Adsorption of aqueous cadmium (II) onto modified multi-walled carbon nanotubes following microwave/chemical treatment

This study evaluates the aqueous cadmium (II) adsorption efficiency of as-grown carbon nanotubes (CNTs) and of those modified by microwave (MW)/H₂SO₄ and MW/H₂SO₄/KMnO₄ processes. The surface area, pH_iep, and FTIR spectra of CNTs were, before and after modification, compared. Aromatic groups, carbonyl groups and hydroxyl groups were herein detected on the surfaces of MW/H₂SO₄ and MW/H₂SO₄/KMnO₄-modified CNTs. At a particular pH, the adsorption capacity of Cd²⁺ of the MW/H₂SO₄/KMnO₄-modified CNTs exceeded that of MW/H₂SO₄-modified CNTs and as-grown CNTs. The kinetic analyses of adsorption were performed and a pseudo second-order model accurately captured the adsorption kinetics. This study suggests that MW/H₂SO₄ and MW/H₂SO₄/KMnO₄ modification not only increased the area of active adsorption sites of CNTs but also reduced the modification period by microwave heating.     

Comparison with as-grown and microwave modified carbon nanotubes to removal aqueous bisphenol A

This study utilized carbon nanotubes (CNTs) to remove bisphenol A (BPA) from aqueous solution. The surfaces of CNTs were modified by SOCl₂/NH₄OH under microwave irradiation. The surface characteristics of as-grown and modified CNTs were analyzed by measuring zeta potential, and using a scanning electron microscope, a surface area analyzer and a Fourier transform infrared spectroscope. The specific surface area of modified CNTs exceeded that of as-grown CNTs. The pH_iep values of as-grown CNTs and modified CNTs were determined to be 4.3 and 6.5, respectively. Some amine functionalities were formed on the surface of modified CNTs; therefore, the surface of the modified CNTs contained more positive charges than that of the as-grown CNTs. The adsorption kinetics were examined using pseudo first- and second-order models, intraparticle diffusion and Bangham's models. The equilibrium data were simulated using Langmuir, Freundlich, Dubinin and Radushkevich (D-R) and Temkin isotherms. The results reveal that the pseudo second-order model and Langmuir isotherm fit the kinetics and equilibrium data, respectively. The adsorption capacity of BPA on the surface of CNTs fluctuates very little with pH in the range of 3-9, suggesting the high stability of CNTs as an adsorbent for BPA over a rather wide pH range. The values of ΔH⁰ and ΔS⁰ were calculated to be − 11.7 kJ/mol and 46.1 J/mol, respectively. The isotherm and thermodynamic simulations indicate that the adsorption of BPA onto as-grown CNTs proceeds by physisorption process.     

Sulfonated multi-walled carbon nanotubes for the removal of copper (II) from aqueous solutions

Sulfonated multi-walled carbon nanotubes (s-MWCNTs) was prepared from purified multi-walled carbon nanotubes (p-MWCNTs) by concentrated H₂SO₄ at elevated temperature. The structure was characterized by SEM, FTIR, Raman, XPS, and BET. It could be dispersed steadily in water at a dosage of 1.0 mg/mL for a week. The adsorption performance of s-MWCNTs toward Cu(II) was investigated including the effects of pH and ionic strength. Results indicated the adsorption was much dependent on pH but not on ionic strength. The adsorption capacity for Cu(II) was enhanced 58.9% via the sulfonation. Moreover, the adsorption mechanisms were carefully analyzed by Freundlich and D-R models.     

Adsorption of Pb(II) ions from aqueous solutions by carbon nanotubes oxidized different methods

This study investigated the heavy metal adsorption of the carbon nanotubes (CNTs) oxidized different methods. Besides the conventional ultrasonication method, the UV-light used as an oxidation agent. The two oxidation methods compared with each other by Pb(II) adsorption. The characterizations of oxidized CNTs were analyzed by FTIR, XRD, DTG, SEM and total acidity capacity analysis. The adsorption capacities of carbon nanotubes were compared with using Langmuir and Freundlich isotherms. Two different kinetic theories were applied to experimental data. These theories are pseudo-second order and intra-particle diffusion models. The adsorption results can be compared using non-linear Langmuir isotherm parameters. For single wall carbon nanotubes (SWCNTs), theoretical adsorption capacity value (Q_m) of UV-light method is 511.99 mg/g and ultrasonication method is 342.36 mg/g. The UV-light increased the surface acidity of the carbon nanotubes more than ultrasonication. After the adsorption experiments, it is apparently seen that the UV-light oxidation method is a useful method for heavy metal adsorption.     

Adsorption characteristics of Ru(II) dye on carbon nanotubes for organic solar cell

We present a study on the adsorption properties of ruthenium(II) dye (Ru(II) dye) on multi-walled carbon nanotubes (MWNTs). To fabricate dye sensitized solar cells (DSCs) using dye coated MWNTs, we have developed a method to form covalently linked adducts of MWNTs and Ru(II) dye. MWNTs were functionalized by sonication in hydrogen peroxide solution. Ru(II) dye can be attached to the functionalized MWNTs by a synthetic route using Thionyl chloride (SOCl₂) followed by ethylenediamine. The adsorption characteristics were affected by parameters such as chemical oxidation of MWNTs, sonication process, processing temperature and time. The amount of adsorbed Ru(II) dye was effectively affected by treatment temperature of SOCl₂ than any other parameters.     

Removal of heavy metal ions from aqueous solution by multi-walled carbon nanotubes modified with 8-hydroxyquinoline: Kinetic study

Carbon nanotubes were modified with 8-hydroxyquinoline and used for the removal of Cu(II), Pb(II), Cd(II), and Zn(II) from aqueous solutions. Different instrumentation parameters and methods of development for determining metal ions from aqueous solutions using differential pulse anodic stripping voltammetry were studied. The adsorption of heavy metals from aqueous solution by the pristine and modified MWCNTs was studied kinetically using different kinetic models, and the results showed that the adsorption process best fitted the pseudo-second-order model and the Elovich model. The mechanism of adsorption was explored using the intra-particle diffusion model and the liquid-film model.     

Removal of antimony (III) by multi-walled carbon nanotubes from model solution and environmental samples

Multi-walled carbon nanotubes (MWCNTs) are used for the adsorption and removal of Sb(III) from model solutions and real environmental samples. The effects of different factors that affect the adsorption process; the results showed that most of the Sb(III) ions were removed from the solution within 30 min, at pH 7.0, using 200 mg MWCNTs. Further, it was observed that the adsorption process was greatly enhanced by lowering the solution temperature, which indicated the exothermic nature of the adsorption. The adsorption study was analyzed kinetically, using different kinetic models, and the results showed that the adsorption of Sb(III) by MWCNTs from the model solution followed pseudo-second-order kinetics with good correlation coefficients. In addition, it was found that the adsorption of Sb(III) occurred in more than one step, including the liquid film diffusion of the Sb(III) ions from the aqueous phase through the liquid film around the MWCNTs and further intra-particle diffusion through the nanotubes’ bundles and aggregates.     

Capacitance properties of multi-walled carbon nanotubes modified by activation and ammoxidation

Catalytic multi-walled carbon nanotubes were modified by KOH activation at 800 °C and/or ammoxidation at 350 °C, and the effect of these treatments on the physicochemical and electrochemical properties was investigated. Whereas texture is moderately changed by ammoxidation, the chemical composition is significantly modified due to the formation of various nitrogen containing groups. The influence of nitrogenated functionality (pyridine, pyridone, NH) on charge accumulation is considered in full electrochemical capacitors, as well as in positive and negative electrodes separately, using acidic (4 mol L⁻¹ H₂SO₄) and alkaline (7 mol L⁻¹ KOH) electrolytes. The presence of nitrogen in the carbon network, especially in the form of pyridone/pyrrolic (N5) and/or pyridine (N6) groups, affects the electron density and enhances the charge affinity of the carbon material. It seems that the nitrogen groups improve particularly the capacitance performance of the negative electrode operating in alkaline medium. Besides the nitrogenated groups, the oxygenated functionality plays also an important role for the ammoxidized nanotubes. Generally, a few-fold increase of capacitance was observed in the N-enriched carbon nanotubular samples. Apart of this capacitance improvement, the presence of nitrogen in the carbon network limits significantly the leakage current and diminishes the self-discharge of supercapacitors.     

Photocatalytic properties of mesoporous TiO2 nanocomposites modified with carbon nanotubes and copper

The photocatalytic activity of mesoporous TiO₂ modified by the addition of carbon nanotubes (CNTs) and Cu is reported. Nanocomposites of carbon nanotubes (CNTs) containing varying amounts of Cu were formed by treatment with Cu²⁺ then reduced to Cu⁰ using NaBH₄ as the reducing agent. The mesoporous TiO₂, synthesized by a sol-gel method from titanium isopropoxide, was combined with the CNT/Cu nanocomposites to form the photocatalysts which were characterized by XRD, SEM, TEM, FTIR, XPS and BET surface area analysis. The photocatalytic properties of the mesoporous TiO₂ composites were studied by measuring the degradation of methyl orange (MO) which was optimal in the sample containing 20 wt% of the Cu-CNT nanocomposite. The degradation efficiency for MO was a synergistic effect of photo-degradation of TiO₂ and may be due to improvement of the electrical conductivity of the system by the presence of the CNT/Cu networks, since the photodegradation of MO and the photocatalytic activity of the photoactive systems increased with increasing copper content.     

Adsorption of cadmium(II) from aqueous solution by surface oxidized carbon nanotubes

Carbon nanotubes (CNTs) were oxidized with H₂O₂, KMnO₄ and HNO₃. Their physicochemical properties were investigated by BET N₂ adsorption, laser particle examination, Boehm’s titration, zeta potential measurement and cadmium(II) adsorption. The experimental results suggest that cadmium(II) adsorption capacities for three kinds of oxidized CNTs increase due to the functional groups introduced by oxidation compared with the as-grown CNTs. The cadmium(II) adsorption capacity of the as-grown CNTs is only 1.1 mg g⁻¹, while it reaches 2.6, 5.1 and 11.0 mg g⁻¹ for the H₂O₂, HNO₃ and KMnO₄ oxidized CNTs, respectively, at the cadmium(II) equilibrium concentration of 4 mg l⁻¹. Adsorption of cadmium(II) by CNTs was strongly pH-dependent and the increase of adsorption capacities for HNO₃ and KMnO₄ oxidized CNTs is more obvious than that of the as-grown and H₂O₂ oxidized CNTs at lower pH regions. The experiments of CNT dosage effect on the cadmium(II) adsorption show that the adsorption capacity for KMnO₄ oxidized CNTs has a sharper increase at the CNT dosage from 0.03 to 0.08 g per 100 ml than the as-grown, H₂O₂ and HNO₃ oxidized CNTs and its removal efficiency almost reaches 100% at CNT dosage of 0.08 g per 100 ml. Analysis revealed that the KMnO₄ oxidized CNTs hosted manganese residuals, and these surely contributed to cadmium sorption to a yet-undefined extent.     

Removal of diclofenac from aqueous solution with multi-walled carbon nanotubes modified by nitric acid☆

Modified multi-walled carbon nanotubes (MWCNTs) were used as adsorbents for removal of diclofenac. The re-action conditions were examined. Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich isotherm models were applied to determine appropriate equilibrium expression. The results show that the experimental data fit the Freundlich equation well. Thermodynamic parameters show that the adsorption process is spontaneous and exothermic. The kinetic study indicates that the adsorption of diclofenac can be well described with the pseudo-second-order kinetic model and the process is controlled by multiple steps.     

Kaolinite, montmorillonite, and their modified derivatives as adsorbents for removal of Cu(II) from aqueous solution

Adsorption of metals by clay minerals is a complex process controlled by a number of environmental variables. The present work investigates the removal of Cu(II) ions from an aqueous solution by kaolinite, montmorillonite, and their poly(oxo zirconium) and tetrabutylammonium derivatives. The entry of ZrO and TBA into the layers of both kaolinite and montmorillonite was confirmed by XRD measurement. The specific surface areas of kaolinite, ZrO-kaolinite, TBA-kaolinite, montmorillonite, ZrO-montmorillonite, TBA-montmorillonite were 3.8, 13.4, 14.0, 19.8, 35.8 and 42.2 m²/g, respectively. The cation exchange capacity (CEC) was measured as 11.3, 10.2, 3.9, 153.0, 73.2 and 47.6 meq/100 g for kaolinite, ZrO-kaolinite, TBA-kaolinite, montmorillonite, ZrO-montmorillonite, TBA-montmorillonite, respectively. Adsorption increased with pH till Cu(II) ions became insoluble in alkaline medium. The kinetics of the interactions suggests that the interactions could be best represented by a mechanism based on second order kinetics (k₂ = 7.7 × 10⁻² to 15.4 × 10⁻² g mg⁻¹ min⁻¹). The adsorption followed Langmuir isotherm model with monolayer adsorption capacity of 3.0–28.8 mg g⁻¹. The process was endothermic with ΔH in the range 29.2–50.7 kJ mol⁻¹ accompanied by increase in entropy and decrease in Gibbs energy. The results have shown that kaolinite, montmorillonite and their poly(oxo zirconium) and tetrabutyl-ammonium derivatives could be used as adsorbents for separation of Cu(II) from aqueous solution.     

Synthesis and characterization of double-walled carbon nanotubes from multi-walled carbon nanotubes by hydrogen-arc discharge

Double-walled carbon nanotubes (DWNTs) were synthesized in a large scale by a hydrogen arc discharge method using graphite powders or multi-walled carbon nanotubes/carbon nanofibers (MWNTs/CNFs) as carbon feedstock. The yield of DWNTs reached about 4 g/h. We found that the DWNT product synthesized from MWNTs/CNFs has higher purity than that from graphite powders. The results from high-resolution transmission electron microscopy observations revealed that more than 80% of the carbon nanotubes were DWNTs and the rest were single-walled carbon nanotubes (SWNTs), and their outer and inner diameters ranged from 1.75 to 4.87 nm and 1.06 to 3.93 nm, respectively. It was observed that the ends of the isolated DWNTs were uncapped and it was also found that cobalt as the dominant composition of the catalyst played a vital role in the growth of DWNTs by this method. In addition, the pore structures of the DWNTs obtained were investigated by cryogenic nitrogen adsorption measurements.     

Separation and purification of functionalised water-soluble multi-walled carbon nanotubes by flow field-flow fractionation

Water-soluble, functionalised multi-walled carbon nanotubes (MWNT) have been separated and purified from amorphous material through direct flow field-flow fractionation. MWNT subpopulations of relatively homogeneous, different length were obtained by collecting fractions of the raw, highly polydispersed (200-5000 nm) functionalised MWNT sample.     

Chitosan functionalized with 2[-bis-(pyridylmethyl) aminomethyl]4-methyl-6-formyl-phenol: equilibrium and kinetics of copper (II) adsorption

The complexation agent 2[-bis-(pyridylmethyl) aminomethyl]-4-methyl-6-formyl-phenol (HL) was immobilized in chitosan in order to obtain a new adsorbent material to be employed in studies on adsorption and pre-concentration of Cu(II). The chitosan modified by the complexation agent was characterized by infrared spectroscopy, DSC and TGA. The studies were conducted as a function of the pH of the medium and the mechanism of Cu(II) adsorption in the solid phase was analyzed utilizing several kinetic models. The parameters for the adsorption of Cu(II) ions by chitosan-HL were determined with a Langmuir isotherm, the maximum saturation capacity of the monolayer being 109.4 mg of Cu(II) per gram of polymer. Electron paramagnetic resonance spectroscopy revealed that Cu^II ions coordinate to the donor atoms of the HL ligand anchored to the surface of the polymer forming a stable chelate complex in the solid state.     

Pore structures of multi-walled carbon nanotubes activated by air, CO2 and KOH

Multi-walled carbon nanotubes (MWNTs) synthesized by the catalytic decomposition of benzene were activated by KOH, CO₂ or air. The adsorption isotherms of the activated MWNTs were analyzed and their pore size distributions were obtained. The results showed that the specific surface areas of the MWNTs activated by KOH, CO₂ and air were increased to 785 m²/g, 429 m²/g and 270 m²/g, respectively. The MWNTs activated by KOH were rich in micropores and mesopores, especially high mesopores having volumes up to 1.04 cm³/g. The CO₂-activated MWNTs also had many micropores while the air-activated MWNTs had a much smaller micropore volume. The morphologies of the activated MWNTs were examined by transmission electron microscopy and high resolution transmission electron microscopy, and the activation mechanisms were discussed.     

Arsenic adsorption using copper (II) oxide nanoparticles

Arsenic poisoning is a major problem in today's life. To reduce its concentration in drinking water, different metal based compounds were explored as arsenic adsorbents. In the present study, copper (II) oxide nanoparticles were prepared by thermal refluxing technique and used as an adsorbent for arsenic removal from water. Characterization of the adsorbent using TEM, BET, XRD and FTIR implied that the prepared adsorbent was in nano size and had excellent adsorption behavior with surface area of 52.11 m²/g. Systematic adsorption experiments were carried out with different process parameters such as contact time, adsorbent mass, pH, temperature and stirring speed and found that copper (II) oxide had very good efficiency towards arsenic adsorption. Thermodynamic parameters and adsorption kinetics were studied in detailed to know the nature and mechanism of adsorption. Results showed that the adsorption process followed pseudo second order kinetic and endothermic behavior. Adsorption equilibrium was studied with Langmuir and Freundlich isotherm models. The adsorption process followed Langmuir isotherm with an adsorption capacity of 1086.2 μg/g. A regeneration study was proposed in order to reuse the adsorbent for better economy of the process. Finally, a process design calculation is reported to know the amount of adsorbent required for efficient removal of arsenic from aqueous medium.     

Mercury(II) removal from aqueous solutions by adsorption on multi-walled carbon nanotubes

Our aim was to test how MWCNTs can be used as a new adsorbent for mercury(II). Multi-walled carbon nanotubes (MWCNTs) have been used for removal of mercury from aqueous solutions. Mercury removal from aqueous solutions by batch adsorption was investigated. Equilibrium isotherms, such as Freundlich, Langmuir, Temkin, Harkins-Jura, were tested. Kinetic studies based on Lagergren first-order, pseudo-second-order and Elovich rate expressions were done. The batch experiments were conducted at three different temperatures (17, 27 and 37 °C) and different pHs of the initial solution. Error function analysis shows that mercury(II) removal obeys pseudo-second order kinetics and Freundlich isotherm equation. Finally, the effects of solution pH and temperature on the adsorption were studied.     

Ion-exchange adsorption of copper(II) ions on functionalized single-wall carbon nanotubes immobilized on a glassy carbon electrode

The cation-exchange property of oxidatively treated carbon nanotubes (CNTs) is newly reported. Single-wall carbon nanotubes (SWNTs), which were oxidatively treated, were immobilized on a glassy carbon surface and, on this CNT-modified electrode, Cu(II) ions were spontaneously adsorbed and their redox waves electrochemically measured. It is suggested that the adsorption of the cationic Cu(II) ions occurs by their electrostatic interaction with the negatively charged carboxylic anions on the CNTs after the ion-exchange with protons. The surface coverage of the adsorbed Cu(II) ions depending on the dipping time, the amount of immobilized CNTs, and the Cu(II) concentration was estimated from the electrochemical chronocoulometric measurements. The effect of the ionic strength on the adsorption of the Cu(II) ions was investigated and the adsorption strengths of various alkali metal cations and protons were compared. It is hoped that this new cation-exchange property of CNT-modified electrodes may extend their range of electrochemical applications.