Predicting solute adsorption on activated carbon: phenol

Activated carbon (AC), the most widely used adsorbent in water and in wastewater treatment, comprises a high surface area of very small, convoluted and interconnected pores. Despite the wide use of AC, there is little fundamental atomic-level understanding of its adsorption capacity and selectivity as well as its pore structure. The purpose of this work is to suggest the methodology for calculation of equilibrium adsorption capacity of common water organic pollutants and use it for phenol as a model. The effects of various functional groups, pore size, and coverage on thermodynamics of phenol adsorption from the gas phase and from water media are calculated using molecular mechanics (MM) and density functional theory (DFT) approaches.     

Electrochemically enhanced adsorption of phenol on activated carbon fibers in basic aqueous solution

Electrosorption isotherms and thermodynamics of phenol on activated carbon fibers (ACFs) in basic solution, as well as the factors (bias potential, initial concentration, and electrolyte) affecting adsorption/electrosorption kinetics, were investigated. The kinetics, which followed the Lagergren adsorption rate law, exhibited a variety of responses depending on bias potential, initial concentration, and electrolyte. The electrosorption isotherms were in agreement with the classical models of Langmuir and Freundlich, but the former gave more satisfactory correlation coefficients. With electrosorption at a bias potential of 700 mV from the basic solution, a nearly 10-fold enhancement of maximum adsorption capacity was achievable. The electrosorption free energy (DeltaG(ads)), enthalpy (DeltaH(ads)), and entropy (DeltaS(ads)) of phenol on the ACFs were calculated from adsorption isotherms at different temperatures. The results indicated that electrosorption of phenol in basic solution was spontaneous and exothermic. Furthermore, it was assessed that electrosorption occurred by dipole-dipole interaction with DeltaH(ads) of -20.14 kJ mol(-1) besides suppositional electrostatic interaction.     

Understanding phenol adsorption mechanisms on activated carbons

The interactions between phenol molecules and activated carbons were investigated in order to understand the adsorption mechanism of this aromatic compound. A series of activated carbons with varied chemical composition but similar porous features were synthesized and submitted to phenol exposure from aqueous phase, followed by thermogravimetric analysis and identification of the desorbed species by temperature programmed desorption coupled with mass spectrometry. Based on these experiments, both physi- and chemisorption sites for phenol were identified on the activated carbons. Our results demonstrate that physisorption of phenol depends strictly on the porosity of the activated carbons, whereas chemisorption depends on the availability of the basal planes in the activated carbons. Thus, oxidation of the carbon can suppress the fraction of chemisorbed phenol since the surface functionalities incorporate to the edges of the basal planes; notwithstanding, hydrophilic carbons may present a small but not negligible contribution of chemisorbed phenol depending on the extent of the functionalization. Moreover, these adsorption sites (chemi-) are recovered by simply removal of the surface functionalities after thermal annealing.     

活性炭表面含氧基团对苯酚吸附平衡及动力学的影响

煤基活性炭分别用烧失处理和HNO3氧化处理后得到不同表面性质的活性炭,采用X射线光电子能谱(XPS)、N2吸附、酸碱滴定及零电荷点(pHPZC)对活性炭表面性质及孔结构进行表征,研究了活性炭表面性质对苯酚吸附平衡和吸附动力学影响.经HNO3氧化后,活性炭表面含氧基团显著增加,烧失处理后,表面含氧基团尤其是羧基显著减少.苯酚最大平衡吸附量随活性炭表面含氧基团的增多而减少,吸附速率常数与碳表面含氧基团的量呈正相关,而吸附活化能与活性炭在一定吸附条件下表面所带电荷多少相关.随着活性炭表面含氧基团增多,吸附活化熵增大(负值减小),苯酚在活性炭表面排列的有序性减小.静态吸附与动态吸附实验结果都表明:在含氧基团较少而碱性更强的活性炭上,发生化学吸附的程度更大.     

Bifunctional activated carbon with dual photocatalysis and adsorption capabilities for efficient phenol removal

Bifunctional activated carbons (AC) with the abilities of both photocatalysis and adsorption were fabricated via the sol?Cgel route combined with hydrothermal treatment and N₂ reactivation method. TiO₂ was located mainly at the entrance of the surface macropores of AC. Under UV light irradiation, efficient removal of phenol was realized by combination of adsorption and photocatalytic degradation for the obtained bifunctional materials. In insufficient light or dark, phenol removal occurred mainly through adsorption. The prepared bifunctional carbon with a mass ratio of 50 TiO₂ per AC ratio exhibited high efficiency for phenol removal. The total phenol removal capacity of 50TiO₂/AC was almost 5 times of that of pure AC and 6 times of that pure TiO₂ after 10 cycles. The prepared bifunctional carbons possess the advantages of high pollutant removal capability and good recyclability, making them promising for the efficient treatment of lightly polluted aqueous solutions.     

甲烷在石墨化热解碳黑和活性炭上的吸附

为研究影响碳基吸附剂吸附超临界温度气体的主要因素,选择石墨化热解碳黑BP280和Ajax活性炭,分析超临界温度高压甲烷在其上的吸附平衡。应用容积法,在压力0~20.5 MPa、温度253 K~313 K测定甲烷的吸附平衡数据,并由等量吸附线标绘和亨利定律常数确定等量吸附热。引入通用吸附等温方程,再由方程的Langmuir标绘确定最大吸附容量,进而通过方程的线性化计算吸附平衡态中甲烷分子的作用能。结果表明,甲烷在两种吸附剂上的最大吸附容量均随温度而变化,并都小于液态甲烷的密度;甲烷在碳黑和活性炭上的等量吸附热分别为11.9 kJ/mol~12.5 kJ/mol和17.5 kJ/mol~22.5 kJ/mol,体现了两种吸附剂不同的表面能量分布;甲烷分子间作用能随吸附量的变化特点反映了超临界温度甲烷以类似于压缩气体状态聚集的特点和吸附剂结构上的差异。碳基吸附剂的比表面积和微孔容积是影响其储存甲烷容量的重要因素。     

Novel three-stage kinetic model for aqueous benzene adsorption on activated carbon

We propose a novel kinetic model for adsorption of aqueous benzene onto both granular activated carbon (GAC) and powdered activated carbon (PAC). The model is based on mass conservation of benzene coupled with three-stage adsorption: (1) the first portion for an instantaneous stage or external surface adsorption, (2) the second portion for a gradual stage with rate-limiting intraparticle diffusion, and (3) the third portion for a constant stage in which the aqueous phase no longer interacts with activated carbon. An analytical solution of the kinetic model was validated with the kinetic data obtained from aqueous benzene adsorption onto GAC and PAC in batch experiments with two different solution concentrations (C(0)=300 mg L(-1), 600 mg L(-1)). Experimental results revealed that benzene adsorption for the two concentrations followed three distinct stages for PAC but two stages for GAC. The analytical solution could successfully describe the kinetic adsorption of aqueous benzene in the batch reaction system, showing a fast instantaneous adsorption followed by a slow rate-limiting adsorption and a final long constant adsorption. Use of the two-stage model gave incorrect values of adsorption coefficients in the analytical solution due to inability to describe the third stage.     

Role of surface chemistry in adsorption of phenol on activated carbons

Two samples of activated carbon of wood origin were oxidized using ammonium persulfate. The structural properties and surface chemistry of the samples and their oxidized counterparts were characterized using sorption of nitrogen and Boehm titration, respectively. Phenol adsorption from solution (at trace concentrations) was studied at temperatures close to ambient without maintaining a specific pH of the solution. The results showed, as expected, that the phenol uptake is dependent on both the porosity and surface chemistry of the carbons. Furthermore, phenol adsorption showed a strong dependence on the number of carboxylic groups due to two factors: (1) phenol reacts with carboxylic groups on the carbon surface, forming an ester bond, and (2) carboxylic groups on the carbon surface remove the pi-electron from the activated carbon aromatic ring matrix, causing a decrease in the strength of interactions between the benzene ring of phenol and the carbon's basal planes, which decreases the uptake of phenol.     

Determination of partial immersion enthalpy in the interaction of water and activated carbon

A way to calculate the enthalpic contributions of each component of the mixture of activated carbon and water to the immersion enthalpy using the concepts of the solution enthalpies is presented. By determining the immersion enthalpies of a microporous activated carbon in water, with values that are between –18.97 and −27.21 Jg⁻¹, from these and the mass ratio of activated carbon and water, differential enthalpies for the activated carbon, ΔH_DIFacH_{{\rm DIF}_{\rm ac}} and water, ΔH_DIFwH_{{\rm DIF}_{\rm w}} are calculated, and values between –15.95 and –26.81 Jg⁻¹ and between –19.14 and –42.45 Jg⁻¹, respectively are obtained. For low ratios of the mixture, the components’ contributions to the immersion enthalpy of activated carbon and water differ by 3.20 Jg⁻¹.     

苯酚及取代酚在碳纳米管上的吸附研究

本文研究了水溶液中碳纳米管(CNTs)吸附苯酚、对甲酚和对甲氧基苯酚的热力学特性,测定了不同温度下的吸附等温线,并探讨了其可能的吸附机理。结果表明:在稀溶液中碳纳米管对三种酚类物质的吸附均符合Freund lich和Langmu ir方程,吸附均为放热、熵增的自发过程,并且都具有物理吸附特征;碳纳米管与三种酚类物质分子之间的л—л共轭作用的强弱决定了碳纳米管对三种酚类物质的吸附能力,顺序依次为:对甲氧基苯酚>对甲酚>苯酚。     

Nitrogen adsorption on fluorinated activated carbon fiber

Nitrogen adsorption isotherms for fluorinated activated carbon fiber (F-ACF) and fluorinated carbon black (F-CB) were measured at 77 K. Surface structures of F-ACF and F-CB were examined by _s -plot analysis using the adsorption data on the nonporous carbon black (CB) and F-CB. The surface energy of F-ACF was lower than that of ACF. The micropore structure of ACF was preserved even after fluorination, although the limiting adsorption amount and the micropore width decreased with fluorination.     

Carbon dioxide-methane mixture adsorption on activated carbon

In this work, we report new experimental data of pure and binary adsorption equilibria of carbon dioxide and methane on the activated carbon RB2 at 273 and 298 K. The pressure range studied were 0–3.5 MPa for pure gases and 0–0.1 MPa for mixtures. The combination of the generalized Dubinin model to describe the pure CO₂ and CH₄ isotherms with the IAST (Ideal Adsorbed Solution Theory) for the mixtures provide a method for the calculation of the binary adsorption equilibria. This formulation predicts with acceptable accuracy the binary adsorption data and can easily be integrated in general dynamic simulation of PSA (pressure swing adsorption process) adsorption columns. It involves only three parameters, independent of the temperature, and directly determined with only one adsorption isotherm of CO₂.     

Immersion enthalpy of benzene/cyclohexane and toluene/cyclohexane binary mixtures into modified activated carbons

Journal of Thermal Analysis and Calorimetry - The interaction between binary mixtures of benzene/cyclohexane and toluene/cyclohexane into three activated carbons with different physical and...     

甲烷在活性炭上吸附平衡模型的研究

比较吸附模型分析甲烷在活性炭上吸附平衡的适用性,为吸附式天然气(ANG)的工程应用提供准确的预测模型。基于在温度268.15～338.15 K、压力0～12.5 MPa测试的甲烷在Ajax活性炭上的吸附平衡数据,选择Langmuir、Langmuir-Freundlich和Toth方程,应用非线性回归拟合方程参数后,确定绝对吸附量和甲烷吸附相态,并比较方程在不同压力区域内的预测精度。结果表明,甲烷吸附相密度随平衡温度和压力变化;由绝对吸附量确定的甲烷在Ajax活性炭上的平均等量吸附热为15.72 kJ/mol,小于由过剩吸附量的标绘结果;Langmuir、Langmuir-Freundlich和Toth方程预测结果在0～0.025 MPa的累积相对误差为6.449 8%、7.918 4%和0.910 0%,在1～10 MPa为0.491 1%、0.161 3%和0.369 4%。Toth方程在整个压力范围内的预测结果最为准确,但Langmuir-Freundlich方程在较高压力区域内具有较高的预测精度。     

Competitive adsorption of toxic metals on activated carbon

Competitive adsorption of zinc and copper on activated carbon is studied in this article. Main aim was to suggest an advanced model for competitive adsorption of both metals considering pH influence and precipitation. A surface-complexation approach was employed for the modeling. Two models were considered: simple adsorption and ion exchange. System “The Geochemists Workbench” was used for calculation of both static and dynamic adsorption tasks. From the batch experiments, concentration of four types of sorbing sites on the carbon surface and its protonation and sorption constants were deduced. Then, batch competitive adsorption experiments were compared with the models’ results. Finally, a column experiment (fixed bed adsorption) was carried out. It was observed that the model of ion exchange can satisfyingly predict both chromatographic effect and increase of zinc concentration in effluent over its initial value, although a quantitative agreement between the model and the experiment was not totally precise.     

Kinetics of salicylic acid adsorption on activated carbon

The adsorption and desorption of salicylic acid from water solutions was investigated in HPLC microcolumns packed with activated carbon. The adsorption isotherm was obtained by the step-up frontal analysis method in a concentration range of 0-400 mg/L and was well fitted with the Langmuir equation. The investigation of rate aspects of salicylic acid adsorption was based on adsorption/desorption column experiments where different inlet concentrations of salicylic acid were applied in the adsorption phase and desorption was conducted with pure water. The concentration profiles of individual adsorption/desorption cycles data were fitted using several single-parameter models of the fixed-bed adsorption to assess the influence of different phenomena on the column behavior. It was found that the effects of axial dispersion and extraparticle mass transfer were negligible. A rate-determining factor of fixed-bed column dynamics was the kinetics of pore surface adsorption. A bimodal kinetic model reflecting the heterogeneous character of adsorbent pores was verified by a simultaneous fit of the column outlet concentration in four adsorption/desorption cycles. The fitted parameters were the fraction of mesopores and the adsorption rate constants in micropores and mesopores, respectively. It was shown that the former rate constant was an intrinsic one whereas the latter one was an apparent value due to the effects of pore blocking and diffusional hindrances in the micropores.     

Kinetics of adsorption and desorption of Pb(II) in aqueous solution on activated carbon by two-site adsorption model

The adsorption and desorption equilibrium and kinetics of lead ions from aqueous solutions on a granular activated carbon (GAC) were examined. Rapid increase followed by slow increase in Pb(II) amount on the GAC was observed as a function of time for the adsorption, while rapid decrease and consecutive very slow decrease was observed in desorption. Based on the experimental results, a two-site adsorption model was proposed for the adsorption and the desorption of Pb(II) under the study conditions. The Pb(II) adsorption on the GAC was estimated to have simultaneously occurred on the strong and the weak adsorption sites. Conventional Langmuir-type kinetic equations were introduced to quantitatively predict the adsorption and desorption with the two-site model by optimizing the parameters to fit the equilibrium and the kinetic experimental results. The equilibrium and kinetic experimental results could be represented by the equations by using one set of the common Langmuir parameters. Resultant kinetic parameters revealed that the adsorption equilibrium constant was two orders of magnitude greater for strong adsorption site than for weak adsorption site, though the maximum number of weak adsorption site was 1.5 times as great as that of strong adsorption site. The strong adsorption equilibrium constant resulted from a small desorption rate constant for the site. The equations were demonstrated to be applicable for predicting other desorption performances as well.     

The adsorption of dyestuffs from aqueous solutions using the activated carbon adsorption model to determine breakthrough curves

A series of breakthrough curves for the adsorption of Deorlene Yellow and Telon Blue on carbon have been used to test a kinetic model. The model is based on the unreacted core theory and assumes resistances due to external mass transfer and pore diffusion, and irreversible adsorption. The experimental and theoretical data are well correlated by the model at low dye flow rates, but the model was modified at high dye flow rates. The effective diffusivities are 4.0 × 10⁻¹⁰ m² s⁻¹ and 3.0 × 10⁻¹⁰ m² s⁻¹ for Deorlene Yellow and Telon Blue respectively.     

Chromium adsorption in olive stone activated carbon

In this work, Cr(III) adsorption on activated carbon obtained from olive stones in an upflow fixed-bed column at 30^∘C was studied. The flow rate influence on the breakthrough curves at a feed concentration of 0.87 meq/L was investigated in an attempt to minimize the diffusional resistances. Breakthrough curves for a flow range of 2–8 mL/min were obtained at 10.5 cm bed height and inlet diameter of 0.9 cm. The mass transfer parameters indicated that the bed minimal resistance was attained at 2 mL/min. Therefore, the data equilibrium was carried out until the bed was saturated at 2 mL/min. The dynamic system generated a favorable isotherm with a maximum chromium uptake of 0.45 meq/g. A column sorption mathematical model was created considering the axial dispersion in the column and the intraparticle diffusion rate-controlling steps. The isotherm was successfully modeled by the Langmuir equation and the mathematical model described the experimental dynamic data adequately for feed concentrations from 0.26 to 3.29 meq/L.