Synthesis of citric acid modified β‐cyclodextrin/activated carbon hybrid composite and their adsorption properties toward methylene blue

Citric acid cross-linked β-cyclodextrins: A review of preparation and environmental/biomedical application

The β-cyclodextrins (β-CD) are biocompatible macrocyclic candidates for the preparation of various composites with enhanced functions. While nontoxic and biodegradable citric acid (CA) is the favorite crosslinking agent for fabricating hierarchical advanced structures. The carboxyl and hydroxyl groups on CA can serve as "structural bridges" and enhance the solubility of β-CD. Leading to the construction of CA cross-linked β-CD with marvelous complicated structures and targeted functions. Here, we directly categorized the grafted composite materials into two main types such as organic and inorganic materials. Particularly, some representative composite materials are listed and analyzed in detail according to their preparation, advantages of unique characteristics, as well as the possible applications in environmental and biomedical fields such as adsorption of pollutants, sensors, and biomedical applications.

Adsorption of Methylene Blue from Aqueous Solution Using Gelatin-Based Carboxylic Acid-Functionalized Carbon Nanotubes@Metal–Organic Framework Composite Beads

Highlights

A new gelatin composite was used to remove methylene blue.

The adsorbent was composed of carbon nanotubes, a metal–organic framework and gelatin.

The adsorbent had a simple preparation process and was friendly to the environment.

The fixation of carbon nanomaterials with gelatin as the substrate avoided secondary pollution.

Using carbon nanotubes as the intermediate improved the adsorption capacity.

Abstract

A novel gelatin-based functionalized carbon nanotubes@metal–organic framework (F-CNTs@MOF@Gel) adsorbent was prepared by the green and simple method for the adsorption of methylene blue (MB). Cu-BTC (also known as HKUST-1) was selected as the MOF type. F-CNTs@Cu-BTC particles were fixed by gelatin, thus avoiding the secondary pollution of carbon nanomaterial particles to the environment. CNTs were used as the connecting skeleton to make more effective adsorption sites exposed on the surface of the internal pore structure of the adsorbent. In this paper, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), powder X-ray diffraction (XRD), thermogravimetry (TGA) and BET analysis methods were used to characterize the new adsorbent. The effects of time, temperature, pH, dosage and initial concentration on the adsorption process were investigated by batch adsorption experiments. The adsorption mechanism was further analyzed by several commonly used kinetic and isotherm models, and the reliability of several fitting models was evaluated by the Akaike information criterion (AIC), Bayesian information criterion (BIC) and Hannan information criterion (HIC). After five regeneration experiments, the adsorbent still had 61.23% adsorption capacity. In general, the new adsorbent studied in this paper has an optimistic application prospect.

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.

Study on the Adsorption Performance of Casein/Graphene Oxide Aerogel for Methylene Blue

Casein (CS) and graphene oxide (GO) were employed for the fabrication of a casein/graphene oxide (CS/GO) aerogel by vacuum freeze drying. Fourier transform infrared spectroscopy, scanning electron microscopy, surface area and micropore analysis (BET), and thermogravimetric analysis were used to characterize the specific surface area, structure, thermal stability, and morphology of the CS/GO aerogel. The influence of experimental parameters such as the GO mass fraction in the aerogel, metering of the adsorbent, pH, contact time, and temperature on the adsorption capacity of the CS/GO aerogel on methylene blue (MB) was also investigated. According to Langmuir isotherm determination, the maximum removal rate of MB from the CS/GO aerogel was 437.29 mg/g when the temperature was 293 K and pH was 8. Through kinetic and thermodynamic studies, it is found that adsorption follows a pseudo-second-order reaction model and is also an exothermic and spontaneous process.

Hydroxyl porous aromatic frameworks for efficient adsorption of organic micropollutants in water

Environmental pollution is an important issue in sustainable human development. People give great importance to environmental protection, especially with regards to increasingly scarce water resources. Water pollution is becoming more and more serious due to the existence of organic micropollutants. As a platform with good stability, porous aromatic frameworks (PAFs) have been widely studied. Because of their high surface area and thermal stability, they are considered to be a good sewage treatment agent. However, the aromatic nature of PAFs makes their skeletons mostly hydrophobic. This characteristic of PAFs seriously affects their diffusion rate in water as an adsorbent, resulting in a low adsorption rate. In this work, we synthesized a series of hydroxyl functionalized porous aromatic frameworks (PAF-80, PAF-81, and PAF-82) via the Sonogashira–Hagihara cross-coupling reaction, which created polar motifs on the hydrophobic surfaces, and carried out adsorption tests on typical organic micropollutants in water such as bisphenol A (BPA), 2-naphthol (2-NO) and p-chloroxylenol (PCMX). Among the three PAFs, PAF-82 exhibited the highest BET surface area, polar active sites, and a high degree of conjugation, which led to the best adsorption performance compared to that of PAF-80 and PAF-81. The Langmuir adsorption capacity of PAF-82 for BPA, 2-NO, and PCMX is 689 mg g⁻¹, 431 mg g⁻¹, and 480 mg g⁻¹, respectively, which surpasses most previously reported adsorbents. In addition, after 5 cycles of regeneration, it still maintained a high removal rate for pollutants. The obtained results reveal that micropollutant adsorption in water is not controlled by a single factor, but is the result of a synergy of multiple factors, including specific surface area, polar functional groups, pore size distribution, and skeleton conjugation. Our study has revealed the great potential of hydroxyl PAFs for efficient adsorption of organic micropollutants in water.

A series of hydroxyl functionalized PAF materials (PAF-80, PAF-81, and PAF-82) were synthesized, which create polar channels to the hydrophobic surfaces and explored as efficient adsorption of organic micropollutants in water.