Preparation of activated carbon fibers from polyvinyl chloride

Waste polyvinyl chloride (PVC) contains high content of chlorine, which is believed to liberate dioxine at its combustion. Efficient removal of chlorine from PVC achieved by selecting the heat-treatment conditions provided free-chlorine PVC based pitch by a two-stage heat-treatment process. The obtained pitch (softening point: 218 °C) was spun, stabilized, carbonized and activated to prepare activated carbon fibers (ACF) whose DeSO_x activity was tested preliminarily and found comparable to other ACF.     

Flue gas desulphurization by activated carbon fibers obtained from polyacrylonitrile by-product

By pyrolysis of a polyacrylonitrile textile by-product, subsequent activation by CO₂ and treatment (at high temperature) with a N₂ flow containing a low percentage of O₂ or of NH₃, three carbonaceous matrices are obtained having a high surface area and surface sites with basic characteristics. The SO₂ sorption properties of these carbon samples (in the temperature range between 100 and 160 °C) from gaseous mixtures having a similar composition to flue gases, seems to be promoted by nitrogen bonded to carbon. The SO₂ adsorbed by the carbons can be divided, by suitable extraction with distilled water, into: (i) desorbable, such as SO₂ or H₂SO₃, (ii) desorbable, such as SO₃ or H₂SO₄, (iii) non-desorbable. Following 10 SO₂ adsorption and desorption cycles, the surface area values of the activated carbons remain practically constant, while both the content of the acidic surface sites and the amount of non-desorbable SO₂ increase; this results in the decrease in the SO₂ carbon sorption property seeming to be even more marked for the carbon sample containing nitrogen.     

Effects of activated carbon surface chemistry and pore structure on the adsorption of organic contaminants from aqueous solution

The objective of this research was to develop activated carbon selection criteria that assure the effective removal of trace organic contaminants from aqueous solution and to base the selection criteria on physical and chemical adsorbent characteristics. To systematically evaluate pore structure and surface chemistry effects, a matrix of activated carbon fibers (ACFs) with three activation levels and four surface chemistry levels was prepared and characterized. In addition, three granular activated carbons (GACs) were studied. Two common drinking water contaminants, relatively polar methyl tertiary-butyl ether (MTBE) and relatively nonpolar trichloroethene (TCE), served as adsorbate probes. TCE adsorbed primarily in micropores in the 7-10 Å width range while MTBE adsorbed primarily in micropores in the 8-11 Å width range. These results suggest that effective adsorbents should exhibit a large volume of micropores with widths that are about 1.3 to 1.8 times larger than the kinetic diameter of the target adsorbate. Hydrophobic adsorbents more effectively removed both TCE and MTBE from aqueous solution than hydrophilic adsorbents, a result that was explained by enhanced water adsorption on hydrophilic surfaces. To assure sufficient adsorbent hydrophobicity, the oxygen and nitrogen contents of an activated carbon should therefore sum to no more than about 2 to 3 mmol/g.     

The effects of picolinic acid and pH on the adsorption of Cu(II) by activated carbon fibers

The adsorption characteristics of Cu(II) on activated carbon fibers (ACFs) from simulated wastewater was investigated in the picolinic acid concentration range from 0.15 to 15 mM by varying pH from 2 to 8. When pH is below 4, the removal fraction of Cu(II) ions decreased with the decrease of pH. The removal fraction of Cu(II) ions is almost constant above pH 4. The removal efficiency of Cu(II) ions increased as the molar ratio of picolinic acid to Cu(II), specific surface area of ACFs and pH of the solution increased. In the case of [Pic]/[Cu(II)]=10, complete adsorption of Cu(II) was performed on ACF, even at pH 2.     

Adsorption of trace polar methy-ethyl-ketone and non-polar benzene vapors on viscose rayon-based activated carbon fibers

The adsorption of polar methy-ethyl-ketone (MEK) and non-polar benzene vapors on viscose rayon-based activated carbon fiber (ACF) was investigated. The pore texture and surface composition of ACF were characterized by nitrogen adsorption at 77 K and X-ray photoelectron spectroscopy (XPS), respectively. Gas adsorption on the samples was measured by the gravimetric method and the Dubinin-Radushkevich (DR) equation was used to fit the experimental adsorption isotherms. The experimental results show that ACF with different pore texture and surface composition exhibited different adsorption and desorption behavior for polar and non-polar vapors. The effect of adsorbate polarity on the adsorption capacity at lower concentrations was more significant in the case of adsorbents with a smaller surface area. It was found that evacuation treatment greatly increased the adsorption rate.     

Effect of air oxidation of Rayon-based activated carbon fibers on the adsorption behavior for formaldehyde

The tailoring of pore surface chemistry of activated carbon fibers is shown to be an effective method for improving the adsorption efficiency of various volatile chemical compounds (VOCs). An oxidation treatment with air resulted in a significant increase in the adsorption capacities and breakthrough time for Rayon-based activated carbon fibers (ACFs) in removal of formaldehyde. The porous structure parameters of Rayon-based ACFs were determined with standard nitrogen adsorption analysis. The pore surface chemistry of samples under study was analyzed by Fourier Transform Infrared spectra (FTIR). Thus to some extent, the relationship between the adsorption properties, porous structure and pore surface chemistry was revealed.     

Preparation and characterization of trilobal activated carbon fibers

The objective of this research was to evaluate the effectiveness of several different methods for controlling the pore size and pore size distribution in activated carbon fibers. Variables studied included fiber shape, activation time, and the addition of small amounts of silver nitrate. Pure isotropic pitch and the same isotropic pitch containing 1 wt.% silver were melt spun to form fibers with round and trilobal cross sections. These fibers were then stabilized, carbonized, and activated in carbon dioxide. Field emission scanning electron microscopy (FE SEM), electron dispersive spectra (EDS), and wavelength dispersive spectra (WDS) were used to monitor the size and distribution of the silver particles in the fibers before and after activation. Each of these analyses showed that the distribution of silver particles was extremely uniform before and after activation. The fibers were also weighed before and after activation to determine the percent burn-off. The BET specific surface areas of the activated fibers were determined from N₂ adsorption isotherms measured at −196 °C. The results showed that round and trilobal fibers with equivalent cross-sectional areas yielded similar burn-off values and specific surface areas after activation. Also, activation rates were found to be independent of CO₂ flow rate. The porosity of the activated fibers depended on the total time of activation and the cross-sectional area of fibers. The N₂ adsorption measurements showed that the activated fibers had extremely high specific surface areas (greater than 3000 m²/g) and high degrees of meso- and macro-porosity. FE SEM was also used to investigate surface texture and size of pore openings on the surfaces of the activated fibers. The photos showed that silver particles generated surface macro- and mesopores, in agreement with the inferences from N₂ adsorption measurements.     

A small-angle neutron scattering study of activated carbon fibers

Phenolic resin based activated carbon fibers are widely used as filter materials in industry. We studied the fiber type ACFY-0204-3-18 which is applied to recover 2-propanol from air. The fibers were characterised by measuring the chemical composition, the apparent density, the wide angle X-ray diffraction data, and the sorption isotherms for nitrogen at 77.4 K and 2-propanol at 308.2 K. The porous structure was studied by small-angle neutron scattering. The two-dimensional contour patterns show an anisotropic intensity distribution I(q) at scattering vectors |q|<0.4 nm⁻¹. In this q-range I(q) is perpendicular to the fiber axis due to a refraction of the neutrons by the fibers, and I(q) increases abruptly with decreasing q. At |q|>0.4 nm⁻¹ scattering is isotropic and I(q) shows a weak interference peak due to the presence of micropores. The mean pore size of ∼2 nm was determined by fitting a monodisperse and a polydisperse Percus-Yevick hard sphere model to the experimental I(q) data. The microstructure of ACFY differs basically from the microstructure of polyacrylonitrile and pitch based carbon fibers.     

Prediction of breakthrough curves for adsorption on activated carbon fibers in a fixed bed

A new model is proposed to describe the removal of volatile organic compounds (VOC) from a gas stream passing through a bed packed with activated carbon fibers (ACFs). Toluene was used as the test compound. Both pore diffusion and surface diffusion are considered in the model. The equilibrium behavior is shown to fit the Dubinin–Radushkevich isotherm with the values of parameters K and W₀ of 1.101 × 10⁻⁹ and 57.73 kg/m³, respectively. The experimental results show that this model can predict VOC breakthrough curve very well.     

Surface properties of oxyfluorinated PAN-based carbon fibers

In this study, the oxyfluorination of PAN-based carbon fibers was undertaken at room temperature using fluorine-oxygen mixtures, and the influence of oxyfluorination on properties such as wettability, surface polarity, surface free energy, conductivity and tensile strength was investigated. As the oxyfluorination time increased at a total pressure of 5 kPa, both the fluorine/carbon and oxygen/carbon ratios increased. The contribution of semicovalent C-F bond to F1s spectra is considerably decreased with increasing total pressure from 5 to 80 kPa, and at the same time, the contribution of covalent C-F bond is increased. As the total pressure of fluorine-oxygen mixtures increased, the contact angle of water significantly decreased and again increased to a similar value to that of as-received carbon fiber. A short oxyfluorination of carbon fibers considerably increased the wettability, that is, hydrophilicity.The electrical conductivity of oxyfluorinated carbon fiber is larger than that of the as-received fiber. This is because the surface region of carbon fiber is fluorinated. An increase in the tensile strength of about 18% after oxyfluorination is observed. The increase in tensile strength of oxyfluorinated carbon fibers can be understood as being due to a decrease in the diameter of the fiber.     

The role of adsorbent pore size distribution in multicomponent adsorption on activated carbon

The impact of adsorbent pore size distribution (PSD) on adsorption mechanism for the multi solute system was evaluated in this study. Anoxic and oxic adsorption equilibrium for the single solute (phenol), binary solute (phenol/2-methylphenol) and ternary solute (phenol/2-methylphenol/2-ethylphenol) systems on one granular activated carbon (GAC) F400 and two types of activated carbon fibers (ACFs), namely, ACC-10 and ACC-15, were determined. F400 has a wide PSD, while ACC-10 and ACC-15 have narrow PSD and their critical pore diameters are 8.0 Å and 12.8 Å, respectively. In single solute adsorption, the increase of adsorptive capacity under oxic conditions as compared to anoxic ones was related to the PSD of the adsorbent. Binary solute adsorption on ACC-10 and ternary solute adsorption on ACC-15 indicated no impact of the presence of molecular oxygen on the adsorptive capacity and the adsorption isotherms were well predicted by the ideal adsorbed solution theory (IAST). Significant differences between oxic and anoxic isotherms were noticed for other multicomponent adsorption systems. The narrow PSD of ACFs was effective in hampering the oligomerization of phenolic compounds under oxic conditions. Such a phenomenon will provide accurate predictions of fixed bed adsorbers in water treatment systems.     

Removal of metal ions from aqueous solution by adsorption onto activated carbon cloths: adsorption competition with organic matter

Activated carbon cloths are recent adsorbents whose adsorption properties are well known for monocomponent solutions of organics or metal ions. However, to treat wastewaters with these materials, their performance has to be determined in multicomponent solution. This work studies adsorption competition between metal ions (Cu²⁺, Pb²⁺) and organic matter (benzoic acid). The first part investigates adsorption equilibrium of monocomponent metal ions solutions and shows the dependence of adsorption capacities on adsorbent porosity and metal ions chemical properties (molecular weight, ionic radius and electronegativity). The influence of pH is also demonstrated. The second part focuses on adsorption competition: (1) between both metal ions (a decrease of adsorption capacities is observed, whose value is related to adsorption kinetics of metal ions); (2) between metal ions and organic matter, in solution or adsorbed onto the activated carbon cloth (a strong influence of pH is shown: when benzoic acid is under benzoate form, in both cases adsorption is increased due to the formation of ligands between adsorbed benzoate ions and metals).     

Surface chemistry of a viscose-based activated carbon cloth modified by treatment with ammonia and steam

The influence of ammonia treatment at 800 °C on the catalytic activity of a viscose-based activated carbon cloth (ACC) was evaluated for the oxidative retention of H₂S or SO₂ at room temperature. Change in the surface chemistry was observed by X-ray spectroscopy of nitrogen (N1s) and by temperature programmed desorption (TPD). Dynamic adsorption of H₂S or SO₂ in moist air onto a packed bed of activated carbon cloth was monitored by measurement of the breakthrough curves at room temperature. ACC modified by ammonia showed noteworthy enhanced SO₂ and H₂S loading relative to the untreated ACC. Improved SO₂ retention rate could be replicated several times after regeneration by washing at room temperature, in contrast to the case with H₂S. The likely reasons for the behavior of H₂S and SO₂ on the ammonia-treated ACC are discussed with reference to the recent literature.     

Surface modification of a granular activated carbon by citric acid for enhancement of copper adsorption

In this study, citric acid was used to modify a commercially available activated carbon to improve copper ion adsorption from aqueous solutions. The carbon was modified with 1.0 M citric acid, followed by an optional step of reaction with 1.0 M sodium hydroxide. It was found that the surface modification reduced the specific surface area by 34% and point of zero charge (pH_pzc) of the carbon by 0.5 units. Equilibrium results showed that citric acid modification increased the adsorption capacity to 14.92 mg Cu/g, which was 140% higher than the unmodified carbon. Higher initial solution pH resulted in higher copper adsorption. The chemical surface modification adversely affected the copper adsorption rate. Adsorption kinetic mechanisms were investigated with an intraparticle diffusion model. It was found that the modification did not change both external diffusion and intraparticle diffusion.     

Preparation and properties of phenolic resin-based activated carbon spheres with controlled pore size distribution

Three kinds of phenolic resin-based activated carbon spheres (P-ACS) with different pore size distribution were prepared successfully by adding pore-forming agents to novolac-type phenolic resin. Polyethylene glycol and polyvinyl butyral, serving as pore-forming agents, evaporated during pyrolysis and left a small amount of carbon residue in the matrix of the phenolic resin-based carbon, thus changing the carbonization and activation behavior of the resin. Mesopores between 3 and 5 nm were created in the P-ACS, which possessed excellent adsorption properties for creatinine. Ferrocene has little effect on the carbonization process of the phenolic resin, but has a great impact on the activation process. Mesopores and macropores with a range from 3-5 to 10-90 nm were produced in the P-ACS, which exhibited large adsorption properties for VB₁₂, a larger molecule than creatinine. P-ACS without pore-forming agents exhibited a small specific surface area and mainly micropores, which resulted in a very small amount of creatinine and VB₁₂ adsorbed.     

Oxidative dehydrogenation of ethylbenzene on activated carbon fibers

Activated carbon fibers from different precursors and with different degrees of activation were used as catalysts for the oxidative dehydrogenation of ethylbenzene. Within each group, the fibers exhibited similar surface chemistries, so that the observed catalytic performances could be interpreted exclusively in terms of their textural properties. Analysis of the catalytic results highlighted common trends. In particular, the fibers with an average micropore width larger than 1.2 nm were found to be the best catalysts for this reaction.     

Removal of volatile organic compound by activated carbon fiber

Experiments were carried out to study adsorption/desorption of volatile organic compound (VOC) on the activated carbon fiber (ACF) under dynamic conditions. The primary objective was to experimentally demonstrate the suitability of ACF in effectively adsorbing VOCs from inert gaseous stream under varying operating conditions, and compare its performance vis-à-vis that of the other commercially available adsorbents, such as granular activated carbon (GAC), silica gel, and zeolites. The adsorption experiments were carried out in a fixed tubular packed bed reactor under various operating conditions including temperature (35-100 °C), gas concentration (2000-10,000 ppm), gas flow rate (0.2-1.0 slpm) and weight of the adsorbent (2-10 g). A mathematical model was developed to predict the VOC breakthrough characteristics on ACF. The model incorporated the effects of the gas-particle film mass transfer resistance, adsorbent pore diffusion and the adsorption/desorption rates within the pore. The experimental data and the corresponding model simulated results were compared and found to be in good agreement. The ACF repeatedly showed a good regeneration capability following desorption by DC electrical heating.     

Copper and strontium adsorption by a novel carbon material manufactured from pecan shells

A novel carbon material (PS276a) was produced from pecan shells, a waste product of the agricultural industry. Preparation of this material involved the impregnation of the pecan shell feedstock with a phosphoric acid solution. Activation was followed by a water wash and a sodium hydroxide treatment. The carbon produced was characterized by adsorption of N₂ and revealed a pore structure with an average pore diameter of 74.8 Å. Equilibrium sorption isotherms prepared for this carbon demonstrate that it has a significantly higher capacity for copper and strontium sorption than that of a commercial material used for comparison. A maximum of 95 mg Cu²⁺ and 180 mg Sr²⁺ are adsorbed per gram of this carbon at pH 3.6 and 8.5, respectively. Demonstrated process advantages of this carbon material and preparation technique include low temperature manufacture, in-situ regeneration potential, and adsorbate recovery capability.