Synthesis of electrically conducting composite adsorbents for wastewater treatment using adsorption & electrochemical regeneration

Electrically conducting adsorbent materials called Nyex™ 1000 & 2000 have already been reported with comparatively low adsorption capacity for various organic, biologically non-degradable and toxic compounds. Two composite adsorbents called CA₁ & CA₂ were synthesized using synthetic graphite-carbon black and expanded graphite-carbon black respectively. The aim of developing the new adsorbents was to increase the adsorption capacity along with good electrical properties. The developed adsorbents were characterized using N₂ adsorption for specific surface area, Boehm surface titration for surface chemistry, bed electrical conductivity, laser size analyzer for average particle size, and scanning electron microscope (SEM) for particle morphology and shape. Then both the composite adsorbents were tested for the adsorption of acid violet 17 followed by an electrochemical regeneration. The adsorption study revealed that both the adsorbents had almost similar kinetic behavior with a significant increase in adsorption capacity for acid violet 17 (300 & 26 mg g⁻¹ respectively) when compared with the adsorption capacity of previously developed electrically conducting materials called Nyex™ 1000 & 2000 (3.5 and 9 mg g⁻¹ respectively). The composite adsorbent CA₂ was successfully electrochemically regenerated by passing an electric charge of 138 C g⁻¹ at a current density of 14 mA cm⁻² for a treatment time of 60 min, whereas, the composite adsorbent CA₁ could not be regenerated successfully. The regeneration efficiencies of CA₂ were obtained at around 120% during five adsorption–regeneration cycles. The amount of actual charge passed of 138 C g⁻¹ for achieving 100% regeneration efficiency was found to be similar with stoichiometrically calculated amount of charge. The amount of electrical energy required to oxidize each mg of adsorbed acid violet onto CA₂ (24 J mg⁻¹) was found to be significantly lower to that of Nyex™ 1000 & 2000 adsorbents (52 J mg⁻¹ & 32 J mg⁻¹ respectively).     

Removal of humic acid from water using adsorption coupled with electrochemical regeneration

A novel and economic waste water treatment technology comprised of adsorption coupled with electrochemical regeneration was introduced at the University of Manchester in 2006. An electrically conducting adsorbent material called Nyex? 1000 (Graphite intercalation based material) was developed for the said purpose. This adsorbent material delivered significantly lower adsorption capacity for the removal of a number of organic pollutants. With the aim to expand the scope of newly developed adsorbent material called Nyex? 2000, we studied the adsorption of humic acid followed by electrochemical regeneration. Nyex? 2000 is a highly electrically conducting material with an adsorption capacity almost twice that of Nyex? 1000 (intercalation based graphite compound) for humic acid. The adsorption of humic acid onto both Nyex? adsorbents was found to be fast enough keeping almost the same kinetics with approximately 50% of the adsorption capacity being achieved within the first twenty minutes. The parameters affecting the regeneration efficiency, including the treatment time, charge passed and current density, were investigated. The regeneration efficiency at around 100% for Nyex? 1000 & 2000 adsorbents saturated with humic acid was obtained using the charge passed of 8 and 22 Cg^?1 at a current density of 7mA cm^?2 during a treatment time of 30minutes, respectively.     

Pre-treatment of adsorbents for waste water treatment using adsorption coupled-with electrochemical regeneration

With the aim to address waste water treatment problems, a novel and economic water treatment technology was introduced at the University of Manchester. It comprised of a unique combination of adsorption and electrochemical regeneration in a single unit. This process successfully eliminated a number of organic pollutants by using an electrically conducting adsorbent material called Nyex? which was a modified form of synthetic graphite. To expand the scope of other graphite types in waste water treatment applications, natural vein and recycled vein graphite materials were selected for electrochemical surface treatment (pre-treatment) in order to evaluate their adsorptive and electrical properties. New graphite based adsorbents were developed and characterized using a laser diffraction particle size analyser, BET surface area, SEM analysis, X-ray (EDS) elemental analysis, X-ray powder diffraction, Boehm surface titration, Zeta potential electrical bed conductivity and bulk density measurements. Boehm surface titration and EDS (X-ray) elemental analysis showed a significant increase in oxygen containing surface functional groups. Although, no significant improvement in bed electrical conductivity was found to occur after electrochemical surface treatment, however, natural vein and recycled vein graphite materials presented highest bed electrical conductivity amongst competing graphite materials. Aqueous solution of acid violet 17 as a standard pollutant was used to evaluate the comparative performance of these adsorbents. The investigations revealed that electrochemical surface treatment contributed to an increase in the adsorption capacity by a factor of two only for natural vein graphite. Un-treated recycled vein graphite adsorbent delivered the same adsorptive capacity (3.0 mg g^?1) to that of electrochemically treated natural vein graphite. The electrochemical regeneration efficiency at around 100% was obtained using a treatment time of 60 and 30 min, current density of 14 mA cm^?2, charge passed of 36 and 18 C g^?1 for synthetic graphite, natural and recycled vein graphite materials respectively. Relatively a small consumption of electrical energy, 24 J g^?1 for regenerating natural vein graphite adsorbent versus 36 J g^?1 for synthetic graphite adsorbent, was found to be required for destruction/oxidation of adsorbed acid violet 17. Multiple adsorption/regeneration cycles presented no loss in adsorptive capacity over 5 adsorption/regeneration cycles. The use of natural and recycled vein graphite adsorbents offered some advantages over graphite intercalation based adsorbents with reduced electrical energy consumption during regeneration and simpler separation of particulate adsorbent.     

Improved phenol adsorption from aqueous solution using electrically conducting adsorbents

The electrically conducting and partially porous graphite based adsorbent (called NyexTM 2000) was tested for its adsorption capacity and electrochemical regeneration ability for the removal of phenol from aqueous solution. Nyex? 2000 was tested in comparison with Nyex? 1000, which is currently being used for a number of industrial waste water treatment applications. Nyex? 1000 exhibited small adsorption capacity of 0.1 mg g^?1 for phenol because of having small specific surface area of 1 m² g^?1. In contrast, Nyex? 2000 with specific surface area of 17 m² g^?1 delivered an adsorption capacity of 0.8 mg g^?1, which was eight-fold higher than that of Nyex? 1000. Nyex? 2000 was successfully electrochemically regenerated by passing a current of 0.5 A, charge passed of 31 C g^?1 for a treatment time of 45 minutes. These electrochemical parameters were comparable to Nyex? 1000 for which a current of 0.5 A, charge passed of 5 C g^?1 for a treatment time of 20 minutes were applied for complete oxidation of adsorbed phenol. The comparatively high charge density was found to be required for Nyex? 2000, which is justified with its higher adsorption capacity. The FTIR results validated the mineralization of adsorbed phenol into CO₂ and H₂O except the formation of few by-products, which were in traces when compared with the concentration of phenol removed from aqueous solution. The electrical energy as required for electrochemical oxidation of phenol adsorbed onto Nyex? 1000 & 2000 was found to be 214 and 196 J mg^?1, respectively. The comparatively low energy requirement for electrochemical oxidation using Nyex? 2000 is consistent with its higher bed electrical conductivity, which is twice that of Nyex? 1000.     

Electrochemical regeneration of a carbon-based adsorbent loaded with crystal violet dye

The removal of colour from a crystal violet dye solution using a non-porous, electrically conducting carbon-based adsorbent was systematically investigated under different operating conditions. Whilst the adsorptive process was very quick (up to 88% of equilibrium capacity could be achieved within 2 min), the adsorptive capacity of the adsorbent was very low (2 mg g⁻¹) compared with activated carbons. This was due to its low surface area. The conductivity of the adsorbent/electrolyte mixture within the anodic compartment of the electrochemical cell was found to be over 13 times greater with the new adsorbent compared with powdered activated carbon. One hundred percent could be achieved in a simple divided electrochemical cell using treatment times as low as 10 min by passing a charge of 25 C g⁻¹ at a current density of 20 mA cm⁻². The efficiency of electrochemical regeneration depends on a range of variables including charge passed, current density, treatment time, electrolyte type and concentration and the adsorbent bed thickness. Multiple adsorption and regeneration cycles indicate that there is little or no loss in adsorbent capacity on regeneration.     

Removal of Tartrazine From Water by Adsorption with Electrochemical Regeneration

An innovative process has been developed at University of the Manchester in order to remove organic contaminants from wastewater using graphite intercalation compounds (GICs) as adsorbents with electrochemical regeneration. The present study has demonstrated the removal of tartrazine, from water by adsorption and electrochemical regeneration. The adsorption of tartrazine onto GIC adsorbent was shown to be a quick process, however, with extremely low adsorption capacity compared to porous adsorbents. Low adsorption capacity of the adsorbent is being compensated by rapid electrochemical regeneration associated with low energy consumption that makes the process cost-effective. Regeneration efficiency of around 100% could be obtained in an electrochemical cell by passing a charge of 18 C g^?1 for 18 min through a 10-mm thick adsorbent bed. A series of adsorption and regeneration cycles showed that there was little loss in adsorbent capacity, demonstrating that tartrazine loaded GIC adsorbent could be effectively regenerated electrochemically.     

Nanocrystalline NiO hollow spheres in conjunction with CMC for lithium-ion batteries

Hollow spherical NiO particles were prepared using the spray pyrolysis method with different concentrations of precursor. The electrochemical properties of the NiO electrodes, which contained a new type of binder, carboxymethyl cellulose (CMC), were examined for comparison with NiO electrodes with polyvinylidene fluoride (PVDF) binder. The electrochemical performance of NiO electrodes using CMC binder was significantly improved. For the cell made from 0.3 mol L⁻¹ precursor, the irreversible capacity loss between the first discharge and charge is about 43 and 24% for the electrode with PVDF and CMC binder, respectively. The cell with NiO–CMC electrode has a much higher discharge capacity of 547 mAh g⁻¹ compared to that of the cell with NiO–PVDF electrode, which is 157 mAh g⁻¹ beyond 40 cycles.     

In situ active chlorine generation for the treatment of dye-containing effluents

This study examined the possibility to remove colour causing-compounds from synthetic effluent by indirect electrochemical oxidation using iridium oxide anode electrodes. Using a high concentration of chloride ions (17.1 mM) and various current densities, it was possible to produce high concentration of active chlorine with a specific production rate of 2.8 mg min⁻¹ A⁻¹. The best performance for acid methyl violet 2B dye (MV2B) decomposition was obtained using Ti/IrO₂ anodes operated at a current density of 15 mA cm⁻² during 40 min of treatment in the presence of 3.42 mM of chloride ions. Under these conditions, more than 99% of MV2B was removed (with a reaction rate apparent constant of 0.20 min⁻¹), whereas COD and TOC removal were 51% and 75%, respectively. The electrolytic cell was then used for the degradation of three other synthetic dye solutions: Eosin yellowish (EOY), Trypan Blue (TRB), Acridine Orange (ACO). TRB was the most difficult dye to remove from solution with a value reaction rate constant of 0.12 min⁻¹, compared to 0.19 min⁻¹ and 0.24 min⁻¹ recorded for ACO and EOY dyes, respectively. More than 99% of these dyes were removed by electrochemical oxidation.     

Chitosan-modified palygorskite: Preparation,characterization and reactive dye removal

Chitosan-modified palygorskite (CTS-modified PA) was prepared by surface grafting of PA with chitosan, and the CTS-modified PA was used as an effective adsorbent for the removal of reactive dye. The effects of various experimental parameters such as initial pH, adsorbent dosage, contact time and initial dye concentration on adsorption were investigated. The adsorption behavior of CTS-modified PA showed that the adsorption kinetics and isotherms were in good agreement with the pseudo-second-order equation and the Langmuir equation, and the maximum adsorption capacity of CTS-modified PA calculated by the Langmuir model was 71.38 mg g^− 1, which was much higher than that of the unmodified PA (6.3 mg g^− 1).     

Mechanical,swelling and adsorptive properties of dry–wet spun chitosan hollow fibers crosslinked with glutaraldehyde

Chitosan (CS) hollow fiber (HF) membranes were successfully prepared according to the dry–wet spinning technique. A post-treatment with glutaraldehyde (GA) aqueous solution was carried out to perform the cross-linking reaction. The effect of GA concentration in the range 50–1000 mg l^?1 on the swelling, mechanical and adsorptive properties was investigated. The morphology and chemical structure of the fibers were examined by means of SEM and FTIR. The degree of swelling and adsorption capacity decreased as the GA concentration increased. CS hollow fibers swelled the most in acidic solution as compared with distilled water and saline solution. The adsorption capacity of CS HFs increased while decreasing of initial pH from 7.5 to 3.5. Desorption experiments showed that CS HFs were reusable as adsorbent. Mechanical properties were strongly affected by GA post-treatment: tensile strength and elastic modulus increased at low GA concentration (50 mg l^?1), to sharply decrease when concentration was ?500 mg l^?1. Breaking elongation decreased with increasing GA concentration.     

Electrochemical pre-treatment of effluents containing chlorinated compounds using an adsorbent

Electrochemical pre-treatment of industrial wastewater to remove refractory and/or toxic organic components, making them more amenable to biological treatment, is likely to be more cost-effective than using a physical or chemical process for complete organic removal. This paper demonstrates the use of a carbon-based adsorbent material that preferentially removes chlorinated organic compounds. Electrochemical regeneration of the adsorbent is shown to be a quick, easy and cheap process, because the adsorbent is both highly electrically conducting and non-porous. High regeneration rates over a number of cycles were obtained by passing a charge of 25 C g⁻¹ through a bed of adsorbent particles, at a current density of 20 mA cm⁻² for 10 min. The energy required to remove a kg of COD from an industrial wastewater was calculated to be 27 kWh.     

Effects of surface area on electrochemical performance of Li[Ni<Subscript>0.2</Subscript>Li<Subscript>0.2</Subscript>Mn<Subscript>0.6</Subscript>]O<Subscript>2</Subscript> cathode material

The effect of surface area on the electrochemical properties and thermal stability of Li[Ni_0.2Li_0.2Mn_0.6]O₂ powders was characterized using a charge/discharge cycler and DSC (Differential Scanning Calorimeter). The surface area of the samples was successfully controlled from ~4.0 to ~11.7 m² g⁻¹ by changing the molar ratio of the nitrate/acetate sources and adding an organic solvent such as acetic acid or glucose. The discharge capacity and rate capability was almost linearly increased with increase in surface area of the sample powder. A sample with a large surface area of 9.6–11.7 m² g⁻¹ delivered a high discharge capacity of ~250 mAh g⁻¹ at a 0.2 C rate and maintained 62–63% of its capacity at a 6 C rate versus a 0.2 C rate. According to the DSC analysis, heat generation by thermal reaction between the charged electrode and electrolyte was not critically dependent on the surface area. Instead, it was closely related to the type of organic solvent employed in the fabrication process of the powder.     

Removal of arsenic from aqueous solution using polyaniline/rice husk nanocomposite

The present study deals with the adsorption of arsenic ions from aqueous solution on polyaniline/rice husk (PAn/RH) nanocomposite. Batch studies were performed to evaluate the influence of various experimental parameters like pH, adsorbent dosage, contact time and the effect of temperature. Optimum conditions for arsenic removal were found to be pH 10, adsorbent dosage of 10 g/L and equilibrium time 30 minutes. Adsorption of arsenic followed pseudo-second-order kinetics. The equilibrium adsorption isotherm was better described by Freundlich adsorption isotherm model. The adsorption capacity (q _max ) of PAn/RH for arsenic ions in terms of monolayer adsorption was 34.48 mg/g. The change of entropy (ΔS⁰) and enthalpy (ΔH⁰) was estimated at −0.066 kJ/(mol K) and −22.49 kJ/mol, respectively. The negative value of the Gibbs free energy (ΔG⁰) indicates feasible and spontaneous adsorption of arsenic on PAn/RH.     

Adsorption characteristics of methylene blue on poplar leaf in batch mode: Equilibrium,kinetics and thermodynamics

Adsorption characteristics of methylene blue (MB) from aqueous solution on natural poplar leaf were investigated. Batch experiments were carried out to study the effects of initial pH, contact time, adsorbent dosage, and initial MB concentration, salt concentration (Ca²⁺ and Na⁺) as well as temperature on MB adsorption. The optimum condition for adsorption was found at pH 6–9 and adsorbent dosage of 2 g L⁻¹. The equilibration time was 240 min. The salt concentration had a negative effect on MB removal. The equilibrium data were analyzed with Langmuir, Freundlich and Koble-Corrigan isotherm models using nonlinear regression method. The adsorption process was more effectively described by Langmuir isotherm based on the values of the correlation coefficient R² and chi-square statistic x². The maximum monolayer adsorption capacity of poplar leaf from the Langmuir model was 135.35 mg g⁻¹ at 293 K. The pseudo second order equation provided a better fit to experimental data in the kinetic studies. Intraparticle diffusion was involved in adsorption process, but it was not the only rate-controlling step. Thermodynamic quantities such as ΔG, ΔH and ΔS were calculated, indicating that the adsorption process was spontaneous and endothermic. Dye-adsorbent interactions were examined by FTIR and SEM analysis. The FTIR results suggested that there were hydroxyl and carboxyl groups on the surface of poplar leaf, which would make MB adsorption possible. The SEM images showed effective adsorption of MB molecules on the adsorbent surface.     

High temperature water electrolysis using metal supported solid oxide electrolyser cells (SOEC)

Metal supported cells as developed according to the DLR SOFC concept by applying plasma deposition technologies were investigated for use as solid oxide electrolyser cells (SOEC) for high temperature steam electrolysis. Cells consisting of a porous ferritic steel support, a diffusion barrier layer, a Ni/YSZ hydrogen electrode, a YSZ electrolyte and a LSCF oxygen electrode were electrochemically characterised by means of i-V characteristics and electrochemical impedance spectroscopy measurements including a long-term test over 2000 h. The cell voltage during electrolysis operation at a current density of −1.0 A cm⁻² was 1.28 V at an operating temperature of 850 °C and 1.4 V at 800 °C. A long-term test run over 2000 h with a steam content of 43% at 800 °C and a current density of −0.3 A cm⁻² showed a degradation rate of 3.2% per 1000 h. The impedance spectra revealed a significantly enhanced polarisation resistance during electrolysis operation compared to fuel cell operation which was mainly attributed to the hydrogen electrode.     

Adsorption kinetics and thermodynamics of azo-dye Orange II onto highly porous titania aerogel

We report herein a kinetic and thermodynamic study of the adsorption of azo-dye Orange II from aqueous solutions onto titania aerogels. Aerogels structure was confirmed by FTIR and N₂ adsorption revealed their specific surface area (500 m²/g), pore volume (2.86 cm³/g) and pore size (mean 13.9 nm). Adsorption tests were conducted in batch reactors under various conditions where the effect of pH, temperature, contact time, dye concentration, and adsorbent dose were studied. Experiments performed at pH 2 show the optimal adsorption due to the best surface charge interactions. The temperature shows a weak influence with a decrease in the adsorption uptake as the temperature increases. Adsorption kinetics is shown to be very fast and follows a pseudo second-order indicating the coexistence of chemisorption and physisorption with the intra-particle diffusion being the rate controlling step. The experimental data fit perfectly with Sips isotherms and reveal the ability of titania aerogel to adsorb 420 mg of Orange II per gram of adsorbent at the optimal conditions. The thermodynamic study reveals the activation energy (42.1 kJ mol⁻¹) and the changes in Gibbs free energy (1.2 kJ mol⁻¹), enthalpy (−16.4 kJ mol⁻¹), and entropy (−58 J mol⁻¹ K⁻¹). The entire regeneration of the titania aerogel adsorption sites at pH 11 and 30 °C shows a total recovery of the dye and the efficient reusability and the economic interest of these adsorbing materials for environmental purposes.     

On the activation and physical degradation of boron-doped diamond surfaces brought on by cathodic pretreatments

The electrochemical activation and physical degradation of boron-doped diamond (BDD) electrodes with different boron doping levels after repeated cathodic pretreatments are reported. Galvanostatic cathodic pretreatment passing up to −14000 C cm⁻² in steps of −600 C cm⁻² using −1 A cm⁻² caused significant physical degradation of the BDD surface, with film detachment in some areas. Because of this degradation, a great increase in the electrochemically active area was observed in Tafel plots for the hydrogen evolution reaction (HER) in acid media. The minimum cathodic pretreatment needed for the electrochemical activation of the BDD electrodes without producing any observable physical degradation on the BDD surfaces was determined using electrochemical impedance spectroscopy (EIS) measurements and cyclic voltammetry: −9 C cm⁻², passed at −1 A cm⁻². This optimized cathodic pretreatment can be safely used when electrochemical experiments are carried out on BDD electrodes with doping levels in the range between 800 and 8000 ppm.     

Synthesis and characterization of 5?V LiCoxNiyMn2?x?yO4 (x?=?y?=?0.25) cathode materials for use in rechargeable lithium batteries

Double doped spinel LiCo _x Ni _y Mn_2−x−y O₄ (x = y = 0.25) have been synthesised via sol–gel method using different chelating agents viz., acetic acid, maleic acid and oxalic acid to obtain 5 V positive electrode material for use in lithium rechargeable batteries. The sol–gel route endows lower processing temperature, lesser synthesis time, high purity, better homogeneity, good control of particle size and surface morphology. Physical characterizations of the synthesized powder were carried out using thermo-gravimetric and differential thermal analysis (TG/DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). The electrochemical behaviour of the calcined samples has been carried out by galvanostatic charge/discharge cycling studies in the voltage range 3–5 V. The XRD patterns reveal crystalline single-phase spinel product. SEM photographs indicate micron sized particles with good agglomeration. The charge–discharge studies show LiCo_0.25Ni_0.25Mn_1.5O₄ synthesized using oxalic acid to be as a promising cathode material as compared to other two chelating agents and delivers average discharge capacity of 110 mA h g⁻¹ with low capacity fade of 0.2 mA h g⁻¹ per cycle over the investigated 15 cycles.     

Adsorption of tannic acid on chitosan-montmorillonite as a function of pH and surface charge properties

Chitosan, a natural biopolymeric cation, is a candidate to modify montmorillonite for the adsorption of anions. As an anionic organic pollutant the adsorption of tannic acid was studied. Because of protonation/deprotonation reactions of both chitosan-montmorillonite and tannic acid, the adsorption process is strongly pH-dependent. The objective of this work is to characterize the pH dependency of adsorption in combination with surface charge determinations.Montmorillonite was modified with different amounts of chitosan, corresponding to 20–1000% of the cation exchange capacity (CEC). The deacetylation degree of chitosan was determined by polyelectrolyte titration and was found to be 74%. The uptake of chitosan was determined by the C-content. The interlayer expansion was investigated by X-ray powder diffraction. The adsorption capacity for tannic acid was investigated with the batch technique at pH 3, 4, 5 and 8. As a measure for the adsorption properties, the electrokinetic surface charge was determined with a particle charge detector.The uptake of chitosan by montmorillonite is up to 152% (1.69 mol_c kg^− 1) of the CEC. The resulting anion exchange capacity of chitosan-montmorillonite calculated from C-content is 0.43 mol_c kg^− 1. At low loadings with chitosan (24.7 and 49.5% uptake), a monolayer is formed in montmorillonite. At an uptake of 96.8%, a bilayer structure is observed, which becomes more dominant at higher loadings. On the external surface, a monolayer of chitosan was formed. From pH 4 to 8, the surface charge of all modified montmorillonites is with − 9 to 8 mmol_c kg^− 1 close to the point of zero charge. The maximal adsorption capacity for tannic acid is found with 240 g kg^− 1 (0.14 mol_c kg^− 1) at pH 4. The adsorption process fits in well with the Freundlich isotherm. At lower as well as higher pH values the adsorption capacity decreases up to about 25%. Most probably the exchange sites in the interlayer do not contribute to the adsorption of tannic acid. The observed surface charge is lower than the adsorbed amount of tannin. It is thought that tannin is adsorbed also by van der Waals forces besides ionic forces.     

Electrooxidation and dischargeability of transition-metal borides as possible anodic materials in neutral aqueous electrolytes

A number of transition-metal borides were studied as anodic materials for neutral aqueous batteries. These borides are shown to have considerably high electrochemical activities in neutral electrolytes. The discharge capacities for TiB₂ reach 1,350 mAh g⁻¹ at a constant current density of 50 mA g⁻¹, exceeding those for all the metal electrodes reported so far. Amorphous CoB_x can deliver a discharge capacity of >650 mAh g⁻¹, and even simply ball-milled FeB_x can also give a discharge capacity of >200 mAh g⁻¹. These results suggest the possible use of boride compounds as a large family of new anodic materials for constructing neutral aqueous batteries with high electrochemical capacity and rate capability.