Statistical condensation adsorption isotherms of gas molecules adsorbed on porous adsorbents, surface monolayer adsorption isotherms and hysteresis phenomena

Condensation adsorption isotherms of type IV (or V) according to BDDT classification on porous adsorbents composed of one or two groups of adsorption sites are derived statistically. When Β_c1 (or Β_c2) is less than unity, the isotherm becomes type IV, and when it is greater than unity, the isotherm becomes type V. It is understood that the negative sign(-) of the additional adsorption energy, q, in the nth layer in deriving those theoretical isotherms plays a decisive role on the horizontally flat approach to the axis of the isotherm near the saturation vapor pressure. The values of the surface area can be calculated easily. The pore radii of all the adsorbents which we have obtained by using the derived isotherms with respect to the appropriately selected experimental data agree with those obtained by using the Kelvin equation. Many surface mono-layer adsorption isotherms are obtained in the process of deriving the various adsorption isotherms. From them we can learn that the surface sites are not adsorbed completely even near the saturated vapor pressure, and we can find the range of error by comparing them with v_ms of the BET equation. We could mention through judging the results of a great deal of the experimental isotherm data of types IV and V that “the cause of hysteresis phenomena of the condensation adsorption-desorption of gases is originated from the deviation from the thermodynamically reversible adsorption-desorption process in the condensation adsorption-desorption of gases”.     

Prediction of binary component isotherms for adsorption on heterogeneous surfaces

The adsorption of three basic dyes onto peat has been studied as single and bisolute equilibrium isotherms. The Freundlich equilibrium isotherm, suitable for heterogeneous adsorption systems has been selected to describe the single component data. Furthermore, the extended Freundlich formula for binary systems has been used to describe binary system equilibrium. A very good agreement is obtained between experiment and theory. The dyes under investigation are: Basic Blue 3, Basic Red 22 and Basic Yellow 21 in their single phases and binary solutions, respectively, all onto peat.     

Nonlinear frequency response functions of a chromatographic column—A critical evaluation of their potential for estimation of single solute adsorption isotherms

Nonlinear frequency response analysis is applied to study single component adsorption in chromatographic columns. Single component adsorption isotherms can be estimated from local derivatives of the adsorption isotherm at specified steady-state concentrations, which are obtained from the low frequency asymptotic behavior of the higher order frequency response functions (FRFs) of a chromatographic column. The functions relevant for the estimation of adsorption isotherms are the phases and the first derivatives of these FRFs. These functions (up to the third order) are analyzed for different types of adsorption isotherm models, different isotherm parameters, steady-state concentrations and numbers of theoretical plates of the chromatographic column. Based on this analysis, some unified characteristics of these functions are observed, which can be used for critical evaluation of the method and to design efficient experimental procedures for its application. Advantages and limitations of this method are discussed.     

Study of adsorption isotherms and kinetic models for Methylene Blue adsorption on activated carbon developed from Egyptian rice hull (Part II)

Adsorption of Methylene Blue (MB) from aqueous solutions on activated carbon prepared from Egyptian rice hulls (ERHA) is studied experimentally. Results obtained indicate that the removal efficiency of Methylene Blue at 25 °C exceeds 99% and that the adsorption process is highly pH-dependent. Results showed that the optimum pH lies between 5 and 9. The amount of Methylene Blue adsorbed form aqueous solution increases with the increase of the initial Methylene Blue concentration and temperature. Smaller adsorbent particle adds to increase the percentage removal of Methylene Blue.The results fit the BET model for adsorption of MB on ERHA, corroborating the assumption of that the adsorbate molecules could be adsorbed in more than one layer thick on the surface of the adsorbent.A comparison of kinetic models at different conditions (pH, Temperature, adsorbent particle size, adsorbent dose and adsorbate concentration) applied to the adsorption of Methylene Blue on the adsorbent was evaluated for the pseudo first-order, the pseudo second-order, Elovich and intraparticle diffusion kinetic models, respectively. Results showed that the pseudo second-order kinetic model correlate the experimental data well.     

Statistically modeled adsorption isotherms for the multilayer gas molecules adsorbed on non-porous solid adsorbents of two and three groups sites

The adsorption isotherms with each saturation vapor pressure factor (c_s1, c_s2 or c_s3) for two groups of sites in two cases of the multilayer and for three groups of sites in one case of the multilayer are derived statistically in heterogeneous non-porous solid adsorbents without interactions among the adsorbed molecules. When some sites of BET isotherm are substituted by less energetic sites, the two-group isotherm obtained by the substitution shows less adsorption over the whole range of relative pressure than the BET isotherm prior to the substitution, at any combined values of f₁ with M₁ of the two-group isotherm with the same saturation vapor pressure factor. A method to get the monolayer sites (v_m) from the ratios of the experimental isotherm to the theoretical isotherm at the whole relative vapor pressure minimizing the standard error is suggested. Our two- or three-group isotherms calculated through many experimental adsorption isotherm data selected appropriately provide larger values of v_m than those obtained from BET isotherms. Differential heat vs. v/v_m and Bose-Condensation heat are mentioned.     

Elucidation of the ion binding mechanism in heterogeneous carbon-composite adsorbents

A two-stage analysis of the mechanism of metal ion binding by a heterogeneous adsorbent was developed. In the first stage, a continuous proton affinity distribution was calculated from potentiometric titration data using the CONTIN method with a Langmuir kernel. Electrostatic effects were accounted for using a diffuse layer model. In the second stage, the parameters obtained from the continuous distribution function (i.e. the number of different types of surface sites, site densities and their protonation constants) were utilized in a discrete distribution to represent the adsorbent in surface complexation and double layer models using the GRFIT speciation code. This information on the surface groups was applied to metal ion potentiometric titration experiments to calculate the surface complexation equilibrium constants of the metal ions and hence elucidate the mechanism of ion binding to these sites. The proposed method was applied successfully to the adsorption of Sr and Cu ions on carbon-composite adsorbents, KAU-mod and SCN-mod. The continuous distribution method (CONTIN) revealed three types of surface sites within these carbon-composite adsorbents with pK values ranging between 3.5–4.1, 5.3–6.3 and 7.7–8.2. The analysis of the metal ion adsorption data using the GRFIT speciation code showed that only the first two surface sites were capable of forming surface complexes with the Sr ions, and that only the first site governed the adsorption of the Cu ions.     

A chromatographic theory based on the concept of a layer of equilibrium adsorption

The LEA model has been analyzed by comparing to other Chromatographic theoretical approaches. In the LEA model a layer of equilibrium adsorption, L_e, serves as an effective kinetic constant. For the layer L_e the concentration issuing from this layer is in equilibrium with the average value of adsorption on the whole layer at any moment of time. The value of L_e is determined from all factors which broaden the Chromatographie zone. Such an approach permits us to combine the features of the mass balance (solute continuity in a Chromatographie column), of the mass transfer phenomenon, and of the adsorption isotherm into a single first order differential equation. Apart from other layer-by-layer models, the LEA model supposes that over the whole column a continuous (real) distribution of adsorption a (L, t) and concentration c (L, t) is observed. The usefulness of the LEA model in developing a Chromatographic theory has been discussed.     

Effectiveness of metal-organic frameworks for removal of refractory organo-sulfur compound present in liquid fuels

Adsorption of organo-sulfur compounds present in liquid fuels on metal-organic framework (MOF) compounds is an efficient alternative to the conventional hydrodesulfurization process. It has been demonstrated that the extent of dibenzothiophene (DBT) adsorption at temperatures close to ambient (304 K) is much higher on MOFs systems than on the benchmarked Y-type zeolite. In addition, the DBT adsorption capacity depends strongly on the MOF type as illustrated by the much higher extent of adsorption observed on the copper- (C300) and Al-containing (A100) MOF systems than on the Fe-containing (F300) MOF counterpart. With the aim to investigate the operation in consecutive cycles, the MOFs used in adsorption experiments were regenerated. In addition, the remaining S-containing compounds were identified and quantified by photoelectron spectroscopy (XPS). Examination of S2p core-level spectrum of the adsorbed S-compounds of regenerated MOFs pointed out that a fraction of these molecules become oxidized into S(VI) species.     

Adsorptive potential of Acacia nilotica based adsorbent for chromium(VI) from an aqueous phase

The objective of this research was to enhance adsorption capacity of Acacia nilotica (keekar) sawdust for the abatement of chromium bearing wastewater and to investigate the effect of process parameters on adsorption capacity. The sawdust was activated by acid wash and functionalized subsequently with formaldehyde. Functionalization of activated sawdust raised its chromium removal efficiency of almost 10% as compared to its adsorption removal efficiency of HCl treated sawdust in a batch adsorption study. Adsorption kinetic data provided better fitting with pseudo second order model. Maximum adsorption capacity calculated through the best fitting Langmuir model was 6.34 mg·g^-1 and 8.2 mg·g^-1 for HCl treated and formaldehyde functionalized sawdust adsorbents, respectively. The adsorption of Cr(VI) was endothermic when studied by varying temperature from 20℃ to 50℃ for both activated and functionalized adsorbents.     

Evaluation of soy oil lutein isotherms obtained with selected adsorbents in hexane miscellas

Bleaching of soy oil hexane/miscellas by adsorption of the lutein was examined. Quantitative differences in the adsorption behavior between a bleaching clay, alkaline rice hull ash and acid rice hull ash were measured by comparing the constants,K andn, of Freundlich isotherms. All the isotherms were dose-dependent withK being inversely related andn directly related, to adsorbent mass. Overall, the bleaching clay performed better than the ashes. Alkaline ash had a largerK value than acid ash, but then value of acid ash was greater. Equations of lutein adsorption were derived based on initial concentrations on lutein and the amount adsorbed per gram. Logarithmic plots produced constants,K ⁱ and ni which were the intercept and slope, respectively. As adsorbent dose increased,K ⁱ decreased andn ⁱ increased for each adsorbent studied. These findings may have applications in the processing of dilute miscellas where low temperature, low viscosity oil refining is desired.     

Method for simultaneous determination of sorption isotherms and diffusivity of cement-based materials

A method for simultaneous determination of the diffusion and sorption properties of cement-based materials is presented. It is a gravimetric method where one small specimen is exposed to stepwise changes in relative humidity while its mass is being measured. As sorption in cement-based materials is slow, the change in relative humidity to the next level is made before final equilibrium has been reached on the previous level. Approximate final (equilibrium) levels are found by extrapolation using an exponential equation, and a factor is applied to correct for the fact that the sorption step does not start at equilibrium conditions. A correction for external mass transfer resistance is also included. The method can be used in desorption as well as absorption mode. Measurements of two materials are presented and compared with the results obtained using conventional methods.     

Adsorption and electrothermal desorption of organic vapors using activated carbon adsorbents with novel morphologies

Novel morphologies of activated carbons such as monolith, beads and fiber cloth can effectively capture organic vapors from industrial sources. These adsorbent materials are also unique because they can undergo direct electrothermal regeneration to recover the adsorbed organic vapors for potential re-use. This investigation compares and contrasts the properties of these adsorbents when using electrothermal-swing adsorption. The adsorption systems consisted of an organic vapor generation system, an electrothermal-swing adsorption vessel, a gas detection unit, and a data acquisition and control system. The activated carbon monolith (ACM) had the lowest pressure drop, highest permeability, highest electrical resistivity and lowest cost as compared to the activated carbon beads (ACB) and the activated carbon fiber cloth (ACFC). ACB had the largest throughput ratio and lowest length of unused bed as compared to the other adsorbents. However, ACFC had the largest adsorption capacity for toluene when compared to ACM and ACB. ACFC was also faster to regenerate and had a larger concentration factor than ACM and ACB. These results describe relevant physical, electrical, adsorption and cost properties for specific morphologies of the adsorbents to more effectively capture and recover organic vapors from gas streams.     

The use of exhausted olive cake ash (EOCA) as a low cost adsorbent for the removal of toxic metal ions from aqueous solutions

The removal characteristics of cadmium (Cd(II)) and nickel (Ni(II)) ions from aqueous solution by exhausted olive cake ash (EOCA) were investigated under various conditions of contact time, pH, initial metal concentration and temperature. Batch kinetic studies showed that an equilibrium time of 2 h was required for the adsorption of Ni(II) and Cd(II) onto EOCA. Equilibrium adsorption is affected by the initial pH (pH₀) of the solution. The pH₀ 6.0 is found to be the optimum for the individual removal of Cd(II) and Ni(II) ions by EOCA. The adsorption test of applying EOCA into synthetic wastewater revealed that the adsorption data of this material for nickel and cadmium ions were better fitted to the Langmuir isotherm since the correlation coefficients for the Langmuir isotherm were higher than that for the Freundlich isotherm. The estimated maximum capacities of nickel and cadmium ions adsorbed by EOCA were 8.38 and 7.32 mg g⁻¹, respectively. The thermodynamic parameters for the adsorption process data were evaluated using Langmuir isotherm. The free energy change (ΔG°) and the enthalpy change (ΔH°) showed that the process was feasible and endothermic respectively. As the exhausted olive cake is discarded as waste from olive processing, the adsorbent derived from this material is expected to be an economical product for metal ion remediation from water and wastewater.     

Adsorptive Removal of Rhodamine B with Activated Carbon Obtained from Okra Wastes

This study aimed at preparing and optimizing an activated carbon (OAC) obtained from dry okra wastes by chemical activation with zinc chloride. Also, Rhodamine B removal performance from aqueous solution was analyzed by using this optimized activated carbon. The characterization of the resultant activated carbon, with a high surface area of 1044?m²/g, was carried out using thermogravimetric analysis, Brunauer–Emmett–Teller model, t-plot, N₂ adsorption/desorption isotherms, density functional theory, elemental analysis, Fourier transform infrared spectroscopy, scanning electron microscopy, and the point of zero charge. Furthermore, the effects of operating conditions (contact time, initial concentration, adsorbent dosage, temperature, and pH) on Rhodamine B adsorption onto OAC were investigated. Langmuir model was determined to be the best adsorption process, and the maximum adsorption capacity was calculated to be 321.50?mg/g at 25°C. Also, the intraparticle diffusion and boundary layer diffusion were involved in RhB adsorption onto OAC. Moreover, OAC adsorption curves of Rhodamine B followed pseudo second-order model. At 25°C, Gibbs free energy, enthalpy, and entropy obtained from thermodynamic studies were determined to be ?27.87?kJ/mol, 13.03?kJ/mol, and 0.15?kJ/mol K, respectively. These thermodynamic values revealed that Rhodamine B adsorption onto OAC was feasible, endothermic, physical, and spontaneous.     

The performance of two adsorption ice making test units using activated carbon and a carbon composite as adsorbents

The available adsorption working pairs applied to adsorption refrigeration system, which utilize activated carbon as adsorbent, are mainly activated carbon-methanol, activated carbon-ammonia, and composite adsorbent-ammonia. The adsorption properties and refrigeration application of these three types of adsorption working pairs are investigated. For the physical adsorbents, consolidated activated carbon showed best heat transfer performance, and activated carbon-methanol showed the best adsorption property because of the large refrigerant amount that can be adsorbed. For the composite adsorbents, the consolidated composite adsorbent with mass ratio of 4:1 between CaCl₂ and activated carbon, showed the highest cooling density when compared to the granular composite adsorbent and to the merely chemical adsorbent. The physical adsorption icemaker that employs consolidated activated carbon-methanol as working pair had the optimum coefficient of refrigeration performance (COP), volume cooling power density (SCP_v) and specific cooling power per kilogram adsorbent (SCP) of 0.125, 9.25 kW/m³ and 32.6 W/kg, respectively. The composite adsorption system that employs the consolidated composite adsorbent had a maximum COP, SCP_v and SCP of 0.35, 52.68 kW/m³ and 493.2 W/kg, respectively, for ice making mode. These results are improved by 1.8, 4.7 and 14 times, respectively, when compared to the results of the physical adsorption icemaker.     

Performance evaluation of carbon adsorbents for automobile canisters

To assess the performance of carbon adsorbents for automobile canisters, we have studied the adsorption and desorption of n-butane and gasoline vapor experimentally in a column charged with pelletized carbons, Car-115 and DP-1. Norit Car-115, which is a well-known carbon adsorbent for canisters, was chosen as a standard adsorbent to compare with a domestic pelletized carbon (DP-1). The use of n-butane as a reference compound for the assessment of adsorbents was proven to be very reliable when compared with the adsorption of real gasoline vapors. According to the results obtained, the carbon adsorbent for automobile canisters requires at least two basic characteristics such as the development of well-defined mesopores and high specific area, which are essential for rapid uptake rate and high working capacity.     

Activated carbons with metal containing bentonite binders as adsorbents of hydrogen sulfide

Wood-based activated carbon was ground and mixed with 10% bentonite binders containing either iron, zinc or copper cations adsorbed within the interlayer space and/or on the external surface of bentonite flakes. To better understand the role of transition metals, carbon was also impregnated with iron, zinc and copper salts. The structure of materials after modification was determined using nitrogen adsorption. The modification resulted in a decrease in porosity, especially in micropore volume, as a result of combined mass dilution effect and adsorption/re-adsorption of metals in small pores. Introduction of bentonite binders containing adsorbed metal increased the capacity of carbon for hydrogen sulfide only in the case of material containing copper. Copper also significantly increases the performance of carbon as an H₂S adsorbent when impregnation is applied whereas the effects of other metals used in this study are much less pronounced. It is likely that copper present in the small pores acts as a catalyst for oxygen activation causing hydrogen sulfide oxidation. As a result of this process, elemental sulfur is formed which, when present in small pores, is oxidized to weakly adsorbed SO₂. The SO₂ is removed from the surface when continuous reaction with hydrogen sulfide occurs. Thus, even though binding carbon with spent bentonites after copper adsorption increases the capacity of carbon toward H₂S removal, the formation of SO₂, another undesirable pollutant, does detract somewhat from the procedure.     

Frequency response of the adsorption vessel loaded with inert core adsorbents

An analytical solution for the frequency response of a semi-batch adsorption vessel with sinusoidal modulation of molar flow rate, loaded with inert core adsorbents is obtained for a linear isotherm coupled with intraparticle diffusion and external film diffusion. The low-frequency limiting values of the in-phase and the out-of-phase characteristic functions of the frequency response are found to be explicit functions of the size of the inert core and the external mass transfer parameter. Simulation results of the in-phase and the out-of-phase characteristic functions show that there exist a crossover frequency and overshoot of the in-phase characteristic function when external mass transfer resistance is present.