Ion-exchange Functional Groups Research Articles

Experimental and modeling of cadmium ions removal by chelating resins

Abstract Nowadays, the concerns regarding cadmium pollution increased steadily, due to its serious damages to the environment and human health. Consequently, stringent limits have been set for this heavy metal ion both in industrial and drinking water. Numerous traditional and unconventional methods were applied to reach these limits. In this paper, two phosphine oxide/phosphine oxide and thiourea chelating resins MPX-310 and MPX-317 produced by Magpie Polymers - Italmatch Chemicals, Italy were comparatively evaluated with a commercial thiourea chelating resin Lewatit® MonoPlus TP 214 produced by Lanxess regarding Cd(II) removal from aqueous synthetic solutions. The physicochemical properties of the resins were studied by FTIR spectroscopy, scanning electron microscopy (SEM), thermogravimetric analysis (TG-DSC) and X-ray Photoelectron Spectroscopy (XPS). Batch tests have been performed to assess the performance of these resins in Cd(II) removal. A value of 3–4 for pH and 600 min of contact time were determined as optimal parameters for the removal process. The experimental results also showed that the Lewatit® MonoPlus TP 214 has a higher removal capacity than MPX-317 and MPX-310 due to the presence of the thiourea and ion-exchange functional groups on the Lewatit® MonoPlus TP 214 backbone, compared with the presence of the phosphine oxide functional groups on the MPX-310 and thiourea and phosphine oxide on MPX-317. A monolayer adsorption on the homogenous adsorbent surface mechanism involved in Cd(II) removal was pointed out by the results regarding adsorption isotherms. The best fit of the pseudo-second kinetic model indicates that the sorption process goes through a mechanism that involves chemical reaction between Cd(II) and functional groups grafted on the resin's backbone. The chelating capacity of the tested resins showed that they are suitable for Cd(II) removal from aqueous synthetic solutions and industrial wastewater.

Modelling of anion-exchange membrane transport properties with taking into account the change in exchange capacity and swelling when varying bathing solution concentration and pH

In this paper we take into account the variation of anion-exchange membrane swelling and effective exchange capacity with changing the concentration and/or pH of the bathing solution when simulating membrane transport characteristics (the electric conductivity, diffusion permeability and ion transport numbers). Two main structural elements of the membrane are distinguished: a microporous polyelectrolyte gel assumed homogeneous, and macropores including structural defects and voids. It is supposed that only microporous gel exerts osmotic pressure, which causes membrane swelling described in our model by the Gregor equation, which employs the mole fractions of free and bound water. Three types of ion-exchange functional groups, namely, the secondary, tertiary and quaternary amino groups are taken into account. The degree of dissociation of these groups depends on the internal solution pH, which is described by equilibriums between protonated and deprotonated forms. The Donnan equations and electroneutrality condition are used to calculate the ion concentration in the membrane. The microheterogeneous model based on the Nernst-Planck equations is used to describe the ion transport. We take into account that the ion diffusion coefficients depend on the degree of gel swelling, this dependence is described by the Mackie-Meares equation. The model parameters include the equivalent volume of dry polyelectrolyte gel, the volume fraction of macropores, the structural parameters and others.The results of calculations are compared with experimental data on the electric conductivity, diffusion permeability and ion transport numbers for two heterogeneous anion-exchange membranes MA-40 and MA-41 (Shchekinoazot, Russia) differed by the fractions of the weakly basic functional groups. A good quantitative agreement between the theory and experiment is found for both membranes using the same set of ion hydration numbers and chemical equilibrium constants for secondary and tertiary amino groups. In particular, it is shown that with increasing pH of the bathing solution, the membrane diffusion permeability passes through a maximum, which is explained by the corresponding maximum of water content. A higher water content results in larger membrane pores, which leads to higher values of ion diffusion coefficients and higher amount of sorbed electrolyte. Together with varying water content, the variation in membrane effective exchange capacity as a function of external solution pH, determines a non-trivial behavior of electric conductivity, diffusion permeability and ion transport numbers in the membrane equilibrated with NaCl solutions of different concentration and pH.

Preparation of Nano-SiO2/Al2O3/ZnO-Blended PVDF Cation-Exchange Membranes with Improved Membrane Permselectivity and Oxidation Stability

Ion exchange membranes are used in practically every industry; however, most of them have defects such as low permeability and poor oxidation resistance. In this paper, cation-exchange membranes were prepared with poly (vinylidene fluoride) (PVDF) blended with nano-SiO2, nano-Al2O3 and nano-ZnO. Sulfonic acid groups were injected into the membrane prepared by styrene grafting and sulfonation. The methods used for characterizing the prepared membranes were Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and electrochemical measurements. Membrane performance, such as the ion exchange capacity (IEC), water uptake (WU), transport number, membrane permselectivity, membrane resistance, functional groups, and morphology were also evaluated. The hydrophilia, IEC, and permselectivity of cation-exchange membranes depended on the nanoparticle content of the membrane matrix. High transport property values were obtained, which increased with increasing nano-SiO2/Al2O3/ZnO weight fractions. Finally, the cation-exchange membranes prepared with 1.5% nano-SiO2, 2.0% nano-Al2O3 or 2.0% nano-ZnO all exhibited excellent membrane properties, including membrane permselectivity (PVDF/2% ZnO-g-PSSA membranes, 94.9%), IEC (PVDF/2% Al2O3-g-PSSA membranes, 2.735 mmol·g−1), and oxidation resistance (PVDF/1.5% SiO2-g-PSSA membranes, 2.33%). They can be used to separate applications in a variety of different areas, such as water treatment, electro-driven separation, heavy metal smelting, or other electrochemical processes.

A simple model for the response of an anion-exchange membrane to variation in concentration and pH of bathing solution

Swelling is an important property of charged gels and membranes. The size of pores and, therefore, membrane properties such as conductivity, diffusion and hydraulic permeability, permselectivity depend on water content and degree of swelling. In this paper we propose a simple model for equilibrium swelling of an ion-exchange membrane, allowing calculation of water content and membrane thickness as functions of the concentration and pH of the bathing solution. The model parameters include the equivalent volume of dry polyelectrolyte gel, the volume fraction of macropores and others. Three types of ion-exchange functional groups, namely, the secondary, tertiary and quaternary amino groups in an anion-exchange membrane are taken into account. The osmotic pressure exerted by micro- and mesopores, appearing in the gel when swelling, is expressed using the Gregor equation, which employs the mole fractions of free and bound water. The equilibria between protonated and deprotonated amino groups are assumed as well as the Donnan and ion-exchange equilibria between the membrane and bathing solution. The results of calculations are compared with experimental data on the membrane thickness and effective exchange capacity obtained for two heterogeneous anion-exchange membranes MA-40 and MA-41 (Shchekinoazot) differed by the composition of functional groups. The procedure of determining membrane structural and thermodynamic parameters is described. A good quantitative agreement between the theory and experiment is found for both membranes using the same set of ion hydration numbers and chemical equilibrium constants for secondary and tertiary amino groups. In particular, it is shown that with increasing pH of the bathing solution, the membrane thickness (and, hence, water content) pass through a local maximum and a local minimum. The simplicity of the model, which does not reduce adequacy, would enable it to be included later in other models describing the transport of ions and water to account for the swelling and change in the membrane structure with a change in pH and concentration of the bathing solution.

Studying effects of ion exchange resin structure and functional groups on Re(VII) adsorption onto Purolite A170 and Dowex 21K

The effects of the functional groups and structures of two different resins, weak base/macroporous and strong base/gel type, Purolite A170 and Dowex 21K on the adsorption properties of Re(VII) ions were investigated experimentally and described by the isotherm, kinetic, and thermodynamic modeling. In this regard, four widely used adsorption isotherm models including Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich (D-R) were subjected to the sorption data in order to describe the reactions involved. Evaluating the correlation coefficients showed that the Freundlich and D-R isotherm models provided the best fit. The Langmuir isotherm capacities (qm) indicated that the perrhenate ion (ReO4-) adsorption was higher for the weak base/macroporous type resin rather than the others (166.67 mg/g and 142.86 mg/g, respectively). Moreover, the results of the EDX studies were in agreement with the previous results. Furthermore, the adsorption kinetics was demonstrated through fitting the data into different mechanisms, among which the pseudo-second-order mechanism was found to be successful for both resins; however, in the case of Dowex 21K, the rate of perrhenate ion uptake was more rapid than that for Purolite A170. Evaluation of the thermodynamic parameters also showed that the reaction mechanism was different for each case and that the adsorption of rhenium on Dowex 21K became more feasible with increase in temperature due to negative values for ΔH.

Novel Ion-Exchange Coagulants Remove More Low Molecular Weight Organics than Traditional Coagulants.

Low molecular weight (MW) charged organic matter is poorly removed by conventional coagulants but contributes to disinfection byproduct formation during chlorination of drinking waters. We hypothesized that CIEX, a new Al-based hybrid coagulant with ion-exchange functional groups, would be new mechanistic approach to remove low MW organic matter during coagulation and would perform better than polyaluminum chloride (PACl) or metal-salt based coagulants. We measured coagulation performance using dissolved organic carbon (DOC) in a high hardness surface water. CIEX achieved excellent turbidity removal and removed 20% to 46% more DOC than FeCl3, Al2(SO4)3, or PACl, depending on dose. The improved DOC removal was attributable to better removal of low MW organic matter (<2 kDa). We further studied removal mechanisms in a model water containing a low MW organic acid (salicylic acid (SA)). CIEX achieved high removal of organic acids (>90% of SA) independent of pH, whereas removal by metal salts was lower (<15%) and was strongly pH dependent. CIEX ion-exchange capability is facilitated by its covalently bound quaternary ammonium group, which conventional coagulants lack. Plus, unlike other cationic polymers that react with chloramines to form N-nitrosodimethylamine (NDMA), CIEX has a low molar yield (9.3 × 10(-7) mol NDMA per mol CIEX-N).

English

Mixed matrix anion exchange membranes (AEMs) were synthesized using dry-wet phase inversion. The casting solutions were prepared by dispersing finely ground anion-exchange resin particles in N,N-dimethylacetamide (DMAc) solutions of polysulfone (PSf). Subsequently, nanosilica particles were introduced into the membranes. The results show that evaporation time (tev) and solution composition contributed to membrane properties formation. A longer tev produces membranes with reduced void fraction inside the membranes, thus the amount of water adsorbed and membrane conductivity are reduced. Meanwhile, the permselectivity was improved by increasing tev, since a longer tev produces membranes with a narrower channel for ion migration and more effective Donnan exclusion. The incorporation of 0.5 %-wt nanosilica particles into the polymer matrix led to conductivity improvement (from 2.27 to 3.41 mS.cm-1). This may be associated with additional pathway formation by hydroxyl groups on the silica surface that entraps water and assists ion migration. However, at further silica loading (1.0 and 1.5 %-wt), these properties decreased (to 1.9 and 1.4 mS.cm-1 respectively), which attributed to inaccessibility of ion-exchange functional groups due to membrane compactness. It was found from the results that nanosilica contributes to membrane formation (increases casting solution viscosity then reduces void fraction) and membrane functional group addition (provides hydroxyl groups).

Development of Coatings for Negative-Electrode Active Materials for Lithium Ion Batteries Using Simulation Technology

Introduction Lithium ion batteries (LIBs) are widely used in portable devices, such as cellular phones and laptop computers, because of their high energy density. Large LIBs for various industrial applications have recently attracted much attention and their development is ongoing [1]. LIBs include a positive electrode, such as LiCoO2, LiNi1-x-yMnxCoyO2, or LiMn2O4, a graphite negative electrode, and an electrolytic solution. The graphite is divided into two classes: natural and artificial graphite. Natural graphite is more cost effective than artificial graphite. But cell performances with natural graphite, for example cyclability and storage characteristics at high temperatures, is lower than with artificial graphite. Many researches suggest that deterioration of cell performance is related to decomposition of the Solid Electrolyte Interface (SEI). To improve cell performance with natural graphite, coating the graphite with ion exchange polymers such as polyacrylic acid (PAA) is effective [2]. The polymer may work as a pseudo SEI. On the other hand, the direct current resistance (DCR) of a cell with natural graphite coated with PAA tends to be high. LIBs, especially for Hybrid electrolytic vehicles require a low DCR, so it is necessary to develop new coating polymers to reduce the DCR. But it is very difficult to find new coating polymers because there are many ion exchange polymers, and we do not know the relationships between the structure of the ion exchange polymers and DCR. In recent years, computational chemistry has been growing; the field of biochemistry, especially medicines, is widely used to promote the efficiency of the research. But there are few examples of developing materials for LIBs, especially coating polymers, using computer simulation. In this research, we have investigated polymer functional groups and DCR values using computer simulation. We also have developed a new coating polymer which can reduce DCR. Experimental 1.Preparation of 18650 cells Natural graphite and a ion exchange polymer (0.5 wt%) were mixed in water as a solvent, and coating graphite was obtained by removing the solvent. Three types of polymers were used (PAA, and two types of polymers containing sulfonate). An 18650 type LIB includes a positive electrode, a polyolefin separator, an electrolytic solution, and a negative electrode which, in this study, was made of coated natural graphite. The DCR values of the 18650 cells were measured at 25°C. 2.Computer simulation In this study, the Gibbs free energy (ΔGr) values of the polymer’s ion exchange functional groups were calculated by ab initio molecular calculations, RB3LYP/6-31G+(d). The polymer molecule is very large, so it is difficult to calculate ab initio. On the other hand, the properties of a polymer reflect the properties of its segments. So in this research, ΔGr was calculated for the polymer segment shown in Fig. 1. 3.Synthesis of a new coating polymer A monomer containing functional groups of Fig. 3 and water as a solvent were mixed (weight ratio of monomer and water = 1:10) and 2,2'-Azobis (2-methylpropionamidine) dihydrochloride as the radical initiator was mixed. The mixture was heated in an oil bath at 60°C for 3 hours. Results and Discussion Fig. 2 shows the relationships between the DCR of the 18650 and ΔGr. The DCR and ΔGr had a well-defined correlation. ΔGr is defined by Eq. (1), where R is the gas constant and T is temperatures, and K is defined by Eq. (2), so ΔGr is related to the degree of disassociation. From the results, highly dissociated polymers existing in the negative electrode are advantageous in decreasing the DCR. The DCR of PAA is higher than that of sulfonate polymers. The reason for the high DCR of PAA is that PAA is low dissociate. To decrease DCR, a polymer which has a functional group that is low ΔGr seems to be effective. So in this study, many polymers which have ion exchange functional groups were assessed. In this research, polymer segment shown in Fig. 3 displayed the lowest ΔGr; the value was 139 kcal/mol. Consequently, a polymer with a segment of Fig. 3 was synthesized, and an 18650 cell was made and its DCR was evaluated. The polymer with segment of Fig. 3 was named Polymer (a). Fig. 4 shows the DCR of Polymer (a). The DCR was lower than that of the other polymers. This result suggests that the ΔGr of Polymer (a) is low; that is, it dissociates to a significant degree, so its DCR can be low.

Novel Anion Exchangers with Spatially Distant Trimethylammonium Groups in Linear and Branched Hydrophilic Functional Layers

Novel polystyrene–divinylbenzene (PS-DVB) based anion exchangers having branched structure of ion exchange functional groups are prepared and characterised. Two proposed synthetic methods include acylation of PS-DVB microspherical particles followed by reductive amination with dimethylamine or methylamine, which results in obtaining tertiary or secondary amino groups on the surface, respectively. Further alkylation of amino groups was provided by reaction with either (3-chloro-2-hydroxypropyl)trimethylammonium chloride or glycidyltrimethylammonium chloride, which allowed the simultaneous insertion of trimethylammonium functional group and hydrophilic spacer into the structure. The other considered option is reaction with diglycidyl ethers, which requires further amination of terminal epoxide rings with trimethylamine. The proposed approaches resulted in preparation of the ion exchangers with extended linear or branched anion exchange structure with trimethylammonium groups located at a distance from the surface and connected via hydrophilic linkers. The ion exchange selectivity and separation efficiency of obtained anion exchangers are studied for the model mixture of inorganic anions (F−, HCOO−, Cl−, NO2−, Br−, NO3−, H\( {\text{PO}}_{ 4}^{2 - } \) and \( {\text{SO}}_{4}^{ 2- } \)) using hydroxide eluents. The adsorbents with branched ion exchange layer having three positively charged sites in the structure of attached groups demonstrate superior column efficiencies and better peak symmetry as compared with anion exchanger having linear structure with two positively charged sites in the chain. In case of branched anion exchangers the calculated values of column efficiencies for polarisable NO3− and Br− are 44,000 and 50,000 N/m, respectively, and significantly higher than 7,000 and 8,000 N/m obtained for anion exchanger with linear structure of the ion exchange layer.

Application of stimuli responsive polymers for sustainable ion exchange chromatography

Ion exchange processes are widely used in the food, bioprocessing and related industries for the isolation of proteins and other ionic species. Traditional ion exchange resins require salts, acids or bases for releasing adsorbed molecules creating a strong saline waste stream with negative environmental and economic impact. Stimuli responsive polymers (SRPs) with ion exchange functional groups can be used to selectively capture and release charged molecules from a complex mixture using physical stimuli to trigger conformational transitions in the polymer. The structural change of the polymers in response to a stimulus may lead to reduced ligand–target molecule interaction resulting in the release of the captured molecule without the use of chemical reagents, thereby reducing the environmental burden associated with ion exchange processes. The use of temperature responsive polymers has already been demonstrated for such applications at analytical scale. However, little progress has been made to extend these discoveries to the development of materials and methods amenable to industrial scale processing. So far, other SRPs such as, electric, magnetic and light responsive polymers remain largely unexplored for such application. This article discusses the potential of temperature responsive and other SRPs for developing sustainable ion exchange processes. It also highlights the material science and engineering challenges that need to be overcome to bring such processes to industrial application.

Physicochemical approach to freshwater microalgae harvesting with magnetic particles

Magnetic harvesting of microalgal biomass provides an attractive alternative to conventional methods. The approach to this issue has so far been pragmatic, focused mainly on finding cheap magnetic agents in combination with harvestable microalgae species. The aim of this work was to study experimentally and theoretically the mechanisms leading to cell-magnetic agent attachment/detachment using real experiments and predictions made by colloidal adhesion (XDLVO) model. Two types of well defined magnetic beads (MBs) carrying ion exchange functional groups (DEAE – diethylaminoethyl and PEI – polyethylenimine) were studied in connection with microalgae (Chlorella vulgaris). Optimal harvesting efficiencies (>90%) were found for DEAE and PEI MBs, while efficient detachment was achieved only for DEAE MBs (>90%). These findings were in accordance with the predictions by XDLVO model. Simultaneously there was found a discrepancy between the XDLVO prediction and the poor detachment of PEI MBs from microalgal surface. This can be ascribed to an additional interaction (probably covalent bonds) between PEI and algal surface, which the XDLVO model is unable to capture given by its non-covalent nature.

Adsorption of dichloromethane from water onto a hydrophobic polymer resin XAD-1600

Dichloromethane is one of the chlorinated volatile organic compounds (CVOCs) that contaminate the waters. Especially, the dichloromethane used as a solvent in polycarbonate synthesis, is dissolved in wastewater with the saturated solubility of 17,220 mg L(-1), which is several times that of other CVOCs. Thus, it is reasonable to recover the dichloromethane dissolved in water instead of destruction based on the economic point of view. To study on the recovery of the dichloromethane, adsorption equilibrium and column dynamics were investigated using a hydrophobic polymer resin (XAD-1600) without the ion-exchange functional groups. In addition, a hydrophilic polymer resin (XAD-7) and an activated carbon (DY-GAC) were chosen for comparison. Conventional two- or three-parameter models such as the Langmuir, Freundlich, or Sis equations could not fit the adsorption equilibrium data of two polymer resins obtained over the entire range of concentration (1-200 mol m(-3)). They were well fitted by a hybrid model consisting of Langmuir and BET (Brunauer-Emmett-Teller) equations. The adsorption amount at high concentration was in the order of XAD-1600>XAD-7>DY-GAC on a mass basis. To confirm the possibility of using resin as a sorbent for the removal of dichloromethane, adsorption breakthrough curves were measured under key operating conditions such as the concentration, the flow rate, and the column length. Moreover, desorption from polymer resins adsorbed with dichloromethane was conducted by using pure water only as a desorbate. A simple dynamic model was also formulated to describe the adsorption breakthrough curves of dichloromethane from XAD-1600, XAD-7 and DY-GAC columns.

Equilibrium model for the selective sorption of gold cyanide on different ion-exchange functional groups

Ion exchange resins are still not used commercially for gold extraction from leached pulps despite the many advantages of ion exchange resins over activated carbon. This is a result of the poor understanding of the chemistry involved and a lack of a rigorous method of modelling, design and scale-up. The present study describes a theoretical model based on the principles of statistical thermodynamics as well as the Metropolis Monte Carlo (MMC) numerical method that has been proposed previously to describe multi-component sorption equilibria. An important aspect of this modelling approach is that it does not use any solely empirical parameters. All parameters used describe only a single characteristic of the sorption system. It is therefore possible to include fundamental information of the system in order to facilitate easier parameter estimation. The model explicitly describes in a mathematical form classical non-ideal sorption phenomena such as selectivity, irreversibility and interaction between sorbed species which previously have only been understood from a chemical viewpoint. The model has been used to successfully describe the multicomponent equilibrium sorption of gold cyanide and copper cyanide onto ion exchange resins containing different aliphatic amino groups. Consequently, by varying the surface properties and comparing the model predictions with experimental data, further insight has been gained into the sorption properties of ion exchange resins for metal cyanide complexes.

Intraparticle diffusion during selective ion exchange with a macroporous exchanger

Chlorophenols, quaternary ammonium compounds, benzene and naphthalene sulfonates and benzene carboxylates are examples of environmentally significant synthetic organic compounds which exist as ions in water over a wide range of pH values. This study discusses the sorption kinetics, and more specifically, intraparticle diffusion behaviors of trace concentrations of pentachlorophenate (PCP −) and other chlorophenates onto a commercially available macroporous polymeric anion exchanger (IRA-900). The anion exchanger is essentially biphasic, i.e. every single exchanger particle contains an enormous number of tiny microgels and an interconnected network of pores. Ion exchange functional groups reside solely within the microgels. For comparison, a gel or microporous anion exchanger is also included in the study. Experimental results reveal a distinctly different type of effect of competing chloride ion concentration on gel and macroporous ion exchangers pertaining to sorption of pentachlorophenate (PCP −). While the effective intraparticle diffusivity of PCP − for gel-type resin remained unaltered with a change in competing chloride concentration, the same increased significantly for the macroporous exchanger with an increase in chloride concentration. Pore diffusion is considered to be the predominant intraparticle transport mechanism for highly preferred PCP − inside the macroporous exchanger. Under the normal hydrodynamic conditions of a fixed bed column run, intraparticle diffusion was found to be the rate-limiting step. For various chlorophenates, the effective intraparticle diffusivities were inversely correlated to the octanol–water partition coefficients of their parent chlorophenols.

Potentiostatic formation of semipenetrable polymeric acrylamideformaldehyde films and the electrochemical behavior of electrodes modified by such films

The potentiostatic formation of an acrylamideformaldehyde copolymer film on a platinum cathode is studied by the voltammetry and impedance methods. As the process is nonstationary in time, the film has a complex nonuniform structure, and the film’s density depends on potential and electrolysis duration. The electrochemical behavior of the electrode-polymeric film composition is examined. The polymer is electrochemically stable at 0.5 to -0.7 V (Ag/AgCl) and contains no electron-exchange or ion-exchange functional groups that could undergo redox conversions in this potential range. The synthesized film is semipenetrable and has ultrafiltration characteristics. The diffusion mobility of depolarizers in the film is lower than that in solution and is determined by the nature and properties of species moving in the electrical field and by the ratio between sizes of depolarizer ions and pores of the film.

Sorption of benzenesulfonates by weak base anion exchange resins

Weak base phenol formaldehyde anion exchange resins were found to have an excellent capacity for removing the anionic organic species benzenesulfonate (BS), p-toluenesulfonate (PTS) and dodecylbenzenesulfonate (DBS) from pH 2 and 4 solutions. The capacity for DBS was generally greater than that of activated carbon and significantly greater than that of a polystyrene-divinylbenzene resin with no ion exchange functional groups. The maximum capacity of the weak base resins appeared to be limited by the exchange capacity of the resin. The mechanism of removal was strongly indicated as being either ion exchange at the amine functional groups in the acid form, or uptake of a proton by the amine groups in the free base form with the organic anion as the associated counter ion. Increasing concentrations of Cl− decreased the amount of organic anion that could be sorbed.The swelling of one of the resins, Duolite A-7, also had a marked effect on its sorption capacity. The pores of the acid chloride or acid organic anion form of the resin were apparently larger than those of the free base form; thus, as conversion of the resin to the acid form took place, a greater amount of uptake was observed. The effect of swelling on the capacity of Duolite S-37 was much less, apparently owing to its larger pore structure, but its capacity for DBS at low pH was lower than that for A-7 because of its lower nitrogen content. Also, BS and DBS were easily removed from the weak base resins by washing them with NaOH. Four bed volumes of 1 m NaOH gave essentially complete regeneration of BS saturated A-7.