Kinetics and equilibrium of dissolved oxygen adsorption on activated carbon

The macroscopic adsorption behavior of dissolved oxygen on a coconut shell-derived granular activated carbon has been studied in batch mode at 301 and 313 K for initial dissolved oxygen concentrations of 10-30 mg/l and oxygen/carbon ratios of 2-180 mg/g. BET (Brunauer, Emmett, and Teller) surface area, micropore volume, and pore size distribution were determined from N₂ isotherm data for fresh and used samples of carbon. The surface groups were characterized using Boehm titrations, potentiometric titrations, and FTIR study. The material is characterized by its high specific surface area , microporocity (micropore volume ), its basic character ( total basic groups) and its high iron content (15,480 ppm Fe). BET n-layer isotherm describes adsorption equilibrium suggesting cooperative adsorption and important adsorbate-adsorbate interactions. Kinetic data suggest a process dependent on surface coverage. At low coverage a Fickian, intraparticle diffusion rate model assuming a local equilibrium isotherm (oxygen dissociation reaction) adequately describes the process. The calculated diffusion coefficients (D) vary between and for initial oxygen concentration of 10 and 20 mg/l, respectively. Sensitivity analysis shows that the oxygen dissociation equilibrium constant determines the equilibrium concentration, whereas the diffusion coefficient controls the kinetic rate of the adsorption process having no effect at the final equilibrium concentration. A combined kinetic mass transfer model with concentration-dependent diffusion (parabolic form) has been developed and successfully applied on the dissolved oxygen adsorption system at high surface coverage. For equilibrium uptake of the estimated mean mass transfer coefficient and adsorption rate constant are and , respectively.     

The importance of finite adsorption kinetics in the sorption of hydrocarbon gases onto a nutshell-derived activated carbon

An activated carbon sample was synthesized from nutshells using a sequential step of carbonization and CO₂ physical activation. Sorption equilibria and kinetics of hydrocarbon gases were measured on this sample at various temperatures and concentrations. The structural heterogeneity of the carbon is characterized by its micropore distribution, which is derived from the analysis of isotherms. The finite kinetics (FK) model [Do, D.D., Wang, K., 1998b. Predictions of adsorption equilibria of non-polar hydrocarbons onto activated carbon. Langmuir 14, 7271-7277; Do, D.D., Wang, K., 1998a. Dual diffusion and finite mass exchange model for adsorption kinetics in activated carbon. A.I.Ch.E. Journal 44, 68.] was employed to describe the sorption kinetics of pure gases and to predict those of binary gas mixtures. The performance of the FK model is compared to that of the HMSD model which assumes the same surface heterogeneity but infinite rates of interchange between the bulk and adsorbed phases. It is demonstrated that the finite kinetics play an important role in the simulation/prediction of desorption kinetics and sorption kinetics of binary gas mixtures.     

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.     

Study of adsorption behavior using activated carbon for removal of colored impurities from 30% caprolactam solution produced by means of SNIA-toluene-technology

We studied the adsorption removal of the colored impurities from caprolactam solution by granular activated carbons. It was observed that removal was favored at lower pH (pH 3.84 or below) and higher temperature. The effects of concentration, dosage of activated carbons, contact time have been also reported. Uptake of colored impurities was very rapid in the first 100 minutes and reached equilibrium after 24 h. The batch adsorption kinetics was found to follow the pseudo-second-order model and the rate constants of adsorption for all these kinetic models have been calculated. Three isotherm expressions Langmuir, Freundlich and Trinomial were shown to fit with the experimental results successfully. The mass transfer coefficientβ and the effective diffusion coefficient in aqueous phase Deff was calculated under a temperature 35–80 °C. The value of the mean free energy of adsorption E signifies that the adsorption of colored impurities onto activated carbon has a physical nature.     

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.     

Electrochemically enhanced adsorption of aniline on activated carbon fibers

For adsorptive separation processes, the adsorption rate and capacity are two important factors affecting the costs. This study describes the anodic polarization of activated carbon fibers (ACFs), which can enhance the adsorption rate and capacity of aniline. The electrosorption kinetics and the affecting factors (bias potential, electrolyte, and pH) of isotherms for aniline on ACFs were investigated. The adsorption/electrosorption of aniline on ACFs follow pseudo-first-order adsorption kinetics, and the adsorption rate improves with increasing bias potential. The electrosorption isotherms, which exhibit a variety of responses depending on bias potential, electrolyte and pH, follow the two classical models of Langmuir and Freundlich. With electrosorption of aniline from aqueous solution, a two-fold enhancement of adsorption capacity is achievable. The initial and saturated ACFs were characterized using scanning electron micrograph (SEM) and Fourier transform infrared spectroscopy (FT-IR). The SEM micrographs show that the surface of ACFs is not oxidized, which is also verified by cyclic voltammetry results. The FT-IR spectroscopy suggests that the interaction between aniline and ACFs is main weak physisorption instead of chemisorption. These experimental results suggest that the electrochemical polarization of ACFs can effectively improve the adsorption rate and capacity of aniline, which may be due to the enhanced affinity between aniline and ACFs instead of the oxidation on the surface of ACFs or in the solution.     

Effects of high relative humidity on the dynamic adsorption of dimethyl methylphosphonate (DMMP) on activated carbon

The effects of high relative humidity (RH) on the breakthrough of the nerve agent simulant dimethyl methylphosphonate (DMMP) vapor in beds of ASC-impregnated, activated carbon were investigated. Maximum concentrations of DMMP at room temperature and RH > 60% were found to be lower by more than an order of magnitude than in dry air. The breakthrough time (t_B) of 1.2 × 10⁻⁴ g l⁻¹ DMMP in pre-humidified beds and humid air of RH = 90% was shortened by a factor of 1.6 relative to adsorption in dry beds and dry air. Analysis of the breakthrough curves according to the Wheeler–Jonas model indicated that the high RH lowered the dynamic adsorption capacity (W_E) but had nearly no effect on the critical bed weight (W_C). The reduction of W_E by humidity correlates with the observed displacement of adsorbed water by DMMP. The use of DMMP for testing filter performance is limited to low and intermediate relative humidities. On the other hand, DMMP in dry air can be used to advantage for testing the capacity of new or used respirator filters and for the detection of filter channeling.     

Removal of metal ions from aqueous solution by adsorption onto activated carbon cloths: adsorption competition with organic matter

Activated carbon cloths are recent adsorbents whose adsorption properties are well known for monocomponent solutions of organics or metal ions. However, to treat wastewaters with these materials, their performance has to be determined in multicomponent solution. This work studies adsorption competition between metal ions (Cu²⁺, Pb²⁺) and organic matter (benzoic acid). The first part investigates adsorption equilibrium of monocomponent metal ions solutions and shows the dependence of adsorption capacities on adsorbent porosity and metal ions chemical properties (molecular weight, ionic radius and electronegativity). The influence of pH is also demonstrated. The second part focuses on adsorption competition: (1) between both metal ions (a decrease of adsorption capacities is observed, whose value is related to adsorption kinetics of metal ions); (2) between metal ions and organic matter, in solution or adsorbed onto the activated carbon cloth (a strong influence of pH is shown: when benzoic acid is under benzoate form, in both cases adsorption is increased due to the formation of ligands between adsorbed benzoate ions and metals).     

Study to investigate the importance of mass transfer of naproxen sodium onto activated carbon

In the present study, the adsorption of naproxen sodium onto activated carbons (BK4, obtained from white polymeric waste and SK4, obtained from black polymeric waste) was investigated by calculating the parameters of pH, contact time, the concentration of naproxen sodium and the temperature. The adsorption data of naproxen sodium onto activated carbon follows the Langmuir isotherm model and its kinetic processes were described by various kinetic adsorption models. It was determined that the pseudo-second-order model was the best choice among all the available kinetic models to describe the adsorption behaviour of naproxen sodium onto activated carbon. During the present study, the intraparticle diffusion rate constant, the external mass transfer coefficient and the film and pore diffusion coefficient were evaluated at various temperatures. In addition, the thermodynamic parameters of the adsorption of naproxen sodium onto activated carbon were also calculated.     

Influence of solvent type on dibenzothiophene adsorption onto activated carbon fiber and granular coconut-shell activated carbon

The adsorption behavior of dibenzothiophene (DBT) on an activated carbon fiber (ACF) and a granular coconut-shell activated carbon (GCSAC) in the solvents n-hexane, n-decane, toluene, and mixture of n-decane and toluene was investigated. The DBT adsorption onto both samples was more active in n-hexane than in n-decane. The lowest DBT adsorption was observed in toluene. Regardless of the type of activated carbons and solvents, all the isotherms fit the Freundlich equation better than the Langmuir equation. At low equilibrium concentrations of <2 mass ppm-S, GCSAC displayed greater capacity for DBT adsorption than did ACF in all the tested solvents. The adsorption kinetics of ACF and GCSAC in all the tested solvents were governed by a pseudo-second-order model.     

Competitive adsorption of a benzene-toluene mixture on activated carbons at low concentration

Previous studies of the adsorption of benzene and toluene at low concentration showed that both porosity and surface chemistry of the activated carbon play an important role. This paper analyses the adsorption behaviour of a mixture of VOCs (benzene-toluene) on AC, due to the lack of information regarding the adsorption of mixtures. Thus, the performance of chemically activated carbons, physically activated carbon with steam and commercial samples is studied. This study shows that chemically activated carbons have better performance than the other samples, showing much higher adsorption capacities, breakthrough times and separation times. Porosity is a key factor and those activated carbons with higher volumes of micropores exhibit higher adsorption capacities and breakthrough times. This work also analyses the state of the adsorbed phase resulting from the mixture adsorption and comparison of the composition of the adsorbed hydrocarbons with that predicted by the ideal adsorption solution theory (IAST), shows good agreement.     

Comparison of adsorption behaviour of light alkanes and alkenes on Kureha activated carbon

Adsorption equilibria of light alkanes and alkenes on Kureha activated carbon were investigated using a volumetric method. Single-component adsorption isotherms are reported at pressures up to 120 kPa and at temperatures in the range from 194 to 338 K for ethane and ethene and from 273 to 358 K for propane and propene. The Tóth model appropriately describes the equilibrium data over the whole range of conditions. The saturation capacity for the alkene extracted by the Tóth model is higher than for the corresponding alkane, attributed to the higher packing efficiency of the alkene molecules inside the micropores. An interesting reversal in alkane/alkene adsorption selectivity with pressure is observed: at low pressures the selectivity towards the alkanes is driven by energetic effects while at high pressures the selectivity is towards the alkenes due to entropic effects.     

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.     

Enhanced adsorption of phenol from aqueous solution by carbonized trace ZIF-8-decorated activated carbon pellets

Trace zeolitic imidazolate framework-8(ZIF-8)-decorated activated carbon(AC) pellets were synthesized by a facile wet impregnation technique. After pyrolysis of the above composite material, the obtained carbon had a large surface area and pore volume, with traces of Zn on its surface. Subsequently, the capacity of the ZIF8/AC samples to adsorb and remove phenol from aqueous media was evaluated in both batch and column experimental setups. The equilibrium adsorption capacity reached 155.24 mg·g~(-1), which was 2.3 times greater than that of the pure AC(46.24 mg·g~(-1)). In addition, adsorption kinetics were examined by pseudofirst and pseudosecond order models, and adsorption isotherms were fitted into Langmuir and Freundlich equations. The adsorbent could be easily filtered from the solution and washed with methanol and water, while maintaining an efficiency N 90% after 4 cycles. The above results make it a potentially reusable candidate for water purification.     

Adsorption and desorption kinetics for hydrophilic and hydrophobic vapors on activated carbon

Adsorption dynamics are of fundamental importance in applications of adsorbents in real situations. The adsorption/desorption characteristics of a series of adsorbates, with varying hydrophilic/hydrophobic and structural characteristics, for activated carbon BAX950, were investigated for temperatures in the range 288-323 K. These data provide a comprehensive kinetic study of adsorption/desorption for an activated carbon. The results are discussed in relation to the adsorbent pore structure and functional group concentration, adsorptive structure and adsorption mechanism. The study provides evidence for a compensation effect where activation energy and ln(pre-exponential factor) parameters obtained from the Arrhenius equation exhibit a linear correlation.