Sorption kinetics of strontium in porous hydrous ferric oxide aggregates I. The Donnan diffusion model

Sorption of ions by hydrous ferric oxide (HFO) often shows a fast initial sorption reaction followed by a much slower sorption process. The second step is diffusion-controlled and can continue for days or months before equilibrium is reached. In this paper, we demonstrate that the diffusion rate may be explained by electrostatic interactions. The internal and external surfaces of HFO are generally positively charged and therefore repel cations. This can result in extremely low cation concentrations in pores, and therefore a significant reduction in pore diffusion rate. The theory is demonstrated here for sorption of Sr(2+) in HFO aggregates. The ion concentrations in the pore space are calculated using a Donnan model and diffusion is calculated from the Donnan concentration and potential gradients. This diffusion model is compared with nonelectrostatic pore diffusion, which does not take electrostatic interactions into account. The Donnan model predicts very low concentrations of Sr(2+) in the pores and diffusion rates that are up to 8000 times lower than predicted with a nonelectrostatic model.     

Characterization of the pores in hydrous ferric oxide aggregates formed by freezing and thawing

Hydrous ferric oxides (HFO) are efficient sorbents for inorganic and organic pollutants and therefore have great potentials in environmental science and engineering applications. Freezing and thawing of HFO suspensions leads to the formation of dense HFO aggregates. It facilitates the handling and increases the drying rate of HFO. In this study, we used a combination of pycnometry, gas adsorption (N(2) gas, water vapor), and small-angle neutron scattering (SANS) to characterize the porosity and pore size distribution of dense HFO aggregates formed by freezing dialyzed HFO suspensions at -25 degrees C and thawing them at room temperature. The crystallinity of the HFO, which was a 2-line ferrihydrite, was not affected by this treatment. Wet sieving and laser diffraction analysis showed that the dense HFO aggregates had a unimodal size distribution with an average diameter of 235+/-35 microm. Increasing the freezing rate by cooling with liquid N(2) (-196 degrees C) resulted in much smaller aggregates with an average diameter of 20 microm. Adding NaNO(3) electrolyte to the HFO suspensions prior to freezing also resulted in the formation of smaller aggregates. The dense HFO aggregates formed at -25 degrees C had a porosity of 0.73+/-0.02 ll(-1). SANS revealed a unimodal size distribution of pores, with an average pore diameter of 2.0 nm. The diameter of the HFO crystallites was estimated by transmission electron microscopy to be 1.9+/-0.5 nm. Geometrical considerations taking into account the unit particle and average pore size suggest that the crystallites retain 1-2 layers of hydration water during the coagulation induced by freezing. Analysis by N(2) gas adsorption showed that drying the dense HFO aggregates induced a reduction in porosity by about 25% and shifted the pore size distribution to smaller diameters. Rewetting during water vapor adsorption did not induce significant changes of the aggregate structure. The specific surface area of the dry HFO aggregates was between 320 and 380 m(2)g(-1).     

Protein adsorption on novel acrylamido-based polymeric ion exchangers. II. Adsorption rates and column behavior

Uptake kinetics and breakthrough behavior were determined for bovine serum albumin (BSA) and alpha-chymotrypsinogen (alphaCHY) in new polymeric ion-exchange media based on acrylamido monomers. Two anion exchangers and a cation exchanger were investigated. As shown in Part I of this work, the two anion exchangers have different morphologies. The first one, BRX-Q, comprises a low-density gel with a matrix of denser polymeric aggregates. While this material has a very low size-exclusion limit for neutral probes, it exhibits an extremely high binding capacity for BSA. The second anion exchanger, BRX-QP, comprises large open pores but has a very low binding capacity. The cation exchanger, BRX-S, also comprises large open pores but exhibits an intermediate capacity; likely as a result of the presence of smaller pores. Dynamic protein uptake experiments showed that the highest mass transfer rates are obtained with BRX-Q. The apparent diffusivity is also highest for this material and increases substantially as the protein concentration is reduced. For these particles, the external film resistance is dominant at very low protein concentrations. Much lower rates and apparent diffusivities are obtained for BRX-QP. Finally intermediate rates and apparent diffusivities are found with BRX-S. The concentration dependence of the apparent pore diffusivity is much less pronounced in this case. The apparently paradoxical result that mass transfer rates are highest for the material with the smallest neutral-probe size-exclusion limit can be explained in terms of a general conceptual model where parallel pore and adsorbed-phase diffusion paths exist in these particles. In the first case, adsorbed phase diffusion in gel pores is dominant, while in the second transport is dominated by diffusion in a macroporous network. In the third case, both contributions are important. The conceptual model provides an accurate prediction of the breakthrough behavior of columns packed with these media using independently determined rate parameters. Dynamic binding capacities of 80-140 mg/ml were observed for BSA on BRX-Q in ca. 1.5 cm columns operated at 300-900 cm/h in agreement with theoretical predictions.     

Determination of sorption capacity of fucoidic sands for Cs+ and Sr2+ under dynamic column conditions

In this paper, the sorption behavior of Cs⁺ and Sr²⁺ on column of fucoidic sands under dynamic flow conditions was investigated, and their sorption capacities (SC) towards these two cations were studied. The determination of SC is based on the construction of respective breakthrough curves using ¹³⁷Cs and ⁸⁵Sr radionuclides as isotopic indicators in laboratory experiments. The samples were taken from several parts of the borehole in the area of interest. Undisturbed cores of 5 cm in diameter and 10 cm long were put in the glass columns and the cores were perfectly tightened using acrylate resin. In this time-dependence study, the so-called cenoman background groundwater was used. A concentration of 10^?6 mol/dm³ of Cs⁺ and Sr²⁺ in liquid phase individually was established using neutral salts of CsNO₃ and Sr(NO₃)₂, respectively. The groundwater was introduced at the bottom of the columns by a multi-head peristaltic pump, at a constant flow-rate of about 4 cm³/h. The results show that the sorption capacity of the investigated fucoidic sands for ¹³⁷Cs and ⁸⁵Sr is 0.1–1.5 and 0.05–0.5 μmol/100 g, respectively, in dependence on the evaluation of corresponding breakthrough curves. Some differences in the behavior of the cores during the experiments have also been observed and explained.     

The uptake of strontium by calcium phosphate phase formed at an elevated pH

The sorption of radioactive strontium by poorly crystalline hydroxyapatite (HA) obtained by the transformation of amorphous calcium phosphate (ACP) at initial pH 10.8, was investigated. Morphological analysis, performed by transmission electron microscopy, has demonstrated that the solid phase consists of nanometer size ultra-micro particles connected into spherical aggregates. A maximum sorption capacity, determined as the asymptote of the sorption isotherm was =(3.8±0.3)·10^–2mol Sr/mol P. Based on this result and the structural conditions, a possible theoretical explanation for the incorporation of a strontium ion into the apatite lattice has been proposed.     

Transport and retention of strontium in surface-modified quartz sand with different wettability

Instead of radioactive ⁹⁰Sr, common strontium chloride was used to simulate the migration of radioactive strontium chloride in surface hydroxylated, silanized, and common quartz sand. The sorption and retardation characteristics of strontium (Sr²⁺) in these surface modified quartz sands were studied by batch tests and column experiments. The equilibrium sorption data for Sr²⁺ on different wettability sands were described by the Langmuir and Freundlich isotherm models, and the Langmuir model has been found to provide better correlation for hydrophilic sand. The breakthrough curves (BTCs) of Sr²⁺ in these media were analyzed with the equilibrium convection–dispersion equation (CDE) and a non-equilibrium two-region mobile–immobile model (TRM) using a nonlinear least square curve-fitting program CXTFIT. The TRM model showed better fit to the measured BTCs of Sr²⁺, and the parameters of the fraction of mobile water indicated that significant preferential flow effected the non-equilibrium transport of Sr²⁺. Although TRM model could not fit the Sr²⁺ BTCs very well, the parameter estimated by TRM model may be more reliable than those obtained from batch experiments because the transport of Sr²⁺ in these kind of sand is non-equilibrium processes.     

pH within pores in plant fiber cell walls assessed by Fluorescence Ratio Imaging

The pH within cell wall pores of filter paper fibers and hemp fibers was assessed by Fluorescence Ratio Imaging (FRIM). It was found that the Donnan effect affected the pH measured within the fibers. When the conductivity of the added liquid was low (0.7 mS), pH values were lower within the cell wall than in the bulk solution. This was not the case at high conductivity (22 mS). The occurrence of the Donnan effect allowed the pH values within pores in normal regions of the cell wall to be compared to the pH in regions with misaligned microfibrils (dislocations) when FRIM was carried out in a low conductivity solution. Surprisingly, no pH difference was observed between normal regions and dislocations, suggesting that pore sizes within the two different regions are approximately the same. In another experiment the Donnan effect was shown to have an effect on hydrolysis of hydrothermally pretreated wheat straw only when conducted in a low conductivity solution and only for xylanase, not cellulases. The hydrolysis experiments indicate that under typical conditions where conductivity is high, the Donnan effect does not lower the pH close to the substrate to an extent that affects enzymatic activity during hydrolysis of lignocellulose.     

Adsorption of BSA and Hemoglobin on Hydroxyapatite Support: Equilibria and Multicomponent Dynamic Adsorption

Interactions of Bovin Serum Albumin and Hemoglobin with an hydroxyapatite gel (HA-Ultrogel, Sepracor), have been studied separately in batch experiments. The adsorption isotherms are of the Langmuir type and can be used directly to scale column operations.For adsorption of hemoglobin alone, in column at pH 6.8 (equal to its isoelectric point) we notice that a classical intraparticle transfer model, based on a constant effective diffusion coefficient represents perfectly the symmetrical breakthrough curve. For acid pH values (pH 5.8), Langmuir isotherms of BSA and hemoglobin adsorptions showed a strong curvature, sign of a quite irreversible adsorption and breakthrough curves obtained under these conditions, exhibit a high dissymmetrical shape for both proteins. In that case, a model of diffusion based on the adsorption on two types of independent sites, with two intraparticle transfer coefficients, gives a good representation of the breakthrough for adsorption of both proteins separately.Binary mixtures of these components were prepared and injected in columns packed with the same support. Competitive Langmuir equation, based on the results obtained in monocomponent batch experiments, give a very good fit to our system. The intraparticle transfer in that case seems to be facilitated, and one effective coefficient alone is enough to predict the breakthrough curves obtained. This behaviour may be the result of an increase of the solution ionic strength, and of the smaller irreversibility feature of the adsorption when proteins are in competition.     

Comparison between mass transfer properties of weak-anion-exchange resins with graft-functionalized polymer layers and traditional ungrafted resins

Glycidylmethacrylate was grafted to Toyopearl HW-65M and subsequently modified with diethylamine to obtain a weak anion exchanger. The degree of grafting was varied from 11 to 50%. The binding capacity for bovine serum albumin was 11 mg/ml for the lowest degree of grafting and 97 mg/ml for the highest degree of grafting. The maximum binding capacity was observed at 27% degree of grafting. The mass transfer properties of the grafted resins and an ungrafted resin(Toyopearl DEAE 650M) were investigated assuming rectangular isotherms. Simple models for reaction kinetics, pore- and surface diffusion and film diffusion were used to describe the concentration-time data in batch mode. The data were best fitted by a pore diffusion model. The estimated pore diffusion coefficients (D(P)) for bovine serum albumin were fitted by a polynome to the degree of grafting with an maximum value at 27% of D(P) = 1.95-10(-11) m2/s. Compared to published data of other ungrafted resins and to the molecular diffusion coefficient of bovine serum albumin in free solution of D(P) = 5.6 10(-11) m2/s, the diffusion in grafted layers seems to be accelerated. The breakthrough curves for columns packed with various resins showed a decrease in sharpness with increasing degree of grafting which could not be described by a simple pore diffusion model using the calculated transport coefficients from batch mode. The shape of the breakthrough curves could be well described by a combined film and pore diffusion model. For the ungrafted Toyopearl DEAE 650M resin the breakthrough curve is more favorable and the influence of film diffusion to the mass transfer is reduced. It can be concluded that grafting will increase the capacity and the pore diffusion in batch mode but in column operation the grafting layer has a film resistance which plays an important role in the overall mass transfer.     

Breakthrough Model of Recombinant Human-Like Collagen in Immobilized Metal Affinity Chromatography

The adsorption of recombinant human-like collagen by metal chelate media was investigated in a batch reactor and in a fixed-bed column. The adsorption equilibrium and kinetics had been studied by batch adsorption experiments. Equilibrium parameters and protein diffusivities were estimated by matching the models with the experimental data. Using the parameters of equilibrium and kinetics, various models, such as axial diffusion model, linear driving force model, and constant pattern model, were used to simulate the breakthrough curves on the columns. As a result, the most suitable isotherm was the Langmuir–Freundlich model, and the ionic strength had no effect on the adsorption capacity of chelate media. In addition, the pore diffusion model fitted very well to the kinetic data. The pore diffusivities decreased with increasing the initial protein concentration, however had little change with the ionic strength. The results also indicated that the models predict breakthrough curves reasonably well to the experimental data, especially at low initial protein concentration (0.3 mg ml⁻¹) and low flow rate (34 cm h⁻¹). By the results, we optimized the experimental conditions of a chromatographic process using immobilized metal affinity chromatography to purify recombinant human-like collagen.     

Transport and sorption of <Superscript>85</Superscript>Sr and <Superscript>125</Superscript>I in crushed crystalline rocks under dynamic flow conditions

Transport and sorption of water-soluble ⁸⁵Sr²⁺ and ¹²⁵I⁻ in the columns with beds of crushed crystalline rocks from synthetic groundwater has been studied under dynamic flow conditions. Samples of crystalline rocks: diorite-I, diorite-II, gabbro, granite and tonalite, having the grain size between 0.25 and 0.80 mm, were used. Plastic syringes of 8.8 cm length and 2.1 cm in diameter were applied as columns. The synthetic groundwater was pumped downward through the columns with a seepage velocity of about 0.2 cm/min and the given radioactive nuclide was added into the water stream individually in a form of a short pulse. In case of ⁸⁵Sr, desorption from diorite-I was also studied using an artificial acid rainfall and then, the longitudinal distribution of the residual ⁸⁵Sr activity along the bed was measured. Retardation, distribution and hydrodynamic dispersion coefficients were determined by the evaluation of respective breakthrough curves. A corrected integral form of a simple advection–dispersion equation was derived and used for fitting the experimental data. The K _d-parameters resulting from dynamic experiments were also compared with the results of static sorption experiments.     

Strontium binding by calcium silicate hydrates

In the present study the binding of strontium with pure calcium silicate hydrates (C-S-H) has been investigated using batch-type experiments. Synthetic C-S-H phases with varying CaO:SiO(2) (C:S) mol ratios, relevant to non-degraded and degraded hardened cement paste, were prepared in the absence of alkalis (Na(I), K(I)) and in an alkali-rich artificial cement pore water (ACW). Two types of experimental approaches have been employed, investigating sorption and co-precipitation processes, respectively. The Sr(II) sorption kinetics were determined as well as sorption isotherms, the effect of the solid to liquid ratio and the composition (C:S ratio) of the C-S-H phases. In addition, the reversibility of the Sr(II) sorption was tested. It was shown that both the sorption and co-precipitation tests resulted in Sr(II) distribution ratios which were similar in value, indicating that the same sites are involved in Sr(II) binding. In alkali-free solutions, the Sr(II) uptake by C-S-H phases was described in terms of a Sr(2+)-Ca(2+) ion exchange model. The selectivity coefficient for the Sr(2+)-Ca(2+) exchange was determined to be 1.2+/-0.3.     

Sorption of the long-lived radionuclides cesium-134, strontium-85 and cobalt-60 on bentonite

The sorption of long-lived radionuclides of cesium, strontium and cobalt (¹³⁴Cs, ⁸⁵Sr and ⁶⁰Co) on bentonite under various experimental conditions, such as contact time, pH, sorbent and sorbate concentrations have been studied. The uptake of Cs and Sr was rapid and equilibrium was reached almost instantaneously in both the cases, while Co sorption was time dependent. The sorption of these nuclides increased by increasing pH. The uptake of Cs, Sr and Co increased with increasing the amount of the bentonite clay. The percentage sorption for Cs, Sr and Co decreased with increasing metal concentrations. The desorption studies with 0.01M CaCl₂ and ground water at low-metal loadings on bentonite showed that about 95% of Cs, 85-90% of Sr and 97% of Co were irreversibly sorbed. These results could be helpful for nuclear waste management, for waste water effluents containing low concentrations of cesium, strontium and cobalt.     

Sorption of strontium by Mexican erionite

The apparent diffusion coefficient of Sr²⁺ in natural Mexican erionite from Sonora at different pH and concentrations were determined. Neutron activation was applied to measure the sorption of strontium. The apparent diffusion coefficient values showed that the mobility of Sr²⁺ through the cavities of the erionite depended on the concentration of strontium and the pH of the solution. As a consequence the sorption of this ion by the erionite was affected by these parameters and the maximum sorption of Sr²⁺ was at pH higher than 3 and 0.0094 mol·l^–1 strontium nitrate solution.     

Microcolumn test and model analysis of activated carbon adsorption of dissolved organic matter after precoagulation: effects of pH and pore size distribution

Microcolumn adsorption experiments were conducted to generate breakthrough profiles of dissolved organic matter (DOM) remaining after coagulation treatment of a naturally colored surface water for three coal-based activated carbons (ACs) and four water pH levels. A plug-flow homogeneous surface diffusion model was applied to determine the intraparticle surface diffusivities of the DOM at different AC-pH combinations. It was found that, for all three ACs, the removal of DOM increased as pH decreased and the increasing extent changed with the ACs used. The pH dependency of the column performance seemed to be attributed more apparently to pH's capability in changing the zeta potential of AC particles. In addition, at all pH levels, the column performance varied markedly with the ACs used. Correlation analyses of the accumulated amounts of DOM onto all three ACs with corresponding pore volumes in several divided pore size regions clearly indicated that pores with sizes 30-100 A were more effective in adsorbing organic macromolecules. Furthermore, based on model simulations, the sensitivity of bed performance to equilibrium and kinetic parameters was quantified by conducting variance analyses with a four-way classification method.     

Separation of strontium from large amounts of calcium, with application to radiostrontium analysis

The elution first of calcium and then of strontium by complex formation with cyclohexanediaminetetra-acetic acid (CyDTA) after sorption on ion-exchange columns, is shown to require careful control of pH and the concentrations of CyDTA and NH(4)(+). A procedure is described for volumetric analysis of calcium and strontium, and for use of this separation procedure before determination of (90)Sr in sea-water. Careful choice of reagents and materials is shown to be necessary for use of such a method in determination of trace amounts of radiostrontium.     

Sorption and desorption of radiostrontium on powdered bentonite: Effect of pH and fulvic acid

The effect of pH and fulvic acid on the sorption of Sr on bentonite was investigated by using batch experiments. The sorption and desorption isotherms of Sr on bentonite were determined at room temperature, at pH 6.0±0.2 and in presence of 0.1M NaCl. It was found that the sorption of Sr is independent at pH<8, and then increases slightly with increasing pH. Fulvic acid increases the sorption of Sr significantly on bentonite at low pH, but decreases the sorption of Sr at pH>8. The sorption of Sr on bentonite can be described by a reversible sorption process and the sorption mechanism consists mainly of ion exchange. This revised version was published online in July 2006 with corrections to the Cover Date.     

Distribution of strontium in milk component

The distribution of strontium between the milk components, i.e., serum, casein micelles, whey and hydroxyapatite was determined. The sorption on hydroxyapatite was investigated using batch method and radiotracer technique. The aqueous phase comprised of either milk or whey. The sorption of strontium on hydroxyapatite depended on the method of its preparation and on the composition of the aqueous phase. The sorption of strontium was increased with an increase of pH. The presence of citrate species resulted in decrease of the sorption of strontium on hydroxyapatite. The sorption of ⁸⁵Sr on hydroxyapatite decreased with the increasing concentration of Ca²⁺ ions. Addition of Ca²⁺ ions to milk resulted in milk pH decrease. The decrease in pH value after calcium addition to milk is related to exchanges between added calcium and micellar H⁺. The average value of strontium sorption on casein micelles in milk with presence of hydroxyapatite was (47.3 ± 5.6) %. The average value of sorption of ⁸⁵Sr on casein micelles in milk without the addition of hydroxyapatite was (68.9 ± 2.2) %.     

Solid phase extraction of copper(II) by fixed bed procedure on cation exchange complexing resins

The efficiency of the metal ion recovery by solid phase extraction (SPE) in complexing resins columns is predicted by a simple model based on two parameters reflecting the sorption equilibria and kinetics of the metal ion on the considered resin. The parameter related to the adsorption equilibria was evaluated by the Gibbs–Donnan model, and that related to the kinetics by assuming that the ion exchange is the adsorption rate determining step. The predicted parameters make it possible to evaluate the breakthrough volume of the considered metal ion, Cu(II), from different kinds of complexing resins, and at different conditions, such as acidity and ionic composition.     

Sorption of strontium on Slovak bentonites

Sorption of Sr on five Slovak bentonites of deposits has been studied with the use of batch technique. In the experiments there have been used natural, chemically modified and irradiated samples, in three different kinds of grain size. The pH influence on sorption of strontium on bentonites, pH change after sorption and influence of competitive ions have been studied. Distribution ratios have been determined for bentonite–strontium solution system as a function of contact time, pH and sorbate concentration. The data have been interpreted in term of Langmuir isotherm. The uptake of Sr has been rapid and the sorption of strontium has increased by increasing pH. The percentage sorption has decreased with increasing metal concentrations. The pH value after sorption for the natrificated forms of bentonite starts already in the alkaline area and moves to the higher values. For the natural bentonites the values occur in the neutral or in the acidic area. Sorption of Sr has been suppressed by presence of competitive cations as follows: Ba²⁺ > Ca²⁺ > Mg²⁺ > NH₄ ⁺ > K⁺ > Na⁺. By sorption on natrificated samples colloidal particles and pH value increase have been formed. The bentonite exposure as a result of interaction of γ-rays has led to expansion of the specific surface, increasing of the sorption capacity and to the change in the solubility of the clay materials.