A Film-Pore-Surface Diffusion Model for the Adsorption of Acid Dyes on Activated Carbon

The sorption of acid dyes from aqueous effluents onto activated carbon has been studied. The effects of initial dye concentration and activated carbon mass on the rate of Acid Blue 80, Acid Red 114 and Acid Yellow 117 removal have been investigated. A three-resistance mass transport model based on film, pore and surface diffusion control has been applied to model the concentration decay curves. The model incorporates an effective diffusion coefficient D _eff, which is dependant on the equilibrium solid phase concentration or fractional surface coverage. The results of the film-pore-surface diffusion model are compared with the data obtained from the basic film-pore diffusion model. It has been found that the film-pore-surface diffusion model provides a major improvement over the data correlated by the film-pore diffusion model. Also, the relationship between surface diffusion and fractional surface coverage has been investigated for the adsorption of acid dyes on activated carbon.     

A Branched Pore Model Analysis for the Adsorption of Acid Dyes on Activated Carbon

A new branched-pore adsorption model has been developed using an external mass transfer coefficient, K _f, an effective diffusivity, D _eff, a lumped micropore diffusion rate parameter, K _b, and the fraction of macropores, f, to describe sorption kinetic data from initial adsorbent-adsorbate contact to the long-term adsorption phase. This model has been applied to an environmental pollution problem—the removal of two dyes, Acid Blue 80 (AB80) and Acid Red 114 (AR114), by sorption on activated carbon. A computer program has been used to generate theoretical concentration-time curves and the four mass transfer kinetic parameters adjusted so that the model achieves a close fit to the experimental data. The best fit values of the parameters have been determined for different initial dye concentrations and carbon masses. Since the model is specifically applicable to fixed constant values of these four parameters, a further and key application of this project is to see if single constant values of these parameters can be used to describe all the experimental concentration-time decay curves for one dye-carbon system.The error analysis and best fit approach to modeling the decay curves for both dye systems show that the correlation between experimental and theoretical data is good for the fixed values of the four fitted parameters. A significantly better fit of the model predictions is obtained when K _f, K _b and f are maintained constant but D _eff is varied. This indicates that the surface diffusivity may vary as a function of surface coverage.     

Vanadium separation with activated carbon and iron/activated carbon nanocomposites in fixed bed column: experimental and modelling study

In this work, iron nanoparticles were impregnated onto a commercial activated carbon surface to produce a novel adsorbent called iron-activated carbon nanocomposite (I-AC). Commercial activated carbon (CAC) and I-AC were used for vanadium separation in a fixed-bed column. The effects of various operating parameters such as inlet vanadium ion concentration, adsorbent dose and volumetric flow rate on vanadium separation performance of CAC were investigated. The performance of both adsorbents was compared in three adsorption/desorption cycles. The experimental breakthrough curves of vanadium ions in the fixed-bed column were modeled using the film-pore-surface diffusion model (FPSDM). The four mass transfer parameters characterizing this model, namely the external mass-transfer coefficient (k _f ), pore and surface diffusion coefficients (D _p and D _s ), and axial dispersion coefficient (D _L ) were evaluated through the model. Modelling and experimental results showed that the I-AC nanocomposite has a better performance for vanadium separation in comparison to AC. Sensitivity analysis on the FPSDM showed that the pore and surface diffusion, external mass transfer and axial dispersion play a significant role in vanadium separation using the I-AC. On the other hand, surface diffusion resulted to be relatively less important when CAC was used.     

Effects of type of adsorption isotherms on parallel diffusion of sulfonated dyes into porous cellulose membrane

Transport phenomenon of three sulfonated azo dyes, C.I. Acid Red 88, C.I. Direct Yellow 12, and C.I. Direct Blue 15 into water-swollen cellulose membranes has been analyzed on the basis of parallel transport theory by surface and pore diffusion. Langmuir equation was applied into the mass balance equation to estimate dye concentration in the pores. The results were compared with the results obtained by applying Freundlich equation in our previous papers. The surface diffusivity (D _s) and the pore diffusivity (D _p) for the parallel diffusion model obtained by applying Langmuir equation agreed with those obtained by applying Freudlich equation. The theoretical concentration profiles for parallel diffusion calculated usingD _s andD _p coincided accurately with the experimental data when we applied either Langmuir or Freundlich equations.     

Effects of affinity and aggregation on parallel diffusion of tri-sulfonated azo compounds into water-swollen cellulose membranes

Adsorption and diffusion of tri-sulfonated azo dyes, C.I. Acid Red 18 and C.I. Acid Red 27 onto waterswollen cellulose membrane has been studied at 25°C. Affinities of these dyes onto cellulose were evaluated by the coefficients of Freundlich equation. Diffusion behavior of these days was analyzed on the basis of a parallel transport theory by surface and pore diffusion. The results could be described by the parallel diffusion model provided that adsorption was stimulated by addition of NaCl. The surface diffusivities for the parallel diffusion model were correlated by the affinity of the dyes, on the other hand, the pore diffusivities for the model were affected by aggregation of the dye depending on its structure and NaCl concentration.     

Adsorption of Pesticides onto Granular Activated Carbon: Determination of Surface Diffusivities Using Simple Batch Experiments

The Homogeneous Surface Diffusion Model (HSDM) has been successfully used to predict the adsorption kinetics for several chemicals inside batch adsorber vessels. In addition to the adsorption equilibrium, this model is based on external mass transfer and surface diffusion. This paper presents the determination of the surface diffusion coefficient (D _s) using a differential column batch reactor (DCBR). The adsorption kinetics for three pesticides onto granular activated carbon have been established experimentally. Their corresponding three diffusion coefficients were determined by fitting the computer simulations to the experimental concentration-time data. The results show that this original apparatus increases by an order of magnitude the range of reachable diffusion coefficient compared to perfectly mixed contactors. Moreover the computed D _s values are more accurate because of the better assessment of the external mass transfer coefficient (k _f) for fixed beds.     

Parallel pore and surface diffusion of levulinic acid in basic polymeric adsorbents

The equilibrium and kinetics of levulinic acid (LA) adsorption on two basic polymeric adsorbents, 335 (highly porous gel) and D315 (macroreticular), were investigated. Experimental adsorption rates in batch stirred vessels under a variety of operating conditions were described successfully by the parallel pore and surface diffusion model taking into account external mass transfer and nonlinear Toth isotherm. The film-pore diffusion model was matched with the rate data and the resulting apparent pore diffusivities were strongly concentration-dependent and approached to a constant value for 335 adsorbent. Thus, the constant value was taken as the accurate pore diffusivity, while the pore diffusivity in D315 was estimated from the particle porosity. The surface diffusivities decreased with increasing initial bulk concentration for both adsorbents. The inverse concentration dependence was correlated reasonably well to the change of isosteric heat of adsorption as amount adsorbed.     

Film-Pore Diffusion Control for the Batch Sorption of Cadmium Ions from Effluent onto Bone Char

The sorption equilibrium and kinetics of cadmium ions from aqueous solution onto bone char have been studied. Equilibrium isotherms for the sorption system were correlated by Langmuir and bi-Langmuir equations. The application of the bi-Langmuir equation was developed because the mechanistic analysis in this research indicated that cadmium removal occurs ion exchange and physical adsorption onto different surface sites. The bi-Langmuir equation provides a better fit to the experimental data. In addition, the removal rates of cadmium ions based on the Langmuir models have been investigated. The effective diffusivity was calculated using the effects of initial metal ion concentration and bone char mass. Two mass-transport models based on film-pore diffusion control have been applied to analyze the concentration decay curves. The film and pore diffusion coefficients using an analytical equation are equal to 1.26x10(-3) cm/s and 5.06x10(-7) cm(2)/s, respectively. The pore diffusion coefficient obtained from the numerical method is 4.89x10(-7) cm(2)/s. A sensitivity analysis showed that the film-pore diffusion model and constant effective diffusivity could be used to describe the mass-transport mechanism of the sorption system with a high degree of correlation. Copyright 2001 Academic Press.     

Study of the influential factors in the simultaneous photocatalytic degradation process of three textile dyes

The influence of several factors in the simultaneous photocatalytic degradation of three textile dyes - Acid Red 97, Acid Orange 61 and Acid Brown 425 - has been studied using a fractional factorial design 2^5-1. The considered factors were: the initial concentration of each dye, the catalyst concentration (TiO₂) and pH.First, we developed a rapid analytical methodology based on recording UV-visible spectra during the degradation process and a data treatment using multivariate curve resolution with alternating least squares (MCR-ALS), which enabled the three dyes to be quantified simultaneously despite the overlap of their spectra.The kinetic constant of degradation for each dye in all the experiments was evaluated. In all cases the degradation followed a first order kinetics. For a significance level of 5%, the most important factor in the photodegradation of each dye is the concentration of Acid Red 97, the degradation is more effective at higher pHs and, in the studied range, the concentration of the catalyst is not important.     

Kinetic and calorimetric study of the adsorption of dyes on mesoporous activated carbon prepared from coconut coir dust

Mesoporous activated carbon has been prepared from coconut coir dust as support for adsorption of some model dye molecules from aqueous solutions. The methylene blue (MB) and remazol yellow (RY) molecules were chosen for study of the adsorption capacity of cationic and anionic dyes onto prepared activated carbon. The adsorption kinetics was studied with the Lagergren first- and pseudo-second-order kinetic models as well as the intraparticle diffusion model. The results for both dyes suggested a multimechanism sorption process. The adsorption mechanisms in the systems dyes/AC follow pseudo-second-order kinetics with a significant contribution of intraparticle diffusion. The samples simultaneously present acidic and basic sites able to act as anchoring sites for basic and acidic dyes, respectively. Calorimetric studies reveal that dyes/AC interaction forces are correlated with the pH of the solution, which can be related to the charge distribution on the AC surface. These AC samples also exhibited very short equilibrium times for the adsorption of both dyes, which is an economically favorable requisite for the activated carbon described in this work, in addition to the local abundance of the raw material.     

Wavelet method to film-pore diffusion model for methylene blue adsorption onto plant leaf powders

In this paper, we have developed an accurate and efficient Haar wavelet method to solve film-pore diffusion model. Film-pore diffusion model is widely used to determine study the kinetics of adsorption systems. To the best of our knowledge, until now rigorous wavelet solution has been not reported for solving film-pore diffusion model. The use of Haar wavelets is found to be accurate, simple, fast, flexible, convenient, and computationally attractive. The power of the manageable method is confirmed. It is shown that film-pore diffusion model satisfactorily describes the kinetics of methylene blue adsorption onto three low-cost adsorbents, Gauva, teak and gulmohar plant leaf powders, used in this study.     

Comparative Adsorption of Crystal Violet and Congo Red onto ZnCl2 Activated Carbon

In this study, the adsorption characteristics of crystal violet (CV) and Congo red (CR) dyes from the aqueous solution onto prepared activated carbon were examined. The activated carbon was prepared from wood apple shell by chemical activation with ZnCl₂. The parameters studied were the effect of contact time, initial dyes concentration, and pH of solution. The experimental equilibrium data were analyzed and fitted to Langmuir, Freundlich, and Temkin isotherms. The maximum monolayer adsorption capacities of CV and CR dyes were found to be 142.85 and 83.33 mg per gram of prepared activated carbon at 298 K. The kinetic data obtained at different concentrations were analyzed using pseudo-first-order, pseudo-second-order, and intra-particle diffusion models. Batch adsorption kinetic studies showed that the adsorption of dyes followed pseudo-second-order kinetics and at four different concentrations of both dyes, indicating that chemisorption is the rate-limiting step. Thermodynamic studies reveal that the removal of dyes from aqueous solution onto activated carbon was a spontaneous, feasible, and endothermic process at a temperature greater than standard equilibrium temperature.     

Removal of phenols from water and petroleum industry refinery effluents by activated carbon obtained from coconut coir pith

Coir pith obtained from the coir industry as waste biomass was used to prepare activated carbon by chemical activation using phosphoric acid (H₃PO₄). The influences of activation temperature and lasting time of activation on specific surface areas (SSA) of the activated carbons were observed. Physical characteristics of the activated carbon were investigated using X-ray diffraction (XRD), infra-red spectroscopy (IR), surface area analyzer, scanning electron microscopy (SEM), thermal analysis and potentiometric titration. The feasibility of using activated carbon for the removal of phenol (P), p-chlorophenol (PCP) and p-nitrophenol (PNP) from water and petroleum refinery industry effluents was investigated. The effects of contact time, adsorbent dose, ionic strength and initial concentration on the adsorption of phenols onto the activated carbon were investigated. The optimum pH for the maximum removal of phenols was 6.0. The equilibrium adsorption data of phenols were correlated to Langmuir and Freundlich isotherm models, the latter being the best fit of the experimental data. Dynamics of the sorption process and mass transfer were investigated using McKay and Urano-Tachikawa models. Adsorption kinetic data fits the Urano-Tachikawa kinetic model. The utility of the adsorbent was tested by using petroleum refinery industry effluent. The adsorbed phenols can be recovered by treatment with 0.1 M NaOH solution.     

Adsorption of acid dyes from aqueous solutions onto acid-activated bentonite

The adsorption of two dyes, namely, Acid Red 57 (AR57) and Acid Blue 294 (AB294), onto acid-activated bentonite in aqueous solution was studied in a batch system with respect to contact time, pH, and temperature. Acidic pH was favorable for the adsorption of these dyes. The surface characterization of acid-activated bentonite was performed using the FTIR technique. The pseudo-first-order and pseudo-second-order kinetic models and the intraparticle diffusion model were used to describe the kinetic data and the rate constants were evaluated. The dynamic data fitted the pseudo-second-order kinetic model well and also followed the intraparticle diffusion model up to 90 min, but diffusion is not the only rate controlling step. The Langmuir and Freundlich adsorption models were applied to describe the equilibrium isotherms and the isotherm constants were determined. The Freundlich model agrees very well with experimental data. The activation energies of adsorption were also evaluated for the adsorption of AR57 and AB294 onto activated bentonite.     

Sorption of some azo-dyes on wool fiber from aqueous solutions

Sorption of some azo-dyes (acid violet 17, acid blue 90, acid red 1 and direct red 80) on locally available wool fiber from aqueous solution has been investigated as a function of shaking time, pH and dyes concentration as a function of temperature. Sorption isotherms for dyes on wool fiber were obtained. It was found that the increasing temperature induces positive effect on the sorption process. Isosteric heat of adsorption (q_st) have also been calculated at different surface coverage indicating that wool fiber used possessed heterogeneous surface with site of different activity. The results also indicate that initial sorption whether chemical or physical means, occurs first on the most active sites and then, as the concentration of dyes is increased, on sites of lesser activity.     

Modeling adsorption rate of organic micropollutants present in landfill leachates onto granular activated carbon

The overall adsorption rate of single micropollutants present in landfill leachates such as phthalic acid (PA), bisphenol A (BPA), diphenolic acid (DPA), 2,4-dichlorophenoxy-acetic acid (2,4-D), and 4-chloro-2-methylphenoxyacetic acid (MCPA) on two commercial activated carbons was studied. The experimental data obtained were interpreted by using a diffusional model (PVSDM) that considers external mass transport, intraparticle diffusion, and adsorption on an active site. Furthermore, the concentration decay data were interpreted by using kinetics models. Results revealed that PVSDM model satisfactorily fitted the experimental data of adsorption rate on activated carbon. The tortuosity factor of the activated carbons used ranged from 2 to 4. The contribution of pore volume diffusion represented more than 92% of intraparticle diffusion confirming that pore volume diffusion is the controlling mechanism of the overall rate of adsorption and surface diffusion can be neglected. The experimental data were satisfactorily fitted the kinetic models. The second-order kinetic model was better fitted the experimental adsorption data than the first-order model.     

Modelling of solid-liquid adsorption: effects of adsorbent heterogeneity

Effects of adsorbent heterogeneity on the adsorption of cobalt phthalocyanine dye on activated carbon have been studied. Adsorption experiments were carried out by varying the temperature and adsorbent mass in batch adsorbers and, in addition, the adsorbent particle size and fluid flow rate in a continuous stirred tank reactor (CSTR)-type adsorber in order to investigate the equilibrium and the kinetics of adsorption.The Brunauer-Emmett-Teller (BET), Langmuir with uniform distribution (LUD) and Langmuir-Freundlich equations are able to represent the equilibrium data with similar accuracy within the range of measurements. Reasonably large values of the heterogeneity parameter (2.69–2.86) show that the carbon surface is energetically heterogeneous.A mathematical model that describes the adsorption dynamics, including film-, pore- and concentration-dependent surface diffusion on an energetically and structurally heterogeneous adsorbent, is presented here and fitted to the experimental concentration vs. time curves obtained in the continuously stirred tank adsorber.Structural heterogeneity of the carbon, if not accounted for in the kinetic model, can be responsible for the very strong concentration dependence of the surface diffusion coefficient and for the variation in the parameter D_o with particle size and adsorber porosity as shown in this work.     

Sulfur fixation on bagasse activated carbon by chemical treatment and its effect on acid dye adsorption

Heteroatoms are known to introduce specific surface functionalities that can enhance the adsorption properties of carbons. Sulfur fixation on bagasse-activated carbon was conducted by a low temperature chemical treatment with sulfuric acid followed by physical activation with CO₂ at 900?°C. The effect of sulfur fixation on the surface chemical properties of bagasse-activated carbons were investigated and on their subsequent acid dye removal (CIBA AB80) behavior. Surface chemical development were examined and followed using Fourier transform infrared spectroscopy (FTIR), heteroatom analysis and carbon surface acidity. Functional group stability with thermal treatment was also investigated. The textural properties of the activated carbons were characterized by nitrogen adsorption. Chemical pre-treatment and gasification was able to fix up to 0.2 wt% of sulfur on the activated carbon. Although the sulfur fixed by chemical treatment is low, this method introduced several advantages in comparison to fixation by thermal methods. The chemical method did not interfere with the textural development of the carbon, as found in thermal methods. In addition, the surface chemistry generated by these levels of sulfur groups was sufficient to increase the uptake of acid blue dyes by more than 700% based on adsorption capacities normalized by the surface area of the carbon.     

Adsorption of dyes on activated carbon and graphitic thermal carbon black

The adsorption of a basic dye (Methylene Blue; MB) and an acidic dye (Acid Orange; AO) has been studied on three activated carbons (ACs; FAS, SKD, and BAU) significantly differing in their porous structures and surface concentrations of ion-exchange groups and on graphitic thermal carbon black (GTCB). The effective specific surface area of FAS, SKD, and BAU determined by dye adsorption is, respectively, 60, 50, and 40% of the BET nitrogen adsorption surface area. The MB uptake on ACs and GTCB increases with rising pH, while the AO uptake decreases. Addition of an electrolyte (0.3 M NaCl) virtually does not effect the adsorption of dyes on ACs and GTCB. It is suggested that hydrophobic interactions, and not ionic ones, are the major contributors to the adsorption of dyes on ACs.     

活性炭催化碳酸二甲酯水解行为的研究

以无氯Cu/AC催化剂中活性炭载体为研究对象,对活性炭上DMC水解反应条件进行了考察,并通过Boehm滴定法和XPS对活性炭上DMC水解活性位点进行了分析。结果表明,温度的升高和水含量的增加对DMC水解有较大的促进作用,较高的压力和CO₂气氛对DMC水解有一定的抑制作用,甲醇量和CO、O₂、N₂气氛对DMC水解影响较小。不同条件处理的活性炭表面基团种类、数量均有较大变化,HNO₃处理使活性炭碱性基团含量下降,羧基等酸性含氧官能团含量明显增加,总酸量最高可达1.88 mmol·g^-1;先HNO₃后NaOH处理的活性炭上酸性基团含量大量减少,表面碱性基团含量则有较大提高,总碱量最高可达1.69 mmol·g^-1。特别地,活性炭表面碱性基团是催化DMC水解的活性位点,在活性炭碱性基团含量由0.16增加到1.69 mmol·g^-1的过程中,DMC水解程度由2.5%增加到了31.7%,而酸处理可以有效降低活性炭表面碱性基团含量,抑制其催化DMC的水解性能。