Hydroxypicolinic acid tethered on magnetite core-silica shell (HPCA@SiO2@Fe3O4) as an effective and reusable adsorbent for practical Co(II) recovery.

The soaring demand and future supply risk for cobalt (Co) necessitate more efficient adsorbents for its recycling from electronic wastes, as a cheaper and less hazardous option for its production. Herein, a magnetic adsorbent covalently tethered with 5-hydroxypicolinic acid (HPCA) as Co(II) ligand was developed. The magnetic component (Fe3O4) was protected with silica (SiO2), then silanized with chloroalkyl linker and subsequently functionalized with HPCA via SN2 nucleophilic substitution (HPCA@SiO2@Fe3O4). Results from FTIR, TGA, EA, and XPS confirm the successful adsorbent preparation with high HPCA loading of 2.62 mmol g-1. TEM-EDS reveal its imperfect spherical morphology with ligands well-distributed on its surface. HPCA@SiO2@Fe3O4 is hydrophilic, water-dispersible and magnetically retrievable, which is highly convenient for its recovery. The Co(II) capture on HPCA@SiO2@Fe3O4 involves monodentate coordination with carboxylate (COO-) and lone pair acceptance from pyridine (aromatic -N = ) moiety of HPCA, with minor interaction from acidic silanols (Si-O-). The binding occurs at 2 HPCA: 1 Co(II) ratio, that follows the Sips isotherm model with competitive Qmax = 92.35 mg g-1 and pseudo-second order kinetics (k2 = 0.0042 g mg-1 min-1). In a simulated LIB liquid waste, HPCA@SiO2@Fe3O4 preferentially captures Co(II) over Li(I) with αLi(I)Co(II)=166 and Mn(II) with αMn(II)Co(II)=55, which highlights the importance of HPCA for Co(II) recovery. Silica protection of Fe3O4 rendered the adsorbent chemically stable in acidic thiourea solution for its regeneration by preventing the deterioration of the magnetic component. Covalent functionalization averted ligand loss, which allowed HPCA@SiO2@Fe3O4 to deliver consistent and reversible adsorption/desorption performance. Overall results demonstrate the potential of HPCA@SiO2@Fe3O4 as a competitive and practical adsorbent for Co(II) recovery in liquid waste sources.

Hyper-crosslinked tetraphenylborate as a regenerable sorbent for Cs+ sequestration in aqueous media through cation-π interactions.

Practical adsorbents that could efficiently collect radioactive Cesium (Cs+) are critically important in achieving proper management and treatment measures for nuclear wastes. Herein, a hyper-crosslinked tetraphenylborate-based adsorbent (TPB-X) was prepared by reacting TPB anions as Cs+ binding sites with dimethoxymethane (DMM) as crosslinker. The most efficient TPB-X synthesis was attained at 1:4 TPB/DMM mole ratio with sorbent yield of 81.75%. Various techniques such as FTIR, TGA-DTG, N2 adsorption/desorption and SEM-EDS reveal that TPB-X is a water-insoluble, thermally stable and highly porous granular sorbent. Its hierarchical pore structure explains its very high BET surface area (1030 m2 g-1). Sequestration of Cs+ by TPB-X involves its exchange with H+ followed by its binding with the phenyl rings of TPB through cation-π interactions. The Cs+ adsorption in TPB-X is endothermic and spontaneous, which adheres to the Hill isotherm model (qm = 140.58 mg g-1) and follows pseudo-second order kinetics (k2 = 0.063 g mg-1 h-1). Calculations from the density functional theory reveal that the binding of TPB anion is strongest for Cs+. Thus, TPB-X was able to selectively capture Cs+ in simulated surface water containing Na+, K+, Mg2+, and Ca2+ and in HLLW containing Na+, Rb+, Sr2+, and Ba2+. Hyper-crosslinking was found beneficial in rendering TPB-X reusable as the sorbent was easily retrieved from the feed after Cs+ capture and was able to withstand the acid treatment for its regeneration. TPB-X exhibited consistent performance with no sign of chemical or physical deterioration. TPB-X offers a practical approach in handling Cs+ contaminated streams as it can be repeatedly used to enrich Cs+ in smaller volume of media, which can then be purified for Cs+ reuse or stored for long-term natural Cs+ decay process.

Chemically Cross-Linked Graphene Oxide as a Selective Layer on Electrospun Polyvinyl Alcohol Nanofiber Membrane for Nanofiltration Application.

Graphene oxide (GO) nanosheets were utilized as a selective layer on a highly porous polyvinyl alcohol (PVA) nanofiber support via a pressure-assisted self-assembly technique to synthesize composite nanofiltration membranes. The GO layer was rendered stable by cross-linking the nanosheets (GO-to-GO) and by linking them onto the support surface (GO-to-PVA) using glutaraldehyde (GA). The amounts of GO and GA deposited on the PVA substrate were varied to determine the optimum nanofiltration membrane both in terms of water flux and salt rejection performances. The successful GA cross-linking of GO interlayers and GO-PVA via acetalization was confirmed by FTIR and XPS analyses, which corroborated with other characterization results from contact angle and zeta potential measurements. Morphologies of the most effective membrane (CGOPVA-50) featured a defect-free GA cross-linked GO layer with a thickness of ~67 nm. The best solute rejections of the CGOPVA-50 membrane were 91.01% for Na2SO4 (20 mM), 98.12% for Eosin Y (10 mg/L), 76.92% for Methylene blue (10 mg/L), and 49.62% for NaCl (20 mM). These findings may provide one of the promising approaches in synthesizing mechanically stable GO-based thin-film composite membranes that are effective for solute separation via nanofiltration.

Understanding D-xylonic acid accumulation: a cornerstone for better metabolic engineering approaches.

The xylose oxidative pathway (XOP) has been engineered in microorganisms for the production of a wide range of industrially relevant compounds. However, the performance of metabolically engineered XOP-utilizing microorganisms is typically hindered by D-xylonic acid accumulation. It acidifies the media and perturbs cell growth due to toxicity, thus curtailing enzymatic activity and target product formation. Fortunately, from the growing portfolio of genetic tools, several strategies that can be adapted for the generation of efficient microbial cell factories have been implemented to address D-xylonic acid accumulation. This review centers its discussion on the causes of D-xylonic acid accumulation and how to address it through different engineering and synthetic biology techniques with emphasis given on bacterial strains. In the first part of this review, the ability of certain microorganisms to produce and tolerate D-xylonic acid is also tackled as an important aspect in developing efficient microbial cell factories. Overall, this review could shed some insights and clarity to those working on XOP in bacteria and its engineering for the development of industrially applicable product-specialist strains. KEY POINTS: D-Xylonic acid accumulation is attributed to the overexpression of xylose dehydrogenase concomitant with basal or inefficient expression of enzymes involved in D-xylonic acid assimilation. Redox imbalance and insufficient cofactors contribute to D-xylonic acid accumulation. Overcoming D-xylonic acid accumulation can increase product formation among engineered strains. Engineering strategies involving enzyme engineering, evolutionary engineering, coutilization of different sugar substrates, and synergy of different pathways could potentially address D-xylonic acid accumulation.

Engineering of xylose metabolism in Escherichia coli for the production of valuable compounds.

The lignocellulosic sugar d-xylose has recently gained prominence as an inexpensive alternative substrate for the production of value-added compounds using genetically modified organisms. Among the prokaryotes, Escherichia coli has become the de facto host for the development of engineered microbial cell factories. The favored status of E. coli resulted from a century of scientific explorations leading to a deep understanding of its systems. However, there are limited literature reviews that discuss engineered E. coli as a platform for the conversion of d-xylose to any target compounds. Additionally, available critical review articles tend to focus on products rather than the host itself. This review aims to provide relevant and current information about significant advances in the metabolic engineering of d-xylose metabolism in E. coli. This focusses on unconventional and synthetic d-xylose metabolic pathways as several review articles have already discussed the engineering of native d-xylose metabolism. This paper, in particular, is essential to those who are working on engineering of d-xylose metabolism using E. coli as the host.

Enhanced glycolic acid yield through xylose and cellobiose utilization by metabolically engineered Escherichia coli.

Microbial biorefinery is a promising route toward sustainable production of glycolic acid (GA), a valuable raw material for various industries. However, inherent microbial GA production has limited substrate consumption using either D-xylose or D-glucose as carbon catabolite repression (CCR) averts their co-utilization. To bypass CCR, a GA-producing strain using D-xylose via Dahms pathway was engineered to allow cellobiose uptake. Unlike glucose, cellobiose was assimilated and intracellularly degraded without repressing D-xylose uptake. The final GA-producing E. coli strain (CLGA8) has an overexpressed cellobiose phosphorylase (cep94A) from Saccharophagus degradans 2-40 and an activated glyoxylate shunt pathway. Expression of cep94A improved GA production reaching the maximum theoretical yield (0.51 g GA g-1 xylose), whereas activation of glyoxylate shunt pathway enabled GA production from cellobiose, which further increased the GA titer (2.25 g GA L-1). To date, this is the highest reported GA yield from D-xylose through Dahms pathway in an engineered E. coli with cellobiose as co-substrate.

Forward osmosis with direct contact membrane distillation using tetrabutylphosphonium p-toluenesulfonate as an effective and safe thermo-recyclable osmotic agent for seawater desalination.

A phosphonium-based ionic liquid (IL) with lower critical solution temperature (LCST) property was assessed as a reusable draw solution (DS) for forward osmosis (FO). Tetrabutylphosphonium p-toluenesulfonate ([P4444]TsO) was successfully synthesized by neutralization reaction. Characterization results reveal its ability to generate sufficient osmotic pressure (14-68 bars for 0.5-2 M DS) to create a gradient across the FO membrane. Its thermal, physico-chemical and other colligative properties are favorable for its application as an osmotic agent. The LCST behavior of [P4444]TsO was found reversible and its phase separation from water can be done above its cloud point temperature Tc ∼57 °C. In vitro cytotoxicity tests from LDH and MTT assay reveal that it can be safely used as DS at an effective concentration EC30 ∼57 mg L-1 as its non-toxic level. Results from FO operations demonstrate that 2 M [P4444]TsO DS can effectively treat saline feed like seawater (0.6 M NaCl) with reasonable Jv = 1.35 ± 0.15 L m-2h-1, low Js = 0.0038 ± 0.00049 mol m-2h-1, and considerably low specific solute flux (Js/Jv âˆ¼ 0.0028 mol L-1). After FO, ∼98% of [P4444]TsO was precipitated by heating the DS at 60 °C and conveniently reused with consistent FO performance. Direct contact membrane distillation (DCMD) was found effective in removing the residual 2% [P4444]TsO in the DS supernatant to finally produce high-quality effluent with concentrations way below the EC30 limit. Cost estimates for the entire process reveal the potential of FO combined with thermo-cyclic [P4444]TsO regeneration with DCMD for desalination application.

Overexpression and characterization of a novel GH16 ß-agarase (Aga1) from Cellulophaga omnivescoria W5C.

OBJECTIVE: To identify and characterize a new ß-agarase from Cellulophaga omnivescoria W5C capable of producing biologically-active neoagarooligosaccharides from agar. RESULTS: The ß-agarase, Aga1, has signal peptides on both N- and C-terminals, which are involved in the type IX secretion system. It shares 75% protein sequence identity with AgaD from Zobellia galactanivorans and has a molecular weight of 54 kDa. Biochemical characterization reveals optimum agarolytic activities at pH 7-8 and temperature 30-45 °C. Aga1 retains at least 33% activity at temperatures lower than the sol-gel transition state of agarose. Metal ions are generally not essential, but calcium and potassium enhance its activity whereas iron and zinc are inhibitory. Finally, hydrolysis of agarose with Aga1 yields neoagarotetraose, neoagarohexaose, and neoagarooctaose. CONCLUSIONS: Aga1 displays unique traits such as moderate psychrophilicity, stability, and synergy with other agarases, which makes it an excellent candidate for biosynthetic production of neoagarooligosaccharides from agar.

Correction to: A pH-responsive genetic sensor for the dynamic regulation of D-xylonic acid accumulation in Escherichia coli.

In the published version, the y-axis data of Fig. 3c was incorrectly inserted (OD600 instead of D-xylonate (g L-1) and the x-axes of Figs. 3b, 3d, 3e and 3f ended at 48 h instead of 72 h. See the correct Fig. 3 below.

A pH-responsive genetic sensor for the dynamic regulation of D-xylonic acid accumulation in Escherichia coli.

The xylose oxidative pathway (XOP) is continuously gaining prominence as an alternative for the traditional pentose assimilative pathways in prokaryotes. It begins with the oxidation of D-xylose to D-xylonic acid, which is further converted to α-ketoglutarate or pyruvate + glycolaldehyde through a series of enzyme reactions. The persistent drawback of XOP is the accumulation of D-xylonic acid intermediate that causes culture media acidification. This study addresses this issue through the development of a novel pH-responsive synthetic genetic controller that uses a modified transmembrane transcription factor called CadCΔ. This genetic circuit was tested for its ability to detect extracellular pH and to control the buildup of D-xylonic acid in the culture media. Results showed that the pH-responsive genetic sensor confers dynamic regulation of D-xylonic acid accumulation, which adjusts with the perturbation of culture media pH. This is the first report demonstrating the use of a pH-responsive transmembrane transcription factor as a transducer in a synthetic genetic circuit that was designed for XOP. This may serve as a benchmark for the development of other genetic controllers for similar pathways that involve acidic intermediates.

Discovering a novel D-xylonate-responsive promoter: the PyjhI-driven genetic switch towards better 1,2,4-butanetriol production.

The capability of Escherichia coli to catabolize D-xylonate is a crucial component for building and optimizing the Dahms pathway. It relies on the inherent dehydratase and keto-acid aldolase activities of E. coli. Although the biochemical characteristics of these enzymes are known, their inherent expression regulation remains unclear. This knowledge is vital for the optimization of D-xylonate assimilation, especially in addressing the problem of D-xylonate accumulation, which hampers both cell growth and target product formation. In this report, molecular biology techniques and synthetic biology tools were combined to build a simple genetic switch controller for D-xylonate. First, quantitative and relative expression analysis of the gene clusters involved in D-xylonate catabolism were performed, revealing two D-xylonate-inducible operons, yagEF and yjhIHG. The 5'-flanking DNA sequence of these operons were then subjected to reporter gene assays which showed PyjhI to have low background activity and wide response range to D-xylonate. A PyjhI-driven synthetic genetic switch was then constructed containing feedback control to autoregulate D-xylonate accumulation and to activate the expression of the genes for 1,2,4-butanetriol (BTO) production. The genetic switch effectively reduced D-xylonate accumulation, which led to 31% BTO molar yield, the highest for direct microbial fermentation systems thus far. This genetic switch can be further modified and employed in the production of other compounds from D-xylose through the xylose oxidative pathway.

Improved cell growth and biosynthesis of glycolic acid by overexpression of membrane-bound pyridine nucleotide transhydrogenase.

The non-conventional D-xylose metabolism called the Dahms pathway which only requires the expression of at least three enzymes to produce pyruvate and glycolaldehyde has been previously engineered in Escherichia coli. Strains that rely on this pathway exhibit lower growth rates which were initially attributed to the perturbed redox homeostasis as evidenced by the lower intracellular NADPH concentrations during exponential growth phase. NADPH-regenerating systems were then tested to restore the redox homeostasis. The membrane-bound pyridine nucleotide transhydrogenase, PntAB, was overexpressed and resulted to a significant increase in biomass and glycolic acid titer and yield. Furthermore, expression of PntAB in an optimized glycolic acid-producing strain improved the growth and product titer significantly. This work demonstrated that compensating for the NADPH demand can be achieved by overexpression of PntAB in E. coli strains assimilating D-xylose through the Dahms pathway. Consequently, increase in biomass accumulation and product concentration was also observed.

Aqueous Synthesis of 14-15-Membered Crown Ethers with Mixed O, N and S Heteroatoms: Experimental and Theoretical Binding Studies with Platinum-Group Metals.

A series of 14-15-membered O, N, and S-containing crown ethers (CEs) was synthesized by cyclization of bis-epoxides with aryl-N or S dinucleophiles using triethylamine as a catalyst and LiCl as a metal template in water. The catalyst dosage, and metal template type and dosage were critical in achieving yields of 56-93 %. Liquid-liquid extraction (LLE) was performed to evaluate the CE complexation with Pd2+ and Pt2+ . Among the CEs, a dioxa-dithia dibenzo CE exhibited the highest Pd2+ selectivity even in the presence of other platinum-group metals (PGMs). Complementary DFT studies reveal that this CE has the most compatible cavity dimension (ØCE =1.58 Å) with Pd2+ (ØPd 2+ =1.56 Å) forming a square-planar S4 geometry. Binding-energy calculations showed the Pd2+ complex has the least energy requirement for structural reorientation during complexation. Overall results highlight the importance of CE cavity dimension and presence of S heteroatoms for the structural design of CEs selective towards PGMs such as Pd2+ .

Everyone loves an underdog: metabolic engineering of the xylose oxidative pathway in recombinant microorganisms.

The D-xylose oxidative pathway (XOP) has recently been employed in several recombinant microorganisms for growth or for the production of several valuable compounds. The XOP is initiated by D-xylose oxidation to D-xylonolactone, which is then hydrolyzed into D-xylonic acid. D-Xylonic acid is then dehydrated to form 2-keto-3-deoxy-D-xylonic acid, which may be further dehydrated then oxidized into α-ketoglutarate or undergo aldol cleavage to form pyruvate and glycolaldehyde. This review introduces a brief discussion about XOP and its discovery in bacteria and archaea, such as Caulobacter crescentus and Haloferax volcanii. Furthermore, the current advances in the metabolic engineering of recombinant strains employing the XOP are discussed. This includes utilization of XOP for the production of diols, triols, and short-chain organic acids in Escherichia coli, Saccharomyces cerevisiae, and Corynebacterium glutamicum. Improving the D-xylose uptake, growth yields, and product titer through several metabolic engineering techniques bring some of these recombinant strains close to industrial viability. However, more developments are still needed to optimize the XOP pathway in the host strains, particularly in the minimization of by-product formation.

Removal of odorous compounds emitted from a food-waste composting facility in Korea using a pilot-scale scrubber.

Monitoring and control of odorous compound emissions have been enforced by the Korean government since 2005. One of the point sources for these emissions was from food waste composting facilities. In this study, a pilot-scale scrubber installed in a composting facility was evaluated for its performance in the removal of malodorous compounds. The exhaust stream contained ammonia and methylamine as the major odorants detected by the threshold odor test and various instrumental techniques (GC-FID, FPD, MS and HPLC/UV). For the scrubber operation, the column was randomly packed with polypropylene Hi-Rex 200, while aqueous sulfuric acid was selected as the scrubbing solution. To achieve 95% removal, the scrubber must be operated by using H2SO4 solution with pH at < 6.5, liquid to gas ratio > 4.5, gas loading rate < 1750 m3/m3-hr and contact time < 0.94 s. The scrubber performance was further evaluated by determining the mass transfer coefficients and then monitoring for 355 days of operation. The pilot-scale scrubber maintained > 95% ammonia and methylamine removal efficiencies despite the fluctuations in the inlet (from composting facility exhaust stream) concentration. The optimum operating conditions and scrubber performance indicators determined in this study provides a basis for the design of a plant-scale scrubber for treatment of composting facility gas emissions.

Draft Genome Sequence of Newly Isolated Agarolytic Bacteria Cellulophaga omnivescoria sp. nov. W5C Carrying Several Gene Loci for Marine Polysaccharide Degradation.

The continued research in the isolation of novel bacterial strains is inspired by the fact that native microorganisms possess certain desired phenotypes necessary for recombinant microorganisms in the biotech industry. Most studies have focused on the isolation and characterization of strains from marine ecosystems as they present a higher microbial diversity than other sources. In this study, a marine bacterium, W5C, was isolated from red seaweed collected from Yeosu, South Korea. The isolate can utilize several natural polysaccharides such as agar, alginate, carrageenan, and chitin. Genome sequence and comparative genomics analyses suggest that strain W5C belongs to a novel species of the Cellulophaga genus, from which the name Cellulophaga omnivescoria sp. nov. is proposed. Its genome harbors 3,083 coding sequences and 146 carbohydrate-active enzymes (CAZymes). Compared to other reported Cellulophaga species, the genome of W5C contained a higher proportion of CAZymes (4.7%). Polysaccharide utilization loci (PUL) for agar, alginate, and carrageenan were identified in the genome, along with other several putative PULs. These PULs are excellent sources for discovering novel hydrolytic enzymes and pathways with unique characteristics required for biorefinery applications, particularly in the utilization of marine renewable biomass. The type strain is JCM 32108T (= KCTC 13157BPT).

Engineering Escherichia coli for glycolic acid production from D-xylose through the Dahms pathway and glyoxylate bypass.

Glycolic acid (GA) is an ⍺-hydroxy acid used in cosmetics, packaging, and medical industries due to its excellent properties, especially in its polymeric form. In this study, Escherichia coli was engineered to produce GA from D-xylose by linking the Dahms pathway, the glyoxylate bypass, and the partial reverse glyoxylate pathway (RGP). Initially, a GA-producing strain was constructed by disrupting the xylAB and glcD genes in the E. coli genome and overexpressing the xdh(Cc) from Caulobacter crescentus. This strain was further improved through modular optimization of the Dahms pathway and the glyoxylate bypass. Results for module 1 showed that the rate-limiting step of the Dahms pathway was the xylonate dehydratase reaction, and the overexpression of yagF was sufficient to overcome this bottleneck. Furthermore, the appropriate aldolase gene for module 1 was proven to be yagE. The results also show that overexpression of the lactaldehyde dehydrogenase gene, aldA, is needed to increase the GA production while the overexpression of glyoxylate reductase gene, ycdW, was only essential when the glyoxylate bypass was active. On the other hand, the module 2 enzymes AceA and AceK were vital in activating the glyoxylate bypass, while the RGP enzymes were dispensable. The final strain (GA19) produced 4.57 g/L GA with a yield of 0.46 g/g from D-xylose. So far, this is the highest value achieved for GA production in engineered E. coli through the Dahms pathway.

Identification and characterization of a thermostable endolytic ß-agarase Aga2 from a newly isolated marine agarolytic bacteria Cellulophaga omnivescoria W5C.

Research on the enzymatic breakdown of seaweed-derived agar has recently gained attention due to the progress in green technologies for marine biomass utilization. The enzymes known as agarases catalyze the cleavage of glycosidic bonds within the polysaccharide. In this study, a new ß-agarase, Aga2, was identified from Cellulophaga omnivescoria W5C. Aga2 is one of four putative agarases from the W5C genome, and it belongs to the glycoside hydrolase 16 family. It was shown to be exclusive to the Cellulophaga genus. Agarase activity assays showed that Aga2 is an endolytic-type ß-agarase that produces tetrameric and hexameric neoagaro-oligosaccharides, with optimum activity at 45°C and pH 8.0. Zinc ions slightly enhanced its activity while manganese ions had inhibitory effects even at very low concentrations. Aga2 has a Km of 2.59mgmL-1 and Vmax of 275.48Umg-1. The Kcat is 1.73×102s-1, while the Kcat/Km is 8.04×106s-1M-1. Aga2 also showed good thermostability at 45°C and above, and retained >90% of its activity after repeated freeze-thaw cycles. Bioinformatic analysis of its amino acid sequence revealed that intrinsic properties of the protein (e.g. presence of certain dipeptides and the relative volume occupied by aliphatic amino acids) and tertiary structural elements (e.g. presence of salt bridges, hydrophobic interactions and H-bonding) contributed to its thermostability.

Performance evaluation of poly-urethane foam packed-bed chemical scrubber for the oxidative absorption of NH3 and H2S gases.

The feasibility of open-pore polyurethane (PU) foam as packing material for wet chemical scrubber was tested for NH3 and H2S removals. The foam is inexpensive, light-weight, highly porous (low pressure drop) and provides large surface area per unit volume, which are desirable properties for enhanced gas/liquid mass transfer. Conventional HCl/HOCl (for NH3) and NaOH/NaOCl (for H2S) scrubbing solutions were used to absorb and oxidize the gases. Assessment of the wet chemical scrubbers reveals that pH and ORP levels are important to maintain the gas removal efficiencies >95%. A higher re-circulation rate of scrubbing solutions also proved to enhance the performance of the NH3 and H2S columns. Accumulation of salts was confirmed by the gradual increase in total dissolved solids and conductivity values of scrubbing solutions. The critical elimination capacities at >95% gas removals were found to be 5.24 g NH3-N/m3-h and 17.2 g H2S-S/m3-h at an empty bed gas residence time of 23.6 s. Negligible pressure drops (< 4 mm H2O) after continuous operation demonstrate the suitability of PU as a practical packing material in wet chemical scrubbers for NH3 and H2S removals from high-volume dilute emissions.

Aerosol Cross-Linked Crown Ether Diols Melded with Poly(vinyl alcohol) as Specialized Microfibrous Li+ Adsorbents.

Crown ether (CE)-based Li+ adsorbent microfibers (MFs) were successfully fabricated through a combined use of CE diols, electrospinning, and aerosol cross-linking. The 14- to 16-membered CEs, with varied ring subunits and cavity dimensions, have two hydroxyl groups for covalent attachments to poly(vinyl alcohol) (PVA) as the chosen matrix. The CE diols were blended with PVA and transformed into microfibers via electrospinning, a highly effective technique in minimizing CE loss during MF fabrication. Subsequent aerosol glutaraldehyde (GA) cross-linking of the electrospun CE/PVA MFs stabilized the adsorbents in water. The aerosol technique is highly effective in cross-linking the MFs at short time (5 h) with minimal volume requirement of GA solution (2.4 mL g-1 MF). GA cross-linking alleviated CE leakage from the fibers as the CEs were securely attached with PVA through covalent CE-GA-PVA linkages. Three types of CE/PVA MFs were fabricated and characterized through Fourier transform infrared-attenuated total reflection, 13C cross-polarization magic angle spinning NMR, field emission scanning electron microscope, N2 adsorption/desorption, and universal testing machine. The MFs exhibited pseudo-second-order rate and Langmuir-type Li+ adsorption. At their saturated states, the MFs were able to use 90-99% CEs for 1:1 Li+ complexation, suggesting favorability of their microfibrous structures for CE accessibility to Li+. The MFs were highly Li+-selective in seawater. Neopentyl-bearing CE was most effective in blocking larger monovalents Na+ and K+, whereas the dibenzo CE was best in discriminating divalents Mg2+ and Ca2+. Experimental selectivity trends concur with the reaction enthalpies from density functional theory calculations, confirming the influence of CE structures and cavity dimensions in their "size-match" Li+ selectivity.