New recyclable and functionalized chitosan-based polyurethane foams for effective and incessant removal of Orange II (OII) and Rhodamine B (RhB) dyes from water

The development of new efficient materials for the removal of water-soluble toxic organic dyes has been one of the focused research areas in the recent past. There is a strong demand for the new materials as most of the reported techniques/materials suffer from serious limitations. In this regard, a series of flexible chitosan-based task-specific polyurethane foams (PUCS-GP, PUCS-CA-GP, PUCS-TA-GP, and PUCS-GA-GP) associated with naturally available hydroxycarboxylic acids was developed. The basis for the preparation of these task-specific and functionalized PU foams is to possess amine groups for trapping the anionic dyes (example: Orange II denoted as OII) and carboxylic acid groups for attracting the cationic dyes (example: Rhodamine B denoted as RhB) under specified pH conditions. Batch adsorption experiments were conducted to assess and improve various parametric conditions. The experimental results revealed that the adsorption kinetics closely agree with the pseudo-second-order model having a maximum sorption capacity of 38.3 mg/g at pH 3 for OII on PUCS-GP and 48.4 mg/g at pH 6 for RhB on PUCS-CA-GP. Furthermore, the adsorption process was described by isotherms, kinetic equations and thermodynamic parameters (ΔG°, ΔH° and ΔS°). Notably, the regeneration of OII and RhB dyes from the exhausted PUCS-GP and PUCS-CA-GP materials was effectively accomplished. The recovered PUCS-GP shows >90 % OII and PUCS-CA-GP displays >70 % RhB removal efficiency even after twelve adsorption-desorption processes under mild conditions, demonstrating excellent recyclability/durability. The advantages of these functionalized foam materials are facile preparation, high adsorption capacity, good reusability, and very efficient removal of organic dyes from wastewater streams.

Two-Dimensional Cu-Based MOF for Selective Staining of the Cellular Nucleus through Fluorescence Imaging and Selective Sorption of Dye Molecules in Aqueous Medium

A two-dimensional copper-based metal-organic framework, [Cu(C23H14O6)(C10H8N2)2]·H2O·DMSO, 1, was synthesized using pamoic acid (C23H16O6) and 4,4'-bipyridine (C10H8N2) as an organic ligand and Cu(II) as a metal ion. Single-crystal structure X-ray diffraction studies of the as-synthesized compound showed a two- dimensional structure with free hydroxyl groups. Upon excitation at 370 nm, the aqueous dispersion of [Cu(C23H14O6)(C10H8N2)2]·H2O·DMSO, 1, showed emission centered at 525 nm resulting from the intraligand energy transfer. Fluorescence microscopic experiments using a human epithelioid cervix carcinoma HeLa cell line were carried out, clearly showing that our compound selectively stained the cellular nucleus. To utilize the porous nature of [Cu(C23H14O6)(C10H8N2)2]·H2O·DMSO, 1, its dye sorption behavior in aqueous solution was determined, and a high affinity for methylene blue (MB) dye was confirmed. Our synthesized compound sorbed 88% MB dye with an initial concentration of 32 mg L-1, and its sorption capacity for MB was found to be 29.79 mg g-1. The possible mechanism of the dye sorption behavior was discussed in terms of the size and charge of dye molecules with respect to molecular-level interactions between the framework and the dye molecules.

Superior Adsorptive Removal of Anionic Azo Dyes from Aqueous Solutions Using Sulfonic Acid Group-Modified MIL-101@Graphene Oxide Composite

It is critical to remove organic contaminants from wastewater released by the printing and dyeing industry for addressing water pollution issue. Therefore, the fabrication of new adsorbents with excellent removal efficiencies is an urgent task. A composite of MIL-101 partially functionalized with -SO3H (MIL-101-SO3H) and graphene oxide (GO) was prepared by assembling MIL-101-SO3H truncated octahedrons on the GO framework. The synthesized MIL-101-SO3H@GO has a superior adsorption efficiency for anionic azo dyes. The maximum adsorption capacities of MIL-101-SO3H@GO-1 for Congo red, methyl orange, acid orange 7, and acid orange G reached 2711.3, 818.8, 551.2, and 319.8 mg/g, respectively, which are considerably higher than those obtained using unmodified MIL-101. This is because additional interactions that promote azo dye adsorption, such as hydrogen bonding between the dye and the sulfonic acid groups of MIL-101-SO3H or the carboxyl groups of GO, were induced, and agglomerate pores that accommodated the dye were formed in the composite. The ultrahigh removal efficiency of the composite for azo dyes is mainly driven by hydrogen bonding, electrostatic interactions, π-π stacking between the MIL-101-SO3H@GO and dye molecules, synergistic interactions at the interface of GO and MIL-101-SO3H microcrystals, and the pore-filling effect. Understanding these driving forces for dye adsorption can contribute to the development of sustainable and functionally modified metal-organic framework composite adsorbents.

Facile fabrication of amino-functionalized MIL-68(Al) metal-organic framework for effective adsorption of arsenate (As(V))

An amino-functionalized MIL-68(Al) metal–organic framework (amino-MIL-68(Al) MOF) was synthesized by solvothermal method and then characterized by FESEM, XRD, FTIR, EDX-mapping, and BET-BJH techniques. In order to predict arsenate (As(V)) removal, a robust quadratic model (R² > 0.99, F-value = 2389.17 and p value < 0.0001) was developed by the central composite design (CCD) method and then the genetic algorithm (GA) was utilized to optimize the system response and four independent variables. The results showed that As(V) adsorption on MOF was affected by solution pH, adsorbent dose, As(V) concentration and reaction time, respectively. Predicted and experimental As(V) removal efficiencies under optimal conditions were 99.45 and 99.87%, respectively. The fitting of experimental data showed that As(V) adsorption on MOF is well described by the nonlinear form of the Langmuir isotherm and pseudo-second-order kinetic. At optimum pH 3, the maximum As(V) adsorption capacity was 74.29 mg/g. Thermodynamic studies in the temperature range of 25 to 50 °C showed that As(V) adsorption is a spontaneous endothermic process. The reusability of MOF in ten adsorption/regeneration cycles was studied and the results showed high reusability of this adsorbent. The highest interventional effect in inhibiting As(V) adsorption was related to phosphate anion. The results of this study showed that amino-MIL-68(Al) can be used as an effective MOF with a high surface area (> 1000 m²/g) and high reusability for As(V)-contaminated water.

Synthesis, characterization and dye adsorption property of a 2D nickel(ii)-coordination polymer constructed from tetracarboxylic acid and 1,3-bis(imidazol-1-yl-methyl)benzene

A new Ni(ii)-coordination polymer, {[Ni2(μ4-L)(μ-mbix)2]·2H2O}n (1), was synthesized; the methylene blue (MB) dye adsorption property of compound 1 was examined and the maximum MB adsorption capacity of the compound is 194.67 mg g−1 at pH = 10 at room temperature.