Contribution of the edge effect to physical adsorption in micropores of activated carbons

The adsorption process in activated carbon micropores can be viewed as a two-dimensional condensation on micropore walls at a critical condensation pressure, p_c, which evolves into volume filling. p_c on a surface of micropore walls is calculated in respect to a critical condensation pressure on the reference nonporous surface, p_c^ref. Although theoretical treatment predicts that p_c^ref should be a parameter of adsorption isotherms, experimental isotherm equations are expressed in terms of the bulk saturation pressure, p_s. Experimental values of p_s/p_c^ref for a graphitized carbon black, which is often considered as a model of activated carbon surfaces, range from 37 to 4000 for typical adsorbates and are by no means close to unity. It is proposed that this apparent discrepancy between theory and experiments has its origin in the edge effects: carbon blacks are modeled by semi-infinite slabs, whereas the magnitudes of thickness and diameters of micropore walls are of the order of a nanometre. Factors associated with edge effects reduce the adsorption energies on surfaces of finite particles resulting in shifts of condensation pressures to higher values. For particles with diameters about 1.4 and 1.8 nm, adsorption energies decrease to ≈0.72ε₁*^∞ and ≈0.81ε₁*^∞, respectively, where ε₁*^∞ is the energy of adsorption on an infinite plane, and p_c approaches p_s. This phenomenon substantiates the application of p_s in adsorption equations and the success of the Dubinin equations probably owes much to this fact.     

Micropore structure of activated carbons and predictions of adsorption equilibrium

A method for calculating micropore size distributions based on a molecular model of adsorption and analytical solution for the adsorption isotherm is presented in this study. Micropore volume filling is considered to be an evolution of two-dimensional condensation, which occurs on micropore walls at the critical condensation pressure. The treatment of adsorption isotherms of propane, propylene, acetylene, ethylene, cyclopentane and benzene shows that the method offers a quantitative approach to determining a reliable carbon texture, which is independent of adsorbate employed and adsorption temperature. The invariant parameters of porous structure coupled with molecular constants of adsorbate provide a prediction of the adsorption equilibrium in a wide range of pressures. Good agreement between experimental and calculated data is demonstrated for adsorption of both gases and vapors.     

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.     

A systematic investigation of the overall rate coefficient in the Wheeler-Jonas equation for adsorption on dry activated carbons

Modelling of the adsorption rate coefficient, k_v of the Wheeler-Jonas equation for estimating the service life of packed carbon beds, is addressed. Current methods for extracting k_v from experimental breakthrough data include approaches that introduce easily propagated errors. The weaknesses of these approaches are analyzed, and a calculation based on multiple points on the breakthrough curve is suggested. Experimental breakthrough data for a representative set of volatile organic compounds (VOCs) has been measured. A systematic investigation of factors influencing k_v, including adsorbate and carbon properties, adsorbate inlet concentration and flow velocity is performed. It is found that flow velocity and carbon particle size have the largest influence, followed by adsorbate properties related to the adsorption capacity. A simple linear empirical model for k_v, including air flow velocity, carbon particle size, and dielectric constant of the adsorbate, is presented. The model is based on the breakthrough range up to 20% of the inlet concentration, for which k_v is shown to be almost constant. The range of the model covers breathing rates valid for a respirator at different work loads. The model can be used to estimate the adsorption rate coefficient for specific carbon particle sizes and various VOCs present in workplace environments.     

Water adsorption by activated carbons in relation to their microporous structure

The present paper examines the adsorption of water by microporous carbons in the absence of specific interactions. The modelling of water adsorption for 293 and 310 K, using variable pore size distributions (PSD), shows that the type V isotherms follow the Dubinin-Astakhov (DA) equation and fulfill the requirement for temperature invariance. Furthermore, the parameters of the DA equation can be related in a simple way to structural properties of the model carbons. For a number of well-characterized carbons, the type V isotherms generated by combining model isotherms with the corresponding PSDs are in good agreement with the limiting isotherms at 293 and 310 K derived on the basis of a recent development of Dubinin’s theory. This approach will provide the basis for further studies including specific interactions.     

Bituminous coal-based activated carbons modified with nitrogen as adsorbents of hydrogen sulfide

Bituminous coal-based activated carbon was modified by impregnation with melamine and heat treatment at 850 °C. Another sample was impregnated with melamine and urea and heat treated at 650 and 850 °C. Chemical and physical properties of the materials were determined using Boehm titration, thermal analysis, sorption of nitrogen and SEM with EDX. Then the H₂S breakthrough capacity tests were carried out and the sorption capacity was calculated. The results revealed that carbons modified with nitrogen-containing species and heat-treated at 850 °C have a hydrogen sulfide removal capacity exceeding more then 10 times the capacity of unmodified sample. H₂S on the surface of these materials is oxidized to sulfuric acid and elemental sulfur and stored in the pore system. New carbons are hypothesized to act as catalytic reactors promoting two different pathways of hydrogen sulfide oxidation in two different locations. In small micropores, where water is present, hydrogen sulfide dissociate to HS⁻ ions and those ions are oxidized to sulfur radicals and sulfur dioxide leading to the formation of sulfuric acid. In larger pores with incorporated nitrogen, basic sites promote dissociation and formation of sulfur polymers, which are resistant to further oxidation.     

Review and comparisons of D/R models of equilibrium adsorption of binary mixtures of organic vapors on activated carbons

Published models and options for predicting equilibrium adsorption capacities of multicomponent mixtures using single component Dubinin/Radushkevich isotherm equations and parameters were reviewed. They were then tested for abilities to predict total and component capacities reported for 93 binary adsorbed mixtures. The best model for calculating molar distributions was the Ideal Adsorbed Solution Theory (IAST), which balances spreading pressures. Combined with the IAST, total and component capacities were best calculated using either the Lewis or original Bering equation with the Ideal Adsorbed Solution (Raoult’s Law) assumption.     

Gas adsorption on activated carbons from PET mixtures with a metal salt

Activated carbons were prepared from carbonized PET by steam activation via pretreatment by mixing PET with a metal salt [Ca(NO₃)₂·4H₂O, Ca(OH)₂, CaCO₃, ZnO, and AlNH₄(SO₄)₂·12H₂O], and with acid treatment after carbonization. The porous properties of the activated carbons were determined by the nitrogen adsorption method. The adsorption isotherms of CO₂, C₂H₆, nC₄H₁₀ and iC₄H₁₀ at 298 K on the prepared activated carbons were measured to determine practical applications and to obtain a better understanding of the porous structure of the prepared carbons. Steam-activated carbons via pretreatment have a larger mesoporosity than carbons with no pretreatment. The metal salt used in the pretreatment for steam activation has no influence on the microporous structure, but it does influence the mesoporous structure of the prepared carbons. Activated carbons prepared via pretreatment show a large adsorption capacity for nC₄H₁₀ and iC₄H₁₀. These carbons are suitable as adsorbents for canisters, etc. Application of the potential theory to adsorption data for the prepared carbons suggests that the pretreatment contributes to the formation of pores larger than 0.50 nm at high burnoff.     

Dynamic adsorption on activated carbons of SO2 traces in air: I. Adsorption capacities

Sulphur dioxide is an atmospheric pollutant which, among numerous others, has to be eliminated by habitacle filters. Breakthrough curves of low concentration SO₂ streams through beds of activated carbons have been obtained. Two carbons were studied, an activated PAN fiber (CF) and a granulated activated carbon (CN) under SO₂ concentrations lower than 100 ppm. Carbon CN used ‘as received’ is able to trap SO₂ in air at concentrations as low as 2.5 ppm. At this concentration, the adsorption of SO₂ is essentially irreversible. The fraction of reversibly adsorbed SO₂ rapidly increases when SO₂ content in air increases from 2.5 to 100 ppm. As expected, the amounts of SO₂ adsorbed per gram of carbon are much smaller than in the case of high SO₂ contents in air (>1000 ppm). The presence of water in carbon micropores enhances both reversible and irreversible adsorption of SO₂. The reversibly adsorbed part is physisorbed while the irreversibly adsorbed part results in oxidation of SO₂ at the carbon surface. This oxidation was evidenced by TPD from carbon samples after adsorption. The mechanism of SO₂ adsorption is discussed in relation to the mechanisms proposed in literature for high SO₂ contents (>1000 ppm).     

Adsorption of Hg(II) ions from aqueous chloride solutions using powdered activated carbons

Activated carbons were developed from Casurina equisetifolia leaves, by chemically treating with sulfuric acid (1:1) or zinc chloride (25%), at low (425 °C) and high (825 °C) temperatures. The resulting powdered activated carbons were applied for removing mercuric ions from aqueous solution at different agitation times and mercuric ion concentrations. The equilibrium data fitted well the Langmuir adsorption isotherm. The Langmuir adsorption capacities were 12.3 and 20.3 mg g⁻¹ for low temperature carbons and 43.9 and 38.5 mg g⁻¹ for high temperature carbons impregnated with H₂SO₄ and ZnCl₂, respectively. Studies of the effects of carbon dosage, NaCl concentrations and solution pH values were carried out for the more effective, high temperature carbons. Increasing NaCl concentration resulted in a significant decrease in the adsorption efficiency. Adsorption was high from solutions with low and neutral pH values and lower for solutions with alkaline pH values for the high temperature carbons.     

The role of dispersive and electrostatic interactions in the aqueous phase adsorption of naphthalenesulphonic acids on ozone-treated activated carbons

The adsorption of 1-naphthalenesulphonic (NS), 1,5-naphthalenedisulphonic (NDS) and 1,3,6-naphthalenetrisulphonic (NTS) acids on ozonated activated carbons was assessed. Commercial activated carbon (Filtrasorb 400) was treated with different ozone doses in order to study the effect of ozone treatment on their surface properties, and investigate the behaviour of these carbon samples in the adsorption of the above naphthalenesulphonic acids. Experimental results showed that the adsorption capacity sharply decreased as the number of sulphonic groups in the aromatic ring increased. As the concentration of oxygenated electron-withdrawing groups on the carbon surface increased, a significant reduction in adsorption capacity of aromatic sulphonic compounds was observed. These results indicate that the adsorption process takes place mainly by π-π dispersion interactions between the aromatic ring electrons of the naphthalenesulphonic acids and the basal plane of the activated carbon. Moreover, in all cases, the adsorption capacity of aromatic sulphonic acids decreased as the pH increased. This indicates that electrostatic interactions between the adsorbate and the carbon surface also took place, as a result of the ionisation of oxygenated surface groups on the carbon surface. Nevertheless, the dispersive (non-electrostatic) forces govern these adsorption processes.     

Liquid-phase adsorption of dyes and phenols using pinewood-based activated carbons

Physical properties of activated carbons prepared from pinewood at different activation times (0.5, 1.5, 2.7, and 4.0 h) in steam at 900 °C were studied. The adsorption equilibria and kinetics of three dyes and three phenols (phenol, 3-chlorophenol, and o-cresol) from aqueous solutions on such carbons were then examined at 30 °C. The adsorption isotherms of phenols could be well fitted by the Freundlich equation, and those of dyes were adequately described by the Langmuir-Freundlich equation. The effect of microporosity of the carbons on adsorption capacity was explored. Four simplified kinetic models including pseudo-first-order equation, pseudo-second-order equation, intraparticle diffusion model, and the Elovich equation were selected to follow the adsorption processes. The adsorption of all six adsorbates could be best described by the Elovich equation. The kinetic parameters of this best-fit model were calculated and discussed.     

Modelling of water adsorption by activated carbons: effects of microporous structure and oxygen content

The present paper examines the adsorption of water by microporous carbons containing various amounts of surface oxygen and a smaller proportion of basic centres. The modelling of water adsorption for 293 and 310 K, using variable pore size distributions (PSD), confirms that the overall type IV isotherm is the sum of a type I isotherm associated with the specific interactions, and a type V isotherm reflecting the non-specific interactions. The principle of temperature invariance is followed by these isotherms, which indicates that modelling leads to the Dubinin-Astakhov equation.The present approach allows the prediction of water adsorption near room temperature, on the basis of the PSD and the density of oxygen present on the surface area of the micropores. It is assumed, to a first and good approximation, that the pores are slit-shaped and the oxygen distribution is random.     

Transport and sorption of water vapour in activated carbons

The adsorption of water vapour on microporous carbons derived from the carbonization of coconut shell has been studied. The adsorption and desorption characteristics of water vapour on the activated carbons were investigated over the pressure range p/p₀ 0–0.95 in a static water vapour system. In these experiments the process of water adsorption/desorption was investigated by both kinetic and equilibrium experimental data. Activated carbons differing by the degree of burn-off have shown the importance of the microstructure. A carbon enriched with nitrogen functions underlined the influence of the surface chemistry.     

Mercury (II) adsorption by activated carbon made from sago waste

The preparation of activated carbon (AC) from sago industry waste is a promising way to produce a useful adsorbent for Hg (II) removal, as well as dispose of sago industry waste. The AC was prepared using sago industry waste with H₂SO₄ and (NH₄)₂S₂O₈ and physico-chemical properties of AC were investigated. Adsorptive removal of mercury (II) from aqueous solution onto AC prepared from sago industry waste has been studied under varying conditions of agitation time, metal ion concentration, adsorbent dose, particle size and pH to assess the kinetic and equilibrium parameters. Adsorption equilibrium was obtained in 105 min for 20 mg l⁻¹ and 120 min for 30, 40, and 50 mg l⁻¹ Hg (II) concentrations. The Langmuir and Freundlich equilibrium isotherm models were found to provide an excellent fitting of the adsorption data, with r² 0.9999 and 0.9839, respectively. The adsorption capacity of Hg (II) (Q_o) obtained from the Langmuir equilibrium isotherm model was found to be 55.6 mg g⁻¹ at pH 5.0 for the particle size range of 125-250 μm. The percent removal increased with an increase in pH from 2 to 10. This adsorbent was found to be effective and economically attractive.     

Prediction of breakthrough curves for adsorption on activated carbon fibers in a fixed bed

A new model is proposed to describe the removal of volatile organic compounds (VOC) from a gas stream passing through a bed packed with activated carbon fibers (ACFs). Toluene was used as the test compound. Both pore diffusion and surface diffusion are considered in the model. The equilibrium behavior is shown to fit the Dubinin–Radushkevich isotherm with the values of parameters K and W₀ of 1.101 × 10⁻⁹ and 57.73 kg/m³, respectively. The experimental results show that this model can predict VOC breakthrough curve very well.     

Effect of adsorption of nitroaromatic compounds on the characteristics of bound water layers in aqueous suspensions of activated carbons

The impact of chemical treatment (oxidation, reduction) of microporous carbon prepared from plum stones on the structural parameters was analysed using the nitrogen adsorption method. The effects of pre-adsorption of 2,4,6-trinitrotoluene (TNT) and 1-chloro-2,4-dinitrobenzene (CDNB) on the characteristics of the interfacial water layers in the aqueous suspensions of these materials were explored using ¹H NMR spectroscopy with freezing out of the bulk water between 210 and 273 K. Substantial variations in the Gibbs free energy of the interfacial water in the aqueous suspension of treated carbons were observed, especially for a reduced sample with pre-adsorbed nitroaromatics, due to changes in the hydrophilic-hydrophobic properties of the systems.