Kinetic modeling of liquid-phase adsorption of reactive dyes on activated carbon

In this paper, adsorption equilibrium and kinetics of three reactive dyes from their single-component aqueous solutions onto activated carbon were studied in a batch reactor. Effects of the initial concentration and adsorbent particle size on adsorption rate were investigated Adsorption equilibrium data were then correlated with several well-known equilibrium isotherm models. The kinetic data were fitted using the pseudo-first-order equation, the pseudo-second-order equation, and the intraparticle diffusion model. The respective characteristic rate constants were presented. A new adsorption rate model based on the pseudo-first-order equation has been proposed to describe the experimental data over the whole adsorption process. The results show that the modified pseudo-first-order kinetic model generates the best agreement with the experimental data for the three single-component adsorption systems.     

Modification of microcrystalline cellulose with pyridone derivatives for removal of cationic dyes from aqueous solutions

Microcrystalline cellulose (MCC) was modified with pyridone derivatives such as pyridone diester (PDE) and pyridone diacid (PDA) by using succinic acid anhydride as a linker. The modified MCCs were characterized by the fourier transform infrared spectroscopy, scanning electron microscopy, thermal gravimetric analysis, elemental analysis and solid state ¹³C NMR. The adsorption capacities of the modified MCCs to cationic dyes were examined by using methylene blue (MB) as a model dye. It was found that the kinetic adsorption data followed the pseudo-second-order kinetic model, and the adsorption equilibriums were reached less than 10 min. The isothermal adsorption data were fitted with the Langmuir isotherm model very well, from which the maximum adsorption capacities of the MCCs modified with PDE and PDA were determined to be 101.01 and 142.86 mg/g, respectively. Further investigation showed that the modified MCCs were pH-dependent for adsorption of MB in aqueous solutions. The modified MCCs could be used for removal of MB from an aqueous solution at pH 8, and reused by regeneration in an acidic solution. It was tested that the modified MCCs had a high reusability for removal of MB from aqueous solutions, and still maintained high adsorption capacities even after multiple cycles of desorption–adsorption processes. Hence, the MCCs modified with PDE and PDA could be an effective and efficient approach to removal of cationic dyes from aqueous solutions.     

Consistent approach to adsorption thermodynamics on heterogeneous surfaces using different empirical energy distribution models

Adsorption on heterogeneous surfaces with three basic energy distribution models (uniform model, exponential model, and normal-like model) is studied. Exact analytical solutions of the adsorption isotherms and the heats of adsorption are derived for the uniform and exponential models, and, with these solutions including a numerical solution for the normal-like model, the behavior of the differential heat of adsorption and the "apparent" standard adsorption entropy concerning the overall surface is described as a function of coverage and temperature. The approximations underlying the isotherms and heats of adsorption in the Temkin, Freundlich, and Langmuir-Freundlich types of adsorption are rationalized. By comparing these empirical formulas to the exact solutions, the level of these approximations is found to be identical, which is similar to the "condensation approximation". Their preconditions are that either the temperature is low enough, or the surface is strongly heterogeneous. Generally, they are suitable for the middle coverage range. The exact solutions provide a method to obtain more information on the heats, entropy, and heterogeneity of the catalyst surface from the calorimetric measurement of the heat of adsorption.     

A quasi-equilibrium theory of protein adsorption

A model is developed to describe the adsorption and desorption of proteins to and from a surface film under quasi-equilibrium conditions. Starting from Fick's first law of diffusion, an equation for the flux of molecules to a surface is derived assuming a gradient in the chemical potential from the bulk to the surface and a potential barrier due to an existing surface film. Protein molecules are modeled as components with varying surface areas to depict the different orientations of molecules with respect to the film. For concentrated solutions, formation of multilayer protein films is described by allowing components with small minimum surface areas. The thermodynamic analysis is based on Butler's equation for the chemical potentials of the components of a Gibbs surface layer and a first-order model for the nonideality of the surface layer enthalpy and entropy. The model assumes reversible adsorption, consistent with globular proteins that show little denaturation or flexible-chain proteins that reversibly denature at the interface. The model predicts the behavior of five different experiments measuring film properties of the serum protein albumin in quasi-equilibrium and equilibrium conditions at over 2 orders of magnitude in concentration using a single set of parameters. This provides a new framework for analyzing interactions and adsorption of protein films. The key new features of this model are an extension of the classical Smoluchowski analysis to calculate the adsorption and desorption rate, a model of multilayers with decreased molecular areas to allow effective densities greater than a close-packed monolayer, and a concentration-dependent layer thickness.     

Adsorption behaviour of sugars versus their activity in single and multicomponent liquid solutions

In this work the activity of three carbohydrates (sucrose, glucose and fructose) in highly concentrated aqueous solutions was studied along with its effect on the adsorption behaviour of the investigated compounds. Activities of individual sugars in aqueous solutions of single solute as well as in binary mixtures were quantified on the basis of solubility properties. Solid–liquid equilibria of sugars were correlated with the NRTL (nonrandom, two liquid) model of activity coefficient formulation. Activities of individual sugars were incorporated into the single component adsorption isotherm model, which reproduced accurately the course of the adsorption equilibria of sugars in aqueous solutions obtained experimentally in previous work using an ion-exchange resin. Activities of sugars determined in binary solute systems along with the single component isotherms were used to predict competitive adsorption equilibria. To calculate adsorbed phase concentrations of individual sugars in binary mixtures the adsorbed solution theory was adopted. The isotherm shapes calculated were compared to the data of competitive adsorption from the former study and found to be able to describe these experimental results.     

MFI zeolite as adsorbent for selective recovery of hydrocarbons from ABE fermentation broths

1-Butanol and butyric acid are two interesting compounds that may be produced by acetone, butanol, and ethanol fermentation using e.g. Clostridium acetobutylicum. The main drawback, restricting the commercialization potential of this process, is the toxicity of butanol for the cell culture resulting in low concentrations of this compound in the broth. To make this process economically viable, an efficient recovery process has to be developed. In this work, a hydrophobic MFI type zeolite with high silica to alumina ratio was evaluated as adsorbent for the recovery of butanol and butyric acid from model solutions. Dual component adsorption experiments revealed that both butanol and butyric acid showed a high affinity for the hydrophobic MFI zeolite when adsorbed from aqueous model solutions. Multicomponent adsorption experiments using model solutions, mimicking real fermentation broths, revealed that the adsorbent was very selective to the target compounds. Further, the adsorption of butyric and acetic acid was found to be pH dependent with high adsorption below, and low adsorption above, the respective pKa values of the acids. Thermal desorption of butanol from MFI type zeolite was also studied and a suitable desorption temperature was identified.     

On the specific adsorption of anions from solutions of low ionic strength

The specific adsorption of anions at positive electrodes from solutions of low ionic strength is examined on the basis of the electrostatic (Stern-Levine) model for specific ionic adsorption. It is shown that as the ionic strength is decreased, the adsorbed charge density falls below the electronic charge density on the metal; for these conditions, high positive potentials are expected at the outer Helmholtz plane. The above model predicts that there is no extended potential region where the diffuse-layer charge is zero. Difficulties in estimating the potential drop across the diffuse layer in dilute solutions are discussed and a possible defect in the simple application of the Stern-Levine model pointed out.     

Low-temperature synthesis of nanoparticle-assembled, transparent, and low-crystallized hydroxyapatite blocks

The mesoporous carbon CMK-3 adsorbent was prepared, characterized, and used for the removal of anionic methyl orange dye from aqueous solution. Adsorption experiments were carried out as batch studies at different contact time, pH, initial dye concentration, and salt concentration. The dye adsorption equilibrium was rapidly attained after 60 min of contact time. Removal of dye in acidic solutions was better than in basic solutions. The adsorption of dye increased with increasing initial dye concentration and salt concentration. The equilibrium data were analyzed by the Langmuir and Freundlich models, which revealed that Langmuir model was more suitable to describe the methyl orange adsorption than Freundlich model. Experimental data were analyzed using pseudo-first-order and pseudo-second-order kinetic models. It was found that kinetics followed a pseudo-second-order equation. Thermodynamic study showed that the adsorption was a spontaneous and exothermic process.     

Kinetic and isothermal studies of lead ion adsorption onto palygorskite clay

The use of a natural palygorskite clay for the removal of Pb(II) from aqueous solutions for different contact times, pHs of suspension, and amounts and particle sizes of palygorskite clay were investigated. The variations of the pH value of Pb(II) solutions on natural palygorskite in the adsorption process were determined. Batch adsorption kinetic experiments revealed that the adsorption of Pb(II) onto palygorskite clay involved fast and slow processes. It was found that the adsorption mechanisms in the lead/palygorskite system follow pseudo-second-order kinetics with a significant contribution from film diffusion. SEM observations demonstrated that an important interaction at the lead-granule interface occurred during the adsorption process. The adsorption isotherms were described by means of the Langmuir and Freundlich isotherms and the Langmuir model represents the adsorption process better than the Freundlich model. The maximum adsorption capacity of Pb(II) onto natural palygorskite was 104.28 mg g(-1).     

Adsorption of polyvinylimidazole onto kaolinite

The adsorption of polyvinylimidazole (PVI) onto kaolinite from aqueous solutions has been investigated systematically as a function of parameters such as calcination temperature of kaolinite, pH, ionic strength, and temperature. According to the experimental results, the adsorption of PVI increases with pH from 8.50 to 11.50, temperature from 25 to 55 degrees C, and ionic strength from 0 to 0.1 mol L(-1). The kaolinite sample calcined at 600 degrees C has a maximum adsorption capacity. Adsorption isotherms of PVI onto kaolinite have been determined and correlated with common isotherm equations such as Langmuir and Freundlich isotherm models. The Langmuir isotherm model appeared to fit the isotherm data better than the Freundlich isotherm model. The physical properties of this adsorbent are consistent with the parameters obtained from the isotherm equations. Furthermore, the zeta potentials of kaolinite suspensions have been measured in aqueous solutions of different PVI concentrations and pH. From the experimental results, (i) pH strongly alters the zeta potential of kaolinite; (ii) kaolinite has an isoelectric point at about pH 2.35 in water and about pH 8.75 in 249.9 ppm PVI concentration; (iii) PVI changes the interface charge from negative to positive for kaolinite. The study of temperature effect has been quantified by calculating various thermodynamic parameters such as Gibbs free energy, enthalpy, and entropy changes. The dimensionless separation factor (RL) has shown that kaolinite can be used for adsorption of PVI from aqueous solutions.     

Model and experimental studies for contact angles of surfactant solutions on rough and smooth hydrophobic surfaces

Despite the practical need, no models exist to predict contact angles or wetting mode of surfactant solutions on rough hydrophobic or superhydrophobic surfaces. Using Gibbs' adsorption equation and a literature isotherm, a new model is constructed based on the Wenzel and Cassie equations. Experimental data for aqueous solutions of sodium dodecyl sulfate (SDS) contact angles on smooth Teflon surfaces are fit to estimate values for the adsorption coefficients in the model. Using these coefficients, model predictions for contact angles as a function of topological f (Cassie) and r (Wenzel) factors and SDS concentration are made for different intrinsic contact angles. The model is also used to design/tune surface responses. It is found that: (1) predictions compare favorably to data for SDS solutions on five superhydrophobic surfaces. Further, the model predictions can determine which wetting mode (Wenzel or Cassie) occurred in each experiment. The unpenetrated or partially penetrated Cassie mode was the most common, suggesting that surfactants inhibit the penetration of liquids into rough hydrophobic surfaces. (2) The Wenzel roughness factor, r, amplifies the effect of surfactant adsorption, leading to larger changes in contact angles and promoting total wetting. (3) The Cassie solid area fraction, f, attenuates the lowering of contact angles on rough surfaces. (4) The amplification/attenuation is understood to be due to increased/decreased solid-liquid contact-area.     

Study on the Surface Property of Surfactant Ionic Liquids Solutions

The surfactant TX-100 can be dissolved in ionic liquid bmimPF6 and decrease the surface tension of 1-buty1-3-methylimidazolium hexafluorophosphate (bmimPF6) solutions. Here, we confirmed that in this new system, the pure solvents need rearrangement at the air-wate rinterface at the initial stage. The dynamic surface tension (DST) study shows that at the initial adsorption stage, the adsorption model of surfactant accords with the diffusion-controlled adsorption mechanism, and the dilute ionic liquids solutions is further close to the diffusion-controlled adsorption.     

A Mechanistic Approach to Modeling Single Protein Adsorption at Solid-Water Interfaces

A kinetic model for single component protein adsorption which can be readily extended to adsorption from multi-protein solutions was developed, and used to simulate adsorption of site-directed, structural stability mutants of bacteriophage T4 lysozyme. The model allows for two different adsorbed "states," distinguished by different binding strengths and different occupied areas. The presence of an increasing energy barrier to adsorption was incorporated into the model by formulating the adsorption rate constants as functions of time. Numerical analysis was performed using the Marquardt method. Estimated model parameters were consistent with the effect of structural stability on adsorption. In particular, kinetic parameters were such that adsorption into the more tightly bound, conformationally altered state was favored by less stable variants. Copyright 1999 Academic Press.     

Use of oxide minerals to abate fluoride from water

The removal of fluoride from aqueous solutions has been investigated using various oxide ores such as refractory grade bauxite, feed bauxite, manganese ore, and hydrated oxides of manganese ores (WAD). The refractory grade bauxite showed promising results. The studies were carried out as functions of contact time, pH, concentration of adsorbents, concentration of adsorbate, and temperature. The adsorption was rapid during the initial 5 min but equilibrium was attained within 120 min. The adsorption followed first-order kinetics. The present system followed the Langmuir adsorption isotherm model. Various thermodynamic parameters such as free energy, enthalpy, entropy, and equilibrium constants were calculated. The isosteric heat calculations showed that the adsorption process followed a heterogeneous model.     

Fixed bed phosphate adsorption by immobilized nano-magnetite matrix: experimental and a new modeling approach

Nano-sized magnetite impregnated charcoal granular activated carbon (nFe-GAC) was utilized for the removal of phosphate from aqueous solutions using a fixed bed column. The dynamic of the phosphate adsorption was analyzed using a new approach to the Thomas model based on a two-step differential sorption rate process. The initial adsorption was found to be external mass transfer controlled, while intra-particle diffusion was the predominant mechanism in the latter stage. Consequently, two kinetic coefficients were calculated for each breakthrough curve resulting in an excellent model prediction. By implementing this approach a transition point, at which diffusion becomes the predominant adsorption mechanism, can be accurately determined. The effect of varying parameters, such as feed flow rates, feed pH, initial phosphate concentrations and adsorbent bed height were examined and described using the modified Thomas model. Reaction rates increased with augmentation of the flow rates from 1 to 40 mL/min while the adsorption capacity and transition point decreased. Similar transition points were obtained for initial phosphate concentrations between 10 and 100 mg/L. The unique characteristics of the nFe-GAC were evident as it exhibited very high phosphate adsorption capacity, at a wide range of pH values (4–9) with negligible effect of competing ions and short critical bed depth.     

Irreversible adsorption of triple-helical soluble collagen monomers from solution to glass and other surfaces

The adsorption from various solutions of triple-helical soluble collagen monomers to solid surfaces was studied by labeling the collagen with ¹²⁵1. Adsorption to glass, siliconized glass, and Teflon, from aqueous solutions at various pH and ionic strength, was determined at collagen concentrations from 2 to 25 μg/ml. Adsorption was shown to be irreversible and little dependent on pH and ionic strength but increasing enormously as the surface is made more hydrophobic. Surface denaturation of the collagen by heat results in a substantial loss of material. The kinetics of adsorption suggest that the adsorption process may be selective and that not all collagen molecules which reach the surface are immediately adsorbed. Checking these results with earlier measurements of adsorbed layer thickness, a model for collagen adsorption is proposed.