Estimation of adsorption energies using the physical characteristics of activated carbons and the molecular properties of volatile organic compounds

Adsorption of volatile organic compounds (VOCs) by granular activated carbons (GACs) is a highly exothermic process and leads to temperature rises, which may reduce the separation efficiency. This study points out the significant characteristics of VOCs and GACs on adsorption energies. Adsorption energies were measured for a wide variety of VOCs, representative of different chemical groups, using 8 different commercial GACs with different porous structures. Afterwards a statistical analysis was applied to the experimental database thus obtained, which enabled one to pinpoint the most significant variables, linked to either VOC molecular properties or the intrinsic characteristics of GACs. Two statistical models have been tested: multi linear regression (MLR) and neuronal networks, and their efficiencies were compared in terms of prediction skill. The best results have been obtained from the MLR approach, which discriminated five different properties of the system. These variables were the polarisability, the heat of vaporization, the ionization potential and the surface tension for adsorbates and the mean micropore radius for GACs. The MLR model enabled one to compute integral adsorption enthalpies with a precision of around 10% and to draw conclusions on the dominant adsorption mechanisms.     

The role of adsorbent pore size distribution in multicomponent adsorption on activated carbon

The impact of adsorbent pore size distribution (PSD) on adsorption mechanism for the multi solute system was evaluated in this study. Anoxic and oxic adsorption equilibrium for the single solute (phenol), binary solute (phenol/2-methylphenol) and ternary solute (phenol/2-methylphenol/2-ethylphenol) systems on one granular activated carbon (GAC) F400 and two types of activated carbon fibers (ACFs), namely, ACC-10 and ACC-15, were determined. F400 has a wide PSD, while ACC-10 and ACC-15 have narrow PSD and their critical pore diameters are 8.0 Å and 12.8 Å, respectively. In single solute adsorption, the increase of adsorptive capacity under oxic conditions as compared to anoxic ones was related to the PSD of the adsorbent. Binary solute adsorption on ACC-10 and ternary solute adsorption on ACC-15 indicated no impact of the presence of molecular oxygen on the adsorptive capacity and the adsorption isotherms were well predicted by the ideal adsorbed solution theory (IAST). Significant differences between oxic and anoxic isotherms were noticed for other multicomponent adsorption systems. The narrow PSD of ACFs was effective in hampering the oligomerization of phenolic compounds under oxic conditions. Such a phenomenon will provide accurate predictions of fixed bed adsorbers in water treatment systems.     

Effect of adsorption deformation on thermodynamic characteristics of a fluid in slit pores at sub-critical conditions

Solvation pressure due to adsorption of fluids in porous materials is the cause of elastic deformation of an adsorbent, which is accessible to direct experimental measurements. Such a deformation contributes to the Helmholtz free energy of the whole adsorbent-adsorbate system due to accumulation of compression or tension energy by the solid. It means that in the general case the solid has to be considered as not solely a source of the external potential field for the fluid confined in the pore volume, but also as thermodynamically noninert component of the solid-fluid system. We present analysis of nitrogen adsorption isotherms and heat of adsorption in slit graphitic pores accounting for the adsorption deformation by means of nonlocal density functional theory.     

Effect of ZnO loading to activated carbon on Pb(II) adsorption from aqueous solution

The effect of zinc oxide loading to granular activated carbon on Pb(II) adsorption from aqueous solution was studied in comparison with zinc oxide particles and oxidized activated carbon. Cu(II), Cd(II) and nitrobenzene were used as reference adsorbates to investigate the adsorption. The BET surface area and point of zero charge (pH_PZC) in the aqueous solution were measured for the adsorbents. The adsorption isotherms were examined to characterize the adsorption of heavy metals and organic molecules. The heavy metal adsorption was improved by both the zinc oxide loading and the oxidation of activated carbon. In contrast, the adsorption of nitrobenzene was considerably reduced by the oxidation, and slightly decreased by the zinc oxide loading. The zinc oxide loading to the activated carbon was found to be effectively used for the Pb(II) adsorption whereas only a part of surface functional groups was used for the zinc oxide particles and the oxidized activated carbon. From the experimental results, the surface functional groups responsible for the Pb(II) adsorption on the zinc oxide loaded activated carbon were considered to be hydroxyl groups that formed on the oxide, while those on the oxidized activated carbon were considered to be carboxylic groups.     

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.     

Adsorption of resorcinol and catechol on granular activated carbon: Equilibrium and kinetics

Investigations were conducted in the batch mode for studying the adsorption behavior of resorcinol and catechol on granular activated carbon from a basic salt medium (BSM) at pH≈7.1 and temperature≈30 °C. The isotherm data were correlated with six isotherm models, namely Langmuir, Freundlich, Redlich–Peterson, Radke–Prausnitz, Toth, and Fritz–Schlunder’s using a nonlinear regression technique. It is observed that the catechol isotherm data may be represented by Redlich–Peterson, Radke–Prausnitz, Toth, and Fritz–Schlunder models with similar accuracy (max. dev. 12%). And the resorcinol data may be represented by Freundlich, Redlich–Peterson, Radke–Prausnitz, and Fritz–Schlunder models equally well (max. dev. 15%). Freudlich being a simple model is recommended for resorcinol. At the conditions investigated in this study, catechol is adsorbed to a greater extent than resorcinol. This is due to the compound’s solubility and position of the –OH group on the benzene aromatic ring. The kinetics of adsorption have been found to be diffusion controlled and the value of effective particle diffusion coefficients is of the order of 10⁻¹³ m²/s. Three distinct phases of kinetics—rapid, medium, and slow—have been observed. These results should be useful for the design of adsorbers for removing these pollutants.     

Estimation of adsorption energies using physical characteristics of activated carbons and VOCs’ molecular properties

Adsorption of volatile organic compounds (VOCs) by granular activated carbons (GACs) is a highly exothermic process and leads to temperature rises, which can be hazardous for high pollutant contents. This study points out the significant characteristics of VOCs and GACs on adsorption energies. For that purpose, adsorption energies were measured for a wide variety of VOCs, representative of different chemical groups, using eight different commercial GACs with different porous structures. Afterwards a statistical analysis was applied to the experimental database thus obtained, which enabled to enlighten the most significant variables, linked to either VOCs’ molecular properties or intrinsic characteristics of GACs. Two statistical models have been tested: multi-linear regression (MLR) and neuron networks (NN), and their efficiencies were compared in terms of prediction skill. The best results have been obtained from the MLR approach, which discriminated five different properties of the system. These explicative variables were the polarizability, the heat of vaporization, the ionization potential and the surface tension for adsorbates and the average micropore radius for GACs. The MLR model enabled to compute integral adsorption enthalpies with a precision around 10%. Thanks to the properties discriminated, we came to some conclusions on the dominant mechanisms of adsorption.     

Performance analysis of an adsorption refrigerator using activated carbon in a compound adsorbent

To improve the performance of the adsorption refrigeration of CaCl₂-ammonia adsorption system, activated carbon has been distributed uniformly in the mass of CaCl₂, thereby helping to enhance mass transfer and uplift the cooling power density. A multifunctional heat pipe adsorption refrigerator, in which activated carbon-CaCl₂ is used as compound adsorbent and ammonia as refrigerant, is designed. Water and acetone are used as working liquids for the heat pipe. This paper presents a study on the adsorption refrigeration performances of this adsorption refrigerator under two different working conditions, ice-maker for fishing boat driven by the waste heat from exhaust gases, and solar ice-maker driven by solar water heating. The obtained average SCP (specific cooling power) and the COP (coefficient of performance) of the refrigerator were measured to be 770.4 W/kg and 0.39 at about −20 °C of evaporating temperature for the former working condition, and they were 161.2 W/kg and 0.12 at about −15 °C of evaporating temperature for the later working condition.     

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.     

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.     

Comparison of a single grain activated carbon and column adsorption system

Results from a single grain activated carbon adsorption study indicate that the effective diffusion coefficient was from 0.65×10⁻⁶ to 7.4×10⁻⁶ cm²/s for H₂S in the concentration range of 20–300 ppmv at 23 °C for both virgin activated carbon (FAC) and impregnated-regenerative activated carbon (IRAC). The effective diffusivity of the IRAC was nearly two times the FAC for H₂S adsorption. The surface reaction of H₂S-impregnated regenerative activated carbon was faster than that of H₂S and virgin activated carbon. The single grain activated carbon kinetic curve and a time scale conversion method were used to predict the breakthrough curve and the adsorption capacity of the column adsorption system. The single grain activated carbon adsorption system measured the breakthrough curve more efficiently than column adsorption. The prediction error was between 10 and 30%. Improvement can be further achieved by enhanced experimental approaches. It has a great potential for scale-up.     

Electrochemical enhancement of adsorption capacity of activated carbon fibers and their surface physicochemical characterizations

The adsorption of activated carbon fibers (ACFs) and their surface characteristics were investigated before and after electrochemical polarization. The adsorption kinetics of m-cresol showed the dependence on polarized potential, and the adsorption rate constant increased by 77.1%, from 6.38 × 10⁻³ min⁻¹ at open-circuit (OC) to 1.13 × 10⁻² min⁻¹ at polarization of 600 mV. The adsorption isotherms at different potentials were in good agreement with Langmuir isotherm model, and the maximum adsorption capacity increased from 2.28 mmol g⁻¹ at OC to 3.67 mmol g⁻¹ at polarized potential of 600 mV. These indicated that electrochemical polarization could effectively improve the adsorption rate and capacity of ACFs. The surface characteristics of ACFs before and after electrochemical polarization were evaluated by N₂ adsorption-desorption isotherms, scanning electron microscope (SEM), zeta potential and Fourier transform infrared spectroscopy (FTIR). The results showed that the BET specific surface area and pore size increased as the potential rose. However, the surface chemical properties of ACFs hardly changed under electrochemical polarization of less than 600 mV. This study was beneficial to understand the mechanism of electrochemically enhanced adsorption.     

Hydrogen adsorption in different carbon nanostructures

Hydrogen adsorption in different carbonaceous materials with optimized structure was investigated at room temperature and 77 K. Activated carbon, amorphous carbon nanotubes, SWCNTs and porous carbon samples all show the same adsorption properties. The fast kinetics and complete reversibility of the process indicate that the interaction between hydrogen molecules and the carbon nanostructure is due to physisorption. At 77 K the adsorption isotherm of all samples can be explained with the Langmuir model, while at room temperature the storage capacity is a linear function of the pressure. The surface area and pore size of the carbon materials were characterized by N₂ adsorption at 77 K and correlated to their hydrogen storage capacity. A linear relation between hydrogen uptake and specific surface area (SSA) is obtained for all samples independent of the nature of the carbon material. The best material with a SSA of 2560 m²/g shows a storage capacity of 4.5 wt% at 77 K.