Adsorption Of Organic Solutes Research Articles

How to avoid mistakes in treating adsorption isotherm data (liquid and solid phases): Some comments about correctly using Radke-Prausnitz nonlinear model and Langmuir equilibrium constant

Two flaws in concepts were identified and discussed in the paper (“Removal of Pb(II) from contaminated waters using cellulose sulfate/chitosan aerogel: Equilibrium, kinetics, and thermodynamic studies”. J. Environ. Manag. 286, 112167; https://doi.org/10.1016/j.jenvman.2021.112167). In the literature, the Radke–Prausnitz model is expressed in different forms, but some of them are incorrect. The first flaw is related to the nonlinear form of the Radke–Prausnitz model. The nonlinear form of this three parameters model is expressed correctly as qe=ARPBRPCeβARP+BRPCeβ–1. The units of two parameters are ARP (L/kg) and BRP [(mol/kg)/(mol/L)β] by considering qe (mol/kg) and Ce (mol/L). The limitation for its exponent is 0≤ β ≤ 1. This model is developed by two authors (Radke and Prausnitz). The correct paper (DOI: 10.1021/i160044a003) cited as reference of this model is “Radke, C.J., Prausnitz, J.M., 1972. Adsorption of organic solutes from dilute aqueous solution of activated carbon. Ind. Eng. Chem. 11, 445–451”. The second is the misconception about the unit of the Langmuir constant (KL; L/mg). The correct unit of KL is litre per milligram of adsorbate (i.e., Pb ions), not litre per milligram of adsorbent (the cellulose sulfate/chitosan aerogel material as reported by Najaflou and co-workers. They proposed a new equation [KL (L/mg) × m/V (mg/L)] to convert the Langmuir constant and then applied it to calculate the thermodynamic parameters of the adsorption process. The m/V is a solid/liquid ratio (g/L or kg/L). However, this conversion and application are mistakes that were thoroughly discussed in this paper. The correction is KEqo=1γAdsorbate×KLLmol×ComolL, with C° (1 mol/L by definition) being the standard state of solute and γAdsorbate (dimensionless) being the activity coefficient of adsorbate in solution. To avoid unexpected mistakes, the present authors suggest that researchers should have a correct citation (citing the original reference instead of using secondary references) and check the consistency of units (i.e., the constants of adsorption models) carefully.

The correlation between structural characteristics of activated carbons and their adsorption of organic solutes from aqueous solutions

Adsorption is controlled by an array of attractive forces between adsorbent, adsorbate, and solvent molecules. Such forces work interactively, making the interpretation and prediction of sorption processes difficult. By carefully designing the experimental matrix, the effects of adsorbent surface chemistry and pore structure on the adsorption of aromatic compounds were isolated from the complicated web of interactions. Two parameters, γ and δ, were created to describe the relative adsorption affinity index of activated carbons to adsorbates and the occupancy rate of activated carbons by active sites that can lead to formation of water clusters. Taking the space availability and the relative adsorption affinity index into account, a correlation between the Freundlich adsorption affinity coefficient and the characteristics of adsorbent was established. With this correlation, if the Freundlich adsorption affinity coefficient of a compound on one carbon is known, its adsorption affinity coefficient on another carbon might be predictable if the surface chemistries and pore structures of both carbons are available.

A thermodynamic approach to assess organic solute adsorption onto activated carbon in water

In this paper, the hydrophobicity of 13 activated carbons is determined by various methods; water vapour adsorption, immersion calorimetry, and contact angle measurements. The quantity and type of oxygen-containing groups on the activated carbon were measured and related to the methods used to measure hydrophobicity. It was found that the water-activated carbon adsorption strength (based on immersion calorimetry, contact angles) depended on both type and quantity of oxygen-containing groups, while water vapour adsorption depended only on their quantity. Activated carbon hydrophobicity measurements alone could not be related to 1-hexanol and 1,3-dichloropropene adsorption. However, a relationship was found between work of adhesion and adsorption of these solutes. The work of adhesion depends not only on activated carbon–water interaction (carbon hydrophobicity), but also on solute–water (solute hydrophobicity) and activated carbon–solute interactions. Our research shows that the work of adhesion can explain solute adsorption and includes the effect of hydrogen bond formation between solute and activated carbon.

Influence of type and position of functional groups of phenolic compounds on NF/RO performance

The performance of commercial reverse osmosis (TFC-HR, BW-30) and nanofiltration (NF-90) membranes was evaluated for phenol and eleven phenolic derivatives in aqueous solution. The correlation between flux decline and organic retention was investigated. Molecular hydrophobicity, related to organic solute adsorption on the membrane surface, was studied in order to describe membrane and solute interactions. The influence of type and position of the substituted functional groups in the benzene ring was analyzed. A systematic decrease in relative flux was obtained when the additional functional group on the aromatic ring shifted from hydroxyl to chloro and finally to nitro. For the same sequence, consistent diminutions in solute retention were found for all membranes. In addition, the membrane retention usually increased following the sequence para → meta → ortho, a trend especially marked for the NF-90 membrane. For polyhydric phenols, the retention also increases as the sequence: mono → di → tri. In both cases, steric hindrance is recognized as the main factor determining the membrane performance.

Catanionic surfactant vesicles for electrostatic molecular sequestration and separation

Mixtures of oppositely charged surfactants, commonly called catanionic mixtures, are one of the most interesting and promising areas of colloidal chemistry. In this paper we review our previous work and report new results on electrostatic adsorption of organic solutes and DNA to the exterior surfaces of catanionic, unilamellar vesicles which form spontaneously in mixtures of sodium dodecylbenzenesulfonate (SDBS) and cetyltrimethylammonium tosylate (CTAT). Our group, along with others, has shown that organic ions and polyelectrolytes will bind to the exterior surface of oppositely charged catanionic vesicles through interactions with unpaired ionic surfactants present in the vesicle bilayer. The electrostatic sequestration of organic ions with catanionic vesicles is extremely efficient with excellent long-term stability and can be used to perform separations on mixtures of charged organic solutes. Using regular solution theory extended to vesicle-forming surfactant mixtures, we can understand how the composition of the bilayer changes with surfactant dilution, and we study this effect using fluorescence correlation spectroscopy (FCS). We employ FCS to make sensitive measurements of bilayer adsorption and compare the adsorption of a small molecular probe with that of a single-stranded, dye-labeled DNA molecule. From these FCS studies, adsorption isotherms can be obtained that report on the relative binding strengths of the two systems. The results show that DNA binds much more strongly to the exterior surface of positively charged catanionic vesicles, and can even stabilize vesicles at very low surfactant concentrations near the critical aggregation concentration (cac).

Characterisation of Surface Ionisation and Adsorption of Phenol and 4-Nitrophenol on Non-Porous Carbon Blacks

The adsorption of phenol and 4-nitrophenol from aqueous solutions by carbon blacks was studied. Particular attention was paid to the characterisation of the surface chemistry and ionisation of the carbon blacks by use of a simple carbon surface ionisation model, as well as the use of a normalised form of the Freundlich equation for the analysis of the adsorption isotherms. The results indicated that the solutes interact directly with the graphene layers and that the adsorption is sensitive to the π-electron density. Phenol has a weaker interaction and is more sensitive to the system conditions. In particular, it is more sensitive to oligomerisation under certain conditions. Comparison of the results with those previously reported for activated carbons with similar points of zero charge prepared from cork confirms the existence of a significant micropore-narrowing effect in the adsorption of organic solutes by activated carbons and which can lead to up to a doubling of the value of the Freundlich exponent, nF.

Molecular simulations to estimate thermodynamics for adsorption of polar organic solutes to montmorillonite

SummaryIdeally, one could use molecular mechanics or quantum mechanics to predict the magnitude of organic solute adsorption from water to soil minerals. Reproduction and/or prediction of mineral and interfacial structures remains challenging, but calculation of meaningful energy relations through computational chemistry techniques is even more difficult than structural calculations. This paper attempts to define the necessary and relevant components for an overall scheme that allows translation of computed interaction enthalpies to experimental adsorption enthalpies and vice versa. While the scheme could be applied to quantum calculations, we test it for the possibility of using empirical molecular mechanics to estimate relative energies for the adsorption of non‐ionic organic solutes in clay mineral‐water‐solute systems. We used molecular dynamics’ simulations to estimate relative clay–organic interaction enthalpies for a series of nitro‐aromatic solutes and hydrated, K‐saturated montmorillonite, for comparison with experimental adsorption isotherm data for the same clay‐nitroaromatic systems. The trend of computed interaction enthalpies (e.g. −234 ± 17 kJ mol−1 for trinitrobenzene and −154 ± 16 kJ mol−1 for p‐nitrobenzene) agreed modestly well with the trend of adsorption maxima from the experiments. Furthermore, we developed several variants on a thermodynamic cycle framework for comparing computed interaction energies with experimentally determined adsorption enthalpies. The algorithms, which include estimates for enthalpy changes both in bulk solution and in the clay interlayer, show promise: for p‐dinitrobenzene and for 1,3,5‐trinitrobenzene, the overall predicted adsorption enthalpies (e.g. −13 ± 22 and −67 ± 23 kJ mol−1, respectively) were in modest agreement with experiments (−18 ± 1 and −28 ± 4 kJ mol−1, respectively). We discuss shortcomings of the methods, in hopes of encouraging better estimates for the various energy terms, improvement of the algorithms, and more valid comparisons between quantum mechanical or molecular mechanical interaction energies and experimental enthalpies.

Kinetics and equilibrium of adsorption of organic solutes on mesoporous carbons

Static and kinetic studies on adsorption of nitrobenzene, 4-nitrophenol and 4-chlorophenol on two mesoporous carbons are performed. The carbon properties are analyzed by means of nitrogen adsorption. The adsorption experiments are performed in acidic buffer solutions in a wide range of concentrations. The static experiments are analyzed by means of Langmuir–Freundlich and Freundlich isotherms. The Lagergren, pseudo-second-order, intraparticle-diffusion and multi-exponent equations are used in the analysis of kinetic equilibria.

Changes in the Surface Chemistry and Adsorptive Properties of Active Carbon Previously Oxidized and Heat-Treated at Various Temperatures. III. Studies of the Adsorption of Organic Solutes from Aqueous Solutions

The dependence between the surface chemistry characteristics of thermally modified active carbon samples with previously oxidized surfaces and their ability to adsorb organic solutes from aqueous solutions was investigated. The characteristics of the surface functional groups and the porous structure of the samples obtained were estimated using various experimental methods. The adsorption isotherms of nitrobenzene and 4-nitrophenol from dilute aqueous solutions onto all the carbon samples obtained were measured. The experimental systems were analyzed in terms of the theory of adsorption onto energetically heterogeneous solids. The relationships between the optimized isotherm parameters, i.e. equilibrium constants and heterogeneity parameters, and the characteristics of the active carbons were discussed.

Adsorption equilibria in the systems: Aqueous solutions of organics—oxidized activated carbon samples obtained from different parts of granules

The adsorption of organics from aqueous solutions on the oxidized activated carbon samples obtained by gradual abrasion of granules is studied. The low temperature nitrogen adsorption–desorption isotherms are applied to determine the parameters of porous structure of investigated materials. The chemistry of adsorbent surface is characterized using XPS technique and Boehm titration method. The adsorption isotherms of three organic substances from dilute aqueous solutions are also measured. The influence of changes of surface properties inside of a carbon particle on organic solute adsorption from aqueous solutions is analyzed and discussed.

Influence of differences in porous structure within granules of activated carbon on adsorption of aromatics from aqueous solutions

The adsorption of organics from aqueous solutions on the activated carbon samples obtained by gradual abrasion of granules is studied. The nitrogen adsorption/desorption isotherms are applied to determine the parameters of porous structure of investigated carbon samples. The adsorption isotherms of four organic substances: nitrobenzene, phenol, 4-nitrophenol, and 4-chlorophenol from dilute aqueous solutions are also measured. The influence of porous structure changes inside of a carbon particle on organic solute adsorption from aqueous solutions is analyzed and discussed. It was stated that the carbon porosity was a function of the distance from granule surface, however, the properties of carbon micropore and mesopore walls were preserved. The similarity of adsorption mechanisms over the whole carbon granule were found for liquid systems.

A General Model for Adsorption of Organic Solutes from Dilute Aqueous Solutions on Heterogeneous Solids: Application for Prediction of Multisolute Adsorption

A general model describing the adsorption process from the single-component and multicomponent dilute aqueous solutions of dissociating organic substances on activated carbon is studied. The theoretical considerations are based on the theory of physical adsorption on energetically heterogeneous solids. The effect of solution pH and ionic strength is also taken into account. The theoretical considerations are illustrated by the experimental data for single-, bi-, and trisolute adsorption systems.

Capillary Phase Separation in Solvent Dehydration by Hygroscopic Porous Adsorbents.

Adsorption equilibria for solvent dehydration, namely, removal of trace water from solvents by inorganic adsorbents such as silica gels and activated alumina, were measured for various combinations of solvents and adsorbents to test the applicability of the capillary phase separation concept to these systems. The concept is that a component in solution with limited mutual solubility can be separated out into narrow pores even at a concentration less than the saturated concentration, by a capillary effect similar to capillary condensation. The results show definite existence of capillary phase separation in these systems, and verify the applicability of the isotherm estimation method proposed earlier by the authors for adsorption of organic solutes from aqueous solution.

Supporting titanium dioxide photocatalysts on silica gel and hydrophobically modified silica gel

This report describes preparation and characterization of TiO 2 supported on silica gels and modified silica gels. The objective is manipulation of the photocatalytic TiO 2 phase on the support, separately from manipulation of the organic solute adsorption and mass transport characteristics of the support. If the two features can be controlled separately, these systems should become good models for elucidation of mechanistic behavior of supported photocatalysts. TiO 2 characteristics and adsorption of substrates are key variables. TiO 2 may be anchored on silica gel 100 by hydrolysis of Ti( OR) 4 ( R = isoprophyl) or heating mixtures of silica gel and TiO 2 crystals (p25). In the first case, TiO 2 penetrates pores of silica gel and forms a phase which is either amorphous or of particle size small enough to give broad lines in powder XRD at angles suggesting anatase. In the second case, anatase particles too large to penetrate pores are anchored on the outer surface. The former method produces a superior photocatalyst. The silica surface can be modified by esterification with octanol to produce a photocatalyst which adsorbs solutes from an aqueous medium in a manner describable by an octanol/water partition coefficient. This modification has little effect on photocatalytic reactivity. A larger particle size silica gel (silica gel 58) produces a photocatalyst with TiO 2 in the interior of the pores which is less active for oxidation of 2,4-dichlorophenol but produces long-lived trapped electron colors in the presence of oxidizable substrates (e.g. propanol). This indicates a way to explore the role of intraparticle diffusion in the larger particles which are attractive for large scale applications.

Adsorption Behaviors of Azulene and Phenol on Surface-Modified Activated Carbons in Organic Solvents

The effect of the surface acidity of activated carbons on the adsorption of organic solutes in organic solvents was examined. Several activated carbons with differentsurface acidity but with the same porosity were prepared by oxidation of commercial activated carbon with nitric acid and heat treatment in hydrogen gas. The adsorbability of azulene, as a nonpolar solute, and phenol, as a polar solute, was measured on the surface-modified activated carbons in organic solvents of different polarities.The adsorbability of azulene decreased with decreasing solvent polarities and showed the lowest value in chloroform, but then increased with further decreasing solvent polarities. Similar behavior was also observed with phenol, though the magnitude of the decrease in the adsorbability was lower, and its adsorbability showed the lowest value in 1-octanol.The adsorbability increased with increasing surface acidity for both azulene and phenol, because increase in the carbon surface acidity caused a decrease in the affinity of the solvent for the activated carbon. The influence of the carbon surface acidity on the adsorption of azulene was larger than that of phenol.

Binding of organic solutes to dissolved humic substances and its effects on adsorption and transport in the aquatic environment

Humic substances constitute a major fraction of dissolved organic matter in natural water and effluents. Their effect on the adsorption of organic contaminants to aquifer material was elucidated, and a model was proposed for the adsorption of organic solutes to aquifer solids in the presence of dissolved humic substances. The model is based on the assumption that organic solute binds to dissolved humic substances in a reversible manner to form a solute-humate complex. Following binding, both free and bound fractions of the organic solute are independently adsorbed onto the solid phase. In order to evaluate the validity of the model, the following parameters were determined: (1) the adsorption coefficient of the organic solute to clay; (2) the binding constant of the solute-humate complex; and (3) the adsorption of humic acid (HA) to clay, assuming that the solute-humate complex is adsorbed similarly to humic acid itself. Using these parameters in the model enabled the effect of dissolved humic substances on adsorption to be evaluated. Experimental results obtained for the adsorption of fluoranthene (a model compound of the PAH group) to clay in the presence of dissolved HA were compared with calculated values derived from the model described above. The sensitivity of the model to various parameters was evaluated and a prediction was made with respect to the effect of dissolved humic substances on the adsorption of a variety of organic solutes. It appears that dissolved humic substances solubilize organic solutes which have higher adsorption coefficients to clay than humic substances, but increase the adsorption of solutes having lower adsorption coefficients relative to humic substances.

Adsorption of Organic Solutes on Membrane Filters during Aqueous Phase Filtration. I. Basic Rate and Equilibrium Studies Using Toluidine Blue

Abstract Membrane filters are made from a wide variety of polymeric materials, in a wide range of pore sizes, and are used in large numbers (several hundred million per year) to filter particulates from many types of solutions. However, during filtration, organic solutes are adsorbed by these membranes, often quite extensively. Thus, the composition of the filtrate can be very different from that of the original solution. A systematic study has been initiated to quantify and explain the adsorption of organic solutes during membrane filtration as affected by membrane type (material, pore size, presence/absence of wetting agents), solute type, pH, ionic strength, and filtration flow rate. Our first studies have employed a dye (toluidine blue) as the dissolved organic solute, and 11 membrane filters made of mixed cellulose esters, polyvinylidene difluoride, polytetrafluoroethylene, and polycarbonate. The results of these studies, which are both equilibrium and kinetic in nature, are presented. These results indicate how adsorption effects may be minimized.

Determination of the acid-base characteristics of clay mineral surfaces by contact angle measurements-implications for the adsorption of organic solutes from aqueous media

The apolar and the polar (electron-acceptor and electron-donor, or Lewis acid-base) surface tension components and parameters of solid surfaces can be determined by contact angle measurements using at least three different liquids, of which two must be polar. With swelling clay minerals (e.g. smectite clay minerals), smooth contiguous membranes can be fabricated, upon which contact angles can be measured directly. With non-swelling clay minerals (e.g. talc), contact angles can be determined by wicking, i.e. by the measurement of the rate of capillary rise of the liquids in question through thin layers of clay powder adhering to glass plates. The apolar and polar (acid-base) surface tension components and parameters thus found for various untreated and quaternary ammonium base-treated clays allowed the determination of the net interfacial free energy of adhesion of human serum albumin onto the various clay particle surfaces immersed in water. The free energies of adhesion, thus found, correlate well with t...

Correlations of heterogeneity parameters for single-solute and multi-solute adsorption from dilute solutions

Solute adsorption from dilute solutions is considered in terms of a new description of physical adsorption from multicomponent mixtures on heterogeneous solids. This description requires that the type of adsorption energy variability is similar for all components of the mixture on different surface sites. The theoretical equations are used to predict the heterogeneity parameter for a multi-solute adsorption system by means of the het-erogeneity parameters which characterize the suitable single-solute adsorption systems. This prediction procedure is examined by using the experimental isotherms of organic solute adsorption from dilute aqueous solutions on activated carbons.

Analysis of a batch adsorber with rectangular adsorption isotherms

This paper deals with an analysis of a batch adsorber with a rectangular adsorption isotherm (i.e., a system operating at the flat part of a Langmuir isotherm). This situation is common in many systems of adsorption of organic solutes from aqueous solution onto activated carbon. The theoretical solutions reveal that the transport parameters, the effective diffusivity and the external mass transfer coefficient, can be easily extracted from two simple linear plots. Two systems of adsorption of phenol onto activated carbon and Victoria Blue dye onto activated carbon are employed to demonstrate the versatility of the theoretical solutions.