Effect of modification of granulated activated carbons with ozone on their properties

Modification of AG-OV-1 activated carbon with an ozone-oxygen mixture was studied. Changes in the porosity, state of the surface, and adsorption capacity of ozonized samples were examined in relation to the oxidation duration. Explanation are suggested for the patterns obtained.     

Complexity of ammonia interactions on activated carbons modified with V2O5

A micro/mesoporous wood-based activated carbon was modified with different loadings of vanadium pentoxide via incipient impregnation with ammonium vanadate solution followed by heating in nitrogen at 500 degrees C. The materials were used as adsorbents for ammonia. Both adsorption and desorption curves were recorded. The initial and exhausted samples were characterized by Fourier transform infrared spectroscopy (FTIR), potentiometric titration, thermal analysis and adsorption of nitrogen. An improvement in ammonia uptake compared to the virgin carbon was observed, and the adsorption capacity was found linearly dependent on the metal content. Water increases ammonia adsorption capacity via dissolution of the gas, but it also competes with ammonia because both of them are preferentially adsorbed on the same vanadium oxide sites (vanadyl oxygens). Even though an increase in the interactions strength between ammonia and the adsorbents' surface has been reached compared to previous studies, some weakly adsorbed ammonia was still released from the surface during air purging.     

Effect of pore characteristics on electrochemical capacitance of activated carbons

Activated carbons (ACs) for electric double layer capacitors (EDLCs) were fabricated from waste tea leaves, activated with the pore-forming substances ZnCl₂ then, carbonized at high-temperature in N₂ atmosphere. The surface texture and porosity of the ACs were determined using transmission electron micros-copy and N₂ adsorption/desorption studies. The surface area of the 20 wt % ZnCl₂ treated sample was found to be 1029 m²g^?1 and had a distribution of micropores and mesopores. The electrochemical properties of the ACs were evaluated by using cyclic voltammetry and galvanostatic charge-discharge studies. ACs from waste tea leaves exhibited excellent specific capacitance as high as 196 F g^?1 in the 0.1 M Na₂SO₄ neutral electrolyte, with rectangular-like cyclic voltammetry curves at a cell potential of 1.5 V and good cyclability with a capacitance retention of 95% at a high current density of 100 mA g^?1 for 2000 cycles. The results show that the pore texture properties and specific surface area of ACs are dominated by changing carbonization temperature and the amount of activating agent ZnCl₂. The electrochemical performance is influenced mainly by surface area, but the pore size distribution becomes a dominating factor for specific capacitance of a carbon electrode material when the pore structure is in range of micropores/mesopores.     

Chemisorption of o-xylene on activated carbons

It was determined volumetrically that at temperature of gas-phase oxidative catalysis (473–573 K) o-xylene is chemisorbed on SKN and oxidized AG-3 carbons. The chemisorption is weak and reversible and is not accompanied by dissociation of o-xylene molecules, The active chemisorption centers are surface heteroatoms of nitrogen (in SKN carbon) or oxygen (in oxidized AG-3 carbon), promoting polarization of the C-H bond of the methyl group of o-xylene.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 22, No. 3, pp. 370–374, May–June, 1986.     

Effect of reduction treatment on copper modified activated carbons on NO(x) adsorption at room temperature

Activated carbon was impregnated with copper salt and then exposed to reductive environment using hydrazine hydrate or heat treatment under nitrogen at 925 °C. On the obtained samples, adsorption of NO(2) was carried out at dynamic conditions at ambient temperature. The adsorbents before and after exposure to nitrogen dioxide were characterized by X-ray diffraction (XRD), thermal analysis, scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM-EDX), X-ray photoelectron spectroscopy (XPS), N(2)-sorption at -196 °C, and potentiometric titration. Copper loading improved the adsorption capacity of NO(2) as well as the retention of NO formed in the process of NO(2) reduction on the carbon surface. That improvement is linked to the presence of copper metal and its high dispersion on the surface. Even though both reduction methods lead to the reduction of copper, different reactions with the carbon surface take place. Heat treatment results in a significant percentage of metallic copper and a reduction of oxygen functional groups of the carbon matrix, whereas hydrazine, besides reduction of copper, leads to an incorporation of nitrogen. The results suggest that NO(2) mainly is converted to copper nitrates although the possibility to its reduction to N(2) is not ruled out. A high capacity on hydrazine treated samples is linked to the high dispersion of metallic copper on the surface of this carbon.     

Analysis on the removal of ammonia nitrogen using peroxymonosulfate activated by nanoparticulate zero-valent iron

Nanoparticulate zero-valent iron (Fe⁰) was used to activate peroxymonosulfate (PMS) to remove low concentration of ammonia nitrogen in the aqueous system. The removal process was investigated under various conditions. It was indicated that the removal of \({\text{NH}}_{4}^{ + }\) followed the pseudo-first-order kinetic model for the initial reactions. The removal rate increased with the ascending of pH and Fe⁰ dosage, while declined with the ascent of initial \({\text{NH}}_{4}^{ + }\) concentration. The existence of nitrogenous compounds would inhibit the reactions, especially for the compounds with carboxyl structure functional groups. The identification of free radical proved that \(\cdot {\text{SO}}_{4}^{ - }\) is the main radical in Fe⁰/PMS for the removal of ammonia nitrogen. The inorganic products including \({\text{NO}}_{2}^{ - }\), \({\text{NO}}_{3}^{ - }\), Fe²⁺ and Fe³⁺ were detected with the detailed mechanism proposed. The results demonstrated that Fe⁰/PMS process was more effective on ammonia removal compared to single Fe⁰, Fe⁰/persulfate and Fe⁰/H₂O₂. This study proposed a cost-effective process for \({\text{NH}}_{4}^{ + }\) removal at very low concentration of sulfate radicals.     

Reactive adsorption of penicillin on activated carbons

A series of activated carbons with varied surface chemistry, obtained by wet oxidation and thermal treatment, was used for the removal of penicillin from low concentration aqueous solution. It was found that the carbon surface chemistry favors the degradation of the antibiotic, giving rise to various intermediates detected both in solution and in the adsorbed phase (deposited with the pore structure of the activated carbons). The confinement of penicillin molecules entrapped in the nanopores of activated carbons of acidic nature accelerates their degradation compared to that one in the bulk solution, which can be linked the strong local pH fall inside the pores. Degradation also takes place in activated carbons of basic pH, although the nature and partition of the intermediates formed differ from those in the acidic carbons. In both cases most of the breakdown products do not present therapeutic activity.     

Understanding phenol adsorption mechanisms on activated carbons

The interactions between phenol molecules and activated carbons were investigated in order to understand the adsorption mechanism of this aromatic compound. A series of activated carbons with varied chemical composition but similar porous features were synthesized and submitted to phenol exposure from aqueous phase, followed by thermogravimetric analysis and identification of the desorbed species by temperature programmed desorption coupled with mass spectrometry. Based on these experiments, both physi- and chemisorption sites for phenol were identified on the activated carbons. Our results demonstrate that physisorption of phenol depends strictly on the porosity of the activated carbons, whereas chemisorption depends on the availability of the basal planes in the activated carbons. Thus, oxidation of the carbon can suppress the fraction of chemisorbed phenol since the surface functionalities incorporate to the edges of the basal planes; notwithstanding, hydrophilic carbons may present a small but not negligible contribution of chemisorbed phenol depending on the extent of the functionalization. Moreover, these adsorption sites (chemi-) are recovered by simply removal of the surface functionalities after thermal annealing.     

Hydrogen adsorption on functionalized nanoporous activated carbons

There is considerable interest in hydrogen adsorption on carbon nanotubes and porous carbons as a method of storage for transport and related energy applications. This investigation has involved a systematic investigation of the role of functional groups and porous structure characteristics in determining the hydrogen adsorption characteristics of porous carbons. Suites of carbons were prepared with a wide range of nitrogen and oxygen contents and types of functional groups to investigate their effect on hydrogen adsorption. The porous structures of the carbons were characterized by nitrogen (77 K) and carbon dioxide (273 K) adsorption methods. Hydrogen adsorption isotherms were studied at 77 K and pressure up to 100 kPa. All the isotherms were Type I in the IUPAC classification scheme. Hydrogen isobars indicated that the adsorption of hydrogen is very temperature dependent with little or no hydrogen adsorption above 195 K. The isosteric enthalpies of adsorption at zero surface coverage were obtained using a virial equation, while the values at various surface coverages were obtained from the van't Hoff isochore. The values were in the range 3.9-5.2 kJ mol(-1) for the carbons studied. The thermodynamics of the adsorption process are discussed in relation to temperature limitations for hydrogen storage applications. The maximum amounts of hydrogen adsorbed correlated with the micropore volume obtained from extrapolation of the Dubinin-Radushkevich equation for carbon dioxide adsorption. Functional groups have a small detrimental effect on hydrogen adsorption, and this is related to decreased adsorbate-adsorbent and increased adsorbate-adsorbate interactions.     

Sorption removal of cephalexin by HNO3 and H2O2 oxidized activated carbons

Cephalexin’s traces within pharmaceutical effluents have toxic impact toward ecological and human health.Low-cost activated carbon derived from Trapa natans husk was oxidized with hydrogen peroxide and nitric acid,and tested for their ability to remove cephalexin from aqueous solutions.Oxidization with H2O2 showed negative effect on the cephalexin sorption,whereas HNO3 oxidization improved the adsorption.The cephalexin adsorption isotherms on the native and HNO3 oxidized carbons correlated well with the Freundlich equation while their kinetics followed the pseudo-second order model.The removal of cephalexin by the native and HNO3 oxidized carbons was found to be most favored at low ionic strength and strong acidic conditions.Based on the thermal and FTIR analyses,the interaction mechanisms of the interaction between cephalexin and the carbons were proposed.Electrostatic attraction,hydrophobic interaction and chemical bonding with surface functional groups were demonstrated as primary mechanisms for cephalexin removal.The nitrogen functionalities on the carbon surface were considered to be an important factor affecting the adsorption process.     

Energetics of methane adsorption on microporous activated carbons

The influence of microporous carbon surface oxidation on energetics of methane adsorption at 308 K is discussed. Obtained adsorption heats and integral molar entropies of the adsorbate show that microporous carbon surface oxidation changes the methane adsorption process. This is probably resulted by the existence of an endothermic effect during adsorption in oxidized carbon micropores.     

Adsorption of simple aromatic compounds on activated carbons

The adsorption of model aromatic compounds (phenol, aniline, nitrobenzene) on modified activated carbons has been investigated. Electrostatic and dispersive adsorbate/adsorbent interactions are involved in this process. Their influence on the uptake of the above mentioned aromatic compounds has been evaluated using different solution pH conditions and activated carbon samples with different surface chemistries. These samples were obtained by modification of a commercial activated carbon by means of chemical treatment with HNO3 (acid sample) and thermal treatment under a flow of H2 (basic sample). The textural properties were not significantly changed after these modifications. The best uptake for all the adsorptives under most of the pH conditions used corresponded to the basic sample, which means that dispersive interactions are the most important in this process. However, electrostatic interactions cannot be neglected, as can be seen from the uptakes for the same sample at different pH. In the case of aniline at pH 2, electrostatic interactions are predominant, and the best uptake corresponds to the acid sample. The influence of textural properties on the adsorption process was also investigated, by comparing with another commercial activated carbon. As expected, for this type of organic compounds the uptake increases with the micropore surface area.     

Adsorption of water vapor on modified activated carbons

Summary The adsorption isotherms of water vapor on modified activated carbons are measured in order to study the role of various surface groups in the primary adsorption of water molecules on these adsorbents. These adsorption isotherms are analysed by means of the Dubinin-Serpinsky and Jovanovic equations, which take into account the special features of water vapor adsorption on microporous activated carbons. Numerical analysis of the measured adsorption isotherms by means of the above mentioned equations showed their limited applicability for interpreting adsorption mechanism of water molecules on activated carbons.

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Adsorption von Wasserdampf auf modifizierter Aktivkohle

Zusammenfassung Die Adsorptionsisothermen von Wasserdampf auf modifizierter Aktivkohle wurden gemessen, um die Rolle verschiedener Oberflächentypen auf die Primäradsorption von Wassermolekülen auf diesen Adsorbenzien zu untersuchen. Die Adsorptionsisothermen wurden mittels der Dubinin-Serpinsky- und Jovanovic-Gleichungen analysiert, welche die speziellen Eigenheiten von Wasser auf mikroporöser Aktivkohle berücksichtigen. Die numerische Analyse der gemessenen Adsorptionsisothermen mittles der genannten Gleichungen zeigte ihre limitierte Anwendbarkeit zur Interpretation von Adsorptionsmechanismen von Wassermolekülen auf modifizierter Aktivkohle.

     

Modified surface chemistry of activated carbons

In this study, immersion calorimetry was used to characterise different samples of commercial granular activated carbon (GAC) which undergo oxidation with HNO₃ (GACOxN) and thermal treatments to modify its superficial group contents, as well as to determine the textural characteristics of the materials through nitrogen adsorption at 77 K and its superficial chemistry by Boehm titration and zero point of charge. Correlations between the immersion enthalpies and the results of the other techniques of characterisation were established. The immersion enthalpies in dichloromethane obtained were greater, which were found to be between ?88.36 and ?155.6 J g^?1, in contrast to those in carbon tetrachloride, which were found to be between ?50.21 and ?94.29 J g^?1. The dependence of the immersion enthalpies in water on the contents of total acidity and basicity surface groups was also established, and a good correlation between the accessible surface area determined by calorimetric technique and the BET area was found.     

Fire hazard evaluation of activated carbons

Journal of Thermal Analysis and Calorimetry - Activated carbons are widely used in the iodine adsorbers in nuclear plants, but little information about their combustibility is available for fire...     

Multicomponent adsorption on activated carbons under supercritical conditions

Adsorption of binary mixtures onto activated carbon Norit R1 for the system nitrogen-methane-carbon dioxide was investigated over the pressure range up to 15 MPa. A new model is proposed to describe the experimental data. It is based on the assumption that an activated carbon can be characterized by the distribution function of elements of adsorption volume (EAV) over the solid-fluid potential. This function may be evaluated from pure component isotherms using the equality of the chemical potentials in the adsorbed phase and in the bulk phase for each EAV. In the case of mixture adsorption a simple combining rule is proposed, which allows determining the adsorbed phase density and its composition in the EAV at given pressure and compositions of the bulk phase. The adsorbed concentration of each adsorbate is the integral of its density over the set of EAV. The comparison with experimental data on binary mixtures has shown that the approach works reasonably well. In the case of high-pressure binary mixture adsorption, when only total amount adsorbed was measured, the proposed model allows reliably determining partial amounts of the adsorbed components.     

Ionic surfactant adsorption onto activated carbons

The adsorption of sodium dodecyl sulfate onto a set of activated carbons from aqueous solutions has been studied in the low concentration range. The adsorption isotherms are reasonably well fitted by a double Langmuir equation but the calorimetry of adsorption enthalpies shows a rather wide distribution of energies. This distribution is related to direct adsorbate-adsorbent interactions in pores of different size, without noticeable contributions from the chemical nature of the surface. The adsorbate-adsorbent interaction free energy through water is evaluated using the model proposed by van Oss and co-workers for the interfacial free energy. The obtained results indicate that the calculated free energy is in good agreement with that found from application of the double Langmuir equation to the adsorption isotherms.     

Prediction of adsorption behavior of activated carbons

A study was made of the effect of temperature on predictive equations recently developed and applied to gas adsorption by beds of activated and impregnated carbons. Adsorption parameters, obtained for the adsorbate DMMP on small gram quantities of impregnated carbon at 25°C and applied to carbon bed breakthru times, were analyzed for changes resulting from direct temperature effects on gas diffusion, adsorption—desorption equilibria, volume expansion, relative pressure, and adsorbate—adsorbent interactions. Modifications in the adsorption parameters, calculated for bed temperatures ranging between 40.3 and 46.7°C, were used in the kinetic equations to predict breakthru times for M10 gas filters, each containing 13,847 g of carbon. The predicted values compared very well with those experimentally determined, the mean deviation in breakthru time being 5.82%, without regard to sign. A general analysis of a 10°C rise in temperature, from 25 to 35°C, for the M10 gas filter under the test conditions used, showed that the breakthru time would be lowered 20.0 min, 87% of this lowering due to a reduced adsorption rate constant, 9% due to a reduced adsorption capacity, and 4% due to volume expansion effects on concentration and flowrate.     

Influence of surface heterogeneity on hydrogen adsorption on activated carbons

This study presents an experimental and theoretical analysis of the effect of surface heterogeneity on the capacity of 20 commercial activated carbons to adsorb hydrogen at 77 and 258 K and for maximum pressures of 20 bar. Some of the samples have been subjected to surface modification by impregnation or by surface oxidation prior to the hydrogen adsorption measurements. All the activated carbons have been analyzed by N₂ adsorption at 77 K using the thermodynamic isotherm presented in a previous study. The hydrogen adsorption capacity of the activated carbons has been well correlated to the micropore volume and the characteristic m₂ parameter of the thermodynamic isotherm accounting for the energy heterogeneity of the material. On the basis of the model presented here, we discuss how surface heterogeneity, in addition to the adsorption strength, might affect the ability of activated carbons and related materials to adsorb hydrogen.