Polymerized Molecular Receptors as Adsorbents to Remove Micropollutants from Water.

Organic micropollutants (MPs) are increasing in number and concentration in water systems as a result of human activities. Often from human origin, these micropollutants build up in the environment because organisms lack the mechanisms to metabolize these substances, which cause negative health, ecological, and economic effects. Adsorption-based remediation processes for these compounds often rely on activated carbon materials. However, activated carbons are ineffective against certain MPs, exhibit low removal efficiencies in the presence of common aqueous matrix constituents, and require energy-intensive activation and regeneration processes. To overcome the deficiencies of traditional technologies, novel adsorbents based on molecular receptors offer promising alternative solutions. This Account describes the recent development of polymer adsorbents based on molecular receptors for removing trace organic chemicals from water. Polymer networks based on molecular receptors have high binding affinities for many MPs but, unlike activated carbons, have a specific molecule-binding mechanism that prevents these polymers from being fouled by matrix constituents such as natural organic matter. The size and hydrophobic pocket of the ß-cyclodextrin receptor preferentially adsorbs target molecules such as organic micropollutants in the presence of matrix constituents, and the nature of the cross-linker tunes the binding affinity and selectivity of the adsorbent for specific classes of MPs, including those of varying charge and hydrophobicity. ß-cyclodextrin polymers also exhibit rapid adsorption kinetics and are easily regenerated. This Account details ß-cyclodextrin polymers made with three different cross-linkers, including a polymer that is postsynthetically transformed from a negatively charged polymer to a positively charged polymer to invert the polymer's micropollutant adsorption profile. Morphological constraints have so far limited these cross-linked polymers' ability to be used in commercial applications, but two methods to create larger and more uniformly sized particles for use in flow-through applications are described here. ß-Cyclodextrin polymers are useful for trapping organic micropollutants such as bisphenol A, perfluorooctanoic acid, and many kinds of pharmaceuticals and pesticides, but their binding pockets are too large to capture micropollutants that are small or of high polarity. Other molecular receptors such as resorcinarene cavitands can target lower-molecular-weight MPs, including halomethane disinfection byproducts and industrial solvents, that are not bound strongly by ß-cyclodextrins. These materials demonstrate the potential of expanding the library of polymers based on molecular receptors. Overall, these emerging adsorbents show promise for the removal of legacy and emerging MPs from water, as well as the ability to rationally tune the adsorbent's structure to target the most persistent and toxic MPs.

ß-Cyclodextrin Polymers with Different Cross-Linkers and Ion-Exchange Resins Exhibit Variable Adsorption of Anionic, Zwitterionic, and Nonionic PFASs.

Per- and polyfluoroalkyl substances (PFASs) occur in groundwater as mixtures of anionic, cationic, zwitterionic, and nonionic species, although few remediation technologies have been evaluated to assess the removal of different types of PFASs. In this study, we evaluated the performance of three ß-cyclodextrin polymers (CDPs), an anion-exchange (AE) resin, and a cation-exchange (CE) resin for the removal of anionic, zwitterionic, and nonionic PFASs from water. We found that a CDP with a negative surface charge rapidly removes all zwitterionic PFASs with log KD values ranging between 2.4 and 3.1, and the CE resin rapidly removes two zwitterionic PFASs with log KD values of 1.8 and 1.9. The CDPs with a positive surface charge rapidly remove all anionic PFASs with log KD values between 2.7 and 4.1, and the AE resin removes all anionic PFASs relatively slowly with log KD values between 2.0 and 2.3. All adsorbents exhibited variable removal of the nonionic PFASs and some adsorption inhibition at higher pH values and in the presence of groundwater matrix constituents. Our findings provide insight into how adsorbents can be combined to remediate groundwater contaminated with complex mixtures of different types of PFASs.

Resorcinarene Cavitand Polymers for the Remediation of Halomethanes and 1,4-Dioxane.

Disinfection byproducts such as trihalomethanes are commonly found in drinking water. Trihalomethanes are formed upon chlorination of natural organic matter found in many drinking water sources. Inspired by molecular CHCl3⊂cavitand host-guest complexes, we designed porous polymers composed of resorcinarene receptors. These materials show higher affinity for halomethanes than a specialty activated carbon used for trihalomethane removal. The cavitand polymers show similar removal kinetics as activated carbon and have high capacity (49 mg g-1 of CHCl3). These materials maintain their performance in drinking water and can be thermally regenerated. Cavitand polymers also outperform commercial resins for 1,4-dioxane adsorption, which contaminates many water sources. These materials show promise for water treatment and demonstrate the value of using supramolecular receptors to design adsorbents for water purification.

Reduction of a Tetrafluoroterephthalonitrile-ß-Cyclodextrin Polymer to Remove Anionic Micropollutants and Perfluorinated Alkyl Substances from Water.

Organic micropollutants (MPs) are anthropogenic substances that contaminate water resources at trace concentrations. Many MPs, including per- and polyfluorinated alkyl substances (PFASs), have come under increased scrutiny because of their environmental persistence and association with various health problems. A ß-cyclodextrin polymer linked with tetrafluoroterephthalonitrile (TFN-CDP) has high affinity for cationic and many neutral MPs from contaminated water because of anionic groups incorporated during the polymerization. But TFN-CDP does not bind many anionic MPs strongly, including anionic PFASs. To address this shortcoming, we reduced the nitrile groups in TFN-CDP to primary amines, which reverses its affinity towards charged MPs. TFN-CDP exhibits adsorption distribution coefficients (log KD values) of 2-3 for cationic MPs and -0.5-1.5 for anionic MPs, whereas the reduced TFN-CDP exhibits log KD values of -0.5-1.5 for cationic MPs and 2-4 for anionic MPs, with especially high affinity towards anionic PFASs. Kinetic studies of the removal of 10 anionic PFASs at environmentally relevant concentrations showed 80-98 % removal of all contaminants after 30 min and was superior to commercial granular activated carbon. These findings demonstrate the scope and tunability of CD-based adsorbents derived from a single polymerization and the promise of novel adsorbents constructed from molecular receptors.

Benchmarking Micropollutant Removal by Activated Carbon and Porous ß-Cyclodextrin Polymers under Environmentally Relevant Scenarios.

The cost-effective and energy-efficient removal of organic micropollutants (MPs) from water and wastewater is challenging. The objective of this research was to evaluate the performance of porous ß-cyclodextrin polymers (P-CDP) as adsorbents of MPs in aquatic matrixes. Adsorption kinetics and MP removal were measured in batch and flow-through experiments for a mixture of 83 MPs at environmentally relevant concentrations (1 µg L-1) and across gradients of pH, ionic strength, and natural organic matter (NOM) concentrations. Performance was benchmarked against a coconut-shell activated carbon (CCAC). Data reveal pseudo-second-order rate constants for most MPs ranging between 1.5 and 40 g mg-1 min-1 for CCAC and 30 and 40000 g mg-1 min-1 for P-CDP. The extent of MP removal demonstrates slower but more uniform uptake on CCAC and faster but more selective uptake on P-CDP. Increasing ionic strength and the presence of NOM had a negative effect on the adsorption of MPs to CCAC but had almost no effect on adsorption of MPs to P-CDP. P-CDP performed particularly well for positively charged MPs and neutral or negatively charged MPs with McGowan volumes greater than 1.7 (cm3 mol-1)/100. These data highlight advantages of P-CDP adsorbents relevant to MP removal during water and wastewater treatment.

A Tunable Porous ß-Cyclodextrin Polymer Platform to Understand and Improve Anionic PFAS Removal.

Cross-linked polymers containing ß-cyclodextrin (ß-CD) are promising adsorbents with demonstrated removal performances for per- and polyfluoroalkyl substances (PFASs) from contaminated water sources. Despite the promising performance of some ß-CD-based adsorbents for PFAS removal, many of these materials are not amenable for rational performance improvement or addressing fundamental questions about the PFAS adsorption mechanisms. These ambiguities arise from the poorly defined structure of the cross-linked polymers, especially with respect to the random substitution patterns of the cyclodextrins as well as side reactions that modify the structures of some cross-linkers. Here, we report a new ß-CD polymer platform in which styrene groups are covalently attached to ß-CD to form a discrete monomer that is amenable to radical polymerization. This monomer was polymerized with styrene and methacrylate comonomers to provide three ß-CD polymers with high specific surface areas and high isolated yields (all >93%). A ß-CD polymer copolymerized with a methacrylate bearing a cationic functional group achieved nearly 100% removal for eight anionic PFASs (initial concentration of 1 µg/L for each compound) in nanopure water at an exceedingly low adsorbent loading of 1 mg L-1, as compared to previous cyclodextrin polymers that required loadings at least 1 order of magnitude higher to achieve an equivalent degree of PFAS removal. Furthermore, when the adsorbents were studied in a challenging salt matrix, we observed that long-chain PFAS adsorption was controlled by a complementary interplay of hydrophobic and electrostatic interactions, whereas short-chain PFASs primarily relied on electrostatic interactions. This approach demonstrates great promise for anionic PFAS removal, and we anticipate that new compositions will be tailored using the versatility of radical polymerization to simultaneously target PFASs and other classes of micropollutants in the future.

Exploring the factors that influence the adsorption of anionic PFAS on conventional and emerging adsorbents in aquatic matrices.

Per and polyfluoroalkyl substances (PFASs) have raised great concern due to their ubiquity in aquatic environments, and adsorption technologies are among the most promising treatment solutions. This study investigated the key factors that influence the adsorption of anionic PFASs on conventional and emerging adsorbents. Batch adsorption experiments were conducted to evaluate the removal of 20 target PFASs at environmentally relevant concentrations by three different activated carbon (AC) materials and two different ß-cyclodextrin polymers (CDPs). Experiments were conducted in Milli-Q water and in groundwater. Major physical properties of the adsorbents were measured, along with general water chemistry parameters for each groundwater sample. Principal component analysis (PCA) was subsequently employed to extract the important associations from the multivariate dataset. The distinct performances of ACs and CDPs were attributed to their different surface chemistry and the distinct nature of their adsorption binding sites. Hydrophobic interactions dominated PFAS adsorption onto ACs while CDPs mostly attracted anionic PFASs via favorable electrostatic interactions. ACs of a smaller average particle size performed better, with our data pointing to an increased external specific surface area as the likely reason. pH and the concentration of cations were the primary contributors to adsorption inhibition in groundwater. Higher pH values limit anionic PFAS adsorption by deprotonating the functional groups on adsorbent surfaces. The elevated levels of cations in some groundwater samples limited the effects of attractive electrostatic interactions. Knowledge of PFAS adsorption mechanisms gained from this study can be used to design more efficient adsorbents and to predict their performance under a range of environmental scenarios.

Evaluating the effects of water matrix constituents on micropollutant removal by activated carbon and ß-cyclodextrin polymer adsorbents.

The performance of adsorbents for the removal of organic micropollutants (MPs) from water can be influenced by the presence of water matrix constituents. The objective of this research was to evaluate the influence of water matrix constituents on the performance of coconut-shell activated carbon (CCAC), porous ß-cyclodextrin polymer (CDP), and CDP coated on cellulose microcrystal (CDP@CMC) adsorbents. MP removals were measured in batch experiments for a mixture of 90 MP at 1 µg L-1 and MP breakthrough was measured in rapid small-scale column test (RSSCT) experiments for a mixture of 15 MP at 500 ng L-1. All experiments were performed first with nanopure water, and subsequently with six different water samples collected from two separate groundwater, surface water, and wastewater effluent sources. The results of batch and RSSCT experiments demonstrate more rapid adsorption kinetics and less adsorption inhibition in the presence of matrix constituents for CDP adsorbents relative to CCAC. Further, the treatment capacity of CDP@CMC in the RSSCT experiments was higher than that of CCAC, particularly in more complex water matrices. Statistical analyses were performed to investigate associations between adsorption inhibition among groups of MPs and the concentrations of specific water matrix constituents. For CCAC, adsorption inhibition was observed for all MPs and was primarily attributed to the presence of dissolved organic matter with molar weight less than 1000 Da. For CDP adsorbents, adsorption inhibition was primarily observed for cationic MPs and was attributed to the screening of the negative surface charge of CDP by inorganic ions in water samples with high ionic strength. These data further demonstrate the value of CDP as an alternative adsorbent to CCAC for the removal of MPs during water and wastewater treatment.

QSARs to predict adsorption affinity of organic micropollutants for activated carbon and ß-cyclodextrin polymer adsorbents.

The removal of organic micropollutants (MPs) from water by means of adsorption is determined by the physicochemical properties of the adsorbent and the MPs. It is challenging to predict the removal of MPs by specific adsorbents due to the extreme diversity in physicochemical properties among MPs of interest. In this research, we established Quantitative Structure-Activity Relationships (QSARs) between the physicochemical properties of a diverse set of MPs and their distribution coefficients (KD) measured on coconut shell activated carbon (CCAC) and porous ß-cyclodextrin polymer (P-CDP) adsorbents. We conducted batch experiments with a mixture of 200 MPs and used the data to calculate KD values for each MP on each adsorbent under conditions of infinite dilution (i.e., low adsorbate concentrations). We used computational software to calculate 3656 molecular descriptors for each MP. We then developed and applied a model-selection workflow to identify the most significant molecular descriptors for each adsorbent. The functional stability and predictive power of the resulting QSARs were confirmed with internal cross validation and external validation. The applicability domain of the QSARs was defined based on the most significant molecular descriptors selected into each QSAR. The QSARs are predictive tools for evaluating adsorption-based water treatment processes and provide new insights into CCAC and P-CDP adsorption mechanisms.

Tetrafluoroterephthalonitrile-crosslinked ß-cyclodextrin polymers for efficient extraction and recovery of organic micropollutants from water.

In this study, we evaluated the performance of a novel tetrafluoroterephthalonitrile-crosslinked ß-cyclodextrin polymer (TFN-CDP) as a solid-phase extraction (SPE) material for the recovery of up to 189 diverse organic micropollutants (MPs) from water. The optimized extraction procedure requires loading of water samples adjusted to a pH of 3 onto 500 mg of TFN-CDP packed into an SPE cartridge. Under these conditions, 88.7% of the MPs have average extraction efficiencies greater than 80%. The optimized recovery procedure requires elution with 15 mL of methanol amended with 15 mg of calcium chloride. Under these conditions, 58.4% of the MPs have average absolute recoveries between 80% and 120%. We compared the performance of the optimized SPE method for TFN-CDP with a previously optimized SPE method employing hydrophilic-lipophilic balance (HLB) adsorbents in nanopure water and in wastewater-impacted surface water. The data indicate that the optimized TFN-CDP method performs as well as or better than the optimized HLB-based SPE method. These findings represent an important step forward in the development of sustainable and inexpensive materials for the extraction and recovery of organic MPs from water.

Phenolation of cyclodextrin polymers controls their lead and organic micropollutant adsorption.

Porous ß-cyclodextrin polymers linked with tetrafluoroterephthalonitrile (TFN-CDPs) have shown promise for adsorbing organic micropollutants (MPs) more quickly and effectively than conventional adsorbents. Prior to their discovery, the nucleophilic aromatic substitution (SNAr) reaction used to prepare TFN-CDP was nearly unknown for the aliphatic alcohol nucleophiles, and the low isolated yields of TFN-CDP motivated model studies of the reaction between TFN and n-butanol. These experiments reveal a previously undescribed substitution reaction of TFN in which a fluorine is substituted by a hydroxyl group. This process is responsible for the low yields of the polymerization and incorporates phenolate groups into the polymer network. Phenolation and polymerization (etherification) are competing processes, and the level of phenolate incorporation was controlled by varying the rate of base addition and initial monomer concentrations. TFN-CDPs with varying phenolate content were prepared and evaluated as adsorbents for both Pb2+ ions and 83 MPs. More heavily phenolated polymers showed increased capacity to bind Pb2+ ions. Phenolation was also correlated with increased binding affinity for almost all of the 83 MPs tested, including neutral, cationic, and anionic substances. These results leverage a newly discovered side reaction during SNAr reactions of electron-poor aryl fluorides to improve both the yield and the uptake affinity for both lead and organic MPs of TFN-CDPs.