Salt-Responsive Fe₃O₄ Nanocomposites and Phase Behavior in Water

The ability to achieve exquisite control over polymer building blocks within multicompartment magnetite nanocomposites (NCs) to afford predictable and ordered packing hierarchical structures remains a significant challenge for the design of NCs. Thus, there is an urgent need to develop new types of nano-dimensional assemblies that undergo responsive shape shift, size, phase, and morphological transitions, especially for processes that are triggered by biologically relevant stimuli such as ionic gradients to meet the demand for diverse applications. Accordingly, we report an unprecedented concept for the preparation of salt-responsive magnetite/polyaniline composite nanoassemblies with chemically distinct dual-compartment structures. The size, shape, and nano-dimensional phase separation of the PANI assemblies within NCs were adjusted in a facile manner with incremental changes in salt gradients using NaCl­(aq). Composition effects bestow desirable diversiform shape, size, and phase behavior of the incorporated conductive polymer via dynamic H-bonding. The size, shape, and superparamagnetic character of iron oxide nanoparticles (IONPs) are unaffected by a “salting-in” process. The mechanism, gradual morphological evolution, interchangeable nanophase separation, and ion-stimulated disassembly of PANI building blocks for these magneto/ion-responsive polymer-composites at elevated ionic strength are strongly supported by DLS, Raman spectroscopy, TEM, and equilibrium dye (MB/MO) recognition studies

Nuclear Magnetic Resonance Investigation of the Fractionation of Water–Ethanol Mixtures with Cellulose and Its Cross-Linked Biopolymer Forms

Cellulose (CE) was cross-linked with epichlorohydrin (EPI) at variable compositions, and the fractionation properties were investigated in binary water–ethanol (W–E) solutions, including the pure solvent systems. The relative uptake of each solvent was measured using quantitative ¹H nuclear magnetic resonance (qNMR) spectroscopy. This study highlights the utility of qNMR as a rapid screening method for estimation of solvent selective fractionation in binary mixtures. The uptake properties of CE–EPI cross-linked polymers with ethanol and water were well-described using the Sips isotherm model. Modeling shows that the monolayer surface coverage (<i>Q</i>_m) of ethanol and water onto the polymer materials covers a range (1.13–2.44 g/g) of values with heterogeneous adsorption behavior, in agreement with the Sips exponential fitting parameter (<i>n</i>_s ≠ 1). The CE–EPI adsorbents display unique fractionation with water and ethanol from binary solutions, as evidenced by the relative selectivity (<i>R</i>_selectivity) value in binary W–E solvent systems. The <i>R</i>_selectivity [<i>Q</i>_m(W)/<i>Q</i>_m(E)] values at saturative conditions varied (from 1.10 to 2.03) and further illustrate that CE materials display molecular selective solvent fractionation in binary W–E solutions. This study provides a greater molecular level understanding for the adsorptive uptake properties of CE that are relevant to developing CE-based adsorbent technology for the fractionation of biofuels and related chemical separations

NMR Investigation of the Fractionation of Water–Ethanol Mixtures with Starch and Its Cross-Linked Forms

The development of low-cost materials and efficient methods for the fractionation of water and ethanol in binary mixtures is of great interest for food processing and biofuels production. Herein, we report a systematic study of a series of linear and branched starch-based biopolymers along with their modified forms via cross-linking with epichlorohydrin (EPI) at variable EPI composition. The fractionation properties of these adsorbent materials were studied in binary ethanol–water solutions and compared against those of neat solvents. ¹H NMR spectroscopy was used to assess the binary solvent composition for the study of isotherms for the respective solvent components. The monolayer adsorption capacity (<i>Q</i>_m) for starch and its cross-linked forms varied from 0.01 to 2.70 g·g^–1 for water and ethanol in binary mixtures according to the Sips isotherm model. The fractionation selectivity [<i>R</i>_selectivity; <i>Q</i>_m(W)/<i>Q</i>_m(E)] of starch-EPI adsorbents for water (W) and ethanol (E) in binary mixtures ranges from 3.8 to 80. At saturative conditions in binary W–E mixtures, the <i>R</i>_selectivity isotherm parameter reveals unique solvent-selective uptake that depends on the amylose versus amylopectin composition and the EPI content of these cross-linked materials. The unique water uptake properties of starch and its cross-linked forms illustrate the role of textural properties and relative hydrophobic character of the polymer network based on the selective adsorption properties. Starch and its modified forms represent a promising class of adsorbent materials and a sustainable technology for the adsorptive-based fractionation of W–E binary mixtures

Magnetite/Polymer Brush Nanocomposites with Switchable Uptake Behavior Toward Methylene Blue

The <i>grafting from</i> approach was used to prepare pH-responsive polyacid brushes using poly­(itaconic acid) (PIA) and poly­(acrylic acid) (PAA) at the amine functional groups of chitosan. Hybrid materials consisting of polymer brushes and magnetite nanoparticles (MNPs) were also prepared. The products were structurally characterized and displayed reversible pH-responsive behavior and controlled adsorption/desorption of methylene blue (MB). Switchable binding of MB involves cooperative effects due to conformational changes of brushes and swelling phenomena in solution which arise from response to changes in pH. Above the p<i>K</i>_a, magnetic nanocomposites (MNCs) are deprotonated and display enhanced electrostatic interactions with high MB removal efficiency (>99%). Below the p<i>K</i>_a, MNCs undergo self-assembly and release the cationic dye. The switchable binding of MB and the structure of the polymer brush between collapsed and extended forms relate to changes in osmotic pressure due to reversible ionization of acid groups at variable pH. Reversible adsorption–desorption with variable binding affinity and regeneration ability was demonstrated after five cycles

Flocculation Optimization of Orthophosphate with FeCl₃ and Alginate Using the Box–Behnken Response Surface Methodology

A coagulation–flocculation process was employed to remove orthophosphate (P_i) in aqueous media using a ferric chloride (FeCl₃) and alginate flocculant system. Jar tests were conducted, and the response surface methodology (RSM) was used to optimize the P_i removal variables. The Box–Behnken design was used to evaluate the effects and interactions of four independent variables: pH, FeCl₃ dose, alginate dose, and settling time. The RSM analysis showed that the experimental data followed a quadratic polynomial model with optimum conditions at pH 4.6, [FeCl₃] = 12.5 mg·L^–1, [alginate] = 7.0 mg·L^–1, and a 37 min settling time. Optimum conditions led to a P_i removal of 99.6% according to the RSM optimization, in good agreement with experimental removal (99.7 ± 0.7%), at an initial concentration of 10.0 mg P_i/L. The isotherm adsorption data at the optimized conditions were analyzed by the pseudo-first-order (PFO) and pseudo-second-order (PSO) kinetic models and several isotherms models (Langmuir, Freundlich, and Sips). The PFO kinetic model and Langmuir isotherm model yielded the best fit to the isotherm results. The maximum adsorption capacity of the flocculant system was 83.6 mg·g^–1. The flocculation process followed electrostatic charge neutralization and an ion-binding adsorption mechanisms

Synthesis and characterization of surface-modified mesoporous silica materials with β-cyclodextrin

<p>Mesoporous silica materials containing microporous cavities provided by surface-bound β-cyclodextrin (CD ICS) were synthesized by co-condensation of a β-CD-functionalized triethoxysilane (CD ICL) with TEOS using dodecylamine, tetradecylamine, or hexadecylamine surfactants as structure directing agents. The incorporation of β-CD within the mesoporous framework was supported by IR, Raman, MALDI TOF MS, ¹³C solids CP-MAS NMR, and TGA results. Small-angle X-ray diffraction and nitrogen adsorption provide evidence of ordered silica mesostructured frameworks. For materials with similar CD loading, the textural properties (surface area and pore volume) doubled as the surfactant changed from dodecylamine (C12) to hexadecylamine (C16). The textural properties decrease with CD loading (2 to 6%). The sorption capacity of gas phase polar and apolar species (nitrogen and methyl chloride) varies along with the adsorption properties in aqueous solution toward <i>p</i>-nitrophenol according to the CD loading (2–6%) and surfactant template employed. Along with gas adsorption of model compounds, the structural effects relate to the surfactant alkyl chain length due to the structure directing effects of the C12 to C16 surfactants. This study reveals the structural contribution of surface modification and framework incorporation of β-cyclodextrin with mesoporous silica framework materials.</p

Preparation and Characterization of a Polymer-Based “Molecular Accordion”

A urethane-based polymer material, denoted HDI-1, was obtained from the addition reaction of β-cyclodextrin (β-CD) with 1,6-hexamethylene diisocyanate (HDI) at the 1:1 mole ratio. In aqueous solution and ambient temperature conditions, HDI-1 adopts a compact (coiled) morphology where the cross-linker units become coiled and are partially self-included in the annular hydroxyl (interstitial) region of β-CD. As the temperature is raised or as <i>p</i>-nitrophenol (PNP) was included within the β-CD cavity and the noninclusion sites of the polymer, an extended (uncoiled) morphology was adopted. The equilibrium distribution between the extended and the compact forms of HDI-1 is thermally and chemically switchable, in accordance with the hydration properties and host–guest chemistry of this responsive polymer system. The molecular structure of this water-soluble urethane polymer and its host–guest complexes with PNP were investigated using spectroscopic (Raman, ¹H NMR, induced circular dichroism), dynamic light scattering (DLS), and calorimetric (DSC) methods in aqueous solution at ambient pH, and compared with native β-CD. This study reports on the unique supramolecular properties of a polymer that resembles a thermally and chemically responsive “molecular accordion”

Modular Cross-Linked Chitosan Beads with Calcium Doping for Enhanced Adsorptive Uptake of Organophosphate Anions

Chitosan beads were cross-linked at variable composition with glutaraldehyde (GA) and epichlorohydrin (EP), respectively. The beads were post-treated by impregnation with a CaCl₂ solution and characterized to evaluate the structure and physicochemical effect of calcium doping. The bead adsorption properties were studied at pH 8.5 with <i>p</i>-nitrophenyl phosphate (PNPP), where beads cross-linked with GA showed higher uptake relative to beads cross-linked with EP. Calcium doping of GA beads showed a 4-fold greater uptake (0.97 mmol g^–1) over non-cross-linked (NCL) beads (0.23 mmol g^–1). By comparison, EP-based beads with calcium doping showed a 2-fold enhancement for the uptake of PNPP (0.90 mmol g^–1) over NCL beads. This work illustrates the utility of cross-linking and calcium doping as modular strategies for tuning the adsorption behavior of chitosan-based beads. Calcium-doped beads cross-linked with glutaraldehyde showed favorable adsorption–desorption properties where the uptake capacity of PNPP remained relatively constant (19.6–17.5%) over several regeneration cycles. The results of this work contribute significantly to the development of advanced materials for the controlled uptake and treatment of waterborne phosphate species

Renewable Starch Carriers with Switchable Adsorption Properties

This Research Article describes a systematic study on the structure and sorption properties of Carnation-based starch-particles (SPs) by various techniques. Structural characterization of the SPs utilized spectroscopy (¹H NMR and FT-IR), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). The sorption properties of the SPs were characterized by solvent swelling and uptake isotherms with cationic adsorbates at equilibrium and kinetic conditions. The surface area (SA; ∼3–588 m²/g) of the SPs was estimated using nitrogen gas and dye adsorption isotherm methods, where the range in SA was related to solvent swelling effects on the textural properties. The SPs contain lipid constituents according to results obtained by ¹H NMR spectroscopy, DSC, and confocal laser microscopy (CLM) with iodine staining. The unique solvent swelling properties of the SPs reveal greater swelling in water over ethanol. SPs display preferential equilibrium uptake of methylene blue (MB; <i>Q</i>_m ≈ 716 mg/g) over cetylpyridinium bromide (CPB; <i>Q</i>_m ≈ 292 mg/g). The uptake of MB was reduced by an order of magnitude (<i>Q</i>_m ≈ 67 mg/g) when the SPs were doped with CPB, further revealing the role of competitive adsorption and similar binding modes for MB and CPB. The doping of SPs with CPB provide a facile approach for alteration of the surface functional properties such as the hydrophile–lipophile character, surface charge, and hydration properties of the SPs. Evidence of monolayer and multilayer adsorption of CPB onto SPs lead to switchable adsorption properties where such amphiphile surface patterning can be harnessed to yield materials with unique controlled-release properties for diverse chemical systems according to tunable surface charge using self-assembly

Biopolymer Flocculants and Oat Hull Biomass To Aid the Removal of Orthophosphate in Wastewater Treatment

This study reports on the removal of orthophosphate (P_i) by coagulation–flocculation with variable combinations of alum, biopolymers, and biomass. The combinatorial effects of these coagulant aids were evaluated for single, binary, and ternary systems. The role of pH, component dosages, and P_i concentration on the coagulation–flocculation efficacy was evaluated. There was an optimal dosage of alum (30 mg/L) while alginate and chitosan were 15 mg/L. P_i removal was 86% for alum and 98% for ternary systems containing chitosan and alginate where [P_i] = 10–11 mg P_i/L. P_i removal for the alum–alginate–chitosan ternary system was more efficient than that for the binary systems, especially at pH 6–7, where reduced efficiency occurred at pH > 7.5. P_i removal was independent of concentration except at lower levels, [P_i] < 10 mg/L. The alum–refined oat hull binary system was 99% effective for P_i removal, especially when [P_i] = 25 mg/L, with greater removal over the use of oat hulls alone