Cyclodextrin/silica hybrid adsorbent for removal of methylene blue in aqueous media

Adsorption studies of the hybrid material obtained from the functionalization of silica with alfa and gamma cyclodextrins

The generation of residues containing dyes by industrial sectors has been mobilizing scientists to develop methodologies capable of treating water containing these contaminants. Adsorption is an option to remove these molecules from the aqueous medium and, for this study, the composites between silica and cyclodextrins alpha (α-CDSI) and gamma (γ-CDSI) were used to capture methylene blue. Adsorption was spontaneous for both composites (ΔG < 0) and characterized as exothermic and of a physical nature, with ΔH of -17.68 and -12.13 kJ mol^-1 for α-CDSI and γ-CDSI, respectively. Adsorption took place over a wide pH range, with an efficiency of approximately 96%, reaching equilibrium at 5 minutes. The adsorption kinetics was described by the pseudo-second-order model (R² > 0.999) and the adsorption isotherms showed that the process must occur mainly by dye complexation in the cyclodextrins cavities. The q_m values obtained were 210.8 and 205.2 mg g^-1 for α-CDSI and γ-CDSI, respectively; the Sips and Temkin models were the ones that best fit the experimental data. The deposition and interactions of the dye with the adsorbent surface were confirmed by the analysis of the IR spectra. Desorption studies showed that the material maintained its adsorption capacity of around 90% until the fourth adsorption/desorption cycle. Thus, the materials produced showed to efficiently remove methylene blue and that composite reuse is a viable process for application in dye removal.

Adsorption of Cationic Pollutants from Water by Cotton Rope Coated with Cyclodextrin Polymers

The contamination from perilous organic compounds (pesticide and dyes) in water generates a significant problem for the environment and humans. A modified textile was prepared by a coating of anionic cyclodextrin polymer, obtained from the cross-linking between citric acid and β-cyclodextrin in the presence of poly (vinyl alcohol), on the cotton cord for cationic pollutant removal from an aqueous solution. Its physicochemical properties were also characterized by gravimetry, titration, stereomicroscopy, SEM, TGA, ¹³C NMR, and ATR-FTIR. The CC2 system exhibited 79.2% coating yield, 1.12 mmol/g COOH groups, 91.3% paraquat (PQ) removal, 97.0% methylene blue (MB) removal, and 98.3% crystal violet (CV) removal for 25 mg/L of initial concentration. The kinetics was fitted to the pseudo-second-order model using 6 h of contact time. The isotherm was suitable for the Langmuir isotherm with a maximum adsorption of 26.9 mg/g (PQ), 23.7 mg/g (MB), and 30.3 mg/g (CV). After 120 h of contact time in water and 5% v/v of HCI in ethanol, the weight loss was 7.5% and 5.6%, respectively. Finally, the recyclability performance reached 84.8% (PQ), 95.2% (MB), and 96.9% (CV) after five reuses.

Adsorption Properties of β- and Hydroxypropyl-β-Cyclodextrins Cross-Linked with Epichlorohydrin in Aqueous Solution. A Sustainable Recycling Strategy in Textile Dyeing Process

β-cyclodextrin (β-CD) and hydroxypropyl-β-cyclodextrin (HP-β-CD) were used to prepare insoluble polymers using epichlorohydrin as a cross-linking agent and the azo dye Direct Red 83:1 was used as target adsorbate. The preliminary study related to adsorbent dosage, pH, agitation or dye concentration allowed us to select the best conditions to carry out the rest of experiments. The kinetics was evaluated by Elovich, pseudo first order, pseudo second order, and intra-particle diffusion models. The results indicated that the pseudo second order model presented the best fit to the experimental data, indicating that chemisorption is controlling the process. The results were also evaluated by Freundlich, Langmuir and Temkin isotherms. According to the determination coefficient (R²), Freunlich gave the best results, which indicates that the adsorption process is happening on heterogeneous surfaces. One interesting parameter obtained from Langmuir isotherm is qmax (maximum adsorption capacity). This value was six times higher when a β-CDs-EPI polymer was employed. The cross-linked polymers were fully characterized by nuclear magnetic resonance (NMR), Fourier transform infrared spectroscopy (FTIR), thermal gravimetric analysis (TGA). Also, morphology and particle size distribution were both assessed. Under optimized conditions, the β-CDs-EPI polymer seems to be a useful device for removing Direct Red 83:1 (close 90%), from aqueous solutions and industrial effluents. Complementarily, non-adsorbed dye was photolyzed by a pulsed light driven advanced oxidation process. The proposed methodology is environmental and economically advantageous, considering the point of view of a sustainable recycling economy in the textile dyeing process.

Structural Variety and Adsorptive Properties of Mesoporous Silicas with Immobilized Oligosaccharide Groups

In this research, we report on the synthesis of mesoporous silicas with various quantities of immobilized oligosaccharide groups and different pore ordering degree. The hydrothermal co-condensation of tetraethyl orthosilicate and β-cyclodextrin-containing organosilane in the presence of cetyltrimethylammonium bromide template was employed. The purpose of this investigation was to show the opportunity of increasing β-cyclodextrin content in silica matrix by changing the molar ratio of initial reagents during organosilane synthesis and to determine whether the enhancing of immobilized groups on the surface influences on model aromatic compound adsorption from water. It was prepared several β-cyclodextrin-organosilanes by modification of (3-aminopropyl)triethoxysilane with oligosaccharide (the molar composition of reaction mixtures were 1:1, 3:1, and 5:1) with using N,N'-carbonyldiimidazole as linking agent. Three types of MCM-41 materials were obtained with 0.018, 0.072, and 0.095 mmol g^-1 β-cyclodextrin-group loading according to chemical analysis of silicas. The IR spectroscopy and potentiometric titration were also performed to confirm the presence of functional groups in the silica matrix. Nitrogen sorptometry experiments exhibited the decrease of high surface area (from 812 to 457 m² g^-1) and the average pore diameter (from 1.06 to 0.60 cm³ g^-1) of synthesized silicas with increasing of immobilized oligosaccharide groups. The influence of β-cyclodextrin-organosilane presence on the forming of hexagonally arranged porous structure of silicas was evaluated by X-ray diffraction and TEM analyses. As the loading of oligosaccharide groups increases in obtained silicas, the (100) reflex in diffraction patterns is even less intense and broader, denoting the decrease of long-range pore ordering. Adsorption experiments were carried out to study the effect of β-cyclodextrin groups' attendance in silica matrix on benzene uptakes from aqueous solutions. Experimental kinetic curves of benzene adsorption on synthesized silicas were compared with theoretical models of Lagergren and Ho-McKay for pseudo-first and pseudo-second-order processes. Langmuir and Freundlich isotherm models were used to evaluate adsorption processes and parameters. Obtained β-cyclodextrin-containing MCM-41 silicas demonstrate adsorption level performance of known samples and could be very promising for benzene uptakes from aqueous solutions in water treatment processes.

Simultaneous removal of five triazole fungicides from synthetic solutions on activated carbons and cyclodextrin-based adsorbents

In this study, an adsorption-oriented process for the removal of fungicides from polycontaminated aqueous solutions was applied. To remove triazole fungicides from aqueous mixtures of propiconazole (PROPI), tebuconazole (TEBU), epoxiconazole (EPOXI), bromuconazole (BROMU) and difenoconazole (DIFENO), several materials used as adsorbents were compared using batch experiments, namely two conventional activated carbons (ACs) and five nonconventional cross-linked cyclodextrin (CD)-based materials (α-CDP, β-CDP, γ-CDP, αβγ-CDP mixture, and hydroxypropyl-β-CDP). This article presents the abatements obtained. As expected, ACs exhibited the highest levels of triazole fungicide removal: the treatment lowered the five azoles by more than 99%, and adsorption was non-selective. Concerning CD-based materials employed for the first time for the removal of fungicides from polycontaminated aqueous solutions, results were interesting in particular for hydroxypropyl-β-CDP: 1 g of adsorbent placed in 1 L of solution containing 1 mg of each of five triazoles (5 mg in total) was able to remove over half of the fungicide amount (2.97 mg). The order obtained was the following: BROMU << PROPI ≅ EPOXI < TEBU << DIFENO. This indicates that, in the mixture studied, strong competition prevailed among fungicides for the binding sites.

Surface immobilization of β-cyclodextrin on hybrid silica and its fast adsorption performance of p-nitrophenol from the aqueous phase

Fast adsorption of p-nitrophenol was achieved through surface immobilization of β-cyclodextrin onto hybrid silica and maintenance of its hydrophobic cavity.