Surface‐Treated Activated Carbon for Removal of Aromatic Compounds from Water

Modifications of commercial activated carbons by chemical treatment with HNO₃ or HCl and HF and the adsorption behavior of simple aromatic compounds (aniline, pyridine, phenol, and benzene) on activated carbon and modified activated carbon were investigated. The results show that the textural properties change a little after these modifications, but the surface acidity (mainly oxygen‐containing groups) of activated carbon modified with HNO₃ increases greatly. The effect of ash of activated carbon on adsorption of the organic compounds mentioned above is insignificant. However, addition of surface acidity (mainly surface oxygen‐containing groups) decreases the adsorption capacity of compounds significantly. The adsorption uptake of compounds on activated carbon with oxidation of HNO₃ is low possibly due to dispersive interaction, water cluster blocking, or competition between water and compounds adsorbed on activated carbon's surface because of hydrophilic increase of the activated carbon surface. The solubility of aromatic compounds in water has an important effect on the adsorption capacity of activated carbon. q_m and K_L (Langmuir adsorption parameters) for the aromatic compounds vary similarly.     

Effects of oxidative modification of carbon surface on the adsorption of sulfur compounds in diesel fuel

This work examines the effects of modification of activated carbons (ACs) by HNO₃ oxidation and gas-phase O₂ oxidation, respectively, on the liquid-phase adsorption of sulfur compounds in diesel fuel. The adsorption characteristics of the oxidized and the original AC samples were evaluated in a fixed-bed flow system by using a model diesel fuel containing 400 parts per million by weight (ppmw) of sulfur as thiophenic compounds and 10 wt% of aromatics in a paraffinic solvent. The pore structure and surface properties of the AC samples were characterized by N₂ adsorption, SEM, FTIR, XPS and surface pH measurements. The adsorptive selectivity factor of the AC samples increases in the order of benzothiophene (BT) ≈ naphthalene (Nap) < 2-methyl naphthalene (2-MNap) < dibenzothiophene (DBT) < 4-methyldibenzothiophene (4-MDBT) < 4,6-dimethyldibenzothiophene (4,6-DMDBT). It was found that the HNO₃ oxidation was an efficient method in improvement of the adsorption performance of the AC for sulfur compounds. The improved adsorption performance upon the HNO₃ oxidation can be attributed mainly to an increase in the acidic oxygen-containing functional groups. However, the improved adsorption capacity upon oxidation is unlikely due to an increase in mesoporous or microporous surface/volume, although such attribution might have been inferred from the literature. An excellent correlation between the concentration of the surface oxygen-containing functional groups and the adsorption capacity per unit area as well as a good relationship between the adsorption capacity and the surface pH value were observed in this work, which suggest that the adsorption of the sulfur compounds over AC from the liquid hydrocarbon fuel may involve an interaction of the acidic oxygen-containing groups on AC with the sulfur compounds.     

Influence of nitric acid concentration on the characteristics of active carbons obtained from a mineral coal

This paper deals with the effect of the concentration of nitric acid solutions on the properties of activated carbons obtained by the oxidation of a parent activated carbon. For this purpose a mineral coal from Algeria has been used as raw material to prepare the parent active carbon AC. This was further treated with nitric acid solutions. The analysis of the samples includes the chemical and textural characterization. The former was carried out by selective titrations and FTIR spectroscopy. The latter, by nitrogen and carbon dioxide adsorption at 77 and 273 K, respectively, and by adsorption of organic probes (benzene, dichloromethane, cyclohexane and 2,2-dimethyl butane) at 303 K. The nitrogen adsorption isotherms have been analysed by using the BET equation, α_s-method and molecular simulation. The Dubinin-Radushkevich approach has been applied to the carbon dioxide and vapours adsorption data. The results show that the treatment with 2 N nitric acid solution is very appropriate because it introduces a large amount of oxygen containing groups with a small change of the textural characteristics of the parent AC. More concentrated nitric acid solutions change in large extent the textural properties although they also introduce large amount of chemical groups.     

Adsorption of single-ringed N- and S-heterocyclic aromatics on carbon nanotubes

Adsorption of single-ringed N- and S-heterocyclic aromatics on single-walled carbon nanotubes (SWCNTs) was examined to explore the potential of using carbon nanotubes (CNTs) as drug carriers and environmental adsorbents. Adsorbates included pyrimidine, 2-aminopyrimidine, 4,6-diaminopyrimidine, thiophene, benzene and aniline. Adsorbents included pristine SWCNTs, oxidized SWCNTs, and nonporous graphite. Adsorption of N- and S-heterocyclic aromatics was significantly enhanced by non-hydrophobic interactions. Particularly, the -NH₂-substituted compounds exhibited much stronger (up to 2 orders of magnitude) adsorption affinities to oxidized SWCNTs than benzene, even though they are much less hydrophobic. The π-π coupling or electron donor-acceptor (EDA) interactions are likely adsorption-enhancement mechanisms for all six compounds. The lone-pair electrons of the N heteroatoms and the -NH₂ group can enable n-π EDA interactions with SWCNT surfaces. Lewis acid-base interactions are another significant adsorption-enhancement mechanism for the -NH₂-substituted compounds (and possibly for pyrimidine) on SWCNTs. For the N-heterocyclic aromatics, adsorption affinity is highly dependent on the O-functionality of the SWCNT surface and on solution pH, due to the speciation reactions of both adsorbates and SWCNT surface O-functional groups, indicating that selective adsorption of N-heterocyclic aromatics is possible by combining the surface functionality of CNTs and solution chemistry.     

Effect of cationic template on the adsorption of aromatic compounds in MCM-41

The effect of cationic template on the adsorption of aromatic compounds in MCM-41 was investigated in the present work. Various MCM-41 samples were prepared through controlling template removal during the synthesis of MCM-41 materials in which cationic surfactant was used as a template. The properties of synthesized samples were characterized with X-ray diffraction (XRD), nitrogen adsorption, FTIR and thermogravimetric analysis etc. The adsorption equilibriums of toluene, cumene and water on MCM-41 samples were measured using a digital microbalance. Compared with parent MCM-41 with template removed completely (denoted as MCM-41), the samples with template partially removed (denoted as C-MCM-41) exhibited moderate adsorption capacity for aromatic compounds. However, the adsorption equilibrium of water showed that the hydrophobicity on the surface of C-MCM-41 was significantly enhanced. The combination of moderate adsorption capacity for aromatics and hydrophobicity for water is desirable for the applications including removal of VOCs. It was found that the adsorption behaviors of aromatic compounds and water in various MCM-41 samples were governed by both porosity and cationic spots generated by cationic templates; and the former played a more important role than the latter when aromatics were adsorbed. The porosity and cation density of the MCM-41 material can be tailored by controlling the removal of the cationic template from the pore structure. The influence of template extraction methods on the structure and adsorption properties of MCM-41 materials has also been addressed.     

Al-MCM-48 as a potential hydrotreating catalyst support: I – Synthesis and adsorption study

The work has been focused on utilizing mesoporous alumina MCM-48 material as a potential hydrotreating catalyst support for light-cycle oil (LCO) and exploring its adsorption ability for large aromatics. A series of Al-MCM-48 samples with various Si/Al ratios was synthesized via a hydrothermal method and characterized with X-ray diffraction (XRD), nitrogen adsorption isotherms, solid NMR, etc. The adsorption equilibrium isotherms of typical aromatics in LCO such as diaromatic 1-methylnaphthalene on Al-MCM-48 and MCM-48 samples were studied by a standard gravimetric technique. Compared with parent MCM-48, Al-MCM-48 samples exhibited higher adsorption capacity and stronger adsorption affinity towards the addressed aromatic compounds. The incorporation of alumina into MCM-48 materials generated acid centers and more hydroxyl groups on the support surface, which can provide enhanced adsorption for the sorbate molecules and dispersion of active phases. It was found that the adsorption behaviors of the aromatic compounds in Al-MCM-48 samples were mainly governed by the acidity of the support. The balance between pore volume and acid strength of Al-MCM-48 can be adjusted by altering Si/Al ratio. The influence of incorporation of Al on the framework and adsorption properties of the materials has also been investigated.     

Sibunit-based catalytic materials for the deep oxidation of organic ecotoxicants in aqueous solution: I. Surface properties of the oxidized sibunit samples

This article presents the first systematic study of the effect of oxidation under various conditions on the surface composition and catalytic properties of a carbon material of the Sibunit family (which is promising for oxidative treatment of industrial wastewater) in deep oxidation of organic ecotoxicants in aqueous solution. Modification of the surface properties and texture of Sibunit-4 by various oxidative treatment procedures using nitric acid, sodium hypochlorite, hydrogen peroxide, or oxygen as the oxidant is reported. The chemical state of the surface of oxidized Sibunit and its texture have been investigated by a combination of physical and chemical methods (XPS, acid-base titration with bases differing in strength, pH slurry measurements, determination of pH at the point of zero charge, and low-temperature N₂ adsorption). By using different oxidation procedures, it is possible to obtain oxidized Sibunit samples with variable concentrations and natures of oxygen-containing surface groups.     

Adsorption of chromium by activated carbon from aqueous solution

Adsorption isotherms of Cr(III) and Cr(VI) ions on two samples of activated carbon fibres and two samples of granulated activated carbons from aqueous solutions in the concentration range 20–1000 mg/l have been studied. The adsorption isotherms have been determined after modifying the activated carbon surfaces by oxidation with nitric acid, ammonium persulphate, hydrogen peroxide and oxygen gas at 350°C and after degassing at different temperatures. The adsorption of Cr(III) ions increases on oxidation and decreases on degassing. On the other hand, the adsorption of Cr(VI) ions decreases on oxidation and increases on degassing. The increase of Cr(III) and the decrease of Cr(VI) on oxidation and the decrease of Cr(III) and the increase of Cr(VI) on degassing have been attributed to the fact that the oxidation of the carbon surface enhances the amount of acidic carbon–oxygen surface groups while degassing eliminates these surface groups. Thus while the presence of acidic surface groups enhances the adsorption of Cr(III) cations, it suppresses the adsorption of Cr(VI) anions.     

Adsorption of H2S or SO2 on an activated carbon cloth modified by ammonia treatment

The aim of this research is to investigate how ammonia treatment of the surface can influence the activity of a viscose-based activated carbon cloth (ACC) for the oxidative retention of H₂S and SO₂ in humid air at 25 °C. Surface basic nitrogen groups were introduced either by treatment with ammonia/air at 300 °C or with ammonia/steam at 800 °C. The pore structure of the samples so prepared was examined by adsorption measurements. Changes in the surface chemistry were assessed by X-ray photoelectron spectroscopy, X-ray absorption spectroscopy and temperature programmed desorption (TPD). The change of ACC activity could not be merely attributed to surface nitrogen groups but to other changes in the support. Ammonia/steam treatment improved ACC performance the most, not only by introducing nitrogen surface groups, but also by extending the microporosity and by modifying the distribution of surface oxygen groups. Successive adsorption-regeneration cycles showed important differences between oxidative retention of H₂S and SO₂ and the subsequent catalyst/support regeneration process.     

Adsorption of p-nitrophenol on an activated carbon with different oxidations

An activated carbon prepared from olive stones has been modified through oxidation by nitric acid or sodium hypochlorite. These treatments introduced large amounts of oxygen groups, which were characterized by mass-spectrometry, temperature-programmed desorption (DTP-MS). Both CO₂- and CO-evolving groups were created by these oxidation treatments. A part of these oxidized samples was then outgassed under vacuum up to 823 K in order to remove most of the CO₂-evolving groups from their surface. Oxidized samples have a smaller surface area than the original sample. The subsequent partial outgassing increases the surface area which, however, does not reach the value it had before oxidation. p-Nitrophenol (PNP) adsorption isotherms from aqueous solutions were determined at 298 K for the original, oxidized, and partly outgassed samples. The results confirm the presence of an intermediate plateau at low equilibrium PNP concentration (at about 10 mg/l). The relative effects of textural versus surface chemistry on PNP uptakes are then discussed. The presence of CO-evolving groups showed no influence on PNP uptakes. The conclusion is that models in which carbonylic groups are basic adsorption sites for substituted phenols can be ruled out for the entire isotherm of PNP obtained with the original carbon. These models are also unlikely for PNP adsorption on oxidized and partly outgassed samples.     

Influence of electronic properties of Na2O/CaO catalysts on their catalytic characteristics for the oxidative coupling of methane

For Na₂O/CaO catalysts of different sodium content the adsorption of oxygen and their electrical properties were studied by transient experiments and measurements of contact potential differences (CPD) as well as electrical conductivity. CPD results show a change of the mechanism of oxygen activation with increasing sodium concentration due to changing the type of ionic conductivity from cationic to anionic. Anion vacancies are formed by incorporation of sodium into the CaO lattice. As CPDs show, the cation conductivity promotes an accumulation of oxygen species on the catalyst surface resulting in a decrease of C₂ product selectivity for the catalyzed oxidative coupling of methane. The anion conductivity favors a dissociation of molecular adsorbed oxygen and a subsequent incorporation into the oxide lattice, hereby, decreasing its concentration on the catalyst surface which favors in term selective formation of ethane and ethylene. This revised version was published online in July 2006 with corrections to the Cover Date.     

Surface-Treated Activated Carbon for Removal of Phenol from Water

Abstract

Adsorption of phenol from dilute solutions has been studied on porous and nonporous carbons, as well as on ion-exchange resins. At a given equilibrium concentration, uptake of phenol on nonporous carbons per unit area is determined by the nature of the carbon surface. Phenol uptake on porous activated carbons decreases sharply upon surface oxidation. However, progressive elimination of chemisorbed oxygen from the oxidized carbon upon heat treatment at increasing temperatures in N₂ increases the phenol adsorption capacity. The capacity is further enhanced if following heat treatment in N₂ at 950 [ddot]C the samples are reacted with H₂ at 300 [ddot]C. The mechanism of phenol adsorption on carbons has been discussed. Activated carbons are more effective adsorbents for phenol than commercial ion-exchange resins.     

Influence of pitch composition and surface properties of petroleum coke on their interaction during the preparation of carbon/carbon composites

The influence of pitch composition and surface properties of petroleum coke on the interaction of pitch and coke in the composite is studied. The adsorption results and EPR data indicate that recombination of paramagnetic centers and interaction of oxygen functional groups occur during the preparation of the composite. As a result of interaction between petroleum coke and pitch, polar compounds are concentrated in the adsorption layer. The baking criterion is used for the characterization of the baking ability of the pitches obtained as a result of different oxidation treatments of commercial coal tar pitch. It is determined that physico-chemical properties of the obtained pitches influence the value of the “baking criterion”. The kind of thermo-oxidation treatment of the parent pitch should be chosen hence achieving the maximum value of the baking criterion. For this purpose, the maximal part of the pitch has to take part in the adsorption layer.     

Investigation of nitric acid treatment of activated carbon for enhanced aqueous mercury removal

In the present work, Hg(II) adsorption of a commercial activated carbon with and without nitric acid treatment was compared in a batch system. Iodine adsorption test and nitrogen adsorption and desorption experiments were carried out to investigate the changes in porous characteristics during acid treatment. Although the results for iodine adsorption of two samples were approximately similar, the increase in porous characteristics during acid treatment was determined by micropore volume and total pore volume of treated and untreated samples. To evaluate the effects of acid treatment on the surface functional groups, FTIR analysis for both types of activated carbons was performed, and showed oxidized surface for treated sample. Furthermore, composition of the gaseous by-product resulted from this treatment has been qualitatively analyzed using a FTIR device. Consequently, NO, NO₂, N₂O₄, N₂O, CO, and CO₂ were detected. Kinetic and equilibrium adsorption studies were performed considering effective parameters, including contact time, initial pH, and initial concentration. It can be seen that nitric acid treatment of activated carbon has enhanced Hg(II) adsorption capacity. Moreover, kinetic studies showed faster adsorption rate for treated activated carbon through changes in external surface rather than internal.     

A study of the effect of oxygen plasma treatment on the interfacial properties of carbon fiber/epoxy composites

In this work, effects of the interface modification on the carbon fiber‐reinforced epoxy composites were studied. For this purpose, the surface of carbon fibers were modified by oxygen plasma treatment. The surface characteristics of carbon fibers were studied by X‐ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), dynamic contact angle analysis (DCAA), and dynamic mechanical thermal analysis (DMTA), respectively. The interlaminar shear strength (ILSS) was also measured. XPS and AFM analyses indicated that the oxygen plasma treatment successfully increased some oxygen‐containing functional groups concentration on the carbon fiber surfaces, the surface roughness of carbon fibers was enhanced by plasma etching and oxidative reactions. DCAA and DMTA analyses show that the surface energy of carbon fibers increased 44.9% after plasma treatment for 3 min and the interfacial bonding intensities A and α also reached minimum and maximum value respectively. The composites exhibited the highest value of ILSS after oxgen plasma treated for 3 min. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Molecular adsorption on V2O3(0001)/Au(111) surfaces

The adsorption of carbon monoxide (CO), propane (C₃H₈) and propene (C₃H₆) on V₂O₃(0001) films grown on Au(111) was studied by Temperature Programmed Desorption (TPD) and X-ray Photoelectron Spectroscopy (XPS). The “oxidized” surface (i.e., as prepared exhibiting V=O termination), the “reduced” surface (i.e., V=O groups being removed by electron irradiation), as well as the oxygen pre-covered reduced surface were investigated. Both TPD and XPS indicate that the oxidized surface has little affinity for CO adsorption, while the reduced surface readily binds CO (CO amount approx. 10 times higher). Accordingly, CO can be used to titrate the presence or absence of vanadyl oxygen (via adsorption on the vanadium atoms) but also of defects like surface oxygen vacancies. For propane and propene, desorption of the parent molecules was the major process, i.e., surface reactions were absent under the applied conditions. When oxygen was pre-adsorbed on the reduced surface, the adsorption properties resembled that of the oxidized surface, i.e., the vanadyl groups were (partially) re-established. TPD and XPS provide a handle to differentiate the binding sites on the V₂O₃ surface. Dedicated to Prof. Konrad Hayek.     

Methanol Adsorption on V2O3(0001)

Well ordered V₂O₃(0001) layers may be grown on Au(111) surfaces. These films are terminated by a layer of vanadyl groups which may be removed by irradiation with electrons, leading to a surface terminated by vanadium atoms. We present a study of methanol adsorption on vanadyl terminated and vanadium terminated surfaces as well as on weakly reduced surfaces with a limited density of vanadyl oxygen vacancies produced by electron irradiation. Different experimental methods and density functional theory are employed. For vanadyl terminated V₂O₃(0001) only molecular methanol adsorption was found to occur whereas methanol reacts to form formaldehyde, methane, and water on vanadium terminated and on weakly reduced V₂O₃(0001). In both cases a methoxy intermediate was detected on the surface. For weakly reduced surfaces it could be shown that the density of methoxy groups formed after methanol adsorption at low temperature is twice as high as the density of electron induced vanadyl oxygen vacancies on the surface which we attribute to the formation of additional vacancies via the reaction of hydroxy groups to form water which desorbs below room temperature. Density functional theory confirms this picture and identifies a methanol mediated hydrogen transfer path as being responsible for the formation of surface hydroxy groups and water. At higher temperature the methoxy groups react to form methane, formaldehyde, and some more water. The methane formation reaction consumes hydrogen atoms split off from methoxy groups in the course of the formaldehyde production process as well as hydrogen atoms still being on the surface after being produced at low temperature in the course of the methanol ?? methoxy + H reaction.     

Chemical transformations of sulfur compounds adsorbed onto activated carbon materials during thermal desorption

Four commercial activated carbon materials, including two fiber cloths, were studied as adsorbents for three organosulfurs in terms of adsorption and thermal regeneration, the organosulfurs being ethyl mercaptan, isopropyl mercaptan and tetrahydrothiophene. Their performance is interpreted using their inherent porosity and surface functional groups characteristics. The most porous granular carbon with acidic surface functional groups performed better than the other adsorbents at a high organosulfur concentration. However, at a low concentration the fiber cloths exhibit greater adsorption than granular materials, thereby confirming their potential to desulfurize various gases. Thermal desorption was performed in nitrogen and oxidizing atmosphere, the major role of oxygen in the degradation process of the organosulfurs has been pointed out. Mercaptans are oxidized to their respective sulfide, disulfide, sulfur dioxide, minor carbonyl compounds and elemental sulfur, whereas tetrahydrothiophene by-products are limited to thiophene, sulfur dioxide, elemental sulfur and traces of sulfone compound. Based on the various observed by-products, the oxidative degradation pathways for the three organosulfurs are presented in this study.     

Modified activated carbons for catalytic wet air oxidation of phenol

This study aims at testing several activated carbons for the catalytic wet air oxidation (CWAO) of phenol solutions. Two commercial activated carbons were used both as received and modified by treatment with either HNO₃, (NH₄)₂S₂O₈, or H₂O₂ and by demineralisation with HCl. The activated carbons were characterised by measuring their surface area, distribution of surface functional groups and phenol adsorption capacity. The parent and treated activated carbons were then checked for CWAO using a trickle bed at 140 °C and 2 bar of oxygen partial pressure. The treatments increase the acidic sites, mostly creating lactones and carboxyls though some phenolic and carbonyl groups were also generated. Only (NH₄)₂S₂O₈ treatment yields a significant decrease in surface area. CWAO tests show that catalytic activity mainly depends on the origin of the activated carbon. The modifications generally had a low impact on phenol conversion, which correlates somewhat with the increase in the acidity of the carbons. Characterisation of the used activated carbon evidences that chemisorbed phenolic polymers formed through oxidative coupling and oxygen radicals play a major role in the CWAO over activated carbon.