Adsorption mechanism in RPLC. Effect of the nature of the organic modifier

The adsorption isotherms of phenol and caffeine were acquired by frontal analysis on two different adsorbents, Kromasil-C18 and Discovery-C18, with two different mobile phases, aqueous solutions of methanol (MeOH/H2O = 40/60 and 30/70, v/v) and aqueous solutions of acetonitrile (MeCN/H2O = 30/70 and 20/80, v/v). The adsorption isotherms are always strictly convex upward in methanol/water solutions. The calculations of the adsorption energy distribution confirm that the adsorption data for phenol are best modeled with the bi-Langmuir and the tri-Langmuir isotherm models for Kromasil-C18 and Discovery-C18, respectively. Because its molecule is larger and excluded from the deepest sites buried in the bonded layer, the adsorption data of caffeine follow bi-Langmuir isotherm model behavior on both adsorbents. In contrast, with acetonitrile/water solutions, the adsorption data of both phenol and caffeine deviate far less from linear behavior. They were best modeled by the sum of a Langmuir and a BET isotherm models. The Langmuir term represents the adsorption of the analyte on the high-energy sites located within the C18 layers and the BET term its adsorption on the low-energy sites and its accumulation in an adsorbed multilayer system of acetonitrile on the bonded alkyl chains. The formation of a complex adsorbed phase containing up to four layers of acetonitrile (with a thickness of 3.4 A each) was confirmed by the excess adsorption isotherm data measured for acetonitrile on Discovery-C18. A simple interpretation of this change in the isotherm curvature at high concentrations when methanol is replaced with acetonitrile as the organic modifier is proposed, based on the structure of the interface between the C18 chains and the bulk mobile phase. This new model accounts for all the experimental observations.     

Heterogeneity of the adsorption mechanism of low molecular weight compounds in reversed-phase liquid chromatography

The retention mechanism in RPLC mode was investigated based on the acquisition of adsorption isotherm data by frontal analysis measurements and their modeling. This work is a review of the results of four years of adsorption data measurements. The data were acquired on a wide variety of brands of C18-silica columns (from Akzo Nobel, Bishoff, Hypersil, Merck, Phenomenex, Supelco, Vydac, and Waters) with several low molecular weight compounds such as phenol (94 g/mol), caffeine (194 g/mol), tryptophan (204 g/mol), sodium 2-naphthalenesulfonate (235 g/mol), and propranololium chloride (295 g/mol). The mobile phase was a mixture of methanol and water at variable composition. The adsorption isotherms were all convex upward (langmuirian), and the degree of heterogeneity of the adsorption system was determined from the calculation of the adsorption energy distribution using the expectation-maximization method. The adsorption isotherm parameters (number of types of adsorption sites, surface concentration of each type of site, and difference between the adsorption energies E(i) - E(j) on sites i and j), obtained from the mathematical fit of the adsorption data to the appropriate multi-Langmuir adsorption isotherm model, were analyzed and compared. The results allow the drawing of general conclusions regarding the relationships between the size of the analyte and the adsorption properties (saturation capacities, adsorption energies) characterizing the retention mechanism in RPLC mode for neutral, anionic, and cationic compounds.     

Effect of the surface heterogeneity of the stationary phase on the range of concentrations for linear chromatography

The range of sample sizes within which linear chromatographic behavior is achieved in a column depends on the surface heterogeneity of the RPLC adsorbents. Two widely different commercial adsorbents were tested, the end-capped XTerra-C18 and the non-end-capped Resolve-C18. Adsorption isotherm data of caffeine were acquired by frontal analysis. These data were modeled and used to calculate the adsorption energy distribution (AED). This double analysis informs on the degree of surface heterogeneity. The best adsorption isotherm models are the bi-Langmuir and the tetra-Langmuir isotherms for XTerra and Resolve, respectively. Their respective AEDs are bimodal and quadrimodal distributions. This interpretation of the results and the actual presence of a low density of high-energy adsorption sites on Resolve-C18 were validated by measuring the dependence of the peak retention times on the size of caffeine samples (20-microL volume, concentrations 10, 1, 0.1, 1 x 10(-2), 1 x 10(-3), 1 x 10(-4), and 1 x 10(-5) g/L). The experimental chromatograms agree closely with the band profiles calculated from the best isotherms. On Resolve-C18, the retention time decreases by 40% when the sample concentration is increased from 1 x 10(-5) to 10 g/L. The decrease is only 10% for Xterra-C18 under the same conditions. The upper limit for linear behavior is 1 x 10(-4) g/L for the former adsorbent and 0.01 g/L for the latter. The presence of a few high-energy adsorption sites on Resolve-C18, with an adsorption energy 20 kJ/mol larger than that of the low-energy sites while the same difference on Xterra is only 5 kJ/mol, explains this difference. The existence of adsorption sites with a very high energy for certain compounds affects the reproducibility of their retention times and a rapid loss of efficiency in a sample size range within which linear behavior is incorrectly anticipated.     

Adsorption on molecularly imprinted polymers of structural analogues of a template. Single-component adsorption isotherm data

The equilibrium adsorption isotherms on two otherwise identical polymers, one imprinted with Fmoc-L-tryptophan (Fmoc-L-Trp) (MIP), the other nonimprinted (NIP), of compounds that are structural analogues of the template were acquired by frontal analysis (FA) in an acetonitrile/acetic acid (99/1 v/v) mobile phase, over a wide concentration range (from 0.005 to 50 mM). These analogues were Fmoc-L-tyrosine, Fmoc-L-serine, Fmoc-L-phenyalanine, Fmoc-glycine (Fmoc-Gly), Fmoc-L-tryptophan pentafluorophenyl ester (Fmoc-L-Trp(OPfp)), and their antipodes. These substrates have different numbers of functional groups able to interact with the 4-vinylpyridine groups of the polymer. For a given number of the functional groups, these substrates have different hydrophobicities of their side groups (as indicated by their partition coefficients (log P(ow)) in the octanol-water system (e.g., from 4.74 for Fmoc-Trp to 2.53 for Fmoc-Gly)). Statistical results from the fitting of the FA data to Langmuirian isotherm models, the calculation of the affinity energy distribution, and the comparison of calculated and experimental band profiles show that all these sets of FA data are best accounted for by a tri-Langmuir isotherm model, except for the data of Fmoc-L-Trp(OPfp) that are best modeled by a simple Langmuir isotherm. So, all compounds but Fmoc-L-Trp(OPfp) find three different types of adsorption sites on both the MIP and the NIP. The properties of these different types of sites were studied systematically. The results show that the affinity of the structural analogues for the NIP is controlled mostly by the number of the functional groups on the substrates and somewhat by the hydrophobicity of their side groups. These two factors control also the MIP affinity toward the enantiomers of the structural analogues that have a stereochemistry different from that of the template. In contrast, the affinity of the highest affinity sites of the MIP toward the enantiomers of these structural analogues that have the same stereochemistry as the template is highest for the imprinted molecule (Fmoc-L-Trp). The separation of the template from the substrates with the same stereochemistry is influenced by the number of the functional groups on the substrates that can interact with the highest affinity sites on the MIP. The separation of the enantiomers of the analogues of the substrates was also achieved on the MIP, and these enantiomeric separations are influenced by the hydrophobicity of the substrates.     

Thermodynamic studies on solvent effects in molecularly imprinted polymers. 2. Concentration of the organic modifier

The effects of the organic modifier concentration on the isotherm parameters of the two enantiomers of Fmoc-tryptophan (Fmoc-L,D-Trp) on an Fmoc-L-Trp-imprinted polymer were investigated over a wide concentration range (0.005-100 mM), using frontal analysis. The modifier was acetic acid; concentrations of 0.2, 0.9, 1.7, and 3.7 M in an acetonitrile-based mobile phase were studied. At each concentration, adsorption isotherm data were acquired for each enantiomer. From these data, the isotherm parameters of each compound were derived from nonlinear isotherm fitting and the affinity energy distributions were calculated independently. We found that three types of sites coexist for Fmoc-L-Trp but only two types of sites for Fmoc-D-Trp, except at the lowest acetic acid concentration (0.2 M), at which three types of sites coexist. Increasing the acetic acid concentration decreases the selectivity and the overall affinity of both enantiomers. The overall affinity of Fmoc-L-Trp is dominated by the contribution of the low-density highest energy sites while that of Fmoc-D-Trp is dominated by the most abundant, low-energy sites. For the low-energy sites, increasing the acetic acid concentration affects the association constant of the enantiomers more than the number of corresponding sites. In contrast, for the highest energy sites (sites that exist only for Fmoc-L-Trp), increasing the concentration of acetic acid affects significantly the number of sites but hardly changes the association constant.     

Surface heterogeneity of six commercial brands of end-capped C18-bonded silica. RPLC separations

Single component adsorption isotherms of phenol and caffeine were measured on six different commercial brands of end-capped C(18)-bonded silica columns (five monomeric bonded phases: Kromasil, Waters Symmetry, Phenomenex, Hypersil, and Chromolith from Merck; one polymeric bonded phase, Vydac) with the same methanol/water solution (30/70, v/v) as the mobile phase. Adsorption data were acquired by frontal analysis (FA) for all these columns in the same way. Depending on their solubility in the mobile phase, the concentrations used ranged between 1 and 100 g/L and between 0.35 and 35 g/L for phenol and caffeine, respectively. Twenty-two adsorption data points were recorded over these ranges. In each case, the best isotherm model accounting for all sets of adsorption data is the bi-Langmuir model, all columns behaving as heterogeneous adsorbents despite the endcapping. Depending on the column, the high-energy sites accounts for between 30 and 40% and between 4 and 7% of the total saturation capacity for phenol and caffeine, respectively. Except for the polymeric phase (Vydac), the ratio of the adsorption constants on the high- and low-energy sites is constant at around 10 for both phenol and caffeine, corresponding to an average adsorption energy difference of 5 kJ/mol between these two sites. The exact nature of the high-energy sites is illustrated by the following properties: (i) they have a very low selectivity for caffeine, with alpha(caffeine/phenol) close to 0.4 for the five monomeric columns, which suggests the complete derivatization of residual silanols; (ii) the high-energy sites account for a large fraction of the surface area of these packing materials (35% for phenol, 6% for caffeine); (iii) there is a small adsorption energy difference between high and low adsorption energy sites (5 kJ/mol); and (iv) the adsorption constants increase with increasing surface coverage of the monomeric columns. Thus, the high energy sites cannot be residual free silanols of the bare silica. More likely, they are related to the local heterogeneity of the C(18)-bonded-layer structure. Caffeine is more strongly retained on the low-energy sites than phenol (the product q(s,) (1)b(1) is larger for caffeine) but the contribution of the high-energy sites (q(s,) (2)b(2)) is markedly lower for caffeine than for phenol, despite the larger value of the adsorption constant, b(2). Because of a larger molecular size, caffeine cannot penetrate as deeply as phenol inside the bonded layer. This explains the paradox of a stronger retention for phenol than for caffeine on end-capped C(18)-bonded stationary phases.     

Effect of ionic strength on the adsorption of copper and chromium ions by vermiculite pure clay mineral

It is important to assess the effects of ionic strength when studying adsorption of metal ions on clay mineral because the background salt may complex metals and compete for adsorption sites. The sorption behavior of vermiculite pure clay mineral has been studied with respect to copper and chromium as a function of ionic strength in single metal ion solutions. Background electrolytes used in these experiments were KCl, NaCl and NH₄Cl. The studies were conducted by a batch method at temperature 25 °C. The adsorption capacity and adsorption energy for each metal ion were calculated from the Langmuir adsorption isotherm.Also the competitive adsorption behavior of some heavy metal ions such as Cr(III), Cu(II), Ni(II) and Co(II) by vermiculite pure clay mineral was studied. The result shows the competition between coexisting heavy metal cations for the same adsorption sites of an adsorbent. However, when trivalent metal was added to the solution it competitively replaced divalent ions that had been previously adsorbed onto the vermiculite pure clay mineral, resulting in the desorption of these metals into the solution.     

Cross-linked quaternary chitosan as an adsorbent for the removal of the reactive dye from aqueous solutions

Adsorption of reactive orange 16 by quaternary chitosan salt (QCS) was used as a model to demonstrate the removal of reactive dyes from textile effluents. The polymer was characterized by infrared (IR), energy dispersive X-ray spectrometry (EDXS) analyses and amount of quaternary ammonium groups. The adsorption experiments were conducted at different pH values and initial dye concentrations. Adsorption was shown to be independent of solution pH. Three kinetic adsorption models were tested: pseudo-first-order, pseudo-second-order and intraparticle diffusion. The experimental data best fitted the pseudo-second-order model, which provided a constant velocity, k₂, of 9.18 × 10⁻⁴ g mg⁻¹ min⁻¹ for a 500 mg L⁻¹ solution and a value of k₂, of 2.70 × 10⁻⁵ g mg⁻¹ min⁻¹ for a 1000 mg L⁻¹ solution. The adsorption rate was dependent on dye concentration at the surface of the adsorbent for each time period and on the amount of dye adsorbed. The Langmuir isotherm model provided the best fit to the equilibrium data in the concentration range investigated and from the isotherm linear equation, the maximum adsorption capacity determined was 1060 mg of reactive dye per gram of adsorbent, corresponding to 75% occupation of the adsorption sites. The results obtained demonstrate that the adsorbent material could be utilized to remove dyes from textile effluents independent of the pH of the aqueous medium.     

Adsorptive removal of phenol from aqueous phase by using a porous acrylic ester polymer

The removal of phenol from aqueous solution was examined by using a porous acrylic ester polymer (Amberlite XAD-7) as an adsorbent. Favorable phenol adsorption was observed at acidic solution pH and further increase of solution pH results in a marked decrease of adsorption capacity, and the coexisting inorganic salt NaCl exerts positive effect on the adsorption process. Adsorption isotherms of phenol were linearly correlated and found to be well represented by either the Langmuir or Freundlich isotherm model. Thermodynamic parameters such as changes in the enthalpy (DeltaH), entropy (DeltaS) and free energy (DeltaG) indicate that phenol adsorption onto XAD-7 is an exothermic and spontaneous process in nature, and lower ambient temperature results in more favorable adsorption. Kinetic experiments at different initial solute concentrations were investigated and the pseudo-second-order kinetic model was successfully represented the kinetic data. Additionally, the column adsorption result showed that a complete removal of phenol from aqueous phase can be achieved by XAD-7 beads and the exhausted adsorbent was amenable to an entire regeneration by using ethanol as the regenerant. More interestingly, relatively more volume of hot water in place of ethanol can also achieve a similar result for repeated use of the adsorbent.     

Adsorption of malachite green from aqueous solution onto carbon prepared from Arundo donax root

Arundo donax root carbon (ADRC), a new adsorbent, was prepared from Arundo donax root by carbonization. The surface area of the adsorbent was determined 158 m²/g by N₂ adsorption isotherm. Batch adsorption experiments were carried out for the removal of malachite green (MG) from aqueous solution using ADRC as adsorbent. The effects of various parameters such as solution pH (3–10), carbon dose (0.15–1.0 g/100 ml) and initial MG concentration (10–100 mg/l) on the adsorption system were investigated. The effective pH was 5–7 and the optimum adsorbent dose was found to be 0.6 g/100 ml. Equilibrium experimental data at 293, 303 and 313 K were better represented by Langmuir isotherm than Freundlich isotherm using linear and non-linear methods. Thermodynamic parameters such as ΔG, ΔH and ΔS were also calculated. The negative Gibbs free energy change and the positive enthalpy change indicated the spontaneous and endothermic nature of the adsorption. The adsorption equilibrium time was 180 min. Adsorption kinetics was determined using pseudo-first-order model, pseudo-second-order model and intraparticle diffusion model. The results showed that the adsorption of MG onto ADRC followed pseudo-second-order model.     

Adsorption of boron from aqueous solutions using fly ash: batch and column studies

In the present paper, boron removal from aqueous solutions by adsorption was investigated. Fly ash particle size used in adsorption experiments was between 250 and 400 microm. During the experimental part of this study, the effect of parameters such as pH, agitation time, initial boron concentration, temperature, adsorbent dosage and foreign ion on boron removal were observed. In addition, adsorption kinetics, adsorption isotherm studies and column studies were made. Maximum boron removal was obtained at pH 2 and 25 degrees C. Thermodynamic parameters such as change in free energy (DeltaG degrees), enthalpy (DeltaH degrees), entropy (DeltaS degrees) were also determined. As a result of the kinetic studies, it was observed that the adsorption data conforms to the second degree kinetics model. In the isotherm studies, Langmuir and Freundlich isotherm models were applied and it was determined that the experimental data conformed to Langmuir isotherm model. Batch adsorbent capacity (q(o)) was calculated as 20.9 mg/g. The capacity value for column study was obtained by graphical integration as 46.2 mg/g. The Thomas and the Yoon-Nelson models were applied to experimental data to predict the breakthrough curves and to determine the characteristics parameters of the column useful for process design.     

In Vitro Adsorption of Propranolol Hydrochloride by Various Antacids

The purpose of this study was to investigate by in vitro methods whether an interaction takes place between propranolol hydro-chloride and adsorbents when antacids are taken concomitantly with the beta-blocker or when excipients having adsorbent properties are present in formulations of the drug products containing propranolol hydrochloride.

Specific surface areas of magnesium trisilicate, magnesium oxide, magnesium hydroxide, dihydroxy aluminum sodium carbonate, magnesium carbonate and kaolin were calculated from nitrogen adsorption isotherm using single pint method and it was found. that magnesium trisilicate has the largest specific surface area.

The adsorption of propranolol hydrochloride to these adsorbents was investigated by in vitro methods. The adsorption isotherms were drawn and the adsorptive capacities of the adsorbents were calculated from the slopes. It was found that magnesium tri-silicate, magnesium hydroxide arid dihydroxy aluminum sodium carbonate possess the highest adsorptive capacities while kaolin and magnesium carbonate possess the lowest.

The results of the adsorption studies indicate that the concomitant use of propranolol hydrochloride and the above mentioned adsorbents could affect the bioavailability of the beta-blocker adversely.     

In Vitro Adsorption of Propranolol Hydrochloride by Various Antacids

The purpose of this study was to investigate by in vitro methods whether an interaction takes place between propranolol hydro-chloride and adsorbents when antacids are taken concomitantly with the beta-blocker or when excipients having adsorbent properties are present in formulations of the drug products containing propranolol hydrochloride.

Specific surface areas of magnesium trisilicate, magnesium oxide, magnesium hydroxide, dihydroxy aluminum sodium carbonate, magnesium carbonate and kaolin were calculated from nitrogen adsorption isotherm using single pint method and it was found. that magnesium trisilicate has the largest specific surface area.

The adsorption of propranolol hydrochloride to these adsorbents was investigated by in vitro methods. The adsorption isotherms were drawn and the adsorptive capacities of the adsorbents were calculated from the slopes. It was found that magnesium tri-silicate, magnesium hydroxide arid dihydroxy aluminum sodium carbonate possess the highest adsorptive capacities while kaolin and magnesium carbonate possess the lowest.

The results of the adsorption studies indicate that the concomitant use of propranolol hydrochloride and the above mentioned adsorbents could affect the bioavailability of the beta-blocker adversely.     

High-concentration band profiles and system peaks for a ternary solute system

High-concentration HPLC band profiles of single solutes and the individual band profiles of the components of binary and ternary mixtures are reported for benzyl alcohol, 2-phenylethanol, and 2-methylbenzyl alcohol. These solutes were eluted from a C18 column by a binary mobile phase (MeOH:H2O = 1:1, v/v). High-concentration system peaks were obtained using mixtures of benzyl alcohol and 2-phenylethanol at different relative concentrations as the feed and 2-methylbenzyl alcohol as the strong mobile phase additive. Band profiles and system peak profiles were calculated using the equilibrium-dispersive model of chromatography. The adsorption equilibrium in the multicomponent system was characterized by the competitive Langmuir model. Excellent quantitative agreement was found between the experimental and the calculated profiles. This work confirms that extremely unusual system peak profiles can be obtained even when the adsorption behavior is quite simple. Under certain circumstances, the use of a properly chosen additive could markedly increase the separation between bands and hence the production rate, the recovery yield, and/or the purity of the fractions.     

Heterogeneity analysis of single-walled carbon nanotubes from the adsorption equilibria of nitrogen and benzene

Nitrogen adsorption/desorption isotherms and gravimetric methods were employed to examine the structural and adsorption properties of selected adsorbent. The equilibrium data of benzene were also obtained at three different temperatures (303.15, 313.15, and 323.15 K) with pressures up to 7 kPa. The results of nitrogen and benzene sorption isotherm revealed that SWCNTs exhibit type II with the features of type I. The Toth and UNILAN models were found to provide a reasonable correlation between the adsorption isotherm data. In addition, the adsorption second virial coefficient and the isosteric heat of adsorption were determined by using these isotherm models. The isosteric heat of adsorption and adsorption energy distribution indicated that SWCNTs have energetically and structurally heterogeneous surfaces.     

Equilibrium, kinetic and sorber design studies on the adsorption of Aniline blue dye by sodium tetraborate-modified Kaolinite clay adsorbent

Raw Kaolinite clay obtained Ubulu-Ukwu, Delta State of Nigeria and its sodium tetraborate (NTB)-modified analogue was used to adsorb Aniline blue dye. Fourier transformed infrared spectra of NTB-modified Kaolinite suggests that modification was effective on the surface of the Kaolinite clay with the strong presence of inner -OH functional group. The modification of Kaolinite clay raised its adsorption capacity from 1666 to 2000 mg/kg. Modeling adsorption data obtained from both unmodified and NTB-modified Kaolinite clay reveals that the adsorption of Aniline blue dye on unmodified Kaolinite clay is on heterogeneous adsorption sites because it followed strongly the Freundlich isotherm equation model while adsorption data from NTB-modified Kaolinite clay followed strongly the Langmuir isotherm equation model which suggest that Aniline blue dye was adsorb homogeneous adsorption sites on the NTB-modified adsorbent surface. There was an observed increase in the amount of Aniline blue adsorbed as initial dye concentration was increased from 10 to 30 mg/L. It was observed that kinetic data obtained generally gave better robust fit to the second-order kinetic model (SOM). The initial sorption rate was found to increased with increasing initial dye concentration (from 10 to 20 mg/L) for data obtained from 909 to 1111 mg kg(-1)min(-1) for unmodified and 3325-5000 mg kg(-1) min(-1) for NTB-modified adsorbents. Thereafter there was a decrease in initial sorption rate with further increase in dye concentration. The linearity of the plots of the pseudo-second-order model with very high-correlation coefficients indicates that chemisorption is involved in the adsorption process. From the design of a single-batch adsorber it is predicted that the NTB-modified Kaolinite clay adsorbent will require 50% less of the adsorbent to treat certain volumes of wastewater containing 30 mg/L of Aniline blue dye when it is compared with the unmodified adsorbent. This will be cost effective in the use of NTB-modified adsorbent for the adsorption of Aniline blue dye from water and wastewater.     

Adsorption equilibria of benzodiazepines on a hybrid polymeric chiral stationary phase

The chromatographic behavior of a series of racemic benzodiazepines was evaluated under linear and nonlinear conditions on a new hybrid polymeric (DACH-ACR) chiral stationary phase (CSP). Differently substituted benzodiazepines were employed as probes to make hypotheses concerning possible molecular interaction mechanisms originating between target compounds and active sites on the CSP. Hydrogen bonds were found to be pivotal for chromatographic retention and chiral selectivity. The competitive effect from a mobile-phase (MP) modifier able to interact with the CSP through H-bonds was investigated. The performance of the polymeric DACH-ACR CSP for preparative purposes was also evaluated. The competitive adsorption isotherms of two benzodiazepines, lorazepam and temazepam, were measured at different MP compositions through the so-called inverse method. The adsorption data were fitted with a competitive bi-Langmuir adsorption isotherm. Enantiomeric separations under nonlinear conditions were modeled by using the equilibrium dispersive (ED) model of chromatography. Theoretical overloaded band profiles (obtained by solving the system of partial differential equations described by the ED model) matched, in a significantly accurate way, the profiles experimentally measured.     

Thermodynamic studies of the solvent effects in chromatography on molecularly imprinted polymers. 3. Nature of the organic mobile phase

Experimental isotherm data of the Fmoc-tryptophan (Fmoc-Trp) enantiomers were measured by frontal analysis on a Fmoc-L-Trp imprinted polymer, using different organic mobile phases, in a wide concentration range. The nonlinear regression of the data and the independent calculation of the affinity energy distributions of the two enantiomers allowed the selection of the isotherm model and the determination of the isotherm parameters. The organic solvents studied were acetonitrile (MeCN), methylene chloride, chloroform, and tetrahydrofuran (THF), all in the presence of the same concentration of acetic acid, used as an organic modifier. It was found that the highest overall affinity and enantiomeric selectivity were obtained in MeCN, which is also the solvent used in the polymerization. In the other solvents, the overall affinity decreases with increasing hydrogen-bonding ability of the solvents but not the enantiomer selectivity. In MeCN, three types of adsorption sites coexist for the two enantiomers on the MIP. The highest energy sites for Fmoc-L-Trp in MeCN are inactive in CH(2)Cl(2), CHCl(3), and THF, and only two types of sites were identified in these solvents. Increasing the acetic acid concentration from 0.2 to 0.9 M causes a large decrease in the association constant of the highest energy sites in CH(2)Cl(2), CHCl(3), and THF but not in MeCN. The overall affinity of Fmoc-L-trp in CH(2)Cl(2), CHCl(3), and THF is dominated by adsorption on the lowest energy sites, the most abundant ones. In contrast, in MeCN, the overall affinity of Fmoc-L-Trp is dominated by adsorption on the highest energy sites, the least abundant sites. In CH(2)Cl(2), CHCl(3), and THF, the number of each type of sites increases with decreasing hydrogen-bonding ability of the solvents while the association constant of the corresponding sites does not change significantly.     

The removal of basic dyes from aqueous solutions using agricultural by-products

The adsorption of two basic dyes, namely, Malachite Green and Methylene Blue onto both agricultural by-products (i.e. rice bran and wheat bran) has been investigated in this study. The adsorption of both basic dyes was solution pH-dependent. The kinetic experimental data were analyzed using four kinetic equations including pseudo-first-order equation, pseudo-second-order equation, external diffusion model and intraparticle diffusion model to examine the mechanism of adsorption and potential rate-controlling step. The best-fit equation was identified using normalized standard deviation. The Langmuir and Freundich isotherms were used to fit the equilibrium data and the results showed that the Langmuir isotherm exhibited a little better fit to the Methylene Blue adsorption data by both adsorbents while the Freundlich isotherm seemed to agree better with the Malachite Green adsorption. The Gibbs free energy changes at 20 degrees C were calculated and the obtained values supported the conclusion that two dyes molecules adsorbed by physical processes. The effects of particle size, adsorbent concentration and solution ionic strength on the adsorption of the two basic dyes were also studied.