Adsorption of Reactive Red M-2BE dye from water solutions by multi-walled carbon nanotubes and activated carbon

Multi-walled carbon nanotubes and powdered activated carbon were used as adsorbents for the successful removal of Reactive Red M-2BE textile dye from aqueous solutions. The adsorbents were characterised by infrared spectroscopy, N₂ adsorption/desorption isotherms and scanning electron microscopy. The effects of pH, shaking time and temperature on adsorption capacity were studied. In the acidic pH region (pH 2.0), the adsorption of the dye was favourable using both adsorbents. The contact time to obtain equilibrium at 298 K was fixed at 1 h for both adsorbents. The activation energy of the adsorption process was evaluated from 298 to 323 K for both adsorbents. The Avrami fractional-order kinetic model provided the best fit to the experimental data compared with pseudo-first-order or pseudo-second-order kinetic adsorption models. For Reactive Red M-2BE dye, the equilibrium data were best fitted to the Liu isotherm model. Simulated dyehouse effluents were used to check the applicability of the proposed adsorbents for effluent treatment.     

Removal of nickel ions from water by multi-walled carbon nanotubes

Multi-walled carbon nanotubes (MWCNTs) were produced by chemical vapor decomposition using acetylene gas in the presence of Ferrocene catalyst at 800 degrees C, and then oxidized with concentrated nitric acid at 150 degrees C. Both (as-produced and oxidized) CNTs were characterized by TEM, Boehm titration, N2-BET and cation exchange capacity techniques. The adsorption capacity for nickel ions from aqueous solutions increased significantly onto the surface of the oxidized CNTs compared to that on the as-produced CNTs. The effects of adsorption time, solution pH and initial nickel ions concentrations on the adsorption uptake of Ni2+ for both the as-produced and oxidized CNTs were investigated at room temperature. Both Langmuir and Freundlich isotherm models match the experimental data very well. According to the Langmuir model the maximum nickel ions adsorption uptake onto the as-produced and oxidized CNTs were determined as 18.083 and 49.261 mg/g, respectively. Our results showed that CNTs can be used as an effective Ni2+ adsorbent due to the high adsorption capacity as well as the short adsorption time needed to achieve equilibrium.     

Adsorption of ionizable organic contaminants on multi-walled carbon nanotubes with different oxygen contents

Multi-walled carbon nanotubes (MWNTs), which are considered to be promising candidates for the adsorption of toxic organics, are released into aqueous environment with their increasing production and application. In this study, the adsorption behaviors of five structurally related ionizable organic contaminants namely perfluorooctane sulfonate (PFOS), perfluorooctanoic acid (PFOA), perfluorooctanesulfonamide (PFOSA), 2,4-dichlorophenoxyacetic acid (2,4-D) and 4-n-nonylphenol (4-NP) onto MWNTs with different oxygen contents (3.84-22.85%) were investigated. The adsorption kinetics was investigated and simulated with pseudo-second-order model. The adsorption isotherms were found to be fitted with Freundlich model and influenced by both the properties of organic chemicals and the oxygen contents of MWNTs. As adsorption capacity decreases dramatically with the increasing of oxygen contents, the MWNTs with the lowest oxygen contents possess the highest adsorption capacity among four MWNTs. For the MWNTs with the oxygen contents of 3.84%, the adsorption affinity related with hydrophobic interaction and π-electron polarizability decreased in the order of 4-NP>PFOSA>PFOS>2,4-D>PFOA. Furthermore, the adsorption characters of five contaminants were affected by solution pH and solute pK(a) considering electrostatic repulse force and hydrogen bonding, which showed the adsorption of MWNTs with lower oxygen content is much sensitive to solution chemistry.     

Adsorption of fluoride from water by aligned carbon nanotubes

Aligned carbon nanotubes (ACNTs), a new type of carbon material, were prepared by catalytic decomposition of xylene using ferrocene as catalyst. The kinetics experiment of ACNTs shows that fluoride adsorption rate is fast in the first 60 min and the adsorption capacity reaches 3.0 mg/g rapidly, then it decreases and adsorption achieves equilibrium gradually in about 180 min. The fluoride adsorption of ACNTs depends slightly on the solution pH value. The highest adsorption capacity of ACNTs occurs at pH 7 and reaches 4.5 mg/g at equilibrium fluoride concentration of 15 mg/l. The experimental results indicate that ACNTs are promising candidate materials for fluoride removal.     

Removal of vanadium from wastewater by multi-walled carbon nanotubes

Nitric acid heating reflux modified multi-walled carbon nanotubes (HMWCNTs) were used for the removal of vanadium(V) in aqueous solution. The removal rate of vanadium(V) decreased with the increase of the initial vanadium(V) concentration and the solution pH, and increased with the increase of reaction time, HMWCNTs amount and solution temperature. The adsorption equilibrium and dynamic kinetics fitted the Langmuir adsorption isotherm and pseudo-second order models. The results obtained by scanning electron micrography and Fourier transform infrared spectroscopy analysis revealed that the hydroxyl and carboxyl functional groups are mainly responsible for the vanadium adsorption. This study showed that the HMWCNTs proved to be a considerable adsorbent for the removal of vanadium from wastewater.     

Magnetic removal of dyes from aqueous solution using multi-walled carbon nanotubes filled with Fe2O3 particles

The Fe₂O₃ nanoparticles have been introduced into the multi-walled carbon nanotubes (MWCNTs) via wet chemical method. The resulting products are characterized by TEM, EDX, XRD and VSM. The magnetic MWCNTs have been employed as adsorbent for the magnetic separation of dye contaminants from water. The adsorption test of dyes (Methylene Blue and Neutral Red) demonstrates that it only takes 60 min to attain equilibrium and the adsorption capacities for Methylene Blue and Neutral Red in the concentration range studied are 42.3 and 77.5 mg/g, respectively. The magnetic MWCNTs can be easily manipulated in magnetic field for desired separation, leading to the removal of dyes from polluted water. The integration of MWCNTs with Fe₂O₃ nanoparticles has great potential application to remove organic dyes from polluted water.     

Structural and magnetic characterization of batch-fabricated nickel encapsulated multi-walled carbon nanotubes

We report on the growth and fabrication of Ni-filled multi-walled carbon nanotubes (Ni-MWNTs) with an average diameter of 115 nm and variable length of 400 nm-1 μm. The Ni-MWNTs were grown using template-assisted electrodeposition and low pressure chemical vapor deposition (LPCVD) techniques. Anodized alumina oxide (AAO) templates were fabricated on Si using a current controlled process. This was followed by the electrodeposition of Ni nanowires (NWs) using galvanostatic pulsed current (PC) electrodeposition. Ni NWs served as the catalyst to grow Ni-MWNTs in an atmosphere of H2/C2H2 at a temperature of 700?°C. Time dependent depositions were carried out to understand the diffusion and growth mechanism of Ni-MWNTs. Characterization was carried out using scanning electron microscopy (SEM), focused ion beam (FIB) milling, transmission electron microscopy (TEM), Raman spectroscopy and energy dispersive x-ray spectroscopy (EDX). TEM analysis revealed that the Ni nanowires possess a fcc structure. To understand the effects of the electrodeposition parameters, and also the effects of the high temperatures encountered during MWNT growth on the magnetic properties of the Ni-MWNTs, vibrating sample magnetometer (VSM) measurements were performed. The template-based fabrication method is repeatable, efficient, enables batch fabrication and provides good control on the dimensions of the Ni-MWNTs.     

Efficient direct water dispersion of multi-walled carbon nanotubes by functionalization with lysine

A small molecule, lysine, has been attached onto the Multi-Walled carbon nanotubes (MWNTs) by producing acyl chloride on the carboxylic groups associated with the nanotubes. Subsequently, highly water-dispersed nanotubes were obtained. Stable concentration as high as 10 mg/ml was obtained in deionized water that was nearly 2 orders of magnitude higher than that obtained from the acidized MWNTs. The functionalized MWNTs can be dispersed in water under a wide range of pH values (5-14) and exhibit pH responsive. Combining the properties of carbon nanotubes and the versatility and biocompatibility of lysine, these lysine functionalized MWNTs could find potential applications in biological applications. And this method could also be used to functionalize other nanomaterials by lysine.     

Incorporation of organic groups on the surface of multi-walled carbon nanotubes using an ultrasonic tip

Multi-walled carbon nanotubes (MWCNTs) have been used to solve problems in different fields due to their mechanical, electrical and nanometric properties. However, the tendency of CNTs to agglomerate, and their poor interactions with other molecules have hindered their use. Accordingly, CNTs have been modified mainly by heating under reflux with strong acids or bases, in some cases for up to 72 h. In this study, the surfaces of MWCNTs were modified with 1,4-diaminobutane dihydrochloride (DB) and oxalic acid dihydrate (OX) using an ultrasonic tip (25 mm) at 20 kHz and a temperature of 67–80°C. Three different ultrasound times were employed: 20, 32 and 64 min. The most significant changes to the MWCNTs were observed when they were modified with DB (N 1s (11.05%) and Cl 2p (1.01%)) and OX (C 1s (74.69%) and O 1s (25.31%)) at 20 min, 68°C and 32 min, 70°C, respectively.     

Adsorption of ciprofloxacin and its role for stabilizing multi-walled carbon nanotubes and characterization

In this study, multi-walled carbon nanotubes (MWCNTs) were dispersed in an aqueous solution using ciprofloxacin (CF) without chemical modification. We found that CF is a useful stabilizer for MWCNTs and the dispersions were stable for more than one month. Scanning electron microscopy, Uv-visible spectroscopy and cyclic voltammetry (CV) showed MWCNTs coated with CF molecules. Dry film of MWCNTs/CF was prepared and characterized by SEM, Uv-vis and CV. MWCNTs/CF dry film can be used as a biocompatible platform for other applications including protein and enzyme immobilization.     

Hydrophobic carbon nanotubes for removal of oils and organics from water

In this work, we synthesized p-phenylenediamine modified carbon nanotubes (P-CNTs) by diazotization reaction. The resulting material shows a BET surface area of 285 m² g^?1 and a total volume of 0.65 cm³ g^?1 with abundant mesopores. Also, the P-CNTs exhibit good surface hydrophobicity with water contact angle of 140.8°, which should be attributed to the cooperation of both surface roughness and hydrophobic chemical compositions (aromatic rings linkages) of P-CNTs. Taking advantages of the intrinsic porosity and surface hydrophobicity, the resulting P-CNTs exhibit a notably selective absorbing ability and good recyclability for removal of organics and oils from water, which makes them the promising candidates for liquid–liquid separation and waste oil treatment.     

Adsorption uptake of synthetic organic chemicals by carbon nanotubes and activated carbons

Carbon nanotubes (CNTs) have shown great promise as high performance materials for adsorbing priority pollutants from water and wastewater. This study compared uptake of two contaminants of interest in drinking water treatment (atrazine and trichloroethylene) by nine different types of carbonaceous adsorbents: three different types of single walled carbon nanotubes (SWNTs), three different sized multi-walled nanotubes (MWNTs), two granular activated carbons (GACs) and a powdered activated carbon (PAC). On a mass basis, the activated carbons exhibited the highest uptake, followed by SWNTs and MWNTs. However, metallic impurities in SWNTs and multiple walls in MWNTs contribute to adsorbent mass but do not contribute commensurate adsorption sites. Therefore, when uptake was normalized by purity (carbon content) and surface area (instead of mass), the isotherms collapsed and much of the CNT data was comparable to the activated carbons, indicating that these two characteristics drive much of the observed differences between activated carbons and CNT materials. For the limited data set here, the Raman D:G ratio as a measure of disordered non-nanotube graphitic components was not a good predictor of adsorption from solution. Uptake of atrazine by MWNTs having a range of lengths and diameters was comparable and their Freundlich isotherms were statistically similar, and we found no impact of solution pH on the adsorption of either atrazine or trichloroethylene in the range of naturally occurring surface water (pH = 5.7-8.3). Experiments were performed using a suite of model aromatic compounds having a range of π-electron energy to investigate the role of π-π electron donor-acceptor interactions on organic compound uptake by SWNTs. For the compounds studied, hydrophobic interactions were the dominant mechanism in the uptake by both SWNTs and activated carbon. However, comparing the uptake of naphthalene and phenanthrene by activated carbon and SWNTs, size exclusion effects appear to be more pronounced with activated carbon materials, perhaps due to smaller pore sizes or larger adsorption surface areas in small pores.     

Dispersion study of long and aligned multi-walled carbon nanotubes in water

Dispersion of nanotubes is a crucial step for many applications. The properties of the final nanotube-based material are strongly dependent on the quality of nanotube suspensions. In this study, long and aligned multi-walled carbon nanotubes obtained by catalytic chemical vapour deposition were dispersed in water with different dispersing agents using high intensity ultrasounds. Among different additives, we selected sodium dodecyl sulfate (SDS) as dispersing agent to prepare suspensions of nanotubes. UV-Visible spectrometry method was used to measure the influence of dispersion parameters (power and duration of sonication) on dispersion state and suspension stability. Therefore, we demonstrated that, even if high intensity ultra-sounds are breaking nanotubes, it is possible to obtain stable water-based suspensions containing MWNTs which exhibit length up to 20 microm.     

Enhancement of polymer luminescence by excitation-energy transfer from multi-walled carbon nanotubes

Carbon nanotubes have been shown to efficiently quench luminescence from conjugated polymers when incorporated in a composite. However, shown here is an up to 100-fold increase in the visible photoluminescence signal from fluorescent chromophores in nylon 10,10 by incorporating multi-walled carbon nanotubes (MWCNTs). Using 325- and 488-nm excitation the optical absorption by MWCNTs embedded within the polymer matrix is demonstrated, followed by efficient excitation-energy transfer to emissive chromophores intrinsic to the polymer but only when the MWCNTs are acid functionalized. Furthermore, the MWCNTs are shown to significantly retard photobleaching of fluorescent centers in the nylon composites. These remarkable properties greatly advance the prospects of utilizing MWCNTs in organic solar cells and electroluminecent devices to improve performance.     

Ho3+ carbon paste sensor based on multi-walled carbon nanotubes: Applied for determination of holmium content in biological and environmental samples

For the first time a novel multi-walled carbon nanotubes (MWCNTs) modified Ho³⁺ carbon paste sensor is introduced. The electrode with a composition containing 20% paraffin oil, 60% graphite powder, 15% N-(1-thia-2-ylmethylene)-1,3-benzothiazole-2-amine (TBA) as an ionophore, and 5% MWCNTs, exhibits a stable potential response to Ho³⁺ ions with a nice Nernstian behavior (19.3 ± 0.3 mV decade^? 1) in a wide dynamic linear concentration range of Ho³⁺ ions (1 × 10^? 8–1.0 × 10^? 2 M). In the absence of MWCNTs, sensitivity of the Ho³⁺ sensor was relatively poor. The proposed modified Ho³⁺ sensor shows very low detection limit (7.0 × 10^? 9 M) and a fast response time (13 s). It has a long life time (more than 2 months) and its response is independent of pH in the range of 3.8–7.5. In term of selectivity, Ho³⁺ sensor has a good selectivity over all lanthanide members and common alkali and alkaline earth metal ions. The Ho³⁺ sensor was applied for the determination of Ho³⁺ ion concentration in water, holmium alloys and synthetic human serum.     

Modification of multi-walled carbon nanotubes by Diels-Alder and Sandmeyer reactions

Random (L) and aligned (A) multi-walled carbon nanotubes (MWNTs) were modified by Diels-Alder (DA) [4+2] cycloaddition, Sandmeyer (SM) reaction and by catalytic oxidation (OX). The properties of modified carbon nanotubes were studied by dispersability tests, elemental analysis, thermogravimetry/mass spectrometry, X-ray photoelectron spectroscopy, and NMR spectroscopy. The cycloaddition reaction could only be successfully performed with the L-MWNTs in molten and in solution state by using an aluminum chloride homogeneous catalyst. The efficiency and thermal stability of the solution phase cycloaddition were much higher than in the case of modification in the molten phase. The functionalization of both types of MWNTs by Sandmeyer reaction was carried out by copper(I) and iron(ll) ions that helped in the radical decomposition of diazonium salts. Successful functionalization of nanotubes is achieved by a long decomposition time of the thermally activated diazonium salts. To the contrary, in the case of radical decomposition of diazonium salts, the time is not a decisive parameter. The dispersability tests have proved the changes in the physical features of modified carbon nanotubes depending on the hydrophobic and hydrophilic character of the solvents. The presence of the modifying groups and their fragments from the functionalized MWNTs has been demonstrated by thermogravimetry/mass spectrometry (TG/MS). Relatively high concentration of sulfur atoms was detected by X-ray photoelectron spectroscopy in nanotubes modified by sulfur substituent groups. In the case of catalytic oxidation, the X-ray photoelectron spectroscopic signal of oxygen bound to nanotubes showed considerable change as compared to pristine nanotubes. Due to the high thermal stability of modified multi-walled carbon nanotubes, the functionalized derivatives are applicable in several industrial fields.     

Adsorption of toluene, ethylbenzene and m-xylene on multi-walled carbon nanotubes with different oxygen contents from aqueous solutions

The purified and oxidized multi-walled carbon nanotubes (MWCNTs) with different oxygen contents are employed as adsorbents to study their physicochemical properties and adsorption behaviors of toluene, ethylbenzene and m-xylene (TEX) in aqueous solutions. The results demonstrate that adsorption capacity is significantly enhanced for 3.2% surface oxygen, but is dramatically reduced for 5.9% oxygen concentration. The adsorption kinetics is investigated and fitted with pseudo-second-order model. The adsorption isotherms are found to be fitted with Langmuir model. More interestingly, with the increasing of surface oxygen content, maximum adsorption capacities firstly increased, and then, began to decrease. In the first stage, dispersion is the most important factor. A better dispersive interaction increases the available adsorption sites, which consequently can be favorable for the aqueous phase adsorption. Therefore, maximum adsorption capacity is remarkably enhanced with the increasing of oxygen content, which is according with our results. However, in the second stage, when oxygen content increases to a certain extent, hydroxyl groups cause water clusters formation on the surface or tube end of MWCNTs, which hinder the interaction between TEX and MWCNTs. Consequently, more oxygen content leads to the decrease in maximum adsorption capacity. The decrease indicates that the formation of water clusters plays a more important role than the better dispersion of MWCNTs for TEX adsorption.     

Investigation of effects produced by chemical functionalization in single-walled and multi-walled carbon nanotubes using Raman spectroscopy

Single-wall carbon nanotubes (SWNTs) as well as multi-wall carbon nanotubes (MWNTs) were characterized by Raman spectroscopy to observe the changes in their physical and structural properties on functionalization. When SWNTs or MWNTs are chemically treated, the defects are created. The analysis of radial breathing mode (RBM) showed that the diameter of the single wall carbon nanotubes changed after functionalization. In the carboxylated sample, the intensity of the disordered band (D-mode) increased more than in the pristine samples. The increase in the D-band intensity in SWNTs after functionalization can be attributed to carbon atoms excited from sp ² to sp ³ hybridization. A higher intensity ratio in D-and G-mode (I _D /I _G ) was observed after functionalization with carboxylic group (COOH). The intensity ratio I _D /I _G increased on acid treatment which was evident from the Raman spectra and their analysis. In case of MWNTs, the intensity of D band became equal to the intensity of G band, which was due to the huge number of defects that had been introduced in the sidewalls. Moreover, it was found in this study that the MWNTs can be much easier chemically functionalized than SWNTs under the same physical conditions.     

Physical activation and characterization of multi-walled carbon nanotubes catalytically synthesized from methane

A series of treatment processes were employed to purify and then physically activate the multi-walled carbon nanotubes obtained using catalytic decomposition of methane. In order to characterize and compare the activation effect, the carbon fibers were also treated by the same activation processes. The results showed that the normal physical activation by CO₂ or steam has not too much effect on the surface area of purified multi-walled carbon nanotubes, in particular, the carbon nanotubes were burned when using the poignant activation conditions. However, the surface area of carbon fibers availably etched in the same activation processes is much increased. In addition, the mechanisms of physical activation on multi-walled carbon nanotubes and carbon fibers have been investigated.     

Dispersion and field emission properties of multi-walled carbon nanotubes by high-energy milling

Multi-walled carbon nanotubes (MWNTs) were high-energy milled for 2 h in texanol after a polycarboxylic acid polymeric dispersant had been added in order to enhance the dispersion. The degree of MWNT dispersion was significantly enhanced by high-energy milling compared to the intact sample, which increased the density of surface-exposed MWNTs with a screen-printed paste. However, the emission properties of high-energy milled MWNTs did not show such a high emission current density relative to their increased surface-exposed density. Further investigation using Raman spectroscopy and X-ray diffraction evidenced the milling-induced MWNT damage, which explained the relatively lower emission current density of high-energy milled MWNTs.