Adsorption of dibenzothiophenes on activated carbons with copper and iron deposited on their surfaces

Three polymer-derived carbons with iron and different amounts of copper on the surface were investigated as adsorbents of dibenzothiophene (DBT) and 4, 6-dimethyldibenzothiophene (DMDBT) from simulated diesel fuel. To characterize the initial and exhausted carbons nitrogen adsorption, elemental analysis, X-ray diffraction (XRD), scanning electron microscopy (SEM) and thermal analysis (TA) were applied. The selectivities for DBT and DMDBT were calculated with reference to naphthalene. In spite of the enhancement in selectivity for DBT and DMDBT removal caused by iron and copper species, the results indicate that the volume of micropores is the main factor governing the amount adsorbed. Oxidation of benzothiophenes is likely promoted by iron oxide and copper/copper oxide highly dispersed on the surface. The oxidation products are then selectively adsorbed on the surface of carbon in larger pores where metal species and heteroatoms are the active centers for adsorption of polar species.     

Catalytic oxidation of hydrogen sulphide on activated carbons

Activated carbon is a suitable adsorbent for removal of hydrogen sulphide from natural, synthesis or other product gases. The process depends predominantly on physical adsorption, though catalytic oxidation is also involved. During catalytic oxidation the H₂S is converted in the presence of oxygen to elemental sulphur, which is adsorbed onto the internal surface of the activated carbon, thus leading to a sulphur load of up to 120% by weight. The oxidation rate depends on the partial pressure of both reactants, H₂S and O₂ and is largely controlled by the characteristics of the activated carbon. The activity of the catalyst can be improved by impregnating the activated carbon with promoters such as iron and iodine. The regeneration of spent carbon is currently carried out using hot gas desorption methods at temperatures around 450 °C.     

Photochemical behaviour of activated carbons under UV irradiation

A series of activated carbons were used to investigate the photochemical behaviour of carbons under UV light as catalysts in the photo-oxidation of phenol in the absence of semiconductor additives. Conventional photocatalytic tests showed an improved photo-oxidation in the presence of activated carbons, beyond the so-called synergistic effect reported in the literature for carbon/titania composites. A novel approach based on UV irradiation of carbons pre-loaded with phenol was used to demonstrate the anomalous photochemical response of carbon materials towards phenol degradation. Analysis of the catalytic reaction from a different standpoint (inside the carbonaceous matrix) demonstrated the catalytic activity of certain carbon materials for phenol photodegradation, without considering photolytic breakdown and adsorption kinetics. The pseudo photochemical quantum yield of several activated carbons was higher than that of photolysis under the same conditions; the nature of the degradation intermediates was also modified in the presence of the carbon materials. Moreover, the degradation of the adsorbed fraction retained inside the pore structure of the carbons has been demonstrated. Our results suggest the occurrence of carbon–photon interactions which could be propagated through the graphene sheets of the materials, and could reach the adsorbed molecules inside the pores.     

Kinetic study of the oxidation resistance of phosphorus-containing activated carbons

Chemical activation of different lignocellulosic materials with phosphoric acid produces activated carbons with higher oxidation resistance than that observed for catalyst free chars obtained at similar conditions from the same biomass, even though the activated carbons have a more developed porous structure. The main reason for such behavior is probably related to the presence of thermally stable phosphorus complexes that remain on the carbon surface after the activation process. XPS analyses point out the oxidation of the phosphorus reduced groups to C–O–PO₃/(CO)₂PO₂ prior to carbon gasification. The latter complexes seem to stabilize the active carbon sites. Moreover, the presence of these phosphorus inhibitors produces a change in the gasification mechanism of the activated carbons with respect to that for char. Char oxidation seems to proceed in the entire available particle surface, while activated carbon gasification is better explained by the shrinking unreacted core model. Such a difference in mechanism is attributed to the inhibition effect of the phosphorus complexes, which reduces the reactivity of carbon active sites, and could act as a physical barrier for oxygen diffusion in the micropores.     

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.     

Influence of oxidation process on the adsorption capacity of activated carbons from lignocellulosic precursors

A set of activated carbon materials non-oxidised and oxidised, were successfully prepared from two different lignocellulosic precursors, almond shell and vine shoot, by physical activation with carbon dioxide and posterior oxidation with nitric acid. All samples were characterised in relation to their structural properties and chemical composition, by different techniques, namely nitrogen adsorption at 77 K, elemental analysis (C, H, N, O and S), point of zero charge (PZC) and FTIR. A judicious choice was made to obtain carbon materials with similar structural properties (apparent BET surface area ∼ 850-950 m²g⁻¹, micropore volume ∼ 0.4 cm³g⁻¹, mean pore width ∼ 1.2 nm and external surface area ∼ 14-26 m²g⁻¹). After their characterisation, these microporous activated carbons were also tested for the adsorption of phenolic compounds (p-nitrophenol and phenol) in the liquid phase at room temperature. The performance in liquid phase was correlated with their structural and chemical properties. The oxidation had a major impact at a chemical level but only a moderate modification of the porous structure of the samples. The Langmuir and Freundlich equations were applied to the experimental adsorption isotherms of phenolic compounds with good agreement for the different estimated parameters.     

Preparation of sulfur-doped microporous carbons for the storage of hydrogen and carbon dioxide

Structurally well ordered, sulfur-doped microporous carbon materials have been successfully prepared by a nanocasting method using zeolite EMC-2 as a hard template. The carbon materials exhibited well-resolved diffraction peaks in powder XRD patterns and ordered micropore channels in TEM images. Adjusting the synthesis conditions, carbons possess a tunable sulfur content in the range of 1.3–6.6 wt.%, a surface area of 729–1627 m² g^?1 and a pore volume of 0.60–0.90 cm³ g^?1. A significant proportion of the porosity in the carbons (up to 82% and 63% for surface area and pore volume, respectively) is contributed by micropores. The sulfur-doped microporous carbons exhibit isosteric heat of hydrogen adsorption up to 9.2 kJ mol^?1 and a high hydrogen uptake density of 14.3 × 10^?3 mmol m^?2 at ?196 °C and 20 bar, one of the highest ever observed for nanoporous carbons. They also show a high CO₂ adsorption energy up to 59 kJ mol^?1 at lower coverages (with 22 kJ mol^?1 at higher CO₂ coverages), the highest ever reported for any porous carbon materials and one of the highest amongst all the porous materials. These findings suggest that S-doped microporous carbons are potential promising adsorbents for hydrogen and CO₂.     

Tailoring activated carbons for enhanced removal of natural organic matter from natural waters

Several pathways have been employed to systematically modify two granular activated carbons (GACs), F400 (coal-based) and Macro (wood-based), for examining adsorption of dissolved natural organic matter (DOM) from natural waters. A total of 24 activated carbons with different physical and chemical characteristics was produced. The impact of carbon treatment on the DOM adsorption was examined by conducting isotherm experiments at a neutral pH using the modified carbons and a DOM isolated from the influent to Myrtle Beach drinking water treatment plant in South Carolina (USA). Adsorption of the DOM by two activated carbon fibers, with relatively uniform pore size distributions, showed that only pores with widths larger than 1 nm were accessible to the DOM macromolecules. Increases in the carbon supermicropore and mesopore volume (i.e., >1 nm) increased the DOM uptake, if the surface chemistry was favorable. The isotherms normalized on a surface area basis showed the significance of carbon surface chemistry on the DOM uptake. At neutral pH, adsorption of negatively charged DOM molecules was favored by basic and positively charged surfaces, while the DOM uptake was minimized when the surface had acidic characteristics. High temperature ammonia treatment of oxidized carbons considerably enhanced the DOM uptake, mainly due to the increase in accessible surface area and surface basicity. Iron-impregnated carbons indicated an enhanced affinity of iron-laden carbon surface toward the DOM species, if the surface was not negatively charged.     

Amination and ammoxidation of activated carbons

Four commercially available carbons were subjected to ammonia and ammonia/oxygen gas mixtures at temperatures between 200 and 420°C. The pore structure of the carbons, as determined by nitrogen porosimetry, was virtually unchanged after the treatments. The surface chemistry of aminated and ammoxidized carbons was found to differ from that of carbons treated with ammonia at high temperatures (600 to 900°C). Probably imide-, amide-, and lactam-type surface species are formed as indicated by FTIR, TPD-MS, acid/base-titration, and elemental analysis. The amide-lactam-imide ratio depends on the modification technique; these surface groups can be converted to aromatic nitrogen species by heat treatment above 600°C. Nitrogen contents and surface fractions are obtainable of 4.4 wt% and 0.14, respectively, for amination and of 3.2 wt% and 0.11 for ammoxidation. A high nitrogen incorporation efficiency on applied ammonia can be reached for ammoxidation (58%). The lactam, imide, and amide groups enhance the absorption of the graphitic plane at 1600 cm⁻¹ in FTIR spectroscopy by induction. The intensity of this absorption is generally related to the amount of oxygen.     

The significance of physical factors on the adsorption of polyaromatic compounds by activated carbons

The adsorption of three synthetic organic compounds (SOCs) (i.e., phenanthrene, biphenyl, and 2-chlorobiphenyl), with similar physicochemical properties but different molecular conformations (i.e., planar and nonplanar), by an activated carbon and an activated carbon fiber was investigated. The physical characteristics of the carbons were obtained from low temperature nitrogen adsorption isotherms using BET, DR, and DFT models. Their surface chemistry was characterized by water vapor adsorption, pH of the point of zero charge, acid/base uptakes, and elemental analysis. The results indicated that adsorbent pore structure characteristics and adsorbate molecular conformation are important in the adsorption of SOCs by porous carbonaceous adsorbents. To predict the adsorption of SOCs by activated carbons, accurate characterization of pore shape and size distribution in primary micropores is important. The results indicated that adsorbate molecules can access and fill more efficiently the slit-shape pores than ellipsoidal pores, whereas the ellipsoidal pores create higher adsorption potential than slit-shape pores. Both molecular conformation and dimensions of adsorbate affect the adsorption. Planar molecules appear to access and pack in slit-shape pores more efficiently as compared to nonplanar molecules. Nonplanar molecular conformation weakens the interactions between adsorbate molecules and carbon surfaces.     

Removal of dibenzothiophenes in kerosene by adsorption on rice husk activated carbon

The capacity of rice husk activated carbon (RHAC) to adsorb refractory sulfur compounds of dibenzothiophenes (DBTs) from commercial kerosene was evaluated in terms of their textural and chemical characteristics. Rice husk activated at 850 °C for 1 h showed an acceptable adsorption capacity for DBTs, despite a much lower specific surface area (473 m²/g) and total pore volume (0.267 cm³/g), when compared to micro-porous activated carbon fiber with a large specific surface area (2336 m²/g) and total pore volume (1.052 cm³/g). The volumes of ultramicropores acting as DBTs adsorption sites, and of mesopores leading DBTs into the ultramicropores were closely related to the DBTs adsorption capacity of the RHACs.     

H2S adsorption/oxidation on unmodified activated carbons: importance of prehumidification

Three microporous activated carbons supplied by Norit^® (of peat and bituminous coal origin) were used in this study as hydrogen sulfide adsorbents. Their surface properties were evaluated by means of nitrogen adsorption, Boehm titration, potentiometric titration, and thermal analysis. The results show that the carbons significantly differ in their pore structure and surface chemistry. This is reflected in their hydrogen sulfide breakthrough capacity. The breakthrough capacity is underestimated when not enough water is adsorbed on the carbon surface. The performance follows the expectations after extensive humidification of the sorbents’ surfaces. Moderately low pH in the acidic range of coal-based carbon, Vapure 612, promotes the oxidation of H₂S to sulfur oxides which is important from the point of view of water regeneration. The high pH of peat-based carbon, RB 4, results in H₂S oxidation to elemental sulfur.     

紫外过硫酸盐法去除水中聚丙烯酰胺的影响因素分析研究

采用紫外过硫酸盐法去除水中聚丙烯酰胺(PAM),研究了过硫酸盐投加量、pH值、PAM初始浓度以及氯离子(Cl^-)等因素对PAM去除效果的影响,并和紫外/TiO_2/H_2O_2法对聚丙烯酰胺的去除效果进行比较。结果表明,在室温20℃、PAM初始质量浓度为100 mg/L、过硫酸盐投加量为0.75 mmol/L、pH为9.0时,通过紫外灯光照,反应120 min后,PAM去除率可达90.5%。     

紫外过硫酸盐法去除水中聚丙烯酰胺的影响因素分析研究

采用紫外过硫酸盐法去除水中聚丙烯酰胺(PAM),研究了过硫酸盐投加量、pH值、PAM初始浓度以及氯离子(Cl~-)等因素对PAM去除效果的影响,并和紫外/TiO_2/H_2O_2法对聚丙烯酰胺的去除效果进行比较。结果表明,在室温20℃、PAM初始质量浓度为100 mg/L、过硫酸盐投加量为0.75 mmol/L、pH为9.0时,通过紫外灯光照,反应120 min后,PAM去除率可达90.5%。     

New developments in the theory and modeling of mercury oxidation and binding on activated carbons in flue gas

Recent advances in the mechanistic understanding of mercury oxidation on a carbon surface in flue gas are reviewed in this paper. Theoretical calculations were performed to determine whether the energetics are feasible for a proposed detailed model for oxidative addition of elemental mercury on a carbon edge structure. The results of the calculation show that mercury complexation with a carbenium ion formed at a zigzag edge carbon has a small positive ΔG, but attack of chloride on the complex will proceed with negative ΔG. The energetics rule out a direct covalent bond formation between mercury and the carbenium ion. Alternative concerted reaction models and double-charged models for the mechanism are also feasible but have not yet been computed.     

Removal of ammonium ions from aqueous solutions with coal-based activated carbons modified by oxidation

The main purpose of this work is to study the possibilities for removal of ammonium ions from aqueous solutions by the two coal-based activated carbons (one obtained from Bulgarian lignite from the Chukurovo deposit, and the second, available as commercial product) and their oxidized modifications. The porous texture and surface chemistry of the adsorbents were characterized. Adsorption of ions was investigated using solutions with different concentrations in the range 35-280 mg l⁻¹ in a static mode. Equilibrium modeling data were fitted to linear Langmuir’s and Freundlich’s equations and maximum adsorption capacities were calculated.     

Iodine adsorption and structure of activated carbons

Studies were conducted on the adsorption of iodine from saturated aqueous solutions and from saturated vapor by eight activated carbons of greatly diverse pore structures. Water adsorption data were used to determine the pore size distribution curves which provided both the distributions of the pore constriction (desorption) and cavity (adsorption) diameters. Adsorption from aqueous phase formed a unimolecular layer on the carbon surface while adsorption from saturated vapor produced pore-filling of micropores (pores less than 30 Å diameter) and surface coverage of the macropores. A great deal of steric interference was present because of the small difference in the diameter of iodine molecule (4.94 Å) relative to the 10–30 Å diameter pores. Good correlation was attained between adsorption and pore structure when corrections were made for the steric effect and the mean diameter distributions of the constrictions and cavities were used. The model for the iodine-on-carbon adsorption resembles packing of spheres into cylinders.