Phenolation of cyclodextrin polymers controls their lead and organic micropollutant adsorption

Discriminative Behavior of Cyclodextrin Polymers against Dissolved Organic Matter: Role of Cavity Size and Sorbate Properties

Cyclodextrin polymers (CDPs) are promising next-generation adsorbents in water purification technologies. The selectivity of the polymer derivate cross-linked with tetrafluoroterephthalonitrile (TFN-CDP) for nonionic and cationic micropollutants (MPs) over dissolved organic matter (DOM) renders the adsorbent also attractive for many analytical applications. The molecular drivers of the observed selectivity are, nonetheless, not yet fully understood. To provide new insights into the sorption mechanism, we (i) synthesized TFN-CDPs with different cavity sizes (α-, β-, γ-CDP); (ii) assessed their extraction efficiencies for selected nonionic MPs in competition with different DOM size fractions (<1, 1-3, 3-10, >10 kDa) to test for size-selectivity; and (iii) performed nontargeted, ultrahigh resolution Fourier transform ion cyclotron resonance mass spectrometry analysis on CDP-extracted DOM compounds (<1 kDa) to probe for molecular sorbate properties governing their selective sorption. First, no evidence of size-selectivity was obtained through either the different CD cavity sizes (i) or the two independent approaches (ii) and (iii). Second, we found a dominant impact of sorbate oxygenation and polarity on the extraction of DOM and MPs, respectively, with relatively oxygen-poor/nonpolar molecules favorably retained on all α-, β-, and γ-CDP. Third, our data indicates exclusion of an anionic matrix, such as carboxylic acids, but preferential sorption of cationic nitrogen-bearing DOM, pointing at repulsive and attractive forces with the negatively charged cross-linker as a likely reason. Therefore, we ascribe TFN-CDP's selectivity to nonpolar and electrostatic interactions between MPs/DOM and the polymer building blocks. These molecular insights can further aid in the optimization of efficient and selective sorbent design for environmental and analytical applications.

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

Cross-linked polymers containing β-cyclodextrin (β-CD) are promising adsorbents with demonstrated removal performances for per- and polyfluoroalkyl substances (PFASs) from contaminated water sources. Despite the promising performance of some β-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 β-CD polymer platform in which styrene groups are covalently attached to β-CD to form a discrete monomer that is amenable to radical polymerization. This monomer was polymerized with styrene and methacrylate comonomers to provide three β-CD polymers with high specific surface areas and high isolated yields (all >93%). A β-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.

Cyclodextrin-Based Nanosponges: Overview and Opportunities

Nanosponges are solid cross-linked polymeric nano-sized porous structures. This broad concept involves, among others, metal organic frameworks and hydrogels. The focus of this manuscript is on cyclodextrin-based nanosponges. Cyclodextrins are cyclic oligomers of glucose derived from starch. The combined external hydrophilicity with the internal hydrophobic surface constitute a unique “microenvironment”, that confers cyclodextrins the peculiar ability to form inclusion host‒guest complexes with many hydrophobic substances. These complexes may impart beneficial modifications of the properties of guest molecules such as solubility enhancement and stabilization of labile guests. These properties complemented with the possibility of using different crosslinkers and high polymeric surface, make these sponges highly suitable for a large range of applications. Despite that, in the last 2 decades, cyclodextrin-based nanosponges have been developed for pharmaceutical and biomedical applications, taking advantage of the nontoxicity of cyclodextrins towards humans. This paper provides a critical and timely compilation of the contributions involving cyclodextrins nanosponges for those areas, but also paves the way for other important applications, including water and soil remediation and catalysis.

Nanostructured Hypercrosslinked Porous Organic Polymers: Morphological Evolution and Rapid Separation of Polar Organic Micropollutants

Nanostructured hypercrosslinked porous organic polymers have triggered immense research interest for a broad spectrum of applications ranging from catalysis to molecular separation. However, it still remains a challenge to tune their nanoscale morphology. Herein, we demonstrated a remarkable variation of morphologies of triptycene-based hypercrosslinked microporous polymers starting from irregular aggregates (FCTP) to rigid spheres (SCTP) to two-dimensional nanosheets (SKTP) from three distinct polymerization methodologies, Friedel-Crafts knitting using an external crosslinker, Scholl reaction, and solvent knitting, respectively. Further, the dramatic role of reaction temperatures, catalysts, and solvents resulting in well-defined morphologies was elucidated. Mechanistic investigations coupled with microscopic and computational studies revealed the evolution of 2D nanosheets of a highly porous solvent-knitted polymer (SKTP, 2385 m² g^-1), resulting from the sequential hierarchical self-assembly of nanospheres and nanoribbons. A structure-activity correlation of hypercrosslinked polymers and their sulfonated counterparts for the removal of toxic polar organic micropollutants from water was delineated based on the chemical functionalities, specific surface area, pore size distribution, dispersity, and nanoscale morphology. Furthermore, a sulfonated 2D sheet-like solvent-knitted polymer (SKTPS) exhibited rapid adsorption kinetics (within 30 s) for a large array of polar organic micropollutants, including plastic components, steroids, antibiotic drugs, herbicides, and pesticides with remarkable uptake capacity and excellent recyclability. The current study provides the impetus for designing morphology-controlled functionalized porous polymers for task-specific applications.

Defining the Macromolecules of Tomorrow through Synergistic Sustainable Polymer Research

Transforming how plastics are made, unmade, and remade through innovative research and diverse partnerships that together foster environmental stewardship is critically important to a sustainable future. Designing, preparing, and implementing polymers derived from renewable resources for a wide range of advanced applications that promote future economic development, energy efficiency, and environmental sustainability are all central to these efforts. In this Chemical Reviews contribution, we take a comprehensive, integrated approach to summarize important and impactful contributions to this broad research arena. The Review highlights signature accomplishments across a broad research portfolio and is organized into four wide-ranging research themes that address the topic in a comprehensive manner: Feedstocks, Polymerization Processes and Techniques, Intended Use, and End of Use. We emphasize those successes that benefitted from collaborative engagements across disciplinary lines.

Identifying the physicochemical properties of β-cyclodextrin polymers that determine the adsorption of perfluoroalkyl acids

Cyclodextrin polymers (CDPs) are emerging adsorbents with demonstrated potential to remove perfluoroalkyl acids (PFAAs) from water. However, little is known about how the physicochemical properties of different types of CDPs determine PFAA adsorption on CDPs. In this study, we investigated the adsorption performance of 34 CDPs which consist of 14 different crosslinkers and exhibit a wide range of physicochemical properties. The performance metrics included adsorption kinetics, equilibrium adsorption density, and adsorption affinity for six PFAAs. We then used complementary bivariate and multivariate analyses to discover relationships between sixteen measurable physicochemical properties of the CDPs and their performance as adsorbents. We found that: (1) CDPs with a less negative or more positive surface charge will exhibit enhanced adsorption of all types of PFAAs; (2) CDPs with greater porosity and surface area will exhibit enhanced adsorption kinetics for all types of PFAAs; (3) CDPs with greater crosslinker content will exhibit enhanced adsorption of short-chain PFAAs; (4) CDPs containing more hydrophobic crosslinkers will exhibit enhanced equilibrium adsorption density and adsorption affinity for longer-chain PFAAs; and (5) CDPs with smaller particle sizes will exhibit enhanced adsorption kinetics and equilibrium adsorption density for all PFAAs. These insights will enable the further development of CDPs and other novel adsorbents to optimize their performance for removing PFAAs during water and wastewater treatment or groundwater remediation.

Macrocycle-Based Porous Organic Polymers for Separation, Sensing, and Catalysis

With the rapid development of materials science, porous organic polymers (POPs) have received remarkable attentions because of their unique properties such as the exceptionally high surface area and flexible molecular design. The ability to incorporate specific functions in a precise manner makes POPs promising platforms for a myriad of applications in molecular adsorption, separation, and catalysis. Therefore, many different types of POPs have been rationally designed and synthesized to expand the scope of advanced materials, endowing them with distinct structures and properties. Recently, supramolecular macrocycles with excellent host-guest complexation abilities are emerging as powerful crosslinkers for developing novel POPs with hierarchical structures and improved performance, which can be well-organized at different spatial scales. Macrocycle-based POPs could have unusual porous, adsorptive, and optical properties when compared to their nonmacrocycle-incorporated counterparts. This cooperation provides valuable insights for the molecular-level understanding of skeletal complexity and diversity. Here, the research advances of macrocycle-based POPs are aptly summarized by showing their syntheses, properties, and applications in terms of separation, sensing, and catalysis. Finally, the current challenging issues in this exciting research field are delineated and a comprehensive outlook is offered for their future directions.

Macrocycle-derived hierarchical porous organic polymers: synthesis and applications

Porous organic polymers (POPs), as a new category of advanced porous materials, have received broad research interests owing to the advantages of light-weight, robust scaffolds, high specific surface areas and good functional tailorability. According to the long-range ordering of polymer skeletons, POPs can be either crystalline or amorphous. Macrocycles with inherent cavities can serve as receptors for recognizing or capturing specific guest molecules through host-guest interactions. Incorporating macrocycles in POP skeletons affords win-win merits, e.g. hierarchical porosity and novel physicochemical properties. In this review, we focus on the recent progress associated with new architectures of macrocycle-based POPs. Herein, these macrocycles are divided into two subclasses: non-planar (crown ether, calixarene, pillararene, cyclodextrin, cyclotricatechylene, etc.) and planar (arylene-ethynylene macrocycles). We summarize the synthetic methods of each macrocyclic POP in terms of the functions of versatile building blocks. Subsequently, we discuss the performance of macrocyclic POPs in environmental remediation, gas adsorption, heterogeneous catalysis, fluorescence sensing and ionic conduction. Although considerable examples are reported, the development of macrocyclic POPs is still in its infancy. Finally, we propose the underlying challenges and opportunities of macrocycle-based POPs.

Tetrafluoroterephthalonitrile-crosslinked β-cyclodextrin polymer as a binding agent of diffusive gradients in thin-films for sampling endocrine disrupting chemicals in water

β-Cyclodextrin (β-CD) is an inexpensive and reproducible material derived from corn starch. It is possible that tetrafluoroterephthalonitrile-crosslinked β-cyclodextrin polymer (TFN-CD), a cheap but efficient adsorbent, could be a suitable binding agent for use in the passive sampling technique, diffusive gradients in thin-films (DGT). Herein, the TFN-CD binding gel was prepared and then evaluated as the binding phase of DGT to sample six endocrine disrupting chemicals (EDCs) in water. The TFN-CD dispersed uniformly in the binding gel due to its hydrophilicity. The quantitative recoveries (99.3%-106%) of EDCs from the TFN-CD binding gel could be conveniently achieved by ultrasonic extraction using 5 mL methanol for 10 min. Compared with the excellent HLB (hydrophilic-lipophilic-balanced resin) binding gel, the TFN-CD binding gel had comparable or even faster adsorption kinetics, although the equilibrium adsorption capacity was slightly lower. The effective adsorption capacities of TFN-CD-based DGT (TFN-CD-DGT) were roughly estimated to enable a 7-days deployment in EDC solution of 25.7-30.0 μg L^-1. Studies of influencing factors showed that the ionic strength (0-0.5 M), pH (3.73-9.13), dissolved organic matter (0-20 mg L^-1) and long-term storage (204 days) had negligible influence on the performance of TFN-CD-DGT. Finally, the TFN-CD-DGT was successfully used to record sudden increases in bulk concentrations during simulated discharge events in pond water. These results demonstrate that TFN-CD is a suitable binding agent for sampling of EDCs, and the low cost of TFN-CD could be conducive to the application of DGT in large-scale sampling.

Thiol/methylthio-functionalized porous aromatic frameworks for simultaneous capture of aromatic pollutants and Hg(II) from water

Simultaneously capturing organic pollutants and heavy metal can greatly reduce the water remediation time and cost, however it is still a great challenge presently. Herein, two novel thiol/methylthio-functionalized porous aromatic frameworks were synthesized as sorbents via the Sonogashira-Hagihara reaction of 1,3,5-triethynylbenzene and 1,3,5-tris(4-bromophenyl) benzene, the subsequent chloromethylation of the phenyl rings, and the final nucleophile substitution of -Cl groups by NaSH/NaSMe. These two sorbents were characterized by FT-IR spectra, energy dispersive X-ray spectra, scanning electron microscope, nitrogen adsorption analysis, thermo-gravimetric analysis, and elemental analyses. Adsorption experiments displayed that new sorbents had high uptake abilities and fast adsorption kinetics for aromatic pollutants and mercury (II) (Hg(II)). The maximum adsorption capacity (Q_max) of toluene and m-xylene on both new sorbents were 531.9-571.4 mg/g with the kinetic binding rate constants (k_obs) of 0.00276-0.02422 g/mg/min, and the Q_max values of Hg(II) were 148.1-180.3 mg/g with k_obs of 0.00592-0.01573 g/mg/min. Moreover, new sorbents indicated high simultaneous uptake abilities for these pollutants with good reusability, and finally they were successfully applied to the simultaneous remediation of these pollutants in two simulated sewages with high and low concentration, indicating their great practical application potential in wastewater remediation.

Zeolite-like performance for xylene isomer purification using polymer-derived carbon membranes

Polymers of intrinsic microporosity (PIMs) have been used as precursors for the fabrication of porous carbon molecular sieve (CMS) membranes. PIM-1, a prototypical PIM material, uses a fused-ring structure to increase chain rigidity between spirobisindane repeat units. These two factors inhibit effective chain packing, thus resulting in high free volume within the membrane. However, a decrease of pore size and porosity was observed after pyrolytic conversion of PIM-1 to CMS membranes, attributed to the destruction of the spirocenter, which results in the "flattening" of the polymer backbone and graphite-like stacking of carbonaceous strands. Here, a spirobifluorene-based polymer of intrinsic microporosity (PIM-SBF) was synthesized and used to fabricate CMS membranes that showed significant increases in p-xylene permeability (approximately four times), with little loss in p-xylene/o-xylene selectivity (13.4 versus 14.7) for equimolar xylene vapor separations when compared to PIM-1-derived CMS membranes. This work suggests that it is feasible to fabricate such highly microporous CMS membranes with performances that exceed current state-of-the-art zeolites at high xylene loadings.

[Recent developments of pesticide adsorbents based on cyclodextrins]

The invention and application of pesticides have greatly increased the yield of crops, greatly contributing to ensuring people's basic livelihoods and gradually improving their livelihoods to a well-off level. However, foods, water sources, and soil, containing high levels of pesticide residues, result in increasingly serious pollution. Pesticide residues usually have the characteristics of micro toxicity, difficult biodegradation, and bioaccumulation, and thus pose serious threat to living organisms and ecosystems. In recent years, pesticide pollution has earned worldwide focus. Thus, methods for the efficient detection of trace pesticides and reduction of the harm caused by pesticide pollution are urgently required. Researchers have used catalysis, electrochemistry, membrane separation, adsorption, and other methods to enrich pesticides from complex matrices. Among these, adsorbents have attracted much attention owing to their advantages of simple operation steps, rapid treatment process, and low amounts of organic solvents required. Research on adsorption materials has always been a very active field, and is also the key to the success of separation and enrichment of pesticides from complex matrices. Development of adsorbents with the advantages of simple synthesis, environment-friendliness, high stability, and strong reusability is of great significance. There has been some progress in the field of pesticide adsorption using supramolecular compounds. Cyclodextrin is a macrocyclic compound with a cavity after crown ether, which can form inclusion complexes via host guest interactions as the main body. Cyclodextrin can also be modified by etherification, esterification, oxidation, and other chemical reactions to improve its adsorption performance. Pesticides can be classified into organic and inorganic substances. One of the most widely used inorganic fungicides is the Bordeaux solution, whose main component is Cu²⁺. Organic fungicides, insecticides, herbicides, and plant growth regulators are basically organic molecules, whose hydroxyl and carboxyl groups can form complexes with Cu²⁺. As a matrix, cyclodextrin not only increases the surface area of the materials, but also provides the binding sites of hydroxyl and carboxyl groups, which guarantees efficient enrichment of Cu²⁺. Organic pesticides with high polarity, high electron density, and strong hydrophobicity could be better adsorbed. In this paper, the application of cyclodextrin-based adsorbents in pesticide adsorption was reviewed, and on this basis, reference to future development directions and application prospects were provided. The adsorption capacity of individual pesticide adsorbents based on cyclodextrin, as reviewed in this paper, is not high enough. Therefore, improving the adsorption capacity is currently a major research target. Some of the above-mentioned adsorbents have unclear degradation mechanisms and can easily cause secondary pollution. Therefore, the development of environment-friendly pesticide adsorbents that are easy to regenerate is a promising research direction for the future. After adsorption, some detection methods are used to determine whether the pesticide residues are up to the standard; however, the detection instruments are expensive. Therefore, the development of a combined detection mechanism that can reduce workload and cost is a promising research direction. Finally, the development of smart cyclodextrin-based adsorbents is also an efficient and rapid method to reduce the cost of detecting residual pesticide concentrations and the risk of pesticide pollution. For example, intelligent materials, whose color changes can be observed by the naked eye, not only adsorb pesticides, but also respond according to the concentration of residual pesticides.

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 β-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. β-cyclodextrin polymers also exhibit rapid adsorption kinetics and are easily regenerated. This Account details β-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. β-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 β-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.

β-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 β-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 K_D values ranging between 2.4 and 3.1, and the CE resin rapidly removes two zwitterionic PFASs with log K_D values of 1.8 and 1.9. The CDPs with a positive surface charge rapidly remove all anionic PFASs with log K_D values between 2.7 and 4.1, and the AE resin removes all anionic PFASs relatively slowly with log K_D 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.

Selective and fast recovery of rare earth elements from industrial wastewater by porous β-cyclodextrin and magnetic β-cyclodextrin polymers

Recovery of rare earth elements (REEs) from industrial wastewater has drawn great attention due to their potential environmental toxicity, as well as their high demand in modern technologies. In this study, we developed a magnetic composite based on the high surface area porous β-cyclodextrin polymer (P-CDP), namely P-CDP@Fe₃O₄. Both P-CDP and P-CDP@Fe₃O₄ rapidly sequester REEs such as Nd, Gd, Eu, and Y, reaching equilibrium in less than 10 min and fitting the Langmuir isotherm model with maximum adsorption capacities ranging from 7.76 to 9.59 mg/g at 25 °C when the highest initial concentration was 100 mg/L. Besides, the recovery of these REEs was not affected by competitive alkali, alkaline earth, and transition metal ions in model studies and industrial wastewater as revealed by the recovery efficiencies, which ranged from 62% to 100% indicating an excellent selectivity on both adsorbents. In addition, both adsorbents can be fully regenerated under mildly acidic conditions for at least five consecutive cycles. Moreover, P-CDP@Fe₃O₄ can be easily isolated by an external magnetic field which simplifies its synthesis and usability. It also overcomes the clogging and high backpressure issues of P-CDP, which facilitates its application for REEs recovery as compared with P-CDP. These characteristics demonstrate the promise of P-CDP and P-CDP@Fe₃O₄ for the pollution control and recovery of REEs.

Polycyclodextrins: Synthesis, functionalization, and applications

Cyclodextrins (CDs) are cyclic oligosaccharides with unique conical structure enabling host-guest inclusion complexes. However, virgin CDs sufferfrom low solubility, lack of functional groups and its inability to strong complexation with the guests. One of the most efficient ways to improve the properties of cyclodextrins is the synthesis of polycyclodextrins. Generally, there are two types of polycyclodextrins: 1) polymers containing CD units as parts of the main backbone; and 2) polymers with CD units as side chains. These polycyclodextrins are produced (i) from direct copolymerization of virgin cyclodextrins or cyclodextrins derivatives with various monomers including isocyanates, epoxides, carboxylic acids, anhydrides, acrylates, acrylamides and fluorinated aromatic compounds, or (ii) by post-functionalization of other polymers with CDs or CD derivatives.. By selecting the proper derivatives of CDs and controlling the polymerization, polycyclodextrins with linear, hyperbranched, and crosslinked structures have been synthesized. Polycyclodextrins have found significant applications in numerous areas, as adsorbents for removal of organic pollutants, carriers in gene/drug delivery, and for preparation of supramolecular based hydrogels. The focus of this review paper is placed on the synthesis, characterization, and applications of CDs so as to highlight challenges as well as the promising features of the future ahead of material developments based on CDs.

Evaluating the effects of water matrix constituents on micropollutant removal by activated carbon and β-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 β-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.

Cross-linked chitosan/zeolite as a fixed-bed column for organic micropollutants removal from aqueous solution, optimization with RSM and artificial neural network

Organic micropollutants (MPs) in low concentrations can affect aquatic ecosystems and human health. Adsorption technique is one of the promising methods to remove MPs. Chitosan and zeolites are environmentally friendly and low-cost adsorbents. Thus, removal of organic MPs (such as bisphenol A (BPA), carbamazepine (CBZ), ketoprofen (KTF) and tonalide (TND) from aqueous solution via cross-linked chitosan/zeolite, as a fixed-bed column, was investigated in the current study. Hydraulic retention time was set at 0.8 h pH and concentration of organic MPs ranged from 4 to 8 and 0.50 mg/L to 2.0 mg/L, and they were considered as factors in optimizing the removal of pollutants via response surface methodology (RSM). Approximately 1.4560 mg/L (89.0%) of BPA, 1.4724 mg/L (90.0%) of CBZ, 1.4920 mg/L (91.2%) of KTF and 1.4118 mg/L (86.3%) of TND were removed at 5.1 pH and 1.636 mg/L initial concentration as the optimum removal efficiency on the basis of RSM. Artificial neural network (ANN) was used to optimise removal effectiveness for each MP. The high R² values and reasonable mean squared errors indicated that ANN optimized MP removal in a logical aspect. Adsorption isotherm studies revealed that organic MP removal through chitosan/zeolite could be explained with Freundlich and Langmuir isotherms.

QSARs to predict adsorption affinity of organic micropollutants for activated carbon and β-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 (K_D) measured on coconut shell activated carbon (CCAC) and porous β-cyclodextrin polymer (P-CDP) adsorbents. We conducted batch experiments with a mixture of 200 MPs and used the data to calculate K_D 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.

β-Cyclodextrin Polymers on Microcrystalline Cellulose as a Granular Media for Organic Micropollutant Removal from Water

Organic contaminants at low concentrations, known as micropollutants, are a growing threat to water resources. Implementing novel adsorbents capable of removing micropollutants during packed-bed adsorption is desirable for rapid water purification and other efficient separations. We previously developed porous polymers based on cyclodextrins that demonstrated rapid uptake and high affinity for dozens of micropollutants (MPs) in batch experiments. However, these polymers are typically produced as powders with irregular particle size distributions in the range of tens of micrometers. In this powdered form, cyclodextrin polymers cannot be implemented in packed-bed adsorption processes because the variable particle sizes yield insufficient porosity packing and consequently generate high back-pressure. Here we demonstrate a facile approach to remove micropollutants from water in a continuous manner by polymerizing cyclodextrin polymer networks onto cellulose microcrystals to provide a core/shell structure. Batch adsorption experiments demonstrate rapid pollutant uptake and high accessibility of the cyclodextrins on the adsorbent. Similarly, column experiments demonstrate rapid uptake of a model pollutant with minimal back-pressure, demonstrating potential for use in packed-bed adsorption processes. Furthermore, the pollutant-saturated columns were regenerated using methanol and reused three times with almost no change in performance. Column experiments conducted with a mixture of 15 micropollutants at environmentally relevant concentrations demonstrated that removal was determined by the affinity of each micropollutant for cyclodextrin polymers. The cyclodextrin polymer grafted onto cellulose microcrystals is more resistant to both anaerobic and aerobic biodegradation as compared to cyclodextrins and unmodified cellulose crystals, presumably due to the aromatic cross-linkers, demonstrating persistence. Collectively, the findings from this study demonstrate a general strategy to incorporate novel cyclodextrin adsorbents onto cellulose substrates to enable rapid and efficient removal of micropollutants during packed-bed adsorption as well as their promising long-term stability and regeneration capabilities.