Remove legacy perfluoroalkyl acids and emerging per- and polyfluoroalkyl ether acids by single-use and regenerable anion exchange resins: Rapid small-scale column tests and model fits

Rapid small-scale column tests (RSSCT) are used to study the removal of per- and polyfluoroalkyl substances (PFAS) for drinking water treatment by ion exchange. Breakthroughs of 15 emerging per- and perfluoroalkyl ether acids and six legacy perfluoroalkyl acid analogs are studied using a single-use PFAS-selective anion exchange resin (AER1) and a regenerable, generic anion exchange resin (AER2). The Bohart-Adams model was used to describe and predict breakthrough, with the modeled results reasonably aligned with RSSCT results in most cases, enabling shorter RSSCT duration for future applications. AER1 exhibited high uptake capacity with no breakthrough for 11 of the 21 tested PFAS during the 144,175 BV continuous operation, allowing compliance with the new National Primary Drinking Water Regulation in many application scenarios. AER2 exhibited much faster breakthroughs for most PFAS and is not a promising option for drinking water treatment. However, the summed PFAS capacity via model fit and total PFAS adsorbed via measurement were only <0.01 % of both resin capacities at full breakthrough, suggesting PFAS could only occupy a tiny portion of the ion exchange sites even for the PFAS-selective AER1. Ether group insertion in the PFAS group leads to later breakthrough, and linear isomers were better captured by the resins than the branched isomers. Overall, PFAS uptake capacity increases and kinetics decrease when the PFAS molecular volume increases. Regeneration using 10 % NaCl solutions partially released PFAS from AER2 but not from AER1, with more short-chain PFAS released than long-chain ones. Ether group insertion decreased the PFAS recoveries during the regeneration of AER2. The regenerated resins showed much faster breakthroughs than the pristine resins, making them unfavorable for drinking water treatment applications. Adsorption displacement of short-chain PFAS by long-chain PFAS was observed in pristine AER1, and post-regeneration leaching occurred for both resins, both phenomena making the resins a possible PFAS source in long-term use.

Important properties of anion exchange resins for efficient removal of PFOS and PFOA from groundwater

Perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) present in various water sources have raised a serious concern on their health risk worldwide. Anion exchange is known to be one of the effective treatment methods but the resin properties suitable for theses contaminants have not been fully understood. We examined four commercially available anion exchange resins with different properties (DIAION™ PA312, HPA25M, UBA120, and WA30) and one polymer-based adsorbent (HP20), for their PFOA and PFOS removal in the batch experiment. All or a part of the selected resins were further characterized for their functional group, surface morphology and pore size distribution. The 72 h batch experiment with the 100 mg/L PFOA or PFOS in the laboratory pure water matrix showed a superior capacity of the strong base anion exchange resins, the porous-type HPA25M and PA312, and the gel-type UBA120, for PFOA removal (92.6-97.9%). Among those resins, the high porous HPA25M was suggested most effective due to its remarkably high reaction rate and effectiveness to PFOS (99.9%). In the groundwater matrix, however, the performance of the those anion exchange resins was generally suppressed, causing up to 71% decrease in their removal rates. The least matrix impact was observed for PFOS removal by HPA25M, which indicated the resin's high selectivity to the contaminant. The physiochemical analysis indicated that the presence of relatively large pores (1 nm-10 nm) over HPA25M played an important role in the PFAS removal.

Resin structure impacts two-component protein adsorption and separation in anion exchange chromatography

The influence of the resin structure, on the competitive binding and separation of a two-component protein mixture with anion exchange resins is evaluated using conalbumin and green fluorescent protein as a model system. Two macroporous resins, one with large open pores and one with smaller pores, are compared to a resin with grafted polymers. Investigations include measurements of single and two-component isotherms, batch uptake kinetics and two-component column breakthrough. On both macroporous resins, the weaker binding protein, conalbumin, is displaced by the stronger binding green fluorescent protein. For the large pore resin, this results in a pronounced overshoot and efficient separation by frontal chromatography. The polymer-grafted resin exhibits superior capacity and kinetics for one-component adsorption, but is unable to achieve separation due to strongly hindered counter-diffusion. Intermediate separation efficiency is obtained with the smaller pore resin. Confocal laser scanning microscopy provides a mechanistic explanation of the underlying intra-particle diffusional phenomena revealing whether unhindered counter-diffusion of the displaced protein can occur or not. This study demonstrates that the resin's intra-particle structure and its effects on diffusional transport are crucial for an efficient separation process. The novelty of this work lies in its comprehensive nature which includes examples of the three most commonly used resin structures: a small pore agarose matrix, a large-pore polymeric matrix, and a polymer grafted resin. Comparison of the protein adsorption properties of these materials provides valuable clues about advantages and disadvantages of each for anion exchange chromatography applications.

Release regularity and cleaning measures of magnetic anion exchange resin during application

Anion exchange resin is responsible for removing harmful anionic contaminants in drinking water treatment, but it may become a significant source of precursors for disinfection byproducts (DBPs) by shedding material during application without proper pretreatment. Batch contact experiments were performed to investigate the dissolution of magnetic anion exchange resins and their contribution to organics and DBPs. Dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) released from the resin were highly correlated with the dissolution conditions (contact time and pH), in which 0.7 mg/L DOC and 0.18 mg/L DON were distributed at exposure time of 2 h and pH 7. The formation potential of four DBPs in the shedding fraction was also revealed that trichloromethane (TCM), dichloroacetonitrile (DCAN), nitrosodimethylamine (NDMA), and dichloroacetamide (DCAcAm) concentrations could reach 21.4, 5.1, 12.1 μg/L, and 69.6 ng/L, respectively. Furthermore, the hydrophobic DOC that preferred to detach from the resin mainly originated from the residues of crosslinkers (divinylbenzene) and porogenic agents (straight-chain alkanes) detected by LC-OCD and GC-MS. Nevertheless, pre-cleaning inhibited the leaching of the resin, among which acid-base and ethanol treatments significantly lowered the concentration of leached organics, and formation potential of DBPs (TCM, DCAN, and DCAcAm) below 5 μg/L and NDMA dropped to 10 ng/L.

Isotherm model discrimination for multimodal chromatography using mechanistic models derived from high-throughput batch isotherm data

In this work, we have examined an array of isotherm formalisms and characterized them based on their relative complexities and predictive abilities with multimodal chromatography. The set of isotherm models studied were all based on the stoichiometric displacement framework, with considerations for electrostatic interactions, hydrophobic interactions, and thermodynamic activities. Isotherm parameters for each model were first determined through twenty repeated fits to a set of mAb - Capto MMC batch isotherm data spanning a range of loading, ionic strength, and pH as well as a set of mAb - Capto Adhere batch data at constant pH. The batch isotherm data were used in two ways-spanning the full range of loading or consisting of only the high concentration data points. Predictive ability was defined through the model's capacity to capture prominent changes in salt gradient elution behavior with respect to pH for Capto MMC or unique elution patterns and yield losses with respect to gradient slope for Capto Adhere. In both cases, model performance was quantified using a scoring metric based on agreement in peak characteristics for column predictions and accuracy of fit for the batch data. These scores were evaluated for all twenty isotherm fits and their corresponding column predictions, thereby producing a statistical distribution of model performances. Model complexity (number of isotherm parameters) was then considered through use of the Akaike information criterion (AIC) calculated from the score distributions. While model performance for Capto MMC benefitted substantially from removal of low protein concentration data, this was not the case for Capto Adhere; this difference was likely due to the qualitatively different shapes of the isotherms between the two resins. Surprisingly, the top-performing (high accuracy with minimal number of parameters) isotherm model was the same for both resins. The extended steric mass action (SMA) isotherm (containing both protein-salt and protein-protein activity terms) accurately captured both the pH-dependent elution behavior for Capto MMC as well as loss in protein recovery with increasing gradient slope for Capto Adhere. In addition, this isotherm model achieved the highest median score in both resin systems, despite it lacking any explicit hydrophobic stoichiometric terms. The more complex isotherm models, which explicitly accounted for both electrostatic and hydrophobic interaction stoichiometries, were ill-suited for Capto MMC and had lower AIC model likelihoods for Capto Adhere due to their increased complexity. Interestingly, the ability of the extended SMA isotherm to predict the Capto Adhere results was largely due to the protein-salt activity coefficient, as determined via isotherm parameter sensitivity analyses. Further, parametric studies on this parameter demonstrated that it had a major impact on both binding affinity and elution behavior, therein fully capturing the impact of hydrophobic interactions. In summary, we were able to determine the isotherm formalisms most capable of consistently predicting a wide range of column behavior for both a multimodal cation-exchange and multimodal anion-exchange resin with high accuracy, while containing a minimized set of model parameters.

Comparative investigation of PFAS adsorption onto activated carbon and anion exchange resins during long-term operation of a pilot treatment plant

Widespread contamination of groundwater with per- and polyfluoroalkyl substances (PFAS) has required drinking water producers to quickly adopt practical and efficacious treatments to limit human exposure and deleterious health outcomes. This pilot-scale study comparatively investigated PFAS adsorption behaviors in granular activated carbon (GAC) and two strong-base gel anion exchange resin (AER) columns operated in parallel over a 441-day period to treat contaminated groundwater dominated by short-chain perfluorocarboxylic acids (PFCA). Highly-resolved breakthrough profiles of homologous series of 2-8 CF₂ PFCA and perfluorosulfonic acids (PFSA), including ultrashort-chain compounds and branched isomers, were measured to elucidate adsorption trends. Sample ports at intermediate bed depths could predict 50% breakthrough of compounds on an accelerated basis, but lower empty bed contact times led to conservative estimates of initial breakthrough. Homologous PFAS series displayed linear (GAC) and log-linear (AER) relationships between chain-length and breakthrough, independent of initial concentration. AERs generally outperformed GAC on a normalized bed volume basis, and this advantage widened with increasing PFAS chain-length. As designed, all treatments would have short full-scale service times (≤142 days for GAC; ≤61 days for AERs) before initial breakthrough of short-chain (2-4 CF₂) PFCA. However, AER displayed far longer breakthrough times for PFSA compared to GAC (>3× treatment time), and breakthrough was not observed for PFSA with >4 CF₂ in AERs. GAC had a finite molar adsorption capacity for total PFAS, leading to a stoichiometric replacement of short-chain PFCA by PFSA and longer-chain PFCA over time. AERs quickly reached a finite adsorption capacity for PFCA, but they showed substantially greater selectivity for PFSA whose capacity was not reached within the duration of the pilot. Breakthrough characteristics of keto- and unsaturated-PFSA, identified in the groundwater by suspect screening, were also evaluated in absence of reference standards. Modified PFAS structures (branched, keto-, unsaturated-) broke through faster than linear and unmodified perfluorinated structures with equal degrees of fluorination, and the effects were more pronounced in GAC compared to AERs. The results highlight that the design of robust PFAS treatment systems should consider facets beyond current PFAS targets including operational complexities and impacts of unregulated and unmonitored co-contaminants.

Bioregeneration of sulfate-laden anion exchange resin

Ion exchange technology removes ionic compounds from waters effectively but treatment of the spent regenerant is expensive. The bioregeneration of sulfate-laden strong base anion exchange resin was successfully tested using both pure and mixed sulfate-reducing bacterial cultures. The resin was first used for removal of sulfate from neutral (pH 6.7 ± 0.5) synthetic sodium sulfate solutions, after which the spent resin was regenerated by incubating with a viable sulfate-reducing bacterial culture in batch and column modes. In the batch bioregeneration tests, the achieved bioregeneration was 36-95% of the original capacity of the fresh resin (112 mg SO₄^2-/g) and it increased with regeneration time (1-14 days). The capacity achieved in the column tests during 24 hours of bioregeneration was 107 mg SO₄^2-/g after the first regeneration cycle. During the bioregeneration, sulfate was mainly reduced by the sulfate-reducing bacteria (approx. 60%), but part of it was only detached from the resins (approx. 30%). The resin-attached sulfate was most likely replaced with ions present in the liquid sulfate-reducing bacterial culture (e.g., HCO₃^-, HS^-, and Cl^-). During the subsequent exhaustion cycles with the bioregenerated resin, the pH of the treated sodium sulfate solution increased from the original 6.7 ± 0.5 to around 9. The study showed that biological sulfate reduction could be used for sulfate removal in combination with ion exchange, and that the exhausted ion exchange resins could be regenerated using a liquid sulfate-reducing bacterial culture without producing any brine.

Pilot study comparison of regenerable and emerging single-use anion exchange resins for treatment of groundwater contaminated by per- and polyfluoroalkyl substances (PFASs)

This study reports the results of an 8-month pilot study comparing both regenerable and emerging single-use anion exchange resins (AERs) for treatment of per- and polyfluoroalkyl substances (PFASs) at a source zone impacted by historical use of aqueous film-forming foam (AFFF). Two regenerable (Purolite A860 and A520E) and three single-use (Purolite PFA694E, Calgon CalRes 2301, and Dowex PSR2+) AERs were tested in parallel, collecting effluent samples after treatment for 30-sec and 2-min total empty bed contact time (EBCT). Results demonstrate that single-use AERs significantly outperform regenerable resins, particularly for treatment of long-chain perfluoroalkyl carboxylic acids (PFCAs) and perfluoroalkyl sulfonic acids (PFSAs). No detectable concentrations of ≥C7 PFCAs or PFSAs were observed within 150,000 bed volumes (BVs) after treatment with the single-use resins (2-min EBCT). Analysis of effluent samples following 30-sec EBCT treatment shows that even the shortest-chain PFSAs do not reach 50% breakthrough within the first 350,000 BVs, though differences in removal of short-chain PFCAs was less dramatic. The regenerable polyacrylic A860 resin performed very poorly compared to all polystyrene resins, with >90% breakthrough of all PFASs occurring within 10,000 BVs. The greater affinity of polystyrene resins is attributed to increased hydrophobic interactions in addition to electrostatic ion exchange. Analysis of breakthrough profiles reveals empirical correlation with ion exchange affinity coefficients (logK_ex) measured in batch experiments. Postmortem analysis of PFASs extracted from spent resins revealed chromatographic elution behavior and competition among PFASs for adsorption to the resins. PFSAs and long-chain PFCAs were preferentially adsorbed to earlier sections in the AER columns, whereas short-chain PFCAs were competitively displaced towards the later sections of the columns and into the effluent, consistent with effluent concentrations of the latter structures exceeding influent values. These results provide insights into the mechanisms that govern PFAS adsorption to AERs in real multisolute groundwater matrices and support findings from other diverse sites regarding PFAS affinity, elution behavior, and competition for exchange sites.

A dual grafted fluorinated hydrocarbon amine weak anion exchange resin polymer for adsorption of perfluorooctanoic acid from water

Perfluorooctanoic acid (PFOA) is a persistent and recalcitrant organic contaminant of exceptional environmental concern, and its removal from water has increasingly attracted global attention due to its wide distribution and strong bioaccumulation. Adsorption is considered an effective technique for PFOA removal and more efficient PFOA sorbents are still of interest. This study developed a dual grafted fluorinated hydrocarbon amine weak anion exchange (WAX) polymeric resin (Sepra-WAX-KelF-PEI) for PFOA removal from water. This polymer was synthesized by a two-step amine grafting reaction procedure involving first the reaction of the Sepra-WAX hydrocarbon polymer with poly(vinylidinefluoride-chlorotrifluoroethylene) (Kel-F 800) and then a second reaction with polyethyleneimine (PEI). Characterization of the synthesized polymers was performed using scanning electron microscopy and elemental analysis (F and Cl) by energy dispersive X-ray spectroscopy. The PFOA adsorption performance evaluations were conducted by packed column flow analyses with on-line detection. The results show the breakthrough of the Sepra-WAX-KelF-PEI synthesized with optimum stoichiometry was two times better than the starting anion exchange polymer Sepra-WAX, and six times better than powdered activated carbon, when using the same column size. The adsorption mechanisms of this novel adsorbent including hydrophobic interaction and electrostatic interaction were also clarified in this study. The adsorption kinetic parameters of the two optimum synthesized sorbents were determined using the Thomas model, the Yoon-Nelson model, and batch isotherm studies, and compared with those found with activated carbon and the starting WAX resin. Good agreement of the batch isotherm and column studies with respect to adsorption capacities trends between all three polymers (Sepra-WAX, Sepra-WAX-KelF, and Sepra-WAX-KelF-PEI) were noted.

Utilization of gel-type polystyrene host for immobilization of nano-sized hydrated zirconium oxides: A new strategy for enhanced phosphate removal

The urgent need for eutrophication control motivated the development of many novel adsorbents for enhanced phosphate polishing removal. Among these, zirconium-based nanomaterial was regarded as an effective kind because of its ability to bind phosphate specifically via inner-sphere complexation. In this study, we proposed a new strategy to improve the efficiency of zirconium oxides (HZO) nanoparticles by immobilizing them onto a gel-type anion exchange resin covalently attached with ammonium groups, denoted as HZO@N201. A previously developed macro-porous polymeric nanocomposite HZO@D201 was used for comparison. The immobilized nanoparticles in HZO@N201 were well dispersed in the gel matrix, manifesting smaller particle size and richer surface hydroxyl groups in comparison to HZO@D201. As a result of the structural merits in collective, HZO@N201 not only exhibited superior phosphate adsorptive capacity and affinity towards phosphate to HZO@D201, but also facilitate phosphate diffusion, based on isotherm, pH and kinetic tests. Mechanistic study by XPS and ³¹P SS-NMR substantiated the selective phosphate adsorption pathway as the formation of inner-sphere complexes by HZO@N201, which exhibited enhanced reactivity than HZO@D201. Lastly, fixed-bed runs of HZO@N201 was conducted, achieving an effective treatable volume of 2000 BV, which was 600 BV more than HZO@D201. Additional adsorption-regeneration cycle confirmed its reusability and potential for practical application. We believe the gel-type polymeric host could facilitate the formation and dispersion of smaller sized nanoparticles, exposing more surface hydroxyl groups highly accessible to phosphate. The results of this paper offer insights to a new strategy for immobilization of functional nanoparticles aiming at enhanced adsorptive removal of phosphate.

Novel Homogeneous Anion Exchange Membranes for Reproducible and Sensitive Nucleic Acid Detection via Current-Voltage Characteristic Measurement

One-pot synthesis of novel hydrogel-based anion exchange membranes (AEMs), with only a single-phase monomer mixture, was used to eliminate surface heterogeneity and generate reproducible electroconvective microvortices in the over-limiting region of the current-voltage characteristic (CVC) curves. Diallyldimethylammonium chloride (DDA) was used as the main component to provide the cation charge groups, and 2-hydroxyethyl methacrylate (HEMA) and ethylene glycol dimethyl acrylate (EGDMA) were used as the auxiliary structure monomers. The uniform membrane structure allowed reproducible and sensitive DNA detection and quantification, as probe-target surface complexes can gate the ion flux and produce large voltage shifts in the over-limiting region. Suppressed membrane curvature due to controlled swelling is a crucial part to avoid the reduction of depletion region for maintaining the influence of target gene hybridization. Fourier-transform infrared (FTIR) spectroscopy verified the synthesized membrane structure, with a residual vinyl group that allows easy carboxylation via additional photografting reaction. Consequently, a significantly higher DNA probe functionalization efficiency is obtained on the homogeneous AEMs, evidenced by the increasing nitrogen element content and bonding via X-ray photoelectron spectroscopy (XPS). The DDA content was optimized to provide a sufficient coulomb force between AEM and nucleic acid backbone to promote the specific binding efficiency but without high dimensional swelling which might change the surface geometry and restrict the voltage shifting for sensing in the over-limiting region, and the optimal DDA/HEMA ratio was found to be 4/10. The synthesized AEM sensor for recombinant 35S promoter sequence identification exhibited a reproducible calibration standard curve with dynamic range between 30 fM and 1 μM and high selectivity with only 0.01 V shift for 1 μM nontarget oligo.

Adsorption of Anthocyanins by Cation and Anion Exchange Resins with Aromatic and Aliphatic Polymer Matrices

This study examines the mechanisms of adsorption of anthocyanins from model aqueous solutions at pH values of 3, 6, and 9 by ion-exchange resins making the main component of heterogeneous ion-exchange membranes. This is the first report demonstrating that the pH of the internal solution of a KU-2-8 aromatic cation-exchange resin is 2-3 units lower than the pH of the external bathing anthocyanin-containing solution, and the pH of the internal solution of some anion-exchange resins with an aromatic (AV-17-8, AV-17-2P) or aliphatic (EDE-10P) matrix is 2-4 units higher than the pH of the external solution. This pH shift is caused by the Donnan exclusion of hydroxyl ions (in the KU-2-8 resin) or protons (in the AV-17-8, AV-17-2P, and EDE-10P resins). The most significant pH shift is observed for the EDE-10P resin, which has the highest ion-exchange capacity causing the highest Donnan exclusion. Due to the pH shift, the electric charge of anthocyanin inside an ion-exchange resin differs from its charge in the external solution. At pH 6, the external solution contains uncharged anthocyanin molecules. However, in the AV-17-8 and AV-17-2P resins, the anthocyanins are present as singly charged anions, while in the EDE-10P resin, they are in the form of doubly charged anions. Due to the electrostatic interactions of these anions with the positively charged fixed groups of anion-exchange resins, the adsorption capacities of AV-17-8, AV-17-2P, and EDE-10P were higher than expected. It was established that the electrostatic interactions of anthocyanins with the charged fixed groups increase the adsorption capacity of the aromatic resin by a factor of 1.8-2.5 compared to the adsorption caused by the π-π (stacking) interactions. These results provide new insights into the fouling mechanism of ion-exchange materials by polyphenols; they can help develop strategies for membrane cleaning and for extracting anthocyanins from juices and wine using ion-exchange resins and membranes.

A non-anhydrous, minimally basic protocol for the simplification of nucleophilic 18F-fluorination chemistry

Fluorine-18 radiolabeling typically includes several conserved steps including elution of the [18F]fluoride from an anion exchange cartridge with a basic solution of K2CO3 or KHCO3 and Kryptofix 2.2.2. in mixture of acetonitrile and water followed by rigorous azeotropic drying to remove the water. In this work we describe an alternative "non-anhydrous, minimally basic" ("NAMB") technique that simplifies the process and avoids the basic conditions that can sometimes limit the scope and efficiency of [18F]fluoride incorporation chemistry. In this approach, [18F]F- is eluted from small (10-12 mg) anion-exchange cartridges with solutions of tetraethylammonium bicarbonate, perchlorate or tosylate in polar aprotic solvents containing 10-50% water. After dilution with additional aprotic solvent, these solutions are used directly in nucleophilic aromatic and aliphatic 18F-fluorination reactions, obviating the need for azeotropic drying. Perchlorate and tosylate are minimally basic anions that are nevertheless suitable for removal of [18F]F- from the anion-exchange cartridge. As proof-of-principle, "NAMB" chemistry was utilized for the synthesis of the dopamine D2/D3 antagonist [18F]fallypride.

Efficient removal of GenX (HFPO-DA) and other perfluorinated ether acids from drinking and recycled waters using anion exchange resins

Carcinogenic GenX chemicals, heptafluoropropylene-oxide-dimer-acid (HFPO-DA), have been recently detected in surface, ground and recycled water sources worldwide. However, GenX removals under the influence of variable characteristics of the organic and inorganic compounds present in the natural water sources, have often been overlooked in scientific literature. This is critically important given that the ionic composition and characteristics of organic matter in natural waters are spatially and seasonally variable. A strongly basic anion exchange (IX) resin was used to remove GenX and two other perfluorinated ether acids (PFEAS) from natural surface and recycled water sources. Factors influencing the uptake behavior included the PFEAS concentrations, resin dosage, and background anion characteristics. The equivalent background compound was employed to evaluate the competitive uptake between natural organic matter (NOM), inorganic ions and PFEAS in natural water matrices. Experimental data were compared with different mathematical and physical models and it was depicted that approximately 4-6% of the initial NOM competed with PFEAS for active exchange sites. Further, IX was able to achieve complete PFEAS removal (C_final<10 ng/L) with simultaneous removal of>60% NOM and >80% inorganic ions. Results of this study indicate that IX exhibits great potential for PFEAS removal from natural drinking water sources.

Differential adsorption of zwitterionic PPCPs by multifunctional resins: The influence of the hydrophobicity and electrostatic potential of PPCPs

Zwitterionic pharmaceuticals and personal care products can interact with adsorbents in different ways due to their various properties. In this work, the effects of hydrophobicity and electrostatic potential were explored through the adsorption of ciprofloxacin (CPX) and tetracycline (TC) onto multifunctional resins. Nonionic surface interaction was dominant for the adsorption on high-surface-area resin GMA10. Thereinto, hydrophobic and π-π interaction dominant for hydrophobic CPX and hydrophilic TC, respectively. Electrostatic interaction played an important role for high-anion-exchange-capacity resin GMA90. Upon their adsorption onto GMA50 resin, the relatively separated positive and negative electrostatic potentials of CPX^+- due to the greater distance (∼12.33 Å) between the anionic and cationic groups led to electrostatic attraction and interaction (Ea = 8.64 ± 0.31 kJ/mol) and the vertical orientation of molecule on the surface. However, TC^+-0 displayed nonionic surface interaction (Ea = 7.96 ± 0.14 kJ/mol) due to its relatively neutral electrostatic potential arising from the adjacent functional groups. Hence, the surface of GMA50 was covered with TC^+-0 molecules adsorbed parallel to the surface, thereby restricting TC^+-0 adsorption. Coexisted with monovalent salts, CPX adsorption was facilitated due to the salting-out effect. By contrast, the salting-out effect for TC was extremely weak, and TC adsorption was restrained due to the competitive adsorption of salts.

The combined influence of hydrophobicity, charge and molecular weight on natural organic matter removal by ion exchange and coagulation

Three different source waters were investigated using virgin and pre-used anion exchange resins, coagulation, and ion exchange combined with coagulation (IEX&Coagulation). The hydrophobicity, size distribution and charge of natural organic matter (NOM) were used to evaluate its removal. Dissolved organic carbon (DOC) removal by pre-used IEX resin was 67-79%. A consistent ratio of different hydrophobicity fractions was found in the removed DOC, while the proportion and quantity of the molecular weight fraction around 1 kDa was important in understanding the treatability of water. For pre-used resin, organic compounds were hypothesised to be restricted to easily accessible exchange sites. Comparatively, virgin resin achieved higher DOC removals (86-89%) as resin fouling was absent. Charge density and the proportion of the hydrophobic fraction were found to be important indicators for the specific disinfection byproduct formation potential (DBP-FP). Treatment of raw water with pre-used resin decreased the specific DBP-FP by between 2 and 43%, while the use of virgin resin resulted in a reduction of between 31 and 63%. The highest water quality was achieved when the combination of IEX and coagulation was used, reducing DOC and the specific DBP-FP well below that seen for either process alone.

Removal of bromide from natural waters: Bromide-selective vs. conventional ion exchange resins

The presence of bromide (Br^-) in water results in the formation of brominated disinfection byproducts (DBPs) after chlorination, which are much more cytotoxic and genotoxic than their chlorinated analogs. Given that conventional water treatment processes (e.g., coagulation, flocculation, and sedimentation) fail to remove Br^- effectively, in this study, we systematically tested and compared the performance of different anion exchange resins, particularly two novel Br-selective resins, for the removal of Br^-. The resins performance was evaluated under both typical and challenging background water conditions by varying the concentrations of anions and organic matter. The overall Br^- removal results followed the trend of Purolite-Br ≥ MIEX-Br > IRA910 ≥ IRA900 > MIEX-Gold > MIEX-DOC. Further evaluation of Purolite-Br resin showed Br^- removal efficiencies of 93.5 ± 4.5% for the initial Br^- concentration of 0.25 mg/L in the presence of competing anions (i.e., Cl^-, NO₃^-, NO₂^-, SO₄^2-, PO₄^3-, and a mixture of all five), alkalinity and organic matter. In addition, experiments under challenging background water conditions confirmed the selectivity of the resins (i.e. Purolite-Br and MIEX-Br) in removing Br^-, with SO₄^2- and Cl^- exhibiting the greatest influence upon the resin performance followed by NOM concentration, regardless of the NOM characteristic. After Br^- removal, both the subsequent formation of brominated DBPs (trihalomethanes, haloacetic acids, and haloacetonitriles), and the total organic halogens (TOX), decreased by ∼90% under the uniform formation conditions. Overall, Br-selective resins represent a promising alternative for the efficient control of Br-DBPs in water treatment plants.

Enhanced selective removal of arsenic(V) using a hybrid nanoscale zirconium molybdate embedded anion exchange resin

Selective removal of trace arsenic is crucial for obtaining safe drinking water. Here, the selective adsorptive performance of arsenate (As(V)) on a hybrid ZMAE (nanoscale zirconium molybdate embedded a macroporous anion exchange resin) was examined. It was found that the As(V) adsorption efficiency of ZMAE was almost retained in the presence of competing ions (NO₃^- or SO₄^2-) up an [SO₄^2-]/[As] or [NO₃^-]/[As] ratio of 150/1, whereas that of bare AE (anion exchange resin) was negligible for [SO₄]/[As] over 15/1. In addition, the As(V) maximum adsorption capacity of ZMAE was found to be 41.2 mg/g, which is in contrast with the negligible adsorption of bare AE under sulfate-rich condition. The enhanced arsenate selectivity of ZMAE can be attributed to the excellent selectivity of ZM NPs (zirconium molybdate nanoparticles), which contributed up to 45% of the adsorption capacity of ZMAE. The behavior of ZMAE towards arsenate was compared with that towards phosphate showing similar adsorption performances between them, which indicates the similar affinity of ZMAE towards arsenate and phosphate. Finally, ZMAE examined for fixed-bed column adsorption for As(V) removal from synthetic As(V) water was effective for up to 5100 BVs, treating As(V) from 0.1 mg/L to below 0.01 mg/L (meeting the WHO guidelines).

Adsorption and regeneration characteristics of phosphorus from sludge dewatering filtrate by magnetic anion exchange resin

Removal and recovery of phosphorus (P) from sewage are essential for sustainable development of P resource. Based on the water quality determination of sludge dewatering filtrate from a wastewater treatment plant in Beijing, this study investigated the adsorption and regeneration characteristics of P by magnetic anion exchange resin (MAEX). The experiments showed that the P adsorption capacity of MAEX could reach a maximum of 2.74 mg/mL when initial P concentration was 25 mg/L and dosage of MAEX was 8 mL/L. The P adsorption on MAEX resin was suitable for large temperature range (283-323 K). However, the adsorption capacity was reduced in various degrees due to the interference of different anions (Br^-, SO₄^2-, Cl^-, NO₃^-, HCO₃^-, CO₃^2-) and organic compounds (bovine serum albumin, humic acid). Kinetics studies indicated that the P adsorption process followed the pseudo-second-order model. The MAEX resin had a rapid P adsorption rate and the P adsorption capacity at 30 min could reach 97.7-99.3% of q_e. Increase of temperature was favorable to P adsorption on MAEX, and the adsorption isotherm data fitted to Langmuir model more than Freundlich model. Meanwhile, the thermodynamics parameters were calculated; it was shown that the adsorption process was an endothermic reaction. Desorption and regeneration experiments showed that NaHCO₃ was a suitable regenerant, and the P adsorption capacity could reach 90.51% of the original capacity after 10 times of adsorption-desorption cycles; this indicated that MAEX resin has an excellent regeneration performance and thus has a very good application prospect of P removal and recovery. Fourier transform infrared spectroscopy (FTIR) analysis confirmed that ion exchange, charge attraction, and hydrogen bonding affected the removal of P by the MAEX resin. The vibrating sample magnetometer (VSM) analysis revealed that MAEX resin was a kind of soft magnetic materials with good magnetism.

Preferential adsorption of nitrate with different trialkylamine modified resins and their preliminary investigation for advanced treatment of municipal wastewater

In this paper, a series of mono-functional and bifunctional anion exchange resins with different kinds of trialkylammonium groups were synthesized and used for adsorption of nitrate from aqueous solution. The obtained resins were systematically characterized by scanning electron microscopy, Fourier transform infrared spectrometry and pore size distribution. Adsorptive behaviors and mechanisms were investigated by batch experiments. The nitrate could be preferentially adsorbed in the presence of chloride, sulfate and humic acid by longer-chain trialkylamine modified resins. Especially, the L20 resin with the triethylammonium functional group was demonstrated to possess high selectivity toward nitrate with the highest distribution coefficient among all tested resins. For both single and bi-solutes systems, the adsorption isotherm data could be well fitted with the Langmuir model, while the experimental kinetic data was well described by both pseudo first-order and second-order kinetic model. The L20 resin could be reused after many adsorption-desorption cycles with most of its virgin adsorption capacity for advanced wastewater treatment, indicating its great potential for the selective and efficient removal of nitrate from large amounts of municipal wastewater or surface water.

Removal of Microcystin-LR from spiked natural and synthetic waters by anion exchange

Cyanobacterial blooms are becoming a serious challenge across the globe due to changing climate and rainfall patterns as a consequence of human activities. In the present study, the fundamental interactions involved during the removal of Microcystin-LR (MCLR), one of the most commonly occurring cyanobacterial toxins, were investigated by employing strongly basic anion exchange (IX) resins. Several factors including the stoichiometric coefficients, competitive fractions and solute affinities were determined under various concentrations of inorganic ions and natural organic matter. The results indicated that suphates were the most competitive fractions with high affinity (α (affinity coefficient) values ~ 9) followed by nitrates (α ~ 4.7) and NOM fractions (α ~ 4.5, p < 0.05). The Equivalent Background Concentration Mode (EBC), that arises from the Ideal Adsorption Solution Theory (IAST), indicated a competitive fraction of ~2 μeq/L NOM, which approximates to <10% of the initial NOM concentrations, indicating a small fraction of the NOM resulting in the competitive effect. Further, studies with natural surface waters indicated that the MCLR uptake could be modeled using the IAST-EBC model and the IX resin could simultaneously removal of >90% of NOM, inorganic ions and MCLR at resin dosages of 3.6 meq/L or higher.

Determination of d-myo-inositol phosphates in 'activated' raw almonds using anion-exchange chromatography coupled with tandem mass spectrometry

BACKGROUND

Activated almonds are raw almonds that have been soaked in water for 12-24 h at room temperature, sometimes followed by a 24 h drying period at low temperature (50 ± 5 °C). This treatment is thought to enhance the nutrient bioavailability of almonds by degrading nutrient inhibitors, such as phytic acid or d-myo-inositol hexaphosphate (InsP₆ ), through the release of phytase or passive diffusion of InsP₆ into the soaking water. Over a wide pH range, InsP₆ is a negatively charged compound that limits the absorption of essential nutrients by forming insoluble complexes with minerals such as iron and zinc. It is hypothesized that hydrating the seed during soaking triggers InsP₆ degradation into lower myo-inositol phosphates with less binding capacity.

RESULTS

Anion-exchange chromatography coupled with tandem mass spectrometry was used to quantify myo-inositol mono-, di-, tris-, tetra-, penta-, and hexaphosphates (InsP_1-6 ) in raw pasteurized activated almonds. At least 24 h of soaking at ambient temperature was required to reduce InsP₆ content from 14.71 to 14.01 µmol g^-1 .

CONCLUSIONS

The reduction in InsP₆ is statistically significant (P < 0.05) after 24 h of activation, but only represents a 4.75% decrease from the unsoaked almonds. © 2018 Society of Chemical Industry.

Antifouling Ion-Exchange Resins

Large quantities of organic ion-exchange resins are used worldwide for water decontamination and polishing. Fouling by microorganisms and decomposition products of natural organic matter severely limits the lifetime of these resins. Much research has thus been invested in polymer-based antifouling coatings. In the present study, poly(4-styrenesulfonate) (PSS) and a co-polymer of PSS and a zwitterionic group were used to spontaneously coat commercial Dowex 1X8 anion-exchange resin. UV-visible spectroscopy provided a precise measure of the kinetics and amount of PSS sorbed onto or into resin beads. When challenged with Chlamydomonas reinhardtii algae, uncoated resin was rapidly fouled by algae. Coating the resin with either the homopolymer of PSS or the co-polymer with zwitterion eliminated fouling. Using narrow- and wide-molecular-weight distribution PSS, a cutoff molecular weight of about 240 repeat units was found, above which PSS was unable to diffuse into the resin. Thus, only one monolayer of added PSS was sufficient to confer a highly desirable antifouling property on this resin while consuming less than 0.1% of the exchanger capacity. Radioactive sulfate ions were used to probe the kinetics of (self)exchange, which were virtually unaffected by the PSS coating. This resin treatment is a fast, ultra-low-cost step for potentially enhancing the lifetime of ion exchangers.

Adsorption characteristics of vanadium on different resin-active carbon composite electrodes in capacitive deionization

Three kinds of anion exchange resins (AERs) (D201, D301, D314) and one kind of cation exchange resin (D860) were used with activated carbon (AC) to fabricated the ion exchange resin-AC (IER/AC) composite electrodes in capacitive deionization (CDI) for selective adsorption of V(V). The characteristics of four kinds of composite electrodes, such as wettability, pore distribution and electrochemical properties, indicates IER/AC composite has great potential as electrode materials for the electro-adsorption in CDI. The pH of solution has apparent influence on the adsorption capacity of the composite electrodes for V(V) because of the various V(V) species in the solution with different pH. The reduction rate of V(V) on IER/AC electrodes mainly relates to the amount of VO₂⁺ in solution. The adsorption capacity of AER/AC electrodes for V(V) is slightly affected by the applied voltage may be due to that the adsorption of V(V) is mainly dependent on ion exchange with AERs and only a minority of V(V) is adsorbed by electrostatic adsorption. The adsorbed V(V) on D860/AC electrode decreases with the rising applied voltage because the pH increases with the increase of voltage. The separation of V(V) from V(V), Al and P indicates that the selective adsorption capability of IER/AC composite electrode is related to the migration rate of V(V), Al, P at different voltages and the selectivity of resins. This study may provide reference for recovering and separating metal ions from aqueous solution with CDI.

Adsorption removal of refractory organic matter in bio-treated municipal solid waste landfill leachate by anion exchange resins

Refractory organic matters (ROM) are the dominant organic matters in the membrane bioreactor -treated MSW landfill leachate, which are usually resistant to microbial degradation. Advanced membrane systems, which are expensive and generally for drinking water treatment, have to be used to make the final effluent meet discharge requirements. Anion exchange approach might be another option to remove ROM from leachate. In this study, the adsorption isotherms and kinetics were performed to illustrate the adsorption mechanism of anion exchange resins, 717 and D301R-Cl, for removing ROM from leachate. The results demonstrated that the adsorption isotherms for both resins were best represented by Langmuir model. The measured adsorption capacities of the 717 and D301-Cl resins were 39.84 and 35.84 mg COD g^-1 dry resin, respectively. The adsorption kinetics of ROM onto both resins followed a pseudo-second-order model and the measured rate constants were 0.00278 and 0.00236 g mg^-1 min^-1 for the 717 and D301R-Cl resins, respectively. Additionally, intra-particle diffusion analysis indicated that the adsorption of ROM was controlled by both film and intra-particle diffusions. Based on the UV/Vis spectra and fluorescence EEM analysis, the UV humic-like substances were preferentially adsorbed on both resins, although more on D301R-Cl resin than 717 resin. Furthermore, column studies showed that the adsorption of ROM on both 717 and D301R-Cl resins can be divided into two phases: monolayer adsorption and multilayer adsorption, while the desorption demonstrated similar pattern but different efficiency due to the unique property of resin. The results suggested that the resins could remove ROM from leachate efficiently, while the practical progress needs to be further optimized.