CN)₆]³⁻) were confined in the film. [Fe(CN)₆]³⁻ ions can be successfully confined in the films from weakly acidic or neutral [Fe(CN)₆]³⁻ solutions, while, in basic solution, [Fe(CN)₆]³⁻ ion was not confined. The [Fe(CN)₆]³⁻ ion-confined Au electrode showed clear redox peaks in the cyclic voltammogram around 0.35 V versus Ag/AgCl. The amounts of [Fe(CN)₆]³⁻ ions confined in the films depended on the thickness of the films or the number of layers in the LbL films. The [Fe(CN)₆]³⁻ ion-confined Au electrode was used for electrocatalytic determination of ascorbic acid in the concentration range of 1-50 mM.     

Changes of electrochemical responses of the bulk solution species by thermotropic permeability of liquid crystalline membranes coated on electrodes

Liquid crystalline/polymer composite membrane-coated electrodes were prepared by casting a 1,2-dichloroethane solution of N-(4-ethoxybenzylidene)-4′-n-butylaniline (EBBA) and polycarbonate (PC) on an electrode surface. The temperature-dependence of the permeability of the EBBA/PC composite membrane on electrodes to Fe(CN)^3?₆ ion as a solution-phase redox ion was investigated by means of hydrodynamic voltammetry at a rotating disk electrode. The permeability changed with temperature over the range of the crystalline-nematic-phase transition temperature of EBBA. It is demonstrated that the observed temperature-dependence of the permeability reflects the thermotropic properties of EBBA in the EBBA/PC composite membrane. Furthermore, the dependence of the limiting current of the steady-state current-potential curves for the reduction of Fe(CN)^—₆ at the EBBA/PC composite membrane-coated electrode upon the membrane thickness, the blend ratio of EBBA and PC and the concentration of Fe(CN)^3?₆ in a bulk solution was examined in order to understand the transport process of Fe(CN)^?3₆ through the EBBA/PC composite membrane from the membrane/solution interface to the electrode/membrane interface. The transport process of Fe(CN)^3?₆ within the membrane was found to obey Fick's Law.     

Equilibrium and kinetic study of novel methyltrimethoxysilane magnetic titanium dioxide nanocomposite for methylene blue adsorption from aqueous media

Novel magnetic titanium dioxide nanoparticles decorated with methyltrimethoxysilane (Fe₃O₄@TiO₂‐MTMOS) were successfully fabricated via a sol–gel method at room temperature. The synthesized material was characterized using Fourier transform infrared spectroscopy, X‐ray diffraction, scanning electron microscopy, energy‐dispersive X‐ray spectroscopy, thermogravimetric analysis and vibrating sample magnetometry. The removal efficiency of the adsorbent was evaluated through the adsorption of methylene blue (MB) dye from water samples. The adsorption isotherm and kinetics were evaluated using various models. The Langmuir model indicated a high adsorption capacity (11.5 mg g^?1) of Fe₃O₄@TiO₂‐MTMOS. The nanocomposite exhibited high removal efficiency (96%) and good regeneration (10 times) compared to Fe₃O₄ and Fe₃O₄@TiO₂ at pH = 9.0. Based on the adsorption mechanism, electrostatic interaction plays a main role in adsorption since MB dye is cationic in nature at pH = 9, whereas the adsorbent acquired an anionic nature. The newly synthesized Fe₃O₄@TiO₂‐MTMOS can be used as a promising material for efficient removal of MB dye from aqueous media.     

Environmentally Friendly g-C3N4/Sepiolite Fiber for Enhanced Degradation of Dye under Visible Light

Herein, novel visible light active graphitic carbon nitride (g-C₃N₄)/sepiolite fiber (CN/SS) composites were fabricated via a facile calcination route, exploiting melamine and thiourea as precursors, and sepiolite fiber as support, for efficient degradation of organic dye methylene blue (MB). The as-prepared CN/SS composites were characterized by various characterization techniques based on structural and microstructural analyses. The effects of CN loading amount, catalyst dosage and initial concentration of dye on the removal rate of dye under visible light were systematically studied. The removal rate of MB was as high as 99.5%, 99.6% and 99.6% over the composites when the CN loading amount, catalyst dosage and initial concentration of dye were 20% (mass percent), 0.1 g, and 15 mg/L in 120 min, respectively. The active species scavenging experiments and electron paramagnetic resonance (EPR) measurement indicated that the holes (h⁺), hydroxyl radical (·OH) and superoxide radicals (·O₂⁻) were the main active species. This study provides for the design of low-cost, environmentally friendly and highly efficient catalysts for the removal of organic dye.     

Improving the electrochemical performances of active carbon-based supercapacitors through the combination of introducing functional groups and using redox additive electrolyte

High-performance and low-cost electrochemical capacitors (ECs) are essential for large-scale applications in energy storage. In this work, the specific capacitance of active carbon (AC) electrode was significantly improved through the combination of introducing functional groups on the surface of AC and adding redox-active molecules (K₃Fe(CN)₆) into 2?M KOH aqueous electrolytes. The surface-oxygen functionalized AC (FAC) was synthesized using HNO₃ echoed as the electrode and 2?M KOH with 0.1?M K₃Fe(CN)₆ as the electrolyte. The surface functional groups of the AC not only contribute to the pseudocapacitance but also increase the active sites of the electrode/electrolyte interface, which enhances the electrochemical activity of the Fe(CN)₆^3?/Fe(CN)₆^4? redox pair, thus leading to high capacitance. In the redox electrolyte, the specific capacitance was much higher in 229.17?F?g^?1 (1?A?g^?1) achieved for those FAC than in raw AC (only 147.06?F?g^?1). Similarly, the FAC electrode suggested high energy density and extended cycling stability in the KOH?+?K₃Fe(CN)₆ electrolyte.