Adsorption of volatile organic compounds onto carbon nanotubes, carbon nanofibers, and high-surface-area graphites

The adsorption of different alkanes (linear and cyclic), aromatics, and chlorohydrocarbons onto different nonmicroporous carbons--multiwalled carbon nanotubes (CNTs), carbon nanofibers (CNFs), and high-surface-area graphites (HSAGs)--is studied in this work by inverse gas chromatography (IGC). Capacity of adsorption was derived from the isotherms of adsorption, whereas thermodynamic properties (enthalpy of adsorption, surface free energy characteristics) have been determined from chromatographic retention data. HSAGs present the highest adsorption capacity, followed by CNTs and CNFs (although CNTs present an intermediate surface area between the two HSAG studied). Among the different adsorbates tested, benzene exhibits the highest adsorption capacity, and the same trend is observed in the enthalpy of adsorption. From surface free energy data, enthalpies of adsorption of polar compounds were divided into dispersive and specific contributions. The interactions of cyclic (benzene and cyclohexane) and chlorinated compounds (trichloroethylene, tetrachloroethylene, and chloroform) with the surfaces are mainly dispersive over all the carbons tested, CNTs being the material with the highest dispersive contribution, as was deduced also from the entropy parameter. Adsorption parameters were correlated with morphological and chemical properties of the materials.     

The preparation and adsorption properties of electrospun aramid nanofibers

The focus of this work is the preparation of aramid nanofibers via electrospinning technology and the study of their adsorption properties. In this article, aramid nanofibers were prepared by electrospinning aramid fibers solution with the addition of lithium chloride (LiCl). It showed a good adsorption capacity when methylene blue (MB) was used as the model target. There were much larger adsorption amounts and faster kinetics of uptaking target species of electrospun aramid nanofibers to MB than that of electrospun polyethersulfone (PES) nanofibers. Compared with activated carbon, aramid nanofibers also have a much faster adsorption rate to MB. Aramid nanofibers were subsequently used to effectively remove endocrine disruptors such as bisphenol A (BPA), phenol (Phe), and p‐hydroquinone (BPhe) from their aqueous solutions. Additionally, molecule imprinted technology enhances aramid nanofibers with much higher adsorption amounts and special adsorption property for endocrine disruptors. These results showed that aramid nanofibers have the potential to be used in environmental applications. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Removal of radiocobalt from aqueous solution by different sized carbon nanotubes

Batch adsorption technique was applied to study the adsorption of radiocobalt on multiwalled carbon nanotubes (CNTs) with deferent sizes. The aim of this work was to examine the effect of contact time, pH, solid content, foreign ions and CNT particle sizes on the removal of Co(II) ions from aqueous solutions by CNTs. The results indicated that the adsorption of Co(II) was strongly dependent on pH and the adsorption capacity was in inverse proportion to the particle sizes of CNTs. The adsorption of Co(II) was weakly affected by ionic strength and foreign ions. Ion exchange and surface complexation were the main adsorption mechanisms. The kinetics of Co(II) adsorption on CNTs was described well by pseudo-second-order model. The Langmuir and Freundlich models were applied to interpret the adsorption data. The results are important to understand the physicochemical behavior of Co(II) with CNTs, and for the application of CNTs in the preconcentration of radiocobalt from large volumes of aqueous solutions.     

Adsorption of SO2 by typical carbonaceous material: a comparative study of carbon nanotubes and activated carbons

Applying activated carbons for SO₂ adsorption and conversion to H₂SO₄, as a dry process, has been considered the development direction of desulfurization technology. Coal-based activated carbon, coconut shell activated carbon, single wall carbon nanotube and multi-wall carbon nanotubes were used as typical carbonaceous materials to study the SO₂ adsorption mechanism. SEM, N₂ adsorption, XPS and fixed-bed reaction system were employed to study the morphology, pore structure, surface functional groups and SO₂ adsorption behaviors of the four adsorbents. The fixed-bed experiment was carried out at normal pressure and SO₂ concentration was set 1,000 ppm. According to SEM and N₂ adsorption results, hierarchical pore structure was an important characteristic of activated carbon. Aggregation was an important characteristic of CNTs. Mesopores and macropores took the dominance of pore structure in CNTs. According the SO₂ adsorption data and correlation analysis, it can be concluded that the dominant adsorption type on activated carbons does not alter with adsorption temperature changing. However, the adsorption type of SO₂ adsorption on CNTs changes with adsorption temperature varying. With adsorption temperature increasing, the dominant adsorption type transforms to chemisorption by physisorption. Higher-density π–π* in carbon nanotubes may be the active sites for the SO₂ chemical adsorption. Micropores with the diameter smaller than 0.7 nm were the best SO₂ adsorption place for both activated carbons and carbon nanotubes. The results provided a profound insight into the microstructure and SO₂ adsorption mechanism of the two kinds of carbonaceous materials.     

Ligand adsorption on an activated carbon for the removal of chromate ions from aqueous solutions

The results presented in this work are related to the design of a guideline to develop specific properties at the surface of an activated carbon (AC). For this, two model aromatic compounds have been synthesized and their electrolytic behavior in aqueous solutions was studied by a potentiometric method. The textural characteristics of the activated carbon were determined by porosimetry methods. The nature of oxygen-carrying functions and the acid-base behavior of the AC surface were characterized by TPD and potentiometric titration methods, respectively. The adsorption and desorption equilibria of the aromatic compounds on activated carbon were measured in aqueous solutions, and the hysteresis between adsorption and desorption, which reveals irreversible adsorption, was discussed on the basis of the frontier orbital theory. HOMO and LUMO orbitals of the adsorbent and adsorbates were calculated, and irreversible adsorption was attributed to the small energy difference between HOMO and LUMO of the aromatic adsorbates and the adsorbent. Adsorption equilibria of K2CrO4 in aqueous solution on the AC alone and on the AC-aromatic ligand adsorbents, respectively, prove the efficient development of specific chemical functions at the carbon surface provided by the adsorbed aromatic compounds.     

Electrochemical detection of phenolic estrogenic compounds at carbon nanotube-modified electrodes

The use of a carbon nanotube-modified glassy carbon electrode (CNT-GCE) for the LC-EC detection of phenolic compounds with estrogenic activity is reported. Cyclic voltammograms for phenolic endocrine disruptors and estrogenic hormones showed, in general, an enhancement of their electrochemical oxidation responses at CNT-GCE attributable to the electrocatalytic effect caused by CNTs. Hydrodynamic voltammograms obtained under flow injection conditions lead to the selection of +700 mV as the potential value to be applied for the amperometric detection of the phenolic estrogenic compounds, this value being remarkably less positive than those reported in the literature using other electrode materials. Successive injections of these compounds demonstrated that no electrode surface fouling occurred. A mobile phase consisting of a 50:50 (v/v) acetonitrile:0.05 mol l⁻¹ phosphate buffer of pH 7.0 was selected for the chromatographic separation of mixtures of these compounds, with detection limits ranging between 98 and 340 nmol l⁻¹. Good recoveries were obtained in the analysis of underground well water and tap water samples spiked with some phenolic estrogenic compounds at a 14 nmol l⁻¹ concentration level.     

Adsorption and desorption of atrazine on carbon nanotubes

The potential impact of carbon nanotubes (CNTs) on human health and the environment is receiving more and more attention. The high surface area of CNTs tends to adsorb a large variety of toxic chemicals, which may enhance the toxicity of CNTs and/or toxic chemicals. In this study, adsorption and desorption of atrazine on carbon nanotubes from aqueous solution were studied through batch reactors. The adsorption equilibrium isotherms were nonlinear and were fitted by Freundlich, Langmuir, and Polanyi-Manes models. It was found that the Polanyi-Manes model described the adsorption process better than other two isotherm models. Together with the "characteristic curve," the Polanyi adsorption potential theory is applicable to describe the adsorption process of atrazine on CNTs. Thermodynamic calculations indicated that the adsorption reaction of atrazine on CNTs is spontaneous and exothermic. The desorption data showed that no significant desorption hysteresis occurred. High adsorption capacity and adsorption reversibility of atrazine on CNTs suggest that CNTs have high health and environmental risks, whereas they have potential applications for atrazine removal from water.     

甲烷在活性炭上吸附平衡模型的研究

比较吸附模型分析甲烷在活性炭上吸附平衡的适用性,为吸附式天然气(ANG)的工程应用提供准确的预测模型。基于在温度268.15～338.15 K、压力0～12.5 MPa测试的甲烷在Ajax活性炭上的吸附平衡数据,选择Langmuir、Langmuir-Freundlich和Toth方程,应用非线性回归拟合方程参数后,确定绝对吸附量和甲烷吸附相态,并比较方程在不同压力区域内的预测精度。结果表明,甲烷吸附相密度随平衡温度和压力变化;由绝对吸附量确定的甲烷在Ajax活性炭上的平均等量吸附热为15.72 kJ/mol,小于由过剩吸附量的标绘结果;Langmuir、Langmuir-Freundlich和Toth方程预测结果在0～0.025 MPa的累积相对误差为6.449 8%、7.918 4%和0.910 0%,在1～10 MPa为0.491 1%、0.161 3%和0.369 4%。Toth方程在整个压力范围内的预测结果最为准确,但Langmuir-Freundlich方程在较高压力区域内具有较高的预测精度。     

Adsorption of benzene,toluene, ethylbenzene and p-xylene by NaOCl-oxidized carbon nanotubes

Multiwalled carbon nanotubes (CNTs) were oxidized by sodium hypochlorite (NaOCl) solution and were employed as adsorbents to study their characterizations and adsorption performance of benzene, toluene, ethylbenzene and p-xylene (abbreviated as BTEX) in an aqueous solution. The physicochemical properties of CNTs such as purity, structure and surface nature were greatly improved after oxidation, which significantly enhanced BTEX adsorption capacity. The adsorption capacity of CNT(NaOCl) increased with contact time and initial adsorbate concentration, but changed insignificantly with solution ionic strength and pH. A comparative study on the BTEX adsorption revealed that the CNT(NaOCl) had better BTEX adsorption as compared to CNTs and granular activated carbon. This suggests that the CNT(NaOCl) are efficient BTEX adsorbents and that they possess good potential for BTEX removal in wastewater treatment.     

Reactivity of silicon and germanium doped CNTs toward aromatic sulfur compounds: A theoretical approach

Adsorption processes of thiophene and benzothiophene on pristine carbon nanotubes (CNTs), and on CNTs doped with Si or Ge, have been modeled with Density Functional. This is the first study on the chemical reactivity of such doped tubes. The calculated data suggest that the presence of silicon or germanium atoms in CNTs increases their reactivity toward thiophene, and benzothiophene. The adsorption of these species on pristine CNTs seems very unlikely to occur, while the addition products involving doped CNTs were found to be very stable, with respect to the isolated reactants, in terms of Gibbs free energy. Several of these adsorption processes were found to be significantly exergonic (ΔG < 0) in non-polar liquid phase. The results reported in this work suggest that Si and Ge defects on CNTs increase their reactivity toward unsaturated species, and could make them useful in the removal processes of aromatic sulfur compounds from oil-hydrocarbons. However, according to our results, CNTs doped with Si atoms are expected to be more efficient as aromatic sulfur compounds scavengers than those doped with Ge. These results also suggest that the presence of silicon and germanium atoms in the CNTs structures enhances their reactivity toward nucleophilic molecules, compared to pristine carbon nanotubes.     

Quantum dot modified multiwall carbon nanotubes

A novel strategy for the fabrication of multiwall carbon nanotube-nanocrystal heterostructures is shown. Different quantum dots (QDs) with narrow size distributions were covalently coupled to carbon nanotubes (CNTs) and silica-coated CNTs in a simple, uniform, and controllable manner. The structural and optical properties of CNT/QD heterostructures are characterized by electron microscopy and photoluminescence spectroscopy. Complete quenching of the PL bands in both QD core and core/shell heterostructures was observed after adsorption to the CNTs, presumably through either carrier ionization or energy transfer. The deposition of a silica shell around the CNTs preserves the fluorescence properties by insulating the QD from the surface of the CNT.     

Applicability of mass spectrometry to detect coeluting impurities in high-performance liquid chromatography

This paper describes the results of selectivity optimization and internal standard prediction for the quantitation of estradiol and levonorgestrel in transdermal patches by reversed-phase liquid chromatography (RPLC) based on the linear solvation energy relationships (LSERs). The patch samples are prepared by swelling with acetonitrile (ACN) and the separation is performed by Zorbax Eclipse XDB ODS columns. A proper retention range is first determined with a binary mobile phase of ACN and water based on the general resolution equation. The interference to estradiol from a levonorgestrel impurity is then eliminated by a ternary mobile phase of acetonitrile-methanol-water with a composition predicted by LSERs. When the resolution is optimized and the "open window" in the chromatogram for an internal standard is selected, LSERs are used to predict the candidate compounds to be evaluated as the internal standard. The approach described in this study can be used, in general, to considerably improve the efficiency of RPLC method development, particularly for neutral samples. Finally, the LSER approach for the selectivity optimization is compared with a statistical response surface methodology (RSM) based on a central composite design (CCD) in terms of the effectiveness and number of experiments. It is concluded that, although the predicted mobile phase composition to achieve the desired selectivity is about the same, the LSER approach is more efficient and fewer experiments are required.     

Nitrogen adsorption characterization of aligned multiwalled carbon nanotubes and their acid modification

Aligned multiwalled carbon nanotube (CNT) arrays were synthesized by using an iron-based sol-gel catalyst and acetylene as the precursor. These CNTs show high purity, uniform diameters and pore-wall thickness. Low temperature nitrogen adsorption was employed to characterize the structural and surface properties of the as-synthesized sample and that modified with boiling concentrated nitric acid. The adsorption characteristics of the as-synthesized and modified CNTs were thoroughly investigated. High-resolution comparative alpha(s)-plot showed that the nitrogen adsorption on CNTs takes place via a multistage mechanism closely related to their structures. It was also found that the acid modification significantly increased the adsorption energy and enhanced the adsorption capacity under low pressures. High-resolution comparative method provided valuable insights about the surface and pore structures of CNTs.     

Microwave assisted preparation of efficient activated carbon from grapevine rhytidome for the removal of methyl violet from aqueous solution

Grapevine rhytidome (the outer layer of bark on trunk), as an abundant and low-cost precursor, was used to prepare granular activated carbon with high surface area for the removal of methyl violet from aqueous solution. Microwave heating source was used to reduce the treatment time and energy consumption. To optimize the preparation, the effects of the different parameters, such as phosphoric acid concentration, acid/precursor weight ratio, impregnation time, microwave power, radiation time, and oven heating time on the ability of the samples for removal of methyl violet were studied. The obtained activated carbon was characterized by N₂ adsorption/desorption, SAXS, TEM and SEM methods. The adsorption of methyl violet onto the activated carbon was studied from both equilibrium and kinetic point of view and the results were compared with the commercial granular activated carbon. The rate of adsorption onto the prepared activated carbon was faster than commercial activated carbon. Different kinetic models were used to analyze the experimental kinetic data. The obtained activated carbon showed higher adsorption capacity (more than twice) for the adsorption of methyl violet in comparison with the commercial one. The equilibrium data were analyzed using different isotherm models. Adsorption was found to be maximum in the pH range 7-9.     

Using ONIOM calculations to investigate the abilities of simple and nitrogen,boron, sulfur-doped carbon nanotubes in sensing of carbon monoxide

In this work, geometries, stabilities, and electronic properties of the carbon monoxide (CO) molecule as an adsorbent in a simple carbon nanotube (CNT) and nitrogen (N), boron (B), sulfur (S)-doped CNTs (NCNT, BCNT, and SCNT) with parallel and perpendicular configurations are fully considered using ONIOM, natural bond orbital, and quantum theory of atom in molecule (QTAIM) calculations. The adsorption energies (E_ad) demonstrate that a CO molecule could be adsorbed on the surface of the simple CNT with parallel configuration and N-doped CNT with perpendicular configuration in an exothermic process. QTAIM calculations showed the close-shell (noncovalent) interactions between the CO molecule and CNT or N, B, S-doped CNTs. In addition, the energy gap (E_g) values between the highest occupied molecular orbital and the lowest unoccupied molecular orbital are calculated. In accordance with the results of energy gap, simple and N-doped CNTs could be used as CO sensors.     

Curvature‐dependent adsorption of water inside and outside armchair carbon nanotubes

The curvature dependence of the physisorption properties of a water molecule inside and outside an armchair carbon nanotube (CNT) is investigated by an incremental density‐fitting local coupled cluster treatment with single and double excitations and perturbative triples (DF‐LCCSD(T)) study. Our results show that a water molecule outside and inside (n, n) CNTs (n = 4, 5, 6, 7, 8, 10) is stabilized by electron correlation. The adsorption energy of water inside CNTs decreases quickly with the decrease of curvature (increase of radius) and the configuration with the oxygen pointing toward the CNT wall is the most stable one. However, when the water molecule is adsorbed outside the CNT, the adsorption energy varies only slightly with the curvature and the configuration with hydrogens pointing toward the CNT wall is the most stable one. We also use the DF‐LCCSD(T) results to parameterize Lennard‐Jones (LJ) force fields for the interaction of water both with the inner and outer sides of CNTs and with graphene representing the zero curvature limit. It is not possible to reproduce all DF‐LCCSD(T) results for water inside and outside CNTs of different curvature by a single set of LJ parameters, but two sets have to be used instead. Each of the two resulting sets can reproduce three out of four minima of the effective potential curves reasonably well. These LJ models are then used to calculate the water adsorption energies of larger CNTs, approaching the graphene limit, thus bridging the gap between CNTs of increasing radius and flat graphene sheets. © 2016 Wiley Periodicals, Inc.     

Oxygen adsorption characteristics on hybrid carbon and boron‐nitride nanotubes

In this work, first‐principles density functional theory (DFT) is used to predict oxygen adsorption on two types of hybrid carbon and boron‐nitride nanotubes (CBNNTs), zigzag (8,0), and armchair (6,6). Although the chemisorption of O₂ on CBNNT(6,6) is calculated to be a thermodynamically unfavorable process, the binding of O₂ on CBNNT(8,0) is found to be an exothermic process and can form both chemisorbed and physisorbed complexes. The CBNNT(8,0) has very different O₂ adsorption properties compared with pristine carbon nanotubes (CNTs) and boron‐nitride nanotube (BNNTs). For example, O₂ chemisorption is significantly enhanced on CBNNTs, and O₂ physisorption complexes also show stronger binding, as compared to pristine CNTs or BNNTs. Furthermore, it is found that the O₂ adsorption is able to increase the conductivity of CBNNTs. Overall, these properties suggest that the CBNNT hybrid nanotubes may be useful as a gas sensor or as a catalyst for the oxygen reduction reaction. © 2014 Wiley Periodicals, Inc.