Water vapor adsorption onto activated carbons prepared from cattle manure compost (CMC)

Activated carbons were prepared from cattle manure compost (CMC) using zinc chloride activation. The structural and surface chemical characteristics of CMC-based activated carbons were determined by N₂ adsorption-desorption and Boehm titration, respectively. The water vapor adsorption properties of the prepared activated carbons with various pore structure and surface nature were examined, and the mechanism of water adsorbed onto activated carbon was also discussed. The results show that the adsorption of water vapor on carbons begins at specific active sites at low relative humidity (RH), followed by micropore filling at medium RH through the formation of pentamer cluster of water molecules in the narrow micropores. The water vapor adsorption capacity of activated carbon is predominantly dependent on its pore volume and surface area. Although capillary condensation is not the mechanism for water adsorption onto activated carbon, water can adsorb on narrow mesopore to some extent.     

Characteristics of adsorption phase with water/organic mixtures at a surface of activated carbons possessing intraparticle and textural porosities

The behaviour of water and water/organic mixtures adsorbed onto activated microporous carbons or a carbon adsorbent with narrow intraparticle micropores and broad mesopores and macropores between nanoparticles was studied using low-temperature adsorption method and ¹H NMR spectroscopy with layer-by-layer freezing-out of liquids at 190-273 K. These investigations revealed concentration-dependent effects of benzene, DMSO, acetone, chloroform, methane and acetonitrile on the characteristics of adsorbed water and the influence of this water on the interfacial behaviour of adsorbed organics. The influence of organics causes the structural and energetic differentiations of adsorbed water. The latter can be displaced by organics from micropores into broader pores and/or form mixture with polar solvents in meso and macropores. Freezing of adsorbed water can affect the adsorbent structure because ice crystallites have a larger size than that of liquid water droplets that lead to changes in the behaviour of adsorbed water/organic mixtures observed by the ¹H NMR and adsorption methods.     

Laboratory simulation of the interaction of water molecules with carbonaceous aerosols in the atmosphere

A number of samples that simulate the chemical composition of carbonaceous aerosols emitted by transport into the atmosphere have been synthesized using the method of deposition of organic compounds and sulfuric acid, which are identified in the particulate coverage of diesel and aircraft engine soot particles, onto the surface of elemental carbon. The analysis of water adsorption isotherms allows one to estimate the influence of the surface chemistry of particles on the degree of their hygroscopicity. Water adsorption measurements show that modification of a particle surface by nonpolar organics (aliphatic and aromatic hydrocarbons) leads to the hydrophobization of a soot surface. The impact of polar oxygen-containing organic compounds (ethers, ketones, aromatic, and aliphatic acids) on adsorption capacity with respect to the water of samples that they modify substantially depends on the nature and composition of the hydrophobic part of the molecules. Among the ionic compounds organic acid salts have the most hydrophilization effect, which is comparable with the adsorption capacity of soot with sulfuric acid deposited on its surface. This observation allows one to quantitatively define how the nature of chemical compounds on soot surface influences water adsorption and to estimate the interaction of water molecules with fossil fuel combustion particles in a humid atmosphere.     

Modification of bamboo-based activated carbon using microwave radiation and its effects on the adsorption of methylene blue

Modification of bamboo-based activated carbon was carried out in a microwave oven under N₂ atmosphere. The virgin and modified activated carbons were characterized by means of low temperature N₂ adsorption, acid-base titration, point of zero charge (pH_pzc) measurement, FTIR and XPS spectra. A gradual decrease in the surface acidic groups was observed during the modification, while the surface basicity was enhanced to some extent, which gave rise to an increase in the pH_pzc value. The species of the functional groups and relative content of various elements and groups were given further analysis using FTIR and XPS spectra. An increase in the micropores was found at the start, and the micropores were then extended into larger ones, resulting in an increase in the pore volume and average pore size. Adsorption studies showed enhanced adsorption of methylene blue on the modified activated carbons, caused mainly by the enlargement of the micropores. Adsorption isotherm fittings revealed that Langmuir and Freundlich models were applicable for the virgin and modified activated carbons, respectively. Kinetic studies exhibited faster adsorption rate of methylene blue on the modified activated carbons, and the pseudo-second-order model fitted well for all of the activated carbons.     

Effect of adsorption of vapours on the electrical conductivity of a series of some naphthyl polyenes: Adsorption and desorption kinetics

This paper deals with the change in semiconductivity of a series of naphthyl polyenes of the type R-(CHCH)_n-R, where R stands for the naphthyl group and n = 1–6, on adsorption of vapours of some organic solvents at a constant sample temperature. Appreciable enhancement in the conductivity is observed. Such enhancement depends on the chemical nature of the solvent and also on the vapour pressure. The desorption is much slower than that of adsorption. Both the adsorption and desorption kinetics follow the modified Roginsky-Zeldovich relation. Kinetics for the compounds having odd-n is distinctly different from those with even-n. The results show that in all these vapour-semiconductor systems, the adsorption is a two-stage process.     

Removal of organic contaminants from aqueous solution by cattle manure compost (CMC) derived activated carbons

The activated carbons (ACs) prepared from cattle manure compost (CMC) with various pore structure and surface chemistry were used to remove phenol and methylene blue (MB) from aqueous solutions. The adsorption equilibrium and kinetics of two organic contaminants onto the ACs were investigated and the schematic models for the adsorptive processes were proposed. The result shows that the removal of functional groups from ACs surface leads to decreasing both rate constants for phenol and MB adsorption. It also causes the decrement of MB adsorption capacity. However, the decrease of surface functional groups was found to result in the increase of phenol adsorption capacity. In our schematic model for adsorptive processes, the presence of acidic functional groups on the surface of carbon is assumed to act as channels for diffusion of adsorbate molecules onto small pores, therefore, promotes the adsorption rate of both phenol and MB. In phenol solution, water molecules firstly adsorb on surface oxygen groups by H-bonding and subsequently form water clusters, which cause partial blockage of the micropores, deduce electrons from the π-electron system of the carbon basal planes, hence, impede or prevent phenol adsorption. On the contrary, in MB solution, the oxygen groups prefer to combine with MB⁺ cations than water molecules, which lead to the increase of MB adsorption capacity.     

Preparation of activated carbons from used tyres by gasification with steam and carbon dioxide

Activated carbons were prepared from waste tyres by gasification with steam and carbon dioxide and their characteristics were investigated. A two-stage activation procedure (pyrolysis at 800 °C in N₂ atmosphere, followed by steam or carbon dioxide activation) was used for the production of activated samples. The effect of the activation temperature (750-900 °C) and the activation time (1-3 h) on the surface characteristics of the prepared carbon was investigated. Carbons produced to different degrees of burn-off were characterized by means of their nitrogen adsorption isotherms at 77 K. In both sets of experiments, the mesopore, micropore volume, and BET surface area increased almost linearly with the degree of activation. For burn-off values lower than 53%, the steam activation produced carbons with a narrower and more extensive microporosity and higher BET and external surface area than the carbon dioxide activation. As the activation proceeds (burn-off > 53%), a strong development of the mesoporosity in the carbons was observed and the micropores size distribution revealed broader micropores, that is, a more heterogeneous distribution.     

Study of the energetic heterogeneity of the adsorption of phenol onto activated carbons by TPD under supercritical conditions

The Temperature Programmed Desorption (TPD) technique with liquid and supercritical water was used to study the heterogeneity of the adsorption of phenol onto different activated carbons. The presence of several desorption peaks in the spectra clearly indicated that phenol was adsorbed onto the activated carbon with different interaction energies. Calculation of the desorption activation energies revealed that most of the phenol was physisorbed, although a small part was always chemisorbed. The study of the textural characteristics of the carbons suggests that the width of the micropores governs the thermal desorption of the physisorbed phenol.     

Characterisation under static and dynamic conditions of commercial activated carbons for their use in wastewater plants

The use of activated carbon for removing organic contaminants in fixed beds is increasing. This is a dynamic process in which the kinetics plays an important role. The aim of this paper is to get more insight into adsorption of p-nitrophenol (PNP) in activated carbon under equilibrium and dynamic conditions. Five commercial activated carbons were studied. The analysis carried out were PNP adsorption isotherms in aqueous solution at 20 °C, N₂ at 77 K isotherms, FT-IR and PNP adsorption under dynamic conditions. The results indicate that the external porous affinity toward the organic contaminants determines in large extent the adsorbents behaviour under dynamic conditions.     

Effects of activated carbon properties on the adsorption of naphthalene from aqueous solutions

The aim of this work was to investigate the role of porous and chemical heterogeneities of activated carbons in the adsorption of naphthalene from aqueous media. A commercially available activated carbon was used as the adsorbent, and its surface heterogeneity was systematically altered by heat treatment at 450 and 850 °C, obtaining a series of carbons with various oxygen contents and similar surface functionalities. The results confirmed that the adsorption of naphthalene depends strongly on the pore size distribution of the adsorbent, particularly narrow microporosity. Moreover, oxygen functionalities reduced the accessibility and affinity of naphthalene to the inner pore structure via formation of hydration clusters. Consequently, the hydrophobic/hydrophilic character of the adsorbent is important, since it dominates the competitive adsorption of water. Adsorbents with a high non-polar character (i.e., low oxygen content) have proven to be more efficient for naphthalene adsorption.     

Adsorption of water—methanol mixtures in carbon and aluminosilicate pores: a molecular simulation study

A theoretical study is reported of the adsorption behaviour of water—methanol mixtures in slit carbon and in uncharged alumino-silicate micropores. The adsorption isotherms are obtained for a pore of width of 2 nm and at a temperature of 298 K from grand canonical ensemble Monte Carlo simulations. The results show that the graphite and uncharged silicate surfaces are covered by a dense layer of flatly adsorbed water and methanol molecules having weaker hydrogen bonding. In the interior of the pore, the fluid exhibits bulk-like behaviour with a stronger hydrogen bonded structure. Solvation forces are also calculated as a function of pore size. The positive values found for the solvation force for all pore sizes reflect the hydrophobic interactions of the mixture with the carbon and uncharged alumino-silicate walls.     

Tailoring the porous texture of activated carbons by CO<Subscript>2</Subscript> reactivation to produce electrodes for organic electrolyte-based EDLCs

Highly microporous carbon obtained by KOH etching of carbohydrates exhibited enhanced specific capacitance due to the increased adsorption of electrolyte ions on its large surface, which renders it a promising electrode material. However, the KOH-activated carbon electrode did not achieve its optimum charge capacity in organic electrolytes due to the limited accessibility of the electrolyte ions to the micropores, which hindered the adsorption of ions. The electrode performance was enhanced by enlarging the micropores of KOH-activated carbon to mesopores via reactivation in a stream of CO₂, which allowed the mesopore/micropore ratio to be increased without compromising the originally high specific surface area. The extended amount of mesopores increased the charge capacity of the electrode by enabling the large organic electrolyte ions to access the porous surface, as compared to untreated KOH-activated carbon.     

有机蒸气的紫外-可见吸收光谱定性检测

采用紫外-可见吸收光谱法定性检测有机物蒸气。用聚乙烯薄膜做介质,用紫外-可见分光光度计检测有机物蒸气,获得大豆油、葵花籽油、花生油、油菜籽油、芝麻油、棉花籽油和油桐种油植物油蒸气,以及丙酮、乙酸乙酯、95%乙醇和冰乙酸有机化合物蒸气的紫外-可见吸收光谱图。结果显示：通过仪器调零,很好地清除聚乙烯薄膜的紫外-可见吸收光谱,获得了所检测植物油蒸气和有机化合物蒸气的紫外-可见吸收光谱,对不同的植物油和不同的有机化合物均有很好的区分性,检测所获得的各植物油蒸气紫外-可见吸收光谱图的λ_max、λ_min、吸收峰的数目、位置、拐点均不一致,可以很好的区分各种植物油,对有机化合物也有很好的检测。方法的重现性好,葵花籽油蒸气10次检测所得的紫外-可见吸收光谱图完全一致。实验结果表明,聚乙烯薄膜作为紫外-可见吸收光谱定性分析用的介质,用于植物油和有机化合物蒸气的检测,具有分析速度快、重现性好、准确可靠、成本低等特点,可提供有机物蒸气的特征信息和有机化合物的结构信息,为化合物和有机物蒸气的鉴定提供新的检测方法。     

The impact of carbon surface chemical composition on the adsorption of phenol determined at the real oxic and anoxic conditions

The results of phenol adsorption-desorption isotherms (at 310 K) measured on the series of activated carbons (D43/1, NORIT RO 0.8, D55/2) are presented. The effect of carbon surface chemical composition on phenol adsorption determined at real oxic and anoxic conditions is discussed. To obtain the real anoxic conditions the two station controlled atmosphere chamber with two catalyst heater units (Plas Labs, Lansing, MI, USA) was applied. It is shown that the adsorption under oxic conditions is always larger than that determined for anoxic ones for all studied carbons. The analysis of those differences shows that in the range of micropore filling they decrease with the equilibrium phenol mole fraction in solution. Contrary they increase after micropores being filled. The average differences between the adsorption properties are the linear function of the concentration of surface acidic groups (assigned from the Boehm's method as “carboxylic”) calculated per the apparent BET surface area of studied carbons.     

Hydrophilic and hydrophobic adsorption on Y zeolites

The uniform large micropores of hydrothermally stable Y zeolites are used widely to confine both polar and non-polar molecules. This paper compares the physisorption of water, methanol, cyclohexane, benzene and other adsorbates over various Y zeolites. These adsorbents are commercial products with reproducibly controllable physical and chemical characteristics. Results indicate that the type I isotherms typical for micropore adsorption can turn into type II or type III isotherms depending on either or both the hydrophobicity of the adsorbent and the polarity of the adsorbate. Methanol produced a rare type V isotherm not reported over zeolites before. Canonical and grand canonical Monte Carlo molecular simulations with Metropolis importance sampling reproduced the experimental isotherms and showed characteristic geometric patterns for molecules confined in Na-X, Na-Y, dealuminated Y, and ZSM5 structures. Adsorbate—adsorbate interactions seem to determine the micropore condensation of both polar and non-polar molecules. Exchanged ions and lattice defects play a secondary role in shaping the adsorption isotherms. The force field of hydrophobic Y appears to exert an as yet unexplored sieving effect on adsorbates having different dipole moments and partial charge distributions. This mechanism is apparently different from both the monolayer formation controlled adsorption on hydrophobic mesopores and macropores and the polarizability and small-pore opening controlled micropore confinement in hydrophobic ZSM5.     

Characterization of synthetic carbons activated with phosphoric acid

The structural heterogeneity of synthetic phosphoric acid activated carbons has been analyzed using pore-size distributions (PSDs) obtained from nitrogen at −196 °C and carbon dioxide at 0 °C isotherms. PSDs where obtained by the BET–Kelvin method. It is shown that the BET–Kelvin method is in good agreement with DFT and provides a fast means for assessment of the porous structure of adsorbents. PSDs obtained by the BET–Kelvin method using different adsorbates give results consistent with each other. Due to the restricted pressure range for carbon dioxide adsorption isotherm the PSD gives information only about pores in the micropore range. The agreement between different methods is better for small micropores.     

BET surface area of carbonaceous adsorbents—Verification using geometric considerations and GCMC simulations on virtual porous carbon models

The applicability of BET model for calculation of surface area of activated carbons is checked by using molecular simulations. By calculation of geometric surface areas for the simple model carbon slit-like pore with the increasing width, and by comparison of the obtained values with those for the same systems from the VEGA ZZ package (adsorbate-accessible molecular surface), it is shown that the latter methods provide correct values. For the system where a monolayer inside a pore is created the ASA approach (GCMC, Ar, T = 87 K) underestimates the value of surface area for micropores (especially, where only one layer is observed and/or two layers of adsorbed Ar are formed). Therefore, we propose the modification of this method based on searching the relationship between the pore diameter and the number of layers in a pore. Finally BET; original and modified ASA; and A, B and C-point surface areas are calculated for a series of virtual porous carbons using simulated Ar adsorption isotherms (GCMC and T = 87 K). The comparison of results shows that the BET method underestimates and not, as it was usually postulated, overestimates the surface areas of microporous carbons.     

The influence of the carbon surface chemical composition on Dubinin-Astakhov equation parameters calculated from SF6 adsorption data-grand canonical Monte Carlo simulation

Using grand canonical Monte Carlo simulation we show, for the first time, the influence of the carbon porosity and surface oxidation on the parameters of the Dubinin-Astakhov (DA) adsorption isotherm equation. We conclude that upon carbon surface oxidation, the adsorption decreases for all carbons studied. Moreover, the parameters of the DA model depend on the number of surface oxygen groups. That is why in the case of carbons containing surface polar groups, SF(6) adsorption isotherm data cannot be used for characterization of the porosity.     

Preparation of activated carbons previously treated with hydrogen peroxide: Study of their porous texture

Cedar wood was used as raw material for the preparation of activated carbons by treatment with hydrogen peroxide of different concentrations. The samples were next carbonised and activated under CO₂ atmosphere. The activated carbons were characterised by means of the adsorption isotherms of N₂ at 77 K, as well as by applying the Density Functional Theory (DFT) method and mercury porosimetry. The experimental results corresponding to the activated samples indicate a more remarkable porous development as a consequence of the treatment with hydrogen peroxide, probably due to the elimination of surface complexes produced during the activation step. The DFT diagrams point out that the activating treatment favours the development of medium and narrow-size micropores whereas the carbonisation process leads to the development of wide micropores of size close to that corresponding to mesopores.