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PubMed2005

Slow adsorption reaction between arsenic species and goethite (alpha-FeOOH): diffusion or heterogeneous surface reaction control.

The slow stage of phosphate or arsenate adsorption on hydrous metal oxides frequently follows an Elovich equation. The equation can be derived by assuming kinetic control by either a diffusion process (either interparticle or intraparticle) or a heterogeneous surface reaction. The aim of this study is to determine whether the slow stage of arsenic adsorption on goethite is more consistent with diffusion or heterogeneous surface reaction control. Adsorption kinetics of arsenate and dimethylarsinate (DMA) on goethite (alpha-FeOOH) were investigated at different pH values and inert electrolyte concentrations. Their adsorption kinetics was described and compared using Elovich (Gamma vs ln time) plots. Desorption of arsenate and DMA was studied by increasing the pH of the suspension from pH 4.0 to pH 10.0 or 12.0. The effective particle sizes and zeta-potential of goethite were also determined. Effective particle size increased rapidly as the pH approached pH(IEP), both in the absence and presence of arsenic. Inert electrolyte concentrations and pH had no effect on the slow stage of arsenate adsorption on goethite, while the kinetics of DMA adsorption on goethite was influenced by both parameters. The slow stage of arsenate adsorption on goethite follows an Elovich equation. Since effective particle size changes with both pH and inert electrolyte concentrations, and effective particle size influences interparticle diffusion, the arsenate adsorption kinetics indicate that the slow adsorption step is not due to interparticle diffusion. DMA also has complex adsorption kinetics with a slow adsorption stage. DMA desorbed completely and rapidly when the pH was raised, in contrast to the slow adsorption kinetics, indicating that the slow adsorption step is not due to intraparticle diffusion. The slow adsorption is not the result of diffusion, but rather is due either to the heterogeneity of the surface site bonding energy or to other reactions controlling arsenic removal from solution.

Journal Article↗
PubMed2006

Probing the mechanism of water adsorption in carbon micropores with multitemperature isotherms and water preadsorption experiments.

The phenomenon of water adsorption in carbon micropores is examined through the study of water adsorption equilibrium in molecular sieving carbon. Adsorption and desorption isotherms are obtained over a wide range of concentrations from less than 0.1% to beyond 80% of the vapor pressure. Evidence is provided in support of a proposed bimodal water adsorption mechanism that involves the interaction of water molecules with functional groups at low relative pressures and the adsorption of water molecules between graphene layers at higher pressures. Decomposition of the equilibrium isotherm data through application of the extended cooperative multimolecular sorption theory, together with favorable quantitative comparison, provides support for the proposed adsorption mechanism. Additional support is obtained from a multitemperature study of water equilibrium. Temperatures of 20, 50, and 60 degrees C were probed in this investigation in order to provide isosteric heat of adsorption data for water interaction with the carbon molecular sieve. At low loading, the derived isosteric heat of adsorption is estimated to be 69 kJ/mol. This value is indicative of the adsorption of water to functional groups. At higher loading, the isosteric heat of adsorption decreases with increasing loading and approaches the heat of condensation, indicative of adsorption between graphene layers. Further support for the proposed adsorption mechanism is derived from carbon dioxide adsorption experiments on carbon molecular sieve that is preadsorbed with various amounts of water. Significant exclusion of carbon dioxide occurs, and a quantitative analysis that is based on the proposed bimodal water adsorption mechanism is employed in this investigation.

Journal Article↗
PubMed2004

Geochemistry of inorganic arsenic and selenium in a tropical soil: effect of reaction time, pH, and competitive anions on arsenic and selenium adsorption.

Factors that can affect As and Se adsorption by soils influence the bioavailability and mobility of these elements in the subsurface. This research attempted to compare the adsorption capacities of As(III), As(V), Se(IV), and Se(VI) on a tropical soil commonly found in Singapore in a single-species system. The effect of reaction time, pH, and competitive anions at different concentrations on the adsorption of both As and Se species were investigated. The As and Se adsorption isotherm were also obtained under different background electrolytes. The batch adsorption experiments showed that the sequence of the As and Se adsorption capacities in the soil was As(V) > Se(IV) > As(III) > Se(VI). The adsorption kinetics could be best described by the Elovich equation. The adsorption of As(V), Se(IV), and Se(VI) appeared to be influenced by the variable pH-dependent charges developed on the soil particle surfaces. Phosphate had more profound effect than SO4(2-) on As and Se adsorption in the soil. The competition between PO4(3-) and As or Se oxyanions on adsorption sites was presumably due to the formation of surface complexes and the surface accumulation or precipitation involving PO4(3-). The thermodynamic adsorption data for As(V) and Se(IV) adsorption followed the Langmuir equation, while the As(III) and Se(VI) adsorption data appeared to be best-represented by the Freundlich equation.

Adsorption↗
PubMed2004

Adsorption behavior of selected monosaccharides onto an alumina interface.

The adsorption of glucose and fructose from their aqueous solutions onto an alumina interface has been carried out spectrophotometrically at room temperature. The adsorption isotherms are characterized as typical L-type and an adsorption mechanism on the basis of dipolar interactions has been suggested. In addition to this, a partial role of metal-saccharide interactions as found in organometallic complexes (OMCs) for the observed adsorption cannot be ruled out. Various kinetic and thermodynamic parameters of the adsorption process have been evaluated. The effects of variation in experimental conditions of the system have also been investigated. The adsorption exhibited a typical response to the pH effect and maximum adsorption was found near the isoelectric point of alumina (pH 9.0). The anionic addition to the suspension affects the adsorbed amount and Cl(-), SO(4)(2-), and PO(4)(3-) affect the adsorption quantitatively. The addition of similar concentration of cations was found to reduce the adsorbed amount. The presence of cationic and anionic detergents was found to influence both the adsorbed amount and the adsorption rate. The temperature was found to have an inverse effect on adsorption. Adsorptive kinetic parameters have revealed that fructose tends to be a better adsorbate than glucose. This is found to be consistent with the chelation behaviour of monosaccharides as found in the OMC of monosaccharides. The thermodynamics of the adsorption model indicates its spontaneous and exothermic nature. The negative values of entropy are an indication of the probability of a favorable nature of adsorption.

Adsorption↗
PubMed2005

Inconsistency in the triple layer model description of ionic strength dependent boron adsorption.

Understanding anion adsorption mechanisms is necessary to allow prediction of anion adsorption behavior. This study was conducted to evaluate the ability of the triple layer model, a chemical surface complexation model, to describe the effect of changes in solution ionic strength (0.01-1.0 M NaCl) and solution pH (3-11) on B adsorption by the iron oxide, goethite, the aluminum oxide, gibbsite, the clay minerals, kaolinite and montmorillonite, and two arid zone soils. Ionic strength dependence of adsorption suggests an inner-sphere adsorption mechanism for goethite, kaolinite, montmorillonite, and the two soils and an outer-sphere adsorption mechanism for gibbsite. The triple layer model, containing an inner-sphere adsorption mechanism, was able to describe B adsorption on goethite, kaolinite, montmorillonite, and the two soils. The model was able to describe B adsorption on gibbsite using an outer-sphere adsorption mechanism. A problematic inconsistency exists in the triple layer model description of ionic strength dependent B adsorption between the type of B surface complex defined in the model and the ionic strength dependence of the model result. That is, postulating an inner-sphere adsorption mechanism in the triple layer model resulted in an ionic strength dependence appropriate for the formation of outer-sphere surface complexes and vice versa. Additional tests of the ability of the triple layer model to describe ionic strength dependent adsorption of additional ions are needed to establish whether the inconsistencies are limited to the B system or are of concern in other triple layer model applications.

Adsorption↗
PubMed2003

Direct observation of intraparticle equilibration and the rate-limiting step in adsorption of proteins in chromatographic adsorbents with confocal laser scanning microscopy.

The adsorption of different proteins in a single biospecific and hydrophobic adsorbent particle for preparative protein chromatography has been observed directly by confocal laser scanning microscopy as a function of time at a constant bulk concentration c(b). The bulk concentration was in the non-linear part of the adsorption isotherm. At all times the concentration of free protein at the particle surface was almost equal to the bulk content indicating that external mass transfer resistance is not rate limiting for the adsorption under these conditions. Inside the particles a distinct maximum in adsorbed and free protein concentration that moved inside to a distance of approximately 0.2 R (R particle radius) from the particle surface, was observed. This is due to a decreasing solid-phase density and adsorptive capacity in the particle between 0.8 R and R indicating that the fraction of macropores (or void space) is larger in the outer than in the inner part of the adsorbent particles. By increasing the bulk concentration by a factor of 10 the equilibration time was reduced by about the same magnitude. This is in agreement with the concentration dependence of the effective pore diffusion coefficient D(p,eff)=D(p)/[epsilon(p)[1+nK/(K +c)(2)]] derived from the mass conservation relations describing the adsorption process. The time dependence protein adsorption up to approximately 90% of the equilibration value q* could be described by a bilinear free driving force model. The rapid equilibration in the outer part of the particle with a half-life time of approximately 100 s in the studied systems accounted for 0.3-0.4 q*. The slower equilibration with a up to ten times longer half-life time, was the adsorption in the inner part of the particle that outside 0.5 R accounts for 0.5-0.6 q*. These data were compared with literature data for batch adsorption of proteins in biospecific, hydrophobic and ion-exchange adsorbents. They could also be described by a bilinear free driving force model, with about the same quantitative results as obtained for similar conditions in the single particle experiments. The static adsorption parameters, maximum binding site concentration n, and dissociation constant for the protein binding to a binding site K, were determined from Scatchard plots. For the same protein-adsorbent system the plots changed from linear to non-linear with increasing n. This change occurred when the average distance between adjacent binding sites become of the same order of magnitude as the size of the binding site or adsorbed protein. This causes a shielding of free binding sites increasing with n and the concentration of adsorbed protein, yielding a concentration dependence in K. These results show that for a high throughput and rapid adsorption in preparative chromatography, the adsorption step should be carried out in the non-linear part of the adsorption isotherm with concentrations up to c(b) where q*/c(b)>/=10 to obtain high protein recoveries. To avoid tailing due to the flow of adsorbed proteins in the inner part of the particles further into the particles at the start of the desorption, and to speed up desorption rates, protein adsorption in the particle within 0.5 R from the particle center should be avoided. This requires the further development of suitable pellicular particles for preparative protein chromatography that meet this requirement.

Adsorption↗
PubMed2002

Adsorption of p-chlorophenol by biofilm components.

Through batch equilibrium experiments under the conditions of temperature 25 degrees C, pH values of 2.7, 5.3 and 6.1, the p-chlorophenol (4-cp) adsorption to biofilm components was investigated in this study. The contributions of biofilm components to 4-cp adsorption were discussed by comparing four adsorption systems, i.e. 4-cp adsorbed by model suspended particulate matter (kaolin) with biofilm coating, bacteria, bacterial exopolysaccharide (EPS) and kaolin, respectively. Langmuir and Freundlich isotherm equations were used to evaluate the experiment data. All the four adsorptions fitted for the two equations. Equilibrium isotherms were obtained for 4-cp adsorption on different adsorbents. The kinetic characteristics of 4-cp adsorption by biofilm components and the effect of pH on the kinetic process were investigated. The time to reach the highest adsorption amount and near equilibrium state in the four systems was different. The 4-cp adsorption by kaolin with biofilm coating reached near equilibrium at 60 min at pH 6.1. The 4-cp adsorption by EPS and kaolin reached near equilibrium at 150 and 180 min, respectively. But the 4-cp adsorption by bacteria showed no evident near equilibrium during 3 h in the experiment. The impact of pH value on the adsorption was also examined. The adsorption amount slightly increased with increasing pH from 2.7 to 6.1 for the adsorption systems of bacterial EPS and kaolin, but it slightly decreased in the systems of kaolin with biofilm coating and bacteria.

Adsorption↗
PubMedDate not supplied

In situ and Ex situ adsorption and recovery of betalains from hairy root cultures of Beta vulgaris.

Various adsorbents were screened for in situ recovery of betalain pigments effluxed from hairy root cultures of red beet, Beta vulgaris. Alumina/silica (1:1) appeared ideal, showing in situ adsorption of 97% in a unit time of 30 min accounting for in situ recovery of 71.39% of the total betalaine effluxed. Other adsorbents such as Amberlite series (XAD-2 and -4), cyclodextrin, maltodextrin, dextrin white, and starches such as wheat starch and corn starch exhibited very poor in situ adsorption properties. Pretreatment of adsorbents with methanol significantly improved the adsorption capacities of some of the adsorbents, with a highest adsorption of 97.2% for alumina followed by alumina/silica (1:1) and higher adsorption by XAD-2 and -4. Complete in situ adsorption equilibrium was reached in 20 min for a solution containing 2.5 mg mL(-)(1) of betalain in adsorbents alumina, silica, and a mixture of alumina and silica. In situ betalain adsorption parameters for alumina/silica were determined using the Langmuir isotherm model where the adsorption capacity was found to be 0.174 mg g(-)(1) and the adsorption energy was 0.9 at pH 5.5 and 25 degrees C. Desorption of pigments from the adsorbents was invariably highest in poor adsorbents, indicating their poor adsorption energy for betalaines. Similarly, recovery by desorption was low in those adsorbents having high adsorption capacity, indicating that adsorbents such as activated ones with highest adsorption capacity with zero desorption property were unsuitable for the recovery of effluxed pigments. Ex situ recovery of betalain done using various combinations of alumina/silica and processed sand and different column geometries indicated that alumina with processed sand at a 2:1 ratio (w/w) and a minimum column material of 2 cm height and 2 cm diameter was good enough to cause 97% pigment adsorption from a solution containing 1.6 mg mL(-)(1). Desorption and recovery of pigments ex situ from columns were affected by various elution mixtures, where a gradient elution with ascending levels of HCl/ethanol in water resulted in 100% recovery of adsorbed pigments in a significantly lesser volume of eluent in a short period of 1 h. Different pigment flow rates of 0.2, 0.3, and 3.1 mL s(-)(1) through a column of alumina/processed sand indicated that a pigment equilibrium concentration of 0.18 mg mL(-)(1) at flow rates of 0.02 and 0.3 mL s(-)(1) resulted in a breakthrough at 110 and 14 min adsorbing 16.9 and 16.91 mg g(-)(1) betalain, respectively. From the breakthrough curves, the column capacities for respective flow rates were calculated as 8.86 and 9.6 mg g(-)(1), and the higher flow rates resulted in earlier breakthrough with lower capacity. Observations made in the present study are useful to develop a process for the on-line recovery of betalains effluxed from hairy roots.

Adsorption↗
PubMed2004

Effects of phosphate on uranium(VI) adsorption to goethite-coated sand.

U(VI)-phosphate interactions are important in governing the subsurface mobility of U(VI) in both natural and contaminated environments. We studied U(VI) adsorption on goethite-coated sand (to mimic natural Fe-coated subsurface materials) as a function of pH in systems closed to the atmosphere, in both the presence and the absence of phosphate. Our results indicate that phosphate strongly affects U(VI) adsorption. The effect of phosphate on U(VI) adsorption was dependent on solution pH. At low pH, the adsorption of U(VI) increased in the presence of phosphate, and higher phosphate concentration caused a larger extent of increase in U(VI) adsorption. Phosphate was strongly bound by the goethite surface in the low pH range, and the increased adsorption of U(VI) at low pH was attributed to the formation of ternary surface complexes involving both U(VI) and phosphate. In the high pH range, the adsorption of U(VI) decreased in the presence of phosphate at low total Fe concentration, and higher phosphate concentration caused a larger extent of decrease in U(VI) adsorption. This decrease in U(VI) adsorption was attributed to the formation of soluble uranium-phosphate complexes. A surface complexation model (SCM) was proposed to describe the effect of phosphate on U(VI) adsorption to goethite. This proposed model was based on previous models that predict U(VI) adsorption to iron oxides in the absence of phosphate and previous models developed to predict phosphate adsorption on goethite. A postulated ternary surface complex of the form of (>FePO4UO2) was included in our model to account for the interactions between U(VI) and phosphate. The model we established can successfully predict U(VI) adsorption in the presence of phosphate under a range of conditions (i.e., pH, total phosphate concentration, and total Fe concentration).

Adsorption↗
PubMed2006

Linear adsorption of nonionic organic compounds from water onto hydrophilic minerals: silica and alumina.

To characterize the linear adsorption phenomena in aqueous nonionic organic solute-mineral systems, the adsorption isotherms of some low-molecular-weight nonpolar nonionic solutes (1,2,3-trichlorobenzene, lindane, phenanthrene, and pyrene) and polar nonionic solutes (1,3-dinitrobenzene and 2,4-dinitrotoluene) from single- and binary-solute solutions on hydrophilic silica and alumina were established. Toward this objective, the influences of temperature, ionic strength, and pH on adsorption were also determined. It is found that linear adsorption exhibits low exothermic heats and practically no adsorptive competition. The solute-solid configuration and the adsorptive force consistent with these effects were hypothesized. For nonpolar solutes, the adsorption occurs presumably by London (dispersion) forces onto a water film above the mineral surface. For polar solutes, the adsorption is also assisted by polar-group interactions. The reduced adsorptive forces of solutes with hydrophilic minerals due to physical separation by the water film and the low fractions of the water-film surface covered by solutes offer a theoretical basis for linear solute adsorption, low exothermic heats, and no adsorptive competition. The postulated adsorptive forces are supported by observations that ionic strength or pH poses no effect on the adsorption of nonpolar solutes while it exhibits a significant effect on the uptake of polar solutes.

Adsorption↗
PubMed2006

Removal of carbonyl sulfide using activated carbon adsorption.

Wastewater treatment plant odors are caused by compounds such as hydrogen sulfide (H2S), methyl mercaptans, and carbonyl sulfide (COS). One of the most efficient odor control processes is activated carbon adsorption; however, very few studies have been conducted on COS adsorption. COS is not only an odor causing compound but is also listed in the Clean Air Act as a hazardous air pollutant. Objectives of this study were to determine the following: (1) the adsorption capacity of 3 different carbons for COS removal; (2) the impact of relative humidity (RH) on COS adsorption; (3) the extent of competitive adsorption of COS in the presence of H2S; and (4) whether ammonia injection would increase COS adsorption capacity. Vapor phase react (VPR; reactivated), BPL (bituminous coal-based), and Centaur (physically modified to enhance H2S adsorption) carbons manufactured by Calgon Carbon Corp. were tested in three laboratory-scale columns, 6 in. in depth and 1 in. in diameter. Inlet COS concentrations varied from 35 to 49 ppmv (86-120 mg/m3). RHs of 17%, 30%, 50%, and 90% were tested. For competitive adsorption studies, H2S was tested at 60 ppmv, with COS at 30 ppmv. COS, RH, H2S, and ammonia concentrations were measured using an International Sensor Technology Model IQ-350 solid state sensor, Cole-Parmer humidity stick, Interscan Corp. 1000 series portable analyzer, and Drager Accuro ammonia sensor, respectively. It was found that the adsorption capacity of Centaur carbon for COS was higher than the other two carbons, regardless of RH. As humidity increased, the percentage of decrease in adsorption capacity of Centaur carbon, however, was greater than the other two carbons. The carbon adsorption capacity for COS decreased in proportion to the percentage of H2S in the gas stream. More adsorption sites appear to be available to H2S, a smaller molecule. Ammonia, which has been found to increase H2S adsorption capacity, did not increase the capacity for COS.

Adsorption↗
PubMed2005

Multivariate analysis of infrared spectra for monitoring and understanding the kinetics and mechanisms of adsorption processes.

Water adsorption onto thin zeolite 3A wafers has been investigated as a function of time, water vapor concentration, and zeolite sample mass using mid-infrared spectroscopy coupled with multivariate data analysis. Principal component analysis (PCA) of the spectral region of the water combination band was used for quantitative characterization of water adsorption onto the zeolite. The kinetics of the adsorption of water are found to be very reproducible and nearly linear with time. The kinetics of water adsorption based on data from different masses of zeolite are consistent with a diffusion/immobilization model for which the interparticle diffusion rate is comparable to the rate of adsorption. The infrared zeolite bands (1340-1550 cm(-1)) change during the adsorption process and yield more detail about the adsorption sites of the material. PCA applied to the zeolite bands was not directly interpretable. However, multivariate curve resolution applied to the spectral region containing the zeolite bands readily demonstrates that zeolite 3A has three water adsorption sites or environments that are sequentially occupied. Potential explanations for the observations of the multivariate curve resolution (MCR) analysis of these infrared (IR) kinetic adsorption experiments are presented. The explanation most consistent with our data suggests that water adsorbs sequentially on the zeolite to form single, double, and triple water adsorption on single zeolite adsorption sites. The combination of infrared spectroscopy and multivariate analysis is therefore demonstrated to be a powerful method to study detailed adsorption kinetics and mechanisms of the adsorption of molecules onto surfaces.

Adsorption↗
PubMed2004

Volatile organic compound adsorption in a gas-solid fluidized bed.

Fluidization finds many process applications in the areas of catalytic reactions, drying, coating, combustion, gasification and microbial culturing. This work aims to compare the dynamic adsorption characteristics and adsorption rates in a bubbling fluidized bed and a fixed bed at the same gas flow-rate, gas residence time and bed height. Adsorption with 520 ppm methanol and 489 ppm isobutane by the ZSM-5 zeolite of different particle size in the two beds enabled the differentiation of the adsorption characteristics and rates due to bed type, intraparticle mass transfer and adsorbate-adsorbent interaction. Adsorption of isobutane by the more commonly used activated carbon provided the comparison of adsorption between the two adsorbent types. With the same gas residence time of 0.79 seconds in both the bubbling bed and fixed bed of the same bed size of 40 mm diameter and 48 mm height, the experimental results showed a higher rate of adsorption in the bubbling bed as compared to the fixed bed. Intraparticle mass transfer and adsorbent-adsorbate interaction played significant roles in affecting the rate of adsorption, with intraparticle mass transfer being more dominant. The bubbling bed was observed to have a steeper decline in adsorption rate with respect to increasing outlet concentration compared to the fixed bed. The adsorption capacities of zeolite for the adsorbates studied were comparatively similar in both beds; fluidizing, and using smaller particles in the bubbling bed did not increase the adsorption capacity of the ZSM-5 zeolite. The adsorption capacity of activated carbon for isobutane was much higher than the ZSM-5 zeolite for isobutane, although at a lower adsorption rate. Fourier transform infra-red (FTIR) spectroscopy was used as an analytical tool for the quantification of gas concentration. Calibration was done using a series of standards prepared by in situ dilution with nitrogen gas, based on the ideal gas law and relating partial pressure to gas concentration. Concentrations up to 220 ppm for methanol and 75 ppm for isobutane were prepared using this method.

Adsorption↗
PubMed1999

The Dynamic Adsorption of Charged Amphiphiles: The Evolution of the Surface Concentration, Surface Potential, and Surface Tension.

In this work the evolution of the surface concentration, surface potential, and surface tension for adsorption of a charged amphiphile at an interface is studied numerically. While the results are of interest for any amphiphile, the simulations are performed for typical surfactant material parameters. The surface potential is related at each time step to the instantaneous surface charge density determined by the surfactant surface concentration using the Gouy-Chapman model. The sublayer concentration at each time step is a Boltzmann distribution in instantaneous equilibrium with the surface potential. At equilibrium, the surfactant is assumed to obey the Davies adsorption isotherm. The model is integrated first for diffusion-controlled adsorption, in which the surfactant diffuses to the sublayer and adsorbs onto the interface in local equilibrium according to the adsorption isotherm. In this limit, since the equilibrium adsorption is strongly reduced by the repulsive electrostatic potential, the time required to deliver the surfactant by diffusion is also reduced. The greater the electrical repulsion, the faster the diffusion-controlled adsorption at a given surfactant concentration. Because less surfactant adsorbs, the surface tension reduces less at equilibrium. Counterions of greater valence than the surfactant are more effective at screening the surface potential. Equilibrium adsorption, surface tension reduction, and diffusion time scales increase. As the surfactant valence increases, so does the repulsion; the opposite trends in surface tension and diffusion time scales are predicted. The model is also integrated including both diffusion and adsorption-desorption kinetic barriers. In experiment, adsorption-desorption kinetic barriers have been shown to control the mass transfer of non-ionic surfactants at elevated bulk concentration. The ability of the interface to deplete the bulk reduces with concentration. Therefore, diffusion time scales are reduced. In these regimes, adsorption-desorption kinetics can be rate determining. In simulation, the occurrence of the shift of the controlling mechanism from pure diffusion control at dilute concentration to mixed kinetic-diffusion control at elevated concentration is strongly influenced by ionic strength and surfactant valence. As the electrostatic adsorption increases, kinetic barriers are apparent at lower concentrations. Finally, a simple time scale argument that has previously proven useful in predicting a priori the time required for diffusion-controlled adsorption to an interface for nonionic surfactant adsorption is extended to include electrostatic effects. Copyright 1999 Academic Press.

Journal Article↗
PubMed2000

Kinetics of Diffusion-Controlled Adsorption of Colloid Particles and Proteins.

A theoretical model was developed for describing localized adsorption kinetics of proteins and colloid particles at solid/liquid interfaces. In contrast to previous approaches the adsorption and desorption rate constants as well as the surface blocking function were evaluated explicitly without using empirical parameters. It was also predicted that irreversible adsorption kinetics can unequivocally be characterized in terms of the adsorption rate constant k(a) and the maximum (jamming) coverage Theta(mx) known for various particle shapes from previous Monte-Carlo simulations. The dimensionless constant k(a) was shown to be inversely proportional to the concentration of particles which is usually very low for protein and colloid adsorption measurements. From the theoretical model it was also deduced that in this case the asymptotic adsorption law for large dimensionless time tau can be expressed as Theta(mx)-Theta approximately 1/tau(1/(n-1)) (where n=3 for spheres, n=4 for side-on adsorption of spheroids, n=5 for randomly oriented spheroid adsorption). It was also shown that this limiting adsorption regime occurs for proteins at surface coverage Theta(l) very close to the jamming value Theta(mx), becoming therefore difficult to detect due to limited experimental accuracy. These analytical predictions were found to be in agreement with numerical calculations performed by using the finite-difference scheme, valid for an arbitrary range of adsorption time. Moreover, it was demonstrated that these numerical results adequately reflected the experimental results of Johnson and Lenhoff who determined the kinetics of colloid particle adsorption using atomic force microscopy. Previously used approaches assuming that particle adsorption flux is reduced by the factor B(Theta) were found to be inadequate. It was also demonstrated that due to the similarity of underlying parameters the results obtained for colloid systems can be exploited as well-defined reference data for estimating the adsorption kinetics of proteins. Copyright 2000 Academic Press.

Journal Article↗
PubMed2006

Frontal analysis for characterizing the adsorption-desorption behavior of beta-lactoglobulin on immunoadsorbents.

High-performance frontal affinity chromatography was employed to study the adsorption-desorption kinetics characterizing the retention of beta-lactoglobulin (beta-LG) onto polyclonal anti-beta-lactoglobulin (anti-beta-LG) chromatographic supports. The adsorption and desorption processes were studied by analyzing two different elution fronts separated by a relatively long rinsing step. The method consists in performing two successive frontal injections of the protein. In between, the column was rinsed with a given volume of mobile phase (buffer alone). During this rinsing stage, a partial desorption may occur and a novel amount of protein could be adsorbed in the second frontal injection step. The whole process (first adsorption, possible desorption, and second adsorption) was simulated by a numerical procedure, in which the column was divided into a large number of slices. A model based on bi-Langmuir type kinetics was used to describe the adsorption of the protein on the support. The model assumes a non-uniform adsorbent with two types of binding sites. At equilibrium the adsorption isotherm is of the bi-Langmuir type. A global adsorption effect was considered which includes the effective binding process and the mass transfer resistances due to the transport to the binding site. Therefore, the column capacity and the kinetic parameters of the model (apparent adsorption and desorption rate constants) were determined from the best fit of the first and second adsorption fronts to the experimental ones. The other parameters of the model are the saturation capacities for the adsorption on each type of sites. The equilibrium affinity constants were estimated in a single experiment from the ratio of the apparent adsorption and desorption rate constants. The high values found (around 10(8) M(-1)) reveal a strong interaction of beta-LG with the immunoadsorbent. Kinetic measurements were carried out at different flow rates. Both the apparent adsorption and desorption kinetics were faster at larger flow rates, indicating an important contribution of the mass transfer resistance in the stagnant fluid at the particle boundary. However, as expected, close values were found for the resulting equilibrium constants calculated from the ratio of the apparent adsorption and desorption rate constant determined at various flow rates.

Chromatography, High Pressure Liquid↗
PubMed2005

Characterizing As(III,V) adsorption by soils surrounding ash disposal facilities.

Leachate derived from unlined coal ash disposal facilities is a potential anthropogenic source of arsenic to the environment. To establish a theoretical framework for predicting attenuation of arsenic by soils subject to ash landfill leachate, which is typically enriched in calcium and sulfate, the adsorption of As(V) and As(III) was characterized from 1mM CaSO4 for 18 soils obtained down-gradient from three ash landfill sites and representing a wide range in soil properties. As(V) consistently exhibited an order of magnitude greater adsorption than As(III). As(V) adsorption was best described by coupling pH with 15s DCB-Fe (R(2)=0.851, alpha=0.001), although pH coupled to clay, DCB-Fe, or DCB-Al also generated strong correlations. For As(III), pH coupled to Ox-Fe (R(2)=0.725, alpha=0.001) or Ox-Fe/Al (R(2)=0.771, alpha=0.001) provided the best predictive relationships. Ca2+ induced increases in As(V) adsorption whereas sulfate suppressed both As(V) and As(III) adsorption. Attenuation of arsenic from ash leachate agreed well with adsorption measured from 1mM CaSO4 suggesting that the use of 1mM CaSO4 in laboratory adsorption tests is a reasonable approach for estimating arsenic behavior in soils surrounding ash landfills. We also showed that the impact of leachate-induced changes in soil pH over time may not be significant for As(V) adsorption at pH<7; however, As(III) adsorption may be impacted over a wider pH range especially if phyllosilicate clays contribute significantly to adsorption. The benefits and limitations of predicting arsenic mobility using linearized adsorption coefficients estimated from nonlinear adsorption isotherms or from the relationships generated in this study are also discussed.

Adsorption↗
PubMed2006

Controlled drug delivery to the lung: Influence of hyaluronic acid solution conformation on its adsorption to hydrophobic drug particles.

This work reports investigations into the interaction and adsorption of the hydrophilic polymer hyaluronic acid (HA) onto the surface of the hydrophobic corticosteroid drug fluticasone propionate (FP). The eventual aim is to formulate a bioadhesive pulmonary drug delivery system with prolonged action that avoids rapid clearance from the lungs by the mucociliary escalator. Adsorption isotherms detailing the adsorption of HA from aqueous HA solution concentrations ranging from 0.14 to 0.0008% (w/v) to a fixed FP particle concentration of 0.1% (w/v) were investigated. The method of preparing FP particles with HA molecules adsorbed on their surfaces (FP/HA particles) involved suspension of the FP either in hydrated HA solution or in water followed by addition of solid HA, centrifugation of the solids to form a pellet, washing the pellet several times with water until no HA was found in the supernatant and then freeze drying the suspension obtained by dispersing the final pellet. The freeze dried powder was then analysed for adsorbed HA using a Stains-all assay. The influence of order of addition of HA to FP, time for the adsorption process, and temperature of preparation on the adsorption isotherms was investigated. The non-equilibrium adsorption isotherms produced generally followed the same trend, in that as the HA solution concentration increased, the amount of HA adsorbed increased to a maximum at a solution concentration of approximately 0.1% (w/v) and then decreased. The maxima in the adsorption isotherms were close to the change from secondary to tertiary conformation in the HA solutions. Below the maxima, adsorption occurred via interaction of FP with the hydrophobic patches along the HA chains in the secondary structures. Above the maxima, secondary HA molecules aggregate in solution to form tertiary network structures. Adsorption from tertiary structure was reduced because strong interactions between the HA molecules limited the availability of hydrophobic patches for adsorption of HA onto FP. The influence of preparation variables on adsorption was also related to the availability of hydrophobic patches for adsorption.

Adsorption↗