Modification of Langmuir isotherm in solution systems--definition and utilization of concentration dependent factor

The Langmuir isotherm, originally derived for the adsorption of gas molecules on solid surfaces, was modified to fit the adsorption isotherm of solutes onto solid surfaces in solution systems. The aim of this modification is based on the fact that direct application of the Langmuir isotherm to solution systems often leads to poor data fitting. In the present communication, it is shown that the level of data fitting to the Langmuir isotherm of literature data can be improved by a simple modification introducing a concentration dependent factor, X. The key concept of the modification lies in that the concentration of solute affects both adsorption and desorption stages. As a first approximation, we adopted a single-term polynomial for both processes of adsorption and desorption. Based on reanalysis of literature data of adsorption in solution, we confirmed that indeed the modified Langmuir isotherm more accurately describes the experimental observations. Furthermore, we proposed that the concentration dependent factor could be associated with the surface heterogeneity index that was introduced in a few other modified Langmuir isotherms. Some advantages and limitations of proposed modified Langmuir isotherm are also discussed.

Anion-Exchange Equilibrium and Phase Segregation in Hydrotalcite Systems: Intercalation of Hexacyanoferrate(III) Ions

The intercalation of hexacyanoferrate(III) in chloride-hydrotalcite was studied as a function of the composition of the equilibrated aqueous phase, the relevant solution variable being the ratio (C(Fe(CN)(6)(3-))(1/3)/C(Cl)-. The isotherm shows that the exchange is markedly nonideal and reveals the segregation of two solid phases of fixed compositions at (C(Fe(CN)(6)(3-))(1/3)/C(Cl)- = 17.1 M(-2/3). This is the first case of solid phase segregation during anion exchange in hydrotalcite systems diagnosed from thermodynamic measurements. In agreement, PXRD patterns of the partially exchanged solids indicate that these phases, with distinct basal spacings, coexist in the range 0.21 < or = x < or = 0.86. The observed behavior is accounted for by classical thermodynamic formalisms of ion exchange. The limitations of other representations commonly used to describe anion exchange in layered double hydroxide systems are stressed. A rationale of the structural changes accompanying the exchange is also offered.

Adsorption characteristics of Fe(III) and Fe(III)-NTA complex on granular activated carbon

The adsorption of Fe(3+) ion on granular activated carbon has been studied in kinetic and equilibrium conditions taking into account the adsorbate concentration, temperature and solution pH as major influential factors. In addition, the effect of nitrilotriacetic acid on adsorption reaction as a complexing agent has been examined. Kinetic studies showed that the adsorption rate was increased as the initial Fe(3+) concentration was raised. The adsorption reaction was estimated to be first-order at room temperature. The adsorption rate and equilibrium adsorption of Fe(3+) increased as the temperature rose. The activation energy for adsorption was approximately 2.23 kJ mol(-1), which implied that Fe(3+) mainly physically adsorbed on activated carbon. Coexistence of nitrilotriacetic acid with Fe(3+) resulted in a decrease of equilibrium adsorption and the extent of decrease was proportional to the concentration of nitrilotriacetic acid. In the presence of nitrilotriacetic acid, the adsorbability of Fe(3+) decreased with pH. However, the trend was reversed in the absence of nitrilotriacetic acid. When activated carbon was swelled by acetic acid, the specific surface area was increased and maximum swelling was achieved at approximately 48 h of swelling time. Thermodynamic parameters such as DeltaG(o), DeltaH(o) and DeltaS(o) for adsorption reaction were estimated based on equilibrium data and in connection with these results the thermodynamic aspects of adsorption reaction were discussed.

Anion specific adsorption on Fe2O3 and AlOOH nanoparticles in aqueous solutions: comparison with hematite and γ-Al2O3

The specific adsorption of radiolabeled sulfate and phosphate ions from perchlorate supporting electrolyte onto nano-AlOOH and nano-Fe(2)O(3) powder has been investigated. The pH dependence of the adsorption of anions onto nanopowders was compared with that of the same ions onto gamma-Al(2)O(3) and hematite. It was demonstrated that the character of the pH dependence of the adsorption is very similar in the comparable cases. It was found, however, that in contrast to the behavior of gamma-Al(2)O(3), nano-AlOOH dissolves at a significant rate at low pH values (pH<2). Thus the study of the pH dependence of the anion adsorption encounters difficulties at these pH values. Disregarding this fact, it can be concluded that no special effects can be observed in the anion adsorption onto the nano-oxides studied.

Application of the Radiotracer Technique for the Study of the Specific Anion Adsorption on Al(2)O(3) in Acidic Medium

The specific adsorption of radiolabeled anions (sulfate, chloride) from perchlorate supporting electrolyte on gamma-Al(2)O(3) powder has been investigated. It was demonstrated that a measurable adsorption occurs only at pH values under pH 6. The adsorption capacity of a given amount of gamma-Al(2)O(3) powder tends to a limiting value with decreasing pH. A Langmuir-like adsorption isotherm describes the concentration dependence of the adsorption of sulfate species. It was found that the experimental results are very similar to those obtained in the case of hematite. Copyright 2000 Academic Press.

On the Applicability of the Langmuir Equation to Estimation of Adsorption Equilibrium Constants on a Powdered Solid from Aqueous Solution

The Langmuir adsorption equation has become popular for summarizing solution-solid adsorption data and estimating adsorption equilibrium constants. The applicability of the Langmuir equation to estimation of adsorption equilibrium constants at solid-aqueous solution interfaces was theoretically examined in terms of (1) comparisons of solid-solution adsorption with solid-gas adsorption, (2) deductions of the Langmuir mathematical expression for the nonelectrolyte/solvent exchange, the phase exchange, the ion exchange, and the simple association reactions, and (3) the fitting procedure. The restrictive assumptions underlying the Langmuir equation for solid-gas adsorption may not be valid for adsorption of solute of small molecules or ions by a powdered solid surface from the aqueous solution phase. The fit of adsorption data to the Langmuir mathematical expression cannot be ascertained solely by the linearity of the commonly used equilibrium concentration/amount adsorbed versus the equilibrium concentration plot and does not provide the actual mechanism of the loss of small molecules or ions from aqueous solution. Therefore, the parameters of fitting to the Langmuir mathematical expression may not provide an unambiguous chemical meaning. In addition, we attempt to bring attention to the importance of the standard states of surface phase and solution phase. Copyright 2000 Academic Press.

Competitive Adsorption of Ferricyanide and Ferrocyanide on gamma-Al(2)O(3) Surface

In the past 3 decades, research has proven the significance of competitive adsorption in the equilibrium of pollutants between solid and liquid phases. However, studies on the competitive adsorption of complex ions are very limited in spite of its important role in transporting pollutants in the natural environment. The objective of this study is to derive the thermodynamic parameters of the competitive adsorption between ferricyanide and ferrocyanide from the modified Langmuir isotherm and the triple-layer model (TLM) to determine the location of adsorption. The effects of pH, temperature, and ion concentration on competitive adsorption onto gamma-Al(2)O(3) were investigated. The results demonstrate that ferrocyanide is more competitive than ferricyanide. By comparing the derived K(app) with K(int), we inferred that the adsorption of ferricyanide and ferrocyanide onto gamma-Al(2)O(3) was achieved through outer-sphere complexation. The negative DeltaH degrees indicated that the adsorption was exothermic. The positive entropy (Delta S degrees ) was caused by the replacement and release of a greater number of smaller surface ions by adsorbed ferricyanide and ferrocyanide ions of larger size. Copyright 2000 Academic Press.

Adsorption Behavior of Iron-Cyanide onto gamma-Al2O3 Interface: A Coagulation Approach

Ferrocyanide and ferricyanide ions have strong coagulation ability in a natural water system due to their high valences. Studies with aluminum oxide turbid waters showed significant differences in coagulation between simple ions (Cl-, SO2-4, Fe(CN)3-6, Fe(CN)4-6) and other species (H2PO-4) that interact chemically with the oxide surface. The evidence suggested that the adsorption of ferrocyanide and ferricyanide on aluminum oxide surface is an outer-sphere reaction. The linear relationship between the logarithm of the significant coagulation concentration and Schultz-Hardy ratios indicated that the coagulation obeyed the DLVO rule. Therefore, it is concluded that the coagulation of aluminum oxide by ferrocyanide and ferricyanide is essentially caused by compression of the electric double layer rather than by charge neutralization. Copyright 1999 Academic Press.

Potentiostatically Enhanced Complexation Model for the Determination of Isopotential Equilibrium Curves

A potentiostatically enhanced complexation model for open systems has been developed that describes the adsorption of heavy metal ions under different pH conditions and forcing electrostatic potentials. This model is based on the so-called complex formation models, and it assumes adsorption of only free ionic species, existence of one type of adsorption site, and limited adsorption site availability. The results show that potentially enhanced adsorption processes can be used for the adsorption of heavy metal ions at pH conditions where adsorption is weak, yet potentials are small enough to avoid reduction and plating, and also electrolysis of water. Moreover desorption can be achieved by turning off the applied potential or by reversing it, thereby avoiding the use of additional chemicals, e.g., a strong acid, to achieve regeneration. Copyright 1998 Academic Press.