The influence of the texture and surface properties of carbon adsorbents obtained from biomass products on the adsorption of manganese ions from aqueous solution

Studies on the adsorption of manganese ions from aqueous solution on carbon obtained from a mixture of biomass products indicate the importance of acidic surface oxides for manganese ion adsorption that is predominantly site specific. The results show that oxygen remaining from the raw material participates in the formation of surface oxides and indicates the possibility of controlling the content of acidic surface sites of the carbon surface by appropriate selection of the precursor composition and surface properties modification. The surface functionalities of oxidized carbon from a mixture of biomass products resembles the behavior of an ion-exchange resin. Oxidized carbon obtained from a 50:50 mixture of tar from steam pyrolysis of apricot stones and furfural contains a balance of surface area and high surface concentration of functional groups favorable for adsorption of positively charged manganese ions.     

Preparation and characterization of carbon adsorbents from furfural

Carbon adsorbents with different properties were obtained from furfural with variations in the activation reagents and conditions of treatment. They possess insignificant ash and sulfur contents. Pore volume, pore size distribution and the chemical character of the surface of the obtained carbon adsorbent depend on the activation reagent and temperature of treatment. Various oxygen-containing groups of acidic character (carboxyl groups, carboxyl groups in lactone-like binding, phenolic hydroxyl and carbonyl groups) and different chemical properties are present on the surface of carbon oxidized with air. The sample activated with water vapour contains predominantly oxygen-containing groups with basic character.     

The influence of oxidation on the texture and the number of oxygen-containing surface groups of carbon nanofibers

The effect of liquid-phase oxidation on the texture and surface properties of carbon nanofibers has been studied using XRD, TEM, SEM, N₂-physisorption, TGA-MS, XPS and acid-base titrations. Oxidation was performed by refluxing the nanofibers in HNO₃ and mixtures of HNO₃/H₂SO₄ for different times. The graphite-like structure of the treated fibers remained intact, however, the specific surface area and the pore volume increased with the severity of oxidation treatment. For the first time it is shown that the most predominant effect that gives rise to these textural modifications is the opening of the inner tubes of the fibers. Moreover, it is demonstrated that both the total oxygen content (O/C=0.02-0.07 at/at) as well as the number of acidic groups (1-3 nm⁻²) are a function of the type of oxidizing agent used and the treatment time. The total oxygen content of the oxidized samples turns out to be substantially higher than can be accommodated in the form of oxygen-containing groups at the exterior surface.     

Catalytic activity of carbon nanotubes in the oxidative dehydrogenation of ethylbenzene

Multi-walled carbon nanotubes (MWNT), before and after different oxidative treatments and hence possessing different oxygen surface groups, were used as catalysts for the oxidative dehydrogenation of ethylbenzene (ODE), and their performances compared to those of activated carbon and graphite samples. MWNT are active catalysts for ODE and show the highest specific activity per initial surface area amongst all the materials tested in this study. Moreover, the main advantage of using MWNT over activated carbons is their higher stability under oxidative conditions. It was shown that pre-oxidised MWNT are more active for ODE during the initial stages of the reaction, highlighting the importance of oxygenated 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.     

Simultaneous adsorption of MIB and NOM onto activated carbon. I. Characterisation of the system and NOM adsorption

The objective of this work was to increase the understanding of the adsorption competition between an odour compound, 2-methylisoborneol, and natural organic material (NOM). Part I describes the characterisation of six commercially available activated carbons, undertaken using nitrogen gas adsorption, surface titrations, and Fourier transform infrared spectroscopy. The natural organic material (NOM) from one raw water and four fractions obtained from an isolation and fractionation procedure undertaken on the same raw water, were characterised using ¹³C NMR, high-performance size exclusion chromatography, UV-visible absorbance and elemental analysis. Simultaneous adsorption of NOM and MIB indicated that the adsorption of the NOM was largely dependent on the pore volume distribution of the activated carbons, and less influenced by the variation in surface chemistry. Larger NOM molecules showed greater relative rates of adsorption where the access to the internal structure of the carbon was restricted by size exclusion, due to the shorter diffusion distances to adsorption sites travelled by the larger molecules. As the concentration of MIB was extremely low compared with that of the NOM in these experiments, no effect of MIB on NOM adsorption was seen. Part II reports the significant effect of the NOM solutions on the adsorption of MIB.     

Influence of carbon-oxygen surface complexes on the surface acidity of tungsten oxide catalysts supported on activated carbons

Tungsten oxide catalysts supported on activated carbons were prepared by using tungsten hexacarbonyl, ammonium tungstate, and tungsten pentaethoxide as precursors. An activated carbon was obtained from olive stone by physical activation. A portion of this activated carbon was oxidized with ammonium peroxydisulfate in order to introduce different oxygen surface complexes. Subsequently, different portions of this oxidized activated carbon were heat treated in nitrogen flow at various temperatures to partially remove the oxygen surface complexes. In this way, activated carbons with different amounts of oxygen surface complexes were obtained, which were then used as supports for the tungsten oxide catalysts. Both the supports and the supported catalysts were pre-treated either in He, dry air or wet air flow at 623 K for 6 h. They were then characterized by X-ray photoelectron spectroscopy, X-ray diffraction, measurements of the pH of the point of zero charge, and activity in the decomposition reaction of isopropanol. Turnover frequencies for the formation of propene were obtained. According to these results, the oxygen surface complexes of the support have a major influence on the total acidity of the tungsten oxide supported catalysts. In some supported catalysts, W(VI) was reduced to W(V) during the decomposition reaction of isopropanol as a consequence of the hydrogen evolution. The results indicate that oxygen surface complexes of the support may play an important role in this reduction process, which was inhibited when the support had high surface oxygen content.     

Preparation of carbon adsorbents with high surface area and a model for calculating surface area

Carbon adsorbents (CAs) were made by heat treatment of a mixture of coal char and KOH under the protection of an inert argon flow and the influence of operating conditions on the properties of adsorbents was investigated. Changes in the characteristics of graphitic crystallites during manufacture by X-ray diffraction (XRD) analysis indicated that disappearance of the peak corresponding to 002 faces correlated to high specific surface area. Based on the experimental results, a new model of graphitic crystallites containing hydrogen atoms is proposed to calculate idealized surface area of CAs. With this model, the idealized surface areas of CA can be precisely calculated with the aid of X-ray diffraction. In this way, some experimental problems can be overcome, such as the difficulty to measure specific surface area with N₂ adsorption when pore diameters are too small and overestimation when surface areas are too large.     

NOx adsorption-temperature programmed desorption and surface molecular ions distribution by activated carbon with chemical modification

Activated carbon, the surface of which has been modified with KOH, was used in this study. The study examined adsorption and desorption behaviors and the accompanied surface reaction mechanism as well as the distribution of molecular ions on the surface. The peaks of NO_x desorption behavior may be classified into three bond categories depending on adsorption strength. NO desorption occurs at the earliest stage as chemical adsorption occurs earlier, in a sort of competition, than physical adsorption due to strong basic OH⁻ ion of surface. It was confirmed that the adequate temperature for NO_x desorption was near 390 °C. The potassium that existed on the surface remained without being consumed even with complete desorption of NO_x.     

Effect of the ozone-carbon reaction on the catalytic activity of activated carbon during the degradation of 1,3,6-naphthalenetrisulphonic acid with ozone

The present work investigated the evolution of catalytic activity of activated carbon in 1,3,6-naphthalenetrisulphonic acid (NTS) ozonation in aqueous phase. Activated carbons pre-treated with ozone showed a reduction in NTS oxidation rate and efficacy of TOC removal that increased with ozone exposure time. The ozone treatment increased the number of surface oxygenated (electron-withdrawing) groups on the carbon, therefore reducing its basic character and its reductive properties. This effect reduced the reactivity of the activated carbon to ozone and, therefore, the extent of the ozone decomposition into highly oxidative species.     

Kinetics of 1,3,6-naphthalenetrisulphonic acid ozonation in presence of activated carbon

Processes based on the simultaneous use of ozone and activated carbon have proven very effective for removing contaminants of high toxicity and low biodegradability. The present study is aimed to determine the kinetic constants involved in this purification process and their relationship with the surface chemistry of the activated carbon. For this purpose, the ozonation of 1,3,6-naphthalenetrisulphonic acid (NTS), selected as model compound, was carried out in the presence of different activated carbons. Determination of the Weisz-Prater parameter (C_WP) revealed that intraparticular diffusion limitations exist in the system for particles >500 μm. The degradation kinetics of NTS in the presence of activated carbon depends on the concentrations of both, the contaminant and the dissolved ozone, with a global reaction order of 2. The heterogeneous reaction constants were determined using a model that allowed quantification of the capacity of the activated carbon to increase the NTS degradation rate and of the chemical surface properties responsible for this increase. The basicity of the activated carbon is mainly responsible for the catalytic activity of the carbon in NTS ozonation, even though, mineral matter contributes positively to the catalytic activity.     

The effect of activated carbon surface moisture on low temperature mercury adsorption

Experiments with elemental mercury (Hg⁰) adsorption by activated carbons were performed using a bench-scale fixed-bed reactor at room temperature (27°C) to determine the role of surface moisture in capturing Hg⁰. A bituminous-coal-based activated carbon (BPL) and an activated carbon fiber (ACN) were tested for Hg⁰ adsorption capacity. About 75-85% reduction in Hg⁰ adsorption was observed when both carbon samples’ moisture (∼2 wt.% as received) was removed by heating at 110°C prior to the Hg⁰ adsorption experiments. These observations strongly suggest that the moisture contained in activated carbons plays a critical role in retaining Hg⁰ under these conditions. The common effect of moisture on Hg⁰ adsorption was observed for both carbons, despite extreme differences in their ash contents. Temperature programmed desorption (TPD) experiments performed on the two carbons after adsorption indicated that chemisorption of Hg⁰ is a dominant process over physisorption for the moisture-containing samples. The nature of the mercury bonding on carbon surface was examined by X-ray absorption fine structure (XAFS) spectroscopy. XAFS results provide evidence that mercury bonding on the carbon surface was associated with oxygen. The results of this study suggest that surface oxygen complexes provide the active sites for mercury bonding. The adsorbed H₂O is closely associated with surface oxygen complexes and the removal of the H₂O from the carbon surface by low-temperature heat treatment reduces the number of active sites that can chemically bond Hg⁰ or eliminates the reactive surface conditions that favor Hg⁰ adsorption.     

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.     

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.     

Performance of fixed-bed KOH impregnated activated carbon adsorber for NO and NO2 removal in the presence of oxygen

KOH-impregnated activated carbon (K-IAC) was used in this study. This paper contains observation the adsorption behavior of NO and NO₂ with/without oxygen and with different bed depths of adsorbent. The paper also defines surface chemical changes due to NO_x adsorption. By using a simple design of adsorber, the packed amount of adsorbent for NO_x abatement for 6 months on a pilot scale was calculated. When oxygen was present, NO and NO₂ had a great improvement in adsorptivity. Adsorption of NO₂ forms a oxide crystal on the surface of the K-IAC and at the same time produces NO, which acts to bring about increased adsorptivity. The higher the bed of adsorbent was, the more NO was produced and the longer the breakthrough time took. The adsorber was designed in a scale-up condition where NO, NO₂ and O₂ were applied to K-IAC. The adsorbate that consumed the least packed amount was NO₂-air followed by NO₂-N₂, NO-air and NO-N₂. The results of the experiment demonstrated that with regard to adsorption of NO and NO₂ on K-IAC, the presence of oxygen and the bed depth of adsorbent were the biggest variables to adsorptivity.     

Catalytic oxidation of Fe(II) by activated carbon in the presence of oxygen.: Effect of the surface oxidation degree on the catalytic activity

The catalytic oxidation of Fe(II) species in aqueous solution by activated carbons with different degrees of surface oxidation is described. The parent activated carbon was oxidized with aqueous solutions of nitric acid or hydrogen peroxide, and submitted to thermal treatment at 373, 523 and 773 K. The activated carbons prepared were characterized by N₂ adsorption and temperature-programmed desorption, and their catalytic behavior was determined by measuring the oxidation rate of Fe(II) to Fe(III) and the generation of hydrogen peroxide. Catalytic activity is a function of the nature of oxygen surface groups generated by oxidation.     

Effects of carbon surface chemistry and solution pH on the adsorption of binary aromatic solutes

Adsorption of p-cresol, nitrobenzene and p-nitrophenol on treated and untreated carbons is investigated systematically. The effects of carbon surface chemistry and solution pH are studied and discussed. All adsorption experiments were carried out in pH-controlled solutions to examine the adsorption properties of the adsorption systems where the solutes are in molecular as well as ionic forms. Using the homogeneous Langmuir equation, the single solute parameters are determined. These parameters are then used to predict the binary solute adsorption isotherms and gain further insights into the adsorption process.     

Adsorption and reaction behavior for the simultaneous adsorption of NO-NO2 and SO2 on activated carbon impregnated with KOH

This study examined the individual and simultaneous adsorption of NO_x (NO-NO₂) and SO₂ on activated carbon impregnated with KOH (KOH-IAC). For individual component adsorption, KOH-IAC showed a higher adsorption capacity in NO-NO₂ rich air than in SO₂-air. In the simultaneous adsorption of NO-NO₂-SO₂, SO₂ showed a greater adsorption affinity than NO-NO₂. The smaller the amount of NO-NO₂ adsorbed, the more SO₂ was adsorbed. XPS analysis of the adsorption of NO-NO₂ rich SO₂-air on KOH-IAC revealed that the adsorbed SO₂ was predominantly found on the external surface, producing mainly K₂SO₄ and, additionally, H₂SO₄ and K₂SO₃. Depth profile analysis showed that the amount of SO₂ adsorbed decreased regularly away from the surface, while the amount of adsorbed NO-NO₂ increased irregularly. We confirmed that the presence of the impregnant in KOH-IAC is a determining factor in the adsorption of NO-NO₂ and SO₂ by chemical reaction, clarifying the surface chemical behavior.