Kinetics of adsorption on activated carbon: application of heterogeneous vacancy solution theory

The kinetics of single component adsorption on activated carbon is investigated here using a heterogeneous vacancy solution theory (VST) of adsorption. The adsorption isotherm is developed to account for the adsorbate non-ideality due to the size difference between the adsorbate molecule and the vacant site, while incorporating adsorbent heterogeneity through a pore-width-related potential energy. The transport process in the bidisperse carbon considers coupled mass transfer in both macropore and micropore phases simultaneously. Adsorbate diffusion in the micropore network is modeled through effective medium theory, thus considering pore network connectivity in the adsorbent, with the activation energy for adsorbate diffusion related to the adsorption energy, represented by the Steele 10-4-3 potential for carbons. Experimental data of five hydrocarbons, CO₂ and SO₂ on Ajax carbon at multiple temperatures, as well as three hydrocarbons on Norit carbon at three temperatures are first fitted by the heterogeneous VST model to obtain the isotherm parameters, followed by application of the kinetic model to uptake data on carbon particles of different sizes and geometry at various temperatures. For the hydrocarbons studied, the model can successfully correlate the experimental data for both adsorption equilibrium and kinetics. However, there is some deviation in the fit of the desorption kinetics for polar compounds such as CO₂ and SO₂, due to the inadequacy of the L-J potential model in this case. The significance of viscous transport in the micropores is also considered here and found to be negligible, consistent with recent molecular simulation studies.     

Theoretical analysis and experimental study on SO2 adsorption onto pistachio‐nut‐shell activated carbon

The adsorption study of SO₂ onto the activated carbon prepared from pistachio‐nut shell was studied theoretically and experimentally. A single‐particle sorption model known as concentration‐dependent surface diffusivity micropore, surface and macropore diffusion control model incorporating micropore, macropore and surface diffusions, together with a nonlinear isotherm at the micropore mouth, has been derived and solved by a finite difference method. The effects of different types of nonlinear isotherms and concentration dependent surface diffusivities have been thoroughly studied. The effects of adsorbate concentration and temperature on adsorption were studied experimentally. Good agreement was found between the model predictions and the experimental results. The value of the tortuosity factor and the extracted diffusion coefficients obtained are consistent with their corresponding values reported. © 2008 American Institute of Chemical Engineers AIChE J 2009     

Adsorption of hydrogen sulphide (H2S) by activated carbons derived from oil-palm shell

Adsorption of hydrogen sulphide (H₂S) onto activated carbons derived from oil palm shell, an abundant solid waste from palm oil processing mills, by thermal or chemical activation method was investigated in this paper. Dynamic adsorption in a fixed bed configuration showed that the palm-shell activated carbons prepared by chemical activation (KOH or H₂SO₄ impregnation) performed better than the palm-shell activated carbon by thermal activation and a coconut-shell-based commercial activated carbon. Static equilibrium adsorption studies confirmed this experimental result. An intra-particle Knudsen diffusion model based on a Freundlich isotherm was developed for predicting the amount of H₂S adsorbed. Desorption tests at the same temperature as adsorption (298 K) and at an elevated temperature (473 K) were carried out to confirm the occurrence of chemisorption and oxidation of H₂S on the activated carbon. Surface chemistries of the palm-shell activated carbons were characterized by Fourier transform infrared spectroscopy and Boehm titration. It was found that uptaking H₂S onto the palm-shell activated carbons was due to different mechanisms, e.g. physisorption, chemisorption and/or H₂S oxidation, depending on the activation agent and activation method.     

Comparison of Cr(VI) Adsorption Using Synthetic Schwertmannite Obtained by Fe3+ Hydrolysis and Fe2+ Oxidation: Kinetics,Isotherms and Adsorption Mechanism

Good sorption properties and simple synthesis route make schwertmannite an increasingly popular adsorbent. In this work, the adsorption properties of synthetic schwertmannite towards Cr(VI) were investigated. This study aimed to compare the properties and sorption performance of adsorbents obtained by two methods: Fe³⁺ hydrolysis (SCH_A) and Fe²⁺ oxidation (SCH_B). To characterise the sorbents before and after Cr(VI) adsorption, specific surface area, particle size distribution, density, and zeta potential were determined. Additionally, optical micrographs, SEM, and FTIR analyses were performed. Adsorption experiments were performed in varying process conditions: pH, adsorbent dosage, contact time, and initial concentration. Adsorption isotherms were fitted by Freundlich, Langmuir, and Temkin models. Pseudo-first-order, pseudo-second-order, intraparticle diffusion, and liquid film diffusion models were used to fit the kinetics data. Linear regression was used to estimate the parameters of isotherm and kinetic models. The maximum adsorption capacity resulting from the fitted Langmuir isotherm is 42.97 and 17.54 mg·g⁻¹ for SCH_A and SCH_B. Results show that the adsorption kinetics follows the pseudo-second-order kinetic model. Both iron-based adsorbents are suitable for removing Cr(VI) ions from aqueous solutions. Characterisation of the adsorbents after adsorption suggests that Cr(VI) adsorption can be mainly attributed to ion exchange with SO₄²⁻ groups.     

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).     

A fundamental study on the removal of air pollutants (sulfur dioxide,nitrogen dioxide and carbon dioxide) by adsorption on activated carbon

Gravimetrically measured adsorption and desorption dynamics of sulfur dioxide, nitrogen dioxide and carbon dioxide on a commercial activated carbon are interpreted by a single-particle model based on three transport processes: macropore, micropore and sorbed-phase diffusion. Additional phenomena, concentration-dependent sorbed-phase diffusivity and sorbent non-isothermality, are incorporated to expand the applicability of the model. The dynamic sorption behaviour of all three gases is adequately described, without resorting to a different particle tortuosity factor for each sorbate. The value of the tortuosity factor (8) and the extracted diffusion coefficients are consistent with literature values. The affinity of the activated carbon for the adsorbates is, in increasing order, CO₂ < SO₂ < NO₂, while the extracted diffusion coefficients show the reverse trend, NO₂ < SO₂ < CO₂.     

Water purification of removal aqueous copper (II) by as-grown and modified multi-walled carbon nanotubes

This study compares aqueous copper (II) adsorbed onto as-grown and modified carbon nanotubes (CNTs), using H₂SO₄ and H₂SO₄/KMnO₄ processes. H₂SO₄ and H₂SO₄/KMnO₄ modifications reduced pH_iep and Fourier Transform Infrared Spectroscopy (FTIR) analysis revealed that some functional groups were formed on modified CNTs. The adsorption capacity of copper (II) onto modified CNTs was greater than that of as-grown CNTs, especially at pH 6. The results demonstrate that the modified processes increased the adsorption capacity because the functional groups were generated on the modified surfaces of the CNTs. Additionally, the adsorption capacity of copper (II) onto as-grown and modified CNTs both increased with temperature, and the results indicated that the Langmuir isotherm fitted the experimental data well. Simulation results indicated that the ΔH⁰ values of as-grown, H₂SO₄-modified CNTs and H₂SO₄/KMnO₄-modified CNTs were 4.83, 14.37 and 29.92 kJ/mol, respectively. Based on ΔH⁰, the adsorption of Cu²⁺ onto H₂SO₄/KMnO₄-modified CNTs is suggested to proceed simultaneously by physisorption and chemisorption but that onto as-grown and H₂SO₄-modified CNTs may proceed only by physisorption.     

Dynamic analysis of changes in diffusion and sorption parameters in the reaction of SO2 with activated soda ash

Variations in pore structure cause significant changes in diffusion resistance and adsorption characteristics during the reaction of SO₂ with activated soda. Breakthrough and moment analysis of the SO₂-activated soda ash reaction showed that the effective diffusion coefficient of SO₂ in the solid product (mostly Na₂SO₃) was about three times smaller than its value in the original activated soda. About 30% of the diffusion flux of SO₂ in the solid product was found to be due to surface diffusion. The adsorption equilibrium constant of SO₂ on the solid product was found to be about half the value of the adsorption equilibrium constant on activated soda. It was also determined that SO₂ was irreversibly adsorbed on the solid product Na₂SO₃ at 200°C and 3.7 atm. The gaseous product CO₂ was not adsorbed on the solid product while it was slightly adsorbed on activated soda.     

Adsorption of sulfur dioxide onto activated carbons prepared from oil‐palm shells impregnated with potassium hydroxide

Adsorption of sulfur dioxide (SO₂), a gaseous pollutant, onto activated carbons prepared from oil‐palm shells pre‐treated with potassium hydroxide (KOH) impregnation was studied. Experimental results showed that SO₂ concentration and adsorption temperature affected significantly the amount of SO₂ adsorbed and the equilibrium time. However, sample particle sizes influenced the equilibrium time (due to effect of diffusion rate) only. Desorption at the same temperature of adsorption and a higher temperature of 200 °C confirmed the presence of chemisorption due to pre‐impregnation. Impregnation with different activation agents was found to have limited effect on the inorganic components of the sample. Compared with the activated carbon pre‐treated with 30% phosphoric acid (H₃PO₄) that had larger BET and micropore surface areas, the sample impregnated with 10% KOH had a higher adsorptive capacity for SO₂, which was closely related to the surface organic functional groups of the sample. In general, the activated carbon prepared from oil‐palm shell impregnated with KOH was more effective for SO₂ adsorption and its adsorptive capacity was comparable to some commercial activated carbons. © 2000 Society of Chemical Industry     

Batch Removal of Crystal Violet from Aqueous Solution by H2SO4 Modified Sugarcane Bagasse: Equilibrium,Kinetic, and Thermodynamic Profile

Batch adsorption studies were carried out using H₂SO₄ modified sugarcane bagasse (HMSB) for the removal of hazardous Crystal Violet (CV) dye from aqueous solutions. The effects of initial solution pH, adsorbent dose, and temperature on the adsorption process were investigated. The Langmuir isotherm model well described the equilibrium dye uptake while the pseudo-second-order kinetic model showed good agreement with the experimental kinetic data. Gibb's free energy change (ΔG⁰) was spontaneous for all interactions, and the adsorption process exhibited endothermic enthalpy values. Results suggest that HMSB is an effective adsorbent for the removal of CV from wastewater.     

Electrochemical study on the effect of bipyridine dicarboxylic acid and its cobalt complex on the corrosion behaviour of carbon steel in acidic medium

The 2,2′-bipyridine-3,3′-dicarboxylic acid (bida) and its cobalt complex (Co-bida) were tested as corrosion inhibitors for N80 carbon steel in sulphuric acid solution by electrochemical polarization and electrochemical impedance spectroscopy method. The results indicate that the complex and ligand inhibit the corrosion of mild steel in H₂SO₄ solutions and the extent of inhibition increases with inhibitor concentration and decreases with temperature. The inhibition efficiency of the inhibitors follows the trend Co-bida > bida. A mixed-inhibition mechanism is proposed for the inhibitive effects of the compounds. The adsorption characteristics of the inhibitors were approximated by Temkin isotherm. Morphological study of the carbon steel electrode surface was undertaken by scanning electron microscope and the interfacial species formed on the surface in the presence of inhibitors analyzed by infrared spectroscopy.     

Phosphate Removal from Aqueous Solution Using Coir-Pith Activated Carbon

The present study deals with the removal of phosphates from aqueous solution using activated carbon developed from coir pith. Batch adsorption experiments were performed to delineate the effect of initial pH, contact time, adsorbent dose and temperature on the removal of phosphates by coir-pith activated carbon (CAC) (activated by H₂SO₄). The removal was found to be maximum in the pH range of 6–10. The kinetics of adsorption showed that the phosphate adsorption onto CAC was a gradual process with a quasi-equilibrium being attained in 3 h. The adsorption equilibrium data followed the Temkin isotherm. Thermodynamic parameters such as ΔG ^o , ΔH ^o , and ΔS ^o were evaluated by applying the Arrhenius and van't Hoff equations, and it was found that the adsorption of phosphate on CAC was spontaneous and endothermic.     

Adsorption of H2S and SO2 on Bigadiç Clinoptilolite

Abstract

H₂S and SO₂ adsorption isotherms of Bigadiç clinoptilolite and its Na?, K?, Ca?, and H?enriched forms were determined in the 0 to 100 kPa range at 25°C by using a constant volume adsorption system. Langmuir, Freundlich, Dubinin-Radushkevich (D-R), and Dubinin-Astakhov (D-A) models were applied to the isotherm data. Higher adsorption capacities and a larger increase in the amount adsorbed at higher pressures were observed for SO₂, in agreement with its higher permanent dipole moment, resulting in stronger ion-dipole and dipole-dipole interactions for this molecule. The Ca-form exhibited a molecular sieving behavior for both gases, originating from the channel blockage caused by the cation locations in the sample. The highest capacities for both gases were obtained with the sample in its H-form, followed by the Na? and K-forms for SO₂, parallel to the decrease in the electronegativity and ionic potential and the increase in the polarizability of the cation. In the case of H₂S, the H-form was followed by the K-form, but the Na-form yielded very low adsorption capacities. Initial dissociative adsorption of H₂S on certain Na sites to yield SH and OH species is thought to contribute to a more effective blocking of the channels, which were already partially blocked in this sample. Of the isotherm models tested, the D-A model explained the variations in the data better than either the Freundlich or D-R models. For the cation-gas combinations with a lower extent of channel blockage though, the Langmuir model was somewhat more representative. Lower E and n values were obtained from the D-A model for H₂S on the Na-form, which may be related to the lower extent of micropore adsorption and to the presence of blocked, almost dead-end shorter channel segments in the sample. Pore volumes close to the theoretical value were estimated from the D-A parameters for SO₂ adsorption.     

Synthesis and Inhibition Effect of a Novel Tri-cationic Surfactant on Carbon Steel Corrosion in 0.5 M H2SO4 Solution

A novel Tri-cationic surfactant was synthesized, purified and characterized. The critical micelle concentration value of the prepared surfactant was determined by surface tension and conductivity measurements. The surface parameters were calculated by surface tension measurements. The relationship between the surface properties and the corrosion inhibition efficiency of the prepared surfactant was discussed. The inhibition effect of the novel Tri-cationic surfactant on carbon steel corrosion in 0.5 M H₂SO₄ was studied by potentiodynamic polarization, electrochemical impedance spectroscopy and weight loss techniques. Potentiodynamic polarization studies revealed that the inhibitor acted as a mixed-type inhibitor. The high inhibition efficiency was attributed to the blocking of active sites on the steel surface through the adsorption of inhibitor molecules. Inhibitor adsorption on the carbon steel surface was in accordance with the Langmuir adsorption isotherm model. Thermodynamic adsorption and kinetic parameters were obtained from weight losses at different temperatures (20–60 °C).     

Evaluation of SO2 oxidation and fly ash filtration by an activated carbon fluidized-bed reactor: The effects of acid modification, copper addition and operating condition

It is expected that the simultaneous removal of acid gases and particles from flue gas, using a single process and at the same temperature, will become an economical, and thus, desirable option. Accordingly, this study investigates the potential for the utilization of a fluidized-bed adsorbent/catalyst reactor for the simultaneous removal of SO₂ and fly ash from simulated flue gas. The operating conditions for the evaluation include: (1) different pre-treatments of the adsorbent/catalyst, (2) the operating parameters of adsorption/filtration and (3) the effects of simultaneous adsorption/filtration through the fluidized-bed reactor. Based on the experimental data gathered, the Brönsted acid sites were formed on the surface of activated carbon (AC) support materials after modification with nitric or sulfuric acid and it acted as anchor. This characteristic accounts for the promotion of the effects of dispersion and adsorption of the adsorbent/catalyst. Moreover, the addition of copper facilitated the oxygen transfer of SO₂ to the carbon matrix. The concentration of SO₂ removed by the fluidized-bed adsorbent/catalyst reactor decreased from 17.9 to 14.2 mg SO₂/g of adsorbent after exposure to a high concentration of fly ash. Therefore, an acid-pre-treatment of the adsorbent/catalyst is required to hasten the removal of SO₂ in the simulated flue gas. Our result shows that the acidic groups may facilitate the adsorbent/catalyst removal of SO₂ when there exist high concentrations of fly ash in the flue gas.     

Evaluating isotherm models for the prediction of flue gas adsorption equilibrium and dynamics

We evaluated isotherm models for the precise prediction of adsorption equilibrium and breakthrough dynamics. Adsorption experiments were performed using pure N₂, CO₂ and their binary mixture with an activated carbon (AC) material as an adsorbent. Both BET and breakthrough measurements were conducted at various conditions of temperature and pressure. The corresponding uptake amount of pure component adsorption was experimentally determined, and parameters of the four different isotherm models, Langmuir, Langmuir-Freundlich, Sips, and Toth, were calculated from the experimental data. The predictive capability of each isotherm model was also evaluated with the binary experimental results of binary N₂/CO₂ mixtures, by means of sum of square errors (SSE). As a result, the Toth model was the most precise isotherm model in describing CO₂ adsorption equilibrium on the AC. Based on the breakthrough experimental result from the binary mixture adsorption, non-isothermal modeling for the adsorption bed was performed. The breakthrough results with all of the isotherm models were examined by rigorous dynamic simulations, and the Toth model was also the most accurate model for describing the dynamics.     

Sorption of mercury by activated carbon in the presence of flue gas components

The purpose of the current study is to evaluate the mercury removal ability of F400 and Norit FGD activated carbons, through fixed bed adsorption tests at inert atmosphere (Hg° + N₂). Additionally, adsorption tests were realized on F400 activated carbon, in the presence of HCl, O₂, SO₂ and CO₂ in nitrogen flow. The obtained results, revealed that F400 activated carbon, with a high-developed micropore structure and increased BET area, exhibit larger Hg° adsorptive capacity compared to Norit. HCl and O₂, can strongly affect mercury adsorption, owing to heterogeneous oxidation and chemisorption reactions, which is in accordance with the assumptions of some researchers. Additionally, SO₂ presence enhances mercury adsorption, in contrast with the conclusions evaluated in other studies. The above result could be attributed to the possible formation of sulphur spaces on activated carbon surface and consist of a clarification for the role of SO₂ on mercury adsorption. On the contrary, the mercury adsorption efficiency of F400 activated carbon showed a decrease at about 25%, with increasing CO₂ concentration from 0 to 12%.     

Effect of α-FeOOH dispersion on adsorption characteristics of pitch-based activated carbon fibers against NO, SO2, and NH3

The effects of dispersion of ultrafine α-FeOOH particles of pitch-based activated carbon fibers (ACF) on the adsorption characteristics against NO, SO₂ and NH₃ were examined at 303 K. Also the micropore structures were studied by N₂ adsorption at 77 K. The adsorption isotherms for not only N₂ but also NO, SO₂ and NH₃ were analyzed by the DR plots. The dispersion of α-FeOOH markedly increased the amounts of adsorption for NO and NH₃ at lower pressure regions, but it did not increase the adsorption capacities for all adsorptives. The a-FeOOH dispersion enhanced the isosteric heat of adsorption for all adsorptives, in particular NH₃.     

Preparation and adsorption properties of a novel superabsorbent based on multiwalled carbon nanotubes–xylan composite and poly(methacrylic acid) for methylene blue from aqueous solution

A novel superabsorbent was prepared by the copolymerization reaction between multiwalled carbon nanotubes (MWCNTs)–xylan composite and poly(methacrylic acid) (PMAA) in the presence of N,N′‐methylenebisacrylamide as cross‐linker and the redox initiation system (NH)₄S₂O₈‐anhydrous Na₂SO₃ as initiator for adsorbing methylene blue (MB) from aqueous solution. First, covalent modification of MWCNTs with a natural polymer xylan was achieved by the nucleophilic substitution reaction. The obtained xylan modified MWCNTs–xylan composite was characterized by transmission electron microscope, Fourier transform infrared spectroscopy, X‐ray photoelectron spectrometer, and thermal gravimetric analysis. The results indicated that the hydroxyl groups of xylan participated in the formation of MWCNTs–xylan composite, and MWCNTs–xylan composite contained 32% xylan and about four molecule chains of xylan were grafted onto 5,000 carbon atoms of MWCNTs sidewalls. Subsequently, a novel superabsorbent based on MWCNTs–xylan composite and PMAA was prepared and characterized by Fourier transform infrared spectroscopy, scanning electron microscope, and swelling test. The results proved that superabsorbent was successfully prepared by copolymerization reaction and found that the superabsorbent evidently presented three‐dimensional network structure. Swelling process of superabsorbent complied with the Fick's law. Adsorption property of superabsorbent for MB was also systematically studied by investigating these parameters, such as superabsorbent dosage, initial MB concentration, contact time, ion strength, and pH. The superabsorbent presented a higher adsorption capacity and removal rate for MB. The adsorption data were fitted by Langmuir, Freundlich isotherm models, and four adsorption kinetic models. Freundlich isotherm model better fitted the equilibrium data than Langmuir, and adsorption kinetics could be well described by pseudo‐second‐order kinetic and intraparticle diffusion. These results indicated that the superabsorbent can be used as an efficient absorbent for removal of MB from aqueous solution. POLYM. COMPOS., 35:1516–1528, 2014. © 2013 Society of Plastics Engineers     

吸附-电催化联合处理苯酚废水及动力学

引言 酚类化合物及其衍生物是重要的、产量较大的有机产品之一,是许多行业所必需的原料,被广泛用于制造涂料、溶剂、塑料、炸药、化肥、化纤、药品、肥皂和燃料等[1-2].